CN113193586B - Flexible direct current compensation control method and system - Google Patents
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Abstract
The invention relates to a flexible direct current compensation control method and a system, when a voltage measurement value of a flexible direct current fault side converter bus is greater than a first voltage set threshold value after a receiving-end power grid fails, calculating an active current compensation quantity and a reactive current compensation quantity of a flexible direct current fault side converter station to obtain a compensated active current instruction value and a compensated reactive current instruction value, controlling the flexible direct current fault side converter station according to the first active current instruction value and the first reactive current instruction value, and improving the reactive power support capability of a flexible direct current system and the capability of the system for resisting phase change failure under alternating current fault by reasonably correcting the active and reactive current instruction values of the flexible direct current converter station; when the voltage measured value is not greater than the first voltage set threshold value, an active current instruction value of the flexible direct current fault side converter station is set as a fault current limit value, and a reactive current instruction value is set as 0, so that overcurrent under the flexible direct current fault is restrained, and fault safe ride-through is realized.
Description
Technical Field
The invention relates to the technical field of flexible direct current control, in particular to a flexible direct current compensation control method and system.
Background
A conventional Direct Current transmission system (Line Committed Converter based High Voltage Direct Current, LCC-HVDC) adopts a power grid commutation Converter, has the advantages of large transmission capacity, controllable transmission power and the like, and is widely applied to large-capacity and long-distance transmission occasions; compared with a conventional Direct-Current transmission system, a flexible Direct-Current transmission system (MMC-HVDC) adopting the Modular Multilevel Converter has the advantages of active and reactive power control decoupling, does not have the risk of commutation failure, and is rapidly developed in recent years. Along with the construction of a flexible direct current transmission project, a hybrid multi-feed direct current transmission system of flexible direct current and original conventional direct current fed into the same alternating current system appears, a receiving-end power grid of the system has a plurality of inverter stations of different types, and the receiving-end power grid can present new operation characteristics through complex interaction among the inverter stations.
Due to the adoption of the semi-controlled device, when a receiving-end power grid fails to work and voltage drops, a conventional direct-current inverter station easily generates the problem of phase change failure, and a converter valve is locked to interrupt power transmission when the problem is serious. The flexible direct current has the four-quadrant power control capability, when the electrical distance between the flexible direct current and a conventional direct current converter station drop point is short, the flexible direct current can be used as reactive compensation to provide support for receiving-end power grid voltage during a fault, a new idea is provided for restraining conventional direct current commutation failure, and a series of related researches are developed in recent years. Most researches are carried out to calculate reactive compensation quantity based on a turn-off angle and a turn-off area during the conventional direct current fault period or the voltage variation of a converter station, the reactive compensation quantity is added into a flexible direct current reactive control loop, reactive power output during the flexible direct current fault period is improved, and the conventional direct current commutation failure suppression is realized. However, when the flexible direct current fails to support the commutation, the converter station needs to increase reactive current, and in order to ensure that the bridge arm current does not overflow, the active current output quantity is reduced under the action of current amplitude limiting control, so that the active power output by the converter station is reduced. Due to unbalanced power, the sub-module capacitor of the flexible direct current converter station is charged, the direct current voltage is increased and exceeds the safe operation range, so that the direct current system is stopped under the action of the protection device, the transient characteristic of a receiving-end power grid under the fault is also deteriorated, and the problems of power angle and frequency stability are caused.
Therefore, when a flexible direct current control strategy is improved, how to reasonably distribute active current and reactive current of flexible direct current in an alternating current fault period is urgently needed to be solved, so that the flexible direct current has the capability of effectively inhibiting conventional direct current commutation failure, and meanwhile, a grid-connected state and a fault can be maintained to safely pass through.
Disclosure of Invention
The invention aims to provide a flexible direct current compensation control method and a flexible direct current compensation control system, which are used for improving the capability of resisting commutation failure of conventional direct current and the capability of passing through flexible direct current and alternating current faults.
In order to achieve the purpose, the invention provides the following scheme:
a flexible dc compensation control method, the method comprising:
judging whether a voltage measurement value of a flexible direct current fault side current conversion bus is larger than a first voltage set threshold value after a receiving end power grid fails, and obtaining a judgment result;
if the judgment result shows that the current value of the direct current voltage of the flexible direct current system is greater than the current value of the direct current voltage of the direct current converter station of the power grid phase-change converter type direct current transmission system, calculating active current compensation quantity and reactive current compensation quantity of the flexible direct current fault side converter station according to the direct current voltage square term variable quantity of the flexible direct current system and the alternating current voltage variable quantity of the direct current converter station of the power grid phase-change converter type direct current transmission system;
obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and determining the compensated active current instruction value and the compensated reactive current instruction value as a first active current instruction value and a first reactive current instruction value;
controlling the flexible direct current fault side converter station according to the first active current instruction value and the first reactive current instruction value until a protection device in a receiving end power grid performs fault removal;
if the judgment result shows that the current is not the fault current limit value, setting the active current instruction value of the flexible direct current fault side converter station as the fault current limit value, setting the reactive current instruction value as 0, and determining the active current instruction value and the reactive current instruction value as a second active current instruction value and a second reactive current instruction value;
and controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is greater than a second voltage set threshold value.
Further, the calculating active current compensation amount and reactive current compensation amount of the flexible dc fault side converter station according to the variation of the dc voltage square term of the flexible dc system and the variation of the ac voltage of the dc converter station of the grid phase-change converter type dc transmission system specifically includes:
according to the variable quantity of the direct current voltage square term of the flexible direct current system, a formula is utilized
Calculating the active current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i dref For an active current compensation, T d2 Is the active current compensation factor and is,
is the variation of the square term of the direct current voltage, and s is a complex variable;
according to the alternating voltage variation of a direct current converter station of a power grid phase-change converter type direct current transmission system, a formula delta i is utilized qref ＝-K q2 ΔV lcc Calculating reactive current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i qref For the amount of reactive current compensation, K q2 Δ V being a reactive current compensation coefficient lcc Is the alternating voltage variable quantity of a conventional direct current converter station.
Further, obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation amount and the reactive current compensation amount, specifically including:
calculating the compensated active current according to the active current compensation quantity, and calculating the compensated reactive current according to the reactive current compensation quantity;
according to the compensated reactive current, based on the dynamic current amplitude limiting control method, using the formula
Obtaining a reactive current instruction value after compensation; wherein i qref For the reactive current command value after compensation,
for reactive current correction, I max Limiting the allowed alternating current of the flexible direct current converter station line;
according to the compensated reactive current instruction value and the compensated active current, based on a dynamic current amplitude limiting control method, a formula is utilized
Obtaining a compensated active current instruction value; wherein i dref For the compensated active current command value,
is the active current compensation quantity.
Further, the controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value specifically includes:
inputting the second active current instruction value and the second reactive current instruction value into an inner ring current controller in the flexible direct current converter station control system, and outputting real current with active current as a fault current limit value and reactive current as 0;
calculating an active power instruction value of the flexible direct current non-fault side converter station according to the voltage measurement value of the flexible direct current fault side converter bus;
and inputting the active power instruction value into an outer ring power controller in a control system of the flexible direct current converter station, and controlling the output power to be the real direct current power of the active power instruction value.
Further, the calculating an active power instruction value of the flexible dc non-fault-side converter station according to the voltage measurement value of the flexible dc fault-side converter bus specifically includes:
when an inverter station in a flexible direct current system determines direct current voltage and a rectifier station determines active power, if an alternating current fault occurs on an inverter side, determining an active power instruction value on a rectifier side by utilizing a relation broken line graph of the voltage and the active power of the rectifier side according to a voltage measurement value of a current conversion bus on the flexible direct current fault side;
when the inverter station in the flexible direct current system determines active power and the rectifier station determines direct current voltage, if alternating current fault occurs at the rectifier side, the active power instruction value at the inverter side is determined by utilizing a relation broken line diagram of the inverter side voltage and the active power according to the voltage measurement value of the flexible direct current fault side current conversion bus.
Further, the controlling the flexible dc fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible dc fault side converter bus is greater than the second voltage setting threshold value, and then further comprising:
setting a second active current instruction value and a second reactive current instruction value to be restored to the values before the fault at the current instruction value restoration rate;
and inputting the active current instruction value before the fault and the reactive current instruction value before the fault into an inner ring current controller in a control system of the flexible direct current converter station, and controlling to output real current of which the active current instruction value before the fault and the reactive current instruction value before the fault are real currents.
A flexible dc compensation control system, the system comprising:
the judgment result obtaining module is used for judging whether the voltage measurement value of the flexible direct current fault side current conversion bus is larger than a first voltage set threshold value after the receiving end power grid fails, and obtaining a judgment result;
the active current compensation quantity and reactive current compensation quantity calculation module is used for calculating active current compensation quantity and reactive current compensation quantity of the flexible direct current fault side converter station according to the direct current voltage square term variable quantity of the flexible direct current system and the alternating current voltage variable quantity of the direct current converter station of the power grid commutation converter type direct current transmission system if the judgment result shows that the current is positive;
the first active current instruction value and first reactive current instruction value determining module is used for obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and determining the compensated active current instruction value and the compensated reactive current instruction value as a first active current instruction value and a first reactive current instruction value;
the first instruction value control module is used for controlling the flexible direct current fault side converter station according to the first active current instruction value and the first passive current instruction value until a protection device in a receiving-end power grid carries out fault removal action;
a second active current instruction value and second reactive current instruction value determining module, configured to set an active current instruction value of the flexible direct current fault-side converter station as a fault current limit value and determine the active current instruction value and the second reactive current instruction value when the determination result indicates that the active current instruction value and the reactive current instruction value are 0;
and the second instruction value control module is used for controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is greater than a second voltage set threshold value.
Further, the active current compensation amount and reactive current compensation amount calculation module specifically includes:
an active current compensation meter operator module for utilizing a formula according to the variation of the DC voltage square term of the flexible DC system
Calculating the active current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i dref For the active current compensation, T d2 Is the active current compensation factor and is,
is the variation of the square term of the direct current voltage, and s is a complex variable;
the reactive current compensation meter operator module is used for utilizing a formula delta i according to the alternating voltage change of a direct current converter station of the power grid phase-change converter type direct current transmission system qref ＝-K q2 ΔV lcc Calculating reactive current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i qref For reactive current compensation, K q2 Is idleCurrent compensation coefficient, Δ V lcc Is the alternating voltage variable quantity of a conventional direct current converter station.
Further, the module for determining the first active current instruction value and the first reactive current instruction value specifically includes:
the compensated reactive current calculation sub-module is used for calculating compensated active current according to the active current compensation quantity and calculating compensated reactive current according to the reactive current compensation quantity;
a compensated reactive current instruction value obtaining submodule for obtaining the reactive current instruction value according to the compensated reactive current based on the dynamic current amplitude limiting control method and by using a formula
Obtaining a reactive current instruction value after compensation; wherein i qref For the reactive current command value after compensation,
for reactive current correction, I max Limiting the allowed alternating current of the flexible direct current converter station line;
the compensated active current instruction value obtaining submodule is used for obtaining the compensated active current according to the compensated reactive current instruction value and the compensated active current based on a dynamic current amplitude limiting control method and by using a formula
Obtaining a compensated active current instruction value; wherein i dref For the compensated active current command value,
is the active current compensation quantity.
Further, the second instruction value control module specifically includes:
the real current output submodule is used for inputting the second active current instruction value and the second reactive current instruction value into an inner ring current controller in the flexible direct current converter station control system and outputting real current with active current as a fault current limit value and reactive current as 0;
the active power instruction value operator module is used for calculating an active power instruction value of the flexible direct current non-fault side converter station according to the voltage measurement value of the flexible direct current fault side converter bus;
and the real direct current power output submodule is used for inputting the active power instruction value to an outer ring power controller in the flexible direct current converter station control system and controlling the output power to be the real direct current power of the active power instruction value.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention provides a flexible direct current compensation control method, which is characterized in that after a receiving end power grid fails, a voltage measurement value of a flexible direct current fault side converter bus is compared with a first voltage set threshold, and when the voltage measurement value is greater than the first voltage set threshold, a control method under slight failure is selected: calculating the active current compensation quantity and the reactive current compensation quantity of the flexible direct current fault side converter station to obtain a compensated active current instruction value and a compensated reactive current instruction value, controlling the flexible direct current fault side converter station according to the first active current instruction value and the first reactive current instruction value, and improving the reactive power support capability of the flexible direct current system under the alternating current fault and the capability of the system for resisting commutation failure by reasonably correcting the active current instruction value and the reactive current instruction value of the flexible direct current converter station; when the voltage measured value is not larger than the first voltage set threshold, selecting a control method under the serious fault: and directly setting the active current instruction value of the flexible direct current fault side converter station as a fault current limit value and the reactive current instruction value as 0, controlling the flexible direct current fault side converter station, inhibiting overcurrent under the flexible direct current fault, and realizing fault safe ride-through.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.
Fig. 1 is a flowchart of a flexible dc compensation control method according to the present invention;
FIG. 2 is a schematic diagram of a flexible DC compensation control method according to the present invention;
FIG. 3 is a block diagram of an outer loop control of a flexible DC converter station according to the present invention;
FIG. 4 is a simplified equivalent triac system diagram provided in accordance with the present invention;
FIG. 5 is a schematic diagram of a dynamic current limit control method according to the present invention;
fig. 6 is a schematic diagram of an active power command value correction module at the rectifying side according to the present invention;
fig. 7 is a schematic diagram of an active power command value modification module on the inverter side according to the present invention;
FIG. 8 is a diagram illustrating a scenario for effect verification according to an embodiment of the present invention;
FIG. 9 is a comparison graph of voltage waveforms of a conventional DC converter bus during a minor fault as provided by an embodiment of the present invention;
FIG. 10 is a comparison of the compliant DC voltage current waveform during a light fault provided by an embodiment of the present invention; fig. 10 (a) is a comparison graph of dc voltage fluctuation during a slight fault, fig. 10 (b) is a comparison graph of dc current fluctuation during a slight fault, and fig. 10 (c) is a comparison graph of effective value fluctuation of bridge arm current during a slight fault;
FIG. 11 is a comparison graph of a soft DC voltage current waveform during a catastrophic failure provided by an embodiment of the present invention; fig. 11 (a) is a comparison diagram of dc voltage fluctuation during a critical fault, fig. 11 (b) is a comparison diagram of dc current fluctuation during a critical fault, fig. 11 (c) is a comparison diagram of effective value fluctuation of bridge arm current during a critical fault, and fig. 11 (d) is a comparison diagram of effective value fluctuation of ac line current of the converter station during a critical fault.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention aims to provide a flexible direct current compensation control method and a flexible direct current compensation control system, which are used for improving the capability of resisting commutation failure by conventional direct current and the capability of passing through flexible direct current and alternating current faults.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
A flexible dc compensation control method, as shown in fig. 1, the method includes:
s101, judging whether a voltage measurement value of a flexible direct current fault side converter bus is larger than a first voltage set threshold value after a receiving end power grid fails, and obtaining a judgment result;
s102, if the judgment result shows that the current is positive, calculating active current compensation quantity and reactive current compensation quantity of the flexible direct current fault side converter station according to the direct current voltage square term variable quantity of the flexible direct current system and the alternating current voltage variable quantity of the direct current converter station of the power grid phase-change converter type direct current transmission system;
s103, obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and determining the active current instruction value and the reactive current instruction value as a first active current instruction value and a first reactive current instruction value;
s104, controlling the flexible direct current fault side converter station according to the first active current instruction value and the first reactive current instruction value until a protection device in a receiving end power grid performs fault removal;
s105, if the judgment result shows that the current is not the fault current limiting value, setting the active current instruction value of the flexible direct current fault side converter station as the fault current limiting value, setting the reactive current instruction value as 0, and determining the active current instruction value and the reactive current instruction value as a second active current instruction value and a second reactive current instruction value;
and S106, controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is larger than a second voltage set threshold value.
Step S102, calculating active current compensation quantity and reactive current compensation quantity of a flexible direct current fault side converter station according to direct current voltage square term variable quantity of the flexible direct current system and alternating current voltage variable quantity of a direct current converter station of a power grid phase-change converter type direct current transmission system, and specifically comprising:
according to the variable quantity of the direct current voltage square term of the flexible direct current system, a formula is utilized
Calculating the active current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i dref For the active current compensation, T d2 Is the active current compensation factor for the current,
is the variation of the square term of the direct current voltage, and s is a complex variable;
according to the alternating voltage variation of a direct current converter station of a power grid phase-change converter type direct current transmission system, a formula delta i is utilized qref ＝-K q2 ΔV lcc Calculating reactive current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i qref For reactive current compensation, K q2 For the compensation coefficient of reactive current, Δ V lcc Is the alternating voltage variable quantity of a conventional direct current converter station.
Step S103, obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and specifically comprising:
calculating the compensated active current according to the active current compensation quantity, and calculating the compensated reactive current according to the reactive current compensation quantity;
according to the compensated reactive current, based on the dynamic current amplitude limiting control method, public current is utilizedIs of the formula
Obtaining a reactive current instruction value after compensation; wherein i qref For the compensated reactive current command value,
as a reactive current correction quantity, I max Limiting the allowed alternating current flowing for the flexible direct current converter station line;
according to the compensated reactive current instruction value and the compensated active current, based on a dynamic current amplitude limiting control method, a formula is utilized
Obtaining a compensated active current instruction value; wherein i dref For the compensated active current command value,
is the active current compensation quantity.
S in FIG. 2 f Indicating the mode of the fault control strategy. Steps S102-S104 correspond to (a) in fig. 2. First in fig. 2 represents a control strategy when no failure occurs.
Step S106, controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value, and specifically comprising the following steps:
inputting the second active current instruction value and the second reactive current instruction value into an inner ring current controller in the flexible direct current converter station control system, and outputting real current with active current as a fault current limit value and reactive current as 0;
according to the voltage measurement value of the flexible direct current fault side converter bus, calculating an active power instruction value of the flexible direct current non-fault side converter station, and specifically comprising the following steps:
when an inverter station in a flexible direct current system determines direct current voltage and a rectifier station determines active power, if an alternating current fault occurs on an inverter side, determining an active power instruction value on the rectifier side by using a relation broken line graph of the voltage and the active power of the rectifier side according to a voltage measurement value of a current conversion bus on the flexible direct current fault side;
when an inverter station in a flexible direct current system determines active power and a rectifier station determines direct current voltage, if an alternating current fault occurs at a rectifier side, determining an active power instruction value at the inverter side by using a relation broken line graph of the inverter side voltage and the active power according to a voltage measurement value of a current conversion bus at the flexible direct current fault side;
and inputting the active power instruction value into an outer ring power controller in a control system of the flexible direct current converter station, and controlling the output power to be the real direct current power of the active power instruction value.
Steps S105-S106 correspond to ((C) of FIG. 2).
Step S106, further including:
setting a second active current instruction value and a second reactive current instruction value to be restored to the values before the fault at the current instruction value restoration rate;
and inputting the active current instruction value before the fault and the reactive current instruction value before the fault into an inner ring current controller in a control system of the flexible direct current converter station, and controlling to output real current of which the active current instruction value before the fault and the reactive current instruction value before the fault are real currents.
The control performed after step S106 corresponds to (r) in fig. 2.
The specific process of the invention is as follows:
step 1: comparing voltage measured value U of flexible direct current fault side current conversion bus M And critical fault crossing the start threshold U Mf Of (c) is used. If U is present M ＞U Mf Skipping to the step 2; if U is present M ＜U Mf And jumping to step 6.
The control strategy selects different control methods according to the voltage drop amplitude of the flexible direct current conversion bus: if the voltage measurement value of the current conversion bus is larger than the fault ride-through starting voltage threshold value, selecting a control method under slight fault, wherein the control purpose is mainly to provide reactive support for conventional direct current and inhibit phase conversion failure; if the voltage measurement value of the current conversion bus is smaller than the serious fault ride-through starting threshold value, a control method under the serious fault is selected, and the control purpose is mainly to inhibit overvoltage and overcurrent under the flexible direct current and alternating current fault and realize the fault safe ride-through.
Wherein, U Mf And selecting a starting threshold value according to serious fault crossing in a flexible direct current and alternating current fault crossing strategy facing to actual engineering.
Step 2: using the square term variation of the DC voltage of a flexible DC system measured during a fault
The variation delta V of the alternating voltage of the conventional direct current converter station lcc Calculating the active current compensation quantity delta i of the converter station at the flexible direct current fault side dref And the reactive current compensation amount delta i qref 。
According to the control block diagram of the outer loop of the flexible direct current converter station after adding additional control under slight fault, as shown in fig. 3, the variation of the square term of the direct current voltage of the flexible direct current system
And the active current compensation quantity delta i dref The relationship of (c) is:
alternating voltage variation delta V of conventional direct current converter station lcc And the reactive current compensation amount delta i qref The relationship of (c) is:
Δi qref ＝-K q2 ΔV lcc (2)
wherein, T d2 Is the active current compensation coefficient, K q2 The current reference direction is from the alternating current system to the flexible direct current converter station, and the variables in the above formula are all taken as per unit values.
U in FIG. 3 acref Indicating a flexible DC-AC voltage command value, U dcref And the flexible direct current voltage command value is shown.
The compensation quantity of active current and reactive current needs to be compensated to the outer ring control through fault linkIn a system, a fault link is used for switching active current and reactive current compensation quantity: setting a fault link to be 1 when a fault is detected, and cutting off the fault or measuring a voltage value U of a current conversion bus M Less than catastrophic fault crossing the onset threshold U Mf Meanwhile, the fault link is set to 0.
Active current compensation coefficient T d2 The calculation method and its derivation are as follows:
when a receiving end power grid fails, the voltage of a converter bus of the flexible direct current converter station can drop, and when the converter station works in an inversion state, the power transmitted to an alternating current system by the converter station can be reduced. At this time, the power input to the converter station by the dc line is greater than the power output to the ac system by the converter station, and the power imbalance may cause the sub-module capacitor of the converter station to charge, and the dc voltage to increase rapidly, resulting in the dc voltage variation Δ U dc . Total energy W stored in sub-module capacitors of converter station C Comprises the following steps:
wherein N is the total number of upper and lower bridge arm submodules of each phase of the converter station, and C sm Is a sub-module capacitor, U dc Is the dc voltage of the flexible dc system, C is the capacitance, and U is the capacitor voltage.
According to the formula (3), the power unbalance amount delta P and the direct current voltage U of the converter station dc The relationship of (c) is:
wherein, P dc Active power P input into the converter station by the DC system ac Is the active power output by the converter station to the ac system.
According to the instantaneous power theory, the active power P output to the alternating current system by the converter station ac Comprises the following steps:
wherein u is sd Is the d-axis component, i, of the commutating bus voltage sd Is the q-axis component of the ac current output by the converter station to the ac system.
Let V be the dc voltage squared term,
and performing per-unit processing on the variables in the formulas (4) and (5) to obtain:
wherein the variable with the wavy line represents the per-unit variable. U shape dcN Is a DC voltage reference value, S, of a flexible DC system acN Is a reference value of the AC system capacity, P dcN Is the power reference value of the flexible direct current system.
Equation (6) can be written in the form of a transfer function:
when the flexible direct current works in a control mode that the direct current voltage of the inverter station and the active power of the rectifier station are fixed, the active power input to the inverter station by the direct current line can be assumed to be constant. Obtaining the variation of the square term of the DC voltage according to the formula (7)
And d-axis component variation of alternating current
The relationship of (1):
from equation (8), it can be determined when the DC voltage changes Δ U dc When needed to compensate forWork current Δ i dref . When the current reference direction is taken to be from the alternating current system to the converter station, the calculation method of the magnitude of the active current compensation quantity is as follows:
wherein, the active current compensation coefficient T d2 The calculation formula of (c) is:
reactive current compensation coefficient K q2 The calculation method and its derivation are as follows:
through network simplification, an alternating current power grid containing flexible direct current and conventional direct current feed-in is equivalently simplified into a three-terminal alternating current system, as shown in fig. 4. jX in FIG. 4 s Represents the reactance in an equivalent alternating current system,
representing the short-circuit voltage generated by the short-circuit current injected into the fault point by the flexible dc converter station,
representing the flexible dc converter bus voltage.
U M ＞U Mf The main purpose of the time control strategy is to suppress the conventional dc commutation failure. When the fault is a three-phase earth fault and occurs on a conventional direct current commutation bus, the conventional direct current is most prone to commutation failure, so that the determination of K in the scene is selected q2 To ensure the control effect.
Setting the grounding reactance of three-phase grounding fault as X k By X k The size of (a) indicates the severity of the fault. According to the superposition theorem, the effective value V of the residual voltage generated by the short-circuit current injected into the fault point of the flexible direct current converter station during the fault period can be obtained by neglecting the line resistance lcc_M ：
Wherein:
X Σ ＝X E +X L +X M (12)
X E 、X L 、X M is the equivalent reactance obtained from the star-delta transformation:
X le is a connection reactance, X, between a conventional direct current conversion bus and an equivalent alternating current system in an equivalent three-terminal network me Is a connection reactance, X, between a conventional direct current conversion bus and an equivalent alternating current system in an equivalent three-terminal network lm The three-terminal network is equivalent to the interconnection reactance between a conventional direct current conversion bus and a flexible direct current conversion bus in a three-terminal network. In addition, B l The equivalent susceptance, X, corresponding to a filter is compensated for the reactive power of the conventional direct current converter station k To a fault earthing reactance, i sd Is d-axis component, i, of the AC current output from the flexible DC converter station sq Is the q-axis component of the ac output current of the flexible dc converter station.
Order to
The full differential of equation (11) is found:
according to formula (16)Knowing that the reactive current output by the flexible DC converter station has a coefficient G q Then provides contribution for the voltage of the conventional direct current conversion bus, so that the voltage drop delta V of the conventional direct current conversion bus can be obtained lcc And in time, the flexible direct current converter station needs increased reactive current compensation quantity. When the current reference direction is taken as the current flowing from the alternating current system to the converter station, the calculation method of the reactive current compensation quantity under the per unit value is as follows:
wherein the variable with the wavy line represents the per-unit variable, S acN Is a reference value of the AC system capacity, P dcN Is the power reference value of the flexible direct current system.
Reactive current compensation coefficient K q2 The calculation formula of (c) is:
and 3, step 3: according to the active current compensation quantity and the reactive current compensation quantity, calculating an active current correction quantity i dref0 +Δi dref And reactive current correction i qref0 +Δi qref (ii) a Inputting the active current correction and the reactive current correction into a dynamic current amplitude limiting link, and outputting an active current instruction value i dref And a reactive current command value i qref 。
Wherein i dref0 And i qref0 And the current instruction values are respectively output by the active outer loop controller and the reactive outer loop controller of the flexible direct current converter station when no additional control is added. Additional control is based on the compensation amount Δ i calculated in step 2 dref And Δ i qref The corrected current command value is i dref0 +Δi dref And i qref0 +Δi qref Inputting the active current into a dynamic current amplitude limiting link for amplitude limiting control to obtain an active current instruction value i input into an inner ring current controller dref And a reactive current command value i qref 。
The flexible direct current traditionally adopts quiescent current amplitude limiting control, and the control can limit the reactive power transmission capability during the fault period, and reduce the inhibition effect on the conventional direct current commutation failure. The dynamic current amplitude limiting control is to give priority to the output of reactive current on the premise of ensuring that the converter station does not overflow, and the control can fully exert the reactive support capability of flexible direct current and improve the phase change failure inhibition effect. A schematic diagram of the dynamic current clipping control stage is shown in fig. 5.
The active current and the reactive current of an input dynamic current amplitude limiting link are assumed to be respectively
And with
The calculation process of the dynamic current amplitude limiting control link is as follows:
wherein,
and
is the input quantity of the dynamic current limiting link, i dref And i qref Is the output quantity of the dynamic current amplitude limiting link, I max Is the limit value of the alternating current allowed to flow by the flexible direct current converter station line.
And 4, step 4: after the fault is removed, the compensation value is set to be 0, and an active current command value i is set dref And a reactive current command value i qref At a rate I cuplim And restored to the pre-failure value.
Under slight fault, at fault cutSetting the fault link to 0 after the fault link is removed, and compensating the active current by the amount delta i dref And the reactive current compensation amount delta i qref And setting 0. The input of a speed limiting link is used for limiting an active current instruction value i dref And a reactive current command value i qref With I cuplim To a pre-failure level. Wherein, I cuplim And selecting according to the current instruction value recovery rate in the flexible direct current and alternating current fault ride-through strategy facing to the actual engineering.
And 5: inputting an active current instruction value and a reactive current instruction value into an inner ring current controller, and controlling the output active current of the flexible direct current fault side converter station to be i dref And a reactive current of i qref The true current of (c).
And 6: setting active current instruction value i of converter station at flexible direct current fault side dref Is shown as I cdF And a reactive current command value i qref Is 0.
Under severe fault, the voltage of a flexible direct current fault side converter bus can drop rapidly, the output current of an alternating current port of a converter station can rise rapidly, and bridge arm current overcurrent can be caused; the output of active power of a converter station can be further suppressed by large voltage drop, so that the power transmitted by the AC side and the DC side of the converter is unbalanced, and the direct-current voltage is over-voltage in severe cases and exceeds the bearing range of devices, so that a direct-current system is locked by protection actions, and the influence caused by faults is worsened. Therefore, the control strategy under the serious fault mainly aims to restrain overvoltage and overcurrent under the flexible direct current fault and realize fault safe ride-through.
When U is formed M ＜U Mf In the process, the flexible direct current system is judged to suffer from more serious fault impact, and an active current instruction value i is limited for restraining the overcurrent of a bridge arm dref Is shown as I cdF Reactive current command value i qref Is 0. Wherein, I cdF The current limit value under the severe fault of the flexible direct current is selected according to the current limit value in the flexible direct current and alternating current fault ride-through strategy facing to actual engineering.
And 7: comparison U M And fault crossing recovery threshold U Mr When the size of U is M ＞U Mr In the meantime, a flexible direct current fault side switch is setStreaming station active current instruction value i dref And a reactive current command value i qref At a rate I cuplim And restored to the pre-failure value.
In severe failure, when U M Is restored to U Mr In the method, an active current instruction value i of the converter station at the flexible direct current fault side is set dref And a reactive current command value i qref From I cdF And 0 with I cuplim To a pre-failure level. Wherein, U Mr Selecting a fault crossing recovery threshold according to the recovery threshold in a flexible direct current and alternating current fault crossing strategy facing to actual engineering, I cuplim And selecting according to the current instruction value recovery rate in the flexible direct current and alternating current fault ride-through strategy facing to the actual engineering.
And step 8: according to the DC voltage measured value U of the flexible DC system dc Calculating the active power instruction value P of the converter station at the non-fault side of the flexible direct current ref 。
Under severe faults, the voltage of a converter bus at the flexible direct current fault side can drop rapidly, active power transmission of a converter station is suppressed, power unbalance is generated on the alternating current side and the direct current side of the converter station, sub-module capacitors of the flexible direct current converter station can be charged and discharged due to the power unbalance, and overvoltage or undervoltage of direct current voltage is caused.
When a fault occurs in a power grid at a fixed direct-current voltage control end, a non-fault end converter station is controlled by fixed active power and does not have direct-current voltage regulation capacity, so that direct-current overvoltage and undervoltage are more likely to occur. Therefore, in order to suppress dc voltage fluctuation during a soft dc fault, a non-fault side converter station employing constant active power control needs to be put into an active power command value correction module.
Fig. 6 is a schematic diagram of an active power command value correction module on the rectifying side, and fig. 7 is a schematic diagram of an active power command value correction module on the inverting side. P in FIG. 6 dc_ref And the flexible direct current active power instruction value is represented.
The active power instruction value correction module has the following action principle:
for a flexible direct current system with constant direct current voltage of an inverter station and constant active power of a rectifier station: when an alternating current fault occurs on the inversion side, the power transmission on the inversion side is blocked, the sub-module capacitor is charged to cause the direct current voltage to rise, the active power transmitted by a direct current line is reduced under the action of the active power instruction value correction module on the rectification side, the amount of power unbalance of the inversion station is reduced, and therefore the fluctuation of the direct current voltage is reduced.
For a flexible direct-current system with constant active power of an inverter station and constant direct-current voltage of a rectifier station: when an alternating current fault occurs on the rectifying side, the power transmitted into the rectifying station by the alternating current system is reduced, the direct current voltage is reduced due to the discharge of the sub-module capacitor, the active power transmitted by the direct current line is reduced under the action of the active power instruction value correction module by the inverter station, the power unbalance amount of the rectifying station is reduced, and therefore the fluctuation of the direct current voltage is reduced.
Designing the active power command value modification module requires determining the variable P dcN 、U dc_min 、U dc_max 、U dc_lim Is a value of (a), wherein P dcN The rated power is transmitted when the flexible direct current normally works; u shape dc_lim The method is characterized in that the method is a direct-current voltage safety limit and is selected according to the upper and lower voltage limits which can be borne by equipment such as a switching device, a capacitor and the like; u shape dc_min Is the lower limit of DC voltage fluctuation, U, of the flexible DC in normal operation dc_max Is the upper limit of the direct current voltage fluctuation when the flexible direct current normally works. According to the working principle of the modularized multi-level converter, the direct current voltage and the output alternating current voltage of the converter have the following relation:
wherein, U ac The effective value of the fundamental frequency alternating current voltage output by the modular multilevel converter is k, the direct current voltage utilization rate of the modular multilevel converter is k, and m is the modulation ratio of the converter. AC line voltage U when system normally operates ac Is + -5%, so U dc_max And U dc_min Optionally (1+ 5%) U dcN And (1-5%) U dcN ，U dcN Is the DC voltage rated value of the flexible DC system under the normal operation condition.
And step 9: the flexible direct current fault side converter inputs an active current instruction value and a reactive current instruction value into the inner ring current controller to control output active current to be i dref A reactive current of i qref The true current of (d); the flexible DC non-fault side converter will have an active power instruction value P ref Input into an outer loop power controller to control the output power to be P ref True dc power.
The control method provided by the invention has the following advantages:
(1) the common commutation failure support control strategy in the existing hybrid multi-feed-in direct current system mainly focuses on the problem of how the flexible direct current supports the conventional direct current to resist commutation failure, and neglects the requirement of the flexible direct current on the safe ride-through fault. The control strategy reasonably corrects the active reactive current instruction value of the flexible direct current converter station, so that the flexible direct reactive support capability under the alternating current fault and the capability of the system for resisting the commutation failure are improved, the fluctuation of the voltage and the current of the flexible direct current converter station is reduced, the problem that the converter station aggravates the fault due to overcurrent and overvoltage locking is avoided, and the fault characteristic of the system is effectively improved.
(2) Compared with the common control strategy of calculating the control quantity by adopting a PI link, the control strategy can obtain the control parameters more quickly and accurately. The derivation of the calculation method may also provide a reference for the design of the relevant control strategy.
The invention further provides an embodiment, and particularly illustrates the technical effect of the control strategy by using a scene that the conventional direct current and the flexible direct current are mixed and fed into the alternating current system.
Embodiment verification is performed in a PSCAD/EMTDC environment, and FIG. 8 is a schematic view of a scene adopted by embodiment effect verification.
In fig. 8, the inversion sides of the regular dc and the soft dc feed the same ac system. The rated direct current voltage of the conventional direct current is 500kV, the rated power is 1000MW, and an CIGRE HVDC standard model carried by PSCAD is adopted; the flexible direct current adopts a modular multilevel converter, the rectification side adopts a constant active power and constant reactive power control mode, the inversion side adopts a constant direct current voltage and constant alternating current voltage control mode, the rated direct current voltage is 500kV, the rated power is 1000MW, the number of bridge arm submodules of each phase is 250, the capacitance voltage of the submodules is 0.01328F, and an independently constructed model is adopted; the rated voltage of a receiving end alternating current system is 230kV, the equivalent internal reactance of the system is 5 omega, the interconnection reactance between an alternating current system bus and a conventional direct current is 7.5 omega, the interconnection reactance between the alternating current system bus and a flexible direct current is 7.5 omega, and the interconnection reactance between the conventional direct current and the flexible direct current is 3 omega.
S in FIG. 8 1 、S 2 、S 3 Respectively representing an LCC-HVDC transmitting end alternating current system, an MMC-HVDC transmitting end alternating current system, a receiving end alternating current system, P 1 -P 4 Respectively representing the active power transmitted by the LCC-HVDC rectifying side, the active power transmitted by the MMC-HVDC rectifying side, the active power transmitted by the LCC-HVDC inverting side and the active power transmitted by the MMC-HVDC inverting side, and Q 1 -Q 4 Respectively representing reactive power transmitted by an LCC-HVDC rectifying side, reactive power transmitted by an MMC-HVDC rectifying side, reactive power transmitted by an LCC-HVDC inverting side and reactive power transmitted by an MMC-HVDC inverting side, wherein the LCC1 represents an LCC-HVDC rectifying station, the LCC2 represents an LCC-HVDC inverting station, the MMC1 represents an MMC-HVDC rectifying station, and the MMC2 represents an MMC-HVDC inverting station.
And calculating the control parameters of the improved control strategy according to the control parameter calculation method. According to the control parameter calculation method, the calculation value of the active current compensation coefficient is 0.03, and the calculation value of the reactive current compensation coefficient is 17; according to a flexible direct current and alternating current fault ride-through control strategy facing engineering, a serious fault ride-through starting voltage threshold is selected to be 0.5p.u., and a recovery threshold is selected to be 0.7 p.u.; in order to prevent the converter station from overcurrent, the active current instruction value I of the converter station at the fault side during serious fault cdF The direct current voltage safety upper limit U of the non-fault side is selected to be 0.6p.u., the reactive current instruction is selected to be 0 dc_lim The safety upper limit of the alternating current of the convertor station is 1.2p.u.
The technical effect of the light fault control strategy is verified by simulation as follows. A three-phase short-circuit fault1 is arranged on a conventional direct current conversion bus, the fault grounding inductance is 0.35H, the fault occurrence time is 7s, the duration is 100ms, at the moment, the voltage of the flexible direct current conversion bus does not drop to a serious fault ride-through starting threshold value, and the fault is judged to be a slight fault. The effect of the improved control strategy is verified under three conditions of not adopting additional control, adopting general commutation failure support control only increasing reactive output during fault and adopting the improved control proposed by the invention. The simulation results under light fault are shown in fig. 9 and fig. 10.
Fig. 9 is a waveform comparison of a conventional dc converter bus voltage, where the solid line represents the system employing the improved control strategy proposed by the present invention and the dashed line represents the system not employing any additional control strategy. As can be seen from fig. 9, when the additional control strategy is not adopted, the voltage of the conventional dc converter bus falls to 0.9081p.u. to the minimum after the voltage fails, and at this time, the conventional dc system fails to perform phase conversion; after the improved control strategy is adopted, the reactive current instruction value of the flexible direct current converter station is increased after compensation, more reactive support is provided for the conventional direct current, the minimum drop is 0.956p.u. after the voltage of the conventional direct current converter bus fails, the level is obviously higher than that without the additional control, and the conventional direct current system does not have commutation failure at the moment. In addition, multiple times of simulation result: the minimum grounding inductance of the system without additional control without commutation failure is 0.38H, and the minimum grounding inductance of the system with improved control without commutation failure is 0.31H, which is obviously smaller than the value without additional control. It can be seen that the improved control strategy under slight fault can significantly improve the voltage level of the conventional direct current commutation bus under fault, enhance the capability of the system for resisting commutation failure, and effectively improve the commutation failure support capability of flexible direct current.
Fig. 10 is a graph comparing the soft dc voltage current waveform during a light fault, where the solid line represents the system employing the improved control strategy proposed by the present invention, and the dashed line represents the system employing a general control strategy that increases only soft dc reactive output to support commutation failure under fault. As can be seen from (a) in fig. 10, when a general control strategy is adopted, the direct-current voltage fluctuates sharply, the highest direct-current voltage fluctuates is 1.053p.u., the lowest direct-current voltage fluctuates is 0.9729p.u., and the fluctuation amplitude reaches 8%; after the improved control strategy is adopted, the active current instruction value of the flexible direct current converter station is increased after compensation, the amount of power unbalance is reduced, the direct current voltage fluctuation is obviously smooth and gentle, the maximum value is 1.038p.u., and the minimum value is 0.9839p.u., the fluctuation amplitude is reduced to 5.4% in the normal fluctuation range of the direct current voltage, and the direct current voltage fluctuation can be effectively reduced through the improved control strategy.
As can be seen from (b) in fig. 10, when the general control strategy is adopted, the dc current fluctuation range is at most 1.078p.u., at most 0.9112p.u., and the fluctuation range is 16.7%, after the improved control strategy is adopted, the highest value and the lowest value of the dc current fluctuation range are respectively 1.068p.u. and 0.925p.u., and the fluctuation range is reduced to 14.3%, which indicates that the improved control strategy can reduce the dc current fluctuation. As can be seen from (c) in fig. 10, when a general control strategy is adopted, the maximum bridge arm current under a fault can be increased to 1.183p.u., and after an improved control strategy is adopted, under the action of current limiting control and direct current fluctuation flattening, the bridge arm current impact is reduced, and the peak value is reduced to 1.149p.u., so that the bridge arm overcurrent of the flexible direct current converter can be effectively inhibited by adopting the improved control strategy. Therefore, under a slight fault, the problem of aggravation of voltage and current fluctuation of the MMC due to the increase of reactive power support conventional direct current during the fault can be effectively solved by improving the control strategy, and the flexible direct current safe ride-through fault is facilitated.
The control effect simulation verification of the serious fault control strategy is as follows. The three-phase short-circuit fault2 is arranged on the flexible direct current conversion bus, the fault grounding inductance is 0.01H, the fault occurrence time is 7s, the duration is 100ms, at the moment, the voltage of the flexible direct current conversion bus falls below a serious fault ride-through starting threshold value, and the fault is judged to be a serious fault. The effect of the improved control strategy is verified under two conditions of not adopting additional control and adopting the improved control proposed by the invention. The simulation results under catastrophic failure are shown in fig. 11.
FIG. 11 is a comparison of a flexible DC voltage current waveform during a catastrophic failure, wherein the solid line represents the system employing the improved control strategy proposed by the present invention and the dashed line represents the system not employing the additional control strategy. As can be seen from fig. 11, when the flexible direct current does not adopt any additional control strategy, the direct current voltage during the fault period may rise to 1.34p.u. at most, the direct current may rise to 1.356p.u. at most, the direct current voltage and the direct current fluctuate seriously, the alternating current line current of the converter station rises under the action of the rising of the active and reactive current instruction of the converter station, and may reach 1.42p.u. at most, far exceeding the upper limit of the alternating current of the converter station, and the bridge arm current may also rise, and may reach 1.3p.u. at most.
After the control strategy provided by the invention is adopted, it can be seen that under the action of the rectification side active power instruction value correction module, the rising amplitude of the direct current voltage is reduced during the fault period and is raised to 1.28p.u. at most, the direct current overvoltage is restrained, the direct current is obviously reduced, and the direct current overcurrent is effectively restrained; under the action of a serious alternating current fault ride-through control strategy on an inverter side, the active reactive current instruction value starts to be limited for 7.024s, the effective value of the alternating current line current of the converter station rapidly drops after rising to 1.2p.u., the overcurrent of the alternating current line of the converter station is effectively inhibited, and the current of a bridge arm drops due to the drop of the direct current and the alternating current line current, so that the overcurrent of the bridge arm is effectively inhibited. Therefore, the fluctuation of voltage and current during the fault period of the flexible direct current converter station can be reduced by improving the control strategy under the serious fault, overvoltage and overcurrent are avoided, and the fault ride-through capability of the flexible direct current and alternating current is improved.
The above-mentioned examples fully verify that the control strategy designed by the invention can realize effective design and coordinated distribution of the flexible direct current active and reactive currents during the alternating current fault period, and can improve the reactive support of the flexible direct current to the receiving-end power grid under the fault, thereby improving the capability of resisting the commutation failure by the conventional direct current, and simultaneously can reduce the fluctuation of the direct current voltage and the bridge arm current during the fault period of the flexible direct current converter station, and avoid overvoltage and overcurrent, thereby ensuring that the flexible direct current safely passes through the alternating current fault.
The invention also provides a flexible direct current compensation control system, which comprises:
the judgment result obtaining module is used for judging whether the voltage measurement value of the flexible direct current fault side converter bus is larger than a first voltage set threshold value after the receiving-end power grid fails, and obtaining a judgment result;
the active current compensation quantity and reactive current compensation quantity calculation module is used for calculating active current compensation quantity and reactive current compensation quantity of the converter station at the flexible direct current fault side according to the direct current voltage square term variable quantity of the flexible direct current system and the alternating current voltage variable quantity of the direct current converter station of the power grid commutation converter type direct current transmission system if the judgment result shows that the current is positive;
the first active current instruction value and first reactive current instruction value determining module is used for obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and determining the compensated active current instruction value and the compensated reactive current instruction value as a first active current instruction value and a first reactive current instruction value;
the first instruction value control module is used for controlling the flexible direct current fault side converter station according to the first active current instruction value and the first passive current instruction value until a protection device in a receiving-end power grid carries out fault removal action;
the second active current instruction value and second reactive current instruction value determining module is used for setting the active current instruction value of the flexible direct current fault side converter station as a fault current limit value and setting the reactive current instruction value of the flexible direct current fault side converter station as 0 to determine the active current instruction value and the second reactive current instruction value if the judgment result shows that the active current instruction value and the reactive current instruction value are not the same;
and the second instruction value control module is used for controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is greater than a second voltage set threshold value.
The active current compensation amount and reactive current compensation amount calculation module specifically comprises:
an active current compensation meter operator module for utilizing a formula according to the variation of the DC voltage square term of the flexible DC system
Calculating the active current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i dref For the active current compensation, T d2 Is the active current compensation factor and is,
is the variation of the square term of the direct current voltage, and s is a complex variable;
the reactive current compensation meter operator module is used for utilizing a formula delta i according to the alternating voltage change of a direct current converter station of the power grid phase-change converter type direct current transmission system qref ＝-K q2 ΔV lcc Calculating reactive current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i qref For the amount of reactive current compensation, K q2 Δ V being a reactive current compensation coefficient lcc Is the alternating voltage variable quantity of a conventional direct current converter station.
The first active current instruction value and the first reactive current instruction value determining module specifically include:
the compensated reactive current calculation sub-module is used for calculating the compensated active current according to the active current compensation amount and calculating the compensated reactive current according to the reactive current compensation amount;
a compensated reactive current instruction value obtaining submodule for obtaining the reactive current instruction value according to the compensated reactive current based on the dynamic current amplitude limiting control method and by using a formula
Obtaining a reactive current instruction value after compensation; wherein i qref For the compensated reactive current command value,
for reactive current correction, I max Limiting the allowed alternating current flowing for the flexible direct current converter station line;
the compensated active current instruction value obtaining submodule is used for obtaining the active current according to the compensated reactive current instruction value and the compensated active current based on the dynamic current amplitude limiting control method and by using a formula
Obtaining a compensated active current instruction value; wherein i dref For the compensated active current command value,
is the active current compensation quantity.
The second instruction value control module specifically includes:
the real current output submodule is used for inputting the second active current instruction value and the second reactive current instruction value into an inner ring current controller in the flexible direct current converter station control system and outputting real current with active current as a fault current limit value and reactive current as 0;
the active power instruction value operator module is used for calculating an active power instruction value of the flexible direct current non-fault side converter station according to the voltage measurement value of the flexible direct current fault side converter bus;
and the real direct current power output submodule is used for inputting the active power instruction value to an outer ring power controller in the flexible direct current converter station control system and controlling the output power to be the real direct current power of the active power instruction value.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the system disclosed by the embodiment, the description is relatively simple because the system corresponds to the method disclosed by the embodiment, and the relevant points can be referred to the method part for description.
The principle and the embodiment of the present invention are explained by applying specific examples, and the above description of the embodiments is only used to help understanding the method and the core idea of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the foregoing, the description is not to be taken in a limiting sense.
Claims (8)
1. A flexible direct current compensation control method is characterized by comprising the following steps:
judging whether a voltage measurement value of a flexible direct current fault side converter bus is larger than a first voltage set threshold value after a receiving end power grid fails, and obtaining a judgment result;
if the judgment result shows that the current value of the direct current voltage of the flexible direct current system is greater than the current value of the direct current voltage of the direct current converter station of the power grid phase-change converter type direct current transmission system, calculating active current compensation quantity and reactive current compensation quantity of the flexible direct current fault side converter station according to the direct current voltage square term variable quantity of the flexible direct current system and the alternating current voltage variable quantity of the direct current converter station of the power grid phase-change converter type direct current transmission system;
obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and determining the compensated active current instruction value and the compensated reactive current instruction value as a first active current instruction value and a first reactive current instruction value;
controlling the flexible direct current fault side converter station according to the first active current instruction value and the first passive current instruction value until a protection device in a receiving end power grid carries out fault removal action;
if the judgment result shows that the current is not the fault current limit value, setting the active current instruction value of the flexible direct current fault side converter station as the fault current limit value, setting the reactive current instruction value as 0, and determining the active current instruction value and the reactive current instruction value as a second active current instruction value and a second reactive current instruction value;
controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is larger than a second voltage set threshold value;
the method for calculating the active current compensation quantity and the reactive current compensation quantity of the converter station at the flexible direct current fault side according to the direct current voltage square term variation of the flexible direct current system and the alternating current voltage variation of the direct current converter station of the power grid phase-change converter type direct current transmission system specifically includes:
according to the variable quantity of the direct current voltage square term of the flexible direct current system, a formula is utilized
Calculating the active current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i dref For an active current compensation, T d2 Is the active current compensation factor and is,
the variation of the square term of the direct current voltage is shown, and s is a complex variable;
according to the alternating voltage variation of a direct current converter station of a power grid phase-change converter type direct current transmission system, a formula delta i is utilized qref ＝-K q2 ΔV lcc Calculating reactive current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i qref For the amount of reactive current compensation, K q2 Δ V being a reactive current compensation coefficient lcc Is the alternating voltage variable quantity of a conventional direct current converter station.
2. The flexible direct current compensation control method according to claim 1, wherein obtaining the compensated active current command value and the compensated reactive current command value of the flexible direct current fault side converter station according to the active current compensation amount and the reactive current compensation amount specifically includes:
calculating the compensated active current according to the active current compensation quantity, and calculating the compensated reactive current according to the reactive current compensation quantity;
according to the compensated reactive current, based on the dynamic current amplitude limiting control method, using the formula
Obtaining a reactive current instruction value after compensation; wherein i qref For the compensated reactive current command value,
as a reactive current correction quantity, I max Limiting the allowed alternating current of the flexible direct current converter station line;
according to the compensated reactive current instruction value and the compensated active current, based on a dynamic current amplitude limiting control method, a formula is utilized
Obtaining a compensated active current instruction value; wherein，i dref For the compensated active current command value,
is the active current compensation quantity.
3. The flexible direct current compensation control method according to claim 1, wherein the controlling the flexible direct current fault side converter station according to the second active current command value and the second reactive current command value specifically includes:
inputting the second active current instruction value and the second reactive current instruction value into an inner ring current controller in the flexible direct current converter station control system, and outputting real current with active current as a fault current limit value and reactive current as 0;
calculating an active power instruction value of the flexible direct current non-fault side converter station according to a voltage measurement value of the flexible direct current fault side converter bus;
and inputting the active power instruction value into an outer ring power controller in a control system of the flexible direct current converter station, and controlling the output power to be the real direct current power of the active power instruction value.
4. The flexible direct current compensation control method according to claim 3, wherein the calculating an active power command value of the flexible direct current non-fault side converter station according to the voltage measurement value of the flexible direct current fault side converter bus specifically includes:
when an inverter station in a flexible direct current system determines direct current voltage and a rectifier station determines active power, if an alternating current fault occurs on an inverter side, determining an active power instruction value on the rectifier side by using a relation broken line graph of the voltage and the active power of the rectifier side according to a voltage measurement value of a current conversion bus on the flexible direct current fault side;
when the inverter station in the flexible direct current system determines active power and the rectifier station determines direct current voltage, if alternating current fault occurs at the rectifier side, the active power instruction value at the inverter side is determined by utilizing a relation broken line diagram of the inverter side voltage and the active power according to the voltage measurement value of the flexible direct current fault side current conversion bus.
5. The flexible direct current compensation control method according to claim 1, wherein the controlling the flexible direct current fault side converter station according to the second active current instruction value and the second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is greater than the second voltage setting threshold value, and thereafter further comprises:
setting a second active current instruction value and a second reactive current instruction value to be restored to the values before the fault at the current instruction value restoration rate;
and inputting the active current instruction value before the fault and the reactive current instruction value before the fault into an inner ring current controller in the flexible direct current converter station control system, and controlling to output real current of which the active current instruction value before the fault and the reactive current instruction value before the fault are real current.
6. A flexible dc compensation control system, the system comprising:
the judgment result obtaining module is used for judging whether the voltage measurement value of the flexible direct current fault side converter bus is larger than a first voltage set threshold value after the receiving-end power grid fails, and obtaining a judgment result;
the active current compensation quantity and reactive current compensation quantity calculation module is used for calculating active current compensation quantity and reactive current compensation quantity of the flexible direct current fault side converter station according to the direct current voltage square term variable quantity of the flexible direct current system and the alternating current voltage variable quantity of the direct current converter station of the power grid phase-change converter type direct current transmission system if the judgment result shows that the current source is positive;
the first active current instruction value and first reactive current instruction value determining module is used for obtaining a compensated active current instruction value and a compensated reactive current instruction value of the flexible direct current fault side converter station according to the active current compensation quantity and the reactive current compensation quantity, and determining the compensated active current instruction value and the compensated reactive current instruction value as a first active current instruction value and a first reactive current instruction value;
the first instruction value control module is used for controlling the flexible direct current fault side converter station according to the first active current instruction value and the first passive current instruction value until a protection device in a receiving-end power grid carries out fault removal action;
a second active current instruction value and second reactive current instruction value determining module, configured to set an active current instruction value of the flexible direct current fault-side converter station as a fault current limit value and determine the active current instruction value and the second reactive current instruction value when the determination result indicates that the active current instruction value and the reactive current instruction value are 0;
the second instruction value control module is used for controlling the flexible direct current fault side converter station according to a second active current instruction value and a second reactive current instruction value until the voltage measurement value of the flexible direct current fault side converter bus is larger than a second voltage set threshold value;
the active current compensation amount and reactive current compensation amount calculation module specifically comprises:
an active current compensation meter operator module for utilizing a formula according to the variation of the DC voltage square term of the flexible DC system
Calculating the active current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i dref For an active current compensation, T d2 Is the active current compensation factor for the current,
is the variation of the square term of the direct current voltage, and s is a complex variable;
the reactive current compensation meter operator module is used for utilizing a formula delta i according to the alternating voltage change of a direct current converter station of the power grid phase-change converter type direct current transmission system qref ＝-K q2 ΔV lcc Calculating reactive current compensation quantity of the converter station at the flexible direct current fault side; wherein, Δ i qref For reactive current compensation, K q2 For the compensation coefficient of reactive current, Δ V lcc Is the alternating voltage variable quantity of the conventional direct current converter station.
7. The flexible direct current compensation control system according to claim 6, wherein the first active current command value and the first inactive current command value determining module specifically includes:
the compensated reactive current calculation sub-module is used for calculating compensated active current according to the active current compensation quantity and calculating compensated reactive current according to the reactive current compensation quantity;
a compensated reactive current instruction value obtaining submodule for obtaining the reactive current instruction value according to the compensated reactive current based on the dynamic current amplitude limiting control method and by using a formula
Obtaining a reactive current instruction value after compensation; wherein i qref For the compensated reactive current command value,
as a reactive current correction quantity, I max Limiting the allowed alternating current flowing for the flexible direct current converter station line;
the compensated active current instruction value obtaining submodule is used for obtaining the active current according to the compensated reactive current instruction value and the compensated active current based on a dynamic current amplitude limiting control method and by using a formula
Obtaining a compensated active current instruction value; wherein i dref For the compensated active current command value,
is the active current compensation quantity.
8. The flexible direct current compensation control system of claim 6, wherein the second instruction value control module specifically comprises:
the real current output submodule is used for inputting the second active current instruction value and the second reactive current instruction value into an inner ring current controller in the flexible direct current converter station control system and outputting real current with active current as a fault current limit value and reactive current as 0;
the active power instruction value operator module is used for calculating an active power instruction value of the flexible direct current non-fault side converter station according to the voltage measurement value of the flexible direct current fault side converter bus;
and the real direct current power output submodule is used for inputting the active power instruction value to an outer ring power controller in the flexible direct current converter station control system and controlling the output power to be the real direct current power of the active power instruction value.
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