CN115051404A - Alternating voltage control method of high-voltage network-building type current converter - Google Patents

Alternating voltage control method of high-voltage network-building type current converter Download PDF

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CN115051404A
CN115051404A CN202210389290.8A CN202210389290A CN115051404A CN 115051404 A CN115051404 A CN 115051404A CN 202210389290 A CN202210389290 A CN 202210389290A CN 115051404 A CN115051404 A CN 115051404A
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voltage
converter
current
sequence
control
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管敏渊
沈建良
楼平
来骏
韩磊
满敬彬
陈攀峰
周开运
胡耀杰
朱开成
李凡
王建锋
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Huzhou Power Supply Co of State Grid Zhejiang Electric Power Co Ltd
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Huzhou Power Supply Co of State Grid Zhejiang Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241—The oscillation concerning frequency
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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  • Power Engineering (AREA)
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Abstract

The invention discloses an alternating voltage control method of a high-voltage network-forming type converter, which introduces a voltage-current proportional relation to construct novel outer ring voltage control of the converter and forms a Grid-forming (Grid-forming) converter control framework together with inner ring current control. The voltage and current proportion outer ring control does not depend on the converter alternating current side parallel capacitive filter, and the converter high-voltage high-capacity multi-level converter is suitable for outputting a high-waveform-quality high-voltage high-capacity multi-level converter, can cancel the parallel capacitive filter, and effectively reduces the system cost; when the power grid fails, the network-forming converter can automatically and quickly enter a positive-sequence current amplitude limiting state and a negative-sequence current inhibiting state under the action of voltage-current ratio control, overcurrent locking and damage of the converter are avoided, and the converter automatically recovers to normal operation after the power grid fault is cleared.

Description

Alternating current voltage control method of high-voltage network-building type current converter
Technical Field
The invention relates to the technical field of converter control, in particular to an alternating-current voltage control method of a high-voltage network type converter.
Background
With the rapid increase of the requirements of direct current transmission, flexible alternating current transmission, distributed energy generation, energy storage and the like, the application of the power electronic converter in a power grid is more and more common. The output alternating voltage of the Grid-following converter is synchronized with an alternating current power Grid through a Phase-Locked Loop (PLL), and the alternating current voltage exchanges active power and reactive power with the power Grid according to instructions. Based on the inner ring current controller, the net-tracking type converter can carry out rapid adjustment and amplitude limiting control on the output alternating current; in addition, when the power grid runs asymmetrically, the negative sequence current controller is added, so that the negative sequence current can be restrained, and the balance of the three-phase current is maintained. Because the current converter has limited overcurrent tolerance capability, the rapid current control capability of the inner-loop positive and negative sequence current controllers can prevent the current converter from overcurrent locking and damage when the power grid is disturbed, and the current converter has important significance for ensuring the safe operation of the current converter. However, the operation of the grid-following type converter depends on an alternating current power source, and the converter may not normally operate under the condition of a passive power grid or a weak power grid.
In recent years, in order to make up for the deficiency of Grid-following converters and to develop a 100% renewable energy Grid, Grid-forming converters that can operate autonomously without depending on an ac Grid have been gaining attention. The main difference between the network-following type converter and the network-constructing type converter lies in the control system and the corresponding operation mode, and the hardware structures of the two converters are the same or similar. A cascade control architecture combining an inner loop current controller and an outer loop voltage controller has become a mainstream control architecture of a grid-type converter. The framework can realize the independent construction of the voltage of the network, and also keeps the inner ring current controller, thereby preventing the current converter from overcurrent locking and damage. The traditional dq outer ring voltage control in the low-voltage grid-connected converter control constructs alternating-current voltage based on the charge-discharge characteristics of a capacitive filter connected in parallel on the alternating-current side of the converter. Currently, practical applications of low-voltage network-building type converters are increasing. On the basis, negative sequence component control is introduced, so that the grid-structured converter has the ride-through capability of asymmetric faults of a power grid.
On the other hand, with the rapid development of the module cascade type multi-level converter in the application of the high-voltage large-capacity converter, the output level of the converter can reach dozens or even hundreds, for example, the level of the flexible direct-current transmission converter can reach hundreds, the output alternating-current voltage and current harmonic content are small, the grid-connected requirement can be met, the high-voltage parallel filter on the alternating-current side can be eliminated, and the system cost is reduced. Therefore, the traditional dq outer ring voltage control cannot be directly applied to a high-voltage network type converter without a parallel capacitive filter, and novel outer ring voltage control needs to be researched.
For example, chinese patent CN113962181A, published 2022, 1 month, 21 days, a method for optimally designing double-loop control parameters of a network-type voltage source converter, which is provided with an inner-loop proportion parameter kc and an outer-loop resonance parameter kr, includes the following steps: drawing a pole point diagram of the closed loop transfer function changing along with the parameters, and finding out a parameter value corresponding to a dominant pole closest to the origin; drawing a baud chart, judging whether the parameter value meets a predefined stability margin, if so, the parameter is an optimized control parameter of the control loop; otherwise the parameter values should be increased or decreased to some extent to meet the requirements. Independently optimizing control parameters of the inner ring and the outer ring, and simultaneously having respective expected stability margin and fastest dynamic performance; directly using a pole point diagram and a baud diagram without complex formula solution; compared with the traditional proportional resonant controller, the resonant controller is only applied to the outer ring, so that the dynamic performance of the system can be obviously improved while the structure and design complexity of the control system are reduced. The scheme also uses a parallel capacitive filter, and the high-voltage parallel filter is not suitable for a control system of the high-voltage grid type converter due to the fact that the use cost of the high-voltage parallel filter is too high.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the existing high-voltage network-building type converter control system has the technical problems that the use cost of a high-voltage parallel filter is too high, and the traditional dq outer ring voltage control cannot be directly applied to the high-voltage network-building type converter without the parallel capacitive filter. The alternating voltage control method of the high-voltage grid-connected type current converter can effectively reduce the system cost without depending on a high-voltage parallel filter on the alternating current side of the current converter.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a method for controlling alternating voltage of a high-voltage network-building type converter comprises the following steps: the outer ring voltage controller is adopted to control the outer ring voltage: and carrying out outer ring voltage control on the high-voltage network-building type converter through voltage-current ratio control.
The invention introduces a novel outer ring voltage controller which is used for controlling the voltage-current ratio to be a high-voltage network type current converter, the voltage control principle of the novel outer ring voltage controller does not depend on a high-voltage parallel filter at the alternating current side of the current converter, and the novel outer ring voltage controller and the novel inner ring current controller can form a cascade control framework. In addition, an inner ring positive sequence current controller can be used for limiting the positive sequence current, and an inner ring negative sequence current controller is used for restraining the negative sequence current, so that the current converter is prevented from overcurrent locking and damage under the condition of power grid faults.
Preferably, in the positive sequence inner loop current control, the command value of the dq axis component of the positive sequence current is determined by a positive sequence outer loop voltage control, and the calculation formula of the positive sequence outer loop voltage control of the converter is as follows:
Figure BDA0003594937290000021
Figure BDA0003594937290000022
wherein the content of the first and second substances,
Figure BDA0003594937290000023
and
Figure BDA0003594937290000024
are respectively the command values of the dq-axis components of the positive sequence alternating voltage at PCC, which is the point of common coupling, u d + And u q + Respectively the dq-axis component of the positive sequence ac voltage at the PCC,
Figure BDA0003594937290000025
and
Figure BDA0003594937290000026
respectively, command values, i, of dq-axis components of the converter output positive-sequence alternating current Ld + And i Lq + Respectively, the dq-axis component of the converter output positive sequence alternating current, and z is a proportionality constant.
And combining with the positive sequence inner loop current control, and giving a command value of the positive sequence inner loop current control by the converter positive sequence outer loop voltage control based on the voltage current ratio control.
Preferably, the converter is protected under grid fault conditions: the inner-loop positive-sequence current controller is used for limiting the positive-sequence current, and the inner-loop negative-sequence current controller is used for suppressing the negative-sequence current.
Preferably, the process of suppressing the negative-sequence current includes: and setting the instruction values of the negative sequence current dq axis components in the negative sequence inner loop current control to be 0, and not adding the negative sequence outer loop voltage control. And controlling the positive sequence dq axis voltage by using voltage-current proportional control and controlling the positive sequence dq axis current by using inner loop current control on the positive sequence component. For the negative sequence component, the negative sequence dq axis current is suppressed by using inner loop current control, and the negative sequence dq axis current command values all take 0.
Preferably, the command value of the dq-axis component of the negative-sequence current in the negative-sequence inner-loop current control is determined by a negative-sequence outer-loop voltage control, and the negative-sequence outer-loop voltage control calculation formula of the converter is as follows:
Figure BDA0003594937290000031
Figure BDA0003594937290000032
wherein the content of the first and second substances,
Figure BDA0003594937290000033
respectively, the command values, u, of the dq-axis components of the negative-sequence alternating voltage at PCC d - And u q - Respectively the dq-axis component of the negative-sequence alternating voltage at the PCC,
Figure BDA0003594937290000034
and
Figure BDA0003594937290000035
respectively, command values, i, of dq-axis components of the negative-sequence alternating current output by the inverter Ld - And i Lq - Respectively, the command values of the dq-axis components of the negative sequence alternating current output by the inverter. When the instruction value of the negative sequence current dq axis component is not set to be 0, negative sequence outer ring voltage control needs to be added, and the construction of the novel outer ring voltage control is independent of the parallel capacitive filter and is suitable for the high-voltage network-forming type current converter without the parallel capacitive filter.
Preferably, under the per unit value system, the value of the proportionality constant z in the positive sequence outer ring voltage control calculation formula of the converter ranges from 0.2 to 5.
Preferably, under the per unit value system, the value range of the proportionality constant z in the negative sequence outer ring voltage control calculation formula of the converter is 0.2 to 5. Note that the proportionality constant z is the only control parameter for voltage-current ratio control, and in a per unit value system, z can be generally selected within the range of 0.5-2, so that the voltage-current ratio relation is in the same order of magnitude, and the proportional control effect of voltage and current is fully exerted.
The substantial effects of the invention are as follows: (1) according to the invention, a voltage-current proportional relation is introduced to construct a novel outer ring voltage control of the converter, and the novel outer ring voltage control and the inner ring current control form a high-voltage network-type (Grid-forming) converter control framework, the voltage-current proportional outer ring control does not depend on the converter alternating current side and is connected with a capacitive filter in parallel, so that the converter is suitable for a high-voltage high-capacity multi-level converter with high output waveform quality, and a capacitive filter can be cancelled in parallel;
(2) when the power grid normally operates and is disturbed, the alternating current voltage and the alternating current of the current converter at the PCC can track the corresponding instruction values under the control action of the voltage-current ratio, and the control target of the network-forming current converter is realized;
(3) when the power grid fails, the network-forming converter can automatically and quickly enter a positive-sequence current amplitude limiting state and a negative-sequence current inhibiting state under the action of voltage-current ratio control, so that overcurrent locking and damage of the converter are avoided, and the converter automatically recovers to normal operation after the power grid fault is cleared;
(4) the voltage-current ratio exception loop controller can realize control over alternating voltage, positive sequence current amplitude limiting and negative sequence current suppression of the converter under the asymmetrical fault of a power grid, and the voltage-current ratio exception loop controller is combined with analog operation of the synchronous machine, so that the network-forming converter can simulate inertia characteristics of the synchronous machine and primary frequency modulation and secondary frequency modulation functions.
Drawings
FIG. 1 is a schematic diagram of a low-voltage grid-type converter system;
fig. 2 is a control block diagram of the network-forming converter according to the present embodiment;
fig. 3 is a schematic structural diagram of a high-voltage grid-type converter system according to the present embodiment;
fig. 4 is a schematic diagram of simulation waveforms of the synchronous machine simulation operation of the converter under the load step;
fig. 5 is a schematic diagram of simulation waveforms of the synchronous machine simulation operation of the converter under the load step;
fig. 6 is a schematic diagram of a simulation waveform of the constant voltage operation of the converter under the condition of single-phase earth fault;
fig. 7 is a second simulation waveform diagram of the constant voltage operation of the converter under the condition of single-phase earth fault;
fig. 8 is a schematic diagram of simulation waveforms of the synchronous machine simulation operation of the converter under the condition of single-phase earth fault;
fig. 9 is a schematic diagram of simulation waveforms of the simulated operation of the converter synchronous machine under the condition of the single-phase ground fault.
Wherein: 1. modular multilevel converter, 2, reactor, 3, transformer, 4, load.
Detailed Description
The following provides a more detailed description of the present invention, with reference to the accompanying drawings.
The embodiment introduces a novel outer ring voltage controller which is used for controlling the voltage-current ratio to be a high-voltage network-forming type converter, the voltage control principle of the novel outer ring voltage controller does not depend on a high-voltage parallel filter on the alternating current side of the converter, and the novel outer ring voltage controller and the novel inner ring current controller can form a cascade control framework; the inner-ring positive-sequence current controller is used for limiting the positive-sequence current, and the inner-ring negative-sequence current controller is used for restraining the negative-sequence current, so that overcurrent locking and damage of the converter under the condition of power grid faults are avoided.
1 conventional dq vector control
1.1 system architecture diagram of a low voltage network type converter for supplying power to a passive network of a low voltage converter system is shown in fig. 1. Wherein v is abc And i abc Three-phase AC voltage and three-phase AC current, u, respectively, output from the inverter abc Is the three-phase ac voltage at the Point of Common Coupling (PCC) of the inverter. The low-voltage network-building type converter can adopt a low-cost two-level or three-level converter, and high-frequency Pulse Width Modulation (PWM) is required to be applied due to the small number of output levels. In order to filter the higher harmonics of the low-voltage inverter output, it is generally necessary to use an LC-type low-voltage inverter as shown in FIG. 1And in the pass filtering system, L and R are equivalent reactance and equivalent resistance of the converter reactor respectively, and C is equivalent capacitance of the parallel filter. U shape d And I d Respectively the dc voltage and the dc current of the inverter.
The voltage-current relationship of the converter system may be expressed as:
L(di abc /dt)＝-Ri abc +v abc -u abc (1)
through dq coordinate transformation, the positive sequence voltage and current of the converter can be represented by the positive sequence dq axis components of equations (2) and (3), and the negative sequence current and voltage of the converter can be represented by the negative sequence dq axis components of equations (4) and (5).
Figure BDA0003594937290000051
Figure BDA0003594937290000052
Figure BDA0003594937290000053
Figure BDA0003594937290000054
Wherein s is a Laplace operator and is the rated angular frequency of the power grid. v. of d + ，v q + And i d + ，i q + The dq-axis components, u, of the converter output positive-sequence AC voltage and current, respectively d + ，u q + Respectively, the dq axis components of the positive sequence ac voltage at the PCC. v. of d - ，v q - And i d - ，i q - The dq-axis components, u, of the negative-sequence AC voltage and current output by the converter d - ，u q - Dq-axis components of negative-sequence alternating voltage at PCC, respectivelyAmount of the compound (A).
The control of the grid-type converter is characterized by the need to control the alternating voltage. As shown in fig. 2, the conventional dq vector control is a cascade control structure, in which the current control is used as an inner loop control for adjusting the output current of the converter, and the voltage control is used as an outer loop control for adjusting the network-side voltage of the converter. The output of the outer loop voltage control is used as the input current command of the inner loop current control.
1.2 inner loop current control:
because the converter transformer usually adopts a Y/delta connection method, when an alternating current power grid has an asymmetric fault, the converter transformer can isolate a zero sequence component on the grid side, and only a positive sequence component and a negative sequence component can be transmitted to the alternating current side of the converter. When the power grid is in a normal symmetrical operation state, the three-phase alternating current voltage and current only have positive sequence components; when the grid is in an asymmetric operating state, the three-phase ac voltage and current will contain positive and negative sequence components. In the inverter shown in fig. 2, the positive-sequence inner-loop current control is represented by equations (6) and (7), and the negative-sequence inner-loop current control is represented by equations (8) and (9).
Figure BDA0003594937290000055
Figure BDA0003594937290000056
Figure BDA0003594937290000057
Figure BDA0003594937290000058
Wherein
Figure BDA0003594937290000059
Respectively, the instruction value of dq-axis component of the converter output positive sequence AC current, whose value is controlled by the outer ring positive sequence voltage of the converterAnd (5) giving out a maker.
Figure BDA00035949372900000510
Respectively, command values of dq-axis components of the negative-sequence alternating current output by the converter; as shown in fig. 2, when the strategy of suppressing the negative sequence current is adopted, the values are all 0; if a negative sequence voltage control strategy is adopted, the value is given by the negative sequence voltage control of the outer ring of the converter. k is a radical of p1 And k i1 Respectively, the proportionality coefficient and the integral coefficient of the inner loop current controller.
1.3 outer Loop Voltage control
The positive sequence outer loop voltage control of the converter in fig. 2 is represented by equations (10) and (11); the negative sequence outer loop voltage control is expressed by equations (12) and (13).
Figure BDA0003594937290000061
Figure BDA0003594937290000062
Figure BDA0003594937290000063
Figure BDA0003594937290000064
Wherein
Figure BDA0003594937290000065
Respectively the command values for the dq-axis component of the positive sequence ac voltage at the PCC,
Figure BDA0003594937290000066
respectively, are the command values for the dq-axis components of the negative sequence alternating voltage at the PCC. i.e. i Ld + ，i Lq + The dq-axis components, i, of positive-sequence alternating currents respectively output from the network side of the parallel capacitors Ld - ，i Lq - Respectively, the dq-axis components of the negative-sequence alternating current output by the parallel capacitor network side. k is a radical of p2 And k i2 Respectively, the proportionality coefficient and the integral coefficient of the inner loop current controller. Since the conventional outer-loop voltage control of the low-voltage network type converter is constructed based on the electrical relationship of equivalent capacitors of the parallel filters, the equivalent capacitors C of the parallel capacitive filters are included in equations (10) to (13).
2 voltage to current ratio control
2.1 high voltage converter system
Fig. 3 is a system configuration diagram of a high voltage network type converter for supplying power to a passive network, including a modular multilevel converter 1, a reactor 2, a transformer 3, and a load 4.
In order to achieve a higher output voltage level, a high-voltage network type converter generally uses a modular multi-level converter, the output level is increased, the harmonic content of the converter is reduced, and a parallel capacitive filter can be eliminated; on the other hand, the cost of the parallel capacitive filter of the high voltage system is significantly higher than that of the parallel capacitive filter of the low voltage system, and the parallel capacitive filter is not usually installed in the high voltage system for improving the economy. L and R are the total equivalent reactance and the total equivalent resistance of the converter and the transformer, respectively.
According to the formulas (10) to (13), the conventional outer-loop voltage control of the low-voltage network-structured converter needs to be constructed based on the equivalent capacitance of the parallel filter, and cannot be used for the outer-loop voltage control of the high-voltage network-structured converter. Therefore, the high-voltage network-building type converter control needs to construct a novel outer-loop voltage control independent of the parallel capacitive filter while keeping the inner-loop current control of the low-voltage network-building type converter, and adjust the current instruction value for the inner-loop current control to realize the control of the network-side alternating-current voltage of the converter.
2.2 combining the novel outer ring voltage control with the positive sequence inner ring current control, the voltage current ratio control based on the voltage and current ratio coupling relation of the converter is proposed as the outer ring voltage control of the grid-type converter. Equations (14) and (15) are based on the converter positive sequence outer loop voltage control of the voltage current ratio control, and give a command value for the positive sequence inner loop current control.
Figure BDA0003594937290000067
Figure BDA0003594937290000068
Similarly, converter negative sequence outer loop voltage control can be constructed as shown in equations (16) and (17).
Figure BDA0003594937290000071
Figure BDA0003594937290000072
Where z is a proportionality constant. Under the voltage-current ratio control, the deviation of the dq-axis component of the positive-sequence alternating current output by the converter relative to the command value of the converter and the deviation of the dq-axis component of the positive-sequence alternating current output by the converter relative to the command value of the converter form a ratio relation.
The compounds of formulae (6), (7), (14) and (15) can be combined to give
Figure BDA0003594937290000073
Figure BDA0003594937290000074
The compounds of formulae (2), (3), (18) and (19) can be combined to give
Figure BDA0003594937290000075
Figure BDA0003594937290000076
u d + ,u q + Respectively, are the command values for the dq-axis components of the positive sequence ac voltage at the PCC.
The construction of a conventional outer loop voltage control is based on a parallel capacitive filter. As shown in fig. 2, the voltage-current ratio control is a new outer loop voltage control, replacing the conventional outer loop voltage control. From the equations (14) to (17), the new outer loop voltage control is constructed without depending on the parallel capacitive filter, and is suitable for a high-voltage network-forming converter without the parallel capacitive filter. In the equations (20) and (21), the alternating voltage and the current have a strong coupling and nonlinear relationship, and the alternating current becomes an interference term in the outer loop voltage control.
To reveal the working mechanism of voltage-current proportional control, the final value theorem is used after equation (20) is processed
Figure BDA0003594937290000077
Wherein
Figure BDA0003594937290000078
Which is used to represent the magnitude of the change of the positive sequence d-axis current component before and after the system disturbance. In a steady state, it can be seen from equation (22) that the d-axis component of the positive sequence ac voltage follows the command value under the voltage-current ratio control. The same analysis can be made that the q-axis component of the positive sequence ac voltage will also track its commanded value.
Note that the proportionality constant z is the only control parameter for voltage-current ratio control, and in a per unit value system, z can be generally selected within the range of (0.5-2), so that the voltage-current ratio relation is in the same order of magnitude, and the proportional control effect of voltage and current is fully exerted. In order to eliminate the coupling relation, a feedforward term can be specially introduced into the outer loop voltage control to cancel the interference term. The present invention does not consider feed forward decoupling in order to simplify the analysis.
2.3 Overall control architecture
The overall control block diagram of the high-voltage grid-connected converter is shown in fig. 2. The synchronous phase of the alternating current component dq axis transformation is:
θ＝∫ω * dt+θ 0 (23)
wherein theta is 0 Is the initial phase; omega * The command value of the angular frequency of the alternating voltage is the rated angular frequency of the power grid, and the common value taking method comprises the following steps:
(1) when the constant voltage operation is adopted, the angular frequency command value of the alternating voltage is equal to the angular frequency rated value of the power grid
ω * ＝ω (24)
(2) When synchronous machine simulation operation is adopted, the alternating voltage angular frequency command value is determined by the following formula,
Figure BDA0003594937290000081
wherein P and P are the command value and the actual value of the active power output by the converter respectively. D is the proportionality coefficient between the active power deviation and the angular frequency deviation, T 1 And T 2 Are two time constants. Equation (25) simulates the inertia characteristics of a synchronous machine and the proportional relationship between frequency deviation and active power deviation in primary frequency modulation.
By varying the initial phase theta 0 The phase angle of the coordinate axis d can be consistent with the phase angle of the phase voltage A output by the converter station, and the instruction value of the dq axis component of the positive sequence voltage output by the converter is as follows:
Figure BDA0003594937290000082
Figure BDA0003594937290000083
wherein U is * Is a command value for the ac voltage amplitude at the inverter PCC. As shown in fig. 2, the dq coordinate inverse transformation is performed on each of equations (6) to (9) to obtain inverter output positive-sequence and negative-sequence three-phase ac voltage modulated waves, and the two are superimposed to obtain an inverter output three-phase ac voltage modulated wave.
In normal steady state, the converter output ac current will track its commanded value, with both the converter ac voltage and current within its limits. The positive sequence inner loop current control has rapid current regulation capability, and the purpose of automatic fault current limiting can be realized by limiting the input positive sequence current instruction value as shown in fig. 2, so that overcurrent locking and damage of the converter during power grid faults are avoided.
The negative sequence outer ring voltage control of the converter can also restrain the negative sequence alternating voltage output by the converter; in view of preventing overcurrent locking and damage of the converter, the negative sequence component control mainly uses a negative sequence current suppression strategy, command values of negative sequence current dq axis components in negative sequence inner loop current control are all set to be 0 as shown in fig. 2, and negative sequence outer loop voltage control is not added.
3 simulation test
A simulation system of a high-voltage network-forming type converter for supplying power to a passive network as shown in fig. 3 is established, system parameters are shown in table 1, the network-forming type converter adopts a modular multilevel converter, and a high-voltage parallel filter is not arranged on an alternating current side. The control block diagram of the grid-type converter is shown in fig. 2, and the control parameters are shown in table 2. And controlling the positive sequence dq axis voltage by using voltage-current proportional control and controlling the positive sequence dq axis current by using inner loop current control on the positive sequence component. For the negative sequence component, the negative sequence dq axis current is suppressed by using inner loop current control, and the negative sequence dq axis current command values all take 0. And the side converter adopts constant direct current voltage control to provide stable direct current side voltage for the side network type converter. The modularized multi-level converter adopts nearest level approximation modulation and sequencing type capacitance voltage balance control, and no circulation current suppression control is configured.
TABLE 1 simulation System parameters
Parameter(s) Numerical value
Rated frequency/Hz of power grid 50
Rated AC voltage/kV of power grid 35
Rated capacity/MVA of converter 20
Maximum overcurrent multiple of current converter 1.5
Rated DC voltage/kV of converter +/-30
Rated AC voltage/kV of converter 31
Bridge arm sub-module number/number 49
Bridge arm reactance/H 0.053
Submodule capacitance/uF 4650
Rated capacity/MVA of converter transformer 30
Transformation ratio of converter transformer 36/31
Leakage reactance/pu of converter transformer 0.08
Active load/MW 25
Reactive load/Mvar 2.5
TABLE 2 control of system parameters
Figure BDA0003594937290000091
3.1 synchronous machine simulation operation example under load step
And under the normal operation state of the power grid, the network-forming type converter is in a normal alternating-current voltage control state. The outer ring voltage control gives a current instruction of the inner ring current control according to an alternating current voltage instruction at the PCC; and determining an alternating voltage command output by the current converter through the inner loop current control, and finally achieving the aim of controlling the alternating voltage at the PCC.
The method is used for testing the system response of the synchronous machine simulation-operated network-building type converter when the load of the alternating current power grid is in normal step. At 3.5 seconds, the AC grid shed a 5.5MW load, testing the system response under disturbance. To eliminate steady-state frequency deviation, the text is thrown in at 4 seconds [15] The synchronous machine simulates the secondary frequency modulation function of operation, and adjusts the active power instruction value of the converter through the integral link of frequency negative feedback, so that the disturbed power grid frequency is gradually restored to the rated value. Fig. 4 and 5 are simulation waveforms of various electrical quantities of the synchronous machine simulation operation of the converter under the load step. U in FIG. 4(a) a And i in FIG. 4(b) a Ac voltage and current at PCC, respectively, fig. 4(c)
Figure BDA0003594937290000101
And
Figure BDA0003594937290000102
respectively, the d-axis voltage and the finger of the converter output positive sequenceLet's value, in FIG. 4(d)
Figure BDA0003594937290000103
And
Figure BDA0003594937290000104
the inverter outputs a positive sequence q-axis voltage and its command value, respectively. FIG. 5(a)
Figure BDA0003594937290000105
And
Figure BDA0003594937290000106
the d-axis current of the inverter output positive sequence and its command value, respectively, are shown in FIG. 5(b)
Figure BDA0003594937290000107
And
Figure BDA0003594937290000108
the inverter outputs a positive-sequence q-axis current and its command value, respectively, as shown in FIG. 5(c)
Figure BDA0003594937290000109
And
Figure BDA00035949372900001010
the current is output by the converter with a negative sequence of d-axis and Q-axis, and in the figure 5(d), P and Q are output active power and reactive power of the converter respectively, and variables in the figure are per unit values. In fig. 5(e), f is an inverter frequency command value.
After 22% of the load of the alternating current power grid is removed, as shown in fig. 5, the active power output by the converter and the positive sequence d-axis current are reduced in corresponding proportion, and the frequency of the converter is gradually increased to 50.03Hz under the relation between the inertia characteristic of the formula (25) and the primary frequency modulation proportion; as shown in fig. 4(c) and (d), the positive and negative sequence dq-axis voltage currents also exhibit some short-term slight disturbance, and the corresponding disturbance is substantially eliminated within 0.1 second. At 4 seconds, after the secondary frequency modulation function of the synchronous machine simulation operation is put into operation, the frequency of the current converter is slowly recovered to the rated frequency as shown in fig. 5(e), and the secondary frequency modulation response is slow, so that the frequency recovery process time is long, and the characteristic is similar to the secondary frequency modulation characteristic of the traditional generator. In summary, when the power grid is in normal operation and in disturbance, the alternating voltage at the PCC of the grid-forming type current converter and the alternating current of the grid-forming type current converter can track the corresponding instruction values under the action of the voltage-current ratio control, and the control target of the grid-forming type current converter is achieved.
3.2 constant Voltage operation example under Single-phase Earth Fault
When the power grid suddenly fails, the normal alternating voltage control target of the converter cannot be realized, at the moment, the positive sequence current instruction value given by the positive sequence outer ring voltage control is limited, the outer ring voltage control fails, and the positive sequence inner ring current control automatically enters a fault current limiting state, so that overcurrent locking and damage of the converter are prevented. The strategy for restraining the negative sequence current can restrain the negative sequence current under the asymmetrical fault of the power grid, but the active power and the reactive power of the converter can generate oscillation components of double power frequency.
The method is used for testing the system response of the network-forming type converter which operates at constant voltage when the alternating-current power grid is in single-phase earth short circuit fault, and the command value of the alternating-current voltage angular frequency of the converter is the rated angular frequency of the power grid. And at 1.1 second, the phase A of the alternating current power grid generates a grounding short circuit fault, and the fault is cleared after lasting for 0.2 second. Fig. 4(a) and (b) are three-phase alternating voltage and current at the PCC, respectively, fig. 4(c) is inverter output positive sequence d-axis voltage and its command value, and fig. 4(d) is inverter output positive sequence q-axis voltage and its command value. Fig. 5(a) shows the converter output positive sequence d-axis current and its command value, fig. 5(b) shows the converter output positive sequence q-axis current and its command value, fig. 5(c) shows the converter output negative sequence d-axis and q-axis currents, and fig. 5(d) shows the converter output active and reactive power.
During a fault, the a-phase voltage drops to near 0 as shown in fig. 4, and the three-phase ac voltage is severely unbalanced. The negative sequence inner loop current control can restrain the negative sequence current, and the three-phase alternating current still maintains balance. Due to the fact that the voltage of the power grid drops seriously, the instruction value of the d-axis current control in the positive sequence inner loop current control shown in fig. 5(a) is limited by the amplitude limiting link, the current converter is prevented from entering overcurrent locking, and the d-axis voltage in the positive sequence outer loop voltage control shown in fig. 4(c) cannot be maintained at the instruction value. Due to the negative sequence voltage, the converter active and reactive power during the fault appear as a double power frequency ripple characteristic as shown in fig. 5 (d). After the fault is cleared, the voltage and current of the converter are gradually restored to the normal operation state. In conclusion, when the power grid fails, the network-forming type converter which operates at constant voltage can automatically and quickly enter a positive sequence current amplitude limiting state and a negative sequence current restraining state under the action of voltage-current ratio control, so that the converter is prevented from overcurrent locking and damage, and automatically recovers to normal operation after the power grid fault is cleared.
3.3 synchronous machine simulation operation example under single-phase earth fault
The method is used for testing the system response of the network-building type converter which operates in a synchronous machine simulation mode when the single-phase grounding short circuit of the alternating-current power grid is in fault, and the command value of the angular frequency of the alternating-current voltage of the converter is determined by an equation (25). And at 1.1 second, the phase a of the alternating current power grid has a grounding short circuit fault, and the fault is cleared after lasting for 0.2 second. Fig. 5(a) and (b) are three-phase alternating voltage and current at the PCC, respectively, fig. 5(c) is inverter output positive sequence d-axis voltage and its command value, and fig. 5(d) is inverter output positive sequence q-axis voltage and its command value. Fig. 6(a) shows the converter output positive sequence d-axis current and its command value, fig. 6(b) shows the converter output positive sequence q-axis current and its command value, fig. 6(c) shows the converter output negative sequence d-axis and q-axis currents, and fig. 6(d) shows the converter output active and reactive power.
Fig. 8 and 9 are simulation waveforms of various electrical quantities of simulation operation of the converter synchronous machine under the condition of single-phase ground fault. U in FIG. 8(a) abc And i in FIG. 8(b) abc Three-phase AC voltage and current at PCC, respectively, in FIG. 8(c)
Figure BDA0003594937290000111
And
Figure BDA0003594937290000112
the inverter outputs a positive sequence d-axis voltage and its command value, respectively, as shown in FIG. 8(d)
Figure BDA0003594937290000113
And
Figure BDA0003594937290000114
the inverter outputs a positive sequence q-axis voltage and its command value, respectively. FIG. 9(a)
Figure BDA0003594937290000115
And
Figure BDA0003594937290000116
the d-axis current of the inverter output positive sequence and its command value, respectively, are shown in FIG. 9(b)
Figure BDA0003594937290000117
And
Figure BDA0003594937290000118
the inverter outputs a positive-sequence q-axis current and its command value, respectively, as shown in FIG. 9(c)
Figure BDA0003594937290000119
And
Figure BDA00035949372900001110
the current is output by the converter with a negative sequence of d-axis and Q-axis, and P and Q in the graph (d) of FIG. 9 are output active power and reactive power of the converter respectively, and variables in the graph are per unit values.
During a fault, the a-phase voltages drop to near 0 as shown in fig. 8, and the three-phase ac voltages are severely unbalanced. Negative sequence inner loop current control also suppresses negative sequence current, and three-phase alternating current remains balanced. Due to the fact that the voltage of the power grid drops seriously, the instruction value of the d-axis current controller in the positive sequence inner loop current control shown in fig. 9(a) is limited by an amplitude limiting link, and the current converter is prevented from entering overcurrent locking, so that the d-axis voltage in the outer loop positive sequence voltage control shown in fig. 8(c) cannot be maintained at the instruction value. Due to the negative sequence voltage, the converter active and reactive power during the fault appear as a double power frequency ripple characteristic as shown in fig. 9 (d). After the fault is cleared, the voltage and current of the converter are gradually restored to the normal operation state. In conclusion, when the power grid fails, the network-building type converter which is simulated to operate by the synchronizer can automatically and quickly enter a positive-sequence current amplitude limiting state and a negative-sequence current inhibiting state under the action of voltage-current ratio control, so that overcurrent locking and damage of the converter are avoided, and the converter automatically recovers to normal operation after the power grid fault is cleared.
Similarly, when a three-phase symmetrical earth fault occurs in a power grid, the network-forming type converter can automatically and quickly enter positive sequence current amplitude limiting under the action of voltage-current ratio control, so that overcurrent locking and damage of the converter are avoided, and the converter automatically recovers to normal operation after the power grid fault is cleared.
In this embodiment, a voltage-current proportional relationship is introduced to construct a novel outer-loop voltage control of the converter, and the novel outer-loop voltage control and the inner-loop current control form a Grid-forming (Grid-forming) converter control architecture. The voltage and current proportion outer ring control does not depend on the converter alternating current side parallel capacitive filter, and the converter high-voltage high-capacity multi-level converter is suitable for outputting a high-waveform-quality high-voltage high-capacity multi-level converter, can cancel the parallel capacitive filter, and effectively reduces the system cost; when the power grid normally operates and is disturbed, the alternating current voltage and the alternating current of the current converter at the PCC can track the corresponding instruction values under the control action of the voltage-current ratio, and the control target of the network-forming current converter is realized; when the power grid fails, the network-forming converter can automatically and quickly enter a positive-sequence current amplitude limiting state and a negative-sequence current inhibiting state under the action of voltage-current ratio control, so that overcurrent locking and damage of the converter are avoided, and the converter automatically recovers to normal operation after the power grid fault is cleared; the voltage-current ratio exception loop controller can control alternating current voltage, and realize positive sequence current amplitude limiting and negative sequence current suppression of the current converter under the asymmetrical fault of a power grid.
The above examples only show some embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.

Claims (7)

1. An alternating voltage control method of a high-voltage network-building type converter is characterized by comprising the following steps:
the outer ring voltage controller is adopted to control the outer ring voltage: and performing outer ring voltage control on the high-voltage network-building type converter through voltage-current ratio control.
2. The method according to claim 1, wherein the command value of the dq-axis component of the positive sequence current in the positive sequence inner loop current control is determined by a positive sequence outer loop voltage control, and the calculation formula of the positive sequence outer loop voltage control of the converter is as follows:
Figure FDA0003594937280000011
Figure FDA0003594937280000012
wherein u is d +* And u q +* Respectively, command values, u, of the dq-axis component of the positive-sequence alternating voltage at PCC d + And u q + The dq-axis components, i, of the positive-sequence AC voltage at PCC, respectively d +* And i q +* Respectively, command values, i, of dq-axis components of the converter output positive-sequence alternating current Ld + And i Lq + Respectively, the dq-axis component of the converter output positive sequence alternating current, and z is a proportionality constant.
3. An ac voltage control method for a high voltage network configuration converter according to claim 1 or 2, characterized in that the converter is protected in grid fault conditions: the inner-loop positive-sequence current controller is used for limiting the positive-sequence current, and the inner-loop negative-sequence current controller is used for suppressing the negative-sequence current.
4. The method according to claim 3, wherein the step of suppressing the negative sequence current comprises: and setting the instruction values of the negative sequence current dq axis components in the negative sequence inner loop current control to be 0, and not adding the negative sequence outer loop voltage control.
5. An ac voltage control method according to claim 1 or 2, wherein the command value of the dq-axis component of the negative sequence current in the negative sequence inner loop current control is determined by a negative sequence outer loop voltage control, and the negative sequence outer loop voltage control of the converter is calculated by:
Figure FDA0003594937280000013
Figure FDA0003594937280000014
wherein u is d -* ,u q -* Respectively, the command values, u, of the dq-axis components of the negative-sequence alternating voltage at PCC d - And u q - Are the dq-axis components, i, respectively, of the negative-sequence alternating voltage at PCC d -* And i q -* Respectively, command values, i, of dq-axis components of the negative-sequence alternating current output by the inverter Ld - And i Lq - Respectively, are command values of the dq-axis components of the negative-sequence alternating current output by the inverter.
6. The method according to claim 2, wherein the value of the proportionality constant z in the forward-sequence outer-loop voltage control calculation formula of the converter ranges from 0.2 to 5 in a per unit value system.
7. The method according to claim 5, wherein the value of the proportionality constant z in the negative sequence outer loop voltage control calculation formula of the converter is in the range of 0.2 to 5 per unit.
CN202210389290.8A 2022-04-13 2022-04-13 Alternating voltage control method of high-voltage network-building type current converter Pending CN115051404A (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115800369A (en) * 2022-12-01 2023-03-14 华中科技大学 Multi-wind-farm negative-sequence current control method and system suitable for flexible direct grid connection
CN115882514A (en) * 2023-02-16 2023-03-31 中国科学院电工研究所 New energy power system and grid-following and network-constructing integrated converter cluster aggregation control method

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115800369A (en) * 2022-12-01 2023-03-14 华中科技大学 Multi-wind-farm negative-sequence current control method and system suitable for flexible direct grid connection
CN115882514A (en) * 2023-02-16 2023-03-31 中国科学院电工研究所 New energy power system and grid-following and network-constructing integrated converter cluster aggregation control method

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