CN110266048B - VSG control method under power grid voltage symmetrical drop fault - Google Patents
VSG control method under power grid voltage symmetrical drop fault Download PDFInfo
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Abstract
The invention discloses a VSG control method under a power grid voltage symmetric drop fault, which comprises the following steps: s1, collecting the network voltage information at the tie line and transmitting the network voltage information to a local controller of the VSG in the microgrid; s2, when the symmetrical drop fault of the grid voltage is detected, calculating an active reference instruction, a reactive reference instruction and a voltage reference instruction; s3, establishing a grid-connected small signal model at the fault operation point and solving control parameters meeting given dynamic performance and stability performance indexes
、
And
synchronously switching active and voltage nominal values to active and voltage reference commands, switching control parameters
、
And
(ii) a S4, the local controller collects local voltage and current information, and establishes a voltage loop control model, a fault current-limiting control model and a current loop control model: and S5, when the fault is eliminated, smooth transition from the fault state to the normal state is realized through phase angle adjustment, and smooth and quick transition from the fault operation state to the normal operation state of the VSG after the grid voltage is recovered can be realized.
Description
Technical Field
The invention belongs to the technical field of new energy power generation grid-connected control, and particularly relates to a VSG control method under a power grid voltage symmetric drop fault.
Background
Distributed Generation (DG) has attracted considerable attention as a power generation technology capable of efficiently utilizing new energy. However, most of the distributed power supplies are connected to a power grid through a power electronic conversion device, and the damping and inertia similar to those of a traditional synchronous machine are lacked, and some distributed power supplies such as wind power and photovoltaic have randomness and volatility in power generation, are difficult to participate in voltage and frequency regulation of a system, and pose a threat to safe and stable operation of the system along with the increase of the permeability of the distributed power supplies. Under the background, the VSG control technology is widely researched because the VSG control technology simulates the operating characteristics of a conventional synchronous machine, and can realize friendly grid connection of distributed power supplies.
At present, the research on grid-connected control of VSG under the condition of a non-ideal power grid is less, the voltage of a power distribution network is often influenced by factors such as load switching, asymmetric operation, short-circuit fault and the like in actual conditions to fluctuate, and the control mechanism of the traditional inverter control strategy and the VSG control strategy is considered to have great difference, the control of the traditional inverter under the non-ideal conditions is difficult to be directly used for the control of a grid-connected inverter based on VSG, the VSG after grid connection faces the problem of reliable operation under the complex power grid working conditions, the problem of low voltage ride through of the VSG is one of the key problems of grid-connected adaptability of the VSG, although the VSG has the damping and inertia output characteristics similar to those of the traditional synchronous machine, the VSG is connected to the power grid through a power electronic converter interface essentially, the overload capacity of the VSG is limited, and if the VSG is not controlled during the voltage drop, will cause over-current and damage the converter equipment; on the other hand, if the VSG is removed from the power grid through the protection device during the voltage sag, in the power grid with high distributed power supply permeability, the removal of a large number of distributed power supplies from the power grid will also result in excessive active and reactive power shortage, and threaten the stability and reliability of the operation of the whole system. Therefore, many countries have come out of the relevant standards, and require that the distributed power supply connected to the power grid has a certain low-voltage ride-through capability, that is, the distributed power supply can continuously operate without being disconnected during the voltage drop of the power grid, and during the fault period, the distributed power supply can provide a certain reactive support for the power grid, and after the fault is eliminated, the distributed power supply can be quickly and stably recovered to a normal operation state.
At present, the research on VSG low voltage ride through control can be divided into a non-control mode switching type and a control mode switching type: for the non-control mode switching type, there are documents that a reference instruction of the VSG output current is obtained by calculating a measured value of the grid voltage by multiplying a variable gain with a VSG power outer loop voltage reference instruction and line impedance, and the calculated VSG output current reference value is prevented from being too large due to grid voltage drop by combining a current limiter, so as to limit the output current of the VSG during fault transient state and steady state; in the prior art, a dynamic voltage feedforward compensation mode is adopted, the peak current is rapidly detected and the voltage compensation quantity is calculated to feed forward and compensate the output voltage of the VSG to limit the peak current output by the VSG at the moment of a fault, but the transient response of the current and the power of the VSG during a voltage drop period is not theoretically analyzed, and the control that the VSG provides active and reactive support to a power grid during the fault period is not concerned; research has proposed that a mode combining virtual resistance current limiting and phasor current limiting is adopted to limit the steady-state fault current and the transient-state impact current of the VSG during the fault period, but the virtual impedance can change the output impedance of the VSG, thereby influencing the output dynamic and steady-state characteristics under the normal operation condition; for the control mode switching type, documents propose to switch the control of the VSG output voltage to the control of the VSG output current before and after the voltage drop of the power grid, which relates to the switching of two control loops, and because the reference phase angle of the voltage control and the current control and the control loops are different, the complicated seamless switching control problem can be faced, the control structure is complicated, and the engineering is difficult to realize; the literature proposes that the balance current control is adopted when the VSG operates normally, the voltage of the power grid is switched to the traditional low-voltage ride-through control after falling, the control objects are all the output currents of the VSG, and although the switching of the control objects is not involved, the control objects control the output currents of the VSG, so that the capability of providing voltage support for the power grid during the normal operation and fault operation of the VSG can be influenced; the method comprises the steps that firstly, the frequency inertia link of the VSG power outer ring is locked in the fault period, the inertia frequency supporting capacity of the VSG to a power grid in the fault period can be influenced, and secondly, the dynamic performance of the VSG in the transient process of fault occurrence and elimination is poor.
After the voltage of the power grid drops, the amplitude and the phase angle of the output voltage of the VSG are kept unchanged in a short time due to the inertia of the VSG, a short-time output peak current can appear to cause a protection action, the VSG is cut off from the power grid, and in the power grid with high permeability of the distributed power supply, the cutting of a large number of distributed power supplies from the power grid can cause overlarge active and reactive power shortage to threaten the stability and the reliability of the operation of the whole system; if the peak current is small enough to cause the protection action, the VSG is stabilized again after a period of transient state adjustment process through the self adjustment function, but because the primary frequency modulation and primary voltage adjustment characteristics of the VSG are set under normal conditions, certain limitation exists, expected active and reactive power support cannot be provided under the voltage drop scenes of different degrees, and the safe and stable operation of the VSG and a parallel power grid during the fault period is not facilitated; for the conventional VSG low-voltage ride-through control, the VSG output current is controlled in the fault period, and the voltage and frequency support is difficult to provide for a power grid in the fault period; after the fault is eliminated, the existing VSG low-voltage ride-through control strategy is less concerned about realizing the control of rapidly and stably recovering to the normal operation state.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides the VSG control method under the power grid voltage symmetrical drop fault, which can provide the active and reactive support functions of the VSG under different power grid voltage drop degrees, improve the safe and stable operation capability of the VSG and the power grid during the fault period, and provide voltage support and inertial frequency support for the power grid during the fault period; the stability and the dynamic performance of the system at different fault operating points can meet set indexes; transient output peak current of the VSG can be effectively limited, and the output characteristic of the VSG in a normal operation state is not influenced; smooth and quick transition of the VSG from a fault operation state to a normal operation state after the grid voltage is recovered can be realized.
In order to achieve the purpose, the invention adopts the technical scheme that: a VSG control method under a power grid voltage symmetric drop fault comprises the following steps:
s1, collecting the network voltage information at the tie line and transmitting the network voltage information to a local controller of a VSG in the microgrid;
s2, carrying out fault detection on the power grid voltage information acquired in the step S1, and calculating active and reactive reference instructions when a power grid voltage symmetrical drop fault is detected;
wherein Q ref And P ref Respectively a reactive power reference instruction and an active power reference instruction of the VSG in the fault period; u shape G Is the per unit value of the voltage amplitude of the power grid; s N Rated installed capacity for the VSG; s max Transmitting active power capacity for the line;
calculating a voltage reference command according to the reactive power reference command:
U ref ＝U g +k Qp (Q ref -Q)+k Qi ∫(Q ref -Q)
wherein, U ref Voltage command for VSG during fault; u shape g Instantaneous measurement value of the amplitude of the phase voltage of the power grid; k is a radical of Qp And k Qi Proportional coefficient and integral coefficient of PI controller; q is the output reactive measurement value of VSG;
s3, establishingGrid-connected small signal model at fault operating point and solving control parameters meeting given dynamic performance and stability performance indexes
And
synchronous switching of active and voltage ratings to active and voltage reference commands, and switching control parameters
And
the steps further include:
s31, establishing a grid-connected small signal model at the fault operation point, wherein the model is as follows:
wherein
Wherein, U g0 For the amplitude of the grid phase voltage after a drop, U g0 The angle 0 is the grid voltage phasor after the drop; u shape s0 The phase voltage amplitude of the VSG output voltage after fault stabilization; theta 0 For the VSG output voltage to lead the phase value of the grid voltage, U s0 ∠θ 0 Is the output voltage phasor of the VSG after the voltage sag; theta, omega and U s Respectively outputting a phase disturbance quantity, an angular frequency disturbance quantity and an amplitude disturbance quantity of a voltage reference command for the VSG; j is moment of inertia; d is a damping coefficient; k is excitation adjusting inertia; k is a radical of ω And k is u Respectively a primary frequency modulation coefficient and a primary voltage regulation coefficient; omega n Is the output angular frequency rating; x is reactance of a VSG grid-connected connection circuit;
s32, setting a constraint condition to enable the VSG to meet given dynamic performance and stable performance indexes at the operation point, wherein the constraint condition is as follows:
wherein S is 1,2 Connecting the dominant pole of the small signal model characteristic equation to the VSG at the corresponding operating point; ζ is the damping ratio; omega s Is a natural frequency; t is t s The time is adjusted for stability; delta is an error band;
s33, calculating the VSG control parameters meeting the given dynamic performance and stability performance indexes according to the step S31 and the step S32
And with
Synchronously switching active and voltage rated values to active and voltage reference commands, switching control parameters
And
s34, calculating the amplitude and phase reference value of the VSG output voltage according to the following control method:
wherein, theta, omega and U are phase reference value, angular frequency reference value and amplitude reference value of VSG output voltage respectively;
and
respectively at the fault operating pointThe VSG control parameter values meeting the given dynamic performance and stability performance indexes are obtained; q n An output reactive rating for the VSG; p and Q are respectively output active and reactive measurement values of the VSG;
s4, gather local voltage and current information through local controller, establish voltage ring control model, trouble current-limiting control model and current ring control model respectively, voltage ring control model is:
wherein i idref And i iqref Respectively representing a d-axis component and a q-axis component of the current reference set value in the dq reference coordinate system; k is a radical of up And k is ui Respectively representing a proportional coefficient and an integral coefficient of the voltage PI controller;
and
a d-axis component and a q-axis component respectively representing voltage reference values in a dq reference frame; u. of od And u oq Respectively representing a d-axis component and a q-axis component of the inverter output voltage in a dq reference frame; c f Represents a capacitance value in an LC filter to which the inverter terminal is connected; the dq reference coordinate system is a direct-current rotating coordinate system obtained by subjecting an abc alternating-current static coordinate system to park transformation; taking a phase reference value theta of the VSG output voltage from a phase angle transformed from an abc reference coordinate system to a dq reference coordinate system;
the fault current-limiting control model is as follows:
wherein, I m The amplitude of the current after amplitude limiting; i is max The maximum peak current value is the trigger protection;
and
d-axis component and q-axis component of the current reference set value in the dq reference coordinate system after current limiting are respectively;
the current loop control model is as follows:
wherein u is id And u iq Respectively representing a d-axis component and a q-axis component of a modulated wave voltage output by the inverter current controller under a dq reference coordinate system; k is a radical of ip And k is ii Respectively representing a proportional coefficient and an integral coefficient of the current PI controller; i.e. i id And i iq A d-axis component and a q-axis component respectively representing the inverter output current value under the dq reference coordinate system; l is f Representing the inductance value in the LC filter to which the inverter terminals are connected;
s5, when a fault cancellation is detected, a smooth transition from the fault state to the normal state is achieved with the following phase angle adjustments:
δθ＝k Pp (P n -P)+k Pi ∫(P n -P)
wherein, δ θ is the VSG output phase angle regulating quantity; k is a radical of Pp And k Pi Proportional coefficient and integral coefficient of PI controller; p n Is the output active rating of the VSG; p is an output active measurement value of the VSG;
switching power and voltage reference commands of the VSG to success and voltage ratings, synchronizing the control parameters
And
dynamically adjusting the voltage to corresponding values J, D and K in a normal operation state, and calculating the amplitude and phase reference value of the VSG output voltage according to the following control equation:
wherein,
the phase angle is referenced for the output voltage of the VSG during fault recovery control.
As an improvement of the present invention, the step S31 modeling process of the small signal model further includes:
the output power of the VSG at the operating point is:
the corresponding output power small signal model is as follows:
the VSG power outer loop control equation in the normal operation state is as follows:
neglecting the influence of the links of the voltage and current double-loop controller and the LC filter circuit, and taking U as U s And obtaining a small signal model of a control equation:
and (4) combining the above formula with an output power calculation formula to obtain a VSG grid-connected small signal model at the fault operation point.
As another improvement of the present invention, in the step S33, if the required control parameters of dynamic performance and stability performance are satisfied
And
do not exist, by adjusting the operating point, or by varying the primary coefficient of modulation k ω Primary voltage regulation coefficient k u VSG grid-connected connection circuit reactance X, damping ratio zeta and stable regulation time t s To obtain a feasible
And
and (4) taking values.
Compared with the prior art, the invention provides a VSG control method under the symmetrical drop of the voltage of the power grid aiming at the defects of the existing VSG low-voltage ride-through control strategy, and based on real-time detection and calculation of the voltage of the power grid and a voltage reference instruction, the active and voltage reference instructions are switched after the fault occurs, the VSG control method can adapt to the VSG with different voltage drop degrees to output active and reactive control, and the safe and stable operation capability of the VSG and the parallel power grid during the fault is improved; calculating control parameters at the fault operation point based on the VSG grid-connected small signal model at the fault operation point and the constraint conditions of dynamic performance and stability performance
And
the dynamic adjustment can meet the requirements of the dynamic performance and stability of the VSG at different operating points; during the fault period, the VSG output voltage is subjected to closed-loop control, and certain voltage and frequency support can be provided for a power grid; a fault current-limiting control model is designed, canThe output peak current of the VSG in the voltage drop period of the power grid can be effectively limited, and the output characteristic of the VSG in the normal operation state is not influenced; finally, aiming at the problem of smooth transition of the VSG from the fault operation state to the normal operation state, smooth and rapid transition of the VSG from the fault operation state to the normal operation state after the voltage of the power grid is recovered can be realized by adopting a phase angle regulating quantity dynamic regulation mode, and the method is simple and easy to operate and has good control effect.
Drawings
FIG. 1 is a flow chart of the steps of a VSG control method under symmetrical sag of grid voltage in accordance with the present invention;
FIG. 2 is a diagram of the relationship between a voltage loop control model, a fault current limit control model and a current loop control model according to the present invention;
fig. 3 is a system control block diagram of embodiment 1 of the present invention.
Detailed Description
The invention will be explained in more detail below with reference to the drawings and examples.
Example 1
A VSG control strategy under a grid voltage symmetric droop fault, as shown in fig. 1, includes the following steps:
step 10), the micro-grid energy management layer collects grid voltage information at a tie line and transmits the grid voltage information to a local controller of a VSG in the micro-grid;
step 20), when the symmetric drop fault of the power grid voltage is detected, the VSG local controller calculates active and reactive reference instructions according to the received power grid voltage information;
wherein Q ref And P ref Reference commands for reactive power and active power, U, of VSG during a fault, respectively G Is the per unit value of the grid voltage amplitude, S N Rated installed capacity, S, of VSG max Active power capacity is transferred for the line.
Calculating a voltage reference command according to the reactive power reference command:
U ref ＝U g +k Qp (Q ref -Q)+k Qi ∫(Q ref -Q) (2)
wherein U is ref Voltage command for VSG during fault, U g For instantaneous measurement of the amplitude of the mains phase voltage, k Qp And k Qi Proportional coefficient and integral coefficient of PI controller, Q is the output reactive measurement value of VSG.
Step 30) supposing that the amplitude of the grid phase voltage falls to U g0 And the phase voltage amplitude of the VSG output voltage after fault stabilization is U s0 VSG output voltage phase leads grid voltage theta after fault stabilization 0 Then VSG is at the point of failure operation (U) s0 ∠θ 0 ,U g0 Angle 0) is:
the corresponding output power small signal model is as follows:
the VSG power outer loop control equation in the normal operation state is as follows:
neglecting the influence of the links of the voltage and current double-loop controller and the LC filter circuit, and taking U as U s And obtaining a small signal model of a control equation:
the combined vertical type (3) and the formula (6) obtain a fault operation point (U) shown in a final formula (7) s0 ∠θ 0 ,U g0 Angle 0) is connected to the VSG grid small signal model:
wherein
In formula (7), θ, ω and U s Phase disturbance quantity, angular frequency disturbance quantity and amplitude disturbance quantity which are respectively VSG output voltage reference instructions, J is rotational inertia, D is damping coefficient, K is excitation adjusting inertia, K is excitation adjusting inertia ω And k is u Primary frequency modulation coefficient and primary voltage regulation coefficient, omega, respectively n For output angular frequency ratings, X is the VSG grid connection line reactance.
In order to make the VSG at the operating point (U) s0 ∠θ 0 ,U g0 The angle 0) meets the given dynamic performance and stability performance indexes, and the following constraint conditions are set:
in the formula S 1,2 Leading poles of VSG grid-connected small signal model characteristic equations at corresponding operating points, wherein zeta is a damping ratio and omega s Is the natural frequency, t s To stabilize the adjustment time, Δ is the error band.
And calculating the value ranges of the VSG control parameters J, D and K meeting the given dynamic performance and stability performance indexes according to the formula (7) and the formula (9). Then, the amplitude and phase reference value of the VSG output voltage are calculated by the VSG power outer loop according to the following control equation:
in the formula, theta, omega and U are respectively a phase reference value, an angular frequency reference value and an amplitude reference value of the VSG output voltage,
and
the specific value of the VSG control parameter J, D meeting the given dynamic performance and stability performance indexes at the fault operation point (Us0 & lttheta 0 & gt, Ug0 & lt0 & gt) and within the K range; q n For the output reactive rating of the VSG, P and Q are the output active and reactive measurements of the VSG, respectively.
Step 40) collecting local voltage and current information through a local controller, respectively establishing a voltage loop control model, a fault current-limiting control model and a current loop control model, as shown in fig. 2, collecting local voltage information by the VSG local controller, and establishing the voltage loop control model:
in the formula i idref And i iqref Respectively representing the d-axis component and the q-axis component, k, of the current reference set-point in the dq reference frame up And k is ui Respectively representing the proportional coefficient and the integral coefficient of the voltage PI controller,
and
a d-axis component and a q-axis component, u, respectively representing a voltage reference value in a dq reference frame od And u oq Respectively representing the d-axis component and the q-axis component of the inverter output voltage in a dq reference frame, C f The capacitance value in an LC filter connected with an inverter terminal is represented, and a dq reference coordinate system is a direct-current rotating coordinate system obtained by subjecting an abc alternating-current static coordinate system to park transformation;
then, establishing the following fault current-limiting control model to limit the fault transient peak current:
in the formula I m For the amplitude of the current after clipping, I max To trigger the maximum peak current value of the protection,
and
the d-axis component and the q-axis component of the current reference set value in the dq reference coordinate system after current limiting are respectively.
Then, collecting VSG local voltage and current information, and establishing a current loop control model:
in the formula u id And u iq Respectively representing d-axis component and q-axis component, k, of modulated wave voltage output by the inverter current controller under dq reference coordinate system ip And k is ii Respectively representing the proportional and integral coefficients, i, of a current PI controller id And i iq Respectively representing d-axis component and q-axis component, L, of the inverter output current value in dq reference frame f Representing the inductance value in the LC filter to which the inverter terminals are connected.
The relationship diagrams of the voltage loop control model, the fault current limiting control model and the current loop control model are shown in fig. 2.
Step 50) when a fault elimination is detected, a smooth transition from the fault state to the normal state is achieved by using the following phase angle adjustment:
δθ＝k Pp (P n -P)+k Pi ∫(P n -P) (14)
where δ θ is the VSG output phase angle adjustment, k Pp And k Pi Proportional and integral coefficients, P, of the PI controller, respectively n And P is the output active rated value of the VSG and the output active measured value of the VSG.
Switching power and voltage reference commands of the VSG to success and voltage ratings, synchronizing the control parameters
And
dynamically adjusting the voltage to corresponding values J, D and K in a normal operation state, and calculating the amplitude and phase reference value of the VSG output voltage according to the following control equation:
in the formula,
the phase angle is referenced for the output voltage of the VSG during fault recovery control.
The VSG control strategy under the power grid voltage symmetrical drop fault is formed, the whole control block diagram is shown in fig. 3, after the fault occurs, active and reactive reference instructions are calculated according to the real-time measured value of the power grid voltage, the voltage reference instruction is calculated, the active and reactive control output under different power grid voltage drop conditions is realized through the switching of the active and voltage reference instructions at the moment of the fault, and the safe and stable operation capacity of the VSG and the power grid during the fault is improved; calculating corresponding control parameters through VSG grid-connected small signal model at fault operation point and constraint conditions of dynamic performance and stability performance
And
and dynamic adjustment is carried out according to different fault operation points, so that the dynamic performance and the stability of the different fault operation points can be metRequiring; during the fault period, certain voltage and frequency support can be provided for a power grid by carrying out closed-loop control on the output voltage of the VSG; the limitation of VSG fault transient state peak current is realized through a fault current limiting control model, and the normal operation output characteristic of the VSG fault transient state peak current is not influenced; after the fault is eliminated, smooth and quick transition of the virtual synchronous machine from the normal running state of the fault running state is realized through the phase angle regulating quantity, the realization is simple, and the effect is obvious.
Example 2
The present embodiment is different from embodiment 1 in that: operating point of failure (U) s0 ∠θ 0 ,U g0 And h 0) is determined according to the grid voltage drop to different degrees in the step 20), and the VSG is set to provide different output active power and reactive power. At the operating point (U) s0 ∠θ 0 ,U g0 Under the condition of less than 0), the primary frequency modulation coefficient and the primary voltage regulation coefficient are respectively k ω And k is u And the control parameters of the dynamic performance and the stability performance meeting the requirements under the condition that the reactance of the VSG grid-connected connection line is X
And
may not exist, if this happens, it is possible to adjust the operating point, or to change the primary modulation factor k ω Primary voltage regulation coefficient k u VSG grid-connected connection circuit reactance X, damping ratio zeta and stable regulation time t s To obtain a feasible
And
and (4) value taking is carried out, so that the steps are continued, and smooth and quick transition of the virtual synchronous machine from the fault operation state to the normal operation state is realized.
In step 50), when the grid fault elimination is detected, smooth recovery control is enabled, fault recovery control of the virtual synchronous machine is realized through the phase angle regulating quantity, when the VSG outputs active power and reactive power to reach normal state values, the normal operation state needs to be maintained for a period of time, and when the set time is met, the recovery control process is completed.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited by the foregoing examples, which are provided to illustrate the principles of the invention, and that various changes and modifications may be made without departing from the spirit and scope of the invention, which is also intended to be covered by the appended claims. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (3)
1. A VSG control method under a power grid voltage symmetric drop fault is characterized by comprising the following steps:
s1, collecting the network voltage information at the tie line and transmitting the network voltage information to a local controller of a VSG in the microgrid;
s2, carrying out fault detection on the power grid voltage information acquired in the step S1, and calculating active and reactive reference instructions when a power grid voltage symmetrical drop fault is detected;
wherein Q is ref And P ref Respectively a reactive power reference instruction and an active power reference instruction of the VSG in the fault period; u shape G Is the per unit value of the voltage amplitude of the power grid; s N Rated installed capacity for the VSG; s max Transmitting active power capacity for the line;
calculating a voltage reference command according to the reactive power reference command:
U ref ＝U g +k Qp (Q ref -Q)+k Qi ∫(Q ref -Q)
wherein, U ref Voltage command for VSG during fault; u shape g For the phase voltage amplitude of the networkA value instantaneous measurement value; k is a radical of Qp And k Qi Proportional coefficient and integral coefficient of PI controller; q is the output reactive measurement value of VSG;
s3, establishing a grid-connected small signal model at the fault operation point and solving control parameters meeting given dynamic performance and stability performance indexes
And
synchronous switching of active and voltage ratings to active and voltage reference commands, and switching control parameters
And
the steps further include:
s31, establishing a grid-connected small signal model at the fault operating point, wherein the model is as follows:
wherein
Wherein, U g0 For the amplitude of the grid phase voltage after a drop, U g0 The angle 0 is the grid voltage phasor after the drop; u shape s0 The phase voltage amplitude of the VSG output voltage after fault stabilization; theta 0 For the VSG output voltage to lead the phase value of the grid voltage, U s0 ∠θ 0 Is the output voltage phasor of the VSG after the voltage sag; theta, omega and U s Respectively outputting a phase disturbance quantity, an angular frequency disturbance quantity and an amplitude disturbance quantity of a voltage reference command for the VSG; j is a rotationInertia; d is a damping coefficient; k is excitation adjusting inertia; k is a radical of ω And k is u Respectively a primary frequency modulation coefficient and a primary voltage regulation coefficient; omega n Is the output angular frequency rating; x is reactance of a VSG grid-connected connection circuit;
s32, setting a constraint condition to enable the VSG to meet given dynamic performance and stable performance indexes at the operation point, wherein the constraint condition is as follows:
wherein S is 1,2 Connecting the dominant pole of the small signal model characteristic equation to the VSG at the corresponding operating point; ζ is the damping ratio; omega s Is a natural frequency; t is t s The time is adjusted for stability; delta is an error band;
s33, calculating the VSG control parameters meeting the given dynamic performance and stability performance indexes according to the step S31 and the step S32
And
synchronously switching active and voltage nominal values to active and voltage reference commands, switching control parameters
And
s34, calculating the amplitude and phase reference value of the VSG output voltage according to the following control method:
wherein, θ, ω and U are phase references of the VSG output voltageA value, an angular frequency reference value and an amplitude reference value;
and
respectively taking values of VSG control parameters meeting given dynamic performance and stability performance indexes at a fault operation point; q n An output reactive rating for the VSG; p and Q are respectively output active and reactive measurement values of the VSG;
s4, gather local voltage and current information through local controller, establish voltage ring control model, trouble current-limiting control model and current ring control model respectively, voltage ring control model is:
wherein i idref And i iqref Respectively representing a d-axis component and a q-axis component of a current reference set value under a dq reference coordinate system; k is a radical of up And k is ui Respectively representing a proportional coefficient and an integral coefficient of the voltage PI controller;
and
a d-axis component and a q-axis component respectively representing voltage reference values in a dq reference frame; u. of od And u oq Respectively representing a d-axis component and a q-axis component of the inverter output voltage in a dq reference frame; c f Represents a capacitance value in an LC filter to which the inverter terminal is connected; the dq reference coordinate system is a direct-current rotating coordinate system obtained by performing park transformation on an abc alternating-current static coordinate system; taking a phase reference value theta of the VSG output voltage from a phase angle transformed from an abc reference coordinate system to a dq reference coordinate system;
the fault current-limiting control model is as follows:
wherein, I m The amplitude of the current after amplitude limiting; i is max The maximum peak current value is the trigger protection;
and
d-axis component and q-axis component of the current reference set value in the dq reference coordinate system after current limiting are respectively;
the current loop control model is as follows:
wherein u is id And u iq Respectively representing a d-axis component and a q-axis component of a modulated wave voltage output by the inverter current controller under a dq reference coordinate system; k is a radical of formula ip And k is ii Respectively representing a proportional coefficient and an integral coefficient of the current PI controller; i.e. i id And i iq A d-axis component and a q-axis component respectively representing the inverter output current value under the dq reference coordinate system; l is f Representing the inductance value in the LC filter to which the inverter terminals are connected;
s5, when a fault cancellation is detected, a smooth transition from the fault state to the normal state is achieved with the following phase angle adjustments:
δθ＝k Pp (P n -P)+k Pi ∫(P n -P)
wherein, δ θ is the VSG output phase angle regulating quantity; k is a radical of Pp And k Pi Proportional coefficient and integral coefficient of PI controller; p n Is the output active rating of the VSG; p is an output active measurement value of the VSG;
switching power and voltage reference commands of a VSGSuccess rate and voltage rating, synchronizing the control parameters
And
dynamically adjusting the voltage to corresponding values J, D and K in a normal operation state, and calculating the amplitude and phase reference value of the VSG output voltage according to the following control equation:
wherein,
the phase angle is referenced for the output voltage of the VSG during fault recovery control.
2. The method for controlling the VSG under the grid voltage symmetric sag fault according to claim 1, wherein the step S31 small-signal model modeling process further comprises:
the output power of the VSG at the operating point is:
the corresponding output power small signal model is as follows:
the VSG power outer loop control equation in the normal operation state is as follows:
neglecting the influence of the links of the voltage and current double-loop controller and the LC filter circuit, and taking U as U s And obtaining a small signal model of a control equation:
and (4) combining the above formula with an output power calculation formula to obtain a VSG grid-connected small signal model at the fault operation point.
3. The method according to claim 2, wherein in step S33, if the required control parameters of dynamic performance and stability performance are met, the method further comprises
And
do not exist, by adjusting the operating point, or by changing the primary coefficient of modulation k ω Primary voltage regulation coefficient k u VSG grid-connected connection circuit reactance X, damping ratio zeta and stable regulation time t s To obtain a feasible
And
and (4) taking values.
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