VSG-based low voltage ride through control method

Abstract

The invention provides a VSG-based low-voltage ride-through control method, which belongs to the field of power electronic technology control and comprises the following steps: monitoring that a voltage per unit value of a power grid module drops to 0.2 pu to 0.9 pu: adjusting an electric potential amplitude E in the VSG to be synchronous with a fault voltage amplitude U of a grid-connected point; meanwhile, an active limit value per unit value is calculated according to the measured power grid voltage per unit value, an active reference value per unit value is set to be the smaller one of the calculated active limit value per unit value and an active reference value before a fault, and a reactive reference value is set to be a reactive reference per unit value calculated according to the measured power grid module voltage per unit value. In the fault period, a voltage adjusting module is arranged in a reactive power-voltage loop, and output voltage is set to be synchronous with power grid fault voltage; a power reference value dynamic adjusting module is arranged in the active-frequency loop, a reactive power reference value is calculated and set, reactive power support is provided for a power grid, active power limitation reaches current limitation, and low-voltage ride-through control is achieved; hardware and a large amount of real-time data are not needed, and the method has the characteristics of high reliability and strong operability.

Description

Technical Field

[0001] The invention relates to the field of power electronic technology control, and in particular to a low voltage ride-through control method based on a VSG. Background Art

[0002] like Figure 1 and 2 As shown, the schematic diagram of the principle structure of the VSG main circuit and its control circuit system and the voltage phasor diagram of the prior art (virtual synchronous generator (VSG)), the system includes a main circuit and a control circuit, the main circuit includes an inverter module, an LC filter module, and a power grid module, wherein the inverter module, the LC filter module, and the power grid module are connected in series in sequence, and the control circuit includes a VSG control module and an SPWM modulation module; the VSG control module is mainly composed of an active-frequency loop, a reactive-voltage loop, and a voltage-current double closed loop. Active-droop control is used to simulate the prime mover speed regulator of the synchronous generator, and the rotor mechanical equation is used to simulate the inertia and damping characteristics of the synchronous generator, and the two together constitute the active-frequency loop; reactive-droop control is used to simulate the excitation voltage regulation part of the synchronous generator to adjust the reactive power output of the VSG; and the voltage-current double closed loop decoupling control and sinusoidal pulse width modulation (SPWM) technology are used to control the inverter, so as to maintain the stability of the system voltage.

[0003] Its working principle is as follows: the three-phase voltage and current output by the inverter module are sampled to calculate the actual output power of VSG; the active power reference value of VSG is subtracted from the active power actually output by VSG, and the system frequency adjustment is generated through the rotor mechanical equation; the system output frequency is updated through the calculated system frequency adjustment; the system output frequency is integrated to obtain the phase angle for SPWM modulation; the reactive power reference value of VSG is subtracted from the actually calculated reactive power output by VSG, and the voltage amplitude is adjusted through the PI controller to generate a VSG reference voltage signal; the VSG reference voltage is compared with the triangular carrier to generate a PWM signal to drive the inverter switch tube; the high-frequency harmonics are filtered out through the LC filter module to output a smooth three-phase voltage.

[0004] In the figure ga 、i gb 、i gc They are the three-phase current output by VSG; Z g =R g +jXL g is the equivalent impedance of the line; the filtering structure consists of the inductor L f , capacitor C f And the resistor R f Composition; e abcis the internal potential of VSG; U dc is a DC voltage source; ω N is the rated angular frequency of the system; ω is the actual angular frequency of the system; Δω is the difference between ω and ω N The difference between p , K q are active-frequency and reactive-voltage droop coefficients respectively; P ref is the set VSG active reference value; P m is the VSG virtual mechanical power; P e is the actual active power output of VSG; J is the virtual moment of inertia of VSG; D is the damping coefficient of VSG; s is the Laplace operator; θ ref is the reference phase of the potential inside the VSG; Q ref VSG output reactive power reference value; Q e is the reactive power actually output by VSG; E is the potential amplitude inside VSG; E ref is the reference amplitude of the potential inside the VSG.

[0005] When analyzing the fault characteristics of a VSG, the reference diagram is as follows Figure 2 As shown in the figure, the phase difference between the VSG output voltage phasor and the grid module voltage phasor is δ 0 (Due to the line impedance Z g is very small, so when the system is working normally 0 Very small, That is, the difference between the VSG output voltage and the grid module voltage caused by the existence of line impedance). However, when the grid module voltage drops, the grid module voltage amplitude will decrease accordingly, and the grid module voltage will increase from the value before the fault. Jump to At this time, the phase difference between the VSG output voltage phasor and the grid module voltage phasor becomes δ F Due to the regulation of the reactive-voltage loop, the VSG output voltage phase changes from becomes Due to the effect of virtual inertia, the phase of the VSG output phase voltage cannot change quickly, so and Still remain in the same phase.

[0006] In actual operation, the distribution network is susceptible to short-circuit faults and grid module voltage drops. At this time, the grid-connected inverter needs to have sufficient low voltage ride-through capability to ensure the stable operation of the grid module under fault conditions. However, the existing VSG control technology cannot suppress overcurrent during voltage drops, and it is difficult to provide sufficient reactive power support to the grid module, resulting in unstable operation of the grid module.

[0007] In response to the above-mentioned problems such as the overcurrent of the inverter caused by the voltage amplitude and phase jump at the VSG grid connection point, the author Zheng Tao published a solution in the article "Research on Low Voltage Ride-Through Control Strategy of Virtual Synchronous Generator Based on Phase Jump Compensation" published in "Power System Technology" Volume 47, Issue 1, January 2023. By switching the VSG voltage amplitude reference value and compensating the reference phase angle, the inverter module port voltage is quickly regulated, thereby achieving VSG overcurrent suppression and VSG power rapid output during faults; in addition, the auxiliary control strategy is used to achieve smooth output of low voltage ride-through control and restore the normal working state of VSG. However, this method requires monitoring a large amount of real-time data to calculate the required physical quantities, and it is necessary to set switch switching modules at multiple different positions of the active-frequency loop and the reactive-voltage loop, which increases the difficulty of actual operation and reduces the accuracy of control. Summary of the invention

[0008] In order to solve the above problems, the present invention provides a VSG-based low voltage ride-through control method with voltage and active power adjustment and reactive power compensation, which is simple to operate and has high reliability, can provide reactive power support for the grid module during the VSG low voltage ride-through period, and solve the problem of excessive voltage difference between the VSG output voltage and the grid module voltage caused by phase change. The method includes:

[0009] When the system monitors the power grid module and detects that the three-phase voltage per unit value drops to the range of 0.2pu to 0.9pu; according to the monitored power grid module voltage per unit value, a voltage adjustment module is set in the VSG reactive-voltage loop to adjust the potential amplitude E in the VSG to be synchronized with the fault voltage amplitude U of the grid connection point; at the same time, according to the monitored power grid module voltage per unit value, an active reference value adjustment module is set in the VSG active-frequency loop to calculate the VSG active limit per unit value during the three-phase short circuit fault, the VSG active reference per unit value is set to the smaller value of the calculated VSG active limit per unit value during the three-phase short circuit fault and the VSG active reference per unit value set by the system before the fault, and the VSG reactive reference per unit value is set to the VSG reactive reference per unit value during the three-phase short circuit fault calculated according to the monitored power grid module voltage per unit value.

[0010] Preferably, the method further comprises:

[0011] When the per-unit value of the three-phase voltage of the system monitoring grid module drops to a range greater than 0.9pu, the per-unit value of the VSG output reactive power reference value is set to 0.

[0012] Preferably, the method for calculating the per-unit value of the VSG active power limit value during a three-phase short circuit fault includes:

[0013] Step 1: According to the monitored grid module voltage per unit value, when it drops to the range of 0.2pu to 0.9pu, the VSG reactive power reference value per unit value during three-phase short circuit fault is calculated by formula (1): :

[0014] Formula (1)

[0015] In the formula, is the VSG reactive power reference value per unit value in the case of three-phase short circuit fault, without unit; K is the VSG reactive power compensation coefficient, which is 1.5; |U| is the fault voltage amplitude of the grid connection point, in volts; U N is the rated voltage of the grid module, in volts;

[0016] Step 2: Divide the VSG reactive current per unit value calculated above by the grid module voltage per unit value to obtain the VSG reactive current per unit value. , that is, formula (2):

[0017] Formula (2)

[0018] In the formula, is the unit value of VSG reactive current; is the per unit value of the grid module voltage;

[0019] Step 3: Subtract the square of the VSG reactive current per unit value calculated above from the square of the preset VSG current limit multiple to obtain the VSG active current per unit value, that is, formula (3):

[0020] Formula (3)

[0021] In the formula, is the per unit value of VSG active current; is the VSG current limit multiple, the value is 1.5;

[0022] Step 4: Multiply the VSG active current per unit value calculated above by the grid module voltage per unit value to obtain the VSG active limit per unit value during a three-phase short circuit fault, that is, formula (4):

[0023] Formula (4)

[0024] In the formula, It is the per-unit value of VSG active power limit in case of three-phase short circuit fault.

[0025] Preferably, the method further comprises:

[0026] When the system monitors that the three-phase voltage per unit value of the power grid module drops to less than 0.2pu, the VSG is ordered to exit the system.

[0027] The present invention provides a VSG low voltage ride-through control method with voltage and active power adjustment and reactive power compensation. By introducing a voltage adjustment module into the VSG reactive power-voltage loop, during the fault of the power grid module, the VSG output voltage is set to be synchronized with the fault voltage of the power grid module. At the same time, an active reference value adjustment module is added to the VSG active power-frequency loop, and the VSG reactive reference value per unit value is calculated according to the real-time monitored power grid module voltage per unit value, providing certain reactive power support to the power grid module; and the VSG active power limit per unit value is calculated according to the real-time monitored power grid module voltage per unit value during a three-phase short circuit fault, and the VSG active power reference value per unit value is set to the smaller value of the calculated VSG active power limit per unit value during a three-phase short circuit fault and the VSG active power reference value per unit value set by the system before the fault, so as to reduce the fault current and complete the low voltage ride-through control of the system. The VSG low voltage ride-through control method provided by the present invention only needs to add a voltage adjustment module in the reactive-voltage loop, and directly introduces a formula to calculate the VSG active reference value in real time. There is no need to add additional hardware equipment, and there is no need to calculate the required physical quantities by monitoring a large amount of real-time data. It has the characteristics of high reliability and strong operability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the principle structure of the VSG main circuit and its control circuit system in the prior art;

[0029] Figure 2 It is the voltage phasor diagram under VSG control in the prior art;

[0030] Figure 3 A schematic diagram of the principle structure of a VSG main circuit and its control circuit system with a low voltage ride-through control function provided by the present invention;

[0031] Figure 4 for Figure 3 The VSG low voltage ride-through control flow chart shown in;

[0032] Figure 5 This is a simulation diagram of the voltage change of the grid module when a three-phase short circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2pu without the low voltage ride-through control method of the VSG;

[0033] Figure 6 This is a simulation diagram of the voltage change of the grid module when the grid module voltage has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu.

[0034] Figure 7This is a simulation diagram of the change of VSG output current when a three-phase short circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2pu without the low voltage ride-through control method;

[0035] Figure 8 This is a simulation diagram of the change in output current of the VSG with a low voltage ride-through control method when a three-phase short circuit occurs in the grid module voltage and the grid connection point voltage drops to 0.2pu;

[0036] Fig. 9 This is a simulation diagram of the change of VSG output active power without low voltage ride-through control method when a three-phase short circuit occurs in the grid module voltage and the grid connection point voltage drops to 0.2pu;

[0037] Fig.10 This is a simulation diagram of the change of active power output of the VSG with a low voltage ride-through control method when a three-phase short circuit occurs in the grid module voltage and the grid connection point voltage drops to 0.2pu;

[0038] Fig.11 It is a simulation diagram of the reactive power change of VSG output without low voltage ride-through control method when the grid module voltage has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu;

[0039] Fig.12 This is a simulation diagram of the reactive power change of VSG output with low voltage ride-through control method when a three-phase short circuit occurs in the grid module voltage and the grid connection point voltage drops to 0.2pu. DETAILED DESCRIPTION

[0040] A VSG low voltage ride through control method with voltage and active power adjustment and reactive power compensation provided by the present invention is described in detail below with reference to specific embodiments and accompanying drawings.

[0041] like Figure 3As shown, it is a schematic diagram of the principle structure of a VSG main circuit and its control circuit system with low voltage ride-through control function provided by the present invention; the system includes a main circuit and a control circuit, the main circuit includes an inverter module, an LC filter module, and a power grid module, wherein the inverter module, the LC filter module, and the power grid module are connected in series in sequence, and the control circuit includes a VSG control module and an SPWM modulation module; the VSG control module is mainly composed of an active-frequency loop, a reactive-voltage loop, and a voltage-current double closed loop; the active-droop control is used to simulate the prime mover speed regulator of the synchronous generator, and the rotor mechanical equation is used to simulate the inertia and damping characteristics of the synchronous generator, and the two together constitute the active-frequency loop; the reactive-droop control is used to simulate the excitation voltage regulation part of the synchronous generator to adjust the reactive power output of the VSG; and the voltage-current double closed loop decoupling control and SPWM technology control are used. Control inverter; the present invention adds a voltage adjustment module in the above-mentioned VSG reactive-voltage loop, and adds an active reference value adjustment module in the above-mentioned VSG active-frequency loop, and its working principle is: sampling the three-phase voltage and current output by the inverter module, calculating the actual output power of the VSG; subtracting the VSG active reference value from the actual active power output of the VSG, and generating the system frequency adjustment amount through the rotor mechanical equation; updating the system output frequency through the calculated system frequency adjustment amount; integrating the system output frequency to obtain the phase angle for SPWM modulation; subtracting the VSG reactive reference value from the actually calculated VSG output reactive power, adjusting the voltage amplitude through the PI controller, and generating a VSG reference voltage signal; comparing the VSG reference voltage with the triangular carrier, generating a PWM signal to drive the inverter switch tube; filtering out high-frequency harmonics through the LC filter module, and outputting a smooth three-phase voltage. The voltage adjustment module added to the reactive power-voltage loop is used to set the output voltage of the VSG to the rated voltage of the grid module during the normal operation of the grid module; when a three-phase short circuit occurs in the grid module, the output voltage of the VSG is adjusted to be synchronized with the fault voltage of the grid connection point, thereby controlling the output voltage value of the VSG. The active power reference value adjustment module added to the active power-frequency loop is used to calculate the VSG active power reference value per unit value in real time according to the designed calculation method when a three-phase short circuit occurs in the grid module, thereby controlling the active power reference value per unit value of the VSG.

[0042] In the figure ga 、i gb 、i gc They are the three-phase current output by VSG; Z g =R g +jXL g is the line equivalent impedance; the filtering structure consists of the inductor L f , capacitor C f And the resistor R f Composition; e abc is the internal potential of VSG; Udc is a DC voltage source; ω N is the rated angular frequency of the system; ω is the actual angular frequency of the system; Δω is the difference between ω and ω N The difference between p , K q are active-frequency and reactive-voltage droop coefficients respectively; P ref is the set VSG active reference value; P m is the VSG virtual mechanical power; P e is the actual active power output of VSG; J is the virtual moment of inertia of VSG; D is the damping coefficient of VSG; s is the Laplace operator; θ ref is the reference phase of the potential inside the VSG; Q ref is the VSG reactive power reference value; Q e is the reactive power actually output by VSG; E is the potential amplitude inside VSG; E ref is the potential reference amplitude in VSG, |U| is the fault voltage amplitude at the grid connection point; U N is the rated voltage of the grid module.

[0043] Here’s how it works:

[0044] During the normal operation of the power grid module, let E = U N =380V, P ref =10 4 W, Q ref =0, that is, VSG operates in the traditional mode;

[0045] Once a three-phase symmetrical short-circuit fault is detected, the VSG low voltage ride-through control method is as follows:

[0046] First, when the grid module voltage per unit value is greater than 0.9pu, the inverter module does not need to provide reactive power support to the grid module, that is, Q ref =0; when the grid module voltage drops to the range of 0.2pu to 0.9pu, the potential amplitude E in the VSG is adjusted to be synchronized with the fault voltage amplitude U at the grid connection point according to the monitored grid module voltage per unit value. At the same time, according to the real-time monitored grid module voltage per unit value, the inverter module needs to adjust the reactive output of the VSG according to formula (1) and limit the active power output of the VSG; when the grid module voltage drops to less than 0.2pu, it indicates that the grid module is in a serious fault state; due to the excessive voltage drop, it is difficult to limit the VSG output overcurrent in a serious fault state, and the VSG should be shut down at this time:

[0047] Formula (1)

[0048] In the formula, is the standard value of VSG reactive power reference value when there is a three-phase short circuit fault; K is the VSG reactive power compensation coefficient, which is 1.5; |U| is the fault voltage amplitude of the grid connection point; U N is the rated voltage of the grid module;

[0049] The specific setting method of the active power limit value output by the above-mentioned VSG is as follows (i.e., the setting method of the per unit value of the active power reference value of the VSG):

[0050] The VSG reactive current per unit value is obtained by dividing the VSG reactive current per unit value obtained by formula (1) during a three-phase short-circuit fault by the grid module voltage per unit value. That is, the VSG reactive current per unit value is calculated according to formula (2):

[0051] Formula (2)

[0052] In the formula, is the unit value of VSG reactive current; is the per unit value of the grid module voltage;

[0053] Then, the square of the VSG reactive current per unit value calculated above is subtracted from the square of the preset VSG current limit multiple to obtain the VSG active current per unit value, that is, the VSG active current per unit value is calculated according to formula (3):

[0054] Formula (3)

[0055] In the formula, is the per unit value of VSG active current; is the VSG current limit multiple, the value is 1.5;

[0056] Next, the VSG active current per unit value calculated by formula (3) is multiplied by the grid module voltage per unit value to obtain the VSG active limit per unit value during a three-phase short circuit fault. That is, the VSG active limit per unit value during a three-phase short circuit fault is calculated according to formula (4):

[0057] Formula (4)

[0058] In the formula, It is the per unit value of VSG active power limit value in case of three-phase short circuit fault;

[0059] Finally, the VSG active reference value per unit value is selected as the smaller of the VSG active limit per unit value calculated by formula (4) during the three-phase short circuit fault and the VSG active reference value per unit value set by the system before the fault, that is:

[0060] Formula (5)

[0061] In the formula, The VSG active reference value is set per unit value; It is the per unit value of the VSG active reference value before the three-phase short circuit fault.

[0062] The reason for this value is that when the VSG active output is small before the three-phase short circuit fault or the voltage drop of the power grid module is light, the VSG active reference value per unit value set by the system before the fault may be lower than its active limit per unit value. At this time, there is no need to impose additional restrictions on the active output of the VSG to ensure that its steady-state fault current does not exceed the set limit. Through this control method, not only can the fault ride-through capability of the power grid module be effectively improved, but also the adverse effects of the power grid module fault on the power system can be minimized to ensure the safe and stable operation of the power system.

[0063] like Figure 4 As shown in the figure, the overall structure diagram of the control method proposed in the present invention coordinates the VSG output voltage and its active reference value, and provides certain reactive power support to the power grid module according to regulations, which helps to limit the VSG steady-state overcurrent to a reasonable range when a three-phase short circuit fault occurs in the power grid module, maintain the stability of the VSG output power, and ensure reliable operation of the system.

[0064] According to the monitored three-phase short-circuit fault grid module voltage per unit value, when it drops to the range of 0.2pu to 0.9pu, the VSG reactive power reference value per unit value is calculated by the following formula: , set the VSG reactive power reference value to the VSG reactive power reference value per unit value calculated according to the monitored grid module voltage per unit value when a three-phase short circuit fault occurs:

[0065] Formula (1)

[0066] In the formula, is the VSG reactive power reference value per unit value in the case of three-phase short circuit fault, without unit; K is the VSG reactive power compensation coefficient (the value is 1.5); |U| is the fault voltage amplitude of the grid connection point, in volts; U N is the rated voltage of the grid module in volts.

[0067] At the same time, under the condition that the voltage drop amplitude of the grid module and the VSG power angle remain constant, the output current level during the fault period can be effectively reduced by dynamically adjusting the output voltage of the VSG. This regulation mechanism aims to reduce the phasor difference between the internal potential of the VSG and the grid module voltage, thereby reducing the steady-state fault current value. To achieve this goal, a voltage dynamic control strategy is adopted. During the normal operation of the grid module, the output voltage E=U NOnce a symmetrical short-circuit fault is detected, E is immediately adjusted to be synchronized with the fault voltage amplitude U at the grid connection point, so as to effectively suppress the steady-state overcurrent caused by the short circuit. The implementation of this voltage dynamic control strategy can not only improve the stability and reliability of the power grid module under fault conditions, but also extend the service life of power equipment and reduce equipment damage caused by overcurrent. In addition, the implementation of this strategy also helps the rapid recovery of the power grid module, reduces the impact of faults on the power system, and improves the overall operating efficiency of the power system.

[0068] At the same time, when the VSG reactive setting value is determined and the grid module voltage drops, it is necessary to limit the active reference value output by the VSG to achieve the purpose of current limiting. The specific setting method of the VSG active reference value is: calculate the VSG reactive reference value per unit value according to the real-time monitored grid module voltage per unit value; obtain the VSG reactive current per unit value by dividing the VSG reactive reference value per unit value by the grid module voltage per unit value; then use the square of the preset VSG current limit multiple (the present invention sets the multiple to 1.5) minus the square of the VSG reactive current per unit value to obtain the square of the VSG active current per unit value; then multiply the VSG active current per unit value by the grid module voltage per unit value to obtain the VSG active limit per unit value; finally, the VSG active reference value per unit value needs to take the smaller value between the calculated VSG active limit per unit value and the VSG active reference value per unit value before the fault. The reason for this value is that when the VSG active output is small before the three-phase short circuit fault of the power grid or the voltage drop of the power grid module is light, the VSG active reference value per unit value set by the system before the fault may be lower than its active limit per unit value. At this time, there is no need to impose additional restrictions on the active output of the VSG to ensure that its steady-state fault current does not exceed the set limit. Through this control method, not only can the fault ride-through capability of the power grid module be effectively improved, but also the adverse effects of the power grid module fault on the power system can be minimized to ensure the safe and stable operation of the power system.

[0069] In order to verify the effectiveness of the VSG low voltage ride-through control method with voltage and active power adjustment and reactive power compensation provided by the present invention, a simulation model was built on the MATLAB / Simulink simulation platform. Figure 3 The system structure shown in the figure, when there is no or no grid module failure, the waveforms of grid module voltage change, VSG output current change, VSG output active power change and VSG output reactive power change simulated by the low voltage ride-through control method provided by the present invention are respectively Figure 5 and 6 , 7 and 8, 9 and 10, 11 and 12.

[0070] Figure 5This is a simulation diagram of the voltage change of the grid module when the grid module voltage drops to 0.2pu after a three-phase short circuit fault occurs in the grid module voltage. The horizontal axis is time in seconds; the vertical axis is the per-unit value of the grid module voltage, without units. It can be seen from the figure that before the control method is added, the grid module voltage will fluctuate slightly at the beginning of the fault 1s and the end of the fault 1.3s.

[0071] Figure 6 This is a simulation diagram of the voltage change of the grid module when the grid module voltage drops to 0.2pu after a three-phase short circuit fault occurs in the grid module voltage. The horizontal axis is time, in seconds; the vertical axis is the per-unit value of the grid module voltage, without units. It can be seen from the figure that after adding the control method, the grid module voltage can reach the rated value in a short time at the corresponding time, which enhances the stability of the system.

[0072] Figure 7 This is a simulation diagram of the change in VSG output current when the grid module voltage has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu without the low voltage ride-through control method. The horizontal axis is time in seconds; the vertical axis is the per-unit value of the VSG output current, without units. It can be seen from the figure that before adding the control method, in the 1-1.3s fault section, the transient overcurrent of the VSG exceeded the rated value by about 5 times, and the steady-state fault current of the VSG reached 2.5 times the rated value before the fault, far exceeding the 1.5 times rated value stipulated by the state.

[0073] Figure 8 This is a simulation diagram of the change in VSG output current when the grid module voltage has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu. The horizontal axis is time, in seconds; the vertical axis is the per-unit value of the VSG output current, without units. As can be seen from the figure, after adding the control method, it can be seen that this method can limit the VSG steady-state fault current to 1.5 times the rated value, which is in line with national regulations, and the VSG transient overcurrent disappears when the fault occurs, and the VSG transient overcurrent at the moment the fault ends is also greatly reduced, which enhances the stability of the system.

[0074] Fig. 9 This is a simulation diagram of the change in VSG output active power when the grid module voltage has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu without the low voltage ride-through control method. The horizontal axis is time in seconds; the vertical axis is the VSG output active power value in watts. It can be seen from the figure that before adding the control method, in the 1-1.3s fault section, the VSG active power output is continuously unstable, and a large oscillation phenomenon occurs instantly after the fault ends, and it takes a long time to recover to stability.

[0075] Fig.10 This is a simulation diagram of the change in the active power output of the VSG with a low voltage ride-through control method when the voltage of the grid module has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu. The horizontal axis is time in seconds; the vertical axis is the active power output of the VSG in watts. It can be seen from the figure that after adding the control method, the active power output of the VSG during the fault period can gradually return to a stable state, and after the fault is over, compared with before adding the control method, the power oscillation is greatly reduced, the time required to return to a stable state is reduced, and the stability of the system is enhanced.

[0076] Fig.11 This is a simulation diagram of the reactive power change of VSG output when the voltage of the grid module drops to 0.2pu after a three-phase short circuit fault occurs in the grid module voltage, and the grid connection point voltage drops to 0.2pu. The horizontal axis is time in seconds; the vertical axis is the reactive power value of VSG output in vars. It can be seen from the figure that before adding the control method, in the 1-1.3s fault section, the reactive power output of VSG is basically 0, and it fails to provide a certain amount of reactive power support to the grid module during the fault period. Moreover, a large oscillation phenomenon occurs instantly after the fault ends, and it takes a long time to recover to stability.

[0077] Fig.12 This is a simulation diagram of the reactive power change of VSG output with low voltage ride-through control method when the voltage of the grid module has a three-phase short circuit fault and the grid connection point voltage drops to 0.2pu. The horizontal axis is time in seconds; the vertical axis is the reactive power value of VSG output in vars. It can be seen from the figure that after adding the control method, during the fault period, VSG can provide reactive power support that meets the regulations to the grid module, the power oscillation is greatly reduced, and the reactive power output of VSG can quickly stabilize, ensuring the reliable operation of the system.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A low voltage ride-through control method based on VSG, characterized in that: The method comprises: When the system monitoring grid module detects that the three-phase voltage per unit value drops to the range of 0.2pu to 0.9pu: according to the monitored grid module voltage per unit value, a voltage adjustment module is set in the VSG reactive-voltage loop to adjust the potential amplitude E in the VSG to be synchronized with the fault voltage amplitude U of the grid connection point; at the same time, according to the monitored grid module voltage per unit value, an active reference value adjustment module is set in the VSG active-frequency loop to calculate the VSG active limit per unit value during the three-phase short circuit fault, and the VSG active reference value per unit value is set to the smaller value of the calculated VSG active limit per unit value during the three-phase short circuit fault and the VSG active reference value per unit value set by the system before the fault, and the VSG reactive reference value is set to the VSG reactive reference value per unit value during the three-phase short circuit fault calculated according to the monitored grid module voltage per unit value.

2. The low voltage ride through control method according to claim 1, characterized in that: Also includes: When the per-unit value of the three-phase voltage of the system monitoring grid module drops to a range greater than 0.9pu, the per-unit value of the VSG reactive power reference value is set to 0.

3. The low voltage ride through control method according to claim 1 or 2, characterized in that: The method for calculating the per unit value of the VSG active power limit value during a three-phase short circuit fault includes: Step 1: According to the monitored grid module voltage per unit value, when it drops to the range of 0.2pu to 0.9pu, the VSG reactive power reference value per unit value during three-phase short circuit fault is calculated by formula (1): : Formula (1) In the formula, is the VSG reactive power reference value per unit value in the case of three-phase short circuit fault, without unit; K is the VSG reactive power compensation coefficient, which is 1.5; |U| is the fault voltage amplitude of the grid connection point, in volts; U N is the rated voltage of the grid module, in volts; Step 2: Divide the VSG reactive current per unit value calculated above by the grid module voltage per unit value to obtain the VSG reactive current per unit value. , that is, formula (2): Formula (2) In the formula, is the unit value of VSG reactive current; is the per unit value of the grid module voltage; Step 3: Subtract the square of the VSG reactive current per unit value calculated above from the square of the preset VSG current limit multiple to obtain the VSG active current per unit value, that is, formula (3): Formula (3) In the formula, is the per unit value of VSG active current; is the VSG current limit multiple, the value is 1.5; Step 4: Multiply the VSG active current per unit value calculated above by the grid module voltage per unit value to obtain the VSG active limit per unit value during a three-phase short circuit fault, that is, formula (4): Formula (4) In the formula, It is the per-unit value of VSG active power limit in case of three-phase short circuit fault.

4. The low voltage ride through control method according to claim 3, characterized in that: Also includes: When the system monitors that the three-phase voltage per unit value of the power grid module drops to less than 0.2pu, the VSG is ordered to exit the system.

Images