A Low-Voltage Ride-Through Control Method Based on VSG

Abstract

The present invention provides a low-voltage ride-through control method based on VSG, belonging to the field of power electronics technology control. It includes monitoring that the per-unit value of the grid module voltage drops within the range of 0.2 pu to 0.9 pu: adjusting the amplitude E of the internal potential of the VSG to be synchronized with the amplitude U of the fault voltage at the grid connection point; at the same time, calculating the per-unit value of the active power limit value from the measured per-unit value of the grid voltage, setting the per-unit value of the active power reference value as the smaller value between the calculated per-unit value of the active power limit value and the active power reference value before the fault, and setting the per-unit value of the reactive power reference value as the per-unit value of the reactive power reference value calculated from the measured per-unit value of the grid module voltage. During the fault, a voltage adjustment module is set in the reactive power-voltage loop, setting the output voltage to be synchronized with the grid fault voltage; and an active power reference value dynamic adjustment module is set in the active power-frequency loop, calculating and setting the reactive power reference value, providing reactive power support to the grid, and limiting the active power to achieve low-voltage ride-through control; there is no need to set up hardware and a large amount of real-time data, and it has the characteristics of high reliability and strong operability.

Description

Technical Field

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

[0002] As Figure 1 and 2 shown, it is a schematic diagram of the principle structure and a voltage phasor diagram of the main circuit and its control circuit system of the existing technology (virtual synchronous generator, abbreviated as 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. Among them, the inverter module, the LC filter module, and the power grid module are connected in series in sequence. 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 governor of the synchronous generator, and the rotor mechanical equation is used to simulate the inertia and damping characteristics of the synchronous generator. The two together form 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 by the VSG. And the voltage-current double closed loop decoupling control and the 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: sample the three-phase voltage and current output by the inverter module, and calculate the actual output power of the VSG; subtract the active reference value of the VSG from the actual output active power of the VSG, and generate a system frequency adjustment amount through the rotor mechanical equation; update the system output frequency through the calculated system frequency adjustment amount; integrate the system output frequency to obtain a phase angle for SPWM modulation; subtract the reactive reference value of the VSG from the actually calculated reactive power output by the VSG, adjust the voltage amplitude through a PI controller, and generate a VSG reference voltage signal; compare the VSG reference voltage with a triangular carrier wave to generate a PWM signal to drive the inverter switching tube; filter out high-frequency harmonics through the LC filter module and output smooth three-phase voltage.

[0004] In the figure, i ga , i gb , i gc are the three-phase currents output by the VSG respectively; Z g =R g +jXL g is the line equivalent impedance; the filter structure is composed of an inductor L f , a capacitor C f and a resistor R f ; e abcis the internal potential of the VSG; U dc is the 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 ; K p 、K q are the active - frequency and reactive - voltage droop coefficients respectively; P ref is the set active reference value of the VSG; P m is the virtual mechanical power of the VSG; P e is the actual active power output by the VSG; J is the virtual moment of inertia of the VSG; D is the damping coefficient of the VSG; s is the Laplace operator; θ ref is the reference phase of the internal potential of the VSG; Q ref is the reactive power reference value output by the VSG; Q e is the actual reactive power output by the VSG; E is the amplitude of the internal potential of the VSG; E ref is the reference amplitude of the internal potential of the VSG.

[0005] When analyzing the fault characteristics of the VSG, the schematic diagram for reference is as Figure 2 shown. The phase difference between the VSG output voltage phasor and the grid - module voltage phasor in the figure is δ0 (due to the very small line impedance Z g , so δ0 is very small during normal system operation, which is the difference between the VSG output voltage and the grid - module voltage caused by the existence of the line impedance), but when a voltage - dip fault occurs in the grid - module voltage, the amplitude of the grid - module voltage will decrease accordingly, and at the same time, the grid - module voltage jumps from before the fault 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 phasor changes from before the fault to . Due to the effect of the virtual inertia, the phase of the VSG output phase voltage cannot change rapidly. Therefore, and still remain in the same phase.

[0006] In actual operation, the distribution network is vulnerable to short - circuit faults, resulting in voltage dips in the grid - module voltage. At this time, it is required that the grid - connected inverter has sufficient low - voltage ride - through ability to ensure the stable operation of the grid - module in the fault state. However, the existing VSG control technology cannot suppress over - current during voltage dips and is also difficult to provide sufficient reactive power support to the grid - module, thus causing the grid - module to operate unstably.

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

[0008] 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 highly reliable, and can provide reactive power support for the grid module during VSG low-voltage ride-through 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 that the per-unit value of the three-phase voltage of the grid module drops within the range of 0.2 pu to 0.9 pu; according to the monitored per-unit value of the grid module voltage, through a voltage adjustment module provided in the VSG reactive-voltage loop, the amplitude E of the internal potential of the VSG is adjusted to be synchronized with the fault voltage amplitude U of the grid connection point; at the same time, according to the monitored per-unit value of the grid module voltage, through an active power reference value adjustment module provided in the VSG active-frequency loop, the per-unit value of the VSG active power limit during three-phase short-circuit faults is calculated, and the per-unit value of the VSG active power reference value is set to the smaller value of the calculated per-unit value of the VSG active power limit during three-phase short-circuit faults and the per-unit value of the VSG active power reference value set by the system itself before the fault, and the per-unit value of the VSG reactive power reference value is set to the per-unit value of the VSG reactive power reference value during three-phase short-circuit faults calculated according to the monitored per-unit value of the grid module voltage.

[0010] Preferably, the method further includes:

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

[0012] Preferably, the calculation method of the per-unit value of the VSG active power limit during three-phase short-circuit faults includes:

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

[0014] Formula (1)

[0015] In the formula, is the per-unit value of the VSG reactive power reference value during a three-phase short-circuit fault, dimensionless; K is the VSG reactive power compensation coefficient, with a value of 1.5; |U| is the amplitude of the grid connection point fault voltage, in volts; U N is the rated voltage of the grid module, in volts;

[0016] Step 2: Divide the per-unit value of the VSG reactive power reference value during the above-calculated three-phase short-circuit fault by the per-unit value of the grid module voltage to obtain the per-unit value of the VSG reactive current, , that is, formula (2):

[0017] Formula (2)

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

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

[0020] Formula (3)

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

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

[0023] Formula (4)

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

[0025] Preferably, the method further includes:

[0026] When the per-unit value of the monitored three-phase voltage of the grid module drops below 0.2 pu, the VSG is commanded 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 period of the power grid module, the output voltage of the VSG is set to be synchronized with the fault voltage of the power grid module. At the same time, an active power reference value adjustment module is added to the VSG active power-frequency loop. The per-unit value of the VSG reactive power reference value is calculated according to the per-unit value of the power grid module voltage monitored in real time, and a certain amount of reactive power support is provided to the power grid module. And the per-unit value of the VSG active power limit value during a three-phase short-circuit fault is calculated according to the per-unit value of the power grid module voltage monitored in real time. The per-unit value of the VSG active power reference value is set to the smaller value of the calculated per-unit value of the VSG active power limit value during the three-phase short-circuit fault and the per-unit value of the VSG active power reference value set by the system itself 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 to the reactive power-voltage loop, and directly introduces a formula to calculate the VSG active power reference value in real time, without adding additional hardware devices, and without calculating the required physical quantities by monitoring a large amount of real-time data, and has the characteristics of high reliability and strong operability. Description of the Drawings

[0028] Figure 1 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 is a voltage phasor diagram under the control of the prior art VSG;

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

[0031] Figure 4 is Figure 3 the VSG low-voltage ride-through control flow chart shown in

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

[0033] Figure 6 is a simulation diagram of the voltage change of the power grid module when the three-phase short-circuit fault occurs in the power grid module voltage and the grid-connected point voltage drops to 0.2 pu with the VSG low-voltage ride-through control method;

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

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

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

[0037] Figure 10 It 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 grid module voltage undergoes a three-phase short-circuit fault and the grid connection point voltage drops to 0.2 pu;

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

[0039] Figure 12 It is a simulation diagram of the change in the reactive power output of the VSG with a low-voltage ride-through control method when the grid module voltage undergoes a three-phase short-circuit fault and the grid connection point voltage drops to 0.2 pu. Specific implementation manners

[0040] The following uses specific embodiments and their accompanying drawings to describe in detail a VSG low-voltage ride-through control method with voltage and active power adjustment and reactive power compensation provided by the present invention.

[0041] Such as Figure 3As shown in the figure, it is a schematic diagram of the principle structure of the main circuit of a VSG with a low-voltage ride-through control function and its control circuit system 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, where the inverter module, the LC filter module, and the power grid module are connected in series in sequence. The control circuit includes a VSG control module and an SPWM modulation module; the VSG control module is mainly composed of an active-power-frequency loop, a reactive-power-voltage loop, and a voltage-current double closed-loop. The active-power droop control is used to simulate the prime mover governor of a synchronous generator, and the rotor mechanical equation is used to simulate the inertia and damping characteristics of a synchronous generator. The two together form the active-power-frequency loop; the reactive-power droop control is used to simulate the excitation voltage regulation part of a synchronous generator to adjust the reactive power output by the VSG; and the voltage-current double closed-loop decoupling control and the SPWM technology are used to control the inverter. In the present invention, a voltage adjustment module is added to the above-mentioned VSG reactive-power-voltage loop, and an active-power reference value adjustment module is added to the above-mentioned VSG active-power-frequency loop. Its working principle is as follows: sample the three-phase voltage and current output by the inverter module, and calculate the actual output power of the VSG; take the difference between the VSG active-power reference value and the actual active power output by the VSG, and generate a system frequency adjustment amount through the rotor mechanical equation; update the system output frequency through the calculated system frequency adjustment amount; integrate the system output frequency to obtain a phase angle for SPWM modulation; take the difference between the VSG reactive-power reference value and the actually calculated VSG output reactive power, adjust the voltage amplitude through a PI controller, and generate a VSG reference voltage signal; compare the VSG reference voltage with a triangular carrier wave to generate a PWM signal to drive the inverter switch tube; filter out high-frequency harmonics through the LC filter module and output 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 value of the power grid module voltage during the normal operation of the power grid module; when a three-phase short-circuit fault occurs in the power grid module, adjust the output voltage of the VSG to be synchronized with the fault voltage at the grid connection point to control 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 per-unit value of the VSG active-power reference value in real time according to the designed calculation method when a three-phase short-circuit fault occurs in the power grid module, so as to control the per-unit value of the VSG active-power reference value.

[0042] In the figure, i ga 、i gb 、i gc are the three-phase output currents of the VSG respectively; Z g =R g +jXL g is the line equivalent impedance; the filtering structure consists of an inductor L f , a capacitor C f and a resistor R f ; e abc is the internal electromotive force of the 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 ; K p , K q are the active - frequency and reactive - voltage droop coefficients respectively; P ref is the set active reference value of the VSG; P m is the virtual mechanical power of the VSG; P e is the active power actually output by the VSG; J is the virtual moment of inertia of the VSG; D is the damping coefficient of the VSG; s is the Laplace operator; θ ref is the reference phase of the internal potential of the VSG; Q ref is the reactive reference value of the VSG; Q e is the reactive power actually output by the VSG; E is the amplitude of the internal potential of the VSG; E ref is the reference amplitude of the internal potential of the VSG, |U| is the amplitude of the grid - connected fault voltage; U N is the rated voltage of the grid module.

[0043] Its working principle is as follows:

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

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

[0046] First, when the per - unit value of the grid module voltage is greater than 0.9 pu, 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 0.2 pu to 0.9 pu, the amplitude E of the internal potential of the VSG is adjusted to be synchronized with the fault voltage amplitude U of the grid - connected point according to the monitored per - unit value of the grid module voltage. At the same time, according to the real - time monitored per - unit value of the grid module voltage, the inverter module needs to adjust the reactive power output of the VSG according to formula (1) and limit the active power output by the VSG; when the grid module voltage drops to less than 0.2 pu, it indicates that the grid module is in a severe fault state; because the voltage drop is too deep, it is difficult to limit the over - current output by the VSG in the severe fault state. At this time, the VSG should be withdrawn from the system operation:

[0047] Formula (1)

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

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

[0050] Divide the per-unit value of the VSG reactive power reference value during a three-phase short-circuit fault obtained from formula (1) by the per-unit value of the grid module voltage to obtain the per-unit value of the VSG reactive current, that is, calculate the per-unit value of the VSG reactive current according to formula (2):

[0051] Formula (2)

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

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

[0054] Formula (3)

[0055] Wherein, is the per-unit value of the VSG active current; is the VSG current limit multiple, and the value is 1.5;

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

[0057] Formula (4)

[0058] Wherein, is the per-unit value of the VSG active power limit during a three-phase short-circuit fault;

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

[0060] Formula (5)

[0061] Wherein, is the per-unit value of the set active power reference value of the VSG; is the per-unit value of the active power reference value of the VSG before the three-phase short-circuit fault.

[0062] The reason for this value selection is that when the active power output of the VSG is small before the three-phase short-circuit fault or the voltage drop of the power grid module is relatively small, the per-unit value of the active power reference value of the VSG set by the system itself before the fault may be lower than its per-unit value of the active power limit. At this time, there is no need to impose additional restrictions on the active power 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 ability of the power grid module be effectively improved, but also the adverse impact on the power system caused by the fault of the power grid module can be minimized, ensuring the safe and stable operation of the power system.

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

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

[0065] Formula (1)

[0066] In the formula, is the per-unit value of the reactive power reference value of the VSG during the three-phase short-circuit fault, dimensionless; K is the reactive power compensation coefficient of the VSG (the value is 1.5); |U| is the amplitude of the fault voltage at the grid connection point, in volts; U N is the rated voltage of the power grid module, in volts.

[0067] At the same time, under the condition that the voltage drop amplitude of the power grid module and the power angle of the VSG remain constant, by dynamically adjusting the output voltage of the VSG, the output current level during the fault can be effectively reduced. This adjustment mechanism aims to reduce the phasor difference between the internal potential of the VSG and the voltage of the power grid module, 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 power grid module, the output voltage E of the VSG is set to U NOnce a symmetrical short - circuit fault is detected, immediately adjust E to be synchronized with the fault voltage amplitude U at the point of common coupling. By this method, the steady - state over - current caused by the short - circuit can be effectively suppressed. The implementation of this voltage dynamic control strategy can not only improve the stability and reliability of the grid module under fault conditions, but also extend the service life of power equipment and reduce the equipment damage caused by over - current. In addition, the implementation of this strategy also helps the rapid recovery of the grid module, reduces the impact of faults on the power system, and improves the overall operation efficiency of the power system.

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

[0069] To verify the effectiveness of a VSG low - voltage ride - through control method provided by the present invention, which has voltage and active power adjustment and reactive power compensation, a system structure as shown in Figure 3 is built on the MATLAB / Simulink simulation platform. The waveforms of the grid module voltage change, VSG output current change, VSG output active power change, and VSG output reactive power change simulated under the condition of no grid module fault or with 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 grid module voltage variation when a three-phase short-circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2 pu. The abscissa is time, with the unit of second; the ordinate is the per-unit value of the grid module voltage, without unit. It can be seen from the figure that before the control method is added, the grid module voltage will show a small fluctuation instantaneously at the start of the 1 s fault and after the end of the 1.3 s fault.

[0071] Figure 6 This is a simulation diagram of the grid module voltage variation when a three-phase short-circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2 pu, with the VSG having a low-voltage ride-through control method. The abscissa is time, with the unit of second; the ordinate is the per-unit value of the grid module voltage, without unit. It can be seen from the figure that after the control method is added, the grid module voltage can reach the rated value within a short time at the corresponding moments above, enhancing the stability of the system.

[0072] Figure 7 This is a simulation diagram of the VSG output current variation when a three-phase short-circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2 pu, with the VSG having no low-voltage ride-through control method. The abscissa is time, with the unit of second; the ordinate is the per-unit value of the VSG output current, without unit. It can be seen from the figure that before the control method is added, in the 1 - 1.3 s fault section, the transient overcurrent of the VSG exceeds about 5 times the rated value, and the steady-state fault current of the VSG reaches 2.5 times the rated value before the fault, far exceeding the national regulation of 1.5 times the rated value.

[0073] Figure 8 This is a simulation diagram of the VSG output current variation when a three-phase short-circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2 pu, with the VSG having a low-voltage ride-through control method. The abscissa is time, with the unit of second; the ordinate is the per-unit value of the VSG output current, without unit. It can be seen from the figure that after the control method is added, it can be seen that this method can limit the steady-state fault current of the VSG to 1.5 times the rated value, meeting the national regulation, and the transient overcurrent of the VSG disappears during the fault, and the transient overcurrent of the VSG at the instant of the end of the fault also decreases significantly, enhancing the stability of the system.

[0074] Figure 9 This is a simulation diagram of the VSG output active power variation when a three-phase short-circuit fault occurs in the grid module voltage and the grid connection point voltage drops to 0.2 pu, with the VSG having no low-voltage ride-through control method. The abscissa is time, with the unit of second; the ordinate is the VSG output active power value, with the unit of watt. It can be seen from the figure that before the control method is added, in the 1 - 1.3 s fault section, the active power output of the VSG is continuously unstable, and a large oscillation phenomenon occurs instantaneously after the end of the fault, and the time to recover to stability is relatively long.

[0075] Figure 10 This is a simulation diagram of the active power change of the VSG with a low-voltage ride-through control method when the grid module voltage undergoes a three-phase short-circuit fault and the grid connection point voltage drops to 0.2 pu. The abscissa is time, with the unit of second; the ordinate is the active power value output by the VSG, with the unit of watt. As can be seen from the figure, after the control method is added, the active power output of the VSG during the fault can gradually recover to a stable state. Moreover, after the fault ends, compared with before the control method is added, the power oscillation is greatly reduced, the time required to recover to the stable state is reduced, and the stability of the system is enhanced.

[0076] Figure 11 This is a simulation diagram of the reactive power change of the VSG without a low-voltage ride-through control method when the grid module voltage undergoes a three-phase short-circuit fault and the grid connection point voltage drops to 0.2 pu. The abscissa is time, with the unit of second; the ordinate is the reactive power value output by the VSG, with the unit of var. As can be seen from the figure, before the control method is added, in the fault section from 1 - 1.3 s, the reactive power output of the VSG is basically 0, and it fails to provide a certain amount of reactive power support to the grid module during the fault. Moreover, after the fault ends, a large oscillation phenomenon occurs instantaneously, and the time to recover to stability is relatively long.

[0077] Figure 12 This is a simulation diagram of the reactive power change of the VSG with a low-voltage ride-through control method when the grid module voltage undergoes a three-phase short-circuit fault and the grid connection point voltage drops to 0.2 pu. The abscissa is time, with the unit of second; the ordinate is the reactive power value output by the VSG, with the unit of var. As can be seen from the figure, after the control method is added, during the fault, the 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 the VSG can quickly tend to be stable, ensuring the reliable operation of the system.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present invention, and they should all be covered within 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 includes: When the system monitoring grid module monitors that the per-unit value of the three-phase voltage drops within the range of 0.2 pu to 0.9 pu: According to the monitored per-unit value of the grid module voltage, through a voltage adjustment module in the VSG reactive power-voltage loop, the amplitude E of the internal potential of the VSG is adjusted to be synchronized with the fault voltage amplitude U of the grid connection point; at the same time, according to the monitored per-unit value of the grid module voltage, through an active reference value adjustment module in the VSG active power-frequency loop, the per-unit value of the active power limit of the VSG during a three-phase short-circuit fault is calculated, and the per-unit value of the active power reference of the VSG is set to the smaller value of the calculated per-unit value of the active power limit of the VSG during a three-phase short-circuit fault and the per-unit value of the active power reference of the VSG set by the system itself before the fault, and the reactive power reference of the VSG is set to the per-unit value of the reactive power reference of the VSG during a three-phase short-circuit fault calculated according to the monitored per-unit value of the grid module voltage.

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

3. The low-voltage ride-through control method according to claim 1 or 2, characterized in that The calculation method of the per-unit value of the active power limit of the VSG during a three-phase short-circuit fault includes: Step 1: According to the per-unit value of the monitored grid module voltage, when it drops within the range of 0.2 pu to 0.9 pu, the per-unit value of the VSG reactive power reference value during a three-phase short-circuit fault is calculated by formula (1). : Formula (1) In the formula, is the per-unit value of the VSG reactive power reference value during a three-phase short-circuit fault, dimensionless; K is the VSG reactive power compensation coefficient, with a value of 1.5; |U| is the amplitude of the fault voltage at the grid connection point, in volts; U N is the rated voltage of the power grid module, in volts; Step 2: Divide the per-unit value of the VSG reactive power reference value during the three-phase short-circuit fault calculated above by the per-unit value of the grid module voltage to obtain the per-unit value of the VSG reactive current , which is Equation (2): Formula (2) Wherein, is the per-unit value of the VSG reactive current; is the per-unit value of the grid module voltage; Step 3: Subtracting the square of the per-unit value of the reactive current of the VSG calculated above from the square of the preset multiple of the VSG current limit, the per-unit value of the active current of the VSG can be obtained, that is, formula (3): Formula (3) Wherein, is the per-unit value of the active current of the VSG; is the multiple of the current limit value of the VSG, and the value is 1.5; Step 4: Multiplying the calculated per-unit value of the active current of the VSG by the per-unit value of the grid module voltage, the per-unit value of the active power limit of the VSG during a three-phase short-circuit fault can be obtained, that is, formula (4): Formula (4) In the formula, is the per-unit value of the active power limit of the VSG during a three-phase short-circuit fault.

4. The low-voltage ride-through control method according to claim 3, wherein It also includes: When the system monitoring grid module monitors that the per-unit value of the three-phase voltage drops within a range less than 0.2 pu, the VSG is made to exit the system.

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