Control method for limiting symmetrical fault current of multi-machine parallel system based on VSG control

Abstract

The present application relates to the control method of the symmetrical fault current limitation of the multi-machine parallel system based on VSG control, and belongs to the technical field of power grid control. The control strategy of the symmetrical fault of the multi-machine parallel system based on VSG control utilizes the rapidity of the current reference value limitation method to the fault current response and the smoothness of the voltage recovery of the virtual impedance at the fault removal moment to realize the current limitation and off-grid operation of the energy storage system of the multi-machine grid type variable flow parallel operation during the fault. The control strategy proposed in the present application makes the energy storage variable flow system working in the VSG mode through the adjustment of the virtual impedance and the current limiting and feedforward coefficient, so that the fault current is effectively inhibited when the device occurs the symmetrical fault. The method not only has the smoothness of the virtual impedance, but also has the rapidity characteristics of the current limiting.

Description

Technical Field

[0001] This invention belongs to the field of power grid control technology, and in particular to a control method for limiting symmetrical fault current in a multi-machine parallel system based on VSG control. Background Technology

[0002] With the rapid development of renewable energy, energy storage technology is playing an increasingly important role in new power systems, and energy storage has become an essential infrastructure for the new energy transformation. The demand for energy storage power station capacity is constantly growing, with a clear trend from megawatts to hundreds of megawatts. In this context, a single power conversion system can no longer meet the needs of energy storage systems. However, larger capacity PCS not only place higher demands on power devices but also increase the risk of failure. Therefore, operating multiple PCS in parallel has become the primary choice for energy storage systems.

[0003] Grid-based energy storage systems play a crucial role in new power systems due to their ability to provide inertia and damping support, autonomously establish system voltage and frequency, and exhibit high stability and robustness in weak grid interconnections. However, the voltage source characteristics of grid-based inverters make their output current highly dependent on external system conditions. When a symmetrical fault occurs at the point of common coupling (PCC), causing a voltage drop, the inverter automatically increases its output current to stabilize the output voltage and maintain internal potential stability. This can lead to sharp fluctuations in grid current and overcurrent phenomena. Traditional synchronous generators can withstand instantaneous overcurrents as high as 5-7 pu, while semiconductor-based inverters struggle to withstand such large instantaneous overcurrents. Therefore, scholars both domestically and internationally have begun to research current limiting methods for VSG-controlled energy storage converters under symmetrical faults. Currently proposed current amplitude limiting methods can be summarized as current reference value limiting and virtual impedance limiting methods. Current reference limiting, by restricting the current reference value generated by the voltage loop output or virtual admittance control, makes the converter behave as a current source. This helps adjust the output current vector angle to meet fault current requirements. However, this method cannot keep the VSG internal potential consistent with the PCC voltage during a fault, and the PCC point voltage will overshoot at the moment of fault clearing. The virtual impedance method, on the other hand, allows the grid-connected converter to maintain voltage source characteristics during severe symmetrical faults, helping to keep the grid-connected converter from disconnecting from the grid during symmetrical fault clearing. The former is faster in responding to symmetrical fault currents than the latter, but voltage surges occur at the moment of fault clearing. The latter is smoother in responding to fault currents, especially avoiding voltage surges at the moment of fault clearing. However, the effectiveness and feasibility of these two methods have only been verified in single-unit equipment, and their application in multi-unit parallel energy storage systems has not been considered. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and propose a control method for limiting symmetrical fault current in a multi-machine parallel system based on VSG control. According to the proposed control strategy, the energy storage converter system operating in VSG mode can effectively suppress fault current during symmetrical faults by adjusting virtual impedance, current limiting, and feedforward coefficients. This invention not only possesses the smoothness of virtual impedance but also the rapidity of current limiting.

[0005] The technical problem solved by this invention is achieved through the following technical solution: A control method for symmetrical fault current limiting in a VSG-controlled multi-machine parallel system includes the following steps: Step 1: Sampling and acquiring data from the grid-connected energy storage converter and the power grid; Step 2: Calculate the output active power of the grid-connected energy storage converter based on the voltage and current of the grid points obtained from the sampled grid data. P e and reactive power Q e The internal electromotive force of the virtual synchronous machine is obtained based on the rotor motion equation and reactive power-voltage control equation. E and phase angle δ The current data of the grid-connected energy storage converter, as well as the grid voltage and current data, are transmitted to the voltage and current dual-loop control through a virtual impedance link to complete the control of the output voltage and output current of the energy storage converter. Step 3: Based on the detected voltage change between the grid-connected energy storage converter PCS and the grid's common connection point PCC, adjust the virtual impedance from Step 2, the current reference value limit of the current loop in the voltage-current dual loop, the feedforward coefficient of the grid current, and the enable signal of the PI regulator controlled by the voltage-current dual loop.

[0006] Furthermore, the grid data in step 1 includes the three-phase line voltage at the grid connection point. u gab , u gbc , u gca The phase transition voltage is obtained after passing through the line voltage. u ga , u gb , u gc Three-phase current on the bridge arm side i fa , i fb , i fc Grid-side current i ga, i gb , i gc Secondly, the corresponding d-axis and q-axis components are obtained through abc / dq transformation. u gd , u gq , i fd , i fq , i gd , i gq .

[0007] Furthermore, in step 2, the output active power of the grid-type energy storage converter is calculated. P e and output reactive power Q e The specific implementation method is as follows: in, u ga , u gb , u gc The three-phase voltage of the power grid. u gab , u gbc , u gca This refers to the three-phase line voltage of the power grid. i ga , i gb , i gc It is a three-phase current.

[0008] Furthermore, the rotor motion equation and reactive power-voltage control equation of the virtual synchronous machine in step 2 are as follows: in, H , K D These are the virtual inertial constant and the damping constant, respectively. w , w pcc These are the angular velocity of the virtual synchronizer and the rated angular velocity of the PCC point, respectively. P ref , P e These are the active power reference value and the active power output, respectively. Qref , Q e These are the reactive power reference value and reactive power output, respectively. U 0 This is the rated voltage value; K Q The reactive power droop coefficient; E The internal potential of the virtual synchronous machine; δ This is the power angle.

[0009] Furthermore, the specific implementation method of transmitting the current data of the grid-connected energy storage converter and the grid voltage and current data to the voltage and current dual-loop control through the virtual impedance link in step 2 is as follows: The grid-side current... i g The d-axis and q-axis components are passed to the virtual impedance, resulting in the voltages generated on it, respectively: Among them, voltage u d_VI and u q_VI The voltage setpoint of the voltage loop is obtained by subtracting the output voltage amplitude from that of the reactive voltage loop. u d_ref = E - u d_VI and u q_ref =- u q_VI The voltage setpoint is used as the voltage reference value for the outer voltage loop input to control the actual voltage output, while the output of the voltage loop is... u DP , u QP Multiply by the feedforward coefficient k f Bridge arm side current i fd , i fq The summation yields the current setpoint for the inner current loop, which is used to control the actual output current.

[0010] Furthermore, the specific implementation method of step 3 is as follows: when a voltage drop at the PCC point is detected, Sen is set to a high level. At this time, the PI regulator in the voltage-current loop stops working. The PCC point voltage is used as a modulation signal and compared with the carrier to generate a PWM signal, thereby controlling the magnitude of the VSG internal potential. After 1ms, Sen is set to a low level, and the voltage-current loop PI regulator starts working. The value of the VSG internal potential is controlled by the reactive power-voltage loop to follow the change of the PCC point voltage. At the same time, in order to prevent overcurrent, the output current is controlled by adjusting the virtual impedance, the current feedforward coefficient, and the current reference value in the current loop.

[0011] The advantages and positive effects of this invention are: This invention proposes a control strategy for symmetrical faults in a multi-machine parallel system based on VSG control. This strategy leverages the rapid response of the current reference value limiting method to fault current and the smoothness of voltage recovery during fault clearance via virtual impedance to achieve current limiting and grid-connected operation of the energy storage system with multiple grid-connected converters operating in parallel during faults. The proposed control strategy, operating in VSG mode, effectively suppresses fault current in the energy storage converter system during symmetrical faults through the adjustment of virtual impedance, current limiting, and feedforward coefficients. This method not only possesses the smoothness of virtual impedance but also the rapidity of current limiting. Attached Figure Description

[0012] Figure 1 A schematic diagram of dual-loop control that introduces virtual impedance and current amplitude limitation in this invention; Figure 2 This is a schematic diagram of the PI controller with enabling function of the present invention; Figure 3 This is a schematic diagram of the parallel operation energy storage system of the two units of the present invention; Figure 4 This is a schematic diagram of the output voltage and current waveforms under symmetrical fault conditions (single-machine system) according to the present invention. Figure 5 This is a schematic diagram of the output voltage and current waveforms of the two-machine parallel system under symmetrical faults according to the present invention; wherein, (a) is the output voltage and current waveform under symmetrical faults (grid-connected mode); (b) is the output voltage and current waveform under three-phase short-circuit faults (off-grid mode). Detailed Implementation

[0013] The present invention will be further described in detail below with reference to the accompanying drawings.

[0014] A control method for symmetrical fault current limiting in a VSG-controlled multi-machine parallel system includes the following steps: Step 1: Sample and acquire electrical signal data from the grid-type energy storage converter and the power grid.

[0015] The grid data in step 1 includes the three-phase line voltage at the grid connection point. u gab , u gbc , u gca The phase transition voltage is obtained after passing through the line voltage. u ga , u gb , u gc Three-phase current on the bridge arm side i fa , i fb , i fc Grid-side current i ga , i gb , i gc Secondly, the corresponding d-axis and q-axis components are obtained through abc / dq transformation. u gd , u gq , i fd , i fq , i gd , i gq The transformation matrix of abc / dq is as follows: Step 2: Calculate the output active power of the grid-type energy storage converter. P e and output reactive power Q e : in: u ga , u gb , u gc The three-phase voltage of the power grid. u gab , u gbc , u gca wei represents the three-phase line voltage of the power grid. i ga , i gb , i gcIt is a three-phase current.

[0016] The rotor motion equations and reactive power-voltage control equations of the virtual synchronous machine are as follows: in: H , K D These are the virtual inertial constant and the damping constant, respectively. w , w pcc These are the angular velocity of the virtual synchronizer and the rated angular velocity of the PCC point, respectively. P ref , P e These are the active power reference value and the active power output, respectively. Q ref , Q e These are the reactive power reference value and reactive power output, respectively. U 0 This is the rated voltage value; K Q The reactive power droop coefficient; E The internal potential of the virtual synchronous machine; δ This is the power angle.

[0017] like Figure 1 As shown, the grid-side current i g The d-axis and q-axis components are passed to the virtual impedance, resulting in the voltages generated on it, respectively: Among them, voltage u d_VI and u q_VI The voltage setpoint of the voltage loop is obtained by subtracting the output voltage amplitude from that of the reactive voltage loop. u d_ref = E - u d_VI and u q_ref =- u q_VI The voltage setpoint is used as the voltage reference value for the outer voltage loop input to control the actual voltage output, while the output of the voltage loop is... u DP , u QP Multiply by the feedforward coefficient k f Bridge arm side current i fd ,i fq The summation yields the current setpoint for the inner current loop, which is used to control the actual output current.

[0018] Step 3: Based on the detected voltage change between the grid-connected energy storage converter (PCS) and the grid's point of common coupling (PCC), adjust the virtual impedance from Step 2, the current reference value limit of the current loop in the voltage-current dual loop, the feedforward coefficient of the grid current, and the enable signal of the PI regulator controlled by the voltage-current dual loop.

[0019] like Figure 2 As shown, when a voltage drop at point PCC is detected, Sen is set to high. At this time, the PI regulator in the voltage-current loop stops working. The voltage at point PCC is used as the modulation signal and compared with the carrier wave to generate a PWM signal, which then controls the magnitude of the potential inside the VSG. After 1ms, Sen is set to low, and the PI regulator in the voltage-current loop starts working. The value of the potential inside the VSG is controlled by the reactive power-voltage loop to follow the change in the voltage at point PCC. At the same time, the virtual impedance is changed according to the output current, and the feedforward coefficient is adjusted. k f Size.

[0020] Based on the aforementioned control method for symmetrical fault current limiting in a multi-machine parallel system controlled by VSG, this invention utilizes a simulation model of a multi-machine parallel energy storage system built on the RT-LAB hardware-in-the-loop simulation experimental platform for testing. Figure 3 As shown, this is to verify the effectiveness of the present invention.

[0021] To ensure the consistency of electrical signals between devices, the main circuit structure parameters are shown in Table 1.

[0022] Table 1 System main circuit structural parameters

[0023] like Figure 4 The figure shows the output voltage and current waveforms of a single grid-connected energy storage converter when a symmetrical fault occurs at the PCC point. As can be seen from the figure, when the energy storage converter operates in grid-connected mode, a symmetrical fault at the PCC point causes the PCC point voltage to drop to 0.5 pu. Within 1 ms of the fault occurring, the PI integrator's disable signal... S enSetting the voltage to high disables the PI regulator. At this point, the voltage setpoint in the energy storage converter control loop is the grid voltage, and the maximum instantaneous three-phase current is 1.464 pu, which is less than the instantaneous overcurrent threshold (1.5 pu) of the energy storage converter. After 1 ms, the PI regulator resumes operation, and the steady-state three-phase current is 1.256 pu, which is less than the steady-state overcurrent threshold (1.3 pu). The fault is cleared after 100 ms. At the moment of fault clearing, neither the PCC voltage nor current exhibits overvoltage or overcurrent, and the equipment operates normally. Therefore, this control strategy effectively limits the current magnitude when a single device experiences a symmetrical fault.

[0024] The data in the diagram illustrates that the analog signal output range of the RTLAB simulation platform is 0~15V, while the analog signal input range received by the device control board is 0~5V. Therefore, it is necessary to convert the variation range of the three-phase voltage and three-phase current to within 0~5V. The conversion formula is as follows: Therefore, the upper limit of the three-phase line voltage variation range is (690×√2) / 650.44+2.5=4.002V, and the lower limit is (-690×√2) / 650.44+2.5=0.9998V. The upper limit of the three-phase current variation range is (1046×√2) / 1662+2.5=3.39A, and the lower limit is (-1046×√2) / 1662+2.5=1.61A. like Figure 5 The figure shows the output voltage and current waveforms of a two-unit parallel energy storage system. A carrier synchronization method is used to ensure that the PWM wave outputs of the two devices remain consistent. Due to the limitation of the oscilloscope's channel count, only [the waveforms shown are not included in the provided text]. u ab and u bc Waveforms and the three-phase currents of the two devices. (From...) Figure 5 (a) It can be seen that when the two devices are working in grid-connected mode, when a symmetrical fault occurs at the PCC point, the voltage at the PCC point drops to 0.5pu, and the maximum instantaneous value of the three-phase current is 1.417pu, which is less than the instantaneous overcurrent threshold (1.5pu) of the energy storage converter device. After entering steady state, the steady-state value of the three-phase current is 1.233pu, which is less than the steady-state overcurrent threshold (1.3pu) of the device. Figure 5(b) The output voltage and current waveforms of the parallel energy storage system with two devices operating in off-grid mode. As shown in the figure, when a three-phase short-circuit fault occurs at point PCC, the voltage at point PCC drops to 0.15 pu, and the maximum instantaneous value of the three-phase current is 1.306 pu, which is less than the instantaneous overcurrent threshold (1.5 pu). After entering steady state, the steady-state value of the three-phase current is 1.285 pu, which is less than the steady-state overcurrent threshold (1.3 pu). After the fault is cleared, there is no surge in the three-phase voltage and current, and the equipment operates normally. In summary, the proposed control strategy can limit the output current under symmetrical faults in the parallel system with two devices, protecting the normal operation of the equipment.

[0025] It should be emphasized that the embodiments described in this invention are illustrative and not limiting. Therefore, this invention includes, but is not limited to, the embodiments described in the specific implementation. Any other implementation methods derived by those skilled in the art based on the technical solutions of this invention also fall within the scope of protection of this invention.

Claims

1. A control method for symmetrical fault current limiting in a multi-machine parallel system based on VSG control, characterized in that: Includes the following steps: Step 1: Sampling and acquiring data from the grid-connected energy storage converter and the power grid; Step 2: Calculate the output active power of the grid-connected energy storage converter based on the voltage and current of the grid points obtained from the sampled grid data. P e and reactive power Q e The internal electromotive force of the virtual synchronous machine is obtained based on the rotor motion equation and reactive power-voltage control equation. E and phase angle δ The current data of the grid-connected energy storage converter, along with the grid voltage and current data, are transmitted to the voltage and current dual-loop control system via a virtual impedance circuit to control the output voltage and current of the energy storage converter. The specific application method of the virtual impedance is as follows: the grid-side current... i g The d-axis and q-axis components are passed to the virtual impedance, resulting in the voltages generated on it, respectively: Step 3: Adjust the virtual impedance in Step 2 according to the detected voltage changes between the grid-connected energy storage converter PCS and the grid's point of common coupling PCC. R v and X v The magnitude of the voltage and current loops, the current reference value limit of the current loop in the voltage and current dual loop, the feedforward coefficient of the grid current, and the enable signal of the PI regulator controlled by the voltage and current dual loops.

2. The control method for symmetrical fault current limiting in a VSG-controlled multi-machine parallel system according to claim 1, characterized in that: The power grid data in step 1 includes the three-phase line voltage at the grid connection point. u gab , u gbc , u gca The phase transition voltage is obtained after passing through the line voltage. u ga , u gb , u gc Three-phase current on the bridge arm side i fa , i fb , i fc Grid-side current i ga , i gb , i gc Secondly, the corresponding d-axis and q-axis components are obtained through abc / dq transformation. u gd , u gq , i fd , i fq , i gd , i gq .

3. The control method for symmetrical fault current limiting in a VSG-controlled multi-machine parallel system according to claim 1, characterized in that: In step 2, the output active power of the grid-type energy storage converter is calculated. P e and output reactive power Q e The specific implementation method is as follows: in, u ga , u gb , u gc The three-phase voltage of the power grid. u gab , u gbc , u gca This refers to the three-phase line voltage of the power grid. i ga , i gb , i gc It is a three-phase current.

4. The control method for symmetrical fault current limiting of a VSG-controlled multi-machine parallel system according to claim 1, characterized in that: The rotor motion equation and reactive power-voltage control equation of the virtual synchronous machine in step 2 are as follows: in, H , K D These are the virtual inertial constant and the damping constant, respectively. w , w pcc These are the angular velocity of the virtual synchronizer and the rated angular velocity of the PCC point, respectively. P ref , P e These are the active power reference value and the active power output, respectively. Q ref , Q e These are the reactive power reference value and reactive power output, respectively. U 0 This is the rated voltage value; K Q The reactive power droop coefficient; E The internal potential of the virtual synchronous machine; δ This is the power angle.

5. The control method for symmetrical fault current limiting in a VSG-controlled multi-machine parallel system according to claim 1, characterized in that: In step 2, the specific implementation method for transmitting the current data of the grid-connected energy storage converter, as well as the grid voltage and current data, to the voltage and current dual-loop control through a virtual impedance link is as follows: The grid-side current... i g The d-axis and q-axis components are passed to the virtual impedance, resulting in the voltages generated on it, respectively: Among them, voltage u d_VI and u q_VI The voltage setpoint of the voltage loop is obtained by subtracting the output voltage amplitude from that of the reactive voltage loop. u d_ref = E - u d_VI and u q_ref =- u q_VI The voltage setpoint is used as the voltage reference value for the outer voltage loop input to control the actual voltage output, while the output of the voltage loop is... u DP , u QP Multiply by the feedforward coefficient k f Bridge arm side current i fd , i fq The summation yields the current setpoint for the inner current loop, which is used to control the actual output current.

6. The control method for symmetrical fault current limiting in a VSG-controlled multi-machine parallel system according to claim 1, characterized in that: The specific implementation method of step 3 is as follows: When a voltage drop at the PCC point is detected, Sen is set to a high level. At this time, the PI regulator in the voltage-current loop stops working. The PCC point voltage is used as the modulation signal and compared with the carrier to generate a PWM signal, which then controls the magnitude of the potential inside the VSG. After 1ms, Sen is set to a low level, and the PI regulator in the voltage-current loop starts working. The value of the potential inside the VSG is controlled by the reactive power-voltage loop to follow the change of the PCC point voltage. At the same time, in order to prevent overcurrent, the output current is controlled by adjusting the virtual impedance, the current feedforward coefficient, and the current reference value in the current loop.

Images