Network construction converter current limiting control method and system based on virtual impedance adjustment

Abstract

The invention discloses a virtual impedance adjustment-based current-limiting control method and system for a network construction converter, and the method comprises the steps: determining a fault transient virtual resistance according to the voltage difference between the terminal voltage of a virtual synchronous machine and the voltage of a grid-connected point in a fault transient state when the system breaks down; in a fault steady state, determining a fault steady state virtual resistance and a fault steady state virtual inductance according to a voltage difference between a terminal voltage of the virtual synchronous machine and a grid-connected point voltage and a current over-limit threshold value of the networking converter; the large value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance; and determining a fault potential difference by using the line current, the fault virtual resistance and the fault virtual inductance, and obtaining a fault current instruction value through a voltage control outer ring according to the difference values of the voltage instruction, the output voltage of the network construction converter and the fault potential difference. The current does not exceed the limit in the fault steady state, and the current impact caused by the superposition of the non-periodic component current and the periodic component current does not exceed the limit in the fault transient state.

Description

Technical Field

[0001] The present invention belongs to the field of grid-type converter control, and in particular, relates to a current limiting control method and system for a grid-type converter based on virtual impedance regulation. Background Art

[0002] With the rapid development of domestic renewable energy, a new power system with renewable energy as the main body will be formed in the future. Since the grid-forming (GFM) grid-connected converter keeps synchronization with the power grid through the power control loop and has external characteristics similar to synchronous generators and voltage source characteristics with low series impedance, it avoids the resonance problem introduced by the phase-locked loop in the grid-following (GFL) power supply control, and shows good adaptability and stability to weak power grids. It can autonomously build system voltage in weak power grids and provide inertia, damping and active support for the power system.

[0003] During a grid fault, due to the limited overcurrent tolerance of the converter, in order to ensure that the converter output current does not exceed the limit, the existing technology proposes to operate the grid VSG control during normal operation, switch to the current source control mode during a fault, and use a phase-locked loop to synchronize with the grid phase. However, the current source control cannot provide voltage support for the grid, and there is an impact current during the switching process, which is not conducive to the stable operation of the system. The existing technology has conducted a lot of research on virtual impedance to limit the inverter current, by increasing linear or nonlinear virtual impedance to reduce the reference voltage of the converter. However, although the traditional adaptive virtual impedance method can suppress the fault steady-state current of the converter, most of them ignore the transient non-periodic component of the fault and the periodic superposition that may cause a larger overcurrent, and cannot ensure that the current limiting effect of the grid converter during the full fault process reaches the expected effect. Summary of the invention

[0004] In order to solve the deficiencies in the prior art, the present invention provides a current limiting control method and system for a grid-connected converter based on virtual impedance regulation, which not only ensures that the current does not exceed the limit in a steady-state fault state, but also effectively avoids the current shock exceeding the limit caused by the superposition of non-periodic and periodic component currents in a transient fault state. The control structure is simple and there is no need for complex mode switching.

[0005] The present invention adopts the following technical solution.

[0006] The present invention proposes a current limiting control method for a grid-connected converter based on virtual impedance regulation. The control system of the grid-connected converter includes virtual synchronous machine control, current and voltage dual-loop control, and an SPWM modulator, wherein the current and voltage dual-loop control includes a voltage control outer loop and a current control inner loop; the virtual synchronous machine control generates a voltage command value according to an actual value of active power, an actual value of reactive power, an active power command value, and a reactive power command value; the voltage command value obtains a current command value through a voltage control outer loop, the current command value obtains a three-phase modulation voltage through a current control inner loop, and the three-phase modulation voltage generates a control pulse through an SPWM modulator;

[0007] When the system is operating normally, the non-fault virtual resistance and non-fault virtual inductance are determined according to the filter resistance and filter inductance of the filter; the non-fault potential difference is determined by using the line current and the non-fault virtual resistance and non-fault virtual inductance, and the difference between the voltage command and the output voltage and non-fault potential difference of the grid-forming converter is used to obtain the non-fault current command value through the voltage control outer loop;

[0008] When a system fault occurs, during the fault transient process, the fault transient virtual resistance is determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage; during the fault steady state process, the fault steady state virtual resistance and the fault steady state virtual inductance are determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid-forming converter, and the larger value of the fault transient virtual resistance and the fault steady state virtual resistance is taken as the fault virtual resistance, and the fault steady state virtual inductance is taken as the fault virtual inductance; the fault potential difference is determined by using the line current and the fault virtual resistance and the fault virtual inductance, and the fault current command value is obtained by the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference through the voltage control outer loop.

[0009] Preferably, the current limiting control method includes:

[0010] Step 1, obtaining the three-phase voltage of the grid connection point and the filter inductance of the grid-connected converter to determine whether the system has a fault; when the system is operating normally, proceed to step 2, and when the system has a fault, proceed to step 3;

[0011] Step 2: When the system operates normally, the filter resistance of the filter is a non-fault virtual resistance, the filter inductance of the filter is a non-fault virtual inductance, and the process goes to step 4;

[0012] Step 3, when a system fault occurs, in the transient process of the fault, according to the voltage difference between the terminal voltage of the virtual synchronous machine and the grid-connected point voltage, the lead-lag link is used to determine the fault transient virtual resistance; in the steady-state process of the fault, according to the voltage difference between the terminal voltage of the virtual synchronous machine and the grid-connected point voltage and the current over-limit threshold of the grid converter, the fault steady-state virtual inductance is determined; based on the impedance ratio of the filter, the fault steady-state virtual inductance is used to determine the fault steady-state virtual resistance; the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance, and the process proceeds to step 4;

[0013] Step 4: When the system is operating normally, the non-fault potential difference is determined using the line current, the non-fault virtual resistance and the non-fault virtual inductance, and the difference between the voltage command and the output voltage of the grid-forming converter and the non-fault potential difference is obtained through the voltage control outer loop to obtain the non-fault current command value; when a system fault occurs, the fault potential difference is determined using the line current, the fault virtual resistance and the fault virtual inductance, and the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference is obtained through the voltage control outer loop to obtain the fault current command value.

[0014] Preferably, in step 1, the per unit value of the filter inductance of the grid-connected converter and the d-axis component of the positive sequence voltage of the system grid connection point are obtained, and whether the system fails is determined according to the following criteria:

[0015] 1) When the per-unit value of the filter inductance is less than the filter inductance threshold, and two consecutive sampling values ​​of any phase voltage at the grid connection point are less than the voltage drop threshold, it is determined that the system has a fault;

[0016] 2) When the filter inductance value is not less than the filter inductance threshold and the d-axis component of the system grid-connected point positive sequence voltage is less than the grid-connected point positive sequence voltage threshold, it is determined that the system has a fault; when the filter inductance value is not less than the filter inductance threshold and the d-axis component of the system grid-connected point positive sequence voltage is not less than the grid-connected point positive sequence voltage threshold, it is determined that the system has not a fault;

[0017] The filter inductance threshold is 0.3pu, the voltage drop threshold is 0.8pu, and the grid-connected point positive sequence voltage threshold is 0.9pu.

[0018] Preferably, in step 3, the fault transient virtual resistance satisfies the following relationship:

[0019]

[0020] In the formula, R vt is the transient virtual resistance of the fault, ΔU is the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage, K t is the gain constant of the lead-lag link, ω cis the crossover frequency of the lead-lag link, s is the integral operator;

[0021] The virtual synchronous machine control outputs the terminal voltage E and the phase angle δ according to the actual value of active power and the actual value of reactive power, the active power command value and the reactive power command value;

[0022] ΔU=EU pcc

[0023] Where, E is the terminal voltage of the virtual synchronous machine, U pcc is the grid connection point voltage.

[0024] Preferably, the fault steady-state virtual inductance and the fault steady-state virtual resistance satisfy the following relationship:

[0025]

[0026] Where, L vt is the fault steady-state virtual inductance, E is the terminal voltage of the virtual synchronous machine, U pcc is the grid-connected point voltage, L0 is the non-fault virtual inductance, R0 is the non-fault virtual resistance, X0 is the non-fault virtual reactance, satisfying X0=ω n L0,ω n is the system rated angular frequency, I max is the current over-limit threshold of the grid converter, R vs is the fault steady-state virtual resistance, σ X / R is the filter impedance ratio.

[0027] Preferably, the fault virtual resistance and the fault virtual inductance satisfy the following relationship:

[0028]

[0029] In the formula, R v is the fault virtual resistance, L v is the fault virtual inductance.

[0030] Preferably, in step 4, when the system operates normally, the dq axis component of the non-fault potential difference satisfies the following relationship:

[0031]

[0032] In the formula, Δu 0d , Δu 0q is the dq axis component of the non-fault potential difference, i ld 、i lq is the dq axis component of the line current, L0 is the non-fault virtual inductance, R0 is the non-fault virtual resistance, ω n is the rated angular frequency of the system;

[0033] When a fault occurs in the system, the dq axis component of the fault potential difference satisfies the following relationship:

[0034]

[0035] In the formula, Δu d , Δu q is the dq axis component of the fault potential difference, R v is the fault virtual resistance, L v is the fault virtual inductance.

[0036] Preferably, the voltage control outer loop adopts a PI regulator, and when the system is operating normally and when a system failure occurs, the input signal of the PI regulator satisfies the following relationship respectively:

[0037]

[0038] In the formula, are the positive sequence voltage difference and negative sequence voltage difference of the input PI regulator respectively, are the dq axis components of the positive sequence voltage command value and the dq axis components of the negative sequence voltage command value, Δu 0dq is the dq axis component of the non-fault potential difference, Δu dq is the dq axis component of the fault potential difference, They are the dq axis component of the actual value of the positive sequence voltage and the dq axis component of the actual value of the negative sequence voltage respectively;

[0039] When the system is operating normally, the PI regulator outputs the dq-axis component of the positive-sequence current command value and the dq-axis component of the negative-sequence current command value under non-fault conditions;

[0040] When a system fault occurs, the PI regulator outputs the dq-axis component of the positive-sequence current command value and the dq-axis component of the negative-sequence current command value under the fault.

[0041] Preferably, based on the method for determining whether a system failure occurs in step 1, when it is determined that the system failure is restored, after a set delay, the gain constant K of the lead-lag link is used. t The slope is , and the fault virtual resistance and the fault virtual inductance are reduced to the non-fault virtual resistance and the non-fault virtual inductance; wherein the delay value is set to 40ms.

[0042] The present invention also proposes a current limiting control system for a grid-connected converter based on virtual impedance regulation. The control system of the grid-connected converter includes virtual synchronous machine control, current and voltage dual-loop control, and an SPWM modulator, wherein the current and voltage dual-loop control includes a voltage control outer loop and a current control inner loop; the virtual synchronous machine control generates a voltage command value according to the actual value of active power, the actual value of reactive power, the active power command value, and the reactive power command value; the voltage command value obtains a current command value through the voltage control outer loop, and the current command value obtains a three-phase modulation voltage through the current control inner loop, and the three-phase modulation voltage generates a control pulse through the SPWM modulator. The current limiting control system also includes:

[0043] A virtual impedance loop connected in series between the virtual impedance virtual synchronous machine control and the current and voltage dual loop control;

[0044] The virtual impedance loop is used to determine the non-fault virtual resistance and non-fault virtual inductance according to the filter resistance and filter inductance of the filter when the system is operating normally; the non-fault potential difference is determined by using the line current and the non-fault virtual resistance and non-fault virtual inductance, and the difference between the voltage command and the output voltage and non-fault potential difference of the grid-forming converter is obtained through the voltage control outer loop to obtain the non-fault current command value;

[0045] The virtual impedance loop is also used when a system fault occurs. During the transient process of the fault, the fault transient virtual resistance is determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage; during the steady-state process of the fault, the fault steady-state virtual resistance and the fault steady-state virtual inductance are determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid-forming converter, and the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance; the fault potential difference is determined by using the line current, the fault virtual resistance and the fault virtual inductance, and the fault current command value is obtained by the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference through the voltage control outer loop.

[0046] The beneficial effects of the present invention are that, compared with the prior art, at least the method proposed by the present invention divides the fault output current of the grid-connected converter into a fault transient impact current and a fault steady-state overload current, and performs virtual impedance adaptive control design for these two fault state currents. For the fault transient impact current, the voltage difference is passed through the lead-lag link to obtain a virtual resistance that only exists during the fault transient period, which effectively reduces the amplitude and attenuation rate of the transient impact current; for the fault steady-state overload current, the virtual impedance is invested after the voltage drop depth and the maximum allowable current are passed through the adaptive control algorithm. The present invention can effectively limit the transient impact current and steady-state overload current of the converter during the fault period, and can be applied to the control system design of the grid-connected converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a main circuit control structure diagram of a grid-connected converter in an embodiment of the present invention;

[0048] Figure 2 It is a control structure diagram of the virtual impedance adaptive adjustment proposed by the present invention;

[0049] Figure 3 It is a voltage and current response change diagram using conventional virtual impedance adaptive control in an example of the present invention;

[0050] Figure 4 It is a voltage and current response change diagram under the proposed control strategy in the example of the present invention;

[0051] Figure 5 It is a diagram showing the changes in the adaptive adjustment of virtual impedance using the control strategy proposed by the present invention in an example of the present invention. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only embodiments of a part of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0053] The main circuit control structure of the grid-connected converter is as follows: Figure 1 and Figure 2 As shown in the figure, the DC side power supply of the grid-building converter is a new energy power supply device. The grid-building converter adopts virtual synchronous machine (VSG) control. The output end of the grid-building converter is connected to the input end of the filter, and then connected to the grid through the transformer and transmission line. g and L g are the system equivalent resistance and equivalent inductance respectively; the control system collects the three-phase voltage V on the AC side of the grid converter a,b,c and three-phase current i a,b,c To perform power calculation, VSG control is based on the calculated actual value of active power and reactive power, as well as the active power command value P ref and reactive power command value Q ref , generate a voltage command value; the voltage command value is obtained by the voltage control outer loop to obtain the current command value, the current command value is obtained by the current control inner loop to obtain the three-phase modulated voltage, and the three-phase modulated voltage generates a control pulse through the SPWM modulator.

[0054] When a system fails, the grid-connected converter is affected by the fault impact current. In order to prevent the current from exceeding the limit and damaging the converter during the fault process, the present invention proposes a current limiting control method for the grid-connected converter based on virtual impedance regulation. The grid-connected converter adopts virtual synchronous machine control to generate a voltage command. The difference between the voltage command and the output voltage of the grid-connected converter is obtained through a PI loop to obtain a current command value; the method comprises:

[0055] When the system is operating normally, the non-fault virtual resistance and non-fault virtual inductance are determined according to the filter resistance and filter inductance of the filter; the non-fault potential difference is determined by using the line current and the non-fault virtual resistance and non-fault virtual inductance, and the difference between the voltage command and the output voltage and non-fault potential difference of the grid-forming converter is used to obtain the non-fault current command value through the PI loop;

[0056] When a system fault occurs, during the fault transient process, the fault transient virtual resistance is determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage; during the fault steady state process, the fault steady state virtual resistance and the fault steady state virtual inductance are determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid converter, and the larger value of the fault transient virtual resistance and the fault steady state virtual resistance is used as the fault virtual resistance, and the fault steady state virtual inductance is used as the fault virtual inductance; the fault potential difference is determined by using the line current and the fault virtual resistance and the fault virtual inductance, and the non-fault current command value is obtained by the difference between the voltage command and the output voltage of the grid converter and the fault potential difference through the PI loop.

[0057] Current limiting control methods include:

[0058] Step 1, obtain the three-phase voltage of the grid connection point and the filter inductance of the grid-connected converter to determine whether the system has a fault; when the system operates normally, proceed to step 2, and when the system has a fault, proceed to step 3.

[0059] Specifically, the per-unit value of the filter inductance of the grid-connected converter and the d-axis component of the positive sequence voltage of the system grid connection point are obtained, and whether the system fails is determined according to the following criteria:

[0060] 1) When the per-unit value of the filter inductance is less than the filter inductance threshold a, and two consecutive sampling values ​​of any phase voltage at the grid connection point are less than the voltage drop threshold, it is determined that the system has a fault; in the embodiment, a is 0.3pu and the voltage drop threshold is 0.8pu;

[0061] 2) When the filter inductance value is not less than the filter inductance threshold a and the d-axis component of the system grid-connected point positive-sequence voltage is less than the grid-connected point positive-sequence voltage threshold b, it is determined that the system has a fault; when the filter inductance value is not less than the filter inductance threshold a and the d-axis component of the system grid-connected point positive-sequence voltage is not less than the grid-connected point positive-sequence voltage threshold b, it is determined that the system has not a fault; in the embodiment, b is taken as 0.9pu.

[0062] The system fault judgment method proposed in the present invention is simple and easy to implement, and can be built into the virtual impedance loop proposed in the present invention, thereby avoiding the complication of the control system structure caused by the introduction of the virtual impedance loop.

[0063] Step 2: When the system is operating normally, the filter resistance R f is the non-fault virtual resistance R0, the filter inductance L f is the non-fault virtual inductor L0, and goes to step 4.

[0064] When there is no fault in the system, the filter resistance and filter inductance of the filter are used as the non-fault virtual resistance and non-fault virtual inductance impedance, both of which are fixed values. In fact, this is to consider that during normal operation, the line current is limited by the filter impedance and will not cause the current of the grid converter to exceed the limit.

[0065] Step 3, when a system fault occurs, during the fault transient process, the lead-lag link is used to determine the fault transient virtual resistance according to the voltage difference between the machine-end voltage of the virtual synchronous machine and the grid-connected point voltage; during the fault steady-state process, the fault steady-state virtual inductance is determined according to the voltage difference between the machine-end voltage of the virtual synchronous machine and the grid-connected point voltage and the current over-limit threshold of the grid converter; based on the impedance ratio of the filter, the fault steady-state virtual inductance is used to determine the fault steady-state virtual resistance; the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance, and the process proceeds to step 4.

[0066] Specifically, step 3 includes:

[0067] Step 3.1, when a system fault occurs, during the fault transient process, according to the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage, the fault transient virtual resistance is determined by using the lead-lag link;

[0068] System faults include fault transient processes and fault steady-state processes. Combining the voltage and current variation trends of the fault transient process and the response characteristics of resistance and inductance, only the fault transient virtual resistance needs to be considered under the fault transient impact current, satisfying the following relationship:

[0069]

[0070] In the formula, R vt is the transient virtual resistance of the fault, ΔU is the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage, K t is the gain constant of the lead-lag link, ω c is the crossover frequency of the lead-lag link, and s is the integral operator.

[0071] according to Figure 2It can be seen that the virtual synchronous machine control outputs the machine terminal voltage E and the phase angle δ according to the actual value of active power and the actual value of reactive power, the active power command value and the reactive power command value.

[0072] ΔU=EU pcc

[0073] Where, E is the terminal voltage of the virtual synchronous machine, U pcc is the grid connection point voltage;

[0074] When the voltage drop at the grid connection point is significant, the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage is large. At this time, the transient virtual resistance of the fault is large, which has a better limiting effect on the transient current of the fault, thus facilitating the current limiting protection of the grid-connected converter. When the fault gradually stabilizes, the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage gradually decreases. At this time, the transient virtual resistance of the fault approaches 0, which has no effect on limiting the transient current of the fault. Therefore, it is necessary to determine the virtual resistance and virtual inductance of the fault steady-state process to avoid the current of the grid-connected converter from exceeding the limit.

[0075] In the fault steady-state process, the fault transient virtual resistance is 0, satisfying the following relationship:

[0076]

[0077] Step 3.2, during the fault steady-state process, the fault steady-state virtual inductance is determined according to the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid-connected converter; based on the impedance ratio of the filter, the fault steady-state virtual inductance is used to determine the fault steady-state virtual resistance; the following relationship is satisfied:

[0078]

[0079] Where, L vt is the fault steady-state virtual inductance, E is the terminal voltage of the virtual synchronous machine, U pcc is the grid-connected point voltage, L0 is the non-fault virtual inductance, R0 is the non-fault virtual resistance, X0 is the non-fault virtual reactance, satisfying X0=ω n L0,ω n is the system rated angular frequency, I max is the current over-limit threshold of the grid converter, R vs is the fault steady-state virtual resistance, σ X / R is the filter impedance ratio;

[0080] When calculating the virtual resistance and virtual inductance of the fault steady-state process, the non-fault virtual resistance and non-fault virtual inductance are introduced as reference values, combined with the current over-limit threshold of the grid converter, so that the calculated virtual resistance and virtual inductance of the fault steady-state process can limit the fault transient current to the allowable level of the grid converter current under normal system operation; and the fault steady-state virtual inductance and the fault steady-state virtual resistance still meet the impedance ratio of the filter, ensuring the normal function of the filter and playing a current limiting role at the same time.

[0081] In step 3.3, the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance.

[0082] Obviously, the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance can play a better role in limiting the current of the grid converter. Therefore, the fault virtual resistance and fault virtual inductance finally obtained satisfy the following relationship:

[0083]

[0084] In the formula, R v is the fault virtual resistance; L v is the fault virtual inductance.

[0085] The method proposed in the present invention divides the fault output current of the grid-connected converter into fault transient impact current (superposition of non-periodic component and periodic component current) and fault steady-state overload current (periodic component current), and performs virtual impedance adaptive control design for these two fault state currents respectively.

[0086] Step 4: When the system is operating normally, the non-fault potential difference is determined using the line current, the non-fault virtual resistance and the non-fault virtual inductance, and the difference between the voltage command and the output voltage of the grid-forming converter and the non-fault potential difference is obtained through the voltage control outer loop to obtain the non-fault current command value; when a system fault occurs, the fault potential difference is determined using the line current, the fault virtual resistance and the fault virtual inductance, and the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference is obtained through the voltage control outer loop to obtain the fault current command value.

[0087] Specifically, when the system is operating normally, the dq-axis component of the non-fault potential difference satisfies the following relationship:

[0088]

[0089] In the formula, Δu 0d , Δu 0q is the dq axis component of the non-fault potential difference, i ld 、i lq is the dq axis component of the line current;

[0090] When a fault occurs in the system, the dq axis component of the fault potential difference satisfies the following relationship:

[0091]

[0092] In the formula, Δu d , Δu q is the dq axis component of the fault potential difference;

[0093] Furthermore, the line current during normal system operation is the transmission current of the grid-connected line, and the line current during system fault is the fault transient impact current; therefore, the non-fault potential difference and the fault potential difference respectively characterize the current limiting capability of the virtual impedance during normal system operation and when the system fails, and the current limiting capability of the virtual impedance is fed back to the voltage control outer loop in the voltage-current dual-loop control, such as Figure 2 As shown, the input signal of the voltage control outer loop includes: the dq axis component of the positive sequence voltage command value and the dq-axis component of the negative sequence voltage command value Based on the dq-axis component Δu of the non-fault potential difference 0dq Or the dq-axis component of the fault potential difference Δu dq , and the dq-axis component of the actual value of the positive sequence voltage and the dq-axis component of the actual value of the negative sequence voltage The voltage control outer loop adopts PI regulator. When the system is operating normally and when a fault occurs, the input signal of the PI regulator satisfies the following relationship respectively:

[0094]

[0095] In the formula, They are the positive sequence voltage difference and negative sequence voltage difference of the input PI regulator respectively;

[0096] When the system is operating normally, the PI regulator outputs the dq-axis component of the positive sequence current command value under non-fault conditions. and the dq-axis component of the negative sequence current command value When a fault occurs in the system, the PI regulator outputs the dq-axis component of the positive sequence current command value under fault and the dq-axis component of the negative sequence current command value As the input signal of the current control inner loop, the input signal is transformed through the current control inner loop and coordinates to obtain a three-phase modulated voltage. After the three-phase modulated voltage is input into the SPWM modulator, a control pulse is generated to control the on and off of the switching power tube in the grid-connected converter.

[0097] Current limiting control methods also include:

[0098] Based on the method of judging whether the system fails in step 1, when the system fails and recovers, after the set delay, the gain constant K of the lead-lag link is used. t The fault virtual resistance and fault virtual inductance are reduced to the non-fault virtual resistance and non-fault virtual inductance to prevent the grid-connected converter from having a current over-limit fault at the moment of fault recovery. In the embodiment, the delay value is set to 40ms.

[0099] The method proposed by the present invention timely limits the output current during the fault period to prevent the converter device from being subjected to excessive current shock, so as to protect safety. The method proposed by the present invention not only designs a virtual impedance adaptive link for the problem of steady-state load overcurrent during the fault, but also further optimizes the problem of transient current shock during the fault, that is, a larger virtual resistor is invested during the transient fault period to quickly reduce the amplitude and attenuation speed of the shock current.

[0100] The present invention also proposes a current limiting control system for a grid-connected converter based on virtual impedance regulation. The control system of the grid-connected converter includes virtual synchronous machine control, current and voltage dual-loop control, and an SPWM modulator, wherein the current and voltage dual-loop control includes a voltage control outer loop and a current control inner loop; the virtual synchronous machine control generates a voltage command value according to the actual value of active power, the actual value of reactive power, the active power command value, and the reactive power command value; the voltage command value obtains a current command value through the voltage control outer loop, and the current command value obtains a three-phase modulation voltage through the current control inner loop, and the three-phase modulation voltage generates a control pulse through the SPWM modulator. The current limiting control system also includes:

[0101] A virtual impedance loop connected in series between the virtual impedance virtual synchronous machine control and the current and voltage dual loop control;

[0102] The virtual impedance loop is used to determine the non-fault virtual resistance and non-fault virtual inductance according to the filter resistance and filter inductance of the filter when the system is operating normally; the non-fault potential difference is determined by using the line current and the non-fault virtual resistance and non-fault virtual inductance, and the difference between the voltage command and the output voltage and non-fault potential difference of the grid-forming converter is obtained through the voltage control outer loop to obtain the non-fault current command value;

[0103] The virtual impedance loop is also used when a system fault occurs. During the transient process of the fault, the fault transient virtual resistance is determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage; during the steady-state process of the fault, the fault steady-state virtual resistance and the fault steady-state virtual inductance are determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid-forming converter, and the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance; the fault potential difference is determined by using the line current, the fault virtual resistance and the fault virtual inductance, and the fault current command value is obtained by the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference through the voltage control outer loop.

[0104] In the embodiment, the rated capacity of the grid-connected converter controlled by the virtual synchronous machine is 2MW, the voltage reference is 0.69kV, the rated frequency of the grid is 50Hz, and the filter reactance L f The per-unit value is 0.1pu, and the filter resistor R f The per unit value is 0.01pu, σ XR is 10, the system short-circuit ratio SCR is set to 5; the virtual inertia time constant is 12, the damping coefficient is 200, and the current over-limit threshold I max The per-unit value is 3pu, and the gain constant is K t Take 10, the crossover frequency ω c Take 1000rad / s, fault recovery signal delay 40ms, fixed slope k c Take 2pu / s, filter inductance threshold a take 0.3pu, grid point positive sequence voltage threshold b take 0.9pu. Initial stable state, active power command value P ref Set to 1pu, reactive power command value Q ref Set to 0pu. Taking the most serious fault as an example, an ABC three-phase metallic grounding fault occurs on the high-voltage side of the transformer at 3s, and the fault duration is 0.5s.

[0105] During the fault period, the three-phase voltage drops close to 0. If the current limiting control strategy is not adopted, the current will inevitably exceed the limit. The three-phase voltage and current changes under the conventional virtual impedance adaptive strategy and the method proposed by the present invention are as follows: Figure 3 and Figure 4 As shown, Figure 3 and Figure 4In the above, Ia_D_pu, Ib_D_pu, and Ic_D_pu are transient impact three-phase currents, Ua_D_pu, Ub_D_pu, and Uc_D_pu are transient impact three-phase voltages, t = 5.0s is the time when the fault occurs, and t = 5.5s is the time when the fault is cleared; when the conventional virtual impedance adaptive strategy is adopted, the current formed by the superposition of the non-periodic component and the periodic component will instantly exceed the limit at the moment of the fault, and the maximum value exceeds 5pu and approaches 6pu. The current over-limit time lasts for nearly one cycle, causing overcurrent damage to the grid-connected converter; and when the method proposed by the present invention is adopted, Figure 5 As shown in the figure, a large virtual resistance Rv_pu will be generated at the time of fault occurrence at t=5.0s, which can effectively limit the transient impact current of the fault. Before the fault is restored and when the fault enters the steady state, a large virtual reactance Xv_pu will be generated. Under the action of the virtual resistance and the virtual reactance, the transient impact three-phase current is well limited to within 2pu, as shown in the figure. Figure 4 As shown, until the fault is cleared, the fault steady state is achieved and the current after the fault is restored does not flow, achieving current limiting and stability under large disturbance faults. The virtual impedance adaptive setting method proposed in the present invention is executed in the grid-type converter control system, and can adaptively change according to the actual situation of the grid fault, thereby coping with overcurrent suppression under grid faults.

[0106] The present disclosure may be a system, a method and / or a computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0107] A computer-readable storage medium may be a tangible device that can hold and store instructions used by an instruction execution device. A computer-readable storage medium may be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples of computer-readable storage media (a non-exhaustive list) include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disk read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination of the foregoing. As used herein, a computer-readable storage medium is not to be interpreted as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through a wire.

[0108] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, optical fiber transmissions, wireless transmissions, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions for storage in the computer-readable storage medium in each computing / processing device.

[0109] The computer program instructions for performing the operation of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages, such as Smalltalk, C++, etc., and conventional procedural programming languages, such as "C" language or similar programming languages. Computer-readable program instructions may be executed completely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or completely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., using an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be customized by utilizing the state information of the computer-readable program instructions, and the electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A current limiting control method for a grid-connected converter based on virtual impedance regulation, wherein the control system of the grid-connected converter includes virtual synchronous machine control, current and voltage dual-loop control, and an SPWM modulator, wherein: The current and voltage dual-loop control includes a voltage control outer loop and a current control inner loop; the virtual synchronous machine control generates a voltage command value according to the actual value of active power, the actual value of reactive power, the active power command value and the reactive power command value; the voltage command value is obtained by the voltage control outer loop to obtain the current command value, and the current command value is obtained by the current control inner loop to obtain the three-phase modulation voltage, and the three-phase modulation voltage generates a control pulse through an SPWM modulator, which is characterized in that: When the system is operating normally, the non-fault virtual resistance and non-fault virtual inductance are determined according to the filter resistance and filter inductance of the filter; the non-fault potential difference is determined by using the line current and the non-fault virtual resistance and non-fault virtual inductance, and the difference between the voltage command and the output voltage and non-fault potential difference of the grid-forming converter is used to obtain the non-fault current command value through the voltage control outer loop; When a system fault occurs, during the fault transient process, the fault transient virtual resistance is determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage; during the fault steady state process, the fault steady state virtual resistance and the fault steady state virtual inductance are determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid-forming converter, and the larger value of the fault transient virtual resistance and the fault steady state virtual resistance is taken as the fault virtual resistance, and the fault steady state virtual inductance is taken as the fault virtual inductance; the fault potential difference is determined by using the line current and the fault virtual resistance and the fault virtual inductance, and the fault current command value is obtained by the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference through the voltage control outer loop.

2. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 1 is characterized in that: Current limiting control methods include: Step 1, obtaining the three-phase voltage of the grid connection point and the filter inductance of the grid-connected converter to determine whether the system has a fault; when the system is operating normally, proceed to step 2, and when the system has a fault, proceed to step 3; Step 2: When the system operates normally, the filter resistance of the filter is a non-fault virtual resistance, the filter inductance of the filter is a non-fault virtual inductance, and the process goes to step 4; Step 3, when a system fault occurs, in the transient process of the fault, according to the voltage difference between the terminal voltage of the virtual synchronous machine and the grid-connected point voltage, the lead-lag link is used to determine the fault transient virtual resistance; in the steady-state process of the fault, according to the voltage difference between the terminal voltage of the virtual synchronous machine and the grid-connected point voltage and the current over-limit threshold of the grid converter, the fault steady-state virtual inductance is determined; based on the impedance ratio of the filter, the fault steady-state virtual inductance is used to determine the fault steady-state virtual resistance; the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance, and the process proceeds to step 4; Step 4: When the system is operating normally, the non-fault potential difference is determined using the line current, the non-fault virtual resistance and the non-fault virtual inductance, and the difference between the voltage command and the output voltage of the grid-forming converter and the non-fault potential difference is obtained through the voltage control outer loop to obtain the non-fault current command value; when a system fault occurs, the fault potential difference is determined using the line current, the fault virtual resistance and the fault virtual inductance, and the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference is obtained through the voltage control outer loop to obtain the fault current command value.

3. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 2 is characterized in that: In step 1, the per unit value of the filter inductance of the grid-connected converter and the d-axis component of the positive sequence voltage of the system grid connection point are obtained, and whether the system fails is determined according to the following criteria: 1) When the per-unit value of the filter inductance is less than the filter inductance threshold, and two consecutive sampling values ​​of any phase voltage at the grid connection point are less than the voltage drop threshold, it is determined that the system has a fault; 2) When the filter inductance value is not less than the filter inductance threshold and the d-axis component of the system grid-connected point positive sequence voltage is less than the grid-connected point positive sequence voltage threshold, it is determined that the system has a fault; When the filter inductance value is not less than the filter inductance threshold and the d-axis component of the system grid-connected point positive sequence voltage is not less than the grid-connected point positive sequence voltage threshold, it is determined that the system has no fault; The filter inductance threshold is 0.3pu, the voltage drop threshold is 0.8pu, and the grid-connected point positive sequence voltage threshold is 0.9pu.

4. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 3 is characterized in that: In step 3, the fault transient virtual resistance satisfies the following relationship: In the formula, R vt is the transient virtual resistance of the fault, ΔU is the voltage difference between the terminal voltage of the virtual synchronous machine and the grid connection point voltage, K t is the gain constant of the lead-lag link, ω c is the crossover frequency of the lead-lag link, s is the integral operator; The virtual synchronous machine control outputs the terminal voltage E and the phase angle δ according to the actual value of active power and the actual value of reactive power, the active power command value and the reactive power command value; ΔU=EU pcc Where, E is the terminal voltage of the virtual synchronous machine, U pcc is the grid connection point voltage.

5. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 4 is characterized in that: The fault steady-state virtual inductance and fault steady-state virtual resistance satisfy the following relationship: Where, L v1 is the fault steady-state virtual inductance, E is the terminal voltage of the virtual synchronous machine, U pcc is the grid-connected point voltage, L0 is the non-fault virtual inductance, R0 is the non-fault virtual resistance, X0 is the non-fault virtual reactance, satisfying X0=ω n L0,ω n is the system rated angular frequency, I max is the current over-limit threshold of the grid converter, R vs is the fault steady-state virtual resistance, σ X / R is the filter impedance ratio.

6. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 5 is characterized in that: The fault virtual resistance and fault virtual inductance satisfy the following relationship: In the formula, R v is the fault virtual resistance, L v is the fault virtual inductance.

7. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 2, characterized in that: In step 4, when the system operates normally, the dq-axis component of the non-fault potential difference satisfies the following relationship: In the formula, Δu 0d , Δu 0q is the dq axis component of the non-fault potential difference, i ld 、i lq is the dq axis component of the line current, L0 is the non-fault virtual inductance, R0 is the non-fault virtual resistance, ω n is the system rated angular frequency; When a fault occurs in the system, the dq axis component of the fault potential difference satisfies the following relationship: In the formula, Δu d , Δu q is the dq axis component of the fault potential difference, R v is the fault virtual resistance, L v is the fault virtual inductance.

8. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 7 is characterized in that: The voltage control outer loop adopts PI regulator. When the system is operating normally and when a fault occurs, the input signal of the PI regulator satisfies the following relationship respectively: In the formula, are the positive sequence voltage difference and negative sequence voltage difference of the input PI regulator respectively, are the dq axis components of the positive sequence voltage command value and the dq axis components of the negative sequence voltage command value, Δu 0dq is the dq axis component of the non-fault potential difference, Δu dq is the dq axis component of the fault potential difference, They are the dq axis component of the actual value of the positive sequence voltage and the dq axis component of the actual value of the negative sequence voltage respectively; When the system is operating normally, the PI regulator outputs the dq-axis component of the positive-sequence current command value and the dq-axis component of the negative-sequence current command value under non-fault conditions; When a system fault occurs, the PI regulator outputs the dq-axis component of the positive-sequence current command value and the dq-axis component of the negative-sequence current command value under the fault.

9. The current limiting control method of a grid-connected converter based on virtual impedance adjustment according to claim 6, characterized in that: Based on the method of judging whether the system fails in step 1, when the system fails and recovers, after the set delay, the gain constant K of the lead-lag link is used. t The slope is , and the fault virtual resistance and the fault virtual inductance are reduced to the non-fault virtual resistance and the non-fault virtual inductance; wherein the delay value is set to 40ms.

10. A current limiting control system for a grid-connected converter based on virtual impedance regulation, the control system of the grid-connected converter comprising virtual synchronous machine control, current and voltage dual-loop control, and an SPWM modulator, wherein: The current and voltage dual-loop control includes a voltage control outer loop and a current control inner loop; the virtual synchronous machine control generates a voltage command value according to the actual value of active power, the actual value of reactive power, the active power command value and the reactive power command value; the voltage command value is obtained by the voltage control outer loop to obtain the current command value, and the current command value is obtained by the current control inner loop to obtain the three-phase modulation voltage, and the three-phase modulation voltage generates a control pulse through an SPWM modulator, characterized in that the current limiting control system includes: A virtual impedance loop connected in series between the virtual impedance virtual synchronous machine control and the current and voltage dual loop control; The virtual impedance loop is used to determine the non-fault virtual resistance and non-fault virtual inductance according to the filter resistance and filter inductance of the filter when the system is operating normally; the non-fault potential difference is determined by using the line current and the non-fault virtual resistance and non-fault virtual inductance, and the difference between the voltage command and the output voltage and non-fault potential difference of the grid-forming converter is obtained through the voltage control outer loop to obtain the non-fault current command value; The virtual impedance loop is also used when a system fault occurs. During the transient process of the fault, the fault transient virtual resistance is determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage; during the steady-state process of the fault, the fault steady-state virtual resistance and the fault steady-state virtual inductance are determined according to the voltage difference between the machine terminal voltage of the virtual synchronous machine and the grid connection point voltage and the current over-limit threshold of the grid-forming converter, and the larger value of the fault transient virtual resistance and the fault steady-state virtual resistance is used as the fault virtual resistance, and the fault steady-state virtual inductance is used as the fault virtual inductance; the fault potential difference is determined by using the line current, the fault virtual resistance and the fault virtual inductance, and the fault current command value is obtained by the difference between the voltage command and the output voltage of the grid-forming converter and the fault potential difference through the voltage control outer loop.

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