A grid-forming inverter fault ride-through method

Abstract

The application discloses a network-constructed inverter fault ride-through method, which effectively improves the transient stability of the network-constructed inverter by increasing system damping and power flexible configuration within the capacity allowable range. The network-constructed inverter controls the grid-connected voltage source inverter through an active power control loop, a reactive power control loop and the network-constructed inverter. The network-constructed inverter fault ride-through method provided by the application changes the controlled power target only when the fault occurs, and the virtual synchronous generator controlled power target can still track the received instruction during the fault duration, thereby enhancing the transient stability of the network-constructed inverter control type voltage source inverter.

Description

Technical Field

[0001] This invention relates to the field of inverters, specifically to a fault ride-through method for grid-connected inverters. Background Technology

[0002] Voltage source converters (VSCs) are widely used as grid interfaces in renewable energy systems, distributed generation systems, and electrified transportation systems to convert direct current (DC) to alternating current (AC).

[0003] Currently, voltage source inverter control strategies can be mainly divided into two categories: grid-connected and grid-connected. Among them, grid-connected voltage source inverters do not contain phase-locked loops and are equivalent to independent voltage sources, capable of independently supplying power to loads. Among the many grid-connected voltage source inverter control methods, virtual synchronous generator (VSG) control is the simplest and most widely used, in which the voltage source inverter is controlled to simulate the damping and inertia of a synchronous generator.

[0004] The flexibility of virtual synchronous generator (VRG) controllers allows for the design of auxiliary control methods to enhance transient stability, provided the transient instability mechanism is understood. Existing technologies propose adding low-pass filters to the reactive power control loop to improve transient stability, but this increases the order of the control system, making system analysis more complex. An adaptive inertia method has also been proposed to reduce frequency deviations during grid faults, thus improving transient stability. However, this method relies on differential components to detect the direction of frequency changes, making implementation complex. A VRG mode switching method has also been proposed, which can change the VRG mode during grid faults, achieving fault ride-through even without a balanced operating point, thus improving transient stability. However, this method requires detecting changes in frequency and active power and also relies on differential components, making practical implementation complex. Another existing method designs a transient damper based on the difference between the grid frequency estimated by a phase-locked loop (PLL) and the angular frequency in the VRG's active power control loop. However, this method relies on the PLL to obtain the grid frequency, and the PLL may lose stability during voltage dips. Summary of the Invention

[0005] The purpose of this invention is to provide a fault ride-through method for grid-connected inverters, which effectively improves the transient stability of voltage source inverters by increasing system damping and flexibly configuring power within the allowable capacity range.

[0006] The beneficial effects of this invention are: the inverter fault ride-through method proposed in this invention only changes the controlled power target when a fault occurs, while maintaining the tracking of the virtual synchronous generator's controlled power target with respect to the received commands during the fault duration, thereby enhancing the transient stability of the grid-connected inverter control voltage source inverter. Attached Figure Description

[0007] Figure 1 This is a control principle diagram of the control method of the present invention;

[0008] Figure 2 This is a simulation waveform diagram of a voltage source inverter (VSI) when the active and reactive power controlled targets remain unchanged;

[0009] Figure 3 The simulation waveform diagram is based on the present invention. Detailed Implementation

[0010] This invention works and is implemented by measuring the three-phase current and voltage at the grid-connected port of the grid-connected voltage source inverter using voltage and current sensors, and then calculating the output active power and output reactive power at the grid-connected port of the inverter, as detailed below:

[0011] (13)

[0012] In the formula, P m This is the actual measured value of the active power output by the voltage source inverter. Q m It is the actual measured value of the reactive power output of the voltage source inverter, i a i b i c These are the three-phase current measurements at the grid-connected port of the voltage source inverter, u a u b u c These are the measured values ​​of the three-phase voltage at the grid-connected port of the voltage source inverter.

[0013] A fault ride-through method for grid-connected inverters involves controlling the grid-connected voltage source inverter through an active power control loop and a reactive power control loop.

[0014] When the active power control loop implements control, the active power control loop will process the reactive power deviation Δ after the dead-time function. Q After multiplying by the frequency deviation Δω, k is amplified or reduced. vw The active power compensation during fault ride can be obtained after multiplying the value by 1.

[0015] Voltage source inverter controlled active power target P ref The difference between the active power compensation and the active power is used to obtain the controlled target P of the virtual synchronous generator active power.cmd ;

[0016] Active power compensation Δ P vw It can be obtained through the following formula.

[0017] (1)

[0018] In the formula, k vw The proportional gain is the active power proportional element, and Δω is the frequency deviation. dead () dead zone function, Q This refers to the actual measured reactive power. Q ref The target can be controlled by reactive power for the virtual synchronous generator; whereby Specifically, it is stated as follows:

[0019] (2)

[0020] In the formula, Q lim Dead zone limit;

[0021] Voltage source inverter controlled active power target P ref Represented as:

[0022] (3)

[0023] In the formula, P cmd For the controlled active power target of the virtual synchronous generator, Δ P vw This is the active power compensation amount.

[0024] When the reactive power control loop implements control, the frequency deviation Δω is used as the reactive power controlled target switching enable term: when the frequency deviation Δω is lower than the allowable frequency deviation value... lim At that time, without any processing, the controlled reactive power target Q received by the virtual synchronous generator is directly transmitted. cmd Converting to a virtual synchronous generator allows for reactive power control of the target Q. ref ;

[0025] When the frequency deviation Δω exceeds the allowable value for frequency deviation lim At that time, the controlled active power target P of the voltage source inverter ref Multiplying the ratio of the transmission line reactance to the resistance yields the initial control target Q of the available reactive power of the virtual synchronous generator. ref0 ;

[0026] While maintaining active power output, the voltage source inverter's remaining converter power capacity and the virtual synchronous generator's available reactive power initial control target Q are... ref0 In comparison, selecting a smaller value as the target Q for reactive power control of the virtual synchronous generator is preferable. ref ;

[0027] (4)

[0028] In the formula, Q cmd X represents the controlled reactive power target received by the virtual synchronous generator. T R is the reactance of the transmission line of the voltage source inverter. T U is the resistance of the transmission line of the voltage source inverter. m S represents the actual measured voltage at the grid-connected port of the voltage source inverter after a voltage drop. N U is the rated capacity of the voltage source inverter. N This is the rated voltage of the voltage source inverter.

[0029] When implementing control through the active power control loop, the virtual power angle reference value of the grid-connected voltage source inverter is obtained. ref The control model formula for the active power control loop is:

[0030] (5)

[0031] (6)

[0032] In the formula, J It is the virtual inertia of the VSC control system. P m This is the actual measured value of the active power output by the voltage source inverter. ω 0 is the reference frequency of the grid-connected port voltage of the voltage source inverter. θ ref It is the virtual power angle reference value of the voltage source inverter, and also the control output of the active power stage.

[0033] When implementing control through a reactive power control loop, the controlled voltage reference value U of the grid-connected voltage source inverter is obtained. ref The control model formula for the reactive power control loop is:

[0034] (6)

[0035] In the formula, U 0 is the reference voltage at the grid connection port of the voltage source inverter. k q It is the reactive power droop coefficient, Q ref The virtual synchronous generator can use reactive power to control the target.Q m This is the actual measured value of the reactive power output of the voltage source inverter. U ref It is the reference value of the controlled voltage of the voltage source inverter, and also the control output of the reactive power link.

[0036] Using the virtual power angle reference value of the voltage source inverter ref In a two-phase dq-axis rotating coordinate system with rotation angle denoted by , the dq-axis voltage commands are written as follows:

[0037] (7)

[0038] In the formula, u d The grid-connected voltage, u, is the d-axis component of the voltage source inverter. q This is the grid-connected voltage of the q-axis component of the voltage source inverter.

[0039] The controlled target of the output current dq axis of the grid-connected voltage source inverter is obtained by using a proportional regulator or a proportional-integral regulator. Specifically, this is expressed as follows:

[0040] (8)

[0041] (9)

[0042] In the formula, i d i is the grid-connected current of the voltage source inverter, which is the d-axis component. q This represents the grid-connected current of the voltage source inverter, specifically the q-axis component. i dref The grid-connected current of the d-axis controlled target of the voltage source inverter is given. i qref The grid-connected current of the q-axis controlled target of the voltage source inverter is given. U md This is the modulation voltage of the d-axis component of the voltage source inverter. U mq G is the modulation voltage of the q-axis component of the voltage source inverter. v (s) is the transfer function of the virtual synchronous generator voltage controller, G i (s) is the transfer function of the virtual synchronous generator current controller;

[0043] (10)

[0044] (11)

[0045] In the formula, sin represents the sine function, cos represents the cosine function, and ia i b i c The measured value of the three-phase current at the grid-connected port of the voltage source inverter is u. a u b u c These are the measured values ​​of the three-phase voltage at the grid-connected port of the voltage source inverter.

[0046] The modulation voltage of the d-axis component of the acquired voltage source inverter U md Modulation voltage of the q-axis component of the voltage source inverter U mq After transformation to a three-phase stationary coordinate system, the grid-connected voltage source inverter is controlled by a pulse width modulation output signal, as shown below:

[0047] (12)

[0048] In the formula, U ma U is the modulation voltage of the a-axis component of the voltage source inverter. mb U is the modulation voltage of the b-axis component of the voltage source inverter. mc This is the modulation voltage of the c-axis component of the voltage source inverter.

[0049] To evaluate the transient performance of the proposed grid-connected inverter-controlled voltage source inverter control method, this invention utilizes Matlab / Simulink to build a system as follows: Figure 1 The simulation model was used for verification. The power grid reference frequency was set as... ω 0 = 50Hz, line voltage RMS value U rms =380V. Virtual synchronous generator active power controlled target. P cmd =40kW, the rated capacity of the voltage source inverter is P N =40kVA, reactive power reference value Q cmd =0, J = D p =8kW / Hz. D q =0.0001V / Var, line impedance Z g =3.288Ω, reactance to resistance ratio is 8.

[0050] Figure 1The medium voltage and current sensors measure the three-phase current and voltage at the grid-connected port of the grid-connected voltage source inverter, and then calculate the output active power and output reactive power at the grid-connected port of the inverter; based on the power difference, the active power compensation amount is determined; combined with the received virtual synchronous generator active power control target P... cmd The controlled reactive power target Q received by the virtual synchronous generator cmd And the compensation amount in formula (2) is used to obtain the target controlled active power P of the voltage source inverter. ref A virtual synchronous generator can use reactive power to control the target Q. ref The virtual power angle reference value of the voltage source inverter is obtained according to formulas (5) and (6). ref and the controlled voltage reference value U of the voltage source inverter ref The grid-connected voltage U of the d-axis component of the voltage source inverter is obtained according to formula (7). d The grid-connected voltage U of the q-axis component of the voltage source inverter q Then, the output voltage modulation command is obtained through the voltage closed-loop formula (8) and the current closed-loop formula (9); according to the phase angle reference value, it is transformed to the three-phase stationary coordinate system, and then the pulse width modulation technology is used to obtain the signal for controlling the switching tube to turn on and off, so as to realize the control of the voltage source inverter.

[0051] Figure 2 The simulation waveforms of the voltage source inverter are shown when the active and reactive power control targets remain constant. The grid three-phase voltage drops to 50%. The simulation waveforms, from top to bottom, represent the grid three-phase voltages. V gabc Three-phase current of the power grid I gabc Active power on the grid side P Frequency deviation Δ ω Virtual power angle δ .

[0052] from Figure 2 As can be seen, in t = At time 0, the grid voltage drops to 50%, and the system becomes transiently unstable. I gabc , P Δ ω and δ Low-frequency oscillations can be observed in all of the waveforms.

[0053] Figure 3 The simulation results using the virtual synchronous generator fault ride-through method of this invention are shown, where the three-phase voltage of the power grid drops to 50%. The simulated waveforms, from top to bottom, represent the three-phase voltage of the power grid. V gabc Three-phase current of the power grid Igabc Active power on the grid side P Frequency deviation Δ ω Virtual power angle δ .

[0054] exist Figure 3 As can be seen, the voltage source inverter controlled by the virtual synchronous generator can still maintain stable operation when the three-phase grid voltage drops; when it reaches steady state, the output power... P =40kW, which shows that the control method proposed in this invention does not change the steady-state output power of the voltage source inverter.

[0055] Therefore, the fault ride-through method for grid-connected inverter-controlled voltage source inverters proposed in this invention only changes the controlled power target when a fault occurs, while maintaining the tracking of the virtual synchronous generator's controlled power target with the received commands during the fault duration, thereby enhancing the transient stability of grid-connected inverter-controlled voltage source inverters.

Claims

1. A fault ride-through method for a grid-connected inverter, characterized by: The grid-connected voltage source inverter is controlled through active power control loop and reactive power control loop. When the active power control loop implements control, the active power control loop will process the reactive power deviation Δ after the dead-time function. Q After multiplying by the frequency deviation Δω, k is amplified or reduced. vw The active power compensation during fault ride can be obtained after multiplying the value by 1. Voltage source inverter controlled active power target P ref The difference between the active power compensation and the active power is used to obtain the controlled target P of the virtual synchronous generator active power. cmd ; Active power compensation Δ P vw It can be obtained through the following formula. （1） In the formula, k vw The proportional gain is the active power proportional element, and Δω is the frequency deviation. dead () dead zone function, Q This refers to the actual measured reactive power. Q ref The target can be controlled by reactive power for the virtual synchronous generator; whereby Specifically, it is stated as follows: （2） In the formula, Q lim Dead zone limit; Voltage source inverter controlled active power target P ref Represented as: （3） In the formula, P cmd For the controlled active power target of the virtual synchronous generator, Δ P vw This is the active power compensation amount.

2. The fault ride-through method for a grid-connected inverter according to claim 1, characterized in that: When the reactive power control loop implements control, the frequency deviation Δω is used as the reactive power controlled target switching enable term: when the frequency deviation Δω is lower than the allowable frequency deviation value... lim At that time, without any processing, the controlled reactive power target Q received by the virtual synchronous generator is directly transmitted. cmd Converting to a virtual synchronous generator allows for reactive power control of the target Q. ref ; When the frequency deviation Δω exceeds the allowable value for frequency deviation lim At that time, the controlled active power target P of the voltage source inverter ref Multiplying the ratio of the transmission line reactance to the resistance yields the initial control target Q of the available reactive power of the virtual synchronous generator. ref0 ; While maintaining active power output, the voltage source inverter's remaining converter power capacity and the virtual synchronous generator's available reactive power initial control target Q are... ref0 In comparison, selecting a smaller value as the target Q for reactive power control of the virtual synchronous generator is preferable. ref ; （4） In the formula, Q cmd X represents the controlled reactive power target received by the virtual synchronous generator. T R is the reactance of the transmission line of the voltage source inverter. T U is the resistance of the transmission line of the voltage source inverter. m S represents the actual measured voltage at the grid-connected port of the voltage source inverter after a voltage drop. N U is the rated capacity of the voltage source inverter. N This is the rated voltage of the voltage source inverter.

3. The fault ride-through method for a grid-connected inverter according to claim 2, characterized in that: When implementing control through the active power control loop, the virtual power angle reference value of the grid-connected voltage source inverter is obtained. ref The control model formula for the active power control loop is: （5） （6） In the formula, J It is the virtual inertia of the voltage source inverter. P m This is the actual measured value of the active power output by the voltage source inverter. ω 0 is the reference frequency of the grid-connected port voltage of the voltage source inverter. θ ref It is the virtual power angle reference value of the voltage source inverter, and also the control output of the active power stage.

4. The fault ride-through method for a grid-connected inverter according to claim 3, characterized in that: When implementing control through a reactive power control loop, the controlled voltage reference value U of the grid-connected voltage source inverter is obtained. ref The control model formula for the reactive power control loop is: （6） In the formula, U 0 is the reference voltage at the grid connection port of the voltage source inverter. k q Q is the reactive voltage droop factor of a voltage source inverter. ref The virtual synchronous generator can use reactive power to control the target. Q m This is the actual measured value of the reactive power output of the voltage source inverter. U ref It is the reference value of the controlled voltage of the voltage source inverter, and also the control output of the reactive power link.

5. The fault ride-through method for a grid-connected inverter according to claim 4, characterized in that: Using the virtual power angle reference value of the voltage source inverter ref In a two-phase dq-axis rotating coordinate system with rotation angle denoted by , the dq-axis voltage commands are written as follows: （7） In the formula, u d The grid-connected voltage, u, is the d-axis component of the voltage source inverter. q This is the grid-connected voltage of the q-axis component of the voltage source inverter.

6. The fault ride-through method for a grid-connected inverter according to claim 5, characterized in that: The controlled target of the output current dq axis of the grid-connected voltage source inverter is obtained by using a proportional regulator or a proportional-integral regulator. Specifically, this is expressed as follows: （8） （9） In the formula, i d i is the grid-connected current of the voltage source inverter, which is the d-axis component. q This represents the grid-connected current of the voltage source inverter, specifically the q-axis component. i dref The grid-connected current of the d-axis controlled target of the voltage source inverter is given. i qref The grid-connected current of the q-axis controlled target of the voltage source inverter is given. U md This is the modulation voltage of the d-axis component of the voltage source inverter. U mq G is the modulation voltage of the q-axis component of the voltage source inverter. v (s) is the transfer function of the virtual synchronous generator voltage controller, G i (s) is the transfer function of the virtual synchronous generator current controller; （10） （11） In the formula, sin represents the sine function, cos represents the cosine function, and i a i b i c These are the three-phase current measurements at the grid-connected port of the voltage source inverter, u a u b u c These are the measured values ​​of the three-phase voltage at the grid-connected port of the voltage source inverter.

7. A fault ride-through method for a grid-connected inverter according to claim 6, characterized in that: The modulation voltage of the d-axis component of the acquired voltage source inverter U md Modulation voltage of the q-axis component of the voltage source inverter U mq After transformation to a three-phase stationary coordinate system, the grid-connected voltage source inverter is controlled by a pulse width modulation output signal, as shown below: （12） In the formula, U ma U is the modulation voltage of the a-axis component of the voltage source inverter. mb U is the modulation voltage of the b-axis component of the voltage source inverter. mc This is the modulation voltage of the c-axis component of the voltage source inverter.

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