CN116961116B - Transient stability lifting method for grid-built inverter based on self-adaptive q-axis voltage feedback - Google Patents

Abstract

The invention discloses a self-adaptive q-axis voltage feedback-based grid-built inverter transient stability lifting method, which relates to the field of grid-built inverter fault ride-through. The invention can not influence the output of the active power of the grid-formed inverter under the normal operation condition, can automatically adjust the equivalent active power reference according to the voltage drop depth of the power grid under the short circuit fault condition, ensures that the system has a stable balance point, realizes the fault ride-through of the system under different voltage drop conditions of the power grid, and can improve the transient stability of the system.

Description

Transient stability improvement method of grid-connected inverter based on adaptive q-axis voltage feedback

Technical field

The present invention relates to the field of grid-type inverter fault ride-through, and in particular to a method for improving the transient stability of a grid-type inverter based on adaptive q-axis voltage feedback.

Background technique

As the power system is developing towards a high proportion of new energy and a high proportion of power electronic equipment, the strength of the power grid is declining, making it difficult for grid-based inverters to actively support system voltage and frequency. Network-type inverters actively support system voltage and frequency, so they have become an important research object. However, network-type inverters are prone to overcurrent problems during short-circuit faults, and current-limiting links are often used to limit them. However, the addition of current-limiting links makes the transient process of the inverter when a short-circuit fault occurs more complicated. The transient stability margin is reduced, and the system is prone to transient instability due to the loss of stable equilibrium point.

At present, in order to solve the problem of lack of stable balance point after grid-connected inverter system fails, the main solutions include switching control strategies, increasing virtual impedance and additional control loops. The switching control strategy is to switch the grid-based control to the grid-following type control when the power grid fails. After the fault is removed, the grid-following type control is switched back to the grid-based control to meet the reactive power demand of the system during the fault. However, this This method requires a backup phase-locked loop, which is more complex to operate in practical applications, making it difficult to ensure the smoothness and stability of the switching process, and has stability problems under weak networks. Increasing virtual impedance can limit the transient overcurrent during a fault, thereby improving the fault ride-through capability of the system, but it is not applicable when the voltage drops significantly. Some studies have introduced a q-axis voltage feedback branch at the grid connection point into the active power synchronization loop of the grid-type inverter. Although the introduction of this branch can reduce the equivalent active power reference and increase the system damping, thus improving the transient stability. However, the q-axis voltage feedback coefficient is a fixed value, which results in the output power being less than the active reference under normal operating conditions. Moreover, the selection principle of the q-axis voltage feedback coefficient is not given, making it difficult to be directly used in actual engineering applications. Therefore, there is a need for a control method that can realize the output of the active power of the grid-type inverter under normal operating conditions, and at the same time, can automatically adjust the equivalent active power reference according to the depth of the grid voltage drop under short-circuit fault conditions. Ensure that the system has a stable equilibrium point and improve the transient stability of the system.

Contents of the invention

The technical problem to be solved by this invention is: it does not affect the active power output of the grid-type inverter under normal operating conditions, and at the same time, it can automatically adjust the equivalent active power reference according to the grid voltage drop depth under short-circuit fault conditions to ensure that the system It has a stable equilibrium point and improves the transient stability of the system. In order to solve the above technical problems, the present invention proposes a method for improving the transient stability of a grid-connected inverter based on adaptive q-axis voltage feedback. The technical solution is as follows:

The transient stability improvement method of network inverter based on adaptive q-axis voltage feedback includes:

Step 1: Construct a network-type inverter control loop based on q-axis voltage feedback

Determine the network-type inverter control based on q-axis voltage feedback: collect the inverter output voltage V _PCC and current I _g , and transmit them to the power calculation module after Parker transformation to obtain the output active power P and output reactive power Q , through The first-order low-pass filter is sent to the power control link. Through the active power-angular frequency droop control and the q-axis component feedback branch of the inverter output voltage V _PCC , the angular frequency ω is integrated to obtain the output phase angle θ of the power synchronization unit, and then After passing through the voltage outer loop and the current inner loop, the voltage modulation signals m _d and m _q are generated, and the voltage modulation signals m _d and m _q are sent to the reverse conversion module to obtain the modulated wave m _abc . Finally, the pulse width modulation module generates the driving signal control Turning on and turning off IGBT;

Step 2: Calculate the adaptive q-axis voltage feedback coefficient based on transient stability constraints

Step 2.1: Calculate the output phase angle θ of the power synchronization unit of the grid-type inverter based on q-axis voltage feedback, and define the virtual power angle δ as the phase angle difference between the d- axis and the grid voltage V _g ;

Step 2.2: According to the circuit relationship and phasor relationship, calculate the output voltage q-axis component V _qu and active power P _u of the grid-type inverter when the current limit is not triggered, and then obtain the equivalent active power reference when the current limit is not triggered. Prefueq , let Prefueq at the moment of _fault equal to zero, thereby solving the q-axis voltage feedback coefficient k _u when the current limit _is not triggered;

Step 2.3: According to the circuit relationship and phasor relationship, calculate the output voltage q-axis component V _{q lim} and active power P _lim of the grid-type inverter when the current limit is triggered, and then obtain the equivalent active power reference P when the current limit is triggered. _reflimeq , make P _reflimeq equal to zero at the moment of fault, thereby solving the q-axis voltage feedback coefficient k _lim when the current limit is triggered;

Step 3: Adaptive control of network-type inverter based on q-axis voltage feedback

Monitor the grid voltage and inverter output current in real time. When the system is in normal operation, set the q-axis voltage feedback coefficient k = 0, and the inverter adopts droop control with a low-pass filter;

When a short-circuit fault occurs in the system, it switches to grid-type inverter control based on q-axis voltage feedback. At this time, it is detected whether the system triggers current limiting;

If the current limit is triggered, the q-axis voltage feedback coefficient k is set to the q-axis voltage feedback coefficient k _lim when the current limit is triggered;

If the current limit is not triggered, set the q-axis voltage feedback coefficient k to the q-axis voltage feedback coefficient k _u when the current limit is not triggered;

After the fault is removed, the voltage feedback coefficient k is reset to 0, that is, it returns to droop control with a low-pass filter.

Further, in step 1, the method for the power calculation module to calculate the output active power P and the output reactive power Q is:

(1)

Among them, P is the output active power, Q is the output reactive power, V _d and V _q are the voltage dq components generated by the Park transform of the output side voltage V _PCC of the grid-type inverter; I _d and I _q are the grid-type inverter output side voltage V PCC. The current dq component generated by the Park transform of the output side current I _g of the inverter;

The calculation method of voltage phase angle θ at angular frequency ω is:

(2)

(3)

(4)

Among them, L ( s ) is the first-order low-pass filter transfer function, ω _c is the cut-off angular frequency, ω is the angular frequency, ω _ref is the rated angular frequency, K _p is the active power-angular frequency droop coefficient, and P _ref is the active power Reference value, θ is the output phase angle of the power synchronization unit, k is the q-axis voltage feedback coefficient, and s is the Laplacian operator.

Furthermore, in step 1, the calculation method of the voltage modulation signals m _d and m _q is:

1) Calculate the voltage reference values V _dref and V _qref obtained by the virtual impedance loop;

2) Calculate the initial current reference values I _{d 1 ref} and I _{q 1 ref} obtained by the voltage loop according to the voltage reference values V _dref and V _qref ;

3) Calculate the current reference values I _dref and I _qref obtained by the current limiting loop under current limiting conditions based on the initial current reference values I _{d 1 ref} and I _{q 1 ref} ;

4) Calculate the voltage modulation signals m _d and m _q obtained from the current loop according to the current reference values I _dref and I _qref under current limiting conditions:

(5)

Among them, V _dc is the DC voltage input to the grid-type inverter, m _d and m _q are the voltage modulation signals generated by the current loop, I _fd and I _fq are the unfiltered current dq components, k _pc and k _ic are the currents respectively. Loop proportional coefficient and integral coefficient, L _f is the filter inductance value.

Furthermore, the droop control with low-pass filter is specifically: collecting the inverter output voltage V _PCC and current I _g , and transmitting them to the power calculation module after Park transformation to obtain the output active power P and output reactive power Q , is transmitted to the droop control link through the first-order low-pass filter. The active power-angular frequency droop control generates the integral of the angular frequency ω to obtain the voltage phase angle θ . After passing through the voltage outer loop and the current inner loop, the modulation signals m _d and m _q are generated and transmitted. At the end of the conversion module, the modulated wave m _abc is obtained, and then the pulse width modulation module generates a driving signal to control the turning on and off of the IGBT.

Furthermore, the output phase angle θ of the power synchronization unit of the mesh-type inverter based on q-axis voltage feedback in step 2.1 is expressed as:

(6)

Among them, ω _c is the cut-off angular frequency, K _p is the Pf droop coefficient, P _ref is the reference value of the output active power, and θ _g is the grid voltage phase angle;

Define the virtual power angle δ as the phase angle difference between the d- axis and the grid voltage V _g :

(7)

The output voltage q-axis component V _qu and active power P _u when the current limit is not triggered in step 2.2 are respectively expressed as:

(8)

(9)

Among them, V _qu and P _u are the q-axis component of the output voltage and the active power respectively when the current limit is not triggered, X _v is the virtual reactance value, X _g is the line impedance; E is the amplitude of the internal voltage source of the inverter; δ _u is the virtual power angle when current limiting is not triggered;

Putting formula (8) and formula (9) representing the q-axis component V _qu of the output voltage when the current limit is not triggered and the active power P _u when the current limit is not triggered into the formula (6), then according to the formula (7), not The virtual power angle δ _u when the current limit is triggered is expressed as:

(10)

Among them, k _u is the q-axis voltage feedback coefficient when the current limit is not triggered;

Derive and simplify both sides of the above equation to obtain the expression of the equivalent active power reference P _refueq when the current limit is not triggered:

(11)

The active power reference P _ref at the fault instant and the virtual power angle δ _u when the current limit is not triggered are respectively expressed as:

(12)

(13)

Among them, δ ₀ is the virtual power angle of the system when the fault occurs;

Substituting the active power reference P _ref represented by formula (12) and formula (13) and the virtual power angle δ _u when the current limit is not triggered into formula (11), and letting P _refueq =0, we get

(14)

Among them, a is the grid voltage drop coefficient, that is, the ratio of the grid voltage after a fault to the rated voltage;

Solve the above equation to obtain the q-axis voltage feedback coefficient k _u when the current limit is not triggered:

(15)

The output voltage q-axis component V _{q lim} and active power P _lim when the current limit is triggered in step 2.3 are respectively expressed as:

(16)

(17)

Among them, V _{q lim} and P _lim are the q-axis component of the output voltage when the current limit is triggered and the active power when the current limit is triggered respectively; δ _lim is the virtual power angle when the current limit is triggered;

Putting formula (16) and formula (17) representing the output voltage q-axis component V _{q lim} when the current limit is triggered and the active power P _lim when the current limit is triggered into the formula (6), then according to the formula (7), the trigger limit The virtual power angle δ _lim during flow is expressed as:

(18)

Among them, k _lim is the q-axis voltage feedback coefficient when current limit is triggered;

Derive and simplify both sides of the above equation to obtain the expression of the equivalent active power reference P _reflimeq when the current limit is triggered:

(19)

The virtual power angle δ _lim when the fault instantaneously triggers the current limit is expressed as:

(20)

Substituting the active power reference P _ref and the virtual power angle δ _lim when the current limit is triggered expressed by formula (12) and formula (20) into formula (19), and let P _reflimeq =0, we get:

(twenty one)

Solve the above equation to get the q-axis voltage feedback coefficient k _lim when triggering current limit:

(twenty two)

Among them, the line impedance X _g is obtained in advance by impedance measurement or based on online impedance estimation method.

Compared with the prior art, the beneficial effects of the present invention are as follows:

The present invention proposes a method for improving the transient stability of a grid-type inverter based on adaptive q-axis voltage feedback, which is used to realize fault ride-through of the grid-type inverter when a short-circuit fault occurs in the power grid. The adaptive q-axis voltage feedback coefficient can automatically Adaptively changing the equivalent active power reference under different grid voltage drop levels ensures the existence of a stable balance point for the system regardless of whether the current limit is triggered or not. In addition, under normal operating conditions, the value of the q-axis voltage feedback coefficient is equal to zero, which has no impact on the output of the system’s active power.

The invention can not affect the active power output of the grid-type inverter under normal operating conditions. At the same time, under short-circuit fault conditions, the equivalent active power reference can be automatically adjusted according to the grid voltage drop depth to ensure that the system has a stable balance point. Realizing the fault ride-through of the system under different grid voltage drops can improve the transient stability of the system.

Description of the drawings

Figure 1 is a schematic structural diagram of the grid-type inverter model including a current limiting link and q-axis voltage feedback of the present invention connected to a three-phase AC power grid.

Figure 2 is a network-type inverter control loop based on q-axis voltage feedback of the present invention.

Figure 3 shows the adaptive control process of the network-type inverter based on q-axis voltage feedback according to the present invention.

Figure 4(a) shows the traditional droop control of the experimental waveform when the grid voltage drops to 0.2pu.

Figure 4(b) shows the experimental waveform adaptive control when the grid voltage drops to 0.2pu.

Figure 5(a) shows the experimental waveform of adaptive control when the grid voltage drops to 0.2pu under different K _p : K _p =0.00002.

Figure 5(b) shows the experimental waveform of adaptive control when the grid voltage drops to 0.2pu under different K _p : K _p =0.001.

Figure 6 shows the experimental waveform of the adaptive control when the grid voltage drops to 0.2pu when the line inductance L _g =10mH.

Detailed ways

The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and examples. So that those skilled in the art can better understand the present invention.

Figure 1 is a schematic structural diagram of the grid-type inverter model including a current limiting link and q-axis voltage feedback of the present invention connected to a three-phase AC power grid. The control module mainly consists of an active power calculation module, an active power synchronization unit, a virtual impedance link, It consists of voltage control loop, current limiter, current control loop and PWM.

The present invention's method for improving the transient stability of a grid-connected inverter based on adaptive q-axis voltage feedback includes the following steps: 1) Constructing a grid-type inverter control loop based on q-axis voltage feedback; 2) Based on Calculation method of adaptive q-axis voltage feedback coefficient with transient stability constraints.

1. Construct a network-type inverter control loop based on q-axis voltage feedback

Figure 2 is a control block diagram of the power synchronization unit of the grid-type inverter based on q-axis voltage feedback of the present invention. It is mainly composed of a droop control loop with a filter and a q-axis voltage feedback branch.

1) Network-type inverter control loop based on droop control: collect the inverter output voltage V _PCC and current I _g , and transmit them to the power calculation module after Park transformation to obtain the output active power P and output reactive power Q , It is sent to the droop control link through the first-order low-pass filter. The active power-angular frequency droop control generates the integral of angular frequency ω to obtain the voltage phase angle θ . After passing through the voltage outer loop and the current inner loop, the modulated signals m _d and m _q are generated and sent to The inverse Park conversion module obtains the modulated wave m _abc , and then the pulse width modulation module generates a driving signal to control the turning on and off of the IGBT.

2) Network-type inverter control loop based on q-axis voltage feedback: Collect the inverter output voltage V _PCC and current I _g , and transmit them to the power calculation module after Parker transformation to obtain the output active power P and output reactive power. Q , is transmitted to the power control link through the first-order low-pass filter. The angular frequency ω is integrated through the active power-angular frequency droop control and the q-axis component feedback branch of V _PCC to obtain the voltage phase angle θ . After the voltage outer loop and the current After the inner loop, the modulated signals m _d and m _q are generated and sent to the inverse Park conversion module to obtain the modulated wave m _abc , and then the pulse width modulation module generates a driving signal to control the turning on and off of the IGBT.

The specific implementation steps of network-type inverter control based on q-axis voltage feedback are:

(1) The output active power P and output reactive power Q are obtained from the power calculation module. The specific formula is:

(1)

Among them, P is the output active power, Q is the output reactive power, V _d and V _q are the voltage and current dq components generated by the Park transform of the output side voltage V _PCC of the grid-type inverter; I _d and I _q are The voltage and current dq components generated by the Park transform of the output side current I _g of the grid-type inverter.

(2) The angular frequency ω and voltage phase angle θ are obtained through droop control. The specific formula is:

(2)

(3)

(4)

Among them, L ( s ) is the first-order low-pass filter transfer function, ω _c is the cut-off angular frequency, ω is the angular frequency, ω _ref is the rated angular frequency, K _p is the active power-angular frequency droop coefficient, and P _ref is the active power Reference value, θ is the output phase angle of the power synchronization unit.

(3) Obtain the voltage reference values V _dref and V _qref from the virtual impedance loop. The specific formula is:

(5)

Among them, V _dref and V _qref are the voltage reference values generated by the virtual impedance loop, E _dref and E _qref are the given voltage reference values, R _v and X _v are the virtual resistance values and virtual reactance values, I _d and I _q are Output current dq component.

(4) Obtain the initial current reference values I _{d 1 ref} and I _{q 1 ref} from the voltage loop. The specific formula is:

(6)

Among them, I _{d 1 ref} and I _{q 1 ref} are the current reference values generated by the voltage loop, V _d and V _q are the dq components of the output voltage, k _pv and k _iv are the proportional coefficient and integral coefficient of the voltage loop respectively, and C _f is Filter capacitor value.

(5) The current reference values I _dref and I _qref under current limiting conditions are obtained from the current limiting loop. The specific formula is:

(7)

Among them, I _dref and I _qref are the current reference values generated by the current limiting loop, and I _max is the maximum allowable value of the current injected into the grid.

(6) Obtain the voltage modulation signals m _d and m _q from the current loop. The specific formula is:

(8)

Among them, m _d and m _q are the voltage modulation signals generated by the current loop, I _fd and I _fq are the unfiltered current dq components, k _pc and k _ic are the current loop proportional coefficient and integral coefficient respectively, and L _f is the filter inductance value.

Finally, m _d and m _q are sent to the Parker inverse transformation module to obtain the modulated wave m _abc , and the modulated wave m _abc is sent to the pulse width modulation module to generate a driving signal to control the turning on and off of the IGBT in the inverter.

2. Calculate the adaptive q-axis voltage feedback coefficient based on transient stability constraints.

The specific implementation steps are:

(1) The output phase angle θ of the power synchronization unit of the grid-type inverter based on q-axis voltage feedback is expressed as:

(9)

Among them, ω _c is the cut-off angular frequency, K _p is the Pf droop coefficient, P and P _ref are the output active power and its reference value respectively; θ _g is the grid voltage phase angle.

Define the virtual power angle δ as the phase angle difference between the d- axis and the grid voltage V _g :

(10)

(2) According to the circuit relationship and phasor relationship, the q-axis component V _qu of the output voltage and the active power P _u when the current limit is not triggered are expressed as:

(11)

(12)

Among them, V _qu and P _u are the q-axis component of the output voltage and the active power respectively when the current limit is not triggered.

Substituting formula (9), formula (11) and formula (12) into formula (10), the virtual power angle δ _u when the current limit is not triggered is:

(13)

Among them, k _u is the q- axis voltage feedback coefficient when the current limit is not triggered.

Derive and simplify both sides of the above equation at the same time:

(14)

(15)

Among them, P _refueq , H _ueq and D _ueq are respectively the equivalent active power, equivalent inertia time constant and equivalent damping coefficient when the current limit is not triggered.

(3) According to the circuit relationship and phasor relationship, the q-axis component V _{q lim} of the output voltage and the active power P _lim when the current limit is triggered are expressed as:

(16)

(17)

Among them, V _{q lim} and P _lim are the q-axis component of the output voltage when the current limit is triggered and the active power when the current limit is triggered respectively, and δ _lim is the virtual power angle when the current limit is triggered.

Substituting formula (9), formula (16) and formula (17) into formula (10), the virtual power angle δ _lim when the current limit is triggered is:

(18)

Among them, k _lim is the q- axis voltage feedback coefficient when the current limit is triggered.

Derive and simplify both sides of the above equation to obtain the transient model after triggering the current limit:

(19)

(20)

Among them, Pref _{lim eq} , H _{lim eq} and D _{lim eq} are the equivalent active power, equivalent inertia time constant and equivalent damping coefficient when the current limit is triggered, respectively.

(4) The expression of the equivalent active power reference P _refueq when the current limit is not triggered is:

(twenty one)

The active power reference P _ref at the fault instant and the virtual power angle δ _u when the current limit is not triggered are respectively expressed as:

(twenty two)

(twenty three)

Among them, δ ₀ is the virtual power angle of the system when the fault occurs.

Substituting the active power reference P _ref and the virtual power angle δ _u expressed by the formula (22) and the formula (23) into the formula (21), and letting P _refueq =0, we get:

(twenty four)

Among them, a is the grid voltage drop coefficient, that is, the ratio of the grid voltage to the rated voltage after a fault.

Solve formula (24) to obtain the q-axis voltage feedback coefficient k _u when the current limit is not triggered:

(25)

(5) The expression of equivalent active power reference P _reflimeq when current limit is triggered is:

(26)

The virtual power angle δ _lim when the fault instantaneously triggers the current limit is expressed as:

(27)

Among them, δ ₀ is the virtual power angle of the system when the fault occurs.

Substituting the active power reference P _ref and the virtual power angle δ _u when the current limit is not triggered expressed by formula (22) and formula (27) into (26), and letting P _reflimeq =0, we get:

(28)

Solve formula (25) to obtain the q-axis voltage feedback coefficient k _lim when triggering current limit:

(29)

Among them, the line impedance X _g can be obtained in advance by impedance measurement, or measured based on online impedance estimation method.

(6) Adaptive control process

As shown in Figure 3, the adaptive control process of the network-type inverter based on q-axis voltage feedback of the present invention is as follows:

First, the grid voltage and inverter output current are monitored in real time. When the grid voltage is above 90% of the rated voltage, the system is considered to be in normal operation. At this time, k = 0, and the inverter adopts droop control with a low-pass filter. When the grid voltage Vg drops to 90% or less of the rated value VgN , the system is considered to have a short-circuit fault and switches to a network-type control based on q _- axis voltage feedback. At this time, _it is detected whether the system triggers the current limit. If the current limit is triggered, Set the value of k according to k _lim , otherwise it is determined according to k _u . After the fault is removed, k is reset to 0, that is, it returns to droop control with a low-pass filter.

The specific parameters of the example are shown in Table 1.

Table 1 Inverter grid-connected system parameters

.

Figure 4(a) shows the experimental waveform of traditional droop control when the grid voltage drops to 0.2pu. The inverter is connected to the grid at t = 0s and runs in a stable state. At t = 3s, the grid has 80% no phase shift. When the three-phase voltage drops, the inverter changes from a voltage source to a current source due to the current limiting link during the fault. The amplitude of the power angle curve drops significantly. There is no intersection between the output active power and the reference active power, and the system loses its balance point. Transient instability occurs. Figure 4(b) shows the experimental waveform when the grid voltage drops to 0.2pu after adopting the adaptive control strategy. The inverter is connected to the grid at t = 0s and runs in a stable state. At t = 3s, 80% of the power grid fails. The phase-shifted three-phase voltage drops. At this time, the q-axis voltage feedback coefficient k is adjusted according to the grid voltage drop, so that the equivalent active power reference of the system at the moment of the fault is reduced to zero, and then as the power angle decreases, the equivalent active power reference increases, allowing the system to operate at a stable equilibrium point during faults. The fault is cleared at t =6s and the system returns to normal operation. The simulation results show that the transient stability of the system can be achieved by using adaptive control strategy.

Figure 5(a) and Figure 5(b) are the experimental waveforms of the grid voltage dropping to 0.2pu when the adaptive control is at K _p =0.00002 and K _p =0.001 respectively. As the droop coefficient K _p increases, the damping and inertia of the system are smaller, and the dynamic performance is better. The system can maintain transient stability during faults, but the oscillation situation intensifies. This result verifies that the proposed adaptive control strategy can maintain good fault ride-through capability of the system under different droop coefficients K _p .

Figure 6 shows the experimental waveform of the adaptive control when the grid voltage drops to 0.2pu when the line inductance L _g =10mH. During the fault, the system can still maintain good transient stability performance at L _g =10mH. This result verifies that the adaptive control strategy proposed in this paper can enable the system to maintain good fault ride-through capabilities under weak networks.

In summary, the present invention can not affect the active power output of the grid-type inverter under normal operating conditions. At the same time, under short-circuit fault conditions, the equivalent active power reference can be automatically adjusted according to the depth of the grid voltage drop, ensuring that the system has stability. The balance point enables the system to ride through faults under different grid voltage drops, which can improve the transient stability of the system.

Claims (5)

1. A method for improving the transient stability of a grid-connected inverter based on adaptive q-axis voltage feedback, which is characterized by: Step 1: Construct a network-type inverter control loop based on q-axis voltage feedback Determine the network-type inverter control based on q-axis voltage feedback: collect the inverter output voltage V _PCC and current I _g , and transmit them to the power calculation module after Parker transformation to obtain the output active power P and output reactive power Q , through The first-order low-pass filter is sent to the power control link. Through the active power-angular frequency droop control and the q-axis component feedback branch of the inverter output voltage V _PCC , the angular frequency ω is integrated to obtain the output phase angle θ of the power synchronization unit, and then After passing through the voltage outer loop and the current inner loop, the voltage modulation signals m _d and m _q are generated, and the voltage modulation signals m _d and m _q are sent to the reverse conversion module to obtain the modulated wave m _abc . Finally, the pulse width modulation module generates the driving signal control Turning on and turning off IGBT; Step 2: Calculate the adaptive q-axis voltage feedback coefficient based on transient stability constraints Step 2.1: Calculate the output phase angle θ of the power synchronization unit of the grid-type inverter based on q-axis voltage feedback, and define the virtual power angle δ as the phase angle difference between the d- axis and the grid voltage V _g ; Step 2.2: According to the circuit relationship and phasor relationship, calculate the output voltage q-axis component V _qu and active power P _u of the grid-type inverter when the current limit is not triggered, and then obtain the equivalent active power reference when the current limit is not triggered. Prefueq , let Prefueq at the moment of _fault equal to zero, thereby solving the q-axis voltage feedback coefficient k _u when the current limit _is not triggered; Step 2.3: According to the circuit relationship and phasor relationship, calculate the output voltage q-axis component V _{q lim} and active power P _lim of the grid-type inverter when the current limit is triggered, and then obtain the equivalent active power reference P when the current limit is triggered. _reflimeq , make P _reflimeq equal to zero at the moment of fault, thereby solving the q-axis voltage feedback coefficient k _lim when the current limit is triggered; Step 3: Adaptive control of network-type inverter based on q-axis voltage feedback Monitor the grid voltage and inverter output current in real time. When the system is in normal operation, set the q-axis voltage feedback coefficient k = 0, and the inverter adopts droop control with a low-pass filter; When a short-circuit fault occurs in the system, it switches to grid-type inverter control based on q-axis voltage feedback. At this time, it is detected whether the system triggers current limiting; If the current limit is triggered, the q-axis voltage feedback coefficient k is set to the q-axis voltage feedback coefficient k _lim when the current limit is triggered; If the current limit is not triggered, set the q-axis voltage feedback coefficient k to the q-axis voltage feedback coefficient k _u when the current limit is not triggered; After the fault is removed, the voltage feedback coefficient k is reset to 0, that is, it returns to droop control with a low-pass filter. 2. The transient stability improvement method of a networked inverter based on adaptive q-axis voltage feedback according to claim 1, characterized in that in step 1, the power calculation module calculates the output active power P and the output reactive power. The method of power Q is: ; Among them, P is the output active power, Q is the output reactive power, V _d and V _q are the voltage dq components generated by the grid-type inverter output side voltage V _PCC after Parker transformation; I _d and I _q are the grid-type inverter output side voltage V PCC. The current dq component generated by the Parker inverter output side current I _g ; The calculation method of the output phase angle θ of the power synchronization unit with angular frequency ω is: ; ; ; Among them, L ( s ) is the first-order low-pass filter transfer function, ω _c is the cut-off angular frequency, ω is the angular frequency, ω _ref is the rated angular frequency, K _p is the active power-angular frequency droop coefficient, and P _ref is the active power Reference value, θ is the output phase angle of the power synchronization unit, k is the q-axis voltage feedback coefficient, and s is the Laplacian operator. 3. The transient stability improvement method of a networked inverter based on adaptive q-axis voltage feedback according to claim 2, characterized in that in step 1, the calculation method of the voltage modulation signals m _d and m _q is : 1) Calculate the voltage reference values V _dref and V _qref obtained by the virtual impedance loop; 2) Calculate the initial current reference values I _{d 1 ref} and I _{q 1 ref} obtained by the voltage loop according to the voltage reference values V _dref and V _qref ; 3) Calculate the current reference values I _dref and I _qref obtained by the current limiting loop under current limiting conditions based on the initial current reference values I _{d 1 ref} and I _{q 1 ref} ; 4) Calculate the voltage modulation signals m _d and m _q obtained from the current loop according to the current reference values I _dref and I _qref under current limiting conditions: ; Among them, V _dc is the DC voltage input to the grid-type inverter, m _d and m _q are the voltage modulation signals generated by the current loop, I _fd and I _fq are the unfiltered current dq components, k _pc and k _ic are the currents respectively. Loop proportional coefficient and integral coefficient, L _f is the filter inductance value. 4. The method for improving the transient stability of a networked inverter based on adaptive q-axis voltage feedback according to claim 1, characterized in that the droop control with a low-pass filter is specifically: collecting the inverter output The voltage V _PCC and current I _g are sent to the power calculation module after Parker transformation to obtain the output active power P and output reactive power Q , which are sent to the droop control link through the first-order low-pass filter. The active-angular frequency droop control generates an angle. After the frequency ω is integrated, the output phase angle θ of the power synchronization unit is obtained. After passing through the voltage outer loop and the current inner loop, the modulated signals m _d and m _q are generated and sent to the reverse conversion module to obtain the modulated wave m _abc , which is then driven by the pulse width modulation module. The signal controls the turning on and off of the IGBT. 5. The method for improving the transient stability of a grid-type inverter based on adaptive q-axis voltage feedback according to claim 1, characterized in that, in step 2.1, the method of the grid-type inverter based on q-axis voltage feedback is The output phase angle θ of the power synchronization unit is expressed as: ; Among them, ω _c is the cut-off angular frequency, K _p is the Pf droop coefficient, P _ref is the reference value of the output active power; θ _g is the grid voltage phase angle; V _q is the grid-type inverter output voltage V _PCC after Parker transformation The voltage q component generated afterwards; Define the virtual power angle δ as the phase angle difference between the d- axis and the grid voltage V _g : ; The output voltage q-axis component V _qu and active power P _u when the current limit is not triggered in step 2.2 are respectively expressed as: ; ; Among them, V _qu and P _u are the q-axis component of the output voltage and the active power respectively when the current limit is not triggered, X _v is the virtual reactance value, X _g is the line impedance; E is the amplitude of the internal voltage source of the inverter; δ _u is the virtual power angle when current limiting is not triggered; Substituting the expressions of the q-axis component V _qu of the output voltage when the current limit is not triggered and the active power P _u when the current limit is not triggered, the virtual power angle δ _u when the current limit is not triggered is expressed as: ; Among them, k _u is the q-axis voltage feedback coefficient when the current limit is not triggered; Derive and simplify both sides of the above equation to obtain the expression of the equivalent active power reference P _refueq when the current limit is not triggered: ; The active power reference Pref at the _fault instant and _the virtual power angle δu without triggering the current limit are respectively expressed as: ; ; Among them, δ ₀ is the virtual power angle of the system when the fault occurs; Substituting the expressions of the active power reference P _ref and the virtual power angle δ _u without triggering the current limit, and letting P _refueq =0, we get ; Among them, a is the grid voltage drop coefficient, that is, the ratio of the grid voltage after a fault to the rated voltage; Solve the above equation to obtain the q-axis voltage feedback coefficient k _u when the current limit is not triggered: ; The output voltage q-axis component V _{q lim} and active power P _lim when the current limit is triggered in step 2.3 are respectively expressed as: ; ; Among them, V _{q lim} and P _lim are respectively the q-axis component of the output voltage when the current limit is triggered and the active power when the current limit is triggered; δ _lim is the virtual power angle when the current limit is triggered; I _max is the maximum allowable value of the current injected into the grid. ; Substituting the expressions of the q-axis component V _{q lim} of the output voltage when the current limit is triggered and the active power P _lim when the current limit is triggered, the virtual power angle δ _lim when the current limit is triggered is expressed as: ; Among them, k _lim is the q-axis voltage feedback coefficient when current limit is triggered; Derive and simplify both sides of the above equation to obtain the expression of the equivalent active power reference P _reflimeq when the current limit is triggered: ; The virtual power angle δ _lim that triggers current limiting at the moment of fault is expressed as: ; Substitute the expressions of the active power reference P _ref and the virtual power angle δ _lim that triggers current limiting, and let P _reflimeq =0, we get: ; Solve the above equation to get the q-axis voltage feedback coefficient k _lim when triggering current limit: ; Among them, the line impedance X _g is obtained in advance by impedance measurement or based on online impedance estimation method.