CN113746108B - T-type three-level SAPF open circuit fault sequence model prediction fault tolerance control method - Google Patents
T-type three-level SAPF open circuit fault sequence model prediction fault tolerance control method Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
- H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
- H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/32—Means for protecting converters other than automatic disconnection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/487—Neutral point clamped inverters
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/20—Active power filtering [APF]
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Abstract
The invention adopts a sequence model to predict fault-tolerant control in order to solve the fault-tolerant control problem of the T-type three-level LCL-type SAPF in the case of horizontal bridge arm or vertical bridge arm faults. Dividing the SAPF under fault operation into three types of healthy state, horizontal unhealthy state and vertical unhealthy state, and selecting different direct current bus voltage reference values and alternative vectors according to the three types of healthy state, horizontal unhealthy state and vertical unhealthy state; through i p ‑i q The method extracts harmonic current of nonlinear load as a reference value of the SAPF output, and simultaneously uses a PI controller to control DC bus voltage; two cost functions of controlling the neutral-point voltage and then controlling the compensation current are designed and operated in cascade, so that the SAPF under fault can obtain excellent current compensation effect under three different states, stability of the direct-current bus voltage and the neutral-point voltage is ensured, and reliability of the active power filter system is improved.
Description
Technical Field
The invention belongs to the technical field of power electronic converter fault control, and particularly relates to a T-shaped three-level SAPF fault-tolerant control method based on sequence model predictive control.
Background
With the large input of nonlinear load, the power grid is injected to have a large number of harmonic waves, so that the quality of electric energy is reduced, and the electric equipment is negatively affected. The parallel active power filter (Shunt Active Power Filter, SAPF) counteracts reactive current or harmonic current of the nonlinear load through a power electronic technology, so that harmonic components of a power grid are obviously reduced, and the parallel active power filter is widely applied. Meanwhile, compared with the traditional two-level topology, the T-type three-level power electronic topology can enable the SAPF to output better compensation current, so that the total harmonic distortion of the power grid is small, and the harmonic treatment function of the SAPF is improved.
In recent years, in order to solve the reliability problem of the power electronic system, the fault-tolerant control problem of the power converter becomes a research hotspot because the power electronic system under fault has the capability of working normally. The SAPF based on T-type three-level topology may cause an open circuit failure of a certain switching device due to thermal cycling and gate driving errors during operation, and may be regarded as that the switching device is always in an off state. This can lead to severe distortion of the compensation current output by the SAPF, allowing the harmonic content of the grid current to increase significantly; it can also cause the dc side capacitor voltage of the SAPF to fluctuate or be unbalanced.
The T-shaped three-level switching device can be divided into a horizontal bridge arm and a vertical bridge arm, wherein the severity of the fault of the horizontal bridge arm is far smaller than that of the fault of the vertical bridge arm. The reason is that the horizontal bridge arm does not affect the space vector modulation range of the topology, and the vertical bridge arm fault reduces the vector modulation range by one half. Therefore, two methods are proposed for the existing fault-tolerant control: firstly, adding a redundant device in an inverter, and when a switch fails, taking over a fault circuit by the redundant circuit to ensure that a new topology has a sufficient modulation range; and secondly, changing a modulation algorithm, improving a reference value of direct-current side voltage, and changing an algorithm of a Space Vector Pulse Width Modulation (SVPWM) mode to enable each output vector to be in a modulation range.
However, the two methods have respective disadvantages: the redundant device method adds additional components such as IGBT, relay, thyristor and the like, which is not economical and the system is heavy; the SVPWM-based modulation algorithm is complex to recalculate the duration of each composite vector at each sampling time.
At present, in fault-tolerant control of SAPF, there is no application of predictive control of a limited control set model. Compared with the traditional control method, the model predictive control does not need PWM modulation, but outputs an optimal switching sequence according to the solution of the optimization problem at each sampling moment; meanwhile, the model predictive control is also suitable for solving the problem of multiple optimization targets such as the SAPF fault-tolerant control.
Disclosure of Invention
The invention aims to solve the technical problems that: the fault-tolerant control method based on the sequence model predictive control is provided for realizing fault-tolerant operation when any one of the switching devices of the T-type three-level LCL type SAPF horizontal bridge arm and the vertical bridge arm has open-circuit fault.
In order to achieve the above purpose, the present invention adopts the following technical scheme.
According to the topological structure diagram shown in fig. 1, three-phase alternating voltage v is acquired by a voltage sensor and a current sensor g Three-phase SAPF output current i 2 Three-phase current i of nonlinear load l Voltage u of two capacitors on DC bus side p And u n . And estimating the three-phase inverter-side current i by a Kalman filter 1 And three-phase filter capacitor voltage v C 。
According to the control block diagram shown in fig. 2, the reactive current and the active harmonic current that the SAPF needs to compensate for are extracted. Firstly, phase information of a power grid is obtained through a phase-locked loop (PLL), three-phase load current is converted into a two-phase static coordinate system through Clark conversion, and then is converted into a two-phase rotating coordinate system through Park conversion, and d-axis phase is consistent with the power grid, so that d-axis component i ld As active current, q-axis component i lq Is reactive current; then, fundamental wave components of active current and reactive current are obtained through a Low Pass Filter (LPF), and then, the fundamental wave components of three-phase load current are obtained through Park inverse transformation and Clark inverse transformation; finally, the fundamental wave component is subtracted by the load current to obtain the harmonic component to be compensated by the SAPF, namely the reference compensation current of the SAPF
If the compensated current is in phase with the power grid, the reactive current switch can be disconnected, and only active current is output.
The control block diagram of fig. 2 also includes a dc voltage control module that performs PI control on the dc voltage error, and the PI control output error current is injected into the d-axis active fundamental current to jointly form the SAPF reference compensation current. The transfer function of the PI controller is:
wherein K is p Is a proportionality coefficient, K i Is an integral coefficient.
The faults of the T-type three-level SAPF can be divided into horizontal bridge arm faults and vertical bridge arm faults. When the horizontal bridge arm fails, the running state can be divided into a healthy state and a horizontal unhealthy state according to different current flow directions of the inverter side; when the vertical bridge arm fails, the running state can be divided into a healthy state and a vertical unhealthy state according to different current flow directions at the side of the inverter. The SAPF in a healthy state is not affected by a fault, while the compensation current and midpoint voltage of the SAPF in a non-healthy state are affected to different extents.
According to different properties of the three states, the minimum direct-current voltage reference value required by guaranteeing the modulation range can be obtained through phasor analysis of the LCL filter circuit and space vector diagrams of the three-state circuit
The empirical formula is as follows:
wherein,,
the maximum value of the modulus of the reference voltage vector is output to the inverter in one power frequency period.
Will be used in the PI control loop in fig. 2.
The sequence model predictive fault-tolerant control targets are two: balance of midpoint voltage, compensation current with respect to reference value thereof
Is a tracking of (a). Defining the midpoint voltage as u n And u p In order to ensure that the two capacitors of the direct current bus always divide the voltage of the direct current power supply equally, the midpoint voltage should be kept as 0 as possible.
Firstly, in order to realize the control of the neutral point voltage balance, a cost function is designed, wherein the cost function formula is as follows:
J 1 ＝|(T s /C 1 )|u(k)| T i 1 (k)+u n (k)-u p (k) I type three
Wherein T is s For sampling time, C 1 U (k) is a variable of the cost function for the upper dc bus capacitance. u (k) will vary from one operating state to another. Since u (k) in equation three has an absolute value, it is possible to take |u (k) | as a variable and substitute all possible cases into J 1 Sequencing the sizes of the three switching vectors to obtain n optimal switching vectors, and recording the n optimal switching vectors as
Secondly, in order to realize the control of the SAPF compensation current, a cost function is designed, and the cost function formula is as follows:
J 2 ＝||C(Ax αβ (k)+Bu(k)+Tv g (k)-x αβref (k+1)) ||four
Wherein|| representative of I vector 2-norm; x is x αβ (k)＝[i 1α (k),i 1β (k),i 2α (k),i 2β (k),v Cα (k),v Cβ (k)] T This is the inverseA state vector formed by combining the converter side current, the SAPF compensation current and the filter capacitor voltage after Clark conversion; x is x αβref (k+1) is a predicted value of the state vector reference value. The prediction of the reference value can be realized by Lagrangian extrapolation and a phase-locked loop; A. b, C, T is a constant matrix and is only related to the parameters of the inverter.
Due to J 1 To find that n optimal switching vectors contain absolute values, it is necessary to add |u opt (k) The absolute value of I is removed to obtain a plurality of J 2 According to the difference in the operation state of the SAPF and |u opt (k) The resulting alternative vectors are different from one another.
Finally, substituting a plurality of switch vectors into the cost function J 1 In (1), find the J 1 The smallest switching vector, denoted u opt (k) A. The invention relates to a method for producing a fibre-reinforced plastic composite This switching vector will be turned on and off to the T-type three-level SAPF switching device at the next sampling instant.
The flow chart of the predictive fault-tolerant control of the sequence model is shown in fig. 3, wherein two cost functions form a cascaded control sequence, and two alternative vectors with different cost functions are selected according to different SAPF running states.
The traditional model predictive control is to make J 1 And J 2 And weighting and combining, wherein the cost function formula is as follows:
J 0 ＝||C(Ax αβ (k)+Bu(k)+Tv g (k)-x αβref (k+1))||+λ np |(T s /C 1 )|u(k)| T i 1 (k)+u n (k)-u p (k) Five kinds of I
Wherein lambda is np Is a weight factor. Three different weight factors need to be selected because of different running performances of the health state, the horizontal non-health state and the vertical non-health state. The weight factor needs repeated trial and error selection, which is a disadvantage of the traditional model predictive control. The invention leads the cost function J of multiple targets to be 0 Conversion into two cascaded cost functions J 1 And J 2 The selection of the weight factors is avoided.
The whole system of the invention is shown in the figure4, wherein the SAPF reference offset current calculation module references the control block diagram of fig. 2. After the working state of the system is determined, the absolute value of the required alternative vector and the reference value of the DC bus voltage can be correspondingly calculated. Wherein the reference value of the DC bus voltage
Will be used in the compensation current reference calculation module, the absolute value of the candidate vector will be used to calculate the cost function J 1 . Two cost functions J 1 And J 2 A cascade control sequence is formed, the midpoint voltage is controlled firstly, then the compensation current is controlled, and finally the optimal switching vector u is controlled opt (k) The feedback control with fault tolerance function is completed by converting the signal into the switching signal required by the switching device of the T-type three-level converter.
Due to the application of the technical scheme, the invention has the following characteristics:
1. the invention adopts model predictive control technology, does not need PWM modulation, can ensure that the T-type three-level SAPF keeps fault-tolerant operation when any one switching device fails, and ensures the quality of a power grid, the balance of midpoint voltage and the stability of direct current voltage;
2. compared with the traditional model predictive control, the invention adopts the sequence model predictive control technology, and avoids the selection of weight factors;
3. the invention divides the fault operation state into a healthy state, a horizontal unhealthy state and a vertical unhealthy state, distributes different control parameters for the three states, and improves the control efficiency.
Drawings
Fig. 1: the T-type three-level LCL-type SAPF topological structure diagram in the invention;
fig. 2: generating a control block diagram of the SAPF reference compensation current and dc voltage control in the present invention;
fig. 3: the invention relates to a flow chart of sequence model prediction fault-tolerant control;
fig. 4: the invention relates to a system block diagram for predicting fault-tolerant control by a sequence model;
fig. 5: nonlinear load circuit diagrams in the invention;
fig. 6: the invention discloses an experimental waveform diagram when no fault occurs, wherein (a) is phase A load current and SAPF compensation current, (b) is phase A power grid current and power grid voltage, (c) is THD of phase A power grid current, and (d) is direct current bus voltage and midpoint voltage;
fig. 7: after the horizontal bridge arm faults occur, the fault-tolerant control strategy is put into an experimental waveform diagram before and after the fault-tolerant control strategy is put into, wherein (a) is three-phase grid current, and (b) is direct-current bus voltage, midpoint voltage and A-phase inverter output voltage.
Fig. 8: after the vertical bridge arm fault occurs, the fault-tolerant control strategy is put into an experimental waveform diagram before and after the fault-tolerant control strategy is put into, wherein (a) is three-phase grid current, and (b) is direct-current bus voltage, midpoint voltage and A-phase inverter output voltage.
Detailed Description
The technical solution will be clearly and completely described below in connection with the preferred examples and the accompanying drawings of the present invention. It should be understood that the preferred examples are illustrative of the present invention and are not intended to limit the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without making any inventive effort are within the scope of the present invention.
The invention provides a fault-tolerant control strategy for T-type three-level SAPF open circuit fault. The working state of the SAPF is distinguished by judging the direction of the current at the inverter side, and then different control parameters are calculated; the predictive control of the sequence model is realized through two cost functions. The integral scheme can realize that the balance of the midpoint voltage and high-quality compensation current can be ensured under the condition of open-circuit fault of a single switching device, and the reliability of the system is improved.
A control block diagram of an embodiment is shown in fig. 4, and the main content of the embodiment includes the following steps (in S a3 The open circuit fault represents the fault of the horizontal bridge arm, S a1 Open circuit fault represents vertical leg fault):
step S1: three-level LCL type SAPF of T type three-level LCL type is gathered through voltage sensor and current sensorPhase ac voltage v g Three-phase network side current i 2 Three-phase nonlinear load current i l Voltage u of two capacitors on DC bus side p And u n 。
Step S2 is as follows: estimating three-phase inverter-side current i by Kalman filter 1 And three-phase filter capacitor voltage v C 。
Step S3: if S a3 Open circuit fault, judging side current i of A-phase inverter 1a If the flow direction is the negative direction, judging the state as a horizontal unhealthy state; if the direction is positive, judging that the patient is in a healthy state; if S a1 Open circuit fault, judging side current i of A-phase inverter 1a If the flow direction is positive, judging that the flow direction is vertical to the unhealthy state; if the direction is positive, judging that the patient is in a healthy state;
step S4: obtaining A-phase power grid voltage v through phase-locked loop ga According to which the three-phase network side current reference value i is determined 2ref . Reversely deducing a reference value i of inverter side current through circuit relation of LCL type grid-connected inverter 1ref Reference value v of filter capacitor voltage Cref Inverter output reference voltage vector
And converts it to a two-phase stationary coordinate system by Clark transformation.
Step S5: calculating reference value of DC bus voltage
The empirical formula is as follows:
wherein,,
the maximum value of the modulus of the reference voltage vector is output to the inverter in one power frequency period. From which the state of health or the horizontal unhealthy state is determinedThe reference value of the current voltage is 800V, and the reference value of the vertical unhealthy state direct current voltage is 1200V.
Step S6: according to the control block diagram of FIG. 2, a reference compensation current for the SAPF is calculated
Wherein +.>
Will be substituted into the PI controller controlling the dc voltage. The transfer function of the PI controller is:
wherein K is p Is a proportionality coefficient, K i Is an integral coefficient.
Step S7: combined state vector x αβ (k)＝[i 1α (k),i 1β (k),i 2α (k),i 2β (k),v Cα (k),v Cβ (k)] T ，x αβref (k) And the same is true. Will x αβref (k) Extrapolation of the normal with a second order Lagrangian extrapolation to obtain the predicted value x αβref (k+1)。
Step S8: step S721: the possible cases of absolute values |u (k) | of candidate vectors in three operating states (see table 1 for details) are substituted into the cost function J 1 In (1), screening to obtain J 1 N optimal solutions at minimum
n is taken to be 4 in the healthy state, 2 in the horizontal unhealthy state and 7 in the vertical unhealthy state. The cost function formula is:
J 1 ＝|(T s /C 1 )|u(k)| T i 1 (k)+u n (k)- u p (k) I type three
Wherein T is s For sampling time, C 1 The upper DC bus capacitance value.
Table 1 shows J in three working states 1 Possible cases of alternative vector absolute values:
| operating state | Cost function J 1 Possible cases of alternative vector absolute values |
| Health status | OOO,OOP,OPO,OPP,POO,POP,PPO,PPP |
| Horizontal unhealthy state | POO,POP,PPO,PPP |
| Vertical unhealthy state | OOO,OOP,OPO,OPP,POO,POP,PPO,PPP |
P in table 1 represents a high level, and O represents a zero level.
Step S9: will J 1 All the n optimal solutions of (a) are absolute values removed and combined into a cost function J 2 Is an alternative vector to (a). The results of the absolute value removal are different according to the working state, and the results of the absolute value removal are shown in tables 2, 3 and 4.
Table 2 is the result of the state of health removal of the absolute value of the optimal solution:
table 3 is the result of the horizontal unhealthy state minus the absolute value of the optimal solution:
table 4 is the result of vertical unhealthy conditions minus the absolute value of the optimal solution:
p in tables 2, 3, 4 represents a high level, O represents a zero level, and N represents a low level.
Step S10: bringing the candidate vector obtained in step S9 into the cost function J 2 In (1), find the J 2 The smallest switching vector, denoted u opt (k) A. The invention relates to a method for producing a fibre-reinforced plastic composite The cost function formula is:
J 2 ＝||C(Ax αβ (k)+Bu(k)+Tv g (k)-x αβref (k+1)) ||four
Wherein A, B, C, T is a constant matrix and is only related to the parameters of the inverter.
Step S11: the switching vector u opt (k) Is converted into a switching signal of a switching device of the T-type three-level converter.
Taking a T-type three-level LCL type SAPF adopting the control strategy of FIG. 4 as an example, the effectiveness of the proposed sequence model predictive fault-tolerant control is verified. The sequence model predictive fault-tolerant control algorithm is implemented by the texas instruments 32-bit floating point digital signal processor TMS320F 28379. The T-shaped three-level converter consists of six FUZI 1MBH50D-060 IGBTs and six FUZI 2MBI150U2A-060 IGBTs. The converter current and grid current were measured using six hall current sensors (HCS-LTS-06A). And adopting a fundamental frequency transformer to perform voltage matching between the converter voltage and the power grid voltage. The nonlinear load circuit is shown in fig. 5.
Table 5 shows some of the parameters of the predictive fault-tolerant control of the T-type three-level SAPF sequence model:
| parameters (parameters) | Description of the invention | Value of | Parameters (parameters) | Description of the invention | Value of |
| C 1 (μF) | DC side capacitor | 500 | R C (Ω) | Damping resistor | 2 |
| L 1 (mH) | Inverter side inductor | 4 | V g (V) | Grid voltage (effective value) | 110 |
| L 2 (mH) | Net side inductance | 2 | I 2ref (A) | Net side reference current amplitude | 10 |
| C(μF) | |
4 | ω(rad/s) | Grid frequency | 314.16 |
| R 1 (Ω) | Inverter side resistor | 0.1 | K p | Scaling factor | 0.05 |
| R 2 (Ω) | Network side resistor | 0.1 | K i | Integral coefficient | 0.1 |
As can be seen from fig. 6, in the case of no failure, the proposed sequence model predictive control of the present invention ensures that the SAPF has excellent current compensation, with THD of only 2.14%; meanwhile, the voltage of the direct current bus is stabilized at 800V, and the fluctuation of the midpoint voltage is kept within 0.5V.
As can be seen from fig. 7, after the horizontal bridge arm fails, when the sequence model prediction fault tolerance control of the invention is put into, the quality of the three-phase power grid current waveform is improved, and the THD is 2.68%; the voltage of the direct current bus is stabilized at 800V, and the midpoint voltage is reduced from 0.8V to 0.5V; the waveform of the output voltage of the inverter is slightly distorted when in fault, the normal three-level output mode is restored after the fault-tolerant control strategy is put into, and the efficiency is improved.
As can be seen from fig. 8, after the fault of the vertical bridge arm, when the sequence model prediction fault tolerance control of the invention is put into, the current waveform quality of the three-phase power grid is obviously improved, and the THD after stabilization is 2.98%; the voltage of the direct current bus is increased from 800V to 1200V and kept stable, and the rising time is 50ms; there is a significant rise in midpoint voltage, but this periodic fluctuation does not affect the proper operation of the dc side capacitor. The waveform of the output voltage of the inverter is slightly distorted when in fault, the normal three-level output mode is restored after the fault-tolerant control strategy is put into, and the efficiency is improved.
The above-described embodiments are provided for illustrating the technical concept and features of the present invention and are intended to enable those skilled in the art to understand the present invention and to implement it accordingly, and are not intended to limit the scope of the present invention, as the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein and is to be accorded the widest scope consistent with the principles and modifications described herein.
Claims (2)
1. A T-type three-level SAPF open circuit fault sequence model predictive fault tolerant control method, the method comprising: when the horizontal bridge arm of the T-type three-level LCL-type SAPF fails, the running state is divided into a healthy state and a horizontal unhealthy state according to different current flows at the side of the inverter;
when the vertical bridge arm fails, the running state is divided into a healthy state and a vertical unhealthy state according to different current flow directions of the inverter side;
according to the different health states, horizontal unhealthy states and vertical unhealthy states, different reference values of direct current bus voltages are distributed to the three states
Cost function J 1 Absolute value of candidate vector of (2), cost function J 1 Outputting the number n of the optimal switching vectors;
through i p -i q The method extracts the reactive current and the active current of the nonlinear loadHarmonic current is used as a reference value of the SAPF output, and a PI controller is used for controlling direct current side voltage;
designing a cost function J for balancing midpoint voltage 1 Substituting the absolute values of the candidate vectors to obtain n optimal switching vectors;
the absolute values of the optimal switching vectors are removed to obtain a cost function J 2 Is a candidate vector for (a);
designing a cost function J for controlling the compensation current 2 Substituting the candidate vector into J 2 An optimal switching vector u is obtained opt (k) Is converted into an input signal of the inverter switching device.
2. The method of claim 1, the sequence model predictive fault tolerant control characterized by:
the sequence model predictive fault-tolerant control method comprises the steps of controlling midpoint voltage and then controlling compensation current, selecting different alternative vectors and reference values of direct current bus voltage under three operating states, and enabling the SAPF under the fault to obtain better current compensation effect and voltage stability under the three different states.
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