CN113746108B - A T-type three-level SAPF open-circuit fault sequence model predictive fault-tolerant control method - Google Patents

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

A T-type three-level SAPF open-circuit fault sequence model predictive fault-tolerant control method

technical field

The invention belongs to the technical field of power electronic converter fault control, and in particular relates to a T-type three-level SAPF fault-tolerant control method based on sequence model predictive control.

Background technique

With the large input of non-linear loads, a large number of harmonics are injected into the power grid, which leads to the decline of power quality and has a negative impact on electrical equipment. The shunt active power filter (Shunt Active Power Filter, SAPF) uses power electronic technology to offset the reactive current or harmonic current of the nonlinear load, so that the harmonic component of the power grid can be significantly reduced, and it has been widely used. At the same time, compared with the traditional two-level topology, the T-type three-level power electronic topology can enable SAPF to output better compensation current, reduce the total harmonic distortion of the power grid, and improve the harmonic control function of SAPF.

In recent years, in order to solve the reliability problem of the power electronic system and make the power electronic system have the ability to work normally under the fault, the problem of fault-tolerant control of the power converter has become a research hotspot. During the operation of the SAPF based on the T-type three-level topology, an open-circuit fault may occur in a switching device due to thermal cycle and gate drive error, which can be regarded as the switching device is always in the off state. This will lead to severe distortion of the compensation current output by the SAPF, which will significantly increase the harmonic components of the grid current; it will also cause voltage fluctuations or imbalances in the DC side capacitors of the SAPF.

T-type three-level switching devices can be divided into two types: horizontal bridge arm and vertical bridge arm, and the severity of faults in the horizontal bridge arm is far less than that in the vertical bridge arm. The reason is that the horizontal leg did not affect the space vector modulation range of the topology, while the vertical leg failure reduced the vector modulation range by a factor of two. Therefore, the existing fault-tolerant control specifically proposes two types of methods: one is to add redundant devices in the inverter. When the switch fails, the redundant circuit will take over the faulty circuit to ensure that the new topology has sufficient modulation range. The second is to change the modulation algorithm, improve the reference value of the DC side voltage, and change the algorithm of the space vector pulse width modulation (SVPWM) mode, so that each output vector is within the modulation range.

However, the above two methods also have their own disadvantages: the redundant device method adds additional components, such as IGBTs, relays and thyristors, etc., which is not economical and the system is cumbersome; Sample time recalculates the duration of each composite vector in a more complex way.

At present, in the fault-tolerant control of SAPF, there is no application of finite control set model predictive control. Compared with traditional control methods, model predictive control does not require PWM modulation, but outputs the optimal switching sequence according to the solution of the optimization problem at each sampling moment; at the same time, model predictive control is also suitable for solving multiple optimization objectives such as SAPF fault-tolerant control question.

Contents of the invention

The technical problem to be solved by the present invention is: in order to realize the fault-tolerant operation when any switching device of the T-type three-level LCL type SAPF horizontal bridge arm and the vertical bridge arm has an open circuit fault, a fault-tolerant control based on sequence model prediction is provided. Control Method.

In order to achieve the above object, the present invention adopts the following technical solutions.

According to the topology diagram shown in Figure 1, the voltage sensor and current sensor are used to collect the three-phase AC voltage v _g , the three-phase SAPF output current i ₂ , the three-phase current i _l of the nonlinear load, and the two capacitors on the DC bus side Voltages u _p and u _n . And use the Kalman filter to estimate the three-phase inverter side current i ₁ and the three-phase filter capacitor voltage v _C .

如果要保证补偿后的电流与电网同相位，可以断开无功电流开关，只输出有功电流。According to the control block diagram shown in Figure 2, extract the reactive current and active harmonic current that SAPF needs to compensate. First, the phase information of the power grid is obtained through a phase-locked loop (PLL), and the three-phase load current is transformed into a two-phase stationary coordinate system through Clark transformation, and then transformed into a two-phase rotating coordinate system through Park transformation. Since the d-axis phase is consistent with the power grid , so the d-axis component i _ld is the active current, and the q-axis component i _lq is the reactive current; after that, the fundamental components of active current and reactive current are obtained through a low-pass filter (LPF), and then through Park inverse transformation and Clark inverse Transform to obtain the fundamental component of the three-phase load current; finally subtract the fundamental component from the load current to obtain the harmonic component that SAPF needs to compensate, that is, the reference compensation current of SAPF

If you want to ensure that the compensated current is in phase with the grid, you can disconnect the reactive current switch and only output active current.

The control block diagram in Figure 2 also has a DC voltage control module, which is realized by performing PI control on the error of the DC voltage. The error current output by the PI control is injected into the active fundamental wave current of the d-axis, which together constitute the SAPF Reference compensation current. The transfer function of the PI controller is:

Among them, K _p is the proportional coefficient, and K _i is the integral coefficient.

The faults of 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 operating state can be divided into two types: healthy state and horizontal unhealthy state according to the flow direction of the inverter side current; Different, the running state is divided into two types: healthy state and vertical unhealthy state. The SAPF in the healthy state is not affected by the fault, while the compensation current and midpoint voltage of the SAPF in the non-healthy state are affected to varying degrees.

经验公式如下：According to the different properties of the three states, the minimum DC voltage reference value required to ensure the modulation range can be obtained through the phasor analysis of the LCL filter circuit and the space vector diagram of the three state circuits

The empirical formula is as follows:

为逆变器输出参考电压矢量的模在一个工频周期内的最大值。

将用于图2中的PI控制环中。in,

The inverter outputs the maximum value of the modulus of the reference voltage vector within a power frequency cycle.

will be used in the PI control loop in Figure 2.

的跟踪。将中点电压定义为u_n和u_p之差，为了保证直流母线的两个电容始终均分直流电源的电压，中点电压应尽量保持为0。The sequence model predicts that there are two objectives for fault-tolerant control: the balance of the midpoint voltage, the compensation current with respect to its reference value

tracking. The midpoint voltage is defined as the difference between u _n and u _p . In order to ensure that the two capacitors of the DC bus always share the voltage of the DC power supply equally, the midpoint voltage should be kept as zero as possible.

First, in order to realize the control of the midpoint voltage balance, a cost function is designed, and the cost function formula is:

J ₁ ＝|(T _s /C ₁ )|u(k)| ^T i ₁ (k)+u _n (k)-u _p (k)|

Among them, T _s is the sampling time, C ₁ is the capacitance value of the upper DC bus, and u(k) is the variable of the cost function. u(k) will vary according to the three operating states. Since u(k) in Equation 3 has an absolute value, |u(k)| can be used as a variable, and all possible situations can be substituted into J ₁ , and their sizes are sorted to find n optimal switch vectors, denoted as

Secondly, in order to realize the control of SAPF compensation current, a cost function is designed, and the cost function formula is:

J ₂ ＝||C(Ax _αβ (k)+Bu(k)+Tv _g (k)-x _αβref (k+1))|| Formula 4

Where |||| represents the vector 2-norm; x _αβ (k)=[i _1α (k),i _1β (k),i _2α (k),i _2β (k),v _Cα (k),v _Cβ (k)] ^T , which is the state vector formed by the inverter side current, SAPF compensation current, and filter capacitor voltage after Clark transformation; x _αβref (k+1) is the prediction of the reference value of the state vector value. The prediction of the reference value can be realized by Lagrangian extrapolation method and phase-locked loop; A, B, C, T are constant matrices, which are only related to the parameters of the inverter.

Since the n optimal switch vectors obtained by J ₁ contain absolute values, the absolute value of |u _opt (k)| needs to be removed to obtain several candidate vectors of J ₂ , according to the different running states of SAPF and |u _opt (k)| is different, and the candidate vectors obtained are also different.

Finally, several switch vectors are substituted into the cost function J ₁ to obtain the switch vector that minimizes J ₁ , denoted as u _opt (k). This switch vector will be converted into the on-off signal of the T-type three-level SAPF switching device and sent in at the next sampling moment.

The flow chart of sequence model prediction fault-tolerant control is shown in Figure 3. Two cost functions form a cascaded control sequence, and two alternative vectors with different cost functions will be selected according to the different running states of SAPF.

The traditional model predictive control is to combine _J1 and _J2 weights, and the cost function formula is:

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)|

Where λ _np is the weight factor. Due to the different operating performances of the healthy state, the horizontal unhealthy state, and the vertical unhealthy state, three different weight factors need to be selected. The selection of weight factors requires repeated trial and error, which is a shortcoming of traditional model predictive control. The present invention converts the multi-objective cost function J ₀ into two cascaded cost functions J ₁ and J ₂ , avoiding the selection of weight factors.

会用于补偿电流参考值计算模块中，备选矢量的绝对值会用于计算代价函数J₁。两个代价函数J₁和J₂组成了一个级联的控制序列，先控制中点电压，后控制补偿电流，最后将最优开关矢量u_opt(k)转化为T型三电平变换器开关器件所需的开关信号，完成了具有容错功能的反馈控制。The overall system block diagram of the present invention is shown in FIG. 4 , and the SAPF reference compensation current calculation module refers to the control block diagram in FIG. 2 . After the working state of the system is determined, the absolute value of the required candidate vector and the reference value of the DC bus voltage can be calculated accordingly. Among them, the reference value of the DC bus voltage

It will be used in the calculation module of the compensation current reference value, and 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 form a cascaded control sequence, which first controls the midpoint voltage, then controls the compensation current, and finally converts the optimal switching vector u _opt (k) into a T-shaped three-level converter switch The switch signal required by the device completes the feedback control with fault tolerance function.

Due to the application of the above-mentioned technical solutions, the present invention has the following characteristics:

1. The present invention adopts model predictive control technology without PWM modulation, and can keep the T-type three-level SAPF in fault-tolerant operation when any switching device fails, ensuring the quality of the power grid, the balance of the midpoint voltage, and the stability of the DC voltage;

2. The present invention adopts the sequential model predictive control technology, which avoids the selection of weight factors compared with traditional model predictive control;

3. The present invention divides faulty operating states into healthy states, horizontal unhealthy states, and vertical unhealthy states, and assigns different control parameters to the three states, thereby improving control efficiency.

Description of drawings

Fig. 1: T-type three-level LCL type SAPF topological structure diagram among the present invention;

Fig. 2: generate the control block diagram of SAPF reference compensation current and DC voltage control in the present invention;

Fig. 3: the flow chart of sequence model prediction fault-tolerant control among the present invention;

Fig. 4: the system block diagram of sequence model prediction fault-tolerant control in the present invention;

Fig. 5: non-linear load circuit diagram among the present invention;

Fig. 6: the experimental waveform diagram when no fault occurs in the present invention, wherein (a) is A-phase load current and SAPF compensation current, (b) is A-phase grid current and grid voltage, (c) is A-phase grid current THD, (d) is DC bus voltage and midpoint voltage;

Fig. 7: After the horizontal bridge arm fault occurs in the present invention, the experimental waveform diagram before and after the fault-tolerant control strategy is put into operation, wherein (a) is the three-phase grid current, (b) is the DC bus voltage, the midpoint voltage and the A-phase inverter The output voltage.

Fig. 8: After the vertical bridge arm fault occurs in the present invention, the experimental waveform diagram before and after the fault-tolerant control strategy is put into operation, wherein (a) is the three-phase grid current, (b) is the DC bus voltage, the midpoint voltage and the A-phase inverter The output voltage.

Detailed ways

The technical solution will be clearly and completely described below in conjunction with preferred examples of the present invention and accompanying drawings. It should be understood that the preferred examples are only for illustrating the present invention, but not for limiting the protection scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The invention provides a fault-tolerant control strategy for T-type three-level SAPF open-circuit fault. By judging the direction of the current on the inverter side, the working state of the SAPF is distinguished, and then different control parameters are calculated; the sequence model predictive control is realized through two cost functions. The overall scheme can realize that SAPF can still ensure the balance of the midpoint voltage and high-quality compensation current under the open circuit fault of a single switching device, which improves the reliability of the system.

A control block diagram of an embodiment as shown in Figure 4, the main content of the present embodiment comprises the following steps (represent horizontal bridge arm fault with S _a3 open circuit fault, S _a1 open circuit fault represents vertical bridge arm fault):

Step S1: Collect the three-phase AC voltage v _g , the three-phase network side current i ₂ , the three-phase nonlinear load current i _l , and the two capacitors on the DC bus side of the T-type three-level LCL type SAPF through the voltage sensor and the current sensor Voltages u _p and u _n .

Step S2: Estimate the three-phase inverter side current i ₁ and the three-phase filter capacitor voltage v _C through the Kalman filter.

Step S3: If S _a3 is an open circuit fault, judge the flow direction of the phase A inverter side current i _1a , if it is in the negative direction, it will be judged as a horizontal unhealthy state; if it is in a positive direction, it will be judged as a healthy state; if S _a1 is open circuit Fault, judge the flow direction of the current i _1a on the inverter side of phase A, if it is in the positive direction, it will be judged as a vertical unhealthy state; if it is in the positive direction, it will be judged as a healthy state;

并将其通过Clark变换转化为两相静止坐标系。Step S4: Obtain the phase information of the grid voltage v _ga of phase A through the phase-locked loop, and determine the current reference value i _2ref of the three-phase grid side based on it. The reference value i _1ref of the inverter side current, the reference value v _Cref of the filter capacitor voltage, and the inverter output reference voltage vector are deduced from the circuit relationship of the LCL type grid-connected inverter

And transform it into a two-phase stationary coordinate system through Clark transformation.

经验公式如下：Step S5: Calculate the reference value of the DC bus voltage

The empirical formula is as follows:

为逆变器输出参考电压矢量的模在一个工频周期内的最大值。据此确定了健康状态或水平非健康状态的直流电压参考值为800V，垂直非健康状态直流电压参考值为1200V。in,

The inverter outputs the maximum value of the modulus of the reference voltage vector within a power frequency cycle. Accordingly, it is determined that the DC voltage reference value of the healthy state or the horizontal unhealthy state is 800V, and the DC voltage reference value of the vertical unhealthy state is 1200V.

其中，步骤S5中所求出的

将代入到控制直流电压的PI控制器中。PI控制器的传递函数为：Step S6: According to the control block diagram described in Figure 2, calculate the reference compensation current of the SAPF

Among them, the obtained in step S5

Will be substituted into the PI controller that controls the DC voltage. The transfer function of the PI controller is:

Among them, K _p is the proportional coefficient, and K _i is the 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) is the same. Linearly extrapolate x _αβref (k) with the second-order Lagrangian extrapolation method to obtain the predicted value x _αβref (k+1).

n在健康状态取4、水平非健康状态取2、垂直非健康状态取7。所述代价函数式为：Step S8: Step S721: Substitute the possible situations of the absolute value |u(k)| of the candidate vectors under the three working states (see Table 1 for details) into the cost function _J1 , and select the n that minimizes _J1 optimal solution

n takes 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)|

Among them, T _s is the sampling time, and C ₁ is the capacitance value of the upper DC bus.

Table 1 shows the possible situations of the absolute value of the _J1 candidate vector under the three working states:

working status <![CDATA[Possible cases of the absolute value of the cost function J₁ alternative vector]]> health status OOO, OOP, OPO, OPP, POO, POP, PPO, PPP level unhealthy POO, POP, PPO, PPP vertical unhealthy OOO, OOP, OPO, OPP, POO, POP, PPO, PPP

P in Table 1 represents high level, and O represents zero level.

Step S9: Remove the absolute values of the n optimal solutions of J ₁ and combine them into candidate vectors of the cost function J ₂ . According to different working conditions, the results of removing the absolute value are also different, and the results of removing the absolute value are shown in Table 2, 3, and 4.

Table 2 is the result of removing the absolute value of the optimal solution in the health state:

Table 3 is the result of removing the absolute value of the optimal solution in the horizontal unhealthy state:

Table 4 is the result of removing the absolute value of the optimal solution in the vertical unhealthy state:

P in Table 2, 3, and 4 represents high level, O represents zero level, and N represents low level.

Step S10: Bring the candidate vector obtained in step S9 into the cost function J ₂ , and obtain the switching vector that minimizes J ₂ , denoted as u _opt (k). The cost function formula is:

J ₂ ＝||C(Ax _αβ (k)+Bu(k)+Tv _g (k)-x _αβref (k+1))|| Formula 4

Among them, A, B, C, and T are constant matrices, which are only related to the parameters of the inverter.

Step S11: converting the switching vector u _opt (k) into a switching signal of the switching device of the T-type three-level converter.

Taking a T-type three-level LCL type SAPF using the control strategy in Figure 4 as an example, the effectiveness of the proposed sequence model to predict fault-tolerant control is verified. The sequence model prediction fault-tolerant control algorithm is realized by the 32-bit floating-point digital signal processor TMS320F28379 of Texas Instruments. The T-type three-level converter consists of six FUZI 1MBH50D-060IGBTs and six FUZI2MBI150U2A-060IGBTs. Six Hall current sensors (HCS-LTS-06A) are used to measure converter current and grid current. A basic frequency transformer is used to match the voltage of the converter to the grid voltage. The nonlinear load circuit is shown in Figure 5.

Table 5 shows some parameters of the T-type three-level SAPF sequence model predictive fault-tolerant control:

parameter describe value parameter describe value <![CDATA[C₁(μF)]]> DC side capacitance 500 <![CDATA[R_C(Ω)]]> Damping resistor 2 <![CDATA[L₁(mH)]]> Inverter side inductance 4 <![CDATA[V_g(V)]]> Grid voltage (effective value) 110 <![CDATA[L₂(mH)]]> Grid side inductance 2 <![CDATA[I_2ref(A)]]> Grid side reference current amplitude 10 C(μF) filter capacitor 4 ω(rad/s) grid frequency 314.16 <![CDATA[R₁(Ω)]]> Inverter side resistance 0.1 <![CDATA[K_p]]> Scale factor 0.05 <![CDATA[R₂(Ω)]]> Grid side resistance 0.1 <![CDATA[K_i]]> Integral coefficient 0.1

It can be seen from Fig. 6 that when no fault occurs, the sequence model predictive control proposed by the present invention enables SAPF to ensure excellent current compensation, and the THD is only 2.14%. At the same time, the DC bus voltage is stable at 800V, and the midpoint voltage The fluctuation is kept within 0.5V.

As can be seen from Fig. 7, after the fault of the horizontal bridge arm, when the sequence model predictive fault-tolerant control of the present invention is put into use, the quality of the current waveform of the three-phase power grid has been improved, and the THD is 2.68%; the DC bus voltage is stable at 800V , the mid-point voltage drops from 0.8V to 0.5V; the waveform of the inverter output voltage is slightly distorted when the fault occurs, and the normal three-level output mode is restored after the fault-tolerant control strategy is put into operation, and the efficiency is improved.

As can be seen from Fig. 8, after the fault of the vertical bridge arm, when the sequence model predictive fault-tolerant control of the present invention is put into use, the quality of the current waveform of the three-phase power grid has been significantly improved, and the THD is 2.98% after stabilization; the DC bus The voltage rises from 800V to 1200V and remains stable, and the rise time is 50ms; the midpoint voltage rises obviously, but this periodic fluctuation does not affect the normal operation of the DC side capacitor. The waveform of the output voltage of the inverter is slightly distorted during a fault, and the normal three-level output mode is restored after the fault-tolerant control strategy is put into operation, and the efficiency is improved.

The above-mentioned embodiments are only to illustrate the technical conception and characteristics of the present invention, and its purpose is to allow people familiar with this technology to understand the content of the present invention and implement it accordingly, and cannot limit the scope of protection of the present invention. The general principles of may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the broadest scope consistent with the principles and novel features disclosed herein, and all equivalent changes or changes made according to the spirit of the present invention Modifications should all fall within the protection scope of the present invention.

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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