# Optimal Control Strategy of Back-to-Back Converter Based on AC/DC Voltage Source Converter

Authors: Haotian Wu; Zhong Wei; Shiwen Liu
Journal: Mobile Information Systems
 (2022)

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## Abstract

The back-to-back converter is a converter system composed of two voltage source converters (VSC). Because VSC can regulate the output of active and reactive power, the back-to-back converter has a good dynamic and static performance. However, not all passive loads are three-phase symmetrical. The existing control strategy may lead to asymmetric output voltage when back-to-back converter is used to supply unbalance load. Usually, an inner loopd/q decoupling controller, a constant DC voltage controller of the rectifier side, and a constant AC voltage controller of the inverter side are established. In this article, in order to improve the rectifier side, fuzzy online self-tuning is used to increase PI parameters on the outer voltage loop. PI control is replaced by PR control on the inner current loop. The improved rectifier control can reduce the overshoot and the output of harmonics. For the inverter side, the three-phase voltage will be imbalanced. Therefore, using PR control, which can track a sinusoidal signal without steady-state error, the control strategy of inverter is changed into “outer loop constant voltage and PR control-inner loop current d/q decoupling control.” This strategy can solve the unbalance problem of three-phase AC output. The simulation results show that the control strategy can solve the problem of asymmetric output voltage of the inverter side effectively.


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## Body

## 1. Introduction

With the development of power electronics technology, AC\DC converter components have undergone a transition from a mercury arc valve to a thyristor. AC\DC converter includes line commuted converter (LCC) and voltage source converter (VSC). However, the traditional AC\DC LCC based on the phase-controlled commutation technology thyristor device has a low frequency and a large commutation loss. In addition, the traditional AC\DC LCC requires an external grid to provide a commutation voltage (i.e., a commutation short circuit ratio). Therefore, it cannot separately supply power to the passive network [1–3]. Different with LCC, VSC is a new high-voltage DC transmission with the modular design. It not only can improve the transmission capacity but also has the advantages of small energy loss, long transmission distance, and large capacity. In addition, it can supply power to the passive network separately [4, 5].The back-to-back converter consists of two VSC components, so it has a good dynamic and static performance. It can control the active and reactive output through control strategy to improve power quality. In order to propose a suitable control strategy for implementing the primary control function of back-to-back converter, many scholars have studied the control strategy. In reference [6], MATLAB simulation model of back-to-back converter to the passive network is established to study the control strategies of the rectifier side and the inverter side. In reference [7], the influence of active load and reactive load on voltage and frequency is studied based on MATLAB simulation model of back-to-back converter supply to passive networks. In reference [8], the inverse system model of back-to-back converter is established by state feedback internalization. Based on the sliding mode variable structure control theory, a new type of controller that back-to-back converter supply to the passive network is established. However, the above research is not systematic. This results in a control strategy and makes it difficult to solve multiple operational problems in the power system.The existing research on the power supply of back-to-back converter to three-phase asymmetric passive network is mainly divided into two aspects. Firstly, different control strategies are usually used to control the output of the inverter to solve the asymmetry problem caused by the three-phase asymmetric load when the back-to-back converter supplies power to the load. Common control strategies included/q instantaneous control and symmetrical component component control. Reference [9] is based on PR control to effectively track a sinusoidal signal at a certain frequency. PR control is introduced into the α/β coordinate system to achieve current balance control of the primary side three-phase current. After that, the imbalance problem of solving the three-phase output current is solved. Second, for the three-phase asymmetric output voltage, the two-phase α/β stationary coordinate system is used to dynamically compensate the three-phase voltage imbalance. In reference [10], the output voltage is separated by positive and negative order through second-order generalized integration. Further, the VUF-based voltage compensation reference generator corrects the reference voltage of the voltage loop. However, references [7–10] do not fully consider the influence of the control strategy of back-to-back converter including the rectifier side, which may lead to the problem of overshooting time and reaction time on the rectifier side and the asymmetry of the inverter output voltage.In order to design the control strategy from the rectifier side to the inverter side when back-to-back converter supplies to the three-phase asymmetric passive network, this paper first analyzes the mathematical model of VSC in the d/q rotating coordinate system. A relational expression of active and reactive power on the AC side is obtained. On the basis of analyzing the common control strategies of the converter, the rectifier side fixed DC voltage controller and the inverter side fixed AC voltage controller are designed. For the rectifier side fixed DC voltage control, the voltage outer loop is improved by combining fuzzy control with PI control. At the same time, no static adjustment of the current is achieved by PR controller to improve the current inner loop. For the power transmission control problem caused by the three-phase unbalanced condition of the inverter side fixed AC voltage control load, the “constant AC voltage” single-loop control is changed to the “outer loop constant voltage-inner loop current decoupling” double closed-loop control. The three-phase distortion of the supply voltage is solved by using PR control to retrofit the voltage outer loop. The realization that back-to-back converter supplies power to unbalance load is realized.


## 2. Topology and Mathematical Model of AC/DC Voltage Source Converter

### 2.1. The Mathematical Model of VSC

The VSC topology diagram is shown in Figure1.Figure 1
The topology diagram of VSC.According to the topology diagram of VSC and Kirchhoff voltage law, the mathematical model of three-phase PWM voltage source converter (VSC) in three-phase static coordinate system (a,b,c) is as follows:(1)Ldiadt+Ria=usa−ua,Ldibdt+Rib=usq−ub,Ldicdt+Ric=usc−uc,Cdudcdt=idc−id.Clarke transform (3/2 transform) is as follows:(2)iαiβ0=231−12−1203232121212iaibc=C3s/2siaibc.Park transformation (2/2 transform) is as follows:(3)idiq=cosθ−sinθsinθcosθ−1iαiβ=cosθ−sinθ−sinθcosθiαiβ=C2s/2riαiβ.After Clarke and Park transformation, the mathematical model of three-phase PWM converter ind/q rotating coordinate system is obtained:(4)Ldiddt+Rid−ωLid=usd−ud,Ldiqdt+Riq+ωLid=usq−uq,Cdudcdt=idc−id,where usd and usq are d/q axis components of grid voltage, respectively; ud and uq are the d/q axis components of the three-phase voltage on the AC side of the converter, respectively; id and iq are the d/q axis components of the three-phase current on the AC side of the converter, respectively; udc and idc are the DC side voltage and current of the converter, respectively; id is the outlet current of DC network; L is filter inductance; and C is the capacitance of DC side.


### 2.2. The Control Principle of VSC Power Decoupling

Under the condition of steady-state operation, ignoring the reactor resistance of VSC and the switching loss of VSC, the following can be obtained:(5)ud=usd+ωLiq,uq=−ωLid.In thed/q coordinate rotating system, the axis is oriented. At the same time, Usq is 0 because the q-axis is 90 degrees ahead of the d-axis. According to the grid voltage orientation control strategy, the following equation can be obtained:(6)Usd=Us,Usq=0.Combined with equations (5) and (6), the expressions of active power and the expressions of reactive power can be received based on the instantaneous power theory:(7)Pac=32udid+uqiq=32usdid=32usid,Qac=32usqid+usdiq=32usdiq=−32usiq.From the above formula,us is invariant, and it can be obtained that the independent control of active power Pac and reactive power Qac can be realized by controlling the value of id and iq.


## 2.1. The Mathematical Model of VSC

The VSC topology diagram is shown in Figure1.Figure 1
The topology diagram of VSC.According to the topology diagram of VSC and Kirchhoff voltage law, the mathematical model of three-phase PWM voltage source converter (VSC) in three-phase static coordinate system (a,b,c) is as follows:(1)Ldiadt+Ria=usa−ua,Ldibdt+Rib=usq−ub,Ldicdt+Ric=usc−uc,Cdudcdt=idc−id.Clarke transform (3/2 transform) is as follows:(2)iαiβ0=231−12−1203232121212iaibc=C3s/2siaibc.Park transformation (2/2 transform) is as follows:(3)idiq=cosθ−sinθsinθcosθ−1iαiβ=cosθ−sinθ−sinθcosθiαiβ=C2s/2riαiβ.After Clarke and Park transformation, the mathematical model of three-phase PWM converter ind/q rotating coordinate system is obtained:(4)Ldiddt+Rid−ωLid=usd−ud,Ldiqdt+Riq+ωLid=usq−uq,Cdudcdt=idc−id,where usd and usq are d/q axis components of grid voltage, respectively; ud and uq are the d/q axis components of the three-phase voltage on the AC side of the converter, respectively; id and iq are the d/q axis components of the three-phase current on the AC side of the converter, respectively; udc and idc are the DC side voltage and current of the converter, respectively; id is the outlet current of DC network; L is filter inductance; and C is the capacitance of DC side.


## 2.2. The Control Principle of VSC Power Decoupling

Under the condition of steady-state operation, ignoring the reactor resistance of VSC and the switching loss of VSC, the following can be obtained:(5)ud=usd+ωLiq,uq=−ωLid.In thed/q coordinate rotating system, the axis is oriented. At the same time, Usq is 0 because the q-axis is 90 degrees ahead of the d-axis. According to the grid voltage orientation control strategy, the following equation can be obtained:(6)Usd=Us,Usq=0.Combined with equations (5) and (6), the expressions of active power and the expressions of reactive power can be received based on the instantaneous power theory:(7)Pac=32udid+uqiq=32usdid=32usid,Qac=32usqid+usdiq=32usdiq=−32usiq.From the above formula,us is invariant, and it can be obtained that the independent control of active power Pac and reactive power Qac can be realized by controlling the value of id and iq.


## 3. Design of Back-to-Back Converter to Three-Phase Passive Network Power Supply Controller

### 3.1. System Structure and Control Strategy Analysis of Back-to-Back Converter Power Supply to Three-Phase Passive Network

The structure diagram of back-to-back converter supplying power to the three-phase load is shown in Figure2. The commutating reactor L1 is used for the exchange of VSC energy on the rectifier side and filters out some higher harmonics. Capacitors C1 and C2 provide the voltage support required for the DC terminal and serve as an inrush current when the buffer arm is turned off. The role of higher harmonics on the DC side is reduced. The inverter side supplies power to the asymmetric load through a single resonant phase. For the characteristics of the whole system, the controllers on the rectifier side and the inverter side are designed separately [11].Figure 2
Block diagram of for back-to-back converter to supply a three-phase passive network.The common control strategies of VSC converter station are generally divided into four types: (1) constant DC voltage control mode; (2) fixed active power control mode; (3) constant AC voltage control; and (4) variable frequency control mode. Different from the back-to-back converter supplying power to the normal three-phase passive network, the back-to-back converter supplies power to the three-phase asymmetric passive network. To maintain the stable operation of the whole system, the two main problems need to be solved: (1) the DC voltage of the DC channel is stable and (2) the inverter side a stable output of the AC voltage.For the first problem, the stability of the DC voltage of the DC channel must be realized by the rectifier side. Therefore, the rectifier side VSC adopts the “outer loop fixed DC voltage-inner loop current decoupling” double closed-loop control mode. The outer loop realizes the fixed DC voltage control target. The inner loop current quickly tracks the reference current input from the outer loop. Then, the current waveform and phase control of the AC side of the inverter will be realized [12, 13]. For the second problem, the stable output of the AC voltage on the inverter side, due to the unbalanced output of the AC voltage caused by the three-phase asymmetric passive network, the traditional constant AC voltage control can no longer meet the needs. In this paper, PR control can track the sinusoidal signal without static difference. And the inverter VSC “fixed AC voltage single-loop control” is changed to “outer loop constant voltage PR control-inner loop current d/p decoupling control.” The three-phase AC voltage on the inverter side remains unchanged regardless of how the three-phase asymmetric passive network changes [14, 15].


### 3.2. Current Inner Loopd/q Decoupling Controller Design

The mathematical model of the three-phase PWM AC/DC converter in the d/q rotating coordinate system shows that there are coupling terms ωLid and ωLiq. Using the input and output linearization control method [16], set(8)Ldiddt+Rid=Δud,Ldiqdt+Riq=Δuq.Set△ud and △up as follows, respectively:(9)Δud=Kpid∗−id+Kidid∗−iddt,Δuq=Kpiq∗−iq+Kidiq∗−iqdt.Equation (9) is subjected to the Laplace transform under zero initial conditions, and the PI controller expression can be obtained:(10)ΔudKp+Kisid∗−id,ΔuqKp+Kisiq∗−iq.The simultaneous (8)–(10) can be obtained:(11)Kp+Kisid∗−id=Δud=Ldiddt+Rid,Kp+Kisiq∗−iq=Δuq=Ldiqdt+Riq.According to equation (4), the mathematical model of the three-phase PWM AC/DC converter in the d/q rotating coordinate system is as follows:(12)Ldiddx+Rid−ωLiq=usd−ud,Ldiddt+Riq−ωLiq=usq−uq,Cdudcdt+idc−id.By combining equations (4) and (11), the d/q decoupling control expression based on current state feedback and voltage feed forward compensation can be obtained as follows [16, 17]:(13)ud=usd−Kp+Kisid∗−id∗+ωLiq,uq=usq−Kp+Kisiq∗−iq+ωLid.The block diagram of the inner loop current decoupling controller is shown in Figure3.Figure 3
Current inner loop decoupling control diagram.According to the controller designed in Figure3, the size control of ud and uq can be control, and the decoupling between id and iq can be realized [17].


### 3.3. Voltage Outer Loop Control of Rectifier Side VSC Based on Fuzzy PI Self-Tuning

Introducing fuzzy control into PI controller, a fuzzy PI self-tuning algorithm is formed. In fuzzy PI control, the proportional coefficientP and integral coefficient I change with the change of external factors, so fuzzy PI control is a kind of nonlinear control. The fuzzy PI control algorithm fully combines the respective advantages of conventional PI control and fuzzy control. It can adjust the proportional coefficient P and integral coefficient I in real time according to the actual changes of the outside world. Compared with the traditional PI control, it has better control effect and control accuracy. The schematic diagram of fuzzy PI control is shown in Figure 4 [18].Figure 4
Block diagram of the fuzzy PI control.For fuzzy PI control algorithm, the control algorithm includes two inputs (e ec) and two outputs (△kp, △ki), which is the system error, ec is the error change rate, △kp is the proportional coefficient variable output, and △ki is the integral coefficient variable output. The universe on fuzzy sets is defined as follows: {−6, −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, 5, 6}. e, ec, △kp, and △ki are the four fuzzy subsets. Let e, ec, △kp, and △ki obey the membership function curve distribution of triangle {NB, NM, NS, ZO, PS, PM, PB}, as shown in Figure 5.Figure 5
The curve of triangular membership function.The fuzzy membership control model is based on the fuzzy membership set and the fuzzy control rule table. Then, finding optimal PI parameter value is used to implement online self-tuning of PI parameter.(14)kp=|kp′+Δkpki=ki′+Δki,where △kp is the proportional gain variation of fuzzy PI self-tuning; △ki is the integral gain variation of fuzzy PI self-tuning; kp′ is the proportional gain setting value of fuzzy PI self-tuning; and ki′ is the integral gain setting value of fuzzy PI self-tuning.In fuzzy PI controller,e is (id∗−id) and ec is (d(id∗−id)/dt, d(qd∗−iq)/dt). The error e and the rate of change ec of the error are both two-dimensional variables. Therefore, the fuzzy controller is a controller with a four-dimensional variable input and a two-dimensional variable (△kp and △ki) output.


### 3.4. Improved Current Inner Loop Control for Rectifier Side VSC Based on PR Control

The principle of PR control: in thedouble closed-loop control system of AC grid connection of permanent magnet fan, the control objectives of voltage outer loop and current inner loop are different. The control goal of the current inner loop is to quickly track the current command and make the control system have good dynamic characteristics. Although the traditional PI control algorithm is relatively simple and reliable, its control effect is not ideal in the actual operation process, which is embodied in the following aspects: (1) if traditional PI control is adopted in the current inner loop, park transformation and Clarke transformation are required for the control quantity, and a large number of coordinate transformations will make the current inner loop control more complex; (2) when using traditional PI control, if you want to realize the decoupling of active and reactive power, it is necessary to introduce feed forward compensationɷLid and ɷLiq. In the actual operation process, the parameter of ɷLid and the parameter of ɷLiq are related to temperature and they are not fixed values; and (3) the proportional coefficient and integral coefficient of traditional PI control are fixed values. In the process of current inner loop control, if the decoupling is not accurate, the control accuracy will be reduced.Therefore, proportional resonance control PR is essentially PI control in two-phased/q rotating coordinate system. Introducing the PR control algorithm into current inner loop control can not only reduce a lot of coordinate transformation but also omit feed forward compensation ɷLid and ɷLiq, without power decoupling. Therefore, by introducing PR control, the current inner loop control at the network side can better achieve the control goal, quickly track the current command, and achieve no static error regulation. The control principle of proportional resonance PR is shown in Figure 6.Figure 6
The principle diagram of PR control.PR controller consists of a proportional link and a generalized integral link. The transfer function is as follows:(15)Gprs=kp+2kiss2+ω02,where kp, ki, and ω0 are the proportional coefficient, integral coefficient, and the resonant frequency, respectively. It can be seen from the above formula that the gain of the ideal PR control at the resonant frequency ω0 is infinite, while the gain at other frequencies is almost zero.In the network side current inner loop control based on traditional PI control, the output power of three-phase PWM inverter is determined by the given current valueid∗and iq∗, in which the size of id∗ determines the output of active power and the size of iq∗ determines the output of reactive power. At the same time, the two-cross compensation ɷLid/ɷLiq and the grid voltage E also affect the stability of the current inner loop control, but in practical engineering, the cross-coupling compensation ɷLid/ɷLiq will change with the change of external temperature, which is not a stable value. The current inner loop control based on the improved PR control in this paper is mainly divided into the following five steps: (1) the detection device is used to detect the AC current signal iabc on the three-phase AC network side; (2) Clarke transform of iabc will be performed to obtain iα and iβ; (3) Park transforms the signal given by the voltage outer loop to obtain the given value iα∗ and iβ∗ of the current inner loop; (4) the difference is made between iα∗ and iα, iβ∗ and iβ, and the two differences are used as the input of PR controller; and (5) the output of PR controller is used as the control command to control the six converter bridge arms of three-phase PWM inverter through SVPWM pulse width modulation.Compared with traditional PI control, PR control has the following two advantages in the inner loop control of grid side current: (1) PR control only needs Clarke transformation of the control quantity, which reduces the complex operation of rotating coordinate system and the difficulty of control algorithm; (2) no cross-coupling compensationɷLid/ɷLiq and Grid voltage E vector orientation are required, and the feed forward decoupling control is eliminated. Therefore, the changes of circuit parameters and grid voltage will not affect the accuracy of the control system, and the robustness and stability of the system will be greatly improved.


### 3.5. Rectifier Side Controller Design

To sum up, based on the double closed-loop decoupling control strategy of grid voltage vector orientation, the control of VSC on the rectifier side is deceptively transformed, and the control strategy of VSC on the rectifier side is established in this chapter. Among them, traditional PI control is transformed into fuzzy self-tuning PI control, which can adjust the proportional coefficientP and integral coefficient I in real time according to the changes of the outside world; AC current is the core of inner loop control. The goal of inner loop control is to track the grid voltage and maintain the same frequency and phase. Traditional PI control is transformed into PR control, which reduces the complex operation of rotating coordinate system, eliminates feed forward decoupling control and grid voltage vector orientation, and improves the stability and anti-interference ability of the system. The integral control diagram of VSC on the rectifier side in this chapter is shown in Figure 7.Figure 7
Integral control diagram of the rectifier side.


### 3.6. Design of Inverter Side Controller

Back-to-back converter is connected to a passive network, in which case a single-loop constant AC voltage control is required.The inverter three-phase AC voltage measurementuabc is converted by d/q coordinates. The measured value is compared with the AC voltage reference value in the d/q coordinate system. Ud, uq, and u0 are obtained through the PI regulator. Finally, the resulting PWM control signal allows the converter to be adjusted. The inverter side fixed AC voltage control block diagram is shown in Figure 8.Figure 8
Common control diagram of the inverter side.


### 3.7. Characteristic Analysis of Output Voltage Distortion under Unbalanced Load

Under three-phase unbalanced conditions, the distorted three-phase AC voltage can be expressed by three quantities: positive sequence voltageUf+, negative sequence voltage Uf—, and zero-sequence voltage Uf0.(16)Uf=Uf++Uf0+Uf−,uaubuc=Ufp+cosωbt+φ+cosωbt+φ−+2π3cosωbt+φ++2π3+Ufp−cosωbt+φ−cosωbt+φ−+2π3cosωbt+φ−+2π3+Ufp0cosωbt+φ0cosωbt+φ0cosωbt+φ0,where the positive, negative, and zero order electromotive force peaks are Ufp+, Ufp−, and Ufp0, respectively; the initial phase angle of the fundamental voltage under positive sequence, negative sequence, and zero sequence are φ+, φ−, and φ0, respectively.Since the three-phase inverter generally has no neutral connection, the zero-sequence electromotive force can be ignored. This meansUfp0=0. Then, equation (16) can be changed to(17)Uf=Uf++Uf0+Uf−,uaubuc=Ufp+cosωbt+φ+cosωbt+φ−+2π3cosωbt+φ++2π3+Ufp−cosωbt+φ−cosωbt+φ−+2π3cosωbt+φ−+2π3.It can be seen from equation (17) that when the alternating current is three-phase distorted, both the positive sequence and the negative sequence need to consider the fundamental wave amount of the alternating voltage. As can be seen from the analytical equation (15), PR control has only a high gain at all and a large attenuation at other frequencies. No static tracking can be achieved when the system gains near infinity at the frequency point. Traditional PR control has only one frequency point. Therefore, the controller is overly sensitive to the signal frequency. However, in practical applications, in order to increase system bandwidth and improve stability, PR controller can be improved to(18)Gprs=kp+ki2ωcss2+2ωcs+ω02,where ωc is the cutoff frequency, Ki = 100, and ω0 = 314 rad/s. When ωc = 2 rad/s and 20 rad/s, the improved PR controller Bode diagram is shown in Figure 9.Figure 9
Bode graph of the improved PR controller.It can be seen from the analysis of equation (18) and Figure 9 that the frequency response of PR controller is improved by ωc. PR controller can simultaneously achieve high gain on both sides ωc − ω0 and ωc − ω0. This is equivalent to performing PI control in both positive and negative sequence coordinates. Therefore, PR control can realize the nonstationary tracking of the three-phase AC quantity and the correction of the three-phase distorted AC quantity.


### 3.8. Improved Voltage Loop Design of PR Controller

To illustrate the principle of PR control and d/q decoupling control, the voltage loop α axis is taken as an example for analysis. Its total transfer function is shown in Figure 10 [19].Figure 10
Current loop transfer function with PR controller.The three-phase supply voltageuabc and the given three-phase voltage uabc∗ are uα and uα∗ obtained by Park transformation. The difference between these two gets the given output by the transfer function of PR controllerGPRs, the transfer function of the PWM link GPWMs, and the transfer function of the resonant link Gos (including the load impedance). Set Ks as the amplification factor of the PWM device. TS and T are the delay time and switching period of the PWM device, respectively. In addition, Ts<T. Normally the PWM switching frequency is large, so the time constant TS is small. PWM device can be simplified to first-order inertia link GPWMs=KS/TSS+1. It can be obtained that the transfer function of the resonant link is Gos=1/Ls+1/Cs+Zr1.The transfer function of theα-axis of the entire voltage flow control loop is as follows:(19)uα=GPRsGPWMsG0s1+GPRsGPWMsG0suα∗.It can be obtained from equation (19) that when the inverter operates at the specified power resonance point, Gos approximates infinity.GPRsGPWMsG0s is much larger than 1. Therefore, uαGPRsGPWMsG0s/1+GPRsGPWMsG0suα∗.


### 3.9. Design of VSC Controller for Inverting Side Power Supply to Three-Phase Unbalanced Load

The passive load is a three-phase asymmetry. Therefore, on the basis of analyzing the traditional double-loop control strategy, the traditional “fixed AC voltage single-loop control” can be modified by using PR control to track the characteristics of the sinusoidal signal without static difference. When back-to-back converter supplies power to the three-phase asymmetric load, the problem of the AC voltage distortion on the inverter side is solved.From the 3.8 summary analysis, PR control can perform no static tracking for a specific three-phase AC signal. For the distorted three-phase AC voltage, if PR control is introduced in the control, the traditional vector-controlled three-phase AC voltage is converted to the two-phase stationary coordinate system, and the primary resonance frequency is set to the PR-controlled resonant frequency, the three-phase AC voltage without the static difference tracking of the command signal will be realized. Finally, the distortion of the three-phase AC voltage is corrected, and the symmetry and stability of the three-phase AC voltage on the inverter side are realized.According to the above control target and control strategy analysis, the traditional inverter side fixed AC voltage single-loop control is changed to “outer loop fixed AC voltage-inner loop current decoupling” double closed-loop control. Further, the external loop using PR control voltage was modified. The stable power supply of back-to-back converter to the three-phase asymmetric passive network is realized. The overall control block diagram of the inverter side is shown in Figure11.Figure 11
Integral control diagram of inverter side of back-to-back converter.


## 3.1. System Structure and Control Strategy Analysis of Back-to-Back Converter Power Supply to Three-Phase Passive Network

The structure diagram of back-to-back converter supplying power to the three-phase load is shown in Figure2. The commutating reactor L1 is used for the exchange of VSC energy on the rectifier side and filters out some higher harmonics. Capacitors C1 and C2 provide the voltage support required for the DC terminal and serve as an inrush current when the buffer arm is turned off. The role of higher harmonics on the DC side is reduced. The inverter side supplies power to the asymmetric load through a single resonant phase. For the characteristics of the whole system, the controllers on the rectifier side and the inverter side are designed separately [11].Figure 2
Block diagram of for back-to-back converter to supply a three-phase passive network.The common control strategies of VSC converter station are generally divided into four types: (1) constant DC voltage control mode; (2) fixed active power control mode; (3) constant AC voltage control; and (4) variable frequency control mode. Different from the back-to-back converter supplying power to the normal three-phase passive network, the back-to-back converter supplies power to the three-phase asymmetric passive network. To maintain the stable operation of the whole system, the two main problems need to be solved: (1) the DC voltage of the DC channel is stable and (2) the inverter side a stable output of the AC voltage.For the first problem, the stability of the DC voltage of the DC channel must be realized by the rectifier side. Therefore, the rectifier side VSC adopts the “outer loop fixed DC voltage-inner loop current decoupling” double closed-loop control mode. The outer loop realizes the fixed DC voltage control target. The inner loop current quickly tracks the reference current input from the outer loop. Then, the current waveform and phase control of the AC side of the inverter will be realized [12, 13]. For the second problem, the stable output of the AC voltage on the inverter side, due to the unbalanced output of the AC voltage caused by the three-phase asymmetric passive network, the traditional constant AC voltage control can no longer meet the needs. In this paper, PR control can track the sinusoidal signal without static difference. And the inverter VSC “fixed AC voltage single-loop control” is changed to “outer loop constant voltage PR control-inner loop current d/p decoupling control.” The three-phase AC voltage on the inverter side remains unchanged regardless of how the three-phase asymmetric passive network changes [14, 15].


## 3.2. Current Inner Loopd/q Decoupling Controller Design

The mathematical model of the three-phase PWM AC/DC converter in the d/q rotating coordinate system shows that there are coupling terms ωLid and ωLiq. Using the input and output linearization control method [16], set(8)Ldiddt+Rid=Δud,Ldiqdt+Riq=Δuq.Set△ud and △up as follows, respectively:(9)Δud=Kpid∗−id+Kidid∗−iddt,Δuq=Kpiq∗−iq+Kidiq∗−iqdt.Equation (9) is subjected to the Laplace transform under zero initial conditions, and the PI controller expression can be obtained:(10)ΔudKp+Kisid∗−id,ΔuqKp+Kisiq∗−iq.The simultaneous (8)–(10) can be obtained:(11)Kp+Kisid∗−id=Δud=Ldiddt+Rid,Kp+Kisiq∗−iq=Δuq=Ldiqdt+Riq.According to equation (4), the mathematical model of the three-phase PWM AC/DC converter in the d/q rotating coordinate system is as follows:(12)Ldiddx+Rid−ωLiq=usd−ud,Ldiddt+Riq−ωLiq=usq−uq,Cdudcdt+idc−id.By combining equations (4) and (11), the d/q decoupling control expression based on current state feedback and voltage feed forward compensation can be obtained as follows [16, 17]:(13)ud=usd−Kp+Kisid∗−id∗+ωLiq,uq=usq−Kp+Kisiq∗−iq+ωLid.The block diagram of the inner loop current decoupling controller is shown in Figure3.Figure 3
Current inner loop decoupling control diagram.According to the controller designed in Figure3, the size control of ud and uq can be control, and the decoupling between id and iq can be realized [17].


## 3.3. Voltage Outer Loop Control of Rectifier Side VSC Based on Fuzzy PI Self-Tuning

Introducing fuzzy control into PI controller, a fuzzy PI self-tuning algorithm is formed. In fuzzy PI control, the proportional coefficientP and integral coefficient I change with the change of external factors, so fuzzy PI control is a kind of nonlinear control. The fuzzy PI control algorithm fully combines the respective advantages of conventional PI control and fuzzy control. It can adjust the proportional coefficient P and integral coefficient I in real time according to the actual changes of the outside world. Compared with the traditional PI control, it has better control effect and control accuracy. The schematic diagram of fuzzy PI control is shown in Figure 4 [18].Figure 4
Block diagram of the fuzzy PI control.For fuzzy PI control algorithm, the control algorithm includes two inputs (e ec) and two outputs (△kp, △ki), which is the system error, ec is the error change rate, △kp is the proportional coefficient variable output, and △ki is the integral coefficient variable output. The universe on fuzzy sets is defined as follows: {−6, −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, 5, 6}. e, ec, △kp, and △ki are the four fuzzy subsets. Let e, ec, △kp, and △ki obey the membership function curve distribution of triangle {NB, NM, NS, ZO, PS, PM, PB}, as shown in Figure 5.Figure 5
The curve of triangular membership function.The fuzzy membership control model is based on the fuzzy membership set and the fuzzy control rule table. Then, finding optimal PI parameter value is used to implement online self-tuning of PI parameter.(14)kp=|kp′+Δkpki=ki′+Δki,where △kp is the proportional gain variation of fuzzy PI self-tuning; △ki is the integral gain variation of fuzzy PI self-tuning; kp′ is the proportional gain setting value of fuzzy PI self-tuning; and ki′ is the integral gain setting value of fuzzy PI self-tuning.In fuzzy PI controller,e is (id∗−id) and ec is (d(id∗−id)/dt, d(qd∗−iq)/dt). The error e and the rate of change ec of the error are both two-dimensional variables. Therefore, the fuzzy controller is a controller with a four-dimensional variable input and a two-dimensional variable (△kp and △ki) output.


## 3.4. Improved Current Inner Loop Control for Rectifier Side VSC Based on PR Control

The principle of PR control: in thedouble closed-loop control system of AC grid connection of permanent magnet fan, the control objectives of voltage outer loop and current inner loop are different. The control goal of the current inner loop is to quickly track the current command and make the control system have good dynamic characteristics. Although the traditional PI control algorithm is relatively simple and reliable, its control effect is not ideal in the actual operation process, which is embodied in the following aspects: (1) if traditional PI control is adopted in the current inner loop, park transformation and Clarke transformation are required for the control quantity, and a large number of coordinate transformations will make the current inner loop control more complex; (2) when using traditional PI control, if you want to realize the decoupling of active and reactive power, it is necessary to introduce feed forward compensationɷLid and ɷLiq. In the actual operation process, the parameter of ɷLid and the parameter of ɷLiq are related to temperature and they are not fixed values; and (3) the proportional coefficient and integral coefficient of traditional PI control are fixed values. In the process of current inner loop control, if the decoupling is not accurate, the control accuracy will be reduced.Therefore, proportional resonance control PR is essentially PI control in two-phased/q rotating coordinate system. Introducing the PR control algorithm into current inner loop control can not only reduce a lot of coordinate transformation but also omit feed forward compensation ɷLid and ɷLiq, without power decoupling. Therefore, by introducing PR control, the current inner loop control at the network side can better achieve the control goal, quickly track the current command, and achieve no static error regulation. The control principle of proportional resonance PR is shown in Figure 6.Figure 6
The principle diagram of PR control.PR controller consists of a proportional link and a generalized integral link. The transfer function is as follows:(15)Gprs=kp+2kiss2+ω02,where kp, ki, and ω0 are the proportional coefficient, integral coefficient, and the resonant frequency, respectively. It can be seen from the above formula that the gain of the ideal PR control at the resonant frequency ω0 is infinite, while the gain at other frequencies is almost zero.In the network side current inner loop control based on traditional PI control, the output power of three-phase PWM inverter is determined by the given current valueid∗and iq∗, in which the size of id∗ determines the output of active power and the size of iq∗ determines the output of reactive power. At the same time, the two-cross compensation ɷLid/ɷLiq and the grid voltage E also affect the stability of the current inner loop control, but in practical engineering, the cross-coupling compensation ɷLid/ɷLiq will change with the change of external temperature, which is not a stable value. The current inner loop control based on the improved PR control in this paper is mainly divided into the following five steps: (1) the detection device is used to detect the AC current signal iabc on the three-phase AC network side; (2) Clarke transform of iabc will be performed to obtain iα and iβ; (3) Park transforms the signal given by the voltage outer loop to obtain the given value iα∗ and iβ∗ of the current inner loop; (4) the difference is made between iα∗ and iα, iβ∗ and iβ, and the two differences are used as the input of PR controller; and (5) the output of PR controller is used as the control command to control the six converter bridge arms of three-phase PWM inverter through SVPWM pulse width modulation.Compared with traditional PI control, PR control has the following two advantages in the inner loop control of grid side current: (1) PR control only needs Clarke transformation of the control quantity, which reduces the complex operation of rotating coordinate system and the difficulty of control algorithm; (2) no cross-coupling compensationɷLid/ɷLiq and Grid voltage E vector orientation are required, and the feed forward decoupling control is eliminated. Therefore, the changes of circuit parameters and grid voltage will not affect the accuracy of the control system, and the robustness and stability of the system will be greatly improved.


## 3.5. Rectifier Side Controller Design

To sum up, based on the double closed-loop decoupling control strategy of grid voltage vector orientation, the control of VSC on the rectifier side is deceptively transformed, and the control strategy of VSC on the rectifier side is established in this chapter. Among them, traditional PI control is transformed into fuzzy self-tuning PI control, which can adjust the proportional coefficientP and integral coefficient I in real time according to the changes of the outside world; AC current is the core of inner loop control. The goal of inner loop control is to track the grid voltage and maintain the same frequency and phase. Traditional PI control is transformed into PR control, which reduces the complex operation of rotating coordinate system, eliminates feed forward decoupling control and grid voltage vector orientation, and improves the stability and anti-interference ability of the system. The integral control diagram of VSC on the rectifier side in this chapter is shown in Figure 7.Figure 7
Integral control diagram of the rectifier side.


## 3.6. Design of Inverter Side Controller

Back-to-back converter is connected to a passive network, in which case a single-loop constant AC voltage control is required.The inverter three-phase AC voltage measurementuabc is converted by d/q coordinates. The measured value is compared with the AC voltage reference value in the d/q coordinate system. Ud, uq, and u0 are obtained through the PI regulator. Finally, the resulting PWM control signal allows the converter to be adjusted. The inverter side fixed AC voltage control block diagram is shown in Figure 8.Figure 8
Common control diagram of the inverter side.


## 3.7. Characteristic Analysis of Output Voltage Distortion under Unbalanced Load

Under three-phase unbalanced conditions, the distorted three-phase AC voltage can be expressed by three quantities: positive sequence voltageUf+, negative sequence voltage Uf—, and zero-sequence voltage Uf0.(16)Uf=Uf++Uf0+Uf−,uaubuc=Ufp+cosωbt+φ+cosωbt+φ−+2π3cosωbt+φ++2π3+Ufp−cosωbt+φ−cosωbt+φ−+2π3cosωbt+φ−+2π3+Ufp0cosωbt+φ0cosωbt+φ0cosωbt+φ0,where the positive, negative, and zero order electromotive force peaks are Ufp+, Ufp−, and Ufp0, respectively; the initial phase angle of the fundamental voltage under positive sequence, negative sequence, and zero sequence are φ+, φ−, and φ0, respectively.Since the three-phase inverter generally has no neutral connection, the zero-sequence electromotive force can be ignored. This meansUfp0=0. Then, equation (16) can be changed to(17)Uf=Uf++Uf0+Uf−,uaubuc=Ufp+cosωbt+φ+cosωbt+φ−+2π3cosωbt+φ++2π3+Ufp−cosωbt+φ−cosωbt+φ−+2π3cosωbt+φ−+2π3.It can be seen from equation (17) that when the alternating current is three-phase distorted, both the positive sequence and the negative sequence need to consider the fundamental wave amount of the alternating voltage. As can be seen from the analytical equation (15), PR control has only a high gain at all and a large attenuation at other frequencies. No static tracking can be achieved when the system gains near infinity at the frequency point. Traditional PR control has only one frequency point. Therefore, the controller is overly sensitive to the signal frequency. However, in practical applications, in order to increase system bandwidth and improve stability, PR controller can be improved to(18)Gprs=kp+ki2ωcss2+2ωcs+ω02,where ωc is the cutoff frequency, Ki = 100, and ω0 = 314 rad/s. When ωc = 2 rad/s and 20 rad/s, the improved PR controller Bode diagram is shown in Figure 9.Figure 9
Bode graph of the improved PR controller.It can be seen from the analysis of equation (18) and Figure 9 that the frequency response of PR controller is improved by ωc. PR controller can simultaneously achieve high gain on both sides ωc − ω0 and ωc − ω0. This is equivalent to performing PI control in both positive and negative sequence coordinates. Therefore, PR control can realize the nonstationary tracking of the three-phase AC quantity and the correction of the three-phase distorted AC quantity.


## 3.8. Improved Voltage Loop Design of PR Controller

To illustrate the principle of PR control and d/q decoupling control, the voltage loop α axis is taken as an example for analysis. Its total transfer function is shown in Figure 10 [19].Figure 10
Current loop transfer function with PR controller.The three-phase supply voltageuabc and the given three-phase voltage uabc∗ are uα and uα∗ obtained by Park transformation. The difference between these two gets the given output by the transfer function of PR controllerGPRs, the transfer function of the PWM link GPWMs, and the transfer function of the resonant link Gos (including the load impedance). Set Ks as the amplification factor of the PWM device. TS and T are the delay time and switching period of the PWM device, respectively. In addition, Ts<T. Normally the PWM switching frequency is large, so the time constant TS is small. PWM device can be simplified to first-order inertia link GPWMs=KS/TSS+1. It can be obtained that the transfer function of the resonant link is Gos=1/Ls+1/Cs+Zr1.The transfer function of theα-axis of the entire voltage flow control loop is as follows:(19)uα=GPRsGPWMsG0s1+GPRsGPWMsG0suα∗.It can be obtained from equation (19) that when the inverter operates at the specified power resonance point, Gos approximates infinity.GPRsGPWMsG0s is much larger than 1. Therefore, uαGPRsGPWMsG0s/1+GPRsGPWMsG0suα∗.


## 3.9. Design of VSC Controller for Inverting Side Power Supply to Three-Phase Unbalanced Load

The passive load is a three-phase asymmetry. Therefore, on the basis of analyzing the traditional double-loop control strategy, the traditional “fixed AC voltage single-loop control” can be modified by using PR control to track the characteristics of the sinusoidal signal without static difference. When back-to-back converter supplies power to the three-phase asymmetric load, the problem of the AC voltage distortion on the inverter side is solved.From the 3.8 summary analysis, PR control can perform no static tracking for a specific three-phase AC signal. For the distorted three-phase AC voltage, if PR control is introduced in the control, the traditional vector-controlled three-phase AC voltage is converted to the two-phase stationary coordinate system, and the primary resonance frequency is set to the PR-controlled resonant frequency, the three-phase AC voltage without the static difference tracking of the command signal will be realized. Finally, the distortion of the three-phase AC voltage is corrected, and the symmetry and stability of the three-phase AC voltage on the inverter side are realized.According to the above control target and control strategy analysis, the traditional inverter side fixed AC voltage single-loop control is changed to “outer loop fixed AC voltage-inner loop current decoupling” double closed-loop control. Further, the external loop using PR control voltage was modified. The stable power supply of back-to-back converter to the three-phase asymmetric passive network is realized. The overall control block diagram of the inverter side is shown in Figure11.Figure 11
Integral control diagram of inverter side of back-to-back converter.


## 4. Simulation and Experimental Verification Based on MATLAB/SIMULINK

### 4.1. Simulation Model and Related Parameters

In this paper, the simulation model of back-to-back converter to three-phase asymmetric load is built by using MTALAB/SIMULINK simulation software. In the simulation model, the relevant control parameters of the rectifier side, the DC side, and the inverter side of back-to-back converter are given in Tables1–3, respectively.Table 1
Related parameters of the rectifier side.
ParametersValueRectification side AC system parameters15 kV/3GVA/50 Hz20 kV/3GVA/50 HzInverter rated capacity50 MWLow pass filter LC parameters5mH + 180μFPWM switching frequency2 kHZModulationSPWMVoltage outer loop Kp, Ki0.015,1.6Current inner loop Kp, Ki5,15Table 2
Related parameters of the DC side.
ParametersValueDC line length10 kmLine resistance0.015 Ω/kmInductance0.792mH/kmDC capacitor30 mFDC line capacitance0.0144μFTable 3
Related parameters of the inverter side.
DC side voltage20 KVInverter rated capacity50 MWPrimary resonant inductor5 mHPrimary resonant capacitor25.3μFThree-phase load resistor84 Ω/50 Ω/24 ΩResonant frequency1 kHZPWM switching frequency2 kHZModulationSVPWMCurrent inner loop kp, ki0.4,500Voltage outer loop kp, Kr5,300Three-phase parallel load after 0.35s50 Ω


### 4.2. Rectification Side Simulation Results and Analysis

The voltage of the rectification side AC system is set to 15 kV/50 Hz and 20 kV/50 Hz, respectively, and the overshoot and reaction time of the AC-DC output signal (DC voltage) are observed.SIMULINK simulation is performed on the rectification side using the conventional fixed DC voltage double closed-loop control, and the results are shown in Figures12 and 13.Figure 12
Simulation results of the common rectifier side when AC voltage is 15 KV.Figure 13
Simulation results of the common rectifier side when AC voltage is 20 KV.Fuzzy PI controller is used to improve the voltage outer loop of the conventional rectifier side double closed-loop control. PR controller is further used to improve the inner loop d/q decoupling controller. The simulation results of SIMULINK are shown in Figures 14 and 15.Figure 14
Simulation results of the improved rectifier side when AC voltage is 15 KV.Figure 15
Simulation results of the improved rectifier side when AC voltage is 20 KV.The result comparison is divided into two parts. (1) In Figures12 and 13, under the conventional PI control, Uabc takes different values from 15 KV to 20 KV; (2) In Figures 14 and 15, under the improved fuzzy PI control, Uabc takes different values from 15 KV to 20 KV.Comparing the two parts 1 and 2, it can be seen that the double closed-loop control of the rectifier improved by fuzzy PI control and PR control can change the PI parameters in real time according to external conditions. Compared with the first part 1, the part 2 has the following characteristics: the DC side voltage, the AC side active power, the AC side reactive power reaching the steady state, and the time taken for the steady state is small; the curve fluctuation is small; the anti-interference ability is stronger; the amount is also smaller; the DC voltage can be stabilized earlier and faster. The control performance of the improved rectifier through fuzzy PI control and PR control has been significantly improved.


### 4.3. Inverter Side Simulation Results and Analysis of Back-to-Back Converter Powered to Three-Phase Asymmetric Passive Network

The control mode usually adopted for the inverter side that supplies power to the passive network is constant AC voltage single-loop control. This article mainly considers the case of supplying power to a three-phase asymmetric passive network. The three-phase asymmetric passive load affects the stable transmission of the three-phase alternating current on the inverter side. In this case, distortion of the three-phase AC voltage on the inverter side affects the stability of the AC voltage. In this paper, the double closed-loop control strategy of “outer loop fixed AC voltage-inner loop current decoupling” is adopted. By using PR control to perform sinusoidal voltage signal without static tracking, the three-phase voltage imbalance problem generated by the asymmetric load is solved. The overall control objective is to control the AC side output and stabilize the three-phase AC voltage. The simulation results in SIMULINK are shown in Figures16–18.Figure 16
Simulation results of the common inverter side.Figure 17
Simulation results of the improved inverter side.Figure 18
The AC voltage contrast of two different inverter sides after amplification.Figure16 shows the three-phase voltage and current wave forms obtained by the conventional constant voltage single-loop control when the inverter supplies power to the three-phase asymmetric load. It can be seen from the waveform that the three-phase voltage has asymmetrical distortion. In addition, the three-phase voltage waveform contains a large number of harmonic components.Figure17 shows the voltage and current simulation wave forms after the double closed-loop control and PR control of the “outer loop fixed AC voltage-inner loop current decoupling,” when the inverter is powered by the three-phase asymmetric load. It can be concluded from the figure that the AC output voltage on the inverter side remains basically unchanged after 0.1 s. After 0.1 s, although the phase of the three-phase current is asymmetrical, the magnitude of the current remains substantially constant and the harmonics are greatly reduced. Under these circumstances, the stable of power is guaranteed.Comparing Figures16 and 17, it can be seen that the improved voltage outer loop control can solve the imbalance problem of the three-phase AC voltage under the three-phase asymmetric load and complete the voltage three-phase symmetry. In addition, by combining the inner loop current d/q decoupling control, the stabilization of the alternating voltage is achieved, and the harmonic content of the alternating voltage is reduced.In order to further analyze the control performance of the “outer loop fixed AC voltage-inner loop current decoupling” double closed-loop control and PR control, a three-phase asymmetric loads ofa, b, and c are 84 Ω, 50 Ω, and 24 Ω  are simulated in the SIMULINK environment (Figure 19, Figure 20, and Figure 21).Figure 19
Simulation results of the common inverter side with load changing at 0.35s.Figure 20
Simulation results of the improved inverter side with load changing at 0.35s.Figure 21
The AC voltage contrast of two different inverter sides after amplification.Comparing Figures19 and 20, it can be seen that the three-phase asymmetric load changes at 0.35s, the conventional fixed AC voltage control strategy loses its effectiveness, not only the three-phase AC voltage is distorted but also the three-phase AC current is distorted. After adopting the “outer loop fixed AC voltage-inner loop current decoupling” double closed-loop control and PR control, even if the three-phase asymmetric load changes at 0.35s, the three-phase AC voltage can maintain three-phase symmetry and amplitude stability. The control target of the “fixed AC voltage” on the inverter side is realized.


## 4.1. Simulation Model and Related Parameters

In this paper, the simulation model of back-to-back converter to three-phase asymmetric load is built by using MTALAB/SIMULINK simulation software. In the simulation model, the relevant control parameters of the rectifier side, the DC side, and the inverter side of back-to-back converter are given in Tables1–3, respectively.Table 1
Related parameters of the rectifier side.
ParametersValueRectification side AC system parameters15 kV/3GVA/50 Hz20 kV/3GVA/50 HzInverter rated capacity50 MWLow pass filter LC parameters5mH + 180μFPWM switching frequency2 kHZModulationSPWMVoltage outer loop Kp, Ki0.015,1.6Current inner loop Kp, Ki5,15Table 2
Related parameters of the DC side.
ParametersValueDC line length10 kmLine resistance0.015 Ω/kmInductance0.792mH/kmDC capacitor30 mFDC line capacitance0.0144μFTable 3
Related parameters of the inverter side.
DC side voltage20 KVInverter rated capacity50 MWPrimary resonant inductor5 mHPrimary resonant capacitor25.3μFThree-phase load resistor84 Ω/50 Ω/24 ΩResonant frequency1 kHZPWM switching frequency2 kHZModulationSVPWMCurrent inner loop kp, ki0.4,500Voltage outer loop kp, Kr5,300Three-phase parallel load after 0.35s50 Ω


## 4.2. Rectification Side Simulation Results and Analysis

The voltage of the rectification side AC system is set to 15 kV/50 Hz and 20 kV/50 Hz, respectively, and the overshoot and reaction time of the AC-DC output signal (DC voltage) are observed.SIMULINK simulation is performed on the rectification side using the conventional fixed DC voltage double closed-loop control, and the results are shown in Figures12 and 13.Figure 12
Simulation results of the common rectifier side when AC voltage is 15 KV.Figure 13
Simulation results of the common rectifier side when AC voltage is 20 KV.Fuzzy PI controller is used to improve the voltage outer loop of the conventional rectifier side double closed-loop control. PR controller is further used to improve the inner loop d/q decoupling controller. The simulation results of SIMULINK are shown in Figures 14 and 15.Figure 14
Simulation results of the improved rectifier side when AC voltage is 15 KV.Figure 15
Simulation results of the improved rectifier side when AC voltage is 20 KV.The result comparison is divided into two parts. (1) In Figures12 and 13, under the conventional PI control, Uabc takes different values from 15 KV to 20 KV; (2) In Figures 14 and 15, under the improved fuzzy PI control, Uabc takes different values from 15 KV to 20 KV.Comparing the two parts 1 and 2, it can be seen that the double closed-loop control of the rectifier improved by fuzzy PI control and PR control can change the PI parameters in real time according to external conditions. Compared with the first part 1, the part 2 has the following characteristics: the DC side voltage, the AC side active power, the AC side reactive power reaching the steady state, and the time taken for the steady state is small; the curve fluctuation is small; the anti-interference ability is stronger; the amount is also smaller; the DC voltage can be stabilized earlier and faster. The control performance of the improved rectifier through fuzzy PI control and PR control has been significantly improved.


## 4.3. Inverter Side Simulation Results and Analysis of Back-to-Back Converter Powered to Three-Phase Asymmetric Passive Network

The control mode usually adopted for the inverter side that supplies power to the passive network is constant AC voltage single-loop control. This article mainly considers the case of supplying power to a three-phase asymmetric passive network. The three-phase asymmetric passive load affects the stable transmission of the three-phase alternating current on the inverter side. In this case, distortion of the three-phase AC voltage on the inverter side affects the stability of the AC voltage. In this paper, the double closed-loop control strategy of “outer loop fixed AC voltage-inner loop current decoupling” is adopted. By using PR control to perform sinusoidal voltage signal without static tracking, the three-phase voltage imbalance problem generated by the asymmetric load is solved. The overall control objective is to control the AC side output and stabilize the three-phase AC voltage. The simulation results in SIMULINK are shown in Figures16–18.Figure 16
Simulation results of the common inverter side.Figure 17
Simulation results of the improved inverter side.Figure 18
The AC voltage contrast of two different inverter sides after amplification.Figure16 shows the three-phase voltage and current wave forms obtained by the conventional constant voltage single-loop control when the inverter supplies power to the three-phase asymmetric load. It can be seen from the waveform that the three-phase voltage has asymmetrical distortion. In addition, the three-phase voltage waveform contains a large number of harmonic components.Figure17 shows the voltage and current simulation wave forms after the double closed-loop control and PR control of the “outer loop fixed AC voltage-inner loop current decoupling,” when the inverter is powered by the three-phase asymmetric load. It can be concluded from the figure that the AC output voltage on the inverter side remains basically unchanged after 0.1 s. After 0.1 s, although the phase of the three-phase current is asymmetrical, the magnitude of the current remains substantially constant and the harmonics are greatly reduced. Under these circumstances, the stable of power is guaranteed.Comparing Figures16 and 17, it can be seen that the improved voltage outer loop control can solve the imbalance problem of the three-phase AC voltage under the three-phase asymmetric load and complete the voltage three-phase symmetry. In addition, by combining the inner loop current d/q decoupling control, the stabilization of the alternating voltage is achieved, and the harmonic content of the alternating voltage is reduced.In order to further analyze the control performance of the “outer loop fixed AC voltage-inner loop current decoupling” double closed-loop control and PR control, a three-phase asymmetric loads ofa, b, and c are 84 Ω, 50 Ω, and 24 Ω  are simulated in the SIMULINK environment (Figure 19, Figure 20, and Figure 21).Figure 19
Simulation results of the common inverter side with load changing at 0.35s.Figure 20
Simulation results of the improved inverter side with load changing at 0.35s.Figure 21
The AC voltage contrast of two different inverter sides after amplification.Comparing Figures19 and 20, it can be seen that the three-phase asymmetric load changes at 0.35s, the conventional fixed AC voltage control strategy loses its effectiveness, not only the three-phase AC voltage is distorted but also the three-phase AC current is distorted. After adopting the “outer loop fixed AC voltage-inner loop current decoupling” double closed-loop control and PR control, even if the three-phase asymmetric load changes at 0.35s, the three-phase AC voltage can maintain three-phase symmetry and amplitude stability. The control target of the “fixed AC voltage” on the inverter side is realized.


## 5. Conclusion

Based on the optimal control strategy of back-to-back converter, this article takes the traditional control strategy of back-to-back converter supplying to passive network as the starting point. And according to the actual situation of back-to-back converter supplying power to three-phase asymmetric passive network, this article analyzes the control objectives of the rectifier side and the inverter side of back-to-back converter, respectively.For the rectifier side converter, the main purpose is to achieve a constant DC voltage. A double closed-loop control strategy of “fixed DC voltage-inner loop current decoupling” is designed. Optimize the voltage outer loop with fuzzy PI control. Optimize the current inner loop with PR control. The system reaction time and error overshoot are reduced, the d/p current coupling term is eliminated, and the anti-interference ability of the system is greatly improved.For the inverter side converter, the double closed-loop control strategy of “fixed AC voltage-internal loop current decoupling” is designed to achieve the fixed AC voltage output as the control target. PR controller can perform the characteristics of no static tracking on a certain frequency signal and optimize the external loop constant voltage control, not only realizes the stable output of the fixed AC voltage but also greatly reduces the harmonic content of the AC voltage. The proposed method can solve the problem of three-phase output voltage distortion when back-to-back converter is supplied to the three-phase asymmetric load and realizes the three-phase balance and stability of the AC output voltage.The innovation points proposed in this article have certain guiding significance for the optimization of control strategy of back-to-back converter, the symmetrical fault ride through of converter supplying to three-phase asymmetric passive network, and the participation of back-to-back converter in the black start process of power grid.



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*Source: 3104149-2022-07-15.xml*