单相SPWM逆变器并联解耦控制策略

英文资料原文来源: 出自JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA,VOL.7,NO.3, SEPTEMBER 2009Decoupling Control Strategy for Single Phase SPWM ParallelInvertersShun-Gang Xu,Jian-Ping Xu,and Tai-Qiang CaoAbstract: A decoupling control strategy of inverter parallel system is proposed based on the equivalent output impedance of single phase voltage source SPWM (sinusoidal pulse width modulation) inverter. The active power and reactive power are calculated in terms of output voltage and current of the inverter, and sent to the other inverters in the parallel system via controller area network(CAN)bus. By calculating and decoupling the circumfluence of the active power and reactive power, the inverters can share load current via the regulation of the reference-signal phase and amplitude.Experimental results of an 110V/2kV A inverter parallel system show the feasibility of the decoupling control strategy.Index Terms-CAN bus, current sharing, inverters, parallel operation.1. IntroductionParallel operation of inverters is an efficient way to enhance the capacity and reliability of inverter systems. The key issue of parallel operation is the distribution of the load current. In an inverter parallel system, the amplitudes and phases of output voltages of all inverters should strictly equal to each other to guarantee that each inverter shares the same load current. Otherwise, the current circumfluence and overload of some inverters in the inverter parallel system may exist. The current circumfluence may also decrease the efficiency and reliability of the inverter parallel system.There are various techniques for the control of inverter parallel operation. Among these techniques, central control and master-slave control are easy to implement and have good current-sharing performance. However, these two control strategies work at the cost ofsystem reliability because of conjunction operation among inverters.In instantaneous-current control of inverter parallel system, there is a current bus to share the current signal among inverters and the instantaneous circumfluence is used to regulate the output current, each inverter has good transient performance and the parallel system has good current sharing performance. However, its analog signal communication is easy to be disturbed and the signal isolation is complicated, which decrease the reliability of the parallel system.Independent control without interconnection droops the output voltage and frequency of inverters, the link among inverters is only via power lines. Thus fewer interconnections are needed and the reliability of inverter parallel systems is improved. Traditionally, this control strategy assumes the output impedance of inverters is mainly inductive due to high inductive component of the line impedance and the large inductor filter. Thus active power-frequency droop and reactive power-voltage droop schemes are adopted. However, this is not always true as the closed-loop output impedance also depends on the control strategy, and the line impedance is predominantly resistive for low voltage cabling. Thus, there is coupling relationship between output active/reactive power and frequency/amplitude of the output voltage. Traditional independence control may lead to instability of inverter parallel systems.In this paper, a decoupling control strategy for inverter parallel systems is proposed. The active power and reactive power of inverters in a parallel system are calculate by their corresponding output voltage and output current, and the output power information is shared by controller area network(CAN)bus communication. Then the active and reactive power circumfluence of each inverter is calculated and applied to regulate its corresponding output voltage and output frequency by decoupling of the power circumfluence, respectively. Thus, the proposed decoupling control strategy overcomes the disadvantages of inverter parallel systems controlled by independence control without intercommunication and instantaneous-current control. The inverter parallel system implemented by this strategy can achieve better current-sharing performance, good stability, and good reliability.2. Analysis of Single Phase PWM InverterDual closed-loop feedback control is usually adopted to control single phase inverters.Fig.1 shows a dual closed-loop feedback control scheme with an inductor-current inner loop and a capacitor voltage outer loop. The capacitor-voltage outer loop adopts proportion-integral control to regulate output voltage, where Pv k and Iv k are proportionalcoefficient and integral coefficient, respectively. The inductor-current inner loop uses proportional control to enhance the transient response of the inverter, Pi k is a proportionalcoefficient.In Fig.1,the power stage includes a full-bridge configuration and an L-C filter,in V is DC link voltage, 1s to 4s are power switches, L and C are filter inductor and capacitor,ref v is a sinusoidal reference voltage signal of the inverter,L r is the sum of inductor equivalent series resistance, switch on-resistance, and connection-line resistance. According to nonlinear control and feedback linearization theory, open-loop averaged output voltage can be characterized by2000002L L in d v d v d i LC r C v L r i V u dt dt dt〈〉〈〉〈〉++〈〉++〈〉=〈〉 (1) where x 〈〉means the average value of x over one switching cycle and u is the control variable, which can take the values 1,0,or-1,depending on the state of switches 1S ,2S ,3S and 4S .For the dual closed-loop feedback control inverter shown in Fig.1,the controller can be characterized by()()000Pv Iv in ref Pi k s k V u v v i Cs v k s +⎛⎫〈〉=-〈〉-〈〉-〈〉 ⎪⎝⎭(2) From (1) and (2) ,the dynamic characteristics of the closed-loop output voltage can be expressed in Laplace domain as()()()()()032203211Pv Pi Pi Iv ref L Pi Pv Pi Pi Iv L Pi L Pi Pv Pi Pi Iv k k s k k v v LCs r C k C s k k s k k Ls r k s i LCs r C k C s k k s k k +=+++++++-+++++ (3)The single phase dual closed-loop inverter can be modeled by two terminal equivalent circuits as00()()ref v G s v Z s i =- (4)()()32()1Pv Pi Pi Iv L Pi Pv Pi Pi Ivk k s k k G s LCs r C k C s k k s k k +=+++++ (5) ()()()232()1L Pi L Pi Pv Pi Pi IvLs r k s Z s LCs r C k C s k k s k k ++=+++++ (6)Fig.1.Block diagram of Single phase dual closed-loop inverter.Frequency (rad/sec)(a)Frequency (rad/sec)(b)Fig.2.Bode diagram of the voltage gain and the equivalent outputimpedance of the dual closed-loop inverter:(a)magnitude vs. frequency and(b)phase vs.frequency.Fig.3.Inverter equivalent circuit.where ()G s is the voltage gain and ()Z s is the equivalent output impedance. The bode diagram of ()G s and ()Z s are shown in Fig.2.From (6), we can know that the equivalent output impedance is closely related to the parameters of the output filter and the feedback control parameters. Let R be the resistive component and X the inductive component of equivalent impedance Z(s).The inverter equivalent circuit can be shown as Fig.3.When 500L H μ=, 10C F μ=, and 0.1L r =Ω,the relations between the impedance ratio R X and the control parameters Pv k ,Pi k , and Iv k are shown in Fig.4.Fig.4.Relations between the impedance ratio R/X and control parameters:(a)R/X vs.Pv k ,(b)R/X vs.Pi k ,and(c)R/X vs.Iv k .From Fig.4, the equivalent output impedance trends to be resistive when PI control parameter Pv k and Pi k are increasing, and trends to be inductive when PI control parameter Iv k is increasing. In the design of dual closed-loop single phase inverter, the PI control parameters must be chosen carefully as they affect both the transient characteristics of the inverter and the current sharing performance of the inverter parallel system.3. Analysis of Inverter Parallel SystemBased on above discussion, the equivalent circuit of inverter parallel system of two inverter modular can be given as Fig.5, where 0E ∠is load voltage, 1111()ref E G s v δ∠=and 111()R jX Z s +=are the output voltage and equivalent output impedance of inverter 1,2222()ref E G s v δ∠=and 222()R jX Z s +=are the output voltage and equivalent output impedance of inverter 2.In the inverter parallel system, the active output power and the reactive output power of the inverter 1 can be expressed as:()()0111101111221101111111012211cos sin cos sin E R E R E X E P R X E X E R E X E Q R X δδδδ-+=+--=+ (7)Due to small difference the phase of output voltage between individual inverters, we can assume that sin ,cos 1i i i δδδ≈≈,12R R R ==and 12X X X ==.Therefore, we have21010101222101010122RE E E E X RE P R X XE E E E R RE Q R X δδ+-=+--=+ (8) Similarly for the inverter 2, we have22020202222202020222RE E E E X RE P R X XE E E E R RE Q R X δδ+-=+--=+ (9)Fig.5.The equivalent circuit of the parallel system of two inverter modular.Fig.6.Structure of parallel operation system.From above analysis we can know that the active/reactive power is related to the amplitude and phase of voltage, and the influence of output voltage amplitude and phase on active and reactive power is closely related to the inductive component and resistive component of the output impedance of the inverter. When resistive component is dominating, active power is mainly depended on the amplitude of output voltage, and reactive power is mainly depended on the phase of the output voltage, and vice versa.4. Control DesignFig.6 shows the structure of inverter parallel system. The digital signal processor TMS320F2812 is adopted in the proposed parallel system; the inverters decouple the active power and the reactive power circumfluence to regulate the amplitude and the phase of the sinusoidal reference voltage signal. Each inverter adopts instantaneous voltage and instantaneous current dual closed-loop feedback control. The inverters can operate not only independently but also in parallel. The CAN bus transfers information of the active power and the reactive power among the inverters.Fig.7.Decoupling control strategy.Fig.8.Experiment wave of inverter parallel system: (a)steady current wave,(b)current wave with a sudden increasing load, and (c)current wave with a sudden decreasing load.In the parallel operation system, the differences between the output active power and reactive power of individual inverter lead to the asymmetry of output current among the inverters. The relation between the active/reactive power and output voltage amplitude/phase is given by (8).In the single phase SPWM inverter which adopts dual closed-loop feedback control, output voltage tracks the amplitude and phase of the sinusoidal reference voltage signal. Thus, the output active and reactive power of the inverter can be controlled by the amplitude and phase of the reference voltage signal. If output active and reactive power equal to each other in the parallel system, the inverters can share the load current well.In the inverter, the output voltage and output current are sampled by digital signalprocessor (DSP) for the calculation of output active and reactive power. All of the inverters share the active and reactive power by the CAN bus, each inverter calculates its corresponding active power circumfluence P∆.These∆and reactive power circumfluence Q circumfluence signals are decoupled to regulate the amplitude and the phase of reference voltage signal as shown in Fig.7.Therefore, each inverter outputs the same active power and reactive power, and the inverters can share the load current in the system.5. Experiment ResultsTwo 2 KV A inverters are used in our experiment. In the parallel system, the output filter inductance is 500μH,the filter capacitance is 10μF,the DC input voltage is 200 V DC, and the AC output voltage is 110 V with 50 Hz. 6N137 is used to isolate the signal between the inverters and the CAN bus, the baud rate of CAN bus is set to 1 Mbps. The closed-loop control, decoupling arithmetic and the SPWM control signal are realized by TMS320F2812 digital signal processor. Experiment results of the inverter parallel system are shown in Fig.8.In the steady state, the two inverters share the current very well and during transient under sudden load variation, the inverter parallel system still can work well. This indicates that excellent load sharing is achieved between these two inverters. 6. ConclusionsThis paper proposes a decoupling control strategy for inverter parallel systems. Theoretical analysis and experimental results verify the feasibility of the proposed control strategy. This control strategy has the following characteristics:1)inverters can work independently or in parallel;2)CAN bus is used for the inverter parallel system; 3)the inverter parallel system supports hot-s operation and has good reliability and expansibility. 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