A Minimal Harmonic Controller for a STATCOM

In this paper, a novel controller with fixed modulation index (MI) and variable dc capacitor voltage reference to minimize voltage and current harmonics is presented for a distribution static synchronous compensator (STATCOM). The STATCOM with the proposed controller consists of a three-phase voltage-sourced inverter and a dc capacitor and is used to provide reactive power compensation and regulate ac system bus voltage with minimum harmonics. A systematic design procedure based on pole-zero cancellation, root locus method, and pole assignment method has been developed to determine proper parameters for the current regulator, the dc voltage controller, and the ac voltage controller of the STATCOM. With the proposed STATCOM controller, harmonic distortions in the inverter output current and voltage can be reduced since the MI is held constant at unity in steady state. In addition, a fast adjustment in the STATCOM output reactive power is achieved to regulate the ac bus voltage through the adjustment of the dc voltage reference during the transient period. Simulation and experimental results for the steady-state operating condition and transient operating conditions for the system subjected to a reactive current reference step change, a three-phase line to neutral fault, and a step load change are presented to demonstrate the effectiveness of the proposed controller.     

Admittance Compensation in Current Loop Control for a Grid-Tie LCL Fuel Cell Inverter

Current loop transfer function of a single-phase grid-tie inverter has been systematically derived with representations of conventional transfer function format using admittance terms for controller design and loop compensation. The power circuit adopts the LCL type filter to allow universal output that can be operated in both standalone and grid-tie modes. The proposed admittance compensation along with a quasi-proportional-resonant controller is designed to achieve high gain at the fundamental frequency while maintaining enough stability margins. The entire current loop controller and admittance compensation have been simulated and tested with a 5-kW fuel cell prototype. Without the admittance path compensation, simulation results indicate that the system cannot start up smoothly and the zero current command cannot be tracked very well. At first simulation cycle, the power flow erratically fed back to the inverter that may cause catastrophe failure. With admittance path compensation, the time-domain current steady-state error can be easily reduced with the loop gain design in frequency-domain. Simulation and experimental results show that the inverter is capable of both standalone and grid-tie connection mode operations and smooth power flow control even with zero current command.      

Parallel processing inverter system

A novel method of instantaneous voltage and power balance control of a parallel processing inverter system is proposed. It consists of a high-speed switching PWM (pulsewidth modulated) inverter with an instantaneous current minor loop controller, a voltage major loop controller, and a power balance controller. This system realizes the following functions with only one inverter: constant AC output voltage control with reactive power control, active filtering to absorb load current harmonics, DC voltage and current control as AC-to-DC converter, and uninterruptible power supply (UPS) for stand-alone operation. This system covers a wide application range, including UPS systems, new energy systems, and active filters with voltage control functions     

基于坐标变换的间隙性功率并网系统解耦控制与设计

针对逆变器电流环的有功电流和无功电流存在耦合的情况,文中采用电流的解耦控制,达到独立控制逆变器的有功功率和无功功率;针对逆变器直流侧电压容易波动,文中采用双环控制,稳定逆变器直流侧的电压。搭建了MA-TALAB/SIMULINK系统仿真实验平台,通过间歇性能源输入逆变器功率变化的实验,验证了该逆变器能独立控制输出的有功功率和无功功率,并且保证了直流侧电压的稳定。     

Decoupling Control Strategy for Single Phase SPWM Parallel Inverters

A deconpling 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/2kVA inverter parallel system show the feasibility of the decoupling control strategy.     

Implementation of a Three-Phase Capacitor-Clamped Active Power Filter Under Unbalanced Condition

A capacitor-clamped voltage-source inverter for active power filter operation under balanced and unbalanced conditions is proposed to suppress current harmonics and compensate the reactive power generated from the nonlinear loads. The adopted voltage-source inverter is based on a three-level capacitor-clamped topology to reduce the voltage stress of power semiconductors. Two control loops are used in the control scheme to achieve harmonic and reactive currents compensation and to regulate the inverter dc side voltage. In the adopted inverter, the neutral point voltage is compensated by a voltage compensator to obtain the balanced capacitor voltages on the dc side. In order to control the flying capacitor voltages, two redundant states in each inverter leg can be selected to compensate the flying capacitor to obtain a better voltage waveform with low harmonic contents on the ac terminals. The balanced and sinusoidal line currents are drawn from the ac source under the balanced and unbalanced conditions. The feasibility of the proposed scheme is confirmed through experimental results     

Integrated power factor compensator without load current measurement

A simple strategy and low cost control for the switching mode rectifier to work simultaneously as a power factor corrector and an active power filter (APF) to reduce current harmonics drawn from the nonlinear load are analysed and presented in this paper. The principal component of the control circuit is an Intel 80196MC microcontroller that performs the dc bus voltage and line current control. The sliding mode control is used in the current loop to achieve fast line current dynamics. The source currents only are measured in the proposed control scheme instead of both the source and load currents needed in the conventional control approach. A simple proportional-integral control is adopted in the voltage loop to achieve slow dc bus dynamics. The proposed control strategy can achieve a high power factor and low current harmonics. No dedicated APF is needed in the proposed control strategy. To demonstrate the effectiveness of the integrated power factor compensator for elimination of reactive power and current harmonics, software simulation and hardware tests are performed.     

Shunt active filter for reactive power compensation

A simple control technique for three-phase shunt active filters without computation of the reactive current component is presented. A current controller with fast dynamics for an active filter is described. Reactive current is directly controlled without the need for sensing and computing the reactive component of the load current, thus simplifying the control system. Current compensation is done in the time domain, allowing a fast time response. The dc voltage control loop keeps the voltage across the dc capacitor constant. High power factor control by an active filter is described. All control functions are implemented in software using a singlechip microcontroller, thus simplifying the control circuit. Any current-controlled synchronous rectifier can be used as a shunt active filter through only the simple modification of the software and the addition of current sensors. It is shown through experimental results that the proposed controller gives good performance for the shunt active filter.     

A New Multilevel Conversion Structure for Grid-Connected PV Applications

A novel scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The scheme is based on two insulated strings of PV panels, each one feeding the dc bus of a standard two-level three-phase voltage-source inverter (VSI). The inverters are connected to the grid by a three-phase transformer having open-end windings on the inverter side. The resulting conversion structure performs as a multilevel power active filter (equivalent to a three-level inverter), doubling the power capability of a single VSI with given voltage and current ratings. The multilevel voltage waveforms are generated by an improved space-vector-modulation algorithm, suitable for the implementation in industrial digital signal processors. An original control method has been introduced to regulate the dc-link voltages of each VSI, according to the voltage reference given by a single maximum power point tracking controller. The proposed regulation system has been verified by numerical simulations and experimental tests with reference to different operating conditions.     

Improved Direct Power Control of Grid-Connected DC/AC Converters

This paper proposes new direct power control (DPC) strategies for three-phase dc/ac converters with improved dynamic response and steady-state performance. As with an electrical machine, source and converter flux, which equal the integration of the respective source and converter voltage, are used to define active and reactive power flow. Optimization of the lookup table used in conventional DPC is outlined first so as to improve power control and reduce current distortion. Then, constant switching frequency DPC is developed where the required converter voltage vector within a fixed half switching period is calculated directly from the active and reactive power errors. Detailed angle compensation due to the finite sampling frequency and the use of an integral controller to further improve the power control accuracy are described. Both simulation and experimental results are used to compare conventional DPC and vector control, and to demonstrate the effectiveness and robustness of the proposed control strategies during active and reactive power steps, and line inductance variations.      

三相四线制无功与谐波全补偿的并联有源电力滤波器的研究

简要分析了并联有源电力滤波器的工作原理,选取含有裂相电容的三桥臂电压源型逆变器作为系统主电路,输出电流采用动态滞环控制。针对三相四线系统,且考虑到电网电压畸变和不对称的情况,给出了一种基于瞬时功率理论实现无功功率和谐波全补偿的控制策略,并使用Matlab对系统进行了仿真。     

三相四线制无功与谐波全补偿的并联有源电力滤波器的研究

简要分析了并联有源电力滤波器的工作原理，选取含有裂相电容的三桥臂电压源型逆变器作为系统主电路，输出电流采用动态滞环控制。针对三相四线系统，且考虑到电网电压畸变和不对称的情况，给出了一种基于瞬时功率理论实现无功功率和谐波全补偿的控制策略，并使用Matlab对系统进行了仿真。     

Performance evaluation of an FPGA controlled soft switched inverter

A field programmable gate array (FPGA) based controller is proposed for a dc link series resonant inverter. The basic operation of the zero current switching inverter is briefly described. A strategy of decoupling the control of the dc link current from the load current is identified and referred as decoupled current control (DCC). The use of gate-controlled devices like metal-oxide-semiconductor field-effect transistor/insulated gate bipolar transistor/MOS-controlled thyristor permits a higher resonance frequency at the link of the inverter. The increased frequency enables the application of pulse density modulation technique with a bang-bang controller to synthesise and control the wave-shapes of current and voltage of the inverter. The DCC strategy eliminates the conventional analogue controller. A digital sequence controller has been designed using the state machine technique for the reliable operation of the inverter. The digital design is implemented on a single chip FPGA. To verify the proposed control strategy and the FPGA controller, a prototype has been built and tested. The test results show that a sinusoidal inverter output voltage is maintained with total harmonic distortion less than 5% and a regulation of about 1% from no-load to full-load, including non-linear and transient loads. The performance of the inverter with the FPGA controller is promising and attractive for uninterrupted power supply applications.     

Output Impedance Design of Parallel-Connected UPS Inverters With Wireless Load-Sharing Control

This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the$P/Q$droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads.     

适用于弱互联电网的光伏逆变器LVRT控制策略研究

目前,光伏装机在电网中占比越来越高,并网光伏逆变器也有严格的低电压穿越要求。现行低电压穿越标准要求逆变器在低电压穿越期间按照曲线给予电网足够的无功电流支撑,但是没有对期间的有功电流响应做出明确要求。弱互联区域电网稳定不仅和无功电流相关,也在很大程度上和有功电流相关,同时低电压穿越期间有功缺额对频率也有较大影响。文章分析了电网电压不平衡时锁相环控制方法,并提出了一种基于解耦双同步坐标系锁相环的低电压穿越期间有功电流无功电流协调控制策略,搭建了基于F28335+CPLD的硬件控制平台,并最终在硬件在环仿真平台中验证了控制策略的有效性。     

A Model-Based Direct Power Control for Three-Phase Power Converters

The direct power control (DPC) technique has been widely used as a control strategy for three-phase power rectifiers due to its simplicity and good performance. DPC uses the instantaneous active and reactive power to control the power converter. The controller design has been proposed as a direct control with a lookup table and, in recent works, as an indirect control with an inner control loop with proportional-plus-integral controllers for the instantaneous active and reactive power errors. In this paper, a model-based DPC for three-phase power converters is designed, obtaining expressions for the input control signal, which allow the design of an adaptive control law that minimizes the errors introduced by parameter uncertainties as the smoothing inductor value or the grid frequency. A controller design process, a stability study of the system, and experimental results for a synchronous three-phase power rectifier prototype are presented to validate the proposed controller.     

DSP-Based Implementation of an LQR With Integral Action for a Three-Phase Three-Wire Shunt Active Power Filter

This paper presents the design and implementation of a Linear Quadratic Regulator (LQR) with Integral action (LQRI) for a three-phase three-wire shunt active filter (SAF). The integral action is added so as to cancel the steady-state errors for reference tracking or disturbance rejection, knowing that the standard LQR provides only proportional gains. The controller is designed to achieve dc bus voltage regulation and harmonics and reactive power compensation. The converter model is set in the $d{-}q$ rotating reference frame. The latter is augmented with the integral of the $q$ component of the SAF currents and dc bus voltage to achieve integral action. The controller's performance depends on the weighting matrix, which is chosen to ensure satisfactory response. The converter is controlled as a whole, i.e., a multi-input–multioutput system and a fixed pulsewidth modulation at 10 kHz is used to generate the gating signals of the power devices. The system is tested for harmonics, reactive power, and load unbalance compensation for balanced/unbalanced loads. The experimental results obtained with a digital signal processor-based implementation of the controller on the DS1104 of dSPACE show good performance in terms of dc bus voltage regulation (small overshoot and very fast time response) and a low total harmonic distortion of ac line currents.      

Direct Power Control of Active Filters With Averaged Switching Frequency Regulation

This paper has developed a direct power control (DPC) structure to improve the performance of an active filter. A control algorithm directly uses the instantaneous power terms as control variables to replace the current and voltage variables that are commonly used in proportional-integral (PI) control systems. Compared to the other DPC schemes that have been reported so far, the proposed algorithm is oriented to harmonic current compensation, for which the switching functions are redefined, the bandwidths of the two hysteresis comparators are dynamically adjusted, and consequently, the pulsewidth modulation (PWM) switching frequencies are regulated to eliminate the unnecessary short switching pulses, and the control system can be used directly and effectively for various types of nonlinear load compensation. With the proposed control scheme, full control of the active filter, including the line current and the dc bus voltage, can be realized within an integrated power control loop. The advantages of the proposed control strategy have been verified by simulation and experimental results on a 2-kVA laboratory prototype.      

Analysis and Design of a Modular Three-Phase AC-to-DC Converter Using CUK Rectifier Module With Nearly Unity Power Factor and Fast Dynamic Response

In this paper, the analysis and design of a modular three-phase ac-to-dc converter using single-phase isolated CUK rectifier modules is discussed based on power balance control technique. This paper analyzes the operation of a modular converter as continuous-conduction-mode power factor correction (CCM-PFC). Design equations, as well as an average small-signal model of the proposed system to aid the control loop design are derived. It is used to obtain the inductor current compensator, thus the output impedance and audio susceptibility become zero, and therefore, the output voltage of the converter presented in this paper is independent of the variations of the dc load current and the utility voltage. The control strategy consists of a single output voltage loop and three-inductor current calculator. The main objective of the proposed system is to reduce the number of stages and improve dynamic response of dc bus voltage for distributed power system. The proposed scheme offers simple control strategy, flexibility in three-phase delta or star-connected, simpler design, fast transient response, good inductor current sharing, and power factor closed to unity. Both simulation and experimental results are presented. They are in agreement with the theoretical analysis and experimental work.      

Hysteresis describing method for control of an active power filter

A hysteresis describing method is proposed to control and design an active power filter. The describing function used to develop this method consists of the linearisation of the non-linear current control loop. The linearisation is done by deriving the hysteresis complex describing function to find the stability condition of the closed-loop current. Under this condition, the frequency and amplitude values of the error signal correspond to the maximum switching frequency and the current ripple, respectively. The proposed method permits calculation of these parameters in a simple algebraic equation as a function of the hysteresis band, dc bus voltage and inductive filter value. Moreover, the compromise between the dc bus voltage and inductor value can be evaluated easily as a function of both switching frequency and ripple requirements. Two design examples are worked out. Experimental results validate the theory.