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.     

Robust current controller for three-phase inverter usingfinite-state automaton

A novel feedback current controller for a three-phase load driven by a power inverter is proposed. The main design specifications are robustness to load electrical parameters, fast dynamical response, reduced switching frequency, and simple hardware implementation. To meet previous specifications a multi-variable hysteresis type controller is proposed, designed as a finite-state automaton and implemented with a programmable logic device. After a general introduction, system analysis is performed, control targets are specified, and the proposed control strategy is presented and discussed. Further, actual controller architecture, based on simple analog-logic hardware, is shown and experimental results are presented using an induction motor as the inverter load. However, this does not limit the wider applicability of the proposed controller that is suitable for different types of three-phase AC loads     

High-performance programmable AC power source with low harmonicdistortion using DSP-based repetitive control technique

This paper proposes a new control scheme based on a two-layer control structure to improve both the transient and steady-state responses of a closed-loop regulated pulse-width-modulated (PWM) inverter for high-quality sinusoidal AC voltage regulation. The proposed two-layer controller consists of a tracking controller and a repetitive controller. Pole assignment with state feedback has been employed in designing the tracking controller for transient response improvement, and a repetitive control scheme was developed in synthesizing the repetitive controller for steady-state response improvement. A design procedure is given for synthesizing the repetitive controller for PWM inverters to minimize periodic errors induced by rectifier-type nonlinear loads. The proposed control scheme has been realized using a single-chip digital signal processor (DSP) TMS320C14 from Texas Instruments. A 2-kVA PWM inverter has been constructed to verify the proposed control scheme. Total harmonic distortion (THD) below 1.4% for a 60-Hz output voltage under a bridge-rectifier RC load with a current crest factor of 3 has been obtained. Simulation and experimental results show that the DSP-based fully digital-controlled PWM inverter can achieve both good dynamic response and low harmonics distortion     

Advanced control of induction motor based on load angle estimation

An advanced control system with load angle adjustment is introduced. The method is based on the action of a phase-locked loop, in which a position synchronization of two vectors to obtain a constant command angle between them is realized. In the system presented, the vectors are stator current and rotor flux. The load angle is kept constant by changing the position of stator current vector as a result of tuning its pulsation. Proportional-integral and fuzzy logic controllers are used to control the load angle. Because of using the load angle controller and simple relations for state variables, the proposed idea does not require exact speed measurement. The discussed control system is not sensitive to motor resistance variations. This idea is realized on a fixed-point digital signal processor and field-programmable gate arrays. Experimental results for the control system fed by a voltage-source inverter and controlled using a predictive current controller are presented.     

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     

Sliding-mode control in voltage source inverter-based higher-order circuits

In this article, a new control methodology of sliding-mode control (SMC) for voltage source inverter (VSI)-based higher-order circuit is proposed. In this method, the SMC is used at the inner terminals for stable tracking of the voltage and current variables. An outer voltage control loop is included to reduce the steady-state error in tracking the reference load voltage. It is shown that when the SMC is applied on the load voltage terminal for higher-order VSI circuits, it leads to the instability. However, it well stabilises the system when the controller is implemented on the inner shunt capacitor terminals. Additional outer voltage control loop with proportional plus integral controller will ensure regulated voltage across the load. It has been shown that the controller is able to achieve good tracking accuracy with an acceptable stability margins. The performance of the proposed SMC has been verified on the fourth-order VSI circuit.     

Design and implementation of an FPGA-based control IC forAC-voltage regulation

This paper presents a field-programmable gate army (FPGA)-based control integrated circuit (IC) for controlling the pulsewidth modulation (PWM) inverters used in power conditioning systems for AC-voltage regulation. We also propose a multiple-loop control scheme for this PWM inverter control IC to achieve sinusoidal voltage regulation under large load variations. The control scheme is simple in architecture and thus facilitates realization of the proposed digital controller for the PWM inverter using the FPGA-based circuit design approach. Bit-length effect of the digital PWM inverter controller has also been examined in this paper. The designed PWM inverter control IC has been realized using a single FPGA XC4005 from Xilinx Inc., which can be used as a coprocessor with a general-purpose microprocessor in application of AC-voltage regulation. Owing to the high-speed nature of FPGA, the sampling frequency of the constructed IC can be raised up to the range that cannot be reached using a conventional digital controller based merely on microcontrollers or a digital signal processor (DSP). Experimental results show the designed PWM inverter control IC using the proposed control scheme can achieve good voltage regulation against large load variations     

Dead beat control of three phase PWM inverter

A novel method for microprocessor control of three-phase sinusoidal-voltage pulse-width-modulated (PWM) inverters is proposed. First, the discretized state equations of the inverter main circuit on the d-q frame are derived. An algorithm for dead beat control with a current minor loop that constrains the inverter current within the safety limit is subsequently developed. To compensate the computing time delay, a second-order prediction method and a novel discretization method using twice the time of the sampling period have been adopted. This method is especially suitable for inverters using high-speed switching devices and digital signal processors. The validity of the control system has been demonstrated by precise simulation using a hybrid computer     

Adaptive repetitive control of PWM inverters for very low THDAC-voltage regulation with unknown loads

An adaptive repetitive control scheme is proposed and applied to the control of a pulsewidth-modulated (PWM) inverter used in a high-performance AC power supply. The proposed control scheme can adaptively eliminate periodic distortions caused by unknown periodic load disturbances in an AC power supply. The proposed adaptive repetitive controller consists of a voltage regulator using state feedback control, a repetitive controller with tuning parameters and an adaptive controller with a recursive least-squares estimator (LSE). This adaptive repetitive controller designed for AC voltage regulation has been realized using a single-chip digital signal processor (DSP) TMS320C14 from Texas Instruments. Experimental verification has been carried out on a 2 kVA PWM inverter. Simulation and experimental results show that the DSP-based adaptive repetitive controller can achieve both good dynamic response and low total harmonic distortion (THD) under large-load disturbances and uncertainties     

A fuzzy self-tuning PI controller for HVDC links

This paper introduces a fuzzy logic-based tuning of the controller parameters for the rectifier side current regulator and inverter side gamma controller in a high voltage direct current (HVDC) power system. A typical point-to-point system has been taken with the detailed representation of converters, transmission links transformers and filters. The current error (and its derivative) and the gamma error (and its derivative) are used as the principal signals to adjust the proportional and integral gains of the rectifier pole controller and the inverter gamma controller, respectively, for optimum HVDC power system performance under various normal and abnormal conditions. Finally, a comparative study has been performed with and without tuning, to prove the superiority of the proposed control scheme     

Discrete sliding-mode control of a PWM inverter for sinusoidaloutput waveform synthesis with optimal sliding curve

This paper presents a discrete sliding-mode control scheme with feedforward compensation for the closed-loop regulation of the pulse-width modulated (PWM) inverter used in an uninterruptible power supply (UPS). The proposed feedforward controller can effectively improve the tracking performance of the PWM inverter. In designing the sliding-mode controller, we have taken load disturbance into consideration to enhance the robustness of the PWM inverter. Moreover, the upper bound of the load disturbance under which the sliding condition can be maintained has also been derived. The sliding curve of the sliding-mode controller is designed such that the behavior of the controlled PWM inverter is optimal subject to the selected cost function. Due to the coordinate transformation proposed in this paper, only the output voltage needs to be measured as feedback for the purpose of closed-loop regulation. Simulation and experimental results are given to show the effectiveness of the proposed control scheme     

Digital control of three-phase PWM inverter with LC filter

A digital control algorithm for the three-phase sinusoidal voltage inverter with an output LC filter has been developed. To take the transient of the LC filter during the discretization time into consideration, a fourth-order matrix state equation of the current and the voltage on the d-q frame is discretized. Precise discrete equations for the inverter are introduced. Using these equations, a deadbeat controller consisting of a d-g current minor loop and a d-q voltage major loop, with precise decoupling of the d-q components, was developed. The voltage major loop controller assures the sinusoidal output voltage and stabilizes the system. A deadbeat controller is used because both the current minor loop and the voltage major loop can used one sampling response. The validity of these techniques is confirmed by simulation studies. This method is expected to be useful for direct digital control of large-capacity sinusoidal voltage inverters using low-switching-frequency devices     

UPS inverter design using discrete-time sliding-mode control scheme

This paper presents a novel discrete-time sliding-mode control algorithm for an uninterruptible power supply (UPS) inverter design. The approach offers a dual-loop design, in which a current predictor utilizes the tracking error of output voltage to estimate the desired inductor current, while a current controller is adopted to regulate the inductor current and, thus, produces a control command to the pulsewidth modulation inverter. An explicit condition for stable controller design is derived. The efficacy of this scheme is validated via a successful implementation on a digital-signal-processor-based UPS inverter. The proposed scheme has shown its robustness on low output voltage distortion, excellent voltage regulation, and it is insensitive to load variation, even under nonlinear loads. Experimental studies were performed to further validate the effectiveness of this scheme     

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.     

Three-phase power quality compensator under the unbalanced sources and nonlinear loads

A three-phase voltage-source inverter for a power quality compensator under the unbalanced mains and nonlinear loads is proposed to provide balanced three-phase source current and improve input power factor. The proposed converter is based on the conventional three-phase voltage-source inverter with three additional ac power switches to achieve three-level pulsewidth modulation. The voltage stress of three ac power switches is clamped to half the dc-link voltage. The balanced reference mains currents are estimated using the dc-bus voltage and load currents. A proportional-integral voltage controller is used in the outer loop to compensate the switching losses of the voltage-source inverter. To perform the integrated power quality compensation, a hysteresis current control scheme is adopted to track the balanced line current command in phase with mains voltage. Three voltage levels are generated on the ac terminal of the proposed inverter. Computer simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.     

3C strategy for inverters in parallel operation achieving an equalcurrent distribution

A circular chain control (3C) strategy for inverters in parallel operation is presented in the paper. In the proposed inverter system, all the modules have the same circuit configuration, and each module includes an inner current loop and an outer voltage loop control. A proportional-integral controller is adopted as the inner current loop controller to expedite the dynamic response, while an H^∞ robust controller is adopted to reach the robustness of the multimodule inverter system and to reduce possible interactive effects among inverters. With the 3C strategy, the modules are in circular chain connection and each module has an inner current loop control to track the inductor current of its previous module, achieving an equal current distribution. Simulation results of two-module and a three-module inverter systems with different kinds of loads and with modular discrepancy have demonstrated the feasibility of the proposed control scheme. Hardware measurements are also presented to verify the theoretical discussion     

Enhanced power quality based single phase photovoltaic distributed generation system

This article presents a novel control strategy for a 1-? 2-level grid-tie photovoltaic (PV) inverter to enhance the power quality (PQ) of a PV distributed generation (PVDG) system. The objective is to obtain the maximum benefits from the grid-tie PV inverter by introducing current harmonics as well as reactive power compensation schemes in its control strategy, thereby controlling the PV inverter to achieve multiple functions in the PVDG system such as: (1) active power flow control between the PV inverter and the grid, (2) reactive power compensation, and (3) grid current harmonics compensation. A PQ enhancement controller (PQEC) has been designed to achieve the aforementioned objectives. The issue of underutilisation of the PV inverter in nighttime has also been addressed in this article and for the optimal use of the system; the PV inverter is used as a shunt active power filter in nighttime. A prototype model of the proposed system is developed in the laboratory, to validate the effectiveness of the control scheme, and is tested with the help of the dSPACE DS1104 platform.     

Discrete pulse modulation strategies for high-frequency invertersystems

High-performance, high-frequency inverter systems for UPS (uninterruptible power system) applications cannot be easily realized using conventional hard-switched PWM inverter topologies. Adoption of typical soft-switched inverters such as the resonant DC link inverter, require the use of discrete pulse modulation strategies. New controller structures are necessary to cope with stringent voltage regulation and distortion constraints in the presence of unbalanced and nonlinear loads. A controller that utilizes a load current feedforward strategy with a cost function current regulator to achieve excellent transient performance characteristics is presented. Voltage regulation is ensured using a synchronous frame regulator. Detailed simulation and experimental results verifying the concepts are presented. Although this work focuses on soft-switching inverters, the control concepts can be applied to conventional hard-switching inverters as well     

Intelligent voltage control strategy for three-phase UPS inverters with output LC filter

This paper presents a supervisory fuzzy neural network control (SFNNC) method for a three-phase inverter of uninterruptible power supplies (UPSs). The proposed voltage controller is comprised of a fuzzy neural network control (FNNC) term and a supervisory control term. The FNNC term is deliberately employed to estimate the uncertain terms, and the supervisory control term is designed based on the sliding mode technique to stabilise the system dynamic errors. To improve the learning capability, the FNNC term incorporates an online parameter training methodology, using the gradient descent method and Lyapunov stability theory. Besides, a linear load current observer that estimates the load currents is used to exclude the load current sensors. The proposed SFNN controller and the observer are robust to the filter inductance variations, and their stability analyses are described in detail. The experimental results obtained on a prototype UPS test bed with a TMS320F28335 DSP are presented to validate the feasibility of the proposed scheme. Verification results demonstrate that the proposed control strategy can achieve smaller steady-state error and lower total harmonic distortion when subjected to nonlinear or unbalanced loads compared to the conventional sliding mode control method.     

An Improved Deadbeat Current Control Scheme With a Novel Adaptive Self-Tuning Load Model for a Three-Phase PWM Voltage-Source Inverter

In this paper, an improved deadbeat current control scheme with a novel adaptive self-tuning load model for a three-phase pulsewidth-modulated (PWM) voltage-source inverter is proposed. First, to achieve high-bandwidth current control characteristics, an improved deadbeat current controller with delay compensation is adopted. The compensation method forces the delay elements, which are caused by voltage calculation, PWM, and synchronous frame rotation, to be equivalently placed outside the closed-loop control system. Hence, their effect on the closed-loop stability is eliminated, and the current controller can be designed with a higher bandwidth. Second, to relax the parameter sensitivity issue of the deadbeat controller and to realize a control scheme with reduced sensors, a novel adaptive self-tuning load model is emerged in the control structure. The adaptive model is designed with low computational demand to estimate in real time the load parameters (R,L) and the back-electromotive-force voltage simultaneously. A unified solution to the present nonlinear estimation problem is presented by adopting a parallel observer structure. Furthermore, the adaptive model has the necessary phase advance of the estimated quantities, which compensates for the total system's delay. Comparative evaluation results are presented to demonstrate the validity and effectiveness of the proposed control scheme