A hysteresis current control for single-phase multilevel voltage source inverters: PLD implementation

In most high-performance applications of voltage source pulse-width modulation inverters, current control is an essential part of the overall control system. In this paper, a hysteresis current control technique for a single-phase five-level inverter with flying-capacitor topology is proposed. Logic controls and a programmable logic device are suitable for handling a large number of switches and implementation of state transitions. This method also considers how to improve unbalanced voltages of capacitors using voltage vectors in order to minimize switching losses. The simulation and experimental results describe and verify the current control technique for the inverter.     

New soft switching techniques for three phase voltage source inverters

Three new composite soft switching configurations for power inverters are presented. Each configuration incorporates an inductor to limit switch di/dt and diode reverse recovery current at turn-on, and realizes zero current switching at turn-off, with minimum active devices or passive components. Different features and applications of the configurations are considered. Simulations of three different inverter configurations are performed using PSpice and circuit switching waveforms are given, and experimental results confirmed analysis and simulations.     

Output voltage integral control technique for compensating nonidealDC buses in voltage source inverters

Output harmonic minimization in standard pulse width modulation (PWM) pattern generators is based on the assumption that the input DC bus voltage is ripple-free. However, in a practical converter system, a nonideal DC bus deteriorates the quality of the inverter output voltage by introducing undesirable low-order harmonics that may be difficult to filter. The existing compensation techniques often use additional and complex circuitry to eliminate the effect of this ripple on the output voltage. This paper presents an online PWM pattern generator that inherently takes into account the DC bus ripple and generates gating signals required to produce high-quality sinusoidal output voltages. The technique is based on integrating the output voltage at a constant frequency on a pulse-by-pulse basis to ensure a sinusoidal volt-sec (V/s) distribution, irrespective of the input DC bus. The principles of operation are explained, and design equations are derived. The features of the proposed PWM pattern generator are illustrated. Comparison of the output voltage waveforms of those standard sinusoidal PWM (SPWM) techniques illustrate, in particular, the effectiveness of the ripple-rejection mechanism. Experimental results obtained on a 3-kVA laboratory prototype confirm the feasibility and features of the proposed pattern generator     

Generalized techniques of selective harmonic elimination andcurrent control in current source inverters/converters

This paper presents generalized techniques for realizing PWM patterns which provide selective harmonic elimination and current magnitude modulation (SHEM) for current source inverters/power converters (CSI/C). A combination of chops and short circuit pulses are positioned in such a way that lower order harmonics are eliminated selectively besides current magnitude modulation with minimum switching frequency. Generalized equations and tables which show the relationship of various PWM-SHEM parameters to the position of short circuit pulses and the number of chops per 30° are provided and discussed in detail     

Concise modulation strategies for four-leg voltage source inverters

The continuous, discontinuous pulse-width modulation (PWM) schemes and a novel space vector modulation methodology are proposed in this paper for four-leg dc-ac inverters. Using a space vector definition that includes the zero sequence voltage component and partitioning the feasible sixteen modes into two separate sets - one set having zero sequence voltages with positive magnitudes and the other set with negative magnitudes - the novel space vector implementation technique is determined as also the discontinuous carrier based PWM scheme. For the continuous carrier based PWM scheme, the indeterminate defining output voltage equations expressed in terms of the existence functions of the switching devices are solved using an optimization technique. The modulation schemes determined are shown by experimental results to synthesis any desirable balanced or unbalanced three-phase voltage sets when operating in the linear modulation region.     

Reduced layer planar busbar for voltage source inverters

Planar busbars are used widely in modern high power inverters and have been shown to provide a good way for minimizing stray inductance and overshoot voltage. Planar busbars, which consist of multilayer copper sheets and dielectric insulators, offer significant advantages such as low impedance, improved thermal characteristic and reduced system cost compared with other interconnection approaches. A common problem of hard switched inverters is stray inductance, which needs to be minimized to reduce the effect of overvoltages. This paper presents a new physical structure for a voltage source inverter with symmetrical planar busbar structure, which minimizes the variation in stray inductance for different switching states. Three-dimensional (3-D) finite element simulations and experimental tests verify the theory     

Direct model-based predictive control scheme without cost function for voltage source inverters with reduced common-mode voltage

This paper presents a direct model-based predictive control scheme for voltage source inverters (VSIs) with reduced common-mode voltages (CMVs). The developed method directly finds optimal vectors without using repetitive calculation of a cost function. To adjust output currents with the CMVs in the range of –V_dc/6 to +V_dc/6, the developed method uses voltage vectors, as finite control resources, excluding zero voltage vectors which produce the CMVs in the VSI within ±V_dc/2. In a model-based predictive control (MPC), not using zero voltage vectors increases the output current ripples and the current errors. To alleviate these problems, the developed method uses two non-zero voltage vectors in one sampling step. In addition, the voltage vectors scheduled to be used are directly selected at every sampling step once the developed method calculates the future reference voltage vector, saving the efforts of repeatedly calculating the cost function. And the two non-zero voltage vectors are optimally allocated to make the output current approach the reference current as close as possible. Thus, low CMV, rapid current-following capability and sufficient output current ripple performance are attained by the developed method. The results of a simulation and an experiment verify the effectiveness of the developed method.     

Novel passive lossless turn-on snubber for voltage source inverters

A novel passive lossless turn-on snubber with a soft-clamped turn-off snubber circuit for voltage source inverters is proposed. The energy trapped in the snubber is recovered into the DC supply and load without any active devices, associated control circuitry, or resistors. The overshoot voltage on the switches is clamped, and the peak switch current is low, making this snubber suitable for use in high-power insulated gate bipolar transistor (IGBT) inverters     

Common- and differential-mode HF current components in AC motors supplied by voltage source inverters

In this paper, an inverter-fed ac motor drive is analyzed in order to investigate the conducted electromagnetic interferences at both the input and output sides of the inverter. HF lumped equivalent circuits for the inverter and the motor stator windings are proposed. The overall circuit model allows time- and frequency-domain analysis to be performed with standard circuit simulators. The proposed model can also predict common- and differential-mode HF current components. The equivalent circuit is verified by experimental tests carried out on a prototype of ac motor drive.     

A voltage control strategy for current-regulated PWM inverters

Alternative voltage control strategies for current-regulated PWM inverters are analyzed, including previously established feedforward and feedforward/feedback controllers and a newly proposed decoupling feedback control strategy. The steady-state and dynamic characteristics of each of these control methods are illustrated and compared for a selected inverter design. It is shown that the feedforward controller exhibits steady-state error and an undesirable overshoot of the output voltages during startup. The addition of a feedback loop eliminates the steady-state error and reduces the overshoot; however, the natural response is underdamped regardless of the choice of feedback gains. A decoupling feedback control strategy that eliminates the disadvantages of the feedforward and feedforward/feedback controllers is described. Using the decoupling feedback controller, it is possible to eliminate the steady-state error and place the closed-loop poles wherever desired. Moreover, if the closed-loop poles are selected appropriately, it is possible to eliminate the overshoot of the output voltages during startup transients     

The generalized discontinuous PWM scheme for three-phase voltage source inverters

This paper presents analytical techniques for the determination of the expressions for the modulation signals used in the carrier-based sinusoidal and generalized discontinuous pulse-width modulation schemes for two-level, three-phase voltage source inverters. The proposed modulation schemes are applicable to inverters generating balanced or unbalanced phase voltages. Some results presented in this paper analytically generalize the several expressions for the modulation signals already reported in the literature and new ones are set forth for generating unbalanced three-phase voltages. Confirmatory experimental and simulation results are provided to illustrate the analyses.     

Amplitude distortion compensation for voltage source dc/ac inverters

The objective of this paper is to propose two different modified sinusoidal pulse width modulation (SPWM) control schemes, MPWM-S and MPWM-T, for the dc/ac inverter with a non-constant dc voltage source. The amplitude distortion of the pulse width modulated waveforms is caused by the input dc voltage source variation and has the most significant impact on the dc/ac inverter’s ac output voltage spectral errors. The proposed MPWM-S and MPWM-T can achieve the desired fundamental amplitude and eliminate the low order harmonic components caused by the dc voltage source variation. The mathematical analysis of the amplitude distortion and the systematic development of the MPWM-S and MPWM-T are presented in this paper. The proposed MPWM-S and MPWM-T are feed-forward compensation methodologies and can be integrated with any practical feedback control schemes. Hardware implementations of the proposed control schemes can be easily accomplished by adding one or two analog multipliers to the traditional SPWM control circuit. The performance of the two proposed modified SPWM control schemes is verified by experimental results.     

Full binary combination schema for floating voltage source multilevel inverters

This paper presents schema of operation for floating voltage source multilevel inverters. The primary advantage of the proposed schema is that the number of voltage levels (and thus power quality) can be increased for a given number of semiconductor devices when compared to the conventional "flying capacitor" topology. However, the new schema requires fixed floating sources instead of capacitors and therefore is more suitable for battery power applications such as electric vehicles, flexible AC transmission systems and submarine propulsion. Alternatively transformer/rectifier circuits may be used to supply the floating sources in a similar way to cascaded H-bridge inverters. Computer simulation results are presented for 4-level, 8-level, and 16-level inverter topologies. A 4-level laboratory test verifies the proposed method.     

Novel current control method for PWM inverters

A new current control scheme is suggested which is suitable for AC servo drives. The scheme has a simple feedback configuration to determine an optimal stator voltage vector. Minimisation of either current deviations or switching frequency is possible without changing the controller configuration.     

Direct self-control and synchronous pulse techniques for high-power traction inverters in comparison

High-power inverters for traction drives employ in the upper range of stator frequency special pulse control methods, optimized synchronous pulsewidth modulation (PWM), mainly in combination with rotor-flux-oriented control schemes, and direct self-control (DSC) which encompasses motor control and pulse pattern generation. The well-known symmetrized sinusoidal PWM is only to be used in the lower speed range. This paper presents and compares exemplarily the stationary behavior of these two control methods; criteria are inverter peak current, motor harmonic losses, torque ripple, and DC-link harmonics. For these low switching frequencies DSC shows, in spite of its simplicity, a very good overall performance, mainly by avoiding the imperfect use of switching frequency by the synchronous pulse patterns.     

A multidimensional variable band flux modulator for four-phase-leg voltage source inverters

This paper presents a multidimensional flux modulator for closed-loop control of the output voltage of a four-phase-leg inverter. The proposed strategy uses flux regulated hysteresis with a rectangular bounding cube in the synchronous d-q-o reference frame, and a simple set of switching rules to ensure the selection of only the nearest active space vectors to the target reference phasor. In addition, the inherent optimal sequencing and central placement of active vectors between successive s result in the modulator having an improved performance as compared to other reported hysteresis schemes. A further enhancement is achieved by adjusting the dimensions of the bounding cube to restrict the switching process to the nearest three active vectors (and vectors) within a constant switching period. Under this constraint, the modulator achieves a harmonic performance virtually identical to the well-known open-loop space vector modulation. But since the modulator is a closed-loop system, it also exhibits excellent robustness to system parameter variations. The performance and practicability of the modulator have been verified both in simulation and experimentally.     

Predictive current control of voltage-source inverters

A new predictive current controller for a voltage-source inverter is presented in this paper. Practical aspects of realizing the new controller in a system with a digital signal processor (DSP) are considered. Delays introduced by measurements are considered and an improved algorithm with one-period prediction of current is presented. The controller was realized in an experimental system with DSP and field-programmable gate array circuits. Results of the simulations and experiments are presented.     

Robust model predictive current control strategy for three-phase voltage source PWM rectifiers

Traditional model predictive control (MPC) strategy is highly dependent on the model and has poor robustness. To solve the problems, this paper proposes a robust model predictive current control strategy based on a disturbance observer. According to the current predictive model of three-phase voltage source PWM rectifiers (VSR), voltage vectors were selected by minimizing current errors in a fixed time interval. The operating procedure of the MPC scheme and the cause of errors were analysed when errors existed in the model. A disturbance observer was employed to eliminate the disturbance generated by model parameters mismatch via feed-forward compensation, which strengthened the robustness of the control system. To solve the problem caused by filter delay in MPC control, an improved compensation algorithm for the observer was presented. Simulation and experimental results indicate that the proposed robust model predictive current control scheme presents a better dynamic response and has stronger robustness compared with the traditional MPC.     

Measuring the real parameters of single-phase voltage source inverters for UPS systems

The design of a voltage source inverter (VSI) control system requires knowledge about its continuous or discrete model. The output filter inductance and the equivalent serial resistance, which is partly the result of power losses in the core, have values at the operating point that are significantly different from the assigned nominal values, particularly for iron-powder cores. Measurements using standard laboratory equipment are not sufficient because it is impossible to get the voltage and current of the operating point. So the best solution is to measure the Bode plots of the inverter control function at the operating point, to estimate the results of the measurements using the proper analytic function and to calculate the real values of the inverter parameters. Knowledge about the equivalent serial resistance enables the estimation of the inverter power losses. Two approaches to measurements are shown – one that does not require any additional equipment and the other that uses a specialised system to automatically measure the Bode plots. The advantages and disadvantages of both approaches are presented. These measurements enable the validation of the coil core magnetic material by taking into account the power losses. The applications of the measurements in the control design are shown.     

A comprehensive study of neutral-point voltage balancing problem inthree-level neutral-point-clamped voltage source PWM inverters

This paper explores the fundamental limitations of the neutral-point voltage balancing problem for different loading conditions of three-level voltage source inverters. A new model in the DQ coordinate frame utilizing current switching functions is developed as a means to investigate theoretical limitations and to offer a more intuitive insight into the problem. The low-frequency ripple of the neutral point caused by certain loading conditions is reported and quantified