A three-level self-synchronizing hysteresis current regulator with constant switching frequency

Grid connected voltage source inverters require both current regulation and synchronisation to control the power flow and to maintain stability during grid disturbances. For this type of application, non-linear current control of VSIs using techniques such as hysteresis current regulation offer advantages compared to linear current regulation, such as inherent over-current protection, robustness to load/filter parameter variation and very rapid dynamic response. However these techniques suffer from variable switching frequency and their implementation can require significant analog circuitry. Furthermore, sensorless grid synchronisation can improve the inverter's robustness to disturbances such as electrical line interference, sensor failure and rectifier commutation voltage notches. This paper presents a fully digital constant frequency three-level hysteresis current regulator for single phase PV inverters that addresses these issues. The proposed control strategy also uses the average inverter voltage to adjust the hysteresis band to maintain a constant frequency, detect the output voltage polarity and estimate the grid voltage. The result is a robust current regulator with increased flexibility and easy adaptability for grid connected inverters.

A novel hysteresis current controller for active power filter (APF) is suggested which is based on optimal voltage vectors and in the meantime with constant switching frequency. In this controller, the location region of the reference voltage vector is detected quickly by a set of hysteresis comparators through one trial-and-error process. Two appropriate switches are then selected to control the corresponding two line to line currents independently with constant switching frequency. The new method has the advantages of quick response, good current tracking accuracy, and constant switching frequency. Therefore it is very efficient and secure in operation. Computer simulation results show that the new current control method can improve APF performance noticeably.

This paper presents a new hysteresis current regulation strategy for a three-phase three-level flying capacitor multilevel inverter, that uses the zero-crossing of the averaged phase leg output voltages to select the switched output level for each inverter phase leg. Consequently each phase leg requires only one hysteresis comparator to generate its switching signals, which significantly improves its performance compared to multiple band strategies.

The aim of this paper is the implementation of a hysteresis current controller with constant switching frequency as a part of an active filtering system control algorithm, based on the indirect current control. The alternative in the literature to the hysteresis current controller is the PI controller that can give better overall performances and constant switching frequency but with the cost of comprehensive parameter tuning. A compromise can be achieved between the classical current controllers which is the hysteresis current controller with constant switching frequency. This can be obtained by combining the hysteresis comparators with the PWM suboptimal modulator. The obtained performances of the active filtering system based on the indirect current control are compared with the corresponding performances obtained by the same system implemented with the well documented PI current controller. The regulator implementation and performance comparative study were accomplished on a comprehensive active compensator system built in the Matlab Simulink environment.

In this paper, a new control scheme for buck converters was proposed. The buck converter utilizes the dual control loop to improve transient response and has the constant switching frequency. The control scheme is mainly as follows: (a) The switch-ON time is regulated by the constant frequency mechanism. (b) The switch-OFF time is regulated by the output voltage. The spec/features of the proposed converter are listed as: (1) The buck converter has an output of 1.0–2.5 V for the input of 3.0–3.6 V. The load current ranges from 100 mA to 500 mA. (2) The actual current sensor is not required. (3) The simulation results show that the recovery time is less than 1.6 μs during load changes. (4) The variation in switching frequency is smaller than 1.05% over the output range of 1.0–2.5 V. (5) This circuit can be fabricated in future by UMC 0.18 μm 1P6M CMOS processes. This paper depicts the control scheme, theoretical analysis, and implementation.

Compared with the widely used constant switching frequency pulse-width-modulation (PWM) method, variable switching frequency PWM can benefit more because of the extra freedom. Based on the analytical expression of current ripple of three-phase converters, variable switching frequency control methods are proposed to satisfy different ripple requirements. Switching cycle $T_{s}$ is updated in DSP in every interruption period based on the ripple requirement. Two methods are discussed in this paper. The first method is designed to arrange the current ripple peak value within a certain value and can reduce the equivalent switching frequency and electromagnetic interference (EMI) noise; the second method is designed to keep ripple current RMS value constant and reduce the EMI noise. Simulation and experimental results show that variable switching frequency control could improve the performance of EMI and efficiency without impairing the power quality.

The Direct Torque Control (DTC) of induction machine has received wide acceptance in many Variable Speed Drive (VSD) applications due to its simple control structure and excellent torque dynamic control performances. However, the conventional DTC which employs a two-level inverter and hysteresis controllers produces two major drawbacks, namely, larger torque ripple and variable switching frequency, which might produce a very high switching frequency (or power loss), particularly at a very low speed operation. This paper highlight the common methods used to provide the constant switching frequency for DTC drives for Induction Motor that able to minimize the power losses and reduced the torque ripple.

Most of the existing finite-control-set model predictive current control (FCS-MPCC) schemes for multi-phase motor suffer from heavy computational burden, inevitable low-order harmonic currents, and variable switching frequencies. Some virtual voltage vector (V3)-based FCS-MPCC schemes can effectively suppress harmonic current by zeroing the harmonic subspace voltage on average during one sampling period. However, they fail when the motor has a nonsinusoidal back electromotive force. In order to solve the aforementioned issues, this paper proposes a constant switching frequency multiple-vector-based FCS-MPCC scheme. Unlike the traditional FCS-MPCC schemes, the proposed scheme selects optimal V3s and their duty ratios in two orthogonal subspaces. Thus, it can simultaneously track the references in both orthogonal subspaces. In this approach, the optimal V3s and their duty ratios are directly obtained from the principle of deadbeat current control without time-consuming enumeration-based state predictions and cost function calculations. In addition, the obtained optimal V3s and their duty ratios are adopted to rearrange the pulse sequence to obtain constant switching frequency and can be simply synthesized by carrier-based pulsewidth modulation. Furthermore, a discrete time disturbance observer is designed to improve the robustness of the proposed FCS-MPCC against parameter mismatch. Finally, comparative experiments with traditional MPCC schemes for five-phase surface-mounted permanent magnet synchronous machine are carried out to verify the effectiveness of the proposed scheme.

This paper proposes a new direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind turbine system. The required rotor control voltage, which eliminates active and reactive power errors within each fixed time period, is directly calculated based on stator flux, rotor position, and active and reactive powers and their corresponding errors. No extra power or current control loops are required, simplifying the system design, and improving transient performance. Constant converter switching frequency is achieved that eases the design of the power converter and the ac harmonic filter. Rotor voltage limit during transients is investigated, and a scheme is proposed that prioritizes the active and reactive power control such that one remains fully controlled while the error of the other is reduced. The impact of machine parameter variations on system performance is investigated and found negligible. Simulation results for a 2 MW DFIG system demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, machine parameters, and wind speed

In this paper, a torque ripple minimization technique with constant switching frequency is proposed for direct torque control (DTC) of an induction motor. Some drawbacks of conventional DTC are the generation of relatively large torque ripple in a low speed region and a variation of switching frequency according to the amplitudes of torque and flux hysteresis bands. In this proposed strategy, an optimal switching instant during one switching cycle is calculated for torque ripple minimization which is derived from the RMS torque ripple equation based on an instantaneous torque equation. The proposed strategy improves the performance of the DTC by combining a low torque ripple characteristic in the steady state and the conventional fast torque dynamic characteristic. Experimental results prove the feasibility of the proposed strategy compared with the conventional method.

The nonlinear and linear controls proposed by Alou and Soto provide very fast transient response (voltage step from 1 to 1.5 V in 2 μs). This nonlinear control is based on hysteretic control of Cout current. This system is very sensitive to effects, like aging, temperature, input and output voltage variation, etc., that modify the switching frequency. This paper proposes a frequency loop to avoid the frequency variation and to adjust the switching frequency to the nominal value by changing the hysteretic band. A 5-MHz buck converter is developed, and experimental results validate the loop design, obtaining the same fast transient response (from 1.5 to 2.5 V in 2 μs) while keeping switching frequency constant in steady state.

Apart from variable switching frequency, a major issue in direct torque controlled (DTC) induction motor drives is the high torque ripple, which can be minimized by incorporating suitable pulsewidth modulation schemes such as space vector pulsewidth modulation (DTC-SVPWM) and duty ratio control DTC. However, the simplicity and robustness of the DTC scheme are compromised due to the high computational complexity and requirement of a number of motor parameters for the control. In this article, a simple and elegant control algorithm for the realisation of the DTC-SVPWM that is less dependent on motor parameters is proposed. The stator flux-linkage space vector angle needed to generate the reference torque for the DTC operation is computed based on the basic torque equation and the reference stator phase voltage needed for the SVPWM is determined in turn in every sampling period. For the modulation, a carrier-based space vector pulsewidth modulation (CSVPWM) scheme is adopted to get low torque ripple at constant switching frequency. The proposed speed control scheme avoids the use of rotor parameters and rotating reference frame transformations. It requires no sector identification and only one PI controller is needed for the speed control. Though the scheme has comparable torque ripple as that of the conventional DTC-SVPWM scheme, it significantly reduces the computational burden. Hardware results on a three-phase, 2 HP, 415 V induction motor are presented to validate the performance of the proposed control scheme.

A discrete-time average model-based predictive control (DTA-MPC) is proposed for a quasi-Z-source inverter (qZSI). As a single-stage inverter topology, the qZSI regulates the dc-link voltage and the ac output voltage through the shoot-through (ST) duty cycle and the modulation index. Several feedback strategies have been dedicated to produce these two control variables, among which the most popular are the proportional–integral (PI)-based control and the conventional model-predictive control (MPC). However, in the former, there are tradeoffs between fast response and stability; the latter is robust, but at the cost of high calculation burden and variable switching frequency. Moreover, they require an elaborated design or fine tuning of controller parameters. The proposed DTA-MPC predicts future behaviors of the ST duty cycle and modulation signals, based on the established discrete-time average model of the quasi-Z-source (qZS) inductor current, the qZS capacitor voltage, and load currents. The prediction actions are applied to the qZSI modulator in the next sampling instant, without the need of other controller parameters’ design. A constant switching frequency and significantly reduced computations are achieved with high performance. Transient responses and steady-state accuracy of the qZSI system under the proposed DTA-MPC are investigated and compared with the PI-based control and the conventional MPC. Simulation and experimental results verify the effectiveness of the proposed approach for the qZSI.

This paper proposes and develops a hybrid QCM/CCM soft-switching scheme for a single-phase PV inverter with two or multiple SiC MOSFETs in parallel. When implemented in a DC-AC converter, conventional soft-switching modulation like critical conduction mode (CRM) suffers from increased condition loss at high load conditions and variable switching frequency. In the proposed modulation technique, a phase-shift modulation scheme is applied to inductor-decoupled parallel half-bridge legs, generating a controllable circulating current flowing through these commutation inductors. Thus, the current of each half-bridge leg is reshaped as quadrilaterals, which enables all the parallel MOSFETs to achieve zero-voltage switching (ZVS). The proposed switched QCM/CCM soft-switching scheme is featured by partial soft-switching, constant switching frequency, seamless and load-dependent mode transition, and high full-power-range efficiency. A 4.4 kW single-phase PV inverter is developed and tested to verify the advantages of the hybrid QCM/CCM operation in power loss reduction and efficiency improvement.

For multi-level flying capacitor (FC) converter in dc-ac applications, variable switching frequency (VSF) control is an attractive solution to regulate the FC voltage ripple compared with the constant switching frequency (CSF) control. However, it is shown that the existing VSF control presents a higher switching loss in the three-phase operation when the identical switching frequency is applied to three phase-legs to reduce the complexity of VSF control implementation. To reduce the device switching losses in three-phase operation, this letter proposes a new strategy for the switching frequency selection, which allows each phase-leg to select its switching frequency individually. The control complexity of the proposed method is also reduced and is very similar to CSF control. The experiment result is also shown to verify the proposed variable switching frequency control strategy.