Reduced Switching Frequency Research Articles

Improved Model Predictive Control Strategy for Modular Multilevel Converter

For reducing switching frequency and decrease computation of the traditional model predictive control (MPC) for modular multilevel converter, an improved MPC strategy is proposed. The control of AC current, circulating current and capacitor voltages are realized synchronously by building multi-objective functions. Combining with capacitor voltage sorting method, MPC calculation is decreased. The switch frequency was lowered by introducing weighting factors. Capacitor voltage difference among the sub-modules is lessened through the allowable value of capacitor voltage deviation between sub-modules. The simulation results verify its correctness.

Cascaded Three Level Inverter Based Shunt Active Power Filter with Modified Three Level Hysteresis Current Control

A Cascaded three level inverter is controlled by using a three level HCC technique. The method has certain advantages compared to a two level HCC, such as a reduced switching frequency and a variation in switching frequency. However, this method suffers from tracking error in current due to dead zones provided between bands. This work aims at the development of modified three level hysteresis current controller (HCC) for a cascaded three level inverter-based shunt active power filter (SAPF) to mitigate the harmonics in a three-phase three-wire systems. Such method significantly reduces the current tracking error, the maximum switching frequency of the inverter as well as the variation in switching frequency. The simulation of the cascaded three level inverter-based shunt active power filter with a modified three level hysteresis current control is carried out in a PSIM environment for 2.5 kVA system. To validate the effect of degree of overlapping between the bands, two cases of a modified three level HCC are proposed. Both the cases are compared with conventional two-level HCC and three level HCC techniques. The simulation results show the effectiveness of the proposed scheme.

Performance and Implementation of Modular Converter Based on Advanced Control Topology

In high power industrial drive use the multi modular converter for high voltage dc transmission system. The reduction of switching frequency, reduce thermal stress and harmonic distortion in the output of the converter by using the modular converter. Synchronous optimal pulse width modulation (SOP) is a developing low gadget exchanging recurrence regulation system for high power converters, which keeps up the nature of converter output current. SOP system has been effectively exhibited for established multilevel topologies and it has been demonstrated that greatest gadget exchanging recurrence can be restricted to evaluated principal recurrence for seven or larger amount inverters without trading off on the nature of yield streams. Be that as it may, execution of SOP procedure for MMC topology is as yet pending. One of the primary difficulties for control of MMC is to keep up drifting capacitor voltages around their ostensible esteem. The objective of our investigation is to propose, break down and actualize improved SOP strategy for MMCs to accomplish low device exchanging recurrence operation, better quality of converter output current and keep up capacitor voltages around their perceived value.

Simplified Space Vector Pulse Width Modulation Based on Switching Schemes with Reduced Switching Frequency and Harmonics for Five Level Cascaded H-Bridge Inverter

This paper presents a simplified control strategy of SVPWM with a three segment switching sequence and 7 segment switch frequency for high power multilevel inverter. In the proposed method, the inverter switching sequences are optimized for minimization of device switching sequence frequency and improvement of harmonic spectrum by using the three most derived switching states and one suitable redundant state for each space vector. The proposed 3-segment sequence is compared with conventional 7-segment sequence similar for five level Cascaded H-Bridge inverter with various values of switching frequencies including very low frequency. The output spectrum of the proposed sequence design shows the reduction of device switching frequency and states current and line voltage. THD this minimizing the filter size requirement of the inverter, employed in industrial applications. Where sinusoidal output voltage is required<em>.</em>

Small-Signal Model of Flyback Converter in Continuous-Conduction Mode With Peak-Current Control at Variable Switching Frequency

Flyback converter is widely used in low-power adaptor applications. To achieve high efficiency for the entire load range of operations, the variable-frequency peak-current-mode (VFPCM) control is often used in the flyback converter to reduce switching frequency at light load. Since there is no small-signal model being developed for the VFPCM control to overcome the unstable problem in a wide range of operations, the present study proposes a novel, accurate small-signal model for VFPCM control operated in continuous-conduction mode. Control behavior and design are analyzed and compared with constant-frequency peak-current-mode control to give engineers insight. A control IC and a flyback converter are built for the experimental validation of the proposed model. Experimental and simulation results verify the proposed model.

Frequency Response Comparison of PI-Based FOC and Cascade-Free MPC using 1 kHz SVM Applied to PMSM drive

Abstract— This paper presents a 1 kHz SVM-FOC (Space Vector Modulation with Field Oriented Control) drive system for a Permanent Magnet Synchronous Motor, using different control strategies. Such strategies are internal model and frequency response designed PI (Proportional and Integral) controllers and a multivariable MPC (Model Predictive Control) controller using a state-space prediction model. This MPC method becomes interesting for improving the closed-loop speed frequency response, since it results in a cascade-free controller. The performance of each controller was evaluated in a qualitative manner through simulations and quantitatively by load torque and speed reference AC sweeps, generating dynamic stiffness curves and Bode diagrams for the utilized techniques. Results show that the MPC approach is useful for enabling fast dynamic responses with the reduced switching frequency, which reduces the drive system cost and improves its efficiency.

An Adaptive Voltage-Balancing Method for High-Power Modular Multilevel Converters

The voltage balancing of the floating capacitors in modular multilevel converter (MMC) is of great importance for the safe operation. However, the existing voltage-balancing algorithms usually suffer from high switching frequency, leading to unnecessary switching losses and device costs. This paper proposes an adaptive capacitor voltage-balancing method for high-power MMCs with reduced switching frequency. The operational characteristics and switching process of the submodules (SMs) are analyzed in detail. And then the relationship between the switching frequency and the capacitor voltage deviation is derived. Based on the analysis, an adaptive method is proposed to make a trade-off between the switching losses and the balancing effect through a closed-loop control of the alternating number of the SMs. The calculation formulas of the switching frequency are also presented. PSCAD/EMTDC simulation and RT-LAB-based real-time control hardware in the loop (HIL) test results demonstrate the effectiveness of the proposed method under different operating conditions.

An improved nearest-level modulation for modular multi-level converters

Voltage balancing and switching frequency reduction are common problems of modular multi-level converters. In this research article, an improved nearest-level modulation strategy, which applies fuzzy control and minimal switching frequency pulse pattern, is proposed to coordinate voltage balancing and switching frequency reduction. The performances of voltage balancing and switching frequency reduction can be regulated by changing the parameters of the fuzzy control and the weighting coefficients of the two parts. The simulations are implemented through a high-voltage direct current system which connects a wind farm. The simulation results validate the practicability of the proposed strategy.

An improved nearest-level modulation for modular multi-level converters

Voltage balancing and switching frequency reduction are common problems of modular multi-level converters. In this research article, an improved nearest-level modulation strategy, which applies fuzzy control and minimal switching frequency pulse pattern, is proposed to coordinate voltage balancing and switching frequency reduction. The performances of voltage balancing and switching frequency reduction can be regulated by changing the parameters of the fuzzy control and the weighting coefficients of the two parts. The simulations are implemented through a high-voltage direct current system which connects a wind farm. The simulation results validate the practicability of the proposed strategy.

Fault-Tolerant Control of Medium-Voltage Modular Multilevel Converters With Minimum Performance Degradation Under Submodule Failures

As the state-of-the-art voltage-source topology, the modular multilevel converter (MMC) is attractive for various medium- or high-power applications. In MMC, reliability and uninterruptable operation are always seen as primary concerns, which requires the MMC can continue operating even though some of the submodules (SMs) are failed. In this paper, a novel fault-tolerant modulation and control strategy is proposed for MMC in medium voltage applications. Compared with previous works, the proposed strategy provides optimum performances under healthy conditions and minimum performance degradation after SM failures. During healthy conditions, the redundant SMs are fully utilized to decrease SM capacitor voltage for higher reliability and to reduce switching frequency for higher efficiency. While during fault-tolerant operation, by appropriate modification of the modulation and controllers, no extra healthy SMs need to be bypassed and the output voltage harmonics of MMC remain unchanged without causing voltage mismatches. Feasibility and effectiveness of the proposed strategy have been verified by simulations in MATLAB/Simulink software and experiments on a downscaled three-phase MMC prototype.

Sliding mode control of DFIG powers in the case of unknown flux and rotor currents with reduced switching frequency

This paper aims to implement a stator power control for a doubly fed induction generator (DFIG) with unknown flux and rotor currents. This goal is achieved based on a high-gain sliding mode control (HG-SMC) and by exploiting the fact that the flux remains near the nominal value. A new implementation scheme has been developed to achieve low switching frequency compared to the conventional method. The advantages of the new scheme over the old one are presented. Some interesting features of the new scheme are: (1) elimination of rotor’s current sensor, which leads to lower cost and failure rate in the drive system; (2) simpler implementation because it does not require any modern method for the flux estimation; (3) longer lifetime of the rotor side-converter. The proposed control scheme is verified by simulations results and experiments on a 7.5 kW DFIG power prototype.

Adaptive Sliding-Mode Position Control for Dielectric Elastomer Actuators

Multilayer stack transducers made from dielectric elastomers (DEs) generate considerable tensile forces and deformations when they are electrically stimulated. Hence, due to their capacitive behavior, they are energy-efficient substitutes, for example, for conventional electromagnetic drives, and enable various completely new applications. Within this contribution, we present the design of a position control for DE stack actuators electrically fed by a bidirectional flyback converter. Due to the unique property of the flyback converter providing an almost constant feeding power for charging and discharging, the sliding-mode control approach is used for the proposed position control. In a first step, a two-point controller is developed and extended afterwards to a three-point controller with hysteresis to significantly reduce the switching frequency. In order to further improve the control behavior and energy efficiency, an adaptation approach for the inner power converter control is carried out that is used to adapt the hysteresis threshold of the three-point controller. Finally, the experimental validations with a prototypic silicone DE stack actuator and bidirectional flyback converter demonstrate that the proposed adaptive three-point controller combines both high dynamics and accuracy with high efficiency due to a significantly reduced switching frequency.

Switching Frequency Dynamic Control for DFIG Wind Turbine Performance Improvement Around Synchronous Speed

In a doubly-fed induction generator (DFIG) wind turbine (WT), large thermal and mechanical oscillations occur around synchronous speed due to the nearly zero-frequency current circulating between rotor windings and rotor-side converter (RSC). A switching frequency dynamic control technique is proposed in this paper to overcome the problem. The basic idea is to dynamically reduce the switching frequency during operation so that both the switching losses and dead-time effect of RSC can be reduced around synchronous speed. According to the characteristic that high-frequency pulse width modulation (PWM) harmonics in DFIG decrease significantly with rotor slip, there would be a new space for switching frequency reduction especially around synchronous speed. The reduced of switching frequency is actually determined by a tradeoff between the increase of both the stator current PWM harmonics and speed PWM ripples and the decrease of insulated-gate bipolar transistor temperature. Compared with the conventional technique with constant switching frequency, the new technique can effectively improve the system performance around synchronous speed including not only better thermal behavior and efficiency of RSC but also smaller current total harmonics and speed total ripples in generator. To clarify the method, first, the effects of switching frequency reduction on a DFIG WT system are investigated. Second, the criteria, control scheme, and procedure of the method are presented. Finally, experimental and simulation studies were carried out, and the results validate its feasibility and effectiveness.

Improved Weighting Factor Selection for Predictive Torque Control of Induction Motor Drive Based on a Simple Additive Weighting Method

A predictive torque control (PTC) technique is emerging as an effective alternative for induction motor drives because of its simple structure, ability to include control parameters directly into the cost function, and fast dynamic response. To implement PTC for induction motor drive, both stator flux and torque errors are predicted for each switching state of the converter and the combined error is determined with the help of cost function using a suitable stator flux weighting factor. However, the selection of suitable weighting factor is a tedious and complex task. In this paper, a simple additive weighting method is introduced in the cost function optimization to simplify the weighting factor selection process. To observe the effectiveness of the proposed control algorithm, both simulation and experimental results are presented for a two-level voltage-source-inverter-fed induction motor drive and compared with conventional PTC under various operating conditions. The proposed control technique achieves low-torque ripple along with reduced switching frequency.

A tuning-less model predictive control for modular multilevel converter capable of unbalanced grid fault

This paper focuses on a tuning-less model predictive control (MPC) strategy with unbalanced fault-ride-through capability for a three-phase modular multilevel converter (MMC). Three individual control stages are designed to cater for the multiple control objectives in MMC without the need for any weighting factor tuning. A conceptually simple power regulation approach which requires neither synchronous coordinate transformation nor grid-voltage phase angle detection is introduced into the proposed MPC. Meanwhile, two alternatives are proposed to the power regulation stage to adapt to the special needs under unbalanced grid fault. Neither of the alternatives involves any tuning work. In addition, a modified sorting algorithm is presented to resolve the problem of unnecessary switching transitions, which is inherent in conventional sorting algorithm. The proposed tuning-less MPC is capable of direct active and reactive power control, circulating current minimization, submodule capacitor voltage balancing, switching frequency reduction, and great resilience under balanced and unbalanced grid conditions. The presented simulation results confirm the effectiveness and feasibility of the proposed control method.

Simplified model predictive control with variable weighting factor for current ripple reduction

Finite control set model predictive control evaluates a predefined cost function for each switching state of power converter. In addition to reference tracking term, cost function can include a term for switching frequency reduction. Small value for the weighting factor of switching reduction term cannot reduce switching frequency effectively and the great value results in reference tracking failure and increase current ripple. In this study a variable weighting factor based on current ripple is obtained and applied to the cost function. The value of weighting factor changes with the position and magnitude of reference voltage. Simulation and experimental results verify the effectiveness of proposed method.

Package-embedded spiral inductor characterization with application to switching buck converters

The design and characterization process of a package-embedded spiral inductor is investigated in this paper via comprehensive electromagnetic simulations. A realistic, multi-layer flip-chip package is considered and modeled in a commercial, full wave electromagnetic simulator. At constant package area, parasitic resistance of a given inductance is minimized. The applicability of the proposed package-embedded spiral inductor to a DC-DC switching buck converter is demonstrated. The flexibility of the package is exploited to obtain a relatively high inductance, enabling a reduced switching frequency. Lower switching frequency minimizes the dynamic loss, thereby increasing the power efficiency. An interleaved multi-phase buck regulator with an array of package-embedded spiral inductors is developed. The extracted layout of the overall circuit is evaluated for performance and primary characteristics. The converter produces an output voltage of 0.9V from 1.2V input voltage while supplying a load current of 1A. Power efficiency of approximately 89% is achieved with 3% output ripple voltage.

Low Switching Frequency-Based Predictive Control of a Grid-Connected Voltage-Sourced Converter

This paper proposes a model predictive direct power control (MPDPC) approach with reduced switching frequency for grid-connected voltage-sourced converters (VSC). This approach is proposed for both a real/reactive power controller (with a fixed input voltage) and a power port controller (with a variable input voltage). It uses two different cost functions: one for the steady state and one for transients. Utilizing these two cost functions leads to having a low switching frequency in the steady state and a fast response during transients. The proposed MPDPC also provides overcurrent protection of the VSC. In applying the proposed MPDPC to a power port, drawbacks of conventional voltage controllers are addressed by employing power estimation and set point modulation in the controller. Simulation and experimental results validate the proposed MPDPC and the voltage controller.

Interleaved LLC Resonant Converter With Hybrid Rectifier and Variable-Frequency Plus Phase-Shift Control for Wide Output Voltage Range Applications

A family of two-phase interleaved LLC (iLLC) resonant converter with hybrid rectifier is proposed for wide output voltage range applications. The primary sides of the two LLC converters are in parallel, and the connection of the secondary windings in the two LLC converters can be regulated by the hybrid rectifier according to the output voltage. Variable frequency control is employed to regulate the output voltage and the secondary windings are in series when the output voltage is high. Fixed-frequency phase-shift control is adopted to regulate the configuration of the secondary windings as well as the output voltage when the output voltage is low. The output voltage range is extended by adaptively changing the configuration of the hybrid rectifier, which results in reduced switching frequency range, circulating current, and conduction losses of the LLC resonant tank. Zero voltage switching and zero current switching are achieved for all the active switches and diodes, respectively, within the entire operation range. The operation principles are analyzed and a 3.5 kW prototype with 400 V input voltage and 150–500 V output voltage is built and tested to evaluate the feasibility of the proposed method.

Integrated model predictive control with reduced switching frequency for modular multilevel converters

The indirect model predictive control (I-MPC) is one of the reduced computational predictive strategies, used to control the modular multilevel converter (MMC). This approach operates at higher switching frequency, which is not desirable for high-power applications. This study proposes an integrated solution for MMC by combining predictive control with the classical energy balancing approach. To implement the predictive algorithm, a detailed three-phase MMC model is presented. The three-phase model includes the zero sequence voltage to reduce the switching frequency of submodules. In addition, the output power quality is enhanced, while operating at reduced switching frequency. The performance of integrated approach is experimentally validated on a laboratory prototype under balanced and unbalanced conditions. In addition, the performance of integrated approach is compared with the existing methodology in terms of output current ripple, switching frequency, computational complexity, and total harmonic distortion.