Capacitor Voltage Balance Control Method Research Articles

Control method of an AC-AC MMC for open-end stator winding wind turbine generation system

With the increasing of power rating and voltage level of wind turbine, the traditaional low voltage converter is being replaced by multilevel converter. This paper proposes a new ac-ac modular multi-level converter (MMC) connecting the open-end stator winding permanent magnet synchronous generator (OW-PMSG) to AC power grid. The working principle of the proposed topology is analyzed. Under different grid conditions, a unified capacitor voltage balance control method and a current control method are proposed. Aiming at the low grid voltage ride through problem, a distributed braking chopper circuit and the corresponding control strategy is given. Simulation is conducted for verification. Compared with the back to back (BTB) MMC for OW-PMSG, the number of capacitors and arm inductors of the proposed topology are reduced by 43% and 33%. Moreover, the number of the state variables of the average capacitor voltages and currents of the proposed topology are reduced by 33% and 47%, reducing the control complexity. The proposed topology also has higher efficiency than the BTB MMC for OW-PMSG. Compared with the other ac-ac MMC topologies for OW-PMSG, the combined power rating of the power devices of the proposed topology is much lower and thus the proposed topology has higher power density.

A reduced switching frequency capacitor voltage balancing control for modular multilevel converters

Modular multilevel converter (MMC) is one of the most promising topologies in high/medium-voltage applications, but high switching losses impair system economy and reliability. For the high-voltage MMC with the nearest level modulation (NLM), there is usually high frequency and uncertain switching actions among submodules (SMs) in each arm, due to the traditional sorting based capacitor voltage balancing control method, which brings additional power losses. In order to solve this problem, the SM switching process and the capacitor voltage sequence of a MMC arm is analyzed, and then a method to predict the capacitor voltage convergence in the next control cycle is proposed in this paper. Based on the predicted voltage sequence, an improved capacitor voltage balancing method is proposed, which can significantly reduce the additional switching loss and improve the operating efficiency of the converter system. MATLAB/simulink simulation and RT-LAB based HIL test results verify the proposed method under different operating conditions.

Study of Modular Multilevel Converter in AC/DC Hybrid Distribution Network

The AC/DC hybrid power system plays an important role in the intelligent distribution network with the advantages of large power supply radius and capacity, low line cost and convenient distributed access. In this paper, a modular multilevel converter based on AC-DC side of AC/DC hybrid distribution network is exploited. An improved MPC sub-module capacitor voltage balance control method is proposed to reduce the calculation burden of MPC. The cost function is defined for phase currents, circulating currents, and sub-modules capacitor voltages. The weight factors of the cost function are eliminated, and the sub-module capacitor voltage control algorithm is improved to reduce the computational complexity of MPC. Not all cost functions for possible switching states combinations of MMC are calculated using improved method. The effectiveness of proposed improved method is evaluated based on 11 levels MMC system using Matlab/Simulink.

A Novel Submodule Level Fault-Tolerant Approach for MMC With Integrated Scale-Up Architecture

A submodule (SM) level fault-tolerant method for modular multilevel converters (MMCs) with a scalable control design for medium- and high-voltage applications is proposed in this article. A large number of SMs, which is typically required in high-power applications, is divided into sets with a smaller number of SMs. A local controller is assigned to each set for the capacitor voltage balancing (CVB) and switching signal generations, and the SM level protection. A CVB control method is proposed to maintain SM voltages among sets within a prespecified band under SM failures, in which an MMC arm can operate with unequal numbers of SMs among sets. The computational and communication burdens are reduced in each local controller. Most importantly, changing the number of SMs is achieved without significant modifications in the control system, which improves the expandability and flexibility of the MMC system to easily scale-up to higher voltage levels. A ±320 kV/1000 MW MMC system has been studied in a real-time digital simulator (RTDS) and field-programmable gate arrays boards to validate the proposed fault-tolerant control method under SM failures.

An Overmodulation Algorithm With Neutral-Point Voltage Balancing for Three-Level Converters in High-Speed Aerospace Drives

In this article, a virtual space vector based overmodulation algorithm is presented for three-level neutral-point (NP) clamped converters in high-speed aerospace motor drives. With the proposed inscribed polygonal-boundary compression technique, the output voltage capability is enhanced under a lower crossover angle and compression coefficient. As a result, it brings an opportunity for the operation of the studied aircraft electric starter/generator (ESG) systems easily extending from the linear modulation range into the overmodulation region. Furthermore, an active capacitor voltage balancing control method is investigated to recover NP potential imbalance in the case of high modulation index and low power factor operating conditions. To simplify the digital implementation of the algorithm, a fast calculation approach is adopted in this work. The modulation performance of the proposed strategy is verified by both simulation and experimental results with a 45 kW, 32 k r/min ESG prototype system.

DC Capacitor Voltage Balance Control Method for High-Power Single-Phase Cascaded H-Bridge Rectifier to Extend the Regulation Range

Single-phase cascaded H-bridge rectifier (CHBR) plays an important role in high-speed trains. The dc capacitor voltage balance control is essential for a single-phase CHBR. However, existing dc capacitor voltage balance control methods suffer from lacking regulation range. To tackle this issue, a balance control method for dc capacitor voltage to extend the regulation range that is readily implemented is proposed in this article, whose regulation range is the widest. The steady-state operating region (SOR) of a single-phase CHBR is analyzed, and the operation loci of different methods in SOR are derived. According to the geometric relationship of operation loci, a simple method to evaluate the regulation range of different balance methods is introduced. Meanwhile, regulation ranges of different balance methods are compared based on this method, from which it can be known that the proposed dc capacitor voltage balance control method has the widest regulation range. Finally, experimental results verify the effectiveness and superiority of the proposed method.

Logic-Form-Equation-Based Active Capacitor Voltage Balancing Control Technique for Stacked Multicell Converters

This paper proposes a novel active capacitor voltage balancing technique for stacked multicell (SM) converters. The proposed active capacitor voltage balancing control method detects the direction of the load current and measures the voltage of flying capacitors (FCs) to generate a proper switching state in order to form the required output voltage level and regulate the FC voltages at their desired reference levels. The proposed active capacitor voltage balancing control technique can modulate the output voltage through any of the pulse-width-modulation (PWM) techniques such as the phase-shifted carrier PWM or the level-shifted-carrier PWM. The foremost advantage of the proposed active control technique is that it does not demand any complex computations because it is not based on the optimization of the FC voltage-deviation cost functions. A thorough set of simulation results and experimental measurements for the five-level and seven-level SM converters are provided to validate the proposed active capacitor voltage balancing control method.

New Active Capacitor Voltage Balancing Method for Flying Capacitor Multicell Converter Based on Logic-Form-Equations

This paper presents a new active capacitor voltage balancing control method for the flying capacitor multicell (FCM) converters which is fully implemented using logic-form equations. The proposed active capacitor voltage balancing control technique, measures output current and flying capacitor (FC) voltages to generate the switching states in order to produce the required output voltage level and to balance FCs’ voltages at their reference values. Output voltage of the FCM converter controlled with proposed active capacitor voltage balancing method can be modulated with any pulse width modulation (PWM) technique such as the phase-shifted-carrier PWM or level-shifted-carrier PWM. An advantage of the proposed active capacitor voltage balancing control method is its simplicity as it does not require any complex computations and tedious optimization calculations. Simulation results and experimental measurements of a three-cell four-level and four-cell five-level FCM converters are presented to verify the performance of the proposed active capacitor voltage balancing control technique.

Zero-Sequence Voltage Injection for DC Capacitor Voltage Balancing Control of the Star-Connected Cascaded H-Bridge PWM Converter Under Unbalanced Grid

This paper presents a dc capacitor voltage balancing control method for the star-connected cascaded H-bridge pulsewidth modulation converter in the static synchronous compensator applications. With the zero-sequence voltage injection as a basis for dc capacitors voltage balancing, this paper investigates the detailed power flow of the converter as a whole and within individual modules under unbalanced grid voltages and then proposes a method to control the reactive power output and the dc capacitors voltage by precisely managing the power flow. This approach enhances the low-voltage ride-through capability, which is very critical as the penetration of distributed energy resources grows rapidly. The proposed control method is verified by a scaled-down prototype in the laboratory.

Research and Design for Cascade SVG Control System

This paper researchs alternating current decoupling control and DC capacitor voltage balance control for cascade SVG. The DC capacitor voltage balance control method has advantages of good stability and wide regulation range which uses the active voltage vectors of each module. The active voltage vector only change the active power of each module without affecting the reactive power. Experimental results show that the system can accurately compensate the reactive load, and has good performance in the steady state and dynamic adjustment.

A Novel DC Capacitor Voltage Balance Control Method for Cascade Multilevel STATCOM

This paper presents a novel dc capacitor voltage balance control method for cascade multilevel static synchronous compensator (STATCOM) and a general analytical method for balance control strategy. Considering that the imbalance of dc capacitor voltage is caused by the inconsistency of active power absorbed and consumed by chain, a balance control strategy based on active voltage vector superposition is proposed, in which an active voltage component is superposed to chain's output voltage to change its absorbed active power. A general analytical method based on vector analysis is also presented, by which the performance of balance control strategy can be analyzed, including stability and regulation capacity. To find out the most appropriate balance control strategy, a comparison still based on vector analysis among the proposed and other two commonly used methods is provided, from which it can be known that the proposed balance control strategy has the advantage of good stability and strong regulation capacity, and simulations are performed to prove it. The effectiveness of proposed control scheme has been verified by experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype.