Christopher D. Townsend

Optimization of Switching Losses and Capacitor Voltage Ripple Using Model Predictive Control of a Cascaded H-Bridge Multilevel StatCom

This paper further develops a model predictive control (MPC) scheme which is able to exploit the large number of redundant switching states available in a multilevel H-bridge StatCom (H-StatCom). The new sections of the scheme provide optimized methods to tradeoff the harmonic performance with converter switching losses and capacitor voltage ripple. Varying the pulse placement within the modulation scheme and modifying the heuristic model of the voltage balancing characteristics allows the MPC scheme to achieve superior performance to that of the industry standard phase shifted carrier modulation technique. The effects of capacitor voltage ripple on the lifetime of the capacitors are also investigated. It is shown that the MPC scheme can reduce capacitor voltage ripple and increase capacitor lifetime. Simulation and experimental results are presented that confirm the correct operation of the control and modulation strategies.

Multigoal Heuristic Model Predictive Control Technique Applied to a Cascaded H-bridge StatCom

A multilevel H-bridge StatCom inherently contains redundancy in the available switching states. This paper develops a variation on the typical model predictive control scheme which is able to exploit this redundancy to simultaneously balance the H-bridge capacitor voltages, provide excellent current reference tracking, and minimize converter switching losses. The scheme consists of a dead-beat current controller that has been integrated with heuristic models of the voltage balancing and switching loss characteristics. The integration of a pulsewidth modulation scheme is also described. Simulation and experimental results are presented that confirm the correct operation of the control and modulation strategies. Comparison with traditional control and modulation schemes is provided in terms of the key performance indicators associated with multilevel H-bridge StatComs.

Control and modulation scheme for a Cascaded H-Bridge multi-level converter in large scale photovoltaic systems

Multi-level Cascaded H-bridge (CHB) converters are ideal for implementing large scale photovoltaic systems. The improved quality of the voltage waveforms, high efficiency and ability to employ multiple Maximum Power Point Tracking (MPPT) algorithms are just some of the advantages. In this paper a three-phase CHB converter supplied with photovoltaic arrays is considered. A control and modulation structure based on Model Predictive Control (MPC) is described. The scheme inherently controls the DC link voltages while also providing the ability to modify any of those voltages to meet MPPT requirements. This avoids the cost and added complexity of extra DC/DC converters that are typically required to keep the DC link voltages uniform. Simulation and experimental results are presented that confirm the correct operation of the proposed approach.

Impact of Practical Issues on the Harmonic Performance of Phase-Shifted Modulation Strategies for a Cascaded H-Bridge StatCom

Phase-shifted carrier (PSC) modulation has become an industry standard in its application to multilevel H-bridge static compensators (H-StatComs). The technique uses the cancellation of harmonics within each phase leg to significantly improve the harmonic performance relative to the switching frequency. This paper investigates subtle practical implementation issues which deteriorate the harmonic performance of this technique. The effects of nonuniform dc bus voltages and capacitor voltage balancing strategies are investigated. Simulation and experimental results are presented which show that the harmonic performance of the PSC technique deteriorates as the number of voltage levels produced by the H-StatCom increases.

A Review of Power Electronics for Grid Connection of Utility-Scale Battery Energy Storage Systems

The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium-voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.

Phase-Shifted Carrier Modulation Techniques for Cascaded H-Bridge Multilevel Converters

Phase-shifted carrier modulation is an industry standard in its application to multilevel H-bridge converters. The major advantage of this scheme over level-shifted and space vector modulation schemes is its inherent ability to evenly distribute losses between semiconductor devices. However, until now, its implementation has necessitated a capacitor voltage balancing scheme that degrades converter harmonic performance. This paper develops two phase-shifted carrier modulation schemes that avoid harmonic degradation by exploiting an extra degree of freedom in the available switching states. Simulation and experimental results are presented that confirm the correct operation of the modulation strategies.

Cascaded H-Bridge Multilevel PV Topology for Alleviation of Per-Phase Power Imbalances and Reduction of Second Harmonic Voltage Ripple

The cascaded H-bridge (CHB) topology is ideal for implementing large-scale converters for photovoltaic (PV) applications. The improved quality of output voltage waveforms, high efficiency due to transformer-less connection, and ability to employ multiple instances of a maximum power point tracking (MPPT) algorithm are just some advantages. An important disadvantage is the required over-rating to ensure balanced three-phase currents at times of unequal PV generation. Unequal generation occurs due to shading, temperature inhomogeneity, faulty H-bridges, etc. Capacitor voltage balancing under such conditions requires zero-sequence voltage injection which increases the required number of series connected H-bridges. However, leakage current and safety requirements often dictate a need for isolation between PV arrays and the cascaded converter. Therefore, this paper proposes a converter topology that avoids the cost of extra series connected H-bridges by extending the function of dc-dc converters that provide isolation. Second harmonic power oscillations seen in typical cascaded topologies can also be eliminated or reduced through use of the proposed topology. Simulation and experimental results are presented that confirm correct operation of the proposed approach.

Comparison of modulation strategies for a cascaded H-bridge StatCom — Part 1: Theoretical background

Phase shifted carrier modulation has become an industry standard in its application to multi-level H-bridge Stat-Coms. The technique uses the cancellation of harmonics within each phase-leg to significantly improve the harmonic performance relative to the switching frequency. The purpose of this paper is to investigate whether the practical implementation of the technique deteriorates the harmonic cancellation. The effects of non-uniform DC bus voltage, transient capacitor voltages and phase shift error are investigated. A comparison with alternative modulation strategies is developed in terms of a defined set of performance indicators. Part 2 of this paper applies the developed analysis to a particular StatCom and investigates the effect of increasing the StatCom level number.

Low-Capacitance Cascaded H-Bridge Multilevel StatCom

This paper introduces a cascaded H-bridge multilevel converter (CHB-MC)-based StatCom system that is able to operate with extremely low dc capacitance values. The theoretical limit is calculated for the maximum capacitor voltage ripple, and hence minimum dc capacitance values that can be used in the converter. The proposed low-capacitance StatCom (LC-StatCom) is able to operate with large capacitor voltage ripples, which are very close to the calculated theoretical maximum voltage ripple. The maximum voltage stress on the semiconductors in the LC -StatCom is lower than in a conventional StatCom system. The variable cluster voltage magnitude in the LC-StatCom system drops well below the maximum grid voltage, which allows a fixed maximum voltage on the individual capacitors. It is demonstrated that the proposed LC-StatCom has an asymmetric V-I characteristic, which is especially suited for operation as a reactive power source within the capacitive region. A high-bandwidth control system is designed for the proposed StatCom to provide control of the capacitor voltages during highly dynamic transient events. The proposed LC-StatCom system is experimentally verified on a low-voltage seven-level CHB-MC prototype. The experimental results show successful operation of the system with ripples as high as 90% of the nominal dc voltage. The required energy storage for the LC-StatCom system shows significant reduction compared to a conventional StatCom design.

Model Predictive Control of a cascaded H-bridge multi-level StatCom

A multi-level H-bridge StatCom inherently contains redundancy in the available switching states. This paper develops a Model Predictive Control (MPC) scheme which is able to exploit this redundancy to optimise three competing objectives. The scheme can simultaneously balance the H-bridge capacitor voltages, provide excellent current reference tracking and optimise converter switching losses. A variation in modulation scheme, consisting of a hybrid of traditional MPC and PWM strategies, is also described. Simulation and experimental results are presented that confirm the correct operation of the control and modulation strategies. Comparison with traditional control and modulation schemes is provided in terms of the key performance indicators associated with multi-level H-bridge StatComs.