Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. The multilevel voltage source converters typically synthesize the staircase voltage wave from several levels of DC capacitor voltages. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. There are several ways of implementing voltage balance in multilevel converters. Without considering the traditional magnetic coupled converters, this paper presents three recently developed multilevel voltage source converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascaded-inverters with separate DC sources. The operating principle, features, constraints, and potential applications of these converters are discussed.

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Multilevel converters-a new breed of power converters

A new multilevel voltage-source inverter with separate DC sources is proposed for high-voltage, high-power applications, such as flexible AC transmission systems (FACTS) including static VAr generation (SVG), power-line conditioning, series compensation, phase shifting, voltage balancing, fuel cell, and photovoltaic utility systems interfacing, etc. The new M-level inverter consists of (M-1)/2 single-phase full bridges in which each bridge has its own separate DC source. This inverter can generate almost sinusoidal waveform voltage with only one time switching per cycle as the number of levels increases. It can solve the size-and-weight problems of conventional transformer-based multipulse inverters and the component-counts problems of multilevel diode-clamp and flying-capacitor inverters. To demonstrate the superiority of the new inverter, an SVG system using the new inverter topology is discussed through analysis, simulation, and experiment.

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A multilevel voltage-source inverter with separate DC sources for static VAr generation

This paper describes an active approach to series line compensation, in which a synchronous voltage source, implemented by a gate turn-off thyristor (GTO) based voltage-sourced inverter, is used to provide controllable series compensation. This compensator, called static synchronous series compensator (SSSC), can provide controllable compensating voltage over an identical capacitive and inductive range, independently of the magnitude of the line current. It is immune to classical network resonances. In addition to series reactive compensation, with an external DC power supply it can also compensate the voltage drop across the resistive component of the line impedance. The compensation of the real part of the impedance can maintain high X/R ratio even if the line has a very high degree of series compensation. Concurrent and coordinated modulation of reactive and real compensation can greatly increase power oscillation damping. The paper discusses the basic operating and performance characteristics of the SSSC, and compares them to those characterizing the more conventional compensators based on thyristor-switched or controlled series capacitors. It also presents some of the results of TNA simulations carried out with an SSSC hardware model.

Static Synchronous Series Compensator: A Solid-State Approach to the Series Compensation of Transmission Lines

Renewable energy sources such as wind turbine generators and photovoltaics produce fluctuating electric power. The fluctuating power can be compensated by installing an energy storage system in the vicinity of these sources. This paper describes a 6.6-kV battery energy storage system based on a cascade pulsewidth-modulation (PWM) converter with focus on a control method for state-of-charge (SOC) balancing of the battery units. A 200-V, 10-kW, 3.6-kWh (13-MJ) laboratory system combining a cascade PWM converter with nine nickel metal hydride (NiMH) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed balancing control.

State-of-Charge (SOC)-Balancing Control of a Battery Energy Storage System Based on a Cascade PWM Converter

Fast acting static synchronous compensator (STATCOM), a representative of FACTS family, is a promising technology being extensively used as the state-of-the-art dynamic shunt compensator for reactive power control in transmission and distribution system. Over the last couple of decades, researchers and engineers have made path-breaking research on this technology and by virtue of which, many STATCOM controllers based on the self-commutating solid-state voltage-source converter (VSC) have been developed and commercially put in operation to control system dynamics under stressed conditions. Because of its many attributes, STATCOM has emerged as a qualitatively superior controller relative to the line commutating static VAR compensator (SVC). This controller is called with different terminologies as STATic COMpensator advanced static VAR compensator, advanced static VAR generator or static VAR generator, STATic CONdenser, synchronous solid-state VAR compensator, VSC-based SVC or self-commutated SVC or static synchronous compensator (SSC or S2C). The development of STATCOM controller employing various solid-state converter topologies, magnetics configurations, control algorithms, switching techniques and so on, has been well reported in literature with its versatile applications in power system. A review on the state-of-the-art STATCOM technology and further research potential are presented classifying more than 300 research publications.

Static synchronous compensators (STATCOM): a review

The availability of high power gate-turn-off (GTO) thyristors has led to the development of controllable reactive power sources, using electronic switching power converters, for use in power transmission systems This new technology has resulted in equipment that is fundamentally different from the conventional thyristor-controlled static VAr compensator (SVC) The new equipment is called a static condenser (STATCON) because its steady state output characteristics are similar to those of the rotating synchronous condenser The paper describes the fundamental operating principles, functional characteristics and basic control approach of the STATCON, with particular reference to a /spl plusmn/100 MVAr prototype planned to be installed at the Tennessee Valley Authority (TVA) Sullivan substation, USA This installation will be the first demonstration of a STATCON under the EPRI flexible AC transmission systems (FACTS) program, and will be the largest installation of its type in the world >

A. Edris

Papers

Development of a /spl plusmn/100 MVAr static condenser for voltage control of transmission systems