Power Electronic Control in Electrical Systems

Electrical power systems

The major elements of an electrical power system are generators, transformers, transmission and distribution lines, loads and protection and control equipment. These elements are interconnected to be able to allow the generation of electrical energy in the optimal locations and in sufficient quantity to meet the customers' demand, to transmit it to the load centres and to supply high quality electrical energy at competitive prices.

The quality of the electricity supply may be measured in terms of:

1. constant voltage magnitude, e.g. no voltage sags
2. constant frequency
3. constant power factor
4. balanced phases
5. sinusoidal waveforms, e.g. no harmonic content
6. lack of interruptions
7. ability to withstand faults and to recover quickly.

Background

The last quarter of the nineteenth century saw the development of the electricity supply industry as a new, promising and fast-growing activity. Since that time electrical power networks have undergone immense transformations (Hingorani and Gyugyi, 2000; Kundur, 1994). Owing to the relative 'safety' and 'cleanliness' of electricity, it quickly became established as a means of delivering light, heat and motive power. Nowadays it is closely linked to primary activities such as industrial production, transport, communications and agriculture. Population growth, technological innovations and higher capital gains are just a few of the factors that have maintained the momentum of the power industry.

Clearly it has not been easy for the power industry to reach its present status. Throughout its development innumerable technical and economic problems have been overcome, enabling the supply industry to meet the ever increasing demand for energy with electricity at competitive prices. The generator, the incandescent lamp and the industrial motor were the basis for the success of the earliest schemes. Soon the transformer provided a means for improved efficiency of distribution so that generation and transmission of alternating current over considerable distances provided a major source of power in industry and also in domestic applications.

For many decades the trend in electric power production has been towards an interconnected network of transmission lines linking generators and loads into large integrated systems, some of which span entire continents. The main motivation has been to take advantage of load diversity, enabling a better utilization of primary energy resources. It may be argued that interconnection provides an alternative to a limited amount of generation thus enhancing the security of supply (Anderson and Fouad, 1977).

Interconnection was further enhanced, in no small measure, by early breakthroughs in high-current, high-power semiconductor valve technology. Thyristor based high voltage direct current (HVDC) converter installations provided a means for interconnecting power systems with different operating frequencies, e.g. 50/60 Hz, for interconnecting power systems separated by the sea, e.g. the cross-Channel link between England and France, and for interconnecting weak and strong power systems (Hingorani, 1996). The rectifier and inverter may be housed within the same converter station (back-to-back) or they may be located several hundred kilometres apart, for bulk-power, extra-long-distance transmission. The most recent development in HVDC technology is the HVDC system based on solid state voltage source converters (VSCs), which enables independent, fast control of active and reactive powers (McMurray, 1987). This equipment uses insulated gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs) 'valves' and pulse width modulation (PWM) control techniques (Mohan et al., 1995). It should be pointed out that this technology was first developed for applications in industrial drive systems for improved motor speed control. In power transmission applications this technology has been termed HVDC Light (Asplund et al., 1998) to differentiate it from the well-established HVDC links based on thyristors and phase control (Arrillaga, 1999). Throughout this book, the terms HVDC Light and HVDC based on VSCs are used interchangeably.

Based on current and projected installations, a pattern is emerging as to where this equipment will find widespread application: deregulated market applications in primary distribution networks, e.g. the 138 kV link at Eagle Pass, interconnecting the Mexican and Texas networks (Asplund, 2000). The 180 MVA Directlink in Australia, interconnecting the Queensland and New South Wales networks, is another example.

Power electronics technology has affected every aspect of electrical power networks; not just HVDC transmission but also generation, AC transmission, distribution and utilization. At the generation level, thyristor-based automatic voltage regulators (AVRs) have been introduced to enable large synchronous generators to respond quickly and accurately to the demands of interconnected environments. Power system stabilizers (PSSs) have been introduced to prevent power oscillations from building up as a result of sympathetic interactions between generators. For instance, several of the large generators in Scotland are fitted with PSSs to ensure trouble-free operation between the Scottish power system and its larger neighbour, the English power system (Fairnley et al., 1982). Deregulated markets are imposing further demands on generating plant, increasing their wear and tear and the likelihood of generator instabilities of various kinds, e.g. transient, dynamic, sub-synchronous resonance (SSR) and sub-synchronous torsional interactions (SSTI). New power electronic controllers are being developed to help generators operate reliably in the new market place. The thyristor-controlled series compensator (TCSC) is being used to mitigate SSR, SSTI and to damp power systems' oscillations (Larsen et al., 1992). Examples of where TCSCs have been used to mitigate SSR are the TCSCs installed in the 500 kV Boneville Power Administration's Slatt substation and in the 400kV Swedish power network. However, it should be noted that the primary function of the TCSC, like that of its mechanically controlled counterpart, the series capacitor bank, is to reduce the electrical length of the compensated transmission line. The aim is still to increase power transfers significantly, but with increased transient stability margins.

A welcome result of deregulation of the electricity supply industry and open access markets for electricity worldwide, is the opportunity for incorporating all forms of renewable generation into the electrical power network. The signatories of the Kyoto agreement in 1997 set themselves a target to lower emission levels by 20% by 2010. As a result of this, legislation has been enacted and, in many cases, tax incentives have been provided to enable the connection of micro-hydro, wind, photovoltaic, wave, tidal, biomass and fuel cell generators. The power generated by some of these sources of electricity is suitable for direct input, via a step-up transformer, into the AC distribution system. This is the case with micro-hydro and biomass generators. Other sources generate electricity in DC form or in AC form but with large, random variations which prevent direct connection to the grid; for example fuel cells and asynchronous wind generators. In both cases, power electronic converters such as VSCs provide a suitable means for connection to the grid.

In theory, the thyristor-based static var compensator (SVC) (Miller, 1982) could be used to perform the functions of the PSS, while providing fast-acting voltage support at the generating substation. In practice, owing to the effectiveness of the PSS and its relative low cost, this has not happened. Instead, the high speed of response of the SVC and its low maintenance cost have made it the preferred choice to provide reactive power support at key points of the transmission system, far away from the generators. For most practical purposes they have made the rotating synchronous compensator redundant, except where an increase in the short-circuit level is required along with fast-acting reactive power support. Even this niche application of rotating synchronous compensators may soon disappear since a thyristor-controlled series reactor (TCSR) could perform the role of providing adaptive short-circuit compensation and, alongside, an SVC could provide the necessary reactive power support. Another possibility is the displacement of not just the rotating synchronous compensator but also the SVC by a new breed of static compensators (STATCOMs) based on the use of VSCs. The STATCOM provides all the functions that the SVC can provide but at a higher speed and, when the technology reaches full maturity, its cost will be lower. It is more compact and requires only a fraction of the land required by an SVC installation. The VSC is the basic building block of the new generation of power controllers emerging from flexible alternating current transmission systems (FACTS) and Custom Power research (Hingorani and Gyugyi, 2000). In high-voltage transmission, the most promising equipment is: the STATCOM, the unified power flow controller (UPFC) and the HVDC Light. At the low-voltage distribution level, the VSC provides the basis for the distribution STATCOM (D-STATCOM), the dynamic voltage restorer (DVR), the power factor corrector (PFC) and active filters.

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