This chapter discusses power transmission control and FACTS.

The invention of the high voltage mercury valve half a century ago paved the way for the development of the HVdc transmission technology. By 1954, the first commercial dc link came successfully into operation and was soon followed by several other schemes orders of magnitude larger. The success of the new technology immediately triggered research and development into an alternative solid state valve, which by the mid-60s had already displaced the use of mercury arc valves in new schemes. Substantial progress made in the ratings and reliability of thyristor valves has increased the competitiveness of dc schemes by reducing converter costs and break-even transmission distances.The inclusion of dc transmission in this book seems to be a contradiction in terms, as often HVdc and FACTS are perceived as competing technologies. The problem arises from a rather restrictive interpretation of the word 'transmission', which often implies long distance, whereas a large proportion of the existing dc links are 'zero distance' interconnectors. This chapter has highlighted the basic components of the HVdc transmission technology as well as the main simulation techniques presently used for their integration in power system analysis. Some of the new concepts under consideration for future HVdc links have also been introduced.

Series capacitors have been successfully used for many years in order to enhance the stability and loadability of high-voltage transmission networks. The principle is to compensate the inductive voltage drop in the line by an inserted capacitive voltage or in other words to reduce the effective reactance of the transmission line.

This chapter discusses unified power flow controllers.

This chapter focuses on the steady state analysis and control of systems with FACTS devices with particular reference to the UPFC. A steady-state UPFC model is proposed and its power injection transformation is derived in rectangular form. The optimal multiplier power flow method for ill-conditioned system is applied to implement the UPFC model. Then a novel method for the steady state control of UPFC for power flow control and voltage support is presented. Essentially, it is based on a power injection model and optimal multiplier power flow. The proposed power injection power flow control can be effectively used to derive UPFC control parameters to achieve the required control objectives. A number of internal limits imposed on the UPFC have been considered, including series injection voltage magnitude, line current through the series inverter, real power transfer between the shunt inverter and series inverter, shunt side current and shunt injection voltage magnitude. The proposed constrained control strategies can co-ordinate the available control freedom to achieve an efficient usage of the UPFC when constrained by the internal limits. The comparison of rating and control flexibility of three major FACTS devices is also included in the numerical examples. The proposed UPFC model and power flow control algorithms have been vigorously tested in a number of systems. The results on two practical systems clearly illustrate the effectiveness of the proposed algorithms.

In this chapter the reader has been introduced to the basic theory of electromechanical transients in power systems. The relative generator rotor motion has been described by the swing equation. The problem of transient stability has been explained on the basis of the well known but, especially from the tutorial point of view, still valuable equal-area criteria. According to this criteria, transient stability of a power system is maintained if the accelerating area (which is in proportion to the additional kinetic energy stored in rotating masses during the fault) equals the decelerating area (which is in proportion to the excess of generator electric power over the mechanical power supplied by turbine) during the 1st rotor swing following the fault clearance. It can be concluded that the most effective device is the UPFC. This has been anticipated, since it combines properties of controllable parallel and series compensation as well as properties of controllable phase shifting transformers. However, a comparison has been carried out on the basis of device ratings which does not necessarily match with the economic comparison. In real systems there are many factors influencing transient stability therefore the presented study should be considered as a demonstration the example results of which should not be generalized.

This chapter reviews the status of power electronics technologies including the development of semiconductor devices for power application, the development of high performance SC converters, and research on FACTS equipment for power system enhancement. Remarkable progress in this field will be useful not only for improving the transmission capability of trunk power systems but also to serve for upgrading the distribution systems through the improvement of power quality. Amidst the business environments such as the deregulation of electric power utilities, global environmental restrictions and energy security for electric power resources, electric power utilities in Japan are facing many social and technical problems. The utilization of power electronics technologies is indispensable for reducing the cost of electric power, for enhancement of transmission capability to adapt to free power wheeling, and for preventing wide area black-out due to inadequate coordination of power system planning and operation. It is also indispensable for the improvement of power quality for meeting higher demand as imposed by the expanded use of upgraded information equipment. The authors expect that the new power electronics devices such as the GCT and IEGT will be used for more advanced SC converters, and that the results from R&D joint projects for interconnection reinforcement in Japan will be useful for developing better power systems in the future.