Power Quality Enhancement Using Custom Power Devices

Modern day power systems are complicated networks with hundreds of generating stations and load centers being interconnected through power transmission lines. An electric power system has three separate components — power generation, power transmission and power distribution. Electric power is generated by synchronous alternators that are usually driven either by steam or hydro turbines. Almost all power generation takes place at generating stations that may contain more than one such alternator-turbine combinations. Depending upon the type of fuel used for the generation of electric power, the generating stations are categorized as thermal, hydro, nuclear etc. Many of these generating stations are remotely located. Hence the electric power generated at any such station has to be transmitted over a long distance to load centers that are usually cities or towns. Moreover, the modern power system is interconnected, i.e., various generating stations are connected together through transmission lines and switching stations. Electric power is generated at a frequency of either 50 Hz or 60 Hz. In an interconnected ac power system, the rated generation frequency of all units must be the same. For example, in the United States and Canada the generation frequency is 60 Hz, while in countries like United Kingdom, Australia, India the frequency is 50 Hz. In Japan both 50 Hz and 60 Hz systems operate and these systems are interconnected by HVDC links.

Power quality problems are not new in power systems, but the general customers’ awareness of these problems has increased in the recent years. Modern technology such as computers and controls are largely responsible for the rise in the impacts of power quality but can also provide a tailor-made solution to these problems. Often these solutions are expensive, and in many cases, the cost has to be borne by the customer. Thus before the application of a power quality solution, the problem has to be analyzed in details and the cost to benefit ratio must also be calculated. The specifics of the analysis of power quality problems are an important issue and will be covered in this chapter.

The concept of custom power was introduced by N. G. Hingorani [1]. Like flexible ac transmission systems (FACTS) for transmission systems, the term custom power (CP) pertains to the use of power electronic controllers for distribution systems. Just as FACTS improves the reliability and quality of power transmission by simultaneously enhancing both power transfer volume and stability, the custom power enhances the quality and reliability of power that is delivered to customers. Under this scheme a customer receives a prespecified quality power. This prespecified quality may contain a combination of specifications of the following

Apart from the breaking and transferring devices, all other power quality (PQ) enhancement devices like DSTATCOM, DVR, UPQC etc. are based on power converters. Furthermore modern FACTS devices like STATCOM, SSSC, UPFC etc. also employ power converters. However, FACTS devices have much higher power rating than PQ enhancement devices since they are used in bulk power transmission systems. Moreover, their operation philosophy is also different as they are assumed to work under balanced sinusoidal conditions. As a consequence, the control strategies of FACTS devices are different from the PQ enhancement or Custom Power devices. Since power converters have an important role to play in modern power systems, we discuss their topologies and control strategies in this chapter. For the background materials in the area of Power Electronics, there are numerous excellent textbooks, e.g., [1‐3].

The benefits of a solid state transfer switch in a Custom Power Park have been outlined in Chapter 4. In addition to the transfer switch a combination of power semiconductor switches and passive elements is used to construct solid state current limiting, breaking and transferring devices. The devices that are discussed in this chapter are

In this chapter we shall discuss shunt compensation of distribution systems. The primary aims of a shunt compensator in a distribution system are to cancel or suppress

In Chapter 7 we have restricted our discussion mainly on the generation of reference currents. We have assumed that the shunt compensator, which tracks the reference currents, is represented by three ideal current sources. In practice however these current sources are implemented using voltage source inverters. The inverter circuit along with interface transformers/inductors is called a distribution static compensator (DSTATCOM). In our discussions in Chapter 7 we have seen that a DSTATCOM may have to inject a set of three unbalanced currents that may also contain harmonics. Therefore, the VSI associated with a DSTATCOM must be able to inject currents in one phase independent of the other two phases. From this point of view the structure of a DSTATCOM attains significance. A DSTATCOM operating as a current source has been termed as DSTATCOM in current control mode in Chapter 4.

In the previous two chapters we have discussed the shunt compensation of power system loads. It has been shown that shunt compensator can be very effective in balancing any unbalance in the load currents and also for cleaning up harmonic pollution in the load, provided that the supply is balanced. In this way, a shunt compensator protects the utility system from the ill effects of customer loads. In this chapter we shall show that a series compensator is the dual of a shunt compensator — it protects a sensitive load from the distortion in the supply side voltage. The basic principle of a series compensator is simple: by inserting a voltage of required magnitude and frequency, the series compensator can restore the load side voltage to the desired amplitude and waveform even when the source voltage is unbalanced or distorted. Usually a series compensator is used to protect sensitive loads during faults in the supply system.

A unified power quality conditioner (UPQC) is a device that is similar in construction to a unified power flow conditioner (UPFC) [1]. The UPQC, like a UPFC, employs two voltage source inverters (VSIs) that are connected to a common dc energy storage capacitor. One of these two VSIs is connected in series with the ac line while the other is connected in shunt with the same line. A UPFC is employed in a power transmission system to perform shunt and series compensation at the same time. Similarly a UPQC can also perform both the tasks in a power distribution system. However, at this point the similarities in the operating principles of these two devices end. Since a power transmission line generally operates in a balanced, distortion (harmonic) free environment, a UPFC must only provide balanced shunt or series compensation. A power distribution system, on the other hand, may contain unbalance, distortion and even dc components. Therefore a UPQC must operate under this environment while providing shunt or series compensation.

Thus far we have considered point compensation and the correction of the voltage or current at a particular location in the network. This chapter considers the voltage profile of lines with distributed loads and the impact of real or reactive power from one or more injection points. The consideration of real power injection is due to a developing trend for distributed generation (DG) and the need to consider their impacts on the distribution system.

There are many developing aspects to the production and delivery of energy. The increased role for power quality will drive an increasing use of compensating devices either as stand alone or in combination with energy delivery. This chapter will explore some of these emerging aspects in the existing system and speculate on the future shape of the delivery and the role of compensators.