**1. Introduction**

Power quality describes the quality of voltage and current. It is an important consideration in industries and commercial applications. Power quality problems commonly faced are transients, sags, swells, surges, outages, harmonics and impulses [1]. Among these voltage sags and extended under voltages have large negative impact on industrial productivity, and could be the most important type of power quality variation for many industrial and commercial customers [1-5].

Voltage sags is mainly due to the fault occurring in the transmission and distribution system, loads like welding and operation of building construction equipment, switching of the loaded feeders or equipments. Both momentary and continuous voltage sags are undesirable in complex process controls and household appliances as they use precision electronic and computerized control.

Major problems associated with the unregulated long term voltage sags include equipment failure, overheating and complete shutdown. Tap changing transformers with siliconcontrolled rectifiers (SCR) are usually used as a solution of continuous voltage sags [6]. They require large transformer with many SCRs to control the voltage at the load which lacks the facility of adjusting to momentary changes. Some solutions have been suggested in the past to encounter the problems of voltage sag [7-11]. But these proposals have not been realized practically to replace conventional tap changing transformers.

Now a day's various power semiconductor devices are used to raise power quality levels to meet the requirements [12]. Several AC voltage regulators have been studied as a solution of voltage sags [13-18]. In [13] the input current was not sinusoidal, in [14-16] the efficiency of the regulator was not analyzed and in [17-18] the input power factor was very low and the efficiency is also found poor. Compact and fully electronic voltage regulators are still unavailable practically.

© 2013 Ahmed and Alam, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Dynamic Voltage Restorer (DVR) is sometimes used to regulate the load side voltage [19- 21]. The DVR requires energy storage device to compensate the voltage sags. Flywheels, batteries, superconducting magnetic energy storage (SMES) and super capacitors are generally used as energy storage devices. The rated power operation of DVR depends on the size and capacity of energy storage device which limits its use in high power applications. Whereas, switching regulator needs no energy storage devices, therefore, can be used both in low power and high power applications.

Power Quality Improvement Using Switch Mode Regulator 123

Control circuit

Feed back Control Circuit

example, if a switch has a switching time of 0.5 us, the oscillator period would be 50 us, which gives the maximum oscillation frequency of 20 KHz. This limitation is due to the switching loss in the switching devices. The switching loss of switching devices increases with the switching frequency. In addition, the core loss of inductor limits the high frequency

Input Output Input

Gating Signal Generator

filter Switch

Power circuit

Output

Reference Voltage

Figure 2 illustrates the circuit of a classical linear power converter. Here power is controlled by a series linear element; either a resister or a transistor is used in the linear mode. The total load current passes through the series linear element. In this circuit greater the difference between the input and the output voltage, more is the power loss in the controlling device

The circuit of Fig. 3 illustrates basic principle of a DC-DC switching-mode power converter. The controlling device is a switch. By controlling the duty cycle, (the ratio of the time in on positions to the total time of on and off position of a switch) the power flow to the load can be controlled in a very efficient way. Ideally this method is 100% efficient. In practice, the efficiency is reduced as the switch is non-ideal and losses occur in power circuits. Hence, one of the prime objectives in switch mode power conversion is to realize conversion with the least number of components having better efficiency and reliability. The DC output voltage to the load can be controlled by controlling the duty cycle of the rectangular wave supplied to the base or gate of the switching device. When the switch is on, it has only a small saturation voltage drop across it. In the off condition the current through the switch

The output of the switch mode power conversion circuit (Fig. 3) is not pure DC. This type of output is applicable in some cases such as oven heating without proper filtration. If constant DC is required, then output of converter has to be smoothed out by the addition

(linear element). Linear power conversion is dissipative and hence is inefficient.

**Figure 1.** Block diagram of Switching-mode power supply (SMPS).

filter

operation.

**2.2. DC-DC converter** 

is zero.

of low-pass filter.

The objective of this chapter is to describe the operation and design procedure of a switch mode AC voltage regulator. Firstly, some reviews of the regulators are presented then the procedure of design and analysis of a switch mode regulator is described step by step. Simulation software OrCAD version 9.1 [22] is used to analyze the regulator. The proposed regulator consists mainly two parts, power circuit and control circuit. The power circuit consist two bi-directional switches which serve as the freewheeling path for each other. A signal generating control circuit is to be associated with the power circuit for getting pulses of the switches. In the control circuit, a commercially available pulse width modulator IC chip SG1524B is used, thus circuit is compact and more viable.
