A Microcontroller-based Static Var Compensator for Unbalanced Loads

The authors report the development of the laboratory model of a microcontroller-based Static Var Compensator (SVC). A prototype of an automatically controlled voltage balancer using thyristors valves to switch the capacitors and inductors has been investigated. A digital control scheme incorporating a microcontroller is developed. At each sampling time, the loading conditions are measured by a voltage transducer and sent to the SVC controller. Accordingly, the microcontroller, the heart of the system, adjusts the voltage to the desired value. This is achieved by controlling a thyristorised static var compensator through microcontroller software. In this paper, experimental results are given. INTRODUCTION The recent attention to exploring the fast switching compensator for mains voltage support and distortion correction has occured for the following major reasons [1]: A focus on power quality, there is a growing concern to try to reduce the power system pollution which has been caused by ever-increasing number of power electronics loads. Recent advances in power semiconductor technology which have significantly improved the speed-power characteristics of switches. The availibility of powerful single-chip microcontrollers. From the distribution authorities point of view, an important factor, which has a significant impact on power quality, is the higher background harmonic levels and unbalanced load in the power systems caused by nonlinear loads [2]. In power system operation, balanced three-phase transmission and distribution systems are normally assumed to be balanced. However, unbalanced load currents resulting from unbalanced loads are usually encountered in distribution systems. A great number of a single load and three-phase Y-connected loads are connected to radial distribution feeders. Both industry and domestic equipment are sensitive to voltage changes above or below a given magnitude. To high a voltage may lead to insulation failure, damage to components or mal-operation of electrical equipments, to low voltage results in unsatisfactory performance [2][3]. the development of thyristor-valves capable of handling large currents, as well as the technique of using them to switch capacitor in and out and control the current trough a reactor, have provide the power system with a new tool to meet reactive power generation and absorption demands [3]. It can be used to achieve one or more of the following task [4][5]: Regulating the supply voltage within the specified limits about the desired steady state value under normal operating condition. Compensation of the voltage fluctuation caused by the reactive power demand of large and fluctuating industrial load. Compensation of the voltage variation under unbalanced load. Power factor correction and harmonic current compensation. The paper discusses the development of a microcontroller based automatic voltage balancer by using low voltage switched capacitors and reactors for reactive power compensation [6][7]. The compensation value of each phase voltage is measured and sent to the microcontroller board which determines the on/off connection of the capacitor or the conductor by firing the corresponding thyristors in each phase. The paper proposes a real time microcontroller-based voltage profile correction scheme for unbalanced load on distribution systems. SYSTEM DESIGN Since the main cause of the voltage variation and unbalance on an LV distribution system is the variation of loads and their unbalance, in this section, the possible method of voltage control and reducing the phase to an acceptable level based on an SVC is introduced [7][8]. The main goal is that the voltage in each phase can be increased or deceased by injection or absorption of reactive power from the network by switching a capacitor or linear inductor respectively. In fact, at time of an excessively low voltage, so as to bring a low voltage to acceptable level, a certain amount of capacitance should be switched in. Meanwhile, in case of a high voltage, an amount of linear inductor should be switched in. So for each phase one inductor and capacitor (or both of them) have to be allocated. In reality, the voltages of all three phases simultaneously becoming too low or to high do not occur. However, the most cases noticed within the Algerian distribution system are either one phase high, one low and the other within C I R E D 19th International Conference on Electricity Distribution Vienna, 21-24 May 2007