09-10-2012, 01:25 PM
AUTOMATIC REACTIVE POWER COMPENSATOR: AN OPEN LOOP APPROACH
AUTOMATIC REACTIVE POWER.pdf (Size: 1.22 MB / Downloads: 70)
ABSTRACT
Over the last few years the sudden increase of the use of non-linear loads such as personal computers and TV sets created a Power Factor (PF) problem. Although such loads consume relatively small amount of power, however the large number of these loads resulted on huge distortion in the power quality. One major element of the power quality is the PF. This thesis proposes an automatic PF correction circuit which can generate a leading as well as lagging reactive current which can be used to improve the PF. The proposed circuit consists of two semiconductor switches and two capacitors and an inductor. Such configuration is capable of generating not only variable reactive current, but also with very low distortion compared to other techniques, which usually generate a high distortion reactive current.
The proposed compensator is controlled by a microprocessor which generates the required switching pulses. The thesis also covers the protection circuits used to protect the semiconductor switches (MOSFETs) during the turn-on and turn-off times.
INTRODUCTION
Problem Overview
In the past, most of the loads were linear loads. A linear load is the load where the voltage across it and the current through follow the same pattern. For example if the voltage is sinusoidal the current is also sinusoidal. Calculations of power, and power factors in such loads were straight forward and most of these loads were fed from sinusoidal voltage and the power factor in these loads is simply cos the angle between the voltage and current. However, over the last 50 years, large non-linear loads such static power converters used in High Voltage DC transmission (HVDC) started to appear and due to the semiconductor devices used in such converters, the linear relationship between the voltage and current is not valid any more. Such large non-linear loads were identifiable and mechanisms for dealing with improving the power factor in such large non-linear loads were fully investigated. The main problem started to appear over the last 20 years where the number of ‘small’ non-linear loads has risen exponentially. For example a typical office building like the Howell building at Brunel University may have less than five PCs in 1985, it is now have more than 500 PCs. Such example is true for most of the buildings in the city. TV sets are another example of such ‘small’ non-linear loads, and it hardly to find a home now without at least two TV sets. This huge increase of ‘small’ non-linear loads comes with a price. The price is a very poor Power Factor (PF) and a much distorted current and voltage waveforms, in another word a very poor power quality. While ‘large’ non-linear loads are identifiable and correction devices can be installed to compensate for a predictable poor power factor, ‘small’ non-linear loads are widely spread and compensation mechanisms which are suitable for ‘large’ non-linear loads cannot be implemented for ‘small’ non-linear loads [2, 3].
Thesis Layout
This thesis is divided into six chapters. This chapter is introduction to the problems of feeding non-linear loads and also it gives a brief idea about compensation as basic solution for improving power factor in power system area.
The second chapter deals with the definition of power factor in linear and non-linear loads. The need for power factor correction was also summarised together with the costing analysis for power factor compensation. A numerical example to illustrate the need for a constantly variable capacitor for a changeable load was introduced. Two well known techniques (MSC and TSC) were briefly summarised. The TSC compensator was analysed in order to see the harmonic contents of such compensator. In some cases, and due to high current harmonics, such compensator will have negative effect on the power factor compensation.
Power Factor Correction
As it can be seen from the previous section, at poor power factor the apparent power is much bigger than the real power. In order to generate amount of power (VA) similar or near similar to the required power (W) the power factor need to be as close as possible to unity. In non linear loads that can be achieved by passive and active filters. In linear loads that can be achieved by capacitors in parallel with the load. In either linear or non-linear loads the cost of poor power factor could be significant [1, 37, 40]. Since utility companies provide both the active and reactive powers to meet the need for industrial consumers, they charge for KVA. That means they charge for poor power factor. The graph shown in Fig. 2.3 illustrates a typical energy charge for a small industrial customer before and after power factor correction.
Thyristor-Switched Capacitor (TSC)
In this technique, as shown in Fig. 2.6, the capacitor is connected in series with either two thyristors back to back or with a Triac. The thyristors (or the Triac) are either in zero or full conduction (α = 0° or α = 180°). The equivalent capacitive reactance in this circuit can be varied in stepwise manner. Although such technique can be effective in some applications, however the triggering of the thyristors in either zero or full conduction can cause the generation of huge amount of current harmonics. So although the displacement factor component in the power factor is improved, the distortion factor (due to the current harmonics) is significantly reduced. Fig. 2.7 illustrates the voltage and current waveforms as well as the frequency spectra of the current waveforms.