Skip to main content

TRADITIONAL SHUNT COMPENSATION

In general, shunt compensators are classified depending on the technology used in their implementation. Rotating and static equipments were commonly used to compensate reactive power and to stabilize power systems. In the last decades, a large number of different static FACTS controllers, using power electronic technologies and digital control schemes have been proposed and developed. There are two approaches to the realization of power electronics-based compensators: the one that employs thyristor-switched capacitors and reactors with tap-changing transformers, and the other group that uses self-commutated static converters. A brief description of the most commonly used shunt compensators is presented below.

  • FIXED OR MECHANICALLY SWITCHED CAPACITORS

Shunt capacitors were first employed for power factor correction in the year 1914. The leading current drawn by the shunt capacitors compensates the lagging current drawn by the load. The selection of shunt capacitors depends on many factors, the most important of which is the amount of lagging reactive power taken by the load. In the case of widely fluctuating loads, the reactive power also varies over a wide range. Thus, a fixed capacitor bank may often lead to either over-compensation or under compensation.

Variable VAR compensation is achieved using switched capacitors. Depending on the total VAR requirement, capacitor banks are switched into or switched out of the system. The smoothness of control is solely dependent on the number of capacitors switching units used. The switching is usually accomplished using relays and circuit breakers. However, these methods based on mechanical switches and relays have the disadvantage of being sluggish and unreliable. Also, they generate high inrush currents, and require frequent maintenance.

  • SYNCHRONOUS CONDENSERS

Synchronous condensers have played a major role in voltage and reactive power control for more than 50 years. Functionally, a synchronous condenser is simply a synchronous machine connected to the power system. After the unit is synchronized, the field current is adjusted to either generate or absorb reactive power as required by the ac system. The machine can provide continuous reactive power control when used with the proper automatic exciter circuit. Synchronous condensers have been used at both distribution and transmission voltage levels to improve stability and to maintain voltages within desired limits under varying load conditions and contingency situations. However, synchronous condensers are rarely used today because they require substantial foundations and a significant amount of starting and protective equipment. They also contribute to the short circuit current and they cannot be controlled fast enough to compensate rapid load changes. Moreover, their losses are much higher than those associated with static compensators, and the cost is much higher as well. Their advantage lies in their high temporary overload capability.

  • THYRISTORIZED VAR COMPENSATORS

As in the case of the synchronous condenser, the aim of achieving fine control over the entire VAR range, has been fulfilled with the development of static compensators but with the advantage of faster response times. Thyristorized VAR compensators consist of standard reactive power shunt elements (reactors and capacitors) which are controlled to provide rapid and variable reactive power. They can be grouped into two basic categories, the thyristor-switched capacitor (TSC) and the thyristor-controlled reactor (TCR).

Comments

Popular posts from this blog

ADVANTAGES AND DISADVANTAGES OF CORONA

Corona has many advantages and disadvantages. In the correct design of a high voltage overhead line, a balance should be struck between the advantages and disadvantages. ADVANTAGES (i) Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors. (ii) Corona reduces the effects of transients produced by surges. DIS-ADVANTAGES (i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line. (ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. (iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.

ADVANTAGES OF PER UNIT SYSTEM

PER UNIT SYSTEM The per-unit system expressed the voltages, currents, powers, impedances, and other electrical quantities basis by the equation: Quantity per unit (pu) = Actual value/ Base value of quantity ADVANTAGES OF PER UNIT SYSTEM While performing calculations, referring quantities from one side of the transformer to the other side serious errors may be committed. This can be avoided by using per unit system. Voltages, currents and impedances expressed in per unit do not change when they are referred from one side of transformer to the other side. This is a great advantage. Per unit impedances of electrical equipment of similar type usually lie within a narrow range, when the equipment ratings are used as base values. Transformer connections do not affect the per unit values. Manufacturers usually specify the impedances of machines and transformers in per unit or percent of name plate ratings. Transformers can be replaced by their equivalent series impedances. ...

ABSOLUTE AND SECONDARY INSTRUMENTS

The various electrical instruments may, in a very broad sense, be divided into (i) Absolute Instruments (ii) Secondary Instruments. Absolute Instruments are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is necessary in their case. The example of such an instrument is tangent galvanometer, which gives the value of current, in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used and the horizontal component of earth’s field.  Secondary Instruments  are those, in which the value of electrical quantity to be measured can be determined from the deflection of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments, which are most generally used in ev...