REACTIVE POWER COMPENSATION IN POWER GRIDS

ABSTRACT

Reactive power compensation in power grids is a problem that needs to be analyzed on several criteria. Reactive power must be constantly monitored in different operating modes of the power grid. Of course, maintaining reactive power at normal values ensures the stability of the energy system. Finding a solution to this problem will allow us to improve voltage modes, minimize power losses, increase line power, improve voltage stability and power factor.

АННОТАЦИЯ

Компенсация реактивной мощности в электрических сетях - проблема, которую необходимо анализировать по нескольким критериям. Реактивную мощность необходимо постоянно контролировать в разных режимах работы электросети. Конечно, поддержание реактивной мощности на нормальных значениях обеспечивает стабильность энергосистемы. Решение этой проблемы позволит нам улучшить режимы напряжения, минимизировать потери мощности, увеличить мощность в сети, улучшить стабильность напряжения и коэффициент мощности.

Keywords: power, reactive power, compensation, voltage, power grid

Ключевые слова: мощность, реактивная мощность, компенсация, напряжение, электросеть.

Almost all consumers of electrical energy not only require active power for operation but also reactive power.

In most cases, this is an inductive reactive power required for creating magnetic fields, which are required in all electrical drives, for example. Like active power, this reactive power must be generated by generators and transmitted to the consumers. Due to the fact that the transmission of reactive power also causes active power losses at the ohmic resistances of the transmission system; the electricity companies try to keep the power factor cosφ of the consumers as high, as possible (i. e. in the vicinity of the value 1). Even a cosφ value less than 0.9 leads to a noticeable rise in the apparent current and, thus, in the active power losses.[1]

There are several ways to compensate for power grids. We will now explore some of them.

Figure 1: Reactive power compensation by means of parallel capacitors and corresponding vector diagram

Parallel compensation:    By means of parallel connection of a capacitance, the power factor of a mixed ohmic – inductive consumer can be improved. The capacitive reactive current of the capacitor compensates the inductive component of the consumer current.

The principle of this parallel compensation, frequently used in practice, is depicted in the following figure for reasons of simplification; the operational capacitance of the line has not been taken into consideration:

The capacitive reactive power  of the capacitor partially or totally compensates the reactive component   of the load current, depending on the value of C. For reasons having to do with tariff scales, complete compensation is not of use to the electricity utilities. Compensation is only carried out to a residual reactive power , at which point a power factor cos φ₂ is just reached, for which no reactive energy need be paid for.

For power values which are uncompensated or partially compensated, the following vector diagrams are valid (P, Q, S = active, reactive, and apparent power of the consumer, cosφ`₂= power factor of the consumer without compensation and the cosφ₂= power factor with compensation):

The compensation power Q_c required for improving the power factor from the angle φ₂ to the angle φ '₂can be deduced from the vector diagram above:[2]

From this the required capacitance of the three individual capacitances of a compensation system in star connection can be deduced :

Figure 2: Power relationships with uncompensated and partially compensated operation

Parallel compensation is dependent on load. In practice, therefore, capacitors are connected or disconnected depending on the load condition. In the determination of the capacitance required in compensation for long overhead lines and cables, half the operational capacitance should taken into consideration if necessary.

If each consumer is individually compensated, then we refer to individual compensation. If a common capacitor system is used for the compensation of a series of consumers - for example, for all drives of an industrial user - then we refer to central or group compensation. Once again, it must be possible to connect and disconnect individual capacitors, in order to adapt the compensation system to varying load conditions.

Series compensation: Another task is fulfilled by the so-called series capacitors. They are used to compensate the inductance of longer lines and thus reduce the voltage drop on the transmission line. However, the transmission losses occurring cannot be reduced using this type of compensation.

To illustrate this principle, it suffices again to study a line without its operating capacitance:

The voltage drop  between  at the beginning of the line and  at the end of the line can be split into a direct-axis component ∆U, and a quadrature component , as shown in Fig. 4; the quadrature component is always much smaller than the quadrature component:[2]

Figure 3: Reactive power compensation using series capacitor and the corresponding vector diagram

The aim of compensation is to achieve a direct-axis voltage drop which is as small as possible. For the capacitance of the series capacitor the following applies when neglecting ∆U_q and under the condition ∆U₁ = 0:[3]

The effect of the series capacitors is independent on the level of the load, but is dependent on its cosφ₂. Because; in the case of consumers of the same type, this does not change very much, a series capacitor, unlike a parallel capacitor, need not be controlled.

The capacitor must be protected against overvoltages, which could appear as a result of high short circuit current caused by a short circuit at the end of the line. For this, elaborate measures – a protective series gap for example - are required, which often make the use of series capacitors uneconomical.

Other types of compensation: A third type of compensation is the possibility of reducing the effect of the operating capacitance of long no load lines, or lines operated with light load, by connecting so-called shunt reactors in parallel in this manner, the danger of a voltage increase by the Ferranti effect is prevented.[3]

Figure 4: division of the voltage drop on a line into a direct-exis and quadrature-axis pompanent

In conclusion, take note that the favorable influencing on transmission losses and reduction in voltage increase of no-load lines can be caused not only by capacitors and reactors (so-called static compensation). For this, synchronous machines are also well suited, which are operated at no-load in the mains and, due to their exciter state, only generate inductive or capacitive reactive power. Since, in this case, the phase angle of the mains can be effected, synchronous machines (normally generators of pumping power stations) are described as rotating phase shifters in this operating mode.

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