Control of Reactive Power and Voltage

Reactive Power Control

In steady state operation both active power balance and reactive power balance must be maintained.    The reactive power generated by synchronous machines and capacitances must be equal to the reactive power of the loads plus the reactive transmission losses. If the active power balance is not kept, the frequency in the system will be influenced, while an imbalance in reactive power will result in that the voltages in the system differ from the desired ones. If the power system is operated in the correct way, the voltage drops on the lines are usually small. The voltages in the nodes of the system will then almost be the same (flat voltage profile). In this case the transmission system is effectively used, i.e. primarily for transmission of active power, and not for transmission of reactive power. As known from the Static Analysis the voltage in a system is strongly affected by the reactive power flow. Consequently the voltage can be controlled to desired values, by control of the reactive power. Increased production of reactive power gives higher voltage nearby the production source, while an increased consumption of reactive power gives lower voltage. Therefore it is of great interest to study which components and devices which can be used to regulate the reactive power in a power system. While the active power is entirely produced in the generators of the system, there are several sources of reactive power. In the other hand the reactive power cannot be transported over long distances in the system, since normally  X ≫ R in a power system.

Important producers of reactive power are:

• Overexcited synchronous machines

• Capacitor banks

• The capacitance of overhead lines and cables

Important consumers of reactive power are:

• Inductive static loads

• Under-excited synchronous machines

• Induction motors

• Shunt reactors

• The inductance of overhead lines and cables

• Transformer inductances

• Line commutated static converters For some of these the reactive power is easy to control, while for others it is practically impossible.

 The reactive power of the synchronous machines is easily controlled by means of the excitation. Switching of shunt capacitors and reactors can also control the reactive power. If thyristors are used to switch capacitors and/or thyristors are used to control the current through shunt reactors, a fast and step-less control of the reactive power can be obtained. Such a device is called SVC (Static Var Compensator). As has been shown earlier it is most effective to compensate the reactive power as close as possible to the reactive load. There are certain high voltage tariffs to encourage large consumers, e.g. industries, and electrical distributions companies to compensate their loads in an effective way. These tariffs are generally designed so that the reactive power is only allowed to reach a certain percentage of the active power. If this percentage is exceeded, the consumer has to pay for the reactive power. The high voltage network is in that way primarily used for transmission of active power. The reactive losses of power lines and transformers depend on the size of the reactance. In overhead-transmission lines the reactance can be slightly reduced by the use of multiple conductors. The only possibility to radically reduce the total reactance of a transmission line is to connect a series capacitor, see 6.5.1 in the part ”Static Analysis”.

Voltage Control

The following factors influence primarily the voltages in a power system:

• Terminal voltages of synchronous machines

• Impedances of lines

• Transmitted reactive power

• Turns ratio of transformersoverhead-transmission lines the reactance can be slightly reduced by the use of multiple conductors.

The only possibility to radically reduce the total reactance of a transmission line is to connect a series

A suitable use of these leads to the desired voltage profile. The generators are often operated at constant voltage, by using an automatic voltage regulator (AVR). The output from this controls the excitation of the machine via the electric field exciter so that the voltage is equal to the set value, see Figure 14.3. The voltage drop caused by the generator transformer is sometimes compensated totally or partly for, and the voltage can consequently be kept constant on the high voltage side of the transformer. Synchronous compensators are installed for voltage control. These are synchronous machines without turbine or mechanical load, which can produce and consume reactive power by controlling the excitation. Nowadays new installations of synchronous compensators are very rare. The impact of the impedances of the lines on the reactive power balance, and thereby the voltage, have been analysed in the Static Analysis. These are generally not used for control of the reactive power. Series capacitors are generally installed to increase the active transmission capacity of a line. From the static analysis it is also known that the reactive power transmitted has a great impact on the voltage profile. Large reactive transmissions cause large voltage drops, thus these should be avoided. Instead, the production of reactive power should be as close as possible to the reactive loads. This can be achieved by the excitation of the synchronous machines, which have been described above. However, there are often no synchronous machines close to the load, so the most cost-effective way is to use shunt capacitors which are switched according to the load variations. An SVC can be economically motivated if fast response or accuracy in the regulation is required. Shunt reactors must sometimes be installed to limit the voltages to reasonable levels. In networks which contain a lot of cables this is also necessary, since the reactive generation from these is much larger than from overhead lines. (C is larger and X is smaller.) An important method for controlling the voltage in power systems is by changing the turns ratio of a transformer. Certain transformers are

equipped with a number of taps on one of the windings. Voltage control can be obtained by switching between these taps, see Figure 14.5. If switching during operation can be made by means of tap changers, this possibility of voltage control is very effective and useful. Normally the taps are placed on the high voltage winding, the upper side, since then the lowest current needs to be switched. If N1 is the number of turns on the upper side and N2 is the number of turns on the lower side, the turns ratio of transformer is defined as

If the voltage decreases on the high voltage side, the voltage on the lower side can be kept constant by decreasing τ , i.e. by switching off a number of windings on the high voltage side. When the transformer is loaded eq. (14.5) is of course not correct, since the load current gives a voltage drop over the leakage reactance of the transformer, zk, but the same principle can still be applied at voltage control. Transformers with automatic tap changer control are often used for voltage control in distribution networks. The voltage at the consumers can therefore be kept fairly constant even though voltage variations occur at the high voltage network. Time constants in these regulators are typically some ten seconds. Sometimes the turns ratio cannot be changed during operation, but just manually when the transformer is off load. In this case one can only change the voltage level in large but not control the voltage variations in the network.