Variable shunt reactors Reason for the application of a shunt reactor R eduction of transmission losses and load independent voltage stabilisation are decisive factors for optimal network operation in the transmission and distribution network. While in heavily loaded state the lines take reactive power from the network; lightly loaded lines feed reactive power into the network. As a result, the voltage varies along the length of the line. As the loads in the network fluctuate sharply according to time, day and season, the voltages along the line accordingly change very sharply too. The unnecessary exchange of reactive power takes place as a result and causes network losses. On one hand, voltage which is too high can lead to insulation problems and short-circuiting; on the other hand, voltage which is too low causes more network losses which the network operator does not want. For this reason, it is necessary to keep the voltage within a permissible voltage band during network operation. The use of shunt reactors is the technically cost-effective solution for reducing the voltage rise in the lightly loaded lines. Many utilities have recognised the necessity for employing shunt reactors and use them in day-to-day operation. In many cases, a non-variable shunt reactor is used with a fixed inductance. This document aims to present the advantages of variable shunt reactors. Advantages of variable shunt reactors Variable shunt reactors are a compact and cost-effective solution to improving network stability and for continuous voltage regulation in low-load operation or no-load operation of the lines. It is possible to dynamically optimise the current network state by using variable shunt reactors. At the same time, the voltage in the network and the losses occurring are controlled with much greater accuracy. Improved voltage profile The reactive power surplus in the network can be optimally compensated by continuously adjusting the power of Equivalent ciruit of a long line with a shunt reactor at the end and corresponding vector diagram 64 the regulated shunt reactor. Compared with non-regulated shunt reactors, regulated shunt reactors significantly reduce the voltage spikes in the network. It is therefore particularly advisable to use variable shunt reactors in networks with a low short-circuit power, which will significantly improve the voltage quality. Regulated network adjusts voltage quality A regulated shunt reactor adjusts the reactive power fed in by the lightly loaded line thus reducing the voltage along the line and consequently results in greater network reliability. By switching over a non-variable shunt reactor, the voltage is changed significantly and abruptly depending on the installed power. This can lead to potential operational malfunctions. The adapted voltage is adjusted gradually using a variable shunt reactor, resulting in significantly greater system safety. If a variable shunt reactor with OLTC is connected at the end of the line, the inductance of the shunt reactor is changed in steps by an on-load tap- changer; consequently the capacitive line current can be compensated entirely with reactor current. The resulting current is inductive in this case. This control process leads to a lower voltage. This voltage along the line can consequently be kept within specified limit values. Compared to non-variable shunt reactors, the reactive power surplus in the network can be optimally compensated by continuously adjusting the power of the variable shunt reactor. Complying with the permissible voltage band It is often a problem for networks with uneven distributed generating systems (DGS) and sharply fluctuating loads to keep all the node voltages in the network within the permissible band at the same time. This is because some node voltages are close to the upper limit and at the same time a few are close to the ESI AFRICA ISSUE 3 2014