Line Compensation – Ideal voltage profile for a transmission line is flat, which can only be achieved by loading the line with its surge impedance loading while this may not be achievable, the characteristics of the line can be modified by line compensators so that
- Ferranti effect is minimized
- Underexcited operation of synchronous generators is not required.
- The power transfer capability of the line is enhanced. Modifying the characteristics of a line(s) is known as line compensation.
Various compensating devices are:
- Capacitors and inductors
- Active voltage source (synchronous generator)
When a number of capacitors are connected in parallel to get the desired capacitance, it is known as a bank of capacitors, similarly a bank of inductors. A bank of capacitors and/or inductors can be adjusted in steps by switching (mechanical).
Capacitors and inductors as such are passive line compensators, while synchronous generator is an active compensator. When solid-state devices are used for switching off capacitors and inductors, this is regarded as active compensation.
Before proceeding to give a detailed account of line compensator, we shall briefly discuss both shunt and series compensation.
Shunt compensation is more or less like load compensation with all the advantages associated with it and discussed in Section 15.3. It needs to be pointed out here that shunt capacitors/inductors can not be distributed uniformally along the line. These are normally connected at the end of the line and/or at midpoint of the line.
Shunt capacitors raise the load pf which greatly increases the power transmitted over the line as it is not required to carry the reactive power. There is a limit to which transmitted power can be increased by shunt compensation as it would require very large size capacitor bank, which would be impractical. For increasing power transmitted over the line other and better means can be adopted. For example, series compensation, higher transmission voltage, HVDC etc.
When switched capacitors are employed for compensation, these should be disconnected immediately under light load conditions to avoid excessive voltage rise and ferroresonance in presence of transformers.
The purpose of series compensation is to cancel part of the series inductive reactance of the line using series capacitors. This helps in (i) increase of maximum power transfer (ii) reduction in power angle for a given amount of power transfer (iii) increased loading. From practical point of view, it is desirable not to exceed series compensation beyond 80%. If the line is 100% compensated, it will behave as a purely resistive element and would cause series resonance even at fundamental frequency. The location of series capacitors is decided by economical factors and severity of fault currents. Series capacitor reduces line reactance thereby level of fault currents.
A detailed discussion on various issues involved in series and shunt compensators now follows.