Read the text. When the load at the receiving end draws a current that is out of phase with the line voltage the current lags behind the voltage and.

Power capacitors

When the load at the receiving end draws a current that is out of phase with the line voltage the current lags behind the voltage and., it is said to be out of phase with the line voltage.

It is sometimes necessary to provide additional equipment, called reactive power compensation equipment, to generate or absorb vars.

The capacitors are used when the load demand is heavy. If they are manually switched in groups the control is slow. During maximum system load, when the system voltage is liable to be low the var output of these capacitors is a minimum. This aggravates the problem. The inductor, which is used when the system load is low, can be a coreless type, a gapped-core type, a saturable reactor or a transductor. The coreless type is linear and is used for the larger ratings and voltages (e.g. 100MYA at 400kv). The gapped-core type is designed to be linear up to a voltage of about 1.3 per unit. Both these types will absorb vars proportional to the square of the terminal voltage (for a given reactance).

It is possible to use a tapped saturable reactor which saturates at selected voltages within the range ±10% of rated voltage. When saturated the vars absorbed by the reactor change rapidly for small changes in voltage.

When saturable reactors and capacitors operate in parallel, the characteristic as the voltage rises, is initially similar to that of the capacitor then swings over to that of the saturable rector at a voltage between 10 and 110% of nominal. Thus at low voltage the equipment exports vars while at high voltage it imports vars.

It is appropriate to mention series-capacitors which are connected in series with the line. They generate vars proportional to the square of current in them. Their primary use is to reduce the series impedance of the line, by canceling about 50% of the line's series inductance in order to raise the power limit and to improve the system stability. A problem with series capacitors is that when they earn fault current the voltage across them generally exceeds the voltage rating of the capacitor.

Synchronous compensators are basically synchronous motors with no mechanical output. The starting sequence is usually under automatic control. When running, the control of the compensator is automatic. The excitation can be set to give automatically, either constant terminal voltage or constant vars, subject to an over-excitation limit and an under-excitation limit (to prevent unstable operation). The methods of obtaining and controlling the D.C. excitation supply of the synchronous compensator are similar to those for alternators. The control of a synchronous compensator is fast and continuous from rated vars export to about half rated vars import. But they contribute current to a system fault (raise the fault level), are liable to become unstable and need relatively high maintenance. Loads such as steel mills and arc furnaces, which fluctuate frequently and rapidly, require fast and automatic compensation equipment placed as close as possible to them. In the case of very long overhead lines synchronous compensators are shunt-connected across the line at intervals of about 150 km.