What is Series Compensation, Formula, Explanation


For reactive power control for one of the method is series compensation. Compensation of a transmission line with the help of series capacitor is known as series compensation. For compensation the series reactance of the line the series capacitor is inserted in the line at a specified point. This type of compensation has provided for long transmission lines having voltages 220kv above.

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The capacitor are connected in series with the line for the compensation of inductive reactance of the line. The circuit resistance R and XL are the total series resistance and inductive reactance in ohms. For XC is the ohmic reactance of a series capacitor VS and VR are the sending and receiving end voltages.

A series capacitor in an ac circuit introduces negative or leading reactance current through this negative reactance causes a voltage drop that leads the current by 900. This drop is opposite to that across an inductive reactance. Thus a series capacitor at rated frequency compensates for the drop or part of the drop through the inductive reactance of the feeder.

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Let neglect the effects of line charging current or shunt capacitance and considering group values of the line constants and the phase to neutral voltage between sending end and receiving end can be written as

With compensation

Drop in voltage Δ V = IR cos ΦR + I (XL-XC) sin ΦR

Without compensation

Drop in voltage Δ V = IR cos ΦR + IXL sin ΦR

At condition XL<XC , XL=XC , XL>XC are referred to as overcompensation full compensation and under compensation respectively. over compensation is not used in transmission systems. But application of this type is sometimes found in distribution systems.

To adjust the quantity XC the regulation of the line can be increased or decreased depending on the requirement provided that sin ΦR ≠ 0.

To obtained change of the voltage takes the form of sudden rise at the capacitor terminal. So the load voltage will be larger than receiving end voltage, when no capacitors are installed. Since the voltage rise is dependent on the load current and the power factor to change in IR produces a change in VC and the capacitor automatically act as voltage regulator.


  • Increase in power transmission capacity of the line
  • Improvement in system stability
  • Improved voltage regulation
  • Load division between parallel circuits
  • Damping effect

Capability of a series compensated line

The transmission line the series capacitors can be connected in various ways. Some of these various arrangements and series capacitors can be located in line sections and switched on as part of the line.

The various factors on which transmission line capability depends are as follows:

Thermal limits of conductor

When heat due to current flow exceeds the allowable limit then we say that thermal limit of conductors is reached. It is experienced on short circuits.

Transient and steady state stability

  • Transient stability is the limit on ling lines while steady state stability is the limit on short lines With series compensation the series reactance of line reduces. So the steady state stability criterion may limit the capability of compensated lines
  • Transmission line capability depends on over voltage at the capacitor terminals of series compensated line.


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