Compensation in Power Transmission Lines

Parallel Compensation in Power Transmission Lines.

Parallel compensation is the standard means of reactive power compensation in the MV and LV systems.

 The aim is to achieve a defined power factor of the load, limitation of the supplied reactive energy and a decrease of the voltage drop. Paral­lel compensation within the high-voltage range uses the same principle to increase the transferable power through lines. The system diagram of Figure (a) is employed. The loss less compensation (capacitor) is placed in the middle of the line. The voltage at the capacitor U_M is equal the voltages at the sending and receiv­ing end of the line, U_A and U_E, if the maximal power is transmitted.

The voltages between the middle and the sending and receiving ends of the line are given by Equation (a1):

with the current I_A according to Equation (a2):

Thus the active power P_K transferable over the line and the reactive power of the line Q_k are calculated according to the following equation,

The figure shown below outlines the dependence of the active and reactive power on the phase-angle.

A parallel compensation in the middle of the line increases the transferable power to up to twice the value of the uncompensated line. Hie reactive power requirement is up to four times in this case and must be made available by the compensation plant and the connected power systems.

With compensation at the sending or receiving end of the line the conditions are worse because the compensation equipment does not here lead to an increase of the transferable power, assuming the same voltages at the line ends of the compensated and the uncompensated case. Compensation at one or both line ends leads to an increase of the transferable power only if higher voltages are achieved.

Serial Compensation of Power Transmission Lines

Serial compensation of lines is realized by reducing the line reactance by serial compensation, for example, a serial capacitor.  The diagram of Figure   (a)   is employed. The line is assumed to be divided into two equal segments. Thus the voltage drop at the longitudinal reactance of the line is compensated with rising current partly by a voltage increase by the serial compensation, so that the total voltage drop of the line becomes smaller than without serial compensation.    The current of the line is calculated according to the following equation,

with the compensation degree k of the compensated line is given by,

Thus the transferable active power PR   and the reactive power QC   produced by the serial compensation are given by,

The active and reactive power depend on the phase - angle of the line as shown in the figure below,

A serial compensated line leads to an increase of the transferable power, but the increase is inversely proportional to the degree of compensation.  The reactive power of the serial compensation plant must be increased to increase the transfer-able active power.