Voltage Control by Synchronous Condenser:

The voltage at the receiving end of a transmission line can be controlled by installing specially designed synchronous motors called synchronous condensers at the receiving end of the line. The Voltage Control by Synchronous Condenser supplies wattless leading kVA to the line depending upon the excitation of the motor. This wattless leading kVA partly or fully cancels the wattless lagging kVA of the line, thus controlling the voltage drop in the line. In this way, voltage at the receiving end of a transmission line can be kept constant as the load on the, system changes.

For simplicity, consider a short transmission line where the effects of capacitance are neglected. Therefore, the line has only resistance and inductance. Let V₁ and V₂ be the per phase sending end and receiving end voltages respectively. Let I₂ be the load current at a lagging power factor of cos Φ₂.

1.Without synchronous condenser: Fig. 15.12 (0 shows the transmission line with resis­tance R and inductive reactance X per phase. The load current I₂ can be resolved into two rectangular components viz I_p in phase with V₂ and I_q at right angles to V₂ [See Fig. 15.12 (ii)]. Each component will produce resistive and reactive drops ; the resistive drops being in phase with and the reactive drops in quadrature leading with the corresponding currents. The vector addition of these voltage drops to V₂ gives the sending end voltage V₁.

2.With synchronous condenser: Now suppose that a synchronous condenser taking a lead­ing current I_m is connected at the receiving end of the line. The vector diagram of the circuit becomes as shown in Fig. 15.13. Note that since I_m and I_q are in direct opposition and that I_m must be greater than I_q, the four drops due to these two currents simplify to :

From the vector diagram, the relation between V₁ and V₂ is given by

From this equation, the value of I_m can be calculated to obtain any desired ratio of V₁/V₂ for a given load current and power factor.