**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 V_{1} and V_{2} be the per phase sending end and receiving end voltages respectively. Let I_{2} be the load current at a lagging power factor of cos Φ_{2}.

**1.Without synchronous condenser:** Fig. 15.12 (0 shows the transmission line with resistance R and inductive reactance X per phase. The load current I_{2} can be resolved into two rectangular components viz I_{p} in phase with V_{2} and I_{q} at right angles to V_{2} [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 V_{2} gives the sending end voltage V_{1}.

**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 V_{1} and V_{2} is given by

From this equation, the value of I_{m} can be calculated to obtain any desired ratio of V_{1}/V_{2} for a given load current and power factor.