Constant Voltage Transmission merits and demerits

Constant Voltage Transmission:

Voltage in a long distance transmission line drops from sending end to receiving end due to the presence of line losses in the system ( except on no load conditions).  Voltage regulation and power handling capability are two important features of the transmission line for delivering the power.Voltage of the transmission line should remain constant even under varying load conditions to improve the performance of the transmission system. Voltage variation from no load to full load condition should not vary more than +5% to -5% of the normal voltage. When the variation in the voltage is beyond the limit, results in increase in line losses. Power handling capability is the amount of power that can be transmitted in the transmission line, this depends on the amount of active power that is possible to deliver in the transmission line.

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 The maintenance of constant voltage between two ends require reactive power control as the line loading increases the reactive power requirement in the line. For constant voltage transmission, specially designed synchronous motors called synchronous phase modifiers are employed at the receiving end which maintains the voltage along the line constant. When the load changes, voltage drop increases and power factor of the system changes results in the change in the reactive power in the line. Synchronous motors employed will deliver required reactive power to transmit the active power in the line and maintains the constant voltage.

Merits:

• Possibility of carrying increased power for a given conductor size in case of long distance transmission system.
• Improvement of power factor under moderate and heavy load conditions.
• Availability of the steady voltages at all loads at line terminals
• Improvement in the system stability due to inertia effect of synchronous phase modifier.

Demerits:

• Increase in the short circuit currents of the system and therefore increase in the circuit breaker ratings. (During short circuit conditions all the motor and generators in power system deliver short circuit current momentarily)
• Increase of the risk of interruption in supply due to loosing of synchronism of synchronous phase modifiers.