Basic Principle Of System Voltage Control
For proper operations of power systems continuous control of voltage levels are needed. In case of sub-transmission and distribution system voltages are controlled by wing tap changing transformer.
While in case of transmission system, voltages are kept constant with the help of reaction power reserves.
Here we assume that the power system operate between medium to heavy loading conditions while near by EHV transmission lines are out of service and system frequency within tolerable limits.
Case-1: Initiated event of disturbances in transmission system :
Let assume that heavily load EHV transmission line gets tripped off. Due this near by transmission lines gets extra loads leading to large reactive power demand. So voltages at the adjacent load centres gets lowered due to this supplied load reduces resulting in change of system frequency marginally uptill governor control is initiated.
If reactive power availability is less, the charging capacity of lines and shunt capacitors throughout the transmission and distribution network.
By Using tap-changing transformer, distribution voltage level is restored to normal value,but it increases the load MW and MVAR, this will increase the series reactive power loss.
This increase in load should be supplied by using generators connected to the system. Increase in reactive power on supply generator can increases series losses in the line and transformer.
Case-2: Initiated event of disturbance in distribution system :
If capacitor bank trips, load bus voltage drops due to shortage of reactive power leading to the fall of charging capacity of lines and cables.
By using on load tap changer we can bring distribution voltage near normal value while main loads but voltage remain lower. So real power demand increases at load bus.
A real power demand increases, reactive power demand also increases reducing reactive charging capacity. If these counter measures are not taken it would leads to the voltage collapse at load end.
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