Voltage Control Methods of Transmission Line And Distribution Line

Voltage Control Methods of Transmission Line And Distribution Line

(a) By Transformer Tap changing :

• Transformer tap changing is the basic and earliest way in various transmission, sub transmission and distribution.
• In high voltage or extra high voltage lines on load tap changing (OLTC) transformers are used while off-load tap changers are used in distribution transformers. It must be notice that transformer does not generate any reactive power actually it consumes some reactive power, it only transfers the reactive power from one side to another by altering the in phase component of the system voltage.



Figure A

• Let the line has two transformers connected one at the sending end with off nominal tap ratio 1 : t_s and another at receiving end with tap-ratio t_r : 1, such that product of t_s and  t_r is unity, when taps are used the actual sending end and receiving end voltages would be  t₂E= (V_S) and the actual receiving end voltage would be t_rV = V_r for nominal sending and receiving end voltages E and V respectively.
• In case of complete line drop compensation, E = V and t_s = √1/[1 – (PR + QX/E²)].
• The above expression shows the tap settings for complete line drop compensation. However, in practice, the compensation is limited for 3.7%
• Beyond that drop, the tap setting requirement is very high and as the transformer does not have any reactive power generation capability hence, for high line drops the tap changing transformers do not improve voltage profile.

(b) Booster Transformer :

• Boosters or regulating transformers are often used when there is inconvenience in having tappings in the main transformers.
• A single phase booster consist of two parts. An exciting transformer connected across the supply along with a series transformer with the supply.
• The output voltage of the series transformer can be added to the input voltage in phase or in reverse phase by changing the position of the switch.
• The output voltage of the regulating transformer can be varied by changing taps of the exciting transformer.
• This transformer is costlier and is not very versatile in use though in some distribution feeder it can be used.

(c) Static VAR System :

Figure B

• Shunt capacitor compensation is required to enhance the system voltage during heavy load condition of transmission line, while reactors are needed to reduce the over voltage of the line at light load conditions.
• A static VAR system (SVS) can perform these two tasks together utilizing the static VAR capacitor.
• The static VAR compensator (SVC) is basically a parallel combination of controlled reactor and fixed static capacitor as shown in Figure B.
• The reactor control is done by an anti-thyristor switch assembly.
• The firing angle of the thyristors governs the voltage across the inductor thus controlling the reactor current. Thus, reactive power absorption by the inductor can be controlled.
• The capacitor in parallel with reactor apples reactive power to the system. Assuming Q_C as the reactive power charging by the capacitor and Q_l the reactive power absorption by the inductor, the net reactive power injection to the bus become Q = Q_C - Q_l 
• In SVC, Q_l is varied and thus Q is controllable. The bus voltage is thus controlled by SVC. During light load period Q_l greater than Q_C while during heavy load condition Q_C greater than Q_l.
• This equipment has got high application in transimission, sub transmission and even in distribution bus voltage control.
• Being a equipment is much more advantageous than the sychronous compensator. Additionally, It improves system stability. voltage stability and reduces power oscillations.