By Compensating Reactive Power Flow (For Reducing Technical Losses)
Q. Explain energy conservation in transmission and distribution system by using compensating reactive power.
An overhead line, when loaded fully, absorbs reactive power given by (I2x) where, I is the line current and X is the reactance of the line in ohms per phase.
- On light loads or at no load the distributed shunt capacitance throughout the line become predominant and the line then supplies charging VAR that is, generates reactive power.
- It is given by (V2ωC ) where, V is the line voltage and w is the system frequency in radians and is the line to earth capacitance.
- Cables generate reactive power. Generation of reactive power by the cables is due to the various cable capacitances.
- The devices that supply reactive power at receiving end bus in transmission system or load bus in distribution systems are called reactive power compensating devices.
- In addition to voltage control the other effects of providing reactive power compensating devices at the load bus are :
(i) Reduction in supply side reactive burden.
ii) Reduction of system copper loss due to reduction of reactive current.
(iii) Decrease in kVA loading of alternators.
(iv) Reduction in investment per kW of load supplied.
Types of Compensators
(A) Shunt Capacitors :
- Small banks of static capacitors are usually connected in star/ delta and used in sub-stations for supplying reactive power to the system.
- It improves the power factor of the load.
- By supplying reactive power to the bus it maintains its voltage profile. In transmission systems, the static capacitor banks are often connected to the tertiary delta winding of the sub-station transformer.
- The reactive power supplied by the capacitor (Qc) is given by
Qc = Vic = V2/Xc = V2ωC VAR/ phase
Xc = Shut
capacitive reactance/ phase
V = Phase voltage
C = Capacitance / phase
- Shunt connected inductors are frequently used at load end of the transmission line to control the over-voltage of receiving end at light load condition which is due to Ferranti effect of lines.
- The inductors are usually coreless type having linear characteristics.
- If X be the reactance per phase of the inductors, with phase voltage V, the reactive power absorbed by induction is given byQL = VIL = – V2/XL =
V2/ωL VAR/ phase - The negative sign indicates reactive power absorption.
(B) Synchronous Compensator (or Synchronous Modifier) :
- It is basically a synchronous motor running at no load. Depending on the value of excitation, it can be generate reactive power.
- However, the losses in the motor are more, wear and tear is high.
- It offers much more flexibility in operation for all load conditions i.e. lagging or leading.
- Hence, its use in high voltage transmission lines helps in either absorption of reactive power or supply of reactive power, under light loads or heavy load conditions respectively.
- Another advantages of this system include improvement in system stability, reduction of the effect of sudden change in load arresting voltage dips and supply of heavy amount of reactive power during short period. Moreover the control is continuous.
- The disadvantages include high cost of installation, contribution of more short-circuit current in the system, more loss and higher maintenance.
(C) Series Capacitors :
- Series capacitors are connected in series with the line conductors and hence reduce the net inductive reactance between the two ends of the line.
- It improves the voltage stable state and supplies reactive power to the receiving end such that voltage profile is maintained.
- In addition to this, due to net decrease of series inductive reactance, the steady state stability improves.
- Power transfer also increases and switched series capacitors can be used in order to improve transient stability.
- However, its use is limited except for EHV systems because of the following reasons :
(i) Severe over voltage during line fault.
(ii) Ferro resonance in series compensated lightly loaded line with transformer at the receiving end.
(iii) Sub-synchronous resonance with the electro-mechanical system of the turbine generator with the causing high torsional stress.
(iv) Economy is affected and its installation is not justified when the load reactive power requirement is small.
(v) Series capacitor compensation may lead to mal-operation of distance relays in the transmission line with the degree of compensation is high.
Figure shows static VAR compensator, in which the inductors and capacitors are switched to the line as per requirements of lagging or leading reactive VA.
Figure shows static VAR compensator, in which the inductors and capacitors are switched to the line as per requirements of lagging or leading reactive VA.
