Compensation does not reduce load reactive demand

Reactive power compensation of a transmission line means supplying or absorbing reactive power at suitable points (usually at the receiving end or along the line) so that the line itself does not have to carry it. Its aims are to keep voltages within limits, reduce line current and losses, and raise the power transfer capability.

Methods of compensation

1. Shunt capacitors at the receiving end/substations: supply lagging VAr to inductive loads; raise voltage at heavy load.
2. Shunt reactors: absorb the charging VAr of long EHV lines at light load; control Ferranti rise.
3. Series capacitors: cancel part of the line reactance $X_L$; reduce voltage drop and raise $P_{max} = V_SV_R/X$.
4. Synchronous condensers: over- or under-excited synchronous motors on no load; continuously variable Q.
5. Static VAr compensators (SVC, TCR/TSC) and STATCOM: fast, thyristor/IGBT-controlled variable Q.

Compensation does not reduce the load's own Q demand

The reactive power a load needs is fixed by the load itself: $Q_L = P\tan\phi_L$. A compensator only changes where that Q comes from.

Example: a load of 10 MW at 0.8 pf lagging at 11 kV, fed by a short line with $X = 5\ \Omega$.

• Load demand: $Q_L = 10\tan(36.87^\circ) = 7.5$ MVAr.

The motor/load still receives 7.5 MVAr in both cases. What changes is that 5 MVAr is now produced locally, so the line carries only 2.5 MVAr. Line current falls from 656 A to 541 A, so $I^2R$ loss falls by about 32 % and the drop $IX$ falls.

 Source ===== line =====+------> Load (10 MW, 7.5 MVAr)
         P=10, Q=2.5    |
                       ===  Capacitor supplies 5 MVAr

Hence compensation relieves the line and source of reactive power; it does not reduce the load's reactive power demand.