Methods of Voltage Control: (b) Tap-Changing Transformers

The basic operation of the tap-changing transformer has been discussed in Chapter 3. By changing the transformation ratio, the voltage in the secondary circuit is varied. Hence voltage and reactive power control is obtained.

In distribution circuits, tap-changing transformers are the primary method of voltage control. In a distribution transformer, the tap-changer compensates for the voltage drop across the reactance of the transformer but also for the variations in the voltage applied to the primary winding caused by changes of load within the high voltage network. In transmission circuits reactive power is dispatched by altering the taps of transformers and this, in turn, controls the network voltages.

Use of Tap-Changing Transformers to Control Voltage in a Distribution System

Consider the 40MVA 132/11 kV transformer with a reactance of 13% on its rating shown in Figure 5.11. It is equipped with an on-load tap-changer that is used to maintain constant voltage at an 11 kV busbar by compensating for variations in the voltage of the 132 kV network and for the voltage drop across the transformer. The variation of network voltage at the 132 kV transformer busbar for heavy and light loading conditions, and the loads of the transformer are given in Table 5.1. Active power losses in the transformer are ignored and it is assumed that the value of the reactance of the transformer is not influenced by the change in the turns ratio.

Tap changing transformer in a distribution circuit

Figure 5.11 Tap changing transformer in a distribution circuit

t is the fraction of the nominal transformation ratios, that is the tap ratio/nominal ratio. For example, a transformer of nominal ratio 132 to 11 kV when tapped to give 144 to 11 kV has a t of 144/132 = 1.09.

Choosing SBAsE of 40 MVA and VBAsEs of 132 kV and 11 kV.

Under heavy loading conditions,

Vr = 11 kV, 1 p.u.

Vs = 120kV, 0.909 p.u.

Q = 13.94 MVAr, 0.3485 p.u.

Under light loading conditions,

Vr = 11 kV, 1 p.u.

Vs = 145kV, 1.1 p.u.

Q = 2.11MVAr, 0.053 p.u.

Table 5.1 Loading of transformer




Desired Voltage


at 132 kV

at 11 kV

of Load

Busbar Vs

Busbar Vr

Heavy loading conditions

32 MVA


120 kV

11 kV

Light loading conditions



145 kV

11 kV

A radial distribution circuit with two tap-changing transformers, is shown in the equivalent single-phase circuit of Figure 5.12. V1 and V2 are the nominal voltages; at

(a) Coordination of two tap-changing transformers in a radial transmission link

Figure 5.12 (a) Coordination of two tap-changing transformers in a radial transmission link (b) and (c) Equivalent circuits for dealing with off-nominal tap ratio, (b) Single transformer, (c) Two transformers the ends of the circuit the actual voltages are tsVi and trV2. It is required to determine the tap-changing ratios needed to compensate completely for the voltage drop in the line. The product tstr will be made unity; this ensures that the overall voltage level remains in the same order and that the minimum range of taps on both transformers is used.

(Note that all values are in per unit; t is the off-nominal tap ratio.)

Transfer all quantities to the load circuit.

The line impedance becomes (R + jX)/tf; Vs = Vj ts and, as the impedance has been transferred Vr = V]ts. The input voltage to the load circuit becomes V s/tr and the equivalent circuit is as shown in Figure 5.10(c). The arithmetic voltage drop


If ts is specified then tr is defined. There are then two values of V2 for a given V:, one low current, high voltage and one high current and low voltage. Only the high voltage, low current solution is useful in a power system.

Example 5.3

A 132 kV line is fed through an 11/132 kV transformer from a constant 11 kV supply. At the load end of the line the voltage is reduced by another transformer of nominal ratio 132/11kV. The total impedance of the line and transformers at 132kV is (25 + /66) V. Both transformers are equipped with tap-changing facilities which are arranged so that the product of the two off-nominal settings is unity. If the load on the system is 100 MW at 0.9 p.f. lagging, calculate the settings of the tap-changers required to maintain the voltage of the load busbar at 11 kV. Use a base of 100 MVA.


The line diagram is shown in Figure 5.13. As the line voltage drop is to be completely compensated, V1 = V2 = 132kV = 1 p.u. Also, ts x tr = 1. The load is 100MW, 48.3 MVAr., that is, 1 + /0.483 p.u.

Using equation (5.7) Schematic diagram of system for Example 5.3

Figure 5.13 Schematic diagram of system for Example 5.3


These settings are large for the normal range of tap-changing transformers (usually not more than ±20% tap range). It would be necessary, in this system, to inject VArs at the load end of the line to maintain the voltage at the required value.

A transformer at the receiving end of a line does not improve the VAr flow in the circuit and the current in the supplying line is increased if the ratio is reduced. In countries with long and inadequate distribution circuits, it is often the practice to boost the received voltage by a variable ratio transformer so as to maintain rated voltage as the power required increases. Unfortunately, this has the effect of increasing the primary supply circuit current by the transformer ratio, thereby decreasing the primary voltage still further until voltage collapse occurs.

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