Electrical Engineering ⇒ Topic : Potential Transformers (PT)

Potential Transformers (P.T)

The primary of the PT is connected across the transmission line whose line voltage may range from 2.4 kV to 220 kV. The secondary voltage is standardized at 110 kV. The load connected to the secondary is referred to as burden [see Figure (a)].

                                 Figure (a) Potential transformer

The requirements of a good PT are:

Accurate turns ratio: The difficulty in maintaining accurate turns ratio is due to the resistance and reactance of the windings and the value of the exciting current of the transformer.

Small leakage reactance: The leakage reactance is due to the leakage of magnetic fluxes of the primary and secondary windings. They can be minimized by keeping the primary and secondary windings as close as possible subject to the insulation problem as the primary is at high voltage.

Small magnetic current: This can be achieved by making the reluctance of the core as small as possible and flux density in the core should also be low, less than 1 Wb/m².

Minimum voltage drop: The resistance of the windings is made as small as possible. Since primary carries high voltage, it should be heavily insulated. Hence, it is immersed in oil and the terminals are brought out to porcelain bushing. Nowadays, synthetic rubber insulation like styrene is used to avoid oil and porcelain. The theory of PT is the same as power transformer vectorially.On no load, the secondary voltage is the same as induced voltage. When the load or burden on the secondary is increased (burden is rated as VA), the secondary current increases with corresponding increase in primary current so that the transformation ratio V_p/V_s, remains the same.

There is a ratio error due to increased loading but it is of negligible magnitude.

Phase angle error: As the power factor of the load is reduced from unity, the phase angle error is reduced (lagging). As the power factor of the burden increases, the ratio error increases.

Effect of frequency: If the frequency is reduced, the flux in the core increases thereby increasing the exciting current and reducing the reactance.

Potential Transformer (P.T.)

A potential transformer (P.T.) is used to measure high alternating potential difference (voltage) in a power system. The primary of this transformer has many turns while the secondary has few turns as shown in Fig.(a). It is clear   from the figure that a potential transformer is simply a well-designed step-down transformer.The stepped down voltage is measured with a low-range a.c. voltmeter. The magnetic core of a potential transformer usually has a shell-type construction for better accuracy. In order to provide adequate protection to the operator, one end of the secondary winding is usually grounded.

                                                           figure (a)

The primary of the potential transformer is connected across the high-voltage line whose voltage is to be measured. A low-range (0-110 V) a.c. voltmeter is connected across the secondary. The line voltage (V_p) and a.c. voltmeter voltage (V_s) are related as:

The primary to secondary voltage ratio (i.e., V_p / V_s) is called P. T. ratio (potential transformation ratio).

Thus if the reading of a.c. voltmeter is 50 V and the P.T. ratio is 100 : 1 (or 100/1), then line voltage = 50 x 100 = 5,000 V Similarly, if the a.c. voltmeter reads 100 V, then line voltage = 100 x 100 = 10,000 V.