Power Transformer Capacitance and Dissipation Factor Test

Transformer insulation system

The capacitance and dissipation factor test is an AC low voltage maintenance test and is very similar to the power factor test.

While the transformer preparation is identical to the power factor test procedure, there is no requiremnts to make connection changes once the initial test set connections are made. High-voltage winding and low-voltage winding test set connection changes are made through a selector switch provided on the test set.

Winding capacitance and dissipation factor test values are obtained by balancing a null meter for each variable at every the measured variable selector switch positions.

Capacitance and Dissipation Factor Test Procedure

(Two winding dry-type transformer)

Step 1.

Isolate the equipment, apply working grounds to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals. Disconnected cables should have sufficient clearance from the switchgear terminals greater that the phase spacing distance.

Use nylon rope to hold cable away from incoming and outgoing terminals as required.

Step 2.

Isolate the neutral bushing connection if applicable from the transformer grounding bar.

Step 3.

Short-circuit all high voltage bushing terminals together.

Step 4.

Short-circuit all low voltage bushing terminals and the neutral bushing terminal together.

Step 5.

Connect the capacitance and dissipation factor test set. Refer to Figure 1 above.

Step 6.

Record the capacitance and dissipation factor values once the null meter is balance for both phasing position. Record values for the five test-variable selector switch position.

Power Transformer Testing – Measuring capacitance and power factor or dissipation factor

The condition of the bushings and the overall insulation of power transformers can be investigated by measuring the capacitance and dissipation factor, also known as the tangent delta, or power factor. Aging and decomposition of the insulation, or the ingress of water, increases the losses and thus more energy is turned into heat in the insulation.

The level of this dissipation is expressed by the dissipation factor or power factor.

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Reference: Substation Commissioning Course – Raymond Lee, Technical Trainer