PF Test Equipment

Posted on Feb 6, 2014

A transformer ratio-arm bridge circuit is used as shown in the simplified diagram of Figure1. The circuit consists of a standard reference capacitor (CS) and the insula- tion under test (CX). A special multiwinding transformer is the characteristic feature of the circuit. A voltage is applied to bothCSandCX. The ratio arms, NSandNXare adjusted to balance capacitive current, and the variable resis- tor (RS) is adjusted to

PF Test Equipment
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balance resistive current. The null indicator is used to determine when the bridge circuit is balanced. The values ofNSandNXare used to determine capacitance and the value ofRScorrelates to power (dissipation) factor of the test insulation. Model CB-100 is a low voltage (LV) bridge which is manually balanced for both capacitance and DF, but it is direct reading. The semiautomatic models require manual balancing for capacitance, but they provide a direct digital readout of DF. Model Delta-3000 is automatic balancing for both capacitance and PF, and can also display DF directly (Figure 2). It is simple to operate, providing automatic balancing and digital display for voltage, current, dielectric loss (in watts), capacitance, and PF. Readings are adjusted to equivalent values of 10 or 2. 5 kV The formulas used for calculating the PF or DF are illustrated by using an example (Figure 3) with an insulation of PF = 1. 0%. The value of PF is given by the cosine of the q angle, or the equation can be written as For specific operating instructions for any Megger test set, the reader is advised to follow the instructions given in the appropriate test set operating manual. This section briefly describes the Doble Engineering Company test equip- ment, its theory, and operation for performing PF tests. The description of theory and operation is based on the MEU test set, but the theory and opera- tion of the other test sets is the same...

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