Page 60 - Energize May 2022

TECHNICAL

What is melting here? Time for an

SFRA test

This case study was generously supplied by the Hitachi Energy Service group in Sweden

INFORMATION FROM MEGGER

22 MVA, 10,5 kV/276 V (voltage ratio, winding resistance, insulation resistance, and sweep frequency response analysis (SFRA).
regulation from 336 to 216 V), Diii Additionally, oil samples were taken for laboratory analysis of dissolved gases in oil (DGA).
A furnace transformer, operating Transformer turns ratio tests were performed with LV excitation from the HV side. A
in heavy use for many years at a steel winding connection Dd0 was used for measurement. Transformer turns ratio tests were
melting plant in Europe was subjected to performed on all taps and acceptable values were obtained for two phases. The test
a complete series of tests after a sudden instrument was unable to perform ratio measurements on phase C. This may have been
failure during operation. The service attributed to a state where the impedance was so low that the test instrument was unable to
contractor evaluated the electrical and build up the excitation voltage configured for the test.
dielectric condition of this transformer. The transformer turns ratio results are presented in Table 1. Winding resistance tests
During service, this type of transformer were then performed on the transformer. Table 2 presents the HV-side winding resistance
has its secondary winding connected measurements performed in the factory (RFAT average) and the field.
externally in a Delta configuration through
a high current bridge. To clarify the vector
designation, a simple diagram showing its
terminals is presented in Figure 1.

Investigation
An outage of a furnace transformer is
devastating for the industrial operator.
Loss of power results in loss of production.
Fortunately, in this case, a spare
transformer was available on-site. This
afforded an opportunity for the service
contractor to perform several tests on the
faulted unit, including transformer turns

Table 1: Turns ratio results on all tap positions

Figure 1: Furnace transformer with winding
configuration Diii Table 2: Factory and field winding resistance measurements (HV winding)

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