doble M4110 - Interpretation of Test Results; Analyzing Initial Test Results

72A-2243-01 Rev. B 9/04 4-1
September 17, 2004
4. Interpretation of Test Results
There are three tabs which show test results: the Results tab shows many of the
measured variables. The main analysis is done with the data in the %
Impedance and % Reactance tabs. Besides the measured and benchmark
values, they contain the Delta Benchmark and Delta Average values. The
analysis depends on whether the test is an initial or a subsequent one. During
the initial test, both the three-phase equivalent and the per-phase
measurements are performed. The subsequent test (with some exceptions,
discussed below) requires only the per-phase measurements.
The factory data are usually expressed in terms of the average short-circuit
impedance. Therefore, they should be compared with the impedance value
measured in the field using the three-phase equivalent measurement. The
comparison between the initial and subsequent tests should be performed on
the basis of the per-phase leakage reactance test results. For transformers
whose nameplate impedance is less than 5% and whose power rating is less
than or equal to 500 KVA, the tester should perform only the per phase test
and its results analysis, and not the three phase equivalent.
Initial Test
The objective of the initial test is twofold: to compare results with factory data
and to establish a benchmark for subsequent tests. The purpose of the
three-phase equivalent test is to produce results for comparison with the
factory data (Delta Benchmark, if the factory nameplate impedance was used
as a benchmark value). When results are not within 3% of the nameplate
values, the explanation may lie in different instrumentation and test setups (a
different de-energized tap was used), difference in flux distribution under the
three- and single-phase excitation, and/or the presence of the winding
distortion. The latter can be confirmed through the per-phase test, where the
comparison between phases (phase pattern) may help to explain the difference
between factory and field results. The average value of the three per-phase
impedances is calculated, and the percentage difference between each
per-phase value and this average is show in the Delta Average column. If each
of the three phases is within 3% of their average value, the winding distortion
most likely is not an issue. For the single-phase units (for obvious reasons), all
the analyses are based on the per-phase measurements only.