Tettex MIDAS 2881, Руководство по эксплуатации

Страница: 140 / 164

130 Applications Guide
According to the number of phases and the vector group the displayed short circuit-impedance must
be multiplied or divided by
√ 3:
Single Phase Star connection Delta connection
R = Rs (Zx=Ls+Rs) R = Rs (Zx=Ls+Rs) / √ 3 R = Rs (Zx=Ls+Rs) · √ 3
X = Ls (Zx=Ls+Rs) X = Ls (Zx=Ls+Rs) / √ 3 X = Ls (Zx=Ls+Rs) · √ 3
where:
R = R
1
+ R
2
X = X
1
+ X
2
R
1
= resistance of primary winding
R
2
= resistance of secondary winding
X
1
= stray resistance of primary winding
X
2
= stray resistance of secondary winding
For a schematic of the current booster unit see chapter "Accessories and Options: Current Booster”
Note: According to the standards; the measurement current should be as close as possible to the
rated current and not lower than half of rated current. This means that only small power (distribution)
transformers with a rating of max. 30 Amps at the High Voltage winding can be measured according
to the standards. Power Transformers with higher current rating can be measured only in order to
compare them with earlier field measurements with similar test equipment (fingerprint comparison).
14.2.6 Excitation Current Measurement
The excitation current measurement can be used to detect short-circuited turns, poor electrical
connections, core de-laminations, core lamination shorts, tap changer problems and other possible
windings and core problems in the transformer. In principle the test measures the current needed to
magnetize the core and generate the magnetic field in the windings.
The excitation current test should be performed before any DC tests. Excitation current tests should
never be conducted after a DC test has been performed on the transformer. Results will be incorrect
because of residual magnetism of the core left from the DC tests.
Measurement Procedure
Excitation current measurements should be performed at the highest test voltage possible within the
range of the test instrument. Nevertheless the test voltage should not exceed the voltage rating of the
windings.
The test voltage is normally applied to the high voltage side of the transformer. This minimizes the
charging current and deterioration or faults in the secondary windings are still detectable.
The secondary winding is always left open.
Due to induced voltages during the excitation current test, caution should be exercised in the vicinity
of all transformer terminals.
Built in current transformers must be shorted during the test and condenser bushing taps should be
earthed.
The test connection for measuring excitation current on a three-phase transformer is outlined below.