Megger DELTA 4000, Справочное руководство

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38 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
High-Voltage turns-ratio
measurements
Ratio measurements on HV transformers are commonly
made using low voltage instruments designed specifically
for that purpose. Those test instruments apply a relatively
low voltage (<100V) to either the primary or the secondary
of the transformer. The resultant voltage is measured and
the voltage ratio is calculated automatically by the test set.
Occasionally there are instances when it is desired to
perform higher voltage ratio tests for diagnostic purposes.
Using a power factor test set like the Delta4000, voltages of
up to 12kV can be applied to a transformer winding, gener-
ating a higher turn-to-turn stress on the winding under test.
It is believed that higher voltage stress on a winding may
break down weak turn-to-turn insulation and help detect
faults that might be overlooked by low voltage test equip-
ment. It is important to recognize that the voltage rating
of the winding being energized must not be exceeded or
damage to good insulation may result.
Test procedure
Determining the ratio of a transformer using the HV TTR
Capacitor involves taking a capacitance measurement of the
HV TTR Capacitor by itself, then taking another measure-
ment with the capacitor connected to the low-voltage wind-
ing of the transformer. The ratio of the capacitance values
is equal to the voltage ratio of the transformer windings.
The figures below will help explain the procedure.
Figure 1 shows the connection used for accurately deter-
mining the capacitance value of the HV TTR Capacitor.
The instrument HV output lead is connected to one side of
the capacitor, and a LV measuring lead is connected to the
other side. Both connections must be isolated from ground,
and the test set measuring configuration should be UST
(Ungrounded Specimen Test). The capacitance value from
this test is C1.
Figure 14: TTR Capacitor measurement
The second step of the procedure is to connect the test set
and the HV TTR Capacitor to the transformer winding to
be tested. Figure 2 shows this connection on a single-phase
transformer. The test set output is connected to one end
of the high voltage winding. The other side of the winding
must be grounded. The HV TTR Capacitor is connected to
one end of the low voltage winding, and is then connected
to the measuring lead of the test set. The other side of the
low voltage winding is grounded as well.
Figure 15: Single-phase transformer
The test set measuring configuration should again be UST.
The value of capacitance from this measurement will be
identified as C2. NOTE: The polarity of the winding con -
nections should be made per the polarity markings shown
on the nameplate of the transformer.
Once the values of C1 and C2 have been established, the
ratio (N) of the transformer (for the tap connection being
measured) is determined as;
N = C1 / C2
The procedure for testing a three-phase transformer is
the same as that for single-phase. Figure 3 shows a typi-
cal three-phase configuration (Δ-Δ). As in the previous
example, connect the HV output lead to the high voltage
winding, and the capacitor plus low voltage measuring lead
to the low voltage winding.
Figure 16: Three-phase Delta-Delta transformer
The test set measuring configuration should again be UST.
The capacitance reading obtained from this measurement is
also identified as C2. Calculate the transformer ratio using
the same formula as the previous example.
For further example, Figure 4 is provided to show a three-
phase delta-wye transformer winding and the connections
that are required.
Figure 17: Three-phase Delta-Wye transformer
Temperature considerations
Due to the design of HV TTR Capacitors, their capacitance
value may be sensitive to changes in temperature. Once the
value of C1 is obtained, it is recommended to promptly
take the C2 measurement without delay. This will ensure