6
User manual - Annex III
High Voltage Instrument Transformers | DDB/DFK/DDN/DFN
MEASUREMENT OF CAPACITANCE AND DIELECTRIC DISSIPATION FACTOR (tan
δ
)
The purpose of this test is to assess the insulation of the HV
transformer.
NOTE: Field testing of tan
δ
may be susceptible to diff erences
in measured values from factory testing due to the following
factors:
› The type of test equipment used and the measurement voltage
level.
› Thermal conditions (high humidity, rain, pollution).
› Temperature of the test object.
› Stress induced to the test object through other equipment.
It is recommended that the surfaces of the transformer (insulators
and others) be clean, to avoid external leakage currents.
Test instruments for fi eld use are usually equipped with solutions
to compensate for external leakage current. The voltage test
for this particular equipment is typically 10 kV for high voltage
insulation and approximately 2.5 kV for low voltage insulation.
Tan
δ
and capacitance measurement can also be performed in
the fi eld using other low voltage test bridges and a transportable
high voltage test set capable of measuring voltages up to the
nominal voltage. The choice and application of various test
equipment is at the discretion and experience of the user.
In addition to the measurement of tan
δ
, the measuring
equipment can usually also measure other parameters such as
capacitance and power factor.
The delta tangent measurement is carried out with the same
layout as the capacitance measurement.
Measurements can be made of tan
δ
and capacitance of the
capacitor unit(s), Cn, C1 and C2.
All measurements must be made with the equipment switched
off and with a test voltage <10 kV for measurements Cn and C1
and <2 kV for measurement C2.
Cn - Total rated capacitance
The rated capacitance Cn of the unit is the capacitance value
between the high voltage terminal (number 1 in fi g. 1) and the
low voltage terminal (number 3 in fi g. 1.)
The capacitor voltage divider is designed to meet the rated
capacitance Cn.
This value, with some tolerance allowed, is engraved on the
nameplate.
The value of Cn is calculated with the following formula:
Cn=( 1
∗
2)/( 1+ 2)
Test procedure (fi g. 2):
› Disconnect the HF connection to ground.
› The (HV) power cable of the measuring equipment must be
connected to the primary terminal (A/H1).
› The measurement cable must be connected to the high
frequency terminal (HF).
C1 - High voltage capacitance
The high voltage capacitance C1 corresponds to the part of the
capacitor connected between the line terminal (number 1 in the
fi g. 1) and the intermediate voltage terminal of the capacitive
divider (number 2 in fi g. 1).
This test can only be performed in the fi eld if the transformer
has a grounding switch
, since point 2 is not directly accessible
because it is inside the lower capacitor unit. For that reason, the
way to measure this capacitance is to close the grounding switch,
thus short-circuiting the electromagnetic unit and connecting
point 2 in the fi gure to ground. The measurement will be made
directly between the high voltage terminal (number 1 in fi g. 1)
and the transformer tank or transformer ground.
Test procedure (fi g. 3):
› Disconnect the HF connection to ground.
› Close the grounding switch.
› The (HV) power cable of the measuring equipment must be
connected to the primary terminal (A/H1).
› The measuring cable must be connected to the ground terminal
of the transformer.
C2 - Capacitance of the intermediate voltage capacitor
The capacitance of the C2 intermediate voltage capacitor
corresponds to the part of the capacitor connected between the
intermediate voltage terminal of the capacitive divider (number
2 in fi g. 1) and the low voltage terminal (number 3 in fi g. 1).
This test can only be performed in the fi eld if the transformer
has a grounding switch
, since point 2 is not directly accessible
because it is inside the lower capacitor unit. For this reason,
the way to measure this capacitance is to close the grounding
switch, thus short-circuiting the electromagnetic unit and
connecting point 2 in the fi gure to ground. The measurement
will be made directly between the low voltage terminal (number
3 in fi g. 1) and the transformer tank or transformer ground.
Test procedure (fi g. 4):
› Disconnect the HF connection to ground.
› Close the earthing switch.
› The power supply cable (HV) of the measuring instrument
must be connected to the high frequency (HF) terminal (low
voltage terminal).
› The measuring cable must be connected to the ground terminal
of the transformer.
CAPACITIVE VOLTAGE TRANSFORMERS AND COUPLING CAPACITORS/
