Voltage Verification
4-44 72A-1230 Rev. F
July 22, 2005
The characteristic charging current (capacitance) and watts-loss of the Liquid
Insulation Cell should be measured whenever a new instrument or cell is
received, and the test data should be recorded for future reference. The test
connections and procedure are the same as for testing a liquid-insulation
sample, except that the test is conducted in both the UST and GST modes. The
cell should be clean and dry, and a stable voltage source should be utilized.
Typically, a clean and dry Liquid Insulation Cell check produces results
approximately as follows by the UST method:
NOTE In the absence of insulating liquid, the dielectric-breakdown strength of
the Liquid Insulation Cell is significantly reduced. Due to the relatively
close spacing of the cell plates, an internal flashover will likely occur
around 7 or 8 kV when the cell is dry.
The charging current measured is a function of the geometric dimensions of
the cell and, therefore, some variation may occur between different cells;
however, the current reading should be repeatable when using the same cell,
properly assembled with electrode spacings unchanged. The measured current
and watts-loss will be slightly higher when the calibration is checked by the
GST method owing to stray capacitive coupling to ground.
Voltage Verification
The factory performs voltage accuracy tests at 5kV using a high voltage RMS
voltmeter (Valhalla, model 4600). The following procedure is a similar
voltage verification test performed at a reduced voltage (500 Volts).
Additional
Instrumentation
Required:
Digital Voltmeter (DVM) with a minimum input capability of 750 VRMS and
an accuracy of +/- 0.5% on the AC Volt scale. The following models are
acceptable for doing a reduced voltage measurement test in the field; Fluke
models 8060A, 8025A, Fluke 85, Fluke 25, Fluke 27 or equivalent.
Test Setup: • Test set must be grounded and the High Voltage Lead connected.
Confirm the center conductor of the High voltage Cable is isolated
from ground.
• Connect the DVM from the High Voltage Lead center conductor to the
Guard circuit (blue terminal on the front panel of the M4100
instrument).
Test Voltage Current Capacitance Watts-Loss
5 kV 400 microamperes 106 pF +0.0020
To Next Page
Loading...
Need help?
General
