HIGH-VOLTAGE, DC TESTS - RAMPED VOLTAGE METHOD
- The ramped voltage test
technique automatically linearizes the
dielectric absorption component of insulation
current eliminating many of the problems
encountered in dc, stepped-voltage testing
methods. Automatic compensation of
absorption current eliminates the need for
extensive absorption current calculations
and complex volt/time testing schedules.
Further improvements have been made
through utilization of state-of-the-art au-
tomatic testing equipment which has
removed the uncertainty of the human factor
in adjusting test voltages and in recording
data. DC testing controllability, sensitivity,
and repeatability are significantly improved
through the use of this test method.
41. Description of Test.
- The ramped
technique of insulation testing uses a
programmable dc, high-voltage test set and
automatically ramps the high voltage at a
preselected rate (usually 1 kV/min).
Insulation current versus applied voltage is
plotted on an x-y recorder providing
continuous observation and analysis of
insulation current response as the test pro-
gresses. To evaluate an insulation, it is no
longer necessary to hand plot insulation
current and resistance versus applied
voltage. Insulation quality can be evaluated
directly from the automatically recorded
insulation current curves, because the
observed insulation current nonlinearities
are directly proportional to leakage current
The test technique was designed so
automated test results are similar in nature
to previous data obtained using USBR's dc,
step testing schedules.
The principal advantages of the ramp test
over the conventional step method are that
it requires only one person to perform the
test and provides that person with better
control and sufficient foresight of impending
failure to avoid damage to the insulation. The
elimination of the human factor from the time,
voltage, and current parameters yields
overall test results which are much more
accurate and repeatable. In addition, the
slow and continuous increase in applied
(17 volts per second) is less apt to damage
insulation than the step-method voltage
increments (approx. 1 kV/s).
Typical ramped-voltage test responses are
curves are a composite of the capacitive
charging, absorption, and leakage currents.
For a full description of the operation and
theory of the ramped-voltage method, refer
to Bureau of Reclamation Report REC-
ERC-78-7, "A Programmable D-C High-
Voltage Ramped Test System for Electrical
42. Ramped DC Test Schedule.
ramp rate of 1 kV/min is normally used to
test stator winding insulation. This rate
produces a current response similar to the
stepped test and is somewhat of a
compromise between maximum sensitivity
and minimum test duration.
The maximum voltage limit is the same as in
the stepped test. However, the increased
sensitivity and continuous-current
monitoring features of the ramped test
provides more information even when the
test is terminated at approximately 85
percent of the maximum voltage limit. A
reduced voltage test can be used to
evaluate very old, weak, or problem
insulation characterized by extensive
corona damage, excessive abrasion, loose
blocking, etc. Under no circumstances
should a reduced voltage test be used for
acceptance testing. Regardless of the
insulation quality, a reduced voltage limit is
not recommended when the dc test is also
to serve as a withstand or proof test. In any
event, to avoid an insulation failure, testing
should be terminated whenever leakage
current starts to become excessive.
The ramp test should be terminated when a
1- to 2-
A, or larger, sharp increase in
insulation current is observed. If the
increase is only a fraction of a
microampere, testing can continue.
However, if the current is unstable or there
are more stepped type of increases directly
following, testing should be halted.
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