Page 37 - Basic PD Theory

On-line PD Tests

5.1.3 Operating Parameters
Because variable operating parameters can impact the PD test results, for tests to be directly comparable you need to test at
similar operating parameters. In reality, it is impractical to think that subsequent tests will be tested at identical operating
parameters; therefore, it is imperative that for each test, the operating parameters be recorded. That way, if there is a change in
the PD since the last test, it may be possible to determine whether the difference is due to changes in these parameters or the
winding. Regardless of the operating temperature, unless checking for Temperature Effect, thermal stabilization is advised.
Environmental conditions such as humidity may have a very noticeable impact, especially if the surface Contamination
becomes partly conductive when damp, so it should be recorded from one test to the next. It is appreciated that at
times it may be very difficult to duplicate test conditions, particularly temperatures; however, the emphasis should always
be on trying to achieve as uniform conditions as possible. This means taking tests when units are hot and the
temperature has stabilized. When unloading a machine to do a reduced or no load test, it is inevitable the temperature
will be in a state of transition through the test. In these cases, it is important to quickly conduct the tests to minimize
the effects of temperature differences.
For direct comparisons on the same couplers over time, the operating parameters during testing should be within the following
recommended ranges (see Section 6.1.3 on the Operating Condition Trend):

Parameter Value
Voltage +/- 2.5%
Load +/- 10%
Temperature +/- 10°C
Hydrogen Pressure ± 5 psi-g, ± 35 kPa, ± 0.35 bar
Humidity (can affect test results) Unknown

5.2 Continuous – Guards & Tracs

5.2.1 Safety Precautions

With the exception of the coaxial cable connections at the junction box, it is imperative that no other connections be removed
from the PD monitoring system while the machine is operational. As with the portable systems, the 50/60Hz AC voltage
between the center pin and the shield of PDA and Bus installs should be 100-600mV AC. If the ground is disconnected it is
possible that this voltage may float to a higher value. SSCs are not electrically connected to the HV, so there is no steady
50/60Hz signal available; however, you should verify both the presence and magnitude of the external reference voltage.

5.2.2 Threshold Alarms

Threshold alarms are either latched or momentarily alarms that can be connected to a plant’s annunciation system to provide
plant personnel with warning that NQN or Qm levels have exceeded a user pre-set value. At Iris we recommend that threshold
alarms are set based on comparing the present PD levels to those of similar machines in the Iris database (see Section 6.2). Our
recommendation is as follows:
 If the present Qm level is less than the 75 percentile, then set the Alarm at the 90 percentile
th
th
 If the present Qm level is greater than the 75 percentile, then set the Alarm at twice the present value
th
 NQN alarms should be 1.5-2 times higher than the Qm alarms
Alarms are phase-specific. PD fluctuates with ambient condition and operating parameters, so caution should be taken before
reacting to alarms. Alarms are only warnings that further investigation with a portable Lite instrument is required. It is the
results of that further investigation that determine what type of maintenance action is required.

www.irispower.com

34 | P ag e