Typical Pd Patterns And Tan Delta Levels On Rotating Machines - Megger Power Diagnostix ICMflex User Manual

ICMflex
Partial discharge and loss factor measurement system

6.2.5 Typical PD patterns and tan delta levels on rotating machines

To simplify the evaluation of all measurement results you can find here some common partial discharge
patterns versus PD origin and tan delta evaluation criteria as known from rotating machines standards and
other related literature. Power Diagnostix has experience in PD diagnostic on rotating machines for more than
20 years. The following table gives an overview regarding regular PD levels and miscellaneous PD phenom-
ena.
In general, partial discharge activity is inherent to epoxy-mica insulation systems. Compared to other insula-
tion systems where the PD level criteria are specified in the lower pC ranges, such as XLPE for medium and
high voltage cables or oil-paper insulation systems for power transformers, the common PD levels for stator
winding insulation systems are higher. Epoxy-mica insulation systems also widely tolerate partial discharge
activity since these systems are developed to be partial discharge resistant for many years. Performing PD
tests on new machines offers a fingerprint for further trending over time and will indicate the presence of
essential design faults, if present. Basically, every significant deviation from regular internal PD can be con-
sidered as a deviation in insulation quality. In this case, the root cause must be investigated.
As the measured apparent charge levels are depending on many different parameters such as the meas-
urement set-up, measurement frequency, location of the coupler, test voltage, insulation system, and origin
of the PD location within the winding, there are currently no specific no-go criteria specified. The levels given
below are indicative values.
PD PHENOMENA
Spherical void discharge
with low availability of starting
electrons
Surface discharge
at the slot exit
the first stage of field grading
problem
64 User guide for ICMflex
TYPICAL FURTHER
RANGE ANALYSIS
RECOMMENDED
0 nC >500 pC
0–2 nC >2 nC
PD PATTERN EXAMPLE
pdix.com