Download OLPD Testing of 10 kV Motors and Generators Following

OLPD Testing of 10 kV Motors and Generators
Following and In-service Failure of a 10 kV Motor
CASE STUDY
Motors and Generators
Oil Processing Facility
HFCT/TEV Sensors
HVPD Longshot™
Project
A gas processing facility based in the North Africa
requested on-line partial discharge (OLPD) testing
of the two (2) 10 kV Gas Turbine Generators and
four (4) process critical 10 kV Motors that had
reached 4 years of service. The testing was
requested as a result of a recent HV stator
insulation failure of 10kV Amine Pump Motor D that
had resulted in operational disruption at the facility.
Previous baseline OLPD diagnostic testing had
been carried out on the rotating HV machines with
the HVPD Longshot™ four years earlier,
immediately after commissioning. Whilst these
baseline tests had shown low levels of OLPD
activity on all of the machines tested, they had not
been tested since, over 4 years of operation in a
harsh desert environment.
The stator insulation of the 10kV Amine Pump
Motor D had failed where the slot section meets
the end winding (the slot-exit section) as shown
opposite, this being a common weak point within
the HV motor stator insulation system. PD-incepted
insulation faults can occur due to insufficient
electrical clearances between rotor and stator, the
formation of condensation and/or the accumulation
of dirt/debritus in this slot-exit area of the stator
windings. These factors can produce localised
conductive areas on the insulation surfaces
producing surface discharges that develop further
to tracking and finally a complete HV insulation
fault to ground. HV insulation degradation can also
be accentuated by a number of operational
stresses, including Thermal (e.g. from variable
loads), Electrical (e.g from voltage spikes),
Ambient (e.g from temp/humidity changes) and
Mechanical (e.g from vibration/out of alignment
issues), the so-called ‘TEAM’ acronym operational
stresses
10 kV Pump Motor
Failure on the 10 kV Pump Motor Stator Winding
Solution
This test project involved the insulation condition evaluation
of 10 kV rotating machines on site. Permanent high-current,
High Frequency Current Transformer (HFCT) OLPD sensors
previously installed in the machine’s cable box (one per
phase) were used. Sensor cables were routed through to a
Sensor POA Termination Box mounted on the side panel of
the generator/motor as shown opposite. The diagnostic
OLPD measurements of the in-service rotating HV machines
were then made using the HVPD Longshot™ diagnostic
OLPD test unit connected to each phase of the machine
under test.
Permanent Sensor Point of Attachment Box
Results
At Commissioning – Baseline Tests - the baseline OLPD test results from the commissioning tests shown
below showed low levels of PD (< 2.0 nC) detected all of the six (6) newly commissioned rotating HV machines
tested. This puts their HV stator insulation of them all in the ‘Excellent/New’ condition category.
Plant Reference
Gas Turbine Generator A (10 kV)
Gas Turbine Generator B (10 kV)
Amine Pump Motor C (10 kV)
Amine Pump Motor D (10 kV)
Amine Pump Motor E (10 kV)
CO2 Compressor Motor F (10 kV)
PD Peak Level (nC)
< 2.0
< 2.0
< 2.0
< 2.0
< 2.0
< 2.0
Condition Assessment
‘New/Excellent’
‘New/Excellent’
‘New/Excellent’
‘New/Excellent’
‘New/Excellent’
‘New/Excellent’
At Test – After 4 Years of Service - the second set of OLPD test results from testing after 4 years of service
(tested after failure and replacement of Amine Pump Motor D) shown below shows that the PD activity had
increased on all machines, apart from Compressor Motor F that remained in the ‘New/Excellent’. The new
replacement Amine Pump Motor D had ‘Average’ condition levels of 9.6nC.
Plant Reference
Gas Turbine Generator A (10 kV)
Gas Turbine Generator B (10 kV)
Amine Pump Motor C (10 kV)
Amine Pump Motor D (10 kV) - replacement
after failure of original Motor D stator fault
Amine Pump Motor E (10 kV)
CO2 Compressor Motor F (10 kV)
PD Peak Level (nC)
13.1
20.5
11.7
Condition Assessment
‘Still Acceptable’
‘Probable Inspection’
‘Still Acceptable’
9.6
‘Average’
12.1
< 2.0
‘Still Acceptable’
‘New/Excellent’
Conclusions and Recommendations
The main conclusion to this case study is that leaving 4 years between OLPD tests is too long!
Due to the difficulty in mobilising test engineers to remote locations (such as this gas facility in North Africa)
an increasing number of facilities in the oil & gas industry are deploying continuous partial discharge
monitoring technology such as the HVPD Kronos® to monitor their ‘mission-critical’ rotating HV machines.
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