Download Diagnostic of CT Failure – Same Location, different Phase

Presentation 7.1
Diagnostic of CT Failure – Same Location, different Phase
Anton Junaidi / Agus Latief, PT. PLN (Persero), Indonesia
Constant, PT. Citra Wahana Sekar Buana, Indonesia
Abstract
The Current Transformer (CT) is one of the most
important equipment in high voltage substations
and it is expected to operate maintenance-free for
over 25 years. The reliability of protection systems
and the accuracy of measurement instruments rely
on the performance of current transformers. A
failure of the current transformer can lead into a
catastrophic damage of the Current Transformer
itself and to the equipment placed nearby. So
almost happened to two CTs installed in
substations in Panakukang. The first failure
occurred on October 13, 2010 on phase T and the
second failure occurred on April 17, 2011 on phase
S. This paper will attempt to determine the possible
cause of the failure by analyzing the test results of
the third CT installed on phase R which has not
failed yet and which is the same type and
manufacturing year from the same brand.
Keywords
Current
Transformer,
Dielectric
Response
Analyzer, CT Analyzer, Dissipation Factor Test,
Partial Discharge Measurement
1. Introduction
PLN AP2B South Sulawesi (Indonesia) operates a
transmission line system with voltage level of
150kV, 70kV and 30kV. It consists of more than
2,200 kilometer transmission line circuits and 34
substations. The total transformer capacity is 1,350
MVA, with an average availability of the system up
to 99% on a annual basis.
Primary Current:
Secondary Current:
Rated Burden:
Class:
Frequency:
800 A
5A
30 VA
5P20
50 Hz
Table 1 Current Transformer Data
2. Failure Phenomenon on
Phases T and S
The first CT failure occurred on October the 13,
2011 on phase T and resulted in a black out in the
Panakukang area. This incident has caused the
total energy loss of 60 MVA. Visual inspection
showed normal temperature conditions at the top
of the current transformer (upper core), and field
investigation did not reveal the presence of any
open secondary circuit.
The second failure occurred on April 17, 2011 on
phase S. The substation operator has seen smoke
coming out from the top of the current transformer
and took immediate action to de-energize the
current transformer. Thanks to this reaction a
further damage and a possible black out could be
avoided. The temperature measurement using
thermal infrared camera was observed and
showed no hot spot on the top of the current
transformer. The wiring verification on the
secondary side of the current transformer could
exclude an open-circuit condition.
PLN AP2B South Sulawesi has comprehensive
testing standards. All high voltage equipment is
tested and the test results thoroughly analyzed to
help to determine possible cause of failures. The
total quantity of current transformers installed is
more than 600 units where most of them are oil
immersed type.
This paper describes the failure that occurred on
two identical current transformers and tries to
diagnose the cause of the failures by analyzing the
test results of a similar unit that has not failed yet.
Fig. 1 Water inside membrane on Phase S CT
© OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum
Presentation 7.2
3. Failure Modes and Effects
Analysis (FMEA)
Fig. 2 Membrane Phase S CT (left) and membrane
Phase T CT (right)
Failure modes and effects analysis (FMEA) is a
procedure in product development and operations
management for analysis of potential failure modes
within a system for classification by the severity
and likelihood of the failures. A successful FMEA
activity helps a team to identify potential failure
modes based on past experience with similar
products or processes, enables the team to design
those failures out of the system with the minimum
of effort and resource expenditure, thereby
reducing development time and costs. Failure
modes are any errors or defects in a process,
design, or item, especially those that affect the
customers, and can be potential or actual. Effects
analysis refers to studying the consequences of
those failures.
PT. PLN has implemented the FMEA for CT failure
consists of:
Fig. 3 CT Phase S inner cover melting
• Defining the system (equipment) and its
function
• Determining the sub-systems and functions
of each subsystem
• Determining the functional failure of each
subsystem
• Determining the failure modes of each
subsystem
•
With FMEA we can design the proper operation
and maintenance guideline according to PLN
standard SE 114 for CT such as:
• In service inspection (visual check)
• In service measurement (Thermo vision)
• Offline measurement (Insulation resistant,
tan delta, oil characteristic, DGA, Oil
characteristic, excitation and grounding.
• Offline treatment.
4. CT Testing on Phase R
Fig. 4 CT Phase T inner cover melting
The two current transformers which failed have
been disassembled to observe the type of damage
that occurred on the inside of the current
transformers. It was found that both failed CTs had
loose connectors at the inner cover of the CT (the
top cover body with the cover secondary winding).
The phase S CT contained water in the membrane
thus decrease the insulation level.
After the failure of this two CTs (phase S and
phase T), the management decided to remove the
CT on phase R from the system as this CT is the
same type and brand and from the same
manufacturing year. Several tests had been done
on this CT to analyze and check for a possible
failure cause of the other two CTs.
© OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum
Presentation 7.3
4.1 CT Analyzer
Several tests can be obtained using OMICRON’s
CT Analyzer such as, ratio, saturation, burden,
polarity and winding resistance. The assessment
according to IEC standard is also possible. The
test result using CT Analyzer of phase R:
• Ratio: OK
• Excitation curve: OK
• ALF (Accuracy Limit Factor): OK
the location of possible fault cause is the cellulose
paper or the oil. So transformer failures can be
avoided and the preventive maintenance can be
on schedule.
New methods are developed for determining the
water content by using the dielectric response
measurements as Polarization Depolarization
Current (PDC) or Frequency Domain Spectroscopy
(FDS).
The Dielectric Response Analysis has been done
to CT phase R. The test results using OMICRON
DIRANA with FDS and PDC method as shown
below:
Table 2 ALF CT Phase R
Fig. 7 CT Measurement Configuration
Fig. 5 Excitation Curve CT Phase R
The DIRANA assessment software consists of a
database containing data from measurements of
real oil-paper-insulation systems, under various
states of moisture content and oil / paper ageing.
Through this basis, the measurement results are
automatically analyzed and the moisture content
then classified according to IEC 60422 between
"dry" and "extremely wet."
Fig. 6 Ratio Error Graph CT Phase R
4.2 Dielectric Response Analysis
One ageing indicator is the water content in the
solid part of the insulation (paper, pressboard).
Water is an ageing product and accelerates the
further deterioration of cellulose through depolymerization. Incremental of water content in oil
may cause bubble formation and lead to an
electrical breakdown.
Method and tools are needed to detect and
measure the water content in the cellulose paper.
Furthermore with the right tool to diagnose the
transformer it can be easily determined as where
Fig. 8 CT Phase R DIRANA Assessment Graph
The software compensates for conductive ageing
by products in the oil and it is well suited for older
Instrument transformers. The DIRANA test result
for Panakukang CT Phase R indicated:
Oil Conductivity:
11pS/m (good)
Cellulose Moisture: 3 .3% (moderately wet)
© OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum
Presentation 7.4
4.3 Dissipation Factor and Capacitance
Test
Another usual approach when performing dielectric
evaluation is tan delta. This test uses AC and
pursues to know loss angle of the tested element.
This measurement technique is off-line measurement and includes information of the moisture,
carbonization, and other forms of contamination of
winding, bushing, and liquid insulation in
Instrument transformers and emulates (greater
voltage) the behavior and voltage aggressions
similar to service ones. It is important to take note
of Instrument transformer temperature and
environmental moisture (surface leakage).
Apparatus insulation system has measurable
electrical parameters, such as capacitance,
dielectric loss, and power factor in addition to other
less well known characteristic. By detecting
changes in these important electrical characteristics, failure hazards can be revealed, thereby
preventing loss of service by permitting orderly
repair or reconditioning of defective insulation.
The CT phase R was tested for the dissipation
factor test using Hot Collar method since the
measuring tap is not available. The test was done
using OMICRON CPC 100 + CP TD1. The
dissipation factor test was done in two different
measurements. First measurement with fixed
frequency of 50 Hz and sweep voltage from 2 kV
up to 12 kV. The second measurement test with
fixed voltage and frequency sweep from 15 Hz up
to 400 Hz. The test results are as follows:
Table 5 Tan Delta Test Result (as function of Voltage)
Table 6 Tan Delta Test Result (as function of Frequency)
4.4 Partial Discharge
In electrical engineering, partial discharge (PD) is a
localized dielectric breakdown of a small portion of
a solid or fluid electrical insulation system under
high voltage stress, which does not bridge the
space between two conductors. While a corona
discharge is usually revealed by a relatively steady
glow or brush discharge in air, partial discharges
within solid insulation system are not visible. PD
can occur in a gaseous, liquid or solid insulating
medium. It often starts within gas voids, such as
voids in solid epoxy insulation or bubbles in
transformer oil. Protracted partial discharge can
erode solid insulation and eventually lead to
breakdown of insulation.
Table 3 CT DF variable voltage fix frequency
Fig. 10 Void and discharge
Table 4 CT DF variable frequency fix voltage
Fig. 9 Graph correlation between DF and Frequency
PD measurements can be taken continuously or
intermittently and detected on-line or off-line. PD
results are used to reliably predict which electrical
equipment is in need of maintenance.
Not only do partial discharge levels provide early
warning of imminent equipment failure, but partial
discharge also accelerates the breakdown
process. The excessive arcing between ground
and conductor within the insulation will, in time,
compromise the dielectric strength and mechanical
integrity of the winding insulation. Once this
happens, a ground fault or a phase-to-phase fault
is inevitable.
The Partial Discharge test on phase R CT was
done using OMICRON MPD 600. The 120 kV
coupling capacitor being used as PD sensor since
the measuring tap is not available on CT. The HV-
© OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum
Presentation 7.5
AC hipot test system up to 100kV being used as
HV source to inject the HV to test object. The PD
measurement being repeated several times since
the noise level from HV source and measurement
test setup is very high. During PD measurement,
the corona activities also observed using DayCor
corona camera. It was observed that the corona
discharge on the CT bushing is small. The PD test
was done by increasing the voltage from 2 kV as
starting point for voltage calibration for MPD 600
test set and voltage increased up to 86.6 kV as
nominal voltage of 150 kV. The MPD 600 test
result shown PRPD as below. The figure 11 shows
PRPD from phase R CT when injection voltage up
to 86 kV is applied. The figure 12 shows ORPD
from HV source only. The figure 11 shows that
there are PD activities which are very high.
4.5. Insulation Resistance Testing
The purpose of this measurement is to detect the
condition of insulation between winding - ground or
between two winding. Common method provided
the DC voltage and represents the condition of
insulation to the units in mega ohm (M). It is also
seen from the measurement of the polarization
index. The aim of the polarization index test is to
ensure the proper equipment to be operated or
even transient overvoltage test. Polarization Index
is the ratio between the 10th minute result to the 1st
minute result, with a constant voltage supply.
Table 7 Polarity Index Result CT Phase R
5. Conclusion
1.
Fig. 11 PD result CT Phase R Test at 86kV
2.
3.
4.
It was observed by disassembling the failed
CTs (phase T and S) that both CTs had
loose connection on its body-ground cable
connection in the CT. It was suspected that
this was caused by aging process or
manufacture defect design. It was also
found that a lot of water trapped inside the
membrane of CT phase S.
The tan delta test for bushing insulation
indicated normal bushing. No suspect on
the bushing failure.
Partial discharge test on phase R CT
indicated high PD activities (over 10 nC).
This might indicated the degradation of
insulation material and/or contact problem
inside the CT.
High moisture content (3.3%) was
measured on phase R CT. The moisture
content may decrease the insulation of the
material and lead to CT explosion.
References
Fig. 12 PD result for HV source only at 86kV
[1] V.K.Prasher, P.J.Thakkar Power Grid India,
EHV Current Transformers - Failure Mode
Analysis & Evaluation Leading to Corrective
and Preventive Actions for Realibility
Enhancment, CIGRE.
[2] OMICRON, DIRANA,
CPC 100 / CP TD1.
CT
© OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum
Analyzer
and
Presentation 7.6
[3] Michael Krüger, Maik Koch, Alexander
Kraetge,
Kay
Rethmeier,
OMICRON
electronics GmbH, Austria Alexander Dierks,
Alectrix, SA - Diagnostic Measurement on
Power Transformer.
[4] Root Cause Failure Analysis Current
TransformerSer. #811282 John Stead, Altalink
Management Ltd.
[5] A.K. Datta, S.C. Singh, S.K. Mishra and S.
Suresh Power Grid Corporation of India
Limited, Dissolved Gas Analysis (DGA) of
Current Transformer (CT) oil – A reliable tool
to identify manufacturing defects.
[6] PLN SE 114: Guidence Maintenance for CT.
About the Authors
Anton Junaidi, born in Surabaya
1978, College graduated Diploma
Degree of Shipbuilding Polytechnic
ITS Surabaya Majoring in Electrical,
then continued Bachelor Degree at
ITATS Engineering Faculty Majoring
Power Electrical. 1999 - 2005
worked at PT. Siam Maspion Polymers as
Electrical & Planning Eng. Started working at PLN
(2005/2006) first position as Transmission Line
Protection PT. PLN (Persero) Wilayah Sulsel,
Sultra dan Sulbar AP2B Sistem Sulel, and recent
position as Manager Transmission and Substation
Panakukang PT. PLN (Persero) Wilayah
SulSelraBar AP2B Sistem SulSel. (7806046 Z)
E-Mail: [email protected]
Agus Latief, Agus Latif (F), was
born in Makassar in 1980,
graduated high school SMKN
makassar majoring in industrial
electrical
construction,
began
working at PLN (2002/2003) as a
substation operator panakukang PT
PLN (Persero) Tragi Panakukang and recent
position as substation maintenance staff PT. PLN
(Persero) Wilayah SulSelraBar AP2B Sistem
Sulsel. (8003044 F)
© OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum