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RHM International The Field Test and Dissection of a New Type
of Composite Insulated Dry Current
Transformer Made of Synthetic Materials
Liang Wenjin
1
Yao Senjing
2
Eric Euvrard
3
Wang Ruzhang
4
1 - Liang Wenjin is with
Guangdong Grid Company Ltd
, Guanzhou, Peoples Republic of China
2 - Yao Senjing is with
Electrical Test and Research Institute of Guangdong Grid Company Ltd
,
Guanzhou, Peoples Republic of China
3 - Eric Euvrard is with
RHM International, LLC
Brookline, MA, USA (e-mail:
eric.euvrard@rhmintl.com
)
4 - Wang Ruzhang is with
TR Electric Company Ltd
and the
Electric Power Research Institute (EPRI)
of
China, Beijing, Peoples Republic of China
IEEE PES 2005-2006 Conference, Dallas, TX, May 23, 2006
RHM
I
nternational 2
Summary
3 units of a new type of Dry High Voltage Current Transformers have been re-
tested and dissected by Guangdong Grid after 3.5 years in the field
The testing shows remarkable insulation margin and sustained low Partial
Discharge and tan
and no sign of aging
The dissection shows equipment without any deterioration or defect, as new
This paper presents the technology and comments on the conditions for such
high performance of a dry type design
Details on the testing procedure are presented and results are commented
Conclusions provide additional views on future developments 3
The Need for Safety and Reliability in the Largest
Chinese T&D Utility Shenzhen Utility has been using a new original dry type
category of Current Transformers, Bushings and Cable
Terminals made by TR Electric Co. for several years. Following an accident in 2002 with a SF6 insulated
transformer Shenzhen requested an emphasis on safer
HV equipments. At the same time, the Province Utility (Guangdong Grid
Company Ltd) was about to request bids for a large
order of current transformers. To assess the value of TR Electrics original Current Transformers (CTs), Guangdong Grid
decided to thoroughly analyze an ABC set of 110 kV dry type CTs that had been in operation
for 3.5 years in the Shenzhen grid. The CTs were subjected to a series of tests against standards and beyond, so as to
investigate operational margins. 4
An Innovation Coming from the Field and Matured
through R&D
The two inventors started to work in the field for 17 years before
joining EPRI of Beijing,
First products developed were wall bushings, implemented in 1990.
First Current Transformers were installed in 1998.
The first expression of the technology was patented by the 2
inventors as early as 1989
TR Electric was created in 2000
HV Dr
y
Shield
Unique advantages versus conventional technologies 5
An original CT Design: Simple and Robust
Grounded
Part
Mechanical link + insulation
The primary winding is
a U-shaped bushing 6
Case study Tests Conditions
Unit 1
AU01129
Unit 2
AU01138
Unit 3
AU01143
In service as an ABC
set for 3.5 years in
Nantou Power Station
Shenzhen Power
Company managed
Outdoor use
Pollution III (coastal
area)
Specific Targeted Parameters:
Insulation resistance
Partial Discharge
Dielectric Dissipation Factor (tan
)
Error Test
AC Voltage Withstand
tan
vs Applied Voltage correlation
Voltage Destructive Test
+ complete dissection of 2 units for visual
inspection of all aspects of the design
Testing Authorities:
Electrical Test & Research Institute, Guangdong Province
For destructive Testing: Wuhan High Voltage Research Institute
New Unit
AU04412
Benchmark new unit
in storage at moment
of study
Initial CT Specifications:
- rated voltage: 110 kV
- 50 Hz
- Accuracy classes: 0.2 ,0.5, 5P20
- withstand voltage: 230kV (1min)
- Weight:: 240kg/529lbs
Reference Standard: IEC 60044-1 7
First, all 3 units passed IEC standard testing for new
products (Final inspection)
Unit 1
AU 01129
Unit 2
AU 01138
Unit 3
AU 01143
New Unit
AU 04412
IEC
standard
Tan, %
@73 kV
0.071
0.137
0.047
0.069
<1
passed
0.5
PD*, pC
@87kV
<1
<1
10
20
Voltage
Withstand
(kV)
passed
passed
passed
185
The products were showing different cleanliness conditions.
However their test results are as good as new products 8
Testing the limits beyond specifications (I)
Dielectric Dissipation factor (tan )
tan
was then monitored as a function
of applied voltage from 20 kV up to
166 kV- 73 kV is the operating
voltage (Um/V3) tan
is about one order of
magnitude lower than the IEC
requested limit of 0.5% no significant variation of tan
even
under a very broad voltage scan
73
0
0.05
0.1
0.15
0.2
0
50
100
150
200
Applied Voltage (kV)
ta
n (%
)
73
Typical conventional type insulation behavior above
operating or rated voltage 9
Magnitude of Partial Discharge (pC)
0
2
4
6
8
10
12
14
0
10
20
30
40
50
60
70
Time (min)
pC
Testing the limits beyond specifications (II)
Partial Discharge (PD) Once cleaned, all three field units presented partial discharge levels lower than when new. Levels are consistently one order of magnitude lower than required standard levels Unique feature of the HV DryShield
TM
insulation is a behavior of self-attenuation of partial
discharge with
operational
time. For instance: on the first prototype of the 256 kV CT developed by TR Electric, PD was
measured vs time (Figure 1) 25% decrease in 50 minutes
175 kV and
measurement of PD (IEC
measurement standard for
PD)
(Figure 1) 10
Testing the limits (III) - Core insulation unaffected by
destructive testing up to 500 kV
0
100
200
300
400
500
600
1
4
7
10
13
16
19
Time (min)
A
p
p
l
ie
d
Vo
lt
a
g
e
(
k
V)
Unit 2 (AUO1138)
Unit 3 (AU01143) First test in air up to 350 kV
before external flashover occurs
(Unit 2) Then tested in oil up to 500 kV
without insulation break down
(Unit 3)
in air
in oil
200 tons of oil 6 meter wide tank
Wuhan High Voltage Research Institute 11
Step by Step Careful Dissection
Testing capabilities
1000 kV
Electrical Test and
Research Institute
of Guangdong Grid
The CT was dirty,
as expected in a
very polluted sea-
coast area 12
A large panel of experts was involved in the analysis
and conclusions 13
Results of Tests and Dissections were Very Positive for
Guangdong Grid After 3.5 years in the field all parameters of the 3 current transformers match
the final inspection testing records before it shipped out of the manufacturing
floor In addition
, HV DryShield
TM
insulation is presenting an opposite behavior to
conventional insulations with self attenuation of PD and low and constant tan
Inability to fail the insulation at very high voltages (x4 the Um voltage)
demonstrated very high operational margins (withstand voltage) which have
been confirmed in harsh field situations since 14
Analysis Electric Stress Control and Large Insulation
Margin
The reasons for those performance are multifold: The combination of the particular composite materials and the very
fine graded capacitor insulation controls the electric field profiles to
the point of avoiding any stress point => field illustration with
conventional HV products with silicone rubber housings showing
discoloration Design leading to a self-attenuation of Partial Discharge Strong understanding of field conditions and needs by the R&D team
in the initial designs to include relevant dimensioning Use of specific composite materials with outstanding performance,
according to specifications developed by the R&D team and relevant
to the field conditions Strong manufacturing know how and process control with
development of specific manufacturing tools
HV DryShield
Conventional Insulator with
darkened silicone rubber housing
- same location - 15
Outstanding Reliability and Resistance has been
Demonstrated in Other Occasions
In February 2001, a very unusual fog developed massive flashover over thousands of square
miles, over several Northeast provinces. Lines and stations from 66 to 500 kV were hit:
433 towers, 504 insulators, 238 lines, 34 power stations experienced flashover that
day.
Switchgears opened 972 times as stated by the official reports issued afterwards. More than 300 products using the HV DryShield
TM
insulation and design were in service
that day. None has suffered a single flashover. Two major Utilities issued a certificate of
excellence for that occasion.
Two major reasons were identified for this strong performance:
very good control of the stress field profile along the compon