Download 7.3 Location by Separation of Electrodes (cont.)

Partial Discharge Detection in HighVoltage Equipment
장성수
11 October, 2003
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7.1 Introduction

Detection of discharged location

Having located it, various courses can be taken ;
.
Change out parts, e.g. the bushing of a transformer
.
Repair, e.g. polishing a sharp edge in G.I.S
.
Accepting the discharge, e.g. nonrelevant corona on top of a cable
terminal

Further interpretation and evaluation are only realistic after
location

Location methods discussed in this chapter locate the discharge
along the length of a sample
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7.2 Nonelectric Location


Nonelectrical discharge detection (Chapter 4)
.
Detecting the presence of discharges
.
Can’t be used for measuring the discharge magnitude
.
Chemical transformation, gas pressure, heat, sound, light
Corona for location
.
.
Discharge in air, like corona and surface discharges can well be
located by acoustical methods (Fig. 4.1 & 4.2)
Location within 100mm can be achieved
Figure 4.1 Location in air with an
ultrasonic microphone
Figure 4.2 Precise location with the aid of
a simple plastic waveguide
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7.2 Nonelectric Location (cont.)

Corona (cont.)
.
Light detection can only be used for surface discharges and
corona
.
Photographic methods take more time but give on-the-spot
locations (Fig. 4.8)
Figure 4.8 Surface discharges, where
a is 3pC, b is 25pC and c is 100pC
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7.2 Nonelectric Location (cont.)

Surface Discharges
.
Some acoustic and photographic methods can be used
.
Sometime discharges can be made accessible.
Figure 4.10 :
-
High voltage cable terminal
-
Surface discharges could be found and both the exact
location and local field strength at which they originated
could be ascertained
Figure 4.10 Photographic detection of
discharges within a cable terminal
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7.2 Nonelectric Location (cont.)

Internal Discharges
.
For location of internal discharge : only acoustic methods
.
In simple and not-too-thick insulation (e.g. Cable), reasonable
location in the order of 50 to 100mm can be achieved
.
-
Figure 4.5 shows the means for conducting experiments
on plastic-insulated cables. The damped oscillation
caused by discharges are displayed on an oscilloscope
-
Figure 4.6 shows that the height of the signal on the
oscilloscope at the place of the discharge
Complicated structures (e.g. Transformer & G.I.S), some
location can be achieved
-
By using several transducers
-
By scanning the tank with a contact microphone
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7.2 Nonelectric Location (cont.)
Figure 4.5 A noise-detection circuit with narrow-band amplifier
having a variable midband frequency
Figure 4.6 Noise detection : variation of
noise signal along a cable
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7.3 Location by Separation of Electrodes

Principles
.
It is possible to distinguish discharges in the edge electrodes from
those in the samples by using balanced detection
.
This principle can be extended by dividing the earth electrode of a
sample in a number of electrodes (Fig. 7.1)
.
In the Figure 7.1,
1.
Two lengths of cable K, connected by a joint M and provided with two
terminals E
2.
The cable sheath is interrupted at a number of places (six divisions are
formed)
3.
Each division can either be connected to the (balanced) detector or to
earth
4.
A good screening is supplied, a high rejection ration of 1000 times or
more can be reached
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7.3 Location by Separation of Electrodes
Figure 7.1 Separation of Electrodes
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7.3 Location by Separation of Electrodes (cont.)

Principles (Conclusions)
.
Discharge can be measured effectively in the joint, even in the
presence of (large) discharges in the other divisions
.
It is possible to discern whether a discharge occurs in the left-hand
or right-hand side of the joint, and to locate where a discharge is
situated to within a few centimeters by further subdividing the joint
.
Similar observations can be made in the cable lengths or the
terminals by alternately connecting divisions to the bridge or to
earth
.
Precise location can be obtained there as well by further
subdivision
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7.3 Location by Separation of Electrodes (cont.)

Separations
.
The required separations can be made in two ways
-
.
.
A hard separation
A Soft separation
Hard separation (Fig. 7.2)
-
In one case, an overlap is created in the low-voltage electrode; this
must be made carefully so that no discharge arise in the separation
itself
-
In the second case, an insulating film is placed between the flanges of a
container; the film must be thin and the flanges well rounded-off to
prevent discharges at the edges
Soft separation (Fig. 7.3)
-
High voltage cables have a semiconductive layer L that serve as a
boundary to the dielectric
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7.3 Location by Separation of Electrodes (cont.)
.
Soft separation (cont.)
-
The metal outer sheath M is simply removed as far as necessary to
create a resistance of about 10 Kilo-Ohms, neither this resistance nor
the required length of separation is critical
-
A soft separation can sometimes be made by applying a
semiconductive paint to the surface of the dielectric
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7.3 Location by Separation of Electrodes
Figure 7.2 Hard Separation
Figure 7.3 Soft Separation with the aid of the Semiconducting layer L
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7.3 Location by Separation of Electrodes (cont.)

Composite Sample
.
Figure 7-4 shows another example of the separation of electrodes
.
Figure 7-4 (a)
.
-
All parts are connected and measured in a straight detection circuit
-
No distinction can be made between discharges in the bushing, in the
coils or in the insulation between coils and earth
Figure 7-4 (b)
-
In the balanced circuit, the electrodes are separated
-
Points 1 to 4 are either connected to the bridge or to earth
-
By skillfully manipulating the connections and the balance of the bridge
the origin of a discharge can be established
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7.3 Location by Separation of Electrodes
Figure 7.4 Composite sample : (a) a straight detection circuit;
(b) separation electrodes in a balanced detection circuit
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7.3 Location by Separation of Electrodes (cont.)

Creation of Electrodes
.
Sometime completely new electrodes are created in order to detect
and locate discharge
.
Figure 7-5 shows the stress cone in a high voltage cable terminal
.
-
The stress cone is cast in epoxy resin and tested for discharges before
being approved for use;
-
For that reason electrodes that follow an equipotential line in the
terminal were developed
Figure 7-6
-
A stress cone is inserted into an electrode placed upside down,
immersed in oil and tested at high voltage
-
The electrode is divided in four parts
-
If a discharge is detected in this layout, the cone can be turned round
and the discharge located
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7.3 Location by Separation of Electrodes (cont.)
.

Figure 7-6 (cont.)
-
Thousands of stress cones have been tested in this way
-
About 2% have been rejected at an acceptance level of 1 to 2 pC
Separation, composite samples and the creation of electrodes are all
examples of one general principle, namely that research problems can
be solved by careful experimentation and electrode design; or routine
tests can be provided
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7.3 Location by Separation of Electrodes (cont.)
Figure 7.5 A synthetic stress cone. The distribution of the
electric field is given by the equipotential lines
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7.3 Location by Separation of Electrodes (cont.)
Figure 7.6 Testing a stress cone against a counter-electrode in the shape of the 43%
equipotential line of Figure 7.5. The counter-electrode is split into four equal parts, the
voltage is applied via a metal cone and the test is done in a dielectric fluid.
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7.4 Location with Electrical Probes

Probes or antennas are used to detect the electromagnetic leakage field
created by partial discharge

Rotating Machines
.
.
A probe is used in large generators and motors
In the Figure 7-7,
1.
2.
The probe
-
Can be constructed conveniently from ferrite
-
Insensitive to electrical disturbance, can be used in electrically
noisy surrounding
A narrow-band amplifier
-
3.
Connected to the probe
Turned to the resonance frequency of the coil and its measuring
lead
A crest voltage meter is used to indicate possible discharges
.
Location of discharges can thus be effected
.
Determining the magnitude fo discharges will be more difficult
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7.4 Location with Electrical Probes (cont.)
Figure 7.7 Pick-up by probe of internal
discharges in machine coils and slots
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7.4 Location with Electrical Probes (cont.)
Figure 7.8 Example of a test on machine coils with the probe of Figure 7.7
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