pd measurement through hot spot

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Hunting for Hot Spots in Gas-Insulated

Switchgear

May 1, 2005 12:00 PMBy Doron Avital, Vladimir Brandenbursky and Alexander Farber, Israel Electric Co.

The reliability of gas-insulated switchgear (GIS) is high, but so are the consequential losses

to system security and a utility's revenue when the system failsespecially if the nominal

GIS voltage is 420 kV or higher. Because extensive damage translates into long and costly

repairs, more attention is being given lately to diagnostic techniques for in-service

maintenance undertaken to improve the reliability and availability of GIS.

Recently, considerable progress has been made in diagnostic techniques that focus on the

GIS-insulation system and are based on partial discharge (PD) measurements.

Following are the three main methods for in-service PD detection in GIS:

The chemical method, which relies on the detection of cracked gas caused by PD. The acoustic method, designed to detect the acoustic emission excited by PD. The electrical method, which is based on the detection of electrical resonance at

ultrahigh frequencies (UHF) up to 1.5 GHz, caused by PD excitation in GIS chambers

(the UHF method).

Keep in mind that these approaches cannot be used for the detection of poor current-carrying

contacts in GIS. This problem does not always produce partial discharges, and it does not

cause gas cracking in the early stages.

In-House Research

Seeking a solution to this problem, the Israel Electric Co. (IEC) conducted an experiment to

examine the opportunity of in-service diagnosis of the poor contact problem in GIS via direct

detection of local heating by using a thermal imaging system. The experiment on part of the

GIS with nominal SF6 pressure examined the following characteristics:

The range of power released in the defective contact that could result in temperaturerise on the surface of enclosure.

Temperature distribution on the surface of enclosure The influence of spacer typedesigned with or without holeson the heat-transfer

process.

The influence of the length of SF6 tubes and their position, horizontal or vertical. The temperature difference between upper and lower parts of the tubes in the

horizontal position.

Practical use of the thermal imaging system for detecting poor contact problems inGIS.

Setup and Test Procedures


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The GIS module IEC used for a series of tests on a single phase of a GIS unit was part of a

170-kV bus bar within an aluminum-welded enclosure comprised of four to six SF6 chambers

separated by spacers. The length of the modules varied from 3.8 m to 6.8 m (12.5 ft to 22 ft).

The GIS module was filled with SF6 gas at nominal pressure (3.5 bar), and experiments were

undertaken on poor contacts of 0.5 m at different power ranges: 100 W, 200 W, 300 W and

400 W. All of the measurements were taken in steady-state temperature conditions after thetemperature rise (for example, after 20 hours).

The inner temperature on the defective contact and the central conductor was measured by

precision centigrade temperature sensor-type LM35D, made by National Semiconductor. The

sensor type has an output voltage that is linearly proportional to Celsius temperature. The

sensor has an accuracy of 0.75C (33F) over a full -40C (-40F) to 110C (230F)

temperature range, and self-heating is less than 0.1C (32.2F).

Temperature measurements on the surface of the enclosure were made using a thermocouple

via the contact method. Simultaneously, the temperature distribution along the surface of

enclosure was obtained by thermal imaging system type SC3000, which is based on QWIPfocal plane technology that gives high-precision temperature measurements 61C (33.8F)

with a thermal sensitivity of less than 0.02C (32F).

Two modules with different lengths3.8 m and 6.8 mwere examined (Fig. 1). The

shorter module included four SF6 compartments, three tubes with a defective contact situated

between two of them (Point 9) and a corner chamber. For the longer module, two tubes of 1-

m (3.3-ft) and 2-m (6.6-ft) lengths were added to the corner chamber. For this module, IEC

tested two types of spacers in the corner chambera solid spacer and a spacer with holes

that permits gas flow. The longer module tests also were repeated in a vertical tube position.

Results and Discussion

Figure 2 shows how the short and long modules were compared when the same amount of

power was released in the contact. The results show that increased length does not change the

temperature distribution along the tubes. The absolute values of temperature vary slightly, but

the temperature difference between the surfaces above the bad contact and a point 1 m away

remains the same and is equal to about 4C (39F).

At the end of the longer module (Point 8), the temperature of the tube is equal to room

temperature. Therefore, we concluded that a longer tube does not change the temperature

distribution received in close proximity to the bad contact.

The temperature distribution obtained by the thermal imaging system for the shorter and

longer modules is shown in Figs. 3 and 4. Thermal pictures reproduce the detected

temperature distribution exactly (Fig. 2.) for these two modules when the power released in

the bad contact is 400 W. The small disturbances on the thermal graphs obtained by the

thermal imaging system correspond to the flange connection of the SF6 tubes.

Figure 5 shows the temperature distribution on the enclosure for the 6.8-m length of the

module for different ranges of power released in the bad contact.


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By decreasing the joule heating from 400 W to 100 W, the temperature of the contact

decreases from 75C (167F) to 38C (100F). Hence, the temperature distribution on the

enclosure surface is flatter. At 1 m from the bad contact position, the temperature difference

is 1C, but the maximum temperature on the enclosure above the bad contact is 4.8C

(40.6F) higher than the remote section. This can be compared with the results for the power

range of 400 W, where the temperature difference at 1 m from the bad contact area is 4.6C(40.3F) and 14.4C (58F) for the remote parts.

The thermal picture for 100-W power range is shown in Fig. 6. For the power of 100 W, the

area of overheating on the surface is relatively small, but at the same time, the temperature

difference between the point above the bad contact and the remote points of the enclosure can

be detected by the thermal imaging system.

The difference in heat between the upper and lower parts of the tubes is illustrated in Fig. 7.

In the area surrounding the bad contact, the temperature of the upper point of the tube is 3C

(37.4F) higher than the lower one. However, this difference is negligible 1 m away.

On the thermal pictures, the temperature difference between the upper and lower sections of

the tubes is obvious for the area of bad contact (Fig. 4). Therefore, this result confirms that

the practical detection in GIS has to be provided on the upper sections of the tubes. The

obtained temperature difference between the upper and lower sections of the tubes shows that

the convection has a vital importance in the heat-transfer process.

The influence of spacer type on the heat transfer is illustrated in Fig. 8. The existence of holes

in the spacer slightly changes the absolute temperatures on the enclosure and the temperature

distribution along the tubes. As expected, the gas flow between the chambers slightly cooled

the bad contact and led to a higher temperature at the remote parts of the enclosure. These

changes do not influence the temperature difference between the point above the bad contact

and the point at 1 m away.

This temperature difference for specified test conditions remains almost the same (about 3C

[37.4F], for P = 300 W) for both types of spacers. Corresponding thermal pictures for the

spacer with holes is shown in Fig. 9.

Figure 10 illustrates the results obtained for the module with vertical arrangement of the

tubes. As a result of convection, a sharp decrease of temperature was recorded for the points

beneath the bad contact, while the maximum temperature appears at a position 30 cm (12

inches) higher than the bad contact position.

The corresponding thermal pictures are shown in Fig. 11, in which the changes in

temperature distribution are clearly detected by the thermal imaging system. The thermal

picture reproduces the temperature distribution on the tubes at both sides of the bad contact

position.

The use of the thermal imaging system was examined on a 420-kV GIS in service. It was

found that for the GIS without defects, the nominal current did not heat the GIS enclosure.

For the bays that are undervoltage with nominal current and those that are not energized, the

enclosure temperature was equal to room temperature. Only the voltage transformers (VTs)

had temperatures that were above room temperature. The corresponding thermal pictures areshown in Fig. 12.


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When VTs are placed in a vertical arrangement, uniform heating of the upper part of the

transformer takes place. However, when VTs are positioned horizontally, heat rises from the

central parts of the transformers, where the core and windings are situated.

Recap of Results

The test results confirm the possibility of overheat detection for in-service GIS units inside

buildings. A few observations can be determined from the results:

Relatively small overheating of the bus bar contact in GIS up to 75C causes significant

temperature differences on the enclosure surface (a local temperature rise up to 4.6C).

According to IEC 60694, contacts in GIS are designed to operate with a maximum

temperature of 105C under normal conditions (a maximum current load and maximum room

temperature of 40C [104F]). The results obtained by using the thermal imaging system

show that it's possible to detect and locate the overheated contact when the contacttemperatures are below the specified design maximum.

Defective contacts that have higher temperatures can easily be identified by a thermalimaging system.

For GIS units inside a building, the temperature distribution obtained with a thermalimaging system reproduced the temperature on the GIS surface with high accuracy.

Convection plays an important role in the heat transfer process, resulting in theenclosure overheating in the upper sections of the tubes as well as those in a vertical

position. Hence, it is more effective to scan the GIS units from above.

The design of the spacerswith or without holesdoes not affect the temperaturedistribution on the enclosure.

A power level of 100 W released as a result of defective contacts is regarded as theminimum power levelthat could be detected using a thermal imaging system in GIS

units having a 170-kV rated voltage.

The use of a thermal imaging system for in-service inspection of GIS units inside abuilding is practically proven during an inspection of 420-kV GIS.

The experimental results reported by IEC verify that the infrared thermo-camera technique is

suitable for identifying and locating poor current-carrying contacts in GIS. The tests proved

that even minor anomalies, such as contact local heating up to a temperature below the

permissible value, are easily detected by the infrared thermo-camera technique.

Doron Avital received a bachelor's of science degree in 1988 and the master's degree in

materials engineering in 2002 from the Israel Institute of Technology. He joined the Israel

Electric Co. (IEC) in 1989, working in its Central Electrical Laboratory. Avital is now

manager of IEC's Thermovision Group, which is responsible for thermovision inspection

tests in T&D substations and power [email protected]

Vladimir Brandenbursky received the Ph.D. in 1984 from the Leningrad Polytechnic

Institute. From 1974-89, he worked as a scientist in the HV Laboratory of Moscow Institute

of Energy on the development of new methods of investigation of discharge phenomena and

HV test techniques. Since 1991, he has worked in IEC's Central Electrical Laboratory in the
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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field of on-site high-voltage testing and the development of new diagnostic techniques for

GIS.

Alexander Farber received the BSEE and MSEE degrees from Leningrad Polytechnic

Institute in Russia. In 1986, he joined Electroapparat, a USSR-based company, where he

worked on the development and design of HV test equipment for GIS. Farber joined IEC in1990, working in the Central Electrical Laboratory in the field of high-voltage test

equipment.

pd measurement through hot spot

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