high voltage pd test on gis

8/4/2019 High Voltage PD Test on GIS

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Presented by Vishal TELANG

Measurement Of PD

&Role Of UHF Sensor

In

Gas Insulated Substation

Vishal Telang

Engineer GIS

Noida, 04th December 2008


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DEFINATION

What is PD?

Partial Discharge (PD), as its name would suggest, is anelectrical discharge that occurs across a portion of theinsulation between two conducting electrodes, withoutcompletely bridging the gap.


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Macroscopic-physical effects by partial discharges

Macroscopic-physical effects by partial discharges

Heat Light Chemical reaction Dielectric losses

Pressure wave, sound High frequency wave (VHF,UHF)


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Some information about PD

Partial discharge is a non distractive test.

Partial discharges can occur in voids in solid insulation(paper, polymer etc), gas bubbles in liquid insulation oraround an electrode in a gas (corona).

The term corona is often used for partial discharges thatoccur in gaseous media around conductor and thereforecorona is a particular kind of partial discharge.

Partial discharge magnitude is measured in pC (Pico

Coulomb) The equivalent circuit of a dielectric incorporating a cavity or

impurty can be modeled as capacitor


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Why should we test for partial discharge

1. By PD test we can detect weakness or any manufacturingdefect of insulating cone.

2. We can observe presence of foreign body particle.

3. For finding out location of defect.

4. To conform dielectric strength of insulating medium(SF6).

5. Critical field intensity or over electric stress point on buscoupling joints.

6. For burning/de ionized potentially harmful dust particle.

7. Find out overall good workmanship during installation.

8. If we didnt find PD point, then it will deteriorated insulationwith time & flashover possible after some time.


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UHF Sensors

The UHF (Ultra High Frequency) sensor mounted on theswitchgear is used to check the dielectric quality of the facility.

The sensor is fitted with aerial located inside gas compartment.

The signal received by the sensor is amplified then analysed.

It detects any internal malfunction.

UHF Sensor


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Role of UHF sensor

When ever PD occurs electromagnetic wave are emittedfrom test object. These signal have frequency ranging from

some 10 MHz to 3GHz i.e. in the band of VHF & UHF.

Around GIS test place due to high voltage induction differenttypes of HF electromagnetic wave are exist which interfereVHF signal and distort it but UFH signal are less effecting

with background noise and gives very good PD pattern.

Due to high S/N ratio forUHF band we are usingvery sensitive UHF sensor.


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How UHF sensor work

UHF sensor is a capacitive disk type sensor and work as ahighpass filter which allows pass only UHF band frequencysignal & block all lower frequency signal.

This signal are sent to frequencyanalyser by HF cable (Coaxial).


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General arrangement of PD test

With HV set


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Testing Kit

Without HV set


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Voltage Injecting kit

Control & monitoring panel

Compensating reactor

Step-up Generating

Transformer

RegulatingTransformer

SLD


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Capacitor formeasurement of PD


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Block Diagram


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Function of Reactor, Transformer & Coupling capacitorIn HV Kit

Variable reactorAs our test object have capacitive load it require highcapacitive reactive power so for compensate or putting PF

near unity, we put variable reactor also it is used to controlthe inrush current in case of a short circuit or flash over in thetested object.

Exciting Transformer

It is a step-up transformer used to generate the requiredhighest test voltage levels .

Coupling capacitor

The coupling capacitor can be used to measure the voltage

level applied to the test object or can be used to couple thepartial discharge pulses into the measuring circuit.


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Points to be remember before starting test

All compartments are filled SF6 gas at rated pressure. All grounding switches of the circuit being tested are in opened position. If VTs are designed for GIS test voltage, one side of the secondary winding

has to be connected to earth and wiring to local cubicle has to be

disconnected

If VTs not designed for GIS test voltage are removed or insulated byopening of a disconnecter .

The secondary winding of all CT must be shorted and grounded.

The Ferro-resonant system in VT should be disconnected. Surge arrestor are disconnected from GIS. Power transformer & cable boxes are isolated from GIS & connect to ground. External electromagnetic disturbance such as switching operations, lighting

equipment, corona or mobile phone may be prevented because they aremain source of noise.

All work on GIS are interrupted and staff is warned that a test will be carriedout.


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Test Sequence for switchgear >300KV

400kv

520kv

291kv

Conditioning

3 min60 s-50HZ

Max test voltage Partial Discharge

30 s

Voltage In KV

Time


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Test sequence

Injection Point


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Injection Point

Excitation

Trafro


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Test Report


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HV Test Report


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Test Sequence for switchgear


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HV Test report


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Signature of PD from spectrum analyser


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Shuffling particle

Moving particle

Jumping particle


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Presented by Vishal TELANG

Thanking you for kind attention

high voltage pd test on gis

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