Appl of ITs-VT

8/2/2019 Appl of ITs-VT

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Voltage Transformers

Voltage Transformers for

- Metering Should Maintain accuracy Between 80 % and 120 %

of Rated Voltage and 25 % to 100 % of Rated Burden

- Protection Should Maintain Accuracy Between 5 % of Rated

Voltage and Voltage Factor times Rated Voltage

Voltage Factors:

- 1.5 for 30 seconds for effectively Earthed System

- 1.9 for 30 seconds for Non-effectively Earthed Systems

_ 1.9 for 8 hours for Resonance Earthed System


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VECTOR DIAGRAM OF VOLTAGE TRANSFORMER

Fig1(C)Fig 1(B)

Fig1(A)


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4/25

ACCURACY REQUIREMENTS OF PROTECTIVE

VOLTAGE TRANSFORMERS

lass Ratio Error % Phase

Displacement

(Minutes)

3P +/- 3.0 +/- 120

6P +/- 6 +/- 240


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Hypothetical 9 MVAr

Capacitor Bank

Number of Groups (X) : 3

No. of capacitor units/

group:(M)

10 of 100 kVAr 6.35 kV


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Rated Residual Voltages

Preferred Values Non-Preferred Values

100 or 110 V 200 V

100/3 or 110 /3 V 200/3 V

100/3 or 110/3 200/3

Taking the Case of Hypothetical Bank shown in theprevious slide:-

Voltage on the remaining units with %Rise Reflected value with 200V

n capacitor removed in a group RVT winding voltage

One unit - 6885 V 108 0.65 V

Two units - 7520 V 118 0.675 V

As the difference is very low for better discrimination

Residual voltages of 380 V or 570 V are used


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N 2

Oil

Single Stage Dead Tank Voltage Transformer


9/25

Schematic Diagram of Layer type Primary Winding

and Multiple Secondary Windings

layers

Inter-layer Capacitance

Earth shield

Secondary Windings


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Multiple Secondary windings-Special Precautions

National and International Standards specify that--

In case of multiple secondary windings the accuracy on individual windings

should be maintained between 25 % and 100 % rated burden,

- while the burden on the other windings is varying between zero to 100% of

their rated value.

In case of a three secondary windings with say,

Winding 1: 200 VA with metering accuracy 0.5

Winding 2 : 200 VA with protection accuracy 5P

Winding 3: 200 VA for residual voltage voltage measurement

-the accuracy of the metering winding 1, will have to be maintained 25% of its

rated burden i.e 50 VA to virtually 300 VA when the other windings are

simultaneously loaded(considering that primary and secondary contributeequally for errors)

As the some of protection windings like winding 3 come into

operation for a short duration under fault condition,it is

advantageous to except load on this winding for purposes of

accuracy measurement on winding 1


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Circuit Diagram

Oil

Two Stage Cascade

Voltage Transformer

N 2


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Four Stage Cascade Voltage Transformer

N 2

Oil


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14/25

FAILURE OF VOLTAGE

TRANSFORMERS DUE TO DISCHARGE OF CAPACITOR CHARGE

THRO THE PRIMARY WINDING

DUE TO FERRORESONANCE

MECHANICAL FAILURE OF PORCELAIN DUE TO

GROUND VIBRATIONS - LIKE

EARTHQUAKEAS


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Discharge of capacitor charges:

q Discharge of residual charge from switched off linesq Discharge of capacitor banksRemedy:

1. Determine magnitude and waveform of discharge current.

2.Refer to manufacturer.

3.Divide capacitor bank into two or more banks


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High Voltage Busbar

Isolator

Isolator

Circuit Breaker

with Grading Capacitors

Voltage

Transformer

Cb - Grading CapacitorsCs -Capacitance to Ground

V.T - Iron Cored Voltage Transformer

V.T


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Fig 26. Points of operation of the circuit(Only first quqdrant shown)

Vl = f(I)

Vc+V Vc-V


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1)

2)

)

Under Ferro-resonance

condition the voltage waveform

exhibits presence of higher

harmonics and increasedvoltage.

The voltage magnitude goes up

to 1.4 P.U.

The current magnitude jumpsup almost hundred-fold,

resulting in severe dielectric

and thermal overloading of V.T

and eventual failure


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FAILURE OF V.Ts DUE TO FERRO RESONANCE

IN INDIA

1. RISHIKESH S/S OF UPSEB2. SHARAVATI S/S OF KSEB

3. MSEB (PROBABLY IN 1987)

ABROAD

1. LABORELEC REPORT MENTIONS OF FAILURES

AT 8 PLACES BETWEEN 1967 AND 1975

FERRO RESONANCE IS KNOWN TO HAVE CAUSED FAILURE OFPOWER TRANSFORMER CONNECTED ON THE IDLE CIRCUIT OF

A DOUBLE CIRCUIT POWER LINE . FERRORESONANCE OCCURREDACROSS THE CAPACITANCE BETWEEN THE LINES AND THE Xr.


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REMEDIES:

1. OPEN THE BREAKER ISOLATOR IMMEDIATELY

AFTER OPENING THE BREAKER ( Isolators to be motor

operated and remotely controlled)

2. USE VTs WITH LINEAR MAGNETISATION

CHARACTERISTIC ACROSS BUSBARS

FERRORESONANCE


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V.T FAILURES ARE KNOWN TO OCCUR BECAUSE OF

INCIDENCE OF HIGH VOLTAGE HIGH FREQUENCYSURGES DURING CAPACITOR SWITCHING

MODE OF OCCURRENCE

1. PD ONSET BECAUSE OF INCIDENCE OF SURGE

2. SUSTAINED PARTIAL DISCHARGES EVEN AFTERRESTORATION OF NORMAL VOLTAGE

3. GRADUAL DETERIORATION OF INSULATION AND EVENTUAL FAILURE

REMEDY:

EMTP STUDY OF SUBSTATIONS WHERE LARGECAPACITOR BANKS ARE LOCATED


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OTHER MECHANICAL REASONS FOR FAILURE

SEALING FAILURE---Allowing free breathing ofatmospheric air

DRY BAND FORMATION--During pre-monsoon showers

and un-seasonal rains OPEN EARTH CONNECTION OF PRIMARY WINDING --

Causing arcing,failure of enamel insulation on windingwire and eventual failure .

FAILURE OF FRAGILE PORCELAIN DUE TO GROUNDVIBRATIONS.

FAILURE OF HIGH VOLTAGE TERMINAL DUETO MECHANICAL FORCES LIKE WIND LOADS -TESTS FOR COMPLIANCE ADDED IN IEC 60044-2 Part 2


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Routine Maintenance

At the time of Installation:

Porcelain:

Inspect Porcelain for Chipping, Cracks and

Leakage of Oil through flanges.

Oil Sampling:- To be kept to the barest minimum

- BDV to be not less than 60 kV


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Routine Maintenance

After Three months of Service :

Nitrogen Pressure to be Checked.

Pressure at the Top through N2 Filling Valve:- to be between 0.2 and 0.3 kg/ sq.cm in the

temperature range of 20 to 30 deg C.

Pressure at the Tank Valve:- to be between 0.38 and 0.48 kg/sq.cm taking into

account static head of oil.


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Periodic Maintenance

Yearly:

-Inspect Porcelain for chipping, cracks and

oil leakage through flanges.

-Clean porcelain with dry cloth

Once in two years:

BDV and moisture content of oil.Depleted

oil can be made up through valve on tank

Documents

Appl of ITs-VT