Instruments Transformers

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Instrument Transformers

- Instrument transformers are designed to

transform voltage or current from the high values

in the transmission and distribution systems to

the low values that can be utilized by low voltagemetering devices and protection.


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Instrument Transformers Primary

Applications:

1. metering- for energy billing and transaction purposes

2. protection control - for system protection and protective

relaying purposes

3. load survey - for economic management of industrial

loads.


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Instrument Transformers Functions

1. To transform current or voltage from high value to handle for protection

relays and instruments.

2. To insulate the metering circuit from the primary high voltage system.

3. To provide possibilities of standardizing the instruments to a few rated

currents and voltages.


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Types Instrument Transformers

1. Current Transformer (CT)

2. Voltage Transformer or Capacitive

Voltage Transformer (CVT)


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Instrument Transformers Typical

Output Level

CT VT

1-5 Amperes 115 120 Volts


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Current Transformers

Are instrument transformers that provide

insulation against the high voltage of the powercircuit and also supply the relays with quantities

proportional to those of the power circuit but

sufficiently reduced in magnitude so that relays

can be made relatively small and inexpensive.

CT usually place near the circuit breaker of anycircuit. IF CT breakdown, the protective

equipment is both Blind and Deaf.


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Manufacturer ABB (Sweden)

Type/Model IMBD 245 A5; A4; A2

Rated Voltage 245 kV

Rated Currents 50-4000 A

Insulation level 460-1050 kV

Highest system voltage 245 kV

Frequency 60 Hz

Insulation level 460-1050 kV

Standard IEC 185

Class: Metering 0.2FS10 and 0.5FS10

Protection 5P20/X

Total mass 1720 kg

Mass of oil 315 kg

Current Transformers

Specifications


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Capacitive Voltage Transformer

A voltage transforming equipment using acapacitance voltage divider connected between

phase and ground of a power circuit. CVT are

placed at the entry point of a

switchyard/substation for voltage indication on

an energized circuit.


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Instrument Transformer Basic Tests

Dos and Donts for Instrument Transformer

1. Never open circuit a current transformer secondary

while the primary is energized.

CTs are intended to be proportional current devices.

Very high voltages can result from open circuiting the

secondary circuit of an energized CT. Even very small

primary currents can cause damage


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Instrument Transformer Basic Tests

Dos and Donts for Instrument Transformer

2. Never short circuit the secondary of an energized VT

VTs are intended to be used as proportional voltage

devices.

Damaging current will result from short circuiting the

secondary circuit of an energized VT.


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Instruments Transformers

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