06 instrument transformer#

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Current Transformer Accuracy class and burden.I Class P I

Specified in terms of: Rated burden Class (5P, 10P) Accuracy Limit Factor (ALF)

Example: 15VA 1OP 20 A.L.F =Multiple of rated current up to which declared accuracy will be maintained with rated

burden connected.Z8 = Rated burden in ohms (!l)=Rated VA /IN 2Knee point voltage: VkValue of the voltage Vk applied to the secondary which, increased by 10%, causes amaximummagnetizing current, lm.

VA *ALFVk=--ln

06 Instrument Transformer

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Magnetizing characteristics

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Current transformer Equivalent circuit (normal/saturate)

CTcaution Don't open CT circuit during load.

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Secondary CT that doesn' t use, should be shorted. CT ratio > full load current.

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POTENTIAL TRANSFORMERType of PT

1. Electromagnet ic PT.

2. Coupling Capacitor VT. (CCVT)



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Potential transformerRequirements for protection VT, according to IEC 186 standard.Rated voltage factor kT:

Defines the maximum primary voltage that the VT can withstand depends on the earthingsystem and VT primary connection.Typical values:kT = 1.9 , 8 hours for phase-to-earth VT.kT = 1.2 unlimited for phase-tophase VT.

Accuracy class3P or6P

3P: 3% error for U between 5% Up and kTUp.6P: 6% error for U between 5% Up and kTUp.Example:Phase-to-phase VT used for protection : 230000V/1OOV 1OOVA cl 2P

VT caution

Don't short VT circuit during load/ Secondary VT thay doesn't use, should be opened. Choose VT burden properly to load. Ferromagnetic resonance

Phenomenon due to saturation of tha VT magnetic core during voltage fluctuations. Following switdhing operations Following a phase-to-earth fault (isolated neutral system)

CONSEQUENCES

Voltage oscillations that may reach 2Un. Increase in the current in the primary of the VT.


06 instrument transformer#

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