Current Transformer Basics - Accuracy Class; Burden; Knee Point; CT Errors

Описание: Current Transformers are classified into two different categories based on their applications in electrical power system network.
1. Protection CT
2. Measuring CT
Protective current transformers are used with relays, trip coils, pilot wires etc.
Metering current transformers are used with Ammeter, Wattmeter etc.

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Errors in CT: There are three types of CT errors as follows:
Ratio Error or Current Error: Ratio error is defined as the percentage error in the magnitude of the CT secondary current.
Phase Angle Error: It is defined as the phase angle between primary current vector & the reversed secondary current vector.
Composite Error: It is defined as the r.m.s. value of the difference (Knis - ip) integrated over one cycle under steady condition.

Accuracy Class of CT: Accuracy class of CT is the class assigned to the CT with the specified limits of ratio error and phase angle error.

CT Burden: The circuit or load connected to the secondary winding of CT is called ‘CT burden’ which is expressed in ‘VA’. The burden on a protective CT comprises of the individual burdens of associated relays, trip coils, connecting leads etc. Total burden is calculated by adding all of these individual loads.

Why CT Secondary is never Open Circuited?
If the CT secondary is open-circuited, the secondary peak voltage may reach up to some kilo volts. The working flux increases and CT core gets saturated. CT primary and core get over-heated and voltages are induced in secondary side by electro-magnetic induction. This is very dangerous for the working personnel on the secondary side. Hence, CT secondary should never be disconnected when current is flowing in the primary side.

CT Magnetization Curve:
Magnetization Curve of CT can be sub-divided into four main regions.
1. Origin to ankle point
2. Ankle point to Knee point
3. Knee region
4. Saturation region

Knee point voltage of CT:
Knee point is defined as the point where 10% increase in the applied voltage will cause 50% increase in excitation current. Above the knee point, CT will mal-operate and will tend to saturate.

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