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CURRENT TRANSFORMER
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CURRENT TRANSFORMER
Definitions
A device used to step down current from high value (primary
current)to a low value (secondary current).
Low value current is easy to measure, no need to develop
highcurrent measuring equipment.
The current ratio (primary current/secondary current) is
substantiallyconstant for a given range of primary current.
The phase angle error (secondary current compared to
primarycurrent) is within specified limit.
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Function
1. ProtectionCurrent transformer connected to protection
equipment(protection relay). The protection relay can be set to
identifywhether the input current (current transformers
secondarycurrent) indicate that fault occur in the system.
2. MeasurementCurrent transformer connected to measurement
equipment(ammeter, energy meter). Measure the current flow in the
circuit.
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Construction Type
1. Ring or WindowThe popular type is ring shape, rectangular
shape and ovalshape.
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Construction Type
2. Split CoreThe shape similar to ring or window construction,
the core aresplit into two part.
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Construction Type
3. Primary Wound (Block)For low current rating. both primary and
secondary are Woundover the core.
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Schematic Diagram
1. Single Ratio
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CURRENT TRANSFORMER
Schematic Diagram
2. Double RatioTransformer with an intermediate tapping on
secondary winding.(terminal S1, S2 & S3) Ratio of tapping as
per designrequirement. During service, unused tapping remain
open.
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CURRENT TRANSFORMER
Schematic Diagram
3. Multi Ratio
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CURRENT TRANSFORMER
Location In The Single Line Diagram
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Connection to Measurement or Protection Devices
Measurement or protective devices are load to the
currenttransformer. The circuit connection shall be in series.
Current transformer not connected to the load when installed to
theswitchgear shall be shorted to ground (secondary terminal
connectedto ground).
Open circuit is a condition when the current transformer is
not
connected to its load. Open circuit during availability of
primarycurrent will result of high voltage appear at the secondary
terminal(>1kV A.C).
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CURRENT TRANSFORMER
Connection to Measurement or Protection Devices
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Typical Current Transformer Description
3000/1A - Current RatioCL.: 0.2 - Accuracy Class15VA - Rated
Burden
3000/1A - Current RatioCL.: 5P20 - Accuracy Class & Accuracy
Limit Factor30VA - Rated Burden
3000/1A - Current RatioCL.: X - Accuracy ClassVkp: 300V - Knee
Point VoltageRct: 12 - Burden (Resistance)
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Term & Definitions
Rated Primary CurrentThe value of primary current which
specified by manufacturer (shallbe based on customer/purchaser
requirements). E.g. 3000A, 1600A,450A, etc.
Secondary CurrentThe value of secondary current which specified
by manufacturer (hasbeen standardized). E.g. 1A or 5a.
Current RatioThe ratio of Rated Primary Current to Rated
Secondary Current. E.g.3000/1A, 1600/5A. etc.
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Term & Definitions
Rated BurdenThe burden assigned by manufacturer at which the
currenttransformer performs with specified accuracy expressed in
VA.Connected burden shall be less than specified rated burden.
BurdenTotal load connected across the secondary of current
transformerexpressed in VA or at rated secondary current.
Accuracy ClassThe class assigned to the current transformer with
the specifiedlimits of ratio error and phase angle error. Accuracy
class specifiesby BS & IEC standard. E.g. X, 5P, 10P, 0.2, 0.5,
1,etc.Class X, 5P, 10P for protection. Class 0.2, 0.5 for
measurement.
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Term & Definitions
Accuracy Class
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Term & Definitions
Accuracy Limit FactorE.g. 10, 20.
Knee Point VoltageThe point at which C.T is saturate where 10%
increase in voltagecauses a 50% increase in magnetizing
current.
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Testing
Polarity TestTo prove that a group of C.Ts has been manufactured
and installedwith the correct polarity.
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Testing
Magnetization CurveTo determine the secondary voltage and
current at which the C.Tmagnetic core begin to saturate. The Result
should be compared tomanufacturers curve.
The point at which saturation is deemed to become significant
isknown as Knee Point Voltage, where 10% increase in voltagecauses
a 50% increase in magnetizing current.
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Testing
Magnetization Curve
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CURRENT TRANSFORMER
Testing
Magnetization Curve
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CURRENT TRANSFORMER
Testing
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Testing
Burden CheckMeasurement of secondary resistance at terminal
SI-S2 with anohmmeter. The value shall be as manufacturer
specification.
Insulation Resistance TestCarried out when the wiring between
C.T and the connectedequipment completed. To measure the C.T
insulation resistance. Thevalue shall be as manufacturer
specification.
Primary InjectionShould be carried out when the wiring is
completed. To enablemeasurements to be made of the secondary
quantities as verificationof the correct installation of the C.T,
the connected wiring andequipment.
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Typical Installation Inside The Gas Insulated Switchgear
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Typical Installation Inside The Gas Insulated Switchgear
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CURRENT TRANSFORMER
Typical C.T Construction
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Typical complete assembly
C.T without outer protection
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Typical C.T Construction
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Typical C.T Construction
CURRENT TRANSFORMER
http://en.wikipedia.org/wiki/Image:Leistungsschalter-110KV.jpg
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Typical C.T winding machine
CURRENT TRANSFORMER
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