CT Sizing Guideline

CURRENT TRANSFORMERS

Copyright© Siemens AG Energy / Transmission / High Voltage Products

Why review CT’s Description of CB and CT Manufacturing Process Issues with CT’s Suggestions for Improvements - CT Specifications - CT Mix - CT Ratings - Other

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Energy / Transmission / High Voltage Products

CT-adapted toto CT-according protection specification requirements

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Energy / Transmission / High Voltage Products

High-Voltage Circuit-Breaker Type 3AP1 DT 72.5 up to 245 kV Product Characteristics
One interrupter unit per pole
Stored energy Spring Drive Mechanism
Rated voltages up to 245 kV
Rated short circuit breaking current up to 63 kA
Type tested according to IEC 62271-100
Equipped with bushing type current transformers
Ambient temperature range from -55 °C up to +50 °C with pure SF6

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Energy / Transmission / High Voltage Products

Current Transformer

Sealing ring

Cowl Cores with secondary winding

Housing

Connecting leads

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Energy / Transmission / High Voltage Products

CT Specifications and Consequences

Standard special very special

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Energy / Transmission / High Voltage Products

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Energy / Transmission / High Voltage Products

Cost Savings per CB Very Special Small Housing [Complying Offer] Ratio:

Quantity

1200-900/5

15 Class:

0.25PX300R0.45 on 900/5 tap [S1-S2] 0.15PX400R0.58 on 1200/5 tap [S1-S3]

Special Small Housing [Alternative Offer] Ratio:

1200-900/5

Class:

0.25PX250R0.4 on 900/5 tap [S1-S2]

15 0.15PX325R0.55 on 1200/5 tap [S1-S3] TOTAL SAVINGS - (Estimated)

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Approx 10k per CB

Energy / Transmission / High Voltage Products

Weight Saving Very Special Small Housing [Complying Offer] Ratio:

1200-900/5

Class:

0.25PX300R0.45 on 900/5 tap [S1-S2] 0.15PX400R0.58 on 1200/5 tap [S1-S3]

Special Small Housing

Weight 150kg per bushing Total 900kg

[Alternative Offer] Ratio:

1200-900/5

Class:

0.25PX250R0.4 on 900/5 tap [S1-S2] 0.15PX325R0.55 on 1200/5 tap [S1-S3]

TOTAL SAVINGS -

129kg per bushing Total 774kg

SAVINGS 126kg per CB

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Energy / Transmission / High Voltage Products

CT Dimension

What do we look at in CT specification

Knee Point

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Burden

Sec. Current

ALF

Accuracy

Energy / Transmission / High Voltage Products

CURRENT TRANSFORMER CONSTRUCTION Current transformers can be constructed in a number of different ways:
WOUND PRIMARY: with a fixed primary with a number of turns around the core;the winding is fully insulated and form an integral part of the assembly
BAR PRIMARY: this has a fixed insulated primary conductor passing through the core and essentially the primary conductor is equal to 1 turn

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Energy / Transmission / High Voltage Products

CURRENT TRANSFORMER CONSTRUCTION( cont.)

BUSHING TYPE:this has no primary winding;it is for use with a fully insulated conductor acting as the primary winding. WINDOW TYPE:this has no primary winding but it normally has insulation in the window so that conductors can be passed through Other types are for HV applications.

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Energy / Transmission / High Voltage Products

BASIC FORMULAE

MAXIMUM SECONDARY WINDING VOLTAGE

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Energy / Transmission / High Voltage Products

BASIC DESIGN CONCEPTS (Cont.)

The more accurate the reflection in the secondary of the primary current the smaller the current and phase angle errors. These errors are dependant on :
Excitation ampere turns
Type of magnetic material used
Configuration of the transformer;in general the smaller the ID and OD the more accurate the transformer for the same excitation Ampere - turns

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Energy / Transmission / High Voltage Products

CT – 1A or 5A Secondary? 1250/5 0.15 PX 100 Rct = 0.5 5P20 20VA Rb = 0.8 SSC = 25kA

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1250/1 0.025 PX 100 Rct = 2.5 5P20 4VA Rb = 4 SSC = 25kA

Energy / Transmission / High Voltage Products

CT – 1A or 5A Secondary? 1250/5 0.15 PX 300 Rct = 0.5 5P20 85VA Rb = 3.4 SSC = 25kA

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1250/1 0.025 PX 250 Rct = 2.5 5P20 13.75VA Rb = 13.75 SSC = 25kA

Energy / Transmission / High Voltage Products

CURRENT TRANSFORMER CHOICE

MEASUREMENT CTS
Require good accuracy up to 120% rated current
Require low saturation levels to protect instruments

PROTECTION CTS
Require accuracy up to many times rated current
Require definite voltage outputs at below saturation levels

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Energy / Transmission / High Voltage Products

CT – Magnetising Curve

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Energy / Transmission / High Voltage Products

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Energy / Transmission / High Voltage Products

MARKET CONSIDERATIONS

THE SPECIFIER
Should ensure that the appropriate class for each application is used;specifying a greater burden than actually required may not give you the accuracy you want!!!
The protection class requirements should be accurately calculated as the specifying of voltages higher than require only result in higher prices

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Energy / Transmission / High Voltage Products

MARKET CONSIDERATIONS (Cont.) THE PURCHASER
Consider carefully the implications of the specification!
Do not use metering CTs for Protection applications:it will cost more later when the end user has problems with its protection system.
Ensure that the CTs you use have the correct mechanical characteristics;ensure that the materials used are adequate to withstand the temperatures of your busbars!!!

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Energy / Transmission / High Voltage Products

TYPICAL EXAMPLES METERING APPLICATION

Customer asks for low ratio with high burden and class Typical 15 VA 1M Result: Large and expensive CT using mumetal

ALTERNATIVE Determine what connected load is; Typical now is electronic with small VA burden typical 2.5 or 5VA

RECOMMEND ALTERNATIVE

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Energy / Transmission / High Voltage Products

TYPICAL EXAMPLE PROTECTION APPLICATION

Customer specifies differential CT’s with large knee point voltage CT size is too large to fit in switchgear Check voltage requirement based on relay recommended formulas!!!! 90% of time customer specified knee point voltage is much higher than needed!! Result : a lower cost ct that can fit in your gear

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Energy / Transmission / High Voltage Products

TYPICAL SWITCHGEAR APPLICATIONS How are dimensions of CTs affected??? Remember the basic formula ,voltage is dependant on: turns; flux density in core and size of core If a customer wants a larger internal diameter then the price will go up due to:
Increased weight of core to compensate for lower flux density; remember flux density is proportional to magnetising force and for a given number of turns the magnetizing force of ampere turns /metre is indirectly proportional to the mean length of core i.e,
The larger the diameter of the core the lower magnetising ampere turns the lower the flux density!!!

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Energy / Transmission / High Voltage Products

CT-Requirements

Old Specification No Calculation Multi-Ratio CT interchangable High Burden

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new adapted Data CT-stability calculation necesary ratios only project specific data adapted burden

Energy / Transmission / High Voltage Products

CT Dimension

What do we look at in CT specification

CT Design Consideration : Knee Point

Burden

Sec. Current

ALF

Accuracy

Don’t re-use old specifications Do your studies to optimise CT considering: •Fault Levels •X/R ratio •Relay response times

Work with the manufacturer . . . . Page 26

Energy / Transmission / High Voltage Products