Download - CT Sizing Guideline

Copyright© Siemens AG

Energy / Transmission / High Voltage Products

CURRENT TRANSFORMERS


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


CT-adapted to protection requirementsCT-according to specification


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


Current Transformer

Cowl

Sealing ring

Cores with secondary winding

Housing

Connecting leads


CT Specifications and Consequences

very special

special

Standard


Cost Savings per CB

Approx$ 10k per CB

TOTAL SAVINGS - (Estimated)

0.15PX325R0.55 on 1200/5 tap [S1-S3]

15Class: 0.25PX250R0.4 on 900/5 tap [S1-S2]

Ratio: 1200-900/5

[Alternative Offer]

Special Small Housing

0.15PX400R0.58 on 1200/5 tap [S1-S3]

Class: 0.25PX300R0.45 on 900/5 tap [S1-S2]

15Ratio: 1200-900/5

Quantity[Complying Offer]

Very Special Small Housing


Weight Saving

TOTAL SAVINGS -

0.15PX325R0.55 on 1200/5 tap [S1-S3]


Ratio: 1200-900/5

[Alternative Offer]

Special Small Housing

0.15PX400R0.58 on 1200/5 tap [S1-S3]


Ratio: 1200-900/5

[Complying Offer]

Very Special Small Housing

Weight

150kg per bushing

Total 900kg

129kg per bushing

Total 774kg

SAVINGS

126kg per CB


What do we look at in CT specification

Knee Point Burden Sec. Current ALF Accuracy

CT

D

ime

nsio

n


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


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 throughOther types are for HV applications.


BASIC FORMULAE

MAXIMUM SECONDARY WINDING VOLTAGE


BASIC DESIGN CONCEPTS (Cont.)

The more accurate the reflection in the secondary of the primarycurrent 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


CT – 1A or 5A Secondary?

1250/5 0.15 PX 100 Rct = 0.55P20 20VA Rb = 0.8SSC = 25kA

1250/1 0.025 PX 100 Rct = 2.55P20 4VA Rb = 4SSC = 25kA


CT – 1A or 5A Secondary?

1250/5 0.15 PX 300 Rct = 0.55P20 85VA Rb = 3.4SSC = 25kA

1250/1 0.025 PX 250 Rct = 2.55P20 13.75VA Rb = 13.75SSC = 25kA


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


CT – Magnetising Curve


MARKET CONSIDERATIONS

THE SPECIFIER

� Should ensure that the appropriate class for each application isused;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


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!!!


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


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


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 ampereturns the lower the flux density!!!


CT-Requirements

Old Specification � new adapted DataNo Calculation � CT-stability calculationMulti-Ratio � necesary ratios onlyCT interchangable � project specific dataHigh Burden � adapted burden


What do we look at in CT specification

Knee Point Burden Sec. Current ALF Accuracy

CT

D

imensio

n

CT Design Consideration :

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 . . . .