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
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
