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 requirements CT-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]
15
Class: 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] 15 Ratio: 1200-900/5 Quantity [Complying Offer]
Weight Saving TOTAL SAVINGS -0.15PX325R0.55 on 1200/5 tap [S1-S3] Class: 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] 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
C T D im e n s io 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 through Other types are for HV applications.
BASIC FORMULAE
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
CT – 1A or 5A Secondary?
1250/5 0.15 PX 100 Rct = 0.5 5P20 20VA Rb = 0.8 SSC = 25kA 1250/1 0.025 PX 100 Rct = 2.5 5P20 4VA Rb = 4 SSC = 25kACT – 1A or 5A Secondary?
1250/5 0.15 PX 300 Rct = 0.5 5P20 85VA Rb = 3.4 SSC = 25kA 1250/1 0.025 PX 250 Rct = 2.5 5P20 13.75VA Rb = 13.75 SSC = 25kACURRENT 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
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
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
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!!
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!!!
CT-Requirements
Old Specification new adapted Data
No Calculation CT-stability calculation
Multi-Ratio necesary ratios only
CT interchangable project specific data
What do we look at in CT specification
Knee Point Burden Sec. Current ALF Accuracy
C T D im e n s io 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
