CT, PT And CVT
It help in measurement of high voltage And current.
* It provide low voltage and current to the electrical device.
* It’s measure application is in the protection system where relay is work on low amount of voltage and current.
3. ▪ Potential Transformer
Principle of operation
Construction ,Classification & Application
▪Electromotive voltage
transformer
Introduction
Construction & Application
▪ Capacitive voltage transformer
Introduction
Construction & Application 3
4. [ Introduction of instrument
transformer ]A electrical device which help in measurement
of high voltage or current.
It provide low range current and voltage to the
protection system.
By using low range ammeter can measure
high current.
By using low range voltmeter can measure
high voltage.
4
5. [ Types of instrument transformer ]
Instrument transformer has two type
1) Current transformer
2)Potential transformer
5
6. [ current transformer ]
It is normally known as CT.
This is a type of step up transformer.
This is step up transformer because when
we step up, the voltage increases and
current decrease.
The current is step down in a known ratio
called current ratio.
6
7. [Construction of current transformer]
7
The current transformer has primary coil
of one or less no. of turns of thick wire
which is connected in series with the
transmission line.
It’s core is made of by rolled steel.
The secondary consist of large number of
turns of fine wire.
The secondary and primary winding are
insulated to each other and oil immersed.
10. [classification of current transformer]
On the basis of primary winding
1) Wound type
2) Bar type
On the basis of secondary winding
1) Ring type
2) Stadium type
3) Rectangular type
On the basis of working
1) Summation CT.
2) Core balance CT.
3) Interposing CT. 10
11. [ Standards ]
The Indian and international standard for CT
o Standard Standard no. Year⁞ ⁞
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
British BS 3938 1973⁞ ⁞
American ANSI C.57.13 1978⁞ ⁞
Australian AS 1675 1986⁞ ⁞
India IS 2705 1992⁞ ⁞
IEC IEC 60044-1 1996⁞ ⁞
(International electro technical commission)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Manufacturing company
BHEL,CGL,ABB,Joti,Siemens… Etc.
11
12. [ Testing of current transformer ]
Different type of test :
Accuracy test
Dielectric insulation test
Temperature rise test
Verification of polarity test
Different method of testing
Mutual induction method
Silsbee’s method 12
13. [ Specification ]
For a 11 kv CT.--------------------------------------------------------------------------------------------
System voltage : 11 kv
Insulation level voltage : 12/28/75 kv
Ratio : 200/1-1-0.577 A
Core 1 : 1 A metering
Core 2 : 1 A protection
Core 3 : 0.577 A, Class power supply
Short time rating 20 kA for 1 sec
Insulation security factor : ISF<10 13
14. [ POTENTIAL
TRANSFORMER ]Potential transformer normally known as
PT.
That is a type of step down transformer.
Step down transformer ,step down the
voltage and rise the current.
The voltage is step down in the known
ratio called voltage ratio.
14
15. [ principle of operation ]
It is always connected in parallel to the
transmission line.
It’s secondary winding is very low with
respect to the primary.
The secondary winding is designed so
that a voltage of 100 to 150 v is delivered
to the instrument load.
Line voltage =voltmeter reading/Ratio
Ratio=no. of turn in secondary/no.of turn in primary
15
18. [ construction of potential
transformer ]
Core size is large and made of by steel.
Primary side large no. of turn.
Secondary side only few no. of turn.
Secondary and primary winding are
insulated to each other and oil immersed.
18
19. [ classification of potential
transformer ]
Types of potential transformer
1) Shell type
2) Dry type
3) Oil type
-----------------------------------------------------------------------------------------------------------------------------------------
Rating Type
------------------------------------------------------------------------------------------------------------------------------------------
Below 5000 v Shell type
5000 – 138000 v Dry or oil type
Above 138000 v Only Oil type
--------------------------------------------------------------------------------------------------------------------------------------------
19
20. [ Standards ]
The Indian and international standard for PT
o Standard Standard no. Year⁞ ⁞
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
British BS 3841 1973⁞ ⁞
American ANSI C.57.13 1978⁞ ⁞
Australian AS 1243 1982⁞ ⁞
India IS 3156 1992⁞ ⁞
IEC IEC 186 1987⁞ ⁞
(International electro technical commission)
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Manufacturing company
BHEL,CGL,ABB,Joti,Siemens… Etc. 20
21. [ Testing of Potential transformer ]
Different type of test :
Accuracy test
Dielectric insulation test
Temperature rise test
Verification of polarity test
Different method of testing
Absolute null method
Employing capacitance divider method
21
22. [ specification ]
For a 11 kv VT :
System voltage : 11kv
Insulation level voltage : 12/28/75 kv
Number of phase : Three
Vector Group : star/star
Ratio : 11 kv/ 110 kv
Burden : 100 VA
22
23. [ Electro motive voltage
transformer ]
o Why not conventional type potential
transformer.
Above 100 kv , size are very large.
Insulation problem
High heating effect
Costly to build
23
24. [ designs ]
1. Insulated casing
Use high voltage insulated oil
2. Moulded rubber potential transformer
Replace oil and porcelain bushing to
the synthetic rubber.
3. Cascaded transformer
For voltage divider cascaded n no. of
transformer.
24
25. [ capacitive voltage
transformer ]
A less expensive alternative of electro
magnetic voltage transformer.
Use no. of capacitor as a voltage
divider.
Less insulation problem.
Very cheaper.
Simple circuit.
25
27. [ Design ]
Use capacitor band.
Use an Auxiliary transformer.
Secondary have very low voltage.
Any bus bar protection system or any
measuring device connect to the
secondary of the auxiliary transformer.
27
29. Application of instrument transformer
* It help in measurement of high voltage or current.
* It provide low voltage and current to the electrical
device.
* It’s measure application is in the protection system
where relay is work on low amount of voltage and current
29
30. REFERENCES :
• Introduction to the electrical machines.
Macmillan.
• Hand book of transformer design and
applications. McGraw-Hill.
• IEEE Transactions on power Apparatus
And System.
30