LOW TENSION CURRENT TRANSFORMERS

It is not possible to use line current directly for measurement and relaying purpose due to its high value and insulation problem. CURRENT TRANSFORMER which is widely known as 'CT' is a piece of electrical equipment which converts line current values which are suitable for the devices to which is connected e.g. measuring instruments, meters, protection relays.

BASIC FUNCTIONS OF CTs ARE :

To reduce line current to a value which is suitable for measuring
instruments, relays etc.
To isolate the measuring instruments, meters, relays etc. from line
voltage of an installation.
To protect measuring instruments against short circuit currents
To sense abnormalities in current and give current signals to protective
relays to isolate the defective system.

INTRODUCTION :

THE MAIN FUNCTION OF A CURRENT TRANSFORMER IS TO

REPRODUCE THE PRIMARY CURRENT ON ITS SECONDARY SIDE & THUS

EXTEND THE RANGE OF INSTRUMENTS/PROTECTIVE DEVICES CONNECTED

IN ITS SECONDARY CIRCUIT. A C.T. BY ITS VERY NATURE OF APLICATION,

ISOLATES THE DDEVICES CONNECTED IN ITS SECONDARY CIRCUIT FROM

THE NETWORK SYSTEM INTO WHICH THE C.T. IS CONNECTED.

THERE ARE TWO BASIC TYPES OF C.T.s -

1 ) BAR PRIMARY &
2 ) WOUND PRIMARY

GENERALLY AS FAR AS POSSIBLE C.Ts. OF THE FORMER

TYPES ARE SELECTED BECAUSE THEY ARE MECHNICALLY STRONGER &

CHEAPER . IN A BAR PRIMARY C.Ts. THE WORKING AMPERETURNS ARE

DETERMINED BY THE PRIMARY CURRENT.

THEREFORE THE ACCURACIES THAT CAN BE OFFERED WITH

THESE C.Ts. BECOME PROGRESSIVELY INFERIOR AS THE RATED PRIMARY

CURRENT DECREASES.IN SUCH CASES , WOUND PRIMARY C.Ts. ARE USED.

A RING TYPE C.T. IS ESSENTIALLY A BAR PRIMARY C.T. WITHOUT THE

PRIMARY BAR . THESE RING TYPE C.Ts. ARE VERY EASY TO CONNECT IN

EXISTING SWITCHGEAR ASSEMBLIES AS THESE ARE MERELY SLIPPED

ON TO THE BUSBARS.

THE INDIAN STANDARD SPECIFICATION 4201 / 1967 (APPLICATION

GUIDE FOR CURRENT TRANSFORMER) AS ALSO VARIOUS INTER-

NATIONAL SPECIFICATIONS RECOMMEND THE FOLLOWING VALUES OF

RATED BURDENS OF VARIOUS TYPES INSTRUMENTS AND THE REQUIRED

CLASS OF ACCURACY FOR VARIOUS METERING APPLICATIONS : -

INSTRUMENT	VA BURDEN
AMMETERS	3
CURRENT COILS OF WATTMETERS, AND POWER FACTOR METERS.	5
CURRENT COILS OF KWH,KVAR METERS	5
RECORDING AMMETERS	3
CURRENT COILS OF RECORDING POWER FACTOR METERS AND WATTMETERS	5

APPLICATION	CLASS OF ACCURACY
FOR PRECISION TESTING OR AS A SUBSTANDARD FOR TESTING LABORATORY CURRENT TRANSFORMERS	0.1
FOR LABORATORY & TEST WORKIN CONJUCTION WITH HIGH ACCURACY INDICATING INSTRUMENTS, INTEGRATING METERS & ALSO SUB-STANDARD FOR TESTING INDUSTRIAL CTs.	0.2
FOR PRECISION INDUSTRIAL METERING	0.5
FOR COMMERTIAL & INDUSTRIAL METERING	0.5 OR 1
FOR USE WITH INDICATING & GRAPHIC WATTMETERS & AMMETERS	1 OR 3
FOR PURPOSES WHERE THE RATIO IS OF LESS IMPORTANCE e.g. AMMETERS WHERE APPROXIMATE VALUES ARE REQUIRED	3 OR 5

THUS IN A FACTORY WHERE AN AMMETER, AN ENERGYMTER & A POWER FACTOR METER IS REQUIRED TO BE INSTALLED THE BURDEN THAT
SHOULD BE SPECIFIED FOR THE C. T.WILL BE 3+5+5+2 (FOR LEADS)
=15 VA & ACUURACY CLASS THAT SHOULD BE 1 .
WHERE ONLY AN AMMETER IS REQUIRED ,A RATED BURDEN OF 5 VA
& ACCURACY CLASS 5 WILL BE SUFFICE.

TYPES OF LOW TENSION CURRENT TRANSFORMER:

TAPE INSULATED RING TYPE C.Ts WITH & WITHOUT MOUNTING ARRANGEMENT.
TAPE INSULATED WOUND PRIMARY C.Ts.
CORE BALANCE C.Ts. : USED IN DETECTING EARTH FAULTS OF VERY SMALL
MAGNITUDE. A THREE PHASE CABLE IS PASSED THROUGH THESE C.Ts &THE
UNBALANCED CURRENT IN THE THREE PHASES SET UP A DIFFERENTIAL
FLUX GIVING RISE TO A CURRENT ON THE SECONDARY SIDE OF THESE C.Ts.
SUFFICIANT TO ACTUATE A RELAY .
INTERPOSING C.Ts. : USED IN MATCHING THE RATIOS OF MAIN FEEDER C.Ts.
FOR DIFFERENTIAL PROTECTION SCHEMES. THESE ALSO FIND APPLICATION
AS SATURABLE C.Ts. USED IN CONJUCTION WITH MAIN FEEDER C.Ts. & THUS
PROTECT THE METERS CONNECTED TO THESE BY LIMITING THE CURRENT
FLOWING THROUGH THE METERS .

SUMMATION C.Ts. : WHEN CURRENT FLOWING IN MORE THAN ONEONE FEEDER IS REQUIRED TO BE METERED , SUMMATION C.Ts . ARE USED . THESE C.Ts . PROVIDED WITH MORE THAN ONE PRIMARY WDG. & ONE COMMON SECONDARY . THE STANDARD PRIMARY & SECONDARY
CURRENTS ARE 5 OR 1 AMP. SUMMATION C.Ts . ARE GENERALLY MANUFACTURE CONFIRMING TO ISS 6949
.BUSHING TYPE OR BUSDUCT C.Ts. : THESE C.Ts . ARE RING TYPE CONSTRUCTION & CAN BE MOUNTED ON BUSDUCT OR BUSHING TURRERT OF POWER TRANSFORMERS .
PRECISION GRADE C.Ts : THESE C.Ts . ARE OF ACCURACY OF 0.1 , 0.2 OR 0.5 & USED AS A STANDARD C.T , TO CHECK ACCURACIES OF OTHER TRANSFORMERS . THESE C.Ts . ARE EITHER WOUND WOUNDPRIMARY OR RING TYPE & MANUFACTURED IN TEAKWOOD CASES .

FOLLOWING ARE GUIDE LINE FOR ORDERING CTR :

* PRIMARY CURRENT
* SECONDARY CURRENT
* VA BURDEN
* ACCURACY CLASS
* INSTRUMENT SECURITY FACTOR
* ACCURACY LIMIT FACTOR
* ENVIRONMENTAL CONDITION
* APPLICATION
* FAULT CUURENT OF THE SYSTEM
* VOLTAGE OF THE SYSTEM
* ANY SPECIAL FEATURES REQUIRED

BAR PRIMARY LOW TENSION C.T. ( BPL )

SR . NO.	RATIO (Amp)	ID (mm) A	OD (mm) B	HT (mm) C	RATED BURDEN (VA)	ACCURACY CLASS
1	50/1	30	85	50	5	5.0
2	60/1	30	85	50	5	5.0
3	75/1	30	85	50	5	5.0
4	100/1	35	85	50	5	1.0
5	150/1	35	85	50	5	1.0
6	200/1	50	110	40	10	1.0
7	250/1	50	100	45	10	1.0
8	300/1	50	105	35	10	1.0
9	400/1	60	105	35	15	1.0
10	500/1	60	105	35	15	1.0
11	600/1	60	105	35	15	1.0
12	800/1	105	140	35	15	1.0
13	1000/1	105	140	35	15	1.0
14	1200/1	105	140	35	15	1.0
15	1500/1	115	165	35	15	1.0
16	1600/1	115	165	35	15	1.0
17	2000/1	115	165	35	15	1.0
18	2500/1	130	190	35	15	1.0
19	3000/1	130	190	35	15	1.0

BAR PRIMARY LOW TENSION C.T. ( BPL )

SR . NO.	RATIO (Amp)	ID (mm) A	OD (mm) B	HT (mm) C	RATED BURDEN (VA)	ACCURACY CLASS
1	50/5	30	85	50	5	5.0
2	60/5	30	85	50	5	5.0
3	75/5	30	85	50	5	5.0
4	100/5	30	85	50	5	1.0
5	150/5	30	85	50	5	1.0
6	200/5	50	110	40	10	1.0
7	250/5	50	100	45	10	1.0
8	300/5	50	100	35	10	1.0
9	400/5	60	105	35	15	1.0
10	500/5	60	105	35	15	1.0
11	600/5	60	105	35	15	1.0
12	800/5	105	140	35	15	1.0
13	1000/5	105	140	35	15	1.0
14	1200/5	105	140	35	15	1.0
15	1500/5	115	165	35	15	1.0
16	1600/5	115	165	35	15	1.0
17	2000/5	115	165	35	15	1.0
18	2500/5	130	190	35	15	1.0
19	3000/5	130	190	35	15	1.0

WPL (WOUND PRIMARY LOW TEBSION C.T.)

RATED BURDEN	15 VA	APPLICABLE STANDARD:
ACCURACY CLASS	1	I.S.S. 2705/1981 OR
RATIO	10/5 TO 200/5 (ANY SINGLE RATIO)	B.S.S. 3938/1973 OR I.E.C. 185/1966

NOTES :-