Instrument Transformer Rev1

INSTRUMENT TRANSFORMERS

CURRENT & VOLTAGE

TRANSFORMERS (CT / PT)


CURRENT / VOLTAGE TRANSFORMER

Produce a scaled down replica of the Primary System Quantities (Current or voltage) to the accuracy expected for the particular measurement.

Performance depends upon ability to transform Primary Quantity in the presence of transient D.C. Off-sets, non-sinusoidal wave forms etc, particularly for protection application.

CT Primary loading is independant of secondary loading, unlike in case of Power / Potential Transformers.


CURRNT TRANSFORMERS – 3 BASIC TYPES of CTs

1. MEASURING CURRENT TRANSFORMER

Used to supply indicating / integrating Instruments. Operating Range : 5% - 120% of Rated Current.

2. PROTECTIVE CURRENT TRANSFORMER

Used with Protective Relays and similar Apparatus. Operation specified over a wide range of Current ( Accuracy

specified at rated to ALF times rated current).

3 DUAL PURPOSE CURRENT TRANSFORMER

Intended for Metering and Protection. Complies with the requirement of both Measuring / Protective CT

in terms of Range / Accuracy.


MEASURING CURRENT TRANSFORMER (IS 2705- 1992 – Part II)

Specified in terms of Accuracy Class , VA Rating and Instrument Security Factor ( e.g. Class 1.0, VA = 15, ISF = 3.0)

Standard Accuracy Class= 0.1, 0.2, 0.5, 1.0, 3.0, 5.0.

Error specified at 5 – 120% of Rated Current. Higher errors permitted at lower currents. For example, Class 1.0 CT has a ratio error of:

(+/-) 1 % at 100-120% of Rated Current (+/-) 1.5% at 20% of Rated current (+/-) 3% at 5 % of rated current.

Error specified with 25-100% of Rated Burden connected.


PROTECTIVE CURRENT TRANSFORMER (IS2705, 1992, PART III)

Specified in terms of Accuracy Class, VA Burden & Accuracy Limit Factor (ALF) e.g. 5P10, 15 VA.

Standard Accuracy Class : 5P, 10P, 15P. Standard VA Ratings : 5, 10, 15, 30. Standard ALF : 5, 10, 15, 20, 30. For economical design VA x ALF should not exceed 150. Error specified at Rated Current & ALF times Rated Current e.g. 5P10, 15 VA CT has a composite error of :

(+/-) 5% at ALF times Rated Current with rated burden connected (+/-) 1% at Rated Current with rated burden connected. For a given CT, burden & ALF are inversely related i.e. If

connected burden is less than rated, ALF is up.

INSTRUMENT TRANSFORMER

PROTECTIVE CURRENT TRANSFORMER

CO-RELATION BETWEEN BURDEN & ALF

RATED BURDEN (15VA) CONNECTED. ALF Times Rated current = 10 x 5 = 50A Total Burden = 0.60 + 0.15 = 0.75 Ohms. Voltage developed by CT at ALF (V) = 50 x 0.75 = 37.5 V


PROTECTIVE CURRENT TRANSFORMER

REDUCED BURDEN (5VA) CONNECTED. Total Burden = 0.20 + 0.15 = 0.35 Ohm. Max. Secondary current developed by CT at limiting voltage of

37.5V = 37.5 / 0.35 = 108A (21.5 times Rated Current) Thus, at reduced burden of 5 VA, ALF increases to 21.5.


PROTECTION CT FOR SPECIAL PURPOSE APPLICATIONS (IS 2705, 1992, PART IV)

Designated as Class “PS” CT(Class “X” as per BS). Used for balanced protections like Differential, REF where required characteristic can not be conveniently expressed in terms of Accuracy Class, VA and ALF.

Specified in terms of :

Knee Point Voltage (Vk) CT Secondary Winding Resistance(Rct) Magnetising Current (Im) Typical Specification : Ratio=200/1A, Vk= 200V, Rct = 2 Ohms, Im

= 30 mA at Vk/2. Low reactance type, Turns ratio error limited to 0.25%.


CLASS PS CT – KNEE POINT VOLTAGE

KNEE POINT – Point on the Magnetisation Characteristic where 10% increase in secondary voltage causes > 50% increase in the Magnetising current.


CURRENT TRANSFORMER

5A CT SECONDARY Used where lead burden insignificant (e.g. CTs used in in-door

metal-clad switch-gear or with high Prim. Rating say10,000/5A). Fine turns ratio adjustment not possible for Bar Prim CT with low

primary ratings (say 25/5A). Comparatively low peak voltage on CT secondary open-circuit.

1A CT SECONDARY Preferred where Cts are Out-door and lead resistance high. Fine turns ratio adjustment possible. Comparatively high peak voltage when CT secondary is open-

circuited.


CURRENT TRANSFORMER (CBCT)

Used for Sensitive Earth Fault / Earth Leakage Protection. Excited by Primary residual current. Hence no out put due to

balanced / unbalanced load.


CURRENT TRANSFORMER (CBCT)

CBCT SPECIFICATIONS- Following parameters to be specified. Min. Primary Earth Fault Current to be detected (say 1A or 2A) Min. Setting of the Sensitive E/F relay (e.g. 10 mA) Ohmic burden of relay at Min. Setting. (say 1 ohm) To and fro lead burden between CBCT & Relay (say 1 ohm) Outer diameter of cable on which CBCT to be mounted.

Note – Source end earthing of Armour to be routed through the window of CBCT and earthed on load side as shown, to prevent earth return current through armour affecting sensitivity.


VOLTAGE TRANSFORMER (VT)

TYPES Metering VT Protection VT Dual Purpose VT Residual VT

SPECIFICATIONS Voltage Ratio Accuracy Class Rated VA Burden Rated Voltage Factor

ANOTHER CLASSIFICATION

Electro-magnetic Voltage Transformer

Capacitor Voltage Transformer (CVT)


VOLTAGE TRANSFORMER (IS 3156 PART II – 1992)ERROR CLASS % RATIO ERROR PHASE ANGLE ERROR (Min)

0.1 (+/-) 0.1% (+/-) 5

0.2 (+/-) 0.2% (+/-) 10

0.5 (+/-) 0.5% (+/-) 15

1.0 (+/-) 1.0% (+/-) 20

3.0 (+/-) 3.0% ----

REFERENCE CONDITIONS Voltage – 80 – 120% Burden 25 – 100% Power factor – 0.8 Lag Frequency - Rated.


PROTECTION VTCLASS % RATIO ERROR PHASE ANGLE ERROR (Min)

3P (+/-) 3% 120

6P (+/-) 6% 240

RESIDUAL VT

5 PR (+/-) 5% 200

10 PR (+/-) 10% -----

REFERENCE CONDITIONS Voltage – 5% to Voltage Factor times rated voltage Burden – 25 – 100% P.F. 0.8 Lag Frequency - Rated.


VOLTAGE TRANSFORMER (VT)

RATED VOLTAGE FACTOR (IS 3156- PART I – 1992)SYSTEM EARTHING RATED V.F. RATED TIME

Effectively Earthed 1.2 Continuous 1.5 30 Seconds

Non-effectively 1.2 Continuous Earthed (With Automatic E/F trip) 1.9 30 Seconds

Non-effectively 1.2 Continuous Earthed (W/O Automatic E/F trip) 1.9 8 Hours In non-effectively earthed system, healthy phase voltage rises to phase to

phase value, in the event of an earth fault. Thus, considering 10% , permissible over voltage,

V (Healthy Phase) = 1.1 x 1.732 = 1.9


VOLTAGE TRANSFORMER

STAR / STAR CONNECTION Both Prim / Secondary neutrals to be

solidly grounded. Requires 3 x 1Phase or 3 Phase, 3

Core VT.

V- CONNECTION Requires 2 x 1Phase PTs (Phase –

Phase connected). Used where Phase – Neutral Voltage

not required (e.g. 3 Phase , 3 Wire Meters)


VOLTAGE TRANSFORMERRESIDUAL CONNECTION (STAR / OPN DELTA)

Primary neutral solidly grounded, secondary connected in open delta. Used for polarising directional E/F relay or detection of earth fault in non-

effectively grounded systems using Neutral Displacement Relay. PTs should be 3 Phase, 5 Limb or 3 x Single Phase.

Instrument Transformer Rev1

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