Current Transformer

Current Transformers (CT)Some Important terms :-1. Burden : The value of load connected across the secondary

of CT , expressed in VA (volt-amps) or ohms at rated secondary current.

2. Errors :(i) Ratio Error : The percentage error in the magnitude of the secondary. K Is - Ip = X 100 Ip K = Rated transformation ratio Is = Secondary current Ip = Primary current

(ii) Phase Angle Error: The phase angle between primary current vector and reversed secondary current vector.

3. Composite Error : The rms value of the difference (KIs - Ip) integrated over one cycle under steady conditions.

4. Accuracy limit factor : The highest value of primary current

assigned,upto which the limits of composite errors are complied with.

5. Instrument Safety Factor : The ratio of the value of current at

which the CT saturates to the rated

current.

6. Magnetising / Exciting current : The current taken by the secondary winding of the CT when sinusoidal voltage of

50HZ is applied , keeping the primary open circuited. This current is utilised in magnetising the core.

7. Saturation : It is that state of a CT when it stops reproducing

the current in the secondary , faithfully. At this

level the magnetic core stops behaving linearly.

Actually the core gets saturated. In this state

the total primary current is utilised in magnetising

the core.

O

Definition of Knee Point :It is that point on the magnetising curve where a 10% increase in the voltage results in 50% rise in the exciting current drawn.

Knee point is an important factor in determining the performance of the CT.

Requirement of Metering CTThe metering CT should be accurate in the entire operating range upto the rated current and even beyond , upto a certain limit.A typical specification :

300/5 ACl : 1.0 , Burden : 15 VAISF 2

This CT must maintain its error within 1% of rated current upto 300 A , definitely & also upto 600A ( rated current X ISF I.e. 300 X 2 ) beyond which it must saturate. The whole idea is faithful reproduction of current upto a certain level & saving the connected meters from over current beyond that level.

Requirement of Protection CTProtective gear is concerned with wide range of currents from fault settings to maximum fault currents which may be many times normal rating. Larger errors may be permitted in such CTs , but it is very important that saturation should be avoided.A typical specification :300/5A , Burden : 15 VAClass : 5P105 is the % error10 is the accuracy limit factor (ALF)This CT must maintain error within 5% of 10 times current (5% of 300 X 10 Amps) upto 10 times current (300X10 = 3000Amps).

CTs for special types of protection like : Differential protection , Restricted Earth Fault Protection etc.

Important parameters are :1. Knee point voltage2. Magnetising current at a particular point of the V-I (or B-H)

curve. 3. CT secondary winding resistance.

A typical specification :Ratio : 300/5 AVk 30(Rct+Rlead)

Im 50 mA at Vk/2

Rct 1 Ohm

Main types of CTs (Construction wise)

1. Ring type or window type or Bar primary typeIn this case no. of turn in primary winding = 1

2. Wound primary type3. Post type (Outdoor)

CURRENT TRANSFORMER

SECTIONAL VIEW OF A SWITCH PANEL

CT

Acceptance / Routine Tests

1. Winding resistance measurement2. IR measurement3. Ratio checking4. Polarity test5. Magnetising characteristic tests6. HV tests (whenever applicable)7. Tan Delta Test (in case of 132 KV outdoor CTs)8. Measurements of errors (in lab, where needed)

List of instrument required for CT Test :1. Multimeter(2nos. 1 for current & 1 for Voltage)2. Low resistance measuring instrument (inductive)3. 1-ph Variac4. Battery5. Primary Injection Set with Primary current measuring CT6. C & tan Delta set (for 132kV & above)7. Ratio & Phase angle error measuring inst. (for Lab.use)8. HV DC Test set (for HV test)9. 1 kV Megger

Voltage Transformer

Voltage Transformers (VT)VTs are used whenever a feedback of primary voltage is

required. It may be for

(i) Metering & Indication(ii) Protection , like Directional / Distance(iii) Synchronising

VTs may be single phase or three phases.A typical specification :Ratio : 33 Kv / 110 V - 110 V 3 3 3Class : 1 /3P -5PBurden : 200 VA - 50 VAVoltage factor : 1.2 Cont. / 1.9 for 8 hoursSuitable for Non-effectively earthed system.Insulation : 36/70/170 KVP

The ratio indicates that it is a single phase VT.There are two secondary windings. One is dual purpose , I.e.

bothfor

(i) metering & (ii) protectionClass 1/3P indicates that it is suitable for both purposes.

“Class 1” is for metering purposes.Error will be within 1% of rated voltage within a range of 80% to 120% of rated voltage and 25% to 100% of rated

burden.

“Class 3P” is for protection purposes.Error will be within 3% within a range of 5% to 190%(Voltage factor) of rated voltage and 25% to 100% of ratedburden.

VTs are normally “Star-Star” connected.For some protection purposes the secondaries are connected inopen delta (residual connection).e.g. Directional E/F protection.In this case the primary must be star connected with earthedneutral.

5 Limbed VTsThe core of such VTs are of a special type. It has two limbs (outer)which have no windings. These two unwound limbs provide path for zero sequence flux. The primary winding neutral must beearthed. Such type of 3 Phase VTs are used when one of thesecondary winding is connected in open delta for E/F protection.

Type of VTs (Construction wise)

1. Electromagnetic VT- Oil filled- epoxy resin encapsulated

2. Capacitor VT (CVT)The CVT is more economical than an electromagnetic VT for system voltage greater than 66 KV.The performance of CVT is inferior compared to electromagnetic types. It’s performance is affected by frequency , switching transients , burden etc.

VOLTAGE TRANSFORMER

VT

VT

Testing of VTs

1. Error measurements ( In Lab)2. Insulation tests3. Induced voltage test4. Polarity test5. HV test.

List of instrument required for VT Test :1. Multimeter(2nos. 1 for current & 1 for Voltage)2. Low resistance measuring instrument (inductive)3. 1-ph Variac4. Battery5. Ratio & Phase angle error measuring inst. (for Lab.use)6. HV AC Test set (for Ratio check)7. 1 kV Megger

Indian Standard :

Current Transformers are covered by IS:2705, 1992 (Reaffirmed

2002).

Voltage Transformers are covered by IS:3156, 1992 (Reaffirmed

2002).

Reference Books : 1. J&P Transformer Handbook2. Application Guide in Protective relaying - GEC

Measurement (ALSTOM)3. Protective relaying - Electricity Council, England

Thank You