Ct Pt Commissioning

COMMISSIONING OF CURRENT TRANSFORMERS POTENTIAL TRANSFORMERS

INSTRUMENT TRANSFORMERS

By Er. ROMIL KUMAR 2

contents   

   



Role of instrument transformers, operating principles Equivalent circuits, errors Design parameters, magnetization, ferro resonance Applicable standards CT &CVT selection parameters Manufacturing and testing Storage, transport, installation, operation do’s and don’ts in commissioning Maintenance and troubleshooting

Instrument Transformer 1. 2.

Instrument Transformers are of two types : Current Transformer (or Series Transformer) Potential Transformer (or Parallel Transformer) Current transformers are used when magnitude of AC currents exceeds safe value of current of measuring instruments. Potential transformers are used where voltage of an AC circuit exceeds 750 V as it is not possible to provide adequate insulation on measuring instruments for voltage more than this.

INSTRUMENT TRANSFORMERS 







Current transformers are used for reducing stepping down a.c. current from higher value to lower value for measurement / protection / control. Typical secondary current is 5A r.m.s. Voltage transformer are used in electrical power system for stepping down a.c. Voltage from higher value to lower value for measurement protection / control . Typical secondary voltage is 110V r.m.s. Actually relays and meters used for protection and metering are not designed for high currents and voltages. High currents or voltages of electrical power system can not be directly fed to relays and meters. 5

CURRENT TRANSFORMER (CT) Definition 



CTs are used for reducing / stepping down ac current from higher value to lower value for measurement / protection / control. Typical secondary current is 5 A rms CT secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.

6

Current Transformer Working principle 



A CT functions with same basic working principle of electrical power transformer but here is some difference. If a electrical power transformer or other general purpose transformer, primary current varies with load or secondary current. In case of CT, primary current is system current and this primary current or system current transforms to CT secondary, hence secondary current or burden current depends upon primary current of current transformer. 7

Instrument Transformer v/s power transformer 



Difference b/w intrument tx and power tx is mainly in their VA rating. Both CTs and VTs have low VA rating (10,30, 100 VA ) and therefore not used for power transfer.

Operating principle of Current Transformer v/s power transformer 



 

In a power transformer , if load is disconnected, only magnetizing current flows in primary. Primary of power transformer takes current from source proportional to load connected with secondary . In CT, primary is connected in series with power line. So current through its primary is current flowing in power line and does not depend upon whether load or burden is connected to secondary or not or what is impedance value of burden. 9

Working principle of Current Transformer or CT 









CT has very few turns in primary where as secondary turns is large in number. Say Np is number of turns in CT primary and Ip is current through primary. Hence primary AT is equal to NpIp AT. If number of turns in secondary and secondary current are Ns and Is respectively then Secondary AT is equal to NsIs AT. In an ideal CT , primary AT magnitude= secondary AT magnitude. So if a CT has one turn in primary and 400 turns in secondary winding, and if it has 400 A current in primary then it will have 1A in secondary . 10 and turn ratio of CT will be 400/1A

Instrument Transformer as CT

Current Transformer ( CT ) 

The primary winding of CT is connected in series with line in which current is to be measured and secondary is connected to ammeter.

Current Transformer   

There are two types of current transformers: 1. Wound primary type 2. Bar primary type.

Wound primary

Bar primary



 



Wound primary: used for smaller currents, low fault level installations due to thermal limitations Bar primary : used for currents > 100 A Protection CTs are most frequently of bar primary, toroidal core with evenly distributed secondary winding type construction. In such CTs, secondary winding can be conveniently put inside high voltage bushings.

CT

CURRENT TRANSFORMER

Construction of Current Transformers 



CT core is made of continuously wound strip of magnetic material. Primary conductor passes thru eye of ring. Secondary is wound on core , turns ratio N2/N1 is very high so that I1 /I2 is very high.

Ring type Ring type core is commonly used when primary current is large. Secondary winding is distributed round ring and primary winding is a single bar. It is a jointless core and there is very small leakage reactance.

Instrument Transformers -CT THERE ARE TWO TYPES OF CT’S

Conductor whose current is to be measured or Bar primary

1

2 Two winding CT.

Terms & Definitions regarding CTs 







Rated primary current: value of primary current on which primary performance of CT is specified . Rated primary current is assigned after conducting heat run tests Rated short time current(primary) : r.m.s value of a.c. component which CT can carry for rated time without damage due to thermal or electro-dynamic stresses. Rated secondary current: value of secondary current , marked on rating plate Current error or ratio error: %age error in magnitude of secondary current is defined in terms of current error.









Phase angle error: phase angle between primary current vector and reversed secondary current vector Accuracy class: class assigned to CT with specified limits of ratio error and phase angle error Over current factor: ratio of rated short time current to rated primary current Insulation level (primary) : insulation level of CT refers to withstand values of --- power frequency withstand voltage --- impulse withstand voltage

Burden of Instrument Transformer 







In current transformer, secondary winding has a very small impedance or “Burden” , so current transformer operates on short circuit conditions. Rated burden of CT is maximum load in volt- amperes which may be applied across secondary terminals without ratio and phase angle errors exceeding permissible limits. Burden depends upon number of instruments or relays connected and their individual burdens. Burden may be expressed as: 0.5 ohm impedance or 12.5 VA at 5 amperes. Let rated burden B volt amperes at rated current Is amp. Then ohmic impedance burden Zb = B/I2 ohms





eg if CT is to feed an indicating meter 1.0VA, a record watt meter 5VA , an o/c relay 6VA . Total burden =12VA If resistance of connecting leads is 0.2 ohm , its burden = I2R = 5VA then total burden on CT = 17VA

Burden of Instrument Transformer 

Burden across secondary of an instrument transformer is also defined as ratio of secondary voltage to secondary current. ZL = secondary voltage/ secondary current =V/I unit of burden are ohms.

secondary windings of different phases of Current Transformers are generally star connected. A typical wiring connection for Core – 1 of Current Transformers in the Bay Marshalling Kiosk / Junction Box

Open circuits of CTs 







CTs generally work at a low flux density. Core is made of very good metal to give small magnetizing current. On open-circuit, secondary impedance becomes infinite and core saturates. This induces a very high voltage in primary up to approximately system volts and corresponding volts in secondary will depend on number of turns ,multiplying up by ratio i.e.volts/turn ×no. of turns). Since CT has much more turns in secondary compared to primary, voltage generated on open-circuited CT will be much more than system volts, leading to flashovers. hence, current transformer must never be operated on open- circuit.

Current Transformers-idle

CT

Short both the terminals and ground

Multi core CT-idle

CT

Ammeter

Short all unused CT terminals and ground

[ Standards ] The Indian and international standard for CT o Standard ⁞ Standard no. ⁞ Year ---------------------------------------------------------------------------- India ⁞ IS 2705 ⁞ 1992  IEC ⁞ IEC 60044-1 2003  British ⁞ BS 3938 ⁞ 1973  American ⁞ ANSI C.57.13 ⁞ 1978  Australian ⁞ AS 1675 ⁞ 1986 (International electro technical commission)  Manufacturing company  BHEL,CGL,ABB,Joti,Siemens… Etc. 32 

Current transformer The basis of all transformers is that: Amp-turns on Primary = Amp-turns on secondary e.g. 100 A × 1 turn = 1 A × 100 turns  Primary current contains two components:  An exciting current, which magnetizes core and supplies eddy current and hysteresis losses, etc.  A remaining primary current component, which is available for transformation to secondary current in inverse ratio of turns.  exciting current is not being transformed and is therefore , cause of transformer errors. 

Causes of error in CT In ideal CT Actual transformation ratio Kc=turn ratio Kt and phase angle would be zero. but due to physical limitations inherent in electric and magnetic ckt , errors are caused. REASONS: 1. CT draws magnetising current Im to provide exciting mmf required by primary winding to produce flux 2. CT draws energy current to supply core losses (eddy current and hysterisis losses) +electrical I2R losses. Thus energy loss component Ie is required to supply losses associated with flux and associated cu loss in winding due to exciting current Io in primary winding 3. Flux density in core is not linear function of magnetising force . Thus ct core gets saturated. 4. There is always a magnetic leakage 

Equivalent circuit of CT

Phasor diagram of CT

Errors introduced by CT 



 



When current is measured by CT, transformation ratio of CT must be fixed and known accurately so that no error in measurement of current. But it is not so-- as it depends on magnetisation and energy components of excitation current, secondary winding load current and its pf. Ratio error = nominal ratio – actual ratio = Kn- Kc actual ratio Kc Kn= rated primary current/ rated secondary current

Kn normally= Kt= no. of turns on secondary/ no. of turns on primary Actual transformation ratio Kc= actual primary current/actual sec current 

Ratio error is +ve when actual ratio of CT< nominal ratio -ve when actual ratio of CT > nominal ratio ratio error can be approx as = -Ie / Kt .Is



Phase angle error 





In an ideal CT , angle between primary and reversed secondary current vector is zero. But in actual CT, there is always a difference in phase between two due to fact that primary current has to supply component of the exciting current. Angle between above two phasors is termed as phase angle error in CT.

Phase angle error

How to Reduce Error in Current Transformer 







1. Using a core of high permeability and low hysteresis loss magnetic materials. 2. Keeping the rated burden to the nearer value of the actual burden. 3. Ensuring minimum length of flux path and increasing cross-sectional area of core, minimizing joint of core. 4. Lowering secondary internal impedance.

Conventional CT core saturates during transient causing delay in high speed relay operation  Secondary resistance  The secondary resistance of a CT is an important factor, as CT has to develop enough voltage to push secondary current through its own internal resistance as well as connected external burden. This should always be kept as low as possible.  Burden : circuit connected to secondary winding. Expressed in voltampere at rated secondary current at rated power factor. CT specification  A CT is normally specified in terms of:  A rated burden at rated current  An accuracy class  An upper limit beyond which accuracy is not guaranteed (known as accuracy limit factor, ALF), which is more vital in case of protection CTs. 

Accuracy class of CT Accuracy class is the class assigned to CT with specified limits of ratio error and phase angle error Limits of error and accuracy class Accuracy class

Current error at rated primary current %

Phase displacement at rated current Minutes

Composite error at rated accuracy limit primary current %

5P

+- 1

+-60

5

10P

+-3

---

10

15P

+- 5

----

15

CTs are marked as: 30/5P 10 First number: output in VA (30) Second number: accuracy class: (5P) Third number: composite error: (10)

Tests on current transformers

Two type of tests on current transformers : 1. Type Test 2. Routine Test

Tests on current transformers Type tests as per IS       



The following tests are type tests a) verification of terminal markings and polarities b) short-time current tests c) temperature rise test d) impulse voltage test on primary e) power frequency voltage withstand test on primary f) error measurement g) overvoltage inter-turn test

Tests on current transformers Routine tests as per IS The following tests apply to each individual CT a) verification of terminal markings and polarities b) high voltage power-frequency voltage withstand test on secondary c) inter-turn overvoltage test d) determination of errors and accuracy class

--- Order of tests is not standardized, but determination of errors shall be performed after other tests

Special tests 

 

 



Following tests are performed upon agreement between manufacturer and purchaser: a) chopped lightning impulse test b) measurement of capacitance and dielectric dissipation factor c) multiple chopped impulse test on primary winding d) mechanical tests e) measurement of transmitted over-voltages

Temperature-rise test 









IS

CT shall be deemed to have attained a steady temperature when rate of temperature rise does not exceed 1 K per hour. Test-site ambient temp. shall be between 10 °C and 30 °C. CT shall be mounted in a manner representative of mounting in service. Temperature rise of windings shall be measured by increase in resistance method, but for windings of very low resistance, thermocouples may be employed. Temperature rise of parts other than windings may be measured by thermometers or thermocouples.

The power-frequency withstand test

as per IEC 60060-1. 

 

Test voltage shall have appropriate value depending on the highest voltage for equipment. The duration shall be 60 s. Test voltage shall be applied between short-circuited primary winding and earth. The short-circuited secondary winding(s), frame, case (if any) and core (if there is a special earth terminal) shall be connected to earth.

Impulse tests on primary winding 







IS

General: Impulse test is performed in accordance with IEC 60060-1. Test voltage is applied between terminals of primary winding (connected together) and earth. frame, core and all terminals of secondary windings are connected to earth. impulse tests consist of voltage application at reference and rated voltage levels. reference impulse voltage shall be between 50 % and 75 % of rated impulse withstand voltage. peak value and wave-shape of impulse shall be recorded. Evidence of insulation failure due to test may be given by variation in wave-shape at both reference and rated withstand voltage.

.

Tests on current transformers 

Error measurement: 1. Direct method– 2 ammeters are used . One for primary current and other for secondary current 2. Comparison method—a sub-standard CT is taken, whose errors are known. CT under test is compared with sub-standard CT.



Turns ratio test: measure magnitudes of primary and secondary currents near rated secondary current with a low value of

secondary burden.

  

Exciting currents: measured for several secondary e.m.f. Apply voltage to secondary winding , primary and other windings being open circuited.

Routine tests Verification of terminal markings and polarity IS   





 

It shall be verified that the terminal markings are correct Polarity test: If at any instant, current is entering primary from P1 ,the current should leave secondary from terminal S1. when key is pressed , current enters the primary thru P1, voltmeter connected should read positive. Polarity of CT and connections: polarity gives relative instantaneous directions of currents in primary and secondary leads. In BS-3938 polarity of CT is marked as P1 and P2 S1and S2 If current flows from P1 to P2 then current will flow from S1 to S2 thru ammeter.

Partial discharge test 



Procedure A: the partial discharge test voltages are reached while decreasing the voltage after the powerfrequency withstand test. Procedure B: the partial discharge test is performed after the power-frequency withstand test. The applied voltage is raised to 80 % of the power-frequency withstand voltage, maintained for not less than 60 s, then reduced without interruption to the specified partial discharge test voltages

 





Insulation tests: these are conducted as per BS/IS/IEC For primary circuit test ,Specified Power frequency voltage is applied to primary for one minute. For secondary circuit test, voltage of 2kv , 50 Hz is applied for one minute between secondary terminals and earth. Impulse tests are conducted on hv CTs.

 



Over voltage inter-turn test: Sec winding is open circuited. Rated frequency ,rated primary current is flown thru primary for one min . Secondary winding is then checked to see if insulation has passed the test.

Rating plate markings   

 



a) the manufacturer’s name b) a serial number c) rated primary and secondary current, i.e.: Kn = Ipn / Isn A (e.g. Kn = 100/5 A) d) rated frequency (e.g. 50 Hz) e) rated output and the corresponding accuracy class, (e.g. 1S, 15 VA, class 0.5; 2S, 30 VA, class 1). f) highest voltage for equipment (e.g. 1,2 kV or 145 kV); g) rated insulation level (e.g. 6/–kV* or 275/650 kV).

Accuracy classes for protective current transformer  

 

12.2.1 Accuracy class designation For protective current transformers, the accuracy class is designed by highest permissible percentage composite error at the rated accuracy limit primary current prescribed for the accuracy class concerned, followed by the letter “P” (meaning protection). 12.2.2 Standard accuracy classes The standard accuracy classes for protective current transformers are: 5P and 10P.

Limits of errors for protective current transformers 





IS

At rated frequency and with rated burden connected, current error, phase displacement and composite error shall not exceed the values given in table 14. For testing purposes when determining current error and phase displacement, burden shall have a power-factor of 0,8 inductive except that, where burden is less than 5 VA, a powerfactor of 1,0 is permissible. For the determination of composite error, burden shall have a power-factor of between 0,8 inductive and unity at discretion of manufacturer.



Potential transformer P.T

Voltage (potential)Transformer VT or PT 



Voltage transformers are much like power transformers operating on very light load. Two types of VTs used for protection equipment. 1.Conventional two-winding, electromagnetic type (commonly referred to as a VT) 2 Capacitive voltage divider type(referred to as a CVT).







electromagnetic type is a step down transformer whose primary (HV) and secondary (LV) windings are connected as :

No. of turns in a winding is directly proportional to opencircuit voltage being measured or produced across it. In three-phase system, one VT per phase is used and they are connected in star or delta depending on method of connection of main power source being monitored.

Potential transformer P.T P2

P1

S1

S2

Voltage Transformers

Medium Voltage

High Voltage

Typical wiring connections of 2 core CVTs and PTs in Bay Marshalling Kiosk / Junction Box are shown below

:

Terms and definitions of PT  





 

Rated voltage: Vol marked on rating plate Rated transformation ratio : Ratio of rated primary vol to rated secondary vol Rated secondary voltage : value of secondary vol marked on rating plate Residual voltage: vector sum of three lines to earth voltages i.e. Vres = V RN + V YN + V BN Residual PT : a three phase pt or a group of 3 single phase residually connected PTs in which residual voltage appears across secondary terminals when 3 phase voltage are applied to primary windings

[ 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. 

70

Advantages of Instrument Transformer

3.

4.

5.

Measuring instruments can be placed for away from high voltage side by connecting long wires to instrument transformer. This ensures safety of instruments as well as operator. This instrument transformers can be used to extend the range of measuring instruments like ammeters and voltmeters. power loss in instrument transformers is very small as compared to power loss due to resistance of shunts and multipliers. By using current transformer with tong tester, current in a heavy current circuit can be measured.

Disadvantages of Instrument Transformer The only draw back is that these instruments can not be used in DC circuits.

18/03/2013

Capacitive voltage transformer (CVT)

Capacitive voltage transformer (CVT) 





For voltages above 100KV, conventional type of PT becomes extremely expensive due to insulation requirements. Capacitor voltage transformer is a combination of a capacitor potential divider and a magnetic potential transformer, known as intermediate transformer of relatively small ratio. A stack of high voltage capacitors forms the potential divider, capacitance of two sections being C1 and C2 respectively, Z is burden









Intermediate transformer primary voltage is usually above 10 Kv Intermediate transformer consist of an inductance L , which may consist of wholly or partly leakage inductance of its winding. Value of L is adjusted to 1/ w2( C1+C2) so that vol drop due to current from divider is compensated. So transformation ratio is independent of burden. Overall ratio is product of divider and transformer ratio.

Capacitive voltage transformer (CVT) 



In EHV national grid networks of utilities, CVTs are commonly used for both protection and communication purposes. CVT serves the purpose of Potential Transformer and Coupling Capacitor

General arrangement of CVT

Electrical performance of CVT

Connections of a CT and a PT to supply, load and relay

CT,VT,CVT

Connections of a CT and a PT to supply, load and relay.

THANKS

THANKS

Typical schematic diagram of CVT

Construction

Accuracy Class of CT: 











CT accuracy is determined by its certified accuracy class which is stamped on nameplate. eg, CT accuracy class of 0.3 means that CT is certified to be accurate within 0.3 % of its rated ratio value for a primary current of 100 percent of rated ratio. CT with a rated ratio of 200/ 5 with accuracy class of 0.3 would operate within 0.45 % of its rated ratio value for a primary current of 100 amps i.e. for a primary current of 100A it is certified to produce a secondary current between 2.489 amps and 2.511 amps. Accuracy is specified as a %age of range, and is given for maximum burden as expressed in VA. Total burden includes input resistance of meter and loop resistance of wire and connections between CT and meter. Example: Burden = 2.0 VA. Maximum Voltage drop = 2.0 VA / 5 Amps = 0.400 Volts. Maximum Resistance = Voltage / Current = 04.00 Volts / 5 Amps =0.080 Ohms. If input resistance of meter is 0.010Ω, then 0.070Ω is allowed for loop resistance of wire, and connections between CT and meter.