07 current transformer applications#

7/28/2019 07 Current Transformer Applications#

1/9

CURRENT TRANSFORMER APPLICATIONS1. TYPES OF CURRENT TRANSFORMERS1.1 Wound CT

Fully insulated CTs. Light or no insulation on busbars. Usually more than one primary turn. Primary and secondary windings from a complete unit, usually insulated. Often usde as auxialiary CTs and for low ratios. Disadvantage of larger electromechanical foece on primary conductor during faults. (Force

proportional to square of number of primary turns)1.2 Ring CT

Faster to manufacture. Easier to handle- lighter and smaller. Greater flexibility. If primary lead is looped seneral times through a ring CT then ratio is reduced. 300/5A CT with

three primary turns behaves as a 100/5A CT.

2. COMMON RATINGS2.1 Rated primary current. Standard values are:10 12.5 15 20 25 30 40 50 60 75 A and decimal multiples.Preferred values are underlined.2.2 Rated secondary current. Standar values are:

! a n d ~ A2.3 Frequency. Standard is 50Hz.2.4 Rated ouput in VA. Standard values are:5 7.5 10 15 and 30 VA.2.5 Output VA equivalent in ohms can be calculated.

Is= Secondary rated current.Vs = Voltaged developed across rated burdenZ8 =Rated burden

2.6 If VA= 15VA Is= SA Z8 = 0.6 ohmsVA=15VA Is= 1A Z8 = 15 ohms

107 Current Transformer Applications


2/9

3. METERING CT.1. Accurarcy Class and Applications.Class Application0.1 High precision and calibration measurement.0.1 Special tariff metering requirements.0.5 Normal tariff metering measurements.1 Wattmeters and power relays.3 Ammeters.5 Regulators, maximum current meters and thermal relays.

2. Limits of Errors.Limits of error for accuracy Classes 0.1 to 1

A percentage current (ratio) A phase displacement at percentage of rated currenterrors at percentage of rated shown belowcurrent shown belowClass minutes centiradians10 up to 20 up to 100 up 10 up to 20 up 100 up 10 up 20 up 100 upbut not but not to 120 but not to but to 120 to but to but to 120incl20 incl100 incl20 not incl not incl not incl100 20 100

0.1 0.25 0.2 0.1 10 8 5 0.3 0.24 0.150.2 0.5 0.35 0.2 20 10 10 0.6 0.45 0.30.5 1.0 0.75 0.5 60 30 30 1.8 1.35 0.91 2.0 1.5 1.0 120 90 60 3.6 2.7 1.8

Limits of error for accuracy Class 3 and Class 5Class A percentage current (ratio) error atpercentage af rated current shownbelow50 1203 3 35 5 5

3. Dual Purpose CTs.Privision in BS 3938 for Cts rated for both measurement and protection. Must be factory tested toensure compliance with both requirements.

07 Current Transformer Applications 2


3/9

4. PROTECTION CT4.1 Accuracy Limit Factor (ALF)Measurement of the primary current up to which the CT will maintain its accuracy.Standard values are: 5 10 15 20 30.4.2 Accuracy Class.Standard values are: 5P 10P.Limits erroe fo accuracy Class 5P and Class 1OPAccuracy class Current error at Phase displacement at rated primary Composite errorrated primary current at rated accuracy

current% minutes centiradians limit primarycurrent%5P 1 60 1.8 5

10P 3 10

4.4 Ratio Errors.

( KNls - lp * 100)Current error = 1 %100 I JT

Composite error= -1 T Knis- ip 2 dt%p 0KN =Rated transformation ratio./pis= Actul primary and secondary currents.ipis =Instantaneous values of primary and secondary currents.T =duration of one cycle.



4/9

4.5 Example of typical Protection CTPrimary current, I P Accuracy ClassIf

300/SA 5P10Secondary current,Is Accuracy limit factor (ALF)

If primary current= 300A, Current error < 1%

Rated burden. In ohms,Z8 =VA= 0.6ohm

15VA

If primary current= lp * ALF = 300 * 10 = 3000A, composite error < 5%Connected burden = 0.6 ohm.If primary current= 3000A,Aecondary current = 50AOutput of CT = g * Z8 =50 2 * 0.6 =1500VAThis is 100 times its rated output.The knee-point of a protection CT is depending on its VA, accuracy limit factor, rated secondarycurrent and internal resistance.Es = ALF x Rated secondary current x (Rs +Z8 ).Where Rs =CT internal resistance (ohms)

Z8 = VA expressed in ohms.



5/9

5. CORE BALANCE CT (ZCT)

Glandinsulation

p

Method of connecting earth bonding strip5.1 All phase and neutral conductors pass through this ring CT.5.2 Used for earth fault protection. (Sums all fluxs to zero under non-earth fault conditions).

5.3 Split or continuous core.5.4 Specifications:

1. Primary current.2. Earth fault setting.3. Relay characteristic.

5.5 Note: Core with earthing.



6/9

6. MARKING CONNECTION AND EARTHING OF CTs.1. Markings of CTs.

1) Primary, secondary and test (or tertiary) windings.2) Relative polarities.

All terminals marked P1, S1, T1 and C1 have same polarity. C markes ratio change connections.2. Test winding.

1) To facilitate testing of CTs which cannot be conveniently tested by primary injection e.g whenmounted in transformer bushing.

2) Test winding rated as 10A.3) Note: Test winding must be open-circuited when not in use.

3. Polarity and Star-Point Arrangement.No standards on such arrangement. Some practices as follows:

1) Differential CTs have star-point towards protected circuit.2) For directional circuits (energy metering, directional protection, etc), more convenient to have

star-point on load side.3) Some practices favour the use of P1 toward source. Others prefer P1 towards busbar.

4. Earthing of CTs.All CTs must be earthed at only one point usualy close to CTs. For multiple CTs e.g busbar protection,practices is to earth common point, usually at relay.



7/9

Test winding terminals

Primary terminals u m~ ~ sr S2 S3Secondary terminals 51 52 51 52(1) Single ratio transformer (2) Transformer with anintermediate tapping onsecondary windingTest windingterminals

Primary terminals

Secondary terminals(3) Transformer withprimary winding in twosections intended forconnections either inseries or in parallel.

p ~ ~ n . t - t - - f - - L - l n ~ . ~ P o 2151 152 251 2S2 3S1 3S2

(6) CT with common primary and multiple cores

'U22LJn1 S2(8) CT with multipleprimary windings(summation CT)


iSI 152 251 2S2(4) Transformer with twosecondary windings eachwith its own magneticcore. (Two alternativemarkings for thesecondary terminals)

PI P2

651 52 53

(5) Transformer with anintermediate tapping onsecondary winding andincorporating test taps.

; n nn 0P251 52 S3 54 SS 5 G

(7) CT with common primary and multiplesecondary windings

7


8/9

7. SOME CONSIDERATIONS OF PROTECTION CTs.1. Knee-point voltage of protection CT is a measure of its rating.At ALF, Es =ALF x I5 (Rs + Z8 ) .300/5A 5P10 15VA R5 = 0.1 ohm.E5 = 10 X 5 X (0.1 + 0.6) = 35V2. lnderect measurement of composite error. For low reactance CTs (e.g ring CTs), composite errorcan be obtained by measuring I8 with calculated E5 applied to the secondary winding.Ec = (AL;xiJ x 00%For the same CT, if IE = 0.20A

0.20Ec = (lOx5) x100 = 0.4% < 5%3. Use of a burden which differs from its rated value affects the performance of protection CT.

P5 +PRActual ALF =Rated ALF x (p p )S + APs =I x Rs (VA) (Internal burden)PRPA =Rated and actual VA burdenI f PR = 15VA. PA = 7.5VA and Rs = 0.1 oh m (Ps = 2.5VA)ActualALF = l O x ( ~ ) = 17.52.5+7.54. A protection CT with secondary terminals shorted will have very high ALF. As aguide, to use 25%minimum burden.

8. OPEN-CIRCUITED CTs1. CTs should never be operated with open-circuited secondary because of high voltagesproduced.2. Open-circuited CT is extreme example of CT saturation. Wiyhout secondary current, primaryampere-turns used ebtirely to magnetise iron core. If current is high, then CT will be saturated.(Typically this occurs at 25% - 30% of rated current). During saturation there is no change of flux andno secondary voltage.3. Area of concern occurs when CT comes out of saturation, i.e, during change from 0sat to -0satThis occurs in very short time t2 Es = -N5d0fdt4. No secondary current flow to control secondary voltage. Very high rate of change of flux(20satin t2) induces very high and peaky seondary voltage, which can be harmful to lives andequipment.



9/9

METERING VTs1. Accuracy Class and Applications.Class Application0.1 Precision testing0.2 Precision I tariff meters0.5 Tariff meters1.0 Commercial grade measuremts3.0 General applications

2. Limits of ErrorsAccuracy class % Ratio error Angle error (ninutes)0.1 0.1 50.2 0.2 100.5 0.5 201.0 1.0 403.0 3.0 Not specified

Error at between 80% and 120% rated voltage and between 25% and 100% rated burden at powerfactor of 0.8 lagging.


07 current transformer applications#

Documents