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CT's are categorized as Protection CT, Special ProtectionCT and Measuring CT. Based on this, the CT's areclassified. Here is the meaning of the CT classes:

Class 5P20:

The letter 'P' indicates it is a protection CT.

The number 5 indicates the accuracy of the CT. Most commonaccuracy numbers are 5 and 10.

The number 20 (called accuracy limit factor) indicates thatthe CT will sense the current with the specified accuracyeven with 20 times of its secondary current flows in thesecondary. This is required for protection CT, because thefault current is high and the CT should be able to sensethe high fault current accurately to protect the system.The common numbers are 10, 15, 20 and 30.

Class PS:

PS is for 'Protection Special'. This class of CT's are usedfor special protection such as differential protection,distance protection etc.

Class 1M:

The letter 'M' indicates it is a measuring CT.

The number 1 indicated the accuracy of the CT. Themeasuring CT's should be more accurate than the protectionCT. The most common accuracy numbers are 0.5 and 1.

Measuring CT's are intended to work in normal and itdoesn't require working with accuracy beyond the CTsecondary current rating, so the accuracy limit factor isnot mentioned here.

Is This Answer Correct ? 149 Yes 6 NoAmit_er1981Answer# 2 in 5p20 5 is 5% of the 20times of the primary current(i.e)in case 100/1A CT 20*100=2000A 5% is 100A ERROR AT 2000A

fault cuurent for instantaneous trip this error isnegligible

Is This Answer Correct ? 23 Yes 4 NoVeera

Answer# 3 K K. MURTY--5P10 & 5P20 Class CTs: They are generally used for Over

current (Instantaneous and IDMT relays) and E/F protectionof Feeders, Transformers etc.(i) 5 signifies the % limit of composite error.

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(ii) P signifies the Protection core,(iii) 10, 20 signify the quantum of fault current asmultiple of the rated current or ALF(Accuracy Limit Factor)up to which the CT shall have the defined percentage ofcomposite error i.e. 5%. During fault condition the faultcurrent is many times the rated current. Standard AccuracyLimit factors as per IS: 2705 and IEC60044-1 are 5, 10, 15,

20 & 30.

Accuracy limit factor is inversely proportional to the ratedVA burden i.e. if the connected VA burden is lower than therated one, the limit of accuracy shall be maintained beyondthe declared one.The formula for actual ALF;ALF(ACT)= ALF(RTD) x {Sin+Sn(Rated Burden)}/{ Sin+Sa (ActualBurden)}.Example:- Calculate actual Accuracy limit factor ifsecondary burden is reduced to 1/3rd, if CT rated as 200/5A,5P10,15VA and Rct =0.15?

ALF Rated=10(From CT data 5P10),Sin (Burden due to internal resistance of the CT) =(5A)squre x 0.15=25 x 0.15=3.75VA,Sn (Rated burden) = 15VA (From CT data), Sa(Actual Burden) =15/3= 5VA,ALF(ACT) = ALF(RTD) x {Sin+Sn(Rated Burden)}/{ Sin+Sa(ActualBurden)}

ALF(ACT) = 10x (3.75+15)/(3.75+5) = 10x18.75/8.75 = 21.42It is clear from the above example that the ALF increases ifthe connected actual burden is lower than the rated one. TheCT in this condition shall maintain accuracy up to faultcurrent of 21.42 times of rated current as against 10 times

the rated current and shall not saturate till then.PS Class CTs: Though abbreviation PS is not elaboratedanywhere, however it may be called as Protection SpecialClassCT core. This core is used particularly where currentbalance is precisely required to be maintained. InDifferential protection, balance is the prime requirementbetween secondary currents of associated CTs of either sideof the equipments. Differential and restricted E/FProtection of Transformers and Overall DifferentialProtection of Generators need such CTS.The 5P10P or 5P20 class CTs cannot match the Characteristicas that of PS class. The core of PS class is such that veryhigh current is needed for saturation of the core. Kneepoint voltage of the CT is of valid importance. Thedeveloped voltage across the relay terminals should be lowerthan the Knee point Voltage of the CTs.From: Er.K.K.Murty, B.E.(Hons)Electrical,FIE.Retd.Chief Engineer(Testing & Commnu),M.P.State ElectricityBoard.Jabalpur.Email ID:kkmurty05@yahoo.com

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Technical information on Kappa productsInstrument transformers are used for measurement and protective application, tog

ether with equipment such as meters and relays. Their role in electrical systemsis of primary importance as they are a means of "stepping down" the current orvoltage of a system to measurable values, such as 5A or 1A in the case of a current transformers or 110V or 100V in the case of a voltage transformer. This offers the advantage that measurement and protective equipment can be standardized on a few values of current and voltage.

Voltage transformers

Current transformers

Kappa has published a reference manual on instrument transformers

Voltage transformers

Principle of operationDefinitionsStandardsTestsTypical Specifications

Principle of operation

The standards define a voltage transformer as one in which "the secondary voltage is substantially proportional to the primary voltage and differs in phase fromit by an angle which is approximately zero for an appropriate direction of the

connections."

This, in essence, means that the voltage transformer has to be as close as possible to the "ideal" transformer. In an "ideal" transformer, the secondary voltagevector is exactly opposite and equal to the primary voltage vector, when multiplied by the turns ratio.

In a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance. One can thus talk of a voltage error,which is the amount by which the voltage is less than the applied primary voltage ,and the phase error, which is the phase angle by which the reversed secondary voltage vector is displaced from the primary voltage vector.

Kappa designs its VT's so that the resistance and reactance drops are minimized.It also uses the best grades of cold rolled grain oriented electrical steels which enables operation at optimum levels of magnetic induction, thus reducing both the size and the cost of the VT.

Definitions

Typical terms used for specifying a voltage transformer (VT)

Rated primary voltage: This is the rated voltage of the system whose voltage is

required to be stepped down for measurement and protective purposes.

Rated secondary voltage: This is the voltage at which the meters and protective

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devices connected to the secondary circuit of the voltage transformer operate.

Rated burden: This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on the VT. This includes the burden imposed by the connecting leads. The VT is required to be accurate at both the rated burden and 25% of the rated burden.

Accuracy class required: The transformation errors that are permissible, including voltage (ratio) error and phase angle error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and Class 3. Both meteringand protection classes of accuracy are specified. In a metering VT, the VT is required to be within the specified errors from 80% to 120% of the rated voltage.In a protection VT, the VT is required to be accurate from 5% upto the rated voltage factor times the rated voltage.

Rated voltage factor: Depending on the system in which the VT is to be used, therated voltage factors to be specified are different. The table below is adoptedfrom Indian and International standards.Rated voltage factor Rated time Method of connecting primary

winding in system1.2 Continuous Between phases in any networkBetween transformer star-point and earth in any network1.21.5 Continuousfor 30 seconds Between phase and earth in an effectively earthed neutral system1.21.9 Continuousfor 30 seconds Between phase and earth in a non-effectively earthed neutral system with automatic fault tripping1.21.9 Continuousfor 8 hours Between phase and earth in an isolated neutral system

without automatic fault tripping or in a resonant earthedsystem without automatic fault trippingTemperature class of insulation: The permissible temperature rise over the specified ambient temperature. Typically, classes E, B and F.

Residual voltage transformer (RVT): RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is novoltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. When using a three phase RVT, the primary neutral should be earthed, as otherwise third harmonic voltages will appear across the residual winding. 3 phase RVTs typically have 5 limb construction.

Metering Units:11kV metering units consist of one 3 phase VT and 2 CT's connected together in a single housing. This can be used for three phase monitoring of energy parameters. It is used with trivector meters and energy meters.

Standards

The Indian and international standard references for VT s are as given in the table below:

Standard Standard Number YearIndian 3156 1992

British BS EN 60044-2 1997British BS 7729 1994International

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Electro technicalCommission (IEC) IEC 60044-2 1997Australian AS 1243 1982Australian AS 60044-2 2007American ANSI C.57.13 1993Kappa manufactures VT s to international standards. Our designs are backed up byextensive type testing at national and international laboratories.

Tests

A number of routine and type tests have to be conducted on VT s before they canmeet the standards specified above. The tests can be classified as:

Accuracy tests to determine whether the errors of the VT are within specified limits

Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, induced over-voltage test , impusetests with 1.2u/50u wave, and partial discharge tests (for voltage>=6.6 kV) to d

etermine whether the discharge is below the specified limits.

Temperature rise testsShort circuit testsVerification of terminal markings and polarityKappa conducts routine tests on each and every VT produced and all designs are type tested.

Typical specification for a 11 kV VT

System voltage: 11 kVInsulation level voltage (ILV) : 12 /28/75 kVNumber of phases: Three

Vector Group: Star / StarRatio: 11 kV/ 110 VBurden: 100 VAAccuracy: Class 0.5Voltage Factor: 1.2 continuous and 1.5 for 30 secondsWith provision for fuseDouble and triple secondary VT s are also manufactured.

Current transformers

Principle of operationDefinitionsStandardsTestsTypical Specifications

Principle of operation

A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." This highlights the accuracy requirement of the current transformer but also important is the isolating function, which means no matter what the system voltage the secondary circuit need be insulated only for a low voltage.

The current transformer works on the principle of variable flux. In the "ideal"current transformer, secondary current would be exactly equal (when multiplied b

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y the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns is utilizedfor magnetizing the core, thus leaving less than the actual primary ampere turnsto be "transformed" into the secondary ampere-turns. This naturally introducesan error in the transformation. The error is classified into two-the current orratio error and the phase error.

Kappa CT s are designed to minimise the errors using the best quality electricalsteels for the core of the transformer. Both toroidal (round) and rectangular CT s are manufactured.

Definitions

Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current.

Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current are 1 A or 5A. In the case of transformer differential protection, secondary currents of 1/

root 3 A and 5/ root 3 A are also specified.

Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)

Accuracy class: In the case of metering CT s, accuracy class is typically, 0.2,0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT sshould pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the

CT.

Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions.

Accuracy limit factor: The value of primary current upto which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15times the rated primary current.

Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1or 3 seconds.

Instrument security factor (factor of security): This typically takes a value ofless than 5 or less than 10 though it could be much higher if the ratio is verylow. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greaterthan 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT's, FS is applicable for the lowest ratio only.

Class PS/ X CT: In balance systems of protection, CT s with a high degree of sim

ilarity in their characteristics are required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the

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knee-point voltage, and the resistance of the CT secondary winding corrected to75C. Accuracy is defined in terms of the turns ratio.

Knee point voltage: That point on the magnetizing curve where an increase of 10%in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).

Summation CT: When the currents in a number of feeders need not be individuallymetered but summated to a single meter or instrument, a summation current transformer can be used. The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding,which feeds a current proportional to the summated primary current. A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5 are to be summated to a single 5A meter.

Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used forearth leakage and earth fault protection. The concept is similar to the RVT. Inthe CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no curren

t flows in the secondary of the CBCT. When there is an earth fault, the residualcurrent (zero phase sequence current) of the system flows through the secondaryof the CBCT and this operates the relay. In order to design the CBCT, the innerdiameter of the CT, the relay type, the relay setting and the primary operatingcurrent need to be furnished.

Interposing CT's (ICT's) : Interposing CT's are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformers.

Standards

The Indian and international standard references for CT s are as given in the ta

ble below:

Standard Standard Number YearIndian 2705 1992British BS EN 60044-1 1999InternationalElectro technicalCommission (IEC) IEC 60044-1 2000Australian AS 1675 1986Australian AS 60044-1 2007American ANSI C.57.13 1993

Kappa manufactures CT s to international standards. Our designs are backed up byextensive up by extensive type testing at national and international laboratories.

Tests

A number of routine and type tests have to be conducted on CT s before they canmeet the standards specified above. The tests can be classified as :

Accuracy tests to determine whether the errors of the CT are within specified limits.Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, inter-turn insulation test at power

frequency voltage, impulse tests with 1.2u/50 wave, and partial discharge tests(for voltage >=6.6kv) to determine whether the discharge is below the specifiedlimits.

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Temperature rise tests.Short time current tests.Verification of terminal markings and polarity.Kappa conducts routine tests on each and every CT produced and all designs are type tested.

Typical specification for a 11 kV CT

System voltage:11 kVInsulation level voltage (ILV) : 12/28/75 kVRatio: 200/1 - 1 - 0.577 ACore 1: 1A, metering, 15 VA/class 1, ISF= 150 V, Imag at Vk/2

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njunction with this book.

Bibliography

If you would like to have to buy a copy of the manual, please contact Kappa.