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The Bureau of Indian Standards (BIS) regulates the technical specification of Current Transformers as per IS 2705:1992 (2nd Revision) guidelines. This standard has been prepared with suitable support taken from IEC185 & BS3938. The Indian Standards on CTs has been published mainly into 4 parts:i) ii) iii) iv) General requirements Measuring current transformers Protective current transformer Protective current transformers for special purpose application
Definitions: In general for the purpose of this standard, the following definition or terminology may be applied: 1) Instrument transformer: A transformer intended to supply measuring instruments, meters, relays & other similar devices 2) Current transformer: An instrument transformer in which the secondary current, in normal condition of use, is substantially proportional to the primary current & differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. The various types of CTs are described as below: a) Wound type CT:- A CT havind primary winding more than 1 full turn wound on the core .
Bar type CT c) Dry type CT:- A CT which does not require the use of liquid or semi-liquid material
Dry Type CT d) Liquid Immersed CT :- A CT which requires the use of oil or other suitable liquid of suitable characteristics
e) Hermetically sealed CT:- A liquid immersed CT which is not in contact with atmospheric air
f) Ring type CT:- A CT which has an opening in the center to accommodate a primary conductor through it
g) Multi-core CT:- A CT having more than one secondary core & winding with a common primary winding
h) Multi-ratio CT:- A CT in which more than one ratio is obtainable by reconnections or tappings, in primary or secondary windings
3) Measuring CT :- A CT intended to supply indicating instruments, integrating meters & similar apparatus. 4) Protective CT :- A CT intended to supply protective relays & similar apparatus. 5) Dual purpose CT :- A CT intended to serve the dual purpose of measuring & protection 6) Primary winding :- The winding through which flows the current to be transformed 7) Secondary winding :- The winding which supplies the current circuits of measuring instrument meters, relays or similar apparatus 8) Secondary circuit :- The external circuit supplied by the secondary winding of a CT 9) Rated primary current :- The value of primary current which appears in the designation of the transformer & on which the performance of the CT is based
10) Rated secondary current :- The value of the secondary current which appears in the designation of the transformer & on which the performance of the transformer is based 11) Actual transformation ratio :- The ratio of the actual primary current to the actual secondary current 12) Rated transformation ratio :- The ratio of the rated primary current to the rated secondary current 13) Current Error (Ratio error) :- The error with a transformer introduces into the measurement of a current & which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio It is expressed as: Current error (%) = ( (Ka*Is - Ip) * 100)/Ip Where, Ka = rated transformation ratio Ip = actual primary current Is = actual secondary when Ip flowing under the conditions of measurement 14) Phase displacement :- The difference in phase between the primary & secondary current vectors, the direction of the vectors so chosen that the angle is zero for a perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually expressed in minutes for sinusoidal currents 15) Accuracy class :- A designation assigned to a current transformer the errors of which remain within specified limits under prescribed conditions of use. 16) Burden :- The impedance of the secondary circuit in ohms which is expressed as apparent power in VA absorbed at a specified power-factor & at rated secondary current 17) Rated Burden :- The value of the burden on which the accuracy of requirements of this specification are based 18) Rated output :- The value of the apparent power (in volt-amperes at a specified power factor) which the current transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it.
19) Highest system voltage :- The highest rms line-to-line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variations due to fault conditions and the sudden disconnection of large loads. 20) Rated Insulation level :- That combination of voltage values (power frequency and lightning impulse, or where applicable, lightning and switching impulse) which characterizes the insulation of a transformer with regard to its capability to withstand dielectric stresses. 21) Isolated Neutral system :- A system which has no intentional connection to earth except through indicating, measuring or protective devices of very high impedance 22) Resonant earthed system :- A system earthed through a reactor, the reactance being of such value that during a single phase-to-earth fault, the power-frequency inductive current passed by this reactor substantially neutralizes the power frequency capacitive component of the earth-fault current 23) Earth-fault factor :- At a selected location of a three-phase system (generally the point of installation of an equipment) for a given system layout, the ratio of the highest rms phaseto-earth power-frequency voltage on a sound phase during a fault to earth (affecting one or more phases at any point) to the rms phase-to-phase power-frequency voltage which would be obtained at the selected location with the fault removed. It should be noted that the earth-fault system should not be greater than 1.4 since at maximum 2 phases may be faulty & the current will be burdened on the third conductor 24) Earthed Neutral System :- A system in which the neutral is connected to earth, either solidly or through a resistance or reactance of low enough value to reduce transient oscillations and to give a current sufficient for selective earth fault protection It should be noted that the earth-fault factor may exceed 1.4 in a non-effectively earthed system. 25) Electrically exposed installation:- An installation in which the apparatus is subject to overvoltages of atmospheric origin 26) Electrically non-exposed installation :- An installation in which the apparatus is not subject to overvoltages of atmospheric origin
27) Rated frequency:- The value of frequency on which the requirements of this standard are based. 28) Rated short-time thermal current (Ith):- The rms value of primary current which the current transformer will withstand for a rated time, with their secondary winding short circuited, without suffering harmful effects a) Rated time:- The time in seconds for which the current transformer shall withstand the rated short-time thermal current in accordance with the specification. b) Rated short-time factor:- The factor which when multiplied by the rated primary current gives the rated short-time thermal current. 29) Rated dynamic current :- The peak value of the primary current which a CT will withstand, without being damaged electrically or mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited 30) Rated continuous thermal current:- The value of current which can be permitted to flow continuously in primary winding, the secondary windings being connected to the rated burdens, without the temperature rise exceeding the specified values. 31) Type tests:- Tests carried out to prove the general qualities and design of a given type of CT in accordance with the requirements of this standard. Tests may be carried out on a prototype which may incorporate special arrangements for the measurements required by this standard 32) Routine test:- Tests carried out on each CT to check requirements likely to vary during production. 33) Optional tests :- Tests which may be in nature of type tests or routine tests and are carried out only as per the agreement between manufacturer & purchaser 34) Instrument safety factor:- The ratio of instrument limit primary current to the rated primary current. 35) Composite Error:- The composite error is defined as percentage of the rms values of the difference between instantaneous primary current & actual secondary current multiplied by the transformation ratio. Ec = ([Integral {K*is - ip}0T dt]/T)^0.5 *100/Ip Where, K= transformation ratio Ip= rms value of primary current ip= instantaneous value of primary current is= instantaneous value of secondary current
36) Exciting current:- The rms value of the current taken by the secondary of a CT when a sinusoidal voltage of rated frequency is applied to the secondary terminals with the primary being open-circuited. 37) Instrument Limit Primary Current:- The value of minimum primary current at which the composite error of the measuring CT is equal to or greater than 10%, the secondary burden being equal to the rated burden. 38) Rated Accuracy Limit Factor:- The value of accuracy limit factor for which accuracy requirements of this specification are met at the rated burden. 39) Rated Accuracy Limit Primary Current :- The value of the highest primary current upto which the transformer will comply with the specified limits of composite error. Part 1 : General requirements The service conditions specified for CT are as follows:a) The maximum ambient, maximum daily average & maximum yearly average temperature should not increase 450, 400 & 350 centigrade respectively while the minimum ambient temperature should not reach below -50C. b) The CT should be installed upto 1000 m above MSL c) The atmospheric condition should not be heavily polluted & should not favour growth of fungi & condensation of moisture with the type of earthing system to be specified
The standard ratings are as follows:a) The standard primary current of single ratio transformer are 10, 12.5, 20, 25, 30, 40, 50, 60, 75 in amperes while 10,15,20,30,50,75 are the most preferred whereas the lowest of the rated current can be taken for multi-core transformer b) The rated secondary current may be 1A or 5A for Star connection in the secondary while for delta connections, divide these ratings by 3^0.5. c) The standard value of the rated output could be 2.5, 5, 7.5, 10, 15 & 30 VA. d) The peak value of the rated dynamic current shall normally be 2.5 times the rated short time thermal current unless or otherwise specified e) The rated insulation levels for the CTs are defined by their highest system voltages classified by up to 72.5kV, above 123kV to 245kV, & above 420kV & shall be designed with various levels of choices of lightning impulse withstand voltages & switching impulse withstand voltage & made in accordance with IS3716:1978 & IS2165:1977.
a) The CTs manufactured shall comply with the specification in o ther clauses of Part 2, 3 & 4 b) The temperature rise test is carried on when primary current equal to the rated continuous thermal current with a unity power factor burden & is limited by the lowest class of insulation. c) The temperature rise is reduced by 0.4% for oil immersed & 0.5% for dry type transformer with increase in altitude for every 100m. d) CTs should also withstand dielectric tests.
1) The terminals should be marked in accordance with proper terminals for single ratio transformer, transformer with immediate tapping on secondary winding ,transformers with primary winding in 2 sections intended for connection either in series or in parallel or transformers with 2 secondary windings each clearly indicating the primary & secondary or the intermediate tappings if any. 2) The rating plate should carry at least the following markings like : a) Manufacturers name & country of origin b) Year of manufacturer c) Serial number & type designation d) Rated primary & secondary currents with operating frequency e) Rated output & corresponding accuracy class f) Highest system voltage & rated insulation class g) Rated dynamic current if it exceeds 2.5 more than rated short time thermal current
The various type tests are: 1) Short-time current test The short time thermal current test & dynamic current test are passed on for a time period of 0.5 to 5 sec with secondary winding as short-circuited & current in the primary to be equal to the rated rms value & peak value of the transformer with a rated short time current not exceeding 180A/mm^2. 2) Temperature-rise test The CT is said to have passed the temperature rise test if its temperature do esnt exceed 10C/hr & is said to have attained the steady state with the maximum allowable temperature with CTs of different voltage levels 3) Lightning impulse test for current transformers for service in electrically exposed installation
With the specified values of highest system voltage & rated insulation level, the CT should withstand 50% to 75% of the reference impulse voltage applied for 5 consecutive full wave pulse for LV & 15 consecutive full wave pulses for HV type CTs & no disruptive discharges or no flashovers should occur for the CT to pass temperature rise test . 4) Switching impulse voltage tests for current transformer of 420 kV & above The test is carried out in the same way as is done for lightning impulse test voltage for 15 consecutive wave pulses. 5) High voltage power frequency wet withstand voltage tests The external insulation level of outdoor CT shall be subjected for 1 min to a power frequency voltage having a peak value equal to 2^0.5 times the ----------------------6) Determination of errors to the requirements of particular design or accuracy class This is carried out by testing individual parts of the CT
*It may be noted that once manufacturers holds a type test certificate, then this practice
can be omitted for testing every transformer of the same design The various routine tests are: a) Verification of terminal marking As per the clause 1 of the Markings b) Power frequency dry withstand test on primary & secondary winding To carry out the test on transformers of up to 245 kV, the test voltage applied should be in accordance with their rated insulation levels & highest system voltages for the time duration of 1 minute under which no disruptive discharge should happen. For CTs above 245kV should be subjected to specified power frequency prestress voltage for 10 sec & then exposed to specified partial discharge voltage for the next 15 min. If it is required to repeat the test the test voltage should be 80% of the prescribed voltage c) Over-voltage inter-turn test Applying a voltage at power frequency to the primary with the windings of secondary kept open, in order to produce a primary current of rms value equal to the rated primary current. The applied voltage should withstand satisfactorily. In a similar manner keeping the primary winding open-circuited a voltage at a frequency not greater than five times the rated frequency shall be applied to the secondary & it should also withstand up to 1 minute satisfactorily. The objective of this test is to check the soundness of inter-turn insulation as open circuiting the secondary may result in over-heating & permanent magnetization in the core which may permanently damage the CT d) Determination of errors with respect to the design or accuracy class
The other optional tests are:a) Chopped lightning impulse test b) Measurement of dielectric dissipation factor for oil immersed CT of 72.5 kV & above c) Commissioning test on new CT up to 36kV Part 2: Measuring Current Transformer Accuracy class:- The accuracy class is designated by the highest permissible percentage current error at rated current for that class The accuracy class of 0.2S may be defined as +/- 0.2% of error at 120% of Ip In addition to that the current error & phase displacement should be within limits when the secondary has a burden between 50% to 100% of the rated burden. In addition to that rated output by accuracy class & instrument security factor should also be marked Test :- The routine test are the same as described in the Part-1 of IS 2705 Whereas the various type test carried out are for a) accuracy class b) For extended current rating greater than 120%, the tests shall be carried out as per the Specified current rating of the transformer c) The Instrument Security Current test is carried out by keeping the primary winding open-circuited & secondary at rated frequency having an rms voltage equal to the secondary limiting emf . The resulting excitation current Iexc should be greater than secondary current multiplied by safety factor shall equate or exceed the rated value of composite by 10%.
Part-3 : Protective Current Transformer
Protective CT :- A CT intended to supply protective devices like relays & trip coils Accuracy class:- the accuracy class shall be designated by highest allowable current error at the rated accuracy limiting primary current prescribed for the accuracy class concerned followed by the letter P Eg:- 5P10 signifies 5% of errors when Ip is 10 times
* The type tests involve Current error & phase displacement, Composite error for direct & indirect testing & routine tests. * Through vectorial representation of secondary current, excitation current, the actual primary could be find out by adding these two vectorially at rated burden. If the resultant & the primary has the zero phase displacement then no error is recorded. If there is a phase displacement then it signifies that there is an error in turns ratio.
Part-4 : Protective CTs for special purpose applications
This CTs are specifically designed for balanced & distance protective system such as Control & relays or automation etc. These are are also applicable to protective cores of multi-core current transformer. These transformers are designated by a special PS class & are transformers of very low reactance. The composite error determined by direct method is less than 1.3 times of the error determined by indirect method. The special class application transformers should be evenly distributed in both the windings. The special requirements for this class are as follows:a) The error in turns ratio shall not exceed +/- 0.2 percent. b) The knee-point voltage shall not be less than the specified rated knee-point voltage. c) The exciting current(s) shall not be greater than the specified value(s) required in accordance with 3.1.3. d) When specified, the resistance of the secondary winding, corrected to 75OC, shall not be greater than the specified value. It is necessary to perform following tests & markings: 1) Secondary winding resistance at 750C 2) Maximum exciting current at specified knee voltage 3) Turns ratio.
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