BS EN 61869-2-2012 pdf download.Instrument transformers Part 2 : Additional requirements for current transformers.
To minimise the effects of variation of cooling-air temperature, particularly during the last test period, appropriate means should be used for the temperature sensors such as heat sinks with a time-constant approximately equal to that of the transformer.
The average readings of two sensors shall be used for the test.
7.2.2.203 Duration of test
The test can be stopped when both of the following conditions are met:
— the test duration is at least equal to three times the current transformer thermal time constant:
– the rate of temperature rise of the windings (and of the top oil of oil-immersed current transformers) does not exceed 1 K per hour during three consecutive temperature rise readings.
The manufacturer shall estimate the thermal time constant by one of the following methods:
— before the test, based on the results of previous tests on a similar design. The thermal time constant shall be confirmed during the temperature rise test.
— during the test, from the temperature rise curve(s) or temperature decrease curve(s) recorded during the course of the test and calculated according to Annex 20.
— during the test, as the point of intersection between the tangent to the temperature rise curve originating at 0 and the maximum estimated temperature rise.
— during the test, as the time elapsed until 63 % of maximum estimated temperature rise.
7.2.2.204 Temperatures and temperature rises
The purpose of the test is to determine the average temperature rise of the windings and, for oil-immersed transformers, the temperature rise of the top oil, in steady state when the losses resulting from the specified service conditions are generated in the current transformer.
The average temperature of the windings shall, when practicable, be determined by the resistance variation method, but for windings of very low resistance, thermometers, thermocouples or other appropriate temperature sensors may be employed.
Thermometers or thermocouples shall measure the temperature rise of parts other than windings. The top-oil temperature shall be measured by sensors applied to the top of metallic head directly in contact with the oil.
The temperature rises shall be determined by the difference with respect to the ambient temperature measured as indicated in 7.2.2.202.
7.2.6.203 Test for composite error of class P and PR protective current transformers
The following two test procedures are given:
a) Compliance with the limits of composite error given in Table 205 shall be demonstrated by a direct test in which a substantially sinusoidal current equal to the rated accuracy limit primary current is passed through the primary winding with the secondary winding connected to a burden of magnitude equal to the rated burden but having, at the discretion of the manufacturer, a power factor between 0,8 inductive and unity (see 2A.4, 2A.5. 2A.6. 2A,7.
The test may be carried out on a transformer similar to the one being supplied, except that reduced insulation may be used, provided that the same geometrical arrangement is retained.
As far as very high primary currents and single-bar primary winding current transformers are concerned, the distance between the return primary conductor and the current transformer should be taken into account from the point of view of reproducing service conditions.
b) For low-leakage reactance current transformers according to Annex 2C, the direct test may be replaced by the following indirect test.
With the primary winding open-circuited, the secondary winding is energized at rated frequency by a substantially sinusoidal voltage having an r.m.s. value equal to the secondary limiting e.m.f. EALF.
The resulting exciting current, expressed as a percentage of I x ALF
shall not exceed
the composite error limit given in Table 205.
The exciting voltage shall be measured with an instrument which has a response proportional to the average of the rectified signal, but calibrated in r.m.s.. The exciting current shall be measured using an r.m.s measuring instrument having a minimum crest factor of 3.
In determining the composite error by the indirect method, a possible correction of the turns ratio need not be taken into account.
7.2.6.204 Test for error at limiting conditions for class TPX, TPY and TPZ protective current transformers
The purpose of the type test is to prove the compliance with the requirements at limiting conditions. For test methods refer to Annex 2B.
If the current transformer is a low-leakage reactance type according to Annex 2C, an Indirect type test may be performed according to 2B.2. otherwise a direct test shall be performed according to 28.3.BS EN 61869-2-2012 pdf download.