BS 7628:1993 pdf free download.Specification for Combined transformers ( Implementation of HD 548.3 S1 ).
BS 7628 covers the requirements and tests in addition to the requirements and tests of IEC Publications 185 and 186 for current and voltage transformers, including capacitor voltage transformers, that are necessary for combined instrument transformers.
2 Definition
combined instrument transformer
a transformer consisting of a current and a voltage transformer in the same casing
Section 2. Limits of error
3 General
The limits of error for combined measuring transformers shall correspond to the requirements for measuring current transformers, indicated in Section 8 of IEC Publication 185, and to the requirements for measuring voltage transformers, indicated in Section 9 of IEC Publication 186. The limits of error for combined protective transformers shall correspond to the requirements for protective current transformers, indicated in Section 12 of JEC Publication 185, and to the requirements for protective voltage transformers, indicated in Section 13, of IEC Publication 186, and to the requirements for capacitor voltage transformers, indicated in Section 19 of IEC Publication 186.
4 Mutual influence
4.1 When operating the current transformer between 5 % of the rated current and the rated continuous thermal current, the voltage transformer shall not exceed the limits of voltage error and phase displacement corresponding to its class within the specified range of burden and between 80 % and 120 % of the rated voltage.
6 Impulse voltage test
In the case of impulse voltage tests for combined instrument transformers, the impulse voltage waves shall be applied to the ahort.circuited primary winding of the current transformer connected to the terminal of the voltage transformer primary winding, which is at high voltage when being operated.
The tests are carried out as indicated in Clause 21 of IEC Publication 185 for current transformers, and in Clauses 15 and 47 of JEC Publication 186 for voltage transformers and for capacitor voltage transformers respectively.
7 Temperature-rise test
A test shall be made to prove compliance with Clause 5. For the purpose of this test, combined instrument transformers shall be considered to have attained a stendy•state temperatore when the rate of temperature rise does not exceed 1 K per hour. The ambient temperature may be between 10 T and 30 C. When there is more than one secondary winding, the tOSts shall be made with the appropriate rated burden connected to each secondary wmding unless otherwise agreed between manufacturer and user. For the test, the transformer shall be mounted in a manner representative of the mounting in service. The prescribed current and voltage are applied simultaneously to the combined instrument transformer. For this purpose, it is necessary that the primary winding and the secondary winding of the transformer generating the high current which excites the current transformers are insulated the one in relation to the other for the full voltage o(the network.
If such a transformer is not available, the combined instrument transformer may be installed insulated. The high voltage is then applied simultaneously to the frame, to the casing, to the terminal of the primary winding usually earthed in service, and to one terminal of each secondary winding, whilst the terminal of the primary winding applied to the mains line in service is earthed. Thus the insulation of the transformer generating the current need not be constructed for high voltage.
The results of the two methods are the same.
The temperature rise of windings shall be measured by the increase in resistance method. For primary windings of the current transformer with very low resistance, thermocouples may be employed. The temperature rise of parts other than windinga may be measured by means of thermometers or thermocouple-s.
8 Tests for accuracy
8.1 The tests for accuracy shall be carried out according to Clauses 29 and 39 of IEC Publication 185. for current transformers. and according to Clauses 2$, 32 and 54) of IF.C Publication 1116, for voltage transformers.
.2 In the case of combined instrument transformers, the influence of the current transformer on the voltage transformer has to be tested as follows:
112.1 First, the voltage error E and the phase displacement .1. olthe voltage transformer are determined with dead current transformer according to Clauses 23. 32 and 50 of LEC Publication 1116 at rated burden and at 25% of thc rated burden (M.-asuremnent 1). Then the current transformer is supplied with the rated continuous thermal current.
The supply line to the current transformer shall form a horizontal loop at the height of the primary terminals (see Figure 1. page 7). The distance of the return conductor shall correspond to the distance of the other phase in the mains line. The primary winding of the voltage transformer is short.ciieuited with a connection as short as possible, which is placed in the vertical plane of the primary terminals of the current transformer.
The voltage induced by the current in the voltage transformer is measured by a millivokmeter or oscilloscope at the secondary terminals- This voltage U is a measure of the maximum variation of the voltage error.
It is recommended to load the voltage transformer with a burden that equals a rated burden of 15 VA to avoid errors by externally influenced voltage (Measurement 2). For protective transformers, it is sufficient to bring the variation .r only into relation to 2 % and for measuring transformers into relation to 110 % of rated secondary voltage.
9 Verification of terminal and rating plate markings
It shall be verified that the terminni markings and the rating plate markings are in accordance with Sections 6. 10 and 14 of IEC Publication 185 for current transformers and Sections 6. 7. 11. 16 and 25 of IEC Publication 186 for voltage transformers,
10 Power-frequency tests
10.1 Primary windings
For combined instrument transformers. the power-frequency teats on primary windings shall he carried out as stated in CLauses 12 and 51 of EEC Publication 18€ for voltage transformers. The induced voltage test fur the voltage transformer is simultaneously the power.frequency lest for the primary winding of the current transformer.
10.2 Secondary windings
The power-frequency test on the secondary winding of the current transformer shall be carried out in accordance with Clause 16 of EEC Publication 185 and the over-voltage inter-turn test of the current transformer in accordance with Clause 17 of EEC Publication 185. The power-frequency test on the secondary winding of voltage transformers shall be carried out in accordance with Clause 13 and Sub-clause 51.3 of IEC PubLication 186.
11 Measurement of partial discharges
Measurement of partial discharges. if called (hr. shall be performed in accordance with
IEC Publication 270. Partial Discharge Measurements and fEC Publication 44-4: Instrument
Transformers. Part 4: Measurement of Partial Discharges.