IEC 62505-3-3 pdf download

IEC 62505-3-3 pdf download Railway applications – Fixed installations – Particular requirements for a.c. switchgear – Part 3-3: Measurement, control and protection devices for specific use in a.c. traction systems – Single-phase inductive voltage transformers
8 Ferro-resonance
8.1 General
The following requirements, additional to those of IEC 60044-2:1 997, apply:
The structure of railway electrification network may easily generate ferro-resonance phenomena. They occur mainly in 25 kV, 50 Hz systems.
When specified by the purchaser, the voltage transformer connected directly to the overhead line shall have the following characteristics in order to avoid the generation of ferro-resonance phenomena and withstand the overvoltages caused by the same.
8.2 Prevention of the generation
The following features shall be adopted:
− minimum resistance of the primary winding: shall be specified by the purchaser;
NOTE Typical resistance values are between 1 5 kΩ and 50 kΩ.
− flux density such that at U max2 (see IEC 60850) the saturation point is not reached.
8.3 Prevention of damages in the voltage transformer
The following features shall be adopted:
− ability to withstand without damage on the primary side a voltage wave obtained by summing a sinusoidal wave at rated frequency with a square wave at 1 /3 of the rated frequency, with a total peak value of approximately 1 ,6 × U n ;
− suitability to operate continuously at 1 /3 of the rated frequency and rated voltage;
− voltage factor of 1 ,9 (1 ,9 × U n for 8 h at rated frequency).
8.4 Ferro-resonance withstand test The voltage transformer will be considered suitable to withstand ferro-resonance if these characteristics are checked and tested, as a special type test named ferro-resonance withstand test. This consists of a temperature-rise test at rated voltage, rated burden and 1 /3 of rated frequency. The standard temperature rises shall not be exceeded.
9 Particular design and construction characteristics The following requirements, additional to those of IEC 60044-2:1 997, apply:
9.1 Pollution The dust pollution on railway tracks (e.g. in tunnels and at the seaside) is often higher than on high voltage transmission lines. For this reason the creepage distance shall not be less than indicated in IEC 62497-1 , Table A.7, for the appropriate pollution degree. NOTE For the choice of the appropriate pollution degree, the degrees used in the associated switchgear (see IEC 62505-1 and IEC 62505-2) should be taken into account.
9.2 External flashover test When specified by the purchaser, outdoor units shall be able to withstand an external flashover of the primary bushing and be capable of continued service. This will be tested as a special type test named the external flashover test by repeating dielectric, partial discharge and accuracy tests after a flashover. The flashover is made by creating a permanent