The Power Voltage Transformer, which is a singlephase unit used for direct conversion of power from high to low voltage, is becoming more and more relevant for substation application. This is why this paper aims to present the properties and advantages of the open-core design concept, which has been functioning very well for a number of years in inductive voltage and combined instrument transformers, enhancing the transformer reliability and operational safety. Furthermore, the intention is to list the known possible application options of these transformers and finally to succinctly point out the guidelines for future development, which would benefit both the manufacturer and the user.

SUMMARYThe methodology and test procedure for the reduction of possible in-service explosions of instrument transformers to negligibly low values is described. As the introduction, an interpretation of statistical results of a survey about different types of failure in instrument transformers is given, showing that about 90% of them develop gradually, taking from several hours to several months. The International Electrotechnical Commission (IEC) is about to adopt a standardised procedure requiring that instrument transformers should withstand the effect of an internal arc. The authors are taking a critical view of the proposal; explaining its main flaws. On the basis of calculations and experimental research on models and 123 kV current transformer it is shown that the mechanism causing explosions of instrument transformers cannot be simulated well enough in the way provided for in the draft of the new IEC standard. A new concept for the reduction of explosions of instrument transformers in service called 'explosion safe' is proposed. It includes additional routine and special tests, and usage of devices for the detection or measurement of oil pressure rise in transformers. The proposed tests are based on the investigation 'Volt-Time Relationships for PD Inception in Oil-Paper Insulation'.

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