Firstly, the G74 standard is UK specific – so it is not well recognised elsewhere in the world. There are two other factors that confuse matters further. To further complicate matters, the latest versions of the IEC 60909 document include more sophisticated / robust ways of calculating fault levels and managing new technology types such as Solar (PV) and Wind Turbines – that were not covered in earlier versions of the document. The current position is now somewhat unusual, as the last version of the G74 standard was issued in 1992, and the IEC 60909 standard has been updated and reissued several times – including most recently 2016 this leads to a scenario where the G74 standard looks rather out of date. The G74 approach usually results in lower fault levels, as the pre-fault voltage is usually around 1.0 pu, while the use of ‘C’ factors creates slightly higher results. In practice the difference is centred around the IEC 60909 use of voltage ‘C’ factors – these ‘C’ factors are used to define the pre-fault voltage, while the G74 approach is based on using a calculated pre-fault voltage based on a loadflow study.
The actual G74 standard was originally developed as an alternative to the IEC 60909 (or 909 standard as it was then), to provide more accuracy for determining fault currents, because the UK operators considered the IEC 60909 standard to be too conservative, and calculations would potentially provide inaccurate results. The IEC 60909 standard for short circuit calculations in three phase AC systems, is a well known and commonly used standard, while the G74 standard refers to a UK Electricity Networks Association (ENA) standard, that is aligned with IEC 60909. The Short Circuit Study IEC 60909 / ENA G74Īn IEC 60909 / G74 short circuit study refers to the process of determining the fault levels (commonly called short circuit levels) in an electrical power system.