Open-phase condition detection
Detection of open phase condition in power transformers
This article deals with open phase events in power transformers, mainly related to nuclear power plants where several cases have led to a global initiative for a prompt solution for this vulnerability. Different solutions are possible, and one of them, which has already been tested at a Spanish nuclear power plant, is presented here. Field test results are presented together with simulations.
Keywords: power transformers; transformer monitoring; open phase operation; nuclear power plants
Power transformers are key components in nuclear power plants and have a clear impact on the operational safety. Some transformers supply critical components in the plant where safety is a first priority. Most transformers are supplied by a three-phase system. For different reasons, one or two phases supplying the power transformer can get to open phase condition while transformer is in operation. This is an Open phase condition event (OPC). Standard protection systems may not detect this situation because the transformer magnetic core restores the voltages in the affected phase, hence not tripping the protection devices. This situation cannot lead to serious damage of the transformer, but if loads like motors are connected to the secondary side, those can end up being supplied by an unbalanced system leading to an over temperature trip or possible damage, depending on their own protection mechanisms.
A relevant open phase condition was first recorded in Byron Station (Illinois, USA) 2012 and several other incidents have been reported since then . This has resulted in an international need to define the problem and design upgraded solutions to avoid it, either at an operational, protection, or electrical level. Subsequently, similar events occurred in several nuclear power plants and, together with a review of the operational experience, it was concluded that open phase problem should be studied in greater depth. This resulted in the WANO (World Association of Nuclear Operators) SOER (Significant Operating Experience Report) 2015-01 “Safety Challenges from Open Phase Events” , in which the possible non-detection of the event by the existing electrical protection systems is identified and how that influences important safety equipment.