ETAP addresses open-phase faults

Tuesday, 9 September 2014

ETAP, the California headquartered software provider, has found a solution to open-phase fault conditions on nuclear power plant distribution systems.

New software, containing single and double open-phase fault analysis capabilities, was introduced in May and a report summarizing the project was issued yesterday.

A reliable source of offsite electric power is critical to the safe shutdown of a nuclear power reactor and is normally supplied from the local transmission system via conductors. An open-phase fault occurs if one or two of the three conductors disconnects, exposing the in-plant safety systems to a potentially undesirable situation that may lead to failure.

In its report, the ETAP Nuclear Utility Users Group (NUUG) said the innovation is the nuclear power industry's first commercially available software "validated to analyse" the effects of open-phase faults. The Institute of Nuclear Power Operations (INPO) and the World Association of Nuclear Operators (WANO) are among the organizations that have recognized the importance of NUUG's effort, ETAP said.

Following an open-phase fault event at Byron Nuclear Power Station near Rockford, in Illinois in 2012, INPO began to raise awareness of this potential nuclear plant design vulnerability in the USA.

ETAP is the most commonly used software in US nuclear power plants - 97% of these plants already use ETAP to analyze their auxiliary power systems, which means that detailed models of these plant's electrical systems already exist. However, at the time of the initial open-phase fault events at Byron, ETAP software did not include the capability to analyze an open-phase fault.

By 2013, four open-phase fault events had occurred at nuclear power plants in the USA, Canada, and Sweden, which prompted WANO to raise awareness outside the USA.

The challenge open-phase faults create for a nuclear power plant is that the loss of a phase on the offsite power source can potentially damage both redundant trains of the emergency core cooling system, but may not be detectable by existing protection systems, NUUG said in the report. Thus, there could be an open-phase fault that goes undetected by nuclear plant operators, it said.

The consequences of an open-phase fault can be entirely different at each nuclear power plant. Research performed by the Electric Power Research Institute has shown that the consequences largely depend on the specific design of the offsite power transformer feeding the plant's nuclear safety systems, NUUG said in the report.

"This means in-depth power system analysis is required to determine the exact consequences for each plant. Thus, governmental, engineering, and industry organizations have been collaboratively focused on developing analytical modeling techniques to study the effects of open-phase faults, with an overall goal of developing any necessary counter-measures to mitigate a design vulnerability," it said.

While there were other software tools available that could be used to simulate an open-phase fault, these were not well-suited for performing a comprehensive analysis of the effects the fault may have on each of the numerous electrical loads throughout various safety systems, NUUG said in the report.

"Early attempts to use some of these software tools emphasized that their use would become cost-prohibitive, especially when iterative analyses were needed," it said. "This was not a viable solution for the nuclear industry because ultimately such high costs for performing engineering analysis would have to be passed along to electric utility customers."

ETAP has enhanced its Unbalanced Load Flow module that provided a comprehensive graphical Open-Phase Fault Analysis capability to study the condition and, in turn, "saved utilities millions of dollars that would have been required by alternative approaches," it said.

In March 2013, NUUG technical chairman Mark Bowman formed a task force of power system analysis experts, which included representatives of Duke Energy, Enercon Services Inc, Exelon Corp, MPR Associates Inc, Sargent & Lundy LLC, Southern Nuclear, and Tennessee Valley Authority. The task force's mission was to assess the ability of the newly-designed ETAP Unbalanced Load Flow module to perform a detailed quantitative analysis of an open-phase fault for a typical nuclear power plant.

"The ETAP NUUG was responding to an industry wide issue with 'open-phase faults'," Bowman told WNN. "Our efforts were well received by the Nuclear Regulatory Commission, Nuclear Energy Institute, and the Institute of Nuclear Power Operations because they saw this as an good example of a nuclear industry group and a supplier (ETAP) pulling together with the ultimate goal of improving nuclear safety," he said.

The report said that this collaborative effort between expert power system analysis engineers from the US nuclear utility industry and expert software designers at ETAP "resulted not only in advancing the capabilities of the software to analyze a new type of fault, but has helped to further improve the safety of nuclear power plants worldwide."

Researched and written
by World Nuclear News