Open Phase Conditions in Electric Power Systems

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Summary of Technical Issue

Nuclear power plant systems are powered by electricity delivered through three lines, or "phases" of the offsite power source. If one phase is lost, or "open," motors and other components can be damaged and emergency power sources might be compromised. The NRC and U.S. nuclear power plants are working to ensure "open phase conditions" are resolved safely.

On Jan. 30, 2012, Unit 2 at the Byron Station nuclear power plant in Illinois shut down safely after an "open phase" event. The shutdown was caused by unbalanced electrical voltage coming into the plant from the regional electric grid. One of the three phases on the plant’s grid connection was no longer transmitting. The plant was not designed, however, to automatically turn off or "trip" circuits to isolate that offsite power source and switch to emergency backup power. Plant operators diagnosed the problem in eight minutes and manually swapped power sources.

This event, and others similar to it, led the NRC and the nuclear power industry to evaluate open phase conditions. Losing one or two phases, with or without ground, on the primary (high voltage) side of a transformer connected to the transmission system can cause unbalanced voltage on the secondary (low voltage) side of the transformer, connected to plant safety equipment. If the condition is not detected, the degraded offsite power line may not be disconnected. Then, the equipment needed to safely shut down the plant may not transfer to another functioning electrical source. Therefore, it may not have enough power to function properly. This condition is possible at 98 of the 99 U.S. commercially operating nuclear reactors.

Every affected U.S. nuclear power plant has temporary measures in place to reduce the risk associated with an open phase condition during normal operation. These provisions include enhancing control room operator awareness and modifying procedures to ensure plants switch to emergency power sources if needed. The NRC staff reviewed and agreed with the temporary measures, and regional inspectors inspected the measures.

Additional Operating Experience

U.S. nuclear power plants have identified related design issues, such as those described in the following event reports:

  • South Texas Project, Unit 2 (Licensee Event Report (LER) 50 499/2001-001, ADAMS Accession No. ML011010017)

  • Beaver Valley Power Station, Unit 1 (LER 50-334/2007-002, ADAMS Accession No. ML080280592)

  • Nine Mile Point, Unit 1 (LER 50-220/2005-04, ADAMS Accession No. ML060620519)

  • James A. Fitzpatrick Power Plant (LER 50-333/2005-06, ADAMS Accession No. ML060610079)

These events involved offsite power circuits with one open phase. In most cases, the condition went undetected for several weeks because offsite power was not set up to power plant safety equipment during normal operation. Instead, this equipment was powered by the plant’s own turbine generator. The operators’ procedures for checking phase-to-phase voltage did not identify the loss of the single phase. At South Texas Project, Unit 2, offsite power was normally supplying plant equipment, so the open phase condition tripped three circulating water pumps. As a result, the operators manually shut down the reactor.

International experience with open phase conditions includes:

  • On Dec. 22, 2012, Unit 1 at Bruce Power Plant in Canada was shut down when a maintenance cooling system pump tripped. Operators tried to manually start both pumps, but could not get them to start because of specific features of the electrical system. Field operators identified a break in one of the three phases of an overhead power line.

  • On May 30, 2013, Forsmark Unit 3 in Sweden reported an event resulting from human error. The plant was in a refueling outage with multiple circuit breakers open for maintenance. When operators were testing the main generator, the remaining offsite power circuit breaker received an erroneous trip signal. One of the three phases failed to open, resulting in a double open phase. Some of the operating equipment tripped due to phase unbalance, while other equipment overheated and failed.

  • On April 27, 2014, the Dungeness B power plant in United Kingdom experienced random tripping of large loads resulting from the loss of one of three phases in the 400 kV electrical supply to the site. The open phase condition was the result of inadequate contact in one pole of the circuit breaker.

In the events discussed above, the protective electrical relay arrangement did not detect the open phase conditions. As a result, degraded power sources continued to supply plant equipment and the onsite diesel generators did not automatically connect to provide appropriate power.

As a result of this issue, the Institute of Nuclear Power Operations (INPO) issued a Level 2 INPO Event Report, which required corrective actions by plant operators.

NRC Response

The NRC alerted reactor operators to the Byron Station operating event by issuing Information Notice 2012-03, "Design Vulnerability in Electric Power System," dated March 1, 2012 (ADAMS Accession No. ML120480170). Then, on July 27, 2012, the staff issued NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System," (ADAMS Accession No. ML12074A115) to confirm that licensees complied with relevant requirements for electric power systems, such as:

The NRC asked for specific information on:
  1. The protection approach to detect and automatically respond to a single phase open circuit or high impedance ground fault on power circuits important to safety.
  2. The operating configuration of engineered safety features buses at power.

The NRC staff documented its review of the licensee responses in a report dated Feb. 26, 2013 (ADAMS Accession No. ML13052A711). The staff recommended that licensees be required to detect and automatically respond to a single-phase open circuit. The NRC staff also obtained additional information to support decision-making through a generic request to all operating reactor licensees (ADAMS Accession No. ML13351A314).

The Nuclear Energy Institute (NEI), on behalf of the nuclear industry, proposed an open phase isolation system that would address the identified issue. The NRC staff sent its response, including four functional criteria that should be achieved when implementing the proposed system, to NEI on Nov. 25, 2014 (ADAMS Accession No. ML14120A203). Furthermore, the NRC staff developed Branch Technical Position 8-9, "Open Phase Conditions in Electric Power System," to provide guidance to staff in reviewing licensees’ and applicants’ proposed solutions to this issue.
The Commission directed the staff in SRM-SECY-16-0068 to verify that licensees have appropriately implemented the voluntary industry initiative.  The staff initially issued a Temporary Instruction, TI-2515/194, "Inspection of the Licensee's Implementation of Industry Initiative Associated with the Open Phase Condition Design Vulnerabilities in Electric Power Systems," (ADAMS Accession No. ML17137A416) to verify implementation of the initiative at four pilot plants.
Some nuclear power plant operators are implementing the permanent solution for this issue either through license amendment requests to modify the protective relaying schemes or  through the installation of open phase isolation systems. The modifications and isolation systems have been installed at several sites.  Some are already providing an active isolation function and others are currently being operated in a monitoring mode to verify that the systems can adequately detect open phase conditions prior to enabling automatic actuation features.
On February 20, 2019, NEI informed the NRC that operating experiences indicated the open phase isolation system circuitry may be sensitive to electric plant transients resulting from circuit breaker switching. As such, there was a potential for spurious actuation that could lead to an inadvertent loss of offsite power. Subsequently, NEI issued Revision 3 to the voluntary industry initiative and developed a guidance document (NEI 19-02) that enables licensees to use a risk-informed approach under which the system would provide an alarm and indication of an open phase condition in the control room. Operators would then diagnose and respond to the alarm by taking manual actions as opposed to an automatic actuation of the system.
The NRC staff issued Revision 1 and Revision 2 of TI-2515/194 (ADAMS Accession Nos. ML19339D067 and ML20230A328, respectively) to provide additional instructions for verifying that the plants employing operator manual actions in lieu of automatic protective features are appropriately implementing this option of the voluntary industry initiative. These inspections are ongoing and are expected to be completed by the end of 2021.   

Page Last Reviewed/Updated Wednesday, April 14, 2021