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Open Phase Protection

Altran has committed resources to addressing open phase conditions in the offsite power supply to nuclear power plants.  Our subject matter experts are prepared to share their in-depth understanding and to provide the basis for rapid, long term resolution of this issue.


Recently offsite power to a nuclear power generating station was compromised when one of the three phase conductors connected to the high voltage side of a station auxiliary transformer opened after a support insulator failed (insulator structural failure).

Generally, nuclear power generating stations have not installed protective relaying schemes that can detect and isolate open phase conditions in the supply to plant auxiliary systems as the possibility of open phase conditions is extremely remote.  Industrial and commercial companies deal with this remote possibility on a limited basis by including phase balance monitoring and phase unbalance tripping in protective relaying schemes applied to individual, large, motors.  The approach used by industrial and commercial companies cannot be used at nuclear power generating stations

unless provisions are made to bypass phase unbalance tripping during various design base events.


Issues with Open Phase Detection

Open phase detection is complicated by the fact that, on one hand, open phase detection schemes need to actuate quickly when open phase conditions occur, but, on the other hand, open phase detection schemes need to be secure from spurious operation during other abnormal conditions, such as when short circuits occur.

The primary indicator of an open phase condition is the presence of negative sequence current.  Unfortunately, negative sequence current is present during unbalanced short circuit conditions, whenever the power system load is unbalanced, and when power system impedances are not closely matched.

Analysis of negative sequence current during open phase conditions is complicated by the fact that negative sequence current is a function of connected load, the type of load (motor or resistive load), and transformer winding connections – at low load conditions, the magnitude of negative sequence current may be so small that detection is problematic.


The Altran Approach

Altran’s independent research has proven that traditional calculations, that is, symmetrical components - the method introduced by Dr. Charles L. Fortescue in 1918 and the basis for short circuit calculations, load flow calculations, etc., is the simplest method to calculate open phase conditions.

Recognizing that sensitive detection of open phase conditions is essential, Altran has researched significant factors, confirmed positive, negative and zero sequence voltage and current signatures for various load and unbalance conditions, developed load models and is prepared to:

  • Develop technical reports with supporting calculations that document the impact of open phase conditions at nuclear power plants.
  • Propose protective relaying schemes that detect open phase conditions and initiate prompt, remedial; action when necessary.
  • Develop design change packages that can be implemented to reliably detect open phase conditions.
  • Simulate open phase conditions in our laboratory in Boston.

Altran’s scheme:

  • Monitors negative sequence current and uses power system data as a weighting factor in the evaluation of open phase conditions.
  • Monitors actual current in each phase and uses phase comparison values in the evaluation of open phase conditions.
  • Monitors positive, negative and zero sequence current and uses ratios of sequence currents in the evaluation of open phase conditions.
  • Alarms when system conditions approach levels of concern.
  • Can be relied upon to actuate when station auxiliary load is above threshold levels.
  • Records data that can be used to verify scheme operability as well as system conditions.

 Altran’s scheme can be implemented using:

  • Microprocessor based protective relay technology.
  • Three phase current inputs to a microprocessor based protective relay.

Altran is prepared to discuss the numerous factors that need to be considered when addressing the potential impact of open phase conditions at nuclear power plants, to present the results of sample calculations, to present the outline of technical reports , and to discuss what Altran can do to help resolve this issue.


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