In our last post, we discussed the necessity of staged fault testing by reviewing three examples of protective relay misoperations. Staged fault testing is a wise preemptive procedure for electric utilities to implement because it will significantly reduce the risk of a wide area blackout.
Several pieces of equipment will be needed; luckily, most equipment will not be permanent, is easily installed, and can be reused to perform staged fault testing at other facilities.
Prescient recommends that electric utilities follow a set procedure to implement staged fault testing, outlined below. Let’s take a closer look at best practices for staged fault testing, and the ways in which these tests can improve the resiliency and reliability of the electric power grid.
Stage Fault Testing Equipment
To conduct a staged fault test, the following equipment is required:
Lowboy trailer to transport staged fault circuit breaker and air break switch
Box truck to transport miscellaneous components
Flexible connectors and ground straps
Barricades to quarantine the test area
Permanent staged fault taps
Staged Fault Testing Sequence
In open air facilities, staged fault testing can be completed in one workday using staged fault circuit breakers mounted on lowboy trailers. To ensure that conductors are not damaged during fault testing, utilities should add staged fault testing taps. This would be the only permanent change that utilities would need to make prior to testing.
Figure 1 shows the proposed arrangement. The circuit breaker is connected to a ground connection and an air break switch which is connected to the stage fault tap.
Part 1: Staged Fault Testing Set Up
To perform staged fault testing, utility workers should:
Install staged fault taps as part of the facility installation.
Receive permission to install temporary staged fault equipment.
Locate the staged fault trailer near the facility that will be tested.
Verify that the staged fault air break switch is open and ready for service.
Verify that the staged fault circuit breaker is open and ready for service.
Connect staged fault equipment to the facility.
Part 2: Staged Single Phase Fault
To conduct a staged single phase fault, following the steps in part 1, utility workers should:
Receive permission to install A phase ground strap.
Receive permission to close staged fault air break switch.
Close staged fault air break switch.
Receive permission to close staged fault circuit breaker (A phase fault).
Staged fault circuit breaker trips immediately when closed.
Verify fault circuit breaker is open.
Receive permission to open staged fault air break switch.
Part 3: Staged Three Phase Fault
To conduct a staged three phase fault, following the steps in part 1 and 2, utility workers should:
Receive permission to install B and C phase ground straps (A phase strap left in place during part 2).
Receive permission to close staged fault air break switch.
Close staged fault air break switch.
Receive permission to close staged fault circuit breaker (three phase fault).
Staged fault circuit breaker trips immediately when closed.
Verify fault circuit breaker is open.
Receive permission to open staged fault air break switch.
Receive permission to remove A, B and C phase ground straps.
Receive permission to remove staged fault taps.
Receive permission to remove staged fault trailer.
Part 4: Analyze Data
Fault data will be retreived from all protective relays and fault recorders that should detect the staged faults. This data will be analyzed as follows:
Primary protective relay actuations before next staged fault.
Redundant protective relay actuations before next staged fault.
Misoperations at other facilities before next staged fault.
Protective relay data at other facilities, within next week.
Staged Fault Testing System
The preceding staged fault testing sequence assures that faults are cleared regardless of whether protective relaying schemes operate as designed. Faults will be cleared in less than 100 milliseconds when the staged fault circuit breaker opens. Testing can be completed at an optimal time, for example, on a Saturday in April when load is light.
Staged Fault Testing Improvements
We predict that staged fault testing will reduce the number of protective relay operations by 75%, as illustrated in Table 1.
The number of misoperations in Table 2 could be reduced to less than 100 by converting existing schemes that operate in racehorse logic, in which the first relay to operate trips a circuit breaker, to voting logic, in which two of three relays must operate to trip a circuit breaker. Implementing voting logic would eliminate misoperations caused by relay failures.
Innovate, Duplicate, Learn from Others
By observing and duplicating the best practices of other industries, the electric utility industry can implement similar reliability standards. For example, the nuclear industry has similar reliability testing practices to the staged fault testing procedures Prescient has proposed here. Nuclear power generating stations use protective relays in applications like the applications used throughout the electric power grid.
Nuclear power generating stations, however, have one additional, important, requirement: the nuclear reactor pressure vessel must be certified for continued operation if 40 full power SCRAMs occur. In other words, the owner of a nuclear power generating station needs to develop protective relaying schemes that include as low as reasonably achievable (ALARA) parameters – one misoperation during 40 years of operation is a reasonable ALARA goal.
It’s time for protective relay engineers and designers to implement ALARA parameters for protective relay applications within the electric utility industry.
Check out Prescient’s next generation blog collection to learn more about our ideas for the fourth generation (4Ge) electric power grid. Contact us to continue to conversation.