50/27 inadvertent energization protection function is an overcurrent function supervised by generator terminal bus voltage derived from system side VTs. Inadvertent or accidental energizing of off-line generators has occurred frequently enough to warrant the use of dedicated protection logic to detect this condition. Operating errors, breaker flashovers, control circuit malfunctions or a combination of these causes have resulted in generators being accidentally energized while off-line.
When a generator is accidentally energized from the power system, it will accelerate like an induction motor. While the machine is accelerating, high currents induced into the rotor can cause significant damage in a matter of seconds. This article explores an actual event, how to best apply this protection and what pitfalls to avoid.
50/27 Inadvertent Energization Scheme Logic
FIGURE 1. 50/27 Scheme Logic
Phase overcurrent armed by phase undervoltage provides this protection. All three phase voltages must drop below the 27 pickup for the undervoltage element to assert. The undervoltage element then arms the overcurrent element to trip following an adjustable time delay on pickup while the generator is off-line. The overcurrent element is disarmed again when the undervoltage element drops out following an adjustable time delay on dropout when the machine is put back in-service.
Setting Considerations
The following points show how to best optimize this protection. Several hard lessons learned during the catastrophic 2003 blackout are incorporated.
27 Pickup
The undervoltage element pickup should be set to 40 – 50 percent of the nominal voltage. Several units tripped during 2003 blackout due a high pickup setting (for example, 80 - 90 percent) for the undervoltage element.
50 Pickup
The overcurrent element pickup should be set simulating one or all three poles of the generator breaker flashes over while the machine is offline. Figure 2 is the circuit for all three poles flashed. The measured current can be as high as 3 per unit nominal current or even higher. If the overcurrent element pickup is set at 125 percent of full load then the scheme should not misoperate during normal operating conditions, which eliminates the need for any blocking elements.
The overcurrent element should operate on the current measured from the generator protection system side CTs, as current is fed from the grid during these events. This makes the scheme more secure as well.
No coordination is required with other protection since this function is only operational when the generator is offline.
FIGURE 2. Short Circuit Diagram for All Three Poles Flashed Over
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Blocking Elements
Some users enable 60 blown fuse (60FL) or loss of potential (LOP) logic to supervise this protection, however it is not necessary since setting the 50 pickup above load prevents operation during normal conditions. Reliability outweighs security for this protection since this type of event can quickly destroy a generator.
Time Delay on Pickup
It is important to consider that the voltage magnitude decreases as the current magnitude increases during a power swing. Therefore, if there is a swing in-progress that has a relatively slow oscillation then it is possible an unwanted trip could occur if the time delay on pickup delay is set too short. Figure 3A and 3B show the relay measurements during a typical power swing. A typical selected value is 5 seconds (i.e., 300 cycles).
Time Delay on Dropout
The dropout time delay is set to 7 seconds (i.e., 420 cycles).
FIGURE 3A. Impedance Loci During Power Swing
FIGURE 3B. Generator Protection Voltage and Current Measurements During Power Swing
50/27 Inadvertent Energization Misoperation
Review of a relay misoperation can shed significant insight into the proper application of this protection. Figure 4 shows the voltage and current measured by the backup relay during an event which both the primary and backup relay tripped on 50/27. It is important to note that the two relays have independent VTs. The trip occurred shortly after the generator breaker closes and the measured current just exceeds the current pickup. Figure 5A shows the original protection settings while Figure 5B shows the settings recommended by the vendor.
FIGURE 4. 50/27 Inadvertent Energization Misoperation
FIGURE 5A. Original 50/27 Inadvertent Energization Settings
FIGURE 5B. Vendor Recommended 50/27 Inadvertent Energization Settings
A misoperation occurred since both relays were measuring balanced nominal three-phase voltage at the time of the trip. It is possible that the voltage circuits dipped for at least 5 seconds to arm the logic and the generator breaker closed before the dropout timer had expired, however this is quite unlikely, as each relay has its own separate VTs, and this has never happened before. Both voltage circuits were thoroughly examined and nothing abnormal was found.
The original settings do not follow the guidelines presented here how to set the 50 pickup, which would have prevented these trips, however it does follow the rest. The vendor recommended settings do follow the guidelines for either timer, their intent was to ensure another misoperation did not occur, however this is not a good practice as if the problem resides with relay this needs to be known so that it can be corrected. The best practice is to keep the original settings and increase the length of the oscillographic recorder so that if there is a voltage dip it would be captured.
CONCLUSION
The article demonstrates how to best set 50/27 inadvertent energization protection for generators. Prevailing system conditions, such as a power swing, must be considered to optimize these settings. An actual misoperation is presented to demonstrate the best practice.
FIGURE 6. Inadvertent Energization due to Breaker Flashover
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