Optimizing 50/27 Inadvertent Energization Protection

Steve Turner, Arizona Public Service CompanyColumns, Relay Column, Summer 2024 Columns

The 50/27 inadvertent energization protection function (Figure 1) is an overcurrent function supervised by generator terminal bus voltage derived from system-side voltage transformers (VTs). Inadvertent or accidental energizing of off-line generators has occurred frequently enough to warrant using 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.

Figure 1: 50/27 Scheme Logic

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 pitfalls to avoid.


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.


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% of the nominal voltage. Several units tripped during the 2003 blackout due to a high pickup setting (for example, 80–90%) for the undervoltage element.

50 Pickup

The overcurrent element pickup should be set to simulate one or all three poles of the generator breaker flashovers while the machine is offline. Figure 2 shows that the circuit for all three poles flashed. The measured current can be as high as three per unit nominal current or even higher. If the overcurrent element pickup is set at 125% of full load, then the scheme should not misoperate during normal operating conditions. This eliminates the need for any blocking elements.

Figure 2: Short-Circuit Diagram for All Three Flashed-Over Poles

The overcurrent element should operate on the current measured from the generator protection system-side current transformers (CTs) as current is fed from the grid during these events. This makes the scheme more secure as well.

No coordination with other protection is required since this function is only operational when the generator is offline.

Blocking Elements

Some users enable 60 blown fuse (60 FL) or loss of potential (LOP) logic to supervise this protection; however, this 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 a swing with relatively slow oscillation is in progress, it is possible for an unwanted trip to occur if the time delay on pickup delay is set too short. Figure 3a and Figure 3b show the relay measurements during a typical power swing. A typical selected value is 5 seconds (i.e., 300 cycles).

Figure 3a: Impedance Loci during Power Swing
Figure 3b: Generator Protection Voltage and Current Measurements during Power Swing
Time Delay on Dropout

The dropout time delay is set to 7 seconds (i.e., 420 cycles).


Reviewing 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 in which the primary and the 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 closed, and the measured current just exceeded the current pickup. 

Figure 4: 50/27 Inadvertent Energization Misoperation

Figure 5a shows the original protection settings; Figure 5b shows the settings recommended by the vendor.

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 on how to set the 50 pickup, which would have prevented these trips; however, the settings do follow the rest of the guidelines. The vendor-recommended settings do not follow the guidelines for either timer; their intent was to ensure another misoperation did not occur. I do not agree with the timer settings recommended by the vendor because they do not properly account for power swings to the magnitude of what was experienced during the last northeast blackout. This is not good practice because if the problem resides with the 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 a voltage dip can be captured.


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). 

Figure 6: Inadvertent Energization Due to Breaker Flashover

Steve Turner is in charge of system protection for the Fossil Generation Department at Arizona Public Service Company in Phoenix. Steve worked as a consultant for two years, and held positions at Beckwith Electric Company, GEC Alstom, SEL, and Duke Energy, where he developed the first patent for double-ended fault location on overhead high-voltage transmission lines and was in charge of maintenance standards in the transmission department for protective relaying. Steve has BSEE and MSEE degrees from Virginia Tech University. Steve is an IEEE Senior Member and a member of the IEEE PSRC, and has presented at numerous conferences.