Generator Differential Trip Analysis

Steve Turner, Arizona Public Service Company Columns, Fall 2022 Columns, Relay Column

A large steam turbine generator tripped on 87 phase differential protection while attempting to synch the machine to the grid. However, only the main generator protection relay operated — not the backup protection. The goal of the analysis is to determine why only one relay operated and what caused the trip to occur.

Figure 1: Power System Configuration


Figure 1 shows the system configuration to sync the generator to the grid. The generator is brought online to full speed; ideally, the generator breaker is then closed when the generator is in sync with the grid. In this case, the main generator protection tripped on 87 phase differential when the generator breaker closed, but the backup protection did not operate.

Table 1 shows the sequence of the event report (SER) captured by the main relay.

Table 1: Event Report

Review of the SER reveals that the total time of the event following the generator breaker closing was approximately 4 cycles, which corresponds to 1 cycle for the main protection to assert the trip contact output and 3 cycles for the generator breaker to open. Review of the SER also reveals that the 87 phase differential protection repeatedly picked up and dropped out over the course of the event. Note that the trip output contact asserts the first time the protection operated since there is no intentional time delay.

Figure 2 shows the oscillography captured by a digital fault recorder (DFR) for the event. The currents shown (Ia gen, Ib gen and Ic gen) are measured on the neutral side of the generator stator winding, which is the current flowing through the generator. Note that these signals are unfiltered and reveal the large DC offset present in these currents.

Figure 2: Generator Currents

At first glance, the event appears to be an A-phase-to-C-phase fault; however, the generator terminal voltages are balanced and nominal magnitude. Note that the generator currents are fully offset during the entire event.

Figure 3 shows the filtered currents flowing through the generator, i.e., the 60-Hz fundamental component for each current waveform. Review of these waveforms reveals that the current magnitude is close to nominal, and the currents are all balanced approximately 120 degrees apart. Thus, no phase fault was present during the event.

Figure 3: Filtered Generator Currents

Figure 4 shows the corresponding 87 phase differential operating characteristics for both the main generator (M-3425A) and backup (SEL-700G) protection. The operating point for the main protection is well within the zone of operation, while the backup protection is outside.

Figure 4: 87 Phase Differential Operating Characteristics, Main and Backup


The root cause of the unwanted trip was a bad sync: The electrical angle across the generator breaker was close to 60 degrees at the time of the closing because of improper timing. The worst case electrically is 180 degrees, while 90 degrees is the worst case mechanically. Improper synchronization can affect the health of the power system and results in electrical and mechanical transients that can damage the prime mover, generator, transformers, and other power system components.

The bad sync was the source of a large DC offset present in the generator currents. It is suspected that the internal relay CTs saturated as a result, which accounts for why the main 87 phase differential protection picked up and dropped out four times during the event.


A large steam turbine generator tripped on 87 phase differential protection while attempting to synch the machine to the grid. However, only the main generator protection relay operated — not the backup protection. The root cause of the unwanted trip was due to a bad sync. Review of the 87 phase differential characteristics illustrates that the main protection is much more sensitive than the backup protection with respect to the operating point for this event. There is no need to change relay settings, and the trip alerted the utility that there was a problem with the sync.

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.