SEL 351-7 fault event report discrepancy? B to ground fault trip event "incorrectly" displays IB=1A and IA= 13248A. Yet, the relay worked as designed

SEL 351-7 fault event report discrepancy? B to ground fault trip event "incorrectly" displays IB=1A and IA= 13248A. Yet, the relay worked as designed

Notation and terms clarification:

IA, IB, and IC in this article signify current magnitudes, not instantaneous currents. In figure 1 current chart, there are 2 notations: IA and IA mag. IA in the chart refers to instantaneous current, and IA mag refers to the current magnitude.

The term event summary represents the upper-right data shown on the rectangle area in figure 1.?

Look at this event report file below in figure 1:

1. The event summary states BG T (B to ground fault trip); however, the currents on the event summary display IB= 1A and IA = 13248A.?
2. Since the only high fault current is on A phase (IA= 13248A), should it not be AG T instead of the present indication BG T??

Figure 1: Event relay showing currents magnitude during the fault.

?Let's discuss the background of this event and the power system.

A substation feeder is rated 34.5 kV and 25 MVA with primary protection an SEL relay 351-7 firmware R511. A fault occurred on that feeder at 13:20, and the relay generated this report in figure 1. The digital chart has been untouched; as a result of this, all the digital elements shown in figure 1 come from the trip and event recorder equations. I removed the voltage charts in figure 1 and did not discuss the feeder's equipment details because they are irrelevant to this article.

In figure 1, the magenta cursor is at the pre-fault location. Before the fault, the magenta cursor shows balanced 3 phase readings Ia=Ib=Ic= 10A, which is about 2% full load, almost no-load conditions.

The orange cursor is at a location during the fault. I randomly placed the orange cursor anywhere between the event record initiation (red dotted line) and the trip initiation. The exact location in that interval did not matter as long as I could show the data during the fault.

During the fault, at the orange cursor, the currents values are IA = 67.33A, IB = 6266.86A, IC = 145.61A.

Let's discuss all the discrepancies and issues:

Issue 1: The event summary declared "Event= BG T." However, on the same event summary, the current on B phase is IB = 1A. Moreover, the current charts show IB = 6266.86A.?

Why does a BG T not show an extremely high IB current on the event summary currents?

Why does IB on the current charts not match with IB on the event summary currents?

Issue 2: The current charts show IA = 67.33A during the fault; however, the event summary currents indicate IA = 13248A.?

Why does IA on the event summary not match with IA on the current charts during the fault?

Why does the event summary indicate BG T instead of AG T, while the only high fault current on the event summary current is IA?

Issue 3:?IEEE buff book recommends no more than 3 cycles for short circuits instantaneous delay. Why did the relay trip in 13.5 cycles?

Short circuits shall be removed by the primary protection relay as fast as possible. This feeder relay was the primary protection (drawings not shown in this article); why did the relay trip on a time overcurrent (51) instead of instantaneous (50)?

?Issue 1 and Issue 2 Explanation:?

When this event occurred, I was stunned and thought this relay was defective. I looked at 600 SEL relays over the last 3 years and never found any discrepancy with an SEL relay, so I assumed the relay was in pristine shape.

After several days staring at this event, I noticed the current charts indicate that IA jumped after 52A deasserted. The orange cursor at that location in figure 2 below displays IA= 13248A. It matches with the event summary that shows IA = 13248A.?

What could cause a high current to be present on the A-phase after 52A deasserted??

At that same timeframe, the current chart shows IB=1A; it matches with the event summary current IB= 1A.

Figure 2: Event relay showing current after 52A deasserts.

Summary??

The relay event appended BG T because chart currents indicated BG fault and IB = 6,200 A (approximate value) during trip initiation. However, immediately after 52A deasserted, something strange happened. Current on IA jumped to 13248A, much higher than the IB fault current of 6266.86A.?

The default (there is an alternative setting) setting of the event summary of the SEL 351-7 relay is to capture currents from the event report row containing the maximum phase current. Since IA strange value that occurred after 52A deasserted was at the row with the highest current values, the event summary report displayed all currents at that time (orange cursor location from figure 2).

In figure 2 current chart at the orange cursor, IA = 13248A and IB = 1.69A. Those values match with currents seen on the event summary.

?Impact on the customer

?????Although this relay operated as designed, the output on the relay screen was not coherent and misled the user.???

The event summary data are the information shown on the relay screen. Electrical workers sometimes rely on the data displayed on the screen to make an initial and quick evaluation.

The engineer/technician incorrectly documented the event as follow:

-????Date: XX-XX-XXXX

-????Event: B to ground fault

-????Currents: IB= 1A and IA =13248A - Misleading and not coherent

-????Follow-up: Investigate A phase splice, A phase bushing- Inaccurate follow-up.

The alternative setting is to capture currents from the event report row used to calculate fault location. I would not recommend using that option.

In some cases, NERC requires documentation of outage, and this documentation sent to NERC would have been inaccurate.

Unanswered questions/issues:

• why did the relay trip outside the guidance of the IEEE protection buff book? (Issue 3 explanation) relay speed was 4 times slower than the recommended speed.
• Why a high fault current was present on the A-phase after the breaker opened (52A deassertion)?
• Why not use the alternative setting for event summary to capture currents from the event reports row used to calculate fault location?

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