Description of Event:
At Beta Hydropower Plant, the “4” relay dropped out and the hydro controller went to a failsafe position of closed. This caused the wicket gates to go closed which called for an open on the 52 G relay and the unit overspeed with vibration.
On Aug 30, 2021, the following annunciation was found at 22:44 PM:
• Unit Excessive Vibration Trip High Vibe Alarm
• Unit Emergency Shutdown with Lockout
• Unit Normal Shutdown with Lockout
• Gen Air Cooler or Gen Bearing Cooling Water Low Flow alarm
• Gen Upper or Lower High Level Oil Trip
• Unit Excess Vibration Alarm
Status as of 06:45 on August 30, 2021
Annunciation:
• Unit Normal Shutdown with Lock Out (LO) (86 N rolled)
• Gen Air Cooler or Gen Bearing Low Flow Alarm
• Generator Upper or Lower bearing oil level Trip
Outstanding question: Why did the 86 E roll and trip 52G?
Actions Taken:
Method & Findings:
An investigation of the following trip devices associated with 86E relay was performed by the team.
Trip Devices associated with 86 E:
51 G – Time overcurrent (generator)
12 – Overspeed
59 – Overvoltage
59 N – Neutral Overvoltage
1 E – E-stop Control Switch
80 – Station Battery Low Voltage
87 T – Transformer Differential
48 E – Incomplete Start Sequence
94 E – Exciter Trip
194 – Transfer Trip from Neighboring Connected Utility
96 L – Excess Vibration
No Flags are set on relays
The generator output voltage (KV) appears to have been decaying before 86 rolled, but this may have been the result of the polling of the current Remote Terminal Unit (RTU). KV is represented as whole numbers and does not offer good resolution. Checked the exciter and the last alarm for an Under Excitation Limit (UEL) was from June 23rd. The appearance of decaying voltage was found to be from the way the trends are graphically represented. The voltage dropped after the breaker opened.
Green = Vibration alarm, Yellow = Generator Frequency, Red, Orange & Brown = Generator Output Voltage (kV)
The vibration alarm indicates a trip signal at 21:17:31 which would be after the 86E indication.
Added Breaker position (52G) = pink indication
The generator breaker appears to have opened before the vibration trip, and before the 86E indication.
Trip signals that will operate 52 G:
• 43 CS (manual switch) and 52 G Trip Switch
• 33 (wicket gate pos. closed) and 4 (start permissive / stop as well for N1, there is not a 5 wired) dropped out – Looks like the most likely condition
• 86E (Emergency Shutdown with LO)
• 87 G (Generator Differential)
• 87 T (Transformer Differential)
• 94 E (Exciter Trip)
• 48 E (Incomplete Start Seq. Gen. BKR closed and exciter off)
8/31/21– Through process of elimination it was determined that the “4” dropped out and the hydro controller went to a failsafe position of closed. This caused the wicket gates to go closed which called for an open on the 52 G. Once the unit began to overspeed and the excess vibration followed. The oil levels and flow rates were a byproduct of the shutdown. The oil level issue is being addressed with a different sensor during next outage.
Due to the lack of annunciation or alarms, and the alarms that came in during the trip, it was determined that the most likely problem was from a limit switch feedback on the Turbine Shutoff Valve (TSV) or a “B” seal on the auxiliary source switch. Heavy amounts of corrosion were found on the auxiliary switch handle. The contacts were cleaned and anti-corrosion compound was added to the connections.
9/15/21– After speaking with operations and the mechanical group, it was determined that the auxiliary source port would not be a likely option if given the ability to provide another energy source to close the TSV. When the plant is down again, technicians will install a hard jumper, hang an information tag, and update the drawings to reflect the change. This will be to avoid another nuisance trip in the future.
Date/Time UNIT RPM Value
8/28/21 6:00:00 PM 303.2776794
8/28/21 6:22:13 PM 303.2776794
8/28/21 8:22:13 PM 303.2776794
8/28/21 9:17:09 PM 322.321228
8/28/21 9:17:10 PM 341.3647766
8/28/21 9:17:12 PM 338.4349976
8/28/21 9:17:13 PM 336.9701233
8/28/21 9:17:14 PM 334.0403442
8/28/21 9:17:15 PM 331.1105652
8/28/21 9:17:16 PM 328.1807861
8/28/21 9:17:17 PM 325.2510071
8/28/21 9:17:18 PM 323.7861328
8/28/21 9:17:19 PM 322.321228
Supporting Materials:
Corrective Actions:
The electrical contacts were cleaned and anti-corrosion compound applied to the auxiliary switch connections.
During the next unit outage, the following tasks will be completed by plant technicians:
Sequence & Completion Dates for Actions Listed:
Lessons Learned:
A lesson learned involved personnel were using line graphs to track the event. Due to the data being event driven or polled on an integrity poll every two hours, the data gave the look as a gradual change. Above when the exported to a .CSV file the data change in real time becomes more apparent.
Recommendations:
1. Real time data monitoring of key operating parameters is critical.
2. In damp areas of the plant, electrical connections for protective devices should be inspected for corrosion during each unit outage.