1

Four LLs were published in February Transmission Relaying – Undesired Blocking 2.Lack of Separation for Critical Control Power Supply Leads to Loss of Multiple Units at a Power Station LLs can be found on the NERC website under “Events Analysis – Lessons Learned”Events Analysis – Lessons Learned A survey is provided for each LL 2

1.Was the Lesson Learned understandable and easy to read? 2.Did this Lesson Learned contain enough technical detail? 3.Did the Lesson Learned result in any actions or changes for your company? 4.How will you use this Lesson Learned in your organization? 5.What additional information or materials would make this Lesson Learned more useful? 6.If NERC publishes another Lesson Learned containing a survey like this, would you be willing to take the survey again? 3

Permanent P-G Fault occurred on Line A, (3-terminal line) All three ends cleared high speed on the initial Fault When Terminal 1 (T-1) reclosed, it cleared in zone two times DFR at T-1 confirmed the high-speed tripping for the initial Fault and delayed tripping for the subsequent Fault DFR indicated trip from T-1 may have been delayed b/c it was receiving a block 4

Causal Analysis: Relays tested - Pilot scheme and carrier transceiver – All tested good with correct relay settings Functional tests done on all three ends of the line and between all combinations of the line terminals – No problems identified Cause: Determined by process of elimination and modeling the event – The misop. resulted from the following : (1) Design of the older electromechanical relay scheme, and (2) Tapped autotransformer(xfmr) at one of the line terminals 5

– No high-side breaker at T-3, so low-side CB of the tapped auto(xfmr) tripped for line Faults – This left the primary of the auto(xfmr) connected to Line A – Pilot scheme interlocks tripping/carrier stop with the CB position – When T-1 reclosed into the faulted line, the ground source contribution from the auto(xfmr) was sufficient to start carrier blocking by the non-directional ground element at T-3 – Since low-side auto(xfmr) breaker was open at T-3, tripping/carrier stop relays (at T-3) could not operate. – This resulted in carrier signal not being stopped, which blocked the remote end from tripping high speed. 6

7 Corrective Actions: The line protection scheme was modified at Terminal 3 to stop carrier when the low-side bank breaker is open Lesson Learned: Important to review conditions that existed during an event after a misop. of a communication-assisted protection scheme If a non-directional start is used in a DCB scheme, the directional unit that stops the blocking signal should not be compromised during a sequence of normal operations If the directional unit cannot perform during a Fault, then modifications to the scheme design should be considered

8 A short circuit in a critical ac control power panel resulted in the trip of a one generator at a large power plant Lack of separation of critical ac power supply to key control sys. and plant instrument air sys. led to loss of 2 add. units at the same plant, led to large frequency dist. Plant has 3 large ST-gen units (near max at time of trip) 7 SBAC (SBACs E and H offline due to maintenance) Plants air is solely dependant on SBACs supplied through common header for – Soot blowing air (280 psi) – Service air (120 psi) – Instrument air (120 psi)

9 Short circuit in critical ac control panel for Unit A, led to control voltage drop on the Furnace Supervisory Safeguard System (FSSS) PLCs and SBACs A, B, and C Both primary and backup FSSS PLCs lost (due to voltage drop) and led Unit A tripping on normal reverse power operation – Investigation indicated that the short circuit originated from the FSSS racks 2 and 3 electrical circuits. The voltage drop caused the PLCs’ power supplies to sense low-control power voltage of less than 97 V for at least 13.6 ms – PLCs then went to a “safe” condition and shut down the systems they controlled

10 Loss of (3) SBACs (~60% plant air) led to degradation of plant service air supply and instrument air pressure for units B and C – All air pressures dropped sharply, showing that air usage at the time was too great for the remaining (2) SBACs – Plant personnel began manually isolating air to non-essential areas, but air pressure continued to decay. – SBAC G tripped (due to high air temp.) approx. 9 minutes after A, B, and C, leaving D running alone – Units B and C tripped when instrument air pressure dropped too low to keep the discharge dampers open on the Primary Air (PA) fans – Low PA pressure caused FSSS to trip all the pulverizers, resulting in unit trips due to loss of all boiler fuel

11 Critical ac control panel: Supply control power to the primary and backup FSSS from two separate critical ac power panels Engineering analysis was completed and correct fuses were installed in the critical ac panels – Fusing in critical ac power panel was not correct size Engineering review completed, and out-of-service and non-critical circuits were removed from the critical ac power panel – Out-of-service circuits & non-critical circuits were previously connected to the critical ac power panel Data loggers installed to aid in future identification of short circuit

12 Plant air supply System: Power supply was re-configured to supply control power to no more than 2 SBACs SBACs from the same panel – 3 SBACs prev. supplied by common critical ac control panel Identification of Non-critical plant air usage: Appropriate methods and isolation points were identified to isolate and remove noncritical air usage during an emergency – After evaluating the usage on all the monitored branches, if the non-essential air had been shut off promptly, Units B and C could have maintained sufficient instrument air pressure to continue operating.

13 Critical ac control panel: Control power for critical primary and backup systems, such as the FSSS and plant air supply, should be supplied from separate critical ac power panels where appropriate Critical ac power panels should be maintained with proper fusing and only necessary circuits should be connected – This will reduce the probability and impact of an equipment failure such as a short circuit Identification of Non-critical plant air usage: Technical and procedural controls should be applied to air supply systems to reduce non-critical air usage when air supply is insufficient to meet critical functions

QUESTIONS? 14 Contact :