ML20248F610

Interim Significant Deficiency Rept L2-88-11 Re Inadequate Degraded Grid Undervoltage Relay Setpoints.Event Discussed W/Engineer Involved to Ensure Procedural Adherence & New Undervoltage Relays Will Be Installed

ML20248F610
Person / Time
Site:	Limerick
Issue date:	04/13/1989
From:	Kowalski S PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	Russell W NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
L2-88-11, NUDOCS 8904130126
Download: ML20248F610 (5)

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e PHILADELPHIA ELECTRIC COMPANY 2301 MARKET STREET P.O. BOX 8699 PHILADELPHIA. PA.19101 (215) 841 4502 S. J. KOWALSKI vics-PassIDENT 10CFR50.55Ce) uu............

Mr. W. T. Russell, Ackninistrator U. S. Nuclear Regulatory Conmission Docket No. 50-353 Attn: Document Control Clerk CPPR-107 Mall Station P1-137 Washington, DC 20555 l

Subject:

Limerick Generating Station - Unit 2 Significant Deficiency Report - SDR No. L2-88-11 Inadequate Degraded Grid Undervoltage Relay Setpoints

Reference:

Interim Report for SDR No. L2-88-11, dated February 7, 1989 Flie:

QUAL 2-10-2 (SDR No. L2-88-11)

Dear Mr. Russell:

As conmitted to in the referenced Interim Report, we are submit-ting the attached Significant Deficiency Report SDR No. L2-88-11 concerning inadequate degraded grid undervoltage relay setpoints.

This condition has been determined to be reportable per 10CFR Part 50.55Ce).

The cause of the condition was a cognitive personnel error by a utility employed engineer who failed to properly follow Implementing procedures. To prevent recurrence, the details of the event have been discussed with the engineer involved who now recognizes the need for strict procedural adherence.

In addition, new undervoltage relays will be installed and transformer tap settings changed to provide compliance with the design basis. This work will be completed prior to Unit 2 Fuel Load.

m mk In conclusion, we consider SDR No. L2-88-11 closed with the 00 50 -

Issuance of this report.

If you have any further questions, please go contact us.

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"4M Copy to:

W. T. Russell, USNRC, Region 1 Administrator R. A. Granm, USNRC, LGS 2 Senior Resident Inspector R. J. Clark, USNRC, LGS 2 Project Manager g

CBT/ds/03298904

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Mr. W. T. Russa11, Administrator J

Dockst No. 50-353, CPPR-107 j

Pagd Two j

bec:

C.'A. McNolli (S26-1) 3 S.

, Kowalski (S25-1) l E. J. Bradley (S23-1)

L. B. Pyrih (N2-1)

E. P. Fogarty (S7-1)

D. R. Helwig (S7-1)

G.-A. Hunger, Jr. (S7-1) j A. J. Marie (N2-1)

H. D. Honan (N2-1)

D. P. Helker (57-1)

G. M. Leitch (LGS)

d. S. Kemper (S25-1)

P. J. Duca, Jr. (LGS)

C. R. Endriss (LGS)

d. M. Corcoran (LGS)

R. M. Krich (N7-1)

P. S. Thurman (N2-1)

R. J. Lees (N2-1)

D. B. Fetters (N4-1)

D. N. Sundt (N2-1)

W. d. Boyer (N2-1)

C. B. Tuttle (N2-1)

W. J. M!ndick N2-1)

DAC (NG-8)

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NUCLEAR ENGINEERING ENGINEERING DIVISION N2-1, 2301 Market Street Significant Deficiency Report SDR No. LD-88-11 Inadequate Degraded Grid Undervoltage Relay Setpoints NRC Construction Pennit No. CPPR-107 Description of Deficiency:

The Linerick Final Safety Analysis Report (FSAR) Section 8.1.6.3.6, "BTP-PSB-1, Adequacy of Station Electric Distribution Systen Voltages,"

cannits to compliance with NRC Branch Technical Position PSB-1,

" Adequacy of Station Electric Distribution System Voltages." Compliance with the guidance provided in Branch Technical Position (BTP)-PSB-1 required a protection schene which would maintain adequate voltage levels to Class 1E equipment at all times.

BTP-PSB-1 recognized the inadequacy of existing undervoltage protection schenes which typically provided only a single level (i.e., a loss of power relay) of undervoltage protection. BTP-PSB-1 states in part that a second level of undervoltage protection with a time delay is needed to protect the Class 1E equipme,t under degraded power supply conditions which would not actuate a loss of power relay typically set at 70% of nominal.

However, as a result of the Unit 2 Independent Design and Construction Assessment (IDCA) being perfonned for Unit 2 and a parallel review of Generic Letter 88-15, " Electric Power Systens - Inadequate Control Over Design Processes," the undervoltage relay setpoints on the Unit 14.16kV safeguard buses were found to be too low to ensure adequate voltage to the 480V Class 1E loads for all operating conditions.

With a degraded grid condition, adequate safeguard bus voltages nay not be assured with the existing protective relay setpoints. This could occur if an equipnent fallure (e.g., failure of the load tap changer)

(LTC) resulted in degradation of the safeguard bus voltage to between 90% and 95.7% of naninal (4160V).

In this range, safeguard bus voltage is too low to ensure proper operation of the 480V Class 1E loads but too high to cause relay trip and subsequent transfer to the alternate power source. The refore, if the safeguard bus voltage degraded to th; range between 90% and 95.7% of nominal, proper operation of the Class 1E loads is not assured. This is contrary to the position in Section 8.1.6.3.6 of the FSAR which states that the plant undervoltage protection design is in conpliance with BTP-PSB-1.

The relay setpoints were calculated to result in compliance with the guidance of BTP-PSB-1.

To ensure adequate voltage levels are

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maintained to the 480V Class 1E loads, a relay setting of 92% of nominal voltage, with a tap changer setting of 5% boost (i.e., secondary voltage i

of 504V vs 480V) on the 4160/480V transformer was calculated to be necessary. The 5% boost tap setting increases the secondary voltage (480V side) by 5% (i.e., 24V). The most accurate undervoltage relay l

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Significant Deficiency Report SDR No. L2-88-11 Page 2 of 3 available at the tine that the calculation was perforned was a Brown Boveri nodel ITE-27D which provides a 12% setpoint tolerance with a 3%

pickup to dropout ratio. The dropout ratio is a relay characteristic which specifies the difference between the trip setpoint and the value recuired for the relay to reset.

Therefore, for a 92% setting this would translate to a nexintm relay actuation at 94% and a neximum reset at 97%. These actuation and reset voltage levels were considered too high since they may have created the possibility for spurious trips, which would be contrary to guidance of BTP-PSB-1.

In addition, the high reset value (i.e., 97%) could prevent relay reset during recovery from an electrical transient and thus cause an unnecessary trip of the supplying offsite power source and transfer to the alternate power source. Considering these facts, a 90% relay setting was chosen by Philadelphia Electric Canpany.

During plant construction, the 4160/480V transformer taps were set at 5% boost, in concert with the previous voltage studies. With these tap settings, there were Instances of equipnent danege due to overvoltage in the power supply to the 480V loads. Therefore, the the 4160/480V transformer taps were reset to nominal boost (0%). This resnited in the primary side voltage potential non-operability range widering from 90% to 92% of nominal, to 90% to 95.7% of nominal. The relay vould have permitted secondary side voltage to drop below the minimun voltage necessary to ensure proper operation of the 480V Class 1E loads. However, further analysis to ensure compliance with the FSAR was mistakenly determined not to be necessary due to the assumption that the undervoltage relay setpoints were based solely on primary side voltage levels and did not consider tap setting effects on the secondary side voltage levels.

Safety Implications:

Degraded voltages on the 4kV safeguard busses in the range between 90% and 95.7% could result in inoperability of 480V, Class 1E loads without the relays operating to isolate the Class 1E power systen from the degraded source.

Corrective Actions:

I The undervoltage relays in question are being replaced with Brown Boveri nodel ITE 27-N relays with a tolerance of 10.2% with a pickup to drop out ratio of 0.5%.

New voltage regulation calculations have been perforned to provide the exact setting of the relay. The tight setpoint tolerance and pickup to dropout ratio of the new relay provides the performance necessary to prevent spurious tripping of the supplying offsite source and also allows re'ay reset at a value only 0.5% above the trip setpoint.

Therefore, an unnecessary transfer of the offsite poaer supply to the alternate source is renote.

In concert with the relay change, the 4160/480V transformer taps will be set to 2-1/2%

boost.

These changes provide compliance with the design basis detailed in FSAR Section 8.1.6.3.6, which cannitted to the guidance of BTP-PSB-1.

These changes provide voltage levels and protection of equipnent in compliance with BTP-PSB-1.

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Significant Deficiency Report

.SDR No. L2-88-11 Page 3 of 3-Actions Taken to Prevent Recurrence:

This event was discussed in detail with the engineer involved, who now recognizes the need for strict procedural adherence.

A review of the Quality Assurance (QA) plan covering construction of Limerick Generating Station and the associated design control procedures in place, showed these procedures to be adequate to preclude recurrence of this event.

CBT/ds/03308901 l

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