ML20205T162

Safety Evaluation Supporting Improvement Plan for Emergency Diesel Generators Transient Voltage Response

ML20205T162
Person / Time
Site:	Sequoyah
Issue date:	11/07/1988
From:	NRC OFFICE OF SPECIAL PROJECTS
To:
Shared Package
ML20205T158	List: ML20205T162 NUREG-1232, Forwards Safety Evaluation Accepting Improvement Plan for Emergency Diesel Generators Transient Voltage Response.Mods Remain Scheduled by TVA for Completions Prior to Restart of Unit 1,Cycle 4 Refueling Outage
References
NUDOCS 8811140157
Download: ML20205T162 (2)

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! SAFETY EVALUATION BY THE OFFICE OF SPECIAL PROJECTS IMPROVEMENT PLAN FOR THE EMERGENCY DIESEL GENERATORS TRANSIENT VOLTAGE RESPONSE TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR POWER PLANT UNITS 1 AND 2

1.0 INTRODUCTION

The Tennessee Valley Authority (TVA) has conducted tests of the emergency h die el generators (EDGs) at the Sequoyah Nuclear Power Plant. These tests are discussed in Section 2.3.3.2.1(4), Diesel Generator Load Analysis, of NUREG-1232, Volume 2, which was issued by letter dated May 18, 1988. These tests were for simulated loss of offsite power and accident conditions. During the tests, pre-programed loads were sequenced started. This starting was by closing their respective load breakers onto the standby bus which was energized by the EDG. The staff observed from the standby bus transient voltage profile that there appeared to be voltage instability. Although the voltage was acceptable, the profile could not be used to bound the original calculations.

In a March 3, 1988 submittal, TVA committed to evaluate the performance of the EDGs to improve the voltage transient profile. This improvement would reduce both under and over voltage conditions. TVA has completed their review of methods to improve the EDGs transient voltage response. This plan was submitted to the staff by letter dated July 8,1988. The staff's evaluation of this plan is given below.

2.0 EVALUATION TVA has selected the following modifications for improving the EDGs voltage transient response:

o Reset the exciter current transfurmer taps to optimize the load current compounding of the existing exciter.

o Replace the existing electro pneumatic-load sequence timers with electronic timers, o Install a voltage overshoot device on the existing exciter.

The staff had noticed that both the steady state and transient load current effects were less thfn the voltose input to the voltage regulator. Further, the effects of the running notors, acting as induction generators, to reduce the voltage cip was reduced by the voltage regulator responding to voltage error. TVA proposes to reset the exciter current transformer taps. This reset g!!40137 e a ncn ovggis%

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4 of the taps would increase the current transformer contribution to the field current to achieve flat compounding. This transformer tap setting should improve the inherent voltage regulation of the excitation system, thereby reducing the amount of the voltage regulator action, This should increase the overall performance of the excitation system and reduce over-voltages.

The staff has noticed from the tost curves that the accuracy of the load sequence electro-pneumatic timers was not : 5% as established bj the vendor.

Depending upon the setpoint, the accuracy ranged from -10% to +15%. This caused load breakers to close when the EDG voltage was less than 100%. The new timers will enhance the EDGs voltage respor.se by increasing the starting time band-width for the loads to allow more tira lor voltage recovery in the load sequence time intervals. This would have the load breakers close at the correct time.

If the above two modifications do not reduce the over voltage conditions TVA proposes a further modification. This modification wculd be to install silicon controlled rectifiers (SCRs) across each phase of the exciter's bridge rectifiers. The SCRs would be triggered by an electronic circuit when it senses an over voltage condition beyond a preset value. This modification would remove the exciter field forcing voltage during excessive transient voltage overshoot conditions and thus reduce the amount of transient soltage overshoot.

TVA believes that although a qualitative evaluation of the proposed modif f-cation indicates that voltage response would be improved, they propose to  ;

perform a post rrodification test (PMT) to verify these improvements. One EDG will be tradified by changing the exciter current transformer taps and the load sequence timers. These rrodifications would then be followeo by a PMT. If the over voltage is not corrected, thea the SCRs will be installed and followed by another PMT. The remaining EDGs would then be modified by the end of the Unit 1

. Cycle 4 refueling outage.

3.0 CONCLUSION

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The staff concludes that the plan submitted by TVA should improve the EDGs ,

transient voltage response. The schedule to complete the modifications by the  ;

i end of the Unit 1 Cycle 4 refueling outage, which is scheduled for about the end of the first quarter of 1990, is acceptable. The staff will monitor these ,

modifications.

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! Principal Contributor: F. Paulitz Dated: November 7. 1988 I

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