ML20246J126
| ML20246J126 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 07/20/1989 |
| From: | Phillips M VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | |
| Shared Package | |
| ML20246J098 | List: |
| References | |
| EE-0021, EE-0021-R00, EE-21, EE-21-R, NUDOCS 8909050042 | |
| Download: ML20246J126 (7) | |
Text
{{#Wiki_filter:- , _ - . r 3 -.- h TECHNICAL REPORT NO. EE-0021 REVISION 0 COORDINATION OF LOAD CENTER CIRCUIT BREAKERS
. E V-l, NORTH ANNA POWER STATION ELECTRICAL ENGINEERING POWER ENGINEERING SERVICES VIRGINIA POWER ,
JULY 20, 1989 Prepared By: /
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QA CATEGORY: SR KEY WORDS: COORDINATION, LOAD CENTER, BREAKERS 8909050042 890825 f' PDR' ADOCK 0500033g O PNU 1 74-TAM-0022-1
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I. INTRODUCTION The North. Anni 480V Emergency Buses are powered from 4160V-480V, -l 1000/1333 KVA, dry-type transformers. The associated 4160V, sources on each Emergency Bus provides primary power to two of these transformers. See Attachment 1 for a one-line diagram of the configuration. The 4160V breakers that power the load centers utilize protective relays for overcurrent protection, which consist of General Electric type IJCV voltage-restraint relays for phase protection, and GE PJC: relays lfor ground protection. (The ground relaying associated with this . system is not an issue,'due to the 4160V-480V transformer Dlta-delta connection. Therefore, only the IJCV phase relays will be discussed in the remainder of this report.) The 480V load center buses consist of a load center main breaker, and several feeder breakers. These breakers are equipped with various 4 sizes of ITE OD-type overcurrent trip units, the size and type depending upon the load being served. The OD units of the main and feeder breakers are set such that electrical coordination exists between them. For example, an electrical fault on the cable leading to a Containment Air Recirc. Fan would trip the fan's associated breaker before-the bus main breaker would trip, for all possible faults. (The bus main breaker 'would trip or provide back-up' protection should the fan's breaker fail to trip.) By providing such selective tripping, a fault on a component has a minimal impact on' the remainder of the electrical-system. The Appendix "R" review for North Anna required .a thorough coordination study of all Appendix "R" equipment and electrical power supplies.to ensure that a fire-induced fault on one circuit in another fire zone would not result in the loss of an entire associated bus. At that time, it was noticed that electrical coordination does not exist in all cases between the 4160V load center feeder breaker's IJCV relays and the 480V load center main breaker's OD '4 trip unit. As such, the 4160V breaker could trip before the 480V bus main breaker for certain faults, causing loss of an entire train (2 transformers) of 480V equipment. See Attachment 2. This was acceptable for the purposes of Appendix "R", since it was concluded that only a fire-induced bus fault could cause this problem. Since both 480V emergency load center buses for a particular unit are in the same fire area, the fire is postulated to disable both buses regardless of the miscoordination. During the recent (February, 1989) Safety System Outage Modification Inspection (SSOMI) conducted by the NRC on North Anna, an inspector identified this problem again. The resulting discussion confimed it is not a violation of Appendix "R" nor a significant safety concern. The NRC expressed concern that it was indicative of poor design l i practice. 1 TAM-0022-2 o _ _
,_ n _ _ _ = _ _ i z.. a p y t' ; V ' As e discu'ssed above, the miscoordination exists only for bus faults in a range of magnitude less than bolted faults. Bus faults are very rare in metal clad switchgear, such as the 480V load centers. The 3 main horizontal bus of the load center is exposed in the back of. the- l equipment. However, 'all work involved inside the load center is j t performed under approved procedures. (It is, of course, to
. work. in this area with the load center de-energized.) preferred The greatest l potential for bus faults occurs when breakers are being racked in or #
out of.. their cubfele, or when the breaker is being opened or closed. Because miscoordinat',on exists for only a " band" of fault current values, and since the initiating event is very rare (bus fault), Engineering considered the condition " safe " though undesirable. The
~NRC inspectors agreed with this assessment.
x This report will provide a discussion of the condition. Revision 0 of this Technical Report provides a status of findings to date. The final resolution of this issue requires additional engineering. This Technical Report will be revised when an engineering solution is firalized. II.- METHODOLOGY The IJCV . relay is a voltage-restraint over. current relay. This relay has two coils'- a current coil and a voltage coil. The time-current-characteristic of the relay depends upon the potential applied to the voltage coil. (For this application, the 4160V Emergency Bus potential is applied to the coil.) Such a relay allows fast tripping when a " weak" or limited source of current exists, such as a diesel generator. The effective relay tap (trip) value of the relay at 0% restraint' voltage is 25% of the tap value at 100% restraint' voltage. This can be seen on Attachment 2. Thereforei depending upon bus voltage during the fault, the relay's characteristics will be somewhere between the two curves shown. The bus voltage during a fault will vary depending upon the source of power to the bus - the Reserve Station Service System (RSS), or the Emergency Diesel Generator (EDG). Tho voltage collapse would be greater if the system is being powered from the EDGs than from the RSE system, which would effectively move the curve closer to the 0% voltage restraint curve. This would cause the miscoordination to apparently be greater when the system is powered froir,the EDG. The 4160V breakers' relays should be set to accomplish each of the following criteria, per References 0 and E. In this
- a. Allow 200% I of the transformer (s). instance it includestwoO333KVAtransformers;
- b. Provide protection against " burn up" for each transformer, detemined from its ANSI thermal-limit capability; and
- c. Coordinate with upstream and downstream devices.
t 74-TAM-0022-3 1
i I In discussions with the Virginia Power System Protection Department, who assists Engineering in protective relay selection and settings, it was noted that the IJCV is not a suitable relay for treasformer t protection since it does not generally allow the above criteria to be met. Their recommended relay for transformer protection is the GE - IAC-53. Its characteristics closely resemble the thermal withstand characteristics associated with dry-type transformers. The existing IJCV relays cannot be set (tapped) at a higher value to allow coordination. The characteristics of these relays are too inverse to allow both coordination and burn-up protection. (As presently set, the IJCV relays provide burn-up protection.) .The miscoordination is for a very limited range of current when the buses are powered from the RSS.- 4160V bus voltage is expected to remain above 85% for all 480V faults. This reduction in voltage-restraint would not drastically shift the relay'e :urve to the left. Miscoordination would not occur for the bol'ed three-phase or phase-to-phase faults which would most likely occur due to bus or breaker faults, since this current would be adequate to allow tripping of the load center breaker. . In order to properly protect and coordinate the Emergency Bus relaying, it might be necessary to change the settings on the load center main breakers (long-time delay time adjustment); feeder breakers of certain loads on the buses (e.g., Containment Air Recire. Fans); and the Emergency Diesel Generator IJCV relays. The entire design necessary to provide a properly protected and coordinated system will be documented in a later revision of this Technical Report after completion of the design basis reconstitution for this relaying. To achieve transformer protection without limiting the load, and better coordination (by verification and/or setting changes, as required) with all associated source (RSS and EDG) and load (load center main and feeder) breakers, the IJCV relays should be replaced with another type, such as the IAC-53. Investigation into the design basis has not produced the reasoning for the existing relay types. This matter will be pursued further prior to final resolution. III. CONCLUSIONS The following conclusions can be drawn from this analysis: l A. Coordination does not exist between the 4160V load center feeder ! t breakers and the load center main breakers for all potential I electrical faults. The safety impact on tha Emergency Electrical Distribution System is minimal becrase of the very high improbability of bus faults. Cocydination exists for all but a small area of available current when the system is pcwered from either the EDG or the RSS system. Single f ailure criteria is not violated, since the opposite train would be unaffected. L
i B. Preliminarily, it appears that better coordination could be maintained on the system if the existing GE IJCV phase relays were replaced with GE IAC 53 relays. In order to achieve total coordination, the load center main breakers' settings may have to - change, which might cause other load center feeder breaker settings to diange. j Engineering will complete further review of the relays, protective devices and fault current associated with the breakers discussed in this report. This involves a detailed study into the settings and their bases. Actual changes to the relaying could be performed under the engineering packages that will 1 implement the Station Blackout modifications, j IV. REFERENCES A. North Anna Appendix "R" Report, Chapter 9 " Electrical Distribution System Coordination Study-Appendix R Reanalysis"
- 8. IEEE Standard 141-1986, "IEEE Recommended Practice for Electric Power Distribution for Industrial Plants."
C. IEEE Standard 242-1986, "IEEE Recommended Practice for Protection and Coordination of I41ustrial and Commercial Power Systems." ' D. Virginia Power Circuit Calculations Methods Manual. E. EPRI Power Plant Electrical Reference Series, Volume 8, " Station Protection." F. IEEE 308-1980, " Standard Criteria fer Class 1E Power Systems for Nuclear Power Generating Stations"
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