Information Notice 2000-06, Offsite Power Voltage Inadequacies

Offsite Power Voltage Inadequacies

ML003695551
Person / Time
Issue date:	03/27/2000
From:	Marsh L Operational Experience and Non-Power Reactors Branch
To:
Petrone C
References
IN-00-006
Download: ML003695551 (6)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 March 27, 2000

NRC INFORMATION NOTICE 2000-06: OFFSITE POWER VOLTAGE INADEQUACIES

Addressees

All holders of operating licenses for nuclear power reactors, except those who have

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform

addressees of experience related to a possible concern regarding the voltage adequacy of

offsite power sources, that is, power from the transmission system grid to nuclear power plants.

It is expected that recipients will review the information for applicability to their facilities and

consider actions as appropriate to avoid similar problems. No specific action or written

response is required by this notice.

Description of Circumstances

On August 11, 1999, the Callaway plant experienced a rupture of a reheater drain tank line. As

a result, the plant operators initiated a manual reactor trip. Since the plant was shutdown, offsite power was required to supply the plant equipment loads. During this period, the grid

conditions were such that a substantial power flow was occurring from north to south through

the local Callaway grid. The licensee stated that the deregulated wholesale market contributed

to conditions in which higher grid power flows are likely to occur. The licensee stated that these

large flows were observed at this time. This power flow, coupled with a high local demand and

the loss of the Callaway generator, resulted in switchyard voltage at the site dropping below the

minimum requirements for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Although offsite power remained available during the

reactor trip transient, the post-trip analysis indicated that in the event that additional onsite

loads would have been in operation at the time of the event, 4-16 kV distribution voltage may

have decreased below the setpoint of the second-level undervoltage relays separating the loads

from offsite power. The NRC conducted a special inspection at Callaway from November 29 to

December 3, 1999, on the circumstances surrounding the event. The inspectors found that

similar conditions existed in 1995 that were undetected by the licensee (Licensee Event Report

(LER) 50-483/99-005 (Accession No. 9909200074); NRC Inspection Report 50-483/99-15 (Accession No. ML003684343), dated February 15, 2000).

The following events identify additional combinations of main generator unavailability, line

outages, transformer unavailability, high system demand, unavailability of other local voltage

support, and high plant load that could result in inadequate voltages. Common among all the

ML003695551

IN 2000-06 March 27, 2000 events is the inability to predict the inadequate voltages through direct readings of plant

switchyard or safety bus voltages, without also considering grid and plant conditions and their

associated analyses.

On July 11, 1989, safety systems at Virgil C. Summer Nuclear Station experienced a sustained

degraded voltage condition and, as a result, the safety buses were automatically transferred

from the offsite power system to onsite standby diesel generators. The degraded condition was

caused by a turbine trip and deficiencies in the offsite power system's transmission network

equipment. The transfer of power supplies was initiated by operation of degraded voltage

protective relays, as designed. Nonsafety system loads remained operable while being

powered for approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> from the degraded offsite power source

(LER 50-395/89-012 (Accession No. 8908140351)).

On November 5, 1991, the licensee for Arkansas Nuclear One, Units 1 and 2, reported that had

its 500-kV auto-transformer been lost during summer peak conditions, the 161-kV system might

not have been able to maintain adequate voltages to support the operation of the safety system

loads of both units (LER 50-313/91-010 (Accession No. 9111150021)).

On December 30, 1993, Northeast Nuclear Energy Company concluded that with the

switchyard at the worst case minimum voltage, Millstone Nuclear Power Station, Unit 1, loss-of- coolant accident (LOCA) mitigation loads could combine with normal loads that are not shed

upon receipt of an accident signal to produce a voltage drop that would actuate degraded

voltage relays resulting in separation from offsite power. The utility determined that this worst

case minimum switchyard voltage could occur after the loss of Millstone Unit 1 generation when

both Millstone Units 2 and 3 are off-line (LER 50-245/94-01 (Accession No. 950920001)).

On February 6, 1995, the licensee for Palo Verde Nuclear Generating Station, Units 1, 2, and 3, reported shortcomings in the plant site voltage regulation. Specifically, if a LOCA occurred with

the switchyard voltage in the lower two-thirds of its operating range, the engineered safety

feature (ESF) loads would begin sequencing onto the preferred offsite power source, and the

house loads would fast transfer to the startup transformer following the main generator or

turbine trip that would accompany the LOCA. The resulting voltage drops at the safety buses

would cause the bus degraded voltage relays to drop out during the ESF load sequencing and

subsequently resequence the loads onto the diesel generators. The licensee identified this

scenario as double sequencing (LER 50-528/93-011-01 (Accession No. 9502160195)).

On August 8, 1995, Pacific Gas & Electric Company (PG&E) reported that during peak system

loading, all transmission lines and a local fossil power plant (Morro Bay) needed to be in service

to meet Diablo Canyon Nuclear Power Plant voltage requirements. A review of the available

data by PG&E on the offsite power supplies identified 47 instances in which the system

configuration could have resulted in a degraded voltage condition between 1990 and 1995.

PG&E identified a potential double sequencing scenario at Diablo Canyon if a LOCA occurred

during these degraded voltage conditions (LER 50-275/95-007-01 (Accession No.

9608140037)).

IN 2000-06 March 27, 2000 On July 22, 1997, the licensee for Clinton Power Station sought an exemption from offsite

power regulatory requirements because of its analysis that offsite power would become

inadequate under certain summer peak conditions following the loss of the nuclear unit. The

exemption request was eventually withdrawn by the licensee.

NRC inspection findings and licensee event reports have indicated instances in which grid

stability analyses had not been updated by the licensees to reflect changes in the grid power

system. An Office of Nuclear Regulatory Research report, The Effects of Deregulation of the

Electric Power Industry on the Nuclear Plant Offsite Power System: An Evaluation, dated

June 30, 1999 (Accession No. 9907120008), recommended that the staff take certain followup

actions to ensure that licensees will continue to maintain their licensing bases in this area.

Discussion

NRC Information Notice (IN) 98-07 discussed the possibility that the changes occurring as a

result of deregulation of the electric utility industry could affect the reliability of the offsite power

systems in nuclear power plants. Offsite power problems highlighted in licensee event reports

were identified as potential sources of concern if not properly managed following the

restructuring that occurs as a result of deregulation. NRC IN 95-37 alerted licensees to

circumstances that could result in inadequate offsite power system voltages during design basis

events.

The most recent problem, which was reported by the licensee for Callaway Unit 1, potentially

tied the inadequate offsite system voltage problem to industry deregulation. The licensee

stated in LER 50-483/99-005 (Accession No. 9909200074) that the magnitude of the power

being transported across the grid during the period had not been previously observed and was

far in excess of typical levels. LER 50-483/99-005 (Accession No. 9909200074) also stated

that the deregulated wholesale power market contributes to conditions in which higher grid

power flows are likely to occur, and these large flows were observed at this time.

Because the Callaway generator was supporting the grid voltage in the vicinity of the plant, the

low grid voltage had not been observed until the Callaway generator voltage support was no

longer available. However, if a design basis event had occurred during the period of high

system demand, the consequential loss of the Callaway generator, combined with the plant

electrical requirements associated with the event, could have actuated the plants degraded

voltage protection and separated safety loads from offsite power, which is the preferred power

supply under these circumstances.

The reports referenced in this notice also identify additional combinations of circumstances than

those seen at Callaway that could result in inadequate offsite voltages. These circumstances

include main generator unavailability, line outages, transformer unavailability, high system

demand, unavailability of other local voltage support, and high plant load. The common

characteristic of these problems is that the true capability of the offsite source cannot

necessarily be verified through direct readings of plant switchyard or safety bus voltages.

IN 2000-06 March 27, 2000 Instead, analyses of grid and plant conditions must be relied upon to determine this capability, considering the postulated occurrence of an event. If these analyses are not accurate and up

to date, licensees could inadvertently operate their plants in regions of inadequate voltages for

some periods of time.

As demonstrated by the Callaway event, industry deregulation can heighten the need to update

the analyses on a more frequent basis. Some utilities have utilized on-line contingency analysis

techniques in their grid control centers and implemented arrangements to be notified when the

offsite system to their plant is in jeopardy of not providing its required capability. When the on- line capability is not available, other utilities have provided for updating of the analyses on a

more frequent basis and have implemented procedures to identify when the plant and grid

conditions are outside the bounds of the assumptions of the analyses, thereby providing the

information to take compensatory actions as necessary.

Maintaining plant operation in a region of adequate offsite voltage is especially important for

licensees that may not have evaluated their plant safety systems for the double-sequencing

scenario identified in the Palo Verde and Diablo Canyon LERs. The safety consequences that

would result if an event occurred during a period of inadequate voltage can, therefore, be

difficult to assess.

This information notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact one of the technical contacts

listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

/RA/

Ledyard B. Marsh, Chief

Events Assessment, Generic Communications

and Non-Power Reactors Branch

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical contacts: Ronaldo V. Jenkins, NRR James J. Lazevnick, NRR

301- 415-2985 301- 415-2782 E-mail: rvj@nrc.gov E-mail: jjl@nrc.gov

Thomas Koshy, NRR Jeffrey L. Shackelford, RIV

301-415-1176 817-860-8144 E-Mail: txk@nrc.gov E-mail: jls2@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

IN 2000-06 March 27, 2000 Instead, analyses of grid and plant conditions must be relied upon to determine this capability, considering the postulated occurrence of an event. If these analyses are not accurate and up

to date, licensees could inadvertently operate their plants in regions of inadequate voltages for

some periods of time.

As demonstrated by the Callaway event, industry deregulation can heighten the need to update

the analyses on a more frequent basis. Some utilities have utilized on-line contingency analysis

techniques in their grid control centers and implemented arrangements to be notified when the

offsite system to their plant is in jeopardy of not providing its required capability. When the on- line capability is not available, other utilities have provided for updating of the analyses on a

more frequent basis and have implemented procedures to identify when the plant and grid

conditions are outside the bounds of the assumptions of the analyses, thereby providing the

information to take compensatory actions as necessary.

Maintaining plant operation in a region of adequate offsite voltage is especially important for

licensees that may not have evaluated their plant safety systems for the double-sequencing

scenario identified in the Palo Verde and Diablo Canyon LERs. The safety consequences that

would result if an event occurred during a period of inadequate voltage can, therefore, be

difficult to assess.

This information notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact one of the technical contacts

listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

/RA by/

Ledyard B. Marsh, Chief

Events Assessment, Generic Communications

and Non-Power Reactors Branch

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical contacts: Ronaldo V. Jenkins, NRR James J. Lazevnick, NRR

301- 415-2985 301- 415-2782 E-mail: rvj@nrc.gov E-mail: jjl@nrc.gov

Thomas Koshy, NRR Jeffrey L. Shackelford, RIV

301-415-1176 817-860-8144 E-Mail: txk@nrc.gov E-mail: jls2@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

ACCESSION #: ML003695551 TEMPLATE #: NRR-052

• See previous concurrence

To receive a copy of this document, indicate in the box C=Copy w/o attachment/enclosure E=Copy with attachment/enclosure N = No copy

OFFICE EEIB EEIB REXB Tech Ed. SC:EEIB C:EEIB

NAME RJenkins* JLazevnick* CPetrone* BCalure* DThatcher* JCalvo*

DATE 02/17/00 02/17 /00 3/15/00 2/17/-3/17/00 02/18/00 3/09/00

OFFICE DD:DE D:DE REXB:DRIP C:REXB:DRIP

NAME RWessman* JStrosnider* TKoshy* LMarsh

DATE 3/ 07/00 3/08/00 3/22/00 3/ 27/00

OFFICIAL RECORD COPY

Attachment

IN 2000-06 March 27, 2000 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

_____________________________________________________________________________________

Information Date of

Notice No. Subject Issuance Issued to

________________________________________________________________________________

2000-05 Recent Medical 3/06/2000 All medical licensees

Misadministrations Resulting

from Inattention to Detail

2000-04 1999 Enforcement Sanctions 2/25/2000 All NRC licensees

for Deliberate Violations of

NRC Employee Protection

Requirements

2000-03 High-Efficiency Particulate Air 2/22/2000 All NRC licensed fuel-cycled

Filter Exceeds Mass Limit conversion, enrichment, and

Before Reaching Expected fabrication facilities

Differential Pressure

2000-02 Failure of Criticality Safety 2/22/2000 All NRC licensed fuel-cycled

Control to Prevent Uranium conversion, enrichment, and

Dioxide (UO2) Powder fabrication facilities

Accumulation

2000-01 Operational Issues Identified in 2/11/2000 All holders of licenses for nuclear

Boiling Water Reactor Trip and power reactors

Transient

99-34 Potential Fire Hazard in the 12/28/99 All holders of licenses for nuclear

use of Polyalphaolefin in reactors and fuel cycle facilities

Testing of Air Filters

99-33 Management of Wastes 12/28/99 All medical licensees

Contaminated With

Radioactive Materials

99-32 The Effect of the Year 2000 12/17/99 All NRC medical licensees

Issues on Medical Licensees

99-31 Operational Controls to Guard 11/17/99 All NRC licensed fuel cycle

Against Inadvertent Nuclear conversion, enrichment and

Criticality fabrication facilities

____________________________________________________________________________________

OL = Operating License

CP = Construction Permit