Information Notice 1995-05, Undervoltage Protection Relay Settings Out of Tolerance Due to Test Equipment Harmonics
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.
20555
January 20, 1995
NRC INFORMATION NOTICE 95-05: UNDERVOLTAGE PROTECTION RELAY SETTINGS OUT OF
TOLERANCE DUE TO TEST EQUIPMENT HARMONICS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
PurDose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees that undervoltage relay settings could be out of
tolerance because of test equipment harmonics. It is expected that recipients
will review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems. However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances
The following occurrences indicate that test equipment harmonics could result
in undervoltage relay actuation settings that are out of tolerance.
Dresden Nuclear Power Station
On April 11,
1994, during routine calibration of the Dresden Unit 3 Asea Brown
Boveri (ABB) Model ITE 27N undervoltage relays, both relays were found to be
out of tolerance. Additional testing of the 27N relay showed that the relay
operating point was influenced by the polarity of the lead wires from the
voltage test source (power supply) producing the sinusoidal voltage signal.
The test source was diagnosed and verified, using an oscilloscope, as having a
distortion on the positive side of the single-phase 120-Vac sine wave.
In
addition, further evaluation showed that the Unit 2 degraded voltage relays
had also been calibrated using the same power supply in October 1992 and
April 1993.
The Dresden 27N relays were originally equipped with a harmonic filter and
time delay circuit. However, in 1992, ABB notified the NRC, pursuant to
Part 21 of Title 10 of the Code of Federal Regulations, that the relay time
delay circuit could fail to initiate the appropriate time delay when exposed
to radiation levels between 22.5 and 45 Gray (Gy) (2.25E03 and 4.5E03 rad). In
addition, ABB indicated that the harmonic filter option could cause the
pickup/dropout operating points to shift high at radiation levels above those
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IN 95-05 January 20, 1995 affecting the time delay, but below 630 Gy (6.3E04 rad). In response to the
notification, the licensee modified the 27N relays in accordance with the
instructions of the manufacturer to remove the harmonic filter and time delay
components. The licensee is now replacing the 27N relays with 27N-R relays
for second-level undervoltage protection. The 27N-R relay is equipped with
radiation-resistant harmonic filter and time delay circuitry.
Before this event, the voltage test source was classified as a 'general usage"
piece of test equipment; its electrical output characteristics, therefore, were not periodically verified. After this event, as a corrective action, the
licensee added the ac voltage test source to a general surveillance program.
The test source output characteristics are now verified annually to meet
manufacturer specifications. In addition the licensee is performing a review
of electrical and electronic measuring and testing equipment for both
certified and general usage to determine if other vulnerabilities exist.
Comanche Peak Steam Electric Station
On May 23, 1994, Comanche Peak Unit 2, was in Mode 5 during its mid-cycle
outage. During the performance of the Train A undervoltage relay calibration
and response time surveillance testing, 14 of 16 relays were found to be
outside their respective allowable technical specification values. The relay
dropouts were found to be 1 to 3 V outside the expected values. These relays
were adjusted to correct setting values.
Following this discovery and
corrective action, the licensee performed a comprehensive root cause
evaluation. This evaluation showed that the testing device used to calibrate
the undervoltage relays did not provide accurate as-found data, because of
harmonic distortion that affected peak values of the ac testing voltage
waveform. From May 26 to May 28, 1994, the licensee reverified both Train A
and Train B relay actuation settings using a clean (low harmonic distortion)
ac power supply. On May 28, 1994, during the reverification process, it
appeared that five of the Train A relays, Types ITE-27N and ITE-27H, were
outside the technical specification values using the most conservative data.
To preclude recurrence, the corrective action taken by the licensee was to
implement a procedural change to require the use of a power source with low
harmonic distortion for testing these types of relays.
Point Beach Nuclear Plant
In February 1994, the Point Beach licensee replaced the installed Type 27D
degraded voltage relays with Type 27N. The 27N relays were more accurate and
had an adjustable reset. These relays were recently recalibrated, and all 12 were found to have "drifted' approximately 0.5 V high (120-V base). Although
the drift did not exceed calibration tolerances, the licensee had not
previously experienced similar calibration shifts for the 27D relays. A
further investigation by the licensee showed that the calibration shift was
attributed to harmonics present in the ac test voltage power source. The
licensee noted that the relays were not sensitive to harmonic frequencies,
IN 95-05 January 20, 1995 but were sensitive to harmonic amplitudes. The licensee measured the harmonic
distortion attendant to the ac voltage test source and the safeguards buses, and concluded that the relay operating point was affected by the harmonic
distortion of both the ac voltage test source and the safeguards buses when
supplied by the offsite power system. Because the system harmonics may vary, the licensee is installing the harmonic filters in all of the 27N degraded
voltage relays. The harmonic filters will attenuate harmonics attendant to
the ac voltage test source and the offsite power system.
Discussion
All three occurrences were attributed to test equipment harmonics. When the
harmonic filters were removed from the 27N relays at Dresden, harmonic
distortion in an ac voltage source waveform had a noticeable effect on relay
operating point. The 27N relay uses a peak detector circuit. This peak
detector circuit samples half of the voltage source waveform and measures its
peak value. Although the peak detector circuit is very accurate, it is
sensitive to harmonic distortion. The relay manufacturer (ABB) recommends an
ac voltage test source with less than 0.3 percent harmonic distortion. As
indicated above, the licensees used ac voltage test sources which were
subsequently found to have high harmonic distortion.
During relay
calibration, a calibrated digital voltmeter (DVM) was used to measure the
relay input voltage to determine its trip setpoint. The DVM measures the root
mean square (RMS) value of the ac voltage test source sinusoidal waveform.
A change in the peak voltage of the ac voltage test source sinusoidal waveform
caused by harmonic distortion does not result in an equivalent change in the
RMS voltage as indicated by the DVM.
The relay operating point is
significantly influenced by its peak voltage detection circuitry. Therefore, if the voltage test source power supply is not included in a calibration
program, the DVM may not accurately reflect the setpoint of the solid-state
relay.
Using harmonic filters in the 27N relays will attenuate both ac voltage test
source harmonics and system harmonics.
However, ABB pointed out that the
harmonic filter option could cause the pickup/dropout operating points to
shift high at radiation levels below the testing end point of 630 Gy
(6.3E04 rad).
In addition, ABB documentation indicates that using the
harmonic distortion filter option increases the repeatability tolerance of the
relays as a function of temperature variations. As documented in ABB
Instructions IB 7.4.1.7-7 Issue D for Type 27N relays, temperature variations, such as -20 to +700C (-4 to 158'F), increase the relay repeatability tolerance
from +/-0.7 to +/-1.5 percent of relay pickup and dropout settings. Therefore, adding the harmonic filter option may necessitate revising the undervoltage
relay setpoint calculations. If harmonic filters are not used, system
harmonics can be determined and the test source harmonic distortion matched to
the system harmonic distortion within the tolerance band to ensure that relays
pick up and drop out at required system setpoints.
IN 95-05 January 20, 1995 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.
Brian K. Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation
Technical contacts:
D. Nguyen, NRR
(301) 504-3202
D. Butler, RIII
(708) 829-9720
F. Ashe, NRR
(301) 504-2785
D. Skeen, NRR
(301) 504-1174 Attachment:
List of Recently Issued
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Information Notices
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IN 95-05 January 20, 1995 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.
Original signed by Brian K. GVimes
Brian K. Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation
Technical contacts:
D. Nguyen, NRR
(301) 504-3202
D. Butler, RIII
(708) 829-9720
F. Ashe, NRR
(301) 504-2785
D. Skeen, NRR
(301) 504-1174 Attachment:
List of Recently Issued Information Notices
DOCUMENT NAME:
95-05.IN
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IN 94-XX
December XX, 1994
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.
Brian K. Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation
Technical contacts:
D. Nguyen, NRR
(301) 504-3202
D. Butler, RIII
(708) 829-9720
F. Ashe, NRR
(301) 504-2785
D. Skeen, NRR
(301) 504-1174 Attachment: List of Recently Issued Information Notices
DOCUMENT NAME:
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DATE 1
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IN 94-XX
Month XX, 1994 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.
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts:
D. Nguyen, NRR
(301) 504-3202
F. Ashe, NRR
(301) 504-2785
D. Butler, RIII
(708) 829-9720
D. Skeen, NRR
(301) 504-1174 Attachment:
List of Recently Issued NRC Information Notices
- PREVIOUS CONCURRENCE
OFC
OECB:DOPS
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PUB:ADM
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NAME
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DOCUMENT NAME:
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Attachment
January 20, 1995 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
95-04
95-03
95-02
95-01
94-90
94-89
94-88
Excessive Cooldown and
Depressurization of the
Reactor Coolant System
Following a Loss of
Offsite Power
Loss of Reactor Coolant
Inventory and Potential
Loss of Emergency Mitiga- tion Functions While in
a Shutdown Condition
Problems with General
Electric CR2940 Contact
Blocks in Medium-Voltage
Circuit Breakers
DOT Safety Advisory:
High Pressure Aluminum
Seamless and Aluminum
Composite Hoop-Wrapped
Cylinders
Transient Resulting in a
Reactor Trip and Multiple
Safety Injection System
Actuations at Salem
Equipment Failures at
Irradiator Facilities
Inservice Inspection
Deficiencies Result in
Severely Degraded Steam
Generator Tubes
01/19/95
01/18/95
01/17/95
01/04/95
12/30/94
12/28/94
12/23/94
All holders of OLs or CPs
for nuclear power reactors.
All holders of Ols or CPs
for nuclear power reactors.
All holders of OLs or CPs
for nuclear power reactors.
All U.S. Nuclear Regulatory
Commission licensees.
All holders of OLs or CPs
for nuclear power reactors.
All U.S. Nuclear Regulatory
Commission irradiator
licensees.
All holders of OLs or CPs
for pressurized water
reactors.
OL - Operating License
CP -
Construction Permit