United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-05: 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.

Purpose

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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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,
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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 +70 C (-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.

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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.

                                                            /S/'D BY BKGRIMES


                              Brian K. Grimes, Director
                              Division of Project Support
                              Office of Nuclear Reactor Regulation

Technical contacts:  D. Nguyen, NRR         D. Butler, RIII
                     (301) 504-3202         (708) 829-9720

                     F. Ashe, NRR           D. Skeen, NRR
                     (301) 504-2785         (301) 504-1174

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