Information Notice No. 87-24: Operational Experience Involving Losses of Electrical Inverters
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
June 4, 1987
Information Notice No. 87-24: OPERATIONAL EXPERIENCE INVOLVING LOSSES
OF ELECTRICAL INVERTERS
All nuclear power reactor facilities holding an operating license or a
This notice is provided to alert recipients of potential problems
involving electrical inverter losses that have led to unplanned plant
transients and/or inoperability or improper functioning of safetyrelated
and other important plant equipment. It is expected that recipients will
review this information for applicability to their facilities and
consider actions, as appropriate, to preclude similar problems from
occurring at their facilities. However, suggestions contained in this
notice do not constitute NRC requirements; therefore, no specific action
or written response is required.
Past Related Correspondence:
Information Notice No. 84-80, Plant Transients Induced By Failure of
NonNuclear Instrumentation Power," November 8, 1984
IE Bulletin 79-27, "Loss of Non-Class IE Insrumentation and Control Power
system BUS During Operation," November 30, 1979
Information Notice No. 79-29, "Loss of Nonsafety-Related Reactor Coolant
System Instrumentation During Operation," November 16, 1979
IE Circular 79-02, "Failure of 120 Volt Vital AC Power Supplies,"
January 11, 1979
Inverters in nuclear power plants provide "uninterruptible" vital ac
electrical power to safety- and non-safety-related instrumentation and
control systems. Generally, loss of this function results in some type of
undesirable system condition and/or plant transient, including
unnecessary actuation of safety systems such as reactor protection and
engineered safeguards systems; loss of indicators that provide plant
status information; system disturbances,
June 4, 1987
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including reactor coolant system transients; improper response of the
feedwater and steam generator water level control systems; loss of
safetyrelated electrical equipment functions; damage to mechanical
equipment; and challenges to operators and the remaining functional
equipment. Such conditions and/or transients clearly have significant
safety implications since they result in challenges to safety equipment
and plant operations and/or a degradation of plant equipment.
The NRC case study report, AEOD/C605 dated December 1986, "Operational
Experience Involving Losses of Electrical Inverters," includes the review
of 94 licensee event reports (LERs), totaling 107 events involving
inverter losses that occurred during 1982 through 1984. The study
includes 35 additional events from the Nuclear Plant Reliability Data
System (NPRDS) that occurred in the same timeframe. These 142 events
occurred at 51 distinct plants: 26 designed by Westinghouse, 11 by
General Electric, 9 by Combustion Engineering, 4 by Babcock & Wilcox, and
1 by General Atomic. The total number of events included in the study for
each of the 3 years along with the number of reactor units which were
operating during each of those years is summarized below.
Parameter 1982 1983 1984
Operating Reactor Units 72 74 82
Inverter Loss Events 34 51 57
Losses/Operating Reactor .47 .69 .70
As indicated above, the NRC has issued information on inverter losses
since 1979; and industry groups have issued approximately 14 reports
related to this issue.
Description of Circumstances:
The NRC case study report identified three potential failure mechanisms
for inverters. One of these involves relatively high ambient temperature
and/or humidity within inverter enclosures. This condition appears to
result in accelerated aging of components that form a part of the
inverter circuitry causing a significant reduction in component life
expectancy and inverter loss.
Another mechanism for inverter failure involves the electrical
interconnecting and physical arrangements for the inverter circuitry
components. In some installations, these arrangements are such that when
certain components fail, other components also may fail or degrade.
The third failure mechanism involves voltage spikes and perturbations.
Many of the electrical loads in a plant have inductive characteristics.
During plant operations that involve energizing and deenergizing these
loads, voltage spikes and perturbations are generated. The solid-state
devices in the inverter circuitry are sensitive to these voltage spikes,
and this has resulted in component failure, blown fuses, and inverter
losses. Additionally, secondary voltage perturbations caused by lightning
strikes or switching surges can have an adverse effect on inverter
June 4, 1987
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The NRC case study report indicates that the failure mechanisms involving
service condition parameters (e.g., ambient temperature and/or humidity
and voltage spikes and perturbations) have common-cause implications.
However, none of the events reviewed and evaluated in the report involved
the simultaneous loss of redundant inverter-powered buses.
The dominant cause of inverter losses was attributed to component
failures. Such components include diodes, fuses, silicon controlled
rectifiers, capacitors, transistors, resistors, printed circuit boards,
transformers and inductors. It also appears that major contributing
factors for the occurrence of component failure events are high ambient
temperature and/or humidity within inverter enclosures and electrical
disturbances at the inverter input/output terminals.
In addition, incorrectly operating circuit breakers, improperly setting up
test equipment, removing the wrong Inverter unit from service, and
improperly transferring power sources for a bus are some personnel
actions that make them the second largest contributor to inverter losses.
It is suggested that licensees consider monitoring of temperature and/or
humidity internal to inverter enclosures and evaluating input and output
voltages of the inverter unit during steady-state and transient conditions
to assure that manufacturer's recommendations are being considered.
Additionally, to minimize the number of inverter loss events resulting
from personnel actions, licensees might consider reviewing related
maintenance and testing procedures and practices for inverters, Further,
specialized training and practice sessions with involved plant personnel
and verification of appropriate sequence of steps to achieve desired
related maintenance and testing activities also may be considered.
No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment Office of Nuclear Reactor
Technical Contact: Vincent D. Thomas, NRR
Attachment: List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015