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Industrial Grade UPS Systems Protect Electric Generating Plant Control Room
Power Generating Plant Uses Industrial Grade UPS System to Back-Up Critical Controls
Glass Making Plant Uses UPS System To Ensure Continuous Operation
Nuclear Power Station Installs New UPS System
Nebraska Nuclear Power Plant Upgrades UPS System
New Hampshire Utility Uses Industrial Grade UPS Systems At Several of Its Power Stations

Nuclear Power Station Installs New UPS System

Safety in operation is of utmost importance to Maine Yankee Atomic Power Company in Wiscasset, Maine. This is why, during a recent refueling outage, the nuclear plant replaced an existing uninterruptible power supply (UPS) system with a new system and a new operating configuration.

The plant, which utilizes a pressurized water reactor design, is owned by a group of ten utility companies in New England. Maine Yankee began commercial operation in December, 1972 and was descend for a 40 year life cycle. The facility employs over 450 people.

The UPS system protects the power supply to vital AC buses. These are connected to distribution panels and provide power to critical instrumentation monitoring reactor operation parameters, as well as other essential equipment and systems.

These AC loads include buses to four different channels that provide power for detectors that monitor neutron levels in the reactor; instrumentation for pressure, temperature, flow rate and level of water in the reactor; power for small motors in equipment such as strip recorders and pumps, warning alarms, and Reactor Protection Systems.

The complete UPS system is made up of battery chargers, DC-AC inverters and back-up batteries. The critical AC loads are powered directly by the DC-AC inverters which receive DC current from the battery chargers during normal operation. The inverter delivers a constantly clean, regulated power supply. In the event of a power outage, the inverter is instantaneously transferred to the DC battery back-up with absolutely no break in current voltage.

The old UPS system was removed and a new system installed during a scheduled eight-week refueling outage, starting in February 1992. The new UPS system was actually installed within four and one-half weeks including functional testing. During the shutdown, a number of other design changes were made, maintenance projects undertaken and one-third of the 512 fuel rods changed out.

There were two unique aspects to this UPS application and installation that made it challenging. First, the plant decided to replace the previous system of five individual inverters and five individual battery chargers with a system incorporating four combination battery charger/inverters and one spare unit. The spare had to have the ability to be brought on-line to replace any of the other four which might be taken out of service. The second major concern was the crucial nature of the timing because of the limited period of the shutdown.

"The implementation of the spare inverter was very important to Maine Yankee," said Wes Frewin, project engineer for CIANBRO Corporation, who designed this aspect of the system.

"Because of our requirements, we needed system specifications as well as equipment specifications."

Frewin's first step was to contact UPS vendors, culled from a utility survey, to see if these companies were still supplying equipment to nuclear plants discovered that most UPS manufacturers were no longer supplying nuclear qualified units because construction of new plants had stopped. RFQs with special technical specifications were sent to four vendors. Maine Yankee preferred ferroresonant-type inverters based on the performance and reliability of the units being replaced, which were manufactured by AMETEK Solidstate Controls. Only two of four vendors responded, one of which was Solidstate Controls, LLC. The others did not offer ferroresonant units.

According to Don Hakkila, senior electrical engineer for Maine Yankee "AMETEK Solidstate Controls put together an excellent bid package. Testing and optional testing were explained. Project schedules and a sequence of performance were provided. Everything in the technical specifications was addressed." Key personnel from AMETEK Solidstate Controls and its local representative who would be involved in the project were also interviewed.

Frewin added, "Maine Yankee wanted some unique features that we warrant quite sure how to address. This is where AMETEK Solidstate Controls helped . . They knew what they were talking about." The new UPS system is made up of five custom- designed Solidstate Controls, LLC.'s 10kVA inverters and five 250 amp battery chargers. Both the battery charger and inverter are housed in the same cabinet, which is a unique design for this type of application. A custom manufactured circuit breaker/distribution cabinet was also installed as an integral part of the system.

The new units replaced AMETEK Solidstate Controls inverters purchased in 1971. These had reached their design life of 20 Separate battery chargers were used in the old configuration. The older units were commercial grade, while the new equipment is Safety Class I E qualified for use in nuclear plants and provides an operating life of 40 years. It also has documented qualification test results which provide assurances of operability during applicable design basis events.

The new system design affected three low-voltage electrical systems: a 480 volt system, 125 VDC system and a 120 VAC instrument power system. The installation of a spare battery charger/inverter, as a ready replacement for any of the other four units, enhances the system's reliability by providing a means to substitute equipment removed from service due to routine maintenance or equipment failure. This is a preferred alternative to closing tie breakers between related channels, since tie breaker usage invokes specific technical specification-related operating restrictions.

Each channel related system interfaces with the space battery charger/inverter unit via the circuit breaker/distribution cabinet. Each channel is electrically isolated, physically separated, and mechanically interlocked from one another to ensure system independence is maintained and to avoid the potential for cross-tie.

Under normal operating conditions, this design provides the ability to remove a channel designated battery charger/inverter unit from service without interruption of power to the respective DC bus and its associated 120 VAC Vital Instrument Bus. This is accomplished through the combined use of the spare battery charger/inverter unit and a make-before- break transfer switch.

Under adverse conditions, such as equipment failure, this system provides the ability to isolate a failed battery charger/inverter and bring into service a one-for-one replacement with very limited disruption to the associated 120 VAC Vital Instrument Bus. This avoids prolonged remedial action associated with such equipment failures and is achieved by the same means as described above, along with the use of input and output circuit breakers to isolate the failed equipment prior to initiating the make-before-break transfer switch. It takes less than one hour to switch over to the spare battery charger/inverter. This is the first time in this type of application, that all battery charger/inverter units have been run through a common circuit breaker cabinet. It is also the first time for using the spare unit in this manner. The NRC audited the plant in April, 1992 and did not identify any deficiencies in their review of the new design. SO assisted in the audit by providing essential system component documentation.

The location of the battery charger/inverter units remains in the Protected Switchgear Room. The new units are comparable in size, electrically to those they replaced. Prior to the removal of the old inverters, it was necessary to redistribute the electrical loads that had previously been on the fifth inverter which was now being eliminated. The spare battery charger/inverter and a spare circuit breaker/distribution cabinet are permanently installed where the fifth inverter had been previously located.

Four battery banks are used in the system, each made up of 59 lead-calcium type cells. Two banks are located in a battery room adjacent to the room housing the inverters, while two additional battery banks are housed in an area directly beneath the other batteries. Two battery banks are also used to start the emergency diesel generators.

A new "human-factored" engineered equipment labeling procedure was also incorporated into the new installation. Color coolant was added to enhance the labeling of channel designated equipment and devices; specifically, battery charger/ inverter unit equipment labels, input circuit breaker labels on the spare battery charger, and both DC and AC circuit breaker cabinets. The use of color in combination with labeling nomenclature provides enhanced recognition at equipment and devices in an effort to improve operator interface.

In combination with alarms monitored in the control room, surveillance is performed every shift by plant personnel to make sure there are no local alarm conditions present on the inverters. Batteries are also checked periodically for proper operation.

Wes Frewin said, "The installation of these battery charger/inverters was one of the most challenging projects handled during the planned shutdown. Maine Yankee was impressed with how easily the equipment was installed. The units performed well at start-up. " AMETEK Solidstate Controls was the better value for Maine Yankee," said Don Hakkila, "They have been responsive and very good to work with."