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New Hampshire Utility Uses Industrial Grade UPS Systems At Several of Its Power Stations

New Hampshire Utility Uses Industrial Grade UPS Systems At Several of Its Power Stations

Since 1926, Public Service of New Hampshire (PSNH) has been serving the power needs of the state. It is the largest electric utility in New Hampshire and supplies electricity to approximately three-quarters of the state's population.

In an effort to provide the highest level of service reliability to its customers, PSNH has upgraded the instrumentation and control systems at three of its generating plants: the Schiller Station, the Merrimack Station and the Newington Station. As part of this upgrade, the company also installed uninterruptible power supply (UPS) systems by Solidstate Controls, Inc. (SCI) to provide continuous operation of critical instrumentation and control systems.

Schiller Station

Located in Portsmouth, New Hampshire, Schiller Station was the first PSNH plant to install a new UPS system from SCI. The oil- fired plant has one 128 MW generator and three 47 MW generators.

A single-phrase 15kVA UPS system, made up of an inverter and a regulated rectifier, was installed in 1984. The UPS is used to back-up a Fischer & Porter ADH (auto data handling) system.

A key reason for PSNH's choice of SCI systems for their other power stations was the excellent performance experienced at Schiller. It also provided equipment standardization and the interchangeability of spare parts for use at all three locations.

Merrimack Station

The Merrimack Station, in Bow, New Hampshire, is capable of producing the most electricity of all of PSNHâs generating stations. The coal-fired plant has a net capability of approximately 433 MW from its two units.

Merrimack's Unit 2, a 320 MW turbine/generator with coal-fired cyclone burners, installed a new SCI 15kVA UPS system in 1986. The single-phase system is made up of an inverter and a regulated rectifier. The UPS system backs up Unit 2's Foxboro Spectrum dis- tributed control system (DCS) with a universal control module (UCM). The SCI system also protects a Fischer & Porter ADH system used by both Unit I and 2 to handle temperature, pressure, level and flow indication.

The new UPS system required more capacity then the previous installation which had become overloaded. It was also necessary for the new units to match the existing station battery voltage of 125 volts.

On October 11, 1986, shortly after the new UPS system was installed, there was a failure on the main bus of Merrimack's Unit 2. The generator was operating at full load at the time. The SCI inverter kept the DCS operating so that an orderly shutdown could be accomplished. The system was kept up long enough until the load could be transferred to an alternate power source. The system could have kept running on the alternate source, but plant procedures dictated a shutdown until the bus was returned to normal service.

All field instrument and control systems for boiler temperature and other functions, which are powered by the inverter, were able to stay on- line. The ADH for both units 1 and 2 also maintained operation during the trip.

Roger Deshaies of the Merrimack Station said, "Maintaining this high level of UPS performance is critical because we really rely on the system to operate dependably under a variety of conditions."

Newington Station

Situated along the banks of the Piscataqua River in New Hampshire, the Newington Station was designed as a "peaking" unit to provide extra generation for those periods during the day when a customer demand reaches peak levels. The 423 MW facility began commercial operation in June of 1974. It has the capability to burn both natural gas and fuel oil.

The station installed a new UPS system in 1986 to back-up its critical control systems. The UPS system is made up of a 25kVA inverter and an unregulated rectifier. The UPS is used to back-up several process controllers, including a Bailey 90 and a Fischer & Porter Series 5000. These control pressure, temperature, level, flow and alarm devices. According to Newington's Bob Descoteaux, "The UPS also backs up a Forney burner management system and is used as our primary source of instrument power."

The Newington UPS system's operational configuration is somewhat unique. The station inverter load is powered directly by an unregulated rectifier rather than batteries. The unregulated rectifier is used to convert 480VAC three-phase current to DC power. This, in turn, is used to supply DC to the inverter which supplies a clean, regulated source of AC power. The batteries are only used as a back- up power source and to supply some small DC loads in the plant. A separate battery charger, not part of the UPS system, is used to maintain the batteries.

Previously, the station used a I 5kVA UPS system. To provide for anticipated growth, however, the new system was specified at 25kVA. Currently, the system is running at about 80% of its rated capacity.

In 1990, a blocking switch was added to the system to keep the inverter from transferring to an alternate power source when voltage deviates. This was necessary because of problems encountered when equalize-charging the station batteries. The slight increase in voltage during this procedure sometimes caused the inverter to transfer to battery power. The switch keeps this from happening during fluctuations of within 10-15 volts.

The quality of power feeding the control systems at all of these stations was also a concern for PSNH. Because they are computer-based, both distributed control systems and PLCs require high quality power for proper operation. Besides providing battery back- up, the UPS systems also provide clean, conditioned power, free of transients that can cause poor performance or damage to sensitive equipment. Power line aberrations such as surges, sags and brownouts can cause errors in critical data readings and signals, as well as premature component failure in the control systems.

The UPS systems used in these applications are industrial grade units, specially designed for harsh environments such as those found in power generation plants. Many UPS systems on the market today are designed for the clean, controlled environments of computer rooms and are very often not suitable for industrial surroundings. These UPS systems employ ferroresonant technology that uses passive electronic and magnetic components in its design. These types of units are better suited to industrial environments than UPS systems using more complicated power devices with complex feedback circuits which must be protected from harsh environments.