Specifying generator control switchgear

06/01/2009


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With daily advances in information technology and other processes and services, the world is becoming a more complicated and power-hungry place. Many industries and services have become increasingly more dependent on a continuous, uninterrupted supply of electric power. However, a continually shrinking electric generation margin reduces the reliability of utility-provided power.

Consequently, the use of backup power control systems and generator control switchgear has grown and will continue to grow—in number, capacity, and complexity—in the coming years. A savvy specifier considers many factors to make sure such equipment performs as needed and its service life is not only long, but virtually trouble-free.

Some facilities cannot be without power for extended periods, but can still tolerate a short outage. For such facilities, a backup power system can be fairly simple. In other applications, however, even a momentary interruption of power could be disasterous. To avoid this, systems should include an uninterruptible power supply (UPS) system to prevent even the slightest “blink.” Generator control switchgear also can be specified to allow closed-transition retransfer or test without power interruption when both sources are available.

Generator control switchgear, typically used as part of a multi-generator electric backup power system at a mission critical facility, should have a life expectancy of at least 15 years. However, another important consideration for maximizing the life expectancy of the switchgear is the possibility of future load growth. Don't just focus on immediate needs.

When extra capacity is specified, future expansions/upgrades can be accommodated more smoothly and at a much lower cost. For example, a hospital's standby electrical load today can be three times what it used to be—75% or more of its total connected load.

Construction and components

Regardless of the switchgear's application, the quality of its construction is key. Start with the cabinets, which should be fabricated from heavy-gauge steel with welded reinforcing gussets for strength and rigidity. For many geographic locations, equipment must be built and tested to withstand a seismic event per International Building Code (IBC) requirements. Enclosures should be protected with a corrosion-resistant electrostatic powder coating.

Inside, the construction of the switchgear should be durable. Switchboard wires should be flame-retardant and should have permanent sleeve markers at both ends. The best wire runs are custom-assembled on chassis (not pre-manufactured) and should include extra wires for faster, simpler repairs and future expansion. Cage clamp-type connectors provide sustained, secure control wiring connections.

Busbar should be formed, cut, and punched before being silver-plated, to ensure integrity of the plating. If the manufacturer buys pre-plated bus and forms it later, the process of bending it can crack the plating and expose raw copper. This can reduce conductivity and cause problems with corrosion and overheating, adversely affecting the performance of the whole system. Where insulated bus is used, the insulation also should be applied after the busbar is formed.

Switchgear should be built and tested to the highest Underwriter Laboratories (UL) requirements such as UL Standard 1558 for switchgear 600 V or less , and the UL category for medium-voltage switchgear (“Circuit Breakers and Metal-Clad Switchgear Over 600 V”). The withstand rating should be a performance value based on actual testing. Keep in mind that, for applications in which the utility and generator sources are paralleled, the withstand and interrupting capacity of all breakers must be greater than the sum of the fault capability available from both power sources.

Controls

Programmable logic controller (PLC)-based digital controls should provide automatic starting, synchronizing, and distribution of standby power upon detection of loss of the utility source. A fully redundant PLC will ensure fail-safe operation. Should one PLC fail, the other one will automatically take over systemwide control. Manual start, paralleling, and load controls are also good ideas in case automatic control is lost.

A color touchscreen operator interface will permit system monitoring and parameter selection on-site. Custom supervisory control and data acquisition (SCADA) will allow for remote monitoring, real-time and historical trending, comprehensive reporting, and remote alarm management. In many cases, the SCADA system for the backup power system can be interfaced with a building's energy management system and other systems via TCP/IP or other Ethernet protocols—to provide a comprehesive overview of power quality and usage and to document energy usage trends and savings.

Long-term considerations

Other concerns are operator safety and ease of maintenance/troubleshooting. Generator control switchgear should have grounded, separately accessible compartments with drawout power circuit breakers located in their own compartments. Controls should be segregated from power bays, and only control voltages should be available in control compartments. For medium-voltage applications, the main bus joints and power connections should be insulated with preformed boots.

A purchaser of generator control switchgear should select a supplier that specializes in the field. When comparing switchgear suppliers, it is best to consult some of their previous customers to learn their track records. A supplier that is experienced in designing complete switchgear systems can contribute important ideas regarding control schemes, sequences of operations, power transfer options, and installation. A top-quality supplier should be able to provide a comprehensive guide specification that deals with most of the considerations discussed here—a specification that can be easily tailored to a customer's unique requirements.

Negotiate a good warranty, too. A supplier that backs its switchgear with a two-year warranty probably builds it with better components than a supplier offering only a one-year warranty. A longer warranty also may help reduce the lifecycle cost of the equipment.

Needless to say, it also helps when the supplier is a service-oriented company. Factory-direct field service is preferable because the technicians are intimately familiar with the equipment, are aware of the latest updates, and have easy access to spare parts. This facilitates equipment repair and reduces downtime.

Managing cost

In some cases, generator control switchgear can actually produce a revenue stream of its own, thus defraying a portion of its lifecycle cost. In fact, standby power systems now are frequently used for peak shaving, with the system's generator controls providing automatic operation. Furthermore, many utilities offer an “interruptible power contract” that gives the utility permission to drop the customer's facility from the electrical grid (with advance notice) during periods of peak demand. In return, for every kilowatt the customer generates while offline, the utility pays the customer a rebate at a rate that, in some cases, is higher than what the utility charges. In certain cases, the facility can get an electricity cost reduction just for being able to provide load-curtailment operation, even if never called upon to do so.

Intitial cost is always a consideration. In the final analysis, however, the lowest first cost solution may not be the lowest total cost solution once installation, commissioning, and maintenance are considered. And, when the switchgear or power control system will be protecting lives (such as at a hospital or airport) or vital electronic records (such as at a data center), the potential losses—in terms of life or money—from an equipment malfunction can be substantial.


<table ID = 'id1511822-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id1513103-0-tbody'><tr ID = 'id1513105-0-tr'><td ID = 'id1513107-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id1513118-3-tr'><td ID = 'id1513120-3-td' CLASS = 'table'> Meuleman is vice president of Russelectric Inc., which designs, builds, and services on-site power control systems. Meuleman has more than 30 years of experience in emergency/backup power systems for mission critical facilities. He holds a degree in electrical engineering and is a member of IEEE, AEE, and NFPA. </td></tr></tbody></table>

Consider the following:

  • Mission critical data centers often process such a staggering volume of transactions that even a short-term loss of power could have devastating consequences.

  • The average hospital's portion of total connected load covered by a backup power system has grown from 25% to 30% in past years to 75% or more today.

  • With facility growth and increased electronic security measures, most airports have significantly increased the load capacity of their backup systems.

  • With the proliferation of electronic controls in manufacturing, building management systems, and security systems, many companies that could have easily weathered a power outage in the past now consider a backup power system vital to their financial survival.



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