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As the recent massive power outages in the Northeastern United States and Canada so vividly demonstrated, maintenance and engineering managers face difficult decisions related to whether and how to have portable generators ready to go when an emergency arises. They also need to consider a range of other issues related to power in their facilities and the availability of equipment on short notice.
Assessing Electrical Loads
Why have portable power generators? In addition to emergency situations, facilities often need power in remote locations but want to avoid the high cost of running long distribution lines and burying cable. Conducting an electrical load assessment is the first step to developing a successful plan for portable power. This assessment must:
- evaluate the total load
- separate the load into critical elements — those requiring redundant or portable power — and non-critical elements
- determine the electrical loads to account for
- provide a basis for selecting the right type, size and quantity of portable power units, along with the proper accessories to tailor them to each use.
To ensure that all loads are considered, the person doing the survey should start at the service entrance and, using system schematics, determine the incoming supply and its distribution through the facility.
During the assessment, note any deviations between the schematics and the in-place system on the print to list modifications or additions to the original system not previously recorded. The updated print serves as the basis for a list of all loads in watts. If electrical load information for an equipment item is not on the print, it is available on the equipment’s nameplate or by using a tong tester to measure amps on the load itself under full load.
The sum of AC and DC wattages is the total load. The sum of all the critical-load wattages is the total portable generator load that a facility needs.
Portable Power Options
A multiple-use example might serve to illustrate the way managers can use portable power options to satisfy needs related to emergencies, security and remote locations.
In this application, a facility has a number of emergency exits, sump pumps in several basements, perimeter lighting, and tamper alarms in remote vehicle storage, warehouses and pump stations.
An emergency portable generator set, or genset, can work well during either a power loss or when motors or generators fail due to mechanical problems. In each application, the manager must consider four general requirements:
- What is the wattage load?
- What type of energy source will drive the genset?
- What optional features are required?
- What is the best balance between cost and robust design?
Emergency power for a lighting panel requires enough wattage for the sum of all wattages that comprise the emergency lighting circuit. Self-contained batteries can supply some emergency exit lights with the balance of the load that is supplied by the emergency lighting genset, which supplies power to a separate feeder circuit on the lighting panel.
Energy sources might be a combination of batteries, fuel cells, propane, natural gas, gasoline, or solar, wind or hydro power, depending on fuel availability and whether the application is inside or outside.
The features of portable generators that managers must consider include whether the unit: has a two-wheel or four-wheel mount; has a skid or pad mount; is towable; and features a manual, either electric- or pull-start, or an automatic transfer switch. Managers also should consider temperature, as sub-zero applications require pan and fuel-line heaters, and altitude, as low oxygen levels affect engine efficiency.
Managers also need to consider operator safety features, including ground fault circuit interruption to meet OSHA/NEC regulations, low-oil alarms, and voltage and service-hour meters to monitor power quality.
The basement sump pumps in the example above are near AC service and normally run on electrical service. Adding battery power at each pump location will provide plenty of backup. The batteries are charged by the AC service when not in use. Indicator lights are green when the batteries are charged and red when they are due for replacement.
Automatic switching enables the battery circuit to energize when AC power is interrupted and de-energize when AC power comes on, preventing flooding when power is interrupted during a storm.
Separate battery-operated uninterruptible power supplies serve computer equipment locations, which normally operate on the AC supply but cannot be interrupted by power failure, due to the possibility of interrupting backups during the night or losing valuable data during daily operations.
Again, these systems have automatic switching and can convert to battery power within a few cycles so an orderly shutdown can occur or the system can operate on battery for several hours until AC power resumes.
Still other needs occur when the power load is remote from the distribution system. Examples include video cameras, lighting and tamper alarm at remote perimeter areas, or at outbuildings.
Solar panels might offer managers options for these locations. In solar systems, arrays of silicon cells convert sunlight into electricity and are connected in one or more photovoltaic (PV) panels. A wiring grid connects the panels to a charge controller. The charge controller prevents solar energy from overcharging the battery bank and prevents stored electricity from dissipating through the solar cells when they are producing energy.
Deep-cycle batteries wired in series, parallel or both, accept the solar panel charge. An inverter changes the stored DC power into AC power, if needed, to power the load. Wiring is sized to carry the maximum current load while maintaining the voltage within acceptable limits. The loads — lights, pumps, tamper alarms, and motion sensors — complete the self-sustained system.
Two factors determine the size of a solar system: daily solar radiation and daily load consumption in ampere-hours of the stored battery charge needed.
Working with Providers
Managers developing plans for renting portable generators will need to give the rental-unit suppliers a complete set of specifications and a description of the application for which the genset will be used. A lighting application, for example, has different needs than the AC motors or the information technology network.
The rental firm also will need to know in advance the wattage, the voltages the genset will drive, brand preferences, delivery method and lead time.
Ensuring Thorough Planning
Managers developing a comprehensive and cost-effective plan for portable power options must consider many additional factors. They include:
- optimum number of separate units
- the amount of copper wire and electric-grade steel to ensure extra power for high-starting loads
- whether low power factor increases load requirements and costs
- heavy-duty circuit diodes to reduce failures
- automatic load balancing to reduce vibration from unbalanced loads
- OSHA/NEC-compliant ground fault circuit interrupters to prevent shock
volt- and run-time meters for monitoring correct voltage and service hours
- overhead valve vs. side-valve design comparison of oil and fuel use and torque delivery
- low harmonic distortion to prevent electrical damage by sine wave distortions greater than 10 percent
- noise levels, because portable power units as quiet as 50 decibels are available
- whether a facility can use a portable generator to reduce costs by interconnecting and slowing the power meter when generating more power than necessary.
It pays for managers to do their homework — to carefully assess facility needs and understand the technical requirements and beneficial uses of portable power — in order to to ensure that planning for portable power is both comprehensive and cost-effective.