EPO: Emergency Power ON

12/01/2008


Over the last several years, continuity of electrical power has become increasingly important not only to individual utility customers, but to the national and world economy. Meanwhile, confidence has eroded in our overstressed and aging utility grid given such events as the Chicago Loop outage of 2000, California’s rolling blackouts of 2001, the Northeast blackout of 2003, and the European outages of 2003 and 2006. To ensure that power is available when needed, many businesses are taking control by installing on-site generation. Here we will examine potential solutions, the nuances of generator system design, and installation and O&M costs.

RECOMMENDED SOLUTIONS

An uninterruptible power supply (UPS) is typically required for critical computer loads, for continuous process manufacturing, or anywhere else where momentary interruptions cannot be endured. A UPS provides ride-through of short-term outages until a generator can come online. Valve-regulated lead-acid (VRLA) batteries, wet-cell lead-acid batteries, and rotary flywheels are among the typical options available for UPS energy storage.

Diesel reciprocating generators are typically selected for standby applications where utility outages are expected to last (and the generator expected to run) less than 100 hours per year. Diesel gensets are attractive for standby applications due to their low initial cost, low maintenance cost, and ease of fuel storage. The low expected annual run times associated with standby applications make the relatively high energy cost associated with diesel fuel a nonissue. In addition, the U.S. Environmental Protection Agency (EPA) emissions permitting is usually rather straightforward.

Natural gas reciprocating generators are better suited for small prime or continuous generation applications such as peak shaving or cogeneration (see discussion below). A natural gas generator requires approximately twice the engine displacement of a diesel engine of the same rating, so installed costs are higher than for diesel. In addition, on-site propane storage may be required as a backup fuel source in the event of a failure of the natural gas utility. However, the lower natural-gas fuel costs and lower emissions make up for the higher installation costs and propane storage issues if the generator is expected to operate more than 2,000 hours per year.

Turbine generators, using steam, natural gas, or oil as a source, can be attractive for large prime or continuous cogeneration applications. Such installations have a relatively high total installed cost, but are reliable, compact, and quiet, and the emissions produced by a cogeneration installation are relatively low. Turbines can take from 30 seconds to several minutes to come up to speed, so they are generally unsuitable for emergency (life-safety) applications.

Fuel cells hold promise for the future due to their low emissions; however, due to their very high installation and operating costs, they have not yet displaced existing reciprocating or turbine technologies. Costs are expected to drop as the technology matures.

Cogeneration—also known as combined cycle; cogen; or combined cooling, heating, and power—should be considered for any generator installation, as it is a means of leveraging a large capital expense (the generator) by using it to reduce operating expenses. A generator on its own may have a thermal efficiency of only 20% to 35%—incorporate it into a cogeneration installation, and the system may achieve a thermal efficiency of 80% or greater. A 30% efficient generator with 70% waste he at also can be viewed as a 70% efficient boiler with free electricity. Moreover, a prime generation installation with N+2 generator redundancy can approach the reliability of a traditional generator-UPS system.

Peak shaving is another way to reduce operating costs. Some utilities offer an “interruptible rate,” whereby the customer is requested to start its generator to unload the utility’s distribution system. In addition, some utilities impose higher peak-demand or time-of-use (TOU) rates, making it financially attractive for customers to transfer over to generators on a daily basis during specific time blocks.

CODES AND STANDARDS

During the design phase, you will need to consider several codes and standards that apply to generator installations:

TIA-942, Telecommunications Infrastructure Standard for Data Centers, can be a useful guide for any generator or UPS installation, and expands greatly upon the hardening and redundancy considerations outlined above.

The NFPA 70: National Electric Code (NEC) includes requirements applicable to any generator or UPS installation. Specific requirements to be reviewed include those under:

• Article 445, Generators

• Article 517, Health Care Facilities

• Article 700, Emergency Systems

• Article 701, Legally Required Standby Systems

• Article 702, Optional Standby Systems

• Article 705, Interconnected Electrical Production Sources.

The NEC makes a clear distinction between emergency systems, legally required standby systems, and optional standby systems. Emergency systems are defined in Article 700 as life-safety systems, and are the highest priority loads per code. Legally required systems, covered under Article 701, are government-mandated (but nonemergency) loads such as high-rise elevators, fire pumps, hospital communications systems, ventilation and smoke removal systems, and sewage disposal systems that, if interrupted, could create hazards or hamper rescue and firefighting operations. Article 702 covers optional standby loads, such as critical business loads and other owner-selected loads that by code are a lower priority than emergency or legally required loads.

NEC 700.6 requires emergency-system automatic-transfer equipment to serve only emergency loads—the generator can be shared with the legally required and optional standby systems, but the automatic-transfer switch (ATS) cannot. NEC 700.27 includes a new requirement for all overcurrent protective devices in the emergency system to be selectively coordinated, which can be difficult to achieve.

Additional guidance is available from NFPA , which publishes several other relevant codes and standards, including:

• NFPA 20: Stationary Pumps for Fire Protection

• NFPA 99: Health Care Facilities

• NFPA 101: Life Safety Code

• NFPA 110: Emergency and Standby Power Systems

• NFPA 1600: Disaster/Emergency Management and Business Continuity Programs.

NFPA 110 and NFPA 1600 are applicable to any generator installation and contain requirements beyond those listed in the NEC; consult the other codes and standards as applicable on a specific project basis.

Another valuable resource to review during any generator installation project is IEEE Standard 446, Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications (Orange Book). Chapter 3 provides a convenient tabulation of general criteria to consider for various business types during the preliminary design phase, and Chapter 4 presents typical system topologies to achieve specific goals. Chapter 7 reviews the grounding of separately derived systems and implies that both cost and complexity often can be reduced by using 3-wire (rather than 4-wire) systems.

ADDITIONAL DESIGN CONSIDERATIONS

Federal, state, and local laws. Beyond codes and standards, businesses must comply with federal, state, and local laws. Noise attenuation may be required for conformance to local noise ordinances. Federal and state EPA requirements for both pollution emissions and fuel storage requirements must be followed. In addition, EPA Spill Prevention, Control, and Countermeasure (SPCC) requirements may dictate that curbing or other fuel containment measures are provided at diesel refueling transfer stations.

Motor-starting requirements. The generator must have a kW rating greater than the critical load it is expected to support. What is less obvious is that the generator motor-starting kVA rating must support the connected motor-starting load.

Motors typically draw six to 10 times their full-load current rating during starting, and the generator’s alternator must be able to provide this additional current. Voltage drop during starting must similarly be considered, noting that NEC and NFPA 20 permit a maximum allowable voltage drop of 15% to fire pumps during starting. Other motors can sustain a greater voltage drop during starting, although motor contactors and VFDs may drop out at 15% to 30% undervoltage.

Staggered motor-starting using time-delay relays or building management system control can reduce both the motor-starting demands on the generator and the experienced voltage dip during starting. Motor-starting requirements can be calculated manually or by using spreadsheets, or, for larger systems, by using software available from most of the generator vendors.

Site selection. Factors to evaluate while selecting a generator site include flooding, vandalism, wind and weather, earthquakes, and chemical spills. It may be possible to select a site with reduced exposure to such threats, or to harden the site to mitigate these risks. Locate generators at an elevation above the 100-year flood plain—or even above the 500-year flood plain. Consider providing seismically rated generator enclosures capable of withstanding 150 mph or greater wind loads. Provide redundant entrances to the site, so fuel delivery will be possible in the event of a closed road or driveway leading to the site.

Redundancy requirements. For extremely critical operations, redundant sites may be warranted. Redundancy in the generator and UPS systems will ensure availability during maintenance or in the event of equipment failure. Consider redundant batteries and starters, especially for single-generator installations. Redundant fuel systems can help protect against water or microbial contamination.

Requirements for open, in-phase, or closed transition, or for extended utility paralleling. Per the IEEE Orange Book, paragraph 4.3.2, “ ... Short-circuit duty will be increased during closed transition.” Given this, make sure you closely evaluate any requirement for open, in-phase, or closed transition, or extended utility paralleling, during the concept development phase, as it will have a significant impact on relaying requirements, available fault current, arc-flash hazard, and overall project cost. Also consider the method of transfer. There are proponents of both breaker-based transfer schemes and ATSs. Bypass should be provided for either scheme on critical systems to permit for maintenance and to facilitate restoration of power in the event of equipment failure. It’s worth repeating that transfer equipment serving emergency systems (i.e., life-safety systems) are required by NEC 700.6 to be dedicated to those emergency loads and must be isolated from legally required or optional standby loads.

Generator exercise under load. Per NFPA 110, light loading creates a condition known as “wet stacking,” whereby low cylinder temperatures permit unburned fuel to enter the valve train and exhaust system. To prevent this, generators should be exercised with a minimum load of 30% of the generator kW nameplate rating to maintain cylinder temperatures. While it certainly may be possible to achieve this by using the building load, many operators do not want to perform unnecessary critical load switching. Therefore, portable or permanent load banks are commonly used. For single-generator installations, it can be convenient and inexpensive to incorporate a load bank into the radiator.

The generator’s mechanical support systems. Fuel tanks need to be sized to provide the required run time. Include provisions for fuel sampling prior to refilling into redundant fuel storage tanks where possible to avoid contamination of the site fuel storage. Consider portable or permanent fuel polishing systems for diesel fuel systems to prevent microbial contamination. Heated, insulated fuel lines may be necessary in cold climates to prevent fuel from gelling. Air intake design should ensure that snow is not sucked into the generator room or enclosure, and that snow does not block the cooling air exhaust. Consider specifying 120 F radiators (rather than the 104 F standard offering of many vendors), especially given the temperature rise above ambient that can occur inside a generator enclosure. Provide heaters for standby generator coolant jackets, alternators, and batteries to prevent condensation and ensure that the generator will start when required.

INSTALLATION AND OPERATING COSTS

Approximate installation and O&M costs (exclusive of energy costs) are summarized in Table 1.

Energy costs for reciprocating generators may be estimated as follows:

• Diesel: Typical generator consumption of 0.07 gal/kW and an off-road diesel fuel cost of approximately $4.00/gal imply an energy cost of $0.28/kWh.

• Natural gas: 13 standard ft3/kWh and a gas cost of $0.012/standard ft3 imply an energy cost of $0.16/kWh.

Turbines are typically incorporated into cogeneration systems; it is difficult to state a generalized energy cost for them, as the thermal efficiency of the system is highly dependent upon its specific configuration. Such systems often can be designed to provide combined heating and power at a very attractive energy cost when compared to both reciprocating engines and utility rates.

CONCLUSIONS

The ever-increasing need for power reliability will continue to drive the installation of local generation. In addition to an array of legal requirements, reliability criteria must be addressed during the design phase to ensure that the generator plant will perform when needed. Keeping the guidelines discussed here in mind during a project’s conceptual and design phases will go far towards ensuring that power is available when needed.

Installed Cost/kW

O&M Cost/kW

Source: Paul Bearn, KlingStubbins project and miscellaneous Internet data

Diesel reciprocating generator

$700/1,500

$0.007/0.020

Natural gas reciprocating generator

$900/2,000

$0.010/0.030

Turbine/microturbine

$800/2,200

$0.003/0.008

Fuel cell

$1,900/3,500

$0.010/0.040


Author Information

Bearn, an electrical services engineer at KlingStubbins, Philadelphia, has extensive experience in data centers, process facilities, laboratories, and commercial installations. Recent mission-critical facility projects include JPMorgan Chase, the City of New York, Roche, Pfizer, and George Mason University.


Power outages at what cost?

According to survey findings presented in an August 2001

The three sectors examined were the digital economy, which includes telecommunications and data centers; continuous process manufacturing; and fabrication and essential services, which includes all other manufacturing, as well as transportation and nonelectrical utilities.

Extrapolated to the entire U.S. economy, the EPRI report suggested the U.S. economy was losing between $104 billion and $164 billion per year to outages and another $15 billion to $24 billion to power quality issues—and these figures did not include the cost of mitigation efforts such as generator and UPS installations.

In regard to frequency of outages, responses varied widely. The largest percentage (24%) reported three per year, although 19% experienced none, and 11% reported seven or more. As for duration, approximately half of the outages were reported to be less than three minutes long, although 23% lasted five minutes to less than an hour, and 15% lasted between one and four hours.



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