Reliability has taken on a new prominence in recent years. An aging and increasingly complex transmission-and-distribution infrastructure has added reliability risks to equipment failure and random system collapse. Users must deal with the operational impacts, and many are reevaluating the reliability of the systems as well as potential replacements for them, including distributed energy. This article discusses how a typical reliability study can unfold to support a business case for distributed energy.

Reliability Studies
A complete power-reliability study should examine the power system both inside and outside a facility. Outside, reliability can be determined by utility "rolling blackouts," outages caused by power shortages rather than equipment failures; equipment redundancy; feeder routing; automatic sectionalizing schemes; utility testing; preventative maintenance practices; and tree-trimming schedules. Reliability of the internal power system might be affected by physical protection of feeders, power-supply redundancy, equipment testing schedules, and preventative maintenance practices.

Required Levels of Reliability
The total money and effort needed to improve power reliability depends on the possible consequences of power outages. The highest level of power reliability is required for "life-safety systems," such as emergency lighting or ventilation, which must operate properly to prevent the loss of human life. The National Fire Protection Association's codes and standards describe life-safety systems and the design criteria and maintenance practices required to provide those systems with reliable power.

The second-highest level of power reliability is required for systems that prevent damage to plant infrastructure (e.g., sump pumps at a wastewater treatment plant), allow monitoring of other systems (e.g., supervisory control and data acquisition, or SCADA, systems), or prevent the loss of vital data during power failures (e.g., at bank data centers) or whose failure to operate could significantly impact public health.

The third-highest level of power reliability is required for processes that would cause sizeable financial losses if power outages occurred. Power outages cause loss of quality control in batch processes—found at microelectronic component manufacturing, food processing, chemical processing, and oil refining facilities—and force owners to discard entire batches. In addition, power losses to processes that operate 24 hours per day, seven days per week—with no openings to recover lost production time—can lead to cancelled orders.

The lowest level of power reliability is adequate for equipment and processes for which operation is not time-critical. Operating this type of equipment, such as a cooling system with a large, cool storage tank, can be deferred to off-peak times; switched to an alternate source, such as an engine generator; or switched to an alternate fuel, such as an electric heating system with fuel-oil backup.

Redundancy
Redundant power supplies do not always improve reliability. If two redundant feeders supply power to an industrial facility but originate at the same utility substation and are carried on the same set of power poles, reliability will be lower than if they originate at separate substations and travel to the site on different sets of power poles. The problem with redundant feeders carried on the same set of poles is that a single-point failure (e.g., a weather-related event, pole fire, or traffic accident) could cause simultaneous outages on both sources.

Assessing Existing Power Reliability
Assessing power reliability at an existing facility requires examination of three or four years of feeder-outage history provided by the serving utility. Causes of outages must be identified, durations of outages must be quantified, and equipment testing schedules and preventive maintenance records must be examined. Written maintenance and testing records provide useful data for determining how reliably power systems can be expected to perform.

Improving Utility Power-System Reliability
Regular tree-trimming will improve the reliability of overhead feeders. Routing overhead feeders away from roads and highways can reduce damage from motor vehicles. Underground feeders might provide higher levels of power reliability in areas with high winds, frequent lightning strikes, heavy forestation, and ice storms.

Most utilities schedule regular testing of power-delivery equipment to ensure a high level of reliability. Testing includes analysis of transformer insulating oil for the presence of water or gases that would indicate pending failures. Power circuit breakers and protective relays should be tested on a regular basis for proper operation and correct calibration. Substation batteries must be tested for proper voltage and capacity. Some utilities now perform nondestructive testing on underground feeder cables to anticipate pending cable failures and proactively replace the cables before faults cause unscheduled outages.

Recently in California, utilities have resorted to rolling blackouts. Previous shortages were very rare or very localized but lately have increased as a result of utility deregulation. Inspired by media coverage of rolling blackouts and dramatic spikes in natural-gas prices, other regions with tight power supplies are evaluating capacity requirements and scrambling to install new generation-energy systems to prevent similar scenarios.

Improving Industrial-Facility Power-System Reliability
Inside an industrial facility, reliability can be improved by protecting electrical equipment from physical damage—for example, by placing underground feeders in concrete-encased duct banks. Redundant feeders and automatic transfer schemes often are used to improve reliability. To minimize the chances of single-point failures causing simultaneous outages, redundant feeders should be routed along different paths within the plant but should not pass through common manholes.

In addition to the regular maintenance performed by utilities, owners need to plan for regular inspection, testing, and maintenance of their breakers, protective relays, and automatic transfer schemes to ensure power reliability. For example, automatic transfer switches should be tested periodically to verify that mechanisms operate freely.

Looking to Distributed Energy
At this point, the owner should decide if the reliability risk has been mitigated sufficiently through the discussed delivery and protection assessments and the potential improvements. This step allows a strong business case to be developed in support of distributed energy.

Some owners opt to improve power reliability by installing standby generation, uninterruptible power supplies (UPS), flywheels, or fuel cells. The chosen system depends on the length of outage the owner can tolerate. Diesel engine generators require about 10 seconds to start, reach rated speed, develop rated voltage, and begin powering loads. Where even momentary outages are unacceptable, owners must install UPS or flywheels. Fuel cells have become commercially available and have performed as highly reliable power sources. A bank data center in Omaha, NE, recently installed fuel cells as its primary power source to maximize reliability and minimize data loss.

Peak Shaving With Onsite Generators
With proper interconnection to an industrial-facility power system, onsite generators can be used for both standby and peak shaving purposes. Taking advantage of peak shaving rates helps justify the capital, operating, and maintenance costs of onsite generators. Owners should be aware, however, that standby and peak shaving requirements are often quite different. Loads requiring emergency power are usually very small and might not be running continuously, whereas ideal loads transferring generator power for peak shaving will be relatively large and will operate continuously.

To determine whether existing standby generators can be used for peak shaving purposes, follow these guidelines:

1. Determine the generator rating (kW capacity).
2. Determine the condition of the prime mover from maintenance records and by completing a physical inspection (i.e., determine whether operating the generator for peak shaving purposes will require an overhaul of the prime mover).
3. Develop a spreadsheet to inventory all existing connected loads.
4. Sort through the connected-loads list to determine which loads are no longer in service.
5. Measure the actual load(s) on the generator(s) to determine whether capacity is available to power additional loads.
6. Review utility peak shaving rates to determine peak shaving benefits.
7. Calculate peak shaving costs from generator fuel-consumption data and maintenance requirements.

If an existing standby generator appears to have capacity available beyond the current requirements for emergency loads, loads should be added with caution to avoid overloading. To ensure that power is always available for critical loads, either standby generators must be oversized or load-shedding equipment must be installed. If load-shedding equipment is installed, loads must be grouped by priority. During utility power failures, the load-shedding equipment always connects the highest-priority loads to the generator first, the second-highest-priority loads next, and so on. If the load-shedding equipment detects loads with kilowatts in excess of the generator rating or decreasing frequency (also indicating generator overload), it quickly sheds loads (starting with the lowest priority) to ensure that reliable power reaches critical loads.

Conclusion
Every facility operator should understand the level of power reliability required for operations. Power-supply reliability is no longer affected only by equipment failure. As is the case in California, power reliability also can be reduced by power shortages. If a facility experiences a utility outage, do the critical processes have backup power to avoid a catastrophic event? Are the existing onsite generators being used to full capacity? Is it possible to maximize onsite generation to its full capability and satisfy the required level of power reliability? From the experiences in California and the Northeast, it is evident that power outages are no longer caused only by equipment failure and that measures need to be taken to assess power reliability at industrial facilities. Consideration of these questions can provide a solid business case for distributed energy as the best option to quickly resolve reliability risks.

MATT CLARK, P.E., BOB THAYER, P.E., and JOHN SPANHAKE, members of HDR Engineering's Power & Energy Group, work nationally in the power industry, providing services in the areas of energy studies, generation system design, transmission and distribution, and the environment.

DE - Jan/Feb 2004