Maximizing MCF standby power
Identify equipment, systems design, and maintenance procedures that contribute to dependable standby power systems for critical facilities.
While standby power system reliability is a concern for any facility, it is especially important for mission critical applications such as hospitals, data centers, telecommunications, government, and municipal water and wastewater treatment facilities. Additionally, numerous organizations rely on standby power systems for business continuity and to reduce exposure to monetary loss resulting from a utility outage.
To maximize reliability, facility managers need to understand and consider the critical factors that go into specifying, installing, and maintaining a standby power system. These factors can be grouped into four categories:
%%POINT%% Genset design and manufacturing quality
%%POINT%% Genset sizing and power system design
%%POINT%% Commissioning and operator training
%%POINT%% Maintenance and periodic testing.
What is “reliability”?
The Institute of Electrical and Electronics Engineers' (IEEE) Reliability Society defines reliability this way:
Reliability is a design engineering discipline, which applies scientific knowledge to assure a product will perform its intended function for the required duration within a given environment. This includes designing in the ability to maintain, test, and support the product throughout its total lifecycle. Reliability is best described as product performance over time.
To a great extent, reliability can be designed into gensets, transfer switches, switchgear, and control systems to increase the likelihood that they function as intended. Of course, the other part of the definition relates to maintenance, testing, and support—all human activities that must be carried out as part of an overall plan to maximize reliability.
For organizations that face life-safety risks or severe financial losses if their standby power system fails, it often is prudent to invest more to attain the highest possible measure of reliability. For example, this often means designing for N+1 redundancy in utility feeds, gensets, and UPS systems as recommended in the Uptime Institute's Tier IV design topology. While this redundant system design approach comes at a higher first-cost, power reliability and availability improve. N+1 redundancy also enables periodic equipment maintenance to be carried out without affecting the availability of the standby power system.
Actual measured availability of power systems in mission critical data center applications ranged from 99.67% to more than 99.99% in a 2006 study by the Uptime Institute. At the higher end of the availability were systems with N+1 redundancy. However, the Uptime Institute noted in its study that actual availability was below the vaunted “Five Nines” (99.999%) sought by many mission critical applications.
Each organization has to determine the level of reliability it can afford or, conversely, the amount of risk it can tolerate. And, while spending more money for redundancy to eliminate single points of failure generally increases reliability, it also increases complexity, which at some point may itself threaten reliability. After determining what level of reliability may be acceptable and affordable, an organization must turn to the selection of equipment and suppliers.
Diesel engines are one of the most reliable prime movers ever designed and are the most popular choice for standby power applications. For the highest reliability, look for gensets with engines that have some measure of reserve horsepower capacity at the alternator's nameplate kW rating and a low brake mean effective pressure (BMEP). ISO 8528-5 identifies larger engine displacement and lower BMEP as key factors in a genset's ability to accept load without an undue drop in output voltage and frequency. Engine manufacturers vary in their approach to compliance with ISO 8528-5. Therefore, when one-step load-acceptance is called for in mission critical applications, select a manufacturer that can provide a generator-drive engine with the highest displacement and lowest BMEP relative to nameplate kW rating.
As a major component in the standby power system, the alternator's ability to supply its rated kVA and resist damage from transients is crucial to the reliability of any power system. While most major manufacturers use standard alternator protection schemes, more recent microprocessor-based controls take transient protection to a higher level.
The type of alternator selected depends not only on the size of the electrical load it must supply, but also the types of loads. Factors to consider when specifying alternators for the most reliable power systems include temperature rise, fault tolerance and reactance issues (especially with large, nonlinear loads such as UPS systems), and large motors. In order to limit voltage distortion and potential system instability caused by nonlinear loads, the sub-transient reactance should be limited to 12% or less.
Genset sizing and system design
Appropriately sizing a genset for the specific application has a major impact on power system reliability. Unless all critical loads are properly supplied within the 10 s as required by NFPA 110, the standby power system cannot be considered to be reliable for mission critical applications. Consult the genset manufacturer during the planning stages to be sure the genset will be capable of providing the expected transient load performance.
Design considerations such as N+1 genset redundancy, transfer switch selection, controls, and ambient conditions play an enormous role in maximizing reliability.
N+1 system design
The Uptime Institute has developed a Tier Classification of I %%MDASSML%% IV to describe the design topology of standby power systems used in mission critical data center applications. Tier I topology (see Figure 1) represents a power system design with no redundancy—typical of most commercial standby power installations. In practice, according to the Uptime Institute, this design scheme results in approximately 99.67% availability annually.
Figure 2 shows a Tier IV topology that is recommended for mission critical data center applications with the greatest need for power availability. With N+1 redundancy in utility feeds, standby generators and UPS systems, such a system is expected to deliver annual availability of approximately 99.99%.
A standby system with multiple gensets (either paralleled or segregated by loads) improves reliability because the scheme increases the likelihood that at least most of the gensets will start and run as intended. In a paralleled N+1 system design, typically all gensets start when there is an interruption in utility service. With proper configuration of the switchgear, the “extra” genset will shut down after a time if all the other gensets start and run normally.
The selection of the transfer switch depends on the types of loads on the system. Choosing the right mode of operation (open, closed, or programmed) for the application can go a long way to minimize the stress of load acceptance on the genset.
Controls have been among the fastest-evolving power system components. Both analog systems and microprocessor-based digital systems offer high reliability, and both continue to be manufactured and used, depending on the application. There is a good argument that the monitoring capability of digital systems enhances reliability of the total system by helping to identify issues before they become problems.
The operating environment must be taken into consideration when designing and installing a standby power system. Power systems in coastal regions are likely to need more frequent maintenance and inspection due to salt air. In areas of the earthquake-prone western United States, power systems used for mission critical applications need to be designed and built to meet the seismic standards of the International Building Code (IBC). Similarly, site altitude and temperatures are important factors in system specification and design that may affect genset rating.
Commissioning and operator training
The purpose of commissioning is to verify that all components in the power system are functioning as designed in the event of a power outage. The genset must start and accept load, and all alarm functions need to be tested and verified. If the system does not function as required, then remedial measures need to be taken. Following a commissioning protocol such as ASHRAE 0-2005 and the manufacturer's guidelines will ensure that the commissioning process will be implemented in a coordinated manner.
Proper training of operating personnel is essential for a reliable standby power system since human error or neglect is responsible for the majority of power system failures. Personnel must be familiar with all the power system components, alarm conditions, and O&M procedures. Frequent retraining is also necessary, along with making sure that personnel maintain an operational history of the power system. Consult your genset manufacturer about factory training opportunities available to customers.
Maintenance and testing
Once a power system has been properly designed and commissioned, the most important factor in its long-term reliability is regular maintenance and system exercise. Preventive maintenance of gensets should include the following operations:
%%POINT%% Oil changes
%%POINT%% Cooling system service
%%POINT%% Fuel system service
%%POINT%% Testing starting batteries
%%POINT%% Regular engine exercise under load.
Like regular maintenance, periodic testing is required by code in mission critical applications. It is best to exercise a genset under the actual facility load it will be expected to supply in emergency conditions. When operated with the actual building load, the entire power system is tested, including the automatic transfer switches and switchgear. Testing under actual facility loads may sometimes risk brief power interruptions during transfer, depending on the type of switchgear and control system. For this reason, many facilities perform full facility load testing only a few times a year.
Operating a genset under no-load conditions can adversely affect its long-term reliability if the generator cannot get up to an exhaust temperature of approximately 650 F before the test is over. It is very important that both the engine and generator reach this minimum operating temperature in order to drive off any accumulated moisture that may have condensed in the system. Under heavy load, diesel engines come up to operating temperature in a matter of minutes, whereas, without load, they may not reach operating temperature even after prolonged operation.
Most manufacturers recommend that gensets be exercised periodically, loaded to at least 30% of rated capacity to prevent the accumulation of moisture and fuel in the exhaust system, commonly called wet stacking. If it is not practical to test with the actual facility load, permanent load banks should be considered in the initial power system design, or a maintenance contract should be considered with a service professional who can bring in a portable load bank to properly load the genset during the exercise period. In addition to setting record-keeping requirements and test conditions, NFPA 110 requires weekly inspections and monthly generator operational testing under a minimum 30% load for 30 min or until the engine reaches a stable operating temperature.
At least once a year, all facilities should exercise the power system under the actual facility load and full-emergency conditions to verify that the system will start, run, and accept the rated load. NFPA 110 specifies an annual test under load of 2 hr continuously. Besides verifying that the genset will start and run, periodic exercise has the benefit of heating up diesel fuel and eliminating accumulated condensation in the fuel tank. Since clogged fuel filters and fuel contamination are among the leading causes of power system malfunctions, the cycling and refreshing of fuel is an important step in ensuring overall system reliability.
Recent air quality laws enacted in the South Coast region of California are restricting some gensets to running a maximum of 30 min per month. This practice may affect the long-term reliability of standby power systems by reducing the frequency of power system testing and possibly damaging gensets by not allowing them to reach minimum operating temperature. Where local codes discourage proper genset exercise due to air quality concerns, consult your genset's manufacturer for recommended exercise procedures.
Dauffenbach is factory training/power gen specialist with MTU Onsite Energy. He has a total of 32 years of experience in the genset business, 26 of which were spent in genset engineering.
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