When you select the appropriate components, engine gen-sets can provide seamless ride-through upon loss of primary power.
By design, engine gen-sets provide full and reliable backup power. If properly set up, they provide a seamless ride-through upon loss of primary power. For this to happen, you must account for several important factors that include deciding which loads the gen-set will support, properly sizing it to those loads, and selecting the appropriate switching equipment. Start this process by identifying for what you will use the gen-set. What is your goal? Are you trying to provide emergency backup power? Do you want a standby source, or are you trying to lower costs through peak shaving or base loading?
Not all situations dictate picking up the entire load. If you are installing a gen-set for emergency loads, segregate the critical elements of the load onto a dedicated emergency bus. Then, power that bus with the gen-set. How you define an emergency load depends on your situation, but you would typically include egress path lighting. You'd also include certain mechanical systems for safety shutdown, communications, security, and fire control. Special cases include other systems. For example, a hospital backs up life-support systems and operating rooms. A data center, on the other hand, is likely to back up everything except designated nonessential loads like outside lighting.
Regardless of which loads you back up, certain principles apply to all gen-sets. Typically, the power output from a gen-set has fewer external influences than grid-supplied power. Thus, when properly applied, these sets produce cleaner waveforms than does the grid. To improve output quality, tune engine gen-sets to local system voltage requirements or demands.
The key to ensuring power quality with engine gen-sets is properly matching the equipment to the load. Once you've decided what your gen-set's mission is and what loads it will support, you are ready to perform a load analysis.
Performing a load analysis
Base your analysis on the requirements of the equipment making up the load. Give careful consideration to adequately addressing the problems caused by your particular concentration and mix of equipment. Loads that present different kinds of power quality problems include electric motors, lighting ballasts, computer power supplies, motor drives, UPSs, and power-factor correction capacitors (photo).
For example, electric motors produce a lagging power factor — which is an increased demand for VARs. Calculate what it takes to start and run your motors. When doing so, consider whether you are starting them sequentially or simultaneously. Lighting ballasts (fluorescent, metal-halide, halogen, and even incandescent) have inductive components you must account for in your evaluation.
Computers, variable-speed drives, and UPSs all have nonlinear power supplies that introduce harmonics into the power system. These harmonics can create problems with the supply. If the power source is a gen-set, you'll see a significant rise in this kind of distortion — as compared to what you'd see if your supply were the utility grid. Why? The voltage regulator for the gen-set cannot accurately read voltage when total harmonic distortion exceeds 20%. Thus, high levels of harmonics can lead to an unstable voltage output from the gen-set. In addition, the input system on a UPS or variable-speed drive requires a 60 Hz sine-wave input and ceases to function properly when the input deviates too far from this.
Most UPSs and motor drives have input filters either as standard equipment or as an option. However, the filter option is expensive and may not be part of a typical bid-spec proposal. Older UPSs may not have filters. The resulting distortion can make the gen-set nonfunctional.
When sizing to a nonlinear load, the gen-set should have a kW rating from 1.5 to 4 times the nonlinear-load kW rating, depending on the degree of filtering, type of load, other voltage-sensitive loads on the bus, and load range of the gen-set. Smaller gen-sets (less than 200kW) are more susceptible to power quality problems because their higher internal impedance tends to amplify, rather than absorb, harmonics. Depending on your load, this may not be a problem. For example, variable-speed drives and computer power supplies can operate with a higher total distortion than UPSs.
Power-factor correction capacitors, especially those with highly inductive loads, require special consideration. Because we normally size gen-sets by kW, any loads that have significant VAR requirements are crucial elements in properly sizing the gen-set.
The load analysis combines all of these factors into a composite that allows you to address the overall needs of the system. Try to match the proper generating equipment to the kVA demands of your facility and equipment. With this composite picture of your load, you can design in preemptive corrective action. For example, you can upsize the alternator beyond minimum specified requirements to reduce certain detriments to power quality, such as voltage dips and harmonics. You can also reduce the effects of incremental loading in large blocks.
Engineering the system
Your design job doesn't end when you've sized the power output to the anticipated load. You must also select the design elements that will assure power quality whenever the gen-sets run, transfer loads, or share loads. A well-engineered system integrates the prime mover and alternator to produce the optimal power output, while minimizing system harmonics. Base your design on industry standards.
The design elements of the prime mover (such as fuel/air delivery system components, intake and exhaust manifolds, and cooling systems) are key considerations. Your choice of fuel will depend on factors like site space and location, local fuel availability, sound-level considerations, exhaust emission constraints, and the anticipated role of the gen-set.
Ancillary equipment can also affect power quality. Specifying particular characteristics for governors, exciters, and voltage regulators will help you ensure a high level of power quality, because these are all integral parts of any high-quality power system.
Select automatic transfer switches and switchgear that are appropriate to the application. Pay extra attention to their synchronization characteristics. Of course, you must size them for the proper amperage and voltage. Closed transition switchgear should include overcurrent protection for both utility and generator breakers, fully automatic synchronizing (frequency, phase-angle, and voltage), and utility-grade protective relaying (for soft-load and peak-shave/base-load systems).
Two types of governor systems are available. Mechanical governors have been the reliable mainstays of the industry, but synchronous electronic governors are gaining popularity for providing tighter frequency control and improving overall performance.
Voltage regulators are moving from analog designs to digital ones. The digital designs offer greater flexibility, and you can more readily integrate them into the management and control of a power system. Permanent magnet brushless excitation, electronic governing, and electronic voltage regulation help you maintain power quality. A system with these features will be more stable and less prone to transient loading than less sophisticated arrangements.
When you select the components properly, engine gen-sets are reliable sources of prime and standby power. By properly sizing and equipping them for your particular load, you can assure a high level of power quality.
Empirical evidence from the field shows that the relationship between generators and power quality is one you cannot ignore. Proper selection, design, and installation can prevent interaction problems while assuring continuous uptime.
Carr is manager of marketing communications and Sitter is an applications engineer at Generac Power Systems, Waukesha, Wis.