Paralleling generators in prime power applications
Paralleling generators can increase efficiency, improve reliability, and save money.
Power: it's a necessity that keeps the lights on and equipment running in every building and on every job site. But in many cases, it's the true lifeblood of an operation, and an outage can be downright costly in critical operations, such as asphalt plants, concrete plants, or mines. In scenarios like these, it's especially important to have predictable and reliable power. And while specifying the right size generator is a good starting point, one might find there's greater dependability, cost savings, and efficiency in numbers.
Since the advent of portable generators, it has been common practice in many prime power applications to choose a single generator with enough power output to operate all of the tools and equipment needed. While this certainly serves the immediate need, operating several smaller generators in parallel might be a more efficient and versatile option (see Figure 1).
The synchronizing process-which at one time involved a vast network of wires, manually adjusting the rpm, and frequently monitoring the loads-is now fairly simple with most portable generators. In fact, some generators feature plug-and-play technology that allows the control to be connected with just one cable. After the connection is made, the user connects the power cables to a common bus, sets a few parameters in the controller, then hits the start button, and the process takes care of itself.
Plug-and-play models feature a special controller that creates a power management system to oversee all of the units. They are often equipped with motorized breakers that allow the units to automatically start and stop as loads fluctuate. Users can parallel as many as 16 generators with some manufacturers' models, and synchronize them much more quickly than they could with older models.
Paralleling generators for use in prime power applications, such as asphalt and concrete batch plants or construction sites, is common practice in many other countries. But is just starting to be used in the U.S. (see Figure 2). Many contractors simply have been unaware of the better power options it presents. As in many common standby applications, paralleling generators on these sites can come with many benefits.
Perhaps the biggest advantage of paralleling two or more single megawatt generators is peace of mind. The operation won't be completely shut down if one unit malfunctions. A complete loss of power can be disastrous. For example, if there is hot asphalt in a hopper and the lone generator on-site goes down, operators can be faced with a tough situation. As the asphalt cools, it begins to harden and can damage equipment. It's also extremely difficult and time-consuming to remove. Boring and drilling applications also are good examples. If a stand-alone generator goes down during drilling, the tool can become stuck and it can be challenging and time-consuming to remove it. On the other hand, if these job sites are equipped with several smaller generators in parallel, operators can simply take one out of service and use the remaining generators with little to no impact on the operation.
Reduced fuel consumption is another significant benefit. For example, if a work site needs 5 MW of power to operate numerous pieces of equipment, perhaps five 1-MW generators operated in parallel can provide the same amount of peak power as two 2.5-MW generators, but can automatically power down as usage decreases and power up as it increases, which reduces fuel consumption. Also, as the load falls below certain levels on some larger generators, so does the fuel efficiency. For example, if the load requirement falls below 750 kW, a 2.5-MW unit would operate at less than 30% load, which is below optimal operating range. But if you use three paralleled 1-MW generators, two will automatically power down and the remaining 1-MW genset will operate in a more efficient range, which can save a considerable amount of fuel.
Many models allow users to customize the power management system to a large range of parameters. For example, if a job site has five paralleled 1-MW generators that will run near full power during periods of the day, but at other times the load is just 0.5 MW, users can program the power management system to operate with the minimum power, plus one additional unit. Adding the 1-MW ensures they'll have continuous, stable power so productivity won't be affected in case of power surges. Operators also can choose to have all of the generators run simultaneously to share the load. Several smaller generators also give site managers the flexibility to use them separately in different areas on the job site or on jobs at other locations. They also can be significantly less costly to purchase or rent than their larger counterparts.
Frequency of features
When choosing generators to be paralleled for prime power applications, follow the same considerations as those used in standby applications: altitude, ambient temperature, amperage, voltage, motor starting method, and surge capacity. However, keep in mind that the power ratings for generators in prime power applications are generally 10% lower than they are in standby applications. It's also a good idea to choose units that are easy to use. Plug-and-play designs are the most user-friendly and are a great alternative to more complicated generators, which must be set up by specialized personnel and can require several cables for connecting units and external switchgear. This all adds up to more costs, hassle, and time.
Generators with tier 2 flex engines are significantly more economical to purchase than those with tier 4 engines. But keep in mind that tier 4 models are more environmentally friendly and the way of the future. If cost is a concern, refurbished machines might be a good alternative to new units, but consider the local emission requirements.
Many manufactures also offer options for use in high altitudes or extreme weather conditions. Some cold-weather features include alternator and coolant heaters, and synthetic 0W-40 engine crankcase oil, which maintains the appropriate viscosity in colder temperatures.
Other items to consider are maintenance-free batteries, such as absorbed glass mat (AGM) batteries. Because they don't require users to replenish electrolyte or use hydrometers to perform specific-gravity checks on the battery's charge, they can save users time and hassle. In addition, they are completely sealed so there is no risk of spills or leaks that can damage equipment and burn skin.
With many pieces of equipment operating simultaneously on job sites, noise pollution can be another concern. Look for units that have factory designed and installed sound attenuation. Be sure the equipment has been tested for the rated sound level.
In addition, rather than placing the generator in a standard ISO shipping container, some manufacturers build them into 20-ft enclosures they design and build from the ground up. This allows them to make the unit structurally sound to accommodate features such as larger openings for maintenance access as well as control panels on the outside of the unit-features that would otherwise weaken the integrity of ISO containers. Instead, they are safer and easier to use while maintaining the same footprint and form factor as ISO containers.
An adjustable exterior control panel gives operators the flexibility to move the control panel up if it's placed at ground level or down if it's placed on a trailer, which will ensure it stays at an ergonomic height for operators. It also eliminates the time and hassle associated with getting on and off a trailer and opening doors to make adjustments, which increases the risk of injury from moving parts and hearing loss from the equipment.
Open for maintenance
Manufacturing their own ISO-spec enclosure also allows manufacturers to incorporate features that make maintenance easier and faster. For example, some generators have all fluids plumbed to the edge of the unit. This eliminates the hassle and time associated with trying to place drain pans in the unit where space is very confined. Instead, users can simply go to one convenient location and can even use hoses to divert the fluids where needed.
Some generators also are equipped with large access doors that swing completely open to give operators more room to work. They may feature one-side accessibility to every maintenance point so users don't need to spend time moving from one side to the other, which helps maintenance go even faster. And to make larger repairs easier, specify models that can be completely removed from the enclosure for full access to maintenance points.
Also consider maintenance intervals. Some manufacturers require preventive maintenance every 250 hr. But in recent years, others have extended that to 500 hr. This is possible because of several features, such as oversized fuel and oil filters, and larger oil pans. Fewer maintenance intervals can have a significant impact on maintenance costs and total cost of ownership, particularly on sites with heavy use. And if site managers are using the generator as a stand-alone unit, this also reduces downtime.
From site to structure
Power is essential to every building and job site. But in critical prime power applications, it can make or break the productivity of an operation. While a single large generator can provide clean, stable power, several smaller paralleled generators add up to even more-from increased efficiency to greater reliability to improved cost savings. Now that's smart power.
Jim Siffring is the product manager of the generator product line for Atlas Copco Construction Equipment and is based at the company's U.S. headquarters in Commerce City, Colo.