The June 29, 2012, derecho storm, which traveled from the Midwest to the Mid-Atlantic, knocked out power to almost 3.5 million customers, some for up to a week. Many people scurried out to purchase generators. While it’s certainly an inconvenience for homeowners to lose power, it’s not as critical as it is for a company that cannot afford downtime. When a Virginia data center run by Amazon Web Services suffered an outage, it disrupted service to companies including Instagram and Netflix. In the world of backup generation, that’s not supposed to happen.
“The goal behind backup generation is to make sure a facility is fully sustainable and redundant and that, no matter what happens on the electrical grid as far as voltage spikes, sags and or brown-outs, the facility won’t see it,” said Tracy McCulloch, senior project manager, Evergreen Power Systems, Seattle.
Selection of a generator
The role of the electrical contractor (EC) in the world of backup generation starts with the selection of a generator.
“Unless contractors are working on a design/build basis, they’ll select a generator based on specifications provided by an engineer,” said Phil Arnold, project executive, Continental Electrical Construction Co., Chicago. “After we get a specification that will list various generator manufacturers, we’ll get quotes from those companies and try to qualify proposals that meet the specifications. If several proposals meet the specifications, then we determine the lowest cost proposal that we have that meets the specifications.”
If an EC is working on a design/build or design/assist basis, the role can vary. In some cases, the EC, after getting the customer’s requirements, will design the engine’s size and the load. In other cases, the customer will ask that an electrical engineer and the EC work together.
“Whatever the customer’s design criteria are for the generators to take control of the bus—N, N+1 or 2N—that would be what is dictated in the design, which is done by the electrical engineer and design/ build electrical contractor and, in some cases, the company or manufacturer,” McCulloch said.
Designs of backup generation systems vary depending on customer needs. Facilities for the large computer-based companies, such as Amazon; telecommunications companies; and data centers want to see total redundancy—total generator backup all the way down to the server on the raised floor—so the server will never see an outage.
“Today, all new servers have dual power sources or dual cords: an A + B or C + D or AB and CD sides. Regardless of what side it is, every server has to have two sources: an A and a B power source, so if one server loses a power supply, the other power supply will keep it stabilized. You want to have so that the power is from different distribution units all the way to the server,” McCulloch said.
And there are different tiers of backup. With what is called N power, a 2-megawatt (MW) load would have 2 MW of backup generation. That might sound sufficient, but what if the backup generators fail? This would not be acceptable in many situations. N+1 would be the minimum backup in a critical data center environment. With N+1, for a load of 2 MW, there would be an infrastructure for 3 MW. With 2N, there would be an infrastructure for 4 MW, doubling the infrastructure for the load.
“And with N+1 or 2N, everything is redundant,” McCulloch said. An A and a B side would each be loaded to 40 percent. If A fails, B takes control at 80 percent.
“If you’re talking about the high end of the generator system game, no one would ever fully load the system because, in a critical data center, everything is redundant, and redundant means that, if I have 4 MW of load, I need 8 MW of generator to cover it at 2N. And the four generators that are 2 MW each (which equals 8) are covering 4 [MW], so they’re all picking up about 1 MW, each paralleling and sharing load across the bus,” he said, adding that it can be simplified with a rule of thumb: only put up to 80 percent on any infrastructure whether it’s the engines or an uninterruptible power supply (UPS). “Everything is loaded at 40 percent with dual-power source all the way down to the raised floor to the servers.”
Why would you use a UPS? Well, in most large data center commercial applications, such as hospitals and data centers, the generators will also have a UPS to carry the critical load-situation where the power cannot be interrupted—e.g., operating rooms, servers in data centers—on batteries until the backup generators take control of the critical load of the facility. Once the generators take control, the UPS goes back off battery to a normal state, and the generator will cover the entire building. While the generators are sized to take control of the whole building (80 percent), the UPS is sized to only cover the critical load, which varies depending on each client’s needs.
Installation and commissioning
Installation of a commercial generator, another EC responsibility, starts from the ground up. Where will the generator be? Will it be indoors or outdoors? ECs installing generators placed outdoors will have to be aware of and meet the decibel (dB) code requirements of their municipalities in terms of sound attenuation, a requirement that is not imposed on indoor installations. For indoor installation, part of the process entails the floor’s design, which must be constructed using rebar to withstand the weight of a chosen generator. The EC provides the structural engineers with the generator’s weight and how much load they’re putting on any particular slab, and the engineers do the structural calculations to design it to carry the load.
Before a generator is transported to a site, it has to go through a factory commissioning process. If the client requires it, the EC and the client will do witness testing per the specifications and stress test the equipment prior to shipment.
The EC’s job handling the generator system installation on-site often involves collaborating with multiple trades and contractors. After the EC solicits pricing from manufacturers and machinery moving/rigging companies, it selects a crane company. Working under the direction of the riggers, the EC will place a generator onto tank rollers that are powered to move the generator to its destination. The moving company will then shore it up, pull out the rollers and drop it on the deck.
“During the installation, we’ll have one person there to make sure that any conduits that are installed are in the right location and don’t get damaged as the machine is put in place. Then we put in a crew of electricians to do the wiring, which varies depending on the manufacturer and location. Is it on a steel structure or the ground or the roof? There are so many variables, and our crew will work with the mechanical [contractor] either as a blended crew as they do fuel systems, cooling and exhaust, or the two crews will work separately,” Arnold said.
The generator is then tested again.
“After installation, we then go through a second commissioning process at the site doing load-stepping, which involves progressively adding load. For example, for a 2-MW engine, we would add 500 kW, then 1,000 kW, then 1,500 kW and then 2 MW. We’d burn in for eight hours to make sure it can handle the whole load in a sustained outage. The last test is across-the-line full load, meaning we put a full 2 MW on the engine to see how it reacts,” McCulloch said.
ECs also have to be aware of issues related to the fuel sources. In larger facilities, diesel engines usually provide generator power, while gasoline-fueled generators are used in some smaller buildings. One issue of fuel source relates to storage.
“You have facilities that hold as much as 120,000 gallons of No. 2 diesel fuel,” McCulloch said. “In one facility where we did the installation, the fuel was stored underground. When needed, it fed to day tanks that fed 2-MW engines that burn about 150 gallons per engine per hour.”
ECs also need to figure out the burn rate, i.e., how quickly the fuel burns, to determine the amount needed for any given time.
“Burn rate is based on kilowatts, e.g., 100 kW of load per engines uses about 7.5 gallons of fuel, which means that for a 2-MW load to be fully loaded, 266 gallons of fuel would be needed. But we would never fully load them. Forty to 45 percent is as much as we’d load it. So if you have 4.5 MW of load and 8 MW of engines, all those engines will take about 1.2 MW, which means they will not burn the whole 266 gallons per hour but perhaps approximately 150 gallons. They will burn less because it’s based on 100 kW of load per engine, and since they are paralleling, they are sharing the total load and burning less fuel,” McCulloch said.
Backup generators provide the power
Show time! What happens in case of an outage? When and how do the backup generators kick into gear?
“If the grid starts getting really unstable, whether in an outage or a spike or multiple sags or spikes, and if it persists for a period of time, the client can elect to go to engines until the storm and/or unstable grid stabilizes or let it happen automatically,” McCulloch said. “When we talk about the generator getting to the bus, it means that the generator’s engines electrically start up, the control system synchronizes them to the utility paralleling board, and the transfer starts. During that process, the UPS is holding the critical load stable while other systems in the building—for example, mechanical—can sustain a momentary outage until the generators have control of the facility, then auto-restart once power is available.
“For example, at one large telecommunications facility we worked on, there were approximately 1,200 to 1,400 control connections and automation needed to make sure that the paralleling switchgear was looking at the utility bus, generator bus and transferring to either source automatically.”
With a stand-alone generator, when the utility source fails, an automatic transfer switch (ATS) sends a start signal to the generator to start and once it is at rpm and reaches the rated voltage, the ATS would send another signal to indicate to transfer to the emergency source.
“When it comes to critical environments, from any aspect, it is imperative to have diverse electrical topology with redundant capacity to serve the critical loads,” McCulloch said. “It comes down to understanding the revenue loss if a company or institution loses their electrical and/or IT infrastructure. Many times clients wait until the system breaks and then say, ‘We should have made this or these systems bullet-proof.’ As an EC, you have to teach your client the importance of protecting their investment and protecting it to stay online 24/7 regardless of whether the electrical utility grid is unstable and or loses power to serve the business needs.”
As a result, maintenance is vital.
“Most hospitals and municipal standards require regular testing programs to make sure they are operating properly. Most are set up to run automatically and programmed to happen once a week,” McCulloch said.