Genset, UPS, or Battery?

Selecting reliable energy sources for critical power systems

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As different industries become more and more reliant on data processing and storage, the importance of robust backup power systems has greatly increased. With the many different components and options that are available that can make up a backup power system, it can be difficult to select the right option to match the need. This article explores the most common forms of backup power equipment and the uses for which they are best suited.

Generators
Generators are the most common form of backup power and are available in a wide array of capacities and voltages. For critical power facilities, the generators could be rated anywhere from 150 kW to 4,000 kW. The capacity of the generator system can also be expanded by paralleling multiple generators. This has the side effect of providing added reliability due to the additional units. For critical loads, it is recommended that a minimum of N+1 generators be provided. When programming generator startup and load shed schemes, it is important to keep this N+1 operation in mind. The load shed scheme should always keep a minimum of one extra generator running than is necessary to support the current load. This will ensure the load is not dropped if a generator fails.

In terms of operations, the major differences between diesel generators and natural gas generators are the startup time, ability to be block loaded, and fuel storage requirements. Most diesel engine generators can start and reach the required speed within 10 seconds. For this reason, diesel generators are used in most life safety installations, which are required by NFPA to restore power to equipment within 10 seconds. Gas generators, on the other hand, can take up to 30 seconds to reach the required speed.

Another factor to consider when deciding between a diesel or gas generator is the block load capability. Most diesel generators can accept a 100% block load. This means that the full load rating of the generator can be placed on it and the generator’s voltage and frequency output will stay within the allowable tolerances. Most gas units are not able to accept a 100% block load. Typically, gas generators cannot handle more than a 65% block load without dropping the voltage out of tolerance. It is possible for gas generators to handle larger block loads, but to do so the footprint and cost will likely increase.

One major advantage of gas generators is that they do not require onsite fuel storage. The fuel input for gas generators normally comes from a gas line directly off the utility service. While this can be helpful for saving space, this also has the potential for reducing the reliability of the system since the gas line could be damaged in the event of a natural disaster when backup power is most likely needed. If a gas generator is being added onto an existing service, the capacity of the existing service should be reviewed to ensure it is sized appropriately for the new generators. With a diesel generator, onsite fuel oil storage is required. For smaller generators, the fuel tank can be located under the generator enclosure; this is referred to as a “belly tank.” This helps to limit the footprint of the system, but belly tanks are typically limited to runtimes of around 24 hours, depending on the size of the generator. For larger generator installations and/or where long run times are essential, a separate fuel oil storage tank will be required.

It is also possible to take advantage of both fuel options by utilizing a combination of diesel and gas generators. By doing so, you are able to limit the amount of onsite fuel storage without sacrificing the block load ability of the diesel generators. When gas and diesel generators are paralleled together, the diesel generators are used to “stiffen” the bus, which makes the generation system more capable of responding to large load swings. This also has the advantage of diversifying the fuel supply to add increased resiliency.

UPS
Uninterruptible Power Supplies (UPSs) are energy storage devices that are used to provide immediate, continuous, and uninterrupted power to equipment in the event of a power outage. In critical power systems, UPSs are utilized to provide power to the load immediately after a power outage for a short duration until the generators reach rated speed and take over. Most commonly, UPSs are used to provide backup power to data center equipment and/or critical control systems. In some instances, UPSs can be used for industrial systems such as HVAC equipment or critical fans/pumps. However, this is not common since this type of equipment can normally sustain a short outage while the generators start without a large impact to the system served. There are two main types of UPSs utilized in critical power systems: static and rotary.

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Static
Static UPSs use a bank of batteries to provide emergency power in the event of a utility outage. The most common type of static UPS is a double conversion UPS. This UPS converts the AC input power into DC and then from DC back to AC. This allows clean power to be provided on the load side of the UPS at all times, even if the power quality on the line side is poor. Most modern UPSs also include some form of an eco (or high-efficiency) mode. The eco mode typically functions by bypassing the double conversion operation by using a static bypass switch. When running in this eco mode, the system efficiency can reach 99%. However, owners should be careful when implementing this technology to ensure that their specific model of UPS can still protect any downstream sensitive electronic equipment from harmful voltage spikes. Currently, not all manufacturers’ UPSs provide switching that is fast enough to switch out of eco mode before damage could occur to downstream equipment.

The main disadvantage with static UPSs is that they produce a large amount of heat and the batteries need to be kept air conditioned, typically to 79 degrees Fahrenheit or lower. This could require large modifications to the building cooling system if adequate cooling is not currently available. In addition, larger capacity static UPSs require a large amount of floor space. A larger UPS system typically consists of multiple UPS modules (usually up to 1 MW), switchgear with input/output and bypass breakers, UPS sections for controls and static bypass, and large DC battery banks. The floorspace for the batteries can vary wildly depending on the type of battery technology that is utilized. Most UPSs can use a range of battery technologies including flooded cell, VRLA, and, more recently, lithium-ion, which have a very small footprint in comparison, but are more expensive.

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Batteries
Flooded cell, Valve-Regulated Lead-Acid (VRLA), and Absorbent Glass Mat (AGM) batteries are all a lead-acid type battery. VRLA batteries were introduced as a “maintenance-free” version of flooded cell batteries. However, despite the descriptor of “maintenance-free,” VRLA batteries still require regular cleaning and monitoring, though they do require less maintenance than flooded cell batteries. A subset of the VRLA battery is the AGM battery, which has similar properties to the VRLA battery but with a smaller footprint. Unlike VRLA and AGM batteries, flooded cell batteries vent hydrogen into the battery room. This needs to be monitored and results in strict air change requirements within the room. In terms of runtime, charge time, and ambient temperature requirements, there is not much of a difference between flooded cell, VRLA, and AGM batteries. However, there is a significant difference in terms of cost. VRLA batteries are now roughly half the cost of flooded cell batteries but take up a similar footprint. AGM batteries cost approximately 30% more than regular VRLA batteries.

Lithium-ion batteries are a newer technology that is more recently being implemented into more modern UPS designs. Lithium-ion batteries take up roughly half of the footprint of other VRLA and flooded cell technologies and weigh approximately 70% less. This also means the runtime of the UPS system could be much greater in roughly the same footprint as lead-acid batteries. In addition, the ambient temperature restrictions are looser for lithium-ion than they are for lead-acid and they can be charged to full capacity more quickly. However, the cost is roughly 20% more for lithium-ion batteries and there is a smaller pool of installed units since this is a newer technology, which adds some risk.

Rotary
Rotary UPSs, like static battery UPSs, still require power conversion equipment to regulate the output AC voltage and frequency. However, in place of the batteries, rotary UPSs utilize a large spinning mass that is constantly rotating while under normal power. When power is lost, the inertia of the mass continues to spin and produces electricity to the output to power the connected load. Rotary UPSs are sometimes used in industrial locations when supplying large motor loads. Due to the large rotating mass, rotary UPSs can more easily ride through voltage spikes, voltage dips, and motor transients without causing damage to the UPS system. In addition, the rotary style UPS does not have the strict ambient temperature conditions that are required for maintaining batteries.

Annual maintenance costs for static and rotary UPSs are similar under ideal situations; however, most maintenance on static UPSs can be performed while the equipment remains live while rotary UPSs require a shutdown. This outage is required to replace the ball bearings on the flywheel.

The major point of consideration when selecting between a static and rotary UPS is the runtime. With a static UPS, a runtime of 10 minutes at full load is typical. This can be extended easily by adding more batteries into the string to get runtimes to 20 minutes or higher. With a rotary UPS, the typical runtime at full load is 12–18 seconds. This can be extended by utilizing static batteries, but that removes the environmental advantage that rotary UPSs have over static systems. For this reason, rotary UPSs are typically installed where fast and reliable generator power is available. In systems where generators need to be paralleled before supplying power, the rotary UPS may drop the load before generator power is available.

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DIESEL-ROTARY UPS (DRUPS)
Another technology on the market that takes advantage of rotary UPS technology is the Diesel-Rotary UPS (DRUPS). A DRUPS unit combines the rotating mass of the rotary UPS and couples it with a diesel generator. In doing so, the footprint of the backup power system is reduced, and overall system efficiency is improved. Another advantage to combining the systems is that the voltage output of the UPS system can be increased to medium voltage, making this technology easier to implement into a centralized backup power system. However, the choke device that is used in the systems to reduce the fault current contribution to the system induces a phase shift which could have an impact on any closed transition switching at the UPS when dealing with multiple power sources. A common complaint with DRUPS systems is the failure rate of the clutch mechanism between the large mass of the flywheel and the alternator. Most reported failures have been catastrophic and render both the generator and the UPS out of service. Newer DRUPS designs have improved the reliability of the clutch mechanism; however, the lifespan of the newer designs cannot yet be fully evaluated due to the relatively recent implementation of improved technologies.

What is Right For You?
While there is no one best backup power setup, selecting the right backup power equipment for your needs is critical to the reliability of the overall system. All characteristics described in this article should be considered in order to best choose a resilient and cost-effective solution.

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