To date, data center design would employ one of two general design concepts: use of single, large kW generator sets with multiple busses, or multiple paralleled generators using traditional paralleling switchgear. With the advances in microprocessor technology, onboard generator controls, and software, most major manufacturers offer the ability to digitally parallel multiple generators without the need for traditional paralleling gear. This is what is referred to as an integrated paralleling system.
In the past, using traditional paralleling gear was the only way to tie multiple generators together to create N+1, N+2, 2N, or other system architecture types designed to increase system reliability. However, due to the high capital cost of traditional paralleling gear along with system complexity and a large physical footprint, these applications were limited to those who could justify the cost and design.
While it did not start this way, the concept of paralleling inverters and chiller systems in the data center industry is commonplace today and something that is perhaps overlooked because paralleling is built-in to this equipment. This same approach is now available for standby generators.
INTEGRATED PARALLELING VS. TRADITIONAL PARALLELING
So, what is integrated paralleling and how does it work? To answer this, we must first know what is required to parallel generators together and discuss how this can be incorporated into an onboard system.
There are four basic requirements when paralleling generators. The following provides a listing of these requirements comparing traditional paralleling and onboard, modular approaches.
Synchronization.Generators must synchronize with each other through identical voltage and frequency. With traditional paralleling gear, this is accomplished through third-party components to drive generator voltage regulators and governor controls. However, with an onboard system, the voltage regulators and governor controls are incorporated into the generator controls.
Load sharing.Paralleled generators need to share load equally with assurance that one is not acting as a motor, pulling load from the other. Because load share modules are located within the gear on traditional systems, the cabling between generators and the gear must be exactly the same length to minimize voltage drop and allow proper load sharing. With an onboard system, the load sharing takes place at the generator set(s), allowing for flexibility in generator placement and design.
Protection.In addition to load sharing, a paralleled system needs to incorporate reverse power, voltage, and overcurrent protection. Similar to synchronization, load share modules and protective relays are third-party components incorporated into traditional paralleling gear. These components are incorporated into the generator controls with an onboard system, resulting in a simpler, more cost-effective and reliable structure.
Point of synchronization.Once generators are synchronized, a point of synchronization or connection to the emergency bus is necessary. With traditional gear, this is accomplished through the use of motorized breakers located within the gear. An integrated, onboard approach allows for greater flexibility utilizing either switches or motorized breakers that can be located on board the generator set, eliminating the need for additional equipment that takes up valuable floor space.
Employing an integrated approach over traditional gear offers several benefits. To begin with, in traditional gear there are several individual components made by separate manufacturers that need to be incorporated in the system. With individual load share modules, protective relays, programmable logic controllers (PLCs), governor controls, and voltage regulators, each component must be tied together with the system, resulting in hundreds of points of wiring, which create many more potential sources of failure. In addition to eliminating this complexity, an integrated approach allows for a single source of supply with everything coming from the generator supplier covered under one warranty. This helps to eliminate finger-pointing that can occur when sourcing items from multiple providers in an application that simply must work.
An integrated system will also carry a shorter lead time. The lead time for a digitally paralleled generator system is no longer than that of a standard generator set, whereas paralleling switchgear manufacturing lead times are typically much longer. In addition, an integrated system can be tested as a fully functioning system by the manufacturer. A system that utilizes traditional gear will only be tested together as a full system once all of the components arrive on the job site.
Another benefit of an integrated approach is the space requirement. Traditional gear will consist of a control cubicle and a cubicle for each generator in addition to distribution. These cubicles are typically 6-ft tall, +36 in. wide, and +48 in. deep. With an integrated approach, all that is needed is emergency distribution, freeing up valuable real estate.
Other factors to consider when comparing traditional paralleling with an integrated approach include commissioning time and ease of expansion. The commissioning time for traditional gear can take weeks and expanding these systems requires a full recommissioning in addition to the extra cost and planning that must take place in advance. With an integrated system, commissioning time is no longer than that of a standard standby generator and expansion is as effortless as adding generators and connecting them via simple control wires.
All of these factors, including less controls components from different manufacturers, shorter lead time, space saving, quicker installation and commissioning time, and the elimination of control and generator cubicles, result in a more cost-effective solution. While cost is not always the driving factor, it is certainly something to consider, as is increased system reliability.
It is agreed upon within the standby power industry that typical generator starting reliability is at about 98% when considering the most common start failures result from fuel and/or starting batteries. However, incorporating multiple generators can result in reliability figures of 99.99% and higher. In the past, a common approach in data center design has been to utilize single, large kW generators per bus; though with this approach, if a single generator fails on that bus, facilities are forced to rely on the other bus and/or generator to pick up the entire load.
USING MULTIPLE SETS IN PLACE OF SINGLE SETS
Providing the ability to parallel multiple generators in an easy and cost-effective manner starts to make sense over using large, single generator sets. Other than increasing reliability, using a modular generator approach provides additional benefits including shorter lead times, reduced capital cost, simplified service, lower operating costs, and scalability. While it may seem counterintuitive, the fact is using two or more small engine generator sets in place of a single large set can actually result in lower up-front capital and operating expenses. This is achieved through the use of over-the-road industrial truck engines used to drive smaller kW generators. Due to economies of scale, these engines are more readily available and carry lower capital cost as opposed to large, single-engine sets, which are custom made. Additionally, these smaller engines typically require less oil and coolant than a larger single-engine set, resulting in lower maintenance costs.
Another reason to consider employing a modular design is the ease of expansion. When using a large single generator set, it is common that the actual load on the generator is less than 30%, introducing the problem of wet stacking where unburnt fuel passes into the engine exhaust. This problem generates a buildup of carbon in the engine and exhaust, which reduces performance and causes potential damage to the engine. To combat this, additional components or service is required. An alternative solution to this problem is to install a smaller single generator set that can be added to as needed.
For example, a data center with a future expected capacity of 1,500 kW, but initial capacity of 500kW, can use a modular approach that has the ability grow with the actual demand. A single 500kW modular generator set can be installed initially, reducing the capital cost over a single 1,500kW set and as the demand grows, additional generators can be added. In this example, a second 500kW could be added as demand increases followed by a third 500kW when and if the demand reaches that level. In this way, if the demand never reaches the 1,500 kW level, you are not put in the situation of having more generators than you actually need.
The majority of larger kW standby generator sets are diesel-fueled. This is due in part to the lower cost of large kW diesel engines to comparable-sized natural gas-fueled engines. With any generator set, the majority of the cost lies within the engine, while the majority of the engine cost is in the block. For applications over 200kW, natural gas engines become very expensive, as the engine block must first be oversized to achieve the horsepower needed. These are typically diesel blocks, which then need to be gasified, further increasing the cost. In comparison, a 1MW natural gas generator set can easily be at least two or three times the cost of its diesel equivalent. Alternatively, smaller gas sets can now be digitally paralleled to provide a more cost-effective solution. To assist in this solution, manufacturers such as Generac® can parallel generators of different kW sizes and fuel types or offer bi-fuel generators that operate on diesel and natural gas — providing the benefits of both fuels.
Another consideration is recent experience with natural disasters such as Hurricane Katrina and Superstorm Sandy. In these events, it became difficult — if not impossible — to get fuel deliveries to standby generators. If these applications had employed natural gas as the fuel source, this concern would have been eliminated.
Further viable solutions include the use of dual-fuel generators. These are natural gas engines that will automatically switch over to propane fuel if the natural gas source fails. Dual-fuel generators allow the benefit of using natural gas with the added security of having on-site fuel storage in the event of a loss of natural gas. These sets can also be digitally paralleled to reach higher kW sizes.
Today’s marketplace is increasingly competitive, yet the demand for reliability never diminishes. One way to address this in your standby generator system is to consider employing a modular and/or integrated paralleled generator design. Ask your generator distributor or manufacturer for more information about these designs and other potential solutions.