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Accommodating Fuel Types Using Paralleling Switchgear

Diesel is presently the most common fuel used by facilities for generating backup power on-site. Rising concern for climate and sustainability issues has increased interest in using natural gas for this purpose. This article summarizes the advantages and disadvantages of each fuel and describes applications where each can be used to an end user’s advantage.

Fuels and Their Availability

There are good reasons why diesel fuel is the most common type for onsite generators. It is readily available throughout many regions of the world for delivery into onsite storage tanks that supply generators. Diesel engines provide powerful and reliable service, and there is a ready workforce with skills needed to install, maintain, and service diesel-powered generator sets.

Although its use is less common than diesel, natural gas is also used to power backup generators. In most industrialized areas, natural gas is available through utility pipeline networks that deliver gas continually. As a result, utility-supplied natural gas offers greater convenience and a streamlined user experience – no one has to schedule deliveries or install and manage tankage. Conversely, natural gas is not available in many rural areas, and may not be available in sufficient quantity at certain sites served by utilities.

In a limited number of installations, waste methane is available for use. Essentially similar to commercial natural gas, facilities located near landfills, wastewater sludge digesters, and other processes involving biodegradation can produce methane for power generation. This is a beneficial use of a resource that might otherwise be dissipated into the atmosphere, where it could contribute to the accumulation of greenhouse gases. Factors that impact methane use for backup power are similar to those for natural gas.

Sustainability initiatives sometimes involve discussions about using hydrogen in operations typically powered by fossil fuels. While a potential “hydrogen economy” has yet to materialize, hydrogen is used in some industrial processes as a fuel stock, an option that could have application in backup power generation, including natural gas-fired gensets. As more alternative sources of power are connected to utility grids, the effects of energy overproduction have the potential to destabilize their operation. The production of hydrogen using excess power could be used to stabilize electric utility grids. If such practice became common, hydrogen could be blended into natural gas distributed by utilities to further reduce greenhouse gas emissions, or generated at sites where alternate energy sources are present.

Energy Density and Engine Performance

The internal combustion engines that power generators operate by burning fuel gases. For a natural gas engine supplied by a utility, the fuel is delivered into the engine in a gaseous state. For a diesel engine, its fuel system vaporizes or atomizes liquid fuel before introducing it into the combustion chambers of engines. The amount of energy contained in a specific volume of each fuel differs, with diesel fuel presenting a greater amount of energy per unit than natural gas. Consequently, diesel engines have historically offered a better response to changes in load than equivalent natural gas engines. Natural gas engines have historically offered less ability to respond quickly to load changes. This can affect how generators accept load increases.

Block Loading Capability

Adding a large electrical load to a genset, an action known as block loading, suddenly will cause it to slow, instantly decreasing the frequency and voltage of the current it produces. A genset will attempt to recover by increasing throttle to introduce more fuel and produce more power. Engines with sufficient reserve power and responsiveness can recover properly from the instant application of large loads. Engines with insufficient responsiveness can produce voltage and frequency excursions beyond acceptable levels for longer-than-accepted durations. In extreme cases, an engine can stall, resulting in an outage.

Genset Startup

Engine responsiveness also plays a role in the starting abilities of engine-generators. To supply acceptable backup power, a genset must accelerate from a cold start condition to full operating speed as quickly as possible. This is especially important for critical loads. For instance, Article 700 of the National Electrical Code requires backup power to be available for life-safety systems of regulated facilities within 10 seconds of the occurrence of an outage. Because natural gas offers a lower energy density than diesel, natural gas engines can be less capable of responding to block loading than diesel engines.

The above-described tendencies have historically been true, and diesels have routinely been selected for this application. Nevertheless, some of the newest natural gas engine-generators have been designed to provide greater responsiveness for better performance when started and block-loaded.

Emissions by Fuel Type

Diesel and natural gas both emit pollutants when burned. Both emit carbon dioxide, a greenhouse gas associated with global warming. However, natural gas emits less carbon dioxide (CO2) and particulate matter than diesel engines. For these reasons and more, natural gas has a reputation as a clean-burning fuel, and it is used in schemes designed to limit environmental impact.

While all of the preceding information is true, the most modern diesel engines run cleaner than prior designs. These can employ cleaner combustion processes as well as a host of pollution control technologies such as diesel particulate filters, exhaust gas recirculation features, and fuel additives that alter combustion chemistry.

Options for Multi-Generator Systems

Because of their performance characteristics and the widespread availability of diesel fuel, diesel-powered generators are the most popular for emergency power, particularly where there is only a single genset. Nevertheless, natural gas-powered generators are successfully used in a variety of applications. They can make sense where generators will see extended planned runtimes. These applications include facilities where two or more generators are used to provide adequate capacity, redundancy, and resilience.

Most facilities with more than one generator distribute power using a single power distribution system. Doing so requires genset synchronization whenever more than one unit is connected to the system. This is accomplished using paralleling switchgear, which synchronizes and connects multiple power sources and loads to one or more sections of the electrical bus. For more information about the design and operation of these systems, see the resources at the end of this article.
Figure 1: Paralleling Switchgear synchronize and connect power sources and manage the connection of loads.
The Role of Paralleling Switchgear

Paralleling switchgear offers flexibility to accommodate a variety of power sources. When utility outages occur, these systems monitor the frequency and voltage of generator power for acceptability before connecting them to a power distribution system, then use custom event sequences to connect them. In particular, they are adaptable to the characteristics of diesel engines, natural gas engines, or other power sources. They can offer advanced power source and load control features that exceed the abilities of onboard paralleling controls provided by generator manufacturers in the following ways:

Fuel Type Independence: Paralleling switchgear can accommodate the performance characteristics of properly sized and configured diesel and natural gas engines.

Manufacturer Independence: Paralleling switchgear can interact with the controls of generators from any popular manufacturer. This can benefit systems where future expansion could occur … using paralleling switchgear can eliminate the limitation of purchasing generators from only one manufacturer.

Hybrid Power Source Options: Paralleling switchgear are designed to connect power sources to power distribution systems. Because of their flexibility, they can connect multiple sources with different operating characteristics. Paralleling switchgear can be used in the following ways to connect diesel and natural gas gensets to power distribution systems that are also served by utility power.

• Life-safety loads can be connected to a fast-starting diesel genset, then other loads are added as natural gas units come online.
• Natural gas units can be used for applications with longer runtimes, including peak shaving applications where utility companies instruct facilities to use backup power to reduce utility demand. Such arrangements can offer significant revenues to facilities that participate in these programs.
• Sustainability initiatives to reduce emissions can benefit from natural gas-powered generators
• Where hydrogen could become available as a fuel stock, paralleling switchgear systems could be designed to accommodate the unique power characteristics that it presents.

Emerging Green Energy Applications: The developing energy landscape will involve more Distributed Energy Resources. These include photovoltaic panels that can supplement utility power. Because paralleling switchgear can connect a variety of power sources, it can be designed to support facility-level and campus-level connectivity for alternate energy inputs. Stated another way, they are well-suited to connecting and managing the power sources that make more sustainable operations possible. Because they connect and disconnect utility feeds, they can play an important role in the development of microgrids that can operate independently of public grids when utility outages occur.
Figure 2: This simplified diagram shows how paralleling switchgear can connect generators, utility feeds, and alternate energy sources to power distributions systems.

Differences in the energy density of fuels produce different performance characteristics in backup power generating sets. Diesel-powered generators offer robust performance well-suited to rapid startup needed for regulatory compliance as well as the demands of block loading. Nevertheless, natural gas-powered gensets can offer adequate performance, clean operation, and a different ownership experience when supplied by a gas utility. Both fuel types can be used in multi-genset applications after carefully considering the application needs, operating risks, and site limitations for each project.

Paralleling switchgear offer broad flexibility for synchronizing and connecting multiple power sources powered by diesel, natural gas, or forms of energy. They also offer manufacturer independence, the ability to use generators powered by different fuels, and can connect power flows from alternate sources of energy. They are an important part of solutions for paralleling onsite power sources with utility power. Because of their flexibility, paralleling switchgear are well-suited to connecting and managing power sources to enhance sustainable facility operations.

The potential advantages of options for fuels and powers sources for specific sites and projects should be evaluated by qualified professionals. For additional information, review the assets below or contact an ASCO representative.


ASCO Technical Brief:
Three Approaches to Grid Stabilization

ASCO White Paper:
Power Control System Basics
Is there a microgrid in Your Facility's Future? Part 1
2020 NEC Changes for Backup Power