INTRODUCTION  
Generators are required in several building types and are specified based on codes and standards as well as on the owner’s project requirements. This brief will answer some of the questions faced when specifying generators.
What trends are you seeing in the specification of generators?
The standby generator market is constantly evolving due to the ever-changing regulatory environment and competitive pressures. From a regulatory standpoint, the impact of National Electric Code (NEC) separation of circuits and selective coordination requirements are apparent within generator specifications. The trend is to have more breakers on the generator, each in their own connection box, feeding different load types (emergency systems, fire pumps, and optional standby). It is quite common for specifications to require two to five output breakers and a mix of electronic and thermal magnetic tripping. Some applications are requiring main lug output in support of downstream fused disconnects and coordinated breakers. From a local market perspective, there is greater pressure for quieter generators. The majority of generators sold are sound attenuated.From a competitive pressure standpoint, the market is trending toward natural gas and integrated paralleling solutions. As more suppliers offer natural gas powered generators, optimized for standby applications, there is an increased customer desire to avoid the pain associated with on-site diesel going bad or running out. Interest is also trending toward The Environmental Protection Agency (EPA) nonemergency configurations that support demand response and utility curtailment programs. Natural gas generators achieve the EPA required emissions cost effectively without the need for selective catalytic reduction necessary with Tier 4 diesel solutions.Integrated generator paralleling has been actively promoted in the market for 15 years with all the major manufacturers, including Generac, offering solutions for the last nine. This has created more competition and market interest. These solutions increased system reliability, provide scalability and enhance serviceability all while maximizing customer value.

In what kilowatt ranges are electrical engineers specifying the most generators?
From a dollar perspective, slightly more money flows into the commercial/industrial generator market 500 kW and below than it does in the larger kW nodes. Looking at the generator market from a unit standpoint, the market from 30 to 500 kW is ten times the size of the market above that point. The vast majority of projects that consulting engineers execute upon are in the 30 to 500 kW segmentation. The largest end of the market is being driven by large data and cloud computing demanding 2.5 and 3.0 MW units. The large urban hospital and wastewater markets tend to utilize 1.5 and 2.0 MW units.

When evaluating the market for specific sweet spot ratings, you discover that natural gas generators dominate the market up to 150 kW rating due to a lower capital cost over diesel solutions. The 150 kW rating is the largest node that utilizes an automotive derivative engine providing the lowest $/KW for natural gas. Diesel generators tend to provide the best $/kW in the 400 to 600 kW range. These ratings are also being utilized extensively in integrated paralleling applications to maximize customer value in larger kW projects.

What challenges are affecting electrical engineers as they specify generators in nonresidential buildings? 
The largest challenges that seem to affect the consulting engineers are selective coordination/separation of circuits, local codes, generator sizing and installation/footprint constraints. The NEC requirements for coordination and circuit separation create more analysis and constraints within the scope of the project. These constraints sometimes require out-of-the-box thinking especially when managing arc flash potential or implementing integrated paralleling solutions. Local codes and local interpretations of national codes always create increased levels of uncertainty and many questions are posed such as:

• Where is the generator located?
• Is sound going to be an issue?
• How much fuel can I have before incurring more constraints?
• Do the tank vents need to be external to the enclosure?
• Will natural gas be accepted as an emergency system fuel?
• Will the generator breaker be acceptable as the disconnect for building entry?

Generator sizing always creates challenges for consulting engineers. They must consider what the largest transient load step could be, what is an acceptable voltage dip, harmonic distortion, the initial expected customer load as well as what load growth is expected. Generator sizing programs are a valuable analytical tool when combined with measurement data and engineering judgement. Load growth and levels of system robustness tend to be extremely subjective, but also extremely costly for the project. Many engineers are utilizing integrated generator paralleling to manage those risks.

Projects always seem to be fighting for an optimal generator location that minimizes installation cost while maximizing system reliability. Generators should not be located in areas prone to flooding or where engine exhaust can be pulled into the building. These challenges become more constraining in high-density urban environments, which may force the generator into the building creating design concerns for fire risks, cooling airflow, engine exhaust piping and fuel transfer.

What are the most critical issues affecting the future of specifying emergency/standby power systems?
The regulatory trends in the emergency generator market continue to add cost and complexity to the product category. Looking back across 20 years, there is a strong advancement of code requirements that constantly raises the bar for emergency system design. There are now requirements for emergency loads to be on their own transfer switch, emergency system breakers must be selectively coordinated and must be in separate vertical sections, emergency systems must have a provision to quickly connect to a mobile generator and there is now a requirement that the generator two-wire start circuit be fail-safe with active monitoring. The National Electric Code (NEC) changes have also affected the optional standby generator market. NEC 702 requires that an optional generator be sized with the same robustness as standard feeder and service calculations instead of the prior sizing requirement of “equipment intended to operate”.

What are the most critical issues affecting the future of specifying emergency/standby power systems? 
The regulatory trends in the emergency generator market continue to add cost and complexity to the product category. Looking back across 20 years, there is a strong advancement of code  requirements that constantly raises the bar for emergency system design. There are now requirements for emergency loads to be on their own transfer switch, emergency system breakers must be selectively coordinated and must be in separate vertical sections, emergency systems must have a provision to quickly connect to a mobile generator and there is now a requirement that the generator two-wire start circuit be fail-safe with active monitoring. The National Electric Code (NEC) changes have also affected the optional standby generator market. NEC 702 requires that an optional generator be sized with the same robustness as standard feeder and service calculations instead of the prior sizing requirement of “equipment intended to operate”.

In addition to the NEC requirements, other regulatory bodies are also consistently raising the bar. The NFPA is tightening requirements for installation and commissioning. The EPA requirements on emission get more stringent. The IBC requirements on seismic and wind are stricter as well as their requirements for product UL/ETL listings. Maintaining alignment with all these regulatory compliance items and the associated local AHJ interpretations adds to the complexity and uncertainty in specifying emergency/standby power systems.

When specifying standby/emergency power, what are some of the key things engineers need to know about generators? 
Generators are a combination of electrical and mechanical systems. Generators need to be positioned where they can be accessed for maintenance, but still be secured against environmental impacts. They need access to fuel, which may be diesel, natural gas or propane. Generators require a significant amount of airflow, breathing in ambient air and expelling hot radiator discharge. Generators produce hot engine exhaust discharge that needs to be directed away from any building air intakes. With these various mechanical elements, most generators are located outdoors in a sound attenuated enclosure. Generators are typically located on the same side of the building as the electrical service to minimize power-cabling costs. They are interconnected into the building electrical system with one or multiple transfer switches. The transfer switch senses loss of utility voltage, provides a signal to the generator to start and transfers the building load onto the generator.

Generators need to be matched to the applications unique situation and customer preferences. Emergency system applications require a thorough understanding of the various code requirements. Sizing is an important step in the application process because generators are not as robust as the normal serving utility. Generators experience greater voltage dips and load induced harmonic voltage distortion. Finally, customers are showing increased preference for evaluating natural gas generator solutions.

Please define the top three items electrical engineers must understand when specifying paralleled generator systems.
Paralleled generation combines multiple generators together to produce a larger back-up system that typically offers various levels of redundancy, scalability and flexibility. The two main approaches to paralleled generation are traditional switchgear or on-generator integrated paralleling. Traditional switchgear is typically implemented with UL891 or UL1558 equipment that utilizes stored energy breakers for switching and load-share modules to control synchronizing and balancing generator power. This system is then sequenced with a custom-programmed PLC. This approach offers the greatest system flexibility, but it comes with a complexity and cost that is not consistent with smaller to medium sized applications.

All manufacturers offer integrated paralleling. Integrated paralleling moves the paralleling functionality into the gen-set controller. Typically, integrated paralleling solutions will also integrate the parallel switching into the generator, though some manufactures position this externally. Load sequencing is typically performed by a small external controller that sequences the systems transfer switches. This approach is often implemented from 750 kW through a few megawatts providing reliability, serviceability and cost advantages over single larger generators and traditional gear.

When designing parallel system, enhanced reliability is often a key design goal. Parallel generation achieves that goal though generator redundancy, provided systemic failures be removed, through good design. Evaluation of all single point system failure modes is always a good system design practice. Common areas of evaluation with parallel generation are failures of the generator load share process (digital communications or analog load share lines), robustness of generator paralleling switching, redundancy of removing a generator from the generator bus and the redundancy of load sequencing. The goal of a well-designed solution is to remove systemic failure modes that could cause the entire system to be lost.

What generator sizing and selection information is needed when designing emergency / standby power? What tools do you offer that will help electrical engineers? 
Generator sizing is critical to the successful implementation of a generator solution. The goal of generator sizing is to identify the load level that the generator will see under peak loading conditions, identify any  load growth needs and then evaluate any load transients and harmonic issues that may limit the match-up of the load to the generator.

Peak loading is evaluated in different ways for different systems. For existing facilities, utility peak demand charges provide a fifteen-minute running average of facility loading. The generator will need to be sized above this point to correct for short duration loading that happens when motors start. For new construction, peak loading is often based on engineering judgement based on square footage and usage. This often places the engineering team in a bind having to error on the high side. The scalability provided by integrated paralleling solutions is a strategy that some engineers are leveraging to offset the risks associated with unknown peak loading and the uncertainty of load growth.

Generator sizing programs do a great job of analyzing the fit between the load requirements and the generators unique characteristics. Generac’s sizing program, Power Design Pro™, offers many complex load types that help in that match-up. Generally, backing up an entire building is straightforward. Problems tend  o be more pronounced when specific loads are isolated onto a generator, making the match-up more important. Common elements of generator sizing focus on managing the voltage transients typically associated with motor  tarting and managing the level of harmonic voltage distorting associated with non-linear loads.

Generator fuel selection is a complex issue. What are the basic fuel options and their associated pros and cons?
Within the stationary generator space, the three main fuel options are diesel, natural gas and propane. Historically, the dominate fuel choice has been diesel, but natural gas utilization is growing significantly. Today, natural gas has an overall market share of 40% while dominating in the 30 to 150 kW with a 65% share. Diesel has a capital cost advantage for applications greater than 150 kW and as an on-site fuel meets the requirements for emergency system loads. Diesel has the disadvantage of needing aggressive maintenance to maintain fuel reliability especially while fueling high pressure, electronically injected engines. This creates a basic problem, too much fuel on-site and it goes bad while too little fuel can lead to refueling issues.

Natural gas has the advantage of an endless supply of clean fuel and no concerns with on-site fuel storage. Natural gas can be utilized as an emergency system fuel, provided the AHJ deems it reliable. Many markets accept natural gas while others will require a letter of reliability from the serving gas utility. For the markets that do not accept natural gas, some applications will configure the generator for dual fuel operation using propane as a backup.

Propane is utilized as an on-site fuel for spark-ignited engines. It is typically utilized when natural gas is not available or in a dual fuel configuration. Propane is typically used in generators up to 150 kW. Larger spark-ignited engines are compression optimized for natural gas and struggle utilizing propane, which is a low octane fuel. As a result, it is much more cost effective to achieve large kW ratings though paralleling 150 kW nodes than applying de-rates to larger engines. Propane also runs into a boil rate constraint in cold climates causing the generators to be configured to utilize propane liquid instead of propane vapor.

Commissioning electrical systems in a commercial building is a key component when bringing them on-line in new construction. What tips can you offer to help commissioning agents and professionals with this task?
The main role of the commission agent is to ensure the performance and cross functionality between different systems supporting the building. NFPA 110 has specific action items that are required during the generator commissioning. These will require the commission agent to work closely with the generator start-up technician. The commissioning agent should ensure that the facilities largest motors could be started while maintaining adequate voltage dips. Often times this largest motor is a fire pump.

Communication between different facilities and power systems is an area of focus during commissioning. If the facility is utilizing a building management system (BMS), any planned interaction with the power generation equipment should be validated. If the application is utilizing parallel generation and load sequencing, all of the load sequence interaction should be verified in normal and generator failure modes.

The commission agent should also validate the basic mechanical interactions of the generator and its surroundings by validating that generator exhaust discharge is not being pulled into the facility and by checking that the generator radiator discharge is not being trapped in the area. In addition, it is necessary to ensure that any fuel transfer schemes are on generator power and fuel transfer schemes should be protected against being easily or unintentionally disabled. Finally, all fuel transfer solutions need to be fail safe with respect to spilling fuel.

In mission critical facilities, transfer switch, system control and other electrical equipment integration is a significant concern. Explain the importance of coordinating these system elements. Mission critical applications like data and healthcare often leverage multiple transfer switches, paralleling switchgear, smart PLC controlled switchboards and BMS. These more complex systems require clearly defined scope-of-supply and responsibilities. The better the interconnection is defined and documented, the easier the commission and long-term supportability will be.

When integrating different systems, it is also necessary to pay greater attention to failure mode operation. One of the concerning failures of highly customized solutions is that complexity and system interaction can lead to unexpected failure modes. It is important to keep in mind that human error is also a significant risk as system complexity increases. A well-designed system should strive for simplicity, but still leverage internal redundancy. Examples may include using communication backed up with hardwire inputs, utilizing bypass transfer switches, building redundancy into the generator and not relying on a single device for load sequencing.

Resiliency of electrical systems within buildings is a central issue. Define how generators can be specified to meet these needs.
Resiliency is based on the Latin word resilio, meaning to spring back. It implies that disruptive events occur and the system must be designed to bounce back. Resiliency must handle both natural and human interaction over time. Put simply, you should plan for things to go wrong. NEC 708.4 requires a risk assessment for critical operation power systems.

Some key ideas for maximizing system resiliency include design, testing and training. Utilizing an integrated paralleling system removes a single generator failure from compromising the system. It is necessary to ensure that your paralleling solution does not have simple common mode failures like loss of communication or no redundant load sequencing. Utilizing bypass isolation transfer equipment protects against a failed transfer switch. Implementing redundant facility systems if possible or dual cord devices in classic 2N architecture provides protection against distribution power path failures.

Systems need to be test with actual facility load periodically to reveal potential failure modes like over loading, leading power factor, harmonics, and load transients. Testing the generators at no load weekly, with facility load monthly, and annually load banking ensures a test process that minimizes uncertainty. Diesel powered generators need a comprehensive fuel maintenance program and not utilizing a common fuel source can protect against a systemic fuel problem. Facility staff should also be adequately trained prior to interacting with the generator system. The facility should implement defined generator testing procedures after making any changes to the electrical loads or power infrastructure.