Understanding the need for paralleled energy converter systems

Energy and power insights

• Paralleled standby power systems are increasingly favored for mission critical facilities, such as hospitals and data centers, due to their modularity, scalability and enhanced redundancy, allowing for seamless energy load management and future growth.
• These systems, guided by NFPA standards, address critical considerations like fuel storage, ventilation, noise control and maintenance, ensuring reliable performance while minimizing disruptions and adapting to evolving energy demands.

The growing complexity and size of mission critical buildings, like hospitals, data centers and industrial plants, has driven an increased demand for large standby power systems. In many cases, paralleled generator systems provide a robust solution to meet these demands in lieu of a single, large generator system.

NFPA 110: Standard for Emergency and Standby Power Systems recognizes more options for designers, including fuel cell systems with a variety of approved fuel types. Multiple energy converters offer increased redundancy and the system can be designed to provide scalability and operational flexibility, ensuring minimal disruption to facility operations during maintenance or expansions.

The case for paralleled energy systems

Paralleled standby energy systems offer the ability to handle larger loads by combining the output of multiple sources. While this capability is essential in facilities where continuous power is critical to operations and any disruption could lead to risk to human life.

Hospitals, as regulated by NFPA 99: Health Care Facilities Code, require reliable backup power systems that ensure continued operation of life safety and critical care systems during a utility power outage. While standby power is required for mission critical facilities, there are other building types that can benefit from large standby energy systems.

One strong argument for standby power is when financial loss is a concern when utility power is lost. If a building is trying to remain operational during an extended outage, the designer and client should determine what building systems are required to remain online. When building heating and cooling systems are added to the system, the capacity requirements are increased.

One of the key advantages of parallel systems is their modularity. By paralleling energy sources, redundancy can be provided for the required branches while optional systems can also be brought online. Designers have the option to size the space and systems for future growth, allowing for initial installations to serve required loads initially. This is a common option when cost is a limiting factor, but the client wants future flexibility. This also allows for additional energy sources as the facility grows or as demand increases.

Figure 1: Mechanical penthouse in a new hospital with redundant equipment branch feeds for each air handling unit (AHU), allowing for 50% of the fans in each AHU to continue operation if one of the two equipment branches is taken out of service or fails for any reason. Courtesy: IMEG

Designing a system for scalability is recommended when long-term planning for universities, hospitals, manufacturing and data centers. Designers can further improve system resiliency by using a variety of standby energy sources, such as combining generator sets with fuel cells and battery systems. In paralleled systems, synchronization of the generators is what ensures a stable and reliable power supply through the ability to have N 1 sizing for emergency and critical loads. While serving a facility with one large generator and a temporary connection is a code-compliant solution, it’s rarely the preferred option due to a lack of redundancy and the hassle of renting a temporary generator for maintenance outages.

Load considerations for standby energy systems

When designing a parallel standby energy system, it is critical to assess both the existing and future electrical load requirements. Facilities that frequently experience growth in power demand over time, due to expansions or increased reliance on technology, commonly need systems that are scalable.

Figure 2: Wiring inside large transfer switches requires coordination with the vendor to determine when rear access is required. Courtesy: IMEG

As industry tackles the push to move buildings off fossil fuels and onto electrical systems for building conditioning, the impact to energy demands on heating systems will force massive changes to the sizing of associated electrical distribution. Where buildings either need or want the heating system to be on standby power, the impact to system design will be substantial. A recent study of a large health care campus revealed the need for 1,000 brake horsepower of heating, which would have required 10 megawatts of generator power if the heating plant moved from natural gas to electric. Many of the heating systems in the northern half of the country are based on natural gas, requiring little electrical demand. As these systems are moved to electric, careful studies are required to evaluate the impact on the electrical systems feeding this equipment.

Environmental and design considerations for paralleled generators

While paralleled energy storage systems offer numerous benefits, their successful implementation requires careful consideration of environmental and design factors. These factors include ventilation, fuel storage, noise control and location selection, all of which are essential to ensuring the long-term reliability and performance of the system.

Figure 3: A new 12.47 kilovolt standby generator is shown with adequate working clearances. Courtesy: IMEG

The 2022 edition of NFPA 110 Chapter 7 outlines the specific environmental considerations that must be accounted for during the design phase, including the need for protection against natural disasters, such as flooding and wind damage. Designers should also be aware of possible damage from building system failures, such as sewer water backup, fire protection systems or firefighting efforts, which means drainage of the space must be a part of the design. This section of NFPA is not limited to energy storage systems but covers Level 1 and Level 2 emergency power supply system (EPSS) equipment, which includes transfer switches.

While it is not common to put generators in basements or below grade, it does exist. In some cases, generators may be located below grade to be physically hardened in areas where flooding is not a typical concern, but tornadoes are. Transfer switches and generator paralleling gear or distribution switchgear are commonly located in separate rooms and tend to be located in basements or lower levels. To comply with Chapter 7, designers should plan for a way to get water out of the room such that pumping systems are not required.

Ventilation and airflow management

The requirement for airflow in the emergency power supply (EPS) room varies based on the type of energy source. Exterior ventilation is a critical design element in generator rooms, particularly for paralleled systems where multiple generators operate simultaneously. Generators produce significant heat during operation and without adequate radiator airflow, their rated output can be severely affected.

According to NFPA 110 Section 7.7, the airflow requirements for generator rooms must be calculated based on the rating, placement and quantity of units. This includes ensuring that intake air pathways are properly sized and positioned to deliver sufficient combustion air and cooling air.

When fuel cells are used, the manufacturer will have recommended temperature ranges that may force the designer to provide cooling systems. Redundancy and scalability of the heating, ventilation and air conditioning (HVAC) system is just as important as the capacity of the system for indoor energy storage and converter systems. Further requirements exist based on the type of fuel being stored in the room.

In facilities with multiple generators, uneven airflow distribution can become a problem, leading to overheating and reduced performance in some units. To prevent this, engineers must design intake and exhaust systems that provide uniform airflow across all generators. This requires coordination with the architectural design team for roof access or exterior wall access and sometimes remote-mounted radiators are needed.

The benefit of a remote radiator is a reduced risk of pulling precipitation or debris into the EPS room. The downside is a potential failure point for the power source of the remote radiator fan. One solution is to power the radiator fan directly from the generator output lugs, so there is no delay in power delivery, in lieu of powering through a transfer switch.

In colder climates, it may be necessary to install heating systems and heat-traced drains to prevent snow and ice from accumulating in air intakes. It is a requirement of 2022 NFPA Chapter 7.7.6 for exterior generators in cold climates to include unit heaters for the enclosure, engine block heaters and battery warmers.

Noise and vibration management

Noise and vibration are common byproducts of generator operation, especially in paralleled systems where multiple units operate at the same time. Excessive noise can disrupt facility occupants, particularly in health care environments where Facility Guidelines Institute sets limits on maximum noise levels in occupied spaces. To address this issue, engineers typically conduct acoustic studies to determine the acceptable noise levels and implement noise abatement strategies accordingly.

Figure 4: Orange conduit indicates equipment branch loads in this mechanical penthouse. Generator load shed schemes were carefully coordinated based on most to least important optional loads. Courtesy: IMEG

The primary sources of noise in generator systems are the radiator fans, exhaust systems and generators. Vibration is another concern, particularly when generators are installed within the building. In such cases, isolation springs and rubber mounts are used on connected piping and conduit to reduce the transmission of vibrations to the building’s structure.

The use of acoustic louvers and sound attenuators is also recommended in environments where site noise control is a concern. When properly designed, these systems not only help maintain the desired airflow, as discussed previously, but help ensure compliance with local noise ordinances. For indoor installations, particularly in multistory buildings, the routing of exhaust piping must be carefully designed to minimize heat and noise concerns within the building.

The proximity of generator exhaust outlets to building air intakes should also be studied to avoid the recirculation of exhaust fumes into the facility’s HVAC systems. While not common, adding sound absorption on generator room walls can help control the overall noise level in that space and adjacent spaces.

Noise control also extends to the placement of the generators themselves. NFPA 110 recommends that generator systems be located away from occupied areas whenever possible. For outdoor installations, this may involve positioning generators at a distance from the main facility, which presents challenges for feeder design and increased feeder costs. All generator location designs should consider noise to neighborhoods, residences and adjacent properties.

Fuel storage and delivery

Fuel storage of standby energy storage systems is yet another important design aspect. The 2022 edition of NFPA 110 Chapter 4.2 specifies Class of emergency power supply systems, which determines runtime, with minimum requirements varying based on the type of facility. For health care facilities, NFPA 99 mandates that emergency power systems maintain a minimum of 24 hours of runtime on-site, with a plan to keep the system online for 96 hours, to ensure continuous operation during extended outages, which is a Class X classification.

Designing fuel storage systems for paralleled generator systems presents unique challenges. In addition to the storage capacity requirements, designers must ensure that the fuel supply lines are routed properly to prevent leaks or blockages. Ideally, fuel return piping should be arranged to allow for gravity drainage, which reduces the need for pumps and solenoid valves.

In facilities where fuel storage is shared with other systems, such as heating boilers, designers must take care to ensure that the generator system’s fuel supply does not interfere with the operation of other systems that rely on the same fuel supply. When expanding the system capacity, designers must evaluate how runtime is impacted as energy production is increased.

In some cases, particularly in regions prone to natural disasters, it may be necessary to install larger fuel storage tanks capable of holding several days’ worth of fuel. These tanks may be located underground or aboveground, depending on the site’s physical constraints and the client’s preference.

To ensure fuel quality over long periods of storage, the designer can incorporate fuel polishing systems to keep the fuel clean by removing water, contaminants and prevent degradation. These systems are particularly important in disaster-prone areas, where emergency fuel supplies must be maintained for extended periods without the possibility of refueling. Refueling locations should be considered, ensuring that in areas where standby power systems are elevated to protect from flooding, the refueling location is also elevated.

Commissioning, testing and maintenance strategies for energy storage systems

The final component of any energy storage or production system design is the commissioning process, which ensures that all systems function as expected before the system is put into full operation. While designers may expect specifications to cover the requirement for functionality, a full commissioning effort will far exceed anything the specifications would cover while ensuring all aspects of the system function, including testing all programmed delays. NFPA 110 emphasizes the importance of commissioning and routine testing for emergency power systems, particularly in mission critical environments where system failure could have impactful consequences.

Figure 5: A unit substation with an oil-filled transformer is used to step-down from medium voltage generators to feed the emergency side of 480/277 V transfer switches. Courtesy: IMEG

Commissioning a paralleled generator system involves a comprehensive series of tests designed to verify the system’s performance under various operating conditions. These tests typically begin with a full load test, during which the generators are run at maximum capacity to ensure that they can handle the facility’s peak power demands. The system’s automatic transfer switches (ATS) also are tested to ensure that they can seamlessly switch between the normal utility power source and the emergency generators without interrupting operations.

During commissioning, the parallel controls are tested to ensure that the generators can start, stop and synchronize correctly under load. The system is also tested for redundancy, ensuring that if one generator fails, the remaining units can continue to supply power. Manual controls should be tested to verify that facility personnel can operate the system manually in the event of an automation failure.

This experiment should be documented and the procedure for manual paralleling should be available to authorized facility staff. Posting this information on the face of the paralleling gear is a recommended process, since access to the EPSS room should already be restricted to authorized personnel.

Routine testing and maintenance are equally important to ensuring the long-term reliability of paralleled generator systems. NFPA 110 mandates regular inspections and testing, including monthly load bank tests and annual full-load tests. These tests help identify potential issues before they lead to system failures, such as fuel contamination or battery degradation. Parallel systems offer the added benefit of allowing maintenance to be performed on individual generators without taking the entire system offline, which is particularly important in facilities that must always remain operational.

Maintenance strategies and redundancy

Regular maintenance is crucial for the long-term reliability of paralleled generator systems. NFPA 110 requires that all components of the system, including the generators, fuel systems, batteries and ATS, be inspected and tested on a regular basis. One of the primary benefits of parallel systems is that maintenance can often be performed without interrupting the facility’s operations. Because parallel generators provide built-in redundancy, individual units can be taken offline for maintenance while the remaining generators continue to supply power.

Maintenance routines typically include checks of fuel quality, battery condition and generator performance under load. Fuel systems require regular inspection to ensure that fuel lines are clear and that tanks are free of contaminants. Fuel polishing systems are often used to maintain fuel quality, particularly in facilities that store large quantities of fuel for disaster preparedness. Battery testing is another critical component of maintenance, as battery failure is one of the leading causes of generator start-up issues.

In mission critical environments, it is important to develop a maintenance schedule that aligns with the facility’s operational needs. For example, maintenance is often performed during off-peak hours or in conjunction with scheduled power outages to minimize disruption to the facility operation. By carefully planning maintenance activities and using the redundancy provided by parallel systems, facility operators can ensure that their emergency power systems always remain operational.

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