With many facilities relying on multiple power sources to ensure continuous operation, the value of integrating and coordinating these electrical systems becomes critical. Integrated power systems can include primary utility transformers, standby engine generators, UPSs (battery or rotary), fuel cells, and other types of power sources. The integration of these electrical systems requires careful planning, communication with other project team members, and support in the field during installation and startup.
Numerous items need to be addressed during the planning phase. One of the first is defining the code-specific classifications of the electrical systems to be integrated. In discussions with the project team, several terms are often used interchangeably, or generically, to describe on-site power systems, which can lead to confusion. These include terms such as standby, backup, normal, primary, emergency, alternate, and others. However, from a code perspective, the NEC identifies specific classifications that should be used. NEC Chapter 7 defines the following types of power systems:
Emergency power: Article 700 states, “Those systems… legally required … by any governmental agency having jurisdiction. Essential for safety or human life.” A prime example of this type of load is egress lighting.
Legally required standby: Article 701 states, “Those systems… classed legally … by any governmental agency having jurisdiction to selected loads ….” An example of this type of load might be smoke removal, firefighting, or rescue operations.
Optional standby system: Article 702 states, “Those systems intended to supply power where life safety does not depend on the performance of the system.” Examples of optional standby loads included data centers, clean rooms, and industrial or commercial process where if power were lost, the result would be a material, product, or financial damage and loss.
Critical operations power systems (COPS): A new, and somewhat controversial, section is part of the recent addition to the NEC—Article 708, Critical Operations Power Systems. This section's requirements originated in facilities that are classed as “mission critical” and related to homeland security. Examples might include police stations, hospitals, fire stations, or 911 facilities, depending on governmental classification. These electrical systems are generally intended to operate longer than emergency power systems, and have notable requirements for testing, commissioning, and protection.
In addition to the traditional on-site power systems discussed in Chapter 7 of the NEC, “green” or alternative energy sources that can be operated in parallel with the utility are becoming more common.
These classifications must be carefully reviewed, as the code defines very different requirements for the systems. Distribution paths, fire ratings, load calculations, startup times, run time, testing, and other requirements vary for these code-defined systems. These code requirements complicate matters when multiple power sources and systems are combined within a single facility.
If your generator, or other standby power source, supplies emergency and non- emergency loads, you will need to separate the code required emergency power distribution systems from the non-emergency paths. You also will need to document the generators' (or other power sources') ability to handle the both the emergency and non-emergency loads simultaneously—or provide load shedding schemes to drop the non-emergency loads.
The size of your power source also is calculated differently, depending on whether your system loads are primary, emergency, standby, or optional. Emergency systems are calculated at full demand and diversification. Simply put, that means you must add up all the name plates with no calculated reductions in demand as you might with do with non-emergency loads. Further, certain loads like fire pumps have very specific voltage drop requirements on emergency systems (maximum of 15%). These load calculations are important as you size and integrate the various types of power sources you will use.
Beyond load demands, additional power systems studies, including short circuit, arc flash, and protective coordination, are essential. These studies must be run under the multiple power source “scenarios” that could occur in an integrated system. The power system characteristics can change notably from being on the utility, to being on a UPS or a generator. Further, the NEC requires a selectively coordinated protection system for emergency or life safety electrical systems. Achieving selective coordination among breakers and fuses can be a challenge; it is beneficial to perform these studies during planning and design rather than after design is complete.
Space planning also is essential for integrating primary and standby electrical systems. Designing rooms for integrated electrical systems is worthy of a detailed discussion not possible here. Questions to consider include:
Will the generators be located inside or outside?
When located inside, how will equipment be moved in and out?
How will ventilation and exhaust be handled?
How will code-mandated separations for emergency systems be accomplished? Will UPS or battery room require special ventilation?
What about environmental permitting for diesel or natural gas generators?
Do you need fire sprinklers in the rooms or special fire rated construction of the rooms?
Additionally, acoustic issues need to be addressed, as noise control is often overlooked.
Planning also should include early definition of the testing and commissioning requirements. Beyond the typical factory or laboratory tests, integrating multiple power sources lends itself to formal acceptance testing and commissioning. The needs for these additional services must be taken into account during the planning phase, as they need to be incorporated into schedules, budgets, and design and specifications.
One of the more challenging aspects of planning for integration of multiple sources is working with utility representatives. Your initial discussions with utility engineers might be delayed by bureaucracy and paperwork. Utility representatives often question the amount of power loading required and automatically apply reducing factors to any power demand requests from the consulting engineer. When planning to use standby power such as UPS or generators, it is helpful, but difficult, to obtain information on local outages or other power quality data to determine suitable backup times. The utility representatives might wish to discuss on-site alternative energy sources if they have concerns about momentary “paralleling” with the utility via transfer switches (make before break) or to discuss the possibility of “interruptible rates” for the client.
Last—and most important during planning—code reviews and meetings with the local authority having jurisdiction are extremely valuable when discussing multiple power systems. The project team should review important topics including environmental permits, sound/noise ordinances, fuel storage limits, ventilation for UPS batteries, and emergency power-off devices.
Most errors and omissions can be attributed to poor coordination and communication among project team members. Electrical engineers are used to waiting for information from others, but they also need to be proactive in reaching out to the rest of the team. Each member of the project team has specific issues when it comes to power systems.
Communication with mechanical engineers typically revolves around ventilation. Most electrical equipment has a space temperature limit of 104 F. As a general rule, electrical spaces should be ventilated or conditioned to not exceed 86 F on average. De-rating of electrical conductors occurs when the ambient temperature is above 86 F. Rooms with UPS batteries should be conditioned to 77 F as battery life decreases above this space temperature. The mechanical engineer will need to get the heat rejection values from the electrical equipment to perform the HVAC design.
The mechanical engineer should review the location of mechanical duct, pipe, or other foreign systems to ensure that they are out of the NEC “no fly zones” over (and in front of) the electrical equipment. The engineer also must carefully coordinate power requirements for things like dampers, ventilation, fuel pumps, and smoke removal fans. There have been instances where the engineer failed to coordinate who will provide power for these miscellaneous mechanical systems, or even neglected to consider that they will likely need backup power. An example of this lack of coordination might be emergency power to louver necessary to ventilate a generator, or a gas booster for natural gas supply to a generator.
The discussion with the architectural team member usually focuses on room sizes. The question electrical engineers often hear from the architectural team member is, “Why do you need so much room?” It is important to review with the architect the requirements for code clearances, arc flash zones, service access, fire ratings, door sizes, and rigging/equipment moving. Wall construction materials can also affect clearance requirements. (See NEC Table 110.26, Condition 2.)
Electrical engineers often neglect to have early discussions with structural engineering team members. Various types of power sources can have notable structural impacts. It is important to share weights and vibrations of equipment with the structural team early in the design. UPS batteries are of particular concern, as are large transformers and standby generators. Review the routing of any significant conduits for hanging loads and where they might penetrate through any structure. The typical electrical one-lines on a plan don't covey the sometimes large amount of space needed by conduits when penetrating walls and floors. Duct banks that pass under exterior foundation walls are another critical item to review with the structural engineering team members.
Key engineering points
A number of key points warrant further technical discussion when integrating primary and standby power systems. Two of these are grounding and power factor (PF).
Grounding is an important and often overlooked consideration in the integration of utility and on-site standby power. Typically, the source of standby power is a generator, and the primary power source is a solidly grounded utility supply transformer. The engineer has the option of:
When using the primary power source ground, a three-pole automatic transfer switch (ATS) with a solidly connected neutral should be specified to maintain the ground path (see Figure 1). When the generator is grounded to create a separately derived source, an ATS with a switched neutral is required (see Figure 2).
The engineer should carefully review the advantages and disadvantages of separately grounding the generator neutral or relying on the primary power source ground. The NEC handbook discusses the issue of ground faults in Section 700.17. However, it is usually recommended that the generator neutral be separately grounded. This approach will improve the application and accuracy of ground fault protection schemes, which are discussed in more detail in ANSI/IEEE Std. 446-1995 .
Electrical engineers are familiar with the basic power triangle showing the relationship of Watts, volt-amps, and vars (see Figure 3). Non-electrical team members, however, often use the common terms kiloWatt (kW) and kiloVoltAmp (kVA) interchangeably and incorrectly. PF describes the relationship between kW and kVA. kVA is most easily described as the total power required or provided and consists of the kW and kVAR components. Further complicating this topic are the separate issues of the PF of the loads and the rated PF of the primary and standby power sources.
With respect to the building loads, most types of facilities have lagging PF. Put simply, kW is less than kVA. The total PF of most modern buildings is 80% to 90% (0.80 to 0.90 PF). Unity PF is 100% (1.0 PF). Facilities with high levels of electronic loads, like data centers, may have PFs in the range of 0.9 to 0.95, or near unity. Data centers under certain load conditions, and other types of facilities with special equipment, may even experience leading PF.
Why does the PF of the load matter when integrating primary and standby electrical systems? It matters because the capacity of each type of power source varies in its ability to handle certain PF loads. Utility transformers have a near unity PF (kVA = kW) and are, therefore, fairly simple to size regardless of the load PF. However, many UPSs may have an output PF rating around 0.9 PF and almost all generators have an output rating of 0.8 PF. Because of their PF ratings, these sources are often called “kW limited” in terms of their capacity. This is why many engineers prefer to focus on the kW rating of UPS and generators rather than kVA to avoid possible under sizing issues. Leading PF loads also may necessitate de-rating with some standby power sources. However, in all cases, the load PF should be reviewed against the PF specifications of the power sources.
Beyond the issue of output (load) PF, the engineer also must consider the input PF, as well as other input specifications, when integrating various standby power sources. A good example of this issue is the concerns when generators are supplying UPSs during primary power loss. The good news is that the old days of over-sizing generators at two to three times the UPS input ratings are gone. Many UPSs have friendlier rectifier designs and high input PFs, which allow you to specify a generator much smaller than before. A review with your generator and UPS vendors will help confirm sizing and compatibility issues during design.
In the field
As a wise engineering mentor once told me, a problem is not a “real problem” unless it gets built incorrectly in the field. No drawings are perfect, and no equipment supplier or installing contractor is perfect. Proper integration must continue during construction in the field to prevent “real problems.” Communication and verification are vital. Integration of primary and standby electrical systems does not stop when the design is done.
Contractors and field personnel are essential members of the project team, and it is important to establish a good relationship between designers and builders. The contractor will be coordinating multiple vendors of equipment, other trades, local authorities, and the design engineers.
When problems arise during the integration of multiple power sources, finger-pointing among the various equipment suppliers, the contractor, and the engineer is a common issue. To avoid this situation the team should review the need for acceptance testing and commissioning during the planning stage.
Acceptance testing is an enhanced level of field testing, outlined by the International Electrical Testing Assn. (NETA), and usually performed by a third party to rigorously verify and test individual electrical components and equipment. You may not need to specify NETA acceptance testing on all parts of the electrical system, but the major parts of primary and standby distribution systems benefit tremendously from these acceptance tests.
Beyond acceptance testing, commissioning is often beneficial and even required in some cases. Commissioning is a system-wide approach to ensure performance and should be implemented during both the design and construction phases for optimal results
Lastly, while in the field, it is important to maintain close communication with the client about relevant issues such as schedule, costs, change orders, field tests, and code inspections. Proper systems training and O&M plan review with the owner are also vital components of a successful project. With this information, the client will be able to maintain a fully integrated primary and standby power system.
| Author Information |
| Rener is licensed professional electrical engineer in the Chicago area. His 20 years of experience include leadership in management, engineering, and operation of commercial, industrial, government, and mission critical facilities. Rener has published or presented numerous papers for the IEEE Industry Applications Society, where he has served as an officer in the Chicago chapter. |
Integrating green power
In general, conventional backup or emergency power sources (like diesel generators or UPSs) are not intended to be operated in parallel, or interconnected, with a building utility sources. However, renewable and sustainable power sources like solar, wind, fuel cells, and biomass generators are being integrated and paralleled into the building utility power systems.
NEC Article 705 covers the interconnection of these types of power sources. A key part of this article is the automatic disconnection of these power sources from the utility (not necessary building system) power connection should utility power be lost. The purpose of this function is to prevent power from being sent back into the dead utility grid connection when repairs or other services might take place to restore and outage.