Electrical design for modern HVAC systems

Modern HVAC systems present both new challenges and new opportunities when integrating with electrical systems.

07/25/2018


Learning objectives

  • Know how to specify proper protection for fan-array motors.
  • Learn how to integrate the building management system (BMS) with electrical equipment for load management.
  • Design electrical systems to account for packaged mechanical equipment.

Mechanical systems have evolved significantly in the past decade. This evolution has occurred not only in the technical aspect, but also in how these systems are procured. Tight project schedules are driving contractors to purchase large mechanical equipment components well in advance of the design completion. 

Electrical engineers should be familiar with the technical differences in the modern HVAC systems, and how to account for packaged mechanical equipment. They must also be familiar with the function and operation of a typical building management system (BMS) and know how to integrate with these advanced controls systems.

Figure 1: The Systems Performance Laboratory was designed with a variety of mechanical and electrical systems for the testing of residential and commercial loads, renewable energy equipment, and cybersecurity power hardware. Courtesy: SmithGroupJJR Fan-array motor protection

Air handling unit (AHU) fan arrays have become increasingly popular in the past decade, and it's not difficult to understand why. Fan arrays provide increased redundancy, a smaller footprint, and less vibration. They also use smaller motors, which are more readily available and easy to replace. With the increased usage of the fan array design, it is important that electrical engineers understand how to provide proper motor protection for these systems. To design the protection schemes for these setups, we must first understand the manual motor protector (MMP).

MMPs are listed under UL 508: Standard for Industrial Control Equipment and consist of a disconnect switch, overload protection, and short-circuit protection. These devices are commonly used with fan arrays because they are compact, relatively inexpensive, and comply with NFPA 70: National Electrical Code (NEC) 430.32(A),which requires separate overload protection for each motor. MMPs for arrays are typically installed in a common enclosure. One specific type of MMP that has features that are well-suited for fan array applications is a UL 508 Type E MMP, also referred to as "self-protected" because its integral short-circuit protection provides the NEC-required branch-circuit protection for an individual motor. Note that Type E MMPs require that specific accessories are installed to maintain this listing, such as line-side insulating barriers and short-circuit trip indicators.

The self-protected characteristic of Type E MMPs makes them ideal for fan-array motors, as they do not have to rely on upstream overcurrent protective devices (OCPDs) for branch-circuit protection (Figure 2). Upstream OCPDs can become a concern when using MMPs that do not have the Type E listing because the installation may need to comply with NEC 450.53, also known as "group motor installation." This section of the NEC has very specific limitations on the motor sizes, conductor sizes, and OCPD sizes when feeding multiple motors from a single branch circuit.

While it is certainly possible to design a system that meets these requirements, it is often difficult to design for a group motor installation because the equipment manufacturers are not yet known, and different manufacturers may have different requirements for group motor installations. Specifying a Type E MMP for fan arrays can eliminate some of this uncertainty and reduce the number of system modifications required once the equipment manufacturers have been selected.

Another desirable feature of Type E MMPs is that they are designed to provide "Type 2 Coordination" per IEC 60947-4-1, which requires that the motor controller sustain no damage during a fault and can be put back into service after a fault without replacing any components. This feature is very important for critical operations, such as hospitals, where downtime to replace a faulted motor must be minimized as much as possible.

UL 508 also contains a Type F classification. Type F MMPs are simply a Type E MMP with the addition of a motor controller (typically a contactor). However, testing requirements for Type F controllers are not as stringent as for Type E, as the contactor is permitted to sustain damage after a fault. A Type F motor controller would typically not be used in conjunction with a fan array, as the motor control component is almost always located upstream of the MMPs.

Figure 2: A typical power distribution and protection scheme for serving fan-array motors from a single power feeder. Courtesy: SmithGroupJJR One of the most important specifications for MMPs is the short-circuit current rating (SCCR). Some MMPs have a rating of only 10 kA. While this may be adequate for some installations, it is likely that the available short-circuit current at the terminals will exceed 10 kA. This is especially true for installations where the AHUs are located near the main electrical service equipment. SCCRs vary widely across different manufacturers (Table 1), so it is best to simply specify the minimum rating required and verify that the manufacturer's shop drawings specify a model that meets this rating.

A short-circuit study should be performed to determine the maximum available fault current at the MMP terminals. If the AHUs are being bid and purchased before the short-circuit study has been performed, specifying an SCCR of 65 kA at 480 V is a reasonable selection, as it can be met by most manufacturers and will be adequate for most installations.

Just as important as specifying the minimum SCCR is knowing where to specify it. Fan-array motor protective devices are typically provided as part of the AHU package, so putting the requirements for MMPs in the electrical specifications or one-line diagram may cause them to be overlooked. It is best to incorporate fan array MMP requirements directly into the AHU specification.

Integrating a BMS for load management

Standby power systems are commonplace for many different building types. The extent of a standby power system can vary widely, from the bare-minimum code-required emergency loads (egress lighting, fire alarm systems, fire pump, etc.) to a full-building standby system with a service entrance-rated transfer switch. Many projects with generators fall somewhere in the middle of these two extremes, with generator capacity designed to accommodate emergency, legally required, and optional standby loads. Because generator size directly impacts cost, determining the appropriate generator rating is often a balancing act between the project budget and the owner's desire to back up optional standby loads that could cause loss of revenue during an extended outage.

Table 1: This table demonstrates how the short-circuit current rating of manual motor protectors can vary widely between manufacturers and frame sizes. Courtesy: SmithGroupJJR One tool available to engineers to maximize the flexibility of their backup power system is load management. The most basic and historic example of load management is a load-add/load-shed system, typically used with paralleled generators. This type of load management is done at the feeder circuit breaker or transfer switch level via hard-wired contacts to the generator controller. However, what if we want to use load management for specific loads, or with only a single generator? This is where the BMS comes in.

The first step to using the BMS for load management is determining how to tell the BMS to go into "emergency power mode." The simplest way to accomplish this is to connect a BMS input module to the "closed on emergency" contacts in the transfer switch that supplies power to the mechanical loads. In the case of multiple transfer switches serving multiple mechanical loads, the BMS will need a contact from each transfer switch and multiple modes (e.g., Emergency Mode A, Emergency Mode B, etc.) so that the system knows how to react if only one transfer switch is closed on the emergency source.

Once the BMS is in emergency power mode, it needs to know what to do in this mode of operation. One possible application for BMS load control is to only permit a single chiller to run when connected to the emergency power source. This can be useful for providing cooling during an outage to a certain percentage of the building spaces while at the same time reducing the load on the generator set by not having to be sized for the full chiller system capacity.

This same concept is applied by elevator equipment manufacturers, as it is common in hospitals and high-rise buildings for the elevator controller to only permit one elevator per bank to operate when connected to the emergency power source.

Figure 3: This power-and-control diagram shows how BMS modules can be interfaced with a transfer switch to provide a specific sequence of operations when the loads are connected to the generator. Courtesy: SmithGroupJJR BMS programming becomes very important when setting up this type of system, as different system parameters, such as setpoints for critical and noncritical spaces, would also need to be adjusted to account for emergency power operation at the reduced capacity. It is also important to note that the power distribution system should be designed to serve all the chillers from the emergency power distribution, even though only one unit will run at a time on generator power. In an N+1 chiller setup (Figure 3), this power system design provides the building owner with the flexibility to have any one chiller down for maintenance without compromising the availability of emergency cooling.

It should be noted that the transfer switch and normal power feeder will need to be sized for the full connected load during normal operating conditions. Though not required, it is also advisable to size the transfer switch emergency feeder the same as the normal-power and load-side feeders; this design provides the most flexibility for the facility in the future.


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