Electrical design for modern HVAC systems
- 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.
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.
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.
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.
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.
Packaged mechanical equipment with VFDs
It is common for certain types of mechanical equipment (chillers, AHUs, pumps) to be purchased in packages, meaning that there are additional components above and beyond the mechanical equipment itself that are included. Motor controllers are typically included in these packages. With the energy savings that can be achieved using variable frequency drives (VFDs), it is not unusual for these packages to include VFDs in place of contactor-based across-the-line motor starters.
While a more efficient mechanical system is obviously a good thing, these VFDs can introduce problems if they are not accounted for in the electrical system.
When a VFD is specified, the electrical engineer must evaluate the impact that the harmonics created by the VFD have on the overall power distribution system. This is true regardless of whether the VFD is provided separately or installed as part of a mechanical equipment package. However, there can be a significant difference in these two scenarios, in terms of specification options available for those VFDs.
When a VFD is purchased separately, the engineer is free to specify any options or VFD types as they see fit; if the impact of harmonics is a concern, then a low-harmonic drive—such as an 18-pulse or active-front-end (AFE) drive—can be specified.
When a VFD becomes part of a mechanical equipment package, some of that flexibility can be lost. The VFDs in a packaged system are typically 6-pulse drives with no bypass. They may or may not have any harmonic filters built-in, though most VFDs serving large motors will have an input-line reactor or dc-link choke to provide at least some harmonic mitigation. While it may be possible to specify different VFD types in mechanical equipment packages, the packages are often designed around a standard 6-pulse drive, and there may not be physical space to accommodate a VFD with improved harmonic performance.
Depending on the building type, load rating, and overall building load, a 6-pulse drive may cause voltage and current distortion above the maximum levels listed in IEEE 519: Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. When reviewing this standard, it is very important to understand the definition of the commonly misunderstood point of common coupling (PCC), which is where the harmonic-distortion values are to be evaluated.
The 2014 update of IEEE 519 clarified that the PCC is the point in the system where the utility could serve other customers (not at the motor-controller terminals or the building service entrance, as it has been interpreted by some in the past). Failure to adhere to this standard could result in power-quality issues within the facility as well as for neighboring utility customers.
When determining if additional harmonic mitigation is required, engineers also need to consider whether the mechanical equipment is connected to a standby generator. The magnitude of the current and voltage distortion increases when the load is fed from a generator; this is because the source impedance of a standby generator is significantly higher than the impedance of a utility source. It is possible to have a scenario where the voltage and current distortion are within IEEE 519 limits, but the generator rating would need to be increased if additional mitigation techniques are not implemented.
If it is determined that the current and/or voltage distortion is above the acceptable levels on either the utility side or the standby-generator side, then the engineer must select a design strategy to address the issue. The possible solutions may depend on the type of mechanical equipment.
For example, a variable-speed chiller with an integral 6-pulse VFD likely does not have options to change to an 18-pulse or AFE-type drive because of the small area that the manufacturers allocate for electrical devices. The chiller is likely to have line reactors on the input side, but it is not a certainty that those reactors will be adequate. In this case, a stand-alone active harmonic filter could be used at the upstream distribution panel to provide additional harmonic mitigation for one or multiple chillers (Figure 5).
Active harmonic filters provide real-time harmonic mitigation and can be applied at a distribution panel to provide correction for multiple loads on a single bus. They are connected to an OCPD on that bus and receive input data from current transformers placed around the cables carrying the nonlinear load current. One downside to the active-filter approach is that it is a single point of failure for harmonic correction of multiple loads.
Another downside is that these devices can have a high heat-rejection load, so additional cooling may be required, especially if they are located in an enclosed electrical room. Some manufacturers’ literature state that active harmonic filters must be used in conjunction with 3% (minimum) line reactors on downstream VFDs, so the use of these filters must be coordinated with the VFD specifications.
AHUs also can be packaged with VFDs for the supply and return fans or fan arrays. In this case, the electrical engineer may have some more flexibility for the VFD specifications, and low-harmonic VFDs may be able to be provided as part of the package. However, it is important for the engineers to consider harmonic-mitigation needs as early as possible; an 18-pulse or active front-end drive can cost three times more than a 6-pulse VFD. The mechanical equipment manufacturer will not be willing to absorb the cost of a low-harmonic VFD if their base bid pricing assumes a standard VFD.
The active harmonic filter solution also could be used in conjunction with packaged AHUs with 6-pulse VFDs, though the costs would need to be compared with the cost of individual low-harmonic VFDs.
Low-harmonic VFDs and active harmonic filters are considered active harmonic mitigation solutions. Passive solutions also are available. Passive harmonic filters consist of inductors and capacitors that are sized to a specific motor’s horsepower rating. Passive filters can be a good solution where the packaged equipment’s VFD specifications are not flexible, low initial cost is a priority, and the electrical-feed structure for the equipment allows for individual connections to the input terminals of the VFDs.
The downside to passive filters is that adding inductance and capacitance to the electrical system can introduce other issues, and they must be used with caution. Luckily, most manufacturers are aware of this issue and offer passive filters with features to address this problem.
For example, a filter can be specified with a contactor to bypass the capacitor section or even the entire filter. This feature is very useful when used in conjunction with a bypass VFD, as it is not desirable to add inductance and capacitance to the system when the motor is run across the line. The specifying engineer also must ensure that the filter is generator-compatible if the load is connected to a standby generator system.
Packaged mechanical equipment accessories
Items often overlooked are the accessories specified as part of mechanical equipment packages. It is common for equipment, such as walk-in rooftop units, to have accessories like electric unit heaters, lights, receptacles, and control panels. These items should be coordinated during the design phase so that the electrical feeders are appropriately sized and the correct quantity of conduits will be brought to the equipment.
Some AHUs also have ultraviolet (UV) lamps. UV lamps are usually specified in health care facilities or any other application where the indoor air quality is critical. The UV lamp circuits come in either 120 or 208 V configurations. The electrical load of the UV lamps is usually much greater than the general lighting load for AHU service lights, so it should always be placed on a dedicated circuit.
Power feeds to mechanical equipment accessories can come in several different electrical configurations. The most common configuration type requires separate connections for each system component, meaning that the electrical contractor will need to bring a separate feed to the equipment for just the accessories. One advantage to this configuration is that the service lights and receptacles will still be energized if the main power feeder to the mechanical unit is disconnected.
This ensures compliance with the last sentence of NEC 210.63, which states the following in reference to the required service receptacle for HVAC equipment: “The receptacle outlet shall not be connected to the load side of the equipment-disconnecting means.”
Another advantage to the individual connections is that accessories can be placed on generator power, even if the mechanical unit itself is not connected to the generator. For example, it may be desirable to put a rooftop unit’s service-corridor heater on generator power for freeze protection during an extended normal power outage.
An alternative way to provide power to accessories is a “single-point connection” for the mechanical equipment. This configuration uses terminal blocks, distribution panels, step-down transformers, and any other components as needed to power all the accessories from a single 3-phase feeder. This simplifies the installation from an electrical standpoint. There may be situations, however, where it is not desirable to disconnect the accessories when the upstream overcurrent device is switched off or trips. These concerns can be partially alleviated by specifying an internal distribution panel with the equipment and coordinating the proper shutdown procedures with the owner’s facilities staff.