Across the United States, cities are facing the challenge of an aging infrastructure. Atlanta experienced this firsthand in 2020, when a cabinet that housed critical electrical equipment at one of its larger pumping stations burned up following a lightning storm. The city had already made major repairs to the station after a flood six months earlier. The electrical accident, and the fact that the aged electrical distribution was original to the building, led the city to declare the station unsafe. It was clear the nearly 50-year-old station would need to be rebuilt, but with the work not scheduled to begin for several years, a solution needed to be found that would enable the station to maintain its critical function.
Rich Pearlstein, managing consulting director for the City of Atlanta Department of Watershed Management, decided an eHouse—a pre-fabricated walk-in enclosure that houses electrical equipment— would be the most cost-effective solution. But since the drives that power and control the electrical components generate a great deal of heat, the air inside the eHouse had to be kept cool in order to preserve those components.
With the need to conserve space, minimize energy expenses and meet the city’s sustainability goals, Pearlstein and his team needed a cost-effective way to keep the structure cool, preferably without a heating, ventilation and air conditioning (HVAC) system.
Back-Channel Cooling
To create an eHouse that would meet the pumping station’s needs, Pearlstein turned to a pump and controls supplier for the city. The company knew that variable frequency drives (VFDs) with back-channel cooling—which remove heat through the back channel or building panels to the outside—would allow for operation in a structure without HVAC. An electrical contractor was also part of this collaborative effort. The contractor handled meticulous specifications of a precast vault (all holes for pipes pre-drilled), which was dropped in below the eHouse. The company also installed dozens of 3- and 4-inch PVC pipes through the vault to connect the utility transformers, 5 kilovolt (kV) switches, generator, eHouse and building before the concrete pour.
Additionally, the drives the pump and controls supplier chose contain a function in their software which allows them to monitor the indoor temperature and, when necessary, switch between recirculating air inside the eHouse and venting heat to the outside. This meant that in the winter, the heat rejected by the drives could be used to heat the eHouse, compensating for the lack of an HVAC system.
Back-channel cooling, available with these drives, is based on a heatsink design, with heat pipes that conduct heat 20,000 times more efficiently than traditional solutions. Using a minimal amount of energy, the concept exploits the heat differentials in materials and air temperature to effectively cool high-performing electronics. In these drives, there is total separation between cooling air and the internal electronics to protect them from dust-borne contaminants.
Efficient heat removal helps prolong product life, increase the overall availability of the system and reduce faults related to high temperatures. Up to 90% of heat losses are exhausted directly outside the enclosure. The drives’ zero-clearance, side-by-side mounting decreases the amount of space required, and the energy consumption related to cooling is brought down to an absolute minimum. Combined installation and energy savings result in up to 30% cost savings in the first year of operation.
Another benefit offered by these drives is that they operate in conjunction with harmonic filtering. Installed as a stand-alone solution at a common point of coupling, the filter ensures optimal harmonic suppression, independent of the number of loads or their individual load profiles. In addition, the active filter corrects the power factor and balances the phase load, providing optimal energy utilization. This improves the system efficiency and increases the grid robustness to avoid downtime. Installing a harmonic filter in the eHouse allowed the pumps and controls supplier to keep the cabinets that contained the drives to the smallest size possible, helping to keep the structure compact and lower total energy costs.
Pearlstein affirmed the equipment was an ideal solution: “The back-channel cooling fit with what we wanted to do, along with the city’s goal of going green and lowering our energy consumption.”
Drives Bring Dividends
An enclosure manufacturer built the eHouse, integrating the drives to work in conjunction with the forced air enclosure. The eHouse was shipped from the enclosure manufacturer to the pumping station, where the on-site contractors connected it to the existing wiring. The drives were programmed so their network communication cards would interface with both the eHouse control panel and external controls, and after several weeks of testing, the drives were brought online.
Since the eHouse began operating in December 2021, Pearlstein has been completely satisfied and reports the electrical equipment has had no problems, a relief for his department. He estimated the drives have resulted in significant energy savings, helping the city fulfill its goals of improving its energy efficiency and lowering costs. With the exception of a smaller motor control center and two panels, all electrical distribution equipment is located in the eHouse. When the pumping station is fully rebuilt, the electrical contractor can tie to existing piping to the eHouse, keeping total electrical costs for the job modest.
Safety as a Primary Design Criteria
One of the main themes that drove the design of this eHouse was pump operator safety, and there were several features that were engineered into the system with safety in mind. Arc flash has become an increasing concern in electrical equipment, and the city is proud to boast that this eHouse has one of the most technologically advanced arc flash mitigation systems available. The basis of design is speed—the faster the fault current is removed, the less the incident energy is capable of building up to damage equipment or harm personnel. This system is engineered to detect the onset of an arc flash and then trip the breaker upstream that is feeding current to the fault. This system does not wait for an instantaneous overcurrent to exist.
This system uses a series of arc flash relays, and these relays continuously monitor for the presence of an arc flash in two concurrent ways—light and amperage. Light sensors are mounted in the main breaker compartment as well as every feeder breaker of the main 4,000-amp switchgear. They are also mounted in each VFD cabinet and in every bucket of the downstream motor control center (MCC). In addition, there are current transformers mounted on the bus system in the main switchgear. All these devices are wired back to relays that are all networked together. The system is designed to detect a flash of light above 8,000 illuminance (lux) and look for a rapid rise in current at the same time. It is important to note that the system is not looking for a specific threshold or target of high current, but merely a rapid change in current that allows for a faster reaction time.
The light sensors can ascertain a flash of light in 500 microseconds and report back to an arc flash relay within 2 milliseconds. The arc flash relay will then send a signal to the breaker upstream of the fault location, and the breaker will trip in about 50 milliseconds. This equates to a total time of 52 milliseconds from arc detection to fault current removal.
Due to the amount of available fault current at this pump station, the electrical equipment would have had a minimum arc rating just under 25 calories per centimenter squared (cal/cm2), which equates to a personal protective equipment (PPE) category 3. This means any person within the working distance to the equipment would be required to have a flash rated jacket, pants or coveralls and a flash suit hood, all rated for minimum 25 cal/cm2, in addition to hand, face, eye, ear and foot protection. However, due to the fact that the arc flash mitigation system can remove the fault current in 52 milliseconds, the incident energy does not have as much time to build up and is thus reduced to below 4 cal/cm2 within the working distance. This is now a PPE category 1, which means an operator would only be required to have a long sleeve shirt, pants or coveralls, all rated for 4 cal/cm2, in addition to hand, face, eye, ear and foot protection. This dramatically increases dexterity and decreases the risk of electric shock due to dropped tools.
Most arc flash incidents occur when a system is being energized or de-energized. Another safety measure employed in the design of this eHouse is the fact that each breaker can be both opened and closed remotely from the human machine interface (HMI) screen. This keeps operators out of the working distance of the breakers during the critical time when loads are being energized or disconnected, thus adding an additional safety measure for pump station personnel.
The electrical equipment in lift stations can present many challenges, and safety measures can often be taken for granted, overlooked or bypassed when problems arise and operators need to get them fixed quickly. All the safety measures built into this eHouse will help to keep the operators safe without any special training or procedures to follow.
