Medium voltage (MV) drives are commonly applied to pump systems in water/wastewater, oil and gas, and power generation industries. Two key application considerations that may not have received due attention are how to manage the heat rejected from the drive, and how to ensure the environment of the drive installation.
A typical MV drive is 96 to 97 percent efficient (including isolation transformer, blower fans and auxiliaries). The rest is rejected as heat to the indoor environment, requiring air conditioning and heater (HVAC) systems to maintain specified temperature, relative humidity and air quality levels. HVACs come with capital expenditure (capex) cost, but the variable costs to run, maintain and service have a material impact on the total cost of ownership (TCO) of the variable frequency drive (VFD) installation. These costs may not be understood initially, but can become significant when assessed over a 25-year service life span.
One alternative thermal management implementation eliminates the need for an HVAC and building/E-house to ensure the drive environment. Recent experience and research in electrical enclosure temperature augmentation techniques have enabled MV drives to be moved outdoors. However, due diligence is necessary when selecting them.
Why Managing Heat & Ensuring Drive Installation Environment Is Critical
VFDs consist of delicate semiconductors, transistors, capacitors and electronics that are susceptible to extreme high/low temperatures, moisture, humidity and air contaminants. The VFD operator is tasked to control the process, production, labor, raw materials and other costs in their plant. But managing the VFD environment is more challenging. Image 2 shows the heat loss and the associated HVAC tonnage required to keep a 1,000 horsepower (hp) or a 5,000-hp indoor VFD in operation.
The conventional method of managing heat for a typical National Electrical Manufacturers Association (NEMA) Type 1 indoor VFD has been to either package the drive in an industrial control building (or E-house) or install in a motor control center (MCC) room.
However, putting drives in an E-house or an MCC building comes with a price tag. At $550 to $600/square foot (E-house option), one is looking at an additional minimum of $55,000 to $60,000 to host a 1,000-hp VFD (100 square foot E-house). Further, the E-house is subject to expensive site install, permitted load transport, building codes, regular HVAC maintenance and filter changes. Beyond the initial capital capex, the year-over-year operational costs (opex) of HVAC itself, maintenance and replacement every seven to 10 years, materially impacts the TCO for large, air-cooled MV VFDs. Image 1 demonstrates the net present value of electricity expense, and Image 3 shows a picture of an E-house with HVAC units.
Managing Heat in MV VFDs
In the past decade, the MV drive industry has introduced several novel solutions that eliminate or drastically reduce the need for an HVAC building and expand the ambient temperature rating and environment in which the VFD is installed. One such solution is the containerization of indoor MV drives. In this method of managing heat, an indoor rated drive is installed in a NEMA Type 3R rated enclosure (Image 4). This treatment makes it suitable for outdoor use.
The roof of the enclosure is used as a plenum to move air in and out of the unit employing standard VFD blower fans (fans not visible in Image 4 due to top covering). A typical indoor drive is suitable for installation in 32 to 104 F ambient temperature or up to 122 F with derate and max 95 percent noncondensing relative humidity.