Grid reliability is a growing industry concern highlighted by an increasing number of brownouts and blackouts. Utility grids and power systems are facing increased demand while adapting to integration of smaller, intermittent renewable generation. In addition, increased weather events such as hurricanes, fires, extreme hot or cold temperatures and even cybersecurity concerns are demonstrating the need for more redundancy to better support infrastructure. For electric motors in pumps, fans, compressors and a variety of other industry equipment, variable frequency drives (VFD) are helping to deliver efficient solutions. They reduce load, provide new levels of protection and can integrate with storage and renewables to provide continuous operation for many critical systems.
Reducing Brownout & Blackout Impact
Brownouts (also referred to as voltage sags or power dips) occur frequently when there is a weak grid or a fault interruption. This can be caused by a large load coming online at a neighboring facility or even a grid failure nearby as the grid pulses to isolate itself. For electric motors, this presents concerns when the incoming line voltage drops and current demand in the motor goes up because the process still has the same power demand. Events like this create risks for overloading or overheating equipment. Brownouts can also trip many systems, including VFDs, and shut down equipment, stressing components or even creating failures.
Although these events are not usually long enough to switch onto generator power, using VFDs that can pull kinetic energy from the motors to keep them online longer can reduce the impact. VFDs are intelligent controllers that can monitor current and prevent overloading conditions, providing increased motor protection. Sometimes they are also set up to trip to prevent pumps from slowing down below a safe speed or to isolate themselves from the situation. Using onboard intelligence, they can be programmed to keep processes running longer or to disconnect quickly when phase losses occur. Overall, they allow users to more accurately monitor and define the preferred response based on their application needs and better maintain the condition of the system.
When the grid fails completely, many systems stop while they wait for generators to come online and then stop again when the grid is restored. This creates two interruptions in service that can lead to production waste, system backup or an interruption in flow that may require a system flushing to restart. To mitigate this, facilities have explored short-term storage or co-generation infrastructure. Often these approaches require intensive capital as they are designed to keep all current and future items online instead of targeting key areas. They may also have an impact on facility efficiency by introducing new mechanical or alternating current (AC) to direct current (DC) power conversion losses.
The Role of VFDs
The variable frequency and voltage VFDs produce are generated from the DC bus within it. Typically, this is created by the integrated three-phase AC to DC rectifier or sometimes from another DC source. Often these units are equipped with DC termination points which allow for several ways to bring in DC energy as an additional power source. This can reduce losses from multiple AC-DC-AC power conversions when considering backup power systems such as a facility-based uninterruptible power supply (UPS), alternate AC co-generation or alternate three-phase energy storage.
With DC power inside the drive, users can also consider directly linking other DC renewable or storage sources to avoid losses in the AC to DC rectifier. These could be solar, batteries or super or ultra-capacitors. We are also seeing increased use of variable speed wind, pumped hydro, compressed air or “power to X” (solutions that use alternative clean fuels such as hydrogen) sources that use VFDs as well for speed and power management.
They often take the variable three-phase generated energy and convert it to a DC source before pushing it back onto the AC grid. This conversion and the associated filters can have a negative impact on system efficiency when compared to taking that DC directly to a VFD to power another motor. Active front ends (AFEs) and emerging DC to DC conversion equipment often found in DC systems are based on standard insulated-gate bipolar transistor (IGBT) technology employed by industry today. This common hardware platform allows for easier access to products, leverages existing competence and provides flexible solutions and sizes to meet application needs.
Investing in Efficiency
Not only are there large investments in new forms of clean energy, storage and new forms of energy recovery (such as recovering gases from waste facilities which are then burned to generate power), there will be more investments in existing infrastructure as well. When investing in new or retrofit solutions, it becomes important to consider ways to reduce energy while also finding ways to create new energy sources. It may also mean rethinking how energy is distributed across the system and managed. One way is to look to flexible DC grids which can increase system efficiency and allow for multiple power generation points and motor controls. Once again, these components are typically designed with the same IGBT hardware used in VFDs—even built on the same platform.
Investments in new ideas are covering a large swath of possibilities for future technology, but many of these solutions have some uncertainties and potential barriers. DC grids pose risks related to fault currents or even have costly installation aspects due to low volume of products on the market today. But as these problems arise, new solutions are forming in industries. For instance, new VFD-based technology is using IGBTs for fast disconnects, clearing faults in microseconds, compared to traditional fuses in milliseconds. This acts as a DC guard, helping to isolate faults and reduce concerns for DC bus short circuits. Also, with increased demand, some DC components are becoming more available and cost effective in the marketplace.
How investments and incentives will impact the energy transition remains somewhat unclear. Yet they are making an impact as many energy providers are focusing heavily on new forms of generation. Using VFDs as intelligent controllers that have flexibility with AC or DC power inputs will allow users to adapt to changing scenarios. They will make it easy to integrate alternate energy or individual storage at facilities. Coordinating with the right partners will ensure you are selecting the right products to meet not only your needs today but also adapt to the systems of the future.