Cost-effective solutions reduce harmonics and improve system reliability, efficiency.
Energy efficiency requirements, regulatory compliance, increasing energy costs and improving system and building reliability are pushing organizations to implement solutions to improve energy efficiency and support sustainability initiatives. Today, saving energy is the right thing to do.
For more than 40 years, variable frequency drives (VFDs) have been a critical component of commercial and industrial systems to improve energy efficiency. Their fundamental usage remains unchanged, while technology has evolved and improved—yielding drives that are better able than ever to reduce energy consumption and improve reliability.
As drives are increasingly incorporated into pumping systems, looking at the harmonic impact on the system and VFD performance is important to insure that the system is the most efficient and can be procured for the lowest initial cost. Traditional and newer technologies are available to minimize harmonics cost effectively. Understanding the Institute of Electrical and Electronics Engineers (IEEE) requirements will help pump operators select a VFD that is both cost effective and optimizes system performance.
Electronically controlling the power supplied to a motor, VFDs control motor speed to closely match power requirements. The goal is to use the energy required, without waste or sacrificing the stability of the motor.
More frequently, pump systems integrate drives to achieve energy savings, precision control and intelligent automation. When applying VFDs to pumping systems, consideration should be given to harmonic mitigation and VFD energy performance.
Harmonics are distorted electrical waveforms that introduce inefficiencies into an electrical system. They produce wasteful heat and cannot be converted into useful energy. The impact of harmonics on a system is comparable to the consequences of putting bad fuel into a car—it burns inefficiently and can net more trips to the service station. The engine works harder, runs hotter and loses efficiency. It causes more pollution and failed emissions testing, resulting in fines. Although it may cost less initially, in the long run, bad fuel will cost more.
Harmonics are introduced into an electrical system as a consequence of nonlinear loads that convert AC line voltage to DC. A range of equipment can introduce harmonics, including VFDs. Too much harmonic distortion can result in slow and steady damage to sensitive electronic equipment and motors.
Know the Standards and Reduce Costs
IEEE Standard 519 addresses harmonic distortion. It is written from the perspective of utilities and establishes the level at which electrical energy is determined to be “clean.” IEEE 519 establishes the requirements for total harmonic distortion (THD). The harmonics caused by VFDs can be mitigated in many ways. This article focuses on what should be considered when initially purchasing VFDs. These considerations are not necessarily the most cost effective ways to improve the harmonics of an installed system that.
VFD Selection Criteria
An important consideration when selecting a VFD is to ensure that the harmonic mitigation solutions are integrated into the VFD. This is important to ensure that the parts of the system have been tested together and to optimize performance. Additionally, using the right mitigation techniques is imperative to meet IEEE Standard 519 cost effectively. In typical pump applications, the IEEE Standard 519 requirement is 5 percent for total harmonic distortion, but that does not mean that every VFD in the system needs to meet this 5 percent THD requirement.
| Some VFD manufacturers have online tools to help select a combination of solutions (based on specific system information) that effectively meet the IEEE standard and are cost effective.|
The IEEE Standard 519 applies to the electrical system—not every component within that system. Further, designing a system that mandates that every VFD meets the 5 percent THD in many cases causes the system to be over designed and introduces unnecessary costs.
Four main harmonic mitigation methods are used today that generate the best harmonic reduction-to-cost ratio. These methods are discussed in this section, in order from lowest cost to highest cost.
Reactors or Chokes
The first approach to reducing harmonics from a drive is to add impedance to the drive, either in the form of an AC line reactor or DC bus choke.
|Some drives use advacned, 18-pulse, clean power technology and are engineered to significantly reduce line harmonics at drive input terminals.|
Typically, this is a low-cost and technically simple solution, but it is likely to reduce only higher order harmonics and has little impact on the 5th and 7th harmonics. Additionally, a voltage drop is associated with these approaches, which means there are limits to the amount of reactance that can be added.