The Definition of Harmonics
Present in both current and voltage distortion, a harmonic is a byproduct of modern day electronics. Harmonics are caused by non-linear loads from devices such as battery chargers, personal computers, uninterruptible power supplies (USPs), and most of all, Variable Frequency Drives (AC and DC). The AC power input to Variable Frequency Drives (VFDs) typically is rectified to DC power at the VFD – and it’s this rectification stage that creates harmonics. The harmonics are characterized by the amount of distortion on the voltage and current input sine waves. Excessive harmonics can lead to a host of issues including overheated transformers and neutral conductors, computer lockups, and PLC problems. In general, they could cause operational problems to electronic tools, machines, computers, and more, as a result of their effect to distort the original fundamental 60Hz.
There are several options available for mitigating issues caused by Variable Frequency Drive harmonics. The best solution, when cost, space required and reliability are considered, are dependent on the application and installation environment. Most available solutions come with a few drawbacks. It is up to you with your solution provider to choose the solution that best fits your facility’s problem.
Steps You Can Take to Mitigate VFD Harmonics Issues
Evaluate System Harmonics: The first step required to prevent or mitigate VFD harmonics issue in an industrial system is the evaluation of system harmonics. This evaluation is required if the facility in question has a history of harmonics issues. You may not know a harmonics problem exists as it may be masked over by “routine” fixing of symptoms, i.e., “routine” replacing of ballasts, sensors, etc. Among other reasons, you should conduct an evaluation if the harmonics injected back into that facility’s system if a harmonic “snap shot” shows total harmonic voltage distortion of over 5% or total harmonic current distortion of over 10%, or if the facility is planning to add a generator, or if a significant amount of harmonics generating equipments have been added to the facility. The harmonic “snap shot” is simply taking a few power quality readings to determine if a system harmonic analysis is required.
Isolation Transformers: Isolation transformer offers the advantage of a step-up / step-down voltage transformer. The cost is relatively efficient when compared to other attenuation methods. While these provide moderate harmonics reduction, they do not offer increased protection for VFD.
Reactors: These provide added protection for variable frequency drives and are also the most cost efficient way to reduce harmonics down to 35-40%. Reactors are a single winding transformer, and while extremely efficient, however, they do generate heat and the possibility of causing a small voltage drop in the DC link circuit. Normally, this is of no consequence.
Multi Pulse Drives – 12 and 18: The configurations on the 12-pulse and 18-pulse drives act to cancel some of the lower level, higher amplitude harmonics such as the 5th, 7th, 11th, etc. As they require a special transformer in addition to multiple rectification circuits, they do have lower efficiency with higher heat losses. They cost many times more than the standard 6-pulse drive, making their price an unfortunate drawback.
Active Harmonic Filters and Active Front End: The recently developed active harmonic filters (AHF) and active front end (AFE) VFDs are single unit solutions to mitigate harmonics to a specified level.
An AFE is comprised of an active filter integral to the VFD. Hence, it is a single VFD solution, and it is the most physical space efficient when compared to other solutions for a single VFD.
An AHF is a single, stand-alone unit that can be installed on the main power circuit to one VFD or several VFDs. Should the AHF go down, the VFDs will continue running as they “don’t care”, hence VFD uptime is not compromised. Some of the benefits of AHF include its flexible harmonic control, power factor control and its almost immediate response to varying loads.
The primary AFE disadvantage is the AFE drive is complex as it’s similar to having a VFD in front of a VFD, thus creating many more potential points of failure. Should the active front end of the VFD have an issue, the VFD is down. Servicing is normally done by the manufacturer. This can result in significant cost and downtime.
The AHF disadvantage is their high cost when used on a single VFD basis when compared to other options. Servicing is normally done by the manufacturer. Also, additional power circuit (circuit breaker, wiring, real estate) requirements and that a high impedance input reactor must be installed on each VFD. This may pose significant problems in a retrofit situation.
Passive Filters: These configurations consist of capacitors, inductors, and at times, resistors used to mitigate harmonic levels to the specified level. Passive means there are no active components like diodes and power switching devices. Their reliability is similar to that of a transformer with no special servicing skills required. They are very power efficient, similar to a reactor. Like an input reactor, they have increased protection for the VFD. As they are a separate device, they can be used with any off the shelf 6 pulse VFD. Should the passive filter need service, it can be quickly bypassed so the VFD can be put back into service. They are normally cost competitive with other single VFD solutions. The disadvantage is their size with the physical space requirement.
In summary, the frequent use of non-linear electrical equipments has lead to a host of VFD harmonics problems. The requirements for mitigating these problems are easy to follow, beginning with the evaluation of system harmonics. Industrial facilities looking to renovate or to add electrical loads, such as VFDs or generators, should plan for a system analysis before the facility is under construction. By improving electrical system reliability with these few steps, and working with a solutions provider for the best solution to mitigate harmonics, the facility can save a great deal of future resources spent on damages and downtime. Sloan can assist.