The Permian Basin in 2020 is full of nonlinear loads. With variable-speed drives dotting their way through fields running submersible pumps, UPS equipment at communication points, and more electronic ballasts in lights at facilities, harmonic distortion is at an all-time high. As the oilfields move to more electronic equipment this will become more apparent as utility distribution systems start to deal with the extra stress that harmonic noise creates in heat, and other inefficiencies.
Linear and non-linear loads
Linear loads are those that are purely resistive, inductive, or capacitive. When dealing with a purely linear load, the current in the circuit follows Ohm's Law (for a specific value of ohms, the relationship of volts and amps is a straight line).
Non-linear loads are created by the use of semiconductors. Due to the nature of these materials, the current in the circuit does not follow in a straight line, but a curve. Every semiconductor device will have its own unique response curve that is different. In an example of a variable speed drive, you will have several unique semiconductors, with several unique response curves, making it almost impossible to know the exact current-voltage relationship without testing.
Voltage and Current Waveform Distortion
Harmonics are a concern because they cause excessive heating and pulsating and reduced torque in motors and generators; increased heating and voltage stress in capacitors; and misoperation in electronics, switchgear, and relaying. These effects can lead to a reduced lifespan of equipment, increased cost of utility bills, and emergency failures if consideration of harmonics is not taken in the design of systems or is not applied correctly.
IEEE wrote standard 519-1992 to provide a basis for limiting harmonics. This standard sets limits for both voltage and current harmonic distortion. Voltage distortion is limited to 5% total harmonic distortion, with each harmonic order limited to 3%. Current harmonic limits vary based on the short circuit strength of the system they are being injected into.
How We Can Correct Harmonic Distortion
The first step in correcting harmonic distortion is testing the system equipment will be installed on, or once a problem is noticed or suspected. We start this by capturing waveform information, as well as collecting information about the equipment and load that is at or will be on location to get a baseline. Once we have the baseline established and can set a target we find a solution that will be appropriate for the site and cost effective for the customer.
The solutions we use are either hardening the system, using harmonic filters, phase-shift transformers, or higher pulse drives. Hardening the system is an effective method for current distortion correction.
By oversizing the electrical system, impedance is added which absorbs harmonic injections.
Harmonic filters can be added in series and are made up of an array of resistors, inductors, and capacitors that are capable of deflecting unwanted currents and can be designed for a specific frequency or frequencies.
Phase-shift transformers reduce harmonic distortion with offset windings to cancel the largest harmonics from the VFD.
12 and 18 pulse drives reduce harmonic distortion by adding a phase-shift or auto-transformer as well as extra inverters. These can be the most effective solutions but are typically much larger than 6-pulse equivalents.
Wrapping Up
Equipment malfunctions from harmonics can cost end users money in repairs and down-time. Planning or correcting harmonic issues before problems occur is something we should consider. For any needs you may have please contact us.
Thanks,
Isaac King
Sources
- "The How's and Why's of Harmonic Distortion.", John DeDad https://www.ecmweb.com/content/article/20901030/the-hows-and-whys-of-harmonic-distortion
- "Application of IEEE Std 519-1992 Harmonic Limits" Thomas M Blooming, P.E. Daniel J Carnovale, P.E. https://www.eaton.com/content/dam/eaton/products/backup-power-ups-surge-it-power-distribution/power-conditioners/harmonic-correction-unit/IEEE-std-519-1992-harmonic-limits.pdf
- Photo - Zbynek Burival https://unsplash.com/@zburival/portfolio