Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.
Most power systems can accommodate a certain level of harmonic currents but will experience problems when they become a significant component of the overall load. As these higher frequency harmonic currents flow through the power system, they can create problems such as:
- Overheating of electrical distribution equipment, such as cables, transformers, standby generators, etc.
- Overheating of rotating equipment, such as electric motors
- High voltages and circulating currents caused by harmonic resonance
- Equipment malfunctions due to excessive voltage distortion
- Increased internal losses in connected equipment resulting in component failure and shortened lifespan
- False operation of protection equipment
- Metering errors
- Lower system power factor preventing effective utilization
- Voltage regulator problems on diesel generators
- Inability of automatic transfer switches to operate in closed transition
Harmonics overheat equipment by several means. For example, in electric machines and transformers, harmonic currents cause additional power losses by (i) increasing the eddy currents that flow in their laminated cores, (ii) through increased leakage currents across insulation and (iii) by producing skin effect in conductors.
The incidence of hot transformers and neutral conductors has been especially common. Even under less than full load conditions, a transformer can run surprisingly hot. One of the reasons is its winding configuration. The overwhelming majority of distribution transformers are DELTA primary, GROUNDED WYE secondary. The delta winding has some undesirable characteristics when significant amounts of 3rd harmonic (and other zero sequence currents) are present on the load side. These harmonics return along the neutral conductor and are trapped in the primary DELTA winding where they circulate causing significant extra heating. They do not flow through to the primary system, but they also are NOT cancelled (Figure 6-1).
Since additional heating will reduce the life-span of a transformer, it must either be derated (not operated at its full nameplate rating), built to tolerate this additional heating (K-rated transformer) or designed to prevent the primary side circulating currents from being induced (harmonic mitigating transformer). A guide for derating has been proposed by CBEMA (Computer and Business Equipment Manufacturers Association) with the intent to provide users the ability to protect existing transformers which service non-linear loads. The relationship is as follows:
Derating Factor = (1.414 x RMS load current) / (PEAK load current)
Since many of today’s multimeters can measure both peak and TRUE-RMS current, the derating factor can be quickly calculated. When a transformer feeds personal computers and other electronic equipment, typical values range from 0.5 to 0.7 meaning that the transformer should be loaded no more than 50 – 70% of its nameplate full-load rating to prevent damage due to premature aging.
The fact that harmonic currents create voltage distortion as they flow through the power system’s impedance makes their impact even more serious. It is voltage distortion, not current distortion, that will affect the connected equipment on the power system.
Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers
This document has been written to provide answers to the more frequently asked questions we have received regarding harmonics and the Harmonic Mitigating Transformer technology used to address them. This information will be of interest to both those experienced in harmonic mitigation techniques and those new to the problem of harmonics. For additional information visit our Website at www.mirusinternational.com.