Problems of Line Side Harmonics

The two main line side harmonic concerns are the problems caused in the facility where they are being generated and the degree of harmonic infection passed to neighboring facilities. The degree of impact for either area depends on the number, size and frequency of use of all linear and non-linear loads . Mainly introduce harmoncs within a facility here.

Within a facility, the ultimate expense of line side harmonics are the costs of frequent downtime or errors, premature equipment replacement and utility expenses. Some of the specific problems a facility may experience include:

•Inadvertent “nuisance” tripping of breakers, fuses andrelays are caused by harmonic current peaks that arehigher than standard waveforms.

•Logic faults in digital devices, malfunctions of automatic control systems and inaccuracy of instrument measurementare possible because harmonics can alter the parametersthese devices are designed to run on.

•Current harmonics create inefficiencies that causeoverheating in the supply transformer and cables feedingthe VFD.

•High harmonics require oversizing, where equipmentis sized larger to accommodate harmonics. This addsadditional cost and may make it difficult, or impossible,to expand facilities.

•Higher utility bills caused by increased total powerconsumption. Additional current harmonics representreactive power (power not actually used to complete work). Since utilities typically bill for total power (active +reactive power), they are both still included in a facility’sutility bill.

Load side harmonic issues are related to differential mode or common mode. In addition to individual problems related to each mode, excess heat generation and audible noise are shared concerns.

Differential mode is associated with the voltage used to power the motor. Elevated differential mode voltages create premature cable and motor failure. The effects worsen as the distance between the VFD and motor increases. As the distance grows, the increasing cable resistance and electricity flow to and from the motor cause the peak voltage and rise time to increase and multiply. This is referred to as creating a reflective wave.

Common mode problems are associated with the flow from line to ground. Utility power produces three smooth waveforms that at any point average a sum of zero. This is an ideal scenario with no common mode effect. VFDs give power through pulses that on average achieve a sinewave. The sum at any point is never zero, which results in damaging common mode voltage and current that travel through the cabling and motor. Over time, common mode voltages can cause pitting to motor bearings and premature motor insulation failure. Common mode ground currents can cause unexpected ground fault trips and erratic behavior on VFD or PLC controller boards

•Passive Harmonic Filters are a combination of reactors andcapacitors that are tuned to eliminate some frequencieswhile passing others through. They lower harmonic effectsinside a facility and often meet IEEE-519 requirements.They have VFD protection characteristics, guaranteedstability and little to no power consumption.

•Active Harmonic Filters monitor harmonic distortion levelsand inject cancellation harmonic current into the line.They are more complex, using active components liketransistors or amplifiers in addition to passive resistors andcapacitors. They are usually designed for an entire systemand are placed near the facility’s power entrance. They will typically ensure IEEE-519 compliance, but do not alwaysensure that clean power is distributed throughout the facilityand can be sensitive to the environment.

Low voltage passive harmonic filter

Low voltage active harmonic filter

Medium voltage passive harmonic filter