Basic Datas of Harmonics and VFD

Harmonic Distortion

Harmonic Distortin is a measure of the amount of deviation from a pure sinusoidal wave form that can be caused by a non-linear load (a VFD is considered a non-linear load because it only draws current from the power line as required).

Distorted Waveform

Distorted Waveform = fundamental (50Hz sinusoidal waveform) + multiples of the fundamental frequency, typically the 5th, 7th, 11th, 13th, and 17th, etc. (e.g. 5 x 50Hz = 250Hz, 7 x 50Hz = 350Hz, etc.)

Voltage Distortion  THDv

Voltage Distortion is the deviation in the supplied voltage from a pure sinusoidal waveform.

Current Distortion  THDi

Current Distortion is the deviation in current drawn by a non-linear load from a purely sinusoidal waveform.

Pulse Width Modulation PWM

PWM is a means by which nearly sinusoidal current can be caused to flow in a motor, at a desired frequency (50, 60Hz for example), by sinusoidally varying the pulse width of a much higher frequency squarewave.

Variable Frequency Drives (VFDs)

Variable Frequency Drives (VFDs) sometimes referred to as “variable speed drives.”

VFDs Add Efficiency…But Create Power Quality Issues

Adding a VFD to electrically control motor speed increases efficiency by limiting energy consumption and reducing equipment wear. However, VFDs inadvertently create power quality issues in the form of harmonics, which are changes or distortions in the waveforms. There are different types of harmonic distortions on the line and load side of a VFD.

On the line side, harmonics are created because the incoming side of a standard six-pulse VFD does not draw current linearly. Instead, it pulls in six distinct pulses that each use some of the current and send the unused portion back to the source as current harmonics. Since current and voltage are proportional, current harmonics ultimately affect voltage, which impacts connected equipment. This is not only a problem for equipment. In the same facility as the offender, but for any consumer who shares a connection to the facility.

On the load side, the VFD does not feed the motor a true voltage waveform, but rather series of electrical pulses in a pattern that replicates an AC sinewave. This is called pulsewidth modulation (PWM) and although it is effective at precisely controlling motor speed, it creates harmonics that can damage motors, cables and bearings. The problems are magnified if the physical distance between the VFD and motor is long.

Harmonic Symptoms/Concerns

Equipment Failure and Misoperation

– Notching (electronic control malfunctioning, regulator misoperation)

– Overheating/Failure (transformers, motors, cables/neutral)

– Nuisance Operation (fuses, breakers, PC lock-ups)

– Insulation deterioration

– Capacitor resonance / failure

Economic Considerations

– Oversizing neutrals, transformers, generators

– Losses/Inefficiencies/PF Penalties

– Inconsistent meter reading

ZDDQ Active Harmonic Filter AHF

ZDDQ-AMS Active harmonic filter is connected in parallel with non-linear loads, and uses one set of CT to detect the load current. It calculates each order harmonic current by FFT algorithms in its DSP microchips, and then generates a compensating current with the same amplitude but opposite phase angles to the detected harmonic current, which cancels out the original load harmonics.Not only eliminates harmonic current from the load side, but it also mitigates harmonic voltage caused by harmonic currents. The APF system can also improve power factor (PF) and correct load imbalances in the power system.