Harmonic distortion of the AC power lines in HVAC systems

             

CAUSE OF POWER LINE DISTORTION

Most adjustable frequency drives operate by using a bridge rectifier to convert the incoming AC 

voltage to DC voltage. An inverter in the drive then converts the DC voltage into a precise output voltage and frequency to control the speed of the motor.

Drives today use a diode bridge rectifier to convert the AC line power into a fixed-voltage DC bus . A DC bus capacitor bank is then used to filter out the AC ripple.

While this results in a very efficient drive, it can cause disturbance on the AC power line due to the way the drive draws AC current.

Current cannot flow from the rectifier into the DC bus until the input voltage is greater than the DC bus voltage . This only happens for a very short period of time for each phase.

This causes a non‐sinusoidal current flow created by the input stage of the drive. In order to transfer the energy required by the motor in such a short period of time, the peak current must be high.

source:http://electrical-engineering-portal.com/