Harmonics and Power Quality

Power Quality relates to the amplitude, frequency and distortion of the supply system. While the amplitude and frequency of the supply is largely controlled by the utility, the distortion of the wave (voltage or current) is attributed to the user (of the power) or the loads. Linear loads like an induction motor, an incandescent bulb, resistive heating or a capacitor bank draw a
current which follows the wave shape of the supplied voltage (sinusoidal). On the other hand, most of the common loads nowadays are non-linear, like VSDs (Variable Speed Drives), rectifiers, UPS-systems, computers, TV sets, energy efficient (fluorescent) lamps, photocopiers etc. These loads draw a current from the source which does not follow the voltage wave shape
and hence introduce distortion.

The distortion is introduced by harmonics generated by the non-linear devices. These harmonics are a growing problem for both electricity suppliers and users.

Harmonics can lead to serious problems:

• overheating of cables, motors and transformers
• damage to sensitive equipment
• tripping of circuit breakers
• blowing of fuses
• premature ageing of the installation
• capacitor overloading and failures
• high current in neutral conductors excitation of network resonance

The ABB Solution:
PQF Power Quality Filter

The ABB Power Quality Filter offers unprecedented ability to clean the network from harmonics. The PQF actively eliminates the harmonics present in the supply system in a controlled way. It is insensitive to large network impedance changes due to change in network topology like paralleling of sources, or switching between mains supply and generator operation.

The PQF monitors the line current in real time and processes the measured harmonics as digital signals in a high-power multi-DSP (Digital Signal Processor) based system. The digital controller generates Pulse Width Modulated (PWM) signals that drive IGBT power modules which through line reactors inject harmonic currents in the network with exactly the opposite phase to the components that are to be filtered.

The PQF also offers communication facilities with the customer?s existing communication network. This feature which uses Modbus RTU, allows the PQF to be easily monitored and controlled from a remote location. The Modbus communication feature can be used by means of an RS-232 to RS-485 converter (optional).