HARMONIC FILTERING
 
In today's electrical system, several types of common equipment generate current distortion's in any electrical network. Those distortions, named harmonics, can be modelized as a collection of sinus current at different frequencies and amplitudes added to the basic network current. They cause many different sorts of problem, such as :
  • Increased losses.
  • Malfunctioning of control system.
  • High currents in neutral phase and neutral.
  • Resonance problems.
  • Interface with telecommunications.
  • Interface with computer traffic.
Further more, modern electrical equipment impose stringent demands on voltage stability and curve shape. Therefore, today's industrial system's and power-distribution network need harmonics filters to have low distortion values and be free from harmonics.
Power harmonics filters are built up from passive RLC components, i.e. resistors, reactors and capacitors which are sized so that they together form a tuned circuit.
Basicaly harmonics filters are built up from a passive LC components (i.e. reactors and capacitors) which are sized in order to shunt off most of harmonics. However, it is never possible to adjust a filter precisely. There are three reasons for this : lack of precision in the original adjustment, possibility of fluctuation in the supply frequency, change in capacitance due to temperature variations. As a result, each filter has an additional resistor which serves to increase the effective bandwidth and make it steadier and easier to adjust.
 
 

DESIGNING OF POWER RESISTORS FOR HARMONICS FILTER
 
The filter resistor as other components of filter (Capacitor, Reactor) must withstand the same stress in terms of voltage, of current and environment conditions with in addition, a thermal stress due to energy dissipation.
This energy is a combination of a permanent (rms) current flow and from time to time of an additionnal overload as a result of a temporary extra current flow.

The variation between Resistance value at ambient temperature and resistance value at operating temperature depends on the Resistance coefficient of the material used.
The temperature of resistor must be limited, so as not destroy resistor or components.
The limitation of temperature to a reasonable limit threshold is therefore an imperative requirement obtained differently according to the partiicular application.

The designing of resistors for harmonics is based on :

1. Power dissipation
This power is partially stored in the metal or the material of the heating element.
The non stored power has to be transferred to the surroundings either by radiation or by convection which depends on :
  • The design of the resistor.
  • The surface of the heating elements.
  • The temperature difference between ambient air and the heating elements.
2. Resistance value
When a resistor is designed from the thermal point of view, the resistance value has to be achieved to adapt the resistor to the required power supply.

3. Voltage resistance
The resistor must be design in accordance with various limits (grid-banks - enclosures).

4. Current resistance
Connections have to be carefully designed and examined because the intervention of high current transients can create undesirable electromagnetic forces.

5. Miscellaneous
Environmental and site conditions.

 
 
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