earlier but included in the equations include v ll which is the line to line voltage which in north america is 480v z which is the percentage of impedance from a normalized per unit set and is the measure of grid u201c stiffness u201d and is an important factor in sizing the filter u221a 3i vsd where i vsd is the current to the variable speed device s and the square root of 3 is used because the illustrated system is 3 phase and when you compute power using v ll it requires the square root of 3 further still q p and q s are the series and parallel quality factors corresponding to the resonance and anti resonance points u03c9 s and u03c9 p are the series and parallel resonance and anti resonance frequencies note that the series resonance is the higher frequency lower impedance portion of fig5 while the parallel resonance is the lower frequency higher impedance referring briefly to fig5 there is illustrated a frequency response graph showing a filter designed with integrated power factor correction capacitor s vs a filter designed with external power factor correction capacitor s the resonance and anti resonance frequencies are selected such that the filter design yields appropriate attenuation of the harmonics most significantly using the design process described with reference to fig4.