Choosing the optimum solution for power-system harmonics

From Electrical Installation Guide

Figure L31 below shows the criteria that can be taken into account to select the most suitable technology depending on the application.

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	Passive filter	Active filter	Hybrid filter
Applications … with total power of non linear loads (variable speed drive, UPS, rectifier…)	Industrial	Tertiary	Industrial
	greater than 200 kVA	lower than 200 kVA	greater than  200 kVA
Power factor correction		No
Necessity of reducing the harmonic distorsion in voltage for sensitive loads
Necessity of reducing the harmonic distorsion in current to avoid cable overload
Necessity of being in accordance with strict limits of harmonic rejected	No

Fig. L31: Selection of the most suitable technology depending on the application

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For passive filter, a choice is made from the following parameters:

• Gh = the sum of the kVA ratings of all harmonic-generating devices (static converters, inverters, speed controllers, etc.) connected to the busbars from which the capacitor bank is supplied. If the ratings of some of these devices are quoted in kW only, assume an average power factor of 0.7 to obtain the kVA ratings
• Ssc = the 3-phase short-circuit level in kVA at the terminals of the capacitor bank
• Sn = the sum of the kVA ratings of all transformers supplying (i.e. directly connected to) the system level of which the busbars form a part

If a number of transformers are operating in parallel, the removal from service of one or more, will significantly change the values of Ssc and Sn. From these parameters, a choice of capacitor specification which will ensure an acceptable level of operation with the system harmonic voltages and currents, can be made, by reference to Figure L32.

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General rule valid for any size of transformer
Standard capacitors	Capacitor voltage rating increased by 10% (except 230 V units)	Capacitor voltage rating increased by 10% + harmonic-suppression reactor
Simplified rule if transformer(s) rating Sn ≤ 2 MVA
			Gh > 0.60Sn
Standard capacitors	Capacitor voltage rating increased by 10% (except 230 V units)	Capacitor voltage rating increased by 10%  + harmonic suppression reactor	Filters

Fig. L32: Choice of solutions for limiting harmonics associated with a LV capacitor bank supplied via transformer(s)