Computer, SMPS power supplies, Single Phase UPS and Electronic devices, and LED lighting create triplen harmonic current, also called third harmonic or zero sequence harmonic; which unlike the normal current and also the industrial harmonic currents, do not get cancelled at neutral but get added and triple at neutral. That's why such load types have very high neutral current. All odd multiples of third harmonics i.e. 3rd, 9th, 15th, 21st, and so on which get generated at zero phase sequence with respect to neutral, are collectively referred to as Triplen harmonic or Third harmonics or Zero sequence harmonic current.

Triplen harmonic filter is installed in parallel at downstream load DB. One filter for each load DB, like for 3ph UPS at output DB, Several 1ph UPS combined at input 3ph DB, LED lighting combined at 3ph supply DB, for Elevator/Lift at 3ph lift-room DB etc. It attenuates zero sequence harmonic current as it offers a least resistance path to its flow. Thus suppresses the high neutral current. Apart from eliminating high neutral current, triplen harmonic filter delivers several other benefit by providing betterment of the electrical power system.

Zero sequence filter is a high efficiency Surge Suppression Device, than the best of best SPD manufacturer could achieve. It takes two phase current (and also voltage), say R and Y-phases in a single limb in two opposite direction winding. Similarly Y and B-phases and B and C-phases in other two limbs. It thus directly and permanently locks down all the three phase voltages with respect to neutral. That eliminates most efficiently, the voltage (and also current) surge. In addition it balances 3phase voltage and current unbalance.  

With regard to momentary voltage fluctuation/ sag, it's an efficient voltage stabiliser than the best of best UPS and voltage stabilizer manufacturers could do. Momentary voltage fluctuation is passed over to load by UPS and voltage stabilizers. Whereas by directly locking down the 3ph voltage wrt neutral, it attenuates any momentary voltage fluctuation/ sag incident. It greatly eliminates tripping and failure with sensitive electronic equipment like computers, servers, lab equipment and diagnostic and bio-medical electronics.    

Summary of Triplen harmonic filter Benefits:

1.    Helps in eliminating building's Electrical Fire Risk. Please refer to above/main article for detailed technical write up.
2.    Eliminates high neutral current from UPS and LED DB. Slashes Energy cost and Enhances UPS battery life.
3.    Eliminates Elevator maloperation- Lift-door not opening. Happens as 3rd harmonic enter lift neutral system.
4.    SPD: The most efficient Surge Protection Device. Permanently locks and balances 3ph Volt & Currents w.r.t. Neutral. Eliminates Surge & fluctuation related Failure of sensitive Electronics/ Server/ Lab equipment.
5.    Saves DG Alternator from 3rd harmonic problems of hunting, unbalance voltage and Stator winding burning.