Technical Articles
Triplen harmonics have frequencies with the multiple of 3, like 3rd harmonic (150 Hz), 9th harmonic (450 Hz), 15th harmonic (750 Hz) and so on. These harmonics have some peculiar characteristics, hence very difficult to handle and mitigate. This article explains about the basics, sources, effects and mitigation techniques of triplen harmonics.
Triplen harmonics are usually generated by single phase non-linear loads that share common neutral. Some of them are,
These types of loads are common in IT parks, office, hospital and other commercial buildings. Following are the typical current harmonic spectrum of SMPS and CFL lamps, where the third harmonic is predominantly high.
All the triplen harmonics are zero sequence in nature and they are exactly in-phase in all the three phases. Whereas the fundamental component (50 Hz) has positive phase sequence and the phases are displaced by 120° between the three phases.
Neutral current is the vector sum of all the three phases (120° phase displaced) and the sum is zero, if the loads are balanced in all the three phases. But the presence of triplen harmonics will result in the flow of neutral current, even if the loads are balanced. This phenomenon explained below:
The accumulated neutral current, because of the additive triplen current, can go up to 200% of the phase current.
Ill effects of triplen harmonics:
Most commonly used passive filter for triplen harmonics is 14% detuned filter. This filter has the tuning frequency of 133 Hz, which is below the 3rd harmonic (150 Hz). This filter is very effective in averting the harmonic amplification caused because of 3rd harmonics & above and thereby protecting power capacitors and other equipments. But detuned filter will not eliminate the harmonics completely.
Ideally zig-zag transformers (or chokes) are used for generating neutral point in three phase three wire systems. The generated neutral point in the transformer helps in trapping the zero sequence currents (triplen harmonics and earth fault current). The zig-zag winding connection is shown in the below figure:
The zigzag transformer contains six coils on three cores/limbs. Each limb carries two windings, similar to primary and secondary. The first coil on each core is connected contrarily to the second coil on the previous/next core. The second coils are then all tied together to form the neutral and the phases are connected to the primary coils.
If three currents, equal in magnitude but 120° out of phase with each other, are applied to the three terminals, the ampere-turns in the windings cannot cancel and the transformer restricts the current flow to the negligible level of magnetizing current. Therefore, the zigzag winding provides an easy path for in- phase currents (triplen harmonics) but does not allow the flow of currents that are 120°out of phase with each other.
The zig-zag transformers are relatively smaller in size and can be installed close to the group of single phase loads that generate triplen harmonics. These transformers are more effective during balanced load condition. Thus the harmonics are trapped at the source and neutral overloading can be avoided.
These two are cost-effective & safe solutions for triplen harmonics mitigation. However, detuned filter or zig-zag transformers do not eliminate the harmonics completely. In order to achieve that, active harmonic filters with three-phase four-wire configuration should be used.
Rajesh R Shirodkar,
DGM-Corporate CommunicationRajesh Shirodkar is a seasoned marketing and business leader with over 20 years of experience in the automation and electrical industry, spanning marketing communications, sales, business development, and software solution selling. He has led successful brand transformation initiatives and high-impact marketing programs, including branding and account-based marketing campaigns, as well as industry events that drive visibility, engagement, and growth. With expertise in brand building, lead generation, solution selling, and sales enablement, Rajesh is known for translating technology and business offerings into compelling value propositions and delivering sustainable growth through strong stakeholder collaboration.
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