Triplen harmonics are the odd multiples of third harmonics. The triplen harmonics orders are 3,9,15,21,. The triplen harmonic frequency is 150Hz,450 Hz,,750Hz etc.
These harmonics are common in power systems with nonlinear loads such as computers, printers, and fluorescent lighting. Unlike other harmonics, triplen harmonics do not cancel out in three-phase systems but instead accumulate in the neutral, leading to overheating and transformer stress. In this article, we’ll explore their meaning, sources, effects, and practical examples.
What are Harmonics in AC Circuits?
Any nonperiodical waveform has a sinusoidal component and integral multiples of the sinusoidal components. The integral multiples of the sinusoidal component or fundamental components are called harmonics. If the current exactly follows the same waveform as the voltage, the current waveform has only fundamental current, and no harmonics exist.
Read detailed article: Harmonics in AC circuit
However, if the current waveform is non-linear to the voltage waveform, the current waveform contains harmonics. The order of harmonics depends on the shape of the current waveform, If the current waveform is symmetrical about the x- axis it does not contain even harmonics and it contains odd harmonics.
The order of harmonics can be categorized as even and odd harmonics. The even harmonic order is 2,4,6, 8, etc., and the odd harmonic order is 3,5,7,9,11, etc. The frequency of harmonics order is;
fn = n x f
Where,
fn = Harmonic frequency
n = Harmonic Order
f = Fundamental frequency
If the fundamental frequency is 50 Hz, the second-order harmonic frequency is 2 x 50 = 100 Hz.
Apart from dividing harmonics into even and odd order, triplen harmonics are another category of harmonic order.
What are Triplen Harmonics?
The odd multiples of third harmonics are triplen harmonics. The order of triplen harmonics orders are 3,9,15,21,.
Sources of Triplen Harmonics
Single-phase nonlinear loads, such as computers, printers, or any single-phase electronic equipment, draw nonlinear current from the sinusoidal voltage source and generate substantial third-order harmonics.
Why Triplen Harmonics are Troublesome?
If the single-phase nonlinear loads are connected to the three-phase four-wire system, the triple harmonic current caused by nonlinear loads does not cancel out. Rather, it gets added. If the triplen harmonic current in each phase of the three-phase four-wire system is 1 ampere, the current in the neutral wire will be 3 amperes. Thus, the current flowing in the neutral overheats the conductor.
Effects of Triplen Harmonics on Power Systems
The third harmonic causes the following adverse effects on the power system.
• Overloading and overheating of neutral conductors (up 173% of phase current).
• Overheating of delta winding in distribution transformers.
• High earth to neutral voltages.
• Distortion of voltage waveform (including ‘flat topping’).
• Poor power factor.
Triplen Harmonic Currents in Neutral Conductor
In a balanced three-phase system, the fundamental currents of each phase are 120° apart, so they cancel out in the neutral. However, triplen harmonics (3rd, 9th, 15th, etc.) are in-phase across all three phases. Instead of canceling, they add up in the neutral conductor.
This means the neutral current can be up to three times the phase harmonic current, which often leads to overheating of the neutral wire.
Practical Example:
- Assume each phase has a 3rd harmonic current of 5 A.
- Because the 3rd harmonic currents are in-phase, they add up in the neutral:
Ineutral =Iphase−A + Iphase−B + Iphase−C
Ineutral =5A + 5A + 5A =15A
Thus, while each phase only carries 5 A of 3rd harmonic current, the neutral is forced to carry 15 A, which can exceed its rated capacity and cause serious overheating.
Triplen Harmonics in Transformers
Triplen harmonics cause serious issues in transformer operation. Since these harmonics are zero-sequence components, they circulate within the delta winding of a transformer. This circulation increases transformer copper losses and causes overheating.
In a delta–star transformer, the delta winding traps most of the triplen harmonic currents and prevents them from flowing into the supply system. However, this protection comes at the cost of extra heating inside the delta winding.
If large amounts of triplen harmonics are present, the transformer may suffer from:
- Overheating of delta windings
- Increased core losses due to flux distortion
- Reduced efficiency and lifespan
Example:
If a transformer supplies single-phase nonlinear loads (like office computers, printers, UPS systems), the 3rd harmonic currents generated in each phase accumulate in the neutral of the star-connected side. Simultaneously, significant triplen currents circulate in the delta winding, stressing the transformer.
How to Mitigate Triplen Harmonics
Triplen harmonics cannot be completely eliminated, but their effects can be reduced through proper system design and corrective measures. Some effective methods include:
1. Use of K-Rated Transformers
K-rated transformers are specially designed to handle non-linear loads and harmonic currents. They have reinforced neutral capacity and additional cooling to withstand the heating caused by triplen harmonics.
2. Harmonic Filters
Passive and active harmonic filters can be installed to absorb or cancel harmonic currents. These filters are particularly effective in reducing 3rd-order and higher harmonic currents.
3. Load Balancing
Balancing single-phase nonlinear loads across different phases reduces the accumulation of triplen harmonics in the neutral conductor.
4. Neutral Conductor Sizing
Oversizing the neutral conductor ensures it can safely carry the higher currents caused by additive triplen harmonics. This prevents overheating and improves system reliability.
Conclusion
Triplen harmonics are the odd multiples of the third harmonic (3, 9, 15, 21, …) that commonly appear in systems with nonlinear single-phase loads. Unlike other harmonics, they do not cancel out in three-phase systems but instead accumulate in the neutral conductor, leading to overheating, transformer stress, and voltage distortion.
Understanding their meaning, sources, and effects is essential for engineers and system designers. By considering mitigation methods such as K-rated transformers, harmonic filters, load balancing, and proper neutral conductor sizing, the harmful impact of triplen harmonics can be significantly reduced.
Triplen harmonics may seem like a hidden issue, but addressing them is critical for power quality, system safety, and long-term equipment reliability.
FAQs on Triplen Harmonics
The 3rd harmonic is a current or voltage component with a frequency three times the fundamental frequency. For a 50 Hz system, the 3rd harmonic occurs at 150 Hz.
Triplen harmonics are zero-sequence components, meaning they are in phase across all three phases. Instead of canceling, they add together in the neutral conductor, causing overheating.
In delta–star transformers, the delta winding traps circulating triplen currents, preventing them from reaching the supply system. However, this circulation causes heating and increased copper losses in the transformer.
Mitigation methods include using K-rated transformers, installing harmonic filters, balancing nonlinear loads, and oversizing the neutral conductor to handle higher currents safely.
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