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5th Harmonics Explained

Definition, causes, measurment and solutions

What is 5th harmonic?

In the ideal case, voltages and currents in AC power systems are pure sine waves with 50 Hz frequency (or 60Hz).

However, voltages (or currents) in the actual power system have additional components, called harmonics, whose frequencies are integral multiples of the power system frequency. Harmonics are multiples of the fundamental frequency and can therefore be expressed as 1f, 2f, 3f, 4f, 5f, etc.

Figure1 is a plot of harmonics (up to 50th order) measured from a power analyzer for an AC Power System of a beverages plant. The yellow bar represents the 5th harmonics.

 Figure 1: Harmonic content measured in AC power system of a Blow Moulding line of a beverages plant

Usually, power analyzers available in the market measure harmonics up to 50th order.

What is the frequency of 5th harmonic?

From above, you may conclude that the 5th harmonic for a 50 Hz power system is 250 Hz. The following illustration shows one cycle of a sinusoid with a peak amplitude of 1.00 (labeled as the fundamental). The other two waveforms shown in the figure are the third harmonic and the fifth harmonic.

5th harmonic explained

Note that the third harmonic completes three cycles during the one cycle of the fundamental and the fifth harmonic completes five cycles.

The 5th harmonic component is added to the fundamental one which yields a distorted signal as shown below:

Fifth Harmonic waveform

What causes 5th order harmonic?

Harmonics are generated due to non-linear loads as explained here. Most of the time, fifth order harmonics are present in distorted signals caused by non-linear loads such as:

  • Variable Frequency Drives (VFDs)
  • Power electronic converters such as rectifiers
  • LED lighting
  • Arcing devices
  • Uninterrupted Power Supplies (UPS)
  • Electric vehicle chargers, etc

Is 5th harmonic a negtaive or positive sequence?

To answer this question, we should define what we mean by positive and negative sequences.

In a balanced three-phase system, we have phase A is at 0 degrees, phase B is shifted by 120 degrees, and phase C by 240 degrees as shown below:

Three Phase System
  • If harmonics of a given order respect the same shifting: Harmonic of phase A, then harmonic of phase B, and after that harmonic of phase C, we say that the harmonic in question is a positive sequence.

The following image shows the 7th harmonic of each phase where they respect the same order: Harmonic of phase A, then harmonic of phase B, and after that harmonic of phase C (yellow, pink, and cyan). This means that the 7th harmonic is a positive sequence.

Positive sequence 7th harmonic

What about the 5th harmonic? is it a positive or a negative sequence? short answer: 5th harmonic is a negative sequence!

Indeed, the 5th harmonic is a negative sequence as shown in the following image where 3 phase harmonics do not respect the order: Harmonic of phase A, then harmonic of phase C, and after that harmonic of phase B (yellow, cyan, and pink).

How do you measure 5th order harmonic?

As we mentioned above, harmonics are measured by a Power Quality Analyzer which can measure up to 50th order of harmonics. It should be noted that some analyzers have the provision to measure harmonics up to 99th order.

Measurement is done in THD (total harmonic distortion) and in an individual order where the THD graph displays the contribution of the 5th harmonic to the total distortion in the power system.

How to eliminate 5th order harmonic?

Many solutions can be adopted to reduce or eliminate the 5th harmonic in the power system. To cite a few:

  • Using passive harmonic filters
  • Using active power filters
  • Using reactors for local loads
  • Adding isolation transformers
  • Using 12 pulse drive rectifiers instead of 6 pulse drive version.

Conclusion

In this short article, we highlighted the 5th harmonic in power systems. This includes its definition, why it is considered as a negative sequence, its causes, and how it can be measured. In the end, we presented some solutions to reduce (or even cancel) the 5th harmonic.

Further reading