The electrical harmonics are sinusoidal components whose frequency is an integer multiple of the fundamental frequency of a periodic signal.
So far we have assumed that alternating current is a pure and perfect sine wave of 50Hz (or 60Hz). That was true 50 years ago, when we only used filament bulbs and heaters.
But today we live in the age of electronics. And electronics don’t like to absorb current smoothly; they like to take it in bites.
This creates noise. And this noise in the electrical wave is called Harmonics.
Harmonics are the “cholesterol” of modern electrical grids. They barely existed before, but today, with so much electronics, they are a plague: cables burning up “for no reason,” computers restarting on their own, and motors whining.
What are harmonics
The name is a mathematical concept in signal analysis, which in turn takes its name from music.
- If you play an A on a piano (440Hz), that is the Fundamental Frequency.
- But it doesn’t sound the same as a violin playing the same note. Why?
- Because the piano also emits “daughter” frequencies at 880Hz, 1320Hz, etc., mixed with the main one. These are the harmonics, and they give the sound its “timbre.”
The same happens in electricity, except that here we don’t want timbre. We want a pure sound.
A harmonic is a parasitic wave whose frequency is an exact multiple of the fundamental frequency.
- Fundamental (Order 1): 50 Hz (Useful energy).
- 3rd Harmonic (Order 3): 50 x 3 = 150Hz.
- 5th Harmonic (Order 5):
. - 7th Harmonic (Order 7): 50 x 7 = 350Hz.
When you add the fundamental wave to these fast parasitic waves, the result is no longer a smooth sine wave. It is a distorted wave, pointy or flattened.
Linear and non-linear loads
Non-linear loads are a common source of current harmonics. Grid impedance and other sources also influence the resulting voltage distortion.
These are loads where the current follows the shape of the voltage. If voltage rises, current rises. Result: a clean wave. Zero harmonics.
Examples: Resistors (heaters, old bulbs), Direct-on-line induction motors.
These are devices that have electronics at their input (rectifiers, diodes, transistors).
These devices wait for the voltage wave to rise and then suddenly open the gate, take a “bite” of current to charge their capacitors, and close abruptly. They don’t absorb current continuously, only in abrupt pulses.
Typical Examples:
- Power Supplies (PC, TV, Chargers): They only draw current at the peak of the wave.
- LED Lighting: Electronic drivers switch at high speed.
- Variable Frequency Drives (VFDs): They control industrial motors by chopping up the wave. They are the world’s biggest harmonic factories.
- Arc Welders.
Effects of harmonics
You might think: “Ok, the wave is a bit ugly. So what?” The problem is physical and economic.
High-frequency current (e.g., 350Hz from the 7th harmonic) does not travel through the entire cable. It tends to travel only on the outer surface (Skin Effect). This reduces the cable’s effective cross-section. A cable that should handle 100A can get red hot with just 80A if it’s full of harmonics.
RCCBs (Residual Current Circuit Breakers) and MCBs (Miniature Circuit Breakers) can get confused. Sometimes they trip for no apparent reason because harmonic peaks fool the tripping mechanism.
Capacitor banks (which we installed to correct the Power Factor in Module 6) are prime victims.
Capacitors offer less resistance as frequency increases (
Triple harmonics and neutral current
This is the most curious and dangerous case in three-phase systems (offices full of computers).
In a balanced three-phase system, we said the Neutral current is ZERO, because the three phases cancel each other out (sum to zero).
But… the third harmonics (150Hz) DO NOT cancel out. By a mathematical quirk, the third harmonics from phase L1, L2, and L3 are in phase. Instead of canceling, they add up in the Neutral.
In a three-phase, four-wire installation, the triple harmonics from single-phase loads can add up in the neutral. Therefore, its current can be high even with balanced phases and must be included in the sizing.
In installations with a lot of electronics (Data Centers, offices), Neutral cables are installed with double the cross-section (twice as thick as the phases) to handle this return flow of harmonics.
Common orders
Harmonics are the electrical “pollution” generated by converting AC to DC abruptly.
| Order | Frequency | Typical Cause | Effect |
|---|---|---|---|
| Fundamental | 50 Hz | Generator | Useful energy. |
| 3rd | 150 Hz | PCs, LED Lights (Single-phase) | Neutral overload. |
| 5th & 7th | 250/350 Hz | Drives, 6-pulse Motors | Motor braking (reverse torque). |
How are they cleaned?
Using Harmonic Filters (active or passive) that inject an “inverse wave” to cancel the noise, much like noise-canceling headphones do.
