Imagine we’re supplying our electrical equipment with a nice, clean sinewave current. In theory, that should be perfect. But what actually comes out isn’t always the same—it’s distorted. That distorted waveform is what we call an electrical harmonic.

Now, here’s the issue: harmonics are like unwanted guests in our power systems. They sneak in at higher frequencies than the fundamental frequency, and that’s when problems begin.

Where Harmonics Come From

So, where do these uninvited harmonics actually come from? They’re usually produced by nonlinear loads—things like electronic ballasts, variable frequency drives (VFDs), battery chargers, or other modern electronic devices.

Whenever these devices convert AC to DC, or the other way around, they rely on high-speed switching. That switching distorts the waveform and creates harmonics.

Now that we’ve got a handle on what harmonics are, let’s look at the different types and see how they affect our electrical systems.

The First Harmonic

We’ll start with the fundamental harmonic. This is the base frequency that comes straight from the generator. Think of it as the rhythm our electrical world dances to.

In most countries, this fundamental frequency—also called the first harmonic—is either 50 Hz or 60 Hz. This isn’t a random choice. It’s a carefully considered standard because all of our appliances are designed to run at this frequency. It’s like a global electrical heartbeat that keeps everything—from our fridges to traffic lights—running in sync.

So, in simple terms, the first harmonic sets the pace. And understanding it helps us see how the other harmonics disrupt the system.

The Second Harmonic

Next up is the second harmonic. This one has twice the frequency of the fundamental. So, if our fundamental is 50 Hz, the second harmonic comes in at 100 Hz.

Here’s where it gets interesting: when the fundamental harmonic reaches zero, the second harmonic is at its peak, and vice versa. This flip in direction creates what we call a negative sequence current.

And let’s be clear—this negative sequence current is no friend to our system. In an induction motor, it works against the rotating magnetic field. The result? Less torque. And less torque means less power to do the work we need.

The Third Harmonic

Now, let’s move on to the third harmonic, also called a triplen harmonic. This one runs at three times the fundamental frequency—so if we’re at 50 Hz, the third harmonic hits at 150 Hz.

Unlike the second harmonic, this one creates a zero sequence current. That means when the fundamental current drops to zero, the third harmonic current does too. But here’s the kicker: when the fundamental current is at its peak, the third harmonic peaks on the negative side.

What does this mean for us? Well, zero sequence current leads to an increase in the neutral voltage. And when the neutral voltage climbs too high, protection devices like relays step in and trip the breaker. Definitely not ideal for a running system.

The Fourth Harmonic

Then there’s the fourth harmonic, four times the fundamental frequency. That makes it 200 Hz if we’re on a 50 Hz system.

This one is more in sync with the fundamental. When the fundamental peaks, the fourth harmonic does too. Same for the troughs. It might sound harmless, but it’s not. This pattern increases the current flow in our conductors.

More current means more heat, and that heat can damage our equipment if it isn’t managed properly. That’s why we need to keep a close eye on fourth harmonics.

And here’s a fun fact: the fourth harmonic is called a “positive harmonic.” Not because it’s good for our systems, but simply because of its phase sequence.

The Fifth Harmonic

Finally, we have the fifth harmonic. At 250 Hz, this one behaves a lot like the second harmonic. When the fundamental drops to zero, the fifth harmonic rises, and vice versa.

What this does is create a reverse phase order. In induction motors, that reverse sequence actually tries to rotate the motor in the opposite direction. So, just like the second harmonic, the fifth can cause braking effects and reduce motor performance.

Wrapping It Up

So, to bring it all together: harmonics are unwanted distortions in our current or voltage waveforms. They can heat up conductors, reduce motor torque, and increase neutral voltage. Unlike positive and negative sequence harmonics that can cancel each other out, triplen harmonics—like the 3rd, 6th, and 9th—don’t cancel. That makes them especially tricky to deal with.

In practice, we often use reactors or filters to manage these harmonics. But the key takeaway is this: the more unwanted harmonic current flows in our system, the less efficient and reliable it becomes.