When Less Load Means More Voltage: Understanding the Ferranti Effect

• July 7, 2025
• Gaurav Joshi

“Sometimes, the absence of a load can cause more trouble than its presence.” It sounds strange, doesn’t it? But that’s exactly what happens in systems that carry high power. One name for this effect is the Ferranti Effect. Say you’re sending electricity at a fixed voltage, like 400 kV, but when it gets to the other end, it shows a shocking 510 kV. In the world of power systems, that’s a red flag. 

It’s important to know why this voltage spike happens when the load is disconnected or very light, whether you’re an electrical engineer working on-site or a design worker using simulations. This doesn’t just happen in theory; it happens in real transmission lines. Let’s figure out the easiest and most useful way to break this down.

Table of Contents

1. What Is the Ferranti Effect?
2. Why Does It Happen?
3. Real-World Example Using Simulation
4. Why Should You Care?
5. How Do We Fix This?
6. Key Takeaways
7. Conclusion

What Is the Ferranti Effect?

When a lengthy transmission line is either lightly loaded or entirely emptied (no-load condition), the voltage at the receiving end rises; this phenomenon is known as the Ferranti Effect.

You read that right the voltage actually increases instead of decreasing.

Think of it like sending a parcel to someone and somehow they receive a much bigger one than you actually sent. Weird? But that’s what happens due to the reactive components, specifically the capacitance present in the transmission line.

Let’s Visualize the Scene

You have a generating station producing electricity at 11 kV, and before transmitting it over a long distance, you step it up to 400 kV (to reduce transmission losses, of course). All good so far.

Now, at the receiving end, you expect the voltage to be close to what you sent. That being said, it’s not 400 kV when you measure it. It’s 510 kV.

Wait, what?

That is not a mistake or a flaw, but rather the Ferranti Effect at work. And when this happens, the substation equipment (designed for 400 kV) is now staring down a dangerous overvoltage situation. Long exposure to this can cause insulation breakdowns, equipment failure, and even system blackouts.

Why Does It Happen?

Get back to the basics if you want to know what caused it.

Every transmission line has:

• Resistance (R) – thanks to impurities in the conductor
• Inductance (L) – because it carries alternating current
• Capacitance (C) – due to the geometry of conductors and the surrounding insulation (including air)

So, a transmission line will have resistance, inductive reactance & capacitive reactance. 

Under normal full-load conditions, the transmission line is drawing full load current (which helps in building the inductive reactance) and also a small current to satisfy the capacitive reactance. In this full load condition, inductive and capacitive effects sort of cancel each other out because they are almost equal & opposite to each other. That’s why the voltage at the sending and receiving ends stays relatively equal.

But remove the load or reduce it significantly, and this balance breaks.

Here’s what happens:

• There’s hardly any current flowing (because there is no load or very less load).
• The inductive voltage drop reduces (since inductors need current to create opposition).
• But the capacitive effect remains, and the transmission line draws a small amount of current to satisfy the capacitive reactance. Currently, there is no inductive reactance as a result, the capacitive reactance dominates the system.
• This capacitive voltage drop adds throughout the transmission line, and this leads to an excess voltage build-up at the receiving end.

This is similar to how a balloon inflates. If you keep pumping air (voltage) into it (the receiving end) and there’s no outlet (load), pressure builds up. Boom! You get a voltage spike.

Real-World Example Using Simulation

In a basic simulation, with the load connected, the sending voltage is 353 kV and receiving voltage is around 356 kV well within acceptable limits.

Now disconnect the load.

Boom! While the sending end voltage stays the same, the receiving voltage surges to 522 kV. This shows how powerful reacting parts really are. And yes, it’s real, not just some academic concept.

Why Should You Care?

If you’re in the electrical industry, especially working with EHV (Extra High Voltage) systems like 400 kV or 800 kV lines, this is a critical effect you need to manage. It’s mostly observed in long-distance lines (200 km and above), and even more so in underground or cable systems where the capacitance is naturally higher.

According to popular belief, “an ounce of prevention is worth a pound of cure.” Perhaps you are familiar with that proverb.

How Do We Fix This?

Enter the reactor. Not the Hollywood kind, but the real MVP in electrical systems a large inductive device added at the receiving end to balance the capacitive effect.

What it does:

• Injects inductive reactance
• Cancels out the excess capacitive charging current
• Brings the receiving voltage back to normal

Reactors are your first line of defense against the Ferranti Effect. They’re especially common in systems above 245 kV, while lines below that usually don’t require them.

Key Takeaways

• The Ferranti Effect occurs due to excessive capacitance when the line is lightly or not loaded.
• It’s more severe in long transmission (above 200kms) lines and cable systems.
• The high voltage can damage expensive switchgear.
• Inductive reactors are recommended for voltage stabilization.

Conclusion

When you’re dealing with power systems, voltage stability is not just a number it’s the safety and longevity of your entire infrastructure. The Ferranti Effect may not ring alarm bells in theory class, but in the field, it’s the silent saboteur.

So the next time you see an unexpected voltage rise without a load, don’t just scratch your head remember: it’s Ferranti, not fantasy.

Still curious about how this voltage rise actually looks in a real system? Check out this video.