Why Are Shunt Reactors Used in Power Systems?

• July 12, 2025
• Gaurav Joshi

Imagine you’re transmitting power at 420 kilovolts (kV) over a 200-kilometer transmission line. By the time it reaches the receiving end, the voltage is not 420 kV anymore, it’s 600 kV. That’s a massive increase, and yes, this happens in real-world power systems. The reason for this voltage rise is a phenomenon called the Ferranti Effect.

Without shunt reactors in the system, the situation can worsen, leading to system instability, damage to equipment, and significant operational challenges. This article explores why shunt reactors are used in power systems, particularly in long-distance high-voltage transmission lines, and how they prevent problems caused by voltage surges.

Table of Contents

• Understanding the Ferranti Effect
• Transmission Lines as Capacitive Networks
• The Balance Between Inductance and Capacitance
• The Problem with No Load or Light Load
• A Real-World Example Explained
• How Shunt Reactors Solve the Issue
• Simulating the Effect: What Happens in Practice
• The Concept of Surge Impedance Loading
• Fixed vs. Variable Shunt Reactors
• Key Takeaways

Understanding the Ferranti Effect

The Ferranti Effect refers to the condition where the voltage at the receiving end of a long transmission line is higher than the voltage at the sending end, particularly when the load is very light or absent. This effect becomes prominent in transmission lines longer than 200 kilometers.

Under light load conditions, the system becomes more capacitive. Capacitive reactance begins to dominate due to the physical configuration of the lines, resulting in higher receiving end voltage.

Transmission Lines as Capacitive Networks

Transmission lines aren’t just conductors. Because they’re suspended above the ground and spaced apart, they form a capacitive network. There are two main types of capacitances at play:

• Line-to-ground capacitance: between the conductor and the earth
• Line-to-line capacitance: between two parallel conductors

These physical structures act just like capacitors in a circuit. Meanwhile, since the conductors carry alternating current (AC), they also introduce inductance. The interplay between these two capacitance and inductance defines how the system behaves.

The Balance Between Inductance and Capacitance

All the way up to full load, the inductive and capacitive effects tend to balance each other out. The reactive powers cancel each other, and the voltage remains stable across the transmission line.

However, when the load is minimal or absent, this balance is lost. Inductive reactance, which is current-dependent, drops significantly because of the low current. Capacitance depends on voltage and stays mostly the same, it continues to affect the system. As a result, the voltage at the receiving end increases a condition that’s neither safe nor acceptable.

The Problem with No Load or Light Load

If there is no load or only a light load on the transmission line, capacitive charging current continues to flow and builds up the voltage at the receiving end. As fewer current passes through the line, the capacitive effect increases and the inductance effect decreases. 

As a result, longer gearbox lines with a large amount of inherent capacitance exhibit a stronger Ferranti Effect. Voltage levels may exceed the equipment’s capacity without any form of compensation, which could result in damage and safety concerns.

A Real-World Example Explained

Let’s consider a real example, adapted from Electrical Machines, Drives and Power System by Theodore Wildi.

• Rated voltage: 735 kV (three-phase)
• Operating voltage: 727 kV
• Transmission line length: 600 km
• Inductive reactance per km: 0.5 ohm → Total: 300 ohms
• Capacitive reactance: 300 kΩ per km → Equivalent: 500 ohms for the system

(Not showing the calculations here for simplicity purpose. You can watch video given below for more details)

Converting this to a single-phase model for simplicity, we get:

• Line-to-neutral voltage (sending end – ES): 420 kV
• Capacitive reactance at each end of the line (XC): 500 ohms + 500 ohms = 1000 ohms
• Inductive reactance (XL): 300 ohms

Using these values, the receiving end voltage (ER) calculates to about 600 kV, a 43% increase. The problem here is clear: capacitive reactance is greater than inductive reactance.

How Shunt Reactors Solve the Issue

To fix this, we introduce an equal and opposite reactance into the system, an inductive one. That’s exactly what shunt reactors are for. They provide inductive reactance that cancels out the excess capacitive effect caused by the transmission line’s structure.

In our example, the reactive power generated by the line capacitance is 176 MVAR (megavolt-ampere reactive) per phase. Therefore, the shunt reactor needs to absorb this much reactive power to stabilize the voltage.

Once installed at both the sending and receiving ends, the reactors help bring the receiving end voltage back down to 420 kV restoring system balance.

Simulating the Effect: What Happens in Practice

Using a simulator helps visualize how this works (you can watch it in the video linked below):

• Initially, with the load disconnected, the sending end voltage is 353 kV (RMS) and the receiving end is 522 kV (RMS) confirming the Ferranti Effect.
• When the load is added, the inductive reactance from the load helps balance the system, and the receiving end voltage decreases.
• Adding a reactor while the line is unloaded brings the receiving end voltage close to the sending end even without load.
• If the reactor remains connected while the load is added again, the inductive reactance can dominate, causing the voltage to drop too low. Disconnecting the reactor then brings voltage back to acceptable levels.

This dynamic behavior demonstrates why shunt reactors need to be either fixed or adjustable depending on how frequently system load changes.

The Concept of Surge Impedance Loading

Surge Impedance Loading (SIL) refers to the specific load level at which the line’s inductive and capacitive reactances perfectly balance each other. At this point:

• No reactors are needed
• Voltage remains consistent
• The line compensates itself naturally

However, achieving SIL consistently isn’t practical in many real-world scenarios due to load fluctuations. That’s where shunt reactors step in to maintain voltage stability.

Fixed vs. Variable Shunt Reactors

There are two main types of shunt reactors used in practice:

• Fixed Shunt Reactors: Installed when the line rarely reaches SIL. These stay connected continuously.
• Variable Shunt Reactors: Used when loads fluctuate frequently. These can be switched in or out of the system as needed.

Choosing the right type depends on your transmission line’s load profile and operational flexibility.

Key Takeaways

• The Ferranti Effect causes voltage rise at the receiving end of long transmission lines during light or no load conditions.
• Transmission lines behave like capacitors, especially over distances greater than 200 kilometers.
• The imbalance between capacitive and inductive reactance is the root cause of the voltage surge.
• Shunt reactors introduce inductive reactance, which cancels out excess capacitive effects.
• Installing reactors at both ends of the line ensures maximum voltage control.
• The concept of Surge Impedance Loading helps identify the balance point between capacitance and inductance.
• Depending on the system needs, either fixed or variable shunt reactors can be used.

Final Thoughts

Shunt reactors are a vital part of high-voltage power transmission systems, especially when dealing with long distances and variable loads. They not only stabilize voltage but also protect the entire system from the harmful effects of overvoltage.

Understanding how these components function provides valuable insight into the dynamics of modern power systems. Whether you’re an engineering student, a power system professional, or just curious, grasping the role of shunt reactors is essential for understanding grid reliability and performance.

Want to see all of this in action? Check out the full video explanation: