Types of Current Limiting Reactors

The power system spans a wide area, and multiple power systems are interconnected through grids. As the power system extends over a larger area, the likelihood of faults occurring also increases. When a fault occurs, circuit breakers in the system must trip and isolate the faulty section immediately. However, a minimum amount of time is required for the circuit breaker to trip. During this interval, short-circuit current continues to flow through the circuit breaker contacts and other equipment. In thins articles we shall explore about types of current limiting reactors in details.

Short-circuit current can reach dangerously high levels. Circuit breakers must be capable of withstanding and interrupting these currents, at least for a short time. To handle such high currents, circuit breakers would need to be designed with higher ratings, which increases their size, cost, and maintenance requirements.

To address this, additional coils known as current limiting reactors are connected in series with the power network. These coils provide reactance that limits the magnitude of short-circuit current, allowing the circuit breakers to operate within their designed limits.

A current limiting reactor is a coil with a specific number of turns, designed to offer high inductive reactance and low resistance when fault current flows through it.

• Under normal conditions, the supply frequency remains nominal, so the reactor presents negligible impedance.
• During fault conditions, the frequency and current may spike, causing the reactor’s inductive reactance to increase proportionally.

The inductive reactance is given by the formula:

\[ X_L = \sqrt{2 \pi f L}\]

Where:

• X_L​ = Inductive reactance
• f= Supply frequency
• L = Inductance of the coil

Current limiting reactors are typically installed in the following locations to reduce fault current:

• In series with generators
• In series with feeders
• In busbars

Reactors are always connected at the entry or exit points of equipment to protect them from faults.

Depending on where they are installed, current limiting reactors are classified into following types:

1. Generator Reactors
2. Feeder Reactors
3. Busbar Reactors

In this configuration, the reactor is connected in series with a generator. The main purpose is to protect the generator from short-circuit faults in the connected feeders.

Advantages:

• Protects the generator before the fault reaches the reactor.

Disadvantages:

• The reactor remains online as long as the generator supplies power, resulting in continuous power loss.
• If a fault occurs close to the busbar, a significant voltage drop can cause the generator to fall out of synchronization.
• Requires separate reactors for each generator, increasing cost and complexity.

Due to these limitations, this configuration is rarely used.

In this configuration, reactors are connected in series with each feeder.

Advantages:

• A fault in one feeder does not disturb the main busbar voltage, reducing the risk of generator desynchronization.
• Faults in one feeder do not affect other feeders, maintaining power continuity.

Disadvantages:

• Offers no protection against faults in the main busbar or generators.
• Reactors remain online, causing continuous power loss.
• The size of the reactor depends on the number of generators connected to the busbar, as well as the fault level in the feeder.

Unlike the previous two, busbar reactors are connected directly to the main busbar to overcome power loss issues in normal conditions.

Types:

• Ring Busbar System
• Tie Busbar System

In this system, each generator and feeder is connected through a section of the busbar, and each section is interconnected using a current limiting reactor.

Advantages:

• Under normal conditions, minimal current flows through the reactors, resulting in low voltage drop and power loss.
• Only the faulty section (generator or feeder) is affected during a fault.
• Other feeders and generators remain in operation, ensuring continuity.

Disadvantage:

• Does not protect the generator connected to the faulted feeder.

This is an improved version of the ring busbar system. It enhances voltage regulation and is particularly suitable for systems where generators are frequently added and operate in parallel.

Configuration:

• Generators are connected to the main busbar through current limiting reactors.
• Feeders are directly connected to the generators.

Advantages:

• Even with an increase in the number of sections, the fault current does not exceed the limit set by individual reactors.
• Switchgear designed for the limited current will operate effectively without damage, even as the system expands.

This configuration is commonly preferred for its scalability and protection efficiency.

Final Thoughts

Current limiting reactors are essential components in modern power systems to ensure safe and efficient operation during faults. By limiting the fault current, they reduce stress on circuit breakers and protect key equipment. Choosing the right reactor configuration depends on system design, reliability requirements, and economic considerations.