What is the function of on-load tap changer?

Detailed Explanation

To understand its function, let's break down why it's needed and how it works.

1. The Core Problem: Voltage Fluctuations

In an electrical power system, the voltage at a given point is not constant. It fluctuates due to:

Varying Loads: When a large industrial motor starts, it draws a huge current, causing a temporary voltage drop (a "sag") for other customers on the same line. Conversely, when load is suddenly removed, voltage can spike.

Line Losses: As electricity travels long distances through power lines, the inherent resistance of the wires causes a voltage drop. The voltage at the end of a long feeder line will be lower than at the substation.

Customers and equipment require a relatively stable voltage to operate correctly and efficiently. Too high or too low voltage can damage equipment or reduce its lifespan.

2. The Solution: Adjusting the Turns Ratio

A transformer works on the principle of electromagnetic induction, where the voltage ratio is proportional to the turns ratio between the primary and secondary windings:

V_primary / V_secondary ≈ N_primary / N_secondary

An OLTC physically changes this turns ratio (N_primary / N_secondary) by connecting to different "taps" (connection points) on the transformer winding. By selecting a different tap, the number of active turns in the winding is effectively increased or decreased, thus raising or lowering the output voltage.

To RAISE the Secondary Voltage: The tap changer selects a tap that reduces the number of turns in the primary winding (or increases them in the secondary).

To LOWER the Secondary Voltage: The tap changer selects a tap that increases the number of turns in the primary winding (or decreases them in the secondary).

3. The Key Challenge: Making the Change "On-Load"

The major technical challenge is that you cannot simply open the circuit to change taps when the transformer is supplying load current. Breaking the circuit under load would create a powerful arc, destroying the tap changer and potentially causing an explosion.

 

How an OLTC solves this:
An OLTC is a complex mechanical switch with a sophisticated transition mechanism. The most common method uses diverter switches and transition resistors.

It briefly bridges two adjacent taps.

It uses resistors to temporarily limit the circulating current between the taps.

It then transfers the load current from the old tap to the new tap.

Finally, it disconnects the old tap.

This entire process happens in a fraction of a second, ensuring a smooth transition without interrupting the power flow.

 

Key Functions and Purposes Summarized:

Voltage Regulation: The primary function. It maintains a stable secondary output voltage despite fluctuations in the primary voltage or the load.

Reactive Power Control: By controlling the voltage, the OLTC indirectly helps control the flow of reactive power in the network, which is crucial for system stability.

Power Quality Assurance: It ensures that customers receive voltage within a specified tolerance (e.g., 230V ±10%), protecting their appliances and machinery.

System Stability: By managing voltage profiles across the power grid, it helps prevent voltage collapse, a condition that can lead to widespread blackouts.

 

Where are OLTCs Used?

OLTCs are almost always found on:

Power Transformers in transmission and distribution substations.

Large Distribution Transformers supplying industrial complexes or large areas.