What is a current transformer?
A current transformer (CT) is a special type of transformer that converts high currents in power systems, which are difficult to measure directly, into a standardized, safe, low current for measurement, metering, and protection.
In simple terms, current transformers are like the "eyes" of the power system, allowing us to safely and conveniently "see" and "control" powerful currents.
1. Core Function and Purpose
Safety Isolation: It electrically isolates the high-voltage, high-current primary circuit from the low-voltage, low-current secondary circuit, protecting operators and measuring equipment from high voltage threats.
Scaling: It scales down high primary currents (e.g., several thousand amperes) to a standardized, low current (usually 5A or 1A).
Standardization: It provides a unified signal standard for subsequent measuring instruments (ammeters, energy meters) and protective devices (relays).
2. Operating Principle: Similar to a Transformer
The operating principle of a current transformer is based on the law of electromagnetic induction, similar to that of a conventional transformer.
Structure: It consists primarily of a closed iron core and two windings:
Primary Winding (N1): It has a small number of turns, typically only one or a few. It is connected in series with the main circuit to be measured and carries the high current I₁.
Secondary winding (N2): This winding has many turns. It connects to a measuring instrument or protective device and outputs a scaled-down standard low current, I₂.
Current transformation relationship:
Ideally, the relationship between the primary current, I₁, and the secondary current, I₂, satisfies the following formula:
I₁ / I₂ = N₂ / N₁ = K
Where K is called the transformation ratio.
For example, a CT with a transformation ratio of 1000/5A means:
When a high current of 1000A flows through the primary, the
secondary outputs a standard current of 5A.
The turns ratio, N₂/N₁, = 1000/5 = 200. This means the secondary winding has 200 times as many turns as the primary.
3. Appearance and Types
You may have seen these types in power facilities. Common types include:
Toroidal (through-hole) type: This type has a central circular hole through which the main circuit cable passes, with the cable itself serving as the primary winding.
Wound type: This type has a primary terminal to which the main circuit conductors are connected.
Pillar type: This type is commonly used in high-voltage switchgear and resembles an insulator.
Summary
| Aspect | Description |
|---|---|
| Essence | A special type of transformer |
| Function | Steps down high current proportionally to a standard, safe, low current (typically 5A or 1A) |
| Core Principle | Electromagnetic induction |
| Key Requirement | The secondary side must be grounded and must never be open-circuited during operation. |
| Main Types | Measuring CTs (high accuracy, weak anti-saturation); Protective CTs (maintains linearity under fault currents) |
| Applications | Measurement, metering, protection, and control in electrical power systems. |