What Is An Indoor Single-phase Current Transformer? How Does It Work?

Current transformers are indispensable core instruments in the metering, protection, and monitoring of power systems. Whether in factory distribution rooms, substations, or building distribution cabinets, they can be found almost anywhere current measurement is required. Indoor single-phase current transformers, as the most widely used type, play a crucial role in converting large currents in high-voltage lines into smaller currents suitable for safe use by instruments and relays. This article will systematically introduce the definition, structure, and working principle of indoor single-phase current transformers, helping electrical professionals and beginners establish a clear understanding.

A current transformer (CT) is a specialized measurement and transformation device. Its core function is to proportionally convert large currents in a power system into standard smaller currents. Typically, it converts a large primary current into a standard 5A or 1A output on the secondary side, ensuring the safe use of secondary equipment such as power metering instruments, ammeters, wattmeters, and relay protection devices.
Why is this conversion necessary? There are two reasons: First, the line current in power systems often reaches hundreds or even thousands of amperes, making direct connection to ordinary instruments impossible. Second, high-voltage lines cannot be directly connected to measuring instruments; otherwise, operators would face the risk of electric shock. Current transformers achieve electrical isolation through the principle of electromagnetic induction, both proportionally reducing the current and completely isolating the high-voltage side from the low-voltage secondary circuit, fundamentally ensuring the safety of measuring personnel and equipment.

Indoor: This refers to a transformer model specifically designed for indoor installation environments. Its insulation structure, enclosure protection level, and creepage distance are designed for indoor conditions and are not suitable for direct use in damp, dirty, or rain-exposed outdoor environments.
Single-Phase: This means that each transformer measures the current of only one phase line, with one unit configured per phase. In a three-phase system, if three-phase current measurement is required, two or three single-phase transformers are typically needed (depending on the wiring method). Compared to three-phase integrated current transformers, single-phase current transformers are smaller and more flexible in installation, facilitating their placement in various distribution cabinets, switchgear, and metering boxes.

Indoor single-phase current transformers consist mainly of the following parts from the outside in:
1. Core
The core is the magnetic circuit core of the transformer, typically made of stacked high-permeability silicon steel sheets, forming a closed magnetic flux loop. The material and cross-sectional area of ​​the core directly affect the accuracy class and error characteristics of the transformer. Metering transformers require high core permeability and low hysteresis loss; protection transformers prioritize the core's ability to prevent premature saturation under high-current faults.
2. Primary Winding (Primary Coil)
The primary winding has very few turns, usually only one to a few turns, sometimes directly formed by a busbar or copper rod passing through the core. The primary winding is connected in series to the measured power line, and the large primary current flows through this winding to generate a magnetic field.
3. Secondary Winding (Secondary Coil) The secondary winding has more turns and is wound around the iron core. It achieves electromagnetic coupling with the primary winding through the iron core's magnetic field, inducing a secondary current. The two terminals of the secondary winding are usually marked S1 and S2 (or K1 and K2) for external instrument and relay connection.
4. Insulator The insulator wraps around the iron core and winding, completely isolating the high-voltage primary circuit from the low-voltage secondary circuit. Common insulation methods for indoor current transformers include epoxy resin casting (monolithic casting, good insulation, compact structure) and insulating paper wrapping with varnish (traditional process, suitable for large-capacity products).
5. Housing and Mounting Bracket The housing is usually a monolithic epoxy resin casting or a metal housing. Installation methods include through-wall, post, and bushing types, with different forms selected depending on the installation location and circuit structure.

The working principle of indoor single-phase current transformers is based on Faraday's law of electromagnetic induction. Its essence is the same as that of a transformer, but the operating conditions are significantly different. The basic working process is as follows:
When the primary current I₁ flows through the primary winding (N₁ turns) in the circuit under test, according to Ampere's law, the current generates an alternating magnetic flux Φ in the iron core. This alternating magnetic flux passes through the secondary winding with N₂ turns, and according to Faraday's law of electromagnetic induction, a secondary electromotive force is induced in the secondary winding, thus generating a secondary current I₂ in the secondary circuit connected to the load (instrument internal resistance).
The key difference between a current transformer and a transformer is: A transformer operates under near-no-load or loaded conditions, and the secondary voltage varies with the load; while a current transformer operates normally with its secondary side in a near-short-circuit state (with minimal instrument internal resistance), and the secondary current has a fixed proportional relationship with the primary current, independent of the load size. Because of this characteristic, the secondary side of a current transformer must never be operated with an open circuit. Once open-circuited, the magnetic flux through the core will increase dramatically due to the loss of demagnetizing ampere-turns on the secondary side, leading to core over-saturation and inducing a dangerously high voltage (up to several kilovolts) on the secondary side, seriously threatening equipment and personnel safety.

After understanding the working principle, it is also necessary to be familiar with the following key parameters:
Rated current ratio (transformation ratio): such as 100/5, 200/5, 400/5, etc., representing the ratio of the rated current on the primary side to the rated current on the secondary side, is the primary parameter for selection.
Accuracy class (precision grade): Commonly seen are 0.1, 0.2, 0.5 (for metering) and 5P, 10P (for protection). The smaller the number, the higher the accuracy. Different grades should be selected for different applications.
Rated Secondary Load (VA): This refers to the apparent power corresponding to the maximum allowable impedance connected to the secondary side of the current transformer. Common values ​​are 5VA, 10VA, 15VA, 30VA, etc. The total impedance of the connected instruments should not exceed this value; otherwise, the error will increase.
Rated Insulation Voltage (Um): This indicates the insulation withstand voltage class of the primary side of the current transformer to ground and to the secondary side, such as 0.72kV, 3.6kV, 7.2kV, etc.

Scenarios Indoor single-phase current transformers are widely used in the following applications: metering and protection circuits in factory power distribution systems; current detection in 10kV switchgear and low-voltage distribution cabinets; current

sampling in substation relay protection devices; energy metering boxes in building distribution rooms; and current monitoring circuits in industrial automation systems.

Q1: Why can't the secondary side of a current transformer be open-circuited?
A: When the secondary side is open-circuited, the magnetic flux in the iron core loses the demagnetizing effect of the secondary current, causing a sharp increase in magnetic flux. This induces a high voltage of several thousand volts on the secondary side, which can easily lead to insulation breakdown of the current transformer and electric shock accidents. The secondary side must be short-circuited before disassembling the instrument.

Q2: Is it safer to choose a larger transformation ratio for the current transformer?
A: Not necessarily. An excessively large transformation ratio will result in an insufficient secondary current under normal load, increasing measurement errors and inaccurate instrument readings. A suitable transformation ratio should be selected based on the actual maximum load current; generally, a normal operating current of 60% to 80% of the rated primary current is preferable.

Q3: Can indoor current transformers be installed outdoors?
A: No. The insulation structure and creepage distance of indoor current transformers are designed for clean indoor environments. Outdoor damp, dirty, and ultraviolet environments will accelerate insulation aging, leading to creepage flashover or insulation breakdown. Dedicated outdoor current transformers must be used.