What is a Current Transformer? MV Current Transformer (Medium Voltage Current Transformer)
A current transformer (CT), also known as an MV current transformer in medium voltage systems, is a critical instrument transformer used to accurately and safely measure high currents in power distribution networks.
In high-voltage and high-current circuits, measuring current directly is both dangerous and impractical. A current transformer steps down the high primary current to a safe, standardized secondary current (usually 1A or 5A) that can be used by measuring devices, protection relays, and energy meters.
MV current transformers provide excellent electrical isolation between the high-voltage primary side and the low-voltage secondary side, ensuring operator safety and protecting sensitive equipment.
These transformers are widely used in medium voltage switchgear (up to 36 kV), transformer substations, distribution panels, and industrial facilities for both metering and protection purposes.
Current Transformer Working Principle
A current transformer operates on the principle of electromagnetic induction, similar to a power transformer.
The primary winding (usually consisting of one or very few turns of thick conductor) carries the high load current. This current creates a magnetic flux in the core, which induces a proportional current in the secondary winding (made of many turns of finer wire).
The current transformer ratio (CT ratio), such as 600/5, defines the relationship between primary and secondary current. This transformation allows accurate monitoring while keeping the measurement circuit safe.
Current Transformer Classification and Selection Criteria
Current transformers are classified according to their accuracy class, burden (VA), and application (metering or protection).
Key selection parameters for MV current transformers include:
- Accuracy Class
- Current Transformer Class (e.g., 0.5, 1, 5P, 10P)
- Rated Burden (VA)
- Thermal and Dynamic Short-Circuit Withstand
- Current Transformer Accuracy
Metering current transformers are designed to saturate quickly during overcurrent conditions to protect instruments. Protection current transformers (typically 5P or 10P) maintain accuracy during fault conditions to ensure reliable relay operation.
The CT ratio must be chosen carefully according to the maximum load and short-circuit currents in the system.
Current Transformer Classes and Ratings
Common accuracy classes for metering current transformers are: 0.1 – 0.2 – 0.5 – 1 – 3
For protection purposes, 5P and 10P classes are most frequently used.
Important technical values:
- Thermal withstand current (Ith): Minimum 100 × rated current
- Dynamic withstand current: 2.5 × thermal withstand current
Safety Note: Never leave the secondary circuit of a current transformer open while the primary is energized. High voltage can be induced, creating serious risk to personnel and equipment. The secondary must always be short-circuited or connected to a load.
Types of Current Transformer
There are several types of current transformer used in medium voltage systems:
- Toroidal Current Transformer: Ring-type design, compact and widely used in MV switchgear. Also known as flexible current transformer in some applications.
- Wound Type / Post Type (Mesnet Type): Epoxy resin cast, excellent insulation, suitable for both metering and protection.
- LPCT (Low Power Current Transformer): Modern digital type that outputs voltage signal (mV) instead of traditional 1A/5A current.
ABB current transformer, Alce current transformer, and other reputable manufacturers offer high-quality solutions in all these categories.
Current Transformer Diagram and Applications
A typical current transformer diagram shows the primary conductor passing through the core and the secondary winding connected to meters or relays.
MV current transformers are essential in:
- Medium voltage modular switchgear
- Transformer substations
- Industrial power distribution systems
- Renewable energy projects
- Protection and control panels
They play a vital role in accurate energy measurement, fault detection, and system protection.
Looking for MV Current Transformer?
Looking for high-quality MV current transformer, toroidal current transformer, ABB current transformer, or other types of current transformer solutions? We provide expert engineering support and reliable products for all your medium voltage needs.
For quotations and requests: info@electricistanbul.com
Whatsapp: +90 501 076 69 91