Internal Design of Current Transformers

A current transformer enables the conversion of high currents to a measurable current range. The relays incorporated in several protection systems are implemented such that the current supplied by a current transformer can be used to actuate them. It is also deemed difficult to measure high magnitude alternating currents using normal low range ammeters and thus, current transformers are used as an intermediate stage for measurement and isolation purposes.


Internal Design of Current Transformers
Figure 1. A variety of current transformers. Image courtesy of Talema Group.


Magnetic components, in general, have been used in different power electronic devices for decades. They are used for the control, transfer, and conditioning of electric power at different stages and in multiple forms. 

Designers are always on the lookout for newer materials, topologies, and processes in order to improve performance. There was a time in history where the design of magnetics was considered more of an art than a science. This is because the design was mostly dependent on trial and error techniques, empirical formulae, and rule-of-thumb considerations. Further, there have been several attempts to standardize the design process to make it more reliable and repeatable. 

Read on to learn more about the working principle of a current transformer and the key concepts involved.


Current Transformers and Circuit Overview

Current transformers are employed to measure the current flowing in high power circuits, typically for measurement or feedback circuits. The use of current transformers is preferred over measuring currents using current shunts in series with the current path, because of a current transformer’s advantages like providing isolation between the power circuit and the measuring circuit, lower contribution to power losses, and good common-mode rejection [1]. In general, a transformer supports AC coupling, voltage and current level transformation along with DC isolation [2]. 


Internal Design of Current Transformers
Figure 2. The use of a current transformer for measuring a high current. [3]

The practical design considerations for current transformers is governed by the ability to conduct necessary values of primary and secondary winding currents effectively. This translates to the right choice of conductors along with the capability of achieving adequate power coupling. Ideally, a tight voltage regulation with no leakage and no current losses — hysteresis or eddy — as well as low overall exciting current distortion are desired. 

If all these design goals need to be met completely, the resulting product might be bulky which again is not intended. Thus, it is an extremely tough balance to strike in terms of achieving the best possible design based on these considerations.


Working Principle

Current transformers belong to the family of current transducers that generates a secondary current proportional to the current flowing through the primary side in magnitude. 

The primary winding is designed such that it consists of one or more turns having a large cross-sectional area and is typically connected in series with the circuit that needs to be sensed for current flow [4]. 

The secondary winding has a higher number of turns and is made of a wire with a smaller cross-sectional area. The secondary winding is connected to the operating coil of the relay or to the current measuring instrument.


Internal Design of Current Transformers
Figure 3. Representation of a current transformer [4]


The application area for a specific type of current transformer is governed by its precision, the ratio of primary to secondary currents, type of insulation employed, mechanical construction, and external operating conditions. 

The working of the current transformer is similar to that of conventional power transformers as they basically function as step-up voltage transformers. Typically, the value of the current will be lower at the high voltage side and vice versa. Thus, when the primary side is energized, the ampere-turns on the primary side will produce a magnetic field in the core. 

An electromotive force is induced in the secondary side due to the generated magnetic flux which in turn drives the secondary current. The ampere-turns are balanced in primary and secondary, and also the voltage drop across primary is much less, making the primary current independent of secondary current.


Design Insights for Current Transformers

The design of the current transformer is a compromise in cost, weight, number of turns in the winding, and its overall performance [1]. Increasing the core area enhances performance, but adversely affects the cost and overall size. Ferrite or steel cores are employed and larger numbers of secondary turns are preferred. Typically, a good current transformer design focuses on the lower voltage on the secondary side, the use of high permeability material, high core area, and large secondary turns.

Usual considerations for choosing core materials include low core loss, low reluctance value, and low flux density. Paper, varnish, tape materials, and their variants are used for insulation purposes.

A current transformer can be wound type or bar type. In the case of wound type low voltage applications, secondary turns are wound on a bakelite followed by primary turns with suitable insulation in between the layers. In bar type, a single bar forms the primary winding and passes through the center of the core. 


Internal Design of Current Transformers
Figure 4. Current transformers can be bar type or wound type.




Key references:

  1. L. Umanand, S.R. Bhat, “Design of Magnetic Components for Switched Mode Power Converters”, Wiley Eastern Limited.
  2. Marian K. Kazimierczuk, “High-Frequency Magnetic Components”, John Wiley and Sons, Ltd.
  3. Marcel Dekkar, “Transformer and Inductor Design Handbook”, 2004.
  4. Current Transformer