Current Transformer (CT) – Construction and Working Principle

Live Tank Current Transformer
In this design of instrument transformers, the tank housing the cores is kept at the system voltage. A live tank CT is shown in the figure. It can be noted that the bushing of this CT is prone to damages in transit as its center of gravity is at large height.
Live Tank Current Transformer
Dead Tank Current Transformer
In Dead Tank design of current transformers, the tank housing the cores is kept at earth potential.
The figure shows a dead tank (single bushing) design which is mounting wise similar to a live tank design but here the center of gravity is low. Hence this type of CT is not damaged in transit.
Dead Tank Current Transformer
The figure depicts dead tank CT (two-bushing) which is very compact in size and can be mounted on a steel structure near the outdoor circuit breakers.
A CT having more than one core and more than one secondary winding is known as a multi-core CT (for example, a CT having metering and protection cores).
A CT in which more than one ratio is obtainable by reconnection or tapings in primary or secondary windings is known as a multi-ratio instrument transformer (e.g. a CT having a ratio of 800-400-200/1 A). In such transformers, tapings in primary windings must be avoided as far as permitted by the design.
An instrument transformer intended to serve the dual purpose of measuring and protection is known as a dual-purpose instrument transformer.
A CT having a split metering core used for the measurement of current in a busbar is known as a split-core CT. The spring action of a split-core CT allows the operator to use this CT for enveloping a current-carrying low tension busbar, without stopping the current flow.
Measurement CT and Protection CT
A CT is similar to a power transformer to some extent since both depend on the same fundamental mechanism of electromagnetic induction but there are considerable differences in their design and operation.
Current Transformer used for metering and indicating circuits are popularly termed as Measuring CT.
Current Transformer used in conjunction with protective devices is termed as Protection CT.
A measurement grade CT has much lower VA capacity than a protection grade CT. A measurement CT has to be accurate over its complete range e.g. from 5% to 125% of normal current. In other words, its magnetizing impedance at low current levels (and hence low flux levels) should be very high.
A metering core CT is designed to work more accurately within the rated current range designated. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. This protects connected metering devices from overloading in the presence of fault level current flows. It buffers the meter from experiencing excessive torques that might be created during those faults.
Metering CT Class
In contrast, for a protection grade CT, the linear response is expected up to 20 times the rated current. Its performance has to be accurate in the range of normal currents and up to fault currents. Specifically, for protection grade current transformers, magnetizing impedance should be maintained to a large value in the range of the currents of the order of fault currents.
Protection CT Class
A protection core is designed to transform a distortion-free signal even well into the overcurrent range. This enables the protective relays to measure the fault current value accurately, even in very high current conditions.
For measuring CTs, the accuracy required is within the normal working range of up to 125 percent of the rated current. For over-current conditions beyond this, accuracy is not desired, rather there must be saturation in the core to relieve the connected instruments of the stresses due to over-current.
Accuracy is not required for currents below the rated Value for protection CTs. But there must be accuracy at all higher values of current up to a maximum primary current equal to the maximum system fault level.
The decision as to whether or not to use dual-purpose CT for measuring and protection depends on various factors such as design, cost and space as also on the ability of the instrument to withstand short time over-current.