What is Current Transformer
Current Transformer is instrument transformer, used for the measurement of high alternating currents of transmission line by stepping down the high value currents to lower value suitable for ammeters.
The primary of current Transformer generally consists single conductor or bus bar while the secondary has large number of turns. The primary current of the current transformer does not depend on secondary load current. Its primary current depends on external load. The current in the secondary is generally of 1A to 5A.
The current Transformer is sometimes called series transformer because the primary winding of the transformer is connected in series with current carrying conductor supplying the load.
There two ratings mentioned on the name plate of current transformer – current transformer (C.T) ratio and Burden.
Current Transformer Ratio: – The C.T. Ratio is the ratio of primary current to the secondary current. If NP is number of primary current, which is generally equal to 1, IP is primary current of current Transformer, Ns is number of secondary turns and Is is secondary current of Current Transformer.
The C.T. Ratio will be
IP NP = Is Ns
IP / Is = Ns / NP
If the C.T. ratio is 100/5 A that means when 100A of current flows in the primary winding of current Transformer, it will be stepped down to 5A in the secondary winding of Current Transformer i.e. the current in the secondary winding of current Transformer will be 5 A.
Burden: – The value of load connected to the secondary of the current transformer is called burden. It is the product of voltage and current on the secondary of current transformer with its maximum rated value of current. Burden is expressed in Volt Ampere (VA).
Why Current Transformer’s secondary is not left open when current is flowing in the primary?
The secondary of current transformer is never left open when primary is energized. If the secondary of current Transformer is left open while the current is flowing in the primary the current transformer acts as a step – up transformer. Very high voltage will be induced in the secondary of current Transformer equal to VP(Ns/Np).
Vp = Voltage on primary side
Vs = Voltage on secondary side
Np = Turns on primary side and Ns = Turns on secondary side.
Suppose we have current transformer whose primary is energized by voltage 11KV, having CT ratio 400/5
IP / Is = Ns / Np = 400 / 5 \;\;\;\;\;\;(1)
Vs / Vp = Ns / Np \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;(2)
From equation (1) and equation (2)
Vs / Vp = 400 / 5 = 20
Vs = 20 VP
Vs = 20 x 11 = 220 KV
Thus extremely high voltage is produce in the secondary of current Transformer that is why the secondary of CT should never opened or left open when current is flowing in the primary. It can be said that Current Transformer never be operated on no load.
If in any case the load has to be removed while current flowing in the primary, secondary terminals of CT should be short circuit first then load should be removed.
Types of Current Transformer
There are two types of current transformers on the basis of application – Indoor Current Transformer and Outdoor Current Transformer.
(A) Outdoor Current Transformer: – These CTs are designed for the outdoor application. The transformer oil or any suitable liquid is used of insulation and cooling for these transformers. Hermetically sealed CTs are the liquid immersed CT which sealed and does not communicate with the atmosphere.
These CTs are further classified as – Live tank type CT and dead tank type CT.
In Live tank type current Transformer, the tank containing the core is kept at system voltage as shown in figure.\\\;\;\;\;\;
In dead tank type current Transformer, the tank housing the core is earthed that means the tank containing core is kept at earth potential as shown in figure.\\\;\;\;\;\;
(B) Indoor Current Transformer: – The current transformers which are mounted inside the metal cubicles are known as indoor current Transformer.
On the basis of construction, the current transformers are further classified as – Bar type CT, Slot/Ring/Window/Toroid type CT and Wound Type CT.
Bar Type CT: In this type of current Transformer, there is only secondary winding. The cable or bus-bar of load circuit acts as primary winding, which usually a single turn. The figure of bar type CT shown below. \\;\;\;\;\;
Slot/Window Type CT: In these Cts the line carrying the current to the network acts as primary winding. They have split core, which allows it to open, installed and close without disconnecting the circuit to which they are attached.\\;\;\;\;\;
Wound Type CT: These CTs have separate primary and secondary winding with different number of turns. The primary winding is connected in series with the circuit carrying the current to be measure. e.g. switch yard high voltage CTs.\\;\;\;\;\;
Uses or Advantages of Current Transformer
There are many uses or advantages of current Transformer
- The larger current values can be easily measured
- Its capability to isolate the high voltage and current to low values ensure safe operations and handling the equipment in a safe manner.
- It can be used as a controlling device to operate protecting devices such as relays and pilot lights
- Numerous instruments can be feeding on a single transformer.