A current transformer is a device which produces an alternating current in its secondary which is proportional to the alternating current in its primary. A current transformer (C.T.) is used where the current is too high to measure directly, or the voltage in the primary circuit is too high, so the galvanic isolation between primary and secondary is important for safety or technical reasons.
Figure 1: Current Transformer
Figure 1 shows a current transformer with a single turn primary. The main conductor is threaded through the transformer ring core.
If the secondary has N turns, then the secondary current is given by:
Isec = Ipri / N
Some C.T.s have more than one turn on the primary. If this is the case, use the relationship:
Ipri x Npri = Isec x Nsec
Most commercially available C.T.s have a specified maximum secondary current, normally standardized to 1A or 5A. So a C.T. rated 4000:5 has a secondary that can supply up to 5A for a primary conductor carrying up to 4000A running through it. From this, we can calculate the number of turns on the secondary, as 4000 / 5 = 800 turns.
Types of Current Transformer
Ring type: Similar to that in Figure 8.14. Can fit a large conductor through the center
Molded plastic type: Similar to Figure 2(a). Usually, have mountings supplied. Centre hole can be supplied ‘stepped’ to enable secure mounting on busbars.
Wound type: This has a wound primary of four or more turns. These are more accurate for primary currents less than around 500A.
Split core type: Can open the core and place around the conductor. Good for existing conductor which you cannot break to thread a conventional C.T. onto.
Bushing type: This is typically used for monitoring current in high-voltage conductors. It is integrated into the insulator bushing to achieve the high voltage insulation required between primary and core.
Clamp Meter (Clip-on Ammeter): This is a split-core C.T. with the meter attached. The jaws can be opened placed around a current-carrying conductor, then re-closed to read the current. Refer to Figure 2(b).
2: Current Transformer Types
The connection of Current Transformer
The current transformer can only be used to monitor alternating currents.
Do NOT operate a C.T. without a burden (load) on its secondary, as the secondary will attempt to drive the current into infinite impedance, causing very high voltages to occur. This may cause insulation breakdown and endanger the operator.
A current transformer should only be installed around one conductor. Placing around a cable carrying phase and neutral conductors, for example, will cancel the flux and indicate zero reading