Current transformers (also refered to as a CT) follow all the same theories that other transformers do, however the way that they are used in circuits is unique and is different from the way transformers are normally used. Power transformers (also known as voltage transformers or potential transformers) are used in situations where the voltage needs to be stepped down or stepped up, usually from distribution reasons (power loss) or for consumption by loads at a particular voltage level. Current transformers on the other hand are used only for current metering.
Current transformers transformers are always connected in series with the source and the load, whereas most other types of transformers are connected in parallel. So why do we connect a current transformer in series? The purpose of a current transformer is to measure current, if we think back to our basic electrical theories, ammeters are always connected in a circuit in series. Power transformers are always connected in parallel to the source and the load. Analyze the two different types of connections in the circuit diagrams shown below
Current Transformers (CT) are connected in series:
Voltage Transformers (VT or PT) are connected in parallel:
From the diagram, V1 is the primary side voltage of the CT and V2 is the secondary side voltage of the CT. The primary winding of a CT is the load current carrying conductor itself, which passes right through the window of the CT. As such the primary side voltage of a current transformer V1 is a very small number , since it is only the voltage drop of the conductor for the portion that it passes through CT window. Relating back to an ammeter, we know that an ammeter’s resistance should be as low as possible, as not to draw current from the circuit which would affect the actual current drawn by the load. For this reason a CT steps up the voltage. Thus V2 the secondary voltage is much larger than V1 (step up), which is accomplished by having much more turns in the secondary winding. From Ohm’s law we know as voltage increases current decreases, thus we are able to keep the current draw low, and connect metering equipment on the secondary side that usually only operate at between 1A to 10A.
Since V2 is usually very high, it is very important that the secondary of a current transformer is never open circuited. It is common practice to connect the leads of the CT to a shorting block before connecting it the meter. The shorting block is just a wiring terminal that has a screw to short the secondary of the CT when the meter is removed. This is where the key difference and confusion lies when it comes to CTs. If you were to short circuit the secondary of a VT/PT, you would cause an increase in current on the primary side.
Never Open Circuit a CT
CTs because they are connected in series, the primary side current is fixed (i.e. the load current) and is not affected by the secondary side load. The secondary side load (meter burden) only affects the voltages on the primary and secondary sides of the CT. The secondary side load (meter burden) directly affects the secondary voltage V2, which by the turns ratio also affects V1. Open circuiting (high resistance thus high voltage) the secondary side causes the voltage of the secondary winding to increase. Short circuiting (low resistance thus low voltage) keeps the secondary side voltage V2 low. The secondary side current is “fixed” and is only affected by the primary side current (load current) via the turns ratio.
Never Short Circuit a VT/PT
VTs and PTs on the other hand are connected in parallel, the primary side voltage is fixed V1 (i.e. the source voltage) and is not affected by the secondary side load. The secondary side load only affects the currents on the secondary and primary sides of the voltage transformer. The secondary side load directly affects the secondary side current, which by the turns ratio also affects the primary side current. Short circuiting (lower resistance thus high current) causes the current on the secondary side to increase. Open circuiting (high resistance thus no current) causes no current to flow in the secondary and thus no current in the primary. The secondary side voltage V2 is “fixed” and is only affected by the primary side voltage V1 via the turns ratio (where V1 is equal to the source voltage Vs).