In the last two articles we discussed about the Voltage Transformer(VT) and Current Transformer(CT). Here we will discuss few more facts about the instrument transformers and compare the two. In industry the voltage transformer is also called as Potential transformer(PT). In a power system representation. CT and VT or PT are shown symbolically. Below are the symbols commonly used for VT and CT. Sometimes VT is also represented by two overlapping circles (as in power transformer). Also similar to power transformer, small round dots are used for identification of polarities of the instruments. Proper polarities identification is important for connection of instrument transformers.
Here below we compare some important characteristics of CT and VT
- A Voltage Transformer(VT) transform high voltage of the primary side to low voltage and Current Transformer(CT) Transform high primary side current to low current. These low values of voltage and current can be readily used by the measuring and protection instruments.
- The instrument transformers insulate the low voltage measuring and protection instruments from the high voltage side. It enhances safety for the personnel at the low voltage control and protection side.
- The instrument transformers make it possible for standardization of instruments and relays etc.
- VT is connected between the line and ground or between the lines(see fig-B). In high voltage application it is usually connected between the line and ground. A CT is connected in series with the line (fig-B).
- When energized from the primary side the secondary of CT should never be kept open and secondary terminals should be shorted. The VT secondary should never be shorted. So a fuse is not inserted in the secondary of CT. But a fuse can be inserted in the primary or secondary side of VT.
- The primary of CT carries the actual current of the Line whose value of current is to be measured or sampled. Hence the primary side of CT is comprised of thick conductor to carry line current and the secondary side has several turns of conductors of thinner cross section. In a VT the primary side voltage is high so there are large numbers of turns in the primary side of thinner conductor. The secondary side of VT has few turns of conductor of large cross sectional area. The secondary side carries large current for supply to the burden.
- For a VT the ideal transformer Law, Vp/Np = Vs / Ns is important. For a CT the transformer Law IpNp =IsNs is important. The design of the VT and CT should be such so that these ideal laws are satisfied to good accuracy for the respective instrument transformers.
CT and PT Connection
You have studied in the school that the ammeter should be connected in series with the load and voltmeter should be connected across the terminals of the load for which the voltage is to be measured. Similarly A CT is connected in series with the load or line and the VT is connected between the line and ground (or between the terminals of load or source). See the figure below. In the figure is a simple system with an AC source and a load connected by a line. This is the HV system shown in thick line. The connection of CT, VT and other measuring and protection instruments are shown with thinner lines. V and I are the voltmeter and ammeter connected to secondary sides of VT and CT respectively for measurement of voltage between the lines and current I flowing through the line. R is the Relay, one is connected across the secondary of VT and another is in series with CT secondary. The relay connected in VT is the voltage operated type and the one connected in the secondary of CT is current operated type. The VT is also shown as connected to the pressure coil of the watt meter and the CT is connected to the current coil of the same watt meter. The VT and CT can be connected to many measuring and/or protection devices. But the sum of the burdens of the devices should not exceed the rated burden of the Instruments.
VT and CT are the measuring instruments and the main purpose is to measure the circuit condition or parameters. So the connection of the instrument transformers should not influence or alter the original circuit condition. It follows that the CT is desired to have very little impedance (or resistance) across its terminals. So that the CT in series with the line should not result in any significant voltage drop across its terminals. The current flowing in the secondary of the CT does not influence the primary side current. The primary side current is solely determined by the load impedance, source voltage and of course the line parameters.
A voltage transformer is connected between line and ground. It is desired to have very high impedance. A low impedance results in comparatively large current flow in VT primary and can considerably alters the original circuit condition which is not desired. Otherwise we can say that the voltage transformer should have negligible loading effect on the main circuit. In figure-B, the values of Iz and I should not show any noticeable difference due to connection of VT and CT.
In fig-C is shown the SLD of the simple system illustrating symbolic connection of CT and VT.