In actual power system, we have to deal with high voltages and current. Let assume that we want to measure the large voltage and current flowing in the transmission line. In that case, ammeter and voltmeter required for the measurement would be of very high rating which are very costly and bulky. Moreover, there will also be huge power losses. Hence it would not be economical as well as not practicable.
In order to provide a convenient way of measuring the large heavy current and voltages, current transformer and potential transformers are used which are known as instrument transformers. Instrument transformers reduces the high voltages and current to such a small reasonable values which can be easily measured by low rating measuring devices. In this article, we will talk about the key difference between current transformer and potential transformer.
If you want to read the full detail of current transformer, kindly read the following article :
What is Current Transformer ? Construction & Types ↗
Difference between Current and Potential Transformer :
Functions :
Current transformer is a step up transformer. It steps up the voltage due to which the value of current is step down by a known ratio. Induce current in the secondary of current transformer is of lower value and it is easy to measure for the low rating ammeter. Potential transformer on the other hand is step down transformer. It reduces the high voltage of primary circuit to a standard low value which can be easily measured by the voltmeter connected at the secondary of potential transformer.
Connections :
Primary winding of current transformer is always connected in series with the line whose current is to be measured. Since primary is connected in series with the load, hence current transformers are also known as series transformer. On the other hand, primary side of potential transformer is always connected in parallel of the voltage to be monitored. Moreover, secondary winding of current transformer is connected with ammeter while secondary of potential transformer is connected to the voltmeter.
Turns :
Primary winding of current transformer consists of single or few turns while its secondary side has a large number of turns. Since voltage level at the primary side of potential transformer is very high, hence number of turns on the primary side of potential transformer is more as compared to its secondary (see the above fig ).
It is to be noted that as the number of turns increases, voltage increases and current reduces. Hence number of turns on the primary and secondary winding of instrument transformers are adjusted in such a way that the current and voltages induce at the secondary side of CT and PT are easy to measure for the low rating ammeters and voltmeters.
Transformation Ratio :
Current transformer converts thousands of primary amp current to a small secondary current in a standard ratio of 5 amp or 1 amp. Potential transformer on the other hand steps down high magnitude voltage to a standard lower voltage with a range up to 110V. The resultant secondary current and voltages are then fed to the measuring devices which in turn measures the primary current and voltages in a particular proportional way.
In normal transformer, electrical load is connected across the secondary from where we get some output. In case of instrument transformers, some measuring and protective instruments like protective relays and meter wires are connected across the secondary. These wires have impedance called as burden.
Increase in the burden of current transformer results in the increase of secondary (output) voltage. A large increase in secondary voltage may lead to core saturation and also increase of phase angle errors. For that reason, load on the secondary of current transformer should have low impedance. Potential transformer on the other hand has high impedance since burden of potential transformers doesn’t affect its output voltage too much.
Output :
Current transformer must have only one output. In other words, current transformer should be install around only one phase conductor. Connecting a CT with multiple phase conductors may result in the cancelling of flux due to which ammeter indicates zero reading. Potential transformer on the other hand may have more than one output.
Safety Measurement :
Secondary of current transformer should never be kept open when its primary is energized. In other words, current transformer can never be operated without a load on its secondary. However, Potential transformer’s secondary can be open circuited during its operation but can never be short circuited.
Types :
Bar CT and Wound CT are two types of current transformer. Potential transformer types includes electromagnetic potential transformers and Capacitive potential transformers.
Conclusion :
Instrument transformers are used for measuring and protective purposes in power system. Large power coming from the transmission line is reduced to a small voltages and current by instrument transformers which can be easily measured by low rating measuring devices. They also provide isolation to the measuring instruments from high high power networks. These transformers are found in substation and grids.