We can not use the protection class CT in place of metering CT & vice versa. Protection class CT and metering class CT are used for different applications. We will discuss the reason for this in this article.
Types of Current Transformers
There are three types of current transformers, metering class CT, protection class CT, and special protection PS class CT, which are widely used for metering and protection. The different applications require different classes of the current transformer.
The protection class current transformers are used for the protection of feeders, transformers, motors, generators, and bus bars are not suitable for metering applications and can’t be used for these applications. The metering class CTs are used for energy metering that needs high measurement accuracy and can not be used for protection applications.
Why can’t we use Metering Class CT for Protection Purpose?
The metering class CTs are used in energy meters to measure the energy accurately. For correct measurement of energy, it is a must that the CTs and energy meter both have a high degree of accuracy. The metering class CT has very low ratio error and phase angle error, the metering CTs read correctly even at low current.
The 0.5,1.0,2.0 metering class CT is widely used for metering applications. The metering class CTs give accurate readings from 100-120 % loading with a permissible rated error. The 0.2 and 0.2 s class CT read very precisely and it measures the current accurately from 20 -100 % loading with a measurement error of +/- 0.2% and can read up to 1 % loading with some more error.
The metering class CT functions in the linearity region of the BH curve of CT. Therefore, they have a very low measurement error because The metering class CT has a very low knee point voltage (The point at which CT saturates), and CT saturates at a lower current. The metering class CTs are designed for lower VA and low knee point voltage, therefore they are smaller in size compared to protection class CTs.
Now, let us discuss the concept of why metering class CTs are not suitable for protection applications. The magnitude of the fault current through the current transformer may be of 20 times the CT-rated current. In this condition, the metering class current transformer CT will go into a saturation state and its output current drops to zero.
The protection relay in this case will not get the current input, and the relay will not trip the circuit breaker. Metering class CT under saturation for a longer period of time will cause insulation failure of CT secondary winding.
Therefore. the metering class CT should not be used for protection applications.
Why can’t we use Protection Class CT for Metering Purposes?
The protection class CT has a higher knee point voltage and operates near the saturation point of the BH curve of CT. The higher knee point voltage of the protection class makes them suitable for protection applications because the CT does not saturate at a high current. The high current flows in CT secondary during the fault.
The 5 P20 protection class CT does not saturate if 20 times the rated primary current flows in the secondary of the CT. Even at this high current, the composite error is +/- 5 %. Thus, we can conclude that the protection class CT has higher knee point voltage with lower measurement accuracy compared to the metering class CT.
Now, what happens if we use a protection class CT in place of a metering class CT? The protection class CT gives a high error when it reads low current. Thus, there will be a high measurement error if we use protection class CT in place of metering class CT. The protection class CT does not saturate when a high current flow through it.
Now, think if the various measuring meters like the current meters, power factor meters, and energy meters are connected to this protection class CT, the mammoth current in the event of a fault will flow through the meters because the protection class CT will not saturate at a high current.
The instruments generally have 10 to 12 times overloading capacity for a finite time, but in the case of fault, the current through the CT is more than the overloading capacity of meters. Thus, in the case of a fault, a large current flowing through protection class CT will damage the meters.
The instrument Safety Factor (ISF) of metering class CT is generally 3-5 and the ISF of protection class CT is 10-20. If the current through the metering class CT is 3 to 5 times, the CT saturates, and it does not output current. Thus instruments connected to metering class CT remain protected.
Therefore, the protection class CT should not be used for metering applications.