Protection Current Transformer (CT) and Metering CT are two different types of CT as per their requirement and hence design. Therefore, Protection class CT should not be used as Metering CT. Similarly, a Metering Current Transformer should not be used for protection purpose. Let the discuss the reason behind this.
Why can’t we use Protection CT for Metering Purpose?
The purpose of a protection class current transformer is measure current during the fault condition and give input to the relay. Based on the current input, the relay then decides for the actuation of trip signal to Circuit Breaker to isolate the healthy section of power system from the faulty section. This simply means that, the main requirement for a protection class CT is that it should measure the current accurately even when the primary current is of the order of 10 or 20 times of the rated current. This fact will be more evident if you look at the accuracy class of protection class CT. It is generally 5P10 or 5P20. This means the CT will maintain its accuracy within ±5% when the current through it 10 or 20 times of the rated current. You may read accuracy limit factor for better understanding.
Another important design feature of a protection class CT is that its core does not get saturated even under the condition of fault. This necessitates for the higher value of Knee Point Voltage. Therefore, such CTs can measure fault current accurately without getting its core saturated.
Considering the above unique features, if the protection class CT is connected to meters then under normal condition, the measurement of meter will not be accurate as desired for revenue meters. Further, under the condition of fault, the secondary current of the current transformer will be very, high around 10 to 20 times of rated current. This results in flow of high value of current through the meter. Since meters are not designed for such a high value of current (10 to 20 times of rated current), this will lead to damage of meter.
Why can’t we use Metering CT for Protection Purpose?
A metering current transformer (CT) is characterized by its ability to measure current accurately (as per its accuracy class) when the primary current is in the range of 1% to 120% of the rated current. Since, the secondary of metering CT is connected to various types of Meters, an Instrument Safety Factor (ISF) is also designated for metering CT. The main purpose of designating ISF is to protect meters connected in the secondary of metering CT during fault condition. Let us assume that the ISF is 3. This means, if the CT primary current is more than 3 times the rated current, the CT core will saturate to limit the current through the meters connected in the CT secondary. Thus, the meters are protected from high value of current under fault condition.
Let us now assume that this metering CT secondary is connected to protection relay. Under fault condition, the CT core will saturate and hence its secondary current output will be heavily distorted. Under such condition, it may happen that the relay may not operate. Thus, the power system stability will be compromised. In other words, the purpose of relay is defeated.
From the above discussion, it is clear that protection class CT should be used for protection purpose and metering CT should be used for metering purpose.