- October 13, 2021
- Posted by: Dyaneshwar Nirmale
- Category: Digital Meters
The accuracy limit factor is a critical factor when choosing protection class current transformers. Protection class current transformers or P class CTs must maintain measuring accuracy at high voltage levels during fault conditions. Here, the CT accuracy limit factor determines the capacity of the current transformer to handle such high levels of current and enable the relay to trip. In the case of metering class CTs, a similar safety function is determined by the Instrument safety factor.
What is the accuracy limit factor?
The CT accuracy limit factor is defined as the ratio of accuracy limit current to the primary current. A typical protection class current transformer is labelled as ‘5P10 or 5P 20. here, 5 denotes the accuracy class of the CT, P denotes that the transformer is protection class, and 20 is the accuracy limit primary current.
if there is a 5P20 CT of 1000/5 Amp, and the primary current increases to 20,000 Amp, (1000×20) the CE accuracy should be around 5% or less. This factor (In x 20)20 is referred to as the accuracy limit factor(ALF). In the case of measurement CTs, if a high current is flowing through the primary winding, it should be limited in the secondary winding. However, in the case of protection CTs, the case is reversed. Here, the secondary current increases as the primary current increases. This in turn enables the relay to detect the high current and trip.
If the current in the primary winding of the current transformer is more than 20 times the primary rated current, the secondary current gets distorted and the CT cannot offer assured protection. Hence, determining the accuracy limit factor is critical when choosing a protection-type current transformer.
Accuracy limit factor calculation
The accuracy limit factor of the current transformer is impacted by the internal burden of the CT secondary coil, and the actual burden of the CT. Depending on these factors, the actual ALF of a current transformer could differ from the rated ALF.
The actual accuracy limit factor of a current transformer can be calculated using the formula:
Conclusion
The highest fault current magnitude at which the CT will deliver the specified accuracy is determined by the accuracy primary limit current. After this level is crossed, the secondary current in the CT is distorted. This could have a severe effect on the performance of the relay. If the relay does not trip, the entire electrical system and human safety are threatened. Hence, the accuracy limit factor is a key consideration I protection-type transformers.