Matching suggestions for current transformers and power meters/protection devices
Mastering the matching principles of current transformers can not only improve the adaptability and application stability of products, but also provide customers with more professional and demand-oriented technical support.
Why is matching critical?
The output of the current transformer is a proportionally reduced current signal for use by the secondary side ammeter, electric energy metering device, relay protection equipment, etc. If the matching is improper, the following problems may occur:
Overload risk: The instrument or protection device is easily damaged or the accuracy decreases when it works for a long time at a current exceeding its rated input
Output distortion: The transformer's insufficient load capacity causes waveform distortion, affecting the relay protection's false operation or non-operation
Accuracy deviation: When the load impedance deviates from the design value, the transformer output error increases, affecting the credibility of the system data
Therefore, ensuring the matching of the secondary load from the source is an important link that cannot be ignored in every power engineering and equipment production link
Key parameters for reasonable matching of current transformers and power meters
1. Transformer ratio (rated primary/secondary current)
The transformation ratio should be selected to meet the maximum operating current of the main circuit, while taking into account the rated input current of the instrument or protection device. The common secondary standard current is 5A or 1A.
Recommendation: The transformation ratio selection should consider 1.2~1.3 times the maximum current of the system as a safety margin.
2. Accuracy level (such as 0.2, 0.5, 1, etc.)
Different instruments or devices have different requirements for measurement accuracy. For example:
Electric energy metering device: It is recommended to use 0.2S, 0.5S high-precision level transformers
Indicator ammeter: 1.0 level transformers can be selected to meet daily readings
Special CT for protection device: Use 5P10, 10P20 and other protection levels, and pay attention to its accuracy limit factor ALF
Matching suggestion: Instrument accuracy ≥ transformer accuracy level to prevent transformer accuracy from limiting overall system performance.
3. Rated load capacity (burden)
The unit is VA (volt-ampere), which indicates the load capacity that the transformer can carry, and needs to match the total impedance of the connected equipment and line.
For example: a 5VA CT, if the total load of the connected instrument + wire exceeds 5VA, it may cause output distortion or reduced accuracy.
Recommendation: The total system load < 80% of the CT rated load, reserve redundancy to ensure linear output.
Special matching requirements for protection devices
The relay protection system requires not only accuracy for the transformer, but also emphasizes that it can act quickly and accurately under extreme current conditions.
Select protection-grade transformers (5P/10P) to cope with high current mutations
Accurate limit factor ALF ≥ actual maximum fault current / rated primary current
Secondary open circuit protection design to prevent the transformer from opening the circuit by mistake during operation and causing overvoltage damage
For suppliers of high-voltage or heavy-load systems, it is recommended to provide CT products with opening alarms, protective shells and high insulation levels to enhance safety.
