What are the consequences of using an unsuitable current transformer?
Jun 17, 2026| 1. Consequences of Measurement and Metering: Accuracy Deviation Leading to Economic Losses
Different types of mismatches can lead to inaccurate metering:
Matching Transformer Ratio Settings: A high transformer ratio will result in overstated measured values and electricity consumption, leading to abnormally high electricity bills and direct economic losses for businesses. A low transformer ratio will result in understated measured values, leading to undercharging and losses for power supply companies.
Low Accuracy Class Selection: Using low-precision transformers in high-precision monitoring scenarios will cause excessive measurement errors, failing to meet the requirements for power quality monitoring and metering settlement. For example, using a 0.5-class transformer in a Class A online monitoring device may result in errors exceeding ±1% under overload conditions, failing to meet specifications.
Incorrect Polarity Wiring: Reversed polarity in the metering circuit will cause undercharging or reverse metering, leading to incorrect electricity billing and economic losses. Power factor judgment will also be incorrect, interfering with power system operation status assessment.
Excessive Secondary Load: Loads exceeding rated allowable values will increase transformer errors, potentially causing magnetic saturation, current waveform distortion, and harmonic measurement errors exceeding ±20%.
2. Consequences Related to Relay Protection: Misoperation/Failure to Operate, Expanding the Scope of the Accident
This is the most dangerous type of consequence, directly impacting power system safety:
Incorrect Transformer Ratio Selection: An oversized transformer ratio amplifies the secondary current, potentially exceeding the protection setting value even under normal load, causing false tripping, leading to faultless power outages, and reducing power supply reliability. An undersized transformer ratio reduces the secondary current, failing to reach the protection setting value during a fault, resulting in protection failure to operate and timely fault isolation. This can cause burnout of main equipment such as switchgear, transformers, and cables, and may also trigger cascading trips of upstream switches, causing widespread power outages.
Incorrect Polarity Connection: This leads to logical confusion in differential protection, zero-sequence protection, and other protection devices, resulting in frequent false tripping during normal operation and inability to properly close and restore power. During a fault, failure to operate prevents fault isolation, delaying power restoration and project progress.
Saturation/Overload: Excessive secondary load or short-circuit current exceeding the rated range can cause transformer saturation. In cases of external faults, this can lead to malfunctioning protection systems; in cases of internal faults, the lack of secondary output can cause protection systems to fail to operate, expanding the scope of the accident.
3. Safety Hazards: These can cause equipment damage and even personal safety risks.
Secondary Open Circuit (due to improper installation/selection): This can generate thousands to tens of thousands of volts at the open circuit terminal, potentially breaking down the insulation and burning out the transformer, and directly endangering the lives of on-site maintenance personnel.
Parameter Mismatch and Long-Term Overload: Prolonged current exceeding the rated value can cause the transformer core to overheat, accelerating insulation aging, reducing service life, and in severe cases, causing insulation breakdown, resulting in short circuits and fires.
Incorrect Environmental Adaptation: Using indoor-type transformers outdoors with insufficient protection levels can lead to water ingress and moisture, causing insulation short circuits, resulting in transformer explosions, threatening equipment and personal safety.
