The reasons why the secondary side of the current transformer must be grounded and the safety hazards caused by not grounding it are analyzed as follows:
Reasons for Grounding
Safety Protection: When a current transformer operates in a high-voltage power system, its primary side is exposed to high voltage. If the insulation between the primary and secondary windings fails, high voltage may penetrate into the secondary side, posing a threat to the safety of personnel and equipment. By grounding the secondary side, when high voltage on the primary side invades the secondary circuit due to insulation damage, the grounding can direct the dangerous voltage to the earth, thereby preventing electric shock to secondary equipment and operators.
Stable Potential: Grounding stabilizes the potential of the secondary circuit and reduces the risk of measurement errors or equipment damage caused by floating potential. After the secondary side is grounded, it attains a reliable zero potential and is no longer in a suspended working state, which is beneficial for the instrument's anti-interference capabilities and helps improve measurement accuracy.
Preventing High Voltage from Injuring People: A current transformer is equivalent to a step-up transformer. Grounding the secondary side mainly prevents the high voltage generated when the secondary side is open-circuited. When the secondary side is open due to a fault, an ultra-high voltage is generated in the coil, which may break down the insulation and pose an electric shock risk. If the secondary coil is grounded, the high voltage is directed into the earth, keeping the secondary coil at ground potential and thus ensuring the safety of personnel and equipment.
Safety Hazards of Ungrounded Secondary Side
Risk of Electric Shock: If the secondary side of the current transformer is not grounded, once the insulation between the primary winding and the secondary winding breaks down, high voltage will connect to the low-voltage side. At this time, if a person touches the secondary side equipment or line, they may suffer an electric shock, resulting in electric burns or even death.
Equipment Damage: High voltage entering the secondary side not only harms personnel but also damages secondary equipment such as ammeters, power meters, and relay protection devices. Damage to these devices affects the normal operation and monitoring of the power system and may even cause more serious failures.
Electric Energy Metering Error: Although an ungrounded current transformer does not directly affect the meter's measurement, the lack of grounding may introduce additional interference currents into the system. These interference currents affect the accuracy of electric energy metering, causing errors and bringing economic losses to power companies and users.
Misoperation of Protection Device: The current transformer is a key component of the relay protection system. If the secondary side is not grounded, it may cause the relay protection device to misoperate or fail to operate. For example, in the event of a fault, the protection device may not correctly identify the fault and take corresponding protection measures, resulting in the expansion of the fault or affecting the stable operation of the power system.
Risk of Voltage Transformer Damage: In an ungrounded system, improper use of the current transformer may cause the burning of the voltage transformer (PT). This is especially prominent in 10kV-35kV ungrounded systems. Damage to the voltage transformer not only affects the accurate measurement of the energy meter but may also cause misoperation of protection devices and safety automatic devices.
Risk of Ferromagnetic Resonance: In some cases, ungrounded systems are more prone to ferromagnetic resonance. Ferromagnetic resonance may cause the primary fuse of the voltage transformer to melt or the voltage transformer to burn, threatening the safety of the power grid.