In power metering and monitoring systems, electricity meters that require external current transformers (CTs) are ubiquitous; they are our "eyes" for accurately sensing massive currents. However, hidden within this sophisticated system is an ironclad rule that must be followed at all times: the secondary side of the current transformer must never be operated with an open circuit. This article will delve into the underlying principles and the dangers involved.
Normal operating principle of current transformer
A CT (Current Transformer) is a special type of transformer that operates based on the principle of electromagnetic induction. Its core design principles are "current reduction" and "isolation."
1. Structure: It typically consists of a closed iron core, a primary winding with fewer turns (connected in series in the main circuit), and a secondary winding with more turns (connected to the electricity meter).
2. Ideal State: In a normal closed circuit, the CT operates in an approximately "short-circuit" state. According to Ampere's circuital law and the law of electromagnetic induction, the primary current I1 will generate an alternating magnetic flux Φ in the iron core, which will then induce a current I2 on the secondary side. The relationship between the two is as follows:
Where N1 and N2 are the number of turns in the primary and secondary windings, and Im is the excitation current. Due to the very large excitation impedance and extremely small Im in the design, it can be simplified to the following under ideal conditions:
Here, Kn refers to the rated transformation ratio, such as 1000/5A. In this case, the large current on the primary side is precisely and proportionally converted into a small current on the secondary side (typically a standard value of 5A or 1A) for safe instrument measurement. Simultaneously, the secondary circuit potential of the current transformer (CT) is very low (usually only a few volts), within a safe range.
Principle analysis when the secondary side is open
When the secondary circuit becomes open due to loose terminals, broken wires, or accidental disconnection during testing, its operating state undergoes a catastrophic change.
| Operating status | Normal closure | Secondary road opening |
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Exists, proportional to I1 |
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Φ |
Effectively suppressed by the demagnetizing flux generated by I2, maintaining a low level | Loss of inhibition, rapid saturation to extremely high values |
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Very low (several volts) | High voltage of thousands or even tens of thousands of volts was induced. |
| Physical nature | Strong coupling, deep negative feedback: I2 strongly resists changes in Φ | Feedback cutoff and energy accumulation: All primary ampere-turns I1 and N1 are used for excitation. |
Its core physical processes are as follows:
1: Disappearance of Demagnetizing Feedback
During normal operation, the magnetic flux generated by the secondary current I2 is always in the opposite direction to the magnetic flux generated by the primary current I1, creating a strong "demagnetizing" effect that confines the combined magnetic flux in the iron core to a low level. After the circuit is opened, I2 = 0, and the demagnetizing effect instantly returns to zero.
2: Rapid saturation of magnetic flux
The unbalanced primary ampere-turns I1 and N1 are entirely converted into magnetizing ampere-turns. Since the core cross-sectional area is designed for low magnetic flux density, the core rapidly enters a deep saturation state at this point.
According to Faraday's law of electromagnetic induction, alternating magnetic flux will induce an electromotive force across the winding. When the magnetic flux increases sharply, an extremely high voltage U2 will be induced across the secondary winding.
3: Generation of high pressure
Under power frequency conditions, for a primary current of several hundred amperes, the induced voltage on the open-circuit secondary side can easily reach several thousand volts, and in extreme cases, it can exceed 10 kilovolts.
The national standard GB/T 20840.2-2014 "Instrument Transformers - Part 2: Supplementary Technical Requirements for Current Transformers" has strict requirements for the insulation performance of instrument transformers, and this sudden high voltage far exceeded its normal design capacity.
Hazards of open circuit on the secondary side of current transformer
The high voltage and accompanying phenomena generated by a secondary open circuit can trigger a series of chain reactions of hazards.
1. Danger of Electric Shock: Thousands of volts of high voltage exist at the secondary terminals, directly posing a serious risk of electric shock. Maintenance and repair personnel who come into contact with this voltage without precautions may suffer electric shock.
2. Equipment Damage:
Insulation Breakdown: High voltage will first break down the inter-turn and inter-layer insulation of the secondary winding, or break down the insulation to ground in the secondary circuit, leading to permanent damage to the current transformer (CT).
Overheating and Burnout: When the iron core is highly saturated, it will generate huge eddy currents and hysteresis losses, causing the iron core to overheat, which may burn out the winding insulation and even cause a fire.
Electric Arc and Explosion: Open circuit points (such as loose terminals) will generate a continuous electric arc under high voltage. The high temperature of the arc may burn out equipment, ignite surrounding combustibles, and the high-temperature gases accumulated in the sealed cabinet may even cause an electrical explosion.
3. System Operation Hazards
Metering Inaccuracy and Failure: For CT-type energy meters, a zero input current will render them unable to measure electricity, resulting in lost electricity and potential trade settlement disputes.
Generation of Dangerous High-Voltage Sparks: This is not only an ignition source, but the resulting strong electromagnetic pulses can also interfere with nearby electronic equipment.
Summarize
An open circuit on the secondary side of a current transformer triggers a violent accumulation of electromagnetic energy, ultimately resulting in a physical catastrophe manifested as high voltage, strong electric arcs, and overheating. Therefore, in all work involving CT circuits, "open circuit prevention" must be strictly adhered to as a procedure.
Meanwhile, the secondary side of the current transformer connected to the electricity meter must be grounded. This, along with "strictly prohibiting open circuits on the secondary side," are the two core ironclad rules of CT operation and maintenance. After grounding, the high voltage that has surged in can be quickly discharged to the ground through the grounding wire, avoiding equipment burnout or electric shock accidents caused by a sudden rise in secondary side potential.