The open circuit of the protection winding of a current transformer (CT) is a very dangerous fault condition.
In simple terms, it refers to: In the normal operation of the secondary winding of a current transformer (i.e., the circuit connected to the protection device and measuring instrument), it suddenly disconnects, causing the circuit impedance to become extremely large, almost infinite.
I. Core principle: Transformation from "current source" to "high-voltage source"
1)During normal operation (closed circuit):
The current transformer is essentially a "current source". The current on the primary side (high current) determines the current on the secondary side (low current).
According to the transformer principle: I₁ / I₂ = N₂ / N₁ (transformer ratio), I₂ is limited to the rated value (usually 5A or 1A). The impedance of the secondary circuit is very small, so although the current is very large, according to U = I × Z, the secondary voltage U is very low (usually less than a few volts).
2) When open circuit occurs:
When the circuit is disconnected, the impedance Z becomes extremely large (approaching infinity).
To maintain electromagnetic induction, the magnetic flux in the iron core of the current transformer will increase sharply, attempting to maintain the secondary current I₂.
According to the formula U₂ = I₂ × Z, when Z is extremely large, a very high voltage (peak value can reach several thousand or even tens of thousands of volts) will be induced on both sides of the open circuit point (peak voltage can be several times or even tens of times the normal voltage).
At this time, the current transformer changes from a "current source" to a dangerous "high-voltage source".
2 .Severe consequences generated
The high voltage and high energy generated during open circuit can cause a series of serious problems:
1)Personal safety accidents:
The extremely high voltage is capable of breaking through the air and causing a fatal electric shock to nearby operators. This is the most direct and dangerous risk.
2)quipment damage:
Insulation breakdown: The high voltage can break through the insulation of the current transformer's windings, the insulation of the secondary circuit wires, and the insulation of the connected protection devices and measuring instruments, resulting in permanent damage to the equipment.
Core saturation and overheating: A sharp increase in magnetic flux will cause the core to be highly saturated, generating huge hysteresis and eddy current losses, causing severe overheating of the core, which may burn out the current transformer and even trigger a fire.
Arc danger: The open circuit point may generate a continuous arc, igniting flammable materials around.
3)System protection failure:
Open circuit of the protection winding will prevent the relay protection device (such as differential protection, overcurrent protection) from receiving the correct current signal, resulting in the protection failing to operate when a real fault occurs in the system, unable to cut off the fault, and possibly causing a wider power outage or equipment damage.
4)Generation of dangerous high-frequency oscillations:
The high voltage generated during open circuit may contain rich harmonics and high-frequency oscillations. These oscillating voltages may be coupled to the secondary system or other equipment through capacitors, causing unexpected interference or damage.
3.How to identify open circuit of current transformer?
Operators or maintenance personnel should pay attention to the following abnormal phenomena:
Abnormal sound: The current transformer itself emits a strong "buzzing" or "squeaking" discharge sound.
Abnormal protection device: The related current meters, power meters show zero or abnormal fluctuations; the protection device may issue "TA disconnection" or similar alarm signals.
Smoke or burnt smell: There is smoke, sparks, or burnt insulation smell at the current transformer or secondary terminal block.
4. Correct handling steps after discovering open circuit (Safety first!)
If it is suspected or confirmed that there is a CT open circuit, it must be immediately handled according to the following principles:
1) Report immediately: Report to the dispatch and relevant responsible persons.
2)Safety isolation: According to the emergency plan, apply for the power outage of related primary equipment (such as circuit breakers, lines).
3)Wear PPE: When handling, must wear insulating gloves and goggles, stand on an insulating pad.
4) Short-circuit treatment: If it is possible to short-circuit at a safe location (such as the connection terminals of the current transformer itself), use short-circuiting clips or insulated wires to reliably short-circuit the secondary winding.
Note: Short-circuit first, then handle the open circuit point; after handling, first remove the open circuit point, then remove the short-circuit line. The sequence must not be wrong.
5)Check and repair: Check the open circuit point (such as loose terminals, broken wires), repair after checking the insulation resistance, and only resume power supply after confirming there are no errors.
6)Strictly prohibit blind operation: Absolutely prohibit touching the open circuit circuit without power outage or effective short-circuit. Absolutely prohibit using wires to forcibly connect the open circuit, live contacts.
The open circuit of the protection winding of the current transformer is a dangerous fault that transforms from a low-voltage small-current state to a high-voltage large-energy state. The primary risk is personal electric shock, followed by equipment burning and protection system failure. Handling such faults must strictly follow safety regulations, prioritize power outage, and perform reliable short-circuiting while taking personal protection measures. The key to prevention lies in regularly checking the tightness and insulation condition of the secondary circuit wiring.
LZZBJ4-35 current transformer by Shaanxi West Power Tongzhong Electrical Co., Ltd. is a kind of sensor used to measure large current, mainly used in substations, power systems and electrical equipment, etc. The transformer adopts magnetic core type DC current transformer, which is characterized by high linearity, strong anti-interference ability, small volume, simple structure, etc., and it is a kind of very high cost-effective current transformer.
LZZBJ4-35 current transformer has many advantages :
Safe and Reliable:
Electrical Isolation: Current transformers effectively isolate high-voltage, high-current primary circuits from low-voltage, low-current secondary measurement and protection circuits, preventing operators from direct contact with high-voltage currents, significantly reducing the risk of electric shock and ensuring personnel safety.
Strong Anti-Interference: Based on the principle of electromagnetic induction, they do not affect or interfere with the measured circuits, ensuring accurate and reliable measurement results. They can operate stably in complex electrical environments, providing a strong guarantee for the safe operation of power systems.
High Measurement Accuracy:
Output signal accuracy can reach Class 0.5, Class 0.2, or even higher, meeting the needs of applications requiring high current signal accuracy. Whether in normal power system operation or during transient conditions during faults, they can accurately measure current, providing precise data support for relay protection, metering, monitoring, and other applications.
Wide Measurement Range:
They are suitable for different voltage and current levels, meeting the needs of various power system and industrial applications. Modern current transformers typically feature multiple adjustable measurement ranges. By adjusting the transformer's transformation ratio, they can accommodate currents of varying magnitudes, offering high versatility and flexibility.
Small Size and Light Weight:
Made of insulating materials and fine-grained conductors, they boast a compact, lightweight design, making them easy to install and maintain. In space-constrained electrical equipment, such as switchgear and distribution boxes, current transformers can be easily installed without excessive burden or space consumption.
Excellent Dynamic Performance:
They can quickly and accurately respond to current changes, exhibiting excellent frequency response and dynamic characteristics. They are suitable for applications with wide current ranges and demanding waveform requirements, such as short-circuit fault detection and protection in power systems. When a short-circuit fault occurs, the current transformer can quickly and accurately reflect the magnitude and changes of the fault current, providing a reliable signal for rapid action of protective devices.
For more information ,pls contact me at any time :
Shaanxi West Power Tongzhong Electrical Co., Ltd.
Contact: Ms.Grace Liu (Director of Sales Department)
Email:xdtz04@westpowerelectric.com
Tel: +86 917 3661109 Fax: +86 917 6739416
Mobile: +86 18091765882(WhatsApp/Wechat/skype )
Website:https://www.xdtzelectrical.com
