Product Overview
The LXK-250/100 Racetrack-Type Zero-Sequence Current Transformer is an epoxy resin cast current transformer designed for indoor 10 kV AC power systems with a maximum system voltage of 12 kV.
The transformer features an elongated oval, or racetrack-shaped, cable aperture specifically designed to accommodate three-phase cables installed side by side. It is mainly used for single-phase earth-fault detection, relay protection, and insulation monitoring in systems with non-effectively earthed neutrals.
The product is widely used in medium-voltage switchgear, ring main units, box-type substations, cable branch boxes, and other power distribution equipment. It is particularly suitable for switchgear cable compartments where multiple cables are arranged in parallel.
Product Features
Insulation and Electrical Performance
- Epoxy resin cast insulation:
The transformer is integrally formed using an epoxy resin vacuum-casting process. The solid insulating body contains no internal air gaps and provides excellent dielectric performance. - Oil-free dry-type construction:
The product contains no insulating oil, eliminating the risks of oil leakage, oil contamination, and oil-related fire hazards. It is suitable for modern indoor oil-free power distribution systems. - High-sensitivity zero-sequence current detection:
The ring core is made of high-quality laminated silicon steel, providing low excitation current and high magnetic sensitivity. It accurately detects zero-sequence current generated during a single-phase earth fault. - Compatible with earth-fault protection systems:
The transformer can supply current signals to low-current earth-fault line-selection devices, zero-sequence protection relays, monitoring devices, and related protection equipment. - Large racetrack-shaped aperture:
The elongated aperture measures 250 × 100 mm and can accommodate three 10 kV cables arranged side by side. This design is particularly suitable for densely arranged cable systems where a conventional circular aperture cannot provide sufficient installation space. - Low partial discharge:
The advanced casting process limits partial discharge to no more than 10 pC, helping reduce insulation ageing and improve long-term operating reliability. - High insulation withstand capability:
The transformer can withstand the specified power-frequency and lightning impulse withstand voltage requirements of 10 kV systems.
Structural and Installation Features
- Fully enclosed dry-type structure:
The magnetic core and secondary winding are enclosed within the epoxy resin cast body. The product provides an IP20 protection rating and contains no exposed internal live components. - Compact construction:
The racetrack-shaped structure is suitable for installation in switchgear cable compartments, cable trenches, and other areas with densely arranged cables. - Through-aperture design:
All three phase cables pass directly through the central aperture, allowing zero-sequence current detection without a conventional primary winding connection. - Simplified primary-circuit arrangement:
The through-type design eliminates additional primary-side wiring, simplifies installation, and reduces the number of connection points and potential faults. - Clearly identified secondary terminals:
The secondary terminals are protected by a transparent cover and clearly marked for convenient wiring. The terminal arrangement is compatible with common zero-sequence protection circuit requirements. - Flexible mounting:
The metal mounting bracket supports secure bolt installation. The transformer can be mounted horizontally or vertically inside indoor switchgear and provides reliable vibration resistance.
Operation and Maintenance Features
- Low-maintenance operation:
The dry-type, oil-free structure presents no oil leakage or oil-related fire risk. The epoxy resin body provides high mechanical strength, impact resistance, and ageing resistance. - Wide operating temperature range:
The transformer can operate at ambient temperatures from -30°C to +70°C. The epoxy resin maintains stable insulation and mechanical properties without becoming brittle or deformed. - Stable signal accuracy:
The high-quality silicon steel core and enamelled copper winding provide low operating losses, limited temperature rise, and stable zero-sequence current detection. - Reliable protection performance:
The transformer can withstand switching overvoltage and short-circuit current effects within its specified ratings, helping ensure dependable earth-fault protection operation.
Technical Specifications
Electrical Parameters
| Parameter | Specification |
|---|---|
| Applicable System Voltage | 10 kV AC, Um = 12 kV |
| Rated Frequency | 50 Hz |
| Rated Primary Zero-Sequence Current | Customized according to the system earth-fault current |
| Rated Secondary Current | 5 A or 1 A, customizable |
| Protection Accuracy Class | 10P10 / 10P15, customizable |
| Power-Frequency Withstand Voltage | 42 kV for 1 minute, without breakdown |
| Lightning Impulse Withstand Voltage | 75 kV peak, without breakdown |
| Partial Discharge | ≤10 pC |
| Insulation Resistance | ≥1,000 MΩ between the primary circuit, secondary circuit, and earth |
| Zero-Sequence Current Sensitivity | Detects earth-fault currents of 0.5 A and above; customizable |
Structural and Material Parameters
| Parameter | Specification |
|---|---|
| Insulating Medium | Fully cast epoxy resin |
| Protection Rating | IP20 |
| Aperture Dimensions | 250 mm long × 100 mm wide, racetrack-shaped |
| Installation Type | Indoor bolt-mounted installation |
| Thermal Class | Class B, 130°C |
Environmental Parameters
| Parameter | Specification |
|---|---|
| Installation Altitude | ≤2,000 m; high-altitude versions available |
| Operating Temperature | -30°C to +70°C |
| Storage Temperature | -40°C to +85°C |
| Relative Humidity | ≤95%, non-condensing |
Applicable Standards
| Standard | Description |
|---|---|
| GB 1208 | Current Transformers |
| GB/T 20840 Series | Instrument Transformers |
| DL/T 866 | Code for Selection and Calculation of Current Transformers and Voltage Transformers |
Applications
Applicable Voltage Level
The LXK-250/100 is suitable for indoor 10 kV AC power systems with a maximum system voltage of 12 kV.
Typical Applications
- Single-phase earth-fault detection and zero-sequence protection in indoor 10 kV medium-voltage switchgear
- Ring main units
- Box-type substations
- Cable branch boxes
- Distribution stations and switching stations
- 10 kV distribution systems in industrial and mining enterprises
- Commercial buildings and industrial parks
- Low-current earth-fault line-selection systems
- Zero-sequence protection devices
- Switchgear cable compartments with densely arranged cables
- Cable trenches and retrofit projects requiring three-phase cables to pass through the transformer side by side
Main Functions
The transformer detects the vector sum of the three-phase currents passing through its central aperture.
Under normal operating conditions, the three-phase current vectors are balanced, and the resulting zero-sequence current is close to zero. When a single-phase earth fault occurs, the current imbalance produces a detectable zero-sequence current signal.
The transformer supplies this signal to:
- Zero-sequence protection relays
- Low-current earth-fault line-selection devices
- Monitoring equipment
- Control systems
- Earth-fault alarm devices
It also provides electrical isolation between the primary power circuit and the secondary protection circuit, helping protect connected equipment and operating personnel.
The racetrack-shaped aperture is especially suitable for three-phase cables arranged side by side.
Operating Conditions
Normal Operating Conditions
- Installation altitude: no more than 2,000 m
- Ambient temperature: -30°C to +70°C
- Severe or sudden temperature changes should be avoided
- Relative humidity: no more than 95%, non-condensing
- The environment should be free from corrosive gases
- Flammable or explosive substances are not permitted
- Severe vibration and mechanical impact should be avoided
- Significant dust and salt-spray contamination should be avoided
- Indoor fixed installation
- Direct sunlight, rain, and condensation should be avoided
Special Operating Conditions
Customized products are required for:
- Installation altitudes above 2,000 m
- Ambient temperatures outside the range of -30°C to +70°C
- Severe pollution or salt-spray environments
- Chemically corrosive environments
- Applications involving strong vibration or mechanical impact
- Non-standard aperture dimensions
- Special sensitivity requirements
- Special protection accuracy classes
Installation and Maintenance
Installation Requirements
- Pre-installation inspection:
Inspect the zero-sequence current transformer and confirm that the epoxy resin body is free from cracks, bubbles, mechanical damage, and other visible defects. Check that the terminal markings are clear and that all supplied accessories are complete. Verify that the model, aperture dimensions, sensitivity, and protection accuracy meet the system design requirements. - Mounting:
Position the transformer at the specified location inside the switchgear and secure it with bolts through the mounting holes in the metal bracket. Ensure that the transformer is firmly installed without looseness or displacement. The mounting position may be horizontal or vertical but must not subject the cast body to uneven mechanical stress. - Primary conductor installation:
Pass all three phase cables through the racetrack-shaped aperture in the same direction. Arrange the three cables evenly inside the aperture wherever possible. Avoid direct contact between the cables and the aperture wall. Do not scratch or damage the insulating surface inside the aperture during cable installation. - Cable earthing arrangement:
Where shielded cables are used, route the cable sheath earthing conductors according to the protection system design to prevent cancellation or incorrect detection of the zero-sequence current signal. - Secondary-side connection:
Connect the secondary terminals to the zero-sequence protection relay or low-current earth-fault line-selection device according to the terminal markings. The secondary circuit must never be left open while the monitored primary circuit is energized. One point of the secondary circuit must be reliably earthed. - Insulation inspection:
After installation, measure the insulation resistance between:- Primary conductors and secondary circuit
- Primary conductors and earth
- Secondary circuit and earth
Maintenance Requirements
- Regularly inspect the epoxy resin body for cracks, damage, discharge marks, and contamination.
- Check the racetrack-shaped aperture for foreign objects, wear, or damage.
- Inspect the secondary terminals for looseness, oxidation, or abnormal heating.
- Check that the three-phase cables do not contact or rub against the internal aperture wall.
- Confirm that the cables remain evenly positioned inside the aperture.
- Check that the secondary wiring remains secure and free from overheating or discoloration.
- Ensure that the secondary circuit remains closed and reliably earthed.
- Periodically test the insulation resistance and zero-sequence current detection accuracy.
- Verify the correct operation of the connected protection and earth-fault line-selection devices.
- Clean dust and contamination from the transformer surface.
- Keep the aperture free from foreign materials.
- Prevent oil and corrosive substances from contacting the epoxy resin body.
- If cracking, electrical discharge, overheating, abnormal detection accuracy, or a secondary-circuit fault is detected, immediately de-energize and inspect the equipment. Replace the transformer where necessary.
Precautions
- The secondary winding must never be left open while the monitored primary circuit is energized. An open secondary circuit may generate hazardous overvoltage and create a serious risk to equipment and personnel.
- During maintenance, the secondary circuit must be securely short-circuited and earthed before disconnection.
- Avoid impact or collision with the epoxy resin body and racetrack-shaped aperture during transportation and installation. Mechanical damage may reduce insulation performance or alter the aperture dimensions.
- Do not scratch the insulating surface inside the aperture when installing the cables.
- All three phase cables must pass through the transformer in the same direction and should be distributed evenly within the aperture.
- Select aperture dimensions suitable for the outside diameter and arrangement of the three-phase cables. Do not force oversized cables through the aperture.
- Prevent the cables from moving and rubbing against the aperture wall during operation, as this may damage the insulation and create a risk of discharge or short circuit.
- The equipment must be reliably earthed. The grounding terminal on the mounting bracket must be securely connected to the switchgear grounding busbar.
- Store the product in a dry, ventilated, and shaded environment. Avoid direct sunlight, high temperatures, moisture, compression, and mechanical impact.
- Installation, commissioning, and maintenance must be carried out by qualified electrical personnel in accordance with applicable electrical safety procedures.
Standard Supply List
| No. | Item | Function |
|---|---|---|
| 1 | LXK-250/100 Racetrack-Type Zero-Sequence Current Transformer | Main device for zero-sequence current transformation, electrical isolation, and signal acquisition |
| 2 | Mounting Bolts and Washers | Used to secure the transformer during installation |
| 3 | Transparent Secondary Terminal Cover | Protects the secondary terminals and helps prevent incorrect connection or short circuits |
| 4 | Product Instruction Manual | Provides installation, operating, maintenance, and cable-routing guidance |
| 5 | Product Certificate and Factory Test Report | Provides product quality and performance documentation, including zero-sequence current ratio, accuracy, and sensitivity test data |
Applicable Standards and Certification
The product complies with the following standards:
- GB 1208, Current Transformers
- GB/T 20840.1, Instrument Transformers—Part 1: General Requirements
- GB/T 20840.2, Instrument Transformers—Part 2: Additional Requirements for Current Transformers
- DL/T 866, Code for Selection and Calculation of Current Transformers and Voltage Transformers
The product has passed the applicable type tests, and its performance meets the earth-fault protection requirements of indoor 10 kV power systems.