Product Overview
The LXK-80–300 Zero-Sequence Current Transformer is an epoxy resin cast current transformer designed for indoor 10 kV and 35 kV AC power systems.
Featuring a ring-type magnetic core and an integrated cast-resin structure, the transformer is mainly used for single-phase earth-fault detection, relay protection, and insulation monitoring in systems with non-effectively earthed neutrals.
It is widely installed in medium-voltage switchgear, ring main units, box-type substations, cable branch boxes, and other power distribution equipment. The product is a key sensing component in power-system earth-fault protection schemes.
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 indoor oil-free distribution systems. - High-sensitivity zero-sequence current detection:
The ring core is made of high-quality laminated silicon steel with low excitation current and high magnetic sensitivity. It accurately detects zero-sequence current generated during single-phase earth faults. - Compatible with earth-fault protection equipment:
The transformer can supply current signals to low-current earth-fault line-selection devices, zero-sequence protection relays, monitoring devices, and other protection equipment. - Wide aperture range:
Available internal aperture diameters range from 80 to 300 mm. Different models can accommodate various three-phase cable bundles or busbar arrangements without requiring major installation modifications. - 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 and 35 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 has no exposed internal live components. - Compact construction:
The ring-type design occupies limited installation space and is compatible with standard switchgear and cable-compartment layouts. - Through-aperture design:
Three-phase cables or busbars pass directly through the central aperture, allowing zero-sequence current detection without a conventional primary winding connection. - Simplified primary-circuit arrangement:
The through-type structure reduces the number of primary-side connections, simplifies installation, and helps minimize potential connection points and related faults. - Clearly identified secondary terminals:
The secondary terminals are protected by a transparent cover and clearly marked for convenient wiring. The terminal layout 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 reliable earth-fault protection operation.
Technical Specifications
Electrical Parameters
| Parameter | Specification |
|---|---|
| Applicable System Voltage | 10 kV / 35 kV AC, Um = 12 kV / 40.5 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 for 10 kV systems; 95 kV for 1 minute for 35 kV systems, without breakdown |
| Lightning Impulse Withstand Voltage | 75 kV peak for 10 kV systems; 185 kV peak for 35 kV systems, 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 |
| Internal Aperture Diameter | 80 / 100 / 150 / 200 / 300 mm |
| Installation Type | Indoor bolt mounting |
| 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 Levels
Suitable for indoor 10 kV and 35 kV AC power systems with maximum system voltages of 12 kV and 40.5 kV.
Typical Applications
- Single-phase earth-fault detection and zero-sequence protection in indoor medium-voltage switchgear
- Ring main units
- Box-type substations
- Cable branch boxes
- Distribution stations and switching stations
- 10 kV and 35 kV distribution systems in industrial and mining enterprises
- Commercial buildings and industrial parks
- Low-current earth-fault line-selection systems
- Zero-sequence protection devices
- Earth-fault monitoring for cable and busbar circuits
Main Functions
The LXK-80–300 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.
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 bolt-mounted 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 diameters
- 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 diameter, 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 surface may be horizontal or vertical but must not place uneven mechanical stress on the cast body. - Primary conductor installation:
Pass all three phase conductors through the central aperture in the same direction. The three-phase cables or busbars should pass concentrically through the aperture wherever possible. Avoid direct contact between the conductors and the aperture wall. Do not scratch or damage the insulating surface inside the aperture during cable or busbar installation. - Cable earthing arrangement:
Where shielded cables are used, the cable sheath earthing conductor must be routed according to the protection 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 internal aperture for foreign objects, wear, or damage.
- Inspect the secondary terminals for looseness, oxidation, or abnormal heating.
- Check that the three-phase cables or busbars do not contact or rub against the internal aperture wall.
- Confirm 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 measurement 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 internal 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 cables or busbars.
- The three-phase conductors must pass through the transformer in the same direction and should be positioned concentrically within the aperture.
- Select an aperture diameter suitable for the outside dimensions of the cable bundle or busbar arrangement. Do not force oversized conductors through the aperture.
- Prevent cables or busbars 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-80–300 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 conductor-routing guidance |
| 5 | Product Certificate and Factory Test Report | Provides product quality and performance documentation, including 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 and 35 kV power systems.
