Product Overview
The LJ-φ75 zero sequence current transformer is an indoor cable-type current transformer designed for earth fault detection and ground protection in small-current grounding systems. It is suitable for AC systems with rated frequency of 50Hz or 60Hz and rated voltage up to 35kV, where the three-phase cable passes through the annular core to detect residual current caused by insulation fault or grounding fault.
The LJ-φ75 series is mainly used with earth fault relays and protection devices. According to the catalogue data, the matching relay type is DD-11/60. The transformer has an annular iron core with secondary winding uniformly wound around the core, and a clip structure is used to clamp and fix the core. The product is suitable for indoor cable feeder protection, non-effectively grounded neutral systems, and ground fault monitoring circuits.
Product Type
| Item | Specification |
|---|---|
| Product name | Zero Sequence Current Transformer |
| Model series | LJ-φ75 |
| Product structure | Indoor annular core zero sequence current transformer |
| Maximum cable outside diameter | φ75 mm |
| Rated voltage application | Up to 35kV system cable grounding protection |
| Rated frequency | 50Hz / 60Hz |
| Rated current ratio | 30/1A |
| Matching relay | DD-11/60 type relay according to catalogue reference |
| Installation location | Indoor |
| Typical applications | Earth fault detection, ground protection, cable residual current monitoring, small-current grounding system protection |
Product Display
Main Applications
- Indoor cable feeder earth fault protection
- Small-current grounding systems up to 35kV
- Non-effectively grounded neutral systems
- Single-phase ground fault detection and alarm circuits
- Residual current monitoring for three-phase cables
- Protection circuits used with DD-11/60 earth fault relay
- Medium-voltage distribution cabinets and cable line protection systems
Key Technical Features
- Zero sequence current detection: Detects residual current generated by unbalanced phase currents during grounding faults.
- φ75 cable window: Suitable for three-phase cables with maximum outside diameter up to 75 mm.
- Annular magnetic core: The circular core allows the cable group to pass through the center for residual current sensing.
- Uniform secondary winding: The secondary winding is evenly wound around the core to support stable signal output.
- Ground protection application: Designed for earth fault relay input and cable grounding protection circuits.
- Indoor fixed installation: The clamp and base structure support installation inside switchgear or protection cabinets.
Working Principle
The LJ-φ75 zero sequence current transformer measures the residual current of a three-phase cable group. Under normal operating conditions, the vector sum of three-phase currents is close to zero, so the transformer output is very small. When a single-phase ground fault occurs, the phase current becomes unbalanced and zero sequence current flows. The transformer secondary winding outputs a signal to the connected relay, allowing the protection device to alarm or trip according to the relay setting.
The cable provides the primary main insulation, so the transformer mainly verifies the insulation performance of the secondary winding. According to the product description, the secondary winding can withstand a 3kV power-frequency test voltage to ground.
Model Designation
The model code can be interpreted as follows:
| Code | Meaning |
|---|---|
| L | Current transformer |
| J | For earth protection / grounding protection |
| φ75 | Maximum outside diameter of the cable: 75 mm |
Technical Data
| Item | Specification |
|---|---|
| Model | LJ-φ75 |
| Application voltage | Indoor systems up to 35kV |
| Rated frequency | 50Hz / 60Hz |
| Rated current ratio | 30/1A |
| Connection line resistance | 1Ω |
| Relay startup current | 0.1A |
| Sensitivity | 6A to 10A |
| Relay scale setting | 0.1A to 0.2A |
| Secondary winding withstand voltage | 3kV power-frequency test voltage to ground |
| Recommended relay type | DD-11/60 |
| Installation location | Indoor |
Relay Matching and Sensitivity Reference
| Type | Connection Line Resistance (Ω) | Relay Startup Current (A) | Sensitivity (A) | Relay Scale Setting (A) | Rated Current Ratio (A) |
|---|---|---|---|---|---|
| LJ-φ75 | 1 | 0.1 | 6–10 | 0.1–0.2 | 30/1 |
Note: Final relay setting, protection sensitivity, wiring resistance, and secondary circuit configuration should be confirmed according to the protection scheme and site conditions.
Service Conditions
- Installation location: indoor
- Applicable system: small-current grounding system up to 35kV
- Rated frequency: 50Hz / 60Hz
- The three-phase cable should pass through the transformer window together.
- The cable shield or grounding conductor should be wired according to the selected grounding protection scheme.
- The installation environment should be free from severe vibration, corrosive gas, heavy moisture, conductive dust, and serious pollution.
- For special cable diameter, relay type, grounding resistance, or protection sensitivity requirements, technical confirmation is recommended before ordering.
Standards and Compliance
The LJ-φ75 zero sequence current transformer is designed for indoor earth fault protection and residual current detection applications. The final protection setting, relay matching, secondary wiring, and test requirements should be confirmed according to the project protection scheme, relay manual, and applicable electrical installation regulations.
Installation and Dimensions
The LJ-φ75 uses an annular core and fixed base structure. The cable passes through the central opening of the transformer. During installation, all three phase conductors should pass through the same window in the same direction, and the secondary terminals should be connected to the relay according to the wiring diagram.
Overall Dimensions Reference
| Dimension Item | Reference Value | Remark |
|---|---|---|
| Window diameter | φ75 mm | Maximum cable outside diameter |
| Overall height | 246 mm | Subject to final drawing |
| Overall width | 193 mm | Subject to final drawing |
| Base width | 160 mm | Subject to final drawing |
| Mounting hole distance | 130 mm | Reference value |
| Mounting hole | 4-φ14 | Reference value |
| Side depth reference | 52 mm | Subject to final drawing |
Unit: mm. Final dimensions, mounting holes, and terminal positions are subject to the approved outline drawing.
Connection and Wiring Notes
- All three phase conductors should pass through the transformer window together in the same direction.
- For a cable with grounding resistance in the protected circuit, connect the relay according to the corresponding grounding scheme.
- For a cable without grounding resistance in the protected circuit, connect the relay according to the corresponding non-resistance grounding scheme.
- Connect the secondary winding terminals to the earth fault relay according to the relay wiring diagram.
- Check the relay startup current, scale setting, wiring resistance, and required protection sensitivity before energizing.
- The secondary circuit should be kept in good contact and should not be disconnected during protection operation.
Installation and Safety Notes
- Confirm the model, window diameter, cable outside diameter, current ratio, relay type, and protection setting before installation.
- Ensure the cable is centered in the window as far as possible and does not mechanically stress the transformer body.
- Fix the transformer securely using the base mounting holes.
- Check secondary terminal tightening and relay wiring before commissioning.
- Perform insulation and protection function checks according to the project commissioning procedure.
- Installation and maintenance should be performed by qualified electrical personnel.
Ordering Information
Please provide the following information when ordering or requesting a quotation:
- Product model: LJ-φ75
- Maximum cable outside diameter
- Rated current ratio: 30/1A or project-specific requirement
- Relay type, such as DD-11/60, or required relay input specification
- Required protection sensitivity and relay setting range
- Connection line resistance or secondary wiring distance
- Installation quantity and switchgear / cable feeder application
- Special requirements for terminals, labels, drawings, inspection documents, certificates, or packaging
Selection Guidelines
- Confirm cable diameter: Select LJ-φ75 when the three-phase cable group can pass through the 75 mm window.
- Confirm grounding system: Use the product for small-current grounding systems requiring earth fault detection.
- Confirm relay matching: Match the transformer with the required relay type and relay startup current.
- Check sensitivity: Confirm that the required protection sensitivity matches the relay and wiring resistance.
- Confirm wiring scheme: Select the correct connection method according to whether grounding resistance is included in the protected circuit.
- Confirm installation space: Check overall dimensions, mounting holes, and cable bending space before cabinet installation.