Overview
The LJ-φ75 is an indoor annular core zero sequence current transformer designed for earth fault detection in AC systems up to 35kV, operating at 50Hz or 60Hz. It features a cast-resin insulated secondary winding and is intended for use with three-phase cables passing through its central aperture.
With a rated current ratio of 30/1A, the transformer provides reliable residual current sensing for ground fault protection in non-effectively grounded and small-current grounding systems. It is typically used with DD-11/60 type earth fault relays and complies with standard requirements for indoor medium-voltage cable feeder protection applications.
Key Ratings Snapshot
| Parameter | Rating |
|---|---|
| Rated voltage | Up to 35kV |
| Rated frequency | 50Hz / 60Hz |
| Rated current ratio | 30/1A |
| Secondary test voltage | 3kV AC (power-frequency) |
| Application system | Small-current grounding, non-effectively grounded neutral |
| Installation environment | Indoor |
| Core type | Annular magnetic core |
| Output compatibility | DD-11/60 earth fault relay |
Model Designation
- L: Current transformer
- J: For earth protection / grounding protection
- φ75: Maximum outside diameter of the cable: 75 mm
Construction
The LJ-φ75 features an annular iron core with the secondary winding uniformly wound around it. The core is held in place by a clip structure that ensures mechanical stability and ease of installation. The secondary winding is encapsulated with cast-resin insulation, providing robust dielectric strength and environmental protection suitable for indoor switchgear environments. The design allows the three-phase cable bundle to pass directly through the center of the core, enabling accurate zero sequence current detection without direct electrical contact. The transformer is mounted inside protection cabinets or medium-voltage distribution enclosures, where it interfaces with earth fault relays for ground fault monitoring and tripping functions.
Technical Parameters
Service Conditions
- Installation location: indoor
- Applicable system: small-current grounding system up to 35kV
- Rated frequency: 50Hz / 60Hz
- Cable routing: The three-phase cable should pass through the transformer window together.
- Grounding wiring: The cable shield or grounding conductor should be wired according to the selected grounding protection scheme.
- Installation environment: Be free from severe vibration, corrosive gas, heavy moisture, conductive dust, and serious pollution.
Technical Parameters
The LJ-φ75 zero sequence current transformer is designed for indoor use in systems up to 35kV with cast-resin insulation. It operates at 50Hz or 60Hz and supports a rated current ratio of 30/1A, suitable for earth fault protection in small-current grounding systems.
| Type (Ratio) |
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 |
Installation & Wiring
Installation Diagram
The LJ-φ75 zero sequence current transformer is installed by passing all three phase conductors through its central window in the same direction. The transformer is mounted securely to a fixed base using the provided mounting holes. Secondary terminals are connected to the earth fault relay according to the specified wiring diagram and grounding scheme.
Selection & FAQs
Selection Guide
- Confirm the system uses small-current or non-effectively grounded neutral protection and requires a 30/1A zero-sequence CT.
- Verify the complete three-phase cable group fits the 75mm window with safe mechanical and insulation clearance.
- Match the CT to the DD-11/60 or equivalent relay and coordinate the 0.1A–0.2A setting with the earth-fault study.
- Keep total secondary connection resistance within the specified 1Ω reference and verify the required 6A–10A sensitivity.
- Confirm conductor and screen routing, fixed mounting, polarity, grounding, and closed secondary circuit before energization.
Frequently Asked Questions
Q1: What is the LJ-φ75 used for?
It detects zero-sequence current for earth-fault protection in indoor cable systems up to 35kV.
Q2: What is the rated current ratio?
The LJ-φ75 uses a fixed 30/1A current ratio.
Q3: What earth-fault sensitivity does it provide?
The published protection sensitivity is 6A–10A when applied with the specified relay circuit and settings.
Q4: Which relay is recommended?
The technical data references the DD-11/60 earth-fault relay with a 0.1A–0.2A scale setting.
Q5: What cable size fits the transformer?
The complete protected cable bundle must fit through the 75mm aperture with adequate clearance.
Q6: How should the three-phase cables be routed?
All three phase conductors pass together through the core in the same direction; screen and grounding conductors follow the approved scheme.
Q7: Why must connection-line resistance be checked?
Excessive secondary resistance can reduce relay current and compromise the specified earth-fault sensitivity.