Product Overview
LJM series busbar-type zero‑sequence current transformers for medium‑voltage ground fault protection.
Functional Definition
The LJM-1, LJM-2, LJM-3, and LJM-3L series busbar-type zero-sequence current transformers are precision electromagnetic instruments designed for ground fault protection in medium-voltage AC power systems. These transformers are engineered for reliable detection of residual current in three-phase systems, providing critical protection against single line-to-ground faults in generators and motors.
Operating at rated frequencies of 50 Hz or 60 Hz, the LJM series is designed for indoor installation in power systems with rated voltage up to 15 kV (commonly applied in 6 kV, 10 kV, and 15 kV networks). The busbar-type integrated structure provides straightforward installation and robust performance for zero-sequence protection applications.
Key Ratings
| Item | Specification (per order / nameplate) |
|---|---|
| System voltage class | 6 kV, 10 kV, 15 kV (indoor ground fault protection applications) |
| Rated frequency | 50 Hz or 60 Hz |
| Rated secondary current | 5 A |
| Application type | Zero-sequence protection (ground fault detection) |
| Primary conductor | Integrated busbar-type structure (cable-through design) |
| Thermal withstand current Ith | 24 kA (LJM-1), 48 kA (LJM-2), 72 kA (LJM-3) |
| Dynamic withstand current Idyn | 2.5 × Ith (peak) |
| Maximum short-circuit impact current | 165 kA (peak) |
| Installation environment | Indoor only (clean, non-corrosive environment) |
| Applicable standards | Q/JB 3380-84 (enterprise standard) |
| Recommended relay | DD11/60 relay for zero-sequence protection coordination |
Working Principle
Zero-sequence current transformers operate on the principle of detecting residual current in three-phase systems. Under normal balanced conditions, the vector sum of three-phase currents is zero, producing no magnetic flux in the transformer core. When a ground fault occurs, unbalanced current (zero-sequence component) flows through the system, generating magnetic flux proportional to the fault current. This flux induces voltage in the secondary winding, producing a standardized output current that activates protective relays when the fault current exceeds preset threshold values.
System Application Position
- Generator Protection: Zero-sequence protection for three-phase AC generators against stator ground faults
- Motor Protection: Ground fault detection for medium-voltage motor installations
- Transformer Neutral Protection: Ground fault monitoring in transformer neutral grounding systems
- Feeder Protection: Cable and overhead line ground fault protection in distribution networks
- Switchgear Integration: Protection coordination with relay systems in 6-15 kV switchgear
Structural Overview
The LJM series features busbar-type construction with integrated primary conductor, enabling direct cable-through installation without external primary connections. The robust mechanical design ensures reliable long-term operation in indoor switchgear environments. When zero-sequence fault current reaches the relay setting value, the transformer outputs accurate secondary current to trigger protective relay operation, providing effective system protection against ground faults.
Model Designation
Model Code Explanation
- L — Current transformer (CT)
- J — Ground protection application (zero-sequence detection)
- M — Busbar-type structure (cable-through design)
- 1 / 2 / 3 — Voltage class code and current rating designation
- 3L — Extended rating variant (5000 A rated primary current)
Service Conditions
The LJM series zero-sequence current transformers are designed for indoor operation under normal service conditions in medium-voltage power systems.
- Installation environment: Indoor installation only in clean, non-corrosive atmosphere
- Altitude: Not exceeding 1000 m above sea level (higher altitude requires engineering confirmation)
- Ambient temperature: −5 °C to +40 °C
- Relative humidity: ≤ 85% at +20 °C reference temperature
- Environmental conditions: Free from corrosive gases, vapors, chemical deposits, explosive or conductive dust; no severe vibration or mechanical shock
Construction
Construction Design
- Structure: Busbar-type configuration with integrated primary conductor
- Core: Ring-type magnetic core optimized for zero-sequence detection
- Insulation: Voltage class-specific insulation system (6 kV / 32 kV, 10 kV / 42 kV, 15 kV / 55 kV)
- System: Cable-through aperture for direct primary conductor installation
- Terminals: Secondary terminal block for relay connection
The busbar-type structure enables straightforward installation by passing three-phase cables or busbars through the transformer aperture. The integrated design eliminates the need for separate primary connections, reducing installation complexity and improving system reliability.
Windings & Terminal Marking
- Primary conductor: Three-phase cables/busbars pass through aperture (no discrete terminals)
- Secondary terminals: K1 / K2 (connection to zero-sequence relay)
- Secondary rated current: 5 A nominal output
Terminal polarity shall be observed during relay connection. Under zero-sequence fault conditions, current flows from K1 to K2, providing proper phase relationship for relay operation. The secondary circuit must remain connected to relay burden during normal operation.
Technical Data
This section provides technical data for the LJM series busbar-type zero-sequence current transformers designed for 6-15 kV AC systems (50 Hz / 60 Hz). Data shown below is intended for model selection based on system voltage class, primary current rating, and short-circuit withstand requirements.
Definitions: Ith is the rated short-time thermal current (1 s duration). Idyn is the rated dynamic current (peak). Unbalance voltage is the maximum residual voltage appearing across open secondary terminals under balanced three-phase primary current conditions.
Data Reference
| Model | LJM-1 | LJM-2 | LJM-3 |
|---|---|---|---|
| Rated primary current (A) | 1750 | 3000 | 4000 |
| Rated secondary current (A) | 5 | 5 | 5 |
| Frequency (Hz) | 50 | 50 | 50 |
| Thermal withstand current Ith (kA) | 24 | 48 | 72 |
| Dynamic withstand current Idyn (kA) | 2.5 × Ith | 2.5 × Ith | 2.5 × Ith |
| Maximum short-circuit impact current (kA) | 165 | 165 | 165 |
| Unbalance voltage (mV) | < 60 | < 85 | < 100 |
| Insulation level (kV) | 6 / 32 | 10 / 42 | 15 / 55 |
Standards & Normative References
| Standard | Title | Application |
|---|---|---|
| Q/JB 3380-84 | Enterprise Standard for Zero-Sequence Current Transformers | Design and performance requirements |
| IEC 61869-1 | Instrument Transformers – Part 1: General Requirements | General CT requirements (reference) |
| IEC 61869-2 | Instrument Transformers – Part 2: Additional Requirements for Current Transformers | CT-specific requirements (reference) |
| GB/T 20840.1 | Instrument Transformers – Part 1: General Requirements | National standard framework |
| GB/T 20840.2 | Instrument Transformers – Part 2: Current Transformers | National CT requirements |
Factory Test Compliance
- Routine tests per applicable Q/JB 3380-84 requirements (polarity verification, ratio verification, insulation resistance)
- Dielectric tests per insulation coordination requirements
- Visual and dimensional inspection including marking and workmanship conformity
- Special tests as required by project specification (thermal withstand, dynamic withstand)
Installation & Dimensions
Installation Environments
- Zero-sequence current transformers shall be installed in indoor switchgear or motor control centers with adequate ventilation and protection from environmental contamination.
- Three-phase cables or busbars shall pass through the transformer aperture in the correct sequence to ensure proper zero-sequence detection.
- Secondary terminals shall be securely connected to zero-sequence relay with appropriate wire gauge and termination method.
- Adequate clearance shall be maintained for insulation integrity, heat dissipation, and maintenance access.
Outlines
LJM-1 (1750 – 4000 A, Indoor Type)
LJM-3L (5000 A, Indoor Type)
Safety Notes
- Secondary circuit must never be disconnected when primary conductors are energized, as dangerous high voltage may appear across open secondary terminals.
- During inspection or relay testing, the secondary circuit shall be short-circuited before disconnecting relay connections.
- One point of the secondary circuit should be reliably grounded in accordance with applicable standards and project specifications.
- All installation and maintenance work shall comply with local electrical safety regulations and utility requirements.
Ordering Information
When placing an order, the required configuration shall be specified according to system voltage class, fault current level, relay coordination requirements, and project technical specification. The following parameters shall be clearly stated for technical confirmation and production release:
- System voltage class (6 kV, 10 kV, or 15 kV)
- Rated primary current (1750 A, 3000 A, 4000 A, or 5000 A based on cable/busbar capacity)
- Insulation level requirements (standard or enhanced if required by project)
- Short-circuit withstand requirements: Ith (1 s) and Idyn (peak) verification against system fault level
- Relay type and setting requirements (recommended DD11/60 or specify project relay)
- Environmental conditions (temperature, humidity, installation configuration)
- Customization requirements (if any special mounting, terminal arrangement, or testing needs apply)
If project-specific requirements apply (cable/busbar arrangement constraints, documentation language, certification requirements, or factory witness testing), specify them at the ordering stage. Special configurations shall be confirmed by technical agreement and final data sheet prior to production.