Product Overview
The LMZC-10 and LMZK1-10 Zero-Sequence Current Transformers are through-type epoxy resin cast current transformers designed for indoor 10 kV AC power systems with a maximum system voltage of 12 kV.
Featuring a toroidal magnetic core and an integrated insulation structure, the transformers are primarily used for single-phase earth-fault detection, relay protection, and insulation monitoring in systems with non-effectively earthed neutrals.
They are widely used in 10 kV medium-voltage switchgear, ring main units, box-type substations, cable branch boxes, and other distribution equipment. The products serve as key signal-acquisition components in power-system earth-fault protection schemes.
Product Features
Insulation and Electrical Performance
- Epoxy resin cast insulation:
The transformer is manufactured using an epoxy resin vacuum-casting process. The solid insulation contains no internal air gaps and provides excellent dielectric performance. - Suitable for 12 kV systems:
The transformer can withstand the specified power-frequency and lightning impulse withstand voltage requirements of 10 kV systems with a maximum system voltage of 12 kV. - 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:
A high-quality toroidal silicon steel core provides low excitation current and high sensitivity. The transformer can accurately detect zero-sequence current generated during a single-phase earth fault. - Compatible with earth-fault protection systems:
The output signal can be supplied to low-current earth-fault line-selection devices, zero-sequence protection relays, and insulation-monitoring systems. - Standard cable aperture:
The central aperture is designed for commonly used 10 kV cables and busbars. Three-phase cables or busbars can pass directly through the aperture without additional primary-side connections. - Low partial discharge:
The advanced casting process limits partial discharge to no more than 10 pC, helping reduce insulation ageing and improve long-term operational reliability.
Structural and Installation Features
- Fully enclosed dry-type structure:
The magnetic core and secondary winding are completely enclosed within the epoxy resin cast body. - IP20 protection rating:
The product provides an IP20 protection rating and is suitable for installation inside indoor medium-voltage equipment. - Compact construction:
Its small footprint reduces the required installation space and is compatible with standard 10 kV switchgear layouts. - Through-type ring structure:
The three-phase cables or busbars pass directly through the central aperture, enabling zero-sequence current acquisition without additional primary-side wiring. - Simplified primary-circuit arrangement:
The through-type design reduces installation procedures and minimizes the number of potential connection and failure points. - Clearly identified polarity:
The cable-entry side is marked P1 to indicate the required current direction and polarity. - Protected secondary terminals:
The secondary terminals are clearly marked and protected by a transparent terminal cover, reducing the risk of incorrect wiring, short circuits, and accidental disconnection. - Flexible installation:
A metal mounting bracket supports horizontal or vertical bolt mounting inside the switchgear. - High mechanical stability:
The mounting structure provides secure installation and reliable vibration resistance.
Operation and Maintenance Features
- Low-maintenance operation:
The dry-type, oil-free structure eliminates leakage and insulating-oil maintenance. 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 under high- and low-temperature conditions. - High-reliability construction:
The magnetic core is made from high-quality laminated silicon steel sheets, while the secondary winding is wound with enamelled copper wire. - Low operating losses:
The optimized core and winding structure provides low losses, limited temperature rise, and stable signal-acquisition accuracy. - Reliable protection operation:
The transformer can withstand switching overvoltage and short-circuit current effects within its specified ratings, supporting dependable earth-fault protection.
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, such as 100–1,000 A |
| 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 | Capable of detecting earth-fault current of 0.5 A or above, customizable |
Structural and Material Parameters
| Parameter | Specification |
|---|---|
| Insulating Medium | Fully cast epoxy resin |
| Protection Rating | IP20 |
| Central Aperture | Standard aperture suitable for 10 kV cables or busbars |
| Installation Type | Indoor fixed 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 Level
The LMZC-10 and LMZK1-10 Zero-Sequence Current Transformers are suitable for indoor 10 kV AC power systems with a maximum system voltage of 12 kV.
Typical Applications
- Single-phase earth-fault detection in indoor 10 kV medium-voltage switchgear
- Zero-sequence protection in ring main units
- Earth-fault monitoring in box-type substations
- Cable branch boxes
- Distribution 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
- Earth-fault monitoring for cable and busbar circuits
Main Functions
The transformer detects the zero-sequence current generated by the three-phase system and supplies a corresponding secondary current signal to:
- Zero-sequence protection relays
- Low-current earth-fault line-selection devices
- Insulation-monitoring equipment
- Protection and control systems
- Power automation equipment
The product supports:
- Single-phase earth-fault detection
- Faulted-feeder identification
- Earth-fault location
- Zero-sequence current protection
- Insulation monitoring
- Protection signal acquisition
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 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 central-aperture dimensions
- Special zero-sequence current sensitivity
- Special 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 and polarity markings are clear and that all supplied accessories are complete. Verify that the model, specifications, and central-aperture dimensions match the system design. Confirm that the specified sensitivity meets the earth-fault protection requirements. - Mounting:
Position the transformer at the specified location inside the switchgear and secure it with bolts through the mounting holes in the base. Ensure that the transformer is firmly installed without looseness or displacement. It may be mounted horizontally or vertically according to the equipment design. Avoid uneven mechanical loading or excessive stress on the cast-resin body. - Primary cable or busbar installation:
Pass all three phase cables or busbars through the central aperture in accordance with the specified P1 polarity direction. Ensure that the cables or busbars pass concentrically through the aperture and do not contact its inner wall. Avoid scratching or damaging the insulating surface inside the aperture during installation. - Secondary-side connection:
Connect the secondary terminals to the zero-sequence protection relay or low-current earth-fault line-selection device according to their markings. The secondary circuit must never be left open while primary current is present. One point of the secondary circuit must be reliably earthed in accordance with the system design and applicable electrical safety requirements. - 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, mechanical damage, discharge marks, and contamination.
- Check the central aperture for foreign objects, wear, or insulation damage.
- Inspect the secondary terminals for looseness, oxidation, or abnormal heating.
- Regularly check the clearance between the three-phase cables or busbars and the aperture wall.
- Confirm that the cables or busbars do not contact, rub against, or discharge to the aperture wall.
- Ensure that all secondary connections remain secure and free from overheating or discoloration.
- Confirm that the secondary-circuit earthing remains reliable.
- Periodically test the insulation resistance and zero-sequence current acquisition accuracy.
- Verify the correct operation of the connected protection and line-selection devices.
- Clean dust and contamination from the transformer surface.
- Keep the central aperture free from foreign materials.
- Prevent oil and corrosive substances from contacting the epoxy resin body.
- If cracking, electrical discharge, overheating, abnormal signal-acquisition accuracy, or an open secondary circuit is detected, immediately de-energise and inspect the equipment.
- Replace the transformer where necessary. Continued operation with a known defect is prohibited.
Precautions
- The secondary winding must never be left open while primary current is present. An open secondary circuit may generate a hazardous overvoltage of several thousand volts, creating 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 central aperture during transportation and installation. Mechanical damage may reduce insulation performance and affect dimensional accuracy.
- Avoid scratching the insulating surface inside the aperture when installing the three-phase cables or busbars.
- The cables or busbars must pass concentrically through the central aperture and must not contact its inner wall.
- Secure the cables or busbars to prevent movement, vibration, or repeated contact with the aperture wall during operation.
- Confirm that the aperture dimensions are compatible with the outside diameter of the cables or busbars. Do not force oversized conductors through the aperture.
- The equipment must be reliably earthed. The grounding terminal on the mounting base 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.
- Secondary wiring must be securely connected to prevent loosening and accidental open-circuit conditions during operation.
Standard Supply List
| No. | Item | Function |
|---|---|---|
| 1 | LMZC-10 / LMZK1-10 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 zero-sequence current transformer during installation |
| 3 | Transparent Secondary Terminal Cover | Protects the secondary terminals and helps prevent incorrect connections or short circuits |
| 4 | Product Instruction Manual | Provides installation, operation, maintenance, wiring, and cable-routing guidance |
| 5 | Product Certificate and Factory Test Report | Provide 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.
