Product Overview
The LZZBJ9-20/24 Current Transformer is a fully enclosed, post-type current transformer designed for indoor 20 kV and 24 kV AC power systems with maximum system voltages of 24 kV and 27.5 kV.
Manufactured using an epoxy resin vacuum-casting process, the current transformer features a multi-shed creepage structure that meets the insulation and creepage-distance requirements of 24 kV systems. The primary winding is available in busbar-type or through-type configurations, while the secondary winding is wound with enamelled copper wire.
The product provides high insulation performance, high short-time thermal and dynamic current withstand capability, and stable accuracy characteristics. It is mainly used for current measurement, energy metering, relay protection, and signal acquisition in power systems. Typical applications include indoor medium-voltage switchgear, box-type substations, distribution stations, and switching stations.
Product Features
High Insulation and Creepage Performance
- Epoxy resin cast insulation: Manufactured through vacuum casting using high-quality epoxy resin. The solid insulating body contains no internal air gaps and provides reliable insulation performance. It can withstand the operating voltage, power-frequency withstand voltage, and lightning impulse withstand voltage requirements of 24 kV systems.
- Multi-shed creepage structure: Multiple external sheds significantly increase the creepage distance, helping meet the insulation requirements of 24 kV systems in high-altitude or polluted environments and reducing the risk of surface flashover.
- 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.
Excellent Electrical Performance
- Wide range of current ratios: Multiple primary-to-secondary current ratios are available, including 10/5 A, 20/5 A, 30/5 A, and ratios up to 2,000/5 A. Accuracy classes can be selected according to metering and protection requirements, including 0.2S, 0.5, 5P, and 10P.
- High short-time thermal and dynamic withstand capability: The transformer can withstand the specified short-time thermal current and dynamic current generated during short-circuit faults in 24 kV systems. It is suitable for high-current and high-short-circuit-capacity applications.
- Stable accuracy characteristics: Metering windings provide low and stable measurement errors and can meet Class 0.2S high-accuracy energy metering requirements. Protection windings provide suitable accuracy limit factors to support reliable relay protection operation and reduce the risk of incorrect or failed tripping.
Structural and Installation Features
- Fully enclosed post-type construction: The one-piece cast structure contains no exposed live components and provides an IP20 protection rating. It is suitable for installation inside indoor switchgear and helps reduce the ingress of dust and moisture.
- Compact modular design: The compact structure occupies limited installation space and is compatible with standard 24 kV switchgear layouts, supporting multi-circuit arrangements inside the cabinet.
- Clearly identified secondary terminals: The secondary terminals are protected by a transparent cover and clearly marked for convenient wiring. The cover helps prevent incorrect connection, short circuits, and accidental opening of the secondary circuit.
- Flexible installation: The metal base includes mounting holes for secure bolt installation. The transformer can be installed horizontally or vertically inside indoor switchgear and provides reliable resistance to vibration.
Operation and Maintenance Features
- Low-maintenance operation: The epoxy resin cast body provides high mechanical strength, impact resistance, and ageing resistance. It contains no insulating oil and therefore presents no leakage risk, helping reduce routine maintenance requirements.
- Wide operating temperature range: The transformer can operate at ambient temperatures from -30°C to +70°C. The epoxy resin maintains stable mechanical and insulation performance under both high- and low-temperature conditions.
- Strong environmental adaptability: The creepage distance and insulation structure can be optimized for high-altitude or relatively polluted power distribution environments.
Technical Specifications
Typical Electrical Parameters
| Parameter | Specification |
|---|---|
| Applicable System Voltage | 20 kV/24 kV AC, Um = 24 kV/27.5 kV |
| Rated Frequency | 50 Hz |
| Rated Primary Current | 10–2,000 A, customizable |
| Rated Secondary Current | 5 A or 1 A |
| Metering Accuracy Class | 0.2S / 0.2 / 0.5 |
| Protection Accuracy Class | 5P10 / 10P10 / 5P20 / 10P20 |
| Power-Frequency Withstand Voltage | 65 kV for 1 minute, phase-to-earth and phase-to-phase, without breakdown |
| Lightning Impulse Withstand Voltage | 125 kV peak, without breakdown |
| Partial Discharge | ≤10 pC |
| Short-Time Thermal Current (Ith) | 20 times the rated current for 1 second, customizable |
| Dynamic Current (Idyn) | 2.5 times Ith, customizable |
Structural and Material Parameters
| Parameter | Specification |
|---|---|
| Insulating Medium | Cast epoxy resin |
| Protection Rating | IP20 |
| Installation Type | Indoor post-type 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 20840.1 | Instrument Transformers—Part 1: General Requirements |
Applications
Applicable Voltage Level
The LZZBJ9-20/24 Current Transformer is suitable for indoor 20 kV and 24 kV AC power systems with maximum system voltages of 24 kV and 27.5 kV.
Typical Applications
- Current measurement, energy metering, and relay protection in indoor 24 kV medium-voltage switchgear
- Current signal acquisition in box-type substations, distribution stations, and switching stations
- 20 kV and 24 kV power distribution systems in industrial and mining enterprises
- Commercial buildings and industrial parks
- Current monitoring and control in power automation systems
- High-altitude or relatively polluted 24 kV power distribution applications
Main Functions
The current transformer converts high primary current into a standardized secondary current of 5 A or 1 A at a specified ratio.
It supplies current signals to:
- Energy meters
- Measuring instruments
- Relay protection devices
- Monitoring equipment
- Control systems
It also provides electrical isolation between the primary and secondary systems, 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 post-type 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 impact
- Non-standard current ratios
- Special accuracy classes
- Non-standard mounting dimensions
Installation and Maintenance
Installation Requirements
- Pre-installation inspection:
Inspect the current transformer and confirm that the cast body is free from cracks, bubbles, mechanical damage, and other defects. Check that all terminal markings are clear and that the supplied accessories are complete. Verify that the model, specifications, current ratio, and accuracy class match the design requirements. - Mounting:
Position the current 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. The mounting surface should be level to avoid uneven mechanical stress. - Primary-side connection:
Connect the primary busbar according to the electrical design. Ensure close contact between the busbar and the primary terminals and tighten all bolts securely to prevent overheating caused by poor contact. - Secondary-side connection:
Connect the secondary winding terminals according to their markings. Metering and protection windings must be connected separately and correctly. The secondary circuit must never be left open while the primary circuit is energized. One point of the secondary circuit must be reliably grounded to prevent hazardous overvoltage. - Insulation inspection:
After installation, measure the insulation resistance between:- Primary and earth
- Secondary and earth
- Primary and secondary
Maintenance Requirements
- Regularly inspect the epoxy resin body for cracks, mechanical damage, discharge marks, and contamination.
- Check the secondary terminals for looseness, oxidation, or overheating.
- Inspect the external sheds for accumulated dust or contamination.
- Regularly check the primary busbar and secondary wiring for secure connections, overheating, or discoloration.
- Ensure that the secondary circuit is not accidentally open-circuited or incorrectly short-circuited.
- Periodically test the insulation resistance and current-ratio error to confirm that the metering and protection accuracy remain within the specified limits.
- Clean dust, oil, and contamination from the transformer surface and keep the equipment clean and dry.
- If cracking, discharge, overheating, or excessive measurement error is detected, immediately de-energize and inspect the equipment. Replace the current transformer where necessary.
Precautions
- The secondary winding must never be left open while the primary circuit is energized. 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 winding must be securely short-circuited and grounded.
- Avoid impact or collision with the cast body and insulating sheds during transportation and installation. Mechanical damage may crack the epoxy resin and reduce creepage and insulation performance.
- The equipment must be reliably grounded. The grounding terminal on the 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.
Standard Supply List
| No. | Item | Function |
|---|---|---|
| 1 | LZZBJ9-20/24 Current Transformer | Main device for current transformation and electrical isolation in 24 kV systems |
| 2 | Mounting Bolts and Washers | Used to secure the current 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, and maintenance guidance |
| 5 | Product Certificate and Factory Test Report | Provides evidence of product quality and performance |
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
The product has passed the applicable type tests, and its performance meets the operating requirements of 20 kV and 24 kV power systems.
