medium voltage circuit breaker
Medium Voltage Circuit Breaker
A medium voltage circuit breaker is a protective device used in circuits with a voltage class ranging from 1kV to 35kV. Its main function is to automatically trip when the circuit experiences overload, short circuit, or other faults, preventing damage to equipment and the expansion of power system failures. The following is an analysis of its core knowledge:
I. Basic Concepts and Classification
1. Voltage Range
- Medium Voltage (MV): It usually refers to a rated voltage where **1kV ≤ Rated Voltage ≤ 35kV**, which is commonly found in industrial power distribution, substations, transmission and distribution networks, and power supply systems for large equipment.
2. Core Functions
- **Fault Protection**: Quickly disconnects the circuit when overload or short-circuit current is detected.
- **Arc Extinguishing Ability**: Extinguishes the arc generated during disconnection (since the arc has high energy, an efficient arc extinguishing medium is required).
- **Control Function**: Can be manually or remotely controlled to switch on and off (such as in conjunction with relays or automation systems).
3. Main Types (Classified by Arc Extinguishing Medium)
| Type | Arc Extinguishing Medium | Features | Application Scenarios |
| Vacuum Circuit Breaker | Vacuum (High Vacuum Environment) | - Fast arc extinguishing speed, small size, long lifespan, suitable for frequent operation | Power distribution systems, industrial motor control, switchgear cabinets |
| - No explosion risk, simple maintenance | |||
| - Voltage Class: 1kV~35kV | |||
| SF₆ Circuit Breaker | Sulfur Hexafluoride Gas (SF₆) | - Extremely strong arc extinguishing ability, excellent insulation performance, suitable for breaking high short-circuit currents | Substations, high-voltage transmission lines, large-capacity equipment |
| - Requires sealing to prevent leakage, and has poor environmental performance (SF₆ is a greenhouse gas) | |||
| - Voltage Class: 1kV~500kV (Applicable to both medium and high voltages) | |||
| Oil Circuit Breaker | Insulating Oil | - Has a long history and relatively strong arc extinguishing ability, but the oil is prone to aging and there is a fire risk | Early industrial power distribution, small substations |
| - Gradually being phased out and is only used in old systems | |||
| - Voltage Class: 1kV~35kV | |||
| Air Circuit Breaker | Compressed Air | - Simple structure, low cost, but weak arc extinguishing ability and large size | Low-voltage expansion scenarios sensitive to cost |
| - Voltage Class: 1kV~24kV |
II. Working Principle
1. **Fault Detection**: Monitors the current through a current transformer (CT) or built-in sensors, and triggers a trip signal when the current exceeds the rated value.
2. **Operation of the Operating Mechanism**:
- During disconnection, the operating mechanism (driven by spring, electromagnetic, or hydraulic means) drives the contacts to separate, generating an arc.
- The arc extinguishing chamber (such as a vacuum interrupter, SF₆ gas chamber) uses the properties of the medium (the insulation strength of vacuum, the strong electronegativity of SF₆) to quickly extinguish the arc and prevent contact erosion.
3. **Insulation Restoration**: After the contacts are disconnected, the medium quickly restores its insulation to prevent the arc from reigniting.
III. Key Technical Parameters
1. **Rated Voltage (kV)**: Needs to match the system voltage (such as 10kV, 20kV, 35kV).
2. **Rated Current (A)**: The maximum allowable current during long-term operation (such as 630A, 1250A, 2500A).
3. **Short-circuit Breaking Capacity (kA)**: The maximum short-circuit current that can be safely interrupted (such as 25kA, 31.5kA, 40kA).
4. **Breaking Time (ms)**: The time from fault detection to complete disconnection of the contacts (usually ≤ 50ms, the faster the better).
5. **Mechanical Life and Electrical Life**:
- Mechanical Life: The number of no-load closing and opening operations (such as 10,000 times).
- Electrical Life: The number of load closing and opening operations (such as 1,000 times, depending on the magnitude of the short-circuit current).
IV. Application Scenarios
1. **Power System**
- Medium voltage side of substations (such as 10kV/35kV busbar protection).
- Sectional and tie switches of distribution lines (in conjunction with automation to achieve fault isolation).
2. **Industrial Field**
- Starting and protection of large motors (such as electric motors, generators).
- Control of impact loads such as electric furnaces and welding machines.
3. **Renewable Energy**
- Confluence and grid connection protection of wind power and photovoltaic power plants (medium voltage collector lines).
4. **Urban Power Distribution**
- Core protection equipment in ring main units and cable branching boxes (such as 10kV power distribution networks).
V. Maintenance and Safety Precautions
1. Daily Maintenance
- **Appearance Inspection**: Check for contact wear, cracks in insulating components, and gas pressure (for SF₆ circuit breakers, the pressure needs to be measured regularly).
- **Operation Test**: Conduct manual/electric closing and opening tests to ensure the mechanism is flexible and free from jamming.
- **Insulation Test**: Use a megohmmeter to detect the insulation resistance (for vacuum circuit breakers, the vacuum degree needs to be measured).
- **Record Data**: Record the number of fault disconnections and action times, and replace the circuit breaker if it exceeds its lifespan.
2. Safe Operation
- Cut off the power and test for electricity, and hang a "Do Not Close the Switch" warning sign.
- For SF₆ circuit breakers, avoid gas leakage (wear protective equipment, ventilate and detect the environmental concentration in case of leakage).
- If the vacuum interrupter of a vacuum circuit breaker shows cracks or color changes, stop using it immediately.
3. Environmental Requirements
- The waste gas of SF₆ circuit breakers needs to be professionally recycled and treated to avoid greenhouse gas emissions.
- Give priority to products without SF₆ and with low environmental impact (such as vacuum circuit breakers).
VI. Standards and Specifications
- **International Standards**: IEC 62271-100 (High-voltage Alternating-current Circuit-breakers), IEC 62271-102 (Medium-voltage Alternating-current Circuit-breakers).
- **Chinese Standards**: GB 1984 *High-voltage Alternating-current Circuit-breakers*, GB/T 11022 *Common Technical Requirements for High-voltage Switchgear and Controlgear Standards*.
Conclusion
Medium voltage circuit breakers are core protective devices in medium voltage power distribution systems. When selecting them, a comprehensive evaluation should be carried out according to the voltage class, short-circuit current, operation frequency, and environmental requirements (such as explosion protection and environmental protection). Vacuum circuit breakers are most widely used in the medium voltage field (especially in the range of 10kV~35kV) due to their maintenance-free nature and strong environmental protection performance; SF₆ circuit breakers have an advantage in higher voltage classes or large-capacity scenarios. Regular maintenance and compliant selection are the keys to ensuring their reliable operation.
