Solution for Medium-Voltage Motor Control and Protection Using Vacuum Contactor-Fuse (VCF) in a Coal Conveying System

1. Project Background

A coal conveying system comprises 15 belt conveyors driven by medium-voltage motors. The system operates under complex conditions, with motors often subjected to heavy loads and frequent starts. To address these challenges and achieve effective control and reliable protection during motor startup, the project comprehensively adopts Vacuum Contactor-Fuse (VCF) combination devices for the 6kV medium-voltage motor power distribution. This solution details the technical features, advantages, and application of VCF, providing a reliable reference for similar working conditions.

2. Core Advantages and Technical Features of VCF

2.1 Advanced Equipment Structure and Insulation Technology

• ​Equipment Type: This solution employs a withdrawable VCF structure for easy installation, maintenance, and replacement.
• ​Core Technology: Utilizing epoxy resin composite insulation and Automatic Pressure Gelation (APG) technology, the vacuum interrupter is directly encapsulated in epoxy resin, significantly enhancing insulation performance, mechanical strength, and environmental stability.
• ​Operating Mechanism: The operating mechanism is designed with precision and features low power consumption.

2.2 Comprehensive Composition and Wide Applicability

• ​Equipment Composition: The VCF consists of an optimized combination of high-voltage current-limiting fuses (capable of interrupting a wide range of short-circuit currents) and frequently operable VCX vacuum contactors, forming a classic F-C circuit solution.
• ​Core Advantages: It offers long operational life, stable performance, and low noise.
• ​Application Scope: Widely used in high-voltage auxiliary power systems in thermal power plants, as well as in metallurgical, petrochemical, and mining industries. It is suitable for controlling and protecting loads such as high-voltage motors, transformers, and induction furnaces.

2.3 High Adaptability and Safety Features

• ​Cabinet Compatibility: The VCF withdrawable unit matches the dimensions and five-prevention interlocking positions of circuit breaker withdrawable units in 800mm-width middle-mounted switchgear, enabling seamless replacement without any modifications to existing switchgear.
• ​Maintenance Convenience: The withdrawable design allows safe and convenient replacement of high-voltage fuses outside the cabinet.
• ​Holding Method: The vacuum contactor can be configured for electrical or mechanical holding based on customer requirements.
• ​Phase-Loss Protection: Equipped with comprehensive phase-loss protection. In the event of a phase loss, the fuse operates and mechanically interlocks to ensure the VCF disconnects the motor circuit, effectively preventing motor damage due to single-phasing.

3. Key Technical Parameters (7.2kV Rating)

4. Protection Control Principle

VCF protection is divided based on current magnitude for optimal performance:

• ​Low Current Range (< 4kA)​: Handled by the vacuum contactor for normal breaking and overload protection.
• ​High Current Range (> 4kA)​: Rapidly interrupted by the high-voltage fuse to address short-circuit faults.
• ​Curve Coordination: The contactor’s protection curve is set below the circuit breaker’s curve to ensure the contactor acts first during overloads. Simultaneously, a properly matched fuse with protection settings lower than the upstream circuit breaker is selected to completely avoid unintended tripping.

5. Advantages of VCF vs. Vacuum Circuit Breaker

For frequently started and stopped motor loads, VCF offers significant advantages over vacuum circuit breakers:

6. Core of VCF Selection: Fuse Selection Guide

The performance of VCF hinges on correct fuse selection, considering the following factors:
Working voltage, working current, motor starting time, starts per hour, motor full-load current, and short-circuit current at the installation point.

6.1 Selection Rules and Steps

1. ​Rated Voltage: The fuse’s rated voltage must not be lower than the system working voltage (7.2kV in this case).
2. ​Rated Current Calculation:
• Use the formula: Iy=N×In×δI_y = N \times I_n \times \deltaIy​=N×In​×δ
• IyI_yIy​: Equivalent current during starting (A)
• NNN: Starting current to full-load current ratio (typically 6)
• InI_nIn​: Motor rated full-load current (A)
• δ\deltaδ: Comprehensive coefficient (based on starts per hour, n, from the table below)

1. ​Curve Matching: Plot the calculated IyI_yIy​ value and the motor’s starting time on the fuse manufacturer’s time-current characteristic curve. Select the fuse rated current corresponding to the curve immediately to the right of this point.
2. ​Additional Check: The selected fuse’s rated current must be ​**> 1.7 times the motor’s full-load current**.

6.2 Selection Example

For a 7.2kV system with a directly started 250kW high-voltage motor:
In=30AI_n = 30AIn​=30A, 16 starts per hour, starting time of 6s.

• Calculation: Iy=6×30A×2.1=378AI_y = 6 \times 30A \times 2.1 = 378AIy​=6×30A×2.1=378A
• Selection: On the fuse time-current curve, locate the curve to the right of the point (378A, 6s), corresponding to a fuse rated current of 100A.
• Verification: 100A > 1.7 × 30A (51A), meeting the requirement. Thus, a 100A or higher-rated high-voltage motor protection fuse can be selected.

7. Conclusion

From a comprehensive cost-performance analysis:

• While vacuum circuit breakers have lower procurement costs, their shorter operational life makes them unsuitable for frequent starts/stops, leading to higher long-term maintenance costs and failure risks.
• The VCF solution combines the advantages of vacuum contactors (long life, low overvoltage, suitability for frequent operation) and fuses (ultra-fast interruption of short-circuit currents), all at an economical overall cost.
• For the coal conveying system and other applications with frequent operation and heavy-load starting characteristics, VCF is an ideal solution offering high performance, reliability, and cost-effectiveness.