Working Principle of Mining Transformers

　　Mining transformers are specialized power equipment designed for harsh underground mining environments (high humidity, dust, vibration, and potential gas/dust explosion risks). Their core working principle is consistent with general transformers—electromagnetic induction—but optimized with structural and functional adaptations to meet miningspecific safety and reliability requirements. Below is a detailed technical breakdown:

　　1. Core Electromagnetic Induction Principle (Fundamental Mechanism)

　　Like all transformers, mining transformers rely on mutual induction between two or more windings to convert voltage levels while maintaining frequency consistency. The key components and induction process are as follows:

　　1.1 Core Components

　　Iron Core: Typically made of coldrolled silicon steel sheets (0.350.5mm thick) stacked with insulated layers to minimize eddy current and hysteresis losses. It serves as the magnetic circuit for flux coupling between windings.

　　Primary Winding: Connected to the underground power grid (e.g., 6kV, 10kV, 35kV), it receives input power and generates alternating magnetic flux when energized.

　　Secondary Winding: Outputs lowvoltage power (e.g., 380V, 660V, 1140V) for mining equipment (coal cutters, conveyors, pumps) or lighting/control systems. The voltage ratio is determined by the turns ratio of primary to secondary windings.

　　1.2 Induction Process

　　1. Flux Generation: When alternating current (AC) flows through the primary winding, it creates an alternating magnetic flux (Φ) in the iron core, which changes periodically with the AC frequency (50Hz in most regions).

　　2. Mutual Induction: The alternating flux cuts through the secondary winding, inducing an electromotive force (EMF) in the secondary winding according to Faraday’s Law of Electromagnetic Induction:

　　\[

　　E = 4.44 f N Φ_m

　　\]

　　Where:

　　\( E \): Induced EMF (V);

　　\( f \): AC frequency (Hz);

　　\( N \): Number of winding turns;

　　\( Φ_m \): Maximum magnetic flux in the iron core (Wb).

　　3. Voltage Conversion: The ratio of primary voltage (\( U_1 \)) to secondary voltage (\( U_2 \)) is approximately equal to the turns ratio of the windings (ignoring copper and iron losses):

　　\[

　　\frac{U_1}{U_2} ≈ \frac{N_1}{N_2}

　　\]

　　Power is conserved (excluding losses), so the secondary current (\( I_2 \)) increases as the voltage decreases (and vice versa):

　　\[

　　\frac{I_1}{I_2} ≈ \frac{N_2}{N_1}

　　\]

　　1.3 Loss Mechanism

　　Copper Loss: Power loss due to resistance in primary/secondary windings (proportional to the square of load current).

　　Iron Loss: Hysteresis loss and eddy current loss in the iron core (constant for a given voltage and frequency, independent of load).

　　2. MiningSpecific Functional Adaptations (Safety & Reliability Enhancements)

　　Mining transformers must comply with strict standards (e.g., IEC 60076, GB 8905 for explosionproof requirements) and integrate specialized designs to address underground hazards:

　　2.1 ExplosionProof Design (Critical for Hazardous Environments)

　　Underground mines may contain flammable gas (methane) or combustible dust (coal dust), so mining transformers (especially those used in gas/dust explosion危险 zones) adopt explosionproof structures:

　　Flameproof Enclosure (Ex d): The tank and joints are designed to withstand internal explosions (e.g., from insulation breakdown) and prevent flame propagation to the external environment. The enclosure is made of highstrength steel with flameproof gaps (≤0.5mm) to cool and extinguish escaping flames.

　　Intrinsically Safe Design (Ex ia): For lowpower control transformers, circuits are designed to limit energy (voltage ≤24V, current ≤0.1A) to prevent spark ignition.

　　2.2 Insulation System Optimization (Resistance to Humidity & Contamination)

　　Underground high humidity (relative humidity ≥95%) and dust can degrade insulation performance. Mining transformers use:

　　HighGrade Insulating Materials: Winding insulation adopts vacuumimpregnated epoxy resin (for drytype mining transformers) or mineral oil with high dielectric strength (for oilimmersed types, with moisture content ≤10ppm).

　　Sealed Structure: Oilimmersed mining transformers use hermetically sealed tanks (with diaphragm conservators) to prevent moisture and dust ingress; drytype transformers use IP54+ protection class enclosures.

　　2.3 Cooling System Adaptation (Heat Dissipation in Confined Spaces)

　　Underground mines have poor ventilation, so cooling systems are optimized for efficient heat removal:

　　OilImmersed Mining Transformers: Equipped with radiators (fintype or tubetype) and forced oil circulation (for largecapacity units, ≥1000kVA) to enhance heat dissipation. Some use natural cooling (ONAN) for smallcapacity units (<500kVA) to reduce maintenance.

　　DryType Mining Transformers: Adopt forced air cooling (AN) with internal fans to circulate air and transfer heat to the enclosure.

　　2.4 Voltage Regulation & Load Adaptability

　　Mining equipment (e.g., coal cutters, hydraulic supports) has variable load characteristics (starting current up to 68x rated current). Mining transformers often include:

　　OnLoad Tap Changer (OLTC): Allows voltage adjustment (±5% or ±10% of rated voltage) under load to compensate for grid voltage fluctuations and maintain stable output for sensitive equipment.

　　ShortCircuit Resistance: Reinforced winding structure (e.g., transposed conductors, rigid bracing) to withstand shortcircuit forces (up to 25kA for 1s) from equipment faults.

　　2.5 Protection & Monitoring Functions (Fault Prevention & Early Warning)

　　Temperature Monitoring: Oilimmersed transformers use oil temperature gauges (with alarm contacts at 85°C and trip contacts at 95°C); drytype transformers use winding temperature sensors (PTC thermistors) to trigger cooling or shutdown.

　　Overcurrent/ShortCircuit Protection: Equipped with builtin current transformers (CTs) and relay protection devices to isolate faults quickly.

　　Leakage Protection: Oilimmersed types have oil level sensors and leak detection alarms; drytype types use insulation resistance monitoring (≥100MΩ at 500V DC).

　　2.6 Mechanical Strength & Portability

　　Underground mines have narrow roadways and frequent transportation. Mining transformers feature:

　　Compact Structure: Smaller volume and lower height (e.g., lowprofile tanks for oilimmersed types) to fit in confined spaces.

　　Rugged Design: Reinforced base frames and shockabsorbing pads to withstand vibration during transportation and operation (vibration resistance up to 3g).

　　3. Typical Application Scenarios & Working Flow

　　3.1 Application Zones

　　Main Underground Transformers: Installed in central substations, converting 35kV/10kV grid voltage to 6kV/1140V for main power supply.

　　Local Transformers: Installed near working faces, converting 6kV/1140V to 380V/220V for coal cutters, conveyors, and lighting.

　　Control Transformers: Used in control cabinets for lowvoltage (24V/12V) power supply to sensors and actuators.

　　3.2 Typical Working Flow

　　1. The primary winding receives highvoltage power from the underground power grid.

　　2. Alternating magnetic flux is generated in the iron core, inducing lowvoltage EMF in the secondary winding.

　　3. The secondary winding outputs stable lowvoltage power to mining equipment, with voltage adjusted via tap changers if needed.

　　4. Monitoring systems (temperature, oil level, insulation resistance) continuously operate; protection devices trigger alarms or shutdowns if parameters exceed safe limits.

　　5. Explosionproof and sealed structures prevent internal faults from causing external hazards, ensuring safe operation in harsh underground environments.

　　4. Key Standards & Performance Indicators

　　Rated Voltage: Primary voltage (6kV, 10kV, 35kV); secondary voltage (380V, 660V, 1140V, 3300V) (1140V is the most common for highpower mining equipment).

　　Rated Capacity: Typically 50kVA ~ 2000kVA (smaller for local use, larger for central substations).

　　ShortCircuit Impedance: 4%6% (to limit shortcircuit current and ensure system stability).

　　Protection Class: IP54+ (drytype), IP65+ (flameproof oilimmersed).

　　Efficiency: ≥95% (for full load, in line with IE2/IE3 energy efficiency standards).

　　By integrating electromagnetic induction principles with miningspecific safety and environmental adaptations, mining transformers reliably convert voltage while ensuring explosion protection, insulation stability, and heat dissipation in harsh underground conditions—critical for maintaining continuous operation of mining equipment and ensuring personnel safety.