Air-core reactor leakage fields have induced substantial circulating currents in ground grids and nearby connected metal. [1] Electrical insulation clearance and magnetic/thermal exposure therefore need separate assessments: a frame outside the electrical gap can still form a linked closed loop.
Draw nearby rails, reinforcing steel, cable screens, fences and support frames. Identify electrically closed loops and the field component threading them. Do not judge risk solely from the nearest point-to-coil distance.
Use two separate installation records
| Boundary | Evidence | Risk assessed |
|---|---|---|
| Electrical clearance | Voltage, insulation coordination and physical spacing | Flashover or contact with live parts |
| Magnetic/thermal clearance | Field map, loop geometry and conductor properties | Induced current, heating and force |
| Support design | Materials, fasteners and continuous metal paths | Unintended shorted loop |
| Personnel boundary | Applicable exposure assessment and access plan | Required occupied-area restrictions |
A loop calculation exposes the mechanism
For a single-turn loop using RMS phasors, linked sinusoidal flux Φ gives e = −jωΦ with the stated orientation. Current is I = e/(R + jωL) and resistive heating is |I|²R. Flux is in webers, voltage in volts RMS, angular frequency in rad/s, R in ohms, L in henries and current in amperes RMS. [2]
The actual flux depends on location, orientation and loop area. Cutting or insulating a loop changes its current path but may also affect structural integrity or protective bonding. Any such change needs a coordinated structural and electrical design, not an ad hoc installation modification.
Any loop modification must retain its verified protective-bonding and structural functions. The cited ground-grid study assessed earth-potential and step/touch consequences alongside the magnetic-current change. [1]
Screen induced voltage in a nearby closed loop
Assumptions: the loop is 2.0 m from the live coil, beyond a project electrical clearance of 1.5 m. Mutual inductance to a 1,000 A RMS, 50 Hz reactor is 2 μH; loop R = 1 mΩ and L = 3 μH at 20 °C. Induced EMF is 0.628 V RMS. Including self-reactance gives 457.24 A and 209.07 W, exceeding an assumed 150 W loop heat budget by 59.07 W. Electrical distance passes but magnetic heating fails. The resistance-only estimate would give 628.32 A and 394.78 W. Hold placement and obtain the actual coupled field/impedance and bonding design. [2]
References
- M. Davarpanah, M. Abedini and S. Mohammadi, Practical solutions for ground grid circulating currents caused by air-core reactor, IET Generation, Transmission & Distribution 14, 3856–3865, 2020. DOI: 10.1049/iet-gtd.2019.1582.
- U.S. Department of Energy, DOE-HDBK-1011/3-92, Electrical Science, Volume 3, June 1992.