Executive summary
At Extra-High/High Tension (EHT/HT), reactors must behave predictably during faults, switching, and harmonic events. Air-core reactors—with no ferromagnetic core to saturate—deliver stable inductance, clean transient response, and tuning accuracy across wide current and frequency ranges. The trade-offs (larger footprint, higher stray field) are manageable with proper field control, making air-core the premium choice for grids that demand reliability.
Saturation risk: the case against iron in dynamic grids
Iron-core designs achieve high inductance density, but the core saturates under high fault currents, DC bias, or deep voltage sags. When saturation occurs, inductance collapses just when you need it most—limiting inrush, shaping fault current, or holding a detuned bank on frequency. Air-core coils store energy in free space; no core = no saturation, so L(I) remains essentially constant through abnormal events.
Where this matters:
- Detuned harmonic filters (MV): Maintains tuning frequency during network swings.
- Series reactors on feeders/cables: Predictable current limiting under faults.
- Drive filters (dv/dt/sine): Inductance holds steady despite rapid current peaks.
Linearity across current and frequency
Air-core inductance varies little with current. It also tracks frequency changes cleanly (important for detuned banks at, say, 189 Hz @ 50 Hz). This linearity underpins:
- Accurate harmonic tuning (avoid pulling the filter off-target as current rises).
- Model–test correlation (digital twin predicts site behavior reliably).
- Simpler thermal design (loss ~ I²R + proximity/skin; no core loss term).
Transient stability and ride-through
Lightning, capacitor switching, reclosing, or converter steps cause fast di/dt events. Air-core’s linear behavior yields short, well-damped responses without the secondary effects of magnetostriction or core reset. That translates to:
- Fewer nuisance trips and alarms.
- Lower mechanical stress on fixtures.
- Cleaner recovery after events.
Harmonics and detuned banks
Air-core avoids the frequency-dependent eddy and core losses that grow quickly in iron when harmonic content is present. For 5th/7th-dominated spectra, the reactor’s loss model stays simple and repeatable; temperature rise is predictable and easier to certify.
EMC and stray field: design it in, not out
The price of no core is higher external magnetic field. Premium air-core solutions manage this at design stage: ring-bus/cage geometries, optimized spacing, field maps, metallic barriers where needed, and a proper earthing/bonding plan for fences and gates. When engineered this way, site induced-voltage risk remains within safe limits and maintenance staff get a clear access envelope.
Acoustic noise and mechanics
With no magnetostriction, air-core avoids dominant tonal hums. Mechanical noise depends on coil bracing, banding tension, and resin system. High-Tg resins, glass banding, and anti-resonant fixtures keep dB(A) low and stable over life.
Materials and manufacturing
- Conductor: copper or aluminum, foil or multi-strand, with edge-radius control to reduce proximity loss.
- Resin/impregnation: VPI or cast systems selected for UV/weathering, crack resistance, and thermal cycles.
- Fixtures & fasteners: anti-loosen strategies (locking features, torque control), corrosion protection, drainage paths.
- Sensors: PT100/NTC at hotspots for heat-run correlation and on-site condition monitoring.
Type tests that actually predict field life
Specify and witness tests that translate to real reliability:
- Heat run at rated current and target harmonic profile.
- Inductance vs current/frequency curve to confirm linearity.
- Impulse/withstand as applicable to voltage class and duty.
- Sound & field mapping with a deliverable you can place on the layout drawing.
- Seismic/wind (where relevant) to IEC/IEEE.
Where air-core is the right answer
- MV/LV detuned harmonic filters where tuning accuracy and thermal predictability are critical.
- Series reactors on feeders and long cable runs needing linear current limiting.
- MV drive filters (dv/dt/sine) that must hold inductance during rapid current steps.
- Weak or renewable-rich grids prone to transients and voltage swings.
When iron-core may still fit
- Tight indoor footprints with benign spectra and well-limited faults.
- Applications prioritizing maximum inductance per volume with strong EMC containment—provided saturation margins and hot-spot risks are addressed.