Grid-tied inverters use output filters to limit switching-frequency current injected into the utility. An LCL network places one inductor on the converter side, one on the grid side, and a capacitor between them. It can provide strong high-frequency attenuation with less total inductance than a simple L filter, but it introduces a resonance that must be damped and controlled. The two LCL filter inductors have different ripple, insulation, loss, and fault duties.
Define the operating envelope
Specify DC-bus range, grid voltage and frequency, power, reactive-power range, switching frequency, modulation, overload, fault ride-through, grid impedance range, and harmonic limits. Weak-grid conditions can move resonance and change control stability.
Do not design from rated current alone. Low grid voltage at full power, reactive operation, unbalance, zero-sequence modes, and fault ride-through can raise phase current or core flux.
Converter-side inductor
The converter-side inductor sees the largest switching ripple because it is directly connected to PWM voltage. Its inductance is selected to limit semiconductor ripple and capacitor current while supporting current-loop bandwidth. Core and AC copper loss can be high.
Voltage to core and chassis includes fast common-mode transitions. Winding capacitance and insulation influence EMI and bearing or leakage currents in connected equipment.
Grid-side inductor
The grid-side inductor sees a smoother waveform but carries grid current and participates in resonance. Utility impedance, transformer leakage, cable inductance, and other filters effectively add to this branch. The design should tolerate the minimum and maximum credible external impedance.
Grid faults and reconnection can impose high current and voltage. Saturation can reduce impedance and increase stress during protection delay.
Resonance and damping
LCL resonance should sit above the useful control bandwidth and sufficiently below switching frequency, with margin for tolerance and grid impedance. Passive damping uses resistors or damped branches and adds loss. Active damping uses control feedback and avoids continuous resistor loss but depends on sensors, delay, software, and stability.
Capacitor tolerance, aging, voltage, and temperature shift resonance. Inductance also changes with bias and temperature. Use worst-case values, not nominal room-temperature components.
Magnetic design
Gapped ferrite, powder core, laminated steel, amorphous, or nanocrystalline structures may be suitable depending on power and switching spectrum. The converter-side inductor often favors low high-frequency loss; the grid-side component may prioritize fundamental current and lower-frequency harmonics.
Three-phase integrated magnetics can exploit flux cancellation but require analysis of zero-sequence, unbalance, and faults. Gap fringing, foil width, litz selection, termination, and cooling determine winding hot spots.
Validation checklist
- Measure both inductor currents and capacitor current across the grid cycle.
- Verify resonance and damping across grid impedance and component tolerance.
- Measure inductance at peak current and hot temperature.
- Check core and winding loss at switching and harmonic frequencies.
- Test weak grid, unbalance, ride-through, reconnect, overload, and faults.
- Verify conducted emissions, common-mode current, and control stability.
BaoHui Tech needs inverter topology, bus and grid ranges, switching waveform, current spectrum, candidate LCL values, resonance and damping method, grid impedance, fault duty, cooling, insulation, and package.
Frequently asked questions
Can grid inductance replace the grid-side inductor?
Relying on uncertain utility impedance can make attenuation and resonance installation dependent. A controlled component is usually needed.
Can one material be used for both inductors?
Yes sometimes, but their ripple spectra and priorities differ, so separate optimization may be better.