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For an off-grid inverter, the worst THD (total harmonic distortion) point is not always at full load. It often appears at light load, also called “small load.” The reason is not that the load is heavier, but that light load amplifies the “fixed errors” in the power stage and control loop.
1. Understanding THD in Off-Grid Inverters
1.1 What Is THD?
Total Harmonic Distortion (THD) measures how much the output voltage waveform deviates from a pure sine wave. A lower THD means a cleaner, more sinusoidal output voltage.
1.2 The Common Misconception
Many engineers assume that the worst THD occurs at full rated load, where the inverter is delivering maximum power. In reality, the worst distortion often appears at light load — typically below 10–20% of rated capacity.
1.3 Why Light Load Matters
Light-load THD is more critical because, at low current levels, the non-ideal behaviors in the power stage and control loop do not scale down proportionally. These fixed errors become a larger fraction of the output, degrading waveform quality.
2. Fixed Errors Take a Larger Share at Low Current
2.1 Output Filter Capacitor Effect
The output filter capacitor of the inverter charges and discharges in every switching cycle. At light load, the load current is small, so the capacitor voltage cannot drop to zero quickly enough before the zero crossing.
This easily creates a flat area around the zero crossing and produces low-order harmonics.
2.2 Dead-Time Error Voltage
Dead time also causes an error voltage: at light load, the fundamental current is small, so the dead-time error becomes larger relative to the fundamental. Near current zero crossing, the current polarity is hard to judge, and zero-current clamping can occur.
2.3 Device Nonlinearity
Device conduction voltage drop and filter inductor nonlinearity also do not shrink linearly with load current. Therefore, the same dead time, voltage drop, and capacitor effect have a more visible impact on the waveform at small current.
3. No Grid Clamping — Harmonics Couple Back
3.1 Grid-Connected vs. Off-Grid Operation
When grid-connected, the large grid has low harmonic impedance. Harmonic currents from nonlinear loads are mainly absorbed by the grid, and voltage distortion at the point of common coupling is held down.
In off-grid operation, the inverter must act as the low-impedance voltage source itself. There is no external grid to absorb harmonic disturbances.
3.2 Nonlinear Load Interaction
Nonlinear loads, such as uncontrolled rectifiers and switching power supplies, draw pulsed currents containing 5th, 7th, and other harmonics. These harmonic currents flow through the inverter output impedance and create harmonic voltage on the output voltage.
At light load, the fundamental current is small, but the “harmonic current share” of a small switching power supply may not be small. As a result, voltage distortion can become more obvious.
This is one of the core mechanisms behind “off-grid inverter waveform distortion.”
4. Control Loop Blind Spots at Light Load
4.1 Limited Harmonic Gain
The voltage closed loop usually has limited gain at the 5th and 7th harmonic frequencies. At light load, the ratio of harmonic disturbance to fundamental changes, and conventional PI control more easily shows insufficient suppression capability.
4.2 Advanced Control Strategies
Repetitive control and resonant control can increase gain at harmonic frequencies. However, if they are not adapted to light-load small current, zero-crossing clamping, and dead-time compensation, light-load THD may become a blind spot outside standard tests.
4.3 Grid-Forming PCS Requirements
As a voltage source, a grid-forming PCS has power quality that directly depends on whether its output impedance at harmonic frequencies is low enough and “stiff” enough.
5. Practical Implications for Users
5.1 What to Look for When Selecting a Model
When selecting a model, do not look only at “rated-load THD.” Off-grid systems often run at light load at night, at low load rates, or in standby.
A better question is: when the load is light, nonlinear, or switching between mixed loads, is the voltage waveform still stable? This is closer to real experience than full-load resistive THD.
5.2 Grid-Forming PCS Capability
For a grid-forming PCS, voltage waveform quality with nonlinear loads is not an add-on function. It is a basic capability of being a voltage source.
5.3 Application Scenarios
Scenario |
Load Type |
Typical Load Rate |
Key THD Concern |
Nighttime standby |
Mixed small loads |
5–15% |
Capacitor flat-top distortion |
Remote telecom tower |
Switching PSU |
10–30% |
Harmonic current coupling |
Island microgrid |
Rectifier + motor |
15–40% |
Dead-time voltage error |
Residential solar |
LED + inverter AC |
10–25% |
Zero-crossing clamping |
Industrial backup |
VFD + PLC |
20–50% |
Output impedance resonance |
6. Conclusion
Light-load THD is more critical because, at light load, the off-grid inverter loses the “dilution effect” that a large current has on non-ideal factors. At the same time, it must alone carry the harmonic impedance responsibility of a voltage source.
When asking about the THD of an off-grid inverter, do not ask only about full load. Ask whether the waveform can hold up under light load and nonlinear loads.
This is the real question behind “off-grid inverter waveform distortion.”
7. Frequently Asked Questions
Q: Why is light-load THD worse than full-load THD in off-grid inverters?
At light load, fixed errors like dead-time voltage drop, capacitor flat-top distortion, and device nonlinearity become a larger fraction of the output waveform. Without a large fundamental current to “dilute” these effects, distortion becomes more visible.
Q: What should I look for when choosing an off-grid inverter regarding THD?
Do not rely solely on the rated-load THD specification. Ask the manufacturer for THD data at 10%, 20%, and 50% load with nonlinear loads. This better reflects real-world operating conditions.
Q: How does grid connection affect THD performance?
When grid-connected, the grid provides a low-impedance path for harmonic currents, effectively absorbing distortion. In off-grid mode, the inverter must handle all harmonic suppression on its own.
Q: What control techniques help improve light-load THD?
Repetitive control, resonant (PR) control, and adaptive dead-time compensation can all improve harmonic suppression. However, these must be specifically tuned for light-load and zero-crossing conditions.
Q: Is light-load THD important for all off-grid systems?
Yes. Most off-grid systems spend significant time at light load — nighttime standby, small electronics, or partial-load operation. Poor light-load THD can affect sensitive equipment and reduce system reliability.
