The integration of large-scale renewable energy systems, particularly solar inverter, into power grids has introduced significant challenges related to voltage fluctuations and harmonic distortions. This paper proposes a refined anti-disturbance control strategy to address these issues, focusing on improving dynamic response, robustness, and harmonic suppression capabilities in grid-forming solar inverter. The methodology combines a Compensating Function Observer (CFO) with a Zero-Phase Repetitive Controller (ZRC), validated through real-time simulations on an IEEE three-machine nine-node system.
1. Key Challenges in Solar Inverter Integration
Solar inverter, as critical interfaces between photovoltaic systems and power grids, face inherent challenges due to non-periodic disturbances, parameter uncertainties, and harmonic-rich loads. Key issues include:
- Voltage Fluctuations: Caused by rapid load changes and low inertia in renewable-dominated grids.
- Harmonic Distortions: Resulting from nonlinear loads and inverter switching dynamics.
- Robustness Deficiencies: Traditional control strategies (e.g., PI, ADRC) exhibit limited performance under parameter mismatches or large disturbances.
2. Proposed Anti-Disturbance Control Framework
The proposed Elegant Anti-Disturbance Control (EAD) integrates two core components:
2.1 Compensating Function Observer (CFO)
The CFO enhances disturbance estimation accuracy and system robustness by addressing non-periodic disturbances and internal parameter uncertainties. The observer dynamics are governed by:{z˙1=z2z˙2=h1e1+h2e2+b0u+d^d^=λs+λd⎩⎨⎧z˙1=z2z˙2=h1e1+h2e2+b0u+d^d^=s+λλd
where z1z1, z2z2 are estimated states, d^d^ is the disturbance estimate, and λλ is the filter cutoff frequency. The stability criterion ensures all poles of the characteristic equation lie in the left-half plane:s3+h2s2+(h1+λh2)s+λh1=0s3+h2s2+(h1+λh2)s+λh1=0
Parameters h1=108ω02h1=108ω02, h2=27ω0h2=27ω0, and λ=4ω0λ=4ω0 are tuned for optimal performance.
2.2 Zero-Phase Repetitive Controller (ZRC)
The ZRC eliminates phase delays inherent in traditional repetitive control, improving harmonic suppression. Its transfer function is:GZRC(z)=krQ(z)z−N1−z−NF(z)GZRC(z)=kr1−z−NQ(z)z−NF(z)
where Q(z)=az+b+az−1Q(z)=az+b+az−1 is a zero-phase low-pass filter with 2a≤b2a≤b. Optimal parameters a=1/4a=1/4, b=1/2b=1/2 ensure minimal phase distortion and enhanced steady-state performance.
3. Stability and Performance Analysis
3.1 Stability Conditions
The EAD system stability requires:
- All roots of 1+GADRC×P(z)=01+GADRC×P(z)=0 lie within the unit circle.
- 1+GZRC(z)×G0(z)≠01+GZRC(z)×G0(z)=0.
These are satisfied by tuning ω0ω0, kpkp, and Q(z)Q(z).
3.2 Voltage Fluctuation Suppression
Under sudden load changes (75 MW at t=0.3 st=0.3s), the EAD strategy outperforms PI and ADRC:
| Control Strategy | Voltage Fluctuation | Settling Time |
|---|---|---|
| PI | 5.5% | 0.1 s |
| ADRC | 2.8% | 0.04 s |
| EAD | 0.4% | 0.02 s |
3.3 Harmonic Suppression
The EAD reduces Total Harmonic Distortion (THD) from 13.43% (PI) to 1.86%, outperforming repetitive control (6.85% THD):
| Harmonic Order | Repetitive Control (%) | EAD (%) |
|---|---|---|
| 5th | 5.02 | 1.36 |
| 7th | 3.44 | 0.97 |
| 11th | 2.28 | 0.62 |
| 13th | 1.92 | 0.52 |
| THD | 6.85 | 1.86 |
3.4 Robustness Under Parameter Mismatch
With inductance reduced by 50%, the EAD maintains superior performance:
| Metric | ADRC | EAD |
|---|---|---|
| Voltage Fluctuation | 2.9% | 0.5% |
| Current THD | 7.68% | 2.53% |
4. Real-Time Validation
The RT1000 platform simulates an IEEE nine-node system with:
- One 100 MVA synchronous generator.
- Two 100 MVA grid-forming solar inverter.
Results confirm the EAD strategy’s efficacy in dynamic tracking, harmonic suppression, and resilience against parameter variations.
5. Conclusion
The proposed EAD control strategy significantly enhances the power quality of solar inverter by:
- Reducing voltage fluctuations to 0.4% (vs. 5.5% for PI).
- Suppressing current THD to 1.86% (vs. 13.43% for PI).
- Maintaining robustness under 50% parameter mismatch.
This methodology provides a viable solution for integrating high-penetration solar inverter into modern power grids while ensuring stability and compliance with grid codes.
Keywords: Solar Inverter, Voltage Fluctuation, Harmonic Suppression, Anti-Disturbance Control, Robustness, Grid-Forming Inverters.
Formula List
- Observer Dynamics:
z˙2=h1e1+h2e2+b0u+d^z˙2=h1e1+h2e2+b0u+d^
- ZRC Transfer Function:
GZRC(z)=krQ(z)z−N1−z−NF(z)GZRC(z)=kr1−z−NQ(z)z−NF(z)
- Stability Criterion:
s3+h2s2+(h1+λh2)s+λh1=0s3+h2s2+(h1+λh2)s+λh1=0