With the rapid integration of renewable energy sources, subsynchronous oscillations (SSOs) in wind farms have become a critical stability challenge. This paper proposes a dual-loop control strategy leveraging energy storage batteries to mitigate SSOs in doubly-fed induction generator (DFIG)-based wind farms through coordinated damping enhancement and resonance suppression.
1. System Modeling and Oscillation Mechanism
The DFIG system with series compensation can be represented by the state-space model:
$$
\dot{\boldsymbol{x}} = \boldsymbol{A}\boldsymbol{x} + \boldsymbol{B}\boldsymbol{u}
$$
Key parameters affecting SSO characteristics include:
| Parameter | Value | Unit |
|---|---|---|
| Rotor Resistance | 0.0137 | pu |
| Stator Leakage Reactance | 0.191 | pu |
| Series Compensation | 4-6% | – |
The damping ratio calculation reveals instability under series compensation:
$$
\xi = -\frac{\sigma_i}{\sqrt{\sigma_i^2 + \omega_i^2}}
$$
where $\sigma_i$ and $\omega_i$ represent real and imaginary parts of eigenvalues.
2. Energy Storage Battery Control Architecture
The proposed control strategy combines proportional-resonant (PR) control with supplementary damping:
| Component | Function | Parameters |
|---|---|---|
| PR Controller | Frequency-selective gain | $K_p=1$, $K_r=30$ |
| Phase Compensator | Delay correction | $T_1=0.02\text{s}$, $T_2=0.01\text{s}$ |
| Limiter | Output saturation | ±20% Prated |
The PR controller transfer function is:
$$
G_{PR}(s) = K_p + \frac{K_r\omega_c s}{s^2 + 2\omega_c s + \omega_r^2}
$$
3. Coordinated Control Implementation
The energy storage battery system implements dual-loop control:
$$
\begin{cases}
P^* = P_{ref} + \Delta P_{SSDC}\\
Q^* = Q_{ref} + \Delta Q_{SSDC}
\end{cases}
$$
Where supplementary signals are generated through:
$$
\Delta P_{SSDC} = \frac{K_d s}{1 + T_f s} \cdot \frac{1 – T_1 s}{1 + T_2 s} \cdot P_{osc}
$$
4. Operational Verification
Simulation results under varying conditions demonstrate the effectiveness of energy storage battery control:
| Scenario | Oscillation Amplitude | Damping Ratio |
|---|---|---|
| Without Control | 0.35 pu | -0.12 |
| With PSSDC | 0.08 pu | 0.18 |
| With P+QSSDC | 0.03 pu | 0.25 |
The energy storage battery demonstrates superior oscillation suppression across different operating points:
$$
\text{THD}_{\text{improvement}} = \frac{\sum|H_{\text{pre}}(f_{\text{sso}})| – \sum|H_{\text{post}}(f_{\text{sso}})|}{\sum|H_{\text{pre}}(f_{\text{sso}})|} \times 100\% > 85\%
$$
5. Conclusion
This paper validates that energy storage battery systems with properly designed supplementary controllers can effectively mitigate SSOs in wind farms through:
- Frequency-selective damping injection
- Active power modulation
- Dynamic impedance shaping
The control strategy maintains effectiveness under varying wind speeds (7.5-10.5 m/s) and series compensation levels (4-6%), proving the energy storage battery’s adaptability in modern power systems.