Abstract
This article addresses the issue of subsynchronous oscillations (SSO) in high-penetration renewable energy power systems, specifically focusing on grid-connected doubly-fed induction generator (DFIG) systems. An effective control strategy using energy storage batteries for additional damping is proposed to mitigate SSO. A system model of the DFIG with series compensation and energy storage battery is constructed, and eigenvalue and time-domain simulations are conducted to analyze the SSO characteristics with and without series compensation capacitors. Based on these analyses, a control strategy tailored to the energy storage battery is formulated, which effectively suppresses SSO. Furthermore, the SSO suppression mechanism is deeply explored, examining both additional damping and proportional resonant (PR) control of the energy storage battery in the DFIG system. An enhanced control strategy is introduced, incorporating an additional SSO damping control on top of the PR control, and its parameters are designed accordingly. Finally, the feasibility and effectiveness of the proposed control strategy are validated through time-domain simulations, offering valuable insights into SSO mitigation.
1. Introduction
The rapid development of renewable energy sources has been accompanied by the emergence of subsynchronous oscillations (SSOs) in power systems with a high penetration of renewable generation. These SSOs can lead to equipment damage, reduced system stability, and even large-scale blackouts. Among various mitigation approaches, electrochemical energy storage batteries have gained significant attention due to their flexibility and high efficiency. This paper reviews existing SSO suppression methods and proposes an innovative control strategy utilizing energy storage batteries for DFIG-based wind farms.
Table 1: Classification of SSO Suppression Methods
| Method | Category | Key Features |
|---|---|---|
| Source-side Suppression | Optimizing DFIG | Adjusting converter control parameters, structural modifications |
| controllers | ||
| Grid-side Suppression | Installing | Cutting off SSO current paths, using FACTS devices |
| inhibiting | (e.g., SMES, SVC) | |
| devices | ||
| Energy Storage Battery-based | Hybrid approach | Flexibility, energy storage capability, additional damping |
| Suppression | control |
2. Analysis of SSO Characteristics in Wind Farm Grid-connected Systems
2.1 Eigenvalue Analysis Method
Eigenvalue analysis is a powerful tool for assessing system stability by solving the eigenvalues of the linearized state-space model of the system. The real part (σi) of an eigenvalue indicates the system’s damping, while the imaginary part (ωi) represents the oscillation frequency. When ωi ≠ 0, a pair of conjugate eigenvalues corresponds to a SSO mode.
Formula for Eigenvalue Analysis:
lambdai=σi+jωi(i=1,2,…,n)
Table 2: Eigenvalue Analysis Parameters
| Parameter | Description |
|---|---|
| λi | Eigenvalue (complex) |
| σi | Real part (damping coefficient) |
| ωi | Imaginary part (oscillation frequency) |
| ξ | Damping ratio (ξ=−σi/ωi if ( \sigma_i < 0 )) |
2.2 SSO Characteristics with and without Series Compensation
The system model consisting of a DFIG with an energy storage battery and series compensation is analyzed through eigenvalue and time-domain simulations.
Without series compensation, the system exhibits positive damping, indicating stable operation without SSO. However, with series compensation, the system experiences negative damping, leading to SSO.
- Without Series Compensation: Stable output power, no SSO.
- With Series Compensation: Clear SSO in output power, instability.
3. Energy Storage Battery Control Strategy for SSO Suppression
3.1 Additional Damping Control Strategy
An additional damping control strategy is proposed for the energy storage battery to mitigate SSO in the DFIG system. This strategy involves injecting damping control variables into the system to suppress oscillations.
The control loop comprises filtering, phase shifting, amplification, and limiting stages. The input and output signals (xin and xout) are processed through these stages, and the resulting damping is injected into the system.
Table 3: Additional Damping Control Parameters
| Parameter | Description |
|---|---|
| xin | Input signal |
| xout | Output signal |
| m | Number of phase shifting stages |
| T1, T2 | Time constants |
| N(s), D(s) | Numerator and denominator of transfer function |
| K | Gain |
3.2 Proportional Resonant (PR) Control
PR controllers offer high gain at resonance frequencies, allowing precise control of SSO frequencies. The transfer function of the PR controller is:
G(s)=Kp+s2+2ζωcs+ωr2Krs
where Kp is the proportional gain, Kr is the resonant gain, ωc is the cutoff frequency, and ωr is the resonant frequency.
Table 4: PR Controller Parameters
| Parameter | Value |
|---|---|
| Kp | 1 |
| Kr | 30 |
| ωr | 42π rad/s |
| ωc | 0.5π rad/s |
The outer loop utilizes a PR controller for optimal gain at SSO frequencies, while the inner loops (active and reactive power) incorporate SSDCs for precise control.
4. Simulation and Verification
4.1 Simulation Setup
A detailed simulation model of the DFIG system with energy storage and series compensation is constructed. The system parameters are summarized in Tables 5 and 6.
Table 5: System Parameters
| Parameter | Value |
|---|---|
| Rated Voltage and Power | 0.69 kV, 201 MW |
| Base Capacity | 150 MV·A |
| Stator Resistance and Reactance | 0.0123 pu, 0.191 pu |
| Excitation Reactance | 11.783 pu |
| Rotor Resistance and Reactance | 0.0137 pu, 0.167 pu |
| Series Compensation | 0.00431 pu |
| Line Inductance and Resistance | 0.0901 pu, 0.107 pu |
| Transformer Reactance | 0.0212 pu |
Table 6: Energy Storage Parameters
| Parameter | Value |
|---|---|
| Rated Power and Capacity | 2 MW, 8 MJ |
| Voltage | 2.4 kV |
| Transformer Type and Ratio | Yd11, 2.4 kV/35 kV |
| DC Capacitor Voltage | 4000 V |
| AC and DC Side Inductances | 0.5 mH, 17.8 mF |
4.2 Simulation Results
Simulations are conducted with and without the additional damping control to demonstrate its effectiveness. The DC capacitor voltage (udc) is used as the input signal for the damping control.
The results show that without damping control, the system exhibits unstable SSO. In contrast, with the proposed damping control, the SSO is effectively suppressed, leading to stable operation.
The addition of both P_SSDC and Q_SSDC further enhances SSO suppression, as evidenced by the reduced oscillations in both active power and terminal voltage.
When only P_SSDC is activated, the terminal voltage remains relatively stable, indicating minimal impact on system dynamics.
The proposed damping control effectively mitigates SSO across various wind speeds and series compensation degrees, ensuring stable operation of the DFIG system.
5. Conclusion
This paper presents an innovative control strategy utilizing energy storage batteries to suppress subsynchronous oscillations in DFIG-based wind farms. Through detailed modeling, eigenvalue analysis, and time-domain simulations, it is demonstrated that the proposed additional damping control, coupled with PR control, can effectively mitigate SSO under various operating conditions. The results provide valuable insights into the design and implementation of energy storage-based SSO suppression systems for renewable energy integration.