The integration of renewable energy sources into power grids has intensified the demand for advanced energy storage systems. Among these, grid-connected energy storage inverter play a pivotal role in maintaining grid stability by balancing power fluctuations. However, traditional control strategies, such as Virtual Synchronous Generator (VSG) methods, often lead to battery overload due to incomplete power controllability under extreme grid frequency or voltage deviations. This paper proposes a hybrid VSG control strategy that combines the advantages of traditional and tracking-type VSG methods to enhance reliability while ensuring grid frequency support.
1. Challenges in Traditional VSG Control
The conventional VSG control mimics synchronous generator dynamics, enabling energy storage inverter to provide inertia and damping. Its active power-frequency and reactive power-voltage droop characteristics are expressed as: [ J \frac{d(\omega – \omega_n)}{dt} = P{\text{set}} – P_e – D_p (\omega – \omega_g) ] [ \frac{dU{\text{ref}}}{dt} = K_u (U_n – U_g) + \frac{K_q}{s} (Q_{\text{set}} – Q_e) ] where (J) is the virtual inertia, (D_p) and (K_q) are droop coefficients, and (P_e/Q_e) represent actual power outputs.
Limitations:
- Steady-state power errors arise due to grid frequency/voltage deviations.
- Excessive power output during severe grid disturbances causes battery overload.
2. Tracking-Type VSG Control
To address power controllability, a tracking-type VSG integrates a PI controller into the active power loop: [ H_p(s) = \frac{k{p}s + k{i}}{s} ] This eliminates steady-state errors and suppresses grid frequency disturbances. For reactive power, the droop feedback is removed, achieving: [ Q_e = Q_{\text{set}} ]
Advantages:
- Zero steady-state error in active/reactive power.
- Prevents battery overload during extreme grid conditions.
3. Hybrid VSG Control Strategy
The hybrid approach combines traditional and tracking-type VSG modes through adaptive switching logic:
Operational Modes:
- Traditional VSG Mode: Activated when grid deviations ((|\Delta f| < f{\text{th}} \text{ or } |\Delta U| < U{\text{th}})) are minor.
- Provides frequency/voltage support.
- Power outputs follow droop characteristics.
- Tracking-Type VSG Mode: Triggered during severe deviations ((|\Delta f| \geq f{\text{th}} \text{ or } |\Delta U| \geq U{\text{th}})).
- Limits power to (P{\text{max}}/Q{\text{max}}).
- Ensures battery protection.
Switching Logic: [ \text{Mode} = \begin{cases} \text{Traditional VSG} & \text{if } |\Delta f| < f{\text{th}} \text{ and } |\Delta U| < U{\text{th}} \ \text{Tracking-Type VSG} & \text{otherwise} \end{cases} ]
4. Mathematical Modeling and Stability
4.1 Power Control Loops
The hybrid VSG’s active power loop combines droop and PI controllers: [ P_e = \frac{K_p}{J s + D_p} P{\text{set}} + \frac{K_p}{J s + D_p} \left( \frac{k{p}s + k{i}}{s} \right) (\omega_n – \omega_g) ] Reactive power control simplifies to: [ Q_e = \frac{K_q}{s + K_q} Q{\text{set}} ]
4.2 Stability Analysis
The characteristic equation for the hybrid system is: [ s^2 + \left( \frac{D_p}{J} + \frac{K_q}{1 + K_q} \right)s + \frac{K_p k_i}{J} = 0 ] Root locus analysis confirms stability under all operational modes.
5. Experimental Validation
Experiments on a 10 kW energy storage inverter validate the hybrid strategy:
5.1 Steady-State Performance
| Condition | Traditional VSG | Tracking-Type VSG | Hybrid VSG |
|---|---|---|---|
| (\Delta f = 0.5) Hz | (P_e = 1.2P_{\text{set}}) | (P_e = P_{\text{set}}) | (P_e = P_{\text{set}}) |
| (\Delta U = 5\%) | (Q_e = 1.1Q_{\text{set}}) | (Q_e = Q_{\text{set}}) | (Q_e = Q_{\text{set}}) |
5.2 Dynamic Response
- Grid Frequency Step (50 Hz → 49 Hz):
- Traditional VSG: (P_e) surges to 120% (overload risk).
- Hybrid VSG: Seamlessly switches to tracking mode, limiting (P_e) to 100%.
- Voltage Sag (220 V → 200 V):
- Reactive power stabilizes within 100 ms, avoiding battery stress.
6. Comparative Analysis
Key Metrics:
| Metric | Traditional VSG | Tracking-Type VSG | Hybrid VSG |
|---|---|---|---|
| Steady-State Error | 10–20% | 0% | 0% |
| Overload Prevention | No | Yes | Yes |
| Frequency Support | Yes | Limited | Yes |
7. Conclusion
The hybrid VSG control strategy effectively balances grid support and battery protection in energy storage inverter. By adaptively switching between traditional and tracking-type modes, it mitigates overload risks while maintaining frequency/voltage regulation. Experimental results confirm its superior performance, making it a robust solution for modern power grids with high renewable penetration.
Future Work:
- Extend the strategy to multi-inverter systems.
- Incorporate AI-based adaptive threshold tuning.
This research underscores the critical role of advanced control strategies in enhancing the reliability of energy storage inverter, paving the way for smarter and more resilient power networks.