Modern power systems face challenges in balancing renewable energy integration with grid stability. Energy storage inverters play a critical role in maintaining frequency and voltage stability through advanced control strategies. This paper proposes a hybrid Virtual Synchronous Generator (VSG) control method that combines traditional and tracking-type VSG techniques to prevent battery overload while ensuring grid support capabilities.
1. Traditional VSG Control Analysis
The fundamental equations governing traditional VSG control for energy storage inverters are:
$$
\begin{cases}
J\frac{d(\omega – \omega_n)}{dt} = P_{set} – P_e – D_p(\omega – \omega_g) \\
U_{ref} = U_n + \frac{k_u}{s}(Q_{set} – Q_e) + D_q(U_n – U_g)
\end{cases}
$$
Where:
$J$ = Virtual inertia
$D_p,D_q$ = Droop coefficients
$P_{set},Q_{set}$ = Power references
$\omega_g,U_g$ = Grid measurements
| Parameter | Traditional VSG | Tracking VSG |
|---|---|---|
| Steady-state Error | Non-zero | Zero |
| Overload Protection | No | Yes |
| Dynamic Response | 0.5-2s | <100ms |
2. Tracking-Type VSG Implementation
The improved control law for energy storage inverters introduces PI compensation:
$$
H_{track}(s) = \frac{k_{p}s + k_i}{s} \cdot \frac{1}{Js + D_p}
$$
This modification enables:
- Active power reference tracking with zero steady-state error
- Automatic overload prevention during grid disturbances
- Enhanced synchronization stability
3. Hybrid Control Architecture
The hybrid VSG strategy for energy storage inverters combines both approaches through adaptive switching:
$$
P_{out} = \begin{cases}
P_{set} + D_p(\omega_n – \omega_g) & \text{if } |\Delta f| \leq 0.5Hz \\
P_{max} \cdot \text{sgn}(\Delta f) & \text{if } |\Delta f| > 0.5Hz
\end{cases}
$$
Key operational modes include:
- Normal Mode: Traditional VSG for primary frequency regulation
- Emergency Mode: Tracking-type VSG for overload prevention
- Transition Mode: Smooth switching between control strategies
4. Experimental Validation
Testing on a 50kW energy storage inverter prototype demonstrated:
$$
\text{THD Improvement} = \frac{18.22\% – 4.98\%}{18.22\%} \times 100\% = 72.7\%
$$
Performance metrics:
- Frequency tracking accuracy: ±0.05Hz
- Mode transition time: <40ms
- Overload reduction: 63.2% during voltage dips
5. Conclusion
The hybrid VSG control strategy significantly enhances energy storage inverter capabilities in grid-connected applications. By combining the grid-support features of traditional VSG with the safety advantages of tracking-type control, this approach addresses critical challenges in modern power systems:
- Maintains grid code compliance during normal operation
- Prevents battery damage during extreme grid events
- Enables seamless integration with renewable generation
Future research directions include adaptive parameter tuning and multi-inverter coordination strategies for large-scale energy storage deployments.