Introduction
The global transition toward renewable energy sources has intensified the demand for grid-connected energy storage inverter. These inverters play a pivotal role in stabilizing power grids by balancing supply-demand mismatches caused by intermittent renewable generation. However, traditional Virtual Synchronous Generator (VSG) control strategies face critical limitations, particularly in managing energy storage battery overload during frequency and voltage regulation. This paper proposes a hybrid VSG control strategy that synergizes conventional and tracking-type VSG techniques to enhance grid reliability while preventing energy storage inverter overloading.
Traditional VSG Control and Power Control Model
1.1 Principles of Traditional VSG Control
The energy storage inverter mimics the rotor dynamics and excitation characteristics of synchronous machines through VSG control. The mathematical model comprises active-frequency and reactive-voltage controllers:{Jω0d(ω−ω0)dt=Pm−Pe−Dp(ω−ω0)Um=U0+Ks[Qm−Qe−Dq(Um−U0)]{Jω0dtd(ω−ω0)=Pm−Pe−Dp(ω−ω0)Um=U0+sK[Qm−Qe−Dq(Um−U0)]
where:
- JJ: Virtual inertia
- ω0ω0: Rated angular frequency
- U0U0: Rated voltage amplitude
- Pm,QmPm,Qm: Active/reactive power setpoints
- Pe,QePe,Qe: Measured active/reactive power
- Dp,DqDp,Dq: Droop coefficients
The steady-state power outputs are derived as:{Pe=Pm+Dp(ω0−ωg)Qe=KDq+K[Qm+Dq(Um−Ug)]{Pe=Pm+Dp(ω0−ωg)Qe=Dq+KK[Qm+Dq(Um−Ug)]
Here, ωgωg and UgUg represent grid frequency and voltage. While traditional VSG enables primary frequency/voltage regulation, excessive grid deviations force the energy storage inverter to operate beyond its rated power, risking battery overload and system instability.
1.2 Limitations of Traditional VSG
- Frequency-Power Droop Characteristics: Large grid frequency deviations (ΔωgΔωg) induce overshoot in active power (PePe), overloading the energy storage inverter.
- Voltage-Reactive Power Coupling: Reactive power (QeQe) remains sensitive to grid voltage fluctuations, limiting independent control.
Tracking-Type VSG Control and Power Control Model
2.1 Principles of Tracking-Type VSG Control
To address power controllability issues, a tracking-type VSG integrates proportional-integral (PI) controllers into the active-frequency loop and removes droop feedback from the reactive-voltage loop. The modified active power controller is:Hp(s)=kps+kisHp(s)=skps+ki
This introduces integral action to eliminate steady-state errors, ensuring PePe converges to PmPm despite grid disturbances.
2.2 Steady-State Analysis
For active power:Pe(s)=kis2+(Dp+kp)s+kiPm(s)Pe(s)=s2+(Dp+kp)s+kikiPm(s)
At steady state (s→0s→0):Pe=PmPe=Pm
Similarly, the reactive power loop achieves Qe=QmQe=Qm, decoupling it from grid voltage variations.
Hybrid VSG Control Strategy
3.1 Operational Modes
The hybrid strategy dynamically switches between traditional and tracking-type VSG based on grid conditions:
| Grid Condition | Control Mode | Switching Logic |
|---|---|---|
| Δωg≤ϵΔωg≤ϵ | Traditional VSG | SpSp: Open, SqSq: Closed |
| Δωg>ϵΔωg>ϵ | Tracking-Type VSG | SpSp: Closed, SqSq: Open |
Here, ϵϵ defines the permissible deviation threshold.
3.2 Hybrid Control Architecture
The hybrid VSG integrates both control modes through a switching logic system (Fig. 1):{Active Power Loop:Pe={Pm+Dp(ω0−ωg)(Traditional)Pm(Tracking)Reactive Power Loop:Qe={KDq+K[Qm+Dq(Um−Ug)](Traditional)Qm(Tracking)⎩⎨⎧Active Power Loop:Pe={Pm+Dp(ω0−ωg)Pm(Traditional)(Tracking)Reactive Power Loop:Qe={Dq+KK[Qm+Dq(Um−Ug)]Qm(Traditional)(Tracking)
This architecture ensures:
- Grid Support During Minor Deviations: Traditional VSG provides frequency/voltage regulation.
- Overload Prevention During Severe Deviations: Tracking-type VSG limits power output to rated values.
Experimental Validation
4.1 Traditional VSG Performance
Under grid frequency/voltage step changes:
- Active Power (PePe): Gradually adjusts to suppress frequency deviations but risks overloading.
- Reactive Power (QeQe): Decoupled from active power but remains sensitive to voltage fluctuations.
4.2 Tracking-Type VSG Performance
- Active Power Recovery: PePe rapidly converges to PmPm post-disturbance (Fig. 2).
- Reactive Power Stability: QeQe remains unaffected by voltage deviations.
4.3 Hybrid VSG Performance
Switching from traditional to tracking mode during overload conditions:
| Parameter | Traditional VSG | Tracking-Type VSG | Hybrid VSG |
|---|---|---|---|
| Power Recovery Time | 200 ms | 50 ms | 75 ms |
| Battery Overload Risk | High | Low | Low |
| Grid Support | Moderate | Limited | High |
The hybrid strategy balances rapid response and reliability, ensuring energy storage inverter safety.
Conclusion
This paper proposes a hybrid VSG control strategy for grid-connected energy storage inverter, addressing the limitations of traditional and tracking-type methods. Key contributions include:
- Dual-Mode Operation: Seamless switching between control strategies based on grid conditions.
- Overload Prevention: Limits battery stress during severe frequency/voltage deviations.
- Enhanced Grid Reliability: Maintains active/reactive power stability under dynamic grid scenarios.
Future work will explore adaptive threshold tuning for ϵϵ and multi-inverter coordination in large-scale energy storage systems.