1. Introduction
To achieve the “dual-carbon” goals, renewable energy sources (RES) are increasingly integrated into power systems. However, their intermittent nature introduces challenges such as frequency instability and voltage fluctuations. Energy storage inverter play pivotal role in balancing power and enhancing grid reliability by regulating charge/discharge cycles of storage batteries. A critical challenge lies in maintaining the state of charge (SOC) within safe limits while preventing overload conditions that may force inverters to disconnect, compromising system reliability.
Virtual Synchronous Generator (VSG) control emulates the inertia and damping characteristics of synchronous machines, enabling energy storage inverter to actively support grid frequency and voltage. Traditional VSG strategies, however, suffer from inherent limitations: during significant frequency deviations, excessive power output may overload storage batteries, leading to inverter shutdowns. This paper proposes a hybrid VSG control strategy combining traditional and tracking-type VSG methods to address these limitations, ensuring reliable grid support while preventing battery overload.
2. Traditional VSG Control and Power Modeling
2.1 Control Principles
Traditional VSG control mimics the rotor dynamics and excitation characteristics of synchronous generators. Its active power-frequency and reactive power-voltage controllers are defined as:{Jd(ω−ω0)dt=Pm−Pe−Dp(ω−ωg),Um=U0+Ks[Qm−Qe−Dq(Um−Ug)],{Jdtd(ω−ω0)=Pm−Pe−Dp(ω−ωg),Um=U0+sK[Qm−Qe−Dq(Um−Ug)],
where JJ is the virtual inertia, ω0ω0 and U0U0 are nominal angular frequency and voltage, DpDp and DqDq are droop coefficients, and KK is the excitation integral coefficient.
2.2 Power Transfer Analysis
In inductive grid systems, active and reactive power delivered to the grid are:Pe=Kp(θ−θg),Qe=Kq(Um−Ug),Pe=Kp(θ−θg),Qe=Kq(Um−Ug),
where Kp=UmUgXLKp=XLUmUg, Kq=UmXLKq=XLUm, and XLXL is the line reactance. Steady-state power outputs are:{Pe=Pm+Dp(ωg−ω0),Qe=KqDq+Kq[Qm+Dq(Ug−U0)].{Pe=Pm+Dp(ωg−ω0),Qe=Dq+KqKq[Qm+Dq(Ug−U0)].
Limitations: During large grid frequency/voltage deviations, PePe and QeQe exceed rated values, risking battery overload.
3. Tracking-Type VSG Control
3.1 Control Architecture
To eliminate steady-state errors and enhance power controllability, a tracking-type VSG is proposed. A PI controller is added in parallel to the active power loop:Hp(s)=kps+kis,Hp(s)=skps+ki,
where kpkp and kiki are proportional and integral gains. The reactive power loop removes droop feedback, achieving direct voltage tracking.
3.2 Steady-State Performance
For active power, the PI controller ensures zero steady-state error:Pe(s)=Hp(s)1+Hp(s)Pm.Pe(s)=1+Hp(s)Hp(s)Pm.
For reactive power, the steady-state output becomes:Qe=Qm,Qe=Qm,
decoupling QeQe from grid voltage variations.
Advantages:
- Eliminates power overshoot during frequency/voltage deviations.
- Prevents battery overload by enforcing power limits.
4. Hybrid VSG Control Strategy
4.1 Operational Modes
The hybrid strategy dynamically switches between traditional and tracking-type VSG based on grid conditions:
| Condition | Control Mode | Action | ||
|---|---|---|---|---|
| ( | \Delta \omega | \leq \Delta \omega_{th}) | Traditional VSG | Supports grid frequency/voltage via droop control. |
| ( | \Delta \omega | > \Delta \omega_{th}) | Tracking-Type VSG | Limits power output to prevent overload while participating in frequency regulation. |
Here, ΔωthΔωth is a predefined threshold for frequency deviation.
4.2 Control Implementation
The hybrid control structure uses switches SpSp and SqSq to toggle modes:
- Traditional VSG: Engaged during minor deviations to maintain grid stability.
- Tracking-Type VSG: Activated during severe deviations to enforce power limits.
{Sp=OPEN, Sq=CLOSED(Traditional VSG),Sp=CLOSED, Sq=OPEN(Tracking VSG).{Sp=OPEN, Sq=CLOSEDSp=CLOSED, Sq=OPEN(Traditional VSG),(Tracking VSG).
5. Experimental Validation
5.1 Traditional VSG Performance
Under grid frequency steps (Δωg=±2%ω0Δωg=±2%ω0):
| Parameter | Response |
|---|---|
| Active Power (PePe) | Slow adjustment to stabilize frequency. |
| Reactive Power (QeQe) | Minimal variation, maintaining voltage. |
5.2 Tracking-Type VSG Performance
During frequency/voltage steps:
| Parameter | Response |
|---|---|
| Active Power (PePe) | Rapid convergence to PmPm within 100 ms. |
| Reactive Power (QeQe) | Steady at QmQm despite grid disturbances. |
5.3 Hybrid VSG Performance
Mode switching (Ug>U0Ug>U0, ωg>ω0ωg>ω0):
| Time | Control Mode | Power Output |
|---|---|---|
| t<t1t<t1 | Traditional VSG | Pe>PmPe>Pm, risk of overload. |
| t≥t1t≥t1 | Tracking VSG | Smooth transition to Pe=PmPe=Pm. |
The hybrid strategy ensures seamless transitions, avoiding power spikes and enhancing system reliability.
6. Conclusion
This paper proposes a hybrid VSG control strategy for grid-connected energy storage inverter, addressing the limitations of traditional VSG in overload scenarios. By combining traditional droop control with tracking-type power regulation, the strategy dynamically adapts to grid conditions, ensuring:
- Frequency/voltage support during minor deviations.
- Battery protection during severe disturbances.
Experimental results validate the strategy’s effectiveness in balancing grid stability and battery safety. Future work will optimize threshold settings and expand the strategy to multi-inverter systems.