As the core component of energy storage systems, the performance of energy storage inverters (PCS) directly affects the efficiency and stability of the entire energy storage system. The Virtual Synchronous Generator (VSG) function, as an advanced control strategy, endows energy storage inverters with the ability to simulate the characteristics of traditional synchronous generators, enabling them to demonstrate unique advantages in various application scenarios. The following discusses the principle of VSG function in energy storage inverters and elaborates on its applications in different fields.
1 VSG functional principle
1. Overview of Virtual Synchronous Generator
Virtual synchronous generator (VSG) is a control strategy that enables energy storage inverters to simulate the operating characteristics of traditional synchronous generators. Through this approach, energy storage inverters not only possess the inertia and damping characteristics of traditional synchronous generators, but also operate stably in both grid connected and off grid modes, providing flexible power support for the power system.
2. Control structure and principle
The VSG control structure is mainly based on the droop control principle, and introduces virtual inertia and damping characteristics on this basis. The core idea is to improve the motion equation of traditional synchronous generators and obtain control equations applicable to power electronic devices. Specifically, VSG control simulates the rotor motion and electromagnetic characteristics of synchronous generators, enabling energy storage inverters to automatically adjust their output power according to changes in system frequency and voltage, thereby achieving effective support for the power grid.
3. Comparison of core control modes
PQ mode: Fixed active/reactive power output, does not respond to changes in grid frequency/voltage (suitable for precise power control).
VSG mode: dynamically responds to power grid disturbances, providing inertia, frequency regulation, and voltage regulation support (suitable for scenarios with high stability requirements).
4. VSG active frequency control
In terms of active frequency control, VSG achieves frequency regulation through a virtual frequency modulator. It calculates the difference between the actual output active power and the rated active power, and then adjusts the virtual torque. This process is similar to the torque difference adjustment in traditional synchronous generators, where the active power equation of the traditional synchronous generator is introduced into the power electronic device through a damping link, thereby obtaining the active frequency modulation equation of the virtual synchronous machine. In this way, when the system frequency changes, VSG can quickly respond and adjust the output active power to maintain the stability of the system frequency.
5. VSG reactive power voltage control
For reactive power voltage control, VSG draws on the excitation system principle of traditional synchronous generators. It first calculates the magnitude of reactive power at the output end of the device, and then calculates the voltage deviation value by the reactive power droop coefficient. This value is combined with the voltage setpoint to obtain the voltage reference value required for the voltage outer loop. The reactive power droop coefficient determines the voltage offset when the reactive power changes, thereby achieving effective control of the system voltage. In off grid mode, the reactive power loop of VSG is consistent with the reactive power loop in droop control, ensuring the reasonable allocation of reactive power and the stability of system voltage.
2 Key application scenarios of VSG
1. Microgrid
Island operation: Independently support voltage and frequency when off grid, ensuring continuous power supply for critical loads such as hospitals and data centers.
Seamless switching between grid connected and off grid: Participate in grid support in grid connected mode, and maintain microgrid stability in islanded mode.
2. High proportion of renewable energy access
Inertia supplement: Compensate for the inertia deficiency of new energy sources such as photovoltaics and wind power, and suppress frequency fluctuations.
Smooth output fluctuations: By suppressing power oscillations through virtual damping, the ability of the power grid to accept renewable energy is enhanced.
3. Intelligent distribution network and weak current network
Dynamic voltage support: Provide reactive power compensation at the end of the distribution network or in weak grid areas to improve voltage stability.
Auxiliary service participation: By utilizing functions such as frequency regulation and reserve capacity, the flexibility of the power grid can be enhanced (such as participating in auxiliary services in the electricity market).
4. Grid connected energy storage system
Enhanced power grid stability: Simulate the characteristics of synchronous generators and participate in frequency regulation (such as primary frequency regulation and inertia response).
Fault ride through capability: When the voltage of the power grid drops temporarily, it helps to restore the power grid through reactive power support (such as low voltage ride through LVRT).
5. Integration of emergency power supply and distributed energy
Rapid response to emergency power supply: In the event of a power grid failure, it serves as an independent power source to support local loads (such as disaster emergency).
Multi DER collaborative control: achieve collaborative optimization of distributed resources such as energy storage and photovoltaics, and improve energy utilization efficiency.
3 VSG's technological advantages
1. Enhance power grid stability: Provide inertia, damping, and frequency and voltage regulation capabilities to adapt to the "low inertia power grid" with a high proportion of power electronic equipment connected.
2. Flexible operation mode: Supports both grid connected and off grid modes, suitable for various scenarios such as microgrids and main grids.
3. Improve power quality: Reduce voltage/current imbalance and harmonic distortion through negative sequence compensation and dynamic control.
4. Economic value enhancement: By participating in auxiliary services such as frequency regulation and backup, the potential revenue of energy storage systems can be increased.
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4 Practical application
Microgrid scenario: During the day, photovoltaics generate electricity through PQ mode, and at night or on isolated islands, energy storage switches to VSG mode to maintain voltage and frequency stability.
Grid connected energy storage: When the grid frequency drops, VSG quickly releases power (inertia response) to suppress frequency drop (such as when the frequency drops from 50Hz to 49.5Hz, the energy storage automatically increases active output).
Weak current network support: In remote areas, VSG improves voltage stability and reduces line losses through reactive power compensation.
5 Conclusion
The VSG function in energy storage inverters, as an advanced control technology, provides powerful support for energy storage systems in various application scenarios by simulating the characteristics of traditional synchronous generators. From islanding operation of microgrids to flexible regulation of intelligent distribution networks, from auxiliary services of grid connected energy storage systems to independent power supply in special scenarios, VSG functions have demonstrated their unique advantages and important roles.