Research on an integrated control strategy for grid-connected and off-grid optical storage microgrids

In order to mitigate the volatility and randomness caused by the switching processes in a photovoltaic storage microgrid, and to enhance its stability, in this paper, the utilization of the PQ+PQ control strategy is proposed for grid-connected operation and the V/f+PQ integrated control strategy is proposed for off-grid operation. Through this approach, a smooth transition from the PQ control of the master inverter to the V/f control is achieved, enabling seamless switching between grid-connected and off-grid modes in the photovoltaic storage microgrid. The proposed integrated control strategy is validated for feasibility and accuracy using a simulation platform built in Matlab/Simulink.


Introduction
The smooth switching of system operation mode is crucial for the safe operation of microgrids.Interconnection with the distribution grid allows microgrids to efficiently utilize distributed power sources and improve power quality [1].The uncertainty of renewable energy and controllable loads needs to be addressed in microgrid energy management [2].In [3], an energy management strategy is proposed for collaborative control of the power generation unit and energy storage unit during the islanded operation of the microgrid.Energy storage plays a critical role in ensuring voltage and frequency stability during different operation modes, such as off-grid, grid-connected, and seamless switching [4].The two operation modes of microgrid are grid-connected mode, where power flows between the microgrid and the larger grid, and off-grid mode, where power only flows within the microgrid [5].

Optical storage microgrid system
The structure of the optical storage microgrid system is shown in Figure 1.In this system, both the photovoltaic battery power generation system and the battery storage system are connected to the power grid.This connection is established through the utilization of a grid-connected inverter, an LC filter circuit, and a grid-connected switch STS [6].
The description of the integrated control strategy is as follows: during grid-connected operation, the control strategy utilizes PQ+PQ master-slave control, while during off-grid operation, it employs V/f+PQ master-slave control [7].The objective of this strategy is to enable a seamless and smooth transition between these two modes.The organic combination of decentralization and unification is realized through the independent control of each converter and the unified setting of switching basis, which makes the system operation more flexible and reliable.The control strategies for on-grid and off-grid are discussed in detail below.
(1) During grid-connected operation, the voltage and frequency of the optical storage microgrid are controlled by the main grid, so there is no need to control the output of its system voltage and frequency, and it is sufficient to supply power to the self-contained loads of the optical storage microgrid and other loads of the main grid as far as possible when the battery of the energy storage system is fully charged.Overall, the PQ+PQ control strategy is an effective way to manage the power output of the microgrid during grid-connected operation.By maintaining power quality within the prescribed limits and continuously meeting the load demand, this strategy enables the microgrid to operate in synchronization with the utility grid and exchange power as needed.
(2) During off-grid operation, the main power inverter of the photovoltaic storage microgrids adopts the voltage-to-frequency (V/f) control strategy to maintain a stable and regulated voltage level, while the auxiliary power inverter adopts the power quality (PQ) control strategy to manage the active and reactive power output to meet the load demand.
If the power control module in PQ and V/f control can be unified into an integrated control module, the seamless connection from PQ to V/f switching can be realized, and the structure diagram is shown in Figure 2.

System simulation analysis
When the photovoltaic storage microgrid is running off-grid, the simulation of the increase and decrease of the local load is conducted (the initial value of the local load power is 20 kW, the load power is increased to 25 kW at the moment of 0.1 s, and the load power is reduced to 15 kW at the moment of 0.2 s), and the simulation results are shown in the waveforms in Figures 3-4.Figures 3-4 illustrate that the output power, U/I of the inverter change with the load, and the waveform of the output produces transient fluctuations but they are quickly stabilized.The proposed control strategy adjusts the active and reactive power outputs of the inverter based on the load demand.This allows the system to respond to changes in load power and maintain stable voltage and frequency.The main inverter must adopt the V/f control strategy while supplementing the power demand of the load, and the slave inverter adopts the PQ control to reach the maximum power output.
The provided information describes that the switching is initiated by turning off the grid-connected switch STS between 0.1-0.2s, resulting in the microgrid transitioning to the off-grid operation state during that period.Figure 5 illustrates the waveforms of the output U/I curves of the main inverter during the switching process.On the other hand, Figure 6 provides a comparison of the waveforms between the main grid voltage and the output voltage of the inverters.Lastly, Figure 7 shows the frequency change waveform at PCC.As can be seen in Figure 5, it is understood that during the microgrid operation state switch between 0.1 s and 0.2 s, there is a control strategy change in the main inverter.At 0.1 s, the control strategy of the main inverter switches from PQ to V/f.Then at 0.2 s, it switches back.At the instant of switching between different modes, the STS is switched off, and the input current in the main grid instantly becomes zero, and the local load power demand is only supplied by the photo-recovery microgrid after the load is removed from the main grid, which makes the current not only reduced but also accompanied by oscillations.At the same time, there are oscillations in the voltage of the photorecovery microgrid.
From Figure 6, it can be seen at 0.1 s moment after the switch is disconnected, the inverter output grid-connected voltage is not controlled by the main grid.The main grid current in 0.1 s moment with the switch disconnected instantly becomes 0, and the phase-locked loop (PLL) input voltage is switched for the inverter output voltage, which ensures that there are no major fluctuations in the phase angle of the turn off-grid operation; 0.2 s after the main grid recovery, with the help of PLL, the large-scale power grid voltage is tracked.After 0.2 s, when the big grid recovers, the PLL is utilized to track the major power grid voltage.This ensures that the phase angle of the inverter output is synchronized with the main grid, enabling the optical storage microgrid to connect to the main grid simultaneously.This synchronization allows for seamless connection and operation between the microgrid and the main grid.According to the information provided, the phase-locked loop control is employed during the transition of the optical storage microgrid from changing operation.This control ensures that the phase angle does not experience any sudden jumps, maintaining a smooth transition.Additionally, Figure 7 demonstrates that there may be some frequency fluctuations during parallel off-grid switching operations, but the frequency mainly remains around 50 Hz, which is the standard frequency for electrical power systems.
The simulation results presented above demonstrate the successful implementation of an integrated control strategy that enables seamless switching operation.These results confirm the feasibility and effectiveness of the integrated control strategy.

Conclusion
In this paper, a comprehensive control strategy is proposed to facilitate seamless and smooth switching between photovoltaic (PV) and storage microgrids during transitions from on-grid to off-grid operation.The core of this strategy lies in the management of power generation from the PV system, load power, and battery output power using a master-slave control strategy based on V/f and PQ control algorithms.This approach allows the microgrid to operate efficiently in two modes, grid-connected and off-grid, while ensuring coordinated control of the master-slave inverter and the grid-connected switch based on the specific operational requirements.The experimental results validating the effectiveness and accuracy of the proposed integrated control strategy for the optical storage microgrid system not only provide valuable insights but also highlight its practical benefits.The successful validation of the control strategy underscores its ability to ensure the stability and reliable operation of the microgrid system.This suggests that the proposed strategy can effectively address the challenges associated with changing operations and contribute to the reliable and efficient functioning of the optical storage microgrid.

Figure 1 .
Photovoltaic cell power generation system

Figure 2 .
Figure 2. Structure of integrated control strategy.

Figure 3 .
Figure 3. V/f versus active power output from PQ inverter.

Figure 4 .
Figure 4. Voltage and current of the main inverter output.

5 Figure 6 .
Figure 6.Comparison waveforms of large grid voltage and inverter output voltage.