Off-grid Operation Research Energy Router Strategy on the Base of Hierarchical Control

To coordinate off-grid control of the Energy Router, the Energy Router topology is first analyzed using isolated bidirectional full-bridge DC/DC inverters for DC ports, boost converters for PV units, and three-phase voltage-based bridge inverters for AC ports. Then the system is layered, with the lower layer developing different control strategies for different ports, the upper layer layering the DC bus voltage and dividing different operating modes in combination with the energy storage SOC, and then developing strategies for each mode. Finally, simulations are built in the MATLAB platform according to the topology to verify the possibility of off-grid operation of the energy router.


Introduction
With the continued use of non-renewable energy resources, the energy crisis is gradually being highlighted.To break the deadlock of the energy crisis, countries are vigorously developing renewable energy sources, especially photovoltaic, wind power, hydropower, and other clean energy sources [1] .This is why experts in various countries have developed the idea of the Energy Internet to reduce the impact of distributed clean energy on the grid and its stability.The proposed energy internet provides an interactive platform for various distributed energy sources to join the IFE grid [2] .However, the traditional inverter is no longer applicable to the access of various distributed power sources, so an energy router with the functions of converting, transforming, and routing energy is proposed.The energy router has the functions of "plug and play," bi-directional flow of energy, and data collection to interconnect multiple energy routers [3] .
The operation strategy of the energy router proposed in the paper is suitable for providing reliable and efficient power supply in remote areas, border areas, islands, and other areas where photovoltaic and wind energy is abundant but cannot be delivered, similar to such areas where the power system is disconnected from the main grid and the system operates off-grid [4] .

Energy router's topology
The proposed energy router topology is shown in Figure 1.The topology is mainly DC port and AC port, DC port for PV and energy storage access, and AC port for AC load and diesel generator/small hydropower [5] .The DC side of the energy storage DC/DC uses an isolated, full-bridge, bi-directional DC/DC inverter.The energy storage system stores excess PV energy and enables peak and trough reduction.The photovoltaic unit uses a Boost converter to maximize PV utilization.Maximum power point tracking (MPPT) of the PV is achieved using an operating strategy [6] .A three-phase voltage bridge converter is used for the AC connection so that the output of the connection is a three-phase 380 frequency voltage available to the load.

Hierarchical control strategy for energy routers
The Energy Router operates in an off-grid mode, powered mainly by solar PV and the energy storage system as the backup power source, ready to be charged and discharged [7] .A hierarchical control strategy is used to ensure port stability, as shown in Figure 2.

AC source port control method
The AC port, as the power supply interface for the AC load, needs to provide stable and safe power and therefore requires a suitable control method.The current control strategy used in the off-grid mode of the inverter is V/F control (constant voltage and constant frequency control) [8] .The control strategy uses a double closed-loop structure with an external voltage control loop and an internal current control loop to achieve a stable output voltage and frequency.The voltage external loop control is achieved by collecting the voltage U from the load, d q transforming it to obtain U d and U q, and making a difference with the specific values U dref and U qref to obtain the current internal loop control values I dre f and I qref by PI regulator.The u d * and u q * are then transformed by the d q coordinate and SPW modulation so that the IGBTs on the three-phase bridge inverter circuit can be switched on and off at tons of times to ensure voltage and frequency stability.

PV unit control method
Photovoltaic systems are the main source of energy for energy routers and are key to maintaining a stable power supply to the load.In most cases, PV systems operate in MPPT (maximum power point tracking) mode to track the maximum power point to charge the energy storage and supply the load with the maximum power.However, there are other situations where a specific power output from the PV is required, and the PV system should be operated in constant voltage mode.
The two operating modes are selected based on the following: when the energy provided by the PV is less than the energy required by the energy router or the energy storage unit SOC value does not reach the set value, the PV unit works again in MPPT mode, and the perturbation observation method is the control method for the PV; when the PV unit energy is greater than the energy required by the energy router and the energy storage cannot be received, and the impact on the bus voltage is not prevented, then the PV unit works in constant voltage mode of operation.

Energy storage battery control methods
The energy storage port adopts a bi-directional BUSK-BOST circuit [9] .In the off-grid operation mode, when the PV power generation is greater than the load, the energy storage is charged to consume the excess power; when the PV power generation is less than the load, the energy storage is discharged to make up for the vacant power of the load.Energy storage SOC cannot be too high or too low; otherwise, it will cause harm to the battery.Energy storage should be protected from over-voltage and under-voltage, and the threshold value of energy storage SOC should be set.If the SOC of the energy storage is greater than 90%, charging should be stopped at that time; if the SOC of the energy storage is less than 20%, discharging should be started. The

Control strategies at the decision-making level
The Energy Router uses a DC microgrid architecture where the DC bus voltage directly determines the whole system balance.Therefore, the control strategy at the decision level uses a DC bus voltage hierarchy, combined with the energy storage SOC for modal division into different operating modes, and only one mode can operate at the same time with voltage fluctuation.The system can only run one mode at a time with voltage fluctuation.The system can flexibly switch according to the variation of the DC bus voltage.Then according to the different modes, we develop different strategies, so that the system's DC bus voltage to maintain stable.

Underlay scheduling strategy based on DC bus voltage stratification
The DC bus system is set to a DC bus voltage of 400 V with a voltage fluctuation value of ±0.05UN.The two adjacent voltage boundary values set are 5% [10] .The selected voltage reference values are shown below: U L2 is the minimum value of the system voltage for stable operation, and U H2 is the maximum value of the system voltage for stable operation; when the system is in the voltage range of U L1 , the line voltage is too low, and the battery needs to be discharged and replenished at this time, so U L1 is the boundary value for discharging the energy storage battery; similarly, U H1 is the boundary value for charging the energy storage battery; U N is the rated voltage of the DC bus of the system.The four DC bus voltage boundary values are shown in Table 1 below.
The five areas of DC bus voltage division are used to divide the operating modes and develop strategies, as shown in Table 2

Simulation validation
To demonstrate the feasibility of the energy router strategy in the off-grid case, a simulation model was built using MATLAB/Simulink.The DC bus voltage is 400 V, the nominal power of the PV system is As night falls or the PV is depleted, the bus voltage continues to fall below the undervoltage threshold, at which point the system switches to mode five from mode two, the PV cells go into standby, and the whole system is controlled by the energy storage for discharge.The experimental waveform in Figure 4 shows that at this point, the bus voltage is maintained at around 390 V and remains stable.

Conclusions
This paper addresses the problem of difficult and unstable power supply in remote areas and islands and proposes a layered control strategy for off-grid lower energy routers.The lower layer control maintains the stable operation of the whole system by controlling each port, and the upper layer is layered using the DC bus voltage, combined with the energy storage SOC to divide the working modes of PV cells and energy storage so that the system DC bus voltage is within a stable range.Simulation experiments are carried out through MATLAB.From the experimental results, the strategy can effectively ensure the stability of the DC bus voltage and effectively guarantee the safety of the power supply.

Figure 2 .
Figure 2. Hierarchical control of energy routers.
PI controller uses the error signal generated by the high voltage side capacitor voltage Uref and the actual DC bus voltage Ub to obtain the inductor current reference value I ref .The error signal formed by I ref and the actual inductor current I b is used by the PI controller to form the PWM switching signal.The charging and discharging of the battery are double closed-loop control.

Table 1 .
DC bus voltage boundary values below.