Coordinated control strategy for microgrid based on improved virtual DC motor

The coordinated control strategy based on the improved virtual DC motor (VDCM) is proposed to address the bus voltage fluctuation of an isolated DC microgrid and the power distribution problem of multiple energy storage units. Firstly, the memory converter control process unit is enhanced by adding the VDCM link and low-pass filtering link. Secondly, considering factors such as photovoltaic power, constant-power loads, and battery charging state, six working modes are defined to ensure stable DC bus voltage in each mode. Finally, a simulation model of the isolated DC microgrid is found by using simulation software to verify the validity of this method.


Introduction
Along with the economic development, how to realize the sustainable use of energy is getting more and more attention.At present, DC microgrids have received widespread attention because of their advantages, for example, high efficiency and easy control, and the absence of high harmonics and frequency variation [1] .Nevertheless, with the increasing share of renewable energy generation in DC microgrids, the voltage of the bus is bound to fluctuate, endangering the stabilization of the power system.In island DC microgrids, when the power fluctuation is great, the bus voltage will fluctuate greatly, which has a serious impact on the security and stability of DC microgrids [2][3] .When there is more than one energy storage device in the microgrid, how to allocate the energy of the storage units is also an urgent problem.
Wu et al. use voltage sag control to allocate unbalanced power, but the system dynamic response is weak [4] .Ribeiro et al. ensure the consistency of the SOC of multiple storage units by comparing the difference between the individual SOC of the storage unit and the average SOC of the entire storage unit to determine the virtual sag factor of the storage unit [5] .Batiyah et al. have suggested an energy management strategy for PV and battery systems in an isolated DC microgrid, considering a nonlinear model with four modes for the PV system and controlling the DC/DC converter for charging and discharging the ESUs by investigating the SOC of the batteries, but not considering the coordinated operation of the multi-storage units of the energy storage system [6] .Kumar et al. put forward an approach to keep power equilibrium and DC bus voltage stability, but the strategy is difficult to provide good suppression of the fluctuation of the load and power supply [7] .Neto et al. propose that based on the virtual inertia technique, the charging and discharging conditions in a battery pack in accordance with the energy flow can be adjusted, and when the energy storage unit is unable to supply or absorb surplus energy, it can supply or absorb surplus energy [8] .
This paper presents a new VDCM control strategy for the Bi-DC/DC converter.The memory converter control process unit is enhanced by adding the VDCM link and low-pass filtering link, which improves the stability of the DC bus voltage, and six operating modes are classified for the power allocation of the multi-output energy storage unit, so that the bus voltage is stable in all operating modes, and finally simulation is carried out to establishment of the appropriate overall system model of the optical storage DC microgrid for verification.

Isolated DC microgrid structure
The structure of an isolated DC microgrid is shown in Figure 1, which consists of a distributed power unit, an energy storage unit, and a constant power load unit.Wherein the distributed power unit is used to provide electrical energy and the energy storage unit is used to store excess electrical energy.

Energy storage converter control strategy
To enhance DC bus voltage stability in isolated microgrids, the control method of the Bi-DC/DC converter for the energy storage unit is improved as follows.

VDCM principle.
Figure 2 shows the virtual DC generator model.The Buck/Boost converter can be equated to a two-port network [9] .The Buck/Boost converter is equivalent to a dual port network with the front end connecting to distributed power supply and the back end connecting to a DC bus.Its equivalent two-port network can simulate the outer properties of a DC generator [10] .

Figure 2. VDCM model.
where E is the armature electromotive force; U is the generator terminal voltage; U bat is the power output voltage; I a is the armature current; U dc is the output voltage of the converter; I dc is the output current of the converter; I bat is the output current of the power supply; R a is the armature resistance; L is the filtering inductance; C is the capacitance; and S 1 and S 2 are the switching tubes.
The mechanical equation for a DC generator is: T e = P e ɘ Τ where D and J are the damping coefficient and rotational inertia of the DC motor; T e and T m are the electromagnetic torque and mechanical torque of the DC motor; P e is the electromagnetic power of the DC motor; and ɘ 0 and ɘ are rated angular velocity and actual angular velocity of the rotor.
The electric potential balance equation is: where E and U are armature-induced electromotive force and machine terminal voltage; R a and I a are armature equivalent resistance and armature current; Ȱ is magnetic flux per pole; and C T is torque coefficient.

Improved virtual DC motor control.
To keep the bus voltage of the microgrid more stable, the Bi-DC/DC converter of the energy storage unit is researched, and a virtual DC motor link and low-pass filtering link are added based on the dual closed-loop control.Figure 3 depicts the specific process of its control chain.The modified VDCM control strategy is composed of DC bus voltage regulation, virtual DC generator control link, and current regulation, and the bus voltage reference is adjusted by adjusting the bus voltage reference to obtain the mechanical power, which is then modulated by PWM for the necessary control signals of the converter after the virtual DC generator link and current regulation.This control process makes the energy storage converter port show the inertia and damping properties, and the low pass filter is introduced into the DC bus voltage regulating system to improve the stability of the DC busbar.where U ref is DC bus reference voltage; u dc and i dc are bus voltage and current; k B is sag coefficient; PI u and PI i are voltage PI controller and current PI controller; οP m is power deviation; οT m is the torque deviation; οɘ is the angular velocity deviation; J and D are inertia coefficients and damping coefficients; E is the induction electromotive force; R a and I a are the armature resistance and armature current; G LPF is the low-pass filtering link, and its expression is: G LPF = ɘ s+ɘ .

DC microgrid mode of operation
To make the power distribution of the energy storage unit more reasonable, the whole DC microgrid system is divided into six modes, and the working status of the PV and ESUs in each of its modes is shown in Table 1.
Among them, Modes 1, 2, and 3 are in the case of sunny days with sufficient light, at this point P pv >P cpl , and the ESUs need to be charged to absorb the surplus power; Modes 4, 5, and 6 are in the period of weak light such as cloudy, rainy, and night, at this point P pv <P cpl , and the ESUs need to be discharged to replenish the power deficit.

Simulation results and analysis
To verify the validity of the proposed coordinated control strategy for the DC microgrid, an islanded DC microgrid simulation model is constructed in simulation software.The simulation model is composed of a group of photovoltaic (PV) power generation units, two groups of energy storage units, and constant power loads.The maximum output power of the PV power generation unit under standard conditions (T=25°C, light intensity of 300 W m 2 Τ ) is 4.65 kW; we set the reference value of DC bus voltage as 750 V; the terminal voltage of the energy storage unit is 700 V, and the capacity of the ESUs is 5.4 Ah; we set the maximum value of SOC for ESUs to 90 percent and the minimum value to 10 percent.Finally, we set the power of the constant power load to 2 kW.Since operating Mode 6 needs to borrow power or remove load to maintain the bus voltage stability, it is not simulated here.
Case 1: The simulation of Mode 1 is shown in Figure 4. Setting the PV power to fluctuate around 300 W m 2 Τ , the original value of SOC of ESU 1 is 65%, and the original value of SOC of ESU 2 is 60%.From the figure, it can be seen that photovoltaic output power is always more than that of the constant load; the bus is kept at 750 V.Both groups of batteries are in the charging state; when ESU 1 reaches the charging at the upper limit, it enters into the shutdown state, and ESU 2 continues to charge.At this time, the microgrid system enters into Mode 2.

Conclusion
This paper puts forward a coordinated control strategy for DC microgrids based on the improved VDCM control, in which firstly, the control strategy of the ESUs is improved to enhanced by adding a VDCM link and low-pass filtering link, effectively improving DC bus voltage stability; and secondly, according to the SOC value of the ESUs and the state of each micro-source in the system, the operational status of the system is consist of six modes, the control strategy of the corresponding unit in each operation mode is determined, bringing the system bus voltage close to reference point, and the power of the ESUs is distributed proportionally to achieve stable operation of the system.Finally, each operating mode is simulated by simulation software, the simulation is carried out, and the results indicate that the control strategy proposed in this paper can make full use of photovoltaic power generation and maintain a stable DC bus voltage.

Figure 4 .
Figure 4. Simulation of Mode 1.(a) The power of PV; (b) The power of the load; (c) The bus voltage; (d) The SOC of ESUs.

Figure 5 .Figure 6 .
Figure 5. Simulation of Mode 2. (a) The power of PV; (b) The power of the load; (c) The bus voltage; (d) The SOC of ESUs.

Figure 7 .Figure 8 .
Figure 7. Simulation of Mode 4. (a) The power of PV; (b) The power of the load; (c) The bus voltage; (d) The SOC of ESUs.

Table 1 .
Control of individual units in different modes.