Modeling and Control Strategy for Multiterminal Flexible DC Distribution Network with Echelon Utilization Power Battery

With the integration of distributed renewable energy to the distribution network and the development of multiterminal ﬂexible DC transmission technology, multiterminal ﬂexible DC distribution network has broad application prospects. At the same time, with the rapid development of new energy vehicles, the echelon utilization of power battery has become a research hotspot. By analyzing the characteristics of ﬂexible DC distribution network and echelon utilization battery, the structure and control strategy of modular multilevel converter (MMC), DC solid-state transformer, photovoltaic power generation, wind power generation, and echelon utilization battery energy storage system are established, respectively, in this paper. To achieve a DC network connection of various types of power supply and load, this paper proposes a starting method of multiterminal ﬂexible DC distribution network and a cooperative control strategy of the wind-solar-storage system. A six-terminal ring-shape DC distribution network model is built in real-time digital simulation (RTDS) platform. The simulation results show that the modeling methods and control strategies of each component in the real-time simulation model meet the operation requirements of the multiterminal ﬂexible DC distribution network, which provides a reference for the construction and research of the ﬂexible DC distribution network.


Introduction
With the development of renewable energy technology and energy storage technology, modern distribution network will contain more and more distributed power and energy storage. e common distributed power sources include photovoltaic cells, fuel cells, wind turbines, and gas turbines, and the power generated by these power sources is DC or can be converted into DC after simple rectification. At the same time, the modern load situation has changed. More and more loads need to use DC power supply mode, such as LCD TV, LED lighting, electric vehicle, personal computer, and mobile phone; the development of DC power grid can greatly reduce the flow change link, reduce the load access cost, improve the power conversion efficiency and power quality, improve the energy utilization efficiency, and fully demonstrate the economic benefits and utilization value of distributed energy. With the increase in new energy generation rate, DC distribution will play an increasingly important role in the future power grid [1].
At present, the research on the flexible DC distribution network system is in the experimental exploration stage at home and abroad [2,3]. For the simulation design of DC distribution network, the paper [4] based on the demonstration project of Shenzhen MMC-DCDS proposes the starting and control strategy for the flexible DC distribution network, which is based on the system operation characteristics. e document [5] proposes the protection control system design scheme and corresponding control protection strategy of the converter station in the DC distribution network, but neither of them involves the research of the control strategy of the distributed power supply. e paper [6] studies the control and protection strategy of DC distribution network and builds the offline simulation model of the flexible DC distribution system based on MATLAB, which is used to verify the correctness of control strategy and the reliability of protection scheme, but offline simulation is not suitable for simulating high switching frequency power electronic devices. e papers [2,7] study and analyze the modeling and real-time simulation technology of the flexible DC distribution network with multiterminal, in which the real-time simulation model of DC distribution network based on the real-time Lab (RT-Lab) is established. e above research studies that involve the modeling of key equipment of the DC distribution system with wind-solar-storage and the coordination control strategy of the system are relatively few.
Based on the analysis of the key equipment of DC distribution network (including distributed generation, DC transformer, converter, and other components), this paper establishes a variety of load types of DC distribution network, such as modular multilevel converter, echelon utilization battery storage system, wind power generation device, photovoltaic power generation device, and synchronous motor, which are based on the control modes of vdc-q, P-Q, VF, and so on, so as to realize the DC network of multiple types of power supply and load. Based on the RTDS simulation platform, a six-terminal "ring" DC distribution network model is built to study the start-up and multisource coordinated control strategy of multiterminal DC distribution network. Finally, the simulation validation is carried out.

Modeling of Components in Flexible DC
Distribution System e multiterminal flexible DC distribution system is mainly composed of converter, DC transformer, distributed power supply (such as photovoltaic power generation system, wind power generation system, and energy storage device), and DC load.

Modular Multilevel
Converter. At present, there are three kinds of voltage source converters used in flexible DC transmission projects: two-level voltage source converter (VSC), three-level VSC, and modular multilevel converter (MMC) [8]. MMC converter has the advantages of easy expansion, small harmonic distortion, low switching loss, no commutation failure, and strong fault handling ability [9][10][11][12]. It has been widely used in recent years. e main circuit topology of three-phase MMC is shown in Figure 1(a). Each phase contains two arms of upper and lower bridge, and there are six bridge arms in all three phases. Each bridge arm is composed of N submodules (SM) and a circulation reactor in series. e structure of SM is shown in Figure 1(b), which consists of a half bridge with switching element IGBT and a DC energy storage capacitor.
MMC modeling is mainly divided into three modules: MMC control module, valve module, and main circuit. e MMC control module is mainly responsible for the active and reactive power control of the converter and then outputs the voltage amplitude and phase of the modulated wave corresponding to the power instruction value [13,14]. e valve module uses the number of submodule corresponding to the former modulation waveform to control the state of each submodule by a set of optimization algorithm so as to realize commutation. In addition, the control of MMC converter also includes some auxiliary control, such as startup, phase-locked, internal circulation suppression, and negative sequence voltage suppression. e control strategy structure of MMC is shown in Figure 2.

DC Solid-State
Transformer. DC solid-state transformer (DCSST) based on power electronics technology can realize the matching of different voltage levels in DC distribution network. In order to improve the power density, a dualactive-bridge (DAB) structure can be used for the applications requiring bidirectional power transmission. DCSST is mainly composed of N identical DAB converters; each DAB converter consists of two full Bridges and one highfrequency isolation transformer, as shown in Figure 3. N DAB converters are connected in series at the high voltage end to connect with the high voltage DC bus and connected in parallel at the low voltage end to connect with the low voltage DC bus so as to increase the voltage level of the high voltage side by n times and the current level of the low voltage end by n times.
DAB in DCSST is controlled by phase shift, and energy transmission is realized by the size of phase shift between two cell bridges. e control block diagram of DAB with constant voltage and low voltage is shown in Figure 4. Among them, V * sec and V sec represent the standard and measured values of the DC voltage on the secondary side of DAB, respectively; δ 1 represents the voltage phase angle at the midpoint of the bridge arm on the primary side; δ 2 represents the voltage phase angle at the midpoint of the bridge arm on the secondary side; and ∆δ is the phase angle difference between two commutator bridge elements.

Photovoltaic Power Generation Device.
Photovoltaic power generation device is a kind of power generation device that converts light energy into electric energy. e photovoltaic power generation in the flexible DC distribution system is mainly composed of photovoltaic panels and Boost links, as shown in Figure 5.
e output voltage and current of photovoltaic panels change with the change of light intensity and cell junction temperature, which has strong nonlinear characteristics, and there is a maximum power output point under a specific working condition. When the light intensity and the cell junction temperature change, the output voltage and output current of the photovoltaic array will also change, so the output power will also change. In order to make full use of solar energy, it is necessary to make the working point of the photovoltaic cell fall to the maximum power point, that is, maximum power point tracking (MPPT) control under the circumstances of light intensity and temperature change [15]. e MPPT algorithm was used to calculate the voltage V * pv of the maximum power point under the corresponding lighting and temperature. e control block diagram of the photovoltaic power generation device is shown in Figure 6. V pv represents the DC voltage output by the photovoltaic 2 Complexity

Wind Power Generation Device.
Wind power generation equipment mainly includes wind turbine, gearbox, generator, and converter. e common wind turbines are mainly doubly fed wind turbines and permanent magnet synchronous generators (PMSGs). PMSG has no rotor winding, no excitation power supply, no collector ring, and carbon brush, which simplifies the structure and improves the conversion efficiency. e structure of PMSG in the flexible DC distribution system is shown in Figure 7. PMSG is a full power converter. Under different wind speeds, the maximum power output speed of the fan is unique. erefore, the maximum mechanical power can be obtained by adjusting the fan speed; that is, MPPT control is adopted to improve the energy utilization rate of the system. e control strategy of PMSG is shown in Figure 8. Among them, ω * and ω, respectively, represent the reference value and measured value of wind turbine speed, i sa , i sb , and i sc , respectively, represent the three-phase current on the generator side, u sa , u sb , and u sc , respectively, represent the converter three-phase reference voltage, ω e represents the angular speed of synchronous motor, L s represents the stator inductance, Ψ f represents the rotor permanent magnet flux, i sd * and i sd represent D-axis reference current and actual current, respectively, i sq * and i sq represent q-axis reference current and actual current, respectively, u sd * and u sd represent d-axis reference voltage and actual voltage, respectively, and u sq * and u sq represent q-axis reference voltage and actual voltage, respectively [17].

Energy Storage of Echelon Utilization Battery.
At present, the commonly used energy storage device is lithium battery, which depends on the concentration difference of Li+ to complete the charge and discharge [18]. e architecture model of echelon utilization battery energy storage in the flexible DC distribution system is shown in Figure 9. In the echelon battery energy storage system, the voltage of each battery pack is inconsistent, so the DC/DC converter is needed to increase or decrease the voltage, and then the energy storage system can be connected to the grid. e DC/DC converter in this architecture adopts multichannel interleaving technology, and each battery pack can be equipped with a DC/DC branch to realize the independent charge and discharge, thus reducing the short board effect and safety problems caused by the poor consistency of battery pack in echelon utilization [19]. e working principle of DC/DC converter in the energy storage system is as follows: when discharging, the buck circuit IGBT turns off, the Boost circuit IGBT works, the DC/DC circuit acts as the Boost circuit, and the current flows from the battery to the load or grid; when charging, IGBT of Boost circuit is off, IGBT of buck circuit works, DC/DC circuit is buck circuit, and current flows from power supply to battery. e operation mode that the energy storage system is connected to grid is constant power control mode [20]. e energy storage converter sends the received output power command to DC/DC converter. e controller of DC/DC converter obtains the SOC of each branch battery pack and sends power command to each DC/DC branch according to the SOC of each branch battery pack. e DC/DC branch power system realizes the command tracking accurately through the power closed-loop control [21].

Modeling of Flexible DC Distribution
System. e sixterminal "ring" flexible DC distribution system is shown in Figure 10 [22][23][24][25]. Terminal 1 and terminal 3 are connected to 10 kV AC distribution network, terminal 2 is connected to AC load, terminal 4 is connected to photovoltaic power generation and energy storage device, terminal 5 is connected to 0.38 kV AC microgrid composed of load and synchronous generator, and terminal 6 is connected to DC load, wind power, and energy storage device. Considering the operation characteristics of each terminal, MMC converter is used for terminals 1, 2, 3, and 5, while DC transformer is used for terminals 4 and 6. e load at terminal 2 is AC resistive load, and the load at terminal 6 is DC load for the plant. e parameters and operation mode of each terminal are shown in Table 1 [26]. suppress the phenomenon of overvoltage and overcurrent. e goal of starting control is to make the DC voltage of the flexible DC system rise quickly to the normal working voltage by means of control methods and auxiliary measures but cannot produce the excessive charging current and voltage overshoot phenomenon.

Control Strategy of
A feasible scheme is to connect the starting resistor in series in the charging loop at start-up, charge the DC capacity through the AC side system voltage, and remove the starting resistor at the end of start-up to reduce the loss, as shown in Figure 11. e whole starting process of the converter station is divided into two areas: uncontrollable current and controllable current, as shown in Figure 12. In the area where the current is not controllable, the converter station realizes current limit through series starting resistor. In the controlled area, the converter station realizes current limiting by controlling the limiting link of the outer loop controller of the system. e starting control flow of a six-terminal "ring" DC distribution system is shown in Figure 13.

Multisource Coordinated Control Strategy.
Multisource coordination is a kind of power supply method which adopts multiple types of distributed power supply and utilizes the complementary characteristics of different distributed power supplies to improve the output characteristics of distributed power supply [27,28]. For example, solar energy and wind energy have strong intermittency and fluctuation, respectively, so the output power of solar power generation device and wind power generation device also has intermittency and fluctuation. is kind of distributed power generation device with unstable output will affect the stable and reliable operation of the distribution system. erefore, photovoltaic power generation and wind power generation devices equipped with an appropriate amount of the energy storage system can improve the output characteristics of distributed power generation, making photovoltaic power generation and wind power generation into controllable and stable power supply.

Simulation Results and Analysis
Based on the real-time digital simulation platform RTDs, a six-terminal "ring" DC distribution system model is built. RTDS has an MMC valve model based on FPGA, which is conducive to simulate the MMC valve with a large number of switching elements and the internal pressure balancing problem of the analog valve. In this six-terminal model, the

e Simulation of Start-Up.
When the system starts up, the auxiliary switches at terminal 1, 2, 3, and 5 are closed to charge the MMC of the whole DC distribution network; when the DC voltage reaches 0.73 p.u. (the reference value is 10 kV), the converter station at the first end unlocks and continues to charge the DC network. Until the DC voltage reaches 1 p.u., the main switch at the first end is closed and the other ends are unlocked so that the system enters the noload operation stage. During system start-up, the waveform of DC voltage and charging current at terminal 1 of constant DC voltage is shown in Figure 14. According to the simulation waveform, it takes about 40°s for terminal 1 voltage to rise from 0 p.u. to 1 p.u., and the maximum charging current peak is 0.04 p.u. (the reference value is 1 kA).
When the terminal 2 that is constant frequency voltage (VF) terminal is merged into the DC system, the main switch at the terminal 2 is closed and the AC side voltage is raised according to the established slope to realize the grid-connected control of the two-terminal converter station. e simulation waveforms of at terminal 1 and 2 are shown in Figure 15. e simulation results show that the voltage of terminal 1 is basically stable at 1 p.u., and the DC power of terminal 1 is 0∼−1 p.u. (the reference value is 10 MW). e effective value of the terminal 2 AC voltage varied from 0 to 0.99 p.u. (AC voltage reference value was 10 kV), and the terminal 2 DC power varied from 0 to 0.94 p.u., which is lasting for 5 s.

Power
Step. Assume that the operation state of the DC distribution network supplied by this double-ended power source is as follows: (1) terminal 1 is the constant voltage terminal; (2) terminal 5 is AC microgrid that includes 3 MW load and 1 MW synchronous generator, which needs to absorb 2 MW active power from the grid; (3) terminal 3 is the constant power terminal, and the output power is 0 MW; (4) terminal 4 is the optical storage access terminal, which is set to transmit 3 MW active power to the power grid; (5) terminal 6 is connected to wind power and energy storage device; and (6) terminal 2 is in the no-load operation stage at this time.

Power
Step of Converter Valve. Suppose that at this time, the power of terminal 3 which is the constant power terminal will step from 0 MW to 6 MW and terminal 6    Figure 11: Structure of AC side converter station with start-up circuit.

Power
Step of Load. Based on the above experiments, the load switching experiment is carried out at terminal 5, and the 3 MW load carried by the terminal 5 is switched off and put into operation within 10 s. e simulation waveform is shown in Figure 17. According to the simulation waveform, when the load switching experiment is carried out at terminal 5, each terminal can quickly return to the stable operation state.

Multisource Coordinated Control Strategy.
When the DC distribution system starts the coordinate control strategy of photovoltaic power generation, energy storage, wind power generation, and energy storage, the terminal 4 and terminal 6 are a stable power source for the whole system. is paper sets the power transmitted to the system at terminal 4 as 3 MW and the temperature as 25°C. e power output of the photovoltaic device changes with the illumination intensity. e output power waveform of the photovoltaic and energy storage at terminal 4 is shown in Figure 18. It can be seen from Figure 18 that the energy storage device can quickly respond to the power change of the photovoltaic device so that the output power of the terminal 4 is stable.
is paper sets the power transmitted to the system at terminal 6 as 4 MW. e output power of the wind power generation device changes with the wind speed and Access to PV, wind turbine, and energy storage direction. e output power waveform of the wind power and energy storage at terminal 6 is shown in Figure 19. As can be seen, the output power of the energy storage device can quickly track the output power of the wind power generation device, thus ensuring that the output power at terminal 6 is constant.

Conclusion
e DC distribution system is the key link to ensure power quality, improve operation efficiency, and innovate service content. e models of modular multilevel converter, DC solid-state transformer, photovoltaic power generation device, wind power generation device, and energy storage device established in this paper are consistent with the basic characteristics of the actual device. e start-up strategy can make the multiterminal DC distribution system put into operation quickly and smoothly. Multisource coordinated control strategy can greatly improve the economic benefits of the DC distribution system operation and make the system operate stably. erefore, the structure and control strategy of each component as well as the system start-up and multisource coordinated control strategy adopted in this paper are correct and effective, which lays a solid foundation for the subsequent control and protection research of the complex DC distribution system. e theoretical and technical problems of planning and design, dispatching control, and relay protection in the DC distribution network need to be further studied.

Data Availability
e data used to support the findings of this study are included within the article.