Study on Energy Control for Low-Voltage DC Distribution Systems

This paper focuses on the study of energy control for LVDC distribution systems based on maintaining a stable DC bus voltage. The system includes photovoltaic (PV) arrays, energy storage, and AC/DC loads. The Boost converter is used in the PV array to switch freely between MPPT and constant voltage modes. The MPPT control is applied to the PV modules to ensure maximum power output, and the energy storage unit adjusts the output to stabilize the DC bus voltage and maintain system power balance. The implementation of the proposed control strategy is in a MATLAB/Simulink simulation model to verify its correctness and effectiveness. The simulation results show that the control strategy can maintain a stable DC bus voltage, and improve the efficiency of LVDC system. The study provides valuable insights into the development of efficient and reliable energy control strategies for LVDC power distribution systems.


Introduction
In recent years, there has been significant interest in low-voltage direct current (LVDC) distribution systems due to their potential to enhance energy efficiency, minimize losses, and facilitate the incorporation of renewable energy sources.[1] [2].LVDC distribution systems have the following advantages compared to conventional alternating current (AC) systems, including lower conversion losses, improved power quality, and better control of energy flow [3] [4].LVDC systems have various applications, including in residential, commercial, and industrial settings.However, the efficient operation of LVDC systems requires effective energy control strategies to ensure optimal utilization of available resources [5].
One of the critical aspects of energy control in LVDC systems is maintaining the stability of the DC bus voltage [6].The DC bus voltage is the primary source of power in the LVDC system, and any fluctuations in the voltage can affect the operation of the system and cause damage to the connected loads.Therefore, it is essential to develop effective energy control strategies to keep a steady DC bus voltage in LVDC power distribution systems.
Several techniques and strategies have been proposed for keeping a steady DC bus voltage in LVDC power distribution systems, including power electronics, energy storage, and control algorithms [7].Power electronics devices such as DC-DC converters and inverters play a crucial role in managing power flows and converting between different voltage levels.To enhance backup power availability and enhance system stability, energy storage systems like batteries and supercapacitors can be integrated into LVDC distribution systems.
These studies provide valuable insights into the development of efficient and reliable energy control strategies for LVDC power distribution systems, and can be used to complement the findings of the present study.By citing these studies, we can demonstrate the current state of research in the field, and highlight the significance of the proposed energy control method for maintaining a stable DC bus voltage in LVDC power distribution systems.
In this paper, the stable control of the DC bus voltage in LVDC distribution systems is studied.The simulation model is constructed with Matlab/Simulink.The simulation results indicate the effectiveness of the proposed energy control method for maintaining a stable DC bus voltage in LVDC power distribution systems.

Structure of an LVDC distribution system
The structure of a low-voltage DC distribution system typically consists of a power source, such as a renewable energy source or a grid connection, a DC-DC converter, which regulates the voltage level and energy flow, and a load, which consumes the energy.The system may also include energy storage devices, such as batteries, to store excess energy and provide backup power.The components of the system are connected through a DC bus, which carries the energy between the components.The system may also include control and monitoring devices to manage the energy flow and ensure system stability and reliability.The structure of a typical LVDC distribution system is shown in Figure 1, including the PV array, energy storage battery, AC /DC loads.

Converter for the PV unit
The PV array adopts a Boost converter that can freely switch between Maximum Power Point Tracking (MPPT) and constant voltage modes.In order to fully utilize solar energy, MPPT control is generally applied to the PV modules to ensure maximum power output.At this time, the energy storage unit adjusts the output to stabilize the bus voltage of the LVDC power distribution system and maintain the system power balance.When the energy storage is fully charged, the PV modules switch to constant voltage control to regulate the LVDC power distribution system power balance and improve system operating efficiency.Figure 2 shows the PV unit's converter and control block diagram.

Converter for the energy storage unit
The energy storage unit is charged and discharged using a bidirectional Buck/Boost converter, which is a power electronic device that can transfer power bidirectionally between two different voltage levels.
The converter employs a constant voltage control algorithm, which consists of an outer voltage loop and an inner current loop.The outer voltage loop is responsible for stabilizing the DC bus voltage by adjusting the duty cycle of the converter, while the inner current loop controls the charging and discharging current to follow the reference value.The voltage loop compares the actual DC bus voltage with the reference value and generates a control signal to adjust the duty cycle of the converter.Figure 3 shows the energy storage unit's converter and control block diagram.

Converter for the inverter unit
A single-phase full-bridge inverter is a power electronic device that converts DC to AC of desired frequency and voltage.The inverter consists of four switches, which are controlled by a PWM signal to generate the desired AC waveform.To achieve better performance and stability of the inverter, a dual closed-loop control algorithm is used.The outer loop controls the output voltage of the inverter by adjusting the reference voltage, while the inner loop controls the current flowing through the load.Figure 4 shows the inverter unit's converter and control block diagram.
S 1

Simulation analysis
In order to test the validity of the proposed energy control strategy, simulation experiments using Matlab/Simulink software are carried out.The simulation model includes the PV array, energy storage, and loads.The output power of the PV array is affected by weather and other factors, and the excess energy is stored by the energy storage device.The simulation model takes into account the effects of power electronics devices such as DC-DC converters and inverters, as well as energy storage devices such as batteries, in managing power flows and ensuring a stable DC bus voltage.
To observe the dynamic response of the system to load step changes, AC load steps from 2800W to 8400W at t =25 us.DC load remains unchanged.The output power of the AC load and the DC load are shown in Figure 5 and Figure 6, respectively.From the figures, it can be observed that when the AC load steps from 2800W to 8400W at t=25us, the AC output power quickly stabilizes, while the DC power experiences a momentary dip due to the impact of the AC load change but recovers quickly and stabilizes.Figure 7 shows the output power of the PV unit.From Figure 7, it can be observed that the PV unit maintains a constant maximum power output, which indicates that the system is effectively utilizing the power generated by the PV unit.The constant output power of the PV unit is an important factor in ensuring a steady DC bus voltage and meeting the load demand.The results of simulation show that the energy control is effective to manage the power flow and make sure that the PV unit is operating at its maximum power.This is crucial for maximizing the efficiency of the PV unit and minimizing energy losses in the system.8, 9, and 10, respectively.It can be seen in Figure 8 and Figure 10, at t=25 us, the SOC shows a decreasing trend after an initial increase, and the output power changes from negative to positive.The simulation results indicate that the battery is charged before discharging.Furthermore, the output power of the battery changes from negative to positive, which indicates that the battery is initially charged and then starts discharging to supply power to the load.11 is a critical indicator of the effectiveness of the proposed energy control strategy.The constant value of 375V maintained by the DC bus voltage is an important factor in ensuring the stable operation of the LVDC power distribution system.Maintaining a constant DC bus voltage is crucial for the proper functioning of the system, as it ensures that the power supplied to the loads is stable and reliable.The energy control strategy proposed in the study plays a critical role in maintaining the constant DC bus voltage by regulating the output of the energy storage unit.Simulation results indicate that the control strategy is effective in stabilizing DC bus voltage and keeping 375V constant.The simulation results of the dynamic characteristics of maintaining a constant DC bus voltage in a DC power distribution system with load variations show that the system is able to keep a steady DC bus voltage despite changes in the load.The simulation results also highlight the importance of energy storage devices in managing energy flows and ensuring the efficient operation of the system.Overall, the simulation results demonstrate the ability of the DC power distribution system to maintain a stable DC bus voltage in the face of load variations, which is critical for the efficient and reliable operation of the system.Through simulation experiments, it can be found that the proposed energy control strategy can effectively control the energy in the LVDC distribution system and ensure the stability and reliability of the system.

Conclusion
In conclusion, this thesis focuses on the research on the energy control of LVDC power distribution system in order to keep steady DC bus voltage.The proposed energy control strategy employs a combination of power electronics devices, energy storage, and control algorithms to manage energy flows and ensure a stable DC bus voltage.Simulation results show that the proposed energy control method is effective in keeping the steady DC bus voltage in the LVDC distribution system.Furthermore, the study highlights the importance of keeping the steady DC bus voltage in LVDC power distribution systems for improved energy efficiency and the integration of renewable energy sources.Future work will focus on optimizing the energy control strategy, studying the impact of different energy storage devices and power converter topologies, and assessing the impact of the system on the power quality of the grid.

Acknowledgment
The research content of this paper is supported by the scientific and technological project of the State Grid (5400-202219175A-1-1-ZN) Corporation of China.

Figure 1 .
Figure 1.Structure of a typical LVDC distribution system.

Figure 2 .
Figure 2. Topology and control block diagram of the PV unit.

Figure 3 .
Figure 3. Converter and control block diagram of the energy storage unit.

Figure 4 .
Figure 4. Converter and control block diagram of the inverter uint.

Figure 5 .
Figure 5. Output power of the AC load.Figure7shows the output power of the PV unit.From Figure7, it can be observed that the PV unit maintains a constant maximum power output, which indicates that the system is effectively utilizing the power generated by the PV unit.The constant output power of the PV unit is an important factor in ensuring a steady DC bus voltage and meeting the load demand.The results of simulation show that the energy control is effective to manage the power flow and make sure that the PV unit is operating at its maximum power.This is crucial for maximizing the efficiency of the PV unit and minimizing energy losses in the system.

Figure 6 .Figure 7 .
Figure 6.Output power of the DC load.

Figure 8 .
Figure 8. SOC of the battery.
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