In recent years, China's rapid economic development, the scale of the power grid is constantly expanding, but due to the development of the power grid in rural areas did not keep pace with the speed of economic development, so that the construction of low-voltage distribution networks in rural areas lag behind the speed of load growth. At present, many rural distribution substation capacity is small, 10 k V trunk lines, branch lines and low-voltage trunk lines, connecting line cross-section are too small, and is located in the power supply radius exceeds the standard desk area. As the load of rural residents grows, although measures have been taken to replace large-capacity transformers, the low voltage phenomenon on the user side is still prominent due to the constraints of the line choke point, especially in remote mountainous areas, where households are dispersed and 10k V lines are far away from sporadic users, and where investment in renovation is too large.

In recent years, China's rapid economic development, the scale of the power grid is constantly expanding, but due to the development of the power grid in rural areas did not keep pace with the speed of economic development, so that the construction of low-voltage distribution networks in rural areas lag behind the speed of load growth. At present, many rural distribution substation capacity is small, 10 k V trunk lines, branch lines and low-voltage trunk lines, connecting line cross-section are too small, and is located in the power supply radius exceeds the standard desk area. As the load of rural residents grows, although measures have been taken to replace large-capacity transformers, the low voltage phenomenon on the user side is still prominent due to the constraints of the line choke point, especially in remote mountainous areas, where households are dispersed and 10k V lines are far away from sporadic users, and where investment in renovation is too large.

In recent years, China's rapid economic development, the scale of the power grid is constantly expanding, but due to the development of the power grid in rural areas did not keep pace with the speed of economic development, so that the construction of low-voltage distribution networks in rural areas lag behind the speed of load growth. At present, many rural distribution substation capacity is small, 10 k V trunk lines, branch lines and low-voltage trunk lines, connecting line cross-section are too small, and is located in the power supply radius exceeds the standard desk area. As the load of rural residents grows, although measures have been taken to replace large-capacity transformers, the low voltage phenomenon on the user side is still prominent due to the constraints of the line choke point, especially in remote mountainous areas, where households are dispersed and 10k V lines are far away from sporadic users, and where investment in renovation is too large.

In order to solve the problem of low voltage in rural distribution networks, the State Grid carried out remediation of low voltage in 2010 and 2014. Generally, the basic methods used by power grid companies to manage low voltage are replacing large-capacity power transformers, building new substations, expanding the conductor radius of transmission lines in distribution grids, and dividing into station areas, etc. Although these methods can solve the problem of low voltage at the end of distribution grids, they are not specific enough to solve the problem. Although these methods can solve the low voltage problem at the end of the distribution network, but the specific implementation, the human, material and financial resources are relatively large, and the construction cycle is long. The general management methods adopted include the installation of reactive power compensation devices or voltage regulators on the lines. Literature[1]A simulation model for reactive power compensation in distribution networks is built on MATLAB/Simulink and the feasibility of the model is verified. Literature[2]The voltage regulation effect of different LV management methods was tested at the end of the distribution network, while active and reactive compensation combined with voltage regulators were proposed for the management of LV. Literature[3]Based on power electronics technology, the low voltage problem of low voltage distribution network is managed. Currently, some researchers start from the root of the problem by connecting distributed power sources, such as photovoltaic wind power, to the end of the distribution network as a way to solve the problem of low voltage. For example, the literature[4]Combined with the construction of distributed photovoltaic, distributed photovoltaic grid-connection is proposed to solve the phenomenon of low voltage in rural distribution networks. Literature[5]A technical solution for low voltage management based on optical storage is proposed. Literature[6]Accessing distributed photovoltaic in the distribution network to manage low voltage is established to minimize the total voltage deviation in the distribution network as an objective function, and particle swarm algorithm is used to find the optimal location of photovoltaic access to the distribution network.

This paper aims to solve the problem of low voltage in rural areas, especially for remote mountainous areas, where the transformation is difficult and the investment is large. The intelligent energy storage low voltage management system developed in this paper combines photovoltaic and energy storage, based on power electronics technology, and designs relevant circuits to connect with the end distribution grid, while the operating status of the system can be remotely understood and controlled through cell phone APP. The intelligent energy storage low voltage management system mainly contains three parts: the DC system, the AC system and the control system, and the schematic diagram of the system is shown in Figure 1.

In the DC system part, it mainly consists of photovoltaic (PV), electromagnetic compatibility (electromagnetic compatiblity, EMC) filter, DC-DC converter circuit and battery pack, as shown in Fig. 2 for the DC part.

In the DC system, there are 2 sets of PVs connected to the DC system of the system, and using the PVs as a starting point, the destination of the power emitted from the PVs can be categorized into 2 cases.

Scenario 1: When the distribution grid has a low voltage state, the DC-AC circuit is in inverter state, and the power from PV is sent to the distribution grid and users through DC-AC inverter, and if there is a surplus of the power provided, it is then charged to the battery pack through the DC-DC converter circuit; if the power provided by the PV is insufficient, the battery pack is charged to the distribution grid and users through the DC-DC converter circuit, DC-AC inverter circuit, and the battery pack is charged to the distribution grid and users through the DC-AC converter circuit, and the battery pack is charged to the distribution grid and users through the DC-AC converter circuit, in turn. distribution grid and users.

Scenario 2: When the voltage of the distribution grid is in the normal range, the power from the PV is used to charge the battery pack through two DC-DC converter circuits. If the PV does not provide enough power and the grid tariff is in a low price, the AC-DC circuit is in a rectified state, and the power from the distribution grid is charged by the AC-DC rectifier circuit, and then by the DC-DC step-down circuit to charge the battery pack.

In the AC system part, as shown in Fig. 2, the AC part is mainly composed of DC-AC converter circuits, voltage and current sensors, EMC filters, ground fault current leakage protectors and smart meters.

In the AC part, if the intelligent energy storage LVM system is considered as a power source, it can be divided into 2 cases.

Situation 1: When the power issued by the intelligent energy storage type low voltage management system is greater than the user load, at this time, the intelligent energy storage type low voltage management system also provides a certain amount of power to the distribution network, the power flow of the smart meter for the distribution network, which can alleviate the load state of the distribution network to lift the voltage of the distribution network, and at the same time, the part of the flow to the distribution network can obtain a certain amount of revenue.

Situation 2: When the power issued by the intelligent energy storage LVM system is less than the user load, at this time, the intelligent energy storage LVM system only supplies power to the user load, and the power flow of the smart meter is directed to the user load, which can lift the voltage of the user terminal, and alleviate the load state of the distribution network to a certain extent.

In the event of a power failure in the distribution network, the intelligent energy storage LVM system can also be used as an emergency power supply (EPS) power source to supply power for a period of time only to the important user loads.

When the intelligent energy storage type low voltage management system is used as a load, at this time, the user terminal voltage should be within the normal range, and both the PV and the distribution grid can charge the battery pack. The role of the AC system part of the sensor is mainly to detect the state of the voltage and current, which is convenient for the system to assess its state and avoid the intelligent energy storage low voltage management system being in an island state.

In the control system part, it mainly includes PV converter control unit, main control module, battery management system (BMS) control module and battery equalization module.

Among them, the main role of the PV converter control unit is to collect the voltage and current signals of the PV output, the voltage and current signals of the DC bus, and the voltage signals of the AC side, evaluate the status on the line according to the collected signals, and control the opening and closing of the DC-DC converter circuit, the DC-AC converter circuit, the relay, and the ground-fault current leakage protector, so as to protect the circuits of the intelligent energy-storage type LVRMS Safety.

The main control module as the main control center of the intelligent energy storage type low voltage management system, equivalent to the human brain, is the signal processing center and send out the center, and its basic function diagram is shown in Figure 3. The user interacts with the operating system and then controls the microcontroller unit (MCU), which sends out corresponding output signals to control the BMS system and the metal-oxide-semiconductor (metal oxide semiconductor, MOS) tubes, and at the same time, the MCU sends out feedback signals by detecting the circuitry and further adjusts the output signals, making the whole system safe and secure. At the same time, the MCU further adjusts the output signal by detecting the feedback signal from the circuit, so that the whole system operates safely and efficiently. Users can not only operate the modules in the energy management system and understand the status information of the equipment through the LCD screen embedded in the intelligent energy storage low voltage management system, but also access to the Internet through Wi Fi or Ethernet, and through the Internet, they can understand the status of the modules in the intelligent energy storage low voltage management system in real time on the cell phone APP, and control the actions of each module, so as to realize remote operation and information interaction. and information interaction.

The main function of the BMS control module is to manage each cell in the battery pack individually, equalize the state of charge (SOC) of each cell, and estimate the state of health (SOH) and state of power (SOP) of each cell, and work with the equalization module to maximize the efficiency of the whole battery pack. Maximize the energy supply efficiency of the whole battery pack; at the same time, the BMS control system can also carry out human-computer interaction, through the liquid crystal display (LCD) screen to understand the state of each battery and the efficiency of the whole battery pack, but also through the interactive interface of the LCD screen to control the specific power of charging and discharging of each battery, and its block diagram is shown in Figure 4. The block diagram is shown in Figure 4.

The intelligent energy storage LVM system developed in this paper is connected to the distribution network in parallel, and in order to simplify the analysis, the intelligent energy storage LVM system can be simplified as a voltage source connected in parallel in the circuit and considered as a compensator. Without adding a compensator, its circuit schematic can be simplified as shown in Fig. 5.

In this case, the steel-core aluminum strand of the distribution network transmission line type LGJ300/25 has a resistivity ρ = 1.3 Ω/km, the length of the line d = 1.54 km, and the total resistance of the line is

Voltage at the outlet of the distribution transformer V_oc= 240 V, user load resistance R_load= 10Ω, then the line current can be calculated to obtain

The voltage at the user side is

The power of the user is

The line losses are

The line loss rate is

With the addition of the compensator, the circuit schematic is shown in Figure 6.

where the output voltage of the compensator V_com= 205 V and the outlet current of the distribution transformer is

The output power of the distribution transformer can be calculated as

The power of the user is

From Eq. (8) and Eq. (9), we can derive the power required to be output by the compensator as

The compensation ratio of the compensator can be calculated from Eq. (9) and Eq. (10) as

The ratio of equipment compensation power to grid compensation power is

Then the loss on the line is

Where: 0.08 is the conversion efficiency of the battery pack and 30 W is the standby power consumption.

The line loss rate is

Then from equation (5) and equation (13), the line loss reduction rate can be obtained as

This paper combines photovoltaic and energy storage devices, and through power electronic circuits connected to the distribution network, realizing the load is large when the voltage is low, the energy storage device to the distribution network power output, which will be the end of the distribution network voltage lift; in the load is small when the voltage is high, the distribution network to the energy storage device for charging, which will be the end of the distribution network voltage is reduced. At present, the intelligent energy storage low voltage management system developed in this paper has been applied to more than a dozen users, at the same time from the 10 k V line more than 2 km station transformation requires nearly one million stations as the application of the site, the system is installed to save the investment of more than 40%, and comprehensively reduce the line loss of more than 30%. At the same time this paper on the low voltage problem for a more effective solution, especially remote mountainous areas, transformation difficulties and large investment in the region, a comprehensive and effective cure, while the new energy access, energy storage technology, microgrid technology, lithium battery management system and lithium battery repair and other areas of technological development has a certain amount of theoretical and technological reserves.