Abstract: With the rapid development of the social economy, the problem of low voltage at the end of distribution networks is becoming increasingly severe, especially in rural and remote mountainous areas. This paper aims to address the issue of low voltage in distribution networks by developing an intelligent energy storage low-voltage management system that combines photovoltaic (PV) and energy storage. The system is applied in practical engineering to achieve local management of low-voltage problems in distribution networks, reducing investment by more than 40% and comprehensive line losses by more than 30%.
Keywords: low-voltage distribution network; low-voltage governance; photovoltaic; energy storage; governance technology
1. Introduction
In recent years, China’s economy has developed rapidly, and the scale of the power grid has continued to expand. However, due to the development speed of rural power grids not keeping pace with economic growth, the construction speed of rural low-voltage distribution networks lags behind the growth rate of load. Currently, many rural distribution transformers have small capacities, and the cross-sectional areas of 10kV main lines, branch lines, low-voltage main lines, and service lines are too small, often located in substations with excessive power supply radii. With the growth of rural resident load, measures such as replacing large-capacity transformers have been taken. However, constrained by line bottlenecks, especially in remote mountainous areas where households are scattered and 10kV lines are far from scattered users, the investment required for transformation is excessive, resulting in prominent low-voltage phenomena on the user side.
2. Research Status Analysis
To address the low-voltage issue in rural distribution networks, State Grid Corporation of China conducted two campaigns to rectify low-voltage problems in 2010 and 2014. Typically, the basic methods used by power grid companies to manage low voltage include replacing high-capacity power transformers, building new substations, expanding the radius of transmission lines in distribution networks, and dividing substation areas. Although these methods can solve the low-voltage problem at the end of distribution networks, their implementation involves significant manpower, material, and financial resources, with long construction cycles. Commonly adopted governance methods include installing reactive power compensation devices or voltage regulators on lines.
Some researchers have approached the root of the problem by connecting distributed power sources such as photovoltaic and wind power to the end of distribution networks to solve the low-voltage issue. For example, one study proposed integrating distributed photovoltaic grid connection to address low voltage in rural distribution networks. Another study presented a low-voltage management technical solution based on photovoltaic and energy storage. A third study introduced distributed photovoltaic into the distribution network to manage low voltage, establishing an objective function to minimize the total voltage deviation in the distribution network and using the particle swarm algorithm to find the optimal location for photovoltaic connection to the distribution network.
3. Principle Introduction of System Components
This paper aims to solve the problem of low voltage in rural areas, especially in remote mountainous regions with significant transformation difficulties and high investment costs. The intelligent energy storage low-voltage management system developed in this paper combines photovoltaic and energy storage, using power electronic technology as the foundation. It designs related circuits to connect with the end of the distribution network and can remotely monitor and control the system’s operating status via a mobile app. The intelligent energy storage low-voltage management system mainly consists of three parts: the DC system, AC system, and control system.
Table 1: System Components and Their Functions
| Component | Function |
|---|---|
| DC System | Combines photovoltaic, EMC filters, DC-DC converters, and battery packs |
| AC System | Includes DC-AC converters, voltage and current sensors, EMC filters, GFCI, and smart meters |
| Control System | Consists of photovoltaic converter control units, main control modules, BMS control modules, and battery balancing modules |
3.1 DC System
The DC system mainly consists of photovoltaic panels, electromagnetic compatibility (EMC) filters, DC-DC conversion circuits, and battery packs. There are two sets of PV connected to the DC system. When the distribution network experiences low voltage, the photovoltaic-generated electricity is inverted by the DC-AC circuit and supplied to the distribution network and users. If there is excess power, it charges the battery pack through the DC-DC conversion circuit. If the PV-provided electricity is insufficient, the battery pack supplies power to the distribution network and users through the DC-DC conversion circuit and DC-AC inversion circuit.
3.2 AC System
The AC system mainly consists of DC-AC conversion circuits, voltage and current sensors, EMC filters, ground fault current leakage protectors, and smart meters. When the power generated by the intelligent energy storage low-voltage management system is greater than the user load, it also supplies power to the distribution network, reducing its load and increasing its voltage. When the system’s power is less than the user load, it only supplies power to the user load, raising the user-end voltage and alleviating the distribution network’s load state. During power outages, the system can serve as an emergency power supply (EPS) for critical user loads.
3.3 Control System
The control system mainly includes photovoltaic converter control units, a main control module, a battery management system (BMS) control module, and a battery balancing module. The photovoltaic converter control unit collects voltage and current signals from the photovoltaic, DC bus, and AC side, assesses line status based on collected signals, and controls the opening and closing of DC-DC conversion circuits, DC-AC conversion circuits, relays, and GFCI to protect circuit safety.
4. Case Study Analysis
The intelligent energy storage low-voltage management system developed in this paper is connected to the distribution network in parallel. For simplified analysis, the system can be modeled as a voltage source paralleled in the circuit, acting as a compensator.
Table 2: Circuit Parameters and Calculations
| Parameter | Value | Calculation |
|---|---|---|
| Line Resistance (R_line) | 1.3Ω/km × 2 × 1.54km = 4.004Ω | (1) |
| Transformer Outlet Voltage (V_oc) | 240V | |
| Load Resistance (R_load) | 10Ω | |
| Line Current (I_line) | 240V / (4.004Ω + 10Ω) = 17.138A | (2) |
| User-end Voltage (V_load) | 17.138A × 10Ω = 171.38V | (3) |
| User Power (P_load) | 17.138A² × 10Ω = 2.937kW | (4) |
| Line Loss (P_loss_line) | 17.138A² × 4.004Ω = 1.176kW | (5) |
| Line Loss Rate (λ2) | 1.176kW / (240V × 17.138A) = 0.286 | (6) |
After adding the compensator, the compensator’s output voltage (V_com) is 205V, and the distribution transformer’s outlet current (I_1) is 8.741A. The distribution transformer’s output power (P_grid) is 1.792kW, and the user’s power (P’_load) is 4.203kW. The compensator’s required output power (P_com) is 2.411kW, with a compensation ratio (λ) of 0.574. The ratio of compensator power to grid power (λ1) is 1.345. The line loss after compensation (P_loss_line1) is 0.529kW, with a line loss rate (λ3) of 0.126. The line loss reduction rate (λ4) is calculated by 1 – P_loss_line1 / P_loss_line.
5. Conclusion
This paper combines photovoltaic and energy storage devices, connecting them to the distribution network through power electronic circuits. When the load is high and the voltage is low, the energy storage device outputs power to the distribution network, raising the end voltage. When the load is low and the voltage is high, the distribution network charges the energy storage device, lowering the end voltage. The intelligent energy storage low-voltage management system developed in this paper has been applied to over a dozen households and a substation area requiring nearly one million yuan in transformation more than 2km away from a 10kV line. After installation, the system saves over 40% in investment and reduces line losses by over 30%. This paper provides a more effective solution to the low-voltage problem, especially in remote mountainous areas with significant transformation difficulties and high investment costs, achieving comprehensive and effective treatment. It also lays a theoretical and technical foundation for the development of technologies in fields such as new energy integration, energy storage technology, microgrid technology, lithium-ion battery management systems, and lithium-ion battery repair.