As an important evaluation index of power quality, voltage is the basic condition to ensure power supply service, which is related to the construction of harmonious power supply and consumption relationship and the ability to serve social and economic development. In recent years, China’s social and economic development has been sustained and steady, and the consumption level of urban and rural residents has been continuously improved. Especially, inspired by a series of policies such as "home appliances going to the countryside", the demand for rural electricity has maintained a rapid growth trend, and the construction and transformation of rural distribution networks have lagged behind, resulting in the low supply voltage in some areas (the voltage value is lower than the lower voltage limit stipulated by national standards, referred to as "low voltage"), which can no longer meet the normal production and living electricity demand of rural residents.
1. Analysis of the causes of low voltage
1.1 Classification of Low Voltage Features
According to the time characteristics of low voltage occurrence and duration, it can be roughly divided into three categories: long-term, seasonal and short-term.
1. Long-term low voltage refers to the phenomenon that the user’s low voltage lasts for 3 months or the daily load peak low voltage lasts for more than 6 months;
2. Seasonal low voltage refers to the phenomenon of low voltage with periodic law that occurs during the summer and winter, spring irrigation and autumn harvest, festivals, baking tea and making cigarettes;
3. Short-term low voltage mainly refers to the periodic irregular low voltage phenomenon that is not long-term and seasonal and caused by the temporary hanging load of rural residents or the electricity load of buildings.
1.2 Distribution of low voltage occurrence periods
1) During centralized irrigation and drainage in rural areas. In January-March, June-September and November-December every year, the agricultural irrigation and drainage load is relatively concentrated, and the electricity consumption is large. Some public distribution transformers with irrigation and drainage load are fully loaded and overloaded for a short time, resulting in low power supply voltage at the end load of low-voltage lines.
2) Peak hours of daily electricity consumption. Due to the rapid development of rural economy, the living standard of farmers has been gradually improved, the number of household appliances has increased rapidly, the power load in rural radio distribution areas has increased rapidly, and the daily electricity consumption in rural areas is relatively concentrated during peak hours.
1.3 Management reasons for low voltage
1) Extensive operation and maintenance management of power supply and distribution facilities. The ledger of medium and low voltage power supply equipment is not perfect or updated in time, and the basic information of grid and equipment is not perfect. The information of marketing, distribution and dispatching data resources cannot be fully shared. There is no effective joint dispatching management mechanism between substations, lines, distribution transformers (distribution transformers for short) and low-voltage users. The tap position of distribution transformers and reactive power compensation equipment are not adjusted in time according to seasonal load conditions and peak and valley conditions of power consumption. Equipment managers are not clear, do not understand and master the equipment operation status and compensation effect, and find, deal with and replace damaged or defective equipment in time.
2) The marketing management in some areas is not fine. In some areas, the management of rural users’ installation and power connection is relatively loose, and there is a phenomenon that a large concentrated load is connected to public distribution transformers or that the production load of rural residents is reported to be small and used to be large, resulting in overload and low voltage of distribution transformers; The administrators in the distribution station area do not distribute the single-phase users in Taiwan area to access the A, B and C phases evenly, and a large number of rural power loads are concentrated in busy farming seasons, such as spring ploughing and autumn harvest, irrigation and drainage, which leads to the serious imbalance of the three phases of power loads on the low-voltage side of the distribution transformer, resulting in low voltage for users in the middle and late stages of heavy load.
3) The voltage monitoring of medium and low voltage distribution network is not comprehensive. According to the general configuration requirements of voltage monitoring points, rural power grid is equipped with one voltage monitoring point for every 100 distribution transformers, and urban power grid is equipped with two voltage monitoring points for every 100 distribution transformers. Rural residents have a wide range of users, and the voltage monitoring at the client is not comprehensive; The representativeness of individual voltage monitoring points is not strong, so it is difficult to accurately grasp the real situation of rural voltage quality based on monitoring data; It is difficult to locate the cause of low voltage problem through systematic correlation analysis because of the uneven quality of operation and status data of monitoring of distribution station area and collection of user electricity information.
4) Low voltage demand side management is not in place. The nature of users’ electricity consumption is not fully grasped, and the predictability of load development in Taiwan area is not enough, which leads to overload operation of distribution transformers in Taiwan area during peak load and has not been effectively monitored and handled in time; Insufficient publicity of users’ electricity knowledge, some users’ indoor lines have not been upgraded synchronously according to the actual electricity load growth, and they have been overloaded over the years, the lines are seriously aging, and the voltage is too low, which makes household appliances unable to work normally; Lack of effective peak-shaving measures for seasonal load of similar agricultural products processing; Lack of publicity and guidance for peak-load users, too concentrated load, failure to transfer load in time, resulting in low voltage problems for users.