Although the DC system of the substation is now equipped with online monitoring equipment, the data can only be monitored locally and cannot be monitored remotely. To meet the requirements of smart grid construction, the monitoring of DC system and battery conditions must be completed in an intelligent and networked manner. The overall goal of this system construction is to establish a consistent long-distance monitoring system for DC equipment in response to the requirements of the construction of a strong smart grid. It aims to standardize the data processing, connection, and control of various DC equipment monitoring devices, complete real-time monitoring and warning functions for DC equipment conditions and critical operating environments, and transmit monitoring data to the equipment condition review hall for centralized monitoring, providing support for the operation of DC equipment.

Key words: battery; long-distance; protective measures

The battery pack is widely used as a direct current power source in the power substation system and plays an extremely important role. There are numerous batteries used in the DC system, and there are often safety hazards such as insufficient capacity, imminent failure, poor contact of connection lines, and abnormal charging and discharging of batteries during operation. Therefore, it is necessary to protect them frequently and the workload is huge. Therefore, whether the network composed of battery packs can operate reliably, stably, continuously, and safely is increasingly being emphasized and valued by people. Due to the large number and wide distribution of power substations, the protection of batteries requires a significant amount of manpower and material resources every year. Additionally, according to the requirements of most battery manufacturers, batteries with float charging should undergo shallow discharge every six months or one year to ensure the battery's lifespan.

1、 The principle of battery

When the battery pack needs protection, a protection command is usually issued in the control room (or actively activated based on the protection situation in the monitoring machine of the DC system). The system adjusts the output voltage of the on-site battery charger according to the voltage of the battery pack and starts discharging. The discharge load of the battery pack is the regular load of the DC system of the substation. At this moment, the battery pack enters the discharge state from float charging. When the battery pack enters the discharge state, the battery inspection host records the voltage and discharge current of the battery pack, together with the voltage and internal resistance of each individual battery in the battery pack. When the voltage value of the battery pack drops to the set value, the discharge process is completed. At this moment, adjust the output voltage and output current of the battery charger to charge the battery pack. The battery pack enters a balanced charging state, and the battery inspection host records the voltage and discharge current of the battery pack, as well as the voltage and internal resistance of each individual battery in the battery pack. When the charging is completed, the battery pack enters the float charging state, and the charging and discharging process is completed. Battery inspection equipment, based on the recorded internal resistance and current of each individual battery during the charging and discharging process, as well as the corresponding voltage of each individual battery, performs curve fitting and calculation through a preset mathematical model to determine the performance of each individual battery, and uploads the results to the control room. After the long-distance active protection of the battery pack is completed, the battery inspection equipment always monitors the internal resistance of each individual battery in the battery pack. The host compares the changes in the internal resistance of each individual battery with the established model and uploads the results to the control room.

2、 Active long-distance protection of DC system in substation

1. The architecture of active long-distance protection system. The active long-distance protection of the DC system in the substation consists of two central parts: the substation measurement and control part and the long-distance monitoring center, which are connected through a local area network. The measurement and control part of the substation mainly includes a switch power module (charger), a power monitoring unit, and a battery monitoring unit. The data communication between the power module and the power monitoring unit, as well as between the power monitoring unit and the battery monitoring unit, is completed through the RS485 communication interface. These devices are located within the substation. The long-distance monitoring center is mainly composed of servers, multiple operator stations, and operator workstations, located in the monitoring center and protection team office.

2. The operating principle of the system. The battery monitoring unit in the substation monitors real-time data such as battery pack current, pack voltage, temperature, cell voltage, and cell resistance. Based on the charge calculation model based on voltage internal resistance, the Kalman filter algorithm is used to estimate the real-time state of charge (SOC), and the data is transmitted in real time to the monitoring center server through the local area network. The monitoring and processing software on the server analyzes the voltage, current, direction, charging and discharging time intervals, and charging and discharging capacity of the battery pack to determine when to perform long-distance online discharge protection on the battery pack. If the battery pack has not undergone useful discharge within the specified time, the server will actively announce an activation command: reduce the output voltage of the power module, the power module will be in standby mode, and the battery pack will supply power to the DC load. The activation command transmission route is: server → LAN → battery monitoring unit → power monitoring unit → power module. During the activation period of the battery, the battery monitoring unit monitors the changes in current, group voltage, temperature, and battery cell voltage in real time, and transmits the data to the monitoring center server in real time. The monitoring and processing software analyzes the discharge capacity in real time, and when it reaches the set value, it actively announces the termination of the activation command. During the activation protection process, if the battery pack voltage, battery cell voltage, or temperature show abnormalities, the activation command will also be automatically announced to be terminated. After receiving the instruction to suspend activation, the power monitoring unit increases the output voltage of the power module, which supplies power to the DC load. The battery pack is in standby mode, and the power module is controlled to perform three-stage charging on the battery pack, namely constant current equalization charging, constant voltage equalization charging, and float charging. During the constant current and constant voltage charging process of the power module, the system actively measures and calculates the charging machine characteristic parameters such as current stability accuracy, voltage stability accuracy, ripple coefficient, etc. of the power module to prevent adverse effects on the battery life caused by unqualified charging machine characteristic parameters.

3、 Long distance active protection technology for substation batteries

1. Proactively monitor online. Online monitoring of various parameters such as voltage, current, internal resistance, temperature, etc. of the battery pack under discharge conditions, analyzing the situation of each battery when it stops through a mathematical model of the battery. Through useful monitoring of the current magnitude, current direction, discharge time interval, and state of charge (SOC) of the battery pack, it is determined which method should be used for long-distance online discharge protection of the battery pack. When the battery is in float charging mode, the voltage of each battery is monitored in real-time, and the difference between the voltage of each battery and the uniform voltage of the battery pack is calculated to determine whether the condition of each individual battery is balanced. Online testing of DC charger characteristic parameters. The primary factors include the voltage stabilization accuracy and ripple voltage coefficient of the charger during float charging and constant voltage current limiting operations; Stable current accuracy and unbalanced charging module current under constant current and voltage limiting operation. These performance indicators of the charging machine are ultimately sent to the CPU of the online monitoring device host for storage and display, and then sent to the monitoring center through the monitoring network for further transmission to the user terminal.

2. Long distance online balance, activated protection. The current standard rule is that valve controlled batteries for new equipment should undergo full capacity verification charging and discharging during inspection. From now on, full capacity verification charging and discharging should be performed every 2-3 years. Valve controlled batteries that have been in operation for 6 years should undergo full capacity verification charging and discharging once a year. If protected according to standards, during the 2-3 year "no protection" period after production, water loss will continuously occur inside the battery, and the electrolyte will remain in concentrated acid until the plates undergo sulfurization. If the battery is in standby mode for a long time or not charged in a timely manner after discharge, the cathode and anode are prone to generate fine crystals of active materials, which affects the reaction process of the charging and discharging electrodes and shortens the lifespan of the battery. Under other long-term float charging conditions, when the voltage of a single battery in the battery pack is unbalanced, such as too high or too low, it can also cause the battery to lose water and undergo sulfation, significantly shortening its lifespan. If there is a voltage difference among the individual cells in the battery pack, it will ultimately lead to the failure of the battery pack. The long-distance active protection system has completed the active balancing of batteries during the "no protection" period, monitoring the voltage of each battery in real time, and calculating the difference between the voltage of each battery and the uniform voltage of the battery pack. If the difference exceeds the set limit, active online balancing will be initiated to discharge batteries with high voltage due to overcharging using high-frequency pulse current to prevent the battery from losing water and reducing its lifespan; Charging batteries with low voltage due to undercharging with high-frequency pulse current can prevent the battery from experiencing a decrease in lifespan due to sulfation, significantly improving the battery's lifespan. Long distance active protection system, actively and regularly providing long-distance online discharge protection for battery packs. During the discharge interval protected by battery regulations, if the battery is always in float charging without effective discharge, the system will actively reduce the output voltage of the charging equipment and supply power to the DC load through the battery pack. At this moment, the power supply of the DC system is mainly based on batteries, and the charging equipment is in standby mode, not supplying current to the outside. When the discharge capacity of the battery reaches the regular level, the system will actively increase the output voltage of the charging equipment to restore it to normal values, and then switch back to the DC system for power supply, with the charging equipment as the main source and the battery pack in standby mode. The entire protection process does not require human intervention and is fully automated through accounting procedures.