The DC system is a crucial component of substations, providing reliable power supply for control, signaling, relay protection, automatic devices, emergency lighting and other systems, and it plays an indispensable role in the reliable operation of substations. In recent years, with the continuous improvement of substation systems, the successive introduction of new equipment, the constant application of new technologies, and the upgrade of automation and digitalization levels, higher requirements have been put forward for the operation, management and maintenance of DC systems.

I. Main Current Maintenance Methods for DC Systems

Discharge Capacity Verification: Conducted manually with a cycle of 1-2 years, with a discharge capacity of 50%-100%*C10. The battery performance is judged by whether the voltage of individual cells reaches the cut-off voltage in advance.

Internal Resistance Measurement: Conducted manually with an annual cycle. The performance of individual cells is judged by the horizontal comparison of the internal resistance of storage batteries.

Voltage Measurement: Conducted manually with a quarterly cycle. The terminal voltage of individual cells under the floating charge state is measured to judge the performance of individual storage batteries.

Online Monitoring: Voltage patrol instruments or other battery monitoring devices measure the real-time voltage of storage batteries and conduct periodic internal resistance measurements.

II. Current Challenges in DC System Maintenance

1. High Technical Difficulty in Maintenance

As battery packs adopt a series structure and remain in a floating charge state for a long time with stable operating voltage and minimal current, conventional DC operation and maintenance methods cannot effectively prevent sudden faults in real time, such as capacity decline of battery packs, open circuit of individual cells, and disconnection of battery packs from the DC bus. The external manifestations of battery performance degradation are not obvious; a systematic analysis of the parameter change trends of individual cells over a period of time and the parameter inconsistencies between different cells is required to determine the degree of battery deterioration, which places high demands on the professional knowledge and skills of maintenance personnel.

2. Heavy Workload of Manual Maintenance

A comprehensive inspection of battery packs is required every year, covering 4 major items and 12 sub-items including terminal voltage and resistance. For individual cells, terminal voltage measurement is required monthly, internal resistance measurement every 6-12 months, and capacity verification discharge test every 1-2 years. Conducting these works entirely manually is time-consuming, labor-intensive and highly difficult to operate.

3. Inaccurate Open Circuit Monitoring Data

Since storage batteries are in a floating charge state for a long time, the overvoltage information reported by voltage patrol instruments may be caused by battery overcharging, making it difficult to detect open-circuit batteries through online voltage monitoring. Offline battery testing can initially determine whether a battery is open-circuited through open-circuit voltage, internal resistance and other parameters, but offline testing of substation batteries can only be carried out periodically, at most once a quarter. The longer the interval between two tests, the higher the risk of battery open circuit during this period.

4. Limited Temporary Remedial Measures After Open Circuit Occurs

In the event of AC power failure in the substation or high current impact, battery pack defects may lead to the capacity collapse of the DC system and further cause the failure of power system protection functions. At present, some substations connect supercapacitors to the DC bus; before the complete power failure of the DC system, the supercapacitors supply power to the DC bus for a short time to complete emergency protection actions. Although this method is effective in the short term, it cannot cope with systematic or long-term faults.

III. Overall Solution for DC Systems

To address the common problems of current battery packs, Wuhan Haomai Electric Power has developed the MDC-1000G Multi-Functional Condition Monitoring and Undervoltage Compensation System for Battery Packs through in-depth research in accordance with relevant standards of the DC industry. Through a modular design, the system integrates functions such as remote capacity verification of DC systems, undervoltage compensation, fault recording, and online monitoring and analysis of individual modules.

△ MDC-1000G Multi-Functional Condition Monitoring and Undervoltage Compensation System for Battery Packs

△ System Composition Structure Diagram

Automation and Remoteization of Repetitive Work

Time-consuming test items such as capacity verification and internal resistance measurement no longer require a large amount of working hours. Managers send capacity verification requests remotely (or locally), and the system automatically deploys the charger to complete the discharge capacity verification test, and saves and uploads the test results. During the test, the system automatically detects the current, voltage, temperature and other information of the entire battery pack and individual cells, records the charge-discharge curves, and identifies underperforming batteries. The capacity verification test adopts an advanced regenerative discharge method, which feeds back electrical energy to the power grid system through inversion.

Intelligentization of Complex Work

The system compares the single test results with historical test data to intelligently determine the battery state and improve the reliability of test results.

DC Bus Open Circuit Monitoring and Undervoltage Compensation

When an open circuit occurs in a part of the DC bus, the system automatically isolates the faulty part, converts the normal part of the bus through DC//DC, supplies power to the undervoltage part, and uploads the state information in real time, gaining more reserved time for fault handling.

Fault Recording of DC Operation Data

Fault data recording is activated under various sudden change conditions to record the changes of various parameters and the entire accident process, facilitating post-accident analysis.

Solution for Two Battery Packs

Including online monitoring module for battery internal resistance, undervoltage compensation and recording module, remote capacity verification module and remote communication module. (Each module is optional according to actual working conditions)

Solution for Single Battery Pack

Including online monitoring and bypass module for battery internal resistance, recording module, remote capacity verification module and remote communication module. (Each module is optional according to actual working conditions)