As a vital component of substations, the DC system safeguards the safe and stable operation of power systems. The battery bank is the core power supply equipment in the DC system; any abnormality of it may lead to power loss of critical devices such as protection equipment, control systems and communication devices, resulting in severe consequences. Therefore, regular performance testing of battery banks is essential. Traditional battery testing methods require on-site operation by staff, who need to continuously record key parameters such as voltage, temperature and internal resistance of batteries during charging and discharging, which is extremely time-consuming and labor-intensive.

△ MDC-2000 Substation Battery Comprehensive Monitoring and Remote Automatic Maintenance System

The MDC-2000 Substation Battery Comprehensive Monitoring and Remote Automatic Maintenance System provides a multi-functional modular integrated monitoring and management system with multi-state monitoring and remote on-line automatic charging/discharging for substation battery banks. It is implemented in accordance with the off-line charging/discharging scheme specified in technical specifications: simulating the manual charging/discharging process, paralleling the busbars, then completely disconnecting the battery bank from the busbars for discharge capacity verification and charging. It fully replaces the manual maintenance mode and realizes remote on-line capacity verification discharge maintenance.

Main Module Units

Inverter Device: Reduces the output voltage of the charger before its equalizing charge, and the on-line discharge device discharges the entire battery bank with load. During discharge, the battery monitoring unit automatically records the charging/discharging curves of the entire bank and single cells, and calculates the charging/discharging capacity. Regular equalizing charge and capacity verification discharge of the battery bank are performed to avoid capacity decline of batteries in the system. It adopts controllable inverter feedback discharge technology to feed back battery energy to the AC side.

AC/DC Monitoring Unit: Monitors the on-site AC voltage, battery bank terminal voltage and current in real time, analyzes various data, and transmits the data to the host computer.

Remote Capacity Verification On-line Monitoring Host: Communicates with each monitoring module to centralize data, realizes local real-time data display through a supporting monitor, and has local functions such as historical data archiving and analysis, curve data archiving, fault alarm, underperforming battery analysis, recorded wave data display and fault analysis display. It controls the inverter device to perform load discharge via instructions, records the charging/discharging curves during discharge, communicates with the remote main station, and uploads data to the remote host computer, realizing remote monitoring and remote capacity verification control, as well as centralized on-line monitoring and visual operation and maintenance management of multiple substations. It is equipped with local and remote alarm outputs, as well as RS485 interface, network port and switch quantity alarm interface outputs.

Isolation Device: Prevents reverse current by utilizing the principle of one-way conduction of diodes.

Battery Cell Monitoring Module and Battery Monitoring Unit: Monitors battery internal resistance, voltage and temperature in real time. Multiple battery cell monitoring modules are connected to the battery monitoring unit via communication cables to monitor the data of each battery cell, analyze internal resistance trends, compare curves, identify underperforming batteries, and issue alarms for abnormal states.

Main Software Functions

△ Main Interface of Remote Capacity Verification for Single Battery Bank

△ Unattended Substation Monitored at the Main Station

△ Curves of Remote Capacity Verification Discharge and Charging Processes of Batteries

Through the remote capacity verification and on-line monitoring of the MDC-2000 system, the battery charging/discharging test can be operated remotely at the main station, and all real-time and historical data of the batteries can be viewed. The AC/DC monitoring unit, battery monitoring unit and battery cell modules feed back the collected data to the monitoring host, which displays the battery bank terminal current, bank terminal voltage, bus voltage, AC voltage of the AC feeder panel and extreme parameters of single battery cells through the monitor.

△ Real-time Battery Data Table

The real-time data table can display all battery data, and draw curves or bar charts based on real-time or historical data. Charging/discharging parameters can be set only after confirming that the data of single battery cells is normal; if the battery voltage exceeds the upper and lower limits, the discharge cannot be executed successfully.

△ Application Site

Field Test

Project Location: An Unattended Substation of a State Grid Electric Power CompanyBasic Battery Information:

Battery Type: 2V

Nominal Capacity: 200AH

Quantity: 104 cells

Test Method: The background of the MDC-2000 system issues a remote discharge command, controls the on-site remote capacity verification equipment to temporarily disconnect the battery bank from the original power supply system and connect it to the capacity test module, and realizes the battery power supply capacity test through the constant current discharge method.

Discharge Parameter Settings:

Set Discharge Current: 20A

Set Discharge Capacity: 200AH

Terminal Voltage Cut-off Value: 187.0V

Single Battery Cell Voltage Cut-off Value: 1.8V

Set Discharge Duration: 10 hours

Discharge Test Results

Index	Result
Actual Discharge Duration	5.5 hours
Reason for Discharge Termination	Single cell voltage reached the preset cut-off value
Terminal Voltage at Termination	203.1V
Actual Discharged Capacity	109.2AH
Maximum Single Cell Voltage	1.92V
Cell Number with Maximum Voltage	No.49
Minimum Single Cell Voltage	1.781V
Cell Number with Minimum Voltage	No.73
Average Voltage of Single Cells	1.953V

Test Conclusions

1. The discharge was automatically terminated after 5.5 hours because the voltage of the No.73 battery cell dropped below 1.8V. The actual discharged capacity was 109.2AH, accounting for 54.6% of the nominal battery capacity.
2. The No.73 battery cell was the underperforming battery in the entire bank, which affected the performance of the whole battery bank.
3. Except for the No.73 battery cell, the voltage of all other cells was above 1.85V at the end of discharge, still retaining a certain discharge capacity.
4. The entire test process was completed via remote operation without on-site attendance. The discharge process data was recorded in full, and the underperforming battery in the bank was detected timely and accurately through the discharge test, enabling the mastery of the real battery capacity and providing effective guidance for maintenance personnel to eliminate potential faults.
5. The indoor temperature of the relay protection room was maintained at about 26℃ during the entire discharge process, and the temperature impact caused thereby was negligible.