The substation DC system is known as the  power heart of a substation, supplying power to switchgear power supplies, relay protection devices, automation equipment and other components in the substation. Its operational status directly affects the safety and reliability of substation operation. Therefore, the operation and maintenance of the DC system are of vital importance. Based on the composition structure and maintenance standards of the substation DC system, this article briefly introduces the operation and maintenance technologies and test methods for the DC system.

Composition of the DC Power Supply System

AC Distribution Unit: An electrical/mechanical interlock device that detects the AC incoming power in the DC cabinet and realizes automatic switching or automatic recovery.

Charging Device: Undertakes the charging of the battery bank, converts AC power to DC power, ensures the output DC voltage is within the required range, and provides necessary protection for the charging device. It guarantees that the technical performance indicators of the DC power supply meet operational requirements, and supplies safe and reliable DC power for daily DC loads and floating charging of the battery bank.

Monitoring Device: Used to manage various parameters and working states of controlled equipment and perform system management, realizing charging module control, battery management and remote background monitoring functions.

Insulation Monitoring Device: Measures and judges the control bus voltage and the insulation resistance of each branch to the ground, and issues an alarm when the voltage exceeds the normal range.

DC Feeder Unit: Used for the regulation, distribution and detection of the DC power supply for the entire substation, distributing the DC output power to each output circuit.

Storage Battery: A device for storing DC electrical energy, which converts electrical energy into chemical energy for storage and converts chemical energy back into electrical energy for supply to DC loads when in use.

Battery Inspection Device: A device that monitors the terminal voltage of a single battery in an operating battery bank. It can also measure the ambient temperature, battery current and internal resistance of a single battery, detect abnormal battery conditions in a timely manner, and effectively avoid extreme situations such as a drop in discharge voltage caused by battery open circuit or poor contact.

Main Test Items of the DC System

Main test items include charger characteristic test, battery charge-discharge test, battery internal resistance test, DC system differential coordination characteristic test, DC circuit breaker time-current characteristic test, etc.

Battery Charge-Discharge Test

The DC system maintenance regulations stipulate that a charge-discharge test shall be conducted on batteries every year. Usually, the station power supply team of the substation management office has a small staff and is responsible for the daily maintenance of multiple substations. The test requires long-term on-site attendance by personnel, resulting in heavy tasks.

The MDC-2000 Substation Battery Comprehensive Monitoring and Remote Automatic Maintenance System provides a multi-functional modular comprehensive monitoring and management system with multi-state monitoring and remote on-line automatic charge-discharge for substation battery banks. It realizes unattended automatic operation and maintenance of substations, greatly saving human and material resources, and eliminating the need for staff to frequently travel to and from each substation site. The system is implemented in accordance with the off-line charge-discharge scheme technology in the technical specifications, that is, simulating the manual charge-discharge process: paralleling the busbars, then completely disconnecting the battery bank from the busbars for discharge capacity verification and charging. It completely replaces the manual maintenance mode and realizes remote on-line capacity verification discharge maintenance.

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

During normal operation, the MDC-2000 system conducts real-time monitoring of the AC and DC parameters of the DC panel and all parameters of the battery (voltage, current, internal resistance, temperature, etc.), fully grasps the performance status of the battery, detects underperforming single cells in a timely manner, and records fault waves when disturbances occur, while transmitting monitoring data to the background remotely. Long-term monitoring, analysis and diagnosis are carried out locally and on the background, and the recorded wave waveforms can be retrieved for accident analysis after a power supply accident occurs.

The system can be networked independently or connected to a remote operation and maintenance platform, with the following core functions:

Monitor the operating status of the battery bank, bank terminal voltage, AC/DC voltage and current, charge-discharge current, status of each switch, ambient temperature, single battery voltage, internal resistance, battery temperature, etc.

Remotely control the switchover of each switch to discharge state, realize busbar parallel connection, with one battery bank charging the busbar and the other isolated for off-line discharge.

Record operating data, charge-discharge data and historical data in real time, which can be remotely transmitted to the background to realize long-term big data management and analysis, and the background monitors the charge-discharge process in real time.

Conduct real-time analysis, detect battery and AC/DC faults and issue alarms in a timely manner, and realize early warning through intelligent analysis.

Differential Coordination Calibration

At present, the DC feeder network of substations mostly adopts a tree structure. From the battery bank to the electrical equipment in the substation, power distribution is generally carried out through three levels, and DC circuit breakers are mostly used as protection electrical appliances for each level of power distribution. DL/T 5044-2014 Technical Code for Design of DC Power Supply Systems in Electric Power Engineering stipulates that the differential coordination scheme of DC circuit breakers in substation DC systems shall meet the requirements of selective protection. Article 12, Clause 8 of the Code for Operation of DC Power Supply Systems stipulates that "DC fuses and air circuit breakers shall adopt qualified products, their fuses or setting values shall be graded and set in accordance with relevant regulations, and verified regularly to prevent the expansion of accidents caused by their incorrect operation".

The correct selection of protection electrical appliances and the coordination of selective protection between upper and lower levels are directly related to whether the fault of the DC power supply can be limited to the minimum range, which is crucial for preventing the expansion of accidents and serious damage to equipment. The DC power supply systems of power plants and substations are designed in accordance with relevant standards to ensure a differential margin of 2 to 4 levels. However, maintenance personnel cannot verify whether the differential coordination of DC circuit breakers in on-site operation meets the requirements of selective protection due to the lack of corresponding test methods and tools, leaving hidden dangers for the safe operation of the power system.

The MDJ-800 DC System Differential Coordination Characteristic Tester is a portable DC power supply protection differential coordination test system with a high degree of automation, suitable for on-site use in substations. It improves the operational reliability of the DC system and ensures the safe and reliable operation of the power grid by calibrating the differential coordination of DC protection electrical appliances.

△ MDJ-800 DC System Differential Coordination Characteristic Tester

The device adopts a master-slave computer combined structure: the slave computer is integrated with the tester, and the master computer software runs on a computer and communicates with the tester through a 100M network cable as the main control system for the test process. Its core features are:

1. High-precision and high-performance data acquisition card with multi-channel parallel acquisition and storage.
2. Can estimate the short-circuit current of the branch through a small on-load current.
3. Can check the table to calculate the predicted value of differential coordination probability.
4. Can display the short-circuit operating current, pre-arcing time, arc extinction time and switch breaking characteristic curve.
5. The load regulation adopts a high-power pure resistance load with good linearity and high precision; the output control adopts multi-stage high-voltage and large-current DC contactors with strong arc extinction capacity and high safety.
6. Equipped with two protection modes: automatic "short connection" software delay and manual emergency stop, ensuring safety and reliability.

Charger Characteristic Test

The characteristic test of the DC charger is an indispensable test item in the handover test and later operation and maintenance of the DC team of the substation. The FDT-220/110 DC System Comprehensive Tester includes a test host, a three-phase electronic automatic voltage regulator, a wireless single battery monitoring system and a load expansion box. It can realize battery discharge, charger ripple coefficient/voltage and current stabilization precision characteristic test, and supports DC systems in 110kV-500kV substations and 2/6/12V batteries. The load expansion box can meet a total discharge current of up to 180A.

△ FDT-220/110 DC System Comprehensive Tester

Its core test functions:

Battery bank discharge test, compatible with DC220V and 110V battery bank testing.

Battery bank charging monitoring, testing the charging curves of the entire bank and single cells.

Can intuitively display the voltage and current stabilization adjustment process and voltage-current change curve, and display the ripple waveform in an oscilloscope mode.

Battery Internal Resistance Test

△ FR-3 Battery Internal Resistance Tester

The internal resistance of a battery increases as the battery capacity decreases, that is, the internal resistance of the battery continues to increase with the continuous aging of the battery and the continuous decrease of capacity. The FR-3 Battery Internal Resistance Tester adopts the AC injection method to conduct continuous section-by-section testing of a single battery or the entire battery bank, and quickly completes the testing of battery internal resistance, voltage, capacity, etc. It can permanently store the parameters of 999 battery groups, the test data records can be read through a USB flash drive, and a battery test report can be generated.