The DC system of a substation is mainly composed of a battery bank, rectifier, DC converter, DC distribution panel and other components. During normal operation, the charger supplies power to the load and performs floating charging for the battery bank at the same time. When AC power is lost, the battery bank switches to discharge mode, and the DC system provides uninterrupted power for emergency lighting, uninterruptible AC power supplies, emergency lubricating oil pumps and other equipment. However, if a fault occurs in the DC system as well, the circuit breakers without DC power supply cannot trip automatically, leading to the expansion of the scope of protection refusal accidents and huge losses to the power system.

△ Typical Wiring Diagram of 220kV DC System

The battery bank is the power source of the substation and is responsible for providing electrical energy for the operation of the power system. As the core component of the DC system, its stable and reliable operation provides a safety guarantee for the substation's operation. Due to the heavy maintenance workload of battery banks and their low automation level, especially the reliance on mobile maintenance equipment for manual capacity verification during maintenance, there are problems such as numerous and heavy equipment, complicated wiring, complex operation and high risk. The maintenance of battery banks has always been a difficult problem for operation and maintenance personnel.

1. Short service life of valve-regulated lead-acid batteries and serious consequences of open circuit.

Valve-regulated lead-acid battery banks are in a floating charge state for a long time with small charge and discharge currents and stable voltage for a long time, making it difficult to detect potential faults of the battery banks. Problems such as battery performance degradation due to insufficient capacity at critical moments may lead to the expansion of power grid accidents.On June 18, 2016, after a cable trench fire in a 330kV substation in a certain province, the accident expanded due to an abnormality in the DC system, eventually causing 3 main transformers to burn out, 8 110kV substations to lose voltage, and a direct load loss of 280,000 kW.

2. The method of testing the voltage of single cells is insufficient to fully grasp the floating charge current status of the battery bank.A set of 500Ah batteries put into operation by a power supply company in a certain province in 2015 passed the capacity verification test in 2018. In May 2019, the on-line internal resistance monitoring device detected that the internal resistance of 2 single cells was close to 1.5mΩ. Subsequently, maintenance personnel removed them, and the actual measurement showed that the 2 cells had an open circuit.

Two sets of batteries in a 110kV substation were put into operation in 2012 and 2013 respectively, both passed the capacity verification test in 2017, were included in the 2019 technical transformation reserve but failed to be put into use, and had successive open circuits in September 2019.

3. There may be open circuit risks during the capacity verification test cycle, with heavy workload and serious shortage of manpower.

Clause 5.3.3.4 of The 18 Major Anti-Accident Measures for Power Grids of State Grid Corporation of China (2018) requires a capacity verification discharge test for newly installed batteries every 2 years, and an annual capacity verification discharge test after 4 years. In the actual implementation process, a single charge and discharge takes at least two days of testing time with a long time span, and the stability of the battery bank during the cycle cannot be guaranteed. The capacity verification discharge test is operated on-site by professional and technical personnel carrying discharge instruments, which consumes a lot of human, material and financial resources, and has potential dangers such as human operation errors.

In response to the above operation and maintenance difficulties, Wuhan Haomai Electric Power has extensively collected the operation experience and opinions of station DC systems, and designed and developed the MDC-2000 Integrated Monitoring and Remote Automatic Maintenance System for Substation Battery Banks in accordance with relevant standards of the DC industry. It replaces the traditional manual maintenance mode and realizes real-time on-line monitoring of battery banks and remote on-line capacity verification discharge maintenance.

△ MDC-2000 Integrated Monitoring and Remote Automatic Maintenance System for Battery Banks

1) On-line Status Monitoring

The monitoring part of the system realizes on-line monitoring of the DC system status by connecting functional modules such as single cell monitoring modules, crossover modules, AC/DC monitoring units and battery bank monitoring units to the DC system. Among them, the single cell monitoring module is installed on the surface of each single cell to monitor important data such as battery internal resistance, voltage and temperature. The single cell crossover module is installed on the pole of the single cell to isolate the cell with abnormal single voltage from the DC system. The AC/DC monitoring unit and the battery bank monitoring unit monitor the status of the charger and the battery bank respectively. The system transmits the monitoring data to the remote management through the on-station communication lines.

2) Remote Feedback Type Capacity Verification Maintenance

The remote capacity verification part of the system consists of an integrated host, a discharge switching device, an inverter feedback discharge module and other auxiliary units. Under normal circumstances, the charging device of the DC panel supplies power to the DC load and performs floating charging for the battery bank at the same time, with the battery bank in a standby state. The status of the battery bank is switched by connecting an on-line battery discharge switching device in series between the DC bus and the battery bank. During discharge, the battery is connected to the DC bus through a diode, at which time the battery cannot be charged, but can supply emergency power to the DC load when AC power is lost. The main control unit controls the source inverter to invert the DC power of the battery into 380V AC power and feed it back to the AC power grid (the current harmonic, power factor and other indicators of the power injected into the grid meet the requirements of GB/T 30427). The capacity verification (half capacity verification/full capacity verification) discharge test of the battery bank is realized by setting the current value during the discharge process.

△ Remote Automatic Maintenance System for Battery Banks

System Characteristics

On-line Shallow Discharge Balancing Maintenance

Long-term floating charging of batteries has a great impact on their service life. Regular shallow discharge of about 10% can extend the service life of batteries and avoid the impact of long-term floating charging on the balance of batteries.

Battery Integrity Detection with Small Current

The battery is judged whether it is off-line by detecting the battery floating charge current, fuse voltage and contacts. The on-line internal resistance test is carried out by the segmented small current DC discharge method, which can effectively monitor the hidden serious defects that are difficult to find during operation such as battery plate fracture, battery open circuit and electrolyte drying up.

Anti-Islanding Protection Function

By detecting the voltage, phase and frequency of the AC side, the inversion angle is automatically adjusted. When AC power is lost, the inversion discharge can be terminated in time to ensure the normal power supply of the DC bus.

Multiple Composite Protection in the Capacity Verification Process

The discharge status of the battery is monitored remotely in real time with multiple protections set. When abnormalities occur in network communication, discharge load communication, uncontrollable current, single cell voltage, total voltage, temperature and other aspects, the discharge can be stopped automatically, and the main discharge circuit can be cut off directly to ensure the safety and reliability of the discharge process.

Visualization of Local and Background Capacity Verification Process

The monitoring terminal software can real-time monitor the simulation diagram of the substation DC system, and dynamically display the on-off status of the DC main circuit switch, bus voltage, battery current, AC voltage and current, AC switch working status, single cell voltage, internal resistance and temperature, effective value of AC inrush and other data. The user management module can add or delete the list of management personnel, and set management permissions such as system administrator, operator and general viewer to prevent misoperation.

△ Application Site of MDC-2000 Integrated Monitoring and Remote Automatic Maintenance System for Substation Battery Banks

At present, the MDC-2000 Integrated Monitoring and Remote Automatic Maintenance System for Substation Battery Banks has been widely applied in major substations across the country. It has realized on-line status monitoring and data analysis of DC systems, as well as remote control of automatic discharge and charge capacity verification of battery banks in each group of each substation, greatly improving the intelligence level of substations and effectively ensuring the safe and stable operation of power systems.