New ideas for dealing with open circuit continuous current of substation battery pack

The battery is an important emergency power source for the DC system of the substation. Once the circuit is open, it will pose a serious threat to the power supply safety of the entire substation. In response to the open circuit problem of the substation battery pack, Hui Lei, Qin Huainian, Huangfu Dezhi, and Wang Hao, researchers from the Qujing Power Supply Bureau of Yunnan Power Grid Co., Ltd., wrote an article in the first issue of "Electrical Technology" magazine in 2020. By analyzing the shortcomings of the existing open circuit freewheeling device, a new idea for realizing real-time open circuit freewheeling of the battery pack was proposed.

The valve-regulated lead acid (VRLA) maintenance-free batteries currently commonly used in substations have many excellent characteristics, such as reliable operation, basically no pollution, easy installation, and easy maintenance. They are suitable for providing reliable DC power for various circuit systems in substations. However, due to the design, production process, and use and maintenance of the battery itself, battery failure often occurs, and its service life is much shorter than the expected life, which seriously affects the safe operation of the DC system.

After a period of use, the capacity of lead-acid batteries will inevitably decrease gradually due to plate corrosion, effective active material shedding, water loss, and sulfation. If you are not careful, it may cause a single battery to fail or the entire battery pack to open circuit. In addition, battery failure is somewhat hidden. The battery pack is in a standby floating charge state most of the time. It is necessary to monitor some technical indicators of the battery to consider the quality and status of the battery.

However, some negative conditions of batteries cannot be detected directly, such as corrosion of the negative busbar. In the floating charge state, because the charging current is very small, the busbar remains connected and the floating charge voltage can basically maintain a normal value. Once the AC power is lost and a large current is required for discharge, the severely corroded negative busbar will be burned out, causing the battery pack to open circuit, which is hard to prevent.

In response to the above-mentioned problems, this paper analyzes the common open-circuit follow-up coping methods and their shortcomings at home and abroad, and proposes a new idea for dealing with open-circuit follow-up of substation battery packs to ensure the safety and reliability of the DC power supply system.

1. Existing battery open circuit freewheeling response method

At present, the way to deal with open-circuit battery follow-up in domestic substations is to install jumper modules on the positive and negative poles of a single battery. When the AC power is lost and the battery pack is supplying power to the outside, if the battery fails, a high-current jumper module can be used to ensure that the battery pack can continue to supply power to the outside, thereby improving the safety and reliability of the DC power supply system.

1.1 Freewheeling diode jumper mode

The most widely used freewheeling method for open-circuit protection of substation battery packs is to connect freewheeling diodes in parallel to the positive and negative poles of a single battery to ensure the continuous power supply capability of the entire battery group after a single battery is open-circuited. The main circuit wiring diagram is shown in Figure 1.

Under normal circumstances, the charger keeps charging the battery pack, and the current direction is shown as i1. When the charging current flows through the positive electrode of the battery, because the positive electrode of the battery is connected to the negative electrode of the freewheeling diode, according to the unidirectional conduction characteristic of the diode, the parallel freewheeling diode circuit is in a high resistance blocking mode. Therefore, the battery pack is charged normally and will not be disturbed by the freewheeling diode circuit.

Figure 1

When the AC power is lost and the battery pack supplies power to the load, the direction of the loop current is shown as i2. At this time, the loop current flows into the negative column of the battery and flows out of the positive column. When a single battery is healthy, since the potential of the positive column of the battery is higher than that of the negative column, the negative potential of the freewheeling diode is higher than that of the positive electrode. At this time, the diode is still in blocking mode, and the battery pack maintains normal discharge; during the discharge of the battery pack, assuming that a single battery Bi is open, the negative electrode of the battery Bi1 and the positive electrode of Bi+1 are equivalent to only a freewheeling diode VDi connected in series, as shown in Figure 2. At this time, VDi is turned on due to the positive voltage on both sides, so that the battery pack maintains power supply to the load.

Figure 2

The response time of the parallel freewheeling diode to the open circuit freewheeling risk of the battery pack is generally in the microsecond level, which can successfully achieve uninterrupted discharge of the battery pack when a single battery is open-circuited, but it also has limitations:

① The open circuit is caused by not tightening the screws of the connecting wire or the corrosion of the connecting strip, which increases the resistance and burns out. The parallel freewheeling diode cannot change the open circuit state of the battery pack;

② After the battery pack is open-circuited, the diode conduction direction is opposite to the battery charging current, and the battery pack cannot be charged. This poses a great risk when the battery itself is self-discharging or has low capacity.

③ When the diode takes on the responsibility of open-circuit freewheeling, a fixed voltage difference V will be formed on both sides of the diode. V is the difference between the charging voltage V1 of the charger and the open-circuit voltage V2 of the battery pack. This forces the design voltage resistance value of the battery pack monitoring device to be greatly increased, greatly increasing the cost of the overall equipment.

1.2 MOS tube jumper mode

At present, there is another method for battery open circuit freewheeling, which is to connect two N-type MOS tubes back to back in parallel on the positive and negative poles of a single battery, and combine the control module and the battery monitoring module to perform remote active cross-connection, while shielding the deteriorated battery to complete the open circuit freewheeling operation. The basic principle diagram is shown in Figure 3.

Figure 3

Under normal circumstances, the monitoring module monitors the battery electrical parameters in real time. The control module receives the data from the monitoring module and determines that the battery is normal, then chooses not to act. The two back-to-back N-type MOS tubes are both in a high-impedance state, thereby blocking the current on both sides of the battery, allowing the battery pack to charge and discharge normally. When the control module receives the monitoring information and determines that the battery i is seriously deteriorated or open-circuited, the same start signal is applied to the N-type MOS tubes VDi1 and VDi2 at the same time, and the two MOS tubes are turned on at the same time, Bi is short-circuited, which is equivalent to shielding Bi in the battery pack. The remaining single batteries have no changes, and the entire battery pack can continue to be charged and discharged.

Compared with the freewheeling diode, the parallel MOS tube can be set to cross-connect and freewheel after the battery deteriorates or opens, which has greater flexibility, but there are also some hidden dangers:

① For the open circuit caused by the connecting wires or connecting bars between the batteries, the open circuit continuous current of the battery pack cannot be guaranteed;

② When the battery is open-circuited, a series of processes such as collection, judgment, control, and action are required to complete the battery pack's freewheeling. The response time is long, and it is difficult to make the battery pack discharge the load uninterruptedly;

③Batteries have electrochemical characteristics. It is not scientific to judge the quality of batteries through online parameter monitoring, and there is a possibility of misjudgment, which will cause healthy batteries to be short-circuited;

④ When the open-circuit battery is bypassed, if the charging voltage is not adjusted in time, the entire battery group will be in an overcharged state and accelerate deterioration;

⑤The entire set of equipment is complex and huge in number, and the possibility of its own failure is also very high.

2 New ideas for dealing with open circuit continuous current of batteries

In view of the harmfulness of substation battery open circuit failure and the limitations of traditional coping methods, this paper proposes a new idea for dealing with substation battery open circuit and continuous current, and the principle diagram is shown in Figure 4. A new battery open circuit and continuous current device is connected between the two independent DC system buses of the substation. When a battery open circuit occurs in one DC system and the AC power is lost, the DC system on the other end will supply power seamlessly, achieving the effect of battery open circuit and continuous current.

Figure 4

2.1 Implementation principle of new ideas

At present, most substations have two isolated DC systems. When the bus tie switch is not connected, the two DC systems operate independently. The two sides of the new battery open circuit freewheeling device in this article are connected to the two DC system buses of the substation. Once the bus loses pressure on one side, the DC/DC module automatically starts, and the DC system on the other side provides backup power supply to maintain the power supply of the load on the fault side.