This article combines the characteristics and applications of valve-regulated batteries that have been widely used for a few days ago, and addresses the problems found in the vertical management of rural electricity, and proposes in depth the problems in the actual operation of DC power systems, describes the technical requirements for understanding and implementing regulations, and analyzes the impact on battery packs. The main factors of service life, combined with the actual situation at the scene, put forward measures for the operation and maintenance of the DC system battery and charging device.
1 Introduction
Recently, according to the requirements of vertical management of agricultural power, the county-level power supply company's special security inspection and several accident analysis have found that the county-level power supply companies in the operation and maintenance of the DC system management problems, but also due to the failure of the DC system, especially the battery Failure of the group caused the accident. On May 1, 2007, a three-phase short circuit of a 10kV cable in the XXX 110kV substation caused a serious drop in the voltage of the 10kV bus in the station. The station's variable AC power dropped to 30%, the station's charging device lost its I-stage power, and the DC output disappeared. The total station protection refused to move, causing a contralateral power line 110kV line protection overstepping tripping, causing the whole station blackout, loss of load 70MVA, after the accident inspection found that the entire group of DC system battery capacity is insufficient, the individual battery pack pole corrosion is serious, charging The device is only connected to one AC power source, and the battery pack's charge and discharge work cannot be properly maintained for a long time.
As a device for protecting automation equipment and monitoring power supply, a DC system is the "heart" of power network protection equipment. Its normal operation or directly affects the safe operation of the power system. With the advancement of technology, DC system equipment has also been greatly developed. The large number of applications of valve-regulated batteries, the intelligentization of charging devices, and the battery sealing technology have greatly reduced the difficulty of operating personnel, but also brought some operational maintenance. The problem: over-believing and relying on new equipment, operation and maintenance work is not in place, the service life of new equipment is not as good as the original equipment, and even the battery is damaged, the DC system of the substation is full, protection is refused, and the substation is completely stopped. The operation and maintenance of the DC system presents some views.
2. Understanding valve-regulated "Maintenance-free" battery (VRLA).
The battery pack is a backup power supply for emergency power supply (EPS) with DC power supply and uninterruptible power supply (UPS). It has been widely used in power system communication and other fields. Usually, the battery is in the state of floating charge for a long time, but the utility power is directly or through conversion. After the load is given, the battery is directly supplied to the load only when the mains power is interrupted or under special circumstances. Recently, a large number of valve-regulated sealed lead-acid batteries (VRLA) have been used due to the use of internal oxygen compound technology, which greatly reduces the loss of electrolyte, and is small in size, stable voltage, no pollution, light weight, high discharge performance, no acid mist Precipitation, without adding water, is often referred to as "Mintenance Free Batteries" and is highly appreciated by professionals. Its true maintenance-free mechanism is:
In valve-regulated lead-acid batteries, in order to prevent the evolution of hydrogen and oxygen at the later stage of charging, the negative electrode capacity is excessive with respect to the positive electrode at the time of design. That is, when charging is carried out to the final stage, the positive plate generates oxygen first, ie
2 H2O → O2 + 4H
Oxygen diffuses through the pores in the separator to the negative electrode plate, and reacts with the active material of the negative electrode plate, sponge-like Pb and H2SO4, to generate PbSO4, and at the same time suppresses the generation of hydrogen gas at the negative electrode plate.
2Pb + O2 → 2PbO
PbO + H2SO4 → PbSO4 + H2O
As it is during charging, the generated PbSO4 is reverted to spongy Pb. The overall result is
O2 + 4H + + 4e - → 2 H2O
Obviously, this is the reverse process of oxygen production by the positive plate, ie the oxygen produced by the electrolyzed water at the charge of the positive plate is composited and reduced to water at the negative plate. Therefore, valve-regulated lead-acid batteries do not emit hydrogen and oxygen during charging, and there is no loss of water. This is why valve-regulated lead-acid batteries do not need to add acid to add water, which is maintenance-free.
However, compared to conventional open type batteries, maintenance-free use of water during the use does not require water, do not need to adjust the proportion of electrolyte, most maintenance personnel because of the misunderstanding of the word maintenance-free, ignored the routine maintenance of valve-operated battery and Management has caused a reduction in the service life of storage batteries, and substations and power plant accidents caused by insufficient capacity or failure have become commonplace.
3. Some Issues Concerning the Implementation of the Technical Requirements of the DC System of the State Grid
3.1 Measurement of Ripple Coefficient of DC System
State Grid DC operation and maintenance technical regulations, the measurement of ripple coefficient is a DC core index, which reflects the pulsation of the output voltage of the DC device. If the value is too large, it will not only affect the service life of the battery, but also cause serious DC system accidents.
3.2 Test of DC monitoring device control program
DC power supply standards for the requirements of the microcomputer monitoring device DC power supply equipment should be able to automatically: constant current limit voltage charging - constant voltage charging - floating charge state or charge - float state automatic conversion. However, the actual on-site inspection found that some equipment is still aging, and the constant voltage can not be automatically converted into a floating state after charging, which is unfavorable to the protection equipment and the battery life.
3.3 Technical Requirements for AC Inputs
The DC power supply technical standards not only require that the charging device should have two AC inputs, which are mutually standby, and that they should be taken from different sections of AC power. It also requires equipment to prevent over-voltage protection and prevent surge surge voltage from entering the charging module. The purpose is that if lightning strikes the lightning wave and invades the DC input through the AC power line, the charging module is damaged, which is damaged by the impact voltage and generates a high DC voltage, resulting in a DC system accident.
3.4 Battery Charge and Discharge
After the regulated valve battery is put into operation, the requirements of the procedure are as follows: a checking charge/discharge is performed every 2-3 years, and a checking charge/discharge is performed once a year after 6 years of operation in order to check whether the battery capacity meets the requirements. The internal chemical of the battery is activated by discharging and charging. Increase battery life.
4. The main factors that affect the safe operation of DC systems
At present, almost all substations use valve-regulated batteries, most of which are 2V battery packs. The capacity is small, generally 100-500Ah, and the design life of 2V battery is about 10 years. This design value is required under a strict operating environment. The theoretical value, the actual life is closely related to the battery's ambient temperature, DC panel parameters, routine maintenance, and battery operation. During the daily inspection, it was also found that some battery rooms did not have air conditioners installed or air conditioners were installed, and normal temperature compensations were not adjusted at the same time, so that the battery life in actual use was greatly shortened.
4.1 Influence of Environmental Temperature
Ambient temperature is an important factor affecting the service life of the battery. As the temperature rises, the discharge capacity of the battery increases. In the environment of 15-25 degrees Celsius, a longer life can be obtained. However, if the temperature rises by 10 degrees, The battery life is reduced by about half, and the higher the temperature, the greater the impact. As soon as the temperature rises, the plate corrosion of the battery will intensify and at the same time it will consume more water, shortening the life of the battery from the page.
The environmental temperature required by general manufacturers is 20±5 degrees. The rated capacity of the battery refers to the capacity at 25 degrees. If the temperature is too low, the discharge capacity of the battery will be affected. It is proved through experiments that the temperature and capacity have a certain degree when the temperature is below 25 degrees. There are relationships.
4.2 Thermal runaway
The initial high battery charge of the battery does not cause thermal runaway problems. This is because all or most of the charge during the initial charging period is used for active material conversion. The current is almost entirely converted to participate in the oxygen composite reaction, and the reaction generates a large amount of heat. In the case of pressure charging, the generated heat will cause the battery to charge more current and will generate more heat. This cycle continues. If the battery is not well ventilated and the current is not limited, it will eventually be airborne. Thermal runaway accident, this is devastating damage to the battery.
4.3 Overcharging
If the battery is overcharged for a long period of time, the positive electrode reacts to the oxygen evolution reaction, water will be consumed, and the concentration of hydrogen ions will increase. As a result, the concentration of acid near the positive electrode will increase, which will increase the corrosion of the grid, and increase the moisture damage. Battery electrolysis will have the danger of drying out, which will greatly shorten the life of the battery. Practice has also proved that when the valve battery temperature is greater than 50 degrees, more than 10 times overcharging will lead to permanent failure of the battery.
4.4 Undercharge
According to the current analysis of some of the bad battery anatomy, one of the main reasons leading to the end of the battery life is the irreversible sulfation of the negative electrode of the battery. This is a typical performance of premature aging of the battery capacity. Under normal conditions, the battery forms crystalline lead sulphate during discharge, which can easily occur when charging the oxygen reduction reaction. The positive electrode is converted to lead dioxide, and the negative electrode is reduced to lead. If the operation and maintenance problems, such as floating charge voltage is too low or often in undercharged state, will cause the floating charge flow to reduce, the relative charge time is extended. If the charge time is insufficient, the battery will be in undercharged state, the battery discharge capacity It will get smaller and smaller.
The float charge voltage is too low and lead sulfate inside the battery plate cannot fully synthesize lead oxide and lead. For a long time, the active substance cannot convert lead and lead oxides at the end, so that the battery can not be discharged.
In addition, in the individual battery cells, once irreversible sulfation occurs, the internal resistance will increase due to the change of the solution concentration in the battery, and a large amount of heat energy generated inside the battery will cause the degree of sulfation to continue to increase, eventually causing short circuiting of the lead sulfate dendrite. Single cell battery plate due to short-circuit current heating and distortion or expansion, usually "drum belly" phenomenon, or even cause the battery shell burst, single cell battery early failure, directly leading to the failure of the entire group of batteries.
Battery failure can be caused by different reasons. Usually, high float voltage will cause thermal runaway and attention of maintenance personnel, such as temperature monitoring of battery room. Actually, the long-term consequences of battery charging due to under-charging development is not easy. Divided by the discovery and attention, so the formation of battery maintenance risks.
4.5 Charging Device Performance and Parameter Settings Impact
Charging devices Some of the charging devices that are currently in progress are phased charging devices, most of which are high frequency power switches. However, the actual operation of the DC charging device steady flow, voltage and constant current limiting voltage charging - constant voltage charging - floating charge status or even charge - floating state automatic conversion and other performance is not desirable, charging device internal float charge, charge voltage Other operating parameter settings are also the key factors affecting the safe operation of the DC system.
5 Technical Measures for Operation and Maintenance of DC System
5.1 Input and use of new battery packs
Under normal circumstances, there will be a period of time before the new battery is shipped from the factory until it is installed. In particular, during the period of infrastructure installation and adjustment, the battery will suffer from different program losses due to prolonged self-discharge, and due to the self-discharge of each battery. The difference causes a non-uniformity in the electrolyte specific gravity of the battery, the battery terminal voltage, and the like. Therefore, before the battery is put into operation, it is necessary to perform a supplementary discharge of the battery. Otherwise, individual batteries will further expand into backward batteries and cause problems for the entire battery.
5.2 Charge and Float Charging of On-Line Running Battery Packs
After the battery is put into use, the battery charging parameters should be set according to the manufacturer's charging requirements. Especially the current DC switching power supply charging equipment, its intelligent methods and procedures are not the same, switching power supply settings parameters are not reasonable, directly leading to battery undercharge, overcharge or over discharge, resulting in shortened battery life.
(1) The power system battery is generally operated in the float charge state, and the float charge operation is the best operating condition of the battery. At this time, the battery is always in the full state of charge. In this condition, the battery will reach the longest service life. Therefore, the float voltage setting is critical to the battery life.
(2) The float voltage of valve-regulated battery is 2.25±0.02V/cell at 25°C. The most recommended value is 2.25V-2.26V/cell, that is, the center value is slightly higher. This is because the battery's standard ambient temperature is 25°C. To ensure that the battery is fully charged during long-term float charging conditions, especially when the old-fashioned phase-switching power supply is used and the device has no self-starting charging function, the proper increase in the float voltage value is a function of operation. advantageous. After the battery is discharged for a short period of time, even if the battery is not fully charged to make up for the electrical energy, since the float charge is normally taken at a relatively high level, after a period of floating charge, it can also make up for the electrical energy.
(3) When the battery is in floating charge operation, the charging voltage should be properly adjusted with the environment. The float voltage value can be calculated according to the temperature compensation system -3.5mV/degree of monomer, ie, the temperature rises by 1 degree and the float voltage drops by 3.5. mV, temperature reduction, floating voltage rise 3.5mV.
5.3 Doing Acceptance of DC Charging Device
When passing and accepting, it is necessary to verify that the charging device has good voltage regulation, steady flow, and current limiting characteristics. To ensure that the battery can operate in the best condition and apply two phases of constant power
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