Problems and solutions of DC system in the hottest

Problems and solutions of DC system in substation

Abstract: the battery DC screen used in small and medium-sized terminal substations has heavy maintenance workload and short battery life. If super capacitors are used to replace batteries, it will have revolutionary progress

key words: DC screen super capacitor I. problems existing in the DC screen of current substations

in China's 110KV, 35kV, 10kV terminal substations, as well as the 6kV power distribution system for plant use, battery DC screen and silicon rectifier capacitor energy storage DC screen are widely used as the power supply for operation, torque control and protection after clearing. For decades, many products have the following problems in operation:

1. Cadmium nickel battery DC screen:

when the DC bus output 220V, it is generally composed of 180 batteries. In the processing and production of batteries, it is impossible to achieve the complete consistency of the charge and discharge characteristics of each battery. Although the manufacturer has made a matching combination when leaving the factory, there is no choice in the hands of users to avoid the problem of falsely high prices or large fluctuations for a long time. In use, the same charging power supply is used and discharged to the same load. Over time, the performance of the whole device is affected by the increasingly different characteristics of individual batteries

nickel cadmium battery has been in floating charge for a long time in operation according to the previous public information. The performance of the charger directly affects the service life of the battery. Generally, manufacturers promise that the battery life is more than 10 years, but in practical application, it is often only 3-5 years. This is because if the floating charge current is too large, the water in the electrolyte will be electrolyzed into hydrogen and oxygen. The mixture of these two gases is a dangerous explosive gas. If the ventilation is poor, there is information that a DC screen explosion accident occurred in an unattended substation (electric world 2000, issue 12)

overcharging will also make the battery liquid. Dark green oxide is generated on the surface of the battery and the connecting piece, which corrodes the components, reduces the insulation, and increases the self discharge

overcharge will also produce redox reaction, generate cadmium oxide on the negative plate, reduce the effective area of the plate and reduce the capacity, which is commonly known as the memory effect. In order to maintain the capacity of the battery, the battery needs to be activated 1-2 times a year. The test must be charged and discharged according to the standard system specified by the manufacturer in order to ensure the efficiency of the battery. As a user and maintainer, it is a headache. Because the nickel cadmium battery has a hard discharge characteristic curve, the voltage drop is not obvious when the discharge amount reaches 80%. A little carelessness will cause the battery to over discharge, reverse polarity and scrap. Because DC screen is the most important equipment in substation, it directly affects the safe operation of substation. In an environment where safety work is more important than Mount Tai, many units include DC screens in daily inspection items for assessment. It is really boring and annoying to replenish 180 (220V) batteries every day, measure the voltage at each end and make records. If all batteries are installed in the cabinet, there is also a certain danger. 2. Sealed lead-acid battery DC screen:

due to the large maintenance amount of cadmium nickel battery, a maintenance free sealed lead-acid battery was made into several orders on the spot, referred to as valve regulated battery or VRLA battery, which began to be widely used. Because it is a fully sealed battery, there is no need to add water, which not only brings many benefits to maintenance, but also brings difficulties to observation and maintenance. The term "maintenance free" brings misunderstanding to users, which leads users to relax their daily maintenance and management of batteries. Since the valve regulated battery has been in China for only about ten years, there is no mature manufacturing and operation experience so far. At the EVC meeting held in Shenzhen last year, experts on the use and manufacture of domestic batteries were gathered to summarize the experience in the manufacture and operation of domestic valve regulated batteries, and the following consensus was reached:

2.1. Service life of valve regulated batteries:

the manufacturer's instructions marked the service life of batteries as 10, 15, 20 years, which is too exaggerated. Both imported and domestic batteries have proved this after actual use. Therefore, the nominal 5 years in the specification is more appropriate; For colloidal batteries, such as German sunshine and Yinbin, they can be used for more than ten years. In addition, the service life indicated in the manufacturer's instructions is based on the premise that it should operate under the specified operating temperature and standard charging and discharging mode [including load size]. In fact, these conditions can only be achieved in the laboratory

2.2. The main factors affecting the service life of threshold controlled batteries are as follows:

a - the service life of valve controlled batteries is very sensitive to temperature. The manufacturer requires that the operating ambient temperature of the battery is 15 ℃ - 25 ℃. When the ambient temperature exceeds 25 ℃, the service life of the battery will be shortened by half for every 10 ℃ increase.

for example, for batteries with a 5-year service life, when the ambient temperature is 35 ℃, the actual service life is only 2.5 years. If the ambient temperature is increased by 10 ℃ to 45 ℃, Its life span is only about 1.25 years. For the vast central and southern China, the annual average temperature will exceed 25 ℃ for more than 3 months. In addition, in the power distribution room where threshold controlled batteries are installed, in order to prevent small animals from entering the room, the doors and windows are relatively closed, and the indoor temperature will rise, which is extremely detrimental to the operation of batteries

b - excessive discharge:

the excessive discharge of the battery is another important factor affecting the service life of the battery. This situation mainly occurs when the battery pack is supplying power to the load after AC power failure or charging module damage. When the battery is over discharged until the output voltage is zero, a large amount of lead sulfate inside the battery will be absorbed to the cathode surface of the battery, forming the sulfation of the battery cathode. As lead sulfate itself is an insulator, its formation will have a bad impact on the charging and discharging performance of the battery. Therefore, the more sulfate formed on the cathode plate, the greater the internal resistance of the battery, the worse the charging and discharging performance of the battery, and the shorter its service life

c-grid corrosion and growth:

grid corrosion is an important reason affecting the service life of batteries. In the open circuit state, the lead alloy is in direct contact with active lead dioxide, and they are immersed in sulfuric acid solution together, and they establish different equilibrium electrode potentials with the solution respectively. The positive grid plate is constantly dissolved, especially in the overcharged state. Due to the oxygen evolution reaction of the positive electrode, water is consumed and h+ increases, resulting in increased acidity near the positive electrode and accelerated anti grid corrosion. If the battery is improperly used and in the overcharged state for a long time, the grid plate of the battery will become thinner, the capacity will be reduced, and the service life will be shortened

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