What is impulse voltage? What are the hazards of too high levels of it, and what factors are related to it?

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1 Overview of Shock Voltage 1.1 Shock Voltage Shock voltage refers to the voltage value displayed on the voltmeter during the electrolyzer’s baking process when current is applied fully; generally, this shock voltage ranges from 3 to 5.5 V. 1.2 Hazards caused by excessively high impulse voltage: (1) It can lead to an uneven distribution of the anode current, resulting in the steel claws turning red, or in damage to those claws as well as cracks in the carbon blocks, thereby making it difficult to start the operation. (2) It easily leads to uneven distribution of the cathode current, resulting in redening of the furnace walls and excessive thermal shock. This causes abnormal temperatures at the local furnace bottom and furnace sides, as well as red-hot leaks, leading to production interruptions. 1.3 Differences between normal and abnormal values of impulse voltage: Based on the practical experience with 300kA large-scale pre-baked cells over the past two years, a normal impulse voltage is generally below 4.5V. A value above 5.0V is considered abnormal, while a value above 5.5V indicates a serious risk; the cause must be analyzed immediately and effective measures taken. 1.4 Measures to take when the impulse voltage is too high: (1) The current increase rate should be slowed down for the first batch of energized cells. (2) In addition to slowing down the rate of current increase, the later-stage energization groove should also increase the shunt current. (3) When laying coke particles, strict requirements should first be placed on the flatness of the bottom surface of the dual anodes; secondly, the proportion of graphite in the coke particles should be increased or a certain amount of graphite should be added as a doping agent. Another thing is to control the thickness of the coke particle layer so that the entire bottom surface of the anode is in contact with the coke particles. 2 Main factors affecting impact voltage 2.1 Calcination method: Due to different calcination methods, different resistive heating materials are used. Molten aluminum has a low impact voltage because of its good electrical conductivity, while coke particles have a high impact voltage due to their high resistance. Graphite has a resistivity that lies between that of molten aluminum and coke particles, so its impact voltage is also relatively intermediate. 2.2 Flatness of the conductive surface: Whether it is aluminum melt roasting or coke particle roasting, if the conductive surfaces beneath each anode group are uneven or have poor contact, it leads to uneven current distribution upon power application and significantly high surge voltages. This necessitates strict control over the process of laying coke particles. 2.3 The impulse voltage when power is supplied to the elevator changes with the rate of increase of current; if the power supply speed is too fast, large differences in local currents lead to an increase in the impulse voltage. 2.4 Shunting by shunt devices: Currently, steel shunt plates are used in the calcination of coke particles. However, depending on the number and thickness of these shunt plates, the amount of current that can be shunted varies, which is reflected in differences in the impact voltage. Yet, with the use of shunt devices, this factor becomes less significant over time. 2.5 Influence of cathode current distribution: The uniformity of conductivity in the cathode carbon blocks directly affects the distribution of current at the anode, which in turn has an impact on the level of impulse voltage. The uniformity of electrical conductivity in the cathode carbon blocks is directly related to the voltage drop resulting from their assembly, which in turn means it is closely associated with the quality of the assembly process.