Safe operation of high-power rectifiers
Safe operation of high-power rectifiers
Manager P.Munk， Senior Engineer P.B
Cooper Boseman
The rectifier is the core equipment of an electrolytic aluminum plant, and once put into operation, it is not allowed to be shut down for a long time. Otherwise, it will cause huge losses and even lead to the cooling and condensation of aluminum liquid in the electrolytic cell. At this point, in addition to the loss of output, the customer also needs to invest heavily in repairing the faulty electrolytic production line. If the selection of fuses is improper, it can cause explosions and also endanger the safety of operators.
This article focuses on the application of fast fuses in high-power rectification equipment in the electrolytic aluminum industry, and also mentions their applications in other industries.
This article first elaborates on the principle of fuse protection for semiconductor rectifiers, and then explains several key parameters involved in selection. The significance of these parameter selections for the safe and stable operation of the rectifier will also be explained one by one. Subsequently, examples will be given to illustrate the problems and characteristics caused by the selection of a lower rated value for fuses.
At the end of the article, the advantages of Cooper Bosman's newly developed No. 5 fast fuse in improving rectifier output capacity, enhancing system efficiency, and optimizing customer investment are introduced.
1. Introduction
High parameter fast fuses are mainly used for the protection of high-power semiconductor equipment. A typical semiconductor device of this type is a high-power rectifier that uses diodes or thyristors to convert AC power into DC. There are many industrial applications of high-power rectification, and this article only mentions a part of them.
2. Application of high-power rectification
Aluminum electrolysis is mainly produced through the electrolysis process. Many aluminum factories around the world are expanding their production scale to improve efficiency and meet the market demand for aluminum.
The process of aluminum electrolysis production is that aluminum oxide (Al2O3) undergoes an electrolytic reaction in the electrolytic cell, producing aluminum and oxygen:
2 Al2O3＋3C→4Al＋3CO2
The electrolytic cell is a large and shallow steel tank lined with a carbon layer on the inside, and different electrolytic cells are electrically connected in series. In each electrolytic cell, direct current flows into the melted alumina electrolyte through the graphite anode, and then flows out through the carbon cathode lined in the electrolytic cell, entering the anode of the next electrolytic cell, and so on. Under the action of electric current, the oxygen element in alumina is separated and combines with carbonization to form carbon dioxide in the upper part of the electrolytic cell, while liquid aluminum precipitates at the bottom of the electrolytic cell. The above generation process is shown in Figure 1.
Chlorine gas electrolysis is usually obtained by electrolyzing saltwater, while generating hydrogen gas and caustic soda. Producing one ton of chlorine gas requires approximately 4000 kWh of electricity. The DC process current required for chlorine electrolysis is between 300-350kA, and the required DC voltage can be as high as 1000Vdc.
Many large steel plants in modern times use direct current arc furnaces to melt and recycle scrap steel. The advantages of using a DC arc furnace are high production capacity, high production efficiency, and low noise. The process current of electric arc furnace for steel smelting is about 100kA, and the voltage is generally higher than 1000Vdc. The electrolysis process is generally clean, and the continuous load current flows steadily through the semiconductor and fuse. The application of electric arc furnaces is different. The characteristics of their production process often result in severe pulse components in the load current, so careful calculation and verification are required when selecting semiconductors and fuses.
The production of many graphite products, such as electrodes, also requires the use of direct current furnaces. Typical process parameters are: voltage generally not higher than 300V, and current may reach up to 160kA.
The process of producing copper, zinc, lead, nickel, cadmium and other metals by smelting other metals also requires the generation of current through high-power rectification.
3. Application of fast fuses
High-power rectifier
The core components of high-power rectifiers are semiconductors (diodes or thyristors) and fast fuses. The effective utilization of power semiconductor components largely depends on how to timely dissipate the heat generated by semiconductor losses.
The solution is forced cooling, and both air and water cooling are standard practices in the industry for heat dissipation. For high-power rectifiers, using deionized water (with ethylene glycol added as antifreeze) for cooling is more effective and is also the choice of most mainstream rectifier manufacturers. Nowadays, double-sided cooling of semiconductor components has become a standard practice in the industry. In order to increase output current, fast melting double-sided cooling has also been adopted more than ever before. In the past, fast fuses mostly used single-sided cooling.
Modern production processes require increasingly high process currents, and to meet this demand, Bussmann has developed the single unit design of the No. 5 fast fuse. This design optimizes the overall heat dissipation performance of the fuse during forced cooling.
快速熔断器的额定电压
执行IEC 60269-1&4标准的快速熔断器，其测试是在110％的额定电压下进行的。额定电压是熔断器选性最重要的参数之一。发生故障时，如果熔断器两端电压高于熔断器额定电压，熔断器极有可能不能正常动作。即使熔断器的测试电压比其额定电压高出10％，我们也从不推荐熔断器用在高于其额定电压的系统中。10％的安全裕度是考虑在恶劣的系统条件下，专门针对系统电压波动而设置的。
在整流应用中，由于没有再生负载，额定电压选择依据整流器输入线电压。如果与3英寸或4英寸二极管或晶闸管配合使用，可以形成的整流器输出为直流2100V，将可允许在一条生产线上有440只电解槽串联，这等效于每年39万吨铝的生产能力。
Rated current of fast fuse
To understand the calculation of the rated current of a fast fuse, it is necessary to first understand how the rated current of the fuse is defined. The international standard IEC 60269-1&4 provides detailed testing standards for determining the rated current of fuses. The steady-state current carrying experiment was conducted in an environment of (20+5) ℃ and converted to a value at 20 ℃. When the ambient temperature deviates from the standard, it is necessary to adjust the maximum allowable current carrying capacity of the fuse accordingly.
When the ambient temperature exceeds 20 ℃, the current allowed to pass through the fuse will be lower than its rated value. The specific value can be obtained by multiplying the rated current by the correction factor (Kt). Neglecting this factor will lead to fatigue of the fuse core, resulting in a fault free operation of the fuse.
Another influencing factor is the size of the copper bars connecting the fast melting and components. According to IEC 60269-1&4, the minimum size of the copper bar is equivalent to a current density of 1.0 to 1.3A/mm2 (calculated based on the rated current of the fuse). When using a small connection bank, the rated current of the fuse should also be multiplied by the corresponding derating factor. The derating coefficient curve can be found in our parameter manual. In extreme cases, if the fuse is used with a corresponding derating, the connecting strip actually serves as a heat source.
The current carrying capacity of fuses can also be improved. When using forced air or water cooling, it can increase the current carrying capacity of the fuse.
There are cyclic loads and overloads in many applications, which may also affect the service life of fuses. These situations need to be carefully analyzed to prevent abnormal operation of the fuse. When overload occurs, the fuse core of the fuse temporarily overheats, approaching the temperature at which the silver sheet melts.
After a period of time, the structure of silver at the narrow neck of the molten core changed. The crystal structure is no longer single, and particles are generated. The silver at the narrow neck will become harder and brittle. This leads to a decrease in the mechanical strength of the melt core, resulting in one or more narrow neck fractures.
Depending on the frequency of overload occurrence and the characteristics of periodic loads, additional derating factors may need to be applied at times. For medium-sized traction stations and mining equipment with obvious periodic load characteristics, the derating factor may be as high as 2.0- that is, when the fuse is used in such situations, only half of its rated current effective value is allowed to be loaded.
During the process of aluminum electrolysis, only a small amount of overload is generated, which generally does not have a significant impact on the selection of fuses. However, when one or more semiconductors in the rectifier are isolated due to faults, other branch components connected in parallel will face the risk of overload.
The coordination between semiconductors and fast melting in rectifiers needs to be carefully considered. A well considered fuse protection scheme has little possibility of semiconductor explosion. On the contrary, improper system design can lead to arcing and fire, damage to the entire rectifier unit, and personal injury to operators. Designing the system according to N-1 mode can protect users from losses caused by partial or complete production line downtime. Therefore, in high-power rectification applications, continuous full load operation is based on N-1 mode. In some applications, customers may even request designs based on N-2- or more, which must be taken into consideration. If the rectifier runs in N-x mode for too long, and the rectifier is not designed according to N-x mode, the result is that all other fuses on the bridge arm where the faulty fuse is located are also damaged.
Semiconductor protection
The function of fast melting in high-power rectifiers is to isolate the faulty branch when an internal short circuit occurs, thereby preventing semiconductor explosions. Therefore, the explosion I2t of the semiconductor should be known, and selecting a fuse with I2t less than this value for 10ms can achieve protection. The explosion I2t of semiconductors can be obtained through their limit testing.
The explosion I2t of fast melting is related to the voltage of the short-circuit system, so in testing, the energy passing through the semiconductor can be controlled by fast melting. The purpose of this test is twofold: first, to find the explosive I2t of the semiconductor, and second, to verify the ability of the fast fuse to prevent semiconductor explosion under high fault currents (up to 300kA).
4. Analysis of the consequences of improper selection of fast melting materials
Improper voltage selection
If the rated voltage of the fuse is lower than the voltage of the faulty system, the arcs ignited at each narrow neck will continue to burn and converge, eventually penetrating the entire fuse. At this point, the ceramic casing of the fuse will not be able to withstand internal pressure and will experience a violent explosion. Fortunately, people usually choose fuses based on the input voltage, and such faults rarely occur on site. In addition, if product testing is strictly carried out in accordance with IEC standards, a 10% safety margin also eliminates the hidden dangers caused by system voltage fluctuations.
I2t value mismatch
Fast acting fuses must limit the allowable energy to below the explosive I2t of the semiconductor, otherwise a violent explosion of the semiconductor will occur. When a semiconductor explosion occurs in the equipment, it is recommended to check the breaking I2t value of the fuse under the fault voltage, as well as the actual explosion I2t of the semiconductor.
Neglecting one or several derating factors in the calculation of rated current
Various derating factors are used to ensure that the fuse will not fail due to fatigue during its lifespan (typically 10-20 years). Following the principle of selecting fast fuses can ensure a service life of approximately 10 years, and in most cases, the actual service life will be longer.
If the derating factor is ignored, or if the actual circuit or load differs significantly from the initially given information, the following situations may occur:
In severe cases, the fuse will be subjected to continuous overload. The protection type of fuses used in high-power rectifiers is mostly aR (according to IEC 60269-1&4, this means used for short-circuit protection of semiconductor components). Therefore, the fuse will only protect against short-circuit faults and cannot interrupt overload currents. When the aR fuse is subjected to continuous overload, the ceramic shell temperature of the fuse will exceed its allowable limit and crack. If a short circuit occurs in the system at this moment, the fuse will explode violently and damage other equipment inside the rectifier cabinet, expanding the accident.
Another situation is that the fuse is frequently overloaded, but it does not cause the shell to break, and the fuse core gradually damages over a relatively long period of three to seven years. One of the characteristics of such damage is the fault free disconnection of one or more fuses.
When multiple aR fuses are connected in parallel, if one of them is disconnected due to overload, the voltage drop between the fuse and the semiconductor connected to it is very small, which may cause the semiconductor to not conduct sufficiently and fail to fuse the trigger line of the indicator. In this case, the indicator of the fuse will not display the status of the fuse correctly, and other fuses connected in parallel to the same bridge arm will actually bear a higher degree of overload, which will accelerate the damage of their fuse cores. Modern rectifier systems are able to monitor this excessive temperature rise, indicating that there must be an abnormality occurring in the bridge arm. Unfortunately, at this point, the harm is highly likely to have already occurred, and the corresponding fuses must be analyzed and diagnosed.
The most common practice afterwards is to replace the disconnected fuse. However, if the fuse cores of other fuses have been damaged, it is highly likely that some of them will also disconnect in the near future. The consequence of this situation is frequent pauses in the production process, resulting in output losses.
The diagnosis and inspection of fuses can be achieved by simply measuring their resistance. If the test value is significantly higher than the printed value on its label, the fuse should be replaced immediately.
This anatomical analysis can be conducted in Bussmann's laboratory, which is particularly useful when customers want to understand the specific causes of fuse failures. By opening the inside of the fuse, we can determine whether it is operating under high short-circuit current or due to low overload, as well as whether it has experienced cyclic loads.
In this case, the only way is to replace all fuses. And welcome to consult our technical application department for professional advice, such as how to prevent the use of fuses with excessive rated current or problems caused by frequent replacement of fuses.
Problems in the mechanical design of rectifiers
The modern production process has increasingly high requirements for the current output capability of rectifiers, which means that the energy density in the rectifier cabinet is increasing. By combining modern high-power semiconductors with No. 5 fast melting, the number of components in the rectifier can be reduced, and the current carried by each component will increase. The fault current level will also increase, and may even reach a level close to 300KA. Under such a large fault current, due to the different directions of the current path, the fuse may have to withstand huge electromagnetic forces.
5. Fast melting of No. 5 and double body No. 4
In modern rectifier design, there are two types of fuses with different shapes to choose from. One is the so-called No. 24, which is composed of two No. 4 fast fuses connected in parallel, and the other is a single No. 5 fast fuse.
Eliminate the problem of uneven flow distribution
One of the advantages of the No. 5 fast fuse compared to the No. 4 fast fuse is that it eliminates the problem of uneven current distribution inside the fuse. In some rectifier designs, it has been found that there is a current imbalance between the two branches of the dual body fast melting, which is determined by the electromagnetic characteristics of the rectifier design. And there is no such problem with the No. 5 fast melting.
By improving the melt design and internal structure, the No. 5 fast fuse can have higher rated voltage and current compared to previous fuses.
6. Overview of Fast Melting No. 5
The No. 5 fast melting is designed specifically for high-power rectifiers and can be used in electrolytic production processes such as aluminum and chlorine gas. To meet the specific needs of our customers, we can customize different products for them. Customizable aspects include mechanical connections and dimensional tolerance requirements.
The optimized design of No. 5 fast melting is more suitable for protecting three inch and four inch semiconductor components under forced cooling in high-power rectifiers. There is data indicating that in some cases, using No. 5 fast melting combined with modern high-power semiconductors can save 30% of components compared to using traditional components. By using fewer components, the efficiency of the rectifier can be improved, thereby reducing the production cost of aluminum.
7. Summary
In the electrolytic aluminum industry, there is a trend of gradually increasing process current and the number of series electrolytic cells. Currently, there are system designs that use 350kA electrolytic current and 1600V DC voltage. In the near future, electrolytic aluminum production lines with process currents up to 500kA will also become a reality.
The safe operation of high-power rectifiers largely depends on the selection of components. Diodes/thyristors and fast melting are the main components used in rectifiers. The trend of high voltage and high current has put forward higher requirements for the safety protection of rectifiers. Carefully selecting semiconductors and fast melting to cope with foreseeable rectifier load fluctuations is crucial for the continuous safe operation of rectifiers and the stability of aluminum production.
The No. 5 fast melting is developed specifically for the development trend of rectifiers. Recent experiments at ABB's high-power laboratory have shown that the rated voltage can reach 1850A at a rated current of 4000A. This progress indicates that the No. 5 fast melting series will be able to meet the requirements of the next generation high-power rectifiers in the aluminum industry. In fact, the rated parameters of No. 5 fast melting can be made higher to provide protection for higher power diodes and thyristors.
8. References
