With the development and progress of the electrolytic technology level, the manufacturing level of rectifier equipment in China has also been greatly developed and improved. In the past 10 years, the electrolytic aluminum project has been built and put into operation. Rectifier unit equipment from DC sensors, DC high current isolation switches to voltage regulating rectifier transformers and rectifiers are all developed, developed and manufactured by domestic manufacturers, which has made great contributions to the development and construction of China's electrolytic aluminum industry.

Rectifier unit as the core equipment of electrolytic aluminum series power supply, its importance can be imagined, once a serious failure not only directly affects the normal production of electrolytic series, but also the direct and indirect losses can not be estimated.

However, in the past 10 years, with the increasing production capacity of single series electrolytic aluminum, the single unit capacity, DC rated output voltage, current, power supply voltage and power grid capacity of rectifier devices have reached a high level, and the design, manufacture, operation and maintenance of rectifier devices are highly required, the requirements for voltage regulating rectifier transformer only stay in the main wiring, type structure, loss, shape and size, etc., while the requirements for rectifier only focus on the parameters of the main components, the loss of the whole machine and the shape and size of the rectifier cabinet, etc., and the requirements for other important details inside the rectifier unit are few. In this way, the overall coordination of voltage regulating rectifier transformer and rectifier, structural manufacturing and selection safety of auxiliary components are determined by the manufacturer, the lack of a unified and standardized constraints also left many problems for the long-term stable operation.

Problems in control, protection and structure of rectifier unit

With the development and application of large-scale electrolytic aluminum series, the series of DC voltage is getting higher and higher, the current is getting larger and larger, the equipment level and design requirements of the rectifier should also be improved accordingly, this should be in-depth study and put forward the corresponding countermeasures, the overall design of the rectifier to be improved. However, due to the large area of electrolytic aluminum plant projects concentrated in the past few years, there was no more detailed work on the design of rectifier unit equipment and rectifier, only to increase the DC voltage and increase the DC current.

In the past two years, through the communication and contact with international well-known companies, we have a certain understanding of some foreign design concepts and technical requirements. In addition, the recent continuous occurrence of rectifier accidents, through the analysis of the causes of failures and the summary of experiences and lessons, I think the design of rectifier we have been doing, especially the design of high-voltage large series electrolytic aluminum rectifier, whether it is the equipment level of the rectifier unit equipment body, the design level and the system design of the rectifier unit, there are some problems in the control, protection, structure and many other aspects. For example:

(1) Set up perfect and reliable DC side reverse flow protection is an important means of rectifier system protection, however, due to the domestic reverse flow protection device is not perfect, the cost of direct purchase of foreign and high, therefore, many aluminum plant did not set the protection;

(2) The arc detection protection in the rectifier cabinet can effectively detect the small accident arc in the rectifier cabinet, and act on the alarm or trip in time to prevent the occurrence of serious arc short circuit fault. Only a little understanding of this technology can be obtained, and further research is needed on how to apply it;

(3) Rectifier cabinet design insulation level is not high, anti-short-circuit ability, anti-arc short-circuit ability is not enough;

(4) The detection and protection of the auxiliary system are not perfect;

(5) Rectifier cabinet shell insulation testing practices are different, there is no uniform industry standard.

The coordination of voltage regulating rectifier transformer and rectifier needs to be improved

The voltage-regulating rectifier transformer and rectifier should be an inseparable whole to form the rectifier unit. The main wiring form and short-circuit impedance value of the voltage-regulating rectifier transformer should be considered in coordination with the component selection, dynamic and thermal stability design and protection of the rectifier. In foreign countries, the design is unified by one manufacturer, while in China, the two manufacturers are completely independent, the design of voltage regulating rectifier transformer does not consider the design and manufacture of rectifier, the design of rectifier is not very understanding of voltage regulating rectifier transformer, the connection between them is realized by the design institute, and the role of the design institute is very limited.

For example, when determining the coordination between fast melting protection and silicon components and the internal dynamic and thermal stability design of rectifiers, various forms of short-circuit parameters in rectifiers need to be calculated in detail. When calculating short-circuit parameters, the main wiring, short-circuit impedance and coil arrangement of voltage regulating rectifier transformers should be understood in detail. However, rectifier manufacturers are about to turn rectifiers into standard products, rectifiers for different users can be manufactured only by slightly changing the number of components, specifications and fast melting parameters, and seldom consider the influence of power supply system and structure formation of voltage regulating rectifier transformer.

Problems in Rectifier Cooling System

The power of the aluminum electrolysis rectifier is very large, and the cooling of the rectifier adopts circulating pure water cooling, that is, using deionized water as a medium to bring the heat of the rectifying element, fast melting, conductive copper bar, etc. to the heat exchanger for cooling.

The cooling system of the rectifier consists of two parts: one is a closed-circuit deionized water system in which the cooling pump unit is directly connected to the rectifier; the other is a closed-circuit circulating water system or cooling fan that cools the heat exchanger.

Cooling pump group and rectifier directly connected to the closed-circuit circulating water system, the water quality requirements are very high, the current domestic use of deionized water, however, because the circulating water system is not completely closed, water quality of the game faster.

The cooling of the heat exchanger, currently the most used in China is a closed-circuit circulating water system, that is, a circulating water cooling system is built near the rectifier. The biggest problem in this way is that when the water pump room fails or the water circuit fails, all the rectifier devices are cut off. Another is to use a fan to directly cool the heat exchanger. This method is a unit independent system with reliable operation, but it is rarely used. Only a few aluminum plants such as Qinghai Aluminum Plant and Henan Jiaozuo Wanfang Aluminum Plant are used.

solutions and countermeasures

Through the above discussion, it can be seen that in recent years, with the development of electrolytic aluminum industry, China's rectifier equipment manufacturing industry has also been rapid development, but also exposed many problems, we need to make great efforts to change the status quo. Through the analysis of the current situation of rectifier equipment and the failure of the rectifier device in the near future, combined with the understanding of some design and manufacturing concepts of foreign rectifier equipment in recent years, it is believed that the following aspects should be improved:

(1) Develop or revise industry standards for the design, manufacture, test and acceptance of rectifying devices as soon as possible;

(2) When providing the technical requirements of the important equipment of the rectifier, it is necessary to further standardize and standardize, and the standards should be improved. It not only puts forward reasonable requirements for the main components, fast melting, loss, etc., but also puts forward requirements for the selection of internal auxiliary components from the specifications and structure of the components;

(3) The structure of the rectifier cabinet should be reasonably configured, for example, the dynamic and thermal stability check of the rectifier cabinet, arc prevention measures, insulation withstand voltage, installation and configuration of auxiliary protection original devices, etc;

(4) Further improve the system control and protection design of the rectifier unit and the rectifier, and add some necessary protection to the system design of both the rectifier device and the rectifier;

(5) as soon as possible to explore the feasibility and necessity of the application of arc detection and protection devices, to find a solution to the rectifier cabinet produced in the early detection of small accident arc measures;

(6) The DC side of the rectifier is provided with perfect and reliable reverse flow protection;

(7) Suggestions for improvement of circulating water system of rectifier: for cooling the circulating deionized water system of rectifier cabinet, it is recommended to adopt a closed circulating deionized water system completely isolated from air to ensure long-term stability of water quality; The objects cooled by the closed circulating deionized water system are all live conductors. The water cooling jacket of domestic components, components and fast-melting copper bars are made of pure copper, which has poor corrosion resistance. Improvement of related materials should be considered; the cooling medium of the heat exchanger is recommended to be cooled by fan, that is, the water air cooling unit, and the centralized circulation cooling water pump house is canceled.

(8) The voltage regulating rectifier transformer shall be provided with a sound and reliable fire extinguishing system;

(9) in the fire water pump room and other important components to set up sufficient capacity to start the diesel generator set.

At present, the overall technology of domestic rectifier has reached a considerable level, but there are still some problems need to be improved, otherwise it is difficult to be in line with international standards, more difficult to enter the international market, it is urgent to establish a series of practical design, manufacturing, testing, acceptance and other standards, improve the manufacturing process and the corresponding test means.