Abstract: With the accession to the WTO, China's low-voltage electrical appliances are facing greater challenges in the 21st century. In order to accelerate the development of low-voltage electrical appliances in China, adapt to the new situation, timely analyze the development trends of low-voltage electrical appliances at home and abroad, and understand and master the new technologies of low-voltage electrical appliances, it is of great significance. This article discusses intelligent appliances, communicable appliances, and information appliances; Simulation technology and virtual appliances; Environmentally friendly electrical appliances; Several aspects such as high performance, miniaturization, and modernization of structural design of electrical appliances were discussed.
With the accession to the WTO, China's low-voltage electrical appliances are facing greater challenges in the 21st century. In order to accelerate the development of low-voltage electrical appliances in China, adapt to the new situation, timely analyze the development trends of low-voltage electrical appliances at home and abroad, and understand and master the new technologies of low-voltage electrical appliances are of great significance. This article discusses intelligent appliances, communicable appliances, and information appliances; Simulation technology and virtual appliances; Environmentally friendly electrical appliances; Several aspects such as high performance, miniaturization, and modernization of structural design of electrical appliances were discussed.
1、 Smart appliances, communicable appliances, and information appliances
The introduction of microprocessors and computer technology into low-voltage electrical appliances not only enables them to have intelligent functions, but also enables low-voltage switchgear, including intelligent circuit breakers and intelligent motor controllers, to achieve bidirectional communication with the central control computer. In the 1990s, with the development of computer information networks, low-voltage distribution systems and motor control centers have unified into intelligent monitoring, protection, and information network systems.
Intelligent low-voltage circuit breakers and motor controllers are the main electrical components for achieving intelligence in low-voltage switchgear and motor control centers. The introduction of microprocessors into low-voltage circuit breakers greatly enhances their protection function. For example, the short delay in its three break protection feature can be set to the I2t characteristic to better match with the subsequent level of protection; Grounding protection can achieve selectivity and can provide memory function for intermittent arc grounding faults.
At present, a large number of soft starters, power electronic speed control devices, and uninterruptible power supplies are used in the electrical network of industrial power consumption. These devices will generate high-order harmonics in the distribution system, and analog electronic releases generally reflect the peak value of fault current. Therefore, high-order harmonics in the power supply can cause the misoperation of circuit breakers. The intelligent circuit breaker with microprocessor reflects the true RMS value of the load current. Its sampling and holding circuit can eliminate high-order harmonics in the input signal, thus avoiding misoperation caused by high-order harmonics. Its central processing unit can ensure the correct operation of the circuit breaker.
Compared with traditional bimetallic thermal relays, intelligent overload relays have a series of advantages. It can protect motors under various starting conditions and has high operational reliability. It can not only protect the motor from overload and phase failure, but also protect against grounding, three-phase imbalance, reverse phase or low current, etc. The intelligent motor controller further developed by intelligent motor protection relays has functions of monitoring, protection, and communication, and its protection function is further enhanced compared to intelligent motor protection relays.
The network system composed of new low-voltage electrical components and central control computer has the following advantages compared to traditional low-voltage distribution systems and motor control centers:
① The implementation of centralized computer control has improved the automation level of low-voltage distribution systems. The bidirectional transmission of data information between low-voltage switchgear and central control computer refers to: on the one hand, intelligent circuit breakers or other switchgear transmit the operating parameters, protection setting values, and fault information of the circuit and protection objects to the central control computer; On the other hand, the central control computer sends operating instructions to intelligent circuit breakers or other switchgear to change and adjust the protection setting value.
② Bring the scheduling and maintenance of low-voltage distribution and control systems to a new level. Through information transmission, the central control computer can store the past operating parameters of the circuit and protected equipment, as well as the first 10 or even the first 50 fault situations. These pieces of information, combined with the powerful comprehensive computing power of computers, help operators predict the occurrence of faults and make corresponding decisions in advance, greatly reducing the downtime and maintenance time of lines and equipment; Information transmission has also improved the power dispatch capability of the distribution system, ensuring safe and reliable power supply to important users during peak electricity consumption periods; Information transmission can achieve inter regional interlocking protection, enabling a reasonable matching of selective protection for the front and rear intelligent circuit breakers.
③ Due to the use of digital new monitoring components, the information provided on the plane of the switchgear is significantly increased. The LCD display panel can measure and display various parameters such as current, voltage, power factor, and power of the phase and line, as well as display the past operating parameters and fault conditions of the protected object and line. It can also display the set circuit breaker protection characteristics in a graphical manner.
④ Compared with traditional indicator and instruction appliances, monitoring components have simple wiring and are easy to install, which improves the reliability of work.
⑤ Can achieve data sharing, reduce information duplication and channels
The development of network communication increasingly requires openness and compatibility between users and devices, so developing a unified communication protocol is a key issue that needs to be urgently addressed. At present, the automation communication network composed of intelligent electrical appliances and central computers through interfaces is developing from centralized control to distributed control. The emergence of fieldbus technology not only provides conditions for constructing distributed computer control systems, but also has the advantages of plug and play, good scalability, and easy maintenance. Therefore, this technology has become a focus of attention both domestically and internationally.
Fieldbus is a fully digital and open bidirectional local communication network that connects intelligent field devices and control rooms. There are many types of fieldbuses, among which the most influential are CAN, Lonworks, and Profibus. They each have their own characteristics and advantages, and have different application values in different fields. But it is precisely the diversity of fieldbus technology and the fragmentation of communication standards that have had a negative impact on the development of this technology. At present, the International Electrotechnical Commission and the Fieldbus Foundation are committed to unifying fieldbus standards, and some units in China are also working in this area. We refer to electrical appliances with communication functions based on fieldbus technology as communicable appliances. At present, fieldbus technology is extending upwards and downwards, with its upper end communicating with the backbone Ethernet, Intranet, and Internet of the enterprise network, and its lower end extending to the industrial control field area. In this regard, there are two bus systems, Siemens' AS interface and Rockwell's Devicenet. These two buses are used in low-level areas and are a low-cost communication connection. They connect industrial control sensors and actuators, including limit switches, photoelectric sensors, motor starters, button indicator lights, multifunctional relays, and other simple control components as slave devices to the network, and communicate with the control computer or programmable logic controller (PLC) as the main device. The contact conversion information of traditional relay contact control systems is converted into binary digital information for communication through serial interfaces and addressing. This has revolutionized traditional low-voltage control components, such as those used in Siemens AS bus systems. The logo multifunctional relay is actually a small PLC, It can perform functions such as delay, latch, counter, and pulse relay.
Further realizing informatization of smart appliances is to enable them to achieve Internet/Intranet functionality at the field level, with the technical core being the implementation of TCP/IP protocol. Embed the TCP/IP protocol into the ROM of smart appliances, so that signal transmission and reception are carried out in TCP/IP mode, further developing the information technology function of smart appliances. By utilizing the functions of Internet/intranet, not only can authorized users of the enterprise's network be authorized, but also on-site information can be shared and remotely controlled, programmed, and configured through browsing in any region where the Internet is enabled. This has brought intelligent appliances into a new era of information appliances.
One important function of low-voltage electrical appliances is their on/off capability. In recent years, another way for the development of intelligent electrical appliances is to improve their on/off performance through the intelligent control of microprocessors. A noteworthy new trend is the new generation DIL-M series contactors developed by Admiralty Moller, which adopt intelligent control technology in current levels above 185A. The wear of the contacts when the contactor is closed is mainly caused by intermittent arcs caused by the bouncing of the contacts. Under the AC-3 working task, the contactor needs to close 6 times the rated current at the rated voltage, so it is the main factor affecting the electrical life of the contacts. The electromagnetic coil of the DIL-M series intelligent control contactor is powered by a pulse rectifier voltage, which can adjust the coil current according to a suction characteristic curve stored in the microprocessor that best matches the reaction characteristics. During the contactor suction stage, a wide pulse voltage is input to ensure safe and reliable suction of the main contact. During the holding stage, a narrow pulse voltage is input to save energy under sufficient contact closure conditions. Due to the use of intelligent control, this type of contactor can achieve an electrical lifespan of 1.3X106 times under the condition of saving contact materials. With the use of intelligent control, the electromagnetic system cross-section and geometric dimensions of different specifications of contactors are consistent, which brings convenience and savings to production and compensates for the additional expenses caused by electronic control.
The development of intelligent electrical appliances has made electromagnetic compatibility EMC an increasingly important issue. The EMC requirement includes two meanings: on the one hand, it requires low-voltage electrical appliances to not be affected by external electromagnetic interference and cause misoperation during operation, and on the other hand, it requires that the electromagnetic field generated by the operation of electrical appliances does not interfere with nearby electronic devices. At present, foreign countries attach great importance to the EMC problem of intelligent electrical appliances and electromechanical products, because electromagnetic interference can cause such systems to malfunction and malfunction, resulting in huge economic losses. Intelligent electrical appliances and their protection and monitoring systems expose sensitive digital electrical components to strong current and high voltage electromagnetic fields, making the electromagnetic anti-interference ability of these devices an indispensable factor in equipment design and operation. Therefore, in the initial design stage of intelligent electrical appliances and their systems abroad, strict electromagnetic compatibility control and management plans are formulated. This plan mainly includes product or system EMC analysis, the development of EMC design technical indicators, design plans, standards, implementation plans, and testing methods, etc., and this plan is regarded as an important part of product or system design. EMC analysis and design are key tasks to meet EMC technical requirements, including analyzing the radiation level and anti-interference ability of electronic circuits, as well as the electromagnetic compatibility performance of system integration. EMC design includes electromagnetic shielding, grounding, determination of wire spacing, and consideration of electromagnetic coupling between printed circuit board wiring. Currently, with the development of high-frequency electromagnetic field numerical analysis and computer hard years, modern simulation technology has replaced traditional testing methods and empirical analysis methods, and has played an increasingly important role in EMC analysis.
2、 Simulation Technology and Virtual Electrical Appliances
In recent years, domestic factories and design institutes have successively introduced 3D computer-aided design software such as UGH and Pro/E. These software can achieve the modeling, assembly, and automatic generation of engineering drawings of components and entities in 3D space, and automatically design molds and generate CNC codes according to the designed components. These software have taken the design of low-voltage electrical appliances in China to a new level, but in order to further meet the requirements of the original technical conditions and achieve the predetermined electrical and mechanical performance, simulation technology is needed.
Design a low-voltage electrical product. After determining the preliminary design scheme and dimensions based on the given technical conditions, engineering analysis or prototype experiments must be conducted to verify whether the design scheme complies with the original technical requirements. For a long time, traditional engineering calculation methods have been used for characteristic analysis, but the accuracy is poor, especially as the main characteristic of low-voltage switchgear, that is, the breaking characteristic cannot be calculated. Therefore, people have to rely on prototype manufacturing and experimental verification to verify the feasibility of the design scheme. This approach requires a lot of manpower and material resources, and prolongs the product development cycle, affecting the market competitiveness of new products.
In order to solve the above problems, computer simulation and simulation technology has developed rapidly in recent years. With this new technology, people can accurately grasp the performance of designed products before prototype production, reduce repeated prototype production and experimental costs, accelerate product development cycles, and improve product performance. This is an important part of the modernization of low-voltage electrical product development methods.
The basic characteristics of low-voltage electrical appliances include breaking capacity, temperature rise, strength of components, electrical and thermal stability, insulation performance, and other electrical properties. This requires simulation and analysis of the physical fields such as electromagnetic field, stress field, and magnetic field of the design object. The progress of computer imitation and simulation technology and the continuous improvement of the performance of commercial finite element analysis software have created conditions for the application of this new technology in low-voltage electrical appliances. The finite element analysis software used in the 1970s and 1980s had very complex pre-processing and post-processing tasks, such as analyzing the electric field of a large transformer and inputting raw data such as the three-dimensional dimensions of various components, which usually required several days or even weeks of hard work. In the 1990s, commercialized finite element analysis software was combined with visualization technology, using feature modeling to input three-dimensional graphics instead of data input for each lock, making input work very simple and intuitive. The post-processing part also makes the output data or three-dimensional graphics easy to observe and analyze. At the same time, with the need to solve complex engineering problems, this simulation and analysis software has been expanded to include fluid dynamics, mechanical vibration, and mechanism dynamics. At present, various precise computer simulation and analysis software are available in the market, which can be divided into two types: general software and specialized software. For example, ANSYS (ANSYS company) and NISA (EMRC company) finite element general simulation and analysis software belong to the first category. These software include stress, temperature field, electromagnetic field, and flow field analysis modules, which can perform single field analysis or comprehensive field analysis. For example, to calculate the protection characteristics of a fuse, the distribution of the current field in the fuse must be calculated first, followed by the calculation of the thermal characteristics. This is a combination of current field and transient thermal field calculation. Specialized software refers to specialized applications, such as ADAMS. The software is used for dynamic analysis of institutions, while the VACTOR FIELDS software is specifically designed for the analysis of various electromagnetic field problems. Both of these software have visual pre - and post-processing modules, and both come in workstation and microcomputer versions. In recent years, well-known foreign companies have regarded computer simulation and analysis of characteristics as an important measure for modernizing product development methods. For example, Fuji Corporation in Japan believes that computer simulation and the use of new materials are the key to achieving high performance of the new series of contactors in the development of SC new series contactors. They have developed a software system for dynamic simulation of the movable part of contactors. They have conducted dynamic simulation of the movable part of contactors, including the iron core and contact system, analyzed the impact of various parameters on the bounce of the iron core and contact after collision, and selected the best design parameters. Mitsubishi Corporation in Japan and Jinzhong Moeller Corporation in Germany both use commercial computer analysis software as the basis and establish their own through secondary development. A computer simulation system for the design and development of low-voltage circuit breakers.
Through computer simulation, a feasible solution for product design can be obtained, which ensures that the design meets the requirements of the established technical conditions. However, it is not yet the optimal solution for economic and technical indicators. In order to achieve the best economic and technical indicators, simulation technology and optimization methods need to be combined. Some commodity simulation software, such as the ANSYS software mentioned above, provide a series of optimization tools that can automatically adjust the geometric shape of the design object to achieve the optimal specified goal. Professor Lindmayer, a renowned low-voltage electrical expert in Germany, used ANSYS software to combine magnetic field simulation and optimization, change the size of the iron core and coil position, and achieve optimized design of the electromagnet.
The development of computer graphics technology and the emergence of a new 3D interactive graphics technology, virtual reality technology, have opened up new horizons for the development of computer simulation technology. Virtual reality is different from traditional interactive 3D graphics technology. The latter allows operators to operate computer graphics in the real world outside the computer, while the former allows operators to enter the virtual world inside the computer to operate in person. By combining this technology with computer simulation, products can be designed inside the computer. This type of product is called a virtual product, and designers can simulate and optimize the product in a "virtual" environment. This is the development direction of computer simulation technology. For a long time, low-voltage electrical products in China have often gathered technical personnel from various research institutes and factories together for joint design, which has caused many inconveniences for most designers to leave the factory environment. The Computer Supported Collaborative Design (CSCD) technology that emerged since the 1990s can provide an environment for remote remote collaborative design. It is a new concept of group design based on the Internet, and recently foreign computer-aided design software such as PRO/E and the Internet have added this feature. With this technology, people can discuss design proposals, draw and modify design drawings together through the Internet at different locations.
3、 Environmentally friendly electrical appliances
With the development of industry and agriculture, environmental protection issues are becoming increasingly serious, which poses new requirements for the widely used low-voltage electrical appliances. Nearly 80% of the materials in low-voltage electrical appliances are plastic. Therefore, for these materials, on the one hand, it is necessary to ensure long service life and the reliability of the electrical appliance itself, and also consider environmental protection requirements, which are pollution-free and recyclable. Currently, flame retardant plastics that do not contain CFC or halogens have been promoted and applied.
For a long time, AgCdO has been widely used in low-voltage electrical appliances, especially as a contact material for control appliances, due to its good resistance to arc erosion. However, in recent years, due to the toxicity of AgCdO materials, the use of AgSnO2 instead of AgCdO materials has been promoted for environmental protection purposes. However, the high temperature rise of AgSnO2 contacts has always been a key technical issue. At present, some international contact material manufacturers are researching how to reduce contact temperature rise by adding additives to prevent the formation of SnO2 film on the contact surface. Degussa AG, a German company, has launched a new grade of AgSnO2SPWIII material with a composite of WO3 and BiO3 as additives. It not only has a temperature rise similar to AgCdO, but also significantly improves the resistance to arc erosion and fusion welding compared to the older grade of AgSnO2 material.
The use of vacuum technology and power electronics technology is an important way to solve environmentally friendly electrical appliances. At present, vacuum technology has taken a dominant position in the field of medium voltage switches, and there has been significant development in the field of low voltage switches in recent years. There is no gas in vacuum, and only a metal vapor arc with less energy can be generated when breaking a faulty circuit. Its strength, arc burning time, and erosion of the contacts are less than in air, so vacuum switches do not require maintenance. From an environmental perspective, the contact system of a vacuum switch is enclosed in a vacuum tube shell, and the arc generated when the contact is disconnected will not affect the environment. Therefore, it can work in harsh and unfavorable working environments. In the field of low-voltage electrical appliances, although its price is higher than that of general air arc extinguishing switch appliances, its excellent performance and environmental protection have increasingly attracted people's attention. Previously, vacuum contactors were generally used in special environments. Currently, due to the progress of vacuum technology and its application, they have been promoted to universal contactors. Recently, the newly launched contactor series by German company Jinzhong Moeller adopts vacuum contactors for rated currents ranging from 580A to 820A. This type of contactor has a very long electrical lifespan, and due to the absence of arcing, it requires less installation space. The viewing window of the shell is easy to observe the wear of the contacts. Vacuum technology is also applied to low-voltage circuit breakers. Siemens first introduced the 3WS1 vacuum low-voltage circuit breaker in the mid-1990s, with a rated current of 630A to 2500A and a breaking capacity of 50KA. At present, domestic research institutions and factories are also actively developing this product, with the main goal of improving its rated current and breaking capacity.
Due to the development of power electronics technology and the emergence of third-generation high-power semiconductor switching devices such as GTO, GTR, and IGBT, solid-state contactless switches have also made significant progress in recent years. Compared with mechanical switches, it is a non arc switch, which has a long lifespan and does not require maintenance. From an environmental perspective, on the one hand, because there is no arc, it will not pollute the environment due to the gasification of contact materials and plastics caused by the arc, and on the other hand, it will not affect contact reliability due to environmental pollution and the formation of oxide film on the contact. Due to the lack of contacts, mechanical switches do not generate heat due to the action of arcs. As a result, the operating frequency of contactless switches can reach a high level. Solid state circuit breakers are limited in their application due to high losses during operation, a lack of power electronic devices with sufficient rated voltage and current, and their large size and high price. However, in the past decade, research work has gradually overcome its shortcomings. Takamu [2] proposed a hybrid solid-state circuit breaker, which is applied to 400V low-voltage lines. The main circuit is composed of a switchable thyristor GTO in parallel with a vacuum switch to ensure that the circuit is connected by a vacuum switch during normal operation, reducing the power loss of the GTO. A fast response overcurrent protection is applied in solid-state circuit breaker circuits with sensitive relays to reduce the overheating effect of short-circuit current on the thyristor GTO. When a fault occurs in the circuit, the signal sent by the overcurrent relay simultaneously opens the vacuum switch and connects the GTO device. The arc voltage on the vacuum switch transfers the fault current to the GTO circuit. Subsequently, the GTO quickly shuts off, causing the circuit breaker to break the fault current. This line reduces the burden on the GTO depending on the rapid action of the sensitive relay. Another frontline router proposed by Brice et al. [3] combines hybrid solid-state circuit breakers with polymer current limiting resistors (PTCs). PTC is a thermistor with positive temperature characteristics, and its reaction speed is very fast. When the temperature rises to the Curie temperature, its resistance increases sharply to limit the short-circuit current. A solid-state circuit breaker with PTC is connected in series with a PTC current limiting resistor to the GTO device. When breaking the fault current, the current limiting effect of PTC is relied on to reduce the burden on the GTO device. Among the above two types of solid-state circuit breakers, metal oxide surge arresters and absorption circuits are used to absorb overvoltage during GTO switching.
4、 High performance, miniaturization, and modernization of structural design
Due to the development of urban power grids and the increase in distribution transformer capacity, high-performance and miniaturization requirements have been put forward for low-voltage switchgear. In order to improve the breaking capacity of low-voltage switchgear, foreign companies have proposed some new arc extinguishing systems and current limiting technologies, among which Mitsubishi of Japan has proposed an ISTAC technology based on the VJC technology invented by the company. This technology increases the electrodynamic force acting on the moving contact rod through the special arrangement of conductive circuits on one hand, and on the other hand, uses narrow slits formed by VJC material to enhance the cooling effect on the arc. Meilan Gelan Company has launched a rotating dual break current limiting structure and proposed a new concept of "energy matching" in terms of the coordination of front and rear protection characteristics. In recent years, PTC thermistor current limiting resistors, as mentioned above, have attracted great interest. Firstly, by using this type of current limiting resistor in series with plastic shell switches, the current limiting effect can be greatly enhanced. Currently, PTC resistors can also be used as current limiting devices alone, and their application scope has been expanded to the medium voltage field.
There are two meanings to miniaturizing switch appliances. On the one hand, the size of the appliance itself should be small, and on the other hand, it is to reduce the arc spraying distance or achieve "no flying arc" to reduce the size of the switchgear for installing such appliances. The new type of contactor proposed by Mitsubishi Company is equipped with a porous metal plate at the outlet of the arc extinguishing chamber to suppress the hot air flow, which can reduce the temperature of the sprayed hot air flow to the original level