Abstract: This paper analyzes and discusses several recent new technologies in low-voltage electrical appliances, such as single-break and double-break circuit breaker breaking technologies and their series of structural schemes, the energy-saving combination of permanent magnet contactors and intelligent operation, the design of a new arc-extinguishing chamber based on arc testing and air blowing mechanism, and the application of virtual prototypes to improve the breaking capacity of circuit breakers. These new technologies contribute to the technological innovation of low-voltage electrical appliances.
Keywords: Disconnection technology; Arc testing; Virtual prototyping
introduction
The development of the power industry places increasingly higher demands on the quality and quantity of low-voltage switchgear, which promotes the development of low-voltage electrical appliances towards high performance and miniaturization. Technological innovation and the application of new technologies are important foundations supporting this development. The application of air-blowing and double-break technologies has promoted the development of a new generation of low-voltage circuit breakers; permanent magnet contactors and their intelligent control have significantly improved contactor performance and achieved energy conservation; new arc testing technologies have helped improve the design of arc-extinguishing chambers; and virtual prototyping technology has established a brand-new R&D platform for low-voltage electrical appliances.
Comparison of single-break and double-break breaking technologies and structural analysis of two low-voltage circuit breaker series
Since the application and promotion of double-break and air-blowing technologies in the mid-1990s, following Schneider Electric's NS series, major international companies such as ABB, GE, LG, and Merrill Lynch have launched their own new series of double-break molded case circuit breakers (MCCBs) in the new century. These include ABB's Tmax, GE's RecordPlus, and Merrill Lynch's MZN. These rotating double-break structures all employ a separate arc-extinguishing chamber for each pole to ensure the sealing of the rear end of the arc-extinguishing chamber, and place gas-generating material in the contact area to achieve air blowing. Figure 1 compares the breaking performance of several companies' new 630A series products with traditional single-break products.
The single-break MCCB1 and MCCB2 are two of the better-performing products currently available on the domestic market. As can be clearly seen from Figure 1, the breaking performance of the double-break type is much higher than that of the single-break type. It can achieve Icu = Ics, especially at 690V, where the breaking performance of the single-break type is even lower.
While double-break structures offer advantages such as small size and high performance, their complex structure and demanding manufacturing processes also present limitations. Firstly, to ensure reliable contact at both points, the contact reaction force is significant, and the short lever arm makes them prone to retraction after contact separation when breaking low-expected short-circuit currents. Two Eaton products with the same rated current—one double-break and one single-break—were tested under low-expected current conditions with an oscillating circuit power supply. The breaking waveforms showed that the single-break structure broke normally, while the double-break structure exhibited a moving contact drop, leading to a prolonged arcing time and, in some cases, causing the moving and stationary contacts to re-close and weld. This phenomenon indicates that the breaking performance of the double-break structure is actually lower than that of the single-break structure when breaking low-expected short-circuit currents. Another issue is whether the contact condition between the two breaks and the breaking process can remain consistent. Xi'an Jiaotong University tested a double-break MCCB without a dedicated arc-extinguishing chamber. Using a two-dimensional fiber array fast imaging system, they obtained images of the arc motion at the two breaks. The images clearly show the inconsistency in the process of the arc entering the grid at the two breaks.
To mitigate the phenomenon of moving contact falling off and the imbalance of contact between the two breaks, rotary double-break circuit breakers need to be designed with a special locking mechanism after the contacts are pushed back, and a rigid connection is not used between the rotating shaft and the moving conductive rod. Each company has its own patents in these two aspects.
To accommodate varying user requirements for breaking performance, MCCB series products from different companies are categorized into economy, standard, high breaking capacity, and overcurrent limiting types. Currently, there are two approaches to selecting different structural types within a series: Schneider Electric, ABB, GE, LG, and Admiralty-Möller adopt a unified dual-break design. When the rotating dual-break structure is used in the economy and standard models, a simplified structure is employed to reduce raw material requirements and lower costs. In contrast, Japanese companies, represented by Mitsubishi, Fuji Electric, and Terasaki, primarily use a single-break structure. For example, the Mitsubishi WS new series MCCB, structures above 250A (see Figure 4) employ a rear-area enclosed structure, improving the breaking performance of the single-break structure. The 250A structure (see Figure 5) uses a separate arc-extinguishing chamber for each phase, with gas-generating material placed within the arc-extinguishing chamber to enhance gas blowing; this structure is known as automatic gas blowing technology using gas-generating material erosion. The super current-limiting type in this series uses a current-limiting head added to a traditional single-break circuit breaker to achieve multi-break disconnection during breaking, reaching Icu＝Ics＝200kA.
Dynamic simulation was performed on a three-air-gap permanent magnet contactor with a coil voltage of 24V. At the critical pull-in voltage, similar to a traditional DC-operated contactor, the contacts of the iron core exhibit strong vibrations. Therefore, permanent magnet contactors are more suitable for integration with intelligent operation to form intelligent permanent magnet contactors. Figure 9 shows a block diagram of an intelligent permanent magnet contactor with current feedback. The AC input is rectified and powered by PWM, supplying power to the coil through a power electronic switch MOSFET. The central control module receives the feedback signal of the coil current and controls the MOSFET. Different coil currents correspond to different modulation duty cycles, thus maintaining a constant current through the coil during electromagnet activation. This avoids the armature jitter phenomenon observed in typical DC electromagnets at the critical pull-in voltage. The current travel curve obtained from the simulation is shown in Figure 10.
Intelligent permanent magnet contactors can further achieve energy saving by changing the duty cycle after engagement. During engagement, the coil current Ix is kept constant, and its corresponding attraction force characteristic is slightly higher than the reaction force characteristic to reduce the kinetic energy of the moving iron core. When the moving iron core is engaged, the coil current is low and maintained at Ib to save energy. Therefore, intelligent permanent magnet contactors can achieve a soft landing of the moving iron core, which greatly improves the life of the contactor and further saves energy.
Application and promotion of virtual prototyping technology
Virtual prototyping technology has revolutionized the traditional design and development model, which relies heavily on experience and imitation. It serves as a new R&D platform for switchgear, enabling the development of new products with independent intellectual property rights and the improvement of older products. For example, a low-voltage electrical appliance manufacturer, through factory-university cooperation, modified the static conductor circuit of an 800A MCCB by changing the original flat-plate inlet static conductor rod to a parallel U-shaped inlet flat static conductor rod (see Figure 16). This improvement maintained the original contact opening distance, strengthened the arc-blowing magnetic field, and significantly increased the breaking capacity from 65kA to 85kA (see Figure 17). The development of new energy sources, the promotion and application of electric power generation, and the miniaturization and high performance of low-voltage electrical appliances have led to increased attention on 690V rated voltage switchgear. Low-voltage appliances at this voltage level can operate at peak voltages as high as 1073V. On the other hand, the miniaturization of electrical appliances involves the issue of heat generation, which directly affects the insulation. Therefore, the design of new electrical appliances must consider both insulation and heat generation simultaneously. For a long time, only high-voltage electrical appliances required electric field simulation and optimization design. However, under the new circumstances, insulation calculations for low-voltage electrical appliances have also become important. Recently, FREI et al. from Rockwell Automation used a simplified calculation method that reduces the three-dimensional field to a two-dimensional field to perform electric field simulation and optimization design of MCCBs. Figure 18 shows the arrangement of electrodes (bimetallic strips and stationary contact conductors, etc.) on the plastic base of the MCCB. Figure 19 shows the two-dimensional electric field distribution obtained from the original design simulation calculation, and Figure 20 shows the two-dimensional electric field distribution after optimization design. Comparing the two, it can be seen that the electric field at the highest point in the original design reached 2.43 kV/mm, while the electric field at the highest point after optimization design has been reduced to 1.24 kV/mm, and the electric field at other locations has also been significantly reduced.
