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
0 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.
1. 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.
In the figure, the single-break MCCB 1 and MCCB 2 are two of the better-performing products currently sold in the domestic market. As can be clearly seen from Figure 1, the breaking performance of the double-break is much higher than that of the single-break, and it can achieve Icu = Ics . Especially at 690V, the breaking performance of the single-break 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 between the two contacts, the contact reaction force is significant, and the short lever arm makes it prone to contact retraction and fall-back when breaking low-expected short-circuit currents. Figure 2 shows one double-break and one single-break product from Eaton Corporation with the same rated current, tested under low-expected current conditions with an oscillating circuit power supply. The breaking waveforms show that the single-break structure breaks normally, while the double-break structure exhibits a moving contact drop-off, 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. The arc motion images of the two breaks were measured using a two-dimensional fiber array fast imaging system (see Figure 3). The figure clearly shows 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 tripping, reaching Icu = Ics = 200kA. Table 1 compares the two schemes.
2. Permanent Magnet Contactors and Intelligent Contactors
Due to energy-saving requirements, permanent magnet operated contactors have received considerable attention both domestically and internationally. Currently, many domestic solutions rely on permanent magnets to maintain the electromagnet's engagement position, which presents a voltage loss protection issue when the power is cut off. Proposed by Schneider Electric, a three-air-gap permanent magnet contactor (see Figure 6) has recently become popular internationally. It relies on a reaction spring for release, thus eliminating the voltage loss protection problem. Figure 6(b) illustrates its working principle. Air gaps 1 and 2 are the main working air gaps used to generate attraction, while air gap 3 is used to generate the release position holding force, i.e., the reaction force. Because the permanent magnet acts as the release holding force, the force of the reaction spring is reduced. Furthermore, the permanent magnet also participates in attraction during engagement, thus saving energy. Secondly, this electromagnetic structure causes the magnetic flux generated by the permanent magnet in the iron core and the coil to cancel each other out, reducing the magnetic flux density in the iron core and allowing for a smaller core size.
Dynamic simulation was performed on a three-air-gap permanent magnet contactor with a coil voltage of 24V. Figures 7 and 8 show the current and travel curves when the coil is subjected to the rated voltage and the critical pull-in voltage of 14V, respectively. As can be seen from the figures, at the critical pull-in voltage, similar to a traditional DC-operated contactor, the contacts of the iron core will vibrate strongly. Therefore, permanent magnet contactors are more suitable for integration with intelligent operation to form intelligent permanent magnet contactors. Figure 9 shows the block diagram of an intelligent permanent magnet contactor with current feedback. The AC input is rectified and generated using 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 common in DC electromagnets at the critical pull-in voltage. The current travel variation curve obtained from the simulation is shown in Figure 10.
Intelligent permanent magnet contactors can further achieve energy savings by changing the duty cycle after engagement. During engagement, the coil current Ix remains constant, and its corresponding attraction force characteristic is slightly higher than its 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 (see Figure 11). Therefore, intelligent permanent magnet contactors significantly improve contactor lifespan and further save energy by achieving a soft landing of the moving iron core.
3 New Arc Measurement Technology and Air Blowing Mechanism
Because the magnetohydrodynamic model of electric arcs is still imperfect, relying on simulation to optimize the design of arc extinguishing chambers is premature. Therefore, the development of new arc extinguishing chambers mainly relies on modern arc testing technologies, the most important of which is the monitoring of arc movement within the chamber. Existing methods utilize high-speed cameras and two-dimensional fiber optic imaging systems. Recently, the Montlucon and Christian Arnoux Electrical Technology Laboratory in France collaborated with Schneider Electric to research a magnetovisual technology based on the inverse problem of magnetic fields. The principle involves placing Hall sensors on both sides of the arc extinguishing chamber. As the arc moves within the chamber, the measured magnetic field is used to calculate the current density distribution in space using the inverse problem, thus determining the arc's position at each instant. In the measurement, the arc is considered as a parallel stack of multiple vertically stacked hexahedral unit volumes, assuming that the current is uniformly distributed within the unit. Based on the magnetic field BA measured by the magnetic sensors, the current density i of each unit can be calculated using the following formula, thereby determining the arc position.
In the formula: N is the number of sensors; g is the coefficient in the matrix.
The laboratory used this method to measure the process of arc transfer from the moving contact to the arc track in a model arc-extinguishing chamber. The expected short-circuit current was 12kA. The test was conducted on three different contact materials: material a was Cu, material c was AgWC, and material c was AgC. Figure 12(a) shows the test setup. The parallel arc track was 80mm long and 4mm wide, with an electrode gap of 20mm. Hall sensors were placed on both sides of the model, with 20 sensors on each side and a 1mm gap between the two sensors. Figure 12 shows the arc images of the three materials at the instant before arc transfer (t1 ) , the instant at the start of transfer ( t2) , and the instant after transfer (t3 ) . The contact material Cu transferred the arc the fastest.
As a novel method, its advantage is that it does not damage the outer shell of the arc-extinguishing chamber, but it is susceptible to the influence of the magnetic conductor inside the arc-extinguishing chamber, and cannot be used when there is magnetic shielding inside the arc-extinguishing chamber. A comparison of the three arc-detecting camera systems is shown in Table 2.
Air blowing technology is now widely used in arc extinguishing of low-voltage circuit breakers. However, many scholars have found through measurements that the pressure below the arc in the arc extinguishing chamber is lower than the pressure in front. This pressure distribution suggests that the arc will move downwards rather than forwards. In recent years, Xi'an Jiaotong University and the University of Brunswick in Germany, through gas dynamics and magnetohydrodynamic analysis and simulation, have proposed that the air blowing effect is caused by the reflection of the shock wave generated by the arc. Xi'an Jiaotong University used a mathematical model combining the conservation equations in fluid dynamics with a chain arc model to simulate the pressure propagation and distribution process in a simple model arc extinguishing chamber (see 13). The arc extinguishing chamber is 0.27m long, with a cross-sectional area of 2.0× 10⁻³ m² , closed at the left end, and has an outlet of 2.0× 10⁻⁴ m² at the right end at x＝b＝0.27m. The initial position of the arc is x＝a＝0.12m. The arc extinguishing chamber is subjected to a constant magnetic field B＝0.001T, pointing outwards from the paper. With a breaking current of 20kA, the simulation results are shown in Figure 14. At t=0.75ms after arcing, left- and right-moving shock waves are visible. At t=0.75ms, the left-moving shock wave is close to the bottom of the arc-extinguishing chamber. At t=0.80ms, the left-moving shock wave generates a pressure wave reflection at the bottom, while the right-moving shock wave reaches the right end. Due to the outlet at the right end, pressure and expansion waves are reflected, and the expansion wave reduces the pressure at the right end of the arc-extinguishing chamber. At t=1.20ms, the reflected waves at both ends form a high-pressure zone in the middle of the arc-extinguishing chamber. This, along with the pressure gradient at the outlet, is the source of the air blow-out, indicating that the air blow-out effect is caused by the reflection of the shock wave generated by the arc.
Figure 15 shows the arc motion image, shock wave reflection, and pressure distribution obtained from an arc motion simulation in a simple arc-extinguishing chamber using an arc magnetohydrodynamic model at the University of Braunschweig, Germany. At t=10μs, the arc ignites, generating a shock wave that moves upwards and downwards, reaching the bottom. At t=15μs, a pressure wave is reflected. At t=25μs, the arc is propelled forward, and this process repeats, causing the arc to move forward repeatedly. The upward-moving shock wave reaches the lower end of the grid at t=35μs. At t=45μs, the first reflected wave reaches the bottom, with a shock wave velocity of 400–600 m/s. The first upward shock wave reaches the top at t=78μs, generating expansion waves (90μs, 100μs), which are reflected downwards, causing a decrease in internal pressure (140μs, 200μs). During the 300μs interval, the arc moves upwards. At 380μs, negative air pressure fills the arc-extinguishing chamber, causing the arc to move in the opposite direction. At 380μs, the pressure distribution reverses again, allowing the arc to continue moving forward. At 550μs, the arc reaches the lower end of the grid plate, then elongates and bends along the plate, and is cooled by it. Under the action of the shock wave, the pressure distribution in the arc-extinguishing chamber changes continuously during the breaking process, affecting the arc's movement. Based on the above-described air-blowing formation mechanism, it can be explained why the novel arc-extinguishing chamber design requires a closed rear end, as this facilitates pressure wave reflection at the bottom.
4. 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.
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This paper discusses and analyzes several new technologies in low-voltage electrical appliances.
