Research and Development of High Voltage Vacuum Full Range Current Limiting Fuse for Protecting Power Capacitors

Wang Jimei, Shanghai Jiaotong University, Shanghai Power Transmission Equipment Co., Ltd. Wang Dezhong, Hangzhou Boda Electric Co., Ltd., Wu Zengsheng, cannot fully meet the requirements of the IEC549 high-voltage fuses for external protection of shunt power capacitors in 1976 and the corresponding national standard requirements. The development of high-voltage vacuum full-range fuses is proposed. necessity. The structure and characteristics of the fuse are described, and the characteristics of the arc fuse's pre-arcing time and expected current are tested and analyzed.

1 Foreword At present, domestic fuses used to protect high voltage shunt capacitors from overload currents are mostly jet-type fuses. The structure is simple, and the protection performance can basically meet the operational requirements. However, the jet fuse has a powerful arc spray during operation, and also produces strong sound and light, therefore,.

Spray-type fuses are only suitable for installation in outdoor open applications. Current-limiting fuses should generally be used in the local compensation device for shunt capacitors. Current-limiting fuses have good short-circuit current opening capability, but due to its The characteristic curve is relatively flat in the small current overload section and does not meet the requirements of the overload protection characteristics of the shunt capacitor. (The national standard for fuses for external protection of shunt capacitors requires that the specified non-melting current be 1.1 times the rated voltage of the fuse part, At this current, the fuse shall be guaranteed to be not blown for 4 hours; at 1.5 times the rated current of the fuse, the fuse shall ensure that the It-characteristic curve of the vacuum fuse has a large steepness in the small-current overload section, which can fully satisfy the external protection of the shunt capacitor. With the requirements of the national standard for fuses, it has excellent overload protection characteristics, and can be used to complete the task of removing the components from the circuit when a breakdown occurs in a high-voltage shunt capacitor without internal fuse, and to avoid the capacitor from forming a penetration breakdown. Fault expansion: The vacuum fuse holder is installed in a vacuum interrupter and its characteristics are very stable. This type of fuse is also connected in series with a high-voltage current-limiting fuse with the symbolic density 1C thermal conductivity SV/m°K resistivity, which can be used to break faults of different short-circuit currents, hence the so-called helium-pressure vacuum full-range current limiting fuse, The current setting of the high-voltage vacuum current-limiting fuse can increase with the increase of the combined capacity of the parallel capacitor. When the vacuum fuse operates, no arc discharges outward and there is no acousto-optic phenomenon, so it can be installed in a high-voltage shunt capacitor. Use in the local compensation cabinet.

At present, high-voltage vacuum full-range fuses have been jointly developed by Xi'an Jiaotong University, Hangzhou Boda Electric Co., Ltd. and Shanghai Electric Motor Co., Ltd. Power capacitor factory, and have been applied in the high-voltage shunt capacitor compensation cabinet with a rated voltage of 6kV.

2 The structure and working principle of the fuse The structure of the helium pressure full-range current-limiting fuse is composed of two parts, a vacuum fuse and a current-limiting fuse. In the figure, 1 is a vacuum fuse, and the vacuum fuse is completely immersed in the epoxy resin of the insulating housing. 2 is a current-limiting fuse, and the melt is buried in the quartz sand extinguishing medium of the insulating housing.

2.1 Vacuum fuse part The main body of the vacuum fuse is a vacuum interrupter as shown. In the figure, a glass case, a 2 shield case, a 3 conductive bar, 4 input terminals, a 5 conductive bar, 6 output terminals, and 7-solution. The degree of vacuum inside the glass enclosure shall not be lower than 1(T3Pa). The shield shall be made of sheet metal. Its main purpose is to prevent the metal vapor generated from the melt from being blown onto the glass cover during the breaking current of the fuse. These deposits The gold particles on the inner surface of the glass shell are likely to cause the conduction between the input end and the output end, and the conductive rod, the input terminal and the output terminal are all made of oxygen-free copper material.The melt can be from the table according to the requirements of different designs. In the 1 selection, this design is suitable for oxygen-free copper as a melt.

Table 1 Physical properties of melts used in vacuum fuses The ribbons are used in the melt. The average current density can be selected from a wide range of 100160a/mm1. For example, the rated current is 50~60A fuse, if choose 4x0.1mm2 section of copper tape, the current density is (5060)/4x.1=125~150A/mm2o and then use the length of the melt to determine the fuser's Rated current and safety characteristics, this fuse is used to protect the rated voltage of 6kV, rated current of 50A and 60A power capacitors, after the calculation of the thermal properties and optimized adjustments after the corresponding melt length of 30mm and 25mm. Specific geometric dimensions such as the melt does not melt 2.2 current limiting fuse part of the current limiting fuse structure shown in Figure 1 an insulating housing, made of ruthenium-strength glass fiber silk epoxy resin. 2 - quartz sand, containing more than 98% of pure quartz sand, after treatment into the insulation shell, as a fuse arcing medium. 3 - Silver-plated copper end cap, as a conductive and sealing fuse, 4 body 99.99% pure silver, its sectional area taken 2.54X0.12 and 2.54x.145mm2, respectively, for rated current 50A and 60A In the fuse tube, a length of 680 mm is wound on a seven-star ceramic pillar, and the fracture arrangement of the melt is as shown. If necessary, an action indicator or striker can be mounted on the end cap.

3 Test and measurement results The following tests and measurement results were analyzed for the full range of pressure-reducing vacuum melting furnaces.

Ampere-second characteristics (characteristics of arc front time vs. expected current) of the vacuum fuse section Test and analysis.

The circuit diagram of the safety-second characteristic of the vacuum fuse is shown in the figure. The figure shows a K-power switch, a large-capacity voltage regulator, a R-stable current resistor, a short-circuit switch, and A-ampere test ammeter. When the test current is more than 2 times the rated current, the memory oscilloscope is used to measure the current, and the VF-measured vacuum fuse is used.

As can be seen from the curve of the expected current characteristic measurement of the pre-arcing time of the prospective current test circuit diagram, it can be seen from these curves that the pre-arcing time vs. expected current curve is very steep at the time of small current overload, which shows that the power capacitor is well-protected. The request.

The characteristics of the current-limiting characteristics of the current-limiting fuse part (arc. time before and expected current) are tested and analyzed.

The pre-arcing time of the current-limiter-disconnector-expected current characteristics of the measured line is as shown, that is, the measurement method is the same as that of the vacuum fuse. The result of the measurement is shown in the figure. It can be seen from the figure that when the overload current of the current limiting fuse is less than 4 times of its rated current (the part indicated by the dashed line in the figure), the fuse has no ability to reliably break the overcurrent. Now.

Determination of the intersection point of the first-phase current (rm.s) characteristic curve of the vacuum fuse's pre-orphanage time From the test results of (1) and (2) above, it can be seen that when the rated current of the vacuum fuse of rated current 50A is greater than 300A, or When the breaking current of the vacuum fuse with a rated current of 60A is more than 500A, it is difficult to meet the requirement of breaking, while the current limiting fuse with a rated current of 50A, when the breaking current is less than 200A, or the current limiting fuse with a rated current of 60A. When the breaking current is less than 300A, it is difficult to meet the requirement of breaking. If they are overlapped on a characteristic curve, they can obtain the continuous characteristics of the intersection point of 280A and 420A, respectively. , as shown. Composition of a fuse with a characteristic from the minimum overload current to the maximum current limiting fuse time-of-flight-time-out characteristic curve (4) Critical breaking characteristics According to China's fuse standard for protecting power capacitors Regulation: The fuse should be able to last for 4 hours without breaking at 1.1 times the rated current, ie 55A and 66A. It should be blown within 75 seconds at 1.5 times the rated current, ie 75A and 90A, at 2.0 times the rated current. , that is, 100A and 120A should be broken in 7.5 seconds, the test can meet the requirements, the specific data shown in Table 2. Table 2 the data obtained by the measurement of the fuse characteristics of the rated current box constant current li times, 4 hours does not fuse constant current 1.5 times, should be blown (S) twice the rated current within 75 seconds, should be main 7.5 seconds 1 blown (S) not blown (5) fracture withstands recovery voltage characteristics after vacuum fuse completes breaking characteristic test, The fracture withstand voltage test was carried out. The withstand voltage was 20kV (rms) and 60S, which all met the requirements. All other tests are in contact with the solution.

4 Conclusion High-voltage vacuum-type full-range current-limiting fuses for the protection of shunted shunt capacitors have no such products at home and abroad. In order to meet the needs of the development of China's reactive power compensation technology, high-voltage vacuum-type full-range protection for power capacitors has been performed. The research and development of the current fuse has obtained the following conclusions: The high-voltage vacuum full-range current-limiting fuse has a good overload current-limiting characteristic: when the rated current of the fuse is 1.1 times, the fuse is not blown for 4 hours and the rated current of the fuse is 1.5 times. When the cutting time is (30~50)s, the melting time of 2.0 times the rated current of the fuse should be specified by the national standard.

Under the action of overload current, the vacuum fuse is able to endure the voltage of the industrial frequency 20kV (rms) on the fracture of the fuse for 60 seconds, which complies with the provisions of IEC549-1976 and corresponding national standards.

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