Performance, advantages and disadvantages of several types of low-voltage capacitor switching devices

in recent years, with the continuous improvement of power supply quality requirements and the need of energy saving and loss reduction, the use of reactive power compensation devices has increased rapidly. Subsequently, various reactive power compensation devices were continuously developed and applied, such as static reactive power compensation device SVC, static reactive power generator SVG and thyristor switching capacitor device TSC. However, due to various factors such as technical maturity or high input cost, the low-voltage reactive power compensation device is still the most widely used and low-input cost. This paper only analyzes the performance, advantages and disadvantages of several types of low-voltage capacitor switching devices existing in China at present, for the reference of users and designers, in order to achieve the effect of rational use, improving the economic benefits of enterprises and saving resources. 1. Performance comparison: At present, the switching elements used in domestic capacitor switching devices can be divided into four categories: 1. In the process of mechanical contactor switching capacitor device (MSC), the initial voltage of the capacitor is zero, and the contact is closed instantly. In most cases, the voltage is not zero, and sometimes it may be at a high peak value (rarely zero), thus generating a very large current, which is often called closing inrush current. The experiment shows that when the inrush current is serious, it can reach 5 times of the rated current of the capacitor. This not only affects the service life of capacitors and contactors, but also impacts the power grid and affects the normal work of other equipment. Therefore, later, series reactors and current limiting resistors were used to suppress inrush current, which can control inrush current within 2 times of rated current, but from the long-term operation, its failure rate is still very high and maintenance costs are high. The overall practical application shows that its performance is as follows: Advantages: low price, low initial investment cost and no leakage current. Disadvantages: large inrush current, short service life, many faults and high maintenance cost. 2. Electronic contactless thyristor switching capacitor device (TSC) The thyristor switching capacitor takes advantage of the fast response speed of the electronic switch. A zero-crossing trigger circuit is used to detect that when the voltage applied to both ends of the thyristor is zero, a trigger signal is sent out and the thyristor is turned on. At this time, the voltage of the capacitor is equal to the grid voltage, so there will be no closing inrush current, which solves the problem of contactor closing inrush current. However, when the thyristor is on, there will be a voltage drop of about one volt between the thyristor junctions. Usually, the rated current of a 15kvar triangular capacitor is 22A, so the power consumed by a thyristor is about 22W. If a 15kvar capacitor cabinet is taken as an example, the power consumed by its SCR switching device can reach 6W during operation, and all of them become heat, which makes the cabinet temperature rise. At the same time, the thyristor has leakage current, and when the capacitor is not connected, even if the thyristor is not conductive, its output terminal is high voltage. Advantages: no inrush current, no contact, long service life, less maintenance and fast switching speed (within 5ms). Disadvantages: high price (the initial investment is about 6 times that of contactor), serious heating, energy consumption and leakage current. 3. Compound Switch Switching Capacitor Device (TSC+MSC) The working principle of the compound switch switching device is that the thyristor is first put into the capacitor when the voltage crosses zero, and then the contacts of the magnetic AC contactor are closed in parallel, and the thyristor exits, and the capacitor runs with the magnetic latching relay contact closed. Therefore, the purpose of running without inrush current and heating is realized. However, in order to reduce the cost, two thyristors with low power and low withstand voltage are usually used in series. Using the characteristic that the current of the thyristors can be overloaded by 1 times the rated current within 2ms, they are put into operation at zero crossing, and then closed by relays. However, the contact of magnetic latching relay is relatively small, and the rated mechanical life is generally 5, times. From the current situation of putting it into the market, the SCR sometimes breaks down, and the magnetic latching relay also gets stuck, which makes the work unstable. Generally speaking, advantages: no inrush current, no heat, energy saving disadvantages: the price is five times that of contactor, the service life is short, there are many faults, there is leakage current, and the switching speed is about .5s. 4. Surge-free Capacitor Switching Device (TSC+MSC) The surge-free Capacitor Switching Device is a patented technology product developed by Shenzhen AIA Yi Company after integrating the advantages of the above switching devices. This capacitor switch is a contactless switch that is put into the capacitor when the voltage crosses zero, and then switched to a special contactor to run without heating. Its characteristic is that the rated current of contactless switch is the same as that of capacitor, and its withstand voltage is 16V V. The mechanical life and electrical life of the special contactor are 1 million times, thus ensuring its reliability and stability. It has been used in the field for nearly one year, which proves that it has strong overload capacity and obvious energy saving effect. Advantages: no inrush current, no heat, energy saving, safety and long service life. Disadvantages: the price is about 3 times that of contactor, and the switching speed is .5s Second, type selection: users choose the type purposefully according to the engineering requirements by comparing the performance of various capacitor switching devices, so as to achieve satisfactory technical and economic performance. Through practice, from the above analysis, the author puts forward the following suggestions: (1) For users who have stable reactive power and don't need to switch capacitors frequently, a contactor with current-limiting resistance can be selected, which is economical and low in price. Due to the few switching times, the corresponding service life is long enough. (2) For users who need to quickly and frequently switch capacitors for compensation, such as electric welding, elevators and other equipment, the contactless thyristor switching capacitor device should be selected to achieve the due compensation effect. (3) For other general factories, communities and ordinary equipment, in areas where the reactive power changes for more than 3s, the inrush-free capacitor switch with no impact on the power grid, energy saving, safety, economy and long service life is considered.