The advantages and disadvantages of three types of capacitor cabinet switching devices

With the wide application of low-voltage reactive power compensation devices, how to select the switching device for capacitor cabinets has become a topic of great concern. Currently, there are three main types of capacitor cabinet switching devices: the switching capacitor contactor, the thyristor switching device, and the composite switch. Each type of capacitor cabinet switching device has its own advantages. It is recommended that users compare the performance of various capacitor switching devices and make targeted selections based on engineering requirements to achieve satisfactory technical and economic performance.

1. Mechanical contactor switching capacitor device
During the operation of the contactor switching process, the initial voltage of the capacitor is zero. At the moment the contacts are closed, in most cases the voltage is not zero; sometimes it may reach the peak value, and rarely it is zero. This results in a very large current, which is commonly referred to as the closing surge current. Experiments have shown that in severe cases, the closing surge current can reach 50 times the rated current of the capacitor. This not only affects the lifespan of the capacitor and the contactor, but also causes an impact on the power grid and affects the normal operation of other equipment. Therefore, later, by connecting an inductor and adding a current-limiting resistor to suppress the surge current, this method was adopted. Although it can control the closing surge current within 20 times the rated current, from the perspective of long-term operation, its failure rate is still very high, and the maintenance cost is relatively high. Overall practical application shows that its price is low, the initial investment cost increases less, there is no leakage, but the disadvantages are large surge current, short lifespan, frequent faults, and high maintenance cost.

2. Electronic non-contact thyristor switching capacitor device
The thyristor switching capacitor utilizes the fast response characteristic of electronic switches. It employs a zero-crossing trigger circuit to detect when the voltage across the thyristor is zero, and then sends a triggering signal, causing the thyristor to conduct. At this point, the voltage of the capacitor is equal to that of the power grid, thus eliminating the inrush current during closing, which solves the problem of inrush current in contactors. However, when the thyristor is conducting, a voltage drop of about one volt will occur between the junctions of the thyristor. For a 15KVAR capacitor connected in a delta configuration with a rated current of 22A, one thyristor consumes approximately 22W of power. If calculated for a 150KVAR capacitor cabinet, the power consumed by the thyristor switching device during operation can reach 600W, and all of this is converted into heat, causing the cabinet temperature to rise. Additionally, the thyristor has leakage current, and even when there is no capacitor connected, the output terminal still has a high voltage. Its advantages are no inrush current, no contacts, long service life, less maintenance, and fast switching speed within 5ms. The disadvantage is that the price is three times that of contactors.

3. Composite Switching Device for Capacitor Switching
The working principle of the composite switching device is as follows: first, the thyristor is turned on when the voltage reaches zero, and then the contacts of the magnetic holding relay are connected in parallel to close, and the thyristor is turned off. The capacitor operates under the condition that the contacts of the magnetic holding relay are closed. Thus, the purpose of achieving zero current input and no heat generation is achieved. However, to reduce costs, usually two small-power, low-voltage-rated thyristors are connected in series. The characteristic of the thyristor is that its current can overload 10 times the rated current within 20ms. Therefore, it is turned on at zero voltage and then the relay closes to operate. However, the contacts of the magnetic holding relay are relatively small and the rated mechanical life is generally 50,000 times. From the current market usage situation, the thyristors sometimes break down, and the magnetic holding relays also have the phenomenon of getting stuck and not operating. The operation is not stable in general. In summary, the advantages are zero current input, no heat generation, energy saving, but the disadvantages are that the price is 5 times that of the contactor, short lifespan, more faults, leakage current, and the switching speed is about 0.5 seconds.

Based on the above analysis, the following suggestions are proposed:
1. For users with relatively stable reactive power and no need for frequent capacitor switching, a contactor with a current-limiting resistor for capacitor switching can be selected. This device is relatively economical and has a low price. Due to the low number of switching times, the corresponding lifespan is sufficient.
2. For users requiring rapid and frequent capacitor switching, such as welding equipment and elevators, a non-contact thyristor switching capacitor device should be selected to achieve the desired compensation effect.
3. For other general factories, residential areas, and ordinary equipment, in areas where the reactive power changes take more than 30 seconds, consider using a non-ripple capacitor switching device that has no impact on the power grid, is energy-saving, safe, economical, and has a long service life.