First, the circuit breakers used in circuit breakers are mostly vacuum switches. The vacuum circuit breaker has the characteristics of fast opening and closing speed. With the continuous improvement of arc extinguishing performance and overvoltage withstand capability, the vacuum circuit breaker has been able to prevent reignition well when breaking the capacitor. Household Electrical Appliances gree , https://www.greegroups.com
However, due to the high cost of the vacuum circuit breaker, the capacitor grouping cannot be made very fine. Therefore, the use of a vacuum circuit breaker as a switching element cannot be invested in an appropriate amount of compensation based on the reactive power shortage. At the same time, since the ordinary three-phase switch of the vacuum circuit breaker can not be separately controlled, it must be turned on and turned on together, and disconnected and disconnected together. It is impossible to select the most suitable phase angle input and cut-off capacitor, that is, at least two capacitors cannot be in the voltage. Input when zero crossing, or cut off when the current crosses zero. The current of the capacitor is proportional to the time of the capacitor voltage. When the initial voltage on the capacitor is not equal to the grid voltage when the breaker is put into operation, the maximum voltage difference can reach 2.8 times the rated voltage. When a large differential pressure is suddenly applied across the capacitor - causing a sudden change in the voltage of the capacitor, the current through the circuit breaker and capacitor will be several tens of times the rated current - the inrush current. This situation is particularly acute in the switching of back-to-back capacitor banks. The inrush current is a great hazard to both the capacitor and the circuit breaker, and the harmonic components contained in the inrush current are amplified by the capacitor. On the other hand, the harmonic current is superimposed on the fundamental current of the capacitor, which increases the effective value of the capacitor current, causing the temperature rise to increase or even overheat, which affects the life of the capacitor. In addition, the maintenance of the vacuum circuit breaker is complicated, and it is not suitable for frequent switching.
Therefore, the vacuum circuit breaker as the switching element is only suitable for the case where the reactive load does not change much, and is used when the low-voltage busbar of the large and medium-sized substation is used for centralized compensation. At this time, the reactive power compensation is basically the compensation of the reactive power consumed by the main transformer.
Second, AC contactor AC contactor has been used as the compensation component of the compensation capacitor device for more than 30 years of operating experience. In terms of cost, the AC contactor has obvious advantages over the vacuum circuit breaker, but like the vacuum switch, the three-phase contacts of the AC contactor must be split at the same time, and a large inrush current is generated when the capacitor is switched. On the one hand, because the cost of the mainstream contactor is relatively low, and the switching element is used as the switching element, the grouping of the capacitor can be made more and finer, and the system reactive power shortage can be compensated more accurately, which solves the vacuum circuit breaker. As a grouping of switching elements, the problem is not detailed enough. On the other hand, since the inrush current is large, the capacitance value of one input is limited, and the value of the capacitance of one input has to be reduced to zero, and the input is performed several times. Taking into account the operating time of the contactor and the decay rate of the inrush current during the input will greatly reduce the response speed of the device. Due to insufficient response speed, the input of the compensation capacitor cannot be synchronized with the change of the reactive load, thus affecting the accuracy of the compensation.
Due to the limited overcurrent capability of the AC contactor contacts, the presence of inrush current often causes the contactor contacts to be soldered, which prevents the contactor from breaking, reducing the reliability and service life of the entire device. Therefore, the contacts must be frequently maintained and replaced during operation to increase operating costs. In order to reduce the inrush current, a special contactor with a pre-discharge resistor is mainly used. A set of auxiliary contacts firstly connect a current limiting resistor provided on the contactor into the circuit to suppress the inrush current, and then the main contact Instantaneous closing shorts the resistor to achieve the input capacitor. However, due to the limitation of the machining accuracy of the components of the contactor and the unavoidable deviation during assembly, the time difference of the switch will inevitably change during long-term operation, causing the input of the resistor to be improperly matched with the main contact. Therefore, the resistance burning of the contactor and the welding of the main contacts often occur. Another method is to increase the inrush current by adding a current limiting coil to the common contactor. Although this method has certain effects in limiting the inrush current, it also has some disadvantages, such as increasing power loss, easily causing resonance, increasing manufacturing cost, complexity, and failure rate, etc., and such products still produce closing. Overcurrent at surge and shutdown. There is also a fuss about the contact material. The alloy with a non-linear resistance characteristic is used as the material of the main contact. When the temperature rises, the resistance rises rapidly, while in the normal state, the resistance is small. Such a contact can effectively limit the current when the inrush current flows through the contact to generate a high temperature rise, and the flow requirement can be satisfied in normal operation. However, this only partially solves the problem of the contactor's ability to withstand the inrush current.
Fundamentally speaking, the use of an AC contactor as a switching element does not solve the problem of inrush current and harmonics.
3. The thyristor switching mode (TSC) currently widely used in high-power thyristors is controlled by industrial computers. Since the capacitor voltage cannot be abruptly changed, the capacitor must be input when the voltage is zero, and the capacitor should be cut off when the voltage is zero, and the high-power thyristor can meet the above requirements. When the IPC detects that the voltage across the capacitor is equal to the grid voltage and the polarity is the same, the microcomputer synchronous phase control technology is used to instantaneously input the capacitor. When the current crosses zero, the thyristor will naturally shut down, and no special discharge resistor or capacitor is needed. Pre-charging, you can put the capacitor at any time, greatly improving the switching speed of the capacitor, and can well adapt to the frequent switching of the capacitor. Because the thyristor automatically turns off when the current crosses zero, the capacitor current only switches between zero current and sinusoidal current. In other words, no harmonics are generated. In this way, the thyristor is used as the switching element to solve the problem of the inrush current and harmonics in principle, which is an ideal choice.
However, in practical applications, thyristor switching components also encounter some problems. The first is that the thyristor has a junction voltage drop when it is turned on, and the power consumption during operation is large, and the resulting heating phenomenon cannot be ignored. The thyristor valve body is cooled by air cooling, oil cooling, water cooling and boiling cooling. It is ideal for large-capacity thyristor water cooling. Regardless of the cooling method used, the investment and complexity of the entire compensation package will be higher. In addition, due to the existence of junction voltage drop, current distortion will occur; when switching the reactive power compensation capacitor bank, the surge current will be caused by the erroneous trigger of the thyristor. Under normal circumstances, the inrush current is 7 times its rated current, and the voltage at this time is more than twice the rated voltage. Under the action of the inrush current, the heat generated by the valve body is more serious.
Since the withstand voltage of a thyristor is generally several kilovolts, it is necessary to connect a plurality of thyristors in series at a voltage level of 10 kV. This has led to the problem of multiple thyristors synchronously triggering series voltage equalization and series protection, which is also a key technology for high voltage level thyristor switching capacitors. The trigger methods currently used include electromagnetic triggering and photoelectric triggering. Electromagnetic triggering is susceptible to electromagnetic interference, while photoelectric triggering avoids electromagnetic interference. The thyristor switching component in operation generally adopts the thyristor and diode anti-parallel mode or the two thyristor anti-parallel mode. In the first wiring mode, when the thyristor is turned off, the thyristor may be subjected to a reverse voltage of about the maximum power supply voltage. Two times the value; in the second wiring mode, if the time constant of the capacitor discharge is sufficiently small, then the maximum reverse voltage that the thyristor is subjected to is only the peak value of the power supply voltage. Obviously, the second method has higher reliability. Even if a thyristor is damaged, it will not cause the capacitor to be mistaken and the response speed is faster, but the problem of synchronous triggering is more prominent and the investment is increased.
Fourth, the composite switch from the above analysis shows that the AC contactor will generate surge current and harmonics during the switching process of the capacitor, but the power consumption is less in the normal positive state; and the thyristor can achieve zero-crossing switching, but in the A large power consumption occurs in a normal normal state. In order to combine the advantages of the two components, a composite switch, a new type of capacitor switching component, has been fabricated. The composite switch is composed of a parallel connection of a crystal tube and a contactor. The thyristor is used to realize over-current switching at the moment of input and cut-off of the capacitor reactive compensation; when the capacitor is put into normal operation, the contact resistance of the mechanical contact is extremely small, and the power consumption is greatly reduced. In order to achieve the above functions, the thyristor and the contactor in the composite switch must have strict timing control. When the composite switch controller receives the closing pulse, the controller detects the voltage between the capacitor and the grid, and sends a trigger pulse to the thyristor at a certain angle before the voltage crosses zero, so that the thyristor is turned on when the voltage crosses zero. The capacitor is inserted and the delay loop of the contactor is activated at the same time. This delay must ensure that the contacts of the contactor can only be closed when the thyristor is turned on. After the contactor is put in, a parallel circuit of the thyristor and the contactor is formed. At this time, since the contact resistance of the contactor is small, the junction voltage drop of the thyristor is limited to be low, so the power consumption thereof is small. After the controller receives the cut-off pulse, the contactor is turned off and the trigger pulse of the thyristor is delayed. At this time, the thyristor is naturally turned off when the anode current crosses zero.
Due to the above advantages, the application of the composite switch has been greatly developed in recent years, and the contactor and the thyristor have been replaced in many fields. However, due to the strict timing requirements of the working principle, there are some problems with the composite switch. If the composite switch is slower, it takes 60 to 160 ms to complete the entire input or removal process. At the same time, since the cooperation between the contactor and the thyristor is affected by the processing precision and assembly technology, the operation timing of the two components will change during operation, and this problem is usually solved by increasing the delay, which further reduces the device. The closing speed of the branch. Because the two elements are connected in parallel, the structure of the device is more complicated. At present, the composite switch is usually applied to low-voltage reactive power compensation. If it is to be applied in the high-voltage field, multiple sets of components must be used in series, the structure of the device will become extremely complicated, and the synchronism of each component is difficult to guarantee. Protection will also be an intractable problem. Therefore, the development of composite switches has also been limited.