Abstract: A comparison of intelligent low-voltage reactive power compensation devices in different ways currently used. As well as a preliminary analysis of some problems in the actual application, the selection of the reactive power compensation scheme and the selection of the device are proposed.
In the distribution network, low-voltage reactive compensation is of great significance for improving power quality and reducing line losses. Has been widely used. However, there are many manufacturers of reactive power compensation devices, and the user conditions vary widely. The actual operation effects of different compensation devices are different, which brings great difficulties to the management of reactive power compensation. Combined with the problems in the actual application, the three parts of the low-voltage reactive power compensation device are briefly analyzed and summarized, and some suggestions are put forward for discussion.
1 switching switch
The switching switch mainly includes three methods: contactor, bidirectional thyristor and composite switch.
1.1 Contactor
Contactors are one of the longest and most widely used methods. Compared with other methods, its biggest advantage is that it is cheap and therefore the most widely used. However, when the contactor switches the capacitor, it is easy to generate a large inrush current, burning the contacts of the contactor, resulting in damage to the capacitor.
The contactor is suitable for reactive power compensation with relatively stable load and low power factor variation frequency, and has good effects.
1.2 Non-contact switch composed of bidirectional thyristor
In this way, the capacitor bank is switched by detecting the zero-crossing point of the voltage and controlling the on and off of the two thyristors in the upper and lower half waves, respectively. Since the investment is made at the zero crossing point, the impact of the inrush current is avoided, and the capacitor is prevented from being affected by the overvoltage.
But one big problem in this way is heat dissipation. The thyristor is theoretically equivalent to a short circuit after conduction, and it is required to pass the rated current for a long time. Actually, the thyristor has a conduction voltage drop, which is generally about 1.5 V. Take a compensation cabinet with a total compensation of 160 kvar as an example. The capacitors are divided into 8 groups. For every 20 kvar, the current flowing in phase A is 52.6 A. The power consumption of each thyristor is P=1.5 & TImes; 52.6=78.9 w, and the two thyristors are nearly 160 w. It is equivalent to two 80-watt bulbs. The case temperature of the thyristor working normally does not exceed 80 ° C. If it is overheated for a long time, the performance of the thyristor may be reduced until it is burned. Therefore, it is necessary to add a radiator, a fan, and a temperature control relay. However, when the outdoor temperature is high in summer, the ventilation room is not ventilated, and there is a heat source such as a transformer, the ambient temperature of the device may reach 50 ° C or more. Even if the heat is dissipated, it is difficult to ensure that the thyristor temperature rise is less than 80 ° C. After adding the radiator, fan and temperature control loop, the required compensation space in the cabinet is larger, the assembly is very troublesome, and the reliability is also reduced. Moreover, harmonic currents are generated when the thyristor is switched, causing the system to be amplified after resonance, resulting in damage to the thyristors.
Therefore, for a reactive power compensation cabinet similar to an outdoor public or special transformer, and a power distribution room with poor heat dissipation, try not to install a non-contact switch composed of such a triac.
1.3 composite switch
This is a new type of capacitor switching switch that combines the advantages of contactors and triacs. Connect the contactor and the triac in parallel. Its working principle is to use the fast characteristics of the thyristor as a switching switch, and the contactor acts as a switch when the current continues to pass. When it is necessary to input the capacitor bank, the thyristor is first put into the voltage zero-crossing to prevent the inrush current from being generated, and then put into the contactor to be in the state of parallel connection with the thyristor; when the system is stable, the thyristor is then withdrawn, The contactor is subjected to the running current; when it is required to exit the capacitor bank, the thyristor is first turned on to be in parallel with the AC contactor; then the AC contactor is disconnected and the operation is performed, and the connection between the capacitor and the grid is short. It is independently undertaken by the thyristor; finally, the trigger signal of the thyristor is cut off, so that the thyristor is naturally turned off when the current crosses zero.
The compensation device for the non-contact switch and the composite switch composed of the triac is less likely to be damaged by the compensation device using the two methods because of the zero-crossing switching. For heavy harmonics, to prevent resonance, if a composite switch is to be used, filter should be considered first.
In order to ensure the safe use of the thyristor, the reverse peak voltage and the on-state current of the VRSM selected by the thyristor are at least 2.5 times or even higher than the rated voltage and current of the system it is subjected to. Of course, it is also limited by cost. The price of the contactless switch and the composite switch composed of the triac is usually 3 to 4.5 times that of the contactor. If the parameters are selected again, the cost will increase dramatically.
2 capacitor
2.1 Self-healing shunt capacitor structural characteristics
Most of the capacitors currently in use are self-healing shunt capacitors. These capacitors are wound from a metallized polypropylene film. This material allows the capacitor to melt and evaporate the surrounding metal layer after being short-circuited by the short circuit, so that the insulation properties are quickly restored and the capacitor can continue to be used. It has a self-healing function.
In practical applications, structural capacitors that use a three-phase angular connection and are integrally packaged are relatively common. The three-phase angular connection facilitates automatic filtering of the third harmonic, and the integrated package facilitates installation. A small resistor is usually connected in parallel across the capacitor as a discharge circuit for the capacitor. It is required that the voltage across the capacitor is less than 65 V after 30 s after the power is turned off. In addition, pressure insurance can be installed to prevent explosion.
But in fact, the capacitor is still damaged, and even caused the accident to expand. The main reason is the overvoltage, overcurrent, and temperature generated when the capacitor is switched. The cause of the overvoltage and overcurrent is the switching time and frequent operation of the capacitor.
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