Series capacitors are widely used in power transmission and distribution systems, especially long-distance, large-capacity power transmission systems, to increase transmission capacity, improve system stability, improve system voltage regulation, and at the same time improve system power factor, reduce line loss.
Parallel capacitor is mainly used to compensate the reactive power of the inductive load of the power system to improve the power factor, improve the voltage quality, reduce the line loss, improve the voltage quality and equipment utilization, and is often used in conjunction with the on-load voltage regulating transformer.
What is the difference between a series capacitor and a parallel capacitor?
First, from the perspective of pressure regulation.
Since the parallel capacitor increases the power factor of the load side to reduce the flow of reactive power to increase the voltage at the receiving end, it is necessary to perform frequent grouping or cutting operations according to the change of the load, and the capacity is proportional to the square of the voltage. When the grid voltage drops, The pressure regulation effect is significantly reduced.
The voltage drop of the series capacitor is directly compensated for the line voltage drop, and the voltage regulation effect is automatically and continuously adjusted with the change of the load, and its voltage regulation effect is much more significant than that of the parallel capacitor. Therefore, only from the point of view of voltage regulation, only when the power factor is large, the ratio of line reactance to load impedance is also large, and considering that the unit capacity price of series capacitors is larger than that of parallel capacitors, it is beneficial to use parallel capacitors.
Second, from the perspective of reducing network loss.
After installing the parallel capacitor compensation, since the reactive power transmission capacity of the transmission line and the transformer is reduced, the parallel capacitor compensation is much better than the series capacitor compensation, so the effect of reducing the network loss is great.
At the same time, under the condition that the maximum transmission current value of the transmission line remains unchanged, the reduced reactive power can be used to transmit more active power accordingly.
The series capacitor compensation basically does not change the reactive power transmission capacity on the transmission line, but only increases the terminal voltage level. Therefore, the series capacitor compensation not only cannot reduce the network loss, but instead increases the load consumption due to the increase of the load voltage. Line loss increases.
Because series compensation will produce abnormal phenomena such as ferromagnetic resonance and self-excitation, which will cause harm to electrical equipment; in ultra-high voltage transmission lines, it may produce a sub-synchronous vibration lower than the power frequency with the generator set, causing shaft torsional vibration. The generator shafting is damaged, so the practical application of series capacitor compensation in the current grid below 220kV is much less than that of parallel capacitor compensation.
Third, the equivalent capacitance is different.
The equivalent capacitance of a parallel capacitor bank is equal to the sum of the capacitances in the capacitor bank, while the reciprocal of the equivalent capacitance of a series capacitor bank is equal to the sum of the reciprocals of the capacitances in the capacitor bank.
The equivalent capacitance of a series capacitor bank is smaller than the capacitance of any one capacitor in the capacitor bank.
Fourth, the difference in capacity.
When capacitors are connected in series, the capacity decreases (to calculate the total capacity after connecting in series, refer to the parallel method of resistors), and the withstand voltage increases. When the capacitors are connected in parallel, the capacity increases (adding each capacity), and the withstand voltage is calculated as the smallest.
Series capacitors: the more the number in series, the smaller the capacitance, but the higher the withstand voltage, the capacity relationship: 1/C=1/C1+1/C2+1/C3
Parallel capacitors: the more the number in parallel, the greater the capacitance, but The withstand voltage remains unchanged, and its capacity relationship is: C=C1+C2+C3.
