The electric energy is the main energy and power for the industrial production. The proper operation of the power supply and distribution systems of the plants is of great significance to the development of the national industry and the growth of the national economy. In this paper, the mode of reactive power capacitor compensation as well as the conventional reactive power capacitor compensation cabinet are introduced. Some effective methods to improve the power factor of the plant power supply and distribution system are obtained.
I.A plant may contains a large number of induction motors, transformers and other inductive equipment. When they are operation, they not only absorb the active power from the power grid so as to perform their work, but also absorb the reactive power in order to establish a magnetic field. This will cause the distribution transformer and transmission line in the plant to generate additional power loss, reduce the natural power factor, and in addition, the reactive power is the main factor that affects the voltage quality. Therefore, it is necessary to strengthen the operation and management of the inductive equipment, improve the power factor and operate the reactive power capacitor compensation cabinet.
II. Summary of the reactive power capacitor compensation
The output power of the power grid is divided into the active power and the reactive power. The active power is the electric power with which the electric equipment converts the electric energy into another form of energy and makes use of it to do the external work. The reactive power refers to the electric power which is needed by the inductive equipment or the capacitive equipment existing in the AC circuit for establishing the alternating magnetic field or the electric field so as to ensure its normal operation, and it does not do any external work.
The power factor is the ratio of the active power P to the apparent power S, and the relationship between the apparent power S and the active power P as well as the reactive power Q is S =
The principle of the reactive power capacitor compensation under the action of the same voltage in the AC circuit, the direction of the inductive current and that of the capacitive current are opposite, with a phase angle difference of 180°, that is to say, the inductive reactive power and the capacitive reactive power can be mutually complementary. Because most of the electric loads in the plant, such as the motors, transformers and so on are inductive loads, so if the reactive power capacitor compensation devices such as the capacitors are installed in parallel in the power supply and distribution system, it is possible to provide certain amount of reactive power to the inductive equipment, reducing the inductive equipment’s demands for the reactive power in the grid. This is the reactive power capacitor compensation.
The purpose of the reactive power capacitor compensation and the significance of the reactive power capacitor compensation: The main purpose of the reactive power capacitor compensation is to improve the power factor of the power supply and distribution system. An improved power factor will have the following benefits to the plant:
1. The electricity fee of the plant can be reduced. The electricity generated by the power supply company is calculated on the basis of the apparent power, but the user’s electricity fee is charged on the basis of the active power consumed by the user. If the plant has absorbed too much reactive power from the public power grid, that is, the power factor of the plant is too low, the cost of the power supply company will be increased. Therefore, at the time when the power supply company charges the electricity bill to the user every month, if the average power factor of the plant is below the prescribed standard, additional electricity fee will be charged in accordance to a certain proportion.
2. In the process of improving the voltage quality, the voltage loss in the system will increase with the increase of the demand for the reactive power. After the reactive power capacitor compensation has been carried out, the power factor will be increased, thus the voltage quality of the system can be improved and the load voltage can be more stable.
3. Reduce the rated capacity of the selected transformer. The rated capacity of the transformer is determined by the apparent power of the secondary side. After the reactive power capacitor compensation has been carried out on the secondary side of the transformer, the apparent power will be reduced. So it will be possible to choose the transformer with a smaller rated capacity. This will reduce the investment outlay.
4. Reduce the electric energy losses. The power loss of the circuit is inversely proportional to the square of the power factor. For example, when the power factor is increased from 0.6 to 0.9, the power loss in the circuit can be reduced by 55.56%. After the reactive power capacitor compensation has been carried out, the current flowing through the transformer winding will decrease, so the loss of the transformer will decrease accordingly.
III. The reactive power capacitor compensation modes and the comparisons among them. Depending on the installation location of the reactive power capacitor compensation cabinet, the reactive power capacitor compensation is divided into the centralized reactive power capacitor compensation and the decentralized reactive power local compensation.
The centralized reactive power capacitor compensation is divided into the centralized reactive power high voltage capacitor compensation cabinet and the centralized reactive low voltage capacitor compensation cabinet.
1. The centralized reactive power high voltage capacitor compensation cabinet: The active power capacitor compensation cabinet is installed on the high voltage bus at the plant substation. This reactive power capacitor compensation method can only be used for making compensation for the reactive power in the whole lines before the high voltage bus. It can not be used for reducing the rated capacity and the power loss of the selected transformer. The advantages of this method are that the initial investment is less, it is convenient to carry out the centralized maintenance and the reactive power capacitor compensation can be made for the reactive power on the high voltage side of the plant, the total power factor of the plant can be improved. This kind of reactive power capacitor compensation is more common in some large and medium-sized plants.
2. The centralized reactive power low voltage capacitor compensation: The low voltage reactive power capacitor compensation equipment is installed on the low voltage bus bar of the plant substation. This reactive power capacitor compensation method can be used for making compensation for the reactive power of all the equipment and lines before the low voltage bus bar of the substation. With this method, it is easy to carry out the maintenance, and it is possible to reduce the rated capacity and the power loss of the selected transformer. This method is quite common among the plants
The decentralized local reactive power capacitor compensation. As to the decentralized reactive power capacitor compensation device, it is installed next to the individual electrical equipment with a large capacity and stable load, which needs the reactive power capacitor compensation. The reactive power capacitor compensation device is generally put into operation and put out of operation at the same time with the electricity consuming equipment. This reactive power capacitor compensation method can make compensation for the reactive power in all the high voltage lines and the low voltage lines as well as the transformers before the installation place. It can reduce the loss of the system and maintain the voltage level of the system in the most appropriate way, so the reactive power capacitor compensation effect is the best. However, the investment for this reactive power capacitor compensation method is large, and the utilization rate of the reactive power capacitor compensation equipment is low.
To sum up, each reactive power capacitor compensation method has its own advantages. In the plants and enterprises, the combination of the centralized reactive power capacitor compensation method and the decentralized local reactive power capacitor compensation method is often used.
IV. The selection of the reactive power capacitor compensation cabinet
Selection of the conventional reactive power capacitor compensation cabinet. At present, the reactive power capacitor compensation cabinets commonly used in the plants and enterprises are the grouping automatic switching reactive power capacitor compensation cabinet type, which is composed of the capacitor banks, the capacitor branch protection, the switching switch and the automatic reactive power capacitor compensation controller. In order to maintain a balance in the operation of the capacitors and the switches in each bank, it is necessary to avoid any premature damage of the capacitor banks caused by the frequent switching on and switching off of the individual capacitor bank. The principle of rotating cycle switching should be adopted among the capacitor banks, that is, the capacitor bank which has been put into operation at an earlier time will be input out of operation at an earlier time, too.
Series reactors. Because of the fluctuation and impact load in the plant, these equipment will generate a large number of harmonics, which will cause fluctuations in the voltage, flickering and imbalance among the three phases in the power grid, which will affect the safe operation of the electrical equipment. Although the capacitor itself has a certain ability to resist the harmonics, it can also produce certain side effect by amplifying the harmonics. When the harmonic content is too large, it will affect the service life of the capacitor, and even will cause the premature damage to the capacitor. Because of the amplification effect brought about to the harmonics by the,capacitor, the harmonic interference in the system will become more serious. Therefore, when there is a large harmonic interference, for a location where the reactive power capacitor compensation needs to be performed, it should be considered to provided more filtering devices.
In the capacitor circuit, the series reactor is an effective method to solve this problem. In addition, the series reactor can also limit the closing inrush current in the capacitor and reduce the impact on the power grid and switching devices, which is brought about by the capacitor inrush current.