I. The theory of equipment compensation
In theory, a capacitor is equivalent to an equipment that produces the reactive current. Its reactive power compensation is mainly implemented by installing in parallel the device with the capacitive power load and the device with the inductive power load in the same capacitor, so that the energy can be effectively exchanged and processed in the two load cases. At this time, the loading capacity of the transformer and the line responsible for the power transmission will be reduced accordingly. At this time, the active power will be increased. When the active power is transmitted in a reasonable manner, the loss in the power supply system will be reduced. Compared with other equipment, the capacitors can reduce the load of the power supply system and the distribution system in a more convenient and effective manner. Therefore, the capacitors are used in the reactive power compensation cabinet and this is an inevitable trend of the development of the times. At present, this method has begun to be used in the electric power industry in many areas.
II. Characteristics of the equipment
Everything has its own advantages and disadvantages, and the equipment is no exception.
First of all, the characteristics of its advantage are as follows. The equipment itself is very convenient for installation and the number of installation places can be reasonably increased or reduced. In addition, its active power loss is very small. The construction time is very short. There is no rotating structure, so it is more convenient to carry out the maintenance and repair works. Even if there is a problem with one of the components, it will not affect the overall operation.
Secondly, the issue of defects. It also has many defects. For example, it can only carry out staged adjustments; it is not able to perform the smooth adjustments; when the operating temperature of the equipment exceeds 70 degrees Celsius, it is easy to explode; moreover, in case of a short circuit, the voltage is not very stable; after a switching off operation, there may be residual load; in addition, the reactive power capacitor compensation cabinet does not have the characteristics of a fine equipment, which has a great impact on the effect, this is not conducive to the development of equipment management. In addition, the relevant units and leaders have not given sufficient attention to the safety and stability shown in its operation.
III. The modes of the reactive power capacitor compensation cabinets
1. The decentralized reactive power high voltage capacitor compensation cabinet
The decentralized reactive power high voltage capacitor compensation cabinet is actually installed on the high voltage side of a single transformer to improve the voltage quality of the power supply. The areas where it is most widely used is the city and it is usually used in the high voltage distribution system.
2.The centralized reactive power high voltage capacitor compensation cabinet
The centralized reactive power high voltage capacitor compensation cabinet refers to the reactive power capacitor compensation cabinet mode in which the capacitors are installed on 6 kV-10 kV high voltage bus bar of the substation or the step down substation of the user. When the user has the corresponding high voltage load, the reactive power loss in the power system can be reduced, in addition, the reactive power compensation ability is also available. Its advantages are as follows: With the centralized reactive power high voltage capacitor compensation cabinet, it is convenient to carry out the automatic switching on and switching off; at the same time, it can effectively increase the power of the user to a certain level, and can make full use of it; the most important thing is that the maintenance is very convenient, and it can reduce the phenomenon of too much compensation and improve the voltage quality in a reasonable manner. however it has its flaws, for example, its economic benefit is not very good.
3.The decentralized reactive power low voltage capacitor compensation cabinet
The decentralized reactive power low voltage capacitor compensation cabinet is to install a single or more low voltage capacitor banks near the electricity consuming equipment according to the amount of reactive power required by the individual electrical equipment in order to make compensation for the reactive power in all the high voltage lines and all the low voltage lines as well as the transformers in front of the installation site. Its advantages are as follows: when the electric device carries on the activity, the reactive power capacitor compensation cabinet also begin to work, and when the electricity consuming equipment stops operation, it can also reduce the reactive power which is contained in the distribution network and the voltage converter equipment, and in this way, the loss is reduced; at the same time, it can also eliminate the issues related to the capacity of the wire surface and the transformer equipment itself. But the decentralized reactive power low voltage capacitor compensation cabinet also has a lot of defects, for example, it is not possible to use this type cabinet in an effective way; in addition, a lot of money needs to be invested and it can not adapt itself to certain equipment.
4.The centralized reactive power low voltage capacitor compensation cabinet
The centralized reactive power low voltage capacitor compensation cabinet refers that the low voltage capacitors is connected to the low voltage bus bar side of the distribution transformer through the low voltage switch, and the switching device of the reactive power capacitor compensation cabinet is used as the control protection device to directly control the switching on and switching off of the capacitors according to the reactive power load on the low voltage bus bar. The switching on and switching off of the capacitors is usually carried out in groups and can not be adjusted in a smooth state. It has many advantages, for example, wiring is very convenient; it is not necessary to carry out the maintenance work at a very large scale; the reactive power can be balanced locally, so that the utilization rate of the distribution transformer is improved, therefor the losses in the network can be reduced in a reasonable way; it has a good economic characteristics; it is the most commonly method used by us at present.
IIV. Safe operation of the power capacitors
First of all, something about the operating current which is allowed; in the daily operation process, the equipment needs to carry out the activities within the specified current limits, and its current should be 1.3 times lower than the specified value, while the difference between the currents in the three phases should not be more than 4%.
Secondly, something about the allowable operating voltage; the capacitor is able to feel the voltage in a very sensitive manner, the main reason for this that the relationship between its loss and the voltage square exists in a positive proportion. When the voltage is too large, it will cause very serious heating on the equipment, and the service life of the equipment will be reduced in an inapparent way, and the phenomenon of electric breakdown will occur in case of serious conditions. Therefore, the device must carry out its operation at the specified value, which usually should not more than one point zero four times the specified value, and should not be greater than one point one time even in the extreme case. In case that the specified valued has been exceeded, the first thing to do is to carry out cooling.
1. The issue related to harmonic
Since the capacitor circuit is an LC circuit, it will cause the phenomenon of harmonics and some problems related to harmonics tend to occur. Under this circumstance, both the current and voltage may rise. In addition, the harmonic current may seriously damage the capacitor, resulting in the short circuit problem of equipment breakdown. Therefore, when the equipment is performing its operations under the normal conditions, it is possible that an electric reactor device can be connected in series to it so as to effectively eliminate this problems related to the electric current.
2. The issue related the relay protection
The relay protection is mainly realized by a complete set of relay protection devices. At present, the technologies used on the relay protection devices produced by several well known electrical manufacturers in China have been very mature. These relay protection devices are safe and stable and they have powerful functions. With the relay protection device, it will be possible to effectively switch off any fault capacitor. The relay protection device is an important means to ensure the safe and stable operation of the power system.
3. The issue related to explosion
During the operation of the capacitor, if any of the following faults has occurred, the capacitor may be damage or exploded: any of the internal components in the capacitor has broken down, the outer shell insulation of the capacitor has been damaged, the sealing is poor, oil leakages are present, the capacitor case has expanded or the any internal component can move freely, the switch is closed with charge, the temperature is too high, the ventilation is poor, the operating voltage is too high, the harmonic components are too large, the operating overvoltage has occurred and other conditions. In order to prevent the capacitor against explosion, normally each capacitor bank can be equipped with a fast fuse, which should have a capacity which is 1.4-2 times that of the current capable of passing through the capacitor bank. If the capacitor has been broken down, the fast fuse will melt and cut off the power supply. The purpose of this is to prevent the capacitor from continuing generating heat. Install an ammeter for each phase of the reactive power capacitor compensation cabinet so as to ensure that the difference between the currents in any two of the three phases does not exceed ±4%. In case that any imbalance has been found, the following measures should be taken: the reactive power capacitor compensation cabinet should stop its operation immediately and check the capacitors; monitor the temperature rise of the capacitor; strengthen the patrol inspection of the capacitor bank so as to avoid any oil leakage and expansion of capacitor case, the purpose of this is to prevent explosion.
4. The allowable operating temperature
When the capacitor is performing normal operation, the rated ambient temperature for the capacitor is generally 40 ℃ ~ -24 ℃, and the temperature of the medium inside it should be less than 64 ℃ and the maximum temperature should not exceed 70 ℃, otherwise a thermal breakdown or expansion will be caused. The temperature of the capacitor casing should be between the temperature of the medium and the ambient temperature, not exceeding 44 ℃ . Therefore, a good indoor ventilation should be available for the capacitor so as to ensure that its operating temperature does not exceed the allowable value.