The condition of traditional reactive power low voltage capacitor compensation cabinet:
1. The single signals are collected, the three-phase capacitor is used. The operation mode is to perform common reactive power compensation for three phases.
This compensation method is suitable for the situation where the load is mainly three-phase load (a motor), but if the current load is mainly resident users, the three-phase load is likely to be unbalanced. Then the reactive power required by each of the three phases are also different, and the use of this compensation method will lead to overcompensation or undercompensation to various extents.
2. AC contactors are often used in the switching switch
The disadvantage is that the response speed is slow, the impact inrush current will be produced in the process of switching on and switching off, and the service life will be short.
3. The strategies for controlling the reactive power
Most of the physical quantities used for control are the voltage, the power factor and the reactive current. The switching modes are: the cyclic switching on and switching off, the coded switching on and switching off. This strategy does not take into consideration the balance relationship between the voltage and the reactive power optimization in that region.
4.The function of distribution monitoring usually is not available
The situation of the intelligent reactive power capacitor compensation cabinets which are in use at present is as follows:
I.The operation mode of the reactive power capacitor compensation cabinet
1. Combination of the fixed reactive power capacitor compensation cabinet and dynamic reactive power capacitor compensation cabinet
With the development of society, the load types are becoming more and more complex, and the reactive power requirements of the power grid are getting higher and higher, so the simple fixed reactive power capacitor compensation cabinet can no longer meet the requirements, and the new dynamic reactive power capacitor compensation cabinet technology can better adapt to the load change.
2. The combination of three-phase reactive power capacitor compensation cabinet and individual phase reactive power capacitor compensation cabinet
The new equipment, especially a large number of power and electronics equipment, lighting equipment and other household equipment, adopt the two-phase power supply. There are more and more imbalances among three phases in the power grid. The reactive power capacitor compensation cabinet which perform three-phase common compensation operation can not solve the problem of the imbalance among three phases. On the other hand, if the reactive power capacitor compensation cabinet which only performs single-phase compensation operation is to be mounted, there will be a large investment. Therefore, the reactive power capacitor compensation cabinet which combines both the common compensation operation and the separate compensation operation is more and more widely used under the new economic conditions, which fully considers the economy according to the load situation.
3. The combination of steady state compensation and fast tracking compensation
The combination of the steady state compensation and fast tracking compensation is a trend in the future. It is mainly aimed at the large iron and steel works and metallurgic enterprises and other enterprises, where the process is complex, the power consumption is large, the load changes are fast, the fluctuations are large. The reactive power capacitor compensation cabinet can not only improve the power factor, reduce the loss and save the energy, but also fully excavate the working capacity of the equipment. It has given full play to the equipment capacity, improved the working efficiency and improved the output and quality. So the economic benefit of the reactive power capacitor compensation cabinet is great.
II. Adopting of the advanced switching switch
At present, there are the following main switching switches that are used.
1.The zero-crossing triggered solid-state relay
The zero-crossing triggered solid-state relay is characterized by the fast dynamic response. During the process of switching, there is no impact on the power grid and no inrush current. Its service long life is long. however it has certain power consumption and harmonic pollution. It is widely used at present.
2. The mechanical-electric intelligent combination switch
The switch is operated by the AC contactor and the solid state relay in parallel, which has combined the advantages of the two switches. It has not only realized fast switching, but also has reduced the power consumption. At present, mainly due to the reasons of cost and reliability, there are less applications.
3. The mechanical-electric intelligent vacuum switch
The switch adopts the low voltage vacuum interrupter and the permanent magnet operating mechanism. With this switch, it is possible to switch on or switch off the capacitor in case of zero crossing. It can also adapt to the switching of the circuit with the capacitors and the reactors in series. It has a long service life and high reliability. At present, it is in the process of commercialization.
III. The control strategy for adopting the intelligent reactive power capacitor compensation cabinet
The three-phase voltage and current signals are collected, the changes in the reactive power in the system are tracked, the reactive power is taken as the control physical quantity, the power factor set by the user is taken as the switching reference limit, and a capacitor combination is selected in an intelligent manner according to the fuzzy control theory. The intelligent reactive power capacitor compensation cabinet has been developed for the occasion where the star-delta connection combination is adopted. The control of the reactive power capacitor compensation cabinet adopts the intelligent control theory, which can automatically and timely switch the reactive power capacitor compensation cabinet to compensate the reactive power capacity. According to the size of reactive power in each phase of the distribution system, a capacitor combination is selected in an intelligent manner and put them into the power grid according to the principle of "leveling and filling" . As the result, the intelligent control on switching on and switching off of the capacitors is realized and the compensation accuracy is high.
1. The conditions for limiting the voltage in a reasonable manner
The over voltage and under voltage protection value can be set. It is also possible to set the voltage values for No switching on (trough high voltage) and No switching off (peak low voltage) . It has the phase missing protection function. The reactive power is used as the switching threshold value.
2.It is possible to set the switching delay
The delay time is adjustable (it supports both the fast tracking reactive power capacitor compensation cabinet and the steady state reactive power capacitor compensation cabinet). The time interval of switching on and switching off operations of the capacitors in the same group can be set, which can be set to zero in case of the fast tracking reactive power capacitor compensation cabinet.
IV. Integrated distribution monitoring function
The comprehensive distribution monitoring function integrates the electrical parameter measurement, memory and communication of the distribution transformer. It is a relatively complete set of distribution operation parameter measurement mechanism, and it is an ideal means to assess the line loss of the unit in low voltage distribution power network. It can provide all kinds of data for the power grid management personnel at any time. It is a reliable management basis for the safe operation and economic operation of the power grid. It is the basic part of the automation system in the distribution network. The main functions are as follows:
Real-time monitoring of the three-phase data of the distribution transformer: voltage, current, power factor, frequency (1 - 3 harmonics); cumulative data recording, whole point data recording and statistical data recording function, cumulative measurement of the active and the reactive power; query, the statistics and analysis functions and generate various reports, curves, bar charts and pie charts according to input conditions. Generally, it is equipped with related background processing software, most of which can realize the network multi-computer operation and the data sharing.
V. Integrated voltage monitoring function
According to the standard of the voltage detector, sampling and data statistical processing are carried out, which is convenient for users for assessing the qualified rate of the voltage. It can be used for voltage monitoring and assessment.
VI. Integrated online harmonic monitoring function
A monitoring terminal with relative better capacity uses DSP as CPU, to apply FFT fast Fourier algorithm, which can accurately calculate and measure the distribution parameters such as voltage, current, power factor, active power and reactive power. It can also analyze 1 - 3 harmonics, so as to realize the on-line harmonic monitoring function. The data can be analyzed and processed on the background software according to the requirements of the user.
Some monitoring terminals with advanced functions fully consider the sustainable use of the equipment. The standard RS232 and RS485 interface can be specially configured with Modem, field bus (Profibus) according to the requirements of users, which can be organically combined with the distribution network automation system. There are the following specific ways of communication, or the combination of one or more ways. Manual meter reading: wired, wireless, electric card and other ways of meter reading.
Direct communication: It is interfaced with distribution automation system to provide users with a variety of solutions so as to adapt to the direct communication between different distribution network automation systems and the substations or the master stations. Communications with FTU: The point-to-multipoint acquisition can be realized through FTU so as to implement the remote data transmission and communication with the distribution automation system interface as well as communication with the centralized reading system, usually carrier or direct connection.
VIII. Modular structure
At present, the widely used modular design structure integrates capacitors, switching switches and protection into one unit to form a standardized unit with various capacity specifications. It is characterized by the modularization of structure and function to form a series of products so as to meet the requirements of different users. At the same time, it is convenient for the maintenance and adjustment of various devices in the field.
In a word, with the rapid development and progress of the science and technologies as well as the increase in the quantity of the high-tech enterprises, the electricity enterprises, while making efforts to meet the increasing needs of users in a better way, should also carry out more comprehensive management and monitoring on the user power grid. In this process, there will be a variety of new technologies, new equipment to be developed. In the future, the reactive power capacitor compensation cabinet technology will be more economical and effective.