The reactive power compensation device for adjusting unbalanced current can compensate reactive power of the system and adjust unbalanced active current at the same time.
The reactive power compensation device for adjusting unbalanced current uses Wang's theorem, that is, capacitors connected across phases can transfer active current between phases. 12 single-phase power capacitors with 400V withstand voltage are installed in the device, and each capacitor can be connected between phase lines or between phase lines and zero lines. Using synchronous grouping switch to change the capacitance connection (patent number: ZL2009200 12 159. x)。
The reactive power compensation device for adjusting unbalanced current can compensate the three-phase power factor to above 0.95, and at the same time adjust the unbalanced current between three phases to within 10% of the rated current of the transformer. Not only can the copper loss of the system be greatly reduced, but also the iron loss of the transformer can be reduced.
The invention is very suitable for three-phase unbalanced rural power grids and urban residential power grids.
2. Synchronous switching technology (patent application number: 200910010913.0)
Synchronous switching technology is the latest development technology in recent years. As the name implies, it is to make the contacts of mechanical switches accurately close or open at the required time. For the synchronous switch of the control capacitor, it needs to be closed at the moment when the voltage at both ends of the switch contact is zero, so as to realize the inrush-free input of the capacitor and open at the moment when the current is zero, so as to realize the arc-free opening of the switch contact.
WSBC-PTK4 synchronous switch adopts the unique patented synchronous switching technology, which makes the magnetic latching relay in the switch perfectly realize the synchronous operation of "voltage zero-crossing input and current zero-crossing cut-off". Compared with the commonly used compound switch, it omits the thyristor component connected in parallel with the magnetic latching relay contact, simplifies the structure, reduces the cost, and avoids the common faults of the thyristor component, thus greatly improving the reliability.
3. Moderate overcompensation technology
Although the reactive power of the transformer itself is not large, there are many transformers, which usually run continuously. In the case of small load at night, the reactive current of the transformer will account for a large part of the system current, so the reactive power of the transformer itself can not be ignored.
The usual compensation device is installed on the low voltage side of the transformer to detect and control the reactive current of the load, but it cannot compensate the reactive current of the transformer itself. Most people always think that the reactive power of transformer can only be compensated on the high voltage side, but it is not. The reactive current of the transformer can also be compensated by overcompensating on the low voltage side. Because transformers belong to ideal components, the so-called ideal components are components with no direction of energy transfer. If the high voltage side is connected to the power supply and the low voltage side is connected to the load, the same transformer is a step-down transformer, and if the low voltage side is connected to the power supply and the high voltage side is connected to the load, the same transformer is a step-up transformer. According to this principle, there is no difference in reactive power compensation between low voltage side and high voltage side. The reactive current of transformer can be compensated by moderate overcompensation on the low voltage side.
In order to realize moderate over-compensation function, it is necessary to design a reactive power compensation controller with over-compensation function, and the amount of over-compensation can be set, and the controller is also required to have high measurement accuracy.
4, 32-bit single chip microcomputer application technology
At present, most of the reactive power compensation controllers are controlled by 8-bit single chip microcomputer, which is slow in speed and small in storage capacity, and cannot achieve high-precision measurement. It can only be controlled relatively simply and will soon be eliminated.
Single-chip microcomputer with 32-bit ARM core is now in full swing, and it has been widely used, and the price is already very cheap, so that the material cost of the controller made with 32-bit single-chip microcomputer may be lower than that made with 8-bit single-chip microcomputer.
The 32-bit single chip microcomputer has powerful functions, fast running speed and large storage capacity, and can realize high-precision measurement and control. Therefore, it is the only way for the development of reactive power compensation technology to manufacture reactive power compensation controller with 32-bit single chip microcomputer.
The only disadvantage of 32-bit single-chip microcomputer is that it is too difficult to develop, and ordinary small companies do not have the ability to develop.
5. Harmonic measurement and protection technology
Nowadays, more and more power electronic components are applied to the power grid, resulting in more and more harmonic currents in the system. The capacitor in reactive power compensation device is very sensitive to harmonic current, which is easy to produce harmonic amplification and cause capacitor damage.
Thermal relays are used to protect capacitors in most reactive power compensation devices.
Capacitor is a steady current element, and its current is only related to voltage and frequency, and has nothing to do with the load current of transformer. Under normal voltage and no harmonics, the capacitor will not be overloaded. When the voltage is too high, the protection function can be realized by the controller, and it is not necessary to realize the protection function by the thermal relay.
When the harmonic exceeds the standard, the capacitor will be overloaded. Although the thermal relay can cut off the capacitor, if the controller can't detect the harmonic, it will continue to put in a new capacitor, resulting in a new overload phenomenon. If the thermal relay is set to the automatic reset state, the capacitors that have been cut off will be put into operation again after a period of time and continue to be overloaded, which will interfere with the operation of the controller, because the controller does not know which capacitors have been cut off by the thermal relay and which capacitors are about to resume operation. If the thermal relay is set to manual reset, all capacitors will be cut off eventually, and even if the harmonics disappear before manual reset, the capacitors cannot be put into operation again. Therefore, in the case of serious harmonics, the protection effect of thermal relay is far less than that of controller with harmonic protection function.
To sum up, the reactive power compensation controller has the functions of harmonic detection and harmonic overload protection, which can not only observe the harmonic content in the system, but also omit the thermal relay, improve the performance and save the cost.
6. Step compensation technology
Because the capacity of the capacitor is fixed, in order to control the compensation amount of the compensation device, several capacitors need to be installed in one compensation device, and the total compensation amount can be controlled by controlling the input number of the capacitors.
The most common design scheme is to use several capacitors with the same capacity. At this time, the step size is the capacity of a single capacitor. If the total compensation is 1, the step size is the reciprocal of the number of capacitors. For example, if 10 20Kvar capacitors are installed in a compensation device, the total compensation amount is 200Kvar and the step size is 20Kvar. According to the unit value, the step size is110. This design scheme is simple, and it is easy to realize the cyclic switching of capacitors. The disadvantage is that the steps are too big. Even if 15 capacitors are installed, the step size is still115, which still can't achieve good compensation effect when the compensated user's load is light.
The best design method is to use several capacitors with the same capacity, and then use a capacitor with the capacity of 1/2 and a capacitor with the capacity of 1/4. For example, 8 capacitors of 20Kvar, 10Kvar 1 capacitor, and 1 capacitor of 5Kvar. At this time, * * * 10 capacitors are used, the total compensation capacity is 175Kvar, the step size is 5Kvar, and the nominal value is 5/ 175= 1/35. The step size of this design scheme is small enough to achieve sufficient compensation accuracy and meet the needs of various occasions. Eight capacitors with the same capacity can be cycled on and off. Although the capacitors with 1/2 capacity and 1/4 capacity may be turned on and off more frequently, it is no problem to turn on and off more frequently because of the small capacity.
7. Software reliability design technology
At present, most reactive power compensation controllers are designed by single chip microcomputer.
When it comes to the reliability of single chip microcomputer, people almost invariably think of watchdog. In fact, the watchdog can only ensure that the microcontroller can generate a reset signal when it crashes, and having a watchdog may not guarantee sufficient reliability.
A computer crash is equivalent to the program going off course and entering an infinite loop. However, if the program does not enter the infinite loop and returns to the normal program loop after escaping, there will be no crash, so the watchdog will not work, but what the program does after escaping is unpredictable, so the reliability design of the software is to ensure that the problems caused by the program after escaping can be made up as much as possible.
The above briefly introduces some new technologies in the field of reactive power compensation. For details, please visit the website of Shenyang Vance Electric Power Technology Research Institute: