Reactive power compensation controllerIt is the core component of the reactive power compensation device, has a pivotal position, most of the manufacturers of the reactive power compensation device are to buy the controller and then assemble the whole machine by themselves, there are not many manufacturers with the ability to design and manufacture the controller, and the manufacturers who can design and manufacture the controller with excellent performance are even rarer.
The existing high-end controllers are controlled based on reactive power, but in addition, the design is often focused on Chinese character display and data communication. In fact, it is quite complicated to really realize the perfect reactive power compensation control, and the realization of perfect reactive power compensation control is the main function of the reactive power compensation controller, and the additional functions can only be considered when the main function is quite perfect.
1. Requirements for measurement accuracy.
Accurate reactive current measurement is essential for accurate reactive power compensation.
Because the voltage variation range is small, the measurement accuracy of the voltage is not high, and usually 1% measurement accuracy is sufficient. Under normal circumstances, good reactive power compensation control can be achieved without measuring the voltage, and the measurement of the voltage is mainly to achieve protection functions such as overvoltage, undervoltage, and phase loss.
The sensitivity of the current measurement is higher. For low-grade controllers using 8-bit microcontrollers, the measurement sensitivity should be more than 1%. Note that the emphasis here is on "measurement sensitivity" rather than "measurement accuracy", and a current measurement sensitivity of 1% is equivalent to being able to distinguish a current change of 1%, for example, the primary current of a current transformer is 500A, which means that the current change from 100A to 105A can be distinguished, and the current measurement value of 100A is not required to be absolutely accurate. For high-end controllers using DSP or 32-bit microcontrollers, the measurement sensitivity should be 0More than 1%, otherwise there will be no talk of high-end. In the same way, the sensitivity of the measurement should reach 01%, which means that the measurement should have 4 significant digits, but again, absolute accuracy is not required. 0A measurement accuracy of 1% is unrealistic and meaningless. However, it is best if the measured values of the controller can be corrected on site.
The sensitivity of the power factor measurement should ideally be 0001。To be precise, it should be the measurement requirement for phase difference, because the measurement of reactive power does not require the use of power factor values. It should be emphasized here that the calculation of reactive current should be calculated by the formula of iq=i sin, and the value of sin should be calculated directly according to the value of the phase difference, and cannot be calculated by the formula of sin = (1-cos 2)1 2, otherwise when the phase difference is around 0 degrees, the small change of cos will lead to a large change in sin, resulting in a large error in the value of sin. For example, cos = 0At 99, the corresponding phase difference is 81 degree, the corresponding SIN value is 014, which means 0-014 other SIN values are not detected.
The measurement of phase difference is required to reach the entire -180-+180 degree range. There are some controllers with the automatic identification function of current transformer reversal, this controller must be positive value to judge the positive and negative of the transformer, which is equivalent to the range of -90-+90 degrees, which may have the following problems:
1) Detection error occurs when the load is in the state of power generation.
2) When the load is pure inductance or pure capacitance, the inductance may be misjudged as capacitance or the capacitance as inductance because the active current is about equal to zero. The state that the load is pure capacitance often occurs, for example, when the load is a single large load and the load is shut down, the reactive power compensation capacitor is still running, so the secondary current of the transformer becomes the pure capacitor current, if the current is misjudged as the inductor current, the controller will continue to put the capacitor until all the capacitors are put into operation, resulting in serious overcompensation.
2. Display selection.
The most commonly used display device is LED digital tube, LED digital tube is low cost and high reliability. It is best to use a multi-position combination of LED digital tubes, which can greatly reduce the wiring of the circuit board and reduce the soldering installation workload.
Many people are more keen to use LCD monitors, which can display Chinese characters and save power in the case of lighting, but the biggest problem with LCD displays is that they have poor low temperature performance, and they usually cannot be displayed normally below -10. Therefore, do not use the LCD unless you can be sure that the ambient temperature of the controller is above -10.
3. Parameter setting function.
For reactive power compensation controllers that are controlled on the basis of reactive current or reactive power, the parameter setting function is required.
When the controller is manufactured, the rated capacity of the capacitor, the conversion ratio of the current transformer and other parameters can not be determined in advance, and can only be set according to the actual situation of the reactive power compensation device and the site situation, so the controller must have the function of parameter setting. The set parameters should ensure that they will not be lost due to power failure.
The most straightforward way to save the set parameters is to use an EEPROM device, such as 24C02. Some microcontrollers have on-chip EEPROM to reduce the number of peripheral components. There are also some microcontrollers that have an application programming function, that is, the contents of the flash program memory on the chip can be modified during the program run. For this kind of microcontroller, the setting parameters can also be saved in the flash program memory, but the program design in the application programming is more complicated.
4. Design of protection function.
The overload of the capacitor is caused by too high voltage or too large harmonics, so it is necessary to design the overvoltage protection function in the controller. If the ability allows, the voltage harmonic detection function should be designed in the controller, because the fundamental cause of the capacitor harmonic overload is voltage distortion, and the detection of voltage harmonics can realize the harmonic overload protection of the capacitor. With overvoltage protection and harmonic overload protection, thermal relays can be eliminated. It not only saves volume and cost, but also reduces the point of failure.
5. Capacitor input and removal control strategy.
Capacitor input and removal should be carried out step by step, and multiple capacitors should not be put in or removed at the same time in a single operation. Otherwise, too large a sudden current change will have a relatively large impact on the system, and it is not conducive to achieving accurate compensation effect.
At the same time, for the compensation device installed with capacitors of different specifications, the switching of the capacitor should be as simple as possible, so as to minimize the number of switching times of the capacitor and meet the compensation requirements as soon as possible. It should not be incremented or decremented by the smallest step step.
For example, there are three specifications of capacitors in the compensation device, namely 10kvar, 20kvar, and 40kvar, if the required reactive power compensation is measured to be more than 40var, a 40var capacitor should be directly put in. In the same way, when the excess reactive power compensation is measured to be more than 30var, a 40var capacitor should be cut off directly.
6. Design of output circuit.
Usually the output of the controller is used to control AC contactors or composite switches, the most common is the 220V AC output. The number of output channels depends on the requirements, usually 10 channels are enough.
The most common output component is the electromagnetic relay, the most important principle of the selection of electromagnetic relay is that the relay armature itself can not be electrically connected with the contact, and the armature of many relays itself is a part of the moving contact, so the relay core is electrified, when there is a problem with the coil insulation, the strong current will enter the control part and cause serious damage. For relays where the armature is not electrically connected to the contact, there will be no strong current channeling into the control part.
When the contact of the electromagnetic relay is disconnected, because the contactor coil is a large inductor current can not be transient, a high arc voltage will be generated, so the resistor-capacitance absorption element must be connected, otherwise serious interference will occur.
The output component can also use an electronic relay, the inside of the electronic relay is a thyristor, because the thyristor can be turned off by the current crossing zero, so there is no need to use the resistor-capacitance absorption element, and the driving voltage and current are very small, it is relatively easy to control. The electronic relay is higher. Poor quality electronic relays are prone to false triggering, resulting in contactor jitter when powering up.
The output circuit can also use a bidirectional thyristor, and the drive circuit of the thyristor is slightly more complex, but the cost is very low, and the reliability can also be done very well.
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Zhejiang Yishuo Electric Co., Ltd. is a high-tech company specializing in R&D, design, production, sales and service of low-voltage power capacitors, smart capacitors, reactors, reactive power compensation controllers, and reactive power compensation devices.
The company adheres to the tenet of "taking science and technology as the driving force and seeking development by quality", and is committed to building a leading brand of smart grid professional services. The company adheres to the tenet of "taking science and technology as the driving force and seeking development by quality", and is committed to building a leading brand of smart grid professional services.
The company has a high-level R&D management team, integrating R&D, production, sales and service, focusing on intelligent reactive power compensation, harmonic control, energy saving and power quality management and other high-tech industries of power automation.
Over the years, we have adhered to the mission of "making products intelligent and services intimate". In line with a high sense of responsibility to the society and users, Yishuo Electric adheres to the road of specialization, continues to innovate and transcend, and strives to make greater contributions to China's smart grid power automation