Reactive power compensation controller design requirements and design methods - Database & Sql Blog Articles

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The reactive power compensation controller is the core component of the reactive power compensation device. It has a pivotal position. Most manufacturers of reactive power compensation devices buy controllers and assemble their own machines. There are not many manufacturers that have the ability to design and manufacture controllers. Manufacturers who can design and manufacture controllers with excellent performance are rare.

1. Requirements for measurement accuracy

To achieve accurate reactive power compensation, accurate measurement of reactive current must be performed.

Since the voltage variation range is small, the measurement accuracy of the voltage is not high, and usually 1% of the measurement accuracy is sufficient. Under normal circumstances, good reactive power compensation control can be realized without measuring voltage. The voltage measurement is mainly to achieve overvoltage, undervoltage, and lack of equal protection.

The measurement sensitivity of the current is required to be higher. For low-end controllers using 8-bit microcontrollers, the measurement sensitivity is above 1%. Note that the emphasis here is on "measurement sensitivity" rather than "measurement accuracy". The 1% current measurement sensitivity is equivalent to a 1% current change. For example, the current transformer's primary current is 500A, which means that it can be distinguished from 100A. The current change to 105A does not require 100A current measurements to be absolutely accurate. For high-end controllers using DSP or 32-bit microcontrollers, the measurement sensitivity should reach 0.1% or more, otherwise it will not be high-grade. By the same token, the sensitivity of the measurement is 0.1%, which means that the measured value should have 4 significant digits, but it is also not absolutely accurate. It is unrealistic and unrealistic to require 0.1% measurement accuracy for the reactive compensation controller. However, the measured value of the controller is preferably calibrated in the field.

The sensitivity to power factor measurement is preferably 0.001. To be precise, it should be a measurement requirement for the phase difference, because measuring the reactive power does not require the use of the power factor value. It should be emphasized here that the calculation of reactive current should be calculated using the formula of Iq=I×sinφ, and the value of sinφ should be directly calculated according to the value of the phase difference. Sinφ=(1-cosφ2)1/2 cannot be used. The formula is calculated, otherwise, when the phase difference is around 0 degrees, a small change in cosφ causes a large change in sinφ, resulting in a value error of sinφ is too large. For example, when cosφ=0.99, the corresponding phase difference is 8.1 degrees, and the corresponding sinφ value is 0.14, which means that other sinφ values ​​between 0 and 0.14 are not detected.

The measurement of the phase difference is required to reach the entire range of -180 to +180 degrees. Some controllers have an automatic identification function of the current transformer reversed. This controller has a positive value of the positive force to judge the positive and negative of the transformer, which is equivalent to the range of -90 to +90 degrees. This may be the following problem:

(1) A detection error occurs when the load is in the power generation state.

(2) When the load is pure inductance or pure capacitance, since the active current is approximately equal to zero, the inductance may be mistakenly judged as a capacitance or the capacitance may be mistakenly judged as an inductance. The state where the load is a pure capacitor often occurs. For example, when the load is a single large load and the load is stopped, the reactive compensation capacitor is still running, so the secondary current of the transformer becomes a pure capacitor current. If this current is mistakenly determined as an inductor Current, the controller will continue to put the capacitor until all the capacitors are put into operation, causing serious overcompensation.

2, the choice of display

The most commonly used display device is the LED digital tube, which is inexpensive and highly reliable. It is best to use a multi-position LED digital tube, which can greatly reduce the wiring of the circuit board and reduce the welding installation workload.

Many people are more enthusiastic about using liquid crystal displays. Liquid crystal displays can display Chinese characters. In the case of lighting, they are also more energy efficient. However, the biggest problem with liquid crystal displays is that the low temperature performance is not good, and usually cannot be displayed normally below -10 °C. Therefore, do not use the LCD monitor unless you can determine that the controller's ambient temperature is above -10 °C.

3, parameter setting function

For reactive power compensation controllers that are controlled based on reactive current or reactive power, the parameter setting function is mandatory.

When the controller is manufactured, the parameters such as the rated capacity of the capacitor and the ratio of the current transformer cannot be determined in advance. It can only be set according to the actual situation of the reactive power compensation device and the site conditions. Therefore, the controller must have the parameter setting function. . The set parameters should be guaranteed not to be lost due to power loss.

The most straightforward way to save set parameters is to use EEPROM devices such as 24C02. Some microcontrollers have an on-chip EEPROM, which reduces the number of peripherals. There are also some microcontrollers that have an application programming function, that is, the contents of the on-chip FLASH program memory can be modified during program execution. For this type of microcontroller, the setting parameters can also be saved in the FLASH program memory, but the programming in the application programming is more complicated.

4, the design of the protection function

The overload of the capacitor is caused by excessive voltage or excessive harmonics, so it is necessary to design an overvoltage protection function in the controller. When the capability allows, the voltage harmonic detection function should be designed in the controller, because the root cause of the harmonic overload of the capacitor is voltage distortion, and the harmonic overload protection of the capacitor can be realized by detecting the voltage harmonic. Thermal relays can be eliminated with overvoltage protection and harmonic overload protection. It saves both volume and cost and reduces the point of failure.

5, capacitor input and cutting control strategy

Capacitor input and resection should be carried out step by step, and multiple capacitors should not be put in or removed simultaneously in one step. Otherwise, excessive current sudden changes will have a relatively large impact on the system, and it is not conducive to achieving accurate compensation effects.

At the same time, for compensating devices with capacitors of different specifications, the switching of the capacitors should be as simple as possible in order to minimize the number of switching of the capacitors and to meet the compensation requirements as quickly as possible. It should not be incremented or decremented step by step by the minimum step.

For example, there are three types of capacitors in the compensation device, namely 10Kvar, 20Kvar, and 40Kvar. If the required reactive compensation amount is 40 var or more, a 40 var capacitor should be directly input. By the same token, when the excess reactive compensation is measured to be 30 var or more, a 40 var capacitor should be cut directly.

6, the design of the output circuit

Usually the output of the controller is used to control the AC contactor or composite switch, the most common is the 220V AC output. The number of outputs is determined by the requirements, usually 10 channels are sufficient.

The most common output component is the electromagnetic relay. The most important principle of using the electromagnetic relay is that the relay armature itself cannot be electrically connected to the contact. Many of the relay armatures are part of the moving contact, so the relay core is charged, when the coil insulation appears. When the problem occurs, the strong electricity will break into the control part and cause serious damage. For a relay that has no electrical connection between the armature and the contact, there is no phenomenon that the strong electric power is intruded into the control part.

When the contact of the electromagnetic relay is disconnected, since the contactor coil is not able to transient due to large inductor current, a high arc voltage is generated, so the RC absorption component 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. Since the thyristor can be turned off by zero current, it is not necessary to use a RC absorbing element, and the driving voltage and current are small, and it is relatively easy to implement control. Good quality electronic relays are more expensive. Electronic relays of poor quality are prone to false triggering, causing contactor jitter during power-up.

The output circuit can also use a bidirectional thyristor. At this time, the driving circuit of the thyristor is slightly more complicated, but the cost is low and the reliability can be well done.