In recent decades, the load characteristics of the urban low voltage power grid have changed dramatically. The previous low-voltage compensation configuration can no longer meet the basic requirements of improving the power quality. Under the promotion of the information age, computer technology and power electronics technology are gradually promoted and applied, and at the same time, the technology of low voltage reactive power compensation device has been promoted Step by step, the research, development, update of compensation device and the application of composite switch are promoted.

1. Capacity of compensation capacitor and related factors

1.1 no load reactive power compensation of power supply transformer

For the no-load reactive power compensation of power supply transformer, the shunt capacitor is generally selected, and the total capacity of the transformer is 3% as fixed compensation, so as to compensate the loss of no-load reactive power of the transformer.

1.2 the capacity gradient of multi-channel compensation needs to be further determined

Firstly, we need to understand the maximum and minimum value of the power load, and analyze the load fluctuation. Then, considering the actual situation, the step length of capacitor and the number of packet paths are further determined, so as to accurately track the reactive power change.

1.3 selection of balance, phase separation and composite compensation

When necessary, we can make field measurement, so as to determine whether the unbalance degree of three-phase load should be selected composite compensation or three-phase balance compensation. If the three-phase phenomenon is seriously unbalanced, the best choice is the appropriate capacity phase separation compensation.

1.4 determine the total capacity of the compensation capacitor

For the natural power factor measurement, the target power factor is determined, and then the total capacity of reactive compensation is determined according to the difference between the two.

If we know that P is equal to active power, cos φ 1 is equal to natural power factor, cos φ 2 is equal to target power factor, then we can calculate the total capacity result of capacitor compensation as follows:

ΔQ＝P（tanφ1－tanφ2）

1.5 determine whether to use harmonic reactive compensation capacitor

If the harmonic of the power grid has a large component, the harmonic should be tested on site. When necessary, special capacitors (matching with the reactor) are also needed to effectively prevent the normal operation of the compensation device or the damage of the capacitor when the harmonic is large.

2 select the reactive power compensation controller

1) Considering that the load fluctuation in the power grid is not large and the three-phase load is in the basic equilibrium state, the goal is to improve the power factor. In order to control the cost of the equipment and reduce it, it is necessary to consider the convenience and simplicity of the reactive power compensation controller and belongs to a single simple operation.

2) For the power grid load with frequent fluctuation and the big gap between the maximum and the minimum load, the three-phase load is still in the basic balance. For the choice of controller, the controller with better performance is preferred. For example, for the control of physical quantity, reactive power or reactive current should be selected, and the threshold of input and cut-off should be set separately, so as to effectively prevent switching oscillation, and at the same time, the under current and over-voltage wind protection functions should be ensured. In the selection of switching mode, it is best to adopt "coding + cycle" to control the program.

3) For the power grid load with frequent fluctuation, and the large gap between the maximum and the minimum load, and the three-phase load is extremely unbalanced, we propose higher and more detailed requirements when the load is lower than the controller. The composite switching function should have "phase separation + balance". The control physical quantity should be a compound type (power factor + reactive power). The specific parameters are as follows:

Sensitivity ≤ 100 mA.

For the action error, the allowable error value of the physical quantity of sampling is as follows:

Reactive current ± 20%;

Power factor ± 2.5%;

Reactive power ± 20%.

Stability range: under the stable operation and the compensation requirements meet, the controller can be selected correctly, so as to prevent the phenomenon of no switching oscillation.

4) In order to better cooperate with the implementation of power grid automation, when the power factor is improved, it can also monitor the parameters of the power grid in real time. In this case, the selection of reactive power compensation controller needs to consider its comprehensive test function.

The controller shall have the functions of composite control physical quantity, composite switch function, high sensitivity and stability, small action error, over voltage and under current, etc. mentioned in the first three items, and it shall also have the functions of real-time online measurement of power grid parameters, data storage, data display, telegraph time correction, power failure data protection, data acquisition and data remote transmission. At the same time, it should be equipped with supporting background software with perfect functions, so as to effectively analyze and visualize the collected data and output various related reports. If GPRS wireless communication is adopted for data transmission, it can also completely eliminate the investment in communication network construction and manual meter reading, and save a lot of financial and human resources.

5) If there is a large harmonic component grid, or there are many nonlinear loads, the selection of reactive power compensation controller should take into account the protection of harmonic overrun and measurement of harmonic, and for the selection of anti harmonic reactor, the device with reasonable parameters shall be selected to form the anti harmonic reactive compensation control device, which is conducive to the emergence of a relatively favorable one When serious harmonic wave is serious, it can still achieve satisfactory effect and ensure reliable operation.

Application of 3 composite switch in low voltage reactive power compensation device

3.1 switch input part

For signal control of input, we usually use +12 V and +5 v. the system will have corresponding phase loss protection function at the same time. It is necessary to better reject the closing of the switch in case of phase shortage, serious overvoltage or undervoltage (if the switch is in closed working state, it will automatically disconnect and cut off the capacitor). No matter the light is closed or disconnected, the switch can work normally only if the signal is controlled under the condition of three-phase basic balance.

3.2.1 delay circuit

When the capacitor is switched on and off, considering the electric shock type switch, there will be strong arc rough marking on the contact, which will easily lead to the contact damage, which may lead to bad phenomenon, and the most serious case may lead to the occurrence of melting; for contactless switches, such as high power or thyristor relay, there is a large power Serious consumption and heat dissipation problems. Considering the problems in practice, the author improves the delay circuit when designing the system. During the opening process, the thyristor must be opened first, and then delayed for two to three cycles. The high-power magnetic field should keep the relay closed. The thyristor in the pipe section is provided with the load working current by the high-power magnetic holding relay. When the switch is off, it is necessary to consider that the magnetic protection relay is disconnected first and then the SCR can be disconnected after two or three weeks.

Through this treatment, the arc pulling and inrush phenomenon of contact will be solved when the relay is disconnected and connected, and the heat dissipation problem without electric shock switch can be eliminated basically, and the service life of switch is greatly prolonged and the reliability of system operation is improved. The whole circuit is composed of digital and analog logic circuits, which can be controlled better in accuracy, and reliable in operation. The most important is relatively stable operation.

3.2.2 pole changing link

The switch part of the system uses a high-power magnetic hold relay, which is driven by DC pulse voltage, and the switching state is changed by reverse pulse voltage. Therefore, the conversion polarity is called the pole changing link.

4 Conclusion

In order to solve the problem of power supply shortage, it is also an effective way to deal with the problem of reactive power compensation besides accelerating the construction speed of power plant. In the process of designing intelligent composite switch, it can integrate the contactless switch and AC contactor into one another. So we need to keep on studying.