Abstract:At present, there are still many problems in the actual operation of the capacitor switching switch that we often use. The new zero crossing switching switch mainly utilizes microelectronics technology, and enterprises have further optimized and adjusted the structure of the circuit, which can fundamentally improve the safety and stability of the switch, making the capacitor bank more accurate and rapid in the process of zero crossing switching. By utilizing cyclic switching control and optimizing the combination of shared compensation, the efficiency of the zero crossing switching process can be further improved, and the service life of capacitors can be further extended. At the same time, the reactive power dynamic compensation of low-voltage power grids becomes more intelligent.

Keywordscapacitor; Switching switch; zero crossing

In recent years, the number of nonlinear loads and inductive charges in the power grid has greatly increased, and the reactive power has also significantly increased. This has led to a decrease in the voltage and power factor at the receiving end, ultimately resulting in a very serious impact on the quality of power supply. The installation of reactive power compensation equipment can effectively meet the problem of reactive power in low-voltage distribution networks. In the process of carrying out reactive power compensation in low-voltage power grids, using capacitors for compensation is a commonly used and effective way. In addition, the rationality of the selection of the switching switch is one of the important links to ensure the normal operation of the relevant compensation work. Starting from the performance of different switching devices, there are still many problems that need to be solved in the commonly used capacitor switching devices, especially in terms of safety and stability, which require further improvement. The circuit structure of the new zero crossing switch has undergone significant changes compared to before, effectively solving the problems of thyristor damage and single touch electromagnetic holding burnout. In addition, under the action of this new type of zero crossing switch, the previously common phenomena of pre ignition and reignition of arcs rarely occur again, ensuring normal long-term operation and avoiding many faults. In addition, the goal of zero crossing without inrush current switching has also been achieved, making the compensation process more accurate and rapid.

1. The principle of zero crossing switching

In situations where zero crossing switching is frequently required, with current technology, contactors are generally chosen. However, when the contactor performs relevant switching actions, due to the lack of a stable phase relationship between the voltage and current of the circuit, current surges or overvoltage phenomena are inevitable in the actual operation process. There have been many discussions and proposed solutions by relevant research institutions regarding this issue, aiming to achieve technological breakthroughs and further improve the efficiency and quality of zero crossing switching work. At present, a new type of capacitor zero crossing switching circuit switch has been developed, which compensates for many problems of the original device and can achieve low energy consumption and high efficiency.

The article first provides a brief introduction to zero crossing switching, which mainly utilizes the characteristics of capacitors. When a capacitor is put into operation, if there is a large voltage difference between the capacitor terminal and the grid voltage, it may cause a sudden change in the capacitor voltage, ultimately leading to the generation of closing inrush current. This inrush current may cause damage to the thyristor. Therefore, in order to avoid related faults, the time to put the capacitor into operation should be grasped when the current terminal voltage is equal to the grid voltage.

2. Zero crossing switching system

The zero crossing switching system of low-voltage power capacitors consists of various components, including voltage and current transformers, filtering circuits, sampling circuits, digital signal processors, microcontrollers, magnetic holding relay drive circuits, and other important parts. In addition, digital signal processors and microcontrollers also have important functions such as capacitor status indication, event recording, and fault alarm. The main components of human-machine interface are display and keyboard, whose main function is to set system parameters and display power parameters on the display for people to use and reference.

2.1 Parameter Sampling and Switching Controller

The zero crossing switching process of low-voltage capacitors requires high performance from processors and microcontrollers. Generally, 32-bit fixed-point processors with good performance and STC89C53 microcontrollers with high performance and low energy consumption are chosen. The STC89C53 microcontroller can obtain three-phase voltage and current parameters of low-voltage power grids through three important channels, including voltage and current transformers, filtering circuits, and sampling circuits. By using DSP, relevant parameters can be calculated in real-time, and then analyzed through the relevant data. The customer's needs determine whether we need to carry out reactive power compensation work on the capacitor bank, and the judgment of this process depends on the microcontroller. The high-speed transmission of information between the microcontroller and the processor requires RS-485 communication to ensure the real-time performance of the system's related work.

2.2 Zero crossing switch

The main components of the zero crossing switching switch for low-voltage capacitors are high-power magnetic holding relays, bidirectional thyristors, and resistance capacitance absorption circuits. The switching of power capacitors requires the use of a microcontroller to control zero point switching. When the capacitor is put into operation, the microcontroller will start issuing relevant instructions based on its basic principle when the voltage at both ends of the bidirectional thyristor is zero, so as to use the trigger pulse to conduct the bidirectional thyristor and smoothly carry out the capacitor input operation. The high-power relay receives relevant instructions to conduct. When the current of the thyristor is zero, the microcontroller will determine whether to disconnect the bidirectional thyristor based on relevant parameters and its own detection results. After conducting, it will send relevant signals to sequentially disconnect the high-power magnetic holding relay and then disconnect the bidirectional thyristor. When the switch is in normal working condition, we need to keep the high-power magnetic holding relay on and off, but we don't need to keep the bidirectional thyristor on and off all the time. It only needs to keep it on and off at the moment of switch switching. The purpose of doing this is to solve the problem of surge current impact during capacitor switching and effectively reduce energy loss during operation.

2.3 Magnetic Holding Relay Drive Circuit

High power magnetic holding relays generally have strong load-bearing capacity and do not consume too much energy during actual operation. The content includes magnetic steel and coils, which all need to follow instructions from the microcontroller during operation. In the selection process of magnetic holding relays, HFE9 type can be considered, which has a dual coil device. When carrying out the closing and opening work, the high level output of the main control chip can make the transistor conductive, which lasts for about 50ms. Afterwards, the main control chip outputs a low level and the magnetic holding relay completes the closing and opening process.

2.4 Thyristor Zero Crossing Detection and Trigger Circuit

During the process of capacitor switching, there may be phenomena such as closing inrush current and overvoltage surge. Therefore, in order to solve this problem and protect the switching process of capacitors, we need to start the switching of capacitors when the voltage or current of the bidirectional thyristor is zero. When selecting chips, taking into account the relevant issues mentioned above, we should try to consider their zero crossing detection and triggering capabilities, and choose ideal chips with obvious advantages in all aspects to carry out driving work on bidirectional thyristors. The MOC3083 driver chip is a chip with strong working ability. Its internal circuit can perform zero crossing detection. When the voltage at both ends of the thyristor is equal to zero, it will emit a relevant trigger pulse signal to turn on or off the thyristor. In addition, the use of varistors in this process solves the problem of abnormal overvoltage in the circuit, which mainly relies on the good ability of varistors to withstand shocks.

2.5 Analysis of Compensation Methods

The compensation method for switching capacitor banks can choose a combination of common and distributed optimization, which has high flexibility and effectively improves the problem of load changes in low-voltage power grids. At the same time, the problem of reactive power compensation for three-phase load imbalance has also been well solved. In load balancing, the method of co compensation can be used, while when the load is not very balanced or there are significant fluctuations, the method of joint compensation first and then separate compensation can be used. For residential electricity consumption, single-phase loads are generally used, so their fluctuations are usually significant. Three phase loads often experience excessive imbalance, and the required compensation capacitance varies for different phases. The difference in reactive power compensation is so significant that we cannot choose the method of joint compensation when selecting compensation, so a separate compensation method is a better solution at this time. This method can effectively reduce resource waste and has excellent rationality in itself.

3. Design of the cutting process

Reactive power and power factor are two important factors affecting the zero crossing switching control of low-voltage power capacitors, among which reactive power has a more significant impact. In addition, the implementation of zero crossing switching control for low-voltage power capacitors needs to be based on both reactive power and power factors. The power operation parameters of the low-voltage power grid need to be obtained through sampling circuits and calculated using DSP. After obtaining the relevant values, they need to be compared with the pre-set values. The capacitance value of the capacitor bank for input or output compensation is determined through our comparison. When selecting capacitors, cyclic switching is a practical and reasonable way to perform different operations based on time. For example, the first capacitor to be put in needs to be withdrawn first, while the second capacitor needs to wait for the previous capacitor to withdraw before withdrawing. This withdrawal method can effectively reduce the temperature of capacitors during the switching process, further improving the service life of capacitors. The cyclic switching method makes the switching process more accurate. In the past, the switching methods we chose were based on a single element, which can easily lead to repeated switching oscillations.

4. Analysis of factors affecting cutting accuracy

The factors that affect accuracy mainly exist in three aspects, namely the delay time of zero crossing square waves, the response time of relay actions, and the half cycle of alternating current. Although these three factors do not play a very important role, they will all produce a certain degree of error, especially some errors that are difficult to compensate for. For certain errors, technical operational issues often have a certain impact on the accuracy of cutting. For the accuracy of zero crossing signals, we have already determined the relevant instruments to be used when formulating the relevant circuit scheme, and the selection of instruments will have a very serious impact on the accuracy. Regardless of the waveform output, the occurrence of delay errors is often unavoidable. So, when designing the circuit for zero crossing detection, it is necessary to fully consider the influence of repeated errors, and try to reduce repeated errors as the main design principle to further improve the stability of the wavelength. In addition, do not use ordinary diodes or optocouplers for zero crossing detection circuits, as these devices often generate significant errors.

5. Introduction to Ankerui capacitor compensation device

The AZC/AZCL series intelligent power capacitor compensation device is a new generation of reactive power compensation equipment used in 0.4kV, 50Hz low-voltage distribution to save energy, reduce line loss, improve power factor and power quality. It consists of an intelligent measurement and control unit, a thyristor composite switch circuit, a line protection unit, and two low-voltage power capacitors that are jointly or separately compensated. It can replace the conventional automatic reactive power compensation device composed of loose parts such as fuses, composite switches or mechanical contactors, thermal relays, low-voltage power capacitors, indicator lights, etc. connected by wires inside and on the cabinet surface. The traditional reactive power compensation device has changed its bulky and cumbersome structural mode, with better compensation effect, smaller size, lower power consumption, cheaper price, more cost savings, more flexible use, more convenient maintenance, longer service life, and higher reliability. It meets the higher requirements of modern power grids for reactive power compensation.

(1) The AZC series intelligent capacitors use thyristor composite switches for switching, with optimal switching points to achieve arc free switching; Complete protection functions integrated into one module, easy to install.

AZC series intelligent capacitor selection:

(2) AZCL is a series connection of appropriate reactance rates based on AZC (7% suitable for harmonic environments above 5th/7th order, 14% suitable for harmonic environments above 3rd/5th/7th order), which can effectively solve harmonics, avoid resonant amplification of harmonics, and protect the life of the capacitor cabinet itself.

AZCL series intelligent capacitor selection:

The above two types of intelligent capacitors use LCD displays, which can display real-time electrical parameters such as three-phase bus voltage, three-phase bus current, three-phase power factor, frequency, number of capacitor circuits and switching status, active power, reactive power, total harmonic voltage distortion rate, capacitor temperature, etc. By using an internal thyristor composite switch circuit, the optimal input (cut-off) point can be automatically found to achieve arc free switching; Ensure zero crossing switching, no inrush current, no sintering of contacts, minimal energy consumption, and no harmonics; At the same time, it has anti-interference, lightning protection, power phase loss, and no-load tripping protection functions, especially suitable for switching capacitors during reactive power compensation, without the need to install a heat sink.

6. Conclusion

The zero crossing switch of low-voltage power capacitors has many advantages, and it also includes the comprehensive advantages of mechanical switches and electronic switches. Therefore, in the actual operation process, its detection results are often more accurate. At the same time, it does not require too much energy consumption and there is no impact of surge current. In addition, it has the characteristics of low cost, low energy loss, and flexible control methods. In addition, the service life of its capacitors has been further improved. In the process of switching capacitors, the compensation method of co distribution and compensation optimization combination has strong flexibility and solves many difficult problems.