Reactive power compensation, harmonic control scheme and selection of components (2)

Abstract: This paper briefly describes the solutions of reactive power compensation and harmonic suppression in low-voltage distribution networks and the selection of components needed in complete sets of equipment.
Keywords: filter reactor filter complete set harmonic control harmonic suppressor
2.5.2 Does the Compensation Device Need to Have Harmonic Management Function (Suppressed or Absorbed)?
With the installation and use of various new types of electronic devices/devices (mainly non-linear loads), a large amount of harmonics are generated at the same time when a large amount of reactive power is consumed, causing great “pollution” to the utility grid. Due to the generation and existence of harmonics, it has brought great harm to the safe operation of the power grid and the safety of the power equipment. The power accidents and economic losses caused by harmonics are obvious to all. "Harmonic" - the pollution of this power grid, has caused more and more attention of the relevant departments.
(1) According to the requirements of GB/T14549-1993 "Power Quality Harmonics in Public Power Networks", it is necessary to limit the harmonic voltage and harmonic current injected into the power grid by various nonlinear loads. When the load generates harmonics or there is harmonic in the power supply system and exceeds the standard, a filter reactive power compensation device needs to be set. On the contrary, it is not needed.
(2) In some occasions, although the harmonics do not exceed the standards, but have affected the normal operation of the system, it is also necessary to set up filter reactive power compensation devices.
(3) In some special occasions, such as hospitals, hotels, office buildings, smart communities, civil aviation, military, scientific research, precision measurement, mines, terminals, metallurgy, etc., filter reactive power compensation devices should be installed, and even special filters need to be configured. Device.
(4) In the same power supply system (same transformer), when the non-linear load (rectification/frequency conversion, medium/high frequency equipment, and generally harmonic source) accounts for more than 25% of the total capacity, it should be configured. Filter reactive power compensation device.
The author believes that with the continuous development of science and technology, the degree of industrialization in our country has gradually increased, and traditional reactive power compensation devices (such a scheme is composed of parallel capacitor banks) will not be able to be used normally in more and more occasions. It will gradually be replaced by a compensation device with a filtering function and a new type of device that is emerging.
2.5.3 What kind of control type should be used for the compensation device (1) When the power supply with reactive power compensation is needed, the power supply quality is better; The reactive power of the load does not change much (load is relatively stable); There is no harmonic or harmonic content in the system Less; Consider relatively fixed control methods (fixed grouping, cyclic switching) - static reactive power compensation devices.
(2) For cases where the reactive power of the load changes rapidly (fast) and frequently; there is no harmonic or less harmonic content in the system; dynamic control is required (non-fixed grouping, fast code switching) - dynamic reactive power Compensation device.
(3) For a single high-power balanced load or a relatively concentrated small device, static in-situ compensation devices may be considered.
2.5.4 What kind of compensation method is used? (1) When three-phase voltage/current is basically balanced, compensating methods of total compensation or partial compensation are used.
(2) When the three-phase voltage/current is unbalanced, adopt a combination of compensation and compensation.
(3) When the three-phase voltage/current is seriously unbalanced, three-phase unbalanced compensation is used.
3 After the compensation plan for the components required by the compensation device is determined,
It is necessary to analyze and select the components needed for the production of the device in terms of function, quality and price. This work is critical to the production of a set of devices with excellent performance and safe and reliable operation. It is a link that cannot be ignored. Because the number of manufacturers of similar components is not the same, the function is not the same, the price is also very different, and the difference between product quality or life is even greater. Therefore, when faced with various products of the same kind, the relevant personnel do not know how to select it. Since the information obtained is incomplete, even more attention is paid only to the price factor of the product, ignoring the quality level, performance indicators, and product features. Due to intrinsic factors, it is often impossible to select the most suitable component, and even mistakenly select components that do not meet the requirements of the functional requirements. This has buried potential problems for the safe operation of the device.
3.1 Selection of Controller The controller is a very important device in the reactive power compensation device. It is the command system of the complete device. Its quality, merits, functions, anti-jamming capability and other indicators determine the quality of the entire device. In unattended operating conditions (for example, box-type substations, etc.), it is particularly important. Care must be taken when selecting models. It must be more careful. More careful.
At present, there are many types of controllers on the market, and the models are not uniform. The difference in functions and prices is very large. To select a suitable one among the controllers of many brands, it is necessary to combine the specific conditions of the devices from the controllers. Considering various functions and controlling physical quantities, we must also pay attention to price factors. The model number and the control physical quantity of the controller have been stipulated in the related industry standard DL/T597-1996. Most companies' products are named according to this standard, and some companies' products are named according to their own enterprise standards. Regardless of how the product model is named, the key is to select its basic function and configuration scheme of the specific device. Otherwise, it is not a function excess (purchasing cost is increased) that is, the function is not enough to affect the use effect.
With regard to the selection of the type of controller, there is a related explanation in the related standard DL/T842-2003. The following methods can be used to automatically control the physical quantity: (1) Reactive power control, power factor limitation; (2) Reactive current control, power factor limitation; (3) Reactive power or reactive current control, voltage limitation.
In addition, after investigating the use of controllers in a compensation device that has been put into operation, it has been found that almost all low-cost controllers use the power factor as a control physical quantity. In a device using this type of controller, the proportion of normal operation is small, the failure rate is high, and some of them have been abandoned and used instead of manual control. This is because such controllers have poor anti-jamming capability. At the same time, due to the use of power factor as the control physical quantity, oscillations and switching are prone to occur, resulting in damage to contactors, capacitors, and other components. Please draw attention to this person.
3.2 Selection of Switching Devices For reactive power compensation devices, what kind of capacitor is selected to switch the actuator is critical to the safe operation of the entire device. There are many kinds of actuators used for capacitor switching. The main ones are:
(1) Ordinary contactor This kind of product is an early solution and has basically been eliminated.
(2) Capacitor-specific contactors This type of product is based on the general contactor to increase the current limiting resistor or current limiting coil program to limit the closing surge current. This product is the leading product on the market due to its easy installation wiring, low operating costs and low price. However, such products still generate over-voltage during turn-on and turn-off and shutdown. Therefore, they are only applicable to occasions where the reactive power of the load does not change much and are infrequent and the system operation is relatively stable. In addition, there are many manufacturers of such products with complicated names and models, and the quality and price are quite different. Please pay attention when selecting.
(3) Non-contact electronic switch This product consists of a thyristor (thyristor), zero-crossing trigger, absorption protection, and other circuits. With the popularization and application of dynamic reactive power compensation devices and the continuous improvement of manufacturing technology, the price of thyristor modules has also fallen drastically, making the application of such switches increasingly gradual. However, the assembly process of such products is complicated, the volume is large, the power consumption is high, and the heat generation is serious (special radiators and exhaust fans need to be configured), and the price is high. In addition, harmonics will be generated during operation, causing some interference and impact on the system.