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We often see low-voltage capacitor banks, compensation cabinets, or high-voltage capacitors in power distribution rooms, and I believe everyone is familiar with them. When the capacitor bank is installed on the high-voltage cabinet side, it is called high-voltage centralized compensation ； Those installed on the low-voltage side are called low-voltage on-site compensation, and low-voltage on-site compensation is the most common type. So what is the function of that capacitor compensation cabinet? What is the principle of a capacitive compensation cabinet? Please keep reading. Capacitor compensation cabinet: yunrun.com.cn/tech/3177.html 1. Starting with the schematic diagram of the capacitor cabinet: http://yunrun.com.cn/upload/202006/29/202006290317190910.png Schematic diagram of low-voltage capacitor cabinets; the type of low-voltage capacitor cabinet is GGD ； The internal components of a low-voltage capacitor bank include fuse-type knife switches, current transformers, circuit breakers, AC contactors, thermal relays, compensation capacitors, and low-voltage surge arresters. The panel of the capacitor compensation cabinet is equipped with three-phase ammeters, voltmeters, voltage conversion switches, and an automatic low-voltage reactive power compensation controller; if the controller has a function to display the power factor, then a separate power factor meter is not necessary ； Conversely, a power factor meter needs to be installed). High-voltage capacitors are similar in nature, but all of their internal components are replaced with those suitable for high voltages; as a result, they appear much larger than low-voltage capacitor cabinets. http://yunrun.com.cn/upload/202006/29/202006290350156438.png Automatic compensator for low-voltage reactive power. yunrun.com.cn/product/2977.html Nowadays, many capacitor banks do not have thermal relays; instead, a small reactor is used in place of the thermal relay, and it serves to suppress sudden changes in line current as well as harmonics, thereby protecting the compensation capacitors at the end of the system. 2. Selection and configuration of the main components in the capacitor compensation cabinet: The following content focuses on the components inside the capacitor compensation cabinet, while no discussion is given regarding the components located on the cabinet’s panel. ①Main switch selection: Changhui Instruments has gained this experience through extensive practical experience in various projects: it is advisable to use a fuse-type knife switch with protective functions as the main switch for capacitor compensation cabinets, as this helps to ensure the safety of the system. When selecting a fuse-type knife switch, it should be noted that a large current flows when the capacitor is switched on or off; therefore, the fuse must be able to withstand the inrush current of the capacitor. ②Capacitor selection: ◆ On the workshop busbar sections, capacitor compensation cabinets with a capacity of 120 kvar or 160 kvar are used for on-site compensation based on the load conditions, while in the low-voltage distribution rooms, capacitors with a capacity of 250 kvar or 300 kvar are employed for centralized compensation. This approach not only meets the compensation requirements but also saves energy; it is easy to locate the capacitors and related spare parts in case of failures, and standard GGD cabinets can be used. ◆The capacity of a single capacitor should not be too large nor too small; depending on the load conditions and installation location, it is recommended that the capacity of each capacitor range from 16 to 30 kvar. ◆Selection of capacitor rated voltage: If the using entity does not have the budget for high-end capacitors, and it is necessary to ensure that the capacitors can continue to function properly under overvoltage conditions, Changhui Instruments recommends choosing a capacitor rated voltage of 0.415 kV on the low-voltage side of the transformer, with the overvoltage protection setting set at 0.42 kV. Capacitors with a rated voltage of 0.4 kV can be used at the end of the circuit; this provides a sufficient margin in terms of rated voltage, which helps to reduce the likelihood of capacitor failures. ③Parameter selection for capacitor switching contactors: Issues such as welded contacts and burned coils in the capacitor compensation cabinets are mainly related to improper configuration of the contactors and to the fact that their actual parameter values are lower than those specified in the manual. Therefore, it is recommended that the corresponding contact selection be upgraded by one level. ④Selection of wiring for individual capacitors: The operating temperature of capacitors can reach up to 50°C; Changhui Instruments recommends choosing a wire current-carrying capacity appropriate for 50°C! The calculated and corrected current-carrying capacity of standard BVR wires at 50°C is shown in the table below: http://yunrun.com.cn/upload/202006/29/202006290331196769.png. The standards for selecting wires for capacitors in compensation cabinets, based on these calculations (with the minimum current-carrying capacity of the wires being 1.5 times the rated current of the capacitors as specified in GB50227-2017; the BZMJ type of capacitor is used as an example), are also presented in the table below; this table can serve as a reference standard for units to inspect the wires used in capacitor compensation cabinets: http://yunrun.com.cn/upload/202006/29/202006290346597864.png. ⑤ Selection of protective devices for individual capacitors: Currently, some manufacturers use miniature circuit breakers instead of fuses as protective devices for capacitors. However, according to numerous field reports, the performance of these miniature circuit breakers is not satisfactory Changhui Instruments recommends that, for capacitor protection devices in low-voltage rooms and workshop circuits, fuses or heating relays be preferred as protection mechanisms. ⑥Regarding the selection of voltage for the secondary circuits in compensation cabinets, Changhui Instruments recommends that AC380V be used uniformly for such circuits, rather than AC220V. The reasons behind this recommendation are not discussed in detail due to space constraints in this article. In summary, for the convenience of maintenance, it is recommended that there not be too many different types of compensation cabinets; instead, 120kvar, 160kvar, 250kvar, and 300kvar capacities should be used consistently. Individual capacitors should have capacities of 16kvar, 20kvar, 25kvar, or 30kvar. Their rated voltage, as well as the associated contactors, wires, and protective devices, can be selected in accordance with the requirements mentioned earlier. For capacitive compensation cabinets, the GGD type should be used; they must have all necessary functions. Where possible, compensation cabinets equipped with contactless switches for switching capacitors can be utilized. Common low-voltage capacitors 3. What is reactive power, and why is reactive power compensation necessary? Reactive power is the power that is only converted from one electrical form to another, rather than being consumed. As we all know, electrical devices have three properties: resistive, capacitive, and inductive. Resistance is easy to understand; it refers to a component in the form of a resistor. When current passes through such a resistor, it is dissipated as heat. For example, in a hair dryer, the resistive wire is used to heat the air that is blown out, thereby converting electrical energy into thermal energy. Capacitive and inductive are different. Capacitance, such as in capacitors, merely stores electrical energy, which can be released when needed ； The same is true for inductors: they convert electrical energy into a magnetic field for storage, and that energy can also be released. So, in a sense, they do no work, which is why they are called \"reactive power\". However, reactive power is not useless; it is actually the foundation of industry. Without reactive power, and with only the active power generated by resistive components, it would be impossible to achieve the industrial prosperity we see today. Most commonly, electric motors need to generate and maintain a rotating magnetic field in order to make the rotor rotate, thereby driving mechanical motion; the rotor magnetic field of an electric motor is created by obtaining reactive power from the power supply. Transformers also require reactive power in order to generate a magnetic field in their primary coil, which in turn induces a voltage in the secondary coil. Therefore, without reactive power, the motor will not rotate, the transformer cannot step up or step down voltage, and the AC contactor will not close. Therefore, under normal conditions, electrical equipment not only consumes active power but also needs to draw reactive power from the power source. If electrical devices do not have sufficient reactive power to establish a normal electromagnetic field, they cannot operate under their rated conditions; the terminal voltage of these devices will drop, thereby affecting their proper functioning. If electrical equipment does not have sufficient reactive power, requesting reactive power from the power grid will increase the losses in the grid and raise the voltage drop along the lines, thereby affecting the proper operation of the entire power grid. Therefore, basically, capacitor compensation cabinets are available in power distribution rooms for on-site compensation ； Moreover, power supply companies generally require the power factor for capacitive compensation to be above 0.9. Of course, power supply companies also do not allow excessive reactive power to be fed into the grid, as this will cause the voltage in the lines to rise. Therefore, capacitive compensation cabinets usually come equipped with a switching switch; by setting it to the “automatic” mode, it is possible to incorporate the required amount of capacitance based on actual conditions. 4. What is an appropriate value for capacitive compensation? Taking a 500 kVA transformer as an example, generally, the reactive power compensation capacity for distribution transformers is around 20%-40% of the transformer’s capacity; designers usually adopt 30% as a standard value, which means that 150 Kvar is chosen as the maximum compensation capacity, or in other words, the installation capacity. Below is the reactive power compensation scheme for a 500kVA transformer: Reactive power compensation capacity: 150kvar, 3×40kvar (20kvar + 20kvar) ； 1×30kvar (20kvar 10kvar); Number of smart capacitors: 3 units, SWL-8MZS/450-20.20 ； For 1 unit of SWL-8MZS/450-20.10, if equipment such as motors and frequency converters are present, the algorithm for reactive power compensation differs slightly from that used for transformers. The required reactive power compensation capacity can also be calculated by comparing the power factors before and after compensation. For example: ① A factory has an active power of 600 kW, a reactive power of 800 kvar, and a apparent power of 1000 kVA; accordingly, the power factor is 600/1000 = 0.6. ②Consult the table of reactive power compensation capacity calculation coefficients, and determine the corresponding coefficient F based on the condition before and after compensation. If it was 0.6 before compensation and 0.95 is desired after compensation, the coefficient F can be found to be 1.005. ③Then, using the calculation formula: active power * coefficient F, this value equals the total amount of reactive power that needs to be compensated. 600×1.005=605kvar. ④Depending on the actual situation, an appropriate amount of margin can be increased to prevent insufficient reactive power after additional equipment is added later.