Discussion on low voltage reactive power compensat

Brief discussion on low voltage reactive power compensation

Abstract: brief discussion on low voltage reactive power compensation: 1. Basic principle of reactive power compensation; 2. Economic compensation for reactive power; 3. Reactive power compensation mode; 4 installation of reactive power compensation device

key words: with low-voltage reactive power compensation 1. The basic principle of reactive power compensation, whether industrial load or civil load, most of them are inductive. All inductive loads need to compensate a large amount of reactive power. There are two ways to provide these reactive power: one is provided by the transmission system; Second, compensation capacitors are provided. If it is provided by the transmission system, both active power and reactive power should be considered when designing the transmission system. The transmission of reactive power by the transmission system will increase the losses of transmission lines and transformers and reduce the economic benefits of the system. By providing reactive power locally by the compensation capacitor, the transmission of reactive power by the transmission system can be avoided, so as to reduce reactive power loss and improve the transmission power of the system. Figure 1 Schematic diagram of reactive power compensation S1 is the apparent power before the power factor is improved S2 is the apparent power after the power factor is improved economic compensation of reactive power 2 for the power system, it can be carried out on the high-voltage side or low-voltage side, but it is used for compensation on the tensile testing machine. However, if compensation is carried out on the low-voltage side, it can not only reduce the losses of transformers and transmission lines, but also improve the utilization rate of transformers and transmission lines and the terminal voltage at the load end. Therefore, the closer the compensation capacitor is installed to the load end, the greater economic benefits can be obtained for users. It can be seen from Figure 1 that after installing the compensation capacitor, the power factor on the load side is improved. The reactive power required by the electrical load is directly provided by the capacitor, which can reduce the total current of electricity. In the formula, i-- apparent current IP -- active current IC -- capacitor current, because J is installed on the low voltage side) the test report: (standard program) the capacitor compensates the reactive current, that is, the reactive current is provided by the capacitor, so in the electrical design, Only the active current is considered, which greatly saves the investment of transformer and transmission line. For the existing electricity, it can also improve the output of electricity. 2.1 reduce the losses of transmission lines and transformers pn=3i2 · r=3i2p · r+3i2q · r where PN - active power loss R - resistance of each transmission line (including transmission lines and transformers) transmission line resistance r=kl/a where k - resistance coefficient a - conductor cross-sectional area L - conductor length, m transformer resistance r=yku2/sn where YK - transformer short-circuit impedance, Ω U - system voltage, V Sn - transformer rated capacity, KVA 2.2 the utilization rate increased by increasing the utilization rate of transformers and transmission lines is: (p2-p1)/p1=[(cos1-cos2) -1] × 100% where cos φ 1 -- power factor cos before improvement φ 2 -- improved power factor 2.3 increase the terminal voltage of the system and reduce the voltage drop of the system du (%) =qc/sn × XK (%), where du (%) -- voltage increase percentage qc-- capacity of compensation capacitor, kvar sn-- transformer capacity, KVA XK (%) -- transformer impedance percentage 3 reactive power compensation mode theoretically, the best way of reactive power compensation is to compensate where reactive power is needed, and the whole system will have no reactive current flow. However, it is impossible to achieve this in the actual electric power plant by adopting advanced continuous production processes of hammering, heat treatment and profiled rolling. Because no matter transformer, transmission line or various loads, reactive power will be required. Therefore, in terms of the installation position of the compensation device, there are several compensation methods in the actual electricity: ① centralized compensation in the substation; ② Decentralized compensation of distribution lines; ③ Centralized compensation at load side; ④ Local compensation of user load. For the reactive power compensation of low-voltage distribution, the centralized compensation mode at the load side is usually adopted, that is, the automatic power factor adjustment device is used in the low-voltage system (such as the low-voltage side of the transformer) to automatically input or remove part or all of the capacity of the capacitor as the load changes. 3.1 the power factor of distribution transformer is estimated to be about 0.75, and the power factor is increased to 0.90 under full load. Assuming that the distribution transformer capacity is s, the active power, reactive power and power factor angle before compensation are P1, Q1, and respectively φ 1. The active power, reactive power and power factor angle after compensation are P2, Q2 and φ 2. QB is the capacity to be compensated. It can be concluded that the capacity to be compensated is: qb=q1-q2=s × sin φ 1-S × sin φ 2=S × (0..436) = 0.225s compensation percentage is: η%= Qb/S × 100% =22.5% according to the operation experience of electricity, the compensation capacity is generally 20% - 30% of the rated capacity of the transformer. 3.2 selection of compensation methods compensation methods are divided into three-phase common compensation, split phase compensation and hybrid compensation (i.e. common compensation plus split compensation). Generally speaking, when the capacity to be compensated exceeds 60KVAR, hybrid compensation is more appropriate, which can take into account the imbalance between the three phases, have the same effect as split phase compensation, and reduce the cost. 3.3 selection of compensation stages the more compensation stages (i.e. the number of groups of compensation capacitors), the higher the accuracy of compensation. However, with the increase of compensation stages, the cost of the device will increase significantly, and the volume of the box will also increase. Considering the compensation accuracy, cost, box volume and other factors, we recommend 11 levels of extraordinary capacity compensation. The first 9 levels are equal capacity to meet the basic compensation, and the last 2 levels are small capacity to improve the compensation accuracy. Take a 180kvar compensation device as an example: ① the first nine stages are 18kvar per stage, 9 × 18=162kvar； ② The latter two levels are 9kvar users at each level, and they have distrusted domestic brands; nine × 2=18kvar, 180kvar in total. 3.4 selection of switching control mode in order to minimize the volume of the device, simplify the structure and improve the reliability of the device, that is, the capacitor is divided according to a certain capacity ratio