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POWER QUALITY phần 3

Vmax và Vmin đại diện cho sự thay đổi trong điện áp trên Vnom điện áp danh định. Ví dụ, nếu điện áp trong một mạch đánh giá ở 120 V danh nghĩa thay đổi từ 122 đến 115 V, nhấp nháy được cho bởi: fv = 100 × (122-115) / 120 = 5,83% Trong giai đoạn đầu của sự phát triển của điện AC | FIGURE 2.7 Voltage swell due to step load rejection. The nominal 480-V generator bus experienced a rise to 541 V that lasted for approximately 18 cycles. where Vmax and Vmin represent the change in voltage over the nominal voltage Vnom. For example if the voltage in a circuit rated at 120 V nominal changed from 122 to 115 V the flicker is given by fv 100 X 122 - 115 120 5.83 In the early stages of development of AC power light flicker was a serious problem. Power generation and distribution systems were not stiff enough to absorb large fluctuating currents. Manufacturing facilities used a large number of pumps and compressors of reciprocating design. Due to their pulsating power requirements light flicker was a frequent problem. The use of centrifugal- or impeller-type pumps and compressors reduced the flicker problem considerably. The flicker problems were not for the most part eliminated until large generating stations came into service. Light flicker due to arc furnaces requires extra mention. Arc furnaces commonly found in many industrial towns typically use scrap metal as the starting point. An arc is struck in the metal by applying voltage to the batch from a specially constructed furnace transformer. The heat due to the arc melts the scrap metal which is drawn out from the furnace to produce raw material for a variety of industrial facilities. Arc furnaces impose large electrical power requirements on the electrical system. 2002 by CRC Press LLC FIGURE 2.8 Voltage changes during elevator operation in a residential multiunit complex. The rate of voltage change causes perceptible light flicker. The current drawn from the source tends to be highly cyclic as arcs are repeatedly struck and stabilized in different parts of the batch. The voltage at the supply lines to an arc furnace might appear as shown in Figure 2.9. The envelope of the change in voltage represents the flicker content of the voltage. The rate at which the voltage changes is the flicker frequency