Voltage Limits and Currents on Power Systems

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The various nonlinear loads like Adjustable Speed Drives (ASD’s), bulk rectifiers, furnaces, computer supplies, etc. draw non sinusoidal currents containing harmonics from the supply which in turn causes voltage harmonics. Current harmonic causes increased power system losses, excessive heating in rotating machinery, interference with nearby communication circuits and control circuits, etc. It has become a vital importance to maintain the sinusoidal nature of voltage and currents in the power system. Various international agencies like IEEE and IEC have issued standards, which put limits on various current and voltage harmonics. The limits for various current and voltage harmonics specified by IEEE-519 for various frequencies are given in Table 2.1 and Table 2.2. Table 2.1 IEEE 519 Voltage Limits Bus Voltage Minimum Individual Harmonic Components (%) Maximum THD (%) 69 kV and below 3 5 115 kV to 161 kV 1.5 2.5 Above 161 kV 1 1.5 The objectives and functions of active power filters have been expanded from reactive power compensation, voltage regulation, etc. to harmonic isolation between utilities and consumers, and harmonic damping throughout the distribution as harmonics propagate through the system. Active power filters are either installed at the individual consumer premises or at substation and/or on distribution feeders. Depending on the compensation objectives, various types of active power filter topologies have evolved, a proper briefing of which is provided in [3-4]. Table 2.2 IEEE 519 Current Limits SCR=Isc/Il h<11 11 to 17 17 to 23 23 to 35 35 <20 4.0 2.0 1.5 0.6 0.3 5.0 20 – 50 7.0 3.5 2.5 1.0 0.5 8.0 50 -100 10.0 4.5 4.0 1.5 0.7 12.0 100 – 1000 12.0 5.5 5.0 2.0 1.0 15.0 >1000 15.0 7.0 6.0 2.5 1.4 20.0... ... middle of paper ... ... IP is captured and "stored" until the next sample of IP is obtained to replace the old one. This action is executed with the help of "Sample and Hold" circuits, which are synchronized with the synchronization pulses to trigger the S&H are generated through the "zero-crossing" signals, obtained from the set of "in-quadrature voltages". These "in-quadrature voltages" are generated in the control block with the DZ0 connection, signal transformer. The control circuit is also has the capability to avoid flickers and transient phenomena in the source, produced by sudden changes in load current. To do this, control system makes soft variation of IP during these moments. However, this action will require to have the energy storage components in the APF. Hence the design of the control system has to take in account the characteristics of the power filter.

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