Voltage Limits and Currents on Power Systems Essay example

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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…show more content…
For reverse voltage blocking a diode is used in series with the self-commutating device (IGBT) . However, GTO-based configurations have restricted frequency of switching they do not need the series diode. They are considered sufficiently reliable, but have higher losses and require higher values of parallel ac power capacitors. Moreover, they cannot be used in multilevel or multistep versions to improve performance in higher ratings. The other converter used is a voltage-fed type Active filter structure, as shown in Fig 2.2. It is having a self-supporting dc voltage bus with a large dc capacitor. It has become very much dominant, since it is lighter, cheaper, and expandable to multilevel and multistep modes, with lower switching frequencies to improve the performance . It is more popular in UPS-based applications, because in the presence of mains, the same Inverter bridge can be used as an Active Filter to eliminate harmonics of critical nonlinear loads. 2.2.2 Topology based Classification Based on the topology used the AF’s can be classified as series or shunt filters, and use a combination of both called unified power quality conditioners. Combinations of active series and passive shunt filtering are known as hybrid filters. Fig 2.3 is an example of an shunt -type AF, which is most widely used to eliminate current harmonics, reactive power compensation (also known as STATCON), and balancing unbalanced currents. It is mainly used at the load end, because
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