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Voltage Limits And Currents On Power Systems

2596 words - 11 pages

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 351000 15.0 7.0 6.0 2.5 1.4 20.0

2.2 Classifications of Active Power Filters
2.2.1 Converter based classification
Current Source Inverter (CSI) (Fig 2.1) and Voltage Source Inverter (VSI) (Fig 2.2) Active Power Filters are two classifications . To meet the harmonics current requirement of the nonlinear loads the Current Source Inverter behaves as a non-sinusoidal current source . 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...

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