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IEEE C57.105-2019 pdf free download
IEEE C57.105-2019 pdf free download.IEEE Guide for Application of Transformer Connections in Three-Phase Electrical Systems.
6.3 Harmonic current flow in a three-phase system
In ungrounded or uni-grounded three-phase supply systems. the flow of the third-harmonic current and its multiples on the primary feeder is suppressed. In symmetrical systems. the muLtiple third-harmonic currents of’ all three phases have a common time phase. Consequently, the sum for all three phases cannot be zero unless each current itself is zero. Since no low-impedance path to neutral is provided by the systems without a neutral conductor, the sum must be zero. Hence the third-harmonic currents are suppressed.
With delta-connected primaries or secondaries, the third harmonic of exciting current circulates in the low- impedance path of the closed delta. Groups of single-phase transformers that are reasonably vell balanced also present a low-impedance path for local circulation of third-harmonic current. However, when three-phase transformers or banks of single-phase transformers arc connected Y-ungrounded-Y-ungrounded. no path exists for the third-harmonic excitation current. A third-harmonic voltage then appears across all three-phase windings in common time phase and results in a displacement of the transformer neutral voltage. The neutral of the transformer connection thus exhibits the triplen third-harmonic voltages that with normal line voltage may be 40% third, 3O% ninth, 20°/o fifteenth. etc.
When the supply is from a source with the neutral conductor, a path does exist for all harmonics regardless of transformer connection except for the Y-ungrounded-Y-ungrounded connection described in the previous paragraph. The iriplen harmonic currents flow through the line conductors and return on the neutral system. In the case of Y-grounded-Y-grounded transforniers, the harmonic current may flow on both primary and secondary system. Other harmonics balance out on the line conductors so that only a residual harmonic current resulting from system asymmetry (e.g. due to un-transposed transmission lines of load unbalance) appears on the neutral.
It is the harmonic currents on the neutral system that cause most problems of interference with communication circuits (see 6.4).
6.4 Power line interference with communication circuits
Harmonic currents flowing on power circuits may cause objectionable noise levels in open-wire telephone, and to a slightly lesser extent in shielded multi-pair telephone cable, circuits paralleling the line route.IEEE C57.105-2019 pdf free download.
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