37pw56gy
Electrical
- Jul 17, 2002
- 14
Steady-state DC currents superimposed on AC power systems produce waveform distortion and 4th harmonics. This occurs as transformer and motor lamina becomes saturated at the crest of each alternate half-cycle. Multi-grounded neutral networks are subject to stray DC where electric railways, cathodic protection systems, DC systems in mines or mills, geomagnetic currents, etc., create differences in earth potential between grounding points.
Steady-state DC currents also may occur where the load is non-linear and asymmetrically. Half-wave rectifiers are one example; a full-wave bridge with one open diode is a practical example of an unintended asymetrical load. Some Halogen-cycle lamps (Sylvania Capsylite, Phillips Masterline) have a hidden diode in their base. Defective phase-controlled devices such as motor drives is another source of load asymmetry.
Faults producing asymmetrical AC waveforms and DC transients are another subject for a separate thread.
Any comments or thoughts on the incidence and severity of the DC problem? What can be done to mitigate stray DC current flow without compromising the effectiveness of grounding? To what extent does auto-cancellation of harmonics in the delta side of a transformer give rise to undesired DC currents elsewhere in the power distribution system?
Has anybody ever seen or heard of a scheme where a power distribution transformer is equipped with a DC bias winding? A controlled, low-level of DC current in this winding serves to cancel flux produced by stray DC in the transformer’s primary or secondary winding. How are large transmission transformers protected against geomagnetic currents?
Steady-state DC currents also may occur where the load is non-linear and asymmetrically. Half-wave rectifiers are one example; a full-wave bridge with one open diode is a practical example of an unintended asymetrical load. Some Halogen-cycle lamps (Sylvania Capsylite, Phillips Masterline) have a hidden diode in their base. Defective phase-controlled devices such as motor drives is another source of load asymmetry.
Faults producing asymmetrical AC waveforms and DC transients are another subject for a separate thread.
Any comments or thoughts on the incidence and severity of the DC problem? What can be done to mitigate stray DC current flow without compromising the effectiveness of grounding? To what extent does auto-cancellation of harmonics in the delta side of a transformer give rise to undesired DC currents elsewhere in the power distribution system?
Has anybody ever seen or heard of a scheme where a power distribution transformer is equipped with a DC bias winding? A controlled, low-level of DC current in this winding serves to cancel flux produced by stray DC in the transformer’s primary or secondary winding. How are large transmission transformers protected against geomagnetic currents?
