Electric power conversion systems – Phase conversion without intermediate conversion to d.c.
Reexamination Certificate
2000-03-23
2001-02-20
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Phase conversion without intermediate conversion to d.c.
Reexamination Certificate
active
06191968
ABSTRACT:
BACKGROUND OF THE INVENTION
To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, several transformer and converter topologies are available. All require installation of extra equipment and add to the total cost. Preferred methods are those which perform well in practical power systems which incorporate voltage unbalance and preexisting harmonic voltages. Examples of these methods are given by Rosa, Meir, and McMurray. Also powerful techniques in which the 3-phase power is first converted to 9-phase power before processing have been described in the literature of U.S. Pat. Nos. 5,124,904, 5,455,759, and 5,619,407. Equipments using these three patents are in use. However, continual improvements are sought to reduce cost and increase the power rating capability. This invention describes another 3-phase to 9-phase transformer method which reduces cost, and increases feasible power capability of a low voltage auto-connected transformer.
BRIEF SUMMARY OF THE INVENTION
A 3-phase to 9-phase auto-transformer with the main windings configured in a wye manner is arranged to provide nine output voltages which are of generally equal amplitude with a phase displacement of generally 40°. Such voltages applied to a 9-phase bridge rectifier arrangement result in low harmonic currents being drawn from the 3-phase AC power source. Thus the rectifier equipment is much better able to conform to specifications such as IEEE 519 which restrict the amount of harmonic current which may be injected into an electrical power system. In its preferred form for 18-pulse operation a single transformer with 7 coils per phase provides nine output voltages of an amplitude such that the converter DC output voltage is generally the same as would be obtained with a 3-phase rectifier bridge connected directly to the 3-phase power source. By this means lower harmonic currents are feasible without affecting the basic design of existing conversion equipments. However, if required, higher or lower voltages can be achieved with suitable tappings on the transformer windings in a manner well known to those familiar with transformer design. The preferred embodiment for 18-pulse operation is shown in FIG.
3
. It includes a basic wye structure auto-transformer such as that taught in U.S. Pat. No. 5,455,759, but includes additional coils connected between terminals 2 and 9 to P1, terminals 3 and 5 to P2, and terminals 6 and 8 to P3. These additional paths reduce the high current pulses which normally flow in the output coils, such as N4C and N5B in phase A, N4A and N5C in phase B, and N4B and N5A, in phase C. They also provide a path for third harmonic currents such that a tertiary winding is not required. Further they allow some flexibility to select coils in a manner favorable to balancing leakage inductance and commutation effects.
REFERENCES:
patent: 4779181 (1988-10-01), Traver et al.
patent: 5053689 (1991-10-01), Woodson et al.
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