Electric power conversion systems – Current conversion – With interphase transformer
Utility Patent
1998-03-26
2001-01-02
Tso, Edward H. (Department: 2838)
Electric power conversion systems
Current conversion
With interphase transformer
C363S154000, C336S005000, C336S010000, C336S012000
Utility Patent
active
06169674
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to controlling harmonic distortion caused by devices connected to an alternating current power source. This invention includes a transformer, a system and a method with which to achieve this control.
One technique for controlling the speed of a three-phase induction motor uses an electronic variable frequency drive (VFD). The VFD has a rectifier circuit that requires multiple phases of alternating current electric power. For example, a six-pulse rectifier needs three phases of electric power to be input so that six pulses are provided by the full-wave rectification. Such multi-phase rectifiers have other uses as well (e.g., for providing direct current for a d.c. motor used in ski lifts or for a d.c. heater used in industrial processes).
Although multi-phase rectifiers are useful, they cause detrimental harmonic currents to flow in the power systems which energize them. For example, the current in a six-pulse VFD is heavily laden with fifth and seventh harmonics. Harmonic currents can cause system components such as transformers and generators to overheat. Harmonic currents also can cause voltage distortion. Voltage distortion can cause electronic devices to malfunction and capacitors to overheat. Multiple rectifiers energized from one power source intensify the harmonic problems because increasing the number of operating rectifiers usually increases the total drive load.
Primary system filters can be used to prevent or attenuate this harmonic distortion. Such filters are, however, designed and applied for a predetermined amount of total drive load, which load cannot always be known with certainty prior to an actual installation. Even when initially predicted, the load may be changed as rectifiers are added to or removed from the system; this can necessitate a change in the filter because the total drive load that can be connected to a filtered system is limited by the design of the filter and not by the capacity of the power system. Additionally, such filters typically are relatively large and expensive.
In view of the foregoing, there is the need for an improved technique for controlling harmonic distortion in a power system without having to use primary system filters.
SUMMARY OF THE INVENTION
The present invention overcomes the above-noted and other shortcomings of the prior art by providing a novel and improved transformer, system and method with which to achieve control of harmonic distortion in power circuits.
The present invention controls harmonic distortion by enabling different phase relationships to be set, and changed, in the field, between the devices being energized and the power source providing the energization. This has particular application, for example, in canceling harmonics caused by multiple six-pulse variable frequency drives used for controlling connected three-phase induction motors that operate electric submersible pumps. The present invention can also be used with twelve-pulse and twenty-four-pulse VFDS, but it can be used with power rectifiers in general and even non-rectifier loads.
The present invention allows the phase relationship primary to secondary of a transformer to be adjusted in the field. In a particular implementation, the phase relationship is adjustable to any one of four different phase angles on 15° increments.
With the present invention, the loads can be distributed approximately equally among the different phase angles during installation. Then later, the load distribution can be re-adjusted to compensate for changes in the number, location and size of the loads. This field adjustability allows the harmonic cancellation to be optimized.
Canceling harmonic currents with the present invention requires relatively little engineering when compared to the design and use of harmonic filters. With the present invention, the load is simply distributed on the available phase angles; and this process continues as additional drives are installed. Primary system filters, on the other hand, must be designed and applied for a predetermined amount of total rectifier load. Often the future load cannot be known with any degree of certainty.
With the present invention, future rectifier additions properly made will not affect the loading of existing units. With primary system harmonic filters, on the other hand, adding rectifier load increases the loading on the existing filters. This is true even if a separate power user adds load on the common power system at a separate location.
With the present invention, the total rectifier load is limited by the system capacity. This is distinguishable from a filtered system in which the total rectifier load that can be connected is limited by the design of the filter and not the capacity of the system.
The present invention can supply three-phase power to rectifier or non-rectifier applications. That is, a transformer of the present invention can be used instead of conventional transformers even if the future use (i.e., device to be energized via the transformer) is unknown. Conversely, some specialized transformers (e.g., “circuit thirty-one,” or delta-wye secondary transformers) used with twelve-pulse rectifiers cannot supply power to a non-rectifier load at full kVA. In the present invention, when a drive is removed from service, permanently or for maintenance, the transformer of the present invention can carry the full kVA, fixed speed, load.
With the present invention, the cost of reducing detrimental harmonic current is reduced. The primary system harmonics can be reduced without the use of filters or special drive rectifier sections. The fifth, seventh, eleventh, thirteenth, seventeenth and nineteenth harmonics can be reduced significantly.
A transformer of the present invention comprises a first winding group having two sets of contact points disposed at respective locations of first phase displacement in the first winding group. The transformer further comprises a second winding group electromagnetically coupled to the first winding group. The second winding group has two sets of contact points disposed at respective locations of second phase displacement in the second winding group. The second phase displacement is different from the first phase displacement. The transformer still further comprises means for connecting either a multiple phase alternating current power source or a multiple phase load device to at least a selected one of the two sets of contact points of the first winding group and means for connecting the other of the multiple phase load device or the multiple phase alternating current power source to at least a selected one of the two sets of contact points of the second winding group. In at least one implementation, this enables any one of four phase relationships between the multiple phase load device and the multiple phase alternating current power source to be selected. In a particular implementation of the transformer, each of the first and second winding groups includes at least one respective plurality of windings connected in a respective closed circuit.
The present invention also includes a three-phase power system providing selectable control over harmonic distortion and comprising a three-phase alternating current power source and a plurality of three-phase loads connected to the power source. Each of the three-phase loads includes a device and a three-phase transformer. Each transformer comprises a first winding group having two sets of contact points disposed at respective locations of first phase displacement in the first winding group, wherein one of the two sets of contact points is connected to the power source. Each transformer also comprises a second winding group electromagnetically coupled to the first winding group, the second winding group having two sets of contact points disposed at respective locations of second phase displacement in the second winding group, wherein the second phase displacement is different from the first phase displacement and wherein one of the two sets of contact points of
Laxton Gary L.
McAfee & Taft
Southwest Electric Company
Tso Edward H.
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