Westinghouse Electric Corporation

Other

The United States of America As Represented by the Secretary of

Electric Power Research Institute Incorporated

Derek Albert Paice

Alan B Shimp

Laszlo Gyugyi

Peter Wood

Charles W Edwards

Eugene C Strycula

Kenneth E Mattern

A polygon connected three-phase autotransformer using six windings per phase provides a reduced power rating nine-phase power source suitable for 18-pulse AC to DC power converters. The windings are selected and connected in a manner that controls harmonic currents and minimizes total kVA rating. When the autotransformer is used to power a nine-phase AC to DC converter its kVA rating is typically less than 48% of the DC load kW. Additional isolated windings can provide means for the invention to operate as an efficient double-wound transformer.

Nine-phase autotransformer

A polygon connected three-phase autotransformer using nine windings per phase provides a reduced power rating fifteen-phase power source suitable for 30-pulse AC to DC power converters. The windings are selected and connected in a manner that accommodates zero-sequence currents, such as the third harmonic, and minimizes total kVA rating. When the autotransformer is used to power a 15-phase AC to DC converter its kVA rating is typically less than 51% of the DC load kW. AC line current distortion is negligible and can satisfy the most exacting practical harmonic specifications. Additional isolated windings can provide means for the invention to operate as an efficient double-wound transformer.

Fifteen-phase autotransformer

An irregular polygon connection of a three-phase autotransformer using only five windings per phase provides a source of nine-phase power suitable for an 18-pulse AC to DC power converter. The windings are connected in a manner to limit the amount of third harmonic current circulating in them. When the autotransformer is used to power a nine-phase AC to DC converter its kVA rating is typically less than 50% of the DC load kW. An additional tap on one of the five windings allows a wide range of AC output voltages to be obtained. Voltages greater than, less than, or equal to the AC input voltage can be obtained. Other multi-phase outputs are feasible. Additional isolated windings can provide means for the invention to operate as an efficient double-wound transformer.

Nine-phase step-up/step-down autotransformer

A polygon connected autotransformer in conjunction with zero sequence blocking inductor(s) enables multipulse AC to DC converters to use lower kVA parts rating by using appropriate phase-shifted voltage sets in conjunction with inductors that extend the conduction period and reduce rms current. Also, lower harmonic voltages in the transformer facilitate use of lower performance magnetic steel. Designs for 12, 18, and 24-pulse use the same conceptual approach. Very efficient high power ratings are feasible. Means are given to limit the maximum no-load DC output voltage. A technique is disclosed that reduces the size of polygon transformers supplying loads with substantial third harmonic.

Low kVA/kW transformers for AC to DC multipulse converters

An 18-pulse converter is constructed using two, high zero-sequence impedance, phase-shifting autotransformers to parallel two 3-phase bridge converters. A third 3-phase bridge converter is directly connected to the power source. Depending on the DC filtering technique, the kVA rating of each autotransformer is 15 to 22% that of the DC load kW. The method facilitates paralleling of 3-phase bridge converters to reduce system AC line current harmonics, and is suitable for 18, 24, 36, and higher pulse numbers. The minimum number of individual autotransformers required is (n&#x2212;1), where n is the number of 3-phase bridge converters. Due to the high zero-sequence impedance of the autotransformers, only a small DC load is needed to limit the DC output voltage. AC line reactance may be included in series with each converter, or in the 3-phase AC power source, to further reduce AC line current harmonics.

Autotransformers to parallel AC to DC converters

In an 18-pulse converter system a three-phase to nine-phase auto-transformer with multiple windings per phase is used to power a nine-phase AC to DC converter. The transformer kVA rating is about 55% of the DC load kW. The basic topology is that of a wye connection, but an active delta path is added to allow circulation of the third harmonics necessary for proper operation. The design requires substantially fewer coils than other three-phase to nine-phase auto-transformers. The voltage ratio can be selected so as to give an average DC output from an 18-pulse converter which is generally the same as that of a conventional three-phase bridge rectifier without a transformer. However, the topology described is also suitable for step up or step down of voltage as required. Where necessary to further reduce high frequency harmonic currents, a three-phase ac line reactor may be connected in series with the source of ac power.

Simplified wye connected 3-phase to 9-phase auto-transformer

In an 18-pulse converter system a 3-phase to 9-phase auto transformer with multiple windings per phase is used to power a 9-phase AC to DC converter. The transformer kVA rating is typically about 55% of the DC load kW. Multiple delta connected coils formed around a basic wye connected structure provide a means to circulate the third harmonics of current necessary for proper operation and reduce the amplitude of load current flowing in individual stub windings. The similar current rating required in several of the multiple coils, coupled with reduced amplitude compared to other topologies, facilitates manufacturing of higher power transformers without the need to parallel conductors. The voltage ratio can be selected so as to give an average dc output from an 18-pulse converter which is generally the same as that of a conventional 3-phase bridge rectifier without transformer, however, the topology described is also suitable for step up or step down of voltage as required. Where necessary to further reduce high frequency harmonic currents, a 3-phase ac line reactor may be connected in series with the source of ac power.

Wye connected 3-phase to 9-phase auto-transformer with reduced winding currents

In a 12-pulse converter system a 3-phase auto transformer with 4 windings per phase is used to power two 6-pulse converter bridges connected in parallel with a large dc filter capacitor. The transformer rating is typically about 40% of the dc kW load. The voltage ratio is typically 1:1 so that the average dc output of a multi-pulse converter is generally the same as that of a conventional 3-phase bridge rectifier without transformer, however, ac input harmonic currents are greatly reduced. A small single-phase transformer is used to block unwanted circulating currents between the two 6-pulse converters. Where necessary to further reduce high frequency harmonic currents, a 3-phase ac line reactor may be connected in series with the source of ac power. Where a smaller degree of harmonic reduction is acceptable, only 3 windings per phase are required on the transformer and the small single-phase transformer is eliminated by raising the zero-sequence impedance of the auto transformer by means of an additional magnetic path. This method provides a higher zero-sequence impedance compared to a conventional 3-limb magnetic structure used in most 3-phase transformers. The 1:1 voltage ratio feasible in this invention facilitates retrofit applications, also the concept can be applied to a greater number of parallel converters such as those giving 18-pulse operation.

Transformers for multipulse AC/DC converters

In a multi-pulse converter system a three-phase transformer with tapped wye-connected primary winding and isolated delta-connected secondary winding is used to power two 6-pulse converter bridges connected in series to give 12-pulse operation. The transformer equivalent kVA rating is a fraction, typically less than 60%, of the total dc power. By adjusting the tapping on the wye-connected primary winding the converter dc output voltage is readily adjusted to provide higher or lower dc voltage than is attainable by a three-phase 6-pulse converter. By these means a reliable and versatile 12-pulse converter is obtained which, without additional hardware, gives excellent performance in the face of practical power sources which may incorporate voltage unbalance and pre-existing harmonic voltages.

Transformer for 12-pulse series connection of converters

In 12-pulse and 18-pulse AC/DC converter systems a single wye connected auto transformer using appropriate segments of voltage from each phase connected in fork fashion is used to develop 6-phase and 9-phase voltages in a symmetrical manner such that inherent compensation is made for the dc converter output voltage to be within 4.2% of that of a 6-pulse converter. Additional design adjustments for step-up or step-down of voltage can be made without compromising the transformer symmetry. An unconnected but closed delta winding provides a path in which third harmonic currents, and multiples thereof can flow.

Symmetrical, phase-shifting, fork transformer

A 12-pulse converter of lower rating and minimal current harmonics is obtained with an auto-connected regular hexagon transformer including a delta winding in its center.

Auto-connected hexagon transformer for a 12-pulse converter

A 18-pulse converter system operates without interphase transformer and neutral point on the DC side by providing three rectifier bridges across the DC terminals, one rectifier bridge being coupled with the AC terminals and the two other rectifier bridges being operated at + and -40 degress phase shift, while the rectifier devices are controlled for 40-degree current conduction.

Optimized 18-pulse type AC/DC, or DC/AC, converter system

In a multiphase AC/DC converter system a single transformer is used having phase shifted tappings drawn from a polygonal succession of long and short windings wherein some of the windings carry only the difference between the outputted currents derived from the tappings.

Multi-pulse converter system

A variable frequency adjustable speed motor drive allows restart of a spinning induction motor by initially increasing the frequency of the associated inverter while keeping the DC voltage source thereof low, then by decreasing the frequency of the inverter while monitoring the effective motor impedance V.sub.DC /I.sub.DC or V.sub.AC /I.sub.AC until the latter jumps at an initial level indicating matching of the frequency with the motor spinning speed. Thereafter, the DC voltage is restored in magnitude and the inverter is increased in frequency to match the desired motor speed.

Coasting AC motor restart system and method

The present invention relates to an AC motor drive including two sets of inverters, wherein one transformer is converted on the output of one of the two inverters, the output of the transformer being connected in series with the output of the other inverter.

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Palm Harbor, FL, United States of America

Derek Albert Paice