Low harmonic content AC to DC power conversion

An AC power conversion system includes a first AC/DC converter to be coupled to a multi-phase AC power supply, a second AC/DC converter coupled in parallel with the first AC/DC converter to the multi-phase AC power supply, a first DC load in series with a first inductor coupled to a first positive DC terminal of the first AC/DC converter and a second negative DC terminal of the second AC/DC converter, and a second DC load in series with a second inductor coupled to a first negative DC terminal of the first AC/DC converter and a second positive DC terminal of the second AC/DC converter.

FIELD OF THE INVENTION

The present invention relates generally to power conversion technologies. More particularly, this invention relates to power conversion circuits with low harmonic content.

BACKGROUND

In a conventional line commutated three phase six-pulse controlled alternating current to direct current (AC/DC) converter (or Graetz bridge), each thyristor is triggered at a nonzero delay angle, denoted alpha, from the zero crossover of the anode-to-cathode voltage of the thyristor yielding an AC current that lags the AC voltage by an angle approximately equal to the angle alpha. The AC current contains a real and reactive component. The DC voltage and current contains a predominantly DC component with a superimposed AC component, sometimes referred to as ripple.

The load on the DC side of a six-pulse controlled converter may be resistive, inductive, or a DC source (e.g. battery) or a combination of all three components, depending on the application. The delay angle, alpha, of the converter can theoretically be between 0 and 180 degrees, depending on the application. A delay angle between 0 and 90 degrees implies real power flows from AC to DC, this is known as rectification. A delay angle between 90 and 180 implies real power flows from DC to AC, this is known as inversion. A delay angle of 90 degrees implies the converter draws purely reactive power (theoretically zero real power). This type of conversion can be referred to as reactive compensation. The AC current in a conventional six pulse converter is polluted with a high amount of harmonics (around 30% or greater) as the waveform is trapezoidal, with a conduction period of 120 degrees in each thyristor.

FIG. 1is a schematic diagram illustrating a typical power converter. Referring toFIG. 1, system100includes an antiparallel DC connection of two six-pulse line commutated three phase thyristor controlled converters101and102, the DC terminals of which are connected in anti-parallel via DC loads103-104, which may include a reactor L, load R and DC source, Edc. An average alpha delay angle (between 0 and 180 degrees) is chosen for the system and each six-pulse converter is triggered continuously at a displacement angle Δα in advance or in delay to the nominal alpha, α.

Each thyristor of converter101is triggered at a displacement angle of 15 degrees advanced of the average delay angle, and each thyristor of converter102is triggered at a displacement angle of 15 degrees delayed of the average delay angle. By triggering in this fashion the DC voltage measured over one period can be calculated in proportion to the average alpha. The AC current in each of the bridges101-102will be displaced on either side by an angle Δα from the nominal alpha, α. The total fundamental AC current drawn by each converter will therefore be out of phase by 30 degrees compared to the other (2*Δα). This is shown in waveforms151and152. By adding vectoraly a harmonic vector from each bridge, taking into account the 30 degree difference in phase shift between fundamental current harmonic components, the magnitude of the vectoral sum will be less than twice the magnitude of the harmonic order of the individual AC bridge currents (see waveform153). This is true for each harmonic order contained in the AC current. As a result, there is a significant reduction of harmonic content in the total AC current.

Using this method of controlling the bridges means the AC ripple voltage “sawtooth” (waveform154) that is superimposed on the DC load103and104becomes larger than in the conventional 6 pulse circuit, and contains harmonics that are of a lower harmonic order. Since the current ripple is determined by the voltage ripple divided by the DC impedance at low frequencies a larger DC reactance is required in order to maintain a low current ripple and continuous DC current. As can be seen for the parameters used in the example ofFIG. 1, waveform155, the DC current ripple is large and only barely maintains continuity. For this application, multiple reactors of different sizing or a single a reactor with a “stepped” gap can be used in order to maintain DC current continuity at low currents however the complexity and size of the DC elements, when taking into account the impact on the control system, can make this option impractical.

Finally, depending on the displacement angle chosen the combined a larger ripple current on the DC side can mean that the AC current remains rich in some harmonic orders and the level of harmonic attenuation for each order is lower than would be theoretically expected.FIG. 2shows waveforms of different components inFIG. 1. Referring toFIG. 2, waveform150is a reference of firing for the thyristors, waveform151shows a current through one phase of inductors107; waveform152shows a current through one phase of inductors106; waveform153shows a current through one phase of inductors105; waveform154shows a voltage across a DC load103; and waveform155shows a current through DC load103.

DETAILED DESCRIPTION

Embodiments of the invention offer methods to greatly reduce the harmonics of the AC current waveform and reduce the necessary sizing of the DC inductance and complexity of design. It also reduces or eliminates completely the need for separately wound commutating reactance that are a necessary feature on the AC side of all thyristor line commutated converters. This can be especially advantageous in high power equipment and at nominal control angle (alpha) close to 90 degrees where the switching voltage of the thyristor is highest and the commutating inductance sizing is the largest and least efficient.

FIG. 3Ais a schematic diagram illustrating an example of a power converter according to one embodiment of the invention. Referring toFIG. 3A, according to one embodiment, a transformer, or coupled inductance (e.g. inductors307-308), with two coils is inserted in series with the DC side of bridges303-304. The coupled inductance has two separate windings coupled (or wound) onto a common core. The purpose of the coupled inductor is to greatly reduce the AC voltage ripple superimposed on the DC voltage across DC loads305-306, therefore reducing the necessary required sizing of the DC reactors305-306and alleviating other disadvantages described above with respect to the circuit ofFIG. 1.

System300includes an antiparallel connection of two six-pulse, three-phase thyristor converters303and304. The AC terminals of each converter are connected in parallel to the three phase line power supply301, the DC terminals of each converter are connected in anti-parallel via a reactor (e.g., DC loads305-306) and coupled inductor (307-308). It is possible to insert a DC source in series with DC loads305and306, though this is not shown. Each thyristor of converter303is triggered at a displacement angle Δα of 15 degrees delayed of the average delay angle of the system, and each thyristor of converter304is triggered at a displacement angle of 15 degrees advanced of the average delay angle of the system. The combined effect of a difference in alpha delay angle and the antiparallel connection of the DC poles of the bridges303-304creates an AC voltage (known as the ripple voltage) superimposed onto the DC voltage appearing across load components305,306,307and308.

Referring toFIG. 3A, in one embodiment, each of converters303-304includes six controllable rectifiers321-326and rectifiers327-332, respectively, for passing respective phases of the AC power supply in turn at a respective firing angle for each circuit. A controller (not shown) is configured to provide control signals for symmetrically controlling the respective rectifiers of converters303and304, such that during any power cycle the firing angles for the respective (or corresponding) rectifiers inside converters303-304are substantially equally but oppositely offset from a base angle.

In one embodiment, converters303-304may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding rectifiers in each converter may be substantially equal but oppositely offset from a nominal delay angle α by, for example, 15° (e.g., Δα=±15°). For example, given a rectifier of converter303, all thyristors in303are fired continuously through each power cycle (or period) at (α+Δα). For the corresponding rectifier304, all thyristors are fired continuously at an equal displacement angle but with opposite polarity (α−Δα) continuously through each power cycle. The magnitude of the offset value (e.g., Δα) can be varied to adjust the cancellation of harmonic current components. Note that although two AC/DC converters are described with respect toFIG. 3A, three or more converters may also be utilized in parallel for the objective of increasing the number of pulses per power cycle.

In one embodiment, converters303-304may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding rectifiers in each converter may be substantially equal but oppositely offset from a nominal delay angle (α) by, for example, 15° (e.g., Δα=±15°) in an alternated manner. For example, given a rectifier of a first AC/DC converter (e.g., converter303), a first firing angle of the rectifier may be (α+Δα) during a first power cycle (or period) and a second firing angle may be (α−Δα) during a second power cycle. The next firing angle is configured back to (α+Δα) in the third power cycle, etc. For the corresponding rectifier of a second or complementary AC/DC converter (e.g., converter304), the corresponding firing angle is (α−Δα) for the first power cycle and (α+Δα) for the second power cycle, and (α−Δα) for the third power cycle etc. The magnitude of the offset value (e.g., Δα) can be varied to adjust the cancellation of harmonic current components. Note that although two AC/DC converters are described with respect toFIG. 3A, three or more converters may also be utilized in parallel for the objective of increasing the number of pulses per power cycle.

In one embodiment, system300includes, but not limited to, AC power supply301having respective phase to neutral voltages381-383, AC/DC converters303-304, DC loads305-306, and mutually coupled inductors307-308coupled to DC loads305-306in series. Supply voltages381-383are connected between phase and neutral (or ground) and are phase displaced from one another. Given381is a reference,382lags381by 120 degrees and383lags381by 240 degrees. For the purpose of illustration, line to line voltages are defined as V1, V2, and V3. V1is defined as the difference in potential between AC node voltage381and383; V2is defined as the difference in potential between AC node voltage382and381; V3is defined as the difference in potential between AC node voltage383and382. Converter303includes rectifiers321-326and converter304includes rectifiers327-332, respectively. In one embodiment, a thyristor can be used as an example of any of rectifiers321-332. A controller (not shown) is configured to provide gate control signals to rectifiers321-332of AC/DC converters303-304. Coupled inductor307and DC load305are connected in series across a negative terminal of converter303and a positive terminal of converter304. Coupled inductor308and DC load306are connected in series across a negative terminal of converter304and a positive terminal of converter303. Coupled inductors307and308are wound onto the same inductor core. There is theoretically no gap in the iron core of coupled inductor307and308, hence it can also be referred to as a transformer with two windings.

Voltages381-383represent respective phases of a three-phase AC power supply301, for example, as supplied by an electrical power utility. In this example, converters303-304are connected in parallel to three-phase AC power supply301. The loads305-306can also include a DC source (battery) or an Inverter. Reactors309-311represent transformer, cabling and connection impedances and are considered to be of sizing typical to a three-phase AC transmission network. For example, the total series reactance is typically approximately (or less than) 5% of the voltage drop at full current rating of the supply.

Three-phase electric power is a common method of AC electric power transmission. It is a type of polyphase systems, and is the most common method used by electric power distribution grids worldwide to distribute power. It is also used to power large motors and other large loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage. In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.

A rectifier is an electrical device that converts AC to DC, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. In one embodiment, a thyristor is used as an example of a rectifier. Turn on of the thyristor may be accomplished by a “positive current” pulse between the gate and cathode terminals. Turn off of a thyristor, in a line commutated three-phase AC/DC converter, is achieved by reversing the current in the thyristor. This can be achieved by firing the next thyristor that shares a common cathode (or anode) in sequence, setting up a circulating current between phases that reverses the current direction and turns off the device. Although a thyristor is used as an example of a rectifier through this application, it will be appreciated that other types of controlled switches can also be utilized.

A controller (not shown, also referred to as a gate trigger unit) may be implemented using analog circuitry or, more preferably, by using a microprocessor or microcontroller (e.g., field programmable gate arrays or FPGAs) which can be more readily programmed or adjusted as required to control the firing of converters303-304. For example, a controller may include a machine-readable storage medium (e.g., memory) to store machine executable instructions that have been programmed according to one or more predetermined algorithms and a processor or processing logic or signal generator to generate proper gate trigger signals having proper firing angles for each of rectifiers321-332based on the programmed algorithms.

In this example, the controller may produce 12 gate trigger signals, one for each of rectifiers321-332. In this example, converter303includes six rectifiers321-326and requires six triggering pulses to drive rectifiers321-326and converter304includes six rectifiers327-332and requires another six gate triggering pulses to drive rectifiers327-332. Converters303-304are also referred to as three-phase bridges. Rectifiers321-323are referred to as the positive group of rectifiers of converter303. Rectifiers327-329are referred to as the positive group of rectifiers of converter304. Rectifiers327-329are considered corresponding rectifiers to321-323, respectively. Rectifiers324-326are referred to as the negative group of rectifiers of bridge303. Rectifiers330-332are referred to as the negative group of rectifiers of bridge304. Rectifiers330-332are considered corresponding rectifiers to324-326, respectively.

Reference to “corresponding rectifier” throughout this application is taken to mean, in the case that multiple bridges that are substantially similar or identical in an embodiment, the rectifier in an alternate bridge that is connected in the same position as the original bridge. Referring toFIG. 3A, for example, rectifier327is the corresponding rectifier of rectifier321. Similarly, rectifier321can be referred to as the corresponding rectifier of rectifier327. Similarly, Rectifier330is the corresponding rectifier of rectifier324. Similarly, rectifier324can also be referred to as the corresponding rectifier of rectifier330, and so on.

According to one embodiment, system300is controlled at a nominal control angle α (e.g. approximately 90 degrees), therefore drawing substantially only reactive power from the AC supply301in this example. Rectifiers inside converter circuits303-304are switched in a complementary fashion, at firing angles which are substantially equally offset (offset, meaning advanced or delayed, or added/subtracted) from the nominal control angle by an offset angle Δα.

FIG. 3Bis a diagram illustrating certain waveforms of certain components ofFIG. 3A. Referring toFIG. 3B, waveform351shows the current of one phase of inductors311; waveform352shows the current of one phase of inductor310; waveform353shows the current through one phase of inductor309; waveform354shows the voltage across the load305; and waveform355shows a current through DC loads305and306.

As can be seen inFIG. 3Bwaveform354, the presence of the coupled inductor307-308in series with the DC load305-306means the ripple voltage across the DC load305is lower in magnitude and the fundamental frequency is of a higher order when compared to waveform154. This means the DC smoothing reactance in load305and306can be smaller in inductance and physical size while achieving the same magnitude of DC current ripple as for converter100. In this example DC reactances305and103are identical in size but comparison of DC current waveforms355and155show that the current ripple in system300is smaller due to the presence of mutual inductance307-308(compare waveforms155and355). The AC current in each 6 pulse bridge is closer to quasi-square in shape due to the lower DC ripple current (compare waveforms353and153). For the displacement angle chosen (15 degrees) the fifth harmonic (250 Hz) is less in magnitude for system300compared to system100.

FIGS. 4 and 5illustrate how the coupled inductance helps to reduce harmonics.FIG. 4Aillustrates a simplified equivalent circuit ofFIG. 3A. The coils of the coupled inductor are connected in series with each DC link. The coils are coupled by the same core, so the coupling coefficient that determines the mutual inductance is assumed to be close to unity. Voltage source VB1represents the voltage ripple existing between the positive pole of bridge303and the negative pole of Bridge304. The load is resistance and inductance R1and L1respectively, and the first coil of the coupled inductance M. Voltage source VB2represents the voltage ripple existing between the positive pole of Bridge304and the negative pole of Bridge303. The load is resistance R2and L2respectively and the second coil of the coupled inductance M. To simplify this explanation the voltage sources V1and V2are considered pure sinusoidal wave of frequency 50 Hz with equal magnitude of AC and DC components. Sources VB1and VB2are in phase.FIG. 4Bshows the waveforms forFIG. 4A. Waveform401shows voltages VB1and VB2, waveform402shows the resultant current flowing through L1and L2. As can be seen402has a very large AC component superimposed onto a small DC component.

FIG. 5Ashows an identical circuit toFIG. 4Aexcept that there is a phase shift of 180 degrees imposed between source voltages VB1and VB2.FIG. 510shows source voltages VB1and VB2, andFIG. 511shows the resultant current flowing through L1and L2. By comparing waveforms511and402it can be seen the AC current through L1and L2is greatly diminished when a phase shift is present between VB1and VB2. This means that current harmonics that are in opposite phase are considerably reduced when a mutual inductance is inserted into the DC path. This is due to the impedance of the self inductance of a mutual coil plus the mutual inductance being inserted in series with the DC reactor L. Given the coupling coefficient between coils of the mutual being ˜1 this means almost twice the self inductance of the mutual is inserted in series with L1and L2in each DC path. For the application of system300(FIG. 3A) the same reasoning can be applied to explain the function of the coupled inductance in reducing the current ripple on the DC side of the converters. The voltage waveform across each DC load is illustrated inFIG. 5C. Waveform512shows the voltage across load components305and307, and waveform513shows the voltage across load components306and308. The waveforms are displayed over a time period of two power cycles. As can be observed waveforms512and513are identical except for a phase shift, φ, between one another. The current harmonics generated due to the phase shift between512and513are attenuated due to the presence of the coupled inductance.

The value of each current harmonic can be calculated by dividing the voltage harmonic by the addition of the self, mutual and load inductance, L. In practice the self inductance of each coil of M is can be designed to be very large in a small volume because there is no DC component of flux to consider due to the coupling between the two DC paths. The coupling coefficient can be designed to be close to unity by ensuring close proximity of the two windings of the coupled inductor.

In principle it has been shown that the coupled inductance will attenuate current harmonics on the DC side of the converters when there is a phase difference present in source voltages. In the circuit ofFIG. 3Athe difference in alpha between the two bridges caused by the introduction of the delay angle Δα in each bridge means that there is a phase shift in DC voltage waveforms across each DC link. The harmonic order magnitudes and phase of each sawtooth shaped source voltage can be found by means of Fourier analysis.

In some applications it may be advantageous to connect the coupled inductor to the AC side of the three phase bridges instead of in series with the DC side of the three phase bridges or the load.FIG. 6Ashows an embodiment of the invention connected to the AC side. Each line of the AC supply voltage source is connected to a center tap of a three winding, single core leg coupled inductance. Each start of each winding is connected to the three phase input of converter303. The finish of each winding is connected to the three phase input of converter304. In this example, line381of power supply301is coupled to a center tap of mutually wound three-terminal inductor501. A first winding terminal of inductor501is coupled to a first phase input of converter303and a second winding terminal of inductor501is coupled to a first phase input of converter304. Line382of power supply301is coupled to a center tap of mutually wound three-terminal inductor502. A first winding terminal of inductor502is coupled to a second phase input of converter303and a second winding terminal of inductor502is coupled to a second phase input of converter304. Line383of power supply301is coupled to a center tap of mutually wound three-terminal inductor503. A first winding terminal of inductor503is coupled to a third phase input of converter303and a second winding terminal of inductor503is coupled to a third phase input of converter304. Note that two additional mutually coupled inductors (e.g., similar to inductors307-308) may also be coupled in series to the DC loads305-306to reduce AC ripple effects on the DC loads305-306, similar to the configuration as shown inFIG. 3A.

FIG. 6Bshows waveforms belonging to the circuit ofFIG. 6A, with the displacement angle Δα equal to 15 degrees. Waveform550is a reference for nominal α. Waveforms551-552show AC currents flowing in each converter through one phase of inductors310-311. Waveform553shows the total AC current of one phase of inductor309. Waveform554shows the voltage across DC load305and306. Waveform555shows a current flowing through the DC load305and306. As can be seen waveforms553,554and555are substantially of a 12 pulse nature, with 12 peaks per power cycle (20 ms).

In this configuration the coupled inductors can, in theory, provide series ‘leakage’ inductance to bridges303and304to limit the rate of change of current during commutation from one thyristor to another eliminating the need for costly, inefficient separately would commutating inductors positioned in the AC line of each bridge. Embodiments of the invention can be used in rectification (0 degree<alpha<90 degree) and inversion (90 degree<alpha<180 degree) mode.

Embodiments of the invention can also be applied to higher order pulsed circuits. By connecting more AC to DC6pulse line commutated controlled converters to the same AC supply point and interconnecting coupled inductors the order of pulse number can be increased to obtain further reduction in AC current harmonics and a lower overall current distortion. For example, a mutually wound inductor can be implemented at both the AC side and DC side of an AC/DC converter, as shown inFIG. 7.

In one embodiment, converters303-304may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding rectifiers in each converter may be substantially equal but oppositely offset from a nominal delay angle (α1) by, for example, 7.5° (e.g., Δα1=±7.5°). For example, given a rectifier of converter303, all thyristors in303are fired continuously through each power cycle (or period) at (α1+Δα1). For the corresponding rectifier304, all thyristors are fired continuously at an equal displacement angle but with opposite polarity (α1−Δα1) continuously through each power cycle. The magnitude of the offset value (e.g., Δα1) can be varied to adjust the cancellation of harmonic current components.

In one embodiment, converters303-304may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding rectifiers in each converter may be substantially equal but oppositely offset from a nominal delay angle (α) by, for example, 7.5° (e.g., Δα=±7.5°) in an alternated manner. For example, given a rectifier of a first AC/DC converter (e.g., converter303), a first firing angle of the rectifier may be (α+Δα) during a first power cycle (or period) and a second firing angle may be (α−Δα) during a second power cycle. The next firing angle is configured back to (α+Δα) in the third power cycle, etc. For the corresponding rectifier of a second or complementary AC/DC converter (e.g., converter304), the corresponding firing angle is (α−Δα) for the first power cycle and (α+Δα) for the second power cycle, and (α−Δα) for the third power cycle etc. The magnitude of the offset value (e.g., Δα) can be varied to adjust the cancellation of harmonic current components.

In one embodiment, converters703-704may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding rectifiers in each converter may be substantially equal but oppositely offset from a nominal delay angle (α2) by, for example, 7.5° (e.g., Δα2=±7.5°). For example, given a rectifier of converter703, all thyristors in703are fired continuously through each power cycle (or period) at (α2+Δα2). For the corresponding rectifier704, all thyristors are fired continuously at an equal displacement angle but with opposite polarity (α2−Δα2) continuously through each power cycle. The magnitude of the offset value (e.g., Δα2) can be varied to adjust the cancellation of harmonic current components.

In one embodiment, converters703-704may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding rectifiers in each converter may be substantially equal but oppositely offset from a nominal delay angle (α) by, for example, 7.5° (e.g., Δα=±7.5°) in an alternated manner. For example, given a rectifier of a first AC/DC converter (e.g., converter703), a first firing angle of the rectifier may be (α+Δα) during a first power cycle (or period) and a second firing angle may be (α−Δα) during a second power cycle. The next firing angle is configured back to (α+Δα) in the third power cycle, etc. For the corresponding rectifier of a second or complementary AC/DC converter (e.g., converter704), the corresponding firing angle is (α−Δα) for the first power cycle and (α+Δα) for the second power cycle, and (α−Δα) for the third power cycle etc. The magnitude of the offset value (e.g., Δα) can be varied to adjust the cancellation of harmonic current components.

The nominal alpha for each pair of bridges is displaced by angle Δα3 or 15 degrees from the nominal alpha for the system700, α3. For this embodiment, the nominal alpha of the system is approximately 90 degrees. For example the nominal alpha for converter pair303-304can be 75 degrees (90−Δα13), and the nominal alpha for converter pair703-704is 105 degrees (90+Δα3).

In one embodiment, converters303,304,703,704may be substantially equal, and the respective loads and their DC currents are substantially equal. The firing angles for the corresponding pairs of converters may be substantially equal but oppositely offset from a nominal delay angle (α3) by, for example, 15° (e.g., Δα3=±15°). For example, given converter pair303-304, the nominal firing angle may be continuously through each power cycle (or period) at angle (α3+Δα3). For the corresponding converter pair703-704, the nominal firing angle may be continuously through each power cycle (α3−Δα3). The magnitude of the offset value (e.g., Δα3) can be varied to adjust the cancellation of harmonic current components.

In one embodiment ofFIG. 7, the firing angles for the corresponding pair of converters may be substantially equal but oppositely offset from a nominal delay angle (α3) by, for example, 15° (e.g., Δα3=±15°) in an alternated manner. For example, given the converter pair303and304, the nominal (or average) firing angle of all the rectifiers may be (α3+Δα3) during a first power cycle (or period) and a second firing angle may be (α3−Δα3) during a second power cycle. The next firing angle is configured back to (α3+Δα3) in the third power cycle, etc. For the corresponding complementary pair of converters, e.g.,703and704, the corresponding nominal (or average) firing angle is (α3−Δα3) for the first power cycle and (α3+Δα3) for the second power cycle, and (α3−Δα3) for the third power cycle etc. The magnitude of the offset value (e.g., Δα3) can be varied to adjust the cancellation of harmonic current components. Note that although AC/DC converters are described with respect toFIG. 7, 5 or more converters may also be utilized in parallel for the objective of increasing the number of pulses per power cycle. AC coupled inductors501,502, and503are wound onto a separate core with respect to coupled inductors701,702, and703. AC coupled inductors501,502, and503are wound onto the same core. AC coupled inductors701,702, and703are wound onto the same core. Coils for DC mutual coupled inductor707,709are wound on a common core. Coils for DC mutual inductor708, and710are wound onto a common core.

FIGS. 8 and 8Bshow the waveforms for the embodiment ofFIG. 7according to certain embodiments. As can be seen802and803show the AC current through one phase of inductors310and311. Waveforms804and805show the AC current through one phase of inductors312and313. The total current supplied by source381, shown in waveform807, is substantially 24 pulse in nature forming a multi-pulse stepped waveform with lower harmonic distortion than previous 12-pulse embodiments. The voltage across305is shown in808. The current through305is shown in809. From808and809it can be deduced that there are 24 pulses of voltage and current ripple per power cycle.

Higher order pulsed AC current is beneficial when connecting high power equipment to power networks as authorities governing these networks are applying more stringent controls of the limits of harmonic content the AC current may have. A 24-pulse system, while having a higher component count, can be more cost effective as it can alleviate the need for additional harmonic filtering.

According to one embodiment, an AC power conversion system includes a first alternating current to direct current (AC/DC) converter to be coupled to a multi-phase AC power supply, the first AC/DC converter having a plurality of rectifiers, a second AC/DC converter coupled in parallel with the first AC/DC converter to the multi-phase AC power supply, the second AC/DC converter having a plurality of rectifiers, a first DC load in series with a first inductor coupled to a first positive DC terminal of the first AC/DC converter and a second negative DC terminal of the second AC/DC converter, and a second DC load in series with a second inductor coupled to a first negative DC terminal of the first AC/DC converter and a second positive DC terminal of the second AC/DC converter. The first inductor and the second inductor are mutually wound to provide mutual inductance with respect to the first DC load and the second DC load. The first inductor comprises a first coil and the second inductor comprises a second coil, and wherein the first coil and the second coil are wound onto the same inductor core. The first inductor and the second inductor form an inter-phase transformer. The first inductor and the second inductor are to reduce AC ripples across the first DC load and the second DC load. The first AC/DC converter comprises at least six rectifiers forming a first rectifier bridge and the second AC/DC converter comprises at least six rectifiers forming a second rectifier bridge. The first AC/DC converter comprises at least six thyristors forming a first rectifier bridge and the second AC/DC converter comprises at least six thyristors forming a second rectifier bridge. The first AC/DC converter is triggered with a first predetermined angle in advanced, while the second AC/DC converter is triggered with a second predetermined angle in delay.

According to another embodiment, an AC power conversion system includes a first alternating current to direct current (AC/DC) converter, the first AC/DC converter having a plurality of rectifiers, a second AC/DC converter, the second AC/DC converter having a plurality of rectifiers, a first inductor coupled in series with the first AC/DC converter to a multi-phase AC power supply, the first AC/DC converter having a plurality of rectifiers, a second inductor coupled in series with the second AC/DC converter to the multi-phase AC power supply, wherein the first inductor and the second inductor are mutually wound onto the same inductor core, a first DC load coupled to a first positive DC terminal of the first AC/DC converter and a second negative DC terminal of the second AC/DC converter, and a second DC load coupled to a first negative DC terminal of the first AC/DC converter and a second positive DC terminal of the second AC/DC converter.

The first and second mutually wound inductors include a first coil coupled to a first phase input of the first AC/DC converter, and a second coil coupled to a first phase input of the second AC/DC converter, where the first coil and the second coil are coupled in series forming a first center tap, where the first center tap is coupled to a first line output of the multi-phase AC power supply, and where the first coil and the second coil are wound onto the same inductor core. The first and second mutually wound inductors further include a third coil coupled to a second phase input of the first AC/DC converter; and a fourth coil coupled to a second phase input of the second AC/DC converter, wherein the third coil and the fourth coil are coupled in series forming a second center tap, and wherein the second center tap is coupled to a second line output of the multi-phase AC power supply, wherein the third coil and the fourth coil are wound onto the same inductor core. The first and second mutually wound inductors further include a fifth coil coupled to a third phase input of the first AC/DC converter; and a sixth coil coupled to a third phase input of the second AC/DC converter, wherein the fifth coil and the sixth coil are coupled in series forming a third center tap, and wherein the third center tap is coupled to a third line output of the multi-phase AC power supply, wherein the fifth coil and the sixth coil are wound onto the same inductor core. The first coil, the second coil, the third coil, the fourth coil, the fifth coil, and the sixth coil are wound onto the same inductor core.

The first AC/DC converter includes at least six rectifiers forming a first rectifier bridge and the second AC/DC converter comprises at least six rectifiers forming a second rectifier bridge. The first AC/DC converter includes at least six thyristors forming a first rectifier bridge and the second AC/DC converter comprises at least six thyristors forming a second rectifier bridge. The first AC/DC converter is triggered with a first predetermined angle in advanced, while the second AC/DC converter is triggered with a second predetermined angle in delay. The system further includes a third inductor coupled in series with the first DC load between the first positive DC terminal of the first AC/DC converter and the second negative DC terminal of the second AC/DC converter, and a fourth inductor coupled in series with the second DC load between the first negative DC terminal of the first AC/DC converter and the second positive DC terminal of the second AC/DC converter. The third inductor and the fourth inductor are mutually wound onto the same inductor core. The third inductor and the fourth inductor form an inter-phase transformer. The third inductor and the fourth inductor are to reduce AC ripples across the first DC load and the second DC load.