EA041589B1 - POWER USE POINT TRACKING INVERTER - Google Patents

Description

State of the art

A single-phase DC/AC converter converts direct current (DC) electrical energy from a DC power source into alternating current (AC) power that meets the grid conditions. Grid conditions dictate that oscillating AC power transmitted over the grid has a sinusoidal waveform with a particular fixed peak voltage and a particular fixed frequency.

A traditional 3-phase DC/AC converter provides AC power to 3 power lines with a 120° phase shift (called A-phase, B-phase and C-phase) between each power line. At its core, a 3-phase DC/AC converter consists of three single-phase DC/AC converters, each extracting and converting to deliver the same time-averaged AC power to the corresponding power line. One of the single-phase DC/AC converters thus provides AC power on the first A-phase power line. The second of the single-phase DC/AC converters thus provides AC power on the second B-phase power line. The third of the single-phase DC/AC converters thus provides AC power on a third C-phase power line. In other words, each set of three single-phase DC/AC converters draws roughly the same amount of DC power; converts this extracted power into AC power with a 120° phase shift compared to AC power provided by other single-phase DC/AC converters; and sends the AC power to the power grid of 3 or 4 power lines. Therefore, each power line carries one single-phase AC power of the same frequency and the same amount of time-averaged power as the AC power of other power lines; but with a phase shift of 120° compared to other lines of force.

The subject matter of this document is not limited to embodiments that address any disadvantages or that only work in the environments described above. Rather, this prior art is provided only to illustrate one exemplary area of technology where some of the embodiments described herein may be practiced.

Brief summary of the invention

The embodiments described herein relate to a serial draw control device for use in a 3-phase DC/AC converter. The 3-phase converter has a first single-phase DC/AC converter containing a first PWM extractor for extracting from the DC power the first AC power that has a first phase and which is suitable for power grid conditions, a second single-phase DC/AC converter containing a second A PWM extractor for extracting a second AC power from the DC power, which has a second phase and which is suitable for power grid conditions, and a third single-phase DC/AC converter, comprising a third PWM extractor for extracting a third AC power from the DC power, which has a third phase and which corresponds to the conditions of the mains. The duty cycle (relative duty cycle) adjustments are made depending on the current section of the AC power cycle.

However, the sequential controller instructs the first PWM extractor to have a first duty cycle during which the first PWM extractor performs extraction, instructs the second PWM extractor to have a second duty cycle during which the second PWM extractor performs extraction, and instructs the third The PWM extractor has a third duty cycle during which the third PWM extractor performs an extraction. Accordingly, unlike traditional 3-phase DC/AC converters, the serial regulator instructs that the first, second, and third duty cycles do not overlap, so that the first, second, and third PWM extractors perform the extraction sequentially rather than simultaneously. This improves the efficiency of extracting power from DC power. The sequential controller further controls the duty cycle adjustments for the first, second and third duty cycles in accordance with the current time of at least one AC power cycle, to thereby regulate one or more of the first AC power, the second AC power and the third AC power. The embodiments described herein are also directed to a 3-phase DC/AC converter that includes such a series regulator.

This summary is provided to introduce a selection of concepts in a simplified form, which are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

Brief description of the drawings

In order to describe the manner in which the above and other advantages and features can be obtained, a more specific description of the various embodiments will be reproduced by reference to the accompanying drawings. Provided that these drawings depict only exemplary embodiments of the invention and are therefore not to be construed as limiting the scope of the invention, the embodiments will be described and explained in further specificity and detail through the use of the accompanying drawings, in which:

fig. 1A depicts modules for a solar power generation sequence to illustrate and clarify the referenced terms power extraction, preparation, matching, DC/AC conversion, and AC power supply;

fig. 1B symbolically shows a sinusoidally time-varying (oscillating) AC voltage sequence representing an AC signal (DC/AC converter output signal) that is sent over specific power lines of an electric power system;

fig. 2A shows a typical circuit of a DC/DC boost converter, which is an example of the mentioned single-phase power extraction/preparation (matching) device;

fig. 2B shows a typical circuit of a DC/DC buck converter, which is an example of the mentioned single-phase power extraction/conversion device;

fig. 2C shows the bridge structure of the switches that control the output polarity of the DC/DC buck converter; thus producing the resulting AC voltage output sequence as shown in FIG. 1B;

fig. 2D symbolically depicts a pulsating AC power sine wave for the DC/AC inverter output power that is sent to the switch bridge;

fig. 2E symbolically depicts a DC energy pulse to be matched by a buck converter in one PWM cycle; which is described as 3 regions: region I representing extracted energy, and region-II and region-III representing excess energy region;

fig. 3A shows diagrams of suitable power extraction devices for a conventional 3-phase DC/AC converter;

fig. 3B symbolically depicts DC input power in one PWM cycle;

fig. 3C symbolically represents the energy simultaneously extracted in one PWM cycle by the 3 energy extraction devices in FIG. 3;

fig. 4 symbolically depicts a configuration in which a power supply supplies DC power P _mx that supports two identical 3-phase DC/AC converters of advertised power rating P _mx , but without a MEUPT device;

fig. 5A shows 3 circuits for extracting A-phase energy, extracting B-phase energy, and extracting C-phase energy of a MEUPT inverter;

fig. 5B symbolically depicts the time sequence in the A-phase, B-phase, and C-phase energy extractions adjusted by the sequential controller of FIG. 5A;

fig. 6A illustrates a block diagram of a power plant in an experiment in which there are two AC power generation units conventionally configured with power and energy meters to measure the AC output of each unit; and fig. 6B illustrates a block diagram of the power plant of FIG. 6A after modification to include decouplers and an energy reservoir, and which has been used to confirm improved power output to the grid.

Detailed description of the invention

Essentially, a 3-phase DC/AC converter consists of 3 sets of single-phase DC/AC converters that perform the functions of extracting and converting, supplying the same time-averaged AC power to 3 pairs of power lines; with a phase shift of 120°. Therefore, to understand the 3-phase DC/AC converter, one should have a good understanding of the single-phase DC/AC converter; especially in the subject under consideration, the extraction of energy. The terms power lines and power cable are used interchangeably in the art and are used interchangeably in this document unless otherwise noted.

Patent publications US2016/0036232 and US2017/0149250 A1 describe the discovery that traditional single-phase converters can only extract and convert less than half of the direct current (DC) input power. These patent publications teach that in order to efficiently extract DC generated electricity for energy use, the characteristics of the power extraction device must match the efficient and productive extraction of DC generated electricity.

In addition, these patent publications teach that related devices must also match the condition and/or delivery of the extracted electricity in order to use energy efficiently. Instead of using a maximum energy utilization point tracker (MPPT) as an optimizer for solar power plants, these publications have suggested using a maximum energy utilization point tracker as an optimizer for power plants; especially PV power plants. Such an optimizer

- 2 041589 will be referred to as the MEUPT optimizer in this document.

According to patent reference publications, the MEUPT optimizer is meant to capture what they call surplus energy or surplus power, which they define as electricity (or power, respectively) that is produced but not extracted and/or delivered to the grid for use. . This definition of excess energy (or excess power) is also used in this document. This excess power has a phase shift of approximately 90° from the mains so that the excess power cannot be sent directly to the mains. The MEUPT optimizer is also meant to temporarily store all captured excess energy in an energy reservoir; and then prepare and deliver this electricity to the grid for use. Thus, the electricity sales revenue for a PV power plant can be improved when the MEUPT optimizer is included in it.

Some specialized knowledge can be applied in DC power extraction, power preparation, power matching and power delivery. The principles described in this document can be used regardless of the source of DC power. However, this invention uses a sequence of elements for generating solar power as an example of a DC power source to illustrate and clarify the terms power extraction, preparation, matching and delivery referred to in this document, i.e. The principles described in this document are not limited to extraction, preparation, matching and delivery in the context of solar energy. Although they have different physical meanings, the terms energy and power are used interchangeably in the art and are used interchangeably herein unless otherwise noted. Also, although they have different physical meanings, the terms AC power sequence and AC voltage sequence are used interchangeably in this document unless otherwise indicated.

Fig. 1A shows the components of a sequence 10 for generating solar power. The sequence starts at the photovoltaic power conversion device (PV solar panel sequence) 101, which generates electricity from light energy (eg, solar energy) as a primary energy source. This electrical energy has a time-varying voltage that depends on cloudiness, the angle of the sun, the efficiency of the PV cells, and many other factors. The electrical power is then prepared and matched by the DC/DC boost converter 201 to a fixed voltage DC power source. By adding the polarity reversal controller 224, the DC/AC conversion module 223 converts the fixed voltage prepared DC power into the sinusoidal time-varying (oscillating) AC voltage sequence shown in FIG. 1B.

As an example, the DC/AC conversion module 223 may be a buck module that is controlled by a pulse width modulator (PWM). A step-down module operating in this way would also be referred to as a DC/AC converter. Fig. 2C illustrates an example of a bridge structure 224, which is referred to as an integral bridge gate transistor (IBGT) in the art and also herein. As shown, bridge structure 224 consists of a set of 4 switches (S1, S2, S3 and S4) that control the synchronism and polarity of the AC output voltage sequence of this DC/AC converter. Here, LOAD represents the load facing transformer 225 from bridge structure 224. The combination of devices 201 and 223 may be referred to as a PWM extractor in this document.

The AC voltage power sequence generated by the bridge structure 224 corresponds to the power grid conditions. The AC power sequence is then fed through transformer 225 to deliver power to grid 300 which is connected to the grid load. Fig. 2A shows an exemplary circuit design of a DC/DC boost converter 201 that can regulate a variable voltage DC power source to a fixed voltage DC source. Fig. 2B shows an exemplary circuit design of a typical PWM controlled step-down module for a single-phase DC/AC inverter 223 to convert a fixed voltage DC source into a sinusoidal AC power train. A switch bridge structure 224 (shown in FIG. 2C) controls the polarity and timing of the output of this single-phase DC/AC converter 223. AC converter 223) can also act as a power extraction/conversion module in a traditional single-phase inverting module of a traditional 3-phase DC/AC converter, which consists of 3 single-phase inverters.

Section 1. An overview of traditional DC/AC conversion.

In general, the voltage at the point of production of maximum power (MPPPV) in a practical sequence of solar panels is time-varying and less than the design peak voltage of the AC power grid. The step-up energy recovery device is essential in PV stations for energy recovery and preparation. The booster voltage 3 041589 power recovery device regulates a time-varying low voltage DC source into a fixed high voltage DC power source.

Fig. 2A is a circuit diagram of a voltage boosting device of the DC/DC boost module 201, which is composed of a choke L; a controlled switch Q controlled by a closed-loop duty cycle controller FCDFA (not shown); diode D; and capacitor C. Switch Q switches at a high frequency (typically about 18 kHz in commercial products) with an adjustable duty cycle. A closed-loop duty cycle controller (FCDFA) controls the adjustable duty cycle so that this DC/DC boost module 201 produces a nearly constant DC output voltage (v ₀ ). In other words, this DC/DC boost module 201 modifies a time-varying voltage DC power source into a power source having a fixed DC voltage v ₀ (typically, v ₀ =v _pk , where v _pk is the AC mains peak voltage), which is suitable for the next device in the sequence (namely, the DC/AC conversion unit 223 in the case of FIG. 1A). This next DC/AC conversion module converts the DC power with the specified peak voltage into AC power in the form of a sinusoidal power sequence that suits the grid conditions.

During the period in which switch Q is on, dedicated inductor L draws power from the input unit (in the case of FIG. 1A, PV solar panel strings 101). In particular, the inductor L is charged with input power for a period of time indicated by the feedback-controlled on duration of the PWM switch. This charging takes place by pushing the voltage v _sw at the terminals of switch Q up towards the input voltage v _in until the voltage v _sw at the terminals of the switch reaches the correct value. During the period in which switch Q is off, current flows from inductor L through diode D to charge the destined capacitor C, thus producing a steady voltage equal to the desired output voltage (in the case of mains connection, v=v ₀ =v _pk ). By using feedback control to adjust the on-duration for the on-off period of the switch Q of a correctly calculated fixed PWM frequency, the output voltage can be increased from v _in to the design peak voltage of the AC mains, v ₀ =v _pk . Thus, this voltage step-up circuit can generate a peak voltage suitable for output to the next DC/AC conversion module. The above circuit is called DC/DC boost converter or boost converter in the technical field.

As described above, the boost converter is intended to modify a time-varying voltage DC source (eg, PV sequences) into a DC source having a nearly constant voltage that can be equal to the AC mains peak voltage. Note that in order to prevent the peak supply voltage from being attenuated in one AC cycle during normal operation, sufficient capacitance is designed for capacitor C of the boost circuit shown in FIG. 2A, i.e. this capacitor C exists to keep the voltage nearly constant for one AC cycle. A capacitor that performs this described function is often referred to as a DC link capacitor in the art. Mains conditions allow very little voltage variation at the ends of the DC link. Therefore, the DC link capacitor is not intended to store large amounts of excess energy, as this would require a huge (and thus expensive) capacitor to stay within the maximum allowable AC mains voltage variation.

Fig. 2B shows a typical DC/AC conversion module 223 which consists of a choke LL, a controlled switch QQ controlled by a duty cycle regulator DFA, a diode DD, and a DC link capacitor CC.

The QQ switch switches at a high frequency (typically around 18 kHz in commercial products) with adjustable duty cycle. The QQ switch (often referred to as a PWM switch) is controlled by a pulse-width modulator (PWM) signal. The on duration of this PWM switch is controlled by the on duration controller DFA so that the AC power sequence produced by this conversion module 223 matches the power grid conditions. The depicted DC/AC conversion module 223 is called a buck converter in the art. A buck converter 223 associated with the DFA can convert a DC power source with a precisely specified peak voltage into a sinusoidal AC power sequence. This pulsed AC power train is sent through the switch bridge structure shown in FIG. 2C (which is an example of the polarity/timing controller 224 in FIG. 1A); and then through a transformer (eg, transformer 225 in FIG. 1A) and into a network (eg, network 300 in FIG. 1A). As described, the bridge structure of the switches acts as a polarity and timing controller.

As shown in FIG. 2C, when switches S1 and S2 are both on and switches S3 and S4 are both off, a positive voltage is applied at the ends of the load. Conversely, when switches S3 and S4 are on and switches S1 and S2 are off, a negative voltage is applied at the ends of the load. When this switching is controlled by a synchronizing regulator (not shown in FIG. 2C), this detects positive/negative voltage transitions (or zero voltage crossings) in the mains, this bridge structure 224 (combined with the duty cycle regulator DFA) can effectively control the output polarity and synchronism of a single-phase DC/AC converter.

The synchronous controller can timely adjust the time-varying adjustment of the PWM duty cycle; so that a pure sinusoidal power waveform is created, which is represented as cos ² (ωt+θ), with the desired AC corner frequency ω, with the desired peak AC voltage v _pk and with its phase θ synchronous with the corresponding mains power line. When combined with a fixed voltage DC input with parasitic inductance and parasitic capacitance in the network, the inductor LL and capacitor CC can be reduced or even omitted in practice. In the technical field, the terms converter and inverter (and for this subject matter, transformation and inversion) are used interchangeably, and thus are used interchangeably in this document.

The duty cycle controller DFA regulates the duty cycle as a function of time according to the design to turn on/off the switch QQ of the buck converter. Thus, with a properly designed circuit and a prepared peak voltage, this conversion module can produce the desired output voltage value, power waveform, frequency and phase that meets the design requirement, including the requirement of AC power grid conditions, and the phase of the corresponding mains power line. In the case of a grid-connected unit, an AC-synchronous regulator (typically built into the DC/AC converter) is used to drift the output value of the prepared AC power along with the mains in the event that the peak voltage of the mains drifts and/or in the event that when the mains frequency drifts. Such a generated AC power (not voltage) signal is shown in FIG. 2E. In other words, with the PWM power extraction device described above, the single-phase DC/AC converter can extract and convert DC power from a fixed voltage DC power source into AC power that meets the power grid conditions.

It is very important to note that the output power P(t) of the above single-phase inverter changes with time in the form of cos ² (ωt+θ). Thus, during a particular period of time, the energy delivered by the mains power line is equal to the sum of its time-varying sequence of output power during that period. The resulting integral value is only half the integration of the constant DC power of the power source over the same period of time. In other words, the above-described conventional single-phase inverter can only extract and convert at most only half of the power provided by the DC power source. The remaining and unused energy is thus equal to more than half of the available input energy. This remaining amount contributes to much of the excess energy described in the aforementioned patent publications.

For the purposes of the following analysis, let us assume that the DC power source has a constant power P _mx over a period of several AC power cycles. Fig. 2E shows the extracted DC energy pulse in one PWM cycle (having a period D). As will be demonstrated, the extracted DC power P _x is less than or equal to the DC power P _mx . The duty cycle d(t)/D in this PWM cycle is adjusted to a value equal to d(t)/D=cos ² (ωt+θ), so that the power produced is almost equal to Pxcos ² ((ωt+θ), which corresponds to the grid conditions , and where θ is the phase of the corresponding field line of the network Fig. 2E (in particular, the lower half of Fig. 2E) also depicts a power-time space (called energy space) with time interval D as one PWM period; - power equal to Pmx and with extracted power equal to P _x .

As shown in FIG. 2E, this space of energy is divided into 3 regions. Region-I represents the extracted DC energy pulse with the extracted power P _x ; and with a duration of time Dcos ² (ωt+θ), which is converted into single-phase AC power P(t)=Pxcos ² (ωt+θ) at an arbitrary time t, which corresponds to the PWM extraction time. Area-I is also called the extracted energy zone or the extracted energy area. The zone between the power Pmx of the power source and the power P _x is the region-W. Region-II is the zone after the extracted energy zone in PWM period D. The combined zone of regions-II and -III represents the excess energy zone in this energy space. The energy in the excess energy zone (area) is not extracted, converted into AC power, and thus not used in the usual way. Instead, this excess energy is eventually absorbed as heat.

Recall that the traditional single-phase DC/AC converter uses a voltage boost module to modify the DC power source with a time-varying voltage in

- 5 041589 DC power source with a practically constant and precisely defined voltage, such as the mains peak voltage. This DC source serves as the DC input source for the PWM extractor to extract and convert DC input DC power into a pulsing DC power signal. When the duty cycle in one PWM cycle is controlled by cos ² (ωt+θ) at time t (where the phase θ is the phase of the corresponding mains power line), the shape of the output power corresponds to the AC power condition. At a high level, the energy space of each PWM cycle consists of two regions, a extracted energy region (eg, region-I in FIG. 2E) and an excess energy region (eg, a combination of region-II and region-III in FIG. 2E). The extracted energy is thus converted into AC power and provided to the corresponding mains power line; while excess energy is converted into heat if not captured and stored in a device such as a MEUPT optimizer.

As described above, reference patent publications teach that the amount of excess energy is at least as large as the extracted energy when integrated over multiple AC time periods. In other words, a conventional single-phase DC/AC converter can only extract and convert at most half of the input DC power, that is, when a conventional single-phase DC/AC converter is used, at least half of the input DC power will become waste power; which is not extracted, not converted, not delivered to the power grid, not used by the load; and eventually turn into heat.

Reference patent publications also emphasize that the root cause of the inefficient power extraction of single-phase DC/AC converters remains in traditional 3-phase DC/AC converters. The reason is that, in essence, a 3-phase DC/AC converter consists of a set of 3 single-phase DC/AC converters that perform the function of extracting and converting, delivering the same time-averaged AC power to 3 pairs of power lines; with phase shifts of 120°.

Section 2. Extraction of energy in traditional 3-phase inverters.

There are three single-phase DC/AC converters built into the traditional 3-phase DC/AC converter. Each of the three single-phase DC/AC converters is equipped with one PWM power extractor. The three power extracting devices are controlled by a simultaneous regulator so that they run at the same frequency (which is called the PWM frequency). Fig. 3A shows three diagrams 301, 302 and 303 of respective PWM power extraction devices. Each circuit 301, 302 and 303 is the same as a single-phase energy recovery device and uses the same operating principles as described above. The single-phase power extractor 301 outputs the AC power of the A-phase and has a switch Q _A ; the single-phase power extraction device 302 outputs AC power of the B-phase and has a switch QB; and the single-phase power extractor 303 outputs C-phase AC power and has a switch Q _C . Simultaneous regulator 310 is applied to simultaneously turn on 3 switches Q _A , QB, Q _C in 3 power extracting devices, so as to start energy extraction at the same time, with the same frequency, but with different on durations.

Assume that the DC power source has a constant DC input power P _mx in one AC cycle. Also, suppose it is true that one PWM period is a small fraction of the entire AC cycle. Fig. 3B depicts the DC input power in one PWM cycle. In one PWM cycle, the energy extracted by the 3 energy extraction devices is symbolically represented in FIG. 3C. The level of extracted power is indicated as P _x in FIG. 3C which is below 1/3 of the DC input power P _mx in FIG. 3B. The duty cycle in this PWM cycle for the A-phase energy extractor is adjusted to cos ² (mt) (or sin ² (mt)), so that the extracted power is Px-cos ² (mt) (or Px-sin ² ( mt)), which corresponds to the condition of single-phase AC power. Similarly, the duty cycle in this PWM cycle for the B-phase power extractor is adjusted to cos ² (mt + 120°) (or sin ² (mt + 120°)), so that the extracted power is Px-cos ² (mt + 120°) (or Px-sin ² (mt + 120°)). Also, the duty cycle in this PWM cycle for the C-phase power extractor is adjusted to cos ² (mt - 120°) (or sin ² (mt - 120°)), so that the extracted power is Px-cos ² (mt - 120°) (or Px-sin ² (mt - 120°)). Also, the phase shift of 3-phase AC output powers is kept at 120° according to 3-phase power grid conditions.

Note that the energy extractions of a typical conventional 3-phase DC/AC converter (shown symbolically in FIG. 3C) overlap in the time domain. An energy extraction that overlaps in the time domain is referred to as a simultaneous energy extraction in this document; and a regulator (shown in FIG. 3A) that controls the simultaneous extraction of power is referred to as a simultaneous regulator herein.

The law of energy conversion (combined with the fact that the characteristics of the source of electrical energy can dynamically change) will dictate that the sum of the levels of three power extractions while extracting power should be no more than the input DC power source P _mx ; or P _mx > P _x + P _x + P _x ; or P _x < (1/3)P _mx . The total sum of conventional 3-phase AC output power is P(t)=P _x (sin ² (ωt) + sin ² (ωt + 120°) + sin ² (ωt - 120°)); or P(t)=P _x (cos ² (ωt) + cos ² (ωt + 120°) + cos ² (ωt - 120°)). It can be shown that (sin ² (ωt) + sin ² (ωt + 120°) + sin ² (ωt - 120°))=(cos ² (ωt) + cos ² (ωt + 120°) + cos ² ( ωt - 120°))=3/2. Thus, P(t)=(3/2)P _x < (3/2)-(1/3)P _mx =1/2 P _mx . Or due to the practical application of the simultaneous extraction of energy, the total amount of traditional

DC/AC converter cannot be more than (1/2)P _mx , half of the DC input power.

In other words, the total AC output power of a traditional 3-phase DC/AC converter (using simultaneous power extraction) cannot be more than half of the DC input power. Or, a traditional 3-phase DC/AC converter can only extract and convert less than half of the power produced in strings of photovoltaic (PV) solar panels when the converter is contained in PV power plants. And at least half of the DC power produced by PV becomes waste energy, which turns into heat at some point; unless this excess energy is captured and stored in a device such as a MEUPT device.

Recall, in fact, the traditional 3-phase DC/AC converter uses three single-phase DC/AC converters to perform the functions of extracting and converting as a single-phase DC/AC converter, delivering similar time-averaged AC power in 3 or 4 pairs of lines of force; with phase shifts of 120°. In other words, a traditional 3-phase DC/AC converter is a DC/AC converter that uses three single-phase DC/AC converters. Each single-phase DC/AC converter draws half of its DC input power (which is equal to 1/3 of its DC input power); converts the amount of extracted DC power into AC power with a phase shift of 120°; and sends three single-phase AC power outputs to a 3- or 4-wire power grid. Each power line carries one single-phase AC power with the same frequency (AC power frequency), with the same time-averaged power; but with phase shifts of 120°. The terms power wires and power cables are used interchangeably in the art and in this document.

According to the result described in the reference patent publications and again confirmed in the theoretical findings above, each single-phase converter (in a 3-phase DC/AC converter) extracts and converts less than half of the DC input power (which is less than or equal to 1/3 of the generated DC -power) in each single-phase AC power output signal. Therefore, the total maximum (extracted and converted) 3-phase AC power output from any conventional 3-phase DC/AC converter at any time can be at most only half the DC power produced; those. P(t)=3-(1/2)-(1/3)Pmx=(1/2)Pmx.

To emphasize, the above theoretical findings reveal a serious implication that is introduced through common design practice for the use of simultaneous power extraction in the traditional 3-phase DC/AC converter industry. This common design practice has been followed in the 3-phase DC/AC converter industry for a long time; even without the knowledge of the industry about the existence of a serious consequence. This practice of simultaneously extracting energy results in a serious consequence, which is revealed for the first time in the above theoretical conclusions. This serious consequence is that the total sum of the 3 AC output powers from a traditional 3-phase DC/AC converter is less than half the DC input power. This discovered common design practice has been validated as a long-term practice in the green renewable electricity industry; particularly in the PV-solar industry.

To put it another way, the traditional PV-solar energy industry puts into practice the simultaneous extraction of energy. In turn, the law of energy conversion (combined with the fact that the characteristics of the electric power source can dynamically change) will dictate that the total sum of the three levels of extracted power P _x must be less than a third of the source PV generated DC power P _mx (i.e. P _x < (1/3)P _mx ). Then it can be shown that the total sum of conventional 3-phase AC output powers is P(t)=(3/2)P _x ; which is equal to (3/2)-(1/3)-P _mx < (1/2)P _mx or less than half of the PV-produced DC power. Therefore, at least half of the PV-produced DC power becomes waste power when using a traditional 3-phase DC/AC converter. This amount of excess energy is eventually converted into heat; unless this excess energy is captured and stored in a device such as the MEUPT optimizer.

As described above, at least half of the DC input power can become wasted power when using a traditional 3-phase DC/AC converter. With this information disclosed in this document, the question may be: Can more than one additional 3-phase DC/AC converter be used to extract, convert excess DC energy to provide AC power?. The answer is no for the reasons described later in this document.

- 7 041589

As shown in FIG. 4, two identical sets of 3-phase DC/AC converters 4210 and 4220 (each with advertised power rating P _mx ) are connected to a 4110 PV generator providing a source of P _mx DC power without a device (such as a MEUPT optimizer) to capture and save excess energy. The power conversion law will allow each of the two 4210 and 4220 DC/AC converters connected in parallel to consume only half of the total DC input power at the power level P _mx (i.e. consume only 1/2 P _mx for each converter as input power). In other words, each of two identical 3-phase DC/AC converters has a DC input power equal to 1/2 P _mx .

Recall that 3 energy extraction devices (in a traditional 3-phase DC/AC converter) are used in practice to extract energy simultaneously. As described above, each of the 3-phase DC/AC converters can only convert half of the DC input power to produce AC output power; which is equal to (1/2)-(1/2)-P _mx , or 1/4 P _mx . The total output AC powers of the two converters are equal to 2-(1/4)P _mx ; which is still (1/2)P _mx . A similar analysis can be performed to reach the same conclusion for cases that are either supplied with a higher power rating or supplied with a large number of DC/AC inverters. Again, energy extraction is the root cause, which results in more than half of the DC power produced becoming waste energy.

The next question might be: can we perform an experiment to prove definitively that half of the power produced by the sun becomes waste energy when extracted through a traditional 3-phase DC/AC converter? The following section describes an experiment to prove that at least half of the power produced by the sun becomes waste energy when extracted through a traditional 3-phase DC/AC converter.

Section 3. Experimental exhaustive proof.

The MEUPT optimizer is designed to capture/use this remaining power, the surplus power. Including the MEUPT optimizer, the following describes experimental setups and step-by-step executions of the experiment, which is intended to conclusively prove that at least half of the power produced by the sun becomes waste energy when extracted by a traditional 3-phase DC/AC converter.

Fig. 6A shows the initial setup of a PV power plant 6000A containing 2 AC power production units 6100A and 6200A. The AC power units 6100A and 6200A each practice blind MPPT matching; and provides 3-phase AC power to the 6600A grid. The 6100A AC power generation unit consists of a 6110A DC power generator (30 kW) and a 6130A 3-phase DC/AC converter (30 kW). The 6200A AC power generation unit consists of a 6220A DC power generator (30kW) and a 6230A 3-phase DC/AC converter (30kW). The 6110A power generator uses 2 parallel-connected PV strings 6111A and 6112A to generate DC electricity. The 6220A power generator uses the other 2 solar panel strings 6221A and 6222A in parallel to generate DC electricity. Each of the 4 PV strings consists of 25 solar panels connected in series; each panel is designed to produce 300 watts of power at high noon and under clear skies.

The 6110A DC power generator supplies DC power to the 6130A 3-phase DC/AC converter; and the 6220A DC power generator supplies DC power to the 6230A 3-phase DC/AC converter. These two 6130A and 6230A converters then convert the applied DC power to 3-phase AC power. In the experiment, the AC power output of the power generation units 6100A and 6200A was measured using two 3-phase AC wattmeters (in kW) 6351A and 6352A, respectively. The AC power production (in kWh) of these two power generation units 6100A and 6200A was also measured by two kWh meters 6361A and 6362A, respectively. The 3-phase AC power produced was then provided to the 6600A network through a 6500A transformer. PV power plant worked; and the power production of two units 6100A and 6200A for AC power production was measured for 7 days.

The readings of the two kWh meters showed equal values every day during this period of time. This provides high confidence that all elements of the two power generation units 6100A and 6200A (including two sets of measurement tools) are nearly identical. After this step, one of the two AC power production units 6200A was kept unchanged, while the other AC power production unit 6100A was modified with a different configuration 6100B, which is shown on the left side of FIG. 6B.

Power generation unit 6200B in FIG. 6B is the unmodified power generation unit 6200A of FIG. 6A. Also, elements 6351B, 6361B, 6352B, 6362B, 6500B, 6600B in FIG. 6B are the same as elements 6351A, 6361A, 6352A, 6362A, 6500A, 6600A, respectively, in FIG.

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6A. In addition, although the configuration of the power production unit 6100B is different in FIG. 6B compared to power generation unit 6100A in FIG. 6A, some of the elements of power generation unit 6100B in FIG. 6B are the same as those included in the power generation unit 6100A in FIG. 6A. For example, PV sequences 6111B and 6112B in FIG. 6B are the same as PV sequences 6111A and 6112A, respectively, in FIG. 6A. Similarly, the DC/AC converter 6130B in FIG. 6B is the same as the DC/AC converter 6130A in FIG. 6A.

The following six (6) steps describe how the power generation unit 6100A has been modified into configuration 6100B and is described relative to the left side of FIG. 6B. Step 1 was to add a set of 6311V decoupling diodes between the 6111V and 6112V solar array strings and the 6130V 3-phase DC/AC converter that implements blind MPPT matching. Step 2 was to add the 6410V energy reservoir to the configuration. Step 3 was to connect the 6410V energy reservoir to the DC terminals of the 6130V DC/AC converter through another set of 6312V decoupling diodes and through switch SW1. Step 4 was to add another 6130S 3-phase DC/AC converter (20kW) to the configuration, with this 6130S converter operating under the control of the dedicated 6420B MEUPT controller. Step 5 was to connect the 6130S DC/AC converter to the 6410V energy tank through another set of 6313V decoupling diodes and through switch SW2. Step 6 was to connect the output terminals of the 6130S transmitter to the 6351B and 6361B Power and Energy Measurement Toolkit via switch SW3. Note that said set of decoupling diodes may be such diodes as are called blocking diodes in the art. Note also that switches SW1, SW2 and SW3 are added as shown in FIG. 6B so that appropriate devices can be introduced (or removed) into experiments at the correct time in the intended experimental steps described below.

On the first night, after the above modification was done, SW1 was turned on while switches SW2 and SW3 were turned off. The 6130B and 6230B converters started working early the next morning. The 6351V and 6352V power meters measuring the two outputs of the 6100V and 6200V power units showed the same reading. The 6410V tank also started to charge as indicated by measuring the high voltage at the 6410V tank terminals. The system worked as described for the entire first day. The measured energy supplied from the two power generation units 6100V and 6200V was the same; as shown on the 6361V and 6362V kW meters. This experimental step demonstrated that the added decoupling diode sets 6311B and reservoir 6410B did not change the power and energy production of the power production unit 6100B.

Switches SW1, SW2 and SW3 were turned on at night after the first day of operation (the second night). The 6130B and 6230B drives started running early in the early morning (second day), while the 6130S drive started running at a lower power conversion level approximately 15 minutes after the 6130B and 6230S drives started running. Thereafter, the 6130S increased its conversion power level approximately every 2 minutes; which is consistent with the design of the controller and the increase in the energy level of the reservoir. The 6351V power meter reading (for the 6100V unit) nearly doubled the reading of the 6352V power meter (for the 6200V unit) for the entire day - almost to sunset. The energy supplied from the two power generation units 6100V and 6200V at the end of the second day was obtained from the readings of two kWh meters. The result showed that the power supplied from the modified 6100V power generation unit was more than twice that of the energy supplied from the unmodified 6200V power production unit. For the next six consecutive days, switches SW1, SW2 and SW3 remained on, and the energy provided from the modified 6100V power generation unit was consistently twice that of the 6200V power production unit each day.

Switches SW2 and SW3 were turned off the next night. The measured energy supplied from the power generation units 6100V and 6200V returned to the same level for the next 5 consecutive days, during which the switches SW2 and SW3 remained off. The next night switches SW2 and SW3 were turned on again. The measured energy production of the power production unit 6100B was again more than double that of the energy production of the power production unit 6200V each day for the next 5 consecutive days with switches SW2 and SW3 remaining on.

As described above, stepping through this experiment definitely proves the existence of excess energy in a PV power plant, as predicted in section 2. In particular, in a PV power plant, when the generated DC power is extracted by a 3-phase DC/AC converter, about half of the PV power generated still exists as residual surplus energy.

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There are two ways to mitigate the unwanted effect described above. The first way is to follow the principles described in patent reference publications to include a MEUPT optimizer in the power system. Another way is to follow the principles described in this document, which propose to put into practice the sequential energy extraction associated with the proposed adjustments of the duration of the inclusion for the A-phase, B-phase and C-phase.

Section 4. Proposed Sequential Energy Extraction.

The principles described in this document propose to practice sequential energy extraction for the A-phase, B-phase and C-phase, which can prevent the occurrence of overlapping energy extraction in time. When applying in practice the sequential extraction of energy in every single PWM period, the A-phase extracts the DC energy first in a timely manner; The B-phase then extracts DC energy in a timely manner immediately after the A-phase is extracted; and finally, the C-phase extracts the remaining DC energy in a timely manner. Thus, the level of energy extraction, P _x in each phase can reach its maximum value, which is equal to P _mx . This consequence is different from the practice of simultaneous energy recovery, in which the energy recovery level may be only one third of (1/3)P _mx .

To make the following analysis intuitive and without loss of generality, assume that the AC frequency is 50 Hz and the PWM frequency is 18 kHz. This assumption makes the AC power phase angle advance exactly 1° during each PWM period. Fig. 5A depicts proposed circuits for this new energy extraction device. The power recovery circuits are similar to those of the conventional power recovery device shown in FIG. 3A. Note that the concurrent controller 310 of the conventional power recovery apparatus shown in FIG. 3A is now being replaced by the serial regulator 510 shown in FIG. 5A.

To emphasize, a power recovery device controlled by a simultaneous regulator is a simultaneous power recovery device, and a power recovery performed by a simultaneous power recovery device is a specific practice of simultaneous power recovery. On the other hand, a power recovery device controlled by a sequential regulator is a power recovery device in series; and the power recovery performed by the sequential power recovery device is a practical application of the proposed sequential power recovery.

As an embodiment, FIG. 5B shows a 3-phase DC/AC converter using a series regulator to control the 3-phase power extractions. In this practical application of energy extraction, A-phase energy extraction is configured to start at the beginning of the PWM cycle with a time duration d _A (t); the energy extraction of the B-phase is performed with the ability to start at the end of the extraction of the A-phase with a duration of time d _B (t); and the C-phase energy extraction is operable to follow the end of the B-phase energy extraction with a duration of time d _C (t). These 3 energy extractions are made to be executed sequentially and non-stop. It can be seen that there is no time-domain overlapping energy extraction as depicted in FIG. 5V. In reality, there may be some time gap between the end of one energy extraction and the start of another energy extraction. However, this time period in each PWM cycle can be kept quite small, and preferably below 33%, 20%, 10% or even 1% of the PWM cycle. Therefore, the power extraction level Px in each phase can be designed to be equal to its maximum value, the DC input power P _mx ; as opposed to the fraction (1/3) of the input power that is used in the simultaneous extraction of energy.

Let's set the duration of one PWM period as D. The duration of switching on for extracting the A-phase is defined as d _A (t)/D; the duration of the inclusion for the extraction of the B-phase is equal to d _B (t)/D; and the duration of the inclusion for the extraction of the C-phase is then equal to d _C (t)/D. In addition, the principles described in this document suggest assigning the values of these three duty factors as: d _A (t)/D=2/3 cos ² (mt), dB(t)/D=2/3 cos ² ( mt + 120°), and dc(t)/D=2/3 cos ² (mt - 120°). The respective time interval for the 3 energy extractions can then be assigned in accordance with the respective duty cycle values. Note that the total time durations of these three energy extractions, dA(t) + d _B (t) + d _C (t), are exactly equal to D, the time interval of one PWM period.

As mentioned above, the time interval of one PWM period is equal to the time interval of 1° phase advance in the AC power cycle; so the 3 phase shifts will be 120° apart plus or minus 1°, which is good within the tolerance of the existing mains. The total amount of power transmitted over three pairs of power lines is then P(t)=PA(t) + PB(t) + PC(t)=Pmx (2/3) (cos ² (mt) + cos ² (mt + 120°) + cos ² (mt - 120°))=P _mx (2/3) (3/2)=P _mx . In other words, the total power transmitted over 3-phase power lines at any time is almost equal to the amount of DC power produced. In other words, when applying in practice the sequential extraction

- 10 041589 energy, there is no (or little) remaining excess energy. In other words, when a series power extraction device is included, a 3-phase DC/AC converter can extract all or substantially all of the DC power produced with zero or substantially zero waste energy.

To reiterate, the principles described in this document suggest starting each 3-phase energy extraction in sequence and without stopping. When the 3 phase power extractions become successive events, the level of the extracted power can be set equal to the input DC power P _mx . The principles described in this document further suggest having 3 duty cycles equal to 2/3 cos ² (ωt) for A-phase, 2/3 cos ² (ωt + 120°) for B-phase, and 2/3 cos ² (ωt -120°) for Sphase. Thus, these 3-phase power extraction events can take place sequentially; and 3 power draw events can end exactly in the same PWM period without problems; and all AC power phase shifts between A-phase, B-phase and C-phase are 120° within an acceptable tolerance of 1°.

Therefore, when the 3-phase DC/AC converter contains the proposed sequential energy extraction device, the designed DC/AC converter can then extract and convert all the power produced by the sun, P _mx without any excess energy remaining; and the produced AC output power can meet the conditions of a 3-phase AC power grid.

Section 5. Design Considerations for a Sequential Regulator.

In one embodiment, a PWM start can be applied to start A-phase extraction with a time duration of (2/3)Dcos(ωt); then apply a change in the A-phase extraction signal (from the on period to the off period) to initiate the start of the B-phase extraction with a time duration of (2/3)Dcos(ωt + 120°); and then apply a switch of the B-phase extraction signal (from the on period to the off period) to initiate the start of the C-phase extraction.

In another embodiment, since one PWM period produces a well-defined AC phase advance (say 1°), a table can be constructed with the well-defined final A-phase extraction time as the first value and the well-defined final B-phase extraction time as second meaning. This table may contain a number of columns to make use of the entire power cycle (say 180 columns for 180°). Note that a 360° voltage cycle creates a power cycle every 180° because power is proportional to the square of the voltage, and squaring the voltage sine waveform creates a power wave with twice the frequency.

With two consecutive time lines in each column of this time table, the project can periodically assign two time lines in one PWM time period as the start time (PWM start) and end time (first time value in the column) to extract A-phase energy; for the start time (the first time value in the column) and the end time (the second time value in the column) to extract the B-phase energy; and for start time (second time value in column) and end time (end of PWM) to extract C-phase energy. The end of 180 columns means the end of DC/AC conversion for one AC power cycle. The process can then be repeated for the next round of DC/AC inversion, and so on. A clock with a time resolution greater than 1/18000 s (or 5 µs) must be used in this embodiment.

The timing module in a traditional 3-phase DC/AC converter can be used to time PWM start at maximum and minimum AC power cycles; so that the phase of the produced AC power can drift along with the phase/frequency drift that occurs in the mains from time to time. The principles described in this document can also use the device to accommodate for phase/frequency wander.

Section 6. Results.

As described in section 1, the traditional single-phase AC converter uses a PWM extractor to extract the DC input power. When the duty cycle in one PWM cycle is controlled by cos ² (ωt) (or sin ² (ωt)) at time t, the AC output power is in accordance with the grid conditions. Note that there are two regions in the energy space of each PWM cycle; one is an area of extracted energy, while the other is an area of excess energy. Reference patent publications teach that the amount of excess energy is at least as large as the extracted energy when integrated over multiple AC time periods. In other words, a single-phase DC/AC converter can only extract and convert at most half of the DC input power. The extracted energy is converted into AC power and provided to the power grid; while excess energy is converted into heat unless captured and stored in a device such as a MEUPT device.

As described in Section 2, a traditional 3-phase DC/AC converter uses three identical single-phase DC/AC converters. Each single-phase DC/AC converter extracts and converts half of its DC input power to AC power. Note that due to the practical application of simultaneous energy extraction, this DC input power can be equal to

- 11 041589 only one third of the generated DC power at most. Therefore, a 3-phase DC/AC converter converts and extracts at most half of the generated DC power. The AC output powers of these three single-phase converters are phase shifted by 120° from each other. Also, these 3 single phase AC outputs are sent to the grid via 3 or 4 power wires.

To put it another way, the traditional solar PV industry uses a traditional DC/AC converter that puts into practice the simultaneous extraction of energy, so that the law of conservation of energy dictates that the total sum of the three extracted power levels P _x must be less than a third of the PV-generated source P _mx DC power (i.e. P _x < (1/3)P _mx ). It can then be shown that the sum of conventional 3-phase AC output powers is P(t)=(3/2)P _x at best, which is less than (1/2)P _mx , or half of the PV-generated DC- power. Therefore, at least half of the PV-produced DC power becomes waste power when using traditional 3-phase DC/AC converters. This amount of excess energy will turn into heat eventually if not captured and stored in a device such as a MEUPT optimizer.

The theoretical conclusions described in Section 2 reveal a major implication that is introduced by the common design practice in the traditional 3-phase DC/AC converter industry. This common design practice has been adopted by the 3-phase DC/AC converter industry for a long time; even without the knowledge of the industry about the existence of a serious consequence. It is common practice that traditional 3-phase DC/AC converters put into practice the simultaneous extraction of energy. This practice can definitely result in a serious consequence, which is revealed for the first time in the industry in the above theoretical findings. This serious consequence is that the total sum of the 3 AC output powers from the 3-phase DC/AC converter is less than half of the DC input power. This discovered common design practice has been validated as a long-term practice in the green renewable electricity industry; particularly in the PV-solar industry.

The two ways to mitigate the above undesirable consequence are 1) following the principles described in patent reference publications to incorporate a MEUPT optimizer into an energy system, or 2) following the principles described herein which propose to put into practice the sequential energy recovery associated with suggested adjustments for the duty cycle for A-phase, B-phase and C-phase.

Section 4 describes the proposed sequential energy recovery with proposed duty cycle adjustments. When applying the described sequential energy extraction in practice in all PWM periods without exception, the A-phase properly extracts the DC energy first in a timely manner; The B phase properly extracts DC energy in a timely manner immediately after the A phase is extracted; and the C-phase properly extracts the DC power in the long run in a timely manner. Thus, the level of energy extraction, P _x in each phase can reach its maximum value, which is equal to the generated power, P _mx . This is different from the consequences resulting from the practice of traditional simultaneous energy extraction; which forces the level of energy extraction, P _x in each phase to reach only 1/3 of P _mx at best.

The principles described in this document assume that the start time of energy extraction for each phase is controlled by a sequential controller. The energy extractions of the 3 phases become successive events; so that the power extraction level can be equal to the input DC power P _mx . The principles described in this document further suggest having 3 duty cycles equal to 2/3 cos ² (ωt) for A-phase, 2/3 cos ² (ωt + 120°) for B-phase, and 2/3 cos ² (ωt - 120°) for the C-phase. These 3-phase power extraction events can then take place consecutively without stopping; and 3 power extraction events can also end in exactly the same PWM period. In addition, AC power phase shifts between A-phase, B-phase and C-phase can be designed to be 120° ± 1°. Therefore, when the 3-phase DC/AC converter contains the proposed series power extraction device, the newly designed DC/AC converter can extract and convert all (or substantially all) of the generated power, Pmx, with no (or little) remaining excess energy. In addition, the output AC powers produced can easily meet the conditions of a 3-phase AC power grid.

The present invention may be embodied in other characteristic forms without departing from its spirit or inherent characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by the preceding description. All changes that appear within the meaning and scope of the equivalence of the claims must be covered by its scope.

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Claims (12)

CLAIM 1. A sequential power recovery control device that is configured to control the DC power recovery sequence and power recovery time duration of three single-phase DC/AC converters that are built into the 3-phase DC/AC converter: the first single-phase converter in the 3-phase DC/AC converter comprises a first pulse-width modulation (PWM) extraction device configured to extract the first AC power from a DC power source such that the first AC power has a frequency and a first a phase that corresponds to the power grid conditions for AC power that exists in the first pair of power supply cables of the 3-phase AC power grid; the second single-phase converter in the 3-phase DC/AC converter comprises a second PWM extractor configured to extract the second AC power from the DC power source such that the second AC power has the same frequency as the first AC power, but with the second a phase that corresponds to the power grid conditions for AC power that exists in the second pair of power-supplying cables of the 3-phase AC power grid; and the third single-phase converter in the 3-phase DC/AC converter comprises a third PWM extractor configured to extract the third AC power from the DC power source such that the third AC power has the same frequency as the first AC power, but with a third phase that corresponds to the AC power conditions that exist in the third pair of power-delivering cables of the 3-phase AC power grid, wherein the sequential power extraction control device comprises a serial controller configured to instruct the first PWM extraction device to have a first duty cycle, during during which the first PWM extractor performs power extraction from the DC power source, instructs the second PWM extractor to have a second on duration during which the second PWM extractor performs power extraction from the DC power source, and instructs the third PWM extractor to to have a third on duration during which the third PWM extractor performs power extraction from the DC power source; so that the first, second, and third on-durations do not overlap in the time domain, and so that the first, second, and third PWM extraction devices perform extraction sequentially; wherein the sequential controller is further configured to control the duty cycle adjustments for the first, second and third duty cycles to thereby regulate one or more of the first AC power, the second AC power and the third AC power, the duty cycle adjustments being made so that the first duty cycle is 2/3 cos ² (wt), the second duty cycle is 2/3 cos ² (wt + 120°) and the third duty cycle is 2/3 cos ² (wt-120°) as a function of time t in the AC power cycle. 2. The sequential power recovery control device according to claim 1, wherein the sequential regulator comprises a clock generator; a time table with a plurality of columns, each column corresponding to a corresponding section of the AC power cycle and representing sufficient information to determine the first, second, and third on-durations for the corresponding section of the AC power cycle; and an activation device configured to use sufficient information to sequentially activate the first, second, and third PWM extraction devices. 3. The sequential power recovery control device according to claim 2, wherein the sufficient information is the start time and the end time for the first duty cycle. 4. The sequential power recovery control device according to claim 2, wherein the time table contains at least two values in each of the plurality of columns; the first value is D(2/3)cos ² (ωt), and the second value is D(2/3)cos ² (ωt + 120°), where D is the PWM period, at is the time in the AC power cycle. 5. The sequential power recovery control device according to claim 1, wherein the sequential regulator is configured to control duty cycle adjustments in response to signals from the control device. 6. The sequential power recovery control device according to claim 1, wherein the interval between the first duty cycle and the second duty cycle is at most one third of the PWM ratio. 7. The sequential power recovery control device according to claim 1, wherein the interval between the first duty cycle and the second duty cycle is at most one fifth of the PWM ratio. - 13 041589 8. The sequential power recovery control device according to claim 1, wherein the interval between the first duty cycle and the second duty cycle is no more than one twelfth of the PWM ratio. 9. The sequential power recovery control device according to claim 1, wherein the interval between the first duty cycle and the second duty cycle is at most one hundredth of the PWM ratio. 10. A serial draw control device for use in a 3-phase DC/AC converter that comprises a first single-phase DC/AC converter comprising a first pulse-width modulation (PWM) draw device to extract the first AC power from the DC power. , which has a first phase and which matches the power grid conditions, a second single-phase DC/AC converter, comprising a second PWM extractor to extract from the DC power a second AC power, which has a second phase and which matches the grid conditions, and a third single-phase A DC/AC converter comprising a third PWM extractor to extract from the DC power a third AC power which has a third phase and which is suitable for mains conditions, the serial extract control device comprising a serial controller which instructs the first PWM extractor to have the first on-time during which th first PWM extractor performs extraction, instructs the second PWM extractor to have a second on duration during which the second PWM extractor performs extraction, and instructs the third PWM extractor to have a third on duration during which the third PWM extractor performs extraction; and so that the first, second, and third duty cycles do not overlap, so that the first, second, and third PWM extractors perform the extraction in sequence, the sequential controller being configured to control the duty cycle adjustments for the first, second, and third duty cycles depending on the current part of an AC power cycle of at least one of the first AC power, the second AC power and the third AC power, the duty cycle adjustments are such that the first duty cycle is 2/3 cos ² (ωt), the second duty cycle is 2/3 cos ² (ωt + 120°) and the third duty cycle is 2/3 cos ² (ωt 120°) as a function of the time t in the AC power cycle. 11. A 3-phase DC/AC converter system comprising A 3-phase DC/AC converter that contains a first single-phase DC/AC converter that contains a first pulse-width modulation (PWM) extraction device for extracting from DC power the first AC power that has a first phase and that matches power grid conditions , a second single-phase DC/AC converter comprising a second PWM extractor for extracting from the DC power a second AC power which has a second phase and which is suitable for power grid conditions, and a third single-phase DC/AC converter comprising a third PWM device extracting for extracting from the DC power a third AC power which has a third phase and which corresponds to the power grid conditions; and a sequential controller that causes the first PWM extraction device to have a first on duration during which the first PWM extraction device performs extraction, instructs the second PWM extraction device to have a second on duration during which the second PWM extraction device performs extraction, and instructs the third PWM extractor to have a third on duration during which the third PWM extractor performs extraction; and so that the first, second, and third duty cycles do not overlap, so that the first, second, and third PWM extractors perform the extraction in sequence, the sequential controller being configured to control the duty cycle adjustments for the first, second, and third duty cycles depending on the current an AC power cycle fragment of at least one of a first AC power, a second AC power, and a third AC power, the sequential controller comprising a clock generator; time table with a plurality of columns, each column corresponding to a corresponding AC power cycle fragment and representing sufficient information to determine the first, second and third duty cycles for the corresponding AC power cycle fragment, the time table containing at least two values in each of the set columns; the first value is D(2/3)cos ² (ωt), and the second value is D(2/3)cos ² (ωt + 120°), where D is the PWM period, at is the time in the AC power cycle; and an activation device configured to use sufficient information to sequentially activate the first, second, and third PWM extraction devices. 12. The 3-phase DC/AC converter system of claim 11, wherein the sufficient information is the start time and end time for the first duty cycle. -