Power factor correction and power balance control of three-phase single-stage AC / DC converter

Abstract

The control strategy of the phase-modularized three-phase single-stage AC / DC converter has the functions of power factor correction and power balance at the same time, and double power frequency ripples generated by unbalance of three-phase input voltage in the output voltage of the converter can be greatly eliminated. Even if unbalanced input voltage and power loss exist, the output capacitance of the three-phase power converter can be very small. A high power factor (greater than 0.99) can be realized under balanced and unbalanced input phase voltage conditions with input voltage harmonic waves and without input voltage harmonic waves.

Description

[0001] Priority claim

[0002] This application is a Chinese translation of the international application PCT / CA2023 / 050867 filed on June 21, 2023, which is filed in China. This application claims priority to U.S. patent application No. 63 / 354,529 filed on June 22, 2022, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention generally relates to a controller and a control method for a three-phase AC-DC converter. Specifically, the present invention relates to a controller and a method, which use a feedback loop for power balancing of a phase-modular three-phase AC-DC converter, and can substantially eliminate the double power frequency ripple in the converter output voltage caused by the converter input power imbalance. Background Art

[0004] Three-phase AC-DC converters play an important role in high-power systems directly connected to the utility grid. The power factor correction (PFC) rectifier provides a sinusoidal current with a power factor close to unity and a stable bus voltage. Traditionally, a three-phase rectifier consists of two independent cascade converters. The first stage is usually a three-phase AC-DC converter for power factor correction, and the second stage is a DC-DC converter that provides voltage isolation and regulation. Many three-phase two-stage AC-DC converters with different topologies have been proposed. The general structure of a two-stage three-phase AC-DC converter with PFC is shown in Figure 1. The first part is usually a non-isolated three-phase boost converter that performs PFC, and the second stage is an isolated DC-DC converter that performs voltage regulation. The unbalanced three-phase input voltage does not harm the output voltage of the two-stage AC-DC converter because the second-stage DC-DC converter can regulate the output voltage to a pure DC level. However, the two-stage approach also has disadvantages, such as low conversion efficiency due to the presence of multiple power processing stages, and low power density due to the presence of large passive components such as boost inductors, DC bus electrolytic capacitors, and a large number of components. Since the life of the DC link electrolytic capacitors is short, typically less than 5000 hours, the reliability of the converter based on the two-stage approach is reduced.

[0005] The use of three separate AC-DC converter modules in a three-phase system, known as phase-modular three-phase power conversion, is an interesting approach to three-phase AC-DC conversion, since all knowledge of single-phase PFC converters can be used in three-phase systems. In addition, the reactive power in a 120-degree phase-shifted three-phase system cancels out, eliminating the need for large output capacitors. The phase-module structure can be implemented with two stages, which typically requires relatively large DC link capacitors on each phase and results in lower power conversion efficiency compared to a single-stage approach. Different single-stage topologies have been investigated in the phase-modular three-phase AC-DC converter structure, including an active clamp boost converter. [1] , Flyback Converter [2] , SEPIC Converter [3] , Zeta Converter [4] , Cuk converter [5] , Full-bridge converter [6] , push-pull converter [7] , Dual Active Bridge (DAB) Converter [8] and LLC resonant converter [9] Most of these references only discuss the implementation and power circuit performance in a four-wire or three-wire three-phase system using conventional current control methods.

[0006] A major challenge faced by three-phase AC-DC converters is the occurrence of double-line frequency output voltage ripple in the case of unbalanced three-phase input voltage. This problem is caused by the unbalanced power sharing between the three phases due to the unbalanced three-phase input voltage and the fixed current reference values ​​of each phase. In a single-stage AC-DC converter, this problem becomes very serious because there is no second-stage DC-DC converter to eliminate the low-frequency voltage ripple caused by the unbalanced input voltage. Double-line frequency voltage ripple at the output end will produce double-line frequency current ripple, which will reduce the efficiency and service life of the DC load. For example, in a battery charging system, this will cause overheating and shorten the service life of the battery pack, which is an expensive part of any system.

[10] Therefore, a single-stage three-phase AC-DC converter requires a power balancing circuit to fully utilize the small output capacitor characteristics.

[0007] There are few studies on the effect of three-phase input voltage imbalance on the output of phase-modular three-phase AC-DC converters. In references

[11] and

[12] , power balancing control is implemented by calculating the inductor current of each phase based on the input voltage, output voltage and output current. By calculating the inductor current, the output voltage of the converter becomes independent of the changes in input voltage and DC load current. Reference [8] implements a simple power control balance in a phase-modular three-phase DAB AC-DC converter. This method reads the output current of each phase through a low-pass filter and then creates a reference current based on the input voltage for power balancing. Therefore, this method has a slow dynamic speed and requires additional sensing circuits.

[0008] In some approaches, input admittance balancing techniques are used for unbalanced input voltages to achieve different goals. In references

[13] and

[14] , input admittance balancing techniques are used in two-stage three-phase AC-DC converters with Y-connected input rectifiers. In these topologies, the input admittance is balanced so that the virtual neutral voltage remains stable even if the grid voltage is unbalanced. In reference

[15] , input admittance balancing is used in three-phase single-stage AC-DC converters to limit the excessive current through each module in the case of unbalanced input voltage. In all these cases, the power distribution between the modules should be unbalanced to achieve the desired goals, and the dynamic speed of the system is slow due to the presence of multiple loops with their own filters in the control system.

[0009] Technical content

[0010] According to one aspect of the present invention, there is provided a controller for a three-phase power converter, comprising: a first circuit for determining the instantaneous input power of each phase; a feedback circuit for determining at least one power control signal according to the output of the three-phase power converter; a second circuit for adjusting the instantaneous input power of each phase using the power control signal to achieve power balance of the three phases; wherein the double power frequency ripple in the output power of the three-phase power converter is substantially eliminated.

[0011] In one embodiment, a first circuit detects the input current and input voltage of each phase and determines the instantaneous input power of each phase; a feedback circuit detects the output voltage of the three-phase power converter and determines the power control signal of each phase; a second circuit uses the power control signal of each phase and the characteristics of the input voltage of each phase to determine the reference power signal of each phase, and compares the reference power signal of each phase with the instantaneous input power of each phase to generate a control signal for each phase; and an output circuit uses the control signal of each phase to generate a drive signal for a switching device of a power circuit of each phase of the three-phase power converter to achieve power balance of the three phases.

[0012] In one embodiment, the input voltage characteristic of each phase is determined according to 1-cos(2ωt).

[0013] In one embodiment, a feedback circuit detects an output voltage and an output current of a three-phase power converter; wherein a controller uses the detected output voltage to determine a power control signal for each phase, and uses the power control signal for each phase and characteristics of the input voltage for each phase to determine a reference current signal for each phase; wherein the controller uses the detected output current and voltage to determine the output power of the three-phase power converter; wherein the output power is used to determine an instantaneous input current for each phase, which is used together with the reference current signal for each phase to generate a control signal for each phase.

[0014] In one embodiment, the input voltage characteristic of each phase is determined according to sin(ωt).

[0015] In one embodiment, the feedback circuit determines the power control signal of each phase by detecting the output voltage and the output current of each phase of the three-phase power converter, and determines the reference power signal of each phase by using the power control signal of each phase and the characteristics of the input voltage of each phase.

[0016] In one embodiment, the input voltage characteristic of each phase is determined according to 1-cos(2ωt).

[0017] In one embodiment, the first circuit includes an average input current calculator for calculating an average input current of each phase in each switching cycle.

[0018] In one embodiment, the feedback circuit determines the average value of the output current of each of the three phases detected within half a power frequency cycle, compares the output current of each phase with the average value of the three-phase current, and places the comparison output of each phase in a compensator to obtain an adjusted power signal for each phase.

[0019] In one embodiment, the input voltage characteristics are determined at the zero crossings of each phase input voltage.

[0020] In one embodiment, a feedback circuit compares a sensed output voltage of a three-phase power converter with a reference voltage and utilizes the comparison output to obtain an appropriate amplitude of at least one power control signal.

[0021] The present embodiment may include an output circuit including a pulse width modulation (PWM) module that generates a switching signal for a switching device of a power converter module of a three-phase power converter.

[0022] The present embodiment may include an output circuit including a pulse frequency modulation (PFM) module that generates switching signals for switching devices of a power converter module of a three-phase power converter.

[0023] This embodiment may be used for a three-phase power converter configured for a three-phase power supply connected to a phase voltage.

[0024] This embodiment may be used in a three-phase power converter for a three-phase power configuration connected to a line voltage.

[0025] According to another aspect of the present invention, a three-phase power converter is provided. The converter includes a controller as described in the present application.

[0026] According to another aspect of the present invention, there is provided a method for controlling a three-phase power converter, comprising: determining the instantaneous input power of each phase; determining a power control signal for each phase according to the output power of the three-phase power converter; determining a reference signal for each phase using the power control signal; adjusting the instantaneous input power of each phase using the reference signal for each phase to achieve power balance of the three phases; wherein the double power frequency ripple in the output power of the three-phase power converter is substantially eliminated.

[0027] One of the implementation schemes includes: detecting the input current and input voltage of each phase of a three-phase power converter and determining the instantaneous input power of each phase; using a feedback circuit to detect the output voltage of the three-phase power converter to determine the power control signal of each phase; using the power control signal of each phase and the characteristics of the input voltage of each phase to determine the reference power signal of each phase; comparing the reference power signal of each phase with the instantaneous input power of each phase to generate a control signal of each phase; using the control signal of each phase to generate a drive signal for the switching device of the power circuit of each phase of the three-phase power converter.

[0028] In one embodiment, the input voltage characteristic of each phase is determined according to 1-cos(2ωt).

[0029] In one embodiment, a feedback circuit detects an output voltage and an output current of a three-phase power converter; the method includes using the detected output voltage to determine a power control signal for each phase, and using the power control signal for each phase and characteristics of an input voltage for each phase to determine a reference current signal for each phase; using the detected output current and voltage to determine the output power of the three-phase power converter; using the output power to determine an instantaneous input current for each phase, which current is used together with the reference current signal for each phase to generate a control signal for each phase.

[0030] In one embodiment, the input voltage characteristic of each phase is determined according to sin(ωt).

[0031] In one embodiment, a feedback circuit detects the output voltage and the output current of each phase of a three-phase power converter to determine a power control signal for each phase; the method includes using the power control signal for each phase and a characteristic of the input voltage for each phase to determine a reference power signal for each phase.

[0032] In one embodiment, the input voltage characteristic of each phase is determined according to 1-cos(2ωt).

[0033] One embodiment includes using pulse width modulation (PWM) to generate switching signals for switching devices of a power conversion module of a three-phase power converter to achieve power balance among the three phases.

[0034] One embodiment includes using pulse frequency modulation (PFM) to generate switching signals for switching devices of a power conversion module of a three-phase power converter to achieve power balance among the three phases.

[0035] According to a specific embodiment, under the conditions of balanced input voltage and unbalanced input voltage and the presence of input voltage harmonics, the double power frequency ripple in the output power of the three-phase power converter is substantially eliminated. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to better understand the present invention and more clearly explain how to implement the present invention, reference will be made to the accompanying drawings, which illustrate embodiments of the present invention, including:

[0037] FIG1 is a general structural block diagram of a three-phase two-stage AC-DC converter in the prior art;

[0038] FIG2A is a general structural block diagram of a phase-modular three-phase single-stage AC / DC converter in the prior art;

[0039] FIG2B is a block diagram of a phase-modular four-wire three-phase (Y-type configuration) AC-DC converter in the prior art;

[0040] FIG2C is a block diagram of a phase-modular three-wire three-phase system (delta configuration) AC-DC converter in the prior art;

[0041] 3A and 3B are schematic diagrams of single-stage PFC converter modules, which respectively include a duty cycle controlled PWM isolated Boost converter (3A) and a frequency controlled PFM LLC resonant converter (3B) according to the prior art;

[0042] Figure 4 The AC input voltage, current and instantaneous power of each module (a, b, c) and the total power of the three-phase power converter are shown;

[0043] Figure 5 Waveforms representing three-phase input voltage, current and power are shown, wherein some harmonics in the grid voltage are taken into account and a power control method according to an embodiment described in the present application is adopted;

[0044] Figures 6A-6C are three control block diagrams of a phase modular three-phase single-stage AC-DC converter PFC and power balancing strategy according to the embodiments described in the present application;

[0045] Fig.6D is a digital implementation schematic diagram of a PFC and power balancing control method of a phase-modularized three-phase single-stage LLC AC-DC converter according to one embodiment;

[0046] Figure 7 is a simulation result diagram of a phase-modularized three-phase single-stage LLC AC-DC converter with a power balance control strategy drawn according to an embodiment, including two cases with input voltage harmonics and without input voltage harmonics;

[0047] Figure 8 This is a simulation result diagram of a phase-modular three-phase LLC AC-DC converter using conventional constant current control when the input voltage V_a has a 15% step drop at 80 milliseconds;

[0048] Fig. 9 is a simulation result diagram of a phase-modular three-phase LLC AC-DC converter with a power balance control strategy according to an embodiment when the input voltage V_a undergoes a 15% step decrease at 80 milliseconds;

[0049] Fig. 10A and 10B is a simulation result diagram of a phase modular three-phase LLC AC-DC converter with a power balancing control strategy, wherein the efficiency of phase C is reduced by 5% according to one embodiment, (A) with only a feedforward power balancing loop, and (B) with both a feedforward and feedback power balancing loop;

[0050] Fig. 10C is a simulation result diagram of a phase-modular three-phase LLC AC-DC converter using a power balance control strategy according to an embodiment when the input AC voltage is unbalanced and contains different harmonics;

[0051] Figures 11A-11D is an oscilloscope screenshot showing the results of an experimental prototype operating at Po = 1.5kW and Vo = 380V, where Fig.11A and 11B They are obtained using traditional constant current control under balanced input voltage and unbalanced input voltage respectively. Fig. 11C and 11D The power balancing control according to the embodiment described in the present application is respectively obtained under balanced input voltage and unbalanced input voltage. DETAILED DESCRIPTION

[0052] When there is a slight voltage imbalance at the input source, the output voltage of the phase-modular three-phase single-stage AC-DC converter will produce a double-power frequency ripple, which is inevitable in a three-phase system. One of the main advantages of the phase-modular three-phase single-stage AC-DC converter is the smaller output capacitor requirement, which is due to the elimination of the double power frequency current of the three-phase system at the output of the converter. However, any voltage imbalance between the phases of the grid voltage will undermine this.

[0053] The present application provides a power control method and a controller for a three-phase single-stage AC-DC converter to simultaneously achieve power factor correction (PFC) and power balancing. According to an embodiment of the present invention, an instantaneous input power calculation is used to create a fast feedforward loop for power balancing. In addition, since the output power may have different correlations with the input power, the average output current of each phase can also be detected so as to be used in the feedback loop to fine-tune the output power balance. According to a specific implementation scheme, only a smaller output capacitor is required, and the phase modular three-phase single-stage PFC AC-DC converter does not require an electrolytic capacitor (E-Cap) even in the case of three-phase imbalance. These characteristics improve the power density and reliability of the phase modular three-phase single-stage rectifier. The present application describes a specific embodiment and describes it as an example of an embodiment including a three-phase single-stage AC-DC converter based on LLC, which is used to verify the performance of the power balancing control method.

[0054] Therefore, the present application introduces a PFC control method and controller with a fast power balancing function for a phase modular three-phase AC-DC structure. In some embodiments, by performing power balancing between AC voltage phases, the twice power frequency ripple of the output current is completely or substantially eliminated. In addition, while implementing PFC, the influence of power imbalance caused by the imbalance of the input voltage of each phase is also solved, so the dynamic speed of the system is very fast. In addition, a feedback loop can be implemented by detecting the output current of each phase to accurately control the output power balance. As described in the present application, different voltage imbalance conditions are simulated and compared with the traditional constant current control method to prove the effectiveness of the power balance control embodiment in reducing the twice power frequency output voltage ripple under unbalanced conditions.

[0055] As used in this application, the term "substantially" means that the characteristics, parameters and / or values ​​do not have to be fully realized, but deviations or changes may occur, including, for example, tolerances, measurement errors, measurement accuracy limitations and other factors known to those of ordinary skill in the art, the number of which does not exclude the effect that the characteristic is intended to provide. Characteristics or features that are significantly reduced or eliminated (for example, double power frequency (such as 120Hz) ripple in the output voltage of a three-phase power converter) can be eliminated or significantly reduced or minimized so that it is within the noise range, below background, below detection capability, or has no effect in practical applications.

[0056] This application describes an embodiment of eliminating or significantly reducing output voltage ripple of a three-phase power converter. However, it should be understood that by eliminating or significantly reducing output voltage ripple, any ripple in the output power will also be eliminated or significantly reduced.

[0057] 1. Impact of grid voltage imbalance in phase modular three-phase single-stage AC / DC converter

[0058] FIG2A shows the general structure of a phase modular three-phase single-stage AC-DC converter. Both a duty cycle controlled pulse width modulation (PWM) converter module and a frequency controlled pulse frequency modulation (PFM) converter module can be used for a phase modular three-phase single-stage AC-DC converter. FIG2B shows a typical Y-type connection of a three-phase AC voltage, where the phase voltage is applied to each single-stage PFC module. FIG2C shows a typical Delta (triangle) connection of a three-phase AC voltage, where the line-to-line voltage is applied to each single-stage PFC module. In the present application, a Y-type connection is used to describe the present embodiment. However, it is understood that the present application is also applicable to a delta-connected three-phase AC voltage, and the present embodiment can be implemented accordingly. FIG3A shows an isolated Boost converter as an example of a PWM converter module, and FIG3B shows an LLC resonant converter as an example of a PFM converter module. The converter module may also adopt other designs, such as, but not limited to, a dual active bridge (DAB) converter, an LCLC converter, an active clamped boost converter, a flyback converter, a SEPIC converter, a Zeta converter, and a Cuk converter. Since the impact of unbalanced input voltage on any single-stage PFC module is similar, in the rest of this application, the LLC resonant converter module will be taken as an example, but it will be understood that this embodiment is not limited thereto.

[0059] Referring to Figure 2A, the rectified AC voltage V in_j (where j = a, b, c) supplies power to the primary side of the PFC module, and the output voltage V o_j (where j = a, b, c) has a DC voltage with a double power frequency ripple component when operating alone. As shown in Figure 2A, when the outputs of the three PFC modules are connected together, the double power frequency ripple component contained in the output of each PFC module is offset. As mentioned above, the structure of the phase modular three-phase AC-DC converter benefits from low output voltage ripple without double power frequency, so no large electrolytic capacitors are required, and the reliability of the AC-DC rectifier is significantly improved. However, if the input voltage is unbalanced, the double power frequency ripple component contained in the output of each PFC module cannot be offset, and double power frequency ripple will appear in the output voltage, which is not desirable.

[0060] Assuming that each phase current is a pure sine wave and the power factor is 1, the voltage and current of the three-phase system can be written in the following form.

[0061]

[0062] Among them, V a , V b and V c is the effective value of the grid voltage; I a , I b and are the effective values ​​of the AC input current, and ω is the angular frequency. Under balanced input current conditions, the effective values ​​of the currents are the same, i.e., I a =I b =I c =I and V a ≠V b ≠V c is the unbalanced grid voltage. The total output current and voltage at twice the power frequency are analyzed as follows. Under unbalanced grid conditions, the instantaneous three-phase input power can be calculated using (1) and (2).

[0063] P 3ph =v a ×i a +v b ×i b +v c ×i c =P in +P 2ω cos(2ωt) (3)

[0064] In the above formula, P in is the average input power, P 2ω It is the amplitude of the pulsating power at twice the power frequency, expressed by (4) and (5) respectively.

[0065] P in =V a I+V b I+V c I (4)

[0066] P 2ω cos(2ωt)=-V a Icos(2ωt)-V b Icos(2ωt-240°)-V c Icos(2ωt+240°)(5)

[0067] The rectified current for each phase has an average value and a high-frequency term. Assuming that the switching frequency is high and the energy stored in the output capacitor related to the switching frequency is negligible, the instantaneous input power is equal to the instantaneous output power in a lossless circuit. Therefore, the high-frequency term of the rectified current can be ignored, and the average output current for each phase can be written as follows.

[0068]

[0069] Among them, V o is the average output voltage considered as DC. Therefore, the instantaneous output current, which is the sum of the three-phase output currents, can be expressed as follows.

[0070] i o =i o_a +i o_b +i o_c =I o +I 2ω cos(2ωt) (7)

[0071] In the above formula, I o is the average output current (DC part), I 2ω is the amplitude of the pulsating current at twice the industrial frequency, expressed by (8) and (9) respectively.

[0072]

[0073]

[0074] In the above formula, if the amplitudes of the three-phase input voltages are inconsistent, the twice power frequency pulsating current term cannot be offset, so the output current and output voltage will produce twice power frequency ripple.

[0075] Figure 4 Displays the input AC voltage (v a , v b , v c ), alternating current (i a ,i b ,i c ) and the instantaneous power of each phase (p a , p b , p c ) and the three-phase total power (p o ) and the three-phase total power. Figure 4 It also shows that at 25 milliseconds, the voltage of phase A drops by 10%, and the voltage of phases B and C rise by 5%. It can be seen from the instantaneous power that once the input voltage is unbalanced (at 25 milliseconds), pulsating power will be generated in the three-phase system and transferred to the load.

[0076] 2. PFC and power balancing control strategy

[0077] As described in the previous section, any imbalance in the amplitude of the three input source voltages will cause a double power frequency ripple to appear at the output of the modular three-phase AC-DC converter. As described in the present application, to eliminate the double power frequency ripple in the output current, the current reference of each phase should be changed so that the product of the voltage and current of each phase in all terms of equation (9) is the same, thereby eliminating the fluctuating current of twice the power frequency. According to certain embodiments, the change in phase current can be achieved by changing the instantaneous power of the phase. For example, if the voltage of one phase decreases, its current will automatically increase. Ignoring the power loss of the converter, the average input power is equal to the average output power of each phase ((P in_j =P o_j Where j = a, b, c), so the relationship between the instantaneous input power and the average output power of each phase can be written as follows:

[0078]

[0079] Considering that the grid voltage is a pure sine wave, if the input power of each phase is controlled to satisfy (10), each phase can obtain a higher power factor, and the average output power of each phase will also be indirectly controlled to be the same as the three phases (i.e., P in (10) o_a =P o_b =P o_c =P o / 3), in the voltage control outer loop, the output of the proportional integral (PI) compensator is the power control signal (P ctrl ) adjusts the output voltage level. Then, the reference power of each phase can be expressed as follows:

[0080]

[0081] Fig. 6A A control block diagram for implementing a control strategy according to an embodiment based on equation (11) is shown. In the figure, the instantaneous power calculator 604 calculates the instantaneous input power of the converter modules 602a, 602b, and 603c. The instantaneous input power of 603c is as follows:

[0082]

[0083] P in =P in_a +p in_b +p in_c (11.1)

[0084] like Fig. 6A As shown, the output voltage V of the three-phase power converter o is sensed at 606 and connected to a reference voltage V o The reference is compared and then a controller 608 (eg, a PI controller) is used to generate a power control signal pctrl The output of the instantaneous power calculator 604 is used to determine a characteristic (eg, a value (1-cos(2ωt))) which is compared at 612 to the power control signal p ctrl Multiply by to generate the power reference signal p ref_a,b,c According to this embodiment, the instantaneous input power p in_j (j = a, b, c) and the reference input power p ref_j (j=a, b, c) are consistent, as shown in formula (11). Controller 614 (such as a PI controller) forces the instantaneous input power (as shown in formula (11.1) to follow the reference input power (as shown in formula 11). Other controllers, such as a PID (proportional-integral-derivative) controller or a QR (quasi-resonant) controller, can also be used. The output of controller 614 is used by modulation block 616 to generate drive signals for the switching devices of three single-phase PFC modules 602a, 602b, and 603c. 603c In steady state, the output power can be used to calculate the reference current of each phase, as shown in the following formula (11.2), and the input AC current of each phase will follow the reference current through controller 614.

[0085]

[0086] P o =V o ×I o (11.2)

[0087] Figure 6B A control block diagram according to another embodiment is shown, which implements a control strategy according to formula (11.2) to control the instantaneous input current of each phase, wherein the instantaneous input current reference value is based on the output power P o and input AC voltage. Figure 6B As shown, this embodiment includes Fig. 6A However, if Figure 6B As shown, the output current I of the three-phase power converter o The output voltage V is sensed and checked with 602 o Multiply to determine the output power P o , which is the input of the current reference calculator 627, which generates the instantaneous input current reference signal i according to formula (11.2) ref_a,b,c Va, Vb and Vc are the RMS values ​​of the phase voltages, calculated by the power balancing block 622. The power control signal p ctrl The generation and Fig. 6A Similar to the embodiment of the present invention, the output of the power balancing block 622 is used to determine a characteristic (e.g., value (sin(ωt))) using, for example, a lookup table 624, which is related to the power control signal p ctrMultiplying at 626, the current reference calculator 627 uses this characteristic to generate an input current reference signal i according to equation (11.2). ref_a,b,c A controller (eg, a PI controller) 628 can force the instantaneous input current represented by equation (11.2) to follow the reference input current. The output of the controller 628 is used by a modulation block 630 to generate drive signals for the switching devices of the three single-phase PFC modules.

[0088] If the power loss of each phase of the three-phase module is not exactly the same, the output power sharing between the three phases cannot be achieved through the same power control signal (i.e., p o_a ≠P o_b ≠P o_c ), as shown in equation (10). In this case, a feedback loop can be used to adjust the power control signal of each phase. To do this, the average output current of each phase can be detected, calculated, and compared with the average of the three output currents. Figure 6C A control block diagram according to one embodiment is shown to implement the control strategy. This embodiment includes Fig. 6A Some features of the embodiment. Figure 6C in,i o_a =i o1 ,i o_b =i o2 ,i o_c =i o3 is the instantaneous output current value of each phase. Assume that I o_a , I o_b , I o_c is the calculated average output current value of each phase. Then

[0089] Io_avg=(Io_a+Io_b+Ioc) / 3 (11.3A)

[0090] Delta_Io_a=Io_avg-Io_a (11.3B)

[0091] Io_b=Io_avg-Io_b (11.3C)

[0092] Delta_Io_c=Io_avg-Io_c (11.3D) The offset power calculation formula for each phase is:

[0093] Padj_a=Delta_Io_a*Vo(11.4A)

[0094] Padj_b = Delta_Io_b*Vo (11.4B)

[0095] Padj_c=Delta_Io_c*Vo(11.4C)

[0096] exist Figure 6C In the embodiment of the present invention, the average output current balancing block 640 performs the above calculation. The calculated output power adjustment p adj_a,b,c is added to the power control signal p ctrl , which is the output of the output voltage controller 642, which can be Fig. 6A After compensation, for example, PI controller 644, p ctrl Can be used with the power adjustment control signal (P adj_j Where j = a, b, c), to generate the power control signal p of each phase ctrl_a,b,c .and Fig. 6A Similar to the embodiment of the instantaneous power calculator 650, the output is used to determine a characteristic (eg, a value (1-cos(2ωt))) using a lookup table 652, which is related to the power control signal p ctrl Multiply by to generate the power reference signal p ref_a,b,c Therefore, considering the output current feedback control loop, (11) can be rewritten as:

[0097]

[0098] Among them, P ctrl_a , P ctrl_b and P ctrl_c is the new control signal with a feedback loop, which is defined as follows:

[0099]

[0100] like Figure 6C As shown, p adj_j (j = a, b and c) is the error between the actual output power and the average output power of phase a, phase b and phase c. Therefore, by adding a feedback loop, the instantaneous output current of each phase will offset the pulsation term of the output current, thereby eliminating the line frequency ripple of the output voltage. In addition, Figure 6C The strategy shown can achieve a power factor close to unity because the input power is controlled by the reference power in equation (12), which can be achieved as long as there is a pure sinusoidal voltage on the grid side. According to the control strategy of the embodiment shown in equations (12) and (13), the output power of each phase is controlled to be substantially the same.

[0101] In the above description, it is assumed that the AC input voltage is a pure sine wave with no harmonics. If the AC input voltage contains harmonics, the instantaneous power balance control strategy described by formula (11) and formula (12) is still valid. In this case, harmonics will also appear in the AC input current of each phase. As described below, the AC harmonic current will significantly reduce the double power frequency component at the output end. Since the harmonic component of the AC voltage is very small in practical applications, the harmonics of the AC current are also very small.

[0102] If the input voltage has harmonics, the maximum achievable power factor will be less than 1, depending on the degree of voltage distortion. According to IEEE 519-1992

[16] , the maximum value of total harmonic distortion (THD) allowed is generally 5%. rd and 5 th etc.), the input AC voltage can be expressed as follows:

[0103]

[0104] Where V j1 , V j3 and V j5 (j=a, b, c) are the effective values ​​of the first harmonic, third harmonic and fifth harmonic of the voltage, respectively. In order to compare the instantaneous power with a reference signal having a DC component and a double power frequency component, the multiplication of the three-phase voltage and current should only contain the above two components, as shown in (3). A solution that can eliminate high-order harmonics in the instantaneous power is that the resulting input current has the same harmonic content as the grid voltage, but with opposite signs. Therefore, in one embodiment of the power-controlled PFC, taking into account the harmonics, the current waveform shown in (14) is as follows:

[0105]

[0106] Among them, I j1 , I j3 and I j5 (j=a, b, c) are the effective values ​​of the first, third and fifth current harmonics respectively.

[0107] Figure 5 The unit representation of the three-phase input voltage (top) is shown, with odd harmonics added to a total harmonic distortion of 5%, and the input current (center) with the same harmonic content but opposite signs. The resulting instantaneous power (bottom) is shown as a solid line, with the markers from equation (10). It can be seen that the markers align with the solid line, proving that only twice the power frequency is retained in the instantaneous power. When the output voltages of the three phases are added, the twice the power frequency voltage is cancelled, leaving only a pure DC voltage.

[0108] Under constant power control, if there are any harmonics in the grid voltage, the same negative harmonics will also appear in the current. By definition, the total harmonic distortion (THD) is the ratio of the square root of the square of the effective value of all harmonic components to the square of the effective value of the first harmonic, and it is insensitive to the sign of the harmonic. Therefore, the total harmonic distortion (THD) in the AC voltage will also be reflected in the AC current. It is worth mentioning that the PFC converter itself may produce some current distortion in different situations, which is not considered here. Therefore, considering the perfect PFC performance of the AC-DC converter, the maximum power factor achievable by constant power control in the presence of harmonics in the grid voltage can be calculated as follows:

[0109]

[0110] When the total harmonic distortion of the grid voltage is 5%, the maximum achievable power factor is 0.9975. In contrast, when constant current control is used, the maximum achievable power factor is 0.9987. As long as the PFC operation of the AC-DC converter is close to unity and the grid voltage harmonic content is within the standard range, the difference in power factor caused by voltage harmonics between the power control method of the present invention and the traditional constant current control method can be ignored. In other words, the power factor will still be high enough to meet the standard requirements.

[0111] Fig.6D Shows Figure 6C Detailed circuit-level digital implementation of, according to one embodiment, in this case a phase modular three-phase LLC AC-DC converter. In the figure, Lin1, Cin1, Lin2, Cin2 and Lin3, Cin3 are high-frequency filters to eliminate switching frequency noise. They do not affect the power balance operation.

[0112] It should be noted that any suitable technology may be used to implement the control strategy embodiments introduced in this application, for example, digital signal controllers (DSC), digital controllers such as microcontroller units (MCUs), field programmable gate arrays (FPGAs), etc., as well as combinations of digital circuits and analog circuits. Fig.6D An MCU is shown.

[0113] See also Fig.6D , through the sensing resistor (R sen1 , R sen2 , R sen3 ) detects the input current of each LLC resonant converter, and the average input current calculator 660 calculates the average input current i in each switching cycle using the instantaneous input current a 、i b 、i c , and then with the sampled input voltage v of each phase a 、v b、v c Multiply them to calculate the instantaneous input power p a 、p b 、p c .

[0114] To generate the reference power signal p refa , p refb , p refc , calling the 1-cos(2ωt) lookup table, such as 664a, 664b, 664c is called starting from the zero crossing point 662 of the input voltage. The three-phase power converter output voltage 668 is sampled and compared with the reference voltage 670, and then the appropriate amplitude power control signal (P) is generated for each phase through the digital PI compensator 672. ctrl_j where j = a, b, c), these control signals are multiplied by the 1-cos(2ωt) term for each phase. Thus, three independent time-varying reference power signals (P ref_j where j = a, b, c), can be compared with the detected instantaneous input power signal p a , p b , p c Make a comparison.

[0115] In the feedback power balance loop, three adjustment power signals (P adj_j Where j = a, b, c) to fine-tune the output power balance. The average output current of the induction output within half the power frequency cycle is averaged by the average output current calculator 674. Then, the output current of each phase is compared with the average value of the three-phase current. After the three-phase current passes through three independent digital PI compensators 676a, 676b, and 676c, an appropriate adjustment power signal is provided for each phase. The speed of these compensators is slower than other control loops. Then, the adjustment power signal (P adj ) and the power control signal (P ctrl ) are added together to achieve accurate power balancing even for unbalanced three-phase modules. The resulting time-varying power reference signal (p ref_j Where j = a, b, c) and the instantaneous input power p of each phase a 、p b 、p c The comparison is then performed and appropriate control signals (i.e. Figures 6A-6CThe frequency shown in the embodiment controls the period or switching period of the PFM converter, and the duty cycle of the PWM converter) for each phase of the PWM module 682a, 682b, 682c, and then generates gate drive signals 884a, 684b, 684c for the converter switching devices (which can be MOSFET, IGBT, HEMT, etc.). It is worth noting that the PWM module (682a, 682b, 682c) can be implemented in an MCU. It generates a frequency signal for a PFM controlled converter (such as an LLC converter), or generates a PWM signal for a PWM controlled converter (such as an isolated Boost converter). Fig.6D In FIG. 1 , period A, period B, and period C represent the switching periods (equivalent to the switching frequency) required by these three-phase power converters at a specific time. Therefore, according to the embodiment, there is a voltage loop to achieve output voltage regulation, three current balance feedback loops to ensure effective output power balance, and each phase has its internal power control loop to achieve PFC operation. As described above, through power factor correction, power balance can be achieved simultaneously in each phase. Therefore, the dynamic speed of the system is very fast, and even in transient conditions, the output voltage ripple of twice the power frequency will not be very large.

[0116] III. Example: Simulation and Experimental Results

[0117] A simulation model of an embodiment was implemented in PSIM software (Powersim Inc., Troy, MI, USA) using a three-phase single-stage LLC AC-DC converter, and a laboratory prototype with digital control implementation was made for actual verification. The parameters used in the simulation and experimental setup are listed in Table 1.

[0118] Table 1. Parameters used for simulation and prototype.

[0119]

[0120] It is worth noting that for a 1500W (380V, 4A) AC-DC rectifier application, the output capacitor value of 60uF is very small. The results show that even with such a small capacitor value, the double power frequency ripple is very small and can be ignored. In practical applications, small capacitors should be used as good engineering practice for noise elimination, impedance matching, etc.

[0121] In order to understand the influence of grid voltage harmonics on the power factor performance of this device, odd harmonics (i.e. 3 rd , 5 th and 7 th ). Figure 7The simulation results of the three-phase pure sinusoidal voltage before 80 milliseconds and 5% total harmonic distortion (THD) after 80 milliseconds are shown. The power factor measured before 80 milliseconds is 0.999, and the power factor measured after 80 milliseconds is 0.995, and the power factors of the two are very close and close to unity. It proves the effectiveness of the power control method embodiment even if there are harmonics in the grid voltage. In addition, the input voltage harmonics have no effect on the output voltage ripple. That is to say, considering that a relatively small (60uF) capacitor is used at the output end, and the output power is 1500W, the voltage is 380V, and the current is 4A, the simulated output double power frequency ripple is very small.

[0122] To observe the double-power frequency output voltage ripple, the traditional method was first used for simulation, with a constant current reference for all phases. In the simulation, only the output voltage V a Reduce the situation by 15%. Figure 8 The input voltage and current of all three phases and the output voltage of the converter are shown. The power factor measured in the simulation is close to unity (i.e. >0.999), the total harmonic distortion (THD) of the input current is less than 2%, and all phases have the same good performance. In addition, the output voltage ripple is small (about 300mV) before 80ms when the input voltage is balanced, and there is no double line frequency ripple before the input voltage step change occurs. Once the input voltage becomes unbalanced due to the step change occurring at 80ms, the output voltage will have double line frequency ripple. The peak-to-peak voltage ripple is as high as 9V, which is 30 times that of the balanced input voltage condition, which requires 30 times larger output capacitance to keep the output voltage ripple at a low level under balanced grid voltage conditions.

[0123] Fig. 9 The simulation results of the power balancing control strategy are shown in Figure 1. It can be seen that under the conditions of balanced and unbalanced input voltage, the output voltage ripple does not appear at twice the power frequency. In this simulation, the power balancing control strategy described by formula (11) is adopted. The power loss of each converter is ignored. Figure 8 The output voltage waveform in Fig. 9 The output voltage waveform shown at the bottom is magnified. It can be seen that in this embodiment, even if there is a large imbalance in the input voltage, the output voltage of the three-phase single-stage LLC AC-DC converter will not have obvious double power frequency ripple, and in the case of unbalanced grid voltage, the power factor is still close to 1.

[0124] In order to clearly understand the impact of unbalanced power loss conditions between the three-phase modules, the power loss of phase C is deliberately increased, making the efficiency of phase C about 5% lower than that of the other two phases. In actual applications, the efficiency difference between different PFC modules is very small, for example, less than 0.5%. The simulation results of the power balancing control method without a power balancing feedback loop are shown in Figure 2. Fig. 10A As shown in the figure, it can be seen that the average output current of phase C is lower than that of the other two phases because the power loss of this phase is larger. In addition, a slight double power frequency voltage ripple can be seen in the output voltage, with a peak-to-peak value of 1.2V, which is caused by a slight imbalance in the output power between phases. Fig. 10B The simulation results of the power balancing feedback loop according to the embodiment (Equations 12 and 13) are shown. It can be seen that the induced average output currents of all three phases are balanced, so the feedback loop effectively eliminates the double power frequency ripple.

[0125] Fig. 10C The simulation results are shown for the following cases: (1) Va is 10% higher, Vb = Vc is 5% lower; (2) Va contains 5% of the third harmonic, Vb contains 3% of the third harmonic, and Vc contains 3% of the fifth harmonic; (3) the control strategy of formula (11) is adopted. It can be seen that there is no double power frequency ripple in the output voltage. The output voltage contains a very small 6 th Harmonics (360Hz), peak to peak value is about 0.2V. This shows that under actual application conditions, the control strategy eliminates the double power frequency ripple.

[0126] A prototype was built to verify the performance of the embodiment of the power balancing control method on output voltage ripple suppression using a three-phase LLC converter with the parameters listed in Table 1. The digital controller uses a low-cost dsPIC Microchip MCU (Microchip Technology Inc.). Fig.11A and Fig. 11B The equilibrium (V a =V b =V c =220V RMS ) and unbalanced (V a =190V,V b =V c =220V RMS ) The experimental results of using traditional constant current control under input voltage. It can be seen that when the input voltage is balanced, the output voltage V o There is basically no double power frequency voltage ripple (close to zero), but in the case of unbalanced input voltage, the output voltage of the AC-DC converter will have a relatively large double power frequency voltage ripple (peak to peak value is about 20V).

[0127] Fig. 11C and Fig.11D The balanced input voltage (V a =V b =V c =220V RMS ) and unbalanced input voltage (V a=190V,V b =V c =220V RMS ) Experimental results of the power balance control embodiment. Fig. 11C and Fig.11D It can be seen that under balanced and unbalanced input voltages, the output voltage ripple is very small, and there is no double power frequency voltage ripple.

[0128] It can be understood that the power balancing control strategy described in the present application can also be applied to a three-phase system with a delta connection, wherein the input voltage of the PFC module is the line voltage.

[0129] In summary, the control strategy embodiments introduced in this application are applicable to phase-modularized three-phase single-stage AC-DC converters with synchronous power factor correction and power balancing functions, which can significantly eliminate the output voltage double power frequency ripple caused by input voltage imbalance. According to the embodiments, even in the presence of unbalanced input voltage and power loss, the output capacitance of the three-phase AC-DC converter can be very small. Under balanced and unbalanced input voltage conditions with and without input voltage harmonics, the power balancing control embodiments can achieve high power factor (i.e., >0.99).

[0130] All cited documents are incorporated herein by reference in their entirety.

[0131] Equivalent

[0132] Those skilled in the art will recognize or be able to determine by routine experimentation equivalents to the embodiments described herein. Such equivalents belong to the scope of the present invention and are encompassed by the appended claims.

[0133] References

[0134] [1] H.Suryawanshi, MRRamteke, KLThakre and VBBorghate, "Unity-Power-Factor Operation of Three-Phase AC-DC Soft Switched Converter Based OnBoost Active Clamp Topology in Modular Approach," IEEE Trans.23, no.1, pp.

[0135] [2] B. Tamyurek and D. A. Torrey, "Three-phase unity power factor single-stage AC-DC converter based on interleaved flyback topology", IEEE Trans. 26, no. 1, pp.

[0136] [3] G. Tibola and I. Barbi, “Isolated three-phase high power factor rectifier based on SEPIC converter operating in discontinuous conduction mode,” IEEE Trans. 28, No. 11, pp. 4962-4969, November 2013.

[0137] [4] B.Singh, S.Singh and G.Bhuvanwswari, "Analysis and Design of a ZetaConverter Based Three-Phase Switched Mode Power Supply," in Proc.InternationalConference on Computational Intelligence and Communication Networks, Mathura, 2012, pp.

[0138] [5] S. Gangavarapu, A.K. Rathore and D.M. Fulwani,"Three-Phase Single-Stage-Isolated Cuk-Based PFC Converter," IEEE Trans. Power Electronics, Vol. 34, No. 2, pp. 1798-1808, February 2019.

[0139] [6] G. Bhuvaneswari, S. Narula and B. Singh, “Three-phase push-pull modular converter based welding power supply with improved power quality,” in Proceedings of the International Conference on Power Electronics (ICPE), Delhi, 2012, pp. 1–5.

[0140] [7] YKEHo, SYRHui and Yim-Shu Lee, "Characterization of single-stage three phase power-factor-correction circuit using modular single-phasePWM DC-to-DC converters," IEEETrans.15, no.1, pp.

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[0142] [9] MMForouzesh, Y.-F. Liu and PCSen, "Implementation of an IsolatedPhase-Modular-Designed Three-Phase PFC Rectifier Based on Single-Stage LLCConverter," in Proc.ECCE, 2021, pages 2266-2273.

[0143]

[10] S. Bala, T. P.Rosenfeld,and F.Delince,"Effect of low-frequency current ripple on the performance of lithium iron phosphate (LFP) battery energy storage system',in Proc.IEEE Energy Conversion Congress Exposition(ECCE),Sep.2012,pp.

[0144]

[11] U. Kamnarn and V. Chunkag, “Analysis and design of parallel and source-splitting configurations using SEPIC modules based on power balancing control techniques,” in Proceedings of the IEEE International Conference on Industrial Technology (ICIT), Hong Kong, China, 2005, pp. 1415-1420.

[0145]

[12] U. Kamnarn and V. Chunkag, "Analysis and design of modular three-phase AC-DC converters using CUK rectifier modules with near-unity power factor and fast dynamic response", IEEE Trans. 24, no. 8, pp.

[0146]

[13] R. Girod and D. Weida, “High-efficiency true three-phase compact switch-mode rectifier modules for telecom power solutions,” in Proceedings of the IEEE International Telecommunications Energy Conference (INTELEC), Smart Power & Efficiency, Hamburg, Germany, 2013, pp. 1–6.

[0147]

[14] L. Huber, M. Kumar and MM Jovanovic, "Analysis, design, and evaluation of three-phase three-wire isolated ac-dc converter implemented with three single phase converter modules," in Proc. IEEE Applied Power Electronic Conference Exposition (APEC), Long Beach, CA, 2016, pp.

[0148]

[15] B. Kim, H. Kim, and S. Choi, “Three-phase on-board charger using three-module single-stage interleaved soft-switching AC-DC converters,” in IEEE Applied Power Electronics Conference and Exhibition (APEC), San Antonio, Texas, 2018, pp. 3405–3410.

[0149]

[16] IEEE Working Group 519. "Institute of Electrical and Electronics Engineers Recommended Practice and Requirements for Harmonic Control in Power Systems." IEEE STD 519-1992, 1992.

Claims

1. A controller for a three-phase power converter, comprising: The first circuit is used to determine the instantaneous input power of each phase; a feedback circuit for determining at least one power control signal based on the output of the three-phase power converter; The second circuit uses the power control signal to adjust the instantaneous input power of each phase to achieve power balance among the three phases; in, The double power frequency ripple in the output power of the three-phase power converter has been basically eliminated.

2. The controller of claim 1, wherein: The first circuit detects the input current and input voltage of each phase and determines the instantaneous input power of each phase; The feedback circuit detects the output voltage of the three-phase power converter and determines the power control signal of each phase; The second circuit determines the reference power signal of each phase by using the power control signal of each phase and the characteristics of the input voltage of each phase, and compares the reference power signal of each phase with the instantaneous input power of each phase to generate the control signal of each phase; The output circuit uses the control signal of each phase to generate a driving signal for the switch device of the power circuit of each phase of the three-phase power converter to achieve power balance of the three phases.

3. The controller of claim 2, wherein the characteristic of each phase input voltage is determined based on 1-cos(2ωt).

4. The controller of claim 2, wherein: The feedback circuit detects the output voltage and output current of the three-phase power converter; The controller determines the power control signal of each phase using the detected output voltage, and determines the reference current signal of each phase using the power control signal of each phase and the characteristics of the input voltage of each phase; wherein the controller uses the detected output current and voltage to determine the output power of the three-phase power converter; The output power is used to determine the instantaneous input current of each phase, and the instantaneous input current is used together with the reference current signal of each phase to generate the control signal of each phase.

5. The controller of claim 4, wherein the characteristic of each phase input voltage is determined according to sin(ωt).

6. The controller as claimed in claim 2, wherein the feedback circuit detects the output voltage of the three-phase power converter and the output current of each phase to determine the power control signal of each phase, and uses the characteristics of the power control signal of each phase and the input voltage of each phase to determine the reference power signal of each phase.

7. The controller of claim 6, wherein the characteristic of each phase input voltage is determined according to 1-cos(2ωt).

8. The controller of claim 1, wherein the first circuit comprises an average input current calculator for calculating an average input current of each phase in each switching cycle.

9. A controller as described in claim 6, wherein the feedback circuit determines the average value of the output current of each of the three phases detected within half a power frequency cycle, compares the output current of each phase with the average value of the three-phase current, and places the comparison output of each phase in the compensator to obtain an adjusted power signal for each phase.

10. The controller of claim 6, wherein the input voltage characteristic is determined at a zero crossing of the input voltage of each phase.

11. The controller of claim 1, wherein the feedback circuit compares the detected output voltage of the three-phase power converter with a reference voltage and uses the output of the comparison to obtain an appropriate amplitude of the at least one power control signal.

12. The controller of claim 1, comprising an output circuit including a pulse width modulation (PWM) module for generating a switching signal for a switching device of a power converter module of a three-phase power converter.

13. The controller of claim 1, comprising an output circuit including a pulse frequency modulation (PFM) module for generating switching signals for switching devices of a power converter module of a three-phase power converter.

14. The controller of claim 1, used in a three-phase power converter of a three-phase power supply configuration connected to a phase voltage.

15. The controller of claim 1, used in a three-phase power converter of a three-phase power configuration connected to a line voltage.

16. A three-phase power converter comprising the controller of claim 1.

17. A method for controlling a three-phase power converter, comprising: Determine the instantaneous input power for each phase; Determine a power control signal for each phase according to the output power of the three-phase power converter; determining a reference signal for each phase using the power control signal; The instantaneous input power of each phase is adjusted by using the reference signal of each phase to achieve three-phase power balance; in, The double power frequency ripple in the output power of the three-phase power converter has been basically eliminated.

18. The method of claim 17, comprising: Detecting the input current and input voltage of each phase of a three-phase power converter and determining the instantaneous input power of each phase; The output voltage of the three-phase power converter is detected by a feedback circuit to determine a power control signal for each phase; Determine a reference power signal for each phase using a power control signal for each phase and an input voltage characteristic for each phase; comparing a reference power signal of each phase with an instantaneous input power of each phase to generate a control signal of each phase; Using the control signal of each phase, a drive signal is generated for a switching device of a power circuit of each phase of the three-phase power converter.

19. The method of claim 18, wherein the characteristic of each phase input voltage is determined based on 1-cos(2ωt).

20. The method of claim 18, wherein the feedback circuit detects an output voltage and an output current of the three-phase power converter; The method includes determining a power control signal for each phase using the detected output voltage, and determining a reference current signal for each phase using the power control signal for each phase and a characteristic of the input voltage for each phase; Determine the output power of the three-phase power converter using the detected output current and voltage; The output power is used to determine the instantaneous input current of each phase, which is used together with a reference current signal for each phase to generate a control signal for each phase.

21. The method of claim 20, wherein the characteristic of each phase input voltage is determined based on sin(ωt).

22. The method of claim 18, wherein the feedback circuit detects an output voltage and an output current of each phase of the three-phase power converter to determine a power control signal for each phase; The method includes determining a reference power signal for each phase using a power control signal for each phase and an input voltage characteristic for each phase.

23. The method of claim 22, wherein the characteristic of each phase input voltage is determined based on 1-cos(2ωt).

24. The method of claim 17, comprising using pulse width modulation (PWM) to generate switching signals for switching devices of a power conversion module of a three-phase power converter to achieve power balance of the three phases.

25. The method of claim 17, comprising using pulse frequency modulation (PFM) to generate switching signals for switching devices of a power conversion module of a three-phase power converter to achieve power balance of the three phases.

26. The method of claim 17, wherein the double power frequency ripple in the output power of the three-phase power converter is substantially eliminated in the case of balanced input voltage and unbalanced input voltage and the presence of input voltage harmonics.

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