44 results about "Vienna rectifier" patented technology

The invention discloses an intelligent charging pile core module based on a VIENNA structure, an electric energy quality management method, system and equipment and a storage medium, and belongs to the field of electric power system energy storage equipment.The intelligent charging pile core module based on the VIENNA structure comprises the steps that an integrated power conversion unit is constructed, the power conversion unit adopts a VIENNA rectifier topology, three-phase alternating current is converted into direct current, and the direct current is converted into direct current; a harmonic suppression structure linked with the control interface is arranged; state information in the operation process is collected and serves as input of the control strategy module; based on the state information, a modulation control instruction is generated in the control strategy module to drive the switching action of the power conversion unit; on the basis of the modulation control instruction, the switch control is executed, meanwhile, the direct-current side midpoint potential of the power conversion unit is synchronously adjusted, and the distribution of direct-current bus capacitor voltage is controlled; and identifying the current operation condition according to the state information, switching to a control template corresponding to the operation condition in the control strategy module, and generating a modulation control instruction.

The application discloses a single-phase rectifier control method, device and system based on PR control, and belongs to the technical field of rectifier control. The method is as follows: collecting operation data of a single-phase rectifier, wherein the operation data comprises an input voltage, an upper capacitor voltage, a lower capacitor voltage and an inductor current; inputting the operation data into a current loop PR controller and a voltage loop PI controller, calculating a voltage loop error and a current loop error, and outputting a current control instruction; calculating a feedforward duty ratio of the single-phase rectifier through a feedforward controller according to the input voltage and a preset voltage reference value; performing pulse width modulation based on the current control instruction and the feedforward duty ratio, outputting a driving signal, and driving a switching tube of the single-phase rectifier to be on or off through the driving signal. Therefore, by implementing the application, the single-phase Vienna rectifier is regulated and controlled through PR control, and beneficial effects such as reduction of harmonics, stabilization of bus voltage and improvement of input power factor are achieved.

The invention relates to a soft start control method for a Vienna rectifier, and the method comprises the steps: calculating a soft start modulation signal for controlling the Vienna rectifier based on a preset modulation signal, an output voltage, a working frequency, and a switching period count of the Vienna rectifier in a process of switching the Vienna rectifier to Vienna control, a switching tube unit of the Vienna rectifier is controlled based on the soft start modulation signal; the switching tube period count is the number of switching periods passed in the process of switching the Vienna rectifier to Vienna control. By implementing the soft start control method of the Vienna rectifier, the Vienna rectifier can be controlled to be slowly turned on according to the switching time, so that the input current when the Vienna control is switched in is effectively reduced, the voltage resistance and current resistance requirements of the switching tube in the switching tube unit of the Vienna rectifier are further reduced, and the switching efficiency of the Vienna rectifier is improved. The service life of the switch tube is prolonged.

The invention provides a PWM wave optimization method and system based on VIENNA rectifier single-period control. The PWM wave optimization method comprises the steps that three-phase input current is sampled; determining the position of the corresponding PWM wave high level in the switching period according to the current direction of each phase; the method specifically comprises the following steps: for each PWM output channel, according to a phase current symbol associated with the PWM output channel, selecting to place a high level at two ends or in the middle of a switching period; and based on the determined PWM high level position and the calculated duty ratio, generating and outputting a PWM driving signal used for controlling a VIENNA rectifier switch tube. According to the method, PWM generation logic is redesigned, a non-optimal voltage vector combination used in traditional synthesis is abandoned, and three vectors (namely vectors corresponding to three vertexes of a small triangle where a target vector is located) closest to the target voltage vector are adopted for synthesis; according to the optimization, on the premise that the switching frequency is not increased and extra hardware cost is not increased, output current harmonics are remarkably reduced.

This invention discloses a general fast modulation method for a VIENNA rectifier, comprising: 1) establishing a mathematical model of nine switches based on the mathematical model of the VIENNA rectifier, obtaining the equality constraints between the duty cycle of the nine switches and the modulation wave; 2) establishing two equations satisfying the control voltage constraint through newly defined variables, and providing a general solution satisfying the equality constraint conditions; 3) obtaining six states based on the different directions of the three-phase current; obtaining inequality constraints under different states based on these six states; and obtaining the upper and lower limits of the free variables based on the inequality constraints under the six states; 4) solving for the compensation amount for neutral point potential balance control starting from the control equation for the capacitor neutral point potential control of the VIENNA rectifier. The modulation model and solution process of the VIENNA rectifier disclosed in this invention have a small computational workload, significantly reducing the computation time compared to the SVPWM modulation technology of three-level converters.

The application discloses a SVPWM modulation method suitable for a staggered parallel Vienna rectifier and belongs to the technical field of power electronics. The method comprises the following steps: judging a sector where a reference voltage vector is located; selecting a corresponding small vector according to the sector, mapping a three-level space to a two-level space to obtain an equivalent reference voltage vector; adopting a traditional two-level SVPWM algorithm to calculate the action time of the equivalent reference voltage vector; introducing a vector distribution factor k to adjust the action time distribution of two switch states of a specific small vector in each sector; and generating a PWM signal. The application simplifies the calculation complexity of the SVPWM through equivalent two-level mapping, simultaneously realizes unified regulation of midpoint voltage balance and input current zero-crossing distortion by using the vector distribution factor k, effectively improves the input current quality, and reduces total harmonic distortion.

This invention discloses a VIENNA rectifier control method based on simplified SVPWM, relating to the field of VIENNA rectifier technology. The method includes: based on a modulation signal and three-phase input current, performing the following operations: determining one of six major sectors where the reference voltage vector is located according to the polarity of the three-phase input current; performing coordinate correction on the modulation signal according to the major sector to obtain the corresponding center correction vector; using the center correction vector to determine the position of the reference voltage vector in a small sector within the corresponding major sector; calculating the corresponding two-phase basic vector action time using only the first small sector within the first major sector as a reference; for small sectors within any other major sector, using a coordinate rotation recursive method to process the corresponding center correction vector to obtain the two-phase basic vector action time of the corresponding sector; and generating a PWM drive signal based on the obtained two-phase basic vector action time to control the switching transistors in the VIENNA rectifier main circuit. This improves the real-time performance and execution efficiency of the algorithm.

The application discloses a variable clamping interval carrier discontinuous modulation method of a three-phase Vienna rectifier. ma , u mb , u mc ; a balanced continuous component u bc , a zero-crossing clamping component u oc and a peak clamping component u pc are calculated; a variable interval clamping coefficient k VAC is set according to different modulation ratios, and a variable interval clamping component u VAC is calculated according to a formula u VAC =k VAC (1-m); a zero sequence voltage u z is calculated, and the zero sequence voltage u z is injected into three-phase normalized sinusoidal waves to obtain three-phase Vienna rectifier variable clamping interval modulation waves u ref_a , u ref_b and u ref_c of A phase, B phase and C phase. The variable clamping interval carrier discontinuous modulation method can reduce terminal voltage error, reduce midpoint potential fluctuation, improve input current quality, and has the advantages of simple calculation and convenient implementation.

The invention discloses an unbalanced power and bipolar voltage cooperative control method for a Vienna rectifier without an electrolytic capacitor, and belongs to the technical field of Vienna rectifier control. The invention aims to solve the problem of system control instability caused by violent bipolar voltage oscillation when a Vienna rectifier is connected with a load with a relatively large power level difference at a direct current side. Comprising the following steps: comparing an unbalanced power boundary with a real-time power unbalance degree, and if the unbalanced power boundary is smaller than the real-time power unbalance degree, selecting a bipolar voltage difference control mode; otherwise, selecting a bipolar voltage balance control mode; in the balance control mode, selecting the optimal clamping mode of intermittent pulse width modulation according to the comparison result of the midpoint voltage at the adjacent next moment and the voltage hysteresis boundary; and under a bipolar voltage difference control mode, calculating an optimal voltage unbalance degree, performing vector synthesis on two adjacent vectors in a voltage vector non-synthesis region, and obtaining a modulation wave after track optimization by calculating a duty ratio. According to the invention, grid-side current harmonics can be reduced.

The invention discloses a control method for a three-phase six-switch VIENNA rectifier, and the method comprises the steps: building a double-closed-loop PI control model of the three-phase six-switch VIENNA rectifier, and enabling the double-closed-loop PI control model to comprise current inner-loop control and voltage outer-loop control; current inner loop control in the double-closed-loop PI control model is improved by adopting a fractional order PI algorithm; and adopting a linear active disturbance rejection controller (LADRC) to improve voltage outer loop control in the double closed loop PI control model. Based on the method, the invention further provides a control system of the three-phase six-switch VIENNA rectifier, and the control system comprises a fractional order PI current inner loop control module, an LADRC voltage outer loop control module, an SVPWM modulation module and a neutral-point potential balance module. According to the invention, the output accuracy of the rectifier in a steady state is improved, the purposes of high steady-state current accuracy and low harmonic distortion rate are achieved, the overshoot of the system is reduced, and the dynamic response speed is high and the robustness is strong.

The application discloses an interleaved Vienna rectifier device and a control method thereof. Positive and negative sequence separation is performed on a positive-negative voltage sequence component and a positive-negative current sequence component, to separate a positive voltage sequence component from a negative voltage sequence component, and a positive current sequence component from a negative current sequence component. The positive voltage sequence component, the negative voltage sequence component, the positive current sequence component and the negative current sequence component are controlled separately. A Proportional-Integral (PI) controller and a Harmonic Elimination (PR) controller are used within a current inner loop to control the positive voltage sequence component, the negative voltage sequence component, the positive current sequence component and the negative current sequence component for generating a plurality of control signals to control the switches of the interleaved Vienna rectifier circuit.

The invention provides a Vienna rectifier control method based on a frequency domain feedback projection sliding mode, and the method comprises the steps: constructing a voltage outer loop controller of a frequency domain feedback projection sliding mode structure according to the mathematical modeling of a Vienna rectifier, and obtaining a system control quantity corresponding to the voltage outer loop controller; designing an inversion sliding mode control law of a current inner loop based on a system equation of the Vienna rectifier; and taking the control quantity output by the voltage loop controller as a reference input value of the current loop to realize a voltage regulation target, thereby generating a PWM control signal for driving a switching device of the Vienna rectifier, and realizing stable tracking of a set value by the direct current side output voltage. Compared with the prior art, a projection function is added into a sliding mode surface to form a projection sliding mode, an amplitude limiting threshold value is set, a frequency domain feedback item is introduced into the projection sliding mode, the frequency selective response capability of the controller is enhanced, and switching buffeting is greatly weakened.

The present application relates to a kind of Vienna rectifier wide load range operation control method mode, proposes a load power estimation algorithm, by collecting three-phase voltage, three-phase current, DC side upper and lower voltage and combining forgetting iterative filtering algorithm and sliding window filtering algorithm, accurately estimate load power and filter out the noise caused by sampling error.Based on the proposed load power algorithm, a new outer ring structure is proposed, which uses capacitor energy storage feedback and load power feedforward control to separate the control of DC side energy storage capacitor and DC load control.When load mutation occurs, the demand of load can be estimated in advance, and the calculation is introduced to the current loop, which improves the response speed of the system and better stabilizes the DC side voltage.The present application solves the problem of parameter design difficulty of the outer ring of traditional control mode of Vienna rectifier, improves the response speed when load mutation occurs, and solves the overvoltage problem during light load start.

The application discloses a Vienna rectifier type EC fan control method and system under power grid unbalanced conditions, three-phase power grid voltage, alternating current side current, DC side filter capacitor voltage and load current are collected first, a phase angle of the power grid is extracted through a phase-locked loop, and abc to dq coordinate transformation is completed. An outer ring voltage sliding mode controller generates an active current instruction according to a DC side voltage error, an ADRC current loop dynamically tracks active and reactive current components, and control output is sent into SVPWM to generate a switching signal through inverse transformation. At the same time, a midpoint voltage balance control is introduced to suppress the asymmetric problem of the DC side capacitor voltage. The system effectively solves the voltage and current control problem under the power grid imbalance, improves the bus stability and power quality, is suitable for the complex working condition operation of the Vienna rectifier, and has strong robustness and control precision.

The utility model provides a power module, which comprises the following steps: the power module comprises a lining plate and a power circuit arranged on the lining plate, the power circuit comprises a Vienna rectifier circuit and a three-phase full-bridge circuit, and the output end of the Vienna rectifier circuit is connected with the input end of the three-phase full-bridge circuit; wherein power devices in the Vienna rectification circuit and the three-phase full-bridge circuit are composed of a plurality of power chips with corresponding functions, and the power chips are silicon carbide power chips. The power factor of the power module can be improved by adopting the Vienna rectifying circuit, meanwhile, the output current of the power module is improved through the plurality of power chips, so that the output power is improved, a silicon carbide-based chip is further adopted to replace an existing silicon-based chip, the size of the chip can be reduced, and the cost is reduced. Therefore, under the condition that higher output power is set, the size of the power module is prevented from being increased.

The invention relates to the technical field of Vienna rectifiers, in particular to a control method for improving harmonic power grid input adaptability of a Vienna rectifier, which comprises the following steps of: performing coordinate transformation on acquired three-phase power grid voltage and input current to obtain voltage and current signals under a fundamental wave rotating coordinate system, extracting a fundamental wave current error signal, and calculating the harmonic power grid input adaptability of the Vienna rectifier; the method comprises the following steps: acquiring a current error component of each harmonic, converting the current error component into a rotating coordinate system corresponding to each specified harmonic to obtain the current error component of each harmonic, and in each harmonic rotating coordinate system, independently controlling the current error component of the corresponding harmonic by adopting a proportional-integral controller to generate a control voltage component of each harmonic; inversely transforming the control voltage component of each harmonic to a fundamental wave rotating coordinate system, and superposing to form a total harmonic compensation voltage; according to the invention, by adopting the control method of combining the self-adaptive loop parameters with the bus lifting strategy, the adaptability of the input current of the rectifier along with the power grid voltage under the harmonic power grid voltage is enhanced, so that the problem of overcurrent of the rectifier under different working conditions is avoided.

The utility model discloses a Vienna rectifier, power equipment and a charging pile. The Vienna rectifier comprises a filter circuit, a full-bridge circuit and a slow start circuit, the input end of the filter circuit is used for being connected with an alternating current source, and the output end of the filter circuit is connected with the midpoint of a three-phase bridge arm of the full-bridge circuit; the slow start circuit is connected in series between the output end of the full-bridge circuit and the output end of the Vienna rectifier; the slow start circuit comprises a resistor and a switch, and the resistor and the switch are connected in parallel. The output end of the full-bridge circuit is connected with the slow start circuit, that is, the direct current side is connected with the slow start circuit, and relays and resistors for slow start do not need to be connected to three phases of alternating current, so that the number of hardware devices can be reduced, the cost is reduced, the number of the relays is reduced, and the control difficulty is also reduced. The slow start circuit can reduce very high surge current generated in the starting process of the Vienna rectifier, and the safety of devices in the full-bridge circuit is protected.

The application belongs to the technical field of power electronics, and discloses a discontinuous carrier modulation method, system and application for reducing switching loss. The method obtains normalized sinusoidal waves of phase A, phase B and phase C according to a three-phase normalization formula of a SPWM modulation strategy of a three-phase Vienna rectifier; calculates a corrected modulation wave; calculates a zero sequence voltage for reducing switching loss, and injects the zero sequence voltage into the three-phase normalized sinusoidal waves to obtain three-phase Vienna rectifier discontinuous modulation waves of phase A, phase B and phase C for reducing switching loss. The application can greatly reduce switching loss and improve the efficiency of a converter. The application does not need to calculate additional modulation coefficients according to different modulation ratios, and only needs one expression to calculate a zero sequence component under different modulation ratios. The application does not need sector judgment, and only needs to inject a zero sequence component once, so the calculation is simple and the calculation burden of a digital controller can be reduced.

This invention discloses a control method for a Vienna rectifier topology based on SiC power devices. First, a mathematical model of a three-phase Vienna rectifier is established; then, the expressions for the Vienna rectifier's midpoint voltage and current are derived; an inner current loop is designed; an outer voltage loop is designed; and finally, midpoint potential balance control is performed. This invention can reduce the size of the Vienna rectifier system, improve the power factor, and reduce system losses.

The invention discloses a Vienna rectifier control algorithm and system based on real-time parameter dynamic adjustment. Belongs to the technical field of power electronic converter control. The method comprises the following steps: S1, acquiring three-phase voltage, load current and direct-current bus voltage of a power grid in real time; s2, judging whether an open-phase fault occurs or not according to the three-phase voltage; s3, if the open phase does not exist, PI controller parameters of a current loop are dynamically adjusted based on the ratio of the load current to the rated current; meanwhile, feedforward compensation is carried out on a direct-current voltage reference value based on power grid voltage fluctuation; and S4, generating a control signal to drive the Vienna rectifier based on the dynamically adjusted parameters or the fault-tolerant control mode under the open phase. According to the method, the problems that traditional fixed parameter PI control is poor in dynamic response and weak in anti-interference capability and shutdown must be carried out under an open-phase fault are effectively solved, and the stability and the reliability of the Vienna rectifier in a wide load range and under a non-ideal power grid condition are remarkably improved.

The application discloses a kind of multi-mode variable frequency control methods of three-phase four-wire Vienna rectifier, it includes carrier reset pulse generation unit and modulation unit;Carrier reset pulse generation unit includes and gate, first integrator and first RS flip-flop;Modulation unit includes or gate, second integrator and second RS flip-flop, and gate output is as the input of or gate first input port.This application uses logic circuit to realize CCM-CRM-DCM multi-mode variable frequency control, improves the input current quality when light load;And by adding gain compensation in the modulation signal voltage in control loop, offset the control loop gain variation caused by variable frequency control, realizes the average current unified control under different working mode, reduces the zero current distortion under DCM mode, avoids the oscillation and impact caused by different mode control switching, further improves the current quality, improves rectifier performance.

The application is suitable for the technical field of power electronic converter control, and provides a Vienna rectifier modulation method and system based on midpoint current difference observation, which comprises the following steps: calculating the midpoint current difference observation value reflecting the instantaneous imbalance trend of the midpoint potential in real time; determining whether the system is in a steady state or a transient state mode through a hysteresis comparator according to the observation value; in the steady state, adopting a full-cycle continuous clamping DPWM strategy covering zero points in combination with PI control to realize low-loss and low-distortion operation; in the transient state, immediately switching to an SVPWM strategy in combination with high-gain P control to strongly suppress the midpoint potential fluctuation. The application eliminates the voltage feedback lag through current feedforward observation, and adaptively integrates the advantages of the two modulation strategies, solves the problem that the efficiency, harmonics and dynamic performance of the Vienna rectifier are difficult to be considered, and significantly improves the overall performance of the system.

The application relates to the technical field of three-level converter, in particular to an interlaced parallel three-level three-switch rectifier topology circuit, which comprises first to third rectifier modules, a first capacitor and a second capacitor, wherein the first rectifier module comprises a first diode assembly and a first switch assembly, the first diode assembly comprises first to fourth diodes; the second rectifier module comprises a second diode assembly and a second switch assembly, the second diode assembly comprises fifth to eighth diodes; and the third rectifier module comprises a third diode assembly and a third switch assembly, the third diode assembly comprises ninth to tenth diodes. Thus, the problem that each phase circuit of the interlaced parallel Vienna rectifier topology is composed of four power switch tubes and the economic burden is increased is solved. The number of switch tube devices is reduced, and the economic cost is reduced.

The invention discloses an electric energy quality optimization method for a Vienna rectifier. The method mainly comprises the following steps: analyzing an input current distortion mechanism and an output voltage duty ratio expression of the Vienna rectifier based on topological properties; a Vienna rectifier steady-state operation region constraint and a phase difference between an input current and a reference voltage are deduced, and an interval clamping control strategy based on negative sequence current adjustment is provided. According to the method, phase characteristics of input current and reference voltage are adjusted by introducing a negative-sequence current adjustment coefficient, and the negative-sequence current adjustment coefficient is calculated by combining steady-state operation region constraints, so that negative-sequence electrofluid adjustment is realized.

The invention discloses a Vienna rectifier, a charging pile, a control method, a device and a medium, and the Vienna rectifier, and is characterized in that the Vienna rectifier comprises a controller, a three-phase boost inductor, a three-phase uncontrolled rectification circuit and a three-phase neutral point clamping switch tube; the first ends of the three-phase boost inductors are used for being connected with three-phase alternating current. The second end of the three-phase boost inductor is connected with the midpoints of the bridge arms of the three-phase uncontrolled rectification circuit. The three-phase neutral point clamping switch tubes are respectively connected between a bridge arm midpoint of the three-phase uncontrolled rectifier circuit and a midpoint of a direct-current bus; the controller is used for increasing the voltage of the direct current bus if the voltage of the direct current bus is smaller than or equal to preset voltage when receiving a reactive power dispatching instruction, and controlling the three-phase neutral point clamping switch tube to enable the Vienna rectifier to output reactive power when the voltage of the direct current bus is larger than the preset voltage. And outputting reactive power.

The application discloses a kind of electrolytic capacitor-free Vienna rectifier model-free predictive control methods, belong to rectifier control field, the electrolytic capacitor-free Vienna rectifier model-free predictive control method described in the application includes the following steps: step S1, constructs the hyperlocal model of Vienna rectifier, and based on hyperlocal model design preset time sliding mode disturbance observer, compared with prior art, the beneficial effects of the application are: the present application constructs hyperlocal model and designs preset time sliding mode disturbance observer, realizes the preset time accurate observation of total disturbance caused by parameter mismatch and current coupling, overcomes the convergence time uncertainty and chattering problem of traditional observer;According to the observed disturbance, the hybrid duty calculation method is used to compensate the grid-side current and the midpoint voltage, accurately synthesize voltage vector, reduce the prediction error, so as to suppress the midpoint voltage fluctuation and grid-side current harmonic under parameter mismatch, improve the system robustness and control accuracy.

The invention provides an interleaved Vienna rectifier device and a control method thereof. A voltage positive-negative sequence component and a current positive-negative sequence component are separated to separate a voltage positive sequence component from a voltage negative sequence component and to separate a current positive sequence component from a current negative sequence component. And respectively controlling the voltage positive-sequence component, the voltage negative-sequence component, the current positive-sequence component and the current negative-sequence component. In a current inner loop, a proportional integral (PI) controller and a harmonic cancellation (PR) controller are used to control the voltage positive sequence component, the voltage negative sequence component, the current positive sequence component and the current negative sequence component to generate a plurality of control signals to control the switches of the interleaved Vienna rectifier circuit.

The application discloses a control method and system of a Vienna rectifier, which is used for controlling switch tubes of the Vienna rectifier when direct current is input into the Vienna rectifier. The control method comprises the following steps: establishing an equivalent circuit when direct current is input into the Vienna rectifier, collecting output voltage values, output voltage reference values, upper capacitor voltage values and lower capacitor voltage values of the equivalent circuit; obtaining first control results through closed-loop control of an output voltage loop according to the output voltage values and the output voltage reference values, obtaining second control results through closed-loop control of a midpoint potential balance loop according to the upper capacitor voltage values and the lower capacitor voltage values, and obtaining first modulation waves and second modulation waves according to the first control results and the second control results; and comparing the first modulation waves and the second modulation waves with first carriers and second carriers respectively, and controlling switch tubes of the equivalent circuit through first driving signals and second driving signals according to comparison results.

The invention relates to the technical field of rectifier control, in particular to a rectifier self-adaptive zero-crossing distortion suppression method and related products, and the method comprises the steps: determining sector misjudgment conditions near input current zero crossing; establishing a vector error prediction model; constructing a multi-target cost function of the prediction model; performing rolling optimization on the multi-target cost function, selecting a switching state enabling the cost function to be minimum as an optimal switching state, and controlling a rectifier by using the optimal switching state; according to the technical scheme provided by the invention, through deep analysis and modeling of a zero-crossing distortion mechanism, dynamic compensation of a voltage vector error is realized by utilizing model predictive control, and the inherent input current zero-crossing distortion problem of the Vienna rectifier is effectively solved.

The application discloses a midpoint voltage DC offset suppression method for a Vienna rectifier and belongs to the technical field of Vienna rectifier control. The application is aimed at the problem that the bipolar DC bus of the Vienna rectifier has midpoint voltage DC offset under the light load condition of unbalanced load. The application comprises the following steps: calculating the maximum q-axis current of odd and even sectors according to the corresponding relationship of grid current vectors and reference voltage vectors in a voltage space vector diagram; simultaneously calculating the q-axis current amplitude; then calculating the q-axis current instruction value and the d-axis current instruction value; further calculating the d-axis reference voltage u d and the q-axis reference voltage; finally, performing dq-axis system to alpha-beta-axis system conversion on the d-axis reference voltage u d and the q-axis reference voltage u q , and combining space vector modulation to obtain the driving signal of the Vienna rectifier, so that the midpoint voltage DC offset suppression is realized. The application is used for suppressing the midpoint voltage DC offset of the Vienna rectifier.