Showing papers on "PWM rectifier published in 2014"

A novel active power decoupling single-phase PWM rectifier topology

Abstract: This paper proposes a new topology for a pulse width modulation (PWM) rectifier which can achieves both sinusoidal input current on the ac side and ripple power decoupling on the DC side without additional switches. The AC inductor of the conventional H-bridge circuit is divided into two parts and only an auxiliary decoupling capacitor is added in the proposed circuit. Control scheme is also proposed for the circuit working as unity power factor rectifier. Compared with other decoupling circuits, this circuit has the same advantage such as high power density. Due to no additional switches, the proposed circuit has higher efficiency. Effectiveness of the circuit is validated by simulation.

An Improved DC-Link Voltage Fast Control Scheme for a PWM Rectifier-Inverter System

Abstract: This paper presents an improved dc-link voltage fast control strategy based on energy balance for a pulse width modulation (PWM) rectifier-inverter system to reduce the fluctuation of dc-link voltage. A conclusion is drawn that the energy in dc-link capacitor cannot be kept constant in dynamic process when the system operates in rectifier mode, even in ideal case. Meanwhile, the minimum dc-link voltage deviation is analyzed. Accordingly, a predictive dc-link voltage control scheme based on energy balance is proposed, while the grid current is regulated by deadbeat predictive control. A prediction method for output power is also introduced. Furthermore, a small-signal model with the control delay is adopted to analyze the stability and robustness of the control strategy. The simulation and experimental results demonstrate both good dynamic and steady-state performances of the rectifier-inverter system with the proposed control scheme.

Active resonance wireless power transfer system using phase shift control strategy

Abstract: Conventionally, both the primary side and the pickup side need series or shunt capacitor compensation to increase transmission efficiency in a wireless power transfer system. In this paper, a wireless power transfer system with an active resonance circuit is proposed. Comparing the diode based rectifier on the pickup side in traditional wireless power transfer systems, a PWM rectifier is exploited for several advantages. The compensation capacitor on the pickup side is replaced by the active resonance circuit for reactive power compensation. Furthermore, the low power transmission efficiency, which is caused by the compensation capacitor malfunction and the resonance tank detuning, could be alleviated in the proposed system. A phase shift control technique is adopted to regulate output power when input power varies. The mathematic model of the propose system is presented, and the influence of parasitic parameters are also analyzed to reach a more precise performance analysis. A 500W prototype is built, and the experimental results are provided to validate the theoretical model and the effectiveness of the system.

Common-Mode and Differential-Mode Active Damping for PWM Rectifiers

Abstract: Modern pulse-width-modulated (PWM) rectifiers use LCL filters that can be applied in both the common mode and differential mode to obtain high-performance filtering. Interaction between the passive L and C components in the filter leads to resonance oscillations. These oscillations need to be damped either by the passive damping or active damping. The passive damping increases power loss and can reduce the effectiveness of the filter. Methods of active damping, using control strategy, are lossless while maintaining the effectiveness of the filters. In this paper, an active damping strategy is proposed to damp the oscillations in both line-to-line and line-to-ground. An approach based on pole placement by the state feedback is used to actively damp both the differential- and common-mode filter oscillations. Analytical expressions for the state-feedback controller gains are derived for both continuous and discrete-time model of the filter. Tradeoff in selection of the active damping gain on the lower order power converter harmonics is analyzed using a weighted admittance function. Experimental results on a 10-kVA laboratory prototype PWM rectifier are presented. The results validate the effectiveness of the active damping method, and the tradeoff in the settings of the damping gain.

Isolated Swiss-Forward three-phase rectifier for aircraft applications

Abstract: Recently an important increase of the electrical equipment in modern aircrafts is leading to an increase in the demand for electrical power. The usual electrical power distribution in aircraft applications is done via a three-phase 115Vac grid. A new trend of DC distribution is emerging employing a 270 Vdc grid. This yields the need for high-efficiency and high power-density AC-DC converters, connecting the two grids while providing galvanic isolation. Traditionally 12-pulse autotransformer passive rectifiers are used in aircraft applications. This converter is robust and highly reliable, however it is a non-controlled topology and it is relatively heavy because of the low frequency power transformer. Two-stage rectifier system approaches, employing three-phase PWM rectifiers or active filters and isolated DC-DC converters, are good alternatives to be applied in aircraft applications, to reduce the weight by using high frequency transformers and inductors. However, two-stage topologies process the energy twice leading to lower efficiency, power density. The reliability is also not very high due to a high semiconductor component count. This paper presents a new isolated single-stage PWM rectifier system, based on the recently presented non-isolated Swiss rectifier topology, called the Swiss-Forward rectifier. The principle of operation of this converter topology is presented together with detailed design guidelines and experimental validation on a 3.3kW 115Vac to 270Vdc prototype.

The study of single-phase PWM rectifier based on PR control strategy

Abstract: Synchronous PI controller is usually used to track current in three-phase PWM rectifier with zero steady-state error which is difficult to achieve in the single-phase system. A novel proportional-resonant (PR) control scheme for single-phase PWM rectifier is proposed in the paper. Compared with traditional PI control and current hystereis control (CHC) methods, the PR control structure is simple and can reduce control time delay significantly. The simulation results verify the feasibility of the proposed control scheme in the disturbance rejection. In addition, sinusoidal current zero static error control can be achieved without a coordinate transformation and the DC voltage can automatically adjust to changes of grid voltage, load value and frequency which contributes to energy conversion and bidirectional flow of electricity.

Control method based on virtual flux linkage orientation and used for voltage-type rectifier

Abstract: The invention discloses a control method based on virtual flux linkage orientation and used for a voltage-type rectifier. The control method comprises the following steps that symmetrical three-phase currents and direct-current bus voltage flowing into the rectifier in a power grid are collected and virtual flux linkage on the alternating-current side is estimated; according to the collected symmetrical three-phase currents and the virtual flux linkage on the alternating-current side, active power and reactive power which are input into the rectifier are worked out, and meanwhile, the flux linkage vector angle is worked out; power prediction is adopted based on a power inner loop to enable the active power of the rectifier and the reactive power of the rectifier to be tracked to set values; a space vector modulation method is adopted to generate switching signals of the three-phase PWM rectifier, and the switching signals are used for driving a power switching device through a driving circuit. By means of the method, a voltage sensor on the alternating-current side is omitted, the reliability of a system is enhanced, the hardware cost of a device is lowered, a PI controller is omitted in the power inner loop, a control structure is simplified and the parameter setting difficulty is lowered.

Optimal Analysis and Design of Power Electronic Distribution Transformer

Abstract: This study proposes a new design of Power Electronic distribution Transformer (PET) by using an intelligent program to get an optimum HF operation, which leads to maximum efficiency. This PET has three stages; the input stage consists of three phases, three level PWM rectifier and isolation stage employing three level half bridge convertor connected with high frequency transformer. The output stage has a matrix converter. This new design of PET has the advantage of improving power factor, voltage sags, voltage swell, reactive power compensation, eliminating harmonic and controlling and Protecting its self (PET) with minimum size, compared to the traditional transformer. The software like MATLAB and Artificial Neural Network will be used to analyze and simulate the PET.

A survey of cascaded multi-level PWM rectifier with VIENNA modules for HVDC system

Abstract: For a Voltage Source Converter based High Voltage Direct Current Transmission(VSC-HVDC)system the most important part is the VSC part because it determines main performance of the whole system. This paper proposed a new topology of PWM AC/DC converter which is cascaded VIENNA rectifier. In addition, in this paper a current controller based on the working modes of three-phase VIENNA rectifier will be introduced to control input currents. By this current controller the input currents can follow the corresponding references to achieve the goal of unit power factor. When this current controller is applied on the proposed cascaded VIENNA rectifier the performance will be investigated too. In order to verify the correctness of the current controller and the cascaded VIENNA rectifier, the prototype of three-phase VIENNA rectifier and single-phase two modules cascaded VIENNA rectifier are developed. The current controller and the proposed topology is validated by the way of analysis of the experimental results.

A New DPC for Three-phase PWM rectifierwith unity power factor operation

Abstract: This paper, presents the performance evaluation for new methods, applied to three-phase PWM rectifier with unity power factor operation, based on virtual flux called Direct Power control (VF-DPC). The principle of this control is based on the instantaneous active and reactive power control loops. The results show the excellent advantages, particularly: nearly sinusoidal input current, regulation of input power factor to unity, low harmonic distortion of line current (THD below 5%), adjustment and stabilization of DC-link voltage (or current), reduced capacitor (or inductor) size due to the continues current.

PWM (Pulse-Width Modulation) rectifier control method and PWM rectifier control device based on model prediction control

Abstract: The invention discloses a PWM (Pulse-Width Modulation) rectifier control method and a PWM rectifier control device based on model prediction control. The PWM rectifier control method comprises the following steps: calculating and predicating active power and reactive power of the next moment; combining the model prediction control and direct current control; taking a negative value conjugated with complex power in a prediction model as a variable, and judging a sector of the variable for one time by adopting a rapid vector selection method to obtain an optimal voltage vector; dividing the judged sector into two small parts by taking the position of a reference complex power vector as the standard; selecting a suboptimum voltage vector according to the position of the reference complex power vector; and calculating acting time of the optimal voltage vector and the suboptimum voltage vector in a control period to obtain a driving signal of a switching tube. The invention further discloses a PWM rectifier control method based on the model prediction control. The PWM rectifier control method and the PWM rectifier control device provided by the invention have the characteristics of small calculation amount, easiness in realization, rapid dynamic responses, good stable-state performance and the like.

The main parameters design based on three-phase voltage source PWM rectifier of voltage oriented control

Abstract: Based on the three-phase voltage source PWM rectifier of voltage oriented control, a method of the main parameters design is proposed. The static and dynamic characteristics of the system are affected by the parameters of the hardware seriously. The mathematical of three-phase PWM rectifier is set up, the simulation of the system is done with Matlab/Simulink, and the application example is realized baseed on the hardware platform of DSP2812.

Three-phase PWM rectifier control method

Abstract: The invention provides a three-phase PWM rectifier control method which includes a direct current capacitor energy storage outer loop control method and a grid-side current inner loop control method. The method includes the steps that direct current capacitor stored energy of a rectifier is fed back as a whole, given capacitor charging power is output after calculation is finished through an energy PI adjuster, grid-side input power is obtained by adding load power to the given capacitor charging power, a given grid-side d-axis current value i*d is obtained by dividing grid-side input power by the grid-side d-axis voltage component, a given q-axis current value i*q is obtained according to the relation between given d-axis current and given q-axis current, a grid-side voltage control value ud and a grid-side voltage control value uq are obtained in sequence according to the grid-side current inner loop control method, given voltage values are obtained through feedforward and decoupling of the ud, uq and the network voltage, given phase voltage output values are obtained through Park inverse transformation and Clark inverse transformation, and switch control signals Sa, Sb and Sc used for driving a PWM rectifier bridge arm electronic switch are obtained finally through space vector pulse width modulation. By means of the method, the dynamic response performance of a three-phase PWM rectifier can be improved greatly, and the method is particularly suitable for fast charging of electric cars and other occasions having high requirements for the response performance of the three-phase PWM rectifier.

Control method for three-phase PWM rectifier suitable for power grid waveform distortion

Abstract: The invention discloses a control method for a three-phase PWM rectifier suitable for power grid waveform distortion. The control method for the three-phase PWM rectifier suitable for the power grid waveform distortion mainly includes that extracting a current reference signal under input voltage distortion and unbalanced disturbance, and actively injecting negative-sequence current, improved dead-beat inner current loop which is forecast through combining with linear extrapolation, and outer direct voltage loop performed with active disturbance rejection control. The control method for the three-phase PWM rectifier suitable for the power grid waveform distortion plays roles of inhibiting the harmonic wave of the input current of the PWM rectifier, injecting proper negative-sequence current to eliminate the secondary power pulse of the output direct current voltage of the PWM rectifier, improving the transient response of the output direct current voltage, and compensating the influences of all kinds of delay on the dead-beat control. The control method for the three-phase PWM rectifier suitable for the power grid waveform distortion is capable of obviously reducing the secondary wave of the output direct current voltage under the disturbance of the input harmonic wave and unbalanced voltage, improving the transient response speed of the PWM rectifier, reducing the transient offset of the direct current voltage, and improving the system control precision.

Variable-speed constant frequency electricity generation control device and electricity generation method based on brushless doubly-fed motor

Abstract: The invention relates to a variable-speed constant frequency electricity generation control device and electricity generation method based on a brushless doubly-fed motor. The device comprises a double-PWM frequency conversion system and an electricity generation control system for controlling the double-PWM frequency conversion system. The electricity generation control system comprises a setting unit, a power winding voltage detection unit, a control winding current detection unit, a voltage outer loop controller, a current inner loop controller and a phase-locked loop controller, and the voltage outer loop controller, the current inner loop controller and the phase-locked loop controller drive a PWM rectifier and a PWM inverter to achieve system variable-speed constant frequency electricity generation in a space vector control mode. By the adoption of the vector control mode, the double-closed-loop feedback control method and the phased-locked loop control method, even though loads or prime power changes and causes fluctuation of rotation speed, current and frequencies of windings can be controlled by accurately adjusting the brushless doubly-fed motor, so that the electricity generation voltage amplitude and the frequencies on the power winding side keep stable, the control process is simple, and the device and the method are applicable to wide use.

Fuzzy logic based direct power control for PWM three-phase rectifier

Abstract: This paper discusses a proposed control strategy based on fuzzy logic for three-phase PWM rectifiers. This technique is called direct control of fuzzy power (DPCF). The objective of this paper is to replace both hysteresis controllers that role for setting the active and reactive power by a single fuzzy controller which adjusts both active and reactive power at the same time. The aim of this work is to eliminate the harmonic currents and therefore reduce the total harmonic distortion of the line current and improve the power factor. This DPCF is tested by simulation studies on a three-phase PWM rectifier.

Space Vector Pulse Width Modulation Technique Based Design and Simulation of a Three-Phase Voltage Source Converter Systems

Abstract: A Space Vector based Pulse Width Modulation control technique for the three-phase PWM converter is proposed in this paper. The proposed control scheme is based on a synchronous reference frame model. High performance and efficiency is obtained with regards to the DC bus voltage and the power factor considerations of the PWM rectifier thus leading to low losses. MATLAB/SIMULINK are used as a platform for the simulations and a SIMULINK model is presented in the paper. The results show that the proposed model demonstrates better performance and properties compared to the traditional SPWM method and the method improves the dynamic performance of the closed loop drastically. For the Space Vector based Pulse Width Modulation, Sine signal is the reference waveform and triangle waveform is the carrier waveform. When the value sine signal is large than triangle signal, the pulse will start produce to high. And then when the triangular signals higher than sine signal, the pulse will come to low. SPWM output will changed by changing the value of the modulation index and frequency used in this system to produce more pulse width. The more pulse width produced, the output voltage will have lower harmonics contents and the resolution increase. Keywords—Power Factor, SVPWM, PWM rectifier, SPWM.

3-Phase AC-DC converter topologies with higher frequency transformer isolation for utility grid interface

Abstract: Two AC-DC converter topologies for 3-phase utility grid interface using high frequency (HF) or medium frequency (MF) transformers have been proposed in this paper. The transformer primary side converter structure is similar in both the topologies (say T-I and T-II), but the secondary side converter structures as well as the modulation schemes are different. The line-frequency AC voltage from the utility is first converted to HF or MF AC using three 1-phase AC-AC converters. The AC-AC converters can be unidirectional if power flow is desired in only one direction. The transformer core can be of silicon steel, amorphous or nano-crystalline material. The secondary side of topology T-I consists of a 3-phase PWM rectifier, while topology T-II consists of three 1-phase boost power factor correction (PFC) stages with a common output DC-bus. Both topologies avoid DC-link capacitors on the primary side and allow for high density power conversion. Potential applications include electric vehicle battery charging, telecom power supply, adjustable speed drives, server power supplies, etc. Simulation results and experimental waveforms from laboratory prototype evince the operation of the topologies.

Current hysteresis control digital implementation system for PWM rectifier

Abstract: The invention discloses a current hysteresis control digital implementation system for a PWM rectifier. TMS320F28069 DSP of Piccolo series of TI is adopted to achieve current hysteresis control, the functions of a chip dual-core processor are fully utilized, logic control, communication and three-phase current given generation for the PWM rectifier are achieved in a main CPU, a current hysteresis control strategy is achieved in a control rate accelerator, PWM pulses are generated to drive a power switch tube of a major loop, and the functions of the PWM rectifier are achieved. The control rate accelerator introduces the concurrent control loop execution function into a C28x series device. In this way, time delay existing from ADC damping to output is greatly reduced, and faster system response and a higher-frequency control loop are achieved. CLA is used for providing services for the control loop with strict time requirements, so that the main CPU freely processes other system tasks such as communication and diagnosis.

Modified one-cycle-controlled three-phase pulse-width modulation rectifiers with a reduced switching frequency to power line frequency ratio

Abstract: In the aircraft power systems with 360-800 Hz line frequency or medium- and high-power three-phase pulse-width modulation (PWM) rectifier applications operating at the utility line frequency (50/60 Hz), the ratio of switching frequency to power line frequency will be reduced because of the increased line frequency or low switching frequency. Therefore the assumptions that the average voltage drop across the input inductor of the PWM rectifiers can be neglected if the conventional one-cycle controlled (OCC) method had been adopted can no longer be true anymore, which will deteriorate the system power factor (PF) dramatically. To solve this issue, a modified OCC scheme is proposed in this study. The proposed control method can improve the input PF of three-phase PWM rectifier without sacrificing the advantages of the conventional OCC scheme, such as no phase-locked loop and constant switching frequency. The control equation of the modified OCC method, which considered the voltage drop across the input inductor, is first derived. The theoretic analysis is supported by the experimental verification performed by a 2.5 kW modified OCC-based three-phase three-switch Vienna rectifier.

Integrated isolation and voltage balancing link of 3-phase 3-level PWM rectifier and inverter systems

Abstract: For a 3-phase pulse width modulated high-bandwidth AC voltage source, this paper presents a series resonant DC-DC converter (SRC) with a high-frequency transformer operated by only two half-bridges interconnecting a 3-phase 3-level rectifier and 3-phase plus neutral conductor 3-level inverter stage. On the primary side, one terminal of the transformer is connected through the resonant capacitor and inductor to one bridge-leg output and the other terminal is connected to the DC-link midpoint. On the secondary side, both terminals directly connect to the second bridge-leg output and the DC-link midpoint. With the proposed SRC, the galvanic isolation and the balancing of the capacitor voltages of the inverter-side split DC-link can be achieved also for an unequal loading of the DC-link capacitors. The AC source needs to supply not only passive symmetrical 3-phase loads, but also passive or active single-phase, 2-phase, DC and asymmetrical 3-phase loads. Hence, the unequal loading of the DC-link capacitors can be generated, for example, in case a DC load is connected to the 4-wire inverter stage. The operation principle of the SRC is described in detail for an unequal loading of the DC-link capacitors. Moreover, design guidelines for the suggested SRC are derived and finally the theoretical analysis is successfully verified by measurements conducted on a 1 kW proof-of-concept SRC prototype.

Improved Virtual-Flux-Linkage Observation Method of PWM Rectifier

Abstract: In order to improve the virtual flux oriented vector control performance of PWM rectifier, an improved observation method of virtual flux-linkage is proposed. To avoid the relevant problems of pure integrator, and to achieve the accurate observation of the grid voltage’s phase, three first order low-pass filters are cascaded to displace the pure integrator. Based on the SVPWM modulation module and the presented algorithm, grid voltage sensorless control system of three-phase PWM rectifier is given. Simulation results have shown that: either in rectifying or inverting status, the amplitude and phase of virtual flux-linkage can be observed accurately, i.e. the control system of PWM rectifier can operate reliably without the grid voltage sensors; the presented estimation method of virtual flux-linkage is effective.

Motor Control Apparatus With Power Failure Determination Unit

Abstract: A motor control apparatus includes: a PWM rectifier; an inverter; a detection unit for detecting an AC voltage value of the PWM rectifier; a calculation unit for calculating a power supply voltage phase; a detection unit for detecting an AC current value of the PWM rectifier; a current loop control unit for generating an AC voltage command to control a power conversion operation of the PWM rectifier; a current loop saturation state determination unit for determining to be in a current loop saturation state when a magnitude of the AC voltage command is equal to or larger than a predetermined voltage prescribed value; an operation determination unit for determining whether the PWM rectifier is in a powering operation or in a regenerative operation and a power failure determination unit for determining a presence or absence of a power failure at the AC power supply side from the determination results.

A low harmonic double star rectifier with current injection at DC side

Abstract: In order to reduce the harmonics in input currents effectively, this paper presents a double star rectifier with an auxiliary PWM rectifier at DC side. The volt-ampere rating of auxiliary PWM rectifier is only 10% of output power of the double star rectifier. The auxiliary PWM rectifier injects a particular triangular wave current into the secondary winding of inter-phase transformer to reduce the THD of input currents. The resulting system draws near sinusoidal utility line currents with less than 5 % THD. The auxiliary PWM rectifier also extracts the harmonic energy and feeds to the load. Detailed analysis of the proposed scheme is presented, and the results of experiment verify the analysis.

Novel reactive power control strategy based on constant DC-capacitor voltage control for reducing the capacity of smart charger for electric vehicles on single-phase three-wire distribution feeders

Abstract: This paper proposes a novel reactive power control strategy to reduce the capacity of the previously proposed smart charger for electric vehicles (EVs) on single-phase three-wire distribution feeders. The proposed reactive power control strategy is based on the constant dc-capacitor voltage control of the grid-connected PWM rectifier. Any calculation blocks of load-side active current are not needed. Thus, we offer the simplest reactive power control strategy. The basic principle of the proposed reactive power control strategy is discussed in detail, and then confirmed by a digital computer simulation using PSIM software. A prototype experimental model is constructed and tested. Experimental result demonstrates that balanced source currents with a power factor of 0.9, which is acceptable value in Japanese home appliances, are obtained on the secondary side of the pole-mounted distribution transformer during battery charging operation in EVs reducing the capacity of the smart charger by 36% as compared with that of the smart charger with the previously proposed control strategy.

Direct power control of PWM rectifier under unbalanced grid voltage

Abstract: PWM rectifier controlled by direct power control (DPC) is characterized by the merits of quick response, no need of inner current tuning and strong robustness, etc. However, conventional table-based DPC cannot work properly when the grid voltage is unbalanced. Various methods have been proposed to mitigate the influence of grid unbalance but they are usually complicated and require positive/negative sequence extraction of grid voltage and/or currents. This paper proposed a novel DPC, which can achieve sinusoidal grid currents and constant active power without using complicated sequence extraction. The simplicity and robustness of conventional DPC is maintained due to the use of a novel reactive power definition. Its effectiveness is validated by simulation and experimental tests.

Vector control of three-phase voltage source PWM rectifier based on fractional-order controller

Abstract: In this paper, the dual closed-loop vector control of the three-phase voltage source pulse-width modulated (PWM) rectifier based on fractional-order controller is studied. The classical proportional integral (PI) controller is used in the inner current loop. The feed-forward decoupling control method is established to regulate active currents and reactive currents. In this way, a power factor of the system is close to one. The fractional-order controller is used in the outer voltage loop to stable the direct current (DC) output voltage. It can enhance the robustness of the system, weaken impacts from the load disturbance and the grid disturbance to the output characteristics of the system, and ensure that the DC voltage remains stable with the parameters changing. Correction method which combining the frequency-domain analysis and flat phase condition is used to tuning the fractional-order controller. Through theoretical analysis and simulation research, we proved that the method enhances the immunity and robustness of the system. With the virtue of faster respond and less overshoot, it can get better control effectiveness.

Current control method for three-phase voltage source type PWM rectifier

Abstract: The invention discloses a current control method for a three-phase voltage source type PWM rectifier. The current control method comprises the steps that in a static coordinate system, a dual-closed-loop vector control strategy is adopted, an alternating-current given value of the rectifier is calculated to be obtained by collecting an alternating-current side voltage, an alternating-current side current and a direct-current voltage of the rectifier, the alternating-current given value and an actual value are compared, calculated and fed into an H-infinity current regulator, an output signal of the H-infinity current regulator and a network voltage feed forward signal are added to obtain a bridge terminal voltage reference signal of the rectifier, and then a switch control signal of the rectifier is generated through PWM and used for controlling an IGBT to be switched on or off so as to achieve the electric energy conversion function. According to the current control method for the three-phase voltage source type PWM rectifier, the H-infinity current regulator is designed to be transformed to solve the H-infinity standard problem by establishing a mathematic model of the rectifier and the filter of the rectifier; due to the influence of the problems of rectifier parameter uncertainty, inherent LC oscillation and the like, a reasonable weight function is constructed to conduct optimal design on the H-infinity current regulator, the output current of the three-phase voltage source type PWM rectifier is regulated through the H-infinity current regulator, and the direct voltage and the output power of the rectifier are controlled.