Three-Phase Diode Bridge Rectifier With Low Harmonics: Current Injection Methods

Three-Phase Diode Rectifiers with Low Harmonics: Current Injection Methods

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The rectifier consists of a three-phase diode bridge, comprising diodes D1 to D6. In the analysis, it is assumed that the impedances of the supply lines are low enough to be neglected, and that the load current I OUT is constant in time. The results and the notation introduced in this chapter are used throughout the book. First, let us assume that the rectifier is supplied by a balanced undistorted three-phase voltage system, specified by the phase voltages:.

Waveforms of the input voltages are presented in Fig. Assuming that I OUT is strictly greater than zero during the whole period, in each time point two diodes of the diode bridge conduct. Since one phase voltage cannot be the highest and the lowest at the same time for the given set of phase voltages specified by 2. This results in an input current equal to zero in the time interval when the phase voltage is neither maximal nor minimal.

The gaps in the phase currents are the main reason for introducing the current injection methods, as they are analyzed in the next chapter. The described operation of the diodes in the diode bridge results in a positive output terminal voltage equal to the maximum of the phase voltages, i. Waveforms of the output terminal voltages specified by 2. These waveforms are periodic, with the period equal to one third of the line period; thus their spectral components are located at triples of the line frequency.

These Fourier series expansions are used frequently in analyses of various current injection methods. Some useful properties of the Fourier series expansions of the output terminal voltages should be underlined here. First, both Fourier series expansions contain spectral components at multiples of tripled line frequency, i.

The corresponding spectral components of v A and vB at odd triples of the line frequency at. On the other hand, the corresponding spectral components at even triples of the line frequency, at 6k0 , have the same amplitudes, but opposite phases. These properties are used in the design of current injection networks described in Chapters 6 and 8. The diode bridge output voltage is given by. Since spectra of v A and vB have the same spectral components at odd triples of the line frequency, these spectral components cancel out in the spectrum of the output voltage.

Thus, the spectrum of the output voltage contains spectral components only at sixth multiples of the line frequency. Another waveform of interest in the analyses that follow is the waveform of the remaining voltage, vC , i. A node in the circuit of Fig. In each point in time, one of the phase voltages equals v A , another one equals vB , while the remaining one equals vC. Thus, the output terminal voltages and the remaining voltage add up to zero.

In the spectrum of the remaining voltage the spectral components are located at odd triples of the line frequency, since the spectral components of v A and vB at even triples of the line frequency cancel out. The spectral components of v AV are located at odd triples of the line frequency, the same as in the spectrum of vC.

After the waveforms of the rectifier voltages are defined and their spectra derived, waveforms of the rectifier currents are analyzed. In the analysis of the rectifier currents, let us start from the states of the diodes.

Values of the diode state functions are summarized in Table , while the waveforms of the diode state functions during two line periods are depicted in Fig. From the data of Table it can be concluded that the rectifier of Fig.

This significantly simplifies the analysis, as seen in Chapter 9, where the discontinuous conduction mode of the diode bridge is analyzed, though with significant mathematical difficulties, since the circuit cannot be treated as a periodically switched linear circuit. Diode state functions. Waveforms of the input currents are presented in Fig.

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From the rectifier input power given by 2. This value for the power factor is reasonably good, and satisfies almost all of the power factor standards. It is significantly better than the power factor value of the rectifier with the capacitive filter connected at the output, which forces the rectifier to operate in the discontinuous conduction mode.

The result is also good in comparison to single-phase rectifiers. Thus, the power factor value of 2. The parameter of the rectifier of Fig.

The fundamental harmonics of the input currents are displaced to the corresponding phase voltages for. This THD value is considered relatively high, and its reduction is of interest in some applications.

Efficient methods to reduce the THD value of the input currents in three-phase diode bridge rectifiers are the topic of this book. Some standards limit amplitudes of particular harmonic components of the input currents. For example, since the passive filter is parallel with the utility system impedance, a resonance condition may result that could cause an over-voltage condition at the point of common coupling.

In addition, besides "shunting" or "sinking" harmonic currents generated from the power conversion system, the passive filter further sinks harmonic currents generated elsewhere in the utility system. Consequently, the power ratings of the passive filter components must be increased to handle the additional load requirements.

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These high power components raise the cost of the filter. Active filters are also connected to the alternating current power utility system when they are used. When active filters are used, harmonic currents generated by the power conversion system from AC-DC or reverse are measured on the utility system. The active filter includes a switch-mode power electronics converter that supplies the harmonic currents drawn by the power conversion system so they are not emanating from the utility system.

However, since the harmonic currents can be almost as large in magnitude as the fundamental frequency current, the power rating of the active filter approaches that of the power conversion system thereby making this filtering technique quite expensive to implement. As opposed to the passive or active filtering techniques discussed above, which compensate for harmonic currents by either shunting or supplying the harmonic currents, respectively, the wave shaping technique attempts to draw from the utility system a current that is sinusoidal at the fundamental frequency 60 Hz in the U.

Three-phase Diode Bridge Rectifier

Commonly, this technique includes a switch-mode interface consisting of six control switches such as power transistors or gate-turn-off thyristors GTO , each with a diode in antiparallel. This is a bidirectional current power interface, and permits the line currents drawn from or supplied to the utility system to be actively shaped to be sinusoidal. In addition, as opposed to the passive and active filters discussed above, this type of power conversion interface allows the DC voltage on the DC power system to be regulated at any desired value greater than the peak line to line voltage.

However, due to the six-switch topology of this interface, and since power flow is typically only in one direction, that is, the switch mode interface is either functioning as a converter or an inverter, this type of interface is considered too expensive to implement. Modulation of the currents on the DC side, at a harmonic frequency, and injecting the modulated current back into the AC side has been demonstrated in the prior art.

However, in the prior art, separate sources for generating the harmonic current are used, and they do not use the direct current. In order to obtain a pure DC current, the prior art devices need large, costly inductors in the DC system, and they require an isolation transformer between the utility system and the DC system for operation and injection of the current which also raises the cost.

Examples of two types of systems that provide a harmonic distortion reduction using injection of a harmonic current are the A.

IEE Vol. Bird, J. Marsh and P. In the Ametani article, current sources are used as shown in FIG. In each instance, a separate source for generating the harmonic current that is injected is provided. An injection of a harmonic current is also shown in the Bird et al.

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The prior art systems do not provide a controlled or regulated DC output, as does the present device, and the prior art devices required transformers for injection of the current for reducing harmonic distortion. In the prior art, for the generation of harmonic currents a 4-quadrant converter will be required whose output voltage as well as current is alternating at the harmonic frequency.

In the present device, the function of harmonic current generation is combined with the DC output regulation. As a consequence, only one quadrant converters are needed, whose output voltage and current both are DC, and always one polarity. The present invention relates to a sinusoidal current interface for a three-phase utility system to reduce the total harmonic distortion caused by converting AC power to DC power and vice versa, by modulating the current outputs of the rectifier at a desired harmonic of the base frequency and applying these as currents to be injected into the AC side of the utility system through an impedance network, such as a set of series connected inductor and capacitor branch, wherein each branch is connected to a line of the three-phase system.

The modulation of the DC system current by a third harmonic current and simultaneous injection of the third harmonic current, which is synchronized and in phase with the AC system and which is at a desired controlled level, will reduce the total harmonic distortion of the utility system currents.

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