The thesis proposes and analyzes a scheme for improving the waveform of the input current of a three-phase bridge rectifier. To reduce ripple in the DC output voltage, a large filter is used at the output of a single-phase rectifier, and a small output filter is used in a three-phase rectifier. In three-phase rectifiers, the input current has the form of a periodic square wave, which is non-sinusoidal and discontinuous in nature.
A fifth-order harmonic injection PWM concept has been developed for the harmonic reduction of the input current of a three-phase diode rectifier. The simulation result shows that the input current of the three-phase rectifier becomes sinusoidal with less than ten percent THD. 15 Figure 2.10 Three-phase rectifier circuit with 5th harmonic injection 16 Figure 2.11 (a) Input line voltages of three phases (b) Input line currents of three.
Introduction
- Background
- Literature review
- Objective and expected result of the thesis
- Thesis Outline
In the diploma thesis, the harmonic injection technique was implemented, which reduces the harmonics of the mains frequency of the three-phase rectifier. As previous studies have shown, the degree of distortion depends on the ratio of the output voltage to the input mains voltage. To alleviate this problem, various modulation techniques have been proposed to reduce the harmonic distortion of the input currents without increasing the output voltage beyond practical levels.
The first approach proposed to improve the harmonic distortion of the input currents involves operating the single boost rectifier in critical mode [17-18]. As a result, the switching frequency becomes variable and the effective modulation of the duty cycle over the line cycle results in reduced THD of the input currents. Another approach to improving the THD of the input currents involves controlling at a constant level the average current of the amplifier diode.
A simple technique that can be used to reduce the harmonic distortion of the input current is the harmonic injection method. Spice-based computer simulation of the overall scheme is discussed and analyzed in the third chapter.
The Parallel Active Filter System
Introduction
Characteristics Analysis and Application consideration of the Parallel Active Filter
- Harmonic producing Nonlinear loads
- Three phase rectifier load with conventional passive filters
- Harmonic Injected Three phase rectifier Load without passive input filter
- Three phase rectifier load with 5 th and 7 th harmonics
- Three phase rectifier load with injection of 5 th harmonic current
- Harmonic injected three phase rectifier Load with passive input filter
- Three phase rectifier load with 5 th and 7 th harmonics injection and input passive filter
- Three phase rectifier load with injection of 5 th harmonic current and Input passive filter
The expression for the Fourier series of the input current of a three-phase rectifier with resistive load is,. Inductors with a value of 30mH and delta-connected three capacitors of the value of 20uF are connected to the input side of the three-phase rectifier. From the earlier calculation, the power factor is reduced compared to that of the three-phase rectifier circuit without input filter.
In the input current of a three-phase diode rectifier, the dominant harmonics are the 5th and 7th harmonics. The three-phase rectifier circuit and the current source circuit of the 5th and 7th harmonic injection are shown in Figure 2.7. Although in a three-phase nonlinear load-like diode rectifier the dominant harmonics are the 5th and 7th harmonics, PSpice simulation shows that the 5th harmonic has a significant effect on improving the THD.
At this point, the power factor and efficiency for a three-phase rectifier with 5th and 7th harmonic current injection and the power factor efficiency for a three-phase rectifier with only 5th harmonic injection can be calculated using equation no. In order to reduce the problem of efficiency and pf degradation, a three-phase rectifier circuit with 5th and 7th harmonic current injection is investigated with a passive filter. Although the 5th and 7th harmonics are the dominant harmonics in the three-phase diode rectifier, PSpice simulation finds that the 5th harmonic has a significant effect on THD improvement by injecting it directly opposite in the input line.
The three-phase rectifier circuit and the 5th harmonic generating current source circuit for injection are shown in Figure 2.17. The power factor and efficiency for the three-phase rectifier with the injection of 5th Harmonic current including Input passive filter and the power factor and Efficiency for 5th harmonic current injected three-phase rectifier with input passive filter are calculated by the equations no.
Topology description of The Parallel Active Filter
Hybrid filters combine passive and active filters in various configurations to reduce the initial cost and increase the efficiency of the filter structure. The basic principle of hybrid filtering is to improve the filtering capacity of a passive filter and to dampen series and parallel resonances with a small rated active filter. In this proposed scheme, combination of a passive filter and active filter is designed to compensate the Total harmonic distortion as recommended in IEEE standards.
To realize a current source for a three-phase three-wire PAF, Figure 2.20 shows a pulse width modulation (PWM) current source converter (CSI) with a DC inductor, where the CSI uses IGBTs with series-connected diodes for feedback blocking capability. The CSI filter creates a current at the point of common connection (PCC) to cancel the harmonic current in the AC system, correct the power factor and balance the load. Therefore, the AC distribution system carries only the active base component of the load current.
The main advantage of the power source inverter is that it increases the voltage to the grid itself, so additional AC/DC and DC/DC converters may not be needed. To properly gate the power switches of a three-phase CSI topology, two important constraints must be met: 1) the AC side is primarily capacitive and thus must not be shorted; this means that at any time a maximum of one upper switch (S1, S3 or S5) and one lower switch (S2, S4 or S6) must be closed and 2) the DC bus is of the current source type and so it cannot be opened; therefore, at least one upper switch and one lower switch must always be closed (Fig. 2.20). Both restrictions can be summarized by saying that only one top switch and one bottom switch may be closed at any time.
The constraints are reduced to nine valid states in three-phase CSIs, where states 7–9 (Table 2.5) produce zero ac line currents. In this case, the DC link current is freewheeling through either the switches S1 and S4, S3 and S6, or S5 and S2.
Fifth Harmonic Current Injection by Active Power Filter
- Introduction
- DC source for the inverter
- Pulse Width Modulator
- Switching Ripple Filter Topology
- Isolator circuit
- The power circuitry of the overall system and result
- The complete circuit of proposed three phase rectifier compensated by Active CSI filter
- Simulation Result
The waveform of the pulse width modulator technique found by PSpice simulation is shown in Figure 3.1 and Figure 3.2. The PWM voltage and current ripple generated by the CSI of the PAF power supply circuit can be propagated to the power line through the PCC where the PAF system is connected to the power supply system. Since the objective of the filter is to sink the switching ripple currents through its path by providing a low impedance, the value of Rf must be low.
By tuning fs of the LCR filter to fsw, the most dominant switching ripple currents in fsw are filtered through the low-impedance path. When a tuned LCR filter is designed to absorb low frequency harmonics of the load current (conventional approach), the filter capacitor Cf is sized according to the reactive power requirement of the load at the fundamental frequency. The SRF, Cf is scaled by the attenuation of the second dominant switching ripple currents at 2fsw.
The Rf value used for the filter is 1mΩ, since the filter is tuned at 5 kHz, the impedance of the filter at 5 kHz is low. As the size of the filter capacitor decreases, the impedance of the filter at 2fsw (10kHz) increases, which means that the attenuation of the second dominant switching ripple currents at 2fsw will be poor. Therefore, the size of the capacitor should be kept as high as possible for better attenuation of switching ripple currents.
A low fp value is undesirable because it increases the chance of oscillations at this frequency, which is explained below. The parallel resonance condition can be explained by considering Figure 3.5, which illustrates the equivalent circuit model of the load, PAF, SRF and AC mains (source) at harmonic frequencies. In Figure 3.5 the load and the PAF are modeled as a current source of IH at harmonic frequencies.
This IH current source models uncompensated current harmonics in the PAF bandwidth range, load current harmonics over the PAF bandwidth, and switching ripple currents of linear current regulators in fsw. The IHS line current is then desired to be the sum of the uncompensated current harmonics over the PAF bandwidth and the load current harmonics over the PAF bandwidth. The necessary circuits used for this purpose are shown in Figure 3.6, where gnd 4 and gnd 1 are separate bases and the gate pulses are connected to switches 1 and 4 of the power circuit.
Once the whole system is modeled in the computer simulation environment, the performance of the CSI circuit as PAF is studied. The modulating signal generator circuit is shown in Figure 3.8, and the pulse width modulator and optocoupler circuit to isolate the switching circuit is shown in Figure 3.9.
Conclusion and Recommendation for Future Works
Conclusion
Future work
Koizumi, "Optimal control of a three-phase boost rectifier for unity power factor and reduced harmonics". 15] S. Chattopadhyay and V. Ramanarayanam, "Digital implementation of a line current shaping algorithm for three-phase high power factor boost rectifier without input voltage sensing". Uceda, “Single switch three-phase power factor controller under variable switching frequency and discontinuous input current.
Zach, "Space Vector Based Analytical Analysis of Input Current Distortion of a Three Phase Switched Mode Boost Rectifier System". Lee, “Harmonic Reduction Techniques for High-Order Harmonic Injection Intermittent Single-Pump Rectifier,” IEEE PESC, no. Jovanovic, "A New Harmonic Input Voltage Supply Injection with Nonlinear Gain Control for the Three-Phase Switched DCM Boost Rectifier".
