05-07-2013, 02:38 PM
THREE PHASE DUAL CONVERTER
[attachment=56486]
Introduction
Thyristor based three-phase controlled rectifiers are widely used in the industry for controlling dc motor drives. Controlled rectifiers offering power conversion from ac to dc are reliable and have higher lifetime compared to other converters. DC motors have higher torque than ac motors and hence are suitable for variable speed and speed reversing applications requiring high torques. Although the operation of controlled rectifiers is simple, the realization of the converter control circuit is complex in nature. The control circuit needs the basic functionalities like : Six state pulse generation and gate drive isolation, Synchronization of the control pulses to the power frequency, Smooth transition in phase angle control, Start-up control, Phase sequence check, field excitation check and over-current protection. If the pulse pattern generation and other control tasks are to be undertaken by a single processor, an ultra high speed processor is needed because of the real-time nature of the system. Moreover, special design interfaces are required for sensing excitation loss and phase sequence checks. This makes the system implementation costly. A new approach is proposed where a hybrid type circuit generates the real time pulses for the converter and a processor supervises the controller functionality. The processor sets the phase angle, monitors the current, phase sequence, excitation condition and external control inputs for start, stop, speed change and speed reversal operations. For compact and cost effective design, instead of using a general purpose microprocessor along with peripheral interfaces, a single microcontroller chip may be used for the implementation.
Project Summary
The block diagram of the three phase dual converter is shown in figure 1. This three phase dual converter basically includes six transformers, three full wave bridge rectifier circuits, three mosfet, firing circuits and the multimeter as shown in figure. The connection of these component are shown in figure 1. Multimeter are used to check the voltage at different-different points. In this project (Three Phase Dual Converter) the three supply is feed from source. The dual converter converts the ac supply into the dc supply and again this dc supply is converts into the ac supply. The output waveform of this dual converter is obtained by connecting the CRO.
Practical Dual Converter
With the firing angles controlled in a manner that α1+ α2= 180° and with both the converter in operation, their average output voltages are equal and have the same polarity. One converter will be operating as a rectifier with firing angle α1 and the other as an inverter with firing angles (180- α1). Through their average output voltages are equal, yet their instantaneous voltages v01 and v02 are out of phase in a practical dual converter. This result in a voltage difference when the two converters are interconnected and as a consequence, a large circulating current flows between the two converters but not through the load. In practical dual converters, this circulating current is limited to a tolerable value by inserting a reactor between the two converters. The circulating current can however, be avoided provided the converters are triggered suitably. In general, a dual converter can operated in the two modes, without circulating current and with circulating current.
Many modern power conversion systems require a bidirectional energy transfer capability as a central part of their system operation. Preferably, such systems should use a single high efficiency power electronic conversion system to reduce size, weight and cost. For higher power bidirectional conversion, the common topology proposed is a dual active bridge structure, where two DC-AC converters are coupled back-to-back through an AC inductor/transformer. Either single-phase or three-phase converters have been proposed for such a system, but to date no clear-cut basis for selecting between these two alternatives has been established. A converter basically consists of an array of on-off electronic switches that use power semiconductor devices. If the switches are considered ideal or lossless (zero conduction drop, zero leakage current, and instantaneous turn-on and turn-off times), the instantaneous and average power will balance at input and output of the converter. Switching mode operation makes the converter nonlinear, thus generating source and load harmonics and also EMI problems. The discrete time switching characteristics cause a delay in signal propagation. Of course, a high switching frequency reduces the propagation delay. A converter can be single stage, or multiple conversions may be involved in a cascaded converter system. Several types of commutation (transferring current from the outgoing device to the incoming device) can be used. Thyristor converters are characterized by line (or natural), load, or forced commutation. Line-commutated converters are used extensively in utility systems, and these will be discussed in this chapter. Force-commutated thyristor converters that require auxiliary transient circuits are practically obsolete. Converters that use devices such as power MOSFETs, GTOs, IGBTs, and IGCTs are characterized by self-commutation. Again, a converter can be based on hard switching or soft-switching. In a soft-switched converter, dv.
Dual Converter for Multi-quadrant operation
Dual converters are used in high-power applications. It can also be suddenly for bringing down the speed of the drive. Four-quadrant operation of a dc motor is required, i.e. reversible monitoring and reversible braking. A single converter needs the addition of either a changeover contact to reverse the armature connections, or a means of reversing the field current in order to change the relationship between the converter voltage and the direction of rotation of the motor. It is the connection of fully controlled converters back to back across the load circuit. Such system is known as Dual Converter as shown in figure. Both voltage and current of either polarity are obtained with dual converter. In full converter, the direction of the current cannot be reverse because of unidirectional property of the thyristor, but polarity of the output voltage can be reversed. Thus the full converter can be operated in first quadrant if firing angle < 90° (both Edc1, Idc1 positive). If firing angle >90°, it can be operated in fourth quadrant, both Edc1 is positive and Idc1 is negative. Therefore in first quadrant, the power flows from ac source to dc source and in fourth quadrant power flows from dc source to ac source.
Principle of Ideal Dual Converter
Dial converters are ideal and they produce pure dc voltage, that is, there is no ripple at the dc output terminal. As shown in figure3, each two quadrant converter is assumed to be a controllable direct voltage source in series with a diode. Diodes D1 and D2 represent the unidirectional current flow characteristics of the converters. The current in load can however flow in either direction. The firing angles of the individual converter of the dual converter are regulated by a firing angle control voltage Ec1 , so that their dc voltages are equal in magnitude but opposite in polarity. Therefore, they can drive the current in opposite direction through the load. Thus, when one converter operates as a rectifier having a dc terminal voltage, the other converter operates as an operates as a inverter with exactly the same voltage. The converter working as a rectifier is called positive group converter and the inverter is called negative group converter
Conclusion
We are trying to design something innovative and easy to understand as well as user friendly. Our aim is to achieve our goal within economic restrictions. We kept in mind the worthiness and reliability of the project. We will make something innovative and compact which will be helpful for college lab. The circuit is simple and easy to understand and result in an user friendly one. This will be a consequence of team work and individual efforts. Today is the era of latest technology and we will not compromise with the compactness and quality of the project. If there will be any possibility of improvisation then we will add it in our work and will achieve further limits. We are going to learn as much as possible about every aspect of dual converter to make it useful in day to day life.
[attachment=56486]
Introduction
Thyristor based three-phase controlled rectifiers are widely used in the industry for controlling dc motor drives. Controlled rectifiers offering power conversion from ac to dc are reliable and have higher lifetime compared to other converters. DC motors have higher torque than ac motors and hence are suitable for variable speed and speed reversing applications requiring high torques. Although the operation of controlled rectifiers is simple, the realization of the converter control circuit is complex in nature. The control circuit needs the basic functionalities like : Six state pulse generation and gate drive isolation, Synchronization of the control pulses to the power frequency, Smooth transition in phase angle control, Start-up control, Phase sequence check, field excitation check and over-current protection. If the pulse pattern generation and other control tasks are to be undertaken by a single processor, an ultra high speed processor is needed because of the real-time nature of the system. Moreover, special design interfaces are required for sensing excitation loss and phase sequence checks. This makes the system implementation costly. A new approach is proposed where a hybrid type circuit generates the real time pulses for the converter and a processor supervises the controller functionality. The processor sets the phase angle, monitors the current, phase sequence, excitation condition and external control inputs for start, stop, speed change and speed reversal operations. For compact and cost effective design, instead of using a general purpose microprocessor along with peripheral interfaces, a single microcontroller chip may be used for the implementation.
Project Summary
The block diagram of the three phase dual converter is shown in figure 1. This three phase dual converter basically includes six transformers, three full wave bridge rectifier circuits, three mosfet, firing circuits and the multimeter as shown in figure. The connection of these component are shown in figure 1. Multimeter are used to check the voltage at different-different points. In this project (Three Phase Dual Converter) the three supply is feed from source. The dual converter converts the ac supply into the dc supply and again this dc supply is converts into the ac supply. The output waveform of this dual converter is obtained by connecting the CRO.
Practical Dual Converter
With the firing angles controlled in a manner that α1+ α2= 180° and with both the converter in operation, their average output voltages are equal and have the same polarity. One converter will be operating as a rectifier with firing angle α1 and the other as an inverter with firing angles (180- α1). Through their average output voltages are equal, yet their instantaneous voltages v01 and v02 are out of phase in a practical dual converter. This result in a voltage difference when the two converters are interconnected and as a consequence, a large circulating current flows between the two converters but not through the load. In practical dual converters, this circulating current is limited to a tolerable value by inserting a reactor between the two converters. The circulating current can however, be avoided provided the converters are triggered suitably. In general, a dual converter can operated in the two modes, without circulating current and with circulating current.
Many modern power conversion systems require a bidirectional energy transfer capability as a central part of their system operation. Preferably, such systems should use a single high efficiency power electronic conversion system to reduce size, weight and cost. For higher power bidirectional conversion, the common topology proposed is a dual active bridge structure, where two DC-AC converters are coupled back-to-back through an AC inductor/transformer. Either single-phase or three-phase converters have been proposed for such a system, but to date no clear-cut basis for selecting between these two alternatives has been established. A converter basically consists of an array of on-off electronic switches that use power semiconductor devices. If the switches are considered ideal or lossless (zero conduction drop, zero leakage current, and instantaneous turn-on and turn-off times), the instantaneous and average power will balance at input and output of the converter. Switching mode operation makes the converter nonlinear, thus generating source and load harmonics and also EMI problems. The discrete time switching characteristics cause a delay in signal propagation. Of course, a high switching frequency reduces the propagation delay. A converter can be single stage, or multiple conversions may be involved in a cascaded converter system. Several types of commutation (transferring current from the outgoing device to the incoming device) can be used. Thyristor converters are characterized by line (or natural), load, or forced commutation. Line-commutated converters are used extensively in utility systems, and these will be discussed in this chapter. Force-commutated thyristor converters that require auxiliary transient circuits are practically obsolete. Converters that use devices such as power MOSFETs, GTOs, IGBTs, and IGCTs are characterized by self-commutation. Again, a converter can be based on hard switching or soft-switching. In a soft-switched converter, dv.
Dual Converter for Multi-quadrant operation
Dual converters are used in high-power applications. It can also be suddenly for bringing down the speed of the drive. Four-quadrant operation of a dc motor is required, i.e. reversible monitoring and reversible braking. A single converter needs the addition of either a changeover contact to reverse the armature connections, or a means of reversing the field current in order to change the relationship between the converter voltage and the direction of rotation of the motor. It is the connection of fully controlled converters back to back across the load circuit. Such system is known as Dual Converter as shown in figure. Both voltage and current of either polarity are obtained with dual converter. In full converter, the direction of the current cannot be reverse because of unidirectional property of the thyristor, but polarity of the output voltage can be reversed. Thus the full converter can be operated in first quadrant if firing angle < 90° (both Edc1, Idc1 positive). If firing angle >90°, it can be operated in fourth quadrant, both Edc1 is positive and Idc1 is negative. Therefore in first quadrant, the power flows from ac source to dc source and in fourth quadrant power flows from dc source to ac source.
Principle of Ideal Dual Converter
Dial converters are ideal and they produce pure dc voltage, that is, there is no ripple at the dc output terminal. As shown in figure3, each two quadrant converter is assumed to be a controllable direct voltage source in series with a diode. Diodes D1 and D2 represent the unidirectional current flow characteristics of the converters. The current in load can however flow in either direction. The firing angles of the individual converter of the dual converter are regulated by a firing angle control voltage Ec1 , so that their dc voltages are equal in magnitude but opposite in polarity. Therefore, they can drive the current in opposite direction through the load. Thus, when one converter operates as a rectifier having a dc terminal voltage, the other converter operates as an operates as a inverter with exactly the same voltage. The converter working as a rectifier is called positive group converter and the inverter is called negative group converter
Conclusion
We are trying to design something innovative and easy to understand as well as user friendly. Our aim is to achieve our goal within economic restrictions. We kept in mind the worthiness and reliability of the project. We will make something innovative and compact which will be helpful for college lab. The circuit is simple and easy to understand and result in an user friendly one. This will be a consequence of team work and individual efforts. Today is the era of latest technology and we will not compromise with the compactness and quality of the project. If there will be any possibility of improvisation then we will add it in our work and will achieve further limits. We are going to learn as much as possible about every aspect of dual converter to make it useful in day to day life.