01-04-2010, 11:23 PM
THREE PHASE DUAL CONVERTER
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.
2 Dc power was obtained from motor-generator (MG) sets or ac power was converted to dc power by means of mercury-arc rectifiers or thyratrons. The advent of thyristors has changed the art of ac to dc conversion. Presently, phase-controlled ac to dc converters employing thyristors are extensively used for changing constant ac input voltage to controlled dc output voltage. In an industry where there is a provision for modernization, mercury-arc rectifier and thyratrons are being replaced by thyristors. In phase controlled rectifiers, a thyristor is turned off as ac supply reverse biases it, provided anode current has fallen to a level below the holding current. The turning-off, or commutation, of a thyristor by supply voltage itself is called natural, or line commutation. I industrial applications, rectifier circuits make use of more than one SCR. In such circuits, when an incoming SCR is turned on by triggering, it immediately reverse biases the outgoing SCR and turns it off. As phase-controlled rectifier need no commutation circuitry, these are simple, less expensive and are therefore widely used in industries where controlled dc power is required. In the study of thyristor systems, SCRs and diodes are assumed ideal switches which mean that: there is no voltage drop across them, no reverse current exists under reverse voltage condition and holding current is zero.
3 For large power dc loads 3 phase ac to dc converters are commonly used. The various types of three-phase phase-controlled converters are 3-phase dual converter. A dual converter is used only when reversible dc drives with power rating of several MW are required. The advantages of three phase converters are in three phase converter, the ripple frequency of the converter output voltage is higher than in single phase converter. Consequently, the filtering requirements for smoothing out the load current are less and the load current is mostly continuous in three phase converters. The load performance, when three phase converters are used, is therefore superior as compares to when single-phase converters are used. Semi- converters are single quadrant converters. This means that over the entries firing range, load voltage and current have only one polarity. In full converters, direction of current cannot reverse because of the unidirectional properties of SCRs but polarity of output voltage can be reversed. Thus a full converter operates as a rectifier in first quadrant and as a inverter in the fourth quadrant. Thus a full converter can operate as a two-quadrant converter. In first quadrant, power flow from ac source to the dc load and in fourth quadrant, power flow from dc circuit to the ac source. In case four quadrant operation is required without mechanical changeover switch, two converters can be connected back to back to the load circuit. Such an arrangement using two full converters in antiparallel and connected to the same dc load is called a dual converter. There are two functional modes of a dual converter, one is non-circulating current modes and the other is circulating current mode. Non-circulating typesof dual converter using single and three phase configurations.
4 Dual converters consist of two converters or bridges. One bridge acts as a rectifier and other acts as inverter. Single-phase full converters allow only two-quadrant operations. In full-controlled converter the output voltage is reversible so the converters operate in first and fourth quadrant. The direction of load current remains same. If two full-controlled converters are connected back-to-back then a four-quadrant operation are possible and such type of converters are called dual converters. It is used in high power variable speed drives. Dual converter is operated with either circulating current or without circulating current. When both the converters conduct at the same time, there would be circulating current and the level of circulating current is restricted by provision of an inductor. It is possible to operate only one converter at any instant, but switching from one converter to the other is carried out after a small delay.
Project Summary
5 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.
Block Diagram of 3-Phase Dual Converter
Figure 1 Block Diagram of Three-Phase Dual Converter
Practical Dual Converter
6 With the firing angles controlled in a manner that a1+ a2= 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 a1 and the other as an inverter with firing angles (180- a1). 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.
7 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
8 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.
Figure 2 General Block Diagram of Dual Converter
Principle of Ideal Dual Converter
9 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
Figure 3 Ideal Dual Converter
Phase Control
10 A Phase control is used to control the phase of an electrical power source and also in addition, often to control the amount of voltage and current or power that a power supply, or power source, feeds to it's load. In the manufacturing and industrial field the majority of the equipment are built to use three phase power. The only way to get three phase power is having the utility company install it in your business, or a much more economical way is to install a phase converter or other phase controller devices. One of the primary large phase control equipment for factories, ship yards, military, and all sorts of large industrial applications is TEMCo. They offer motor generator sets and other types of digital phase control technologies. Phase control will cover all aspects of power phases and the different phase controllers on the market today and the benefits and disadvantages of each one. Controlling phase power is sometimes a challenge when dealing with all the different phase controllers on the market today. Phase control will touch base on all the factors that are important to know. Single phase power is basically distributed to residential areas around the world today and with the equipment we use in today's industry, you need three phase power. Here you will learn how to obtain three phase control. Learn about variable frequency drives and what they can do for phase motor control. A VFD controls the rotational speed of an alternating current electrical motor by controlling the electrical power that is supplied to the motor. Variable frequency drives are used with many applications. Some form of phase control is needed when single phase power needs to be converted to three phase power. A phase converter is the most common method of changing single phase power into three phase power. Three phase power is now a very common form of electrical power and it has many properties that makes three phase power desirable for three phase power distribution. Phase control will review the different types of phase converters on the market today. There are several different types, rotary and static being the most common. There are other methods of phase control on the market such as inverters, transformers and generators as other alternatives to your phase control solutions.
11 Phase control (PFC), also called phase cutting, is a method of pulse width modulation (PWM) for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform. Phase fired control is often used to control the amount of voltage, current or power that a power supply feeds to its load. It does this in much the same way that a pulse width modulated (PWM) supply would pulse on and off to create an average value at its output. If the supply has a DC output, its time base is of no importance in deciding when to pulse the supply on or off, as the value that will be pulsed on and off is continuous.PFC differs from PWM in that it addresses supplies that output a modulated waveform, such as the sinusoidal AC waveform that the national grid outputs. Here, it becomes important for the supply to pulse on and off at the correct position in the modulation cycle for a known value to be achieved; for example, the controller could turn on at the peak of a waveform or at its base if the cycle's time base were not taken into consideration. Phase fired controllers take their name from that fact that they trigger a pulse of output at a certain phase of the input's modulation cycle. In essence, a PFC is a PWM controller that can synchronise itself with the modulation present at the input. Most phase fired controllers use thyristors or other solid state switching devices as their control elements. Thyristor based controllers may utilise Gate Turn Off (GTO) thyristors, allowing the controller to not only decide when to pulse the output on but also when to turn it off, rather than having to wait for the waveform to pass within the element's Zero Cross Point.
12 Previously, extremely expensive and heavy multi-tapped transformers were used as the supplies for such elements, with the corresponding winding tap being connected to the element to produce the desired temperature. This limited the temperature resolution to the number of tap combinations available. They often find their way into controllers designed for equipment such as electric ovens and furnaces. In modern, usually high power, equipment, the transformer is replaced with phase fired controllers connecting the load directly to the mains, resulting in a substantially cheaper and lighter system. However, the method is usually limited to use in equipment that would be unrealistic without it. This is because removal of the mains transformer means that the load is in direct galvanic contact with the input. For industrial ovens and furnaces the input is often the national grid AC, which is itself galvanically referenced to the Earth. With the controller's output referenced to the Earth, a user need only be in contact with the Earth and one of the output terminals to risk receiving an electrical shock. With many high power pieces of equipment running from three phase 415 V, high current capable inputs and having the entirety of any metallic housing or framework present Earthed (grounded), this is a serious risk that must be assessed with care.
Conclusion
13 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.
14 In case when four quadrant operation is required without mechanical changeover switch, two converters can be connected back to back to the load circuit. Such an arrangement using two full converters in antiparallel and connected to the same dc load is called a dual converter. The controller functions properly as per theoretical expectations and offer smooth operation during steady state and dynamic conditions. A new technique for designing the controller of a converter is reported in this project. The controller is implemented using mixed analog digital and memory circuitry centered on a single chip microcontroller. This project is helpful to understand the operation of the rectifiers, invertors and practical implementation of the theoretical studies and gives the various waveforms of the dual convertors in different operation. This project is also useful for the AIT Labs, to learn student about Dual Converter practically and see the waveform of dual converter on CRO which is they only study theoretically.
realated article
http://buet.ac.bd/eee/icece2004/P086.pdf
