INTRODUCTION
1.1 BACKGROUND
Power Electronics deals with various types of converters such as dc-dc, ac-dc, ac-ac, dc-ac converters. This converters controlled by some sort of gate pulses, which are use to generate the desired signals and also for controlling abnormality in the required wave shapes. Here we will focus on only three phase inverter gate pulses and three phase SCR full converter gate pulses generation. In three phase inverter we generate gate pulses for a typical IGBT module and for controlled rectifier we generate gate pulses for a typical SCR module.
AC/DC rectifiers are the first group of the power switching circuits applied in industrial applications. In the 1940s, Mercury-arc rectifiers were very popular in DC power supply. In 1960s, semiconductor manufacture development brought power devices, such as power diode, thyristor (or silicon controlled rectifier, SCR), gate turn-off (GTO), Triac, bipolar transistor (BT), insulated gate bipolar transistors (IGBT) and metal oxide semiconductor field effected transistor (MOSFET) and so on, into the DC power supply. The DC power supply equipment is totally changed. The corresponding control circuit is gradually changed from analog-to-digital control system since 1980s. The mathematical modeling for all AC/DC rectifiers is discussed widely in worldwide. [1]
We know the general acceptance and growth of AC power systems, but the need continued for DC in electrochemistry, variable speed motors, and traction systems. Although George Westinghouse demonstrated AC traction motors, they were operated at 25 Hz rather than the prevailing 60 Hz used for utility systems. Metropolitan transit systems, however, required DC for the traction motors. The New York subway system generated and distributed 25-Hz power but ran the cars on DC through rotary converters. With the predominately 60 Hz power systems, the problem became one of converting 60 Hz AC to DC. The terms rectifier and converter are often used interchangeably to denote a system of any sort that converts AC to DC. Present-day usage seems to favor rectifier to denote an uncontrolled conversion system and converter to denote a controlled system. [2]
DC/AC inverters are a newly developed group of the power switching circuits applied in industrial applications in comparison with other power switching circuits. Although choppers were popular in DC/AC power supply long time ago, power DC/AC inverters were used in industrial application since later 1980s. Semiconductor manufacture development brought power devices, such as gate turn-off thyristor, Triac, bipolar transistor, insulated gate bipolar transistor and metal-oxide semiconductor field effected transistor (GTO, Triac, BT, IGBT, MOSFET, respectively) and so on, in higher switching frequency (say from thousands Hz upon few MHz) into the DC/AC power supply since 1980s. Due to the devices such as thyristor (silicon controlled rectifier, SCR) with low switching frequency, the corresponding equipment is low power rate. Square-waveform DC/AC inverters were used in early ages before 1980s. In those equipment thyristors, GTOs and Triacs could be used in low-frequency switching operation. High-frequency/high-power devices such as power BTs and IGBTs were produced in the 1980s. The corresponding equipment implementing the PWM technique has large range of the output voltage and frequency, and low total harmonic distortion (THD). Nowadays, most DC/AC inverters are DC/AC PWM inverters in different prototypes.
The power DC/AC inverter produces variable frequency and voltage to implement the ASD. The power devices used for ASD can be thyristors, Triacs and GTOs in the 1970s and early 1980s. Power IGBT was popular in the 1990s, and greatly changed the manufacturing of DC/AC inverters. The DC/AC power supply equipment is totally changed. The corresponding control circuit is gradually changed from analog control to digital control system since late 1980s. The mathematical modeling for all AC/DC rectifiers is well discussed widely in worldwide. [3]
The IGBT is a fairly recent invention. The first-generation devices of the 1980s and early 1990s were relatively slow in switching, and prone to failure through such modes as latchup and secondary breakdown. Second-generation devices were much improved, and the current third-generation ones are even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads. Three phase inverters are used for variable-frequency drive applications and for high power applications such as HVDC power transmission. Modern Power Electronics makes generous use of IGBTs in most applications and, if the present trend is any indication, the future will see more and more applications making use of IGBTs. It operates as a MOSFET with an injecting region on its Drain side to provide for conductivity modulation of the Drain drift region so that on-state losses are reduced, especially when compared to an equally rated high voltage MOSFET.
1.2 IMPORTANCE AND APPLICATIONS OF VARIOUS CONVERTERS
Three-phase controlled rectifiers have a wide range of applications, from small rectifiers to large high voltage direct current (HVDC) transmission systems. They are used for electrochemical processes, many kinds of motor drives, traction equipment, controlled power supplies, and many other applications. [4]
Linear power regulators, whose principle of operation is based on a voltage or current divider, are inefficient. This is because they are limited to output voltages smaller than the input voltage, and also their power density is low because they require low frequency (50 or 60 Hz) line transformers and filters. Linear regulators can, however, provide a very high-quality output voltage. Electronic devices in linear regulators operate in their active (linear) modes, but at higher power levels switching regulators are used. Switching regulators use power electronic semiconductor switches in on and off states. Because there is a small power loss in those states (low voltage across a switch in the on state, zero current through a switch in the off state), switching regulators can achieve high energy conversion efficiencies.
Modern power electronic switches can operate at high frequencies. The higher the operating frequency, the smaller and lighter the transformers, filter inductors, and capacitors. In addition, the dynamic characteristics of converters improve with increasing operating frequencies. [5]
Step-down choppers find most of their applications in high performance dc drive systems, for example, electric traction, electric vehicles, and machine tools. The dc motors with their winding inductances and mechanical inertia act as filters resulting in high-quality armature currents. The average output voltage of step-down choppers is a linear function of the switch duty ratio. Step-up choppers are used primarily in radar and ignition systems. The dc choppers can be modified for two-quadrant and four-quadrant operation. Two-quadrant choppers may be a part of autonomous power supply systems that contain battery packs and such renewable dc sources as photovoltaic arrays, fuel cells, or wind turbines. Four-quadrant choppers are applied in drives in which regenerative breaking of dc motors is desired, for example, transportation systems with frequent stops. The dc choppers with inductive outputs serve as inputs to current-driven inverters. The addition of filtering reactive components to dc choppers results in PWM dc-dc converters. The dc-dc converters can be viewed as dc transformers that deliver to the load a dc voltage or current at a different level than the input source. This dc transformation is performed by electronic switching means, not by electromagnetic means such as in conventional transformers. The output voltages of dc-dc converters range from one volt for special VLSI circuits to tens of kilovolts in X-ray lamps.
The dc-dc converters are building blocks of distributed power supply systems in which a common dc bus voltage is converted to various other voltages according to requirements of particular loads. Such distributed dc systems are common in space stations, ships and airplanes, as well as in computer and telecommunication equipment. It is expected that modern portable wireless communication and signal processing systems will use variable supply voltages to minimize power consumption and to extend battery life. Low-output voltage converters in these applications utilize the synchronous rectification arrangement.
Another major area of dc-dc converter applications is related to the utility ac grid. For critical loads, if the utility grid fails, there must be a backup source of energy, for example, a battery pack. This need for continuous power delivery gave rise to various types of uninterruptible power supplies (UPSs). Thus dc-dc converters are used in UPSs to adjust the level of a rectified grid voltage to that of the backup source. Because during normal operation the energy flows from the grid to the backup source and during emergency conditions the backup source must supply the load, bidirectional dc-dc converters are often used. Moreover dc-dc converters are also used in dedicated battery chargers. Power electronic loads, especially those with front-end rectifiers, pollute the ac grid with odd harmonics. Thus dc-dc converters are used as intermediate stages, just after a rectifier and before the load-supplying dc-dc converter, for shaping the input ac current to improve power factor and decrease the harmonic content. The boost converter is especially popular in such power factor correction (PFC) applications.
Another utility grid-related application of dc-dc converters is in interfaces between ac networks and dc renewable energy sources such as fuel cells and photovoltaic arrays. In isolated dc-dc converters, multiple outputs are possible with additional secondary windings of transformers. Only one output is regulated with a feedback loop, but other outputs depend on the duty ratio of the regulated one and on their loads. A multiple-output dc-dc converter is a convenient solution in applications where there is a need for one closely regulated output voltage and for one or more other noncritical output voltage levels. [6]
DC/AC inverters are used for inverting DC power source into AC power applications. They are generally used in following applications:
1. Variable voltage/frequency AC supplies in adjustable speed drives (ASDs), such as induction motor drives, synchronous machine drives and so on.
2. Constant regulated voltage AC power supplies, such as uninterruptible power supplies (UPSs).
3. Static var compensations.
4. Active filters.
5. Flexible AC transmission systems (FACTSs).
6. Voltage compensations.
Adjustable speed induction motor drive systems are widely applied in industrial applications. These systems requested the DC/AC power supply with variable frequency usually from 0 to 400 Hz in fractional horsepower (HP) to hundreds of HP. [7]
1.3 OBJECTIVES
We study only about three phase inverter as a DC to AC converter and three phase Thyristor full converter as a controlled rectifier. The objectives for these are –
a) To study about different types of gate pulses.
b) To study the architecture of Microcontroller PIC16f72 and its application as a programmable gate pulse generator.
c) To study the architecture of comparator LM 324 for using as sensing device for the triggering point of three phase sine wave.
d) Generation of programmable gate pulses for driving a three phase IGBT module.
e) Generation of programmable gate pulses for driving a typical SCR module.
1.4 METHODOLOGY
a) First we have to decide which type of pulses should be generated.
b) For three phase IGBT module we have to calculate the pulse widths for a sine PWM generation with specific modulation index.
c) Then we have to generate six pulses with 600 phase shift with the consecutive pulses.
d) For controlled rectifier we have to sense the intersection points for different phase of the sine wave using comparator.
e) Then generate pulses for different gates of the SCR module with specific delay.
1.5 ADVANTAGES OF PROGRAMMABLE GATE PULSES
a) The main advantage of programmable gate pulses is the opportunity to change program according to the requirements.
b) Same device can be used for different types of gate pulses.
c) Controlling the abnormality of the power devices using feedback.
d) Wide range of control over frequency.
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