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					ASN WOMEN’S ENGENEERING COLLEGE TENALI


    POWER ELECTRONICS AND DRIVES




                                    SUBMITTED BY

                                    G. CHIDRUPI

                                    K. SRI LAKSHIMI
ABSTRACT:

Power electronics has gone through rapid technological evolution in the recent years, and its
applications are fast expanding in industrial, commercial, residential, military, aerospace and utility
environments. Many innovations in power semiconductor devices, converter topologies, analytical and
simulation techniques, electrical machine drives, and control and estimation methods have contributed
this advancement.The frontier of this complex and interdisciplinary technology has been further
advanced by the artificial intelligence (AI) techniques, such as fuzzy logic, neural networks and genetic
algorithm, thus bringing more challenge to power electronic engineers. In the global industrial
automation, energy generation, conservation and environmental pollution control trends of the 21st
century, the widespread impact of power electronics is inevitable. The paper begins with a discussion on
global energy generation scenario and the corresponding environmental issues. The mitigation of energy
and environmental problems is then discussed with particular emphasis of power electronics
applications. A brief but comprehensive review of recent advances in power electronics is incorporated
in the paper.

INTRODUCTION:

Power electronics originated at the beginning of this century. Many technical articles and several
books on the subject were published during the period from 1930-1947. These dealt primarily
with the application of grid-controlled gas-filled tubes. Because of the limitations of the
mercury-arcrectifier and gas-filled thyratrons, only a relatively limited number of equipments
were manufactured. The invention of the bipolar junction transistor in 1948 was the beginning of
semiconductor electronics. This device and semiconductor diodes spawned a revolution
electronics. At the same time, in 1948, Shockley and Pearson tried fabricating a rudimentary
FET using evaporated layers of germanium on dieletric. However, it was not until Bardeen
theoried on the surface state phenomenon and Shockley published his theoretical analysis of the
unipolar field-effect transistor (Shockley 1952).

POWER SEMICONDUCTOR DEVICES AND DEVELOPMENT TRENDS

There are many additional special thyristor and transistor devices. These include multiple bipolar
transistors fabricated in a single package, chip-type (unpackaged) thyristors, power FETs, and
Schottky diodes. In addition, the continued development of IC components and microprocessors
is having a most important impact on control circuits for power electronic systems.

The SIT stands for Static Induction Transistor or Static Induction Thyristor. SIT was first
invented by Dr. Nishizawa (Japan) in 1950s who also invented the PIN diode. This new SIT
technology may challenge SCRs, GTOs, MOSFETs, IGBTs, magnetrons and most other power
semiconductors (Ohmi 1979). SITs have extremely low forward voltage drop, very fast swithing
speeds and greater radiation tolerance than most semiconductors. They operate at thousands of
volts, hundreds of amperes, frequency up to 10 GHz and levels up to one megawatt. It is believed
that this new technology will bring revolution in power electronics industry.
SWITCHING OPERATION:

When semiconductors are used in high-power applications, they are generally operated as
switches. There are only a very few exceptions to this, such as linear amplifiers and series or
shunt transistor-regulated linear power supplies. When the semiconductor is used as a switch, it
is possible to control large amounts of power to a load with a relatively low power dissipation in
the switching device. For example, in an ideal switch with zero voltage drop when "on," zero
leakage current when "off," and zero switching time, there is no power dissipation in the
switching device regardless of the on-state voltage drop across the switching devices limits the
on-state current because of device dissipation limits. All practical power semiconductor devices
also have a limited off-state voltage rating.

 When high-frequency switching is employed, there also may be significant switching loss due to
the large instantaneous device power dissipation because of the finite times of practical power
semiconductors. Practical semiconductors always have some losses. In the following section
power losses in a switching power transistor are considered. This can be treated as a reference
for the analysis of power losses of other power semiconductor devices.
COMMUTATION:

Thyristor commutation is the process to turn off a thyristor. The circuit that is used to realize
commutation process is called the commutation circuit. The duration for which the commutation
circuit is able to apply reverse bias across the thyristor is called the "circuit turn-off time"
which should be greater than the "thyristor turn-off time". In general, complete commutation
may involve a number of events. The most important of these are:



(1) The reduction of forward current to zero in one power semiconductor switching elements.
(2) The delay of reapplication of forward voltage to this element until it has regained its forward-
blocking capability.

(3) The build-up of forward current in the next element which is to conduct.

POWER SEMI CONDUCTOR DEVICES CHARACTERISTICS:

The characteristics of semiconductor devices are mainly electrical and thermal. When
semiconductors are used for power applications, the device characteristics of great importance
are the on-state voltage drop, the on-state current handling ability, the off-state voltage blocking
ability, the switching speed, the rate of recovery of blocking capability, the power dissipation
limits, and the power gain. The detailed characteristics and ratings of each power semiconductor
device are very involved and need a separated paragraph to discuss. In the followings, however,
brief description of power semiconductor characteristics are introduced. These are mainly in
terms of thyristor; it is to be assumed that it is equally valid, where applicable, to diodes,
transistors, GTOs, and MOSFETs.

EFFECT S OF TEMPERATURE:

Temperature affects semiconductor devices in two ways: firstly it affects their electrical
characteristics, and secondly it may have a direct effect on the materials used or inadvertently
included in their construction.

VOLTAGE RATINGS :

Absolute voltage limits are set to the permissible operating voltage by the breakover voltage in
the forward direction, and by the sharp increase in reverse current in the avalanche region. The
"breakover voltage" is defined as the voltage on the voltage-current characteristic for which the
differential resistance is zero and where the voltage applied to the thyristor increases a maximum
value. In practice a maximum instantaneous reverse voltage rating is usually determined by
reference to a particular value of leakage current, based on the known spread of characteristics
for a particular type of cell; this constitutes a peak transient or non-repetitive rating, related to a
specified duration: e.g. the amplitude of a 10 msec half-sine-wave. Lower values of voltage,
arrived at by applying an empirical ratio or testing to a lower current limits, may be specified as
repetitive instantaneous or dc ratings, limiting the average or continuous reverse loss to a
reasonable, low level.

FORWARD CURRENT RATING:

There are three main considerations which may set limits to the permissible forward currents
namely

(1) The requirement that the rated junction temperature should not be exceeded as a result of
    losses.
(2) The safe current rating of the external connections, particularly flexible conductors and
    crimps.
(3) The temperature gradients within the cell, which should not be such as to cause excessive
    stress through differential expansion

GATE FIRING CHERACTRISTICS AND GATE RATING:

The direct effect of applying a positive gate current to a thyristor can be considered to be a
reduction in the breakover voltage, typically as illustrated by the graph of Fig. 5.5. In a practical
application, the minimum effective firing current is that which will reduce the breakover voltage
below the voltage applied to the cell, and to cover the majority of applications it is defined with
respect to a suitable low anode-to-cathode voltage-typically five volts. The figure quoted is the
maximum limit of what is normally a fairly wide production spread, at a particular temperature,
together with the corresponding maximum gate firing voltage.




di/dt Effects:

The rate of rise of current, commonly referred to as 'di /dt', for which a thyristor can safely be
rated varies greatly according to its operating conditions, being reduced by factors which
increase the energy dissipated and increased by those which improve the thyristor's switching
characteristics; Thus the rating is reduced for

(a) an increasing forward blocking voltage immediately before switching;

(b) an increasing peak forward current;

(c) an increasing designed voltage rating, which generally implies a thicker silicon element and
higher forward voltage drop; and is increased by
(d) an increased gate overdrive i.e., the ratio between the gate current and the firing current limit
for the cell (since the latter varies with temperature, the minimum operating junction temperature
is also a factor)

(e) a reduced gate current rise.

dv/dt Effects:

A thyristor may be triggered as a result of a high rate of rise of forward voltage, even though no
gate current is applied. The effect may be thought of as due to capacitive current within the
element performing the same function as a gate current, albeit not necessarily in the region of the
gate. The rate of rise, commonly 'dv/dt', at which a thyristor may break over is basically a
characteristic, but, as in the case of breakover due to excessive forward voltage, it may
effectively constitute a rating.

APPLICATIONS:

 The power circuits, the output of which may be a variable direct or alternating voltage or current
source or may be an alternating source of variable voltage and frequency.

 The digital circuits, which in response to the signals from the controlling system switch the power
switching devices of the power circuits on and off at appropriate instants.

 The controlled system, which may simply be a rotating machine and driven load with appropriate
feedback output, or may be something considerably more complicated.

 The controlling system, which in response to the command and feedback signals issues the
appropriate control signals to the digital circuits.

FUTURE DEVELOPMENT ON POWER ELECTRONICS :
Lower voltages, higher current and better regulation are needed for the coming generation of VLSI.
Highly regulated voltages on the order of three volts, or less, currents in tens of hundreds of amperes
and load dynamics of 100 A/sec will be required. Narain G. Hingorani, vice president of the Electrical
System Division for EPRI (Electric Power Research Institute, Palo Alto, CA, USA), says power electronics
should be a national priority. He points out (1987) that over the next 20 years the development of
semiconductor devices for high-voltage, high current applications will bring about a major
transformation in industrial system. He says this "second electronics revolution" has always begun and is
being used to make AC/DC converters for high-voltage DC (HVDC) transmission, static VAR
compensators, UPS to protect sensitive equipment and drives for adjustable speed motors (Miller 1987).
ADVANTAGES

--The electric motor is not a "heat engine (2)" and can achieve efficiencies over 90%
--full torque at 0 RPM means that no transmission is required.
--Power configurations can be done in software rather than in the hardware of the engine:

DISADVANTAGES
-- the electric motor can be heaver than alternatives like the air engine or turbine.
-- the electric motor accepts only one fuel: (electricity, although this can be made in a number of
ways)
-- Although there are many ways to make an electric motor some require "rare earths" although
"rare earths" are not "rare" sources in the US, Canada and Greenland have not been exploited.
This may cause delays in some production.

CONCLUSION:

When designing a system to protect equipment from lightning the best method is to use all of the
different types of devices available for lightning protecton.just as a fighter goes in to the ring
with all the tools and skills he can muster, lighting protection should include all economical
means possible to protect the damage . some type of method should be employed to move the
lightning away from the equipment,and if a strik does happen, then devices should employed to
dissipate the voltage and current to ground.

				
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