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单相到三相功率转换的常规电路
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2024-03-17
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功率半导体器件构成了现代电力电子的核心,并以开关矩阵的形式广泛应用于电力电子转换器中,有助于将功率从一种形式转换为另一种形式。 有四种基本的转换功能可以实现,即:;交流到交流,交流到直流,直流到交流和直流到直流。开关模式的功率转换效率很高,但缺点是由于开关的非线性,在电源和负载侧都会产生谐波。这些开关并不理想,并且它们具有导通、导通和关断开关损耗。尽管功率半导体驱动器的成本可能很难超过20-30%,但总设备成本和性能可能会受到用于功率转换的电路拓扑结构的高度影响。随着电力电子技术在电力转换中的发展,交流调速驱动器在工业应用中越来越受欢迎。这种设备提高了能源效率,但也存在一些关键问题,如效率和线路谐波注入,这会影响功率因数和系统的总体成本,在将任何驱动器用于工业或商业目的之前,都需要考虑这些问题。
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CHAPTER 1
INTRODUCTION
1.1 Introduction
Power semiconductor devices constitute the heart of the modern power
electronics, and are being extensively used in power electronic converters in the form
of a matrix of on -off switches, and help to convert power from one form to another.
There are four basic conversion functions that can be implemented namely; ac to ac,
ac to dc, dc to ac and dc to dc. The switching mode power conversion gives high
efficiency but the disadvantage is that due to the non- linearity of the switches,
harmonics are generated in both the supply and load sides. The switches are not ideal
and they have conduction, turn-on and turn off switching losses. Although the cost of
the power semiconductor drives, may hardly exceed 20-30 percent, the total
equipment cost and performance may be highly influenced by the topology of the
circuit used for power conversion. Owing to the development of power electronics in
power conversion, AC adjustable speed drives are becoming more and more popular
for industrial applications. This equipment improves energy efficiency, but there are
key issues, like efficiency and harmonic injection into the line, which affects the
power factor, and the overall cost of the system, and these issues need to be
considered before any drive is used for industrial or commercial purpose.
One way to increase the efficiency of the drive is by reducing the losses at
possible places such as in the converter used along with the ac motor. These losses
are computed as switching losses and conduction losses. It may also be improved as
1
the number of circuit elements is minimized, because as the number of devices
reduces the associated amount of switching reduces and so the losses are minimized.
Proliferation of nonlinear loads, such as three-phase rectifiers, adjustable
speed drives and uninterruptible power supplies are prone to high harmonic injection
into the utility, which powers them [1]. To reduce harmonic injections, improvement
in displacement factor is considered and so power factor correction equipment like
capacitors and filters are installed in the system. Harmonic currents cause resonance
between utility and harmonic–producing loads or among multiple harmonic
producing loads. These harmonic related phenomena result in de-rating of the system
equipment such as transformers, higher transmission line loss and reduced system
stability margin. Since electrical motors consume around 56% of the total consumed
electrical energy the improvement in power factor of electrical drives as seen by the
utility connection has been of major concern. Another consideration is the need to
increase the VA capacity of motor drives, so that the full utilization of the isolated
real power is possible [2].
In order to solve some of these problems, a large variety of control
techniques and converter topologies have appeared in the literature [3]. Since good
quality power factor systems are becoming more and more mandatory, power factor
improvement is one of the key issues in designing a system.
Several methods have been attempted in order to obtain a satisfactory
power quality from the supply mains. The use of terminal capacitors across the
machine windings is very common, due to its low cost and simplicity. However, this
method is often not often recommended for the adjustable speed drives employing
2
inverters which are PWM operated, as the capacitor may draw high harmonic currents
due to the harmonics present in the PWM terminal voltages, and the motor may
experience self-excitation, which might cause over-voltages in its terminals [4].
In rural electric systems, the cost of bringing three-phase power to a remote
location is often high due to high cost for a three-phase extension. Furthermore the
rate structure of a three-phase service is higher than that for single-phase service.
Therefore, single-phase to three-phase power converters are excellent choices for
situations where three-phase power is not available. Such converters have a wide
range of applications in which a three-phase motor is a main component and the
available supply is single-phase. Other factors that influence the choice of a static
converter and three–phase motor combination are listed as follows [5]:-
1. Three–phase motors are more efficient and economical than their single-phase
counter parts.
2. Starting and inrush currents in a three-phase motor are less severe than in a
single-phase motor.
Owing to wide applications of power converters, it is essential to develop single to
three-phase converters, which are efficient, cost effective and give high quality
performance. Presently, available converters for such applications are classified as
rotary type; autotransformer with switched capacitors and lastly, the static converter
type. The first two types of converters as given in [5] employ bulky magnetic
components of considerable size and weight. The third category that employs static
semiconductor devices for direct conversion of single-phase to three-phase is by far
the most active research area in which the bulky magnetic part can be eliminated and
3
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