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OPTIMAL CONTROL OF POWER ELECTRONIC TRANSFORMERS WITH
SINGLE-PHASE MATRIX CONVERTER
Hakan AÇIGÖZ, Mustafa ŞEKKELİ, Ö.Fatih KEÇECİOĞLU
(Institutional Information of Authors)
Transformer is a static electrical device that alternates the voltage applied to one of its
coils through electromagnetic induction, thus enabling us to obtain different voltage and
current values at the same frequency and power from the other coils. Transformers are
widely used in energy transmission and distribution and they constitute one of the most
important components of the energy transmission systems and power electronic systems.
Conventional transformers (50/60 Hz) are the most expensive and heaviest parts of the
electric power systems. There are many studies carried out to eliminate such
disadvantages of the conventional transformers and increase their performance. As it is
well known, the size of a transformer is inversely proportional to the operating frequency.
This feature can be advantageous in that the transformer size can be reduced by increasing
the frequency. In recent years, rapid developments in micro-processors and power
electronic circuit elements prompted many studies on special-purpose high-frequency
transformers composed of electronic power components. These new types of transformers
are called power electronic transformers (PET). With this new type of transformer, it
becomes possible to improve voltage transformation, galvanic isolation and the power
quality on a single circuit. The most important feature of the PETs is the ability to use
electronic circuits on the primary and secondary sides of the transformer for different
purposes. In cases of voltage transformation, voltage drop and rise on the primary and
secondary sides of the transformer, stabilization of DC voltage and correction and
improvement of unity power factor can be achieved. There are many studies carried out on
PETs in the literature. Matrix converters, known as direct AC–AC Power Converters,
were first presented by Venturini and Alesina in 1979 and these converters has a great
deal of advantages when compared to the DC-link converters. In matrix converters, AC
voltage is directly converted into a variable frequency and voltage without using any DClink. The most significant advantage of these types of converters is that they have bidirectional power flow, sinusoidal input and output voltage and high unity power factor.
Such an arrangement is generally produced with a combination of unidirectional
semiconductor switches. Such converter applications generally use controlled
semiconductor switching devices such as insulated-gate bipolar transistors (IGBT) and
metal-oxide semiconductor field-effect transistors (MOSFET). In the bi-directional switch
arrangement with antiparallel IGBT diode pair shown in Figure-1, two diodes and two
IGBTs are connected in anti-parallel. The diodes are included in order to block the
reverse voltage. Such an arrangement enables the independent control of the flow
direction in bi-directional switching applications. In the PETs arranged with a matrix
converter, the purpose of the matrix converters in the primary and secondary sides of the
transformer is to achieve the desired input and output waveform. Therefore, both matrix
converters have different switching signals. On the primary side, a voltage and current of
50 Hz in frequency is converted into a voltage and current of a higher frequency and
transmitted to the higher frequency transformer. Depending on the conversion ratio of the
high-frequency transformer, a high-frequency voltage is obtained on the secondary side.
The obtained high-frequency voltage is converted with a matrix converter on the
secondary side in a way to obtain 50 Hz frequency at the output by means of using proper
switching configuration. This study examines the PET systems on which many studies and
different control topologies are introduced nowadays. Since the designed PET system
does not require any DC-link capacity, both primary and secondary sides of the PET are
controlled through matrix converters. The matrix converters on the primary and secondary
sides of the transformer were switched with a technique that operates at a fixed switching
frequency. Performance of the PET systems can be increased by means of controlling the
matrix converters with different switching techniques. The main purpose of the use of
PETs is to reduce the size of the transformers, to control the unity power factor, to
improve the power quality and to reduce the Total Harmonic Distortion values. In the
study, the unity power factor was determined to be 1 and THD value to be 1.09% which is
satisfactory in terms of the quality of power. Moreover, it is important to keep the THD
value of the load current under 5% in accordance with the international standards. As a
result, by changing the switching techniques, it can be possible to increase the
performance of matrix converters and efficiency of the PET systems.
Key Words: Transformers, Single-Phase Matrix Converter, Power Electronic
Transformer