Copyright © 2018 Dr. Raaed Faleh Hassan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. International Journal of Engineering & Technology, 7 (3) (2018) 1776-1782 International Journal of Engineering & Technology Website: www.sciencepubco.com/index.php/IJET doi: 10.14419/ijet.v7i3.16423 Research paper Design and software implementation of solid state transformer Dr. Raaed Faleh Hassan * Department of Control and Automation Engineering Techniques Electrical Engineering Technical College Middle Technical University *Corresponding author E-mail: drraaed_alanbaki@eetc.mtu.edu.iq Abstract The work presented in this paper concerned with the analysis, design and software implementation of the Solid State Transformer as an alternative to the conventional power transformer. The proposed transformer aims to perform the same task as the conventional one with additional facilities and advantages. Three stages are considered to configure the Solid State Transformer. The first stage which is known as input stage and implemented using Vienna rectifier which converts the AC voltage of the main supply to a DC voltage. The second stage (isolation stage) step down the DC voltage to a lower level DC voltage. This stage consists of a single – phase five-level diode clamped inverter, 1 KHz step – down transformer and fully controlled bridge rectifier. The output stage (third stage) is a three-phase three-level diode clamped inverter which converts the low level DC voltage to a three-phase, 50 Hz AC voltage. Model Predictive Current Control has been employed for driving transformer’s stages. The gating signal is produced directly when the given cost fun ction is minimized, therefore there is no need of any modulator. Behavior of the proposed structure is achieved by simulation which shows high quality power conversion with low Total Harmonic Distortion. Keywords: Solid-State Transformer; Model Predictive Current Control; Vienna Rectifier; Diode Clamped Converter and High Frequency Transformer. 1. Introduction Power transformer plays important role in the electric power sys- tems. It is enables these systems to transfer the generated electrical power for long distance with high efficiency [1-4]. In order to achieve low transmission losses, power transformers providing voltage boosting at the generation side of the power system. How- ever, at the distribution side, another power transformer acts to step- ping down the voltages to the values used by industrial, commer- cial, and residential applications [4]. Although it is important part in the power systems, power transformer has some drawbacks like bulky size, heavy weight, sensitive to the load variations, unable to compensate power factor, etc. [1-3], [5-7]. Power transformers dominate 25% of the overall size of the power system and more than 30% of the overall weight. The size of the power transformer is determined by the value of the saturation flux density of the ma- terial implements the transformer core. As the maximum opera- tional flux density is inversely proportional with the operating fre- quency, then increasing this frequency will lead to reducing the size of the transformer. With continuous advancement of power elec- tronics devices and circuits, a considerable possibility to develop promising technique that performs the function of the power trans- former and overcomes the transformer drawbacks became at hand. The technique that has attracted researcher’s attention as an alter- native to the conventional transformer is the Solid State Trans- former (SST) or Power Electronics Transformer (PET) [2]. In addi- tion to performing the same task as traditional transformer, employ- ing SST opens up a new horizon in the utilization of electrical power systems [5]. The proposed structure of the SST allows of adopting high operating frequency which leads to valuable reduc- tion in the weight and size of this new transformer [8-11]. SST en- ables a perfect isolation of induced harmonics from passing through power system [1]. It is also performing power conversion between different formats with any desired frequencies [12], keeps instanta- neous voltage regulation [13], reactive power compensation, and potential current limitation [10]. SST can be considered as the back- bone of the future intelligent power systems [13]. The major task of the SST is to perform the voltage stepping up/down based on me- dium – to – high frequency isolation, therefore to achieve a consid- erable reduction in its size and weight. In order to realize this task, the 50/60 Hz ac voltage is transformed to a high frequency by means of rectifier / inverter set. Then this high frequency ac voltage is either stepped up or down using high frequency transformer. Fi- nally, the output of the high frequency transformer is shaped back into 50/60 Hz through another rectifier / inverter set [4]. The pres- ence of the rectifiers and inverters in the composition of these trans- formers provides the possibility to achieve voltage and current reg- ulation, power flow control, fault current limitation. Also the con- figuration of the SST provides a scope to supply different voltage forms (ac and dc) at the load side which enables the new era in the power systems. Penetration of the SST into distribution power sys- tems needs its structure to employ power devices which have the ability to work reliably in high voltages and frequencies environ- ment [4]. Recently, the researcher’s efforts concerned with adopt- ing either multi – modules for the SST structure [1] [5-7] [11] [15- 19], or using multilevel configuration [2] [12]. Employing the high voltage semiconductor devices based on wide – bandgap substances like 4H – SiC, leads to the appearance of the third version of the SST configuration [9] [13] [20-21]. Various control methods are presented for controlling the power converters, and Fig. (1) sum- marize the most familiar ones [22].