Optimal Analysis and Design of Power Electronic Distribution Transformer

This study proposes a new design of Power Electronic distribution Transformer (PET) by using an intelligent program to get an optimum HF operation, which leads to maximum efficiency. This PET has three stages; the input stage consists of three phases, three level PWM rectifier and isolation stage employing three level half bridge convertor connected with high frequency transformer. The output stage has a matrix converter. This new design of PET has the advantage of improving power factor, voltage sags, voltage swell, reactive power compensation, eliminating harmonic and controlling and Protecting its self (PET) with minimum size, compared to the traditional transformer. The software like MATLAB and Artificial Neural Network will be used to analyze and simulate the PET.


INTRODUCTION
Transformers are important equipment in power distribution system as well as in power electronic system. They can step down high voltages in transmission at substations or step up currents to the needed level of end-users. Additionally, many functions, like, isolation, noise decoupling or phase-shifting can be achieved through transformers. The traditional distribution transformer has many disadvantages including being heavy and bulky, secondary voltage varies with the load and input voltage, lack of self protection, having no ability of voltage and current control, not having an intelligent function and the expensive components in electrical power system (Ratanapanachote, 2004).
In this study, a new type of Power Electronics distribution Transformer (PET) is proposed, which realizes the voltage transformation, galvanic isolation and power quality improvements in one device. The proposed PET design is fundamentally different and more complete approach by using power electronics on the primary and secondary sides of the transformer. Several features such as instantaneous voltage regulation, voltage sags compensation and power factor correction can be combined into PET. In addition, the PET is environmentally friendly since no liquid dielectrics are used for cooling. PET can be equipped with advanced communication interface that includes smart metering, diagnostics and distance control features (Subramanya and Anjaneyulu, 2012). The distribution transformer can be classified as shown in Fig. 1.
The prominent challenges in the new design of PET are to achieve smaller core size, higher power quality, higher efficiency and minimum losses. The main challenge is how to increase the efficiency of PET. The general objective of this research is to introduce and elaborate a new design approach for a PET system that can improve the performance of commercial power electronic transformers. In particular this study aims at: • Design a new PET • Optimize the design to achieve best trade-offs of highest efficiency, highest power quality, minimum size and minimum losses • Prototyping and experimenting the design

LITERATURE REVIEW
This section presents the different topologies for PET, in recent 30 years. The first idea of a power electronic transformer has been introduced by Navy researchers 1980, that consisted of an AC/AC buck converter as shown in Fig. 2 to reduce the input voltage to a lower one (Brooks, 1980). This was followed in 1995 by a similar Electrical Power Research Institute (EPRI) sponsored effort (Resischi, 1995). Both of these efforts yielded working prototypes, but they operated at power and primary voltage levels that were orders of magnitude below utility distribution levels.
In1996 the next attempt of a High Frequency (HF) AC link, termed an electronic transformer, was proposed. In this approach, the line side AC waveform is modulated into a HF square wave coupled to the  (Ronan et al., 2002) secondary of the HF transformer and again is demodulated to its AC form by a synchronous converter. The schematic (Shown in Fig. 3) has a benefit of reducing the transformer size and weight and the stress factor more reasonably. But it does not provide any benefit in term of control and power factor improvement and may not protect critical loads from momentary power interruptions due to the lack of an energy storage system (Harada, 1996;Kang et al., 1999;Krishnaswami and Ramanarayanan, 2005). Figure 4 shows that the other type is a three-part design that utilizes an input stage, an isolation stage and an output stage, as addressed in Ronan et al. (2002), Iman-Eini and Farhangi (2006), Iman-Eini et al. (2008, Maitra et al. (2009), Wang et al. (2005 and Jih-Sheng et al. (2005). The first stage is an AC/DC converter which is utilized to form the input current, to correct the input power factor and to regulate the voltage of the primary DC bus. The second stage is an isolation stage which provides the galvanic isolation between the primary and secondary side. In the isolation stage, the DC voltage is converted to a high frequency square wave voltage, coupled to the secondary of the HF transformer and is rectified to form the DC link voltage. The output stage is a voltage source inverter which produces the desired AC waveforms. These types enhance the flexibility and functionality of the electronic transformers owing to the available DC links. This approach can perform different power quality functions and can provide galvanic isolation but they need either too much power electronic converters or DC-link electrolytic capacitors. The main disadvantage of this topology is the large number of components which results in possibly lower efficiency and reliability. Moreover in the near future it may be possible to be commercialized. The literature review data are summarized in the Table 1.

METHODOLOGY
This study is suggested to be carried out in three steps, which are: Phase A: • Literature review: The state of the art and basic concepts of knowledge will be revised while related literate and relevant data of PET will be collected. Consequently, the main feature of the proposed work will be pointed out.

Phase B:
• Simulation and theoretical analysis: MATLAB ANN software will be used to obtain the optimum PET design parameter and then the efficiency result will be checked against the traditional transformer. • Design the closed loop control for PFT.
• Testing: PET system will be tested under different load condition. • The full scale prototype will be made for testing and the result will be compared with theoretical analysis and simulation.

Phase C:
• Analysis of the result: The results arising from step B above are tabulated and analyzed. • Conclusion and recommendations will be drawn from the analysis.

PROPOSED TOPOLOGY
The new proposed topology of power electronic transformer will have a three stages including; an input stage, isolation stage and output stage, as shown in Fig. 5. The input stage consists of three phase, three level PWM rectifier that reduces the stress ratio of the component, where the reactive power and high-voltage dc-link voltage closed-loop controls provide the ability to adjust the reactive power and stabilize the dc-link voltage in the high-voltage side and improve the efficiency. The isolation stage employs three level half bridge convertor connected with high frequency transformer. The output stage has a matrix converter that is used to replace the rectifier and the inverter to reduce the isolation stage component resulting in an efficiency improvement. On the other hand, the optimum design of HF transformer is done by ANN to reduce the losses.
PET mathematical model: By analyzing the new topology of PET, the structure consists of the input stage, middle stage and output stage ( Fig. 6 and 7).
Before developing the PET models, we assume that the power system is symmetrical and operates under a three phase balanced condition. By neglecting the VSCs losses, the dynamic differential equations of VSC-1, VSC-2 and the modulating-demodulating block are (Wang et al., 2007): ) cos( ) ) 3 where, d E1j (j = a, b or c) and d E2j (j = a, b or c) are the duty ratio switching functions for VSC-1 and VSC-2, m and ߠ the amplitude modulation index and phase angle of the control signal of each VSC, respectively and k the transformation ratio of the high-frequency transformer. By applying Park's transformation, the dynamic model of EPT in the d-q rotating reference frame can be given by: A new topology model of power electronic transformer, after development is shown in Fig. 8. The input stage mathematical models are: The output stage mathematical models are: The mathematical models of the isolation stage are:

Controlling input rectifier:
The main functions associated with the rectifier control are shaping the input current, controlling the input power factor and keeping the DC-link voltage at the desired reference value. The control scheme developed in rotating d-q reference frame would lead to better performance (Wang et al., 2007). The mathematical models of the input stage are: To implement the above functions, in the outer-loop control, the feedback DC voltage is compared to the reference value. The difference is then passed through the PI controlled to get the reference value of d-axis current. When the reference value of q-axis current is set to zero, the rectifier can achieve unity power factor operation. In the inner-loop, three-phase input currents are transformed into d-axis current and q-axis current components in the synchronous rotating d-q reference frame. The components are compared with the reference values of both d-axis current and q-axis current, the differences then formed the modulated wave signal by the PI controller. The three-level rectifier control principle is shown in Fig. 9.
The output stage control: In the output stage, threephase matrix converter outputs the constant-amplitude and constant-frequency voltage to the load through the output voltage closed-loop control. When the load changes within a certain range, the PET should maintain the amplitude of the output voltage. The control principle diagram is shown in Fig. 10. The three-phase output voltages are converted to the d-axis u E2d and qaxis u E2q in the synchronous rotating d-q reference frame, respectively. The voltage is then compared with the reference values of u E2 dref and u E2 qref . The modulated signal is formed by the PI controller. The PWM control algorithms realize the control of inverter switches.
HFT design procedure: The starting point of transformer design procedures is rating design requirement (V ୮ and I ୮ ). The variables parameter are core area, conductor area, flux density, current density and frequency operation. The voltage ampere rating, S of transformer defined as: And the primary voltage, V P : The primary current, I P : Then:  (37) where, N the number of turns, A cor , A w , A cu the core area, winding area and copper area respectively, f the frequency, B flux density and J rms the current density. Core and winding losses and transformer temperature rise are expressed in terms of geometry, material, winding and excitation parameters. Hysteresis loss (core loss): where R dc the DC resistance of the winding and R ac the net resistance of the winding. The power dissipated due to thermal, P s : where, T S = Maximum surface temperatures T a = Ambient temperatures  where, cos φ is the power factor.

SIMULATION RESULTS
In this section, the load characteristics of the PET are investigated by simulations. The simulation is based on MATLAB/SIMULINK. In the simulation, the  primary voltage of PET is 6.6 KV and the output voltage is 415V. The parameters are shown in Table 2 phase output phase voltage in the load for PET and the three phase current in the load also show in (e) and (f) shows the output voltage and current of PET in phase. As we can see from Fig. 11 and Table 3 the output voltage is 236 V and the output current is 56A. In terms of electrical performance the proposed electronic transformer and conventional transformer are identical, but the power electronic transformer not only used to step down the voltage, but also to improve the power quality of distribution system, that lead to wide application prospect.

CONCLUSION
A new power electronic distribution transformer is proposed, the simulation result shows that the output current and voltage have a better waveform that supports the proposed power electronic transformer system. The optimization of the whole system will be achieved by using close loop control, best core size, optimal frequency operation and reducing the number of components.