Design and Implementation of an Amorphous High Frequency Transformer Coupling Multiple Converters in a Smart Micro Grid

 

Abstract

Recent improvements in magnetic material characteristics and switching devices have generated a  possibility to replace the electrical buses with high  frequency magnetic links in micro-grids. Multi-winding transformers (MWTs) as magnetic links can effectively reduce the number of conversion stages of renewable energy system by adjusting turn ratio of windings according to the source voltage level. Other advantages are galvanic isolation, bidirectional power flow capability and simultaneous power transfer between multiple ports. Despite the following benefits, design and characterization of MWTs are relatively complex due to their structural complexity and cross coupling effects. This paper presents all stages of numerical design, prototyping and characterization of a MWT for micro-grid application. To design the transformer for certain value of parameters, reluctance network method is employed. Due to the iterative nature of transformer design it presented less computation time and reasonable accuracy. A prototype of designed transformer is implemented using amorphous magnetic materials. A set of experimental tests are conducted to measure the magnetic characteristics of the core and series coupling and open circuit tests are applied to measure the transformer parameters. A comparison between the simulation and experimental test results under different loads within the medium frequency range  validated both design and modeling procedures.

EXISTING SYSTEM:

Multiwinding transformers (MWTs) can provide a common magnetic bus for integrating renewable energies in the form of magnetic flux. Their application in multi-active bridge phase  shift converter makes it possible to simply integrate the  sources of different voltage levels using different turn ratios. Other advantages are galvanic isolation, bidirectional  power flow capability, faster control, and simultaneous power transfer a the ports. Design of MWTs for certain value of inductances is relatively complex due to their  complex structure and cross coupling effects.  Research on MWTs mainly is focused on their  characterization and modeling and there is not much research  on design methods. This paper provides a complete  discussion on design, prototyping and experimental tests of a  high frequency toroidal MWT. The transformer is designed for certain values of inductances, using reluctance network method (RNM). A prototype has been fabricated using amorphous magnetic materials to validate the accuracy of proposed design method. To measure the transformer parameters and extract the equivalent electrical model, the open circuit, differential and cumulatively coupled tests are conducted on the prototype transformer. The short circuit test is excluded as it did not provide reliable results due to relatively high leakage inductances. The prototype transformer is tested for a wide frequency range under different load conditions and the results are compared with the simulation based on extracted transformer model.

PROPOSED SYSTEM: 

MWTs have been used as the common magnetic links in multi-active bridge phase shift converters to integrate the renewable energies effectively. The converter designed in this research includes four ports connected to the load, fuel cell, battery and PV as illustrated in Fig.1. The Hbridge units produce high frequency ac square wave from dc buses linked to the DC sources. The power flow between the ports one, two and three is controlled by using the phase shift technique. To apply the technique, port one is selected as the reference and ports two and three are shifted for a leading or lagging phase angle to send or receive power to port one. A  duty cycle control is applied to port three for the maximum  power point tracking of PV panel.  As illustrated in the figure, port one is a bi-directional port transferring power from renewable sources or battery to the inverter and further to the load and grid. It can also transfer the power reversely from the grid to the battery (using port two and four). Port two is connected to the dc bus linking directly to the fuel cell and a  bi-directional buck-boost converter linked to the 24 V battery bank. The voltage of the dc bus is 50-70 V as it is connected directly to the fuel cell stack. It operates in the buck and boost modes to charge and discharge the battery respectively. Fuel cell in this system can be used as back up energy source and in normal operation mode, power transfers from PV (port three) as preferred source to the inverter and load (port one). Battery can be charged by fuel cell, PV or grid and also is used to balance the voltage of dc bus due to low dynamic response of fuel cell during fuel cell operation.

 

CONCLUSION    

Due to the application of MWTs as a common magnetic link in integration of renewable energy sources, they have attracted  considerable research interest. This paper covered three stages of design, prototyping and experimental test of a high frequency toroidal MWT. The RNM was used to design the   transformer based on the required specifications due to the low computation time. A prototype transformer was developed using amorphous magnetic materials and the transformer parameters including leakage and mutual inductances and resistances are measured applying open circuit and series coupling tests. A comparison between the simulation and experimental test results under different loads within the medium frequency range validated both design and modeling procedures.

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