SOLID STATE TRANSFORMERS: NEW APPROACH AND NEW OPPORTUNITY

Transcription

1 SOLID STATE TRANSFORMERS: NEW APPROACH AND NEW OPPORTUNITY 1 SHILPAKALA G. BANSODE, 2 PRASAD M. JOSHI 1 Research Scholar; Elect Dept, GCE, Karad, 2 Professor, Elect. Department, GCE, Karad Abstract- The advancement in semiconductor technology has provided a new alternative to the hundred year old conventional transformer technology by providing an elegant solution using Solid State Transformer (SST). The SST is applied semiconductor technology for changing the voltage ratio. The SST can achieve high power density as well as operation at high frequency, thus reducing the size and the cost. This has provided a new opportunity for researchers, world over, to suggest new topologies, use of new material and experimentation in different environment and area of application. In this paper a review of the SST technology is carried out in respect of its application as distributed transformer. The comprehensive literature review is reported. Different topologies of semiconductor devices used in SST are discussed. The advantages and limitations of the technology, as reviewed, are presented. Index Terms- Solid State Transformer, Microgrid, Conventional Transformer. I. INTRODUCTION The step down transformers are one of the pivotal power system apparatus at the distribution level. More than 35% of the total cost of distribution network is for distribution transformers. Moreover, the regular failures and maintenance cost of these transformers is an additional burden for the utility. Though robust in design, the conventional distribution transformers have following limitations: Sensitive to harmonics Voltage drop under load No control mechanism from system disruptions and overloads Environmental concerns regarding mineral oil Poor performance under dc-offset load unbalances No Power factor improvement. In recent years, the complexity of the electrical grid has grown due to the increased use of renewable energy and other distributed generation sources. To cope with this complexity, new technologies are required for better control and a more reliable operation of the grid [1]. One of such technologies is the solid-state transformer (SST). Solid State Transformer is the advanced technology suggested to replace the conventional distribution transformers. In this paper SST technology and its current state of affairs is elaborated. The paper is organized as follows: The second section describes structure, features & configurations of SST. Third section discusses the SST technologies appear in the reviewed literature. The fourth section deals with advancement in materials used in SST. Fifth section gives comparison of SST with Conventional Transformer (CT). Sixth section describes the specification of SST. The applications of SST are given in seventh section. Finally, the concluding section comments on the future of the SST technology. II. SST TOPOLOGY A. Structure of SST Solid State Transformer is firstly introduced by Navy researchers, in the form of AC/AC buck converter having two switches. It gives straight forward approach to ac to ac power conversion at normal frequency. The main drawback of this SST is that each switch must be able to block full primary voltage and have capacity to conduct full secondary current. Due to this, it requires high cost to design SST. To overcome this drawback, based on power electronic technology, SST with high frequency (HF) transformer is discovered. The basic structure of a SST is shown in Fig.1 [1]. SST works on the same principle as that of conventional transformer, but at high frequency. The incoming voltage is converted into a high frequency AC through the use of power-electronics based converters before applied to the primary side of the HF transformer. The opposite process is performed on the HF transformer secondary side to obtain an AC and/or DC voltage for the load. The power transfer at higher frequency helps in reduction in weight, as well as size, of a transformer. 15

2 topologies proposed for SST as well as for general AC-AC power conversion have been studied [5]. Fig.1 Basic Structure of SST B. Features of SST With the addition of semiconductor devices, SST does much more than just changing a voltage ratio. The SST with three stage topology is more popular in distribution system to supply power to the consumer. The power can be supplied in ac as well as dc form as per requirements [4]. Generally, the SST includes following three stages: The rectification stage first converts a high-voltage ac to dc at high voltage dc bus. In second stage, high-frequency transformation is used to convert higher dc voltage to lower level; generally this is called as dc/dc converter stage. At the output of this stage (at low voltage dc bus) a regulated low dc voltage at desired level is available. The last inversion stage helps to produce a desired, regulated low ac voltage (ac bus). Therefore, the SST is called as a three-port energy router and power exchanger [4]. It can integrate the distribution system, residential ac system, and envisioned dc system. In order to improve the system efficiency, the dc type sources and dc load are connected to dc port, whereas the ac type sources and ac load are connected to ac port. The three-port characteristics of SST make it very suitable to enable a new microgrid that exhibits better performance compared with conventional ac and dc microgrids [1]. An approach to classify the SST topologies and select the appropriate configuration according to the specific needs is discussed [14]. Three SST configurations that cover all the possible SST topologies are identified as follows: 1) Two-stage with low voltage DC (LVDC) link - This is simplest form of SST. It has low voltage dc link required to convert high level ac to lower one. Therefore it is widely used in low voltage applications (as shown in Fig.3). 2) Two stage with high voltage DC (HVDC) links The DC link of the third configuration is not appropriate for DES and DER integration since it is high voltage and has no isolation from the grid; therefore, topologies under that classification are not practical for SST implementation (as shown in Fig.4). 3) Three-stage with both HVDC and LVDC links Of the four possible classifications, the three-stage architecture, with two DCs, is the most feasible because of its high flexibility and control performance. The DC links decouple the HV-from the LV-side, allowing for independent reactive power control and input voltage sag ride-though. This topology also allows better control of voltages and currents on both primary and secondary side. It consists of an AC-DC conversion stage at the HV-side, a DC-DC conversion stage with high-frequency transformer for isolation and a DC-AC conversion stage at the LV-side( as shown in Fig.5)[22]. Fig.3 Two-stage with low voltage DC (LVDC) link Fig.2 Functional Diagram of SST C. Configurations of SST The selection of the appropriate topology for SST implementation is a key aspect. The main issue is addressed by comparing some of the potential topologies that support bidirectional power flow as a minimum requirement. In order to select these potential topologies for comparison, a number of Fig.4 Two stage with high voltage DC (HVDC) link Fig.5 Three-stage with both HVDC and LVDC links 16

3 III. LITERATURE REVIEW Solid-state transformer (SST) has been regarded as one of the ten most emerging technologies in 2011 by MIT technology review and is now promoted by Future Renewable Electrical Energy Delivery and Management (FREEDM) Systems Center at North Carolina State University, Raleigh. Nearly 20 years ago, Navy researchers proposed a power-electronic transformer that consisted of an ac/ac buck converter to reduce the input voltage to a lower one. This was followed in 1995 by a similar EPRI sponsored effort [1]-[4]. In 1970, W. McMurray form G.E. first introduced a high frequency link AC/AC. In 1980, Navy researchers proposed a power-electronic transformer that consisted of an AC/AC buck Converter [2]. In 1997, a high-power AC/AC conversion was proposed (SST without DC link). In 1999, a new three-stage SST structure was introduced. [2], [5]. power quality and harmonics of the ac side current meets utility requirement. Gangyao, Huang, Subhashish Bhattacharya had done comparison between 6.5kV 25A Si IGBT and 10-kV SiC MOSFET in Solid-State Transformer Application with consideration of switching characteristics, switching loss etc.. The results illustrate that the 10kV MOSFET has much higher frequency switching capability [10],[11],[12]. After that Haifeng Fan and Hui Li proposed high-frequency transformer isolated bidirectional dc dc converter modules with high efficiency over wide load range designed for 20 kva SST [8]. The proposed converter modules are connected in ISOP modular structure to enable the use of low-voltage MOSFETs, featuring low on-state resistance and resulted low conduction losses, to address MV power conversion [8]. Edward R. Ronan, Scott D. Sudhoff also worked on the design 10 KVA Power Electronic-Based Distribution Transformer which consists of three stages as Input Stage, Isolation Stage and Output Stage [4]. Last two stages are depending on Input stage. This Power Electronic Transformer is also known as Solid State Transformer. In this case, design of 10KVA, 7.5KV PET which shows much more ripple in input current and voltage results into fluctuation in output voltage, and indirectly decrease the proportion of output power. Hengsi Qin and Jonathan W. Kimball proposed application of an ac-ac dual active bridge converter for solid state transformer [5]. This topology consists of two active H-bridges and one high frequency transformer. They discuss 3.3 KVA 2.4KV/120V ac-ac DAB converter based SST with Zero voltage Switching techniques by using phase shift modulation. It is found that SST is most efficient if integrated with DC microgrid. DC microgrid is very useful for various types of zonal loads like colleges, schools, small industries, societies etc. DC zonal micro-grid architecture and control is proposed by Xu She and Srdjan, Lukic [3]-[4]. Alex Hung studied use of the SST in the FREEDM (Future Renewable Electric Energy Delivery and Management System system) is to achieve compatibility and flexibility [4]. As FREEDM components are added to (or removed from) the distribution network, the system must continue to function properly. The SST acts as a smart plug-and play interface for transforming and distributing electric energy. SST control also ensures that the Electrical Power Research Institute (EPRI) has been researching the Intelligent Universal Transformer (IUT) [7] since it is proposed to replace conventional distribution transformers with the multilevel power electronic systems. In 1996, Koosuke Harada proposed a new intelligent transformer [13], which significantly reduce the size of transformers by performing high frequency link. Various functions, such as constant voltage and constant power are realized by phase control. She Xu and Alex hung also studies on the worldwide review of SST [18]. They also proposed SST integrate microgrid with power management algorithm [1]. This reviews that SST has received increasing attention from both industry and academia for the applications in smart grid and traction system. They point out the SST converter design used in industry with comparative ratings, with state of art technology for every converters presented in SST. Four well known high voltage SST designs, namely the UNIFLEX, EPRI, GE, and ABB, are highlighted with future scope of SST and it s applications of SST in distribution system. SST has great scope in technical area in future. Furthermore, new developing research directions were also presented, opening the path to new horizons [15]. IV. ADVANCEMENT IN MATERIAL OF SST As compared to CT, SST has power electronic converters with high frequency transformer. In these power electronic converters, power switches are used such as MOSFET, IGBT etc. In order to design SST, mainly high frequency transformer design plays important role. In case of design, it examines mainly efficiency of high frequency transformer depending 17

4 on the operating condition, wire and core selection and electromagnetic analysis to have a required magnetizing and leakage inductance for converter [6]. Even though the high operating frequency makes the transformer compact, there are many restraints which have to be considered, such as insulation, power loss and cost as well. The transformer losses are strongly related to frequency. These losses contribute to the economics of the system in which they operate. There are two losses mainly contributing to the total transformer losses [20] are: 1) The core loss (which represents the no load loss). 2) The winding or copper loss (which represent the load loss). The core loss i. e. the power dissipated in the core consists of Eddy current and hysteresis losses. Hysteresis loss It is consumed in the continuous reversal of the magnetic field due to the changing direction of the magnetizing current. This loss is easier to control through the design stage than the eddy current loss. Eddy current loss It is caused by circulating currents in the body of the core. This current is produced due to the induced voltage when the magnetic flux is changing. In principle, the induced voltage per turn in the core is the same as at in the secondary winding. The direction of this current is normal to the magnetic flux direction that produced it. At low frequency, the eddy currents can be reduced by laminating the core in the direction of the induced voltage. As frequency rises the required laminations become impracticable, and research is made to find alternative material which naturally have low eddy currents by virtue of the granular structure. The core loss, which is determined by the core materials and the design, is a function of the amplitude and frequency of the applied voltage [17], [19]. Core manufacturers have gradually improved core material properties, including the Ferrites which are widely used at present. Also different types of core material characteristics specification described in Table I as: Characteristics\Materia l Saturation flux density B sat (T) Curie temperature Tc (ºC) Max. Operation temp. (ºC) TABLE I Nano crystallin e Fine met FT-3M Ferrit e 3F Super alloy 0.79~0.8 7 Amorphou s 2605SA The design of high frequency transformer under high voltage condition requires more accurate electromagnetic analysis and concern from the control point of view and insulation point as well. High frequency transformers in solid state transformer point out for the performance and overall efficiency of SST system, so it is important to select right materials and optimize the design to fulfill all requirements in the operating condition. Depending on the frequency, High frequency transformer material is categorized IN Table II [19], [21]: TABLE II Frequency Material Winding Core type 3 KHz Typical solenoid Meglas C-Core made up of amorphous alloy 20 KHz or Above 20 KHz coaxial cores Nano crystalline toroidal High frequency transformer is designed as dry-type for environmental and safety issues. IV. COMPARISON OF SST AND CT The Conventional Transformer (CT) has been used since the introduction of AC systems for voltage conversion and isolation. The widespread use of this device has resulted in a cheap, efficient, reliable and mature technology and any increase in performance are marginal and come at great cost. Despite its global use, the CT suffers from several disadvantages. Some of these are: Bulky size and heavy weight Transformer oil can be harmful when exposed to the environment Core saturation produces harmonics, which results in large inrush currents Unwanted characteristics on the input side, such as voltage dips, are represented in output waveform Harmonics in the output 18

5 current has an influence on the input. Depending on the transformer connection, the harmonics can propagate to the network or lead to an increase of primary winding losses. Relative high losses at their average operation load. Transformers are usually designed with their maximum efficiency at near to full load, while transformers in a distribution environment have an average operation load of 30%. All CTs suffer from non-perfect voltage regulation. The voltage regulation capability of a transformer is inversely proportional to its rating. At distribution level, the transformers are generally small and voltage regulation is not very good. The Solid State Transformer (SST) provides an alternative to the CT. It should be noted that the SST is not a 1:1 replacement of the CT, but rather a multi-functional device, where one of its functions is transforming one AC level to another. Other functions and benefits of the SST which are absent in the CT are: High controllability due to the use of power electronics. 1. Reduced size and weight because of its high-frequency transformer. The transformer size is inverse proportional to its frequency; hence a higher frequency results in a smaller transformer. 2. Unity power factor because the AC/DC stage acts as a power correction device. Unity power factor will usually increase the available active power by 20%. 3. Not being affected by voltage swell or sag as there is a DC link in the solid state transformer. 4. Capability to maintain output power for a few cycles due to the energy stored in the DC link capacitor. 5. Function as circuit breaker. Once the power electronics used in the solid state transformer are turned off, the flow of electricity will stop and the circuit is interrupted. 6. Fast fault detection and protection V. SPECIFICATIONS OF SST With incorporation of the solid-state technology into the distribution transformer, many new specifications [14], [16] can be realized as: 1) Voltage sag compensation - When the input source voltages compensate for the deficit and maintain constant output voltage. The total period of compensation, as a function of the amount of energy storage, can be adapted to the specific need of the customer. 19 2) Outage compensation - Similar to voltage sag compensation, the SST can provide full voltage compensation or the period needed by the built-in energy storage. 3) Instantaneous voltage regulation - If the input source voltage fluctuates due to power system transient or other load effects, the SST will maintain constant output voltage because it has the energy buffer. 4) Fault isolation - The SST can act as a circuit breaker to isolate the power grid from load fault and vice versa. 5) Power factor correction (and reactive power compensation) - The SST can maintain a unity power factor within its power rating. The SST can also generate or absorb reactive power as required by the system. 6) Harmonic isolation - Nonlinear loads produce harmonic-distorted current that tends to propagate back to the primary side of the transformer. The SST will maintain a clean input current with a unity power factor. 7) DC output - In addition to the 120/240V AC voltage, the SST has 400V DC output, which allows easier connection to distributed energies. 8) Metering or advanced distribution automation - The SST has advanced monitoring capabilities including instantaneous voltage, current, power factor, harmonic percentage, kwh and fault current or voltage information as well. 9) Environmental benefit - Unlike the conventional liquid immersed transformer, the solid state transformer is an oil-free transformer and friendly to environment. VI. APPLICATIONS OF SST A SST can be used instead of the conventional transformer (CT) in any electrical system, but because of its additional advantages and functions[15], the application of the SST in certain areas is much more attractive. Examples of these applications are: 1. Locomotives and other traction systems The transformer used in current locomotive vehicles is 16.7Hz and is ±15% of the total weight of the locomotive. The SST can provide a significant weight reduction. Additionally, the SST is also able to improve the efficiency, reduce EMC, harmonics and acoustic emissions. 2. Desired energy generation Offshore generation, whether from wind, tidal or any other source, can benefit from the reduction in weight and size. This reduction leads to smaller and thus cheaper offshore platforms. Another advantage is that the SST

6 can achieve unity power factor, thus increasing the efficiency in power transmission. 3. Smart Grids In future power systems, the usage of renewable generation is expected to increase, and will require an energy management scheme that is fundamentally different from the classic methods. For fast and efficient management of the changes in different loads and sources, the SST can be used to dynamically adjust the energy distribution in the grid[17]. The SST will manage the flow of energy. For this reason, the SST is sometimes also called an energy router. 4. DC Source for Power Delivery The SST concept is ideally suited to extend the use of DC, both in MV and LV applications [4]. The difficulty in interrupting a DC feeder under fault conditions is often cited as a major hurdle in the acceptance of DC distribution in MV applications. The use of the power electronic interface (SST) to generate the DC is a means of controlling the system and interrupting fault currents. 5. Integration with other systems The LV DC link in the SST topology provides a good and readily accessible integration point for renewable energy systems into the distribution grid. A unidirectional converter could be used when the load demand is much bigger than the renewable energy generation capabilities. Where the peak generation capabilities exceed the load demand during certain periods, the excess power could be fed back into the grid by using a bidirectional converter. 6. Application between generation source and load or distribution grid In this scenario, the SST can enable constant voltage and frequency at its output if the input voltage and frequency are variable. The SST can also allow the energy transport between source and load or grid to occur at unity power factor. This results in better utilization of the transmission lines and increased flow of active power. Another function, which the SST can provide is to improve system damping during the transient state. 7. Application between two distribution grids - One of the features of the SST is that it does not require both grids to have the same voltage level, frequency or to operate synchronously. The SST can be used to control the active power flow between both 20 grids. It can also be used as a reactive power compensator for both grids. Special application is made, when considering the commercial side of power systems. During periods when energy in grid 2 is cheaper than in grid 1, the operator of grid 1 can reduce its own generation and buy the energy from grid. 8. Connection between the MV-and LV-grid - In contrast to the CT, the SST can accurately control the amount of active power flowing from the MV-to the LV-grid. This is useful if the LV-side also has generation sources such as PV-panels. The SST can limit the amount of energy that flows back and forward through certain parts of the grid, to avoid overload of transmission lines with limited current carrying capacity. 9. Connection between MV-grid and loads - LV-loads are often unbalanced which can lead to harmonics disturbances in the voltage and asymmetrical voltages. A neutral wire is added in order to eliminate these disturbances and achieve a more symmetrical voltage. When the imbalance is large or consists of many non-linear loads, the addition of a neutral wire might not nullify the disturbances completely. In this case, the SST can help by generating a voltage that hardly suffers from unbalanced and non-linear loads. 10. Application as interface for distributed generation and smart grids - Distributed energy sources, such as photovoltaic arrays and wind turbines, provide a variety of electric sources. These sources often have a varying voltage or frequency or can even be a DC voltage. The SST is flexible enough to allow connection of these sources to the traditional grid. CONCLUSION Finally, we conclude that the basic study of SST (Solid State Transformer) shows the bidirectional power flow required for grid with basic topology. It also shows importance of SST over CT in considerations with size, volume and operating frequency. SST gives future challenges like improvement in efficiencies, Reduction in power conversion stages, Optimum control algorithms, and Suitable modulation techniques to reduce switching losses, Reduction in cost etc. SST has great opportunities in future such as Hybrid vehicles, Electric vehicle charging station, Ships and submarines, Traction locomotives, Microgrids, Distribution systems due to its advance features and

7 configurations etc. This paper also explains the applications of SST in Industrial area which shows the benefits for future development. REFERENCES [1] Xu She, Alex Q.Huang, Srdjan Lukic, and Mesut E. Baran On integration of solid-state transformer with zonal DC microgrid IEEE transactions on smart grid, vol. 3, no. 2, June [2] E. R. Ronan, S. D. Sudhoff, S. F. Glover, and D. L. Galloway, A power electronic- based distribution transformer, IEEE Trans. Power. Del., vol. 17, no. 2, pp , Apr [3] X. She, S. Lukic, and A. Q. Huang, DC zonal micro grid architecture and control, in Proc. IEEE IECON, 2010, pp [4] A. Q. Huang, M. L. Crow, G. T. Heydt, J. P. Zheng, and S. J. Dale, The future renewable electric energy delivery and management system: The energy internet, Proc. IEEE, vol. 99,no. 1, pp , Jan [5] Hengsi Qin and Jonathan W. Kimball Ac-Ac Dual Active Bridge Converter for Solid State Transformer Department of Electrical and Computer Engineering Missouri University of Science and Technology 301 W. 16th St., Rolla, MO USA. [6] Moonshik Kang, Prasad N. Enjeti, and Ira J. Pitel, Fellow, IEEE Analysis and Design of Electronic Transformers for Electric Power Distribution System IEEE transactions on power electronics, vol. 14, no. 6, November [7] J.-S. Lai, A. Maitra, A. Mansoor and F. Goodman, Multilevel Intelligent Universal Transformer for Medium Voltage Applications, Conf. Rec. IEEE/IAS Annu. Meeting. vol. 3, pp: , Oct, [8] Haifeng Fan, and Hui Li, High-Frequency Transformer Isolated Bidirectional DC DC Converter Modules with High Efficiency over Wide Load Range for 20 kva Solid-State Transformer IEEE transactions on power electronics, VOL. 26, NO. 12, DECEMBER [9] Jianjiang Shi, Wei Gou, Hao Yuan, Tiefu Zhao, and Alex Q. Huang, Research on Voltage and Power Balance Control for Cascaded Modular Solid-State Transformer IEEE transactions on power electronics, VOL. 26, NO. 4, APRIL [10] S. Bhattacharya, T. F. Zhao, G. Y.Wang, S. Dutta, S. Baek, Y. Du, B. Parkhideh, X. H. Zhou, and A. Q. Huang, Design and development of generation-i silicon based solid state transformer, in Proc. IEEE APEC, 2010, pp [11] G. Y. Wang, X. Huang, J. Wang, T. F. Zhao, S. Bhattacharya, and A. Q. Huang, Comparisons of 6.5 KV 25 A Si IGBT and 10 KV Sic MOSFET in solid state transformer application, in Proc. IEEE ECCE, 2010, pp [12] S. Baek, Y. Du, G. Y. Wang, and S. Bhattacharya, Design considerations of high voltage and high frequency transformer for solid state transformer application, in Proc. IEEE IECON,2010, pp [13] S Yogendra Reddy, S Bharat A modular power electronic transformer for medium voltage application Electrical Engineering. PNC & VIET, Guntur, India. [14] Xu She, Alex Q. Huang, and Gangyao Wang, 3-D Space Modulation With Voltage Balancing Capability for a Cascaded Seven-Level Converter in a Solid-State Transformer IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 12, DECEMBER [15] Xunwei Yu, Xu She, Alex Huang Hierarchical Power Management for DC Microgrid in islanding mode and Solid State Transformer Enabled Mode Electrical Engineering Department North Carolina State University Raleigh, NC,US. [16] D.K. Rathod Solid State Transformer (SST) Review of Recent Developments Department of Electrical Engineering, Institute of Diploma Studies, Nirma University, S.G. Highway, Ahmedabad, Gujarat, INDIA. [17] D. Aggeler, J. Biela, J. W. Kolar Solid-State Transformer based on SiC JFETs for Future Energy Distribution Systems ETH Zurich, Power Electronic Systems Laboratory, Switzerland. [18] Xu She1, Alex Huang1, Rolando Burgos, Review of the Solid State Transformer Technologies and its Application in Power Distribution System FREEDM, NC 27606, US Center for Power Electronics Systems (CPES), Virginia Tech, Blacksburg, VA 24061, US. [19] G. Chin, Review of magnetic properties of Fe-Ni alloys, Magnetics, IEEE Transactions on, Vol. 7, Issue 1, Mar 1971, pp [20] B.L. theraja ELECTRICAL TECHNOLOGY II S chand publications. [21] Alex m. Leary,1,3 paul r. Ohodnicki,2 and michael e. Mchenry1, soft magnetic materials in high-frequency, high-power Conversion applications [22] N Mohan, Undeland, Robbins book of POWER ELECTRINICS converters, Applications and Designs. [23] Wei Shen, Design of High-density Transformers for High-frequency High-power Converters July, 2006 Blacksburg, Virginia. 21