ACTIVE POWER ELECTRONIC TRANSFORMER A STANDARD BUILDING BLOCK FOR SMART GRID

Transcription

1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14) ISSN (Print) ISSN (Online) Volume 5, Issue 12, December (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJEET I A E M E ACTIVE POWER ELECTRONIC TRANSFORMER A STANDARD BUILDING BLOCK FOR SMART GRID SHARUN JOHN 1, BRINTA N.R 2 1 P G Scholar, Department of EEE, Sree Narayana Gurukulam College of Engineering Kadayiruppu, India 2 Assistant Professor, Department of EEE, Sree Narayana Gurukulam College of Engineering Kadayiruppu, India ABSTRACT Nowadays distribution transformers are considered among the huge and expensive equipment because of their massive iron core and heavy copper windings. Since the size of the transformers and the maximum power transition are reversely related, a new family of electronic power transformer has come into existence which increases the transformer frequency, using power electronic converters. In addition to voltage transformation and good isolation which they bring about, these transformers are also associated with significant advantages, including considerable reduction in the size, power quality improvement, voltage profile improvement, etc. This paper attempts to introduce a new structure for Active Power Electronic Transformer. Active power electronic transformers, composing an important part of the smart grid, could have many roles in that variable system. Active power electronic transformer is used later as a standard building block in more complex systems to guarantee the power quality on the HV grid side. The performance of the base module was verified by computer-aided simulations. Keywords: Active Power Electronic Transformer, power electronic converters, power quality improvement, smart grid, variable system I. INTRODUCTION Today s distribution grid operators face new challenges. Distributed generation and renewable energy sources increase the uncertainty factor in the grid and make a centralized control impossible. New operation and performance objectives will be defined that can come under the term smart grid concept. Transformers are widely used in electric power system to perform the primary functions, such as voltage transformation and isolation. Transformers are one of the heaviest and most expensive devices in an electrical system because of the large iron cores and heavy copper windings in the composition [1]. Power generation, transmission and distribution are three main parts of the modern power system, in which the power transformers play a significant role [2]. The power electronics has offered enabling technologies for the power quality enhancements in the transmission and distribution systems. Such systems are for example the flexible AC transmission systems, static VAR compensators, static synchronous compensators, unified power flow controller etc. The APET is a new type of transformer that realizes voltage transformation, galvanic isolation and power quality enhancements in a single device. The APET is suitable for the use in the power systems that comprise renewable energy sources, energy storage devices as well as different type of loads. Thus the bi-directional power flow is the most important requirement that the APET has to fulfill. Additional expected characteristic is the reduction of the volumes by means of increasing the operating frequency. Since transition power of transformer and their size are reversely related to 178

2 the frequency, increasing the frequency provides for using magnetic steel core in the transformers, thereby considerably reducing its size. Since there is no energy storage source in the conventional transformers, when there is disturbance in the transformer input the output loads are disturbed as well. Similarly when the load is troubled with transient states, harmonics and power disturbances the conventional transformers reflect all this problems toward the grid. In order to overcome such problems power electronic technology could be an option, which act as energy buffer, thereby preventing the mutual effect of grid and load on one another. That's why the new family of electronic power transformers have come into existence which, using electronic converters, increasing the frequency of AC signals and thus reduce the transformer sizes [3]. Different topologies have been presented for realizing the PET, in recent years, the research states that the three stage PET topology seems to be the most promising. This topology consists of the controllable input (AC/DC), the isolation (DC/DC) and output (DC/AC) power electronic stages (see Fig. 1). First stage is an AC/DC converter which is utilized to shape the input current, to correct the input power factor, and to regulate the voltage of primary DC bus. Second stage is an isolation stage whichh 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 (MF) transformer and is rectified to form the DC link voltage. The output stage is a voltage source inverter which produces the desired AC waveforms. Fig. 1. Three stage topology of PET. There is no unique structure for an APET. Depending on the application, it could be with multiple input/output ports, integrating high voltage (HV) and low voltage (LV) ports, AC and dc ports, single-phase and three-phase ports, some of them connected to the grid or consumers, other to renewable energy plants or to energy storage systems. The structure of the smart grid could be very complex and hardly predictable. Thus, an APET could have many roles in that stochastic and variable system. The aim was to develop a base module of the APET that could be used later as a standard building block in more complex systems. To provide flexibility and universal properties, the base module was designed as a symmetrical topology with the transfer ratio 1:1. The power quality on the HV grid side of the base module of the APET is guaranteed by the PFC functionality. The performance of the proposed base module of the APET was verified by computer-aided simulations using MATLAB/SIMULINK. In this project a single phase three-stage topology of APET was selected as a base module. II. SELECTION OF THE TOPOLOGY Active power electronic transformer (APET) can be classified into single-stage, two-stage and three-stage topologies. Active Power Electronic Transformer Single-stage Two-stage Three-stage With LV dc-link With HV dc link Fig. 2. General Classification of Active Power Electronic Transformer Topologies. 179

3 Single-stage actually means a topology without a dc-link. It includes direct ac-ac or matrix converters. The lack of the dc-link strongly limits their functionality, integration of renewable energy sources and energy storage devices. Thus, single-stage topologies are not considered suitable candidates for the APET in the smart grid applications. Two-stage topologies (Fig. 2) are divided into topologies with a HV dc-link and with a LV dc-link. The LV dc-link is required for connection of distributed energy storages (DES). Thus, two-stage topologies with the HV dc-link do not comply with smart grid requirements. Two-stage topologies with the LV dc-link cannot compensate the reactive power on the HV side and will also have a larger voltage ripple in the dc-link [4]. Nevertheless, availability of the LV dc-link improves the output power quality and allows connection of distributed energy sources and distributed energy storage devices. Thus, this configuration is considered a better choice for the APET than the two-stage topology with the HV dclink. The most feasible configurationn of the APET is the three-stage configuration (Fig. 1), i.e. topologies with both HV and LV dc-links available [5]. The voltage or current can be separately controlled in each stage. It is possible to add distributed energy storage devices or distributed energy sources. HV and LV dc-links enhancee the ride-through capability of the APET and allow power quality improvement in the input and in the output. Multilevel converter topologies can be implemented in each stage and optimize the APET for high-voltage or high-power applications. The three-stage topology was selected in the current project to build the base module. III. PROPOSED TOPOLOGY In general, a three-stage topology includes an input stage, an isolation stage and an output stage. In the input stage, there is a converter, which converts the input AC voltage to DC voltage. The second part of the converter is formed by a DC/AC converter. This part of the converter contains the MF transformer with the high insulation capability. In the output part, the high frequency voltage is revealed as a power-frequency voltage. In this paper, a three part design is introduced. It is a new configuration based on the matrix converter with new function shown in Fig. 3. It can provide desired output voltage. In addition, it performs power quality functions, such as sag correction, reactive power compensation and is capable to provide three-phase power from a single phase system. The APET has three stages and each stage can be controlled independently from the other one. Many advantages of the APET such as output power quality and power factor correction depend on appropriate close-loop control, and correlativee research is necessary. The reliability of a system is indirectly proportional to the number of its components. The main purpose of proposed APET is reduction of the power delivery stage (AC/DC and DC/AC links) in PET with DC-link. Fig. 3. Structure of a Three-Stage Topology IV. PROPOSED ACTIVE POWER ELECTRONIC TRANSFORMER The electrical circuit diagram of the proposed base module is shown in Fig. 4. The model is divided into four parts: input stage (L1, H-Bridge, C1), isolation stage (inverter, medium frequency transformer, rectifier), output stage (LC filter, H- Bridge, LC filter), and load (diode rectifier with an inductive load). 180

4 Fig. 4. Electrical Circuit Diagram of the Base Module In the fig 5 shows MATLAB / SIMULINK modeling of proposed Active power proposed solution allows bidirectional energy control. electronic transformer. The INPUT STAGE ISOLATION STAGE OUTPUT STAGE Fig 5. Mat lab / Simulink modeling of proposed active power electronic transformer A. Input stage The input stage is a three or single phase PWM rectifier, which is used to convert the primary low frequency voltage into the DC voltage. 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. Many control methods are presented for control of input stage in conventional PET. Fig. 6. Shows single phase rectifier with input inductances. A single phase PWM rectifier is used in this paper, which operates same as input stage of conventional PET [6]-[7]. 181

5 Figure 6 Structure of the Proposed Input Stage B. Isolation Stage The isolation stage consists of the inverter, MF transformer and rectifier (Fig. 7). Isolation stage is contained a single-phase high frequency voltage source converter (VSC), which converts the input DC voltage to AC square voltage with high (or medium) frequency and HF (MF) transformer. The main functions of the HF (MF) transformer are such as voltage transformation and isolation between source and load. Figure 7 Structure of the Proposed Isolation Stage C. Output Stage The output stage is represented in Fig. 8. The proposed converter generates desired output voltage with suitable shape and frequency. The output stage consists of LC filter 1, sine wave modulator; H-Bridge inverter and LC filter 2. Figure 8 Circuit of Output Stage 182

6 D. Modeling Load To simulate a nonlinear load, a diode rectifier was used with an inductive load in the output. To simulate a resistive load, the inductive load was replaced by a resistor. V. SIMULATION RESULTS To evaluate the expected performance of the APET, the design was simulated to predict steady state performance. A prototype based on the proposed topology is simulated using MATLAB/SIMULINK. Operation of proposed APET is described by Fig. 9. Fig. 9(a) shows input line voltage of APET. As it can be seen in Fig.9(b), the output voltage of the input rectifier. Fig. 9(c) depicts the output voltage of inverter stage that transforms DC voltage to medium frequency (2 KHz) AC voltage as the transformer primary voltage. In the output stage, the medium frequency voltage is revealed as a 50 Hz waveform followed by a rectifier & inverter stage. Fig. 9(d) depicts the output of step down transformer. Fig. 9(e) & Fig. 9(f) implies the output of, the AC/DC and DC/AC converters respectively. Figure 9 (a) Input voltage (b) DC-link voltage (c) MF transformer primary voltage (d) MF Transformer secondary voltage (e) Secondary rectifier output voltage and (f) output voltage Symbol Uin L1 rl1 C1 rc1 N L2 rl2 C2 rc2 Udc1 L3 rl3 C3 rc3 Lload rload Uout TABLE 1 : SIMULATION PARAMETERS Parameter Value Input voltage (RMS) 230V,50 Hz Input inductor 100 mh Series resistor of the inductor 10 µω HV dc-link capacitor 10 mf Shunting resistor of the capacitor 100 kω Turns ratio of isolation transformer 1:1 Inductance of the LV dc-link inductor 30 mh Series resistor of the inductor 200 mω LV dc-link capacitor 500 µf Shunting resistor of the capacitor 10 kω HV dc-link voltage 400 V dc Output filter inductor 50 mh Series resistor of the inductor 500 mω Output filter capacitor 100 µf Shunting resistor of the capacitor 10 kω Load inductor 500 mh Load resistor 100 Ω Output voltage of APET (RMS) 20V, 50 Hz 183

7 VI. HARDWARE IMPLEMENTATION The system parameters are as follows: 1 Utility grid side: input voltage 12 V AC, 50Hz 1 primary dc link side: capacitor 1000 µf, 1 high-frequency transformer: ratio of transformation is 1:1, frequency 10 khz, 1 secondary DC link side: filter capacitance 1000 µf, 1 load side: filter inductance 14 mh, filter capacitor 2.5 µf, reference value of load voltage is 12 V AC, 50Hz. The DC voltage from the input stage is modulated into the high-frequency square wave by the full-bridge inverter, and the amplitude of high-frequency square wave voltage is half of DC high voltage. Figure 10 Hardware circuit model VII. CONCLUSION An active power electronic transformer (APET) could have many roles in the smart grid. The base module of the APET that could be used later as a standard building block in more complex systems was proposed and evaluated. The base module is a single-phase system consisting of different converters. In addition to providing galvanic isolation between the input and the output, it isolates input from output distortions. Thus, the input voltage disturbances have no effect on the output ones, and vice versa. In general, the power quality is improved in both ports: input and output. The performance of the proposed base module of the APET was verified by computer-aided simulations using MATLAB/SIMULINK. REFERENCES [1] Kang, P.N. Enjeti, I.J. Pitel, Analysis and design of electronic transformers for electric power distribution system, IEEE Trans. On Power Electronics, vol. 14, no. 6, pp , November [2] She, Xu; Burgos, Rolando; Wang, Gangyao; Wang, Fei; Huang, Alex Q.;, "Review of solid state transformer in the distribution system: From components to field application," Energy Conversion Congress and Exposition (ECCE), 2012 IEEE, vol., no., pp , Sept [3] H. Krishnaswami, V. Ramanarayanan, Control of high frequency ac link electronic transformer, IEE Electric Power Applications, vol. 152, no. 3, pp , May [4] Falcones, S.; Xiaolin Mao; Ayyanar, R.; "Topology comparison for Solid State Transformer implementation," Power and Energy Society General Meeting, 2010 IEEE, pp.1-8, July [5] J. R. Rodr ıguez, J. W. Dixon, J. R. Espinoza, J. Pontt, and P. Lezana, PWM regenerative rectifiers: State of the art, IEEE Trans. Ind. Electron., vol. 52, no. 1, pp. 5 22, Feb [6] D. Wang, C. Mao, J. Lu, S. Fan, F.Z. Peng, Theory and application of distribution electronic power transformer, Electric. Power Syst. Res, vol. 77, pp , March [7] D. Wang, C. Mao, J. Lu, Coordinated control of EPT and generator excitation system for multidouble-circuit transmission-lines system, IEEE Trans. Power Deliver. vol. 23, no.1, pp , 2008 [8] Dr. Damanjeet Kaur, Smart Grids and India International Journal of Electrical Engineering & Technology (IJEET), Volume 1, Issue 1, 2010, pp , ISSN Print: , ISSN Online: