See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/304411258 Flexible power distribution unit — A novel power electronic transformer development and demonstration for distribution system Conference Paper · November 2015 DOI: 10.1109/IECON.2015.7392155 CITATIONS READS 9 66 7 authors, including: Duan Qing Jianhua Wang China Electric Power Research Institute Southeast University (China) 8 PUBLICATIONS 34 CITATIONS 47 PUBLICATIONS 542 CITATIONS SEE PROFILE All content following this page was uploaded by Jianhua Wang on 20 July 2019. The user has requested enhancement of the downloaded file. SEE PROFILE Flexible Power Distribution Unit－A Novel Power Electronic Transformer Development and Demonstration for Distribution System Qing Duan1, Jianhua Wang 2, Chunyan Ma1, Binshi Gu2, 1 China Electric Power Research Institute & Beijing Key Laboratory of Distribution Transformer Energy-saving Technology, Baojian Ji3, Peng Qiu3, Jun You2 2 Jiangsu Provincial Key Laboratory of Smart Grid Technology & Equipment, School of Electrical Engineering Beijing, China Southeast University, Nanjing, China Email: firstname.lastname@example.org 3 College of Automation and Electrical Engineering Nanjing Tech University, Nanjing, China Abstract—Power electronic transformer (PET) is an emerging new type of power converter in recent years. It has not only the basic functions of power transformation and isolation, but also the extra functions of power quality control. A novel power electronics transformer for distribution system named flexible power distribution unit is proposed in this paper, and the energy exchange mechanism between the network and load is revealed. Finally, a 100kW 600Vac/220Vac/110Vdc medium frequency isolated prototype is developed and demonstrated in laboratory. The experimental results show that the proposed structure and control strategy are feasible. Keywords—Power electronic transformer; system; flexible power distribution unit distribution I. INTRODUCTION Distribution transformer is the most important and common equipment in power distribution network, which is mainly responsible for voltage transformation and isolation. Traditional distribution transformer is very reliable, but it is bulky, cumbersome and harmonics could not be isolated between primary and secondary sides. And extra equipment is needed to monitor and protect itself for possible breakdown issue. Now days these drawbacks are the real concerns in academics and industries . Therefore, the power electronics based transformer called PET (Power electronic transformer), IUT(Intelligent Universal Transformer)，SST(Solid State Transformer), ST (Smart transformer) and others has gradually become an emerging issue over the last 10 years, especially for railway traction and smart grid-. ABB, Alstom, Bombardier, Siemens have done a lot of work in traction applications. The pilot installation from ABB was completed in mid-2011, and homologation with the Swiss Federal Office for Transport (FOT) was achieved by the end of the year . At the same time, scientists and researchers in the UNIFLEX-PM (Advanced Power Converters for Universal and Flexible Power Management in Future Electricity Networks), FREEDM (the Future Renewable Electric Energy Delivery and Management), MEGA Cube and Project Supported in part by the Basic and Prospective Science and Technology Project of State Grid Corporation of China (EPRIYDKJ(2014)3317), by the National Natural Science Foundation of China (51207023), by the Cooperative Innovation Fund of Jiangsu Province--the Prospective and Joint Research Project (BY2015070-18),by the Science and Technology Pillar Program of Zhenjiang City(GY2014029) HEART (The Highly Efficient And Reliable smart Transformer (HEART), a new Heart for the Electric Distribution System) Projects lead by the leading universities are still continuously investigating various issues of PET for smart grid, partly because of that the existing 50/60Hz power system is more complicate than the 16.67Hz traction electric system-. Besides USA EPRI IUT projects, China EPRI also starts its PET project for distribution power system. This paper would like to introduce its Gen-I PET project, a flexible power distribution unit for future distribution system. A novel PET structure for distribution system is proposed and described here, and the prototype is also developed and demonstrated in laboratory for detail tests. The experimental results show that the proposed structure and control strategy are feasible. II. THE BASIC PRINCIPLE OF PET PET is mainly composed of the power electronic converter and medium/high frequency isolation transformer, as shown in figure 1, in which the power electronic converter mainly complete the conversion, control, protection of electrical energy. Medium/high frequency isolation transformer is mainly responsible for the transformation of the galvanic isolation. Due to the operation frequency of transformer is inversely proportional to its volume, the high-frequency transformer can drastically reduce the volume and weight and improve the capacity and efficiency of the transformer. High frequency isolation transformer Source Power electronic converter Power electronic converter Load Figure.1 Basic structure of PET According to whether contains dc link can PET be divided into two categories: one is direct AC/AC PET which does not contain dc link in the process of transformation, another is AC/DC/AC PET that contains dc link. Figure 2 shows the typical structure of AC/DC/AC PET . This structure can realize the input power factor correction and suppress the bidirectional harmonic flow. The use of integral modularized transformation method creates the concise structure. Furthermore, the proposed PET topology in this paper improves the structure with a muti-winding transformer. AC/DC High frequency transformer AC/DC DC/AC Lg es ed e a r ao h h l T p ea DC/AC eb ec Input Isolation Output Figure.2 AC/DC/AC topology of PET The working principle of the PET is accessible. Firstly, input AC/DC converter converts high voltage power frequency alternating current (ac) to high voltage direct current (dc). Secondly, the isolation DC/AC inverter converts direct current into high frequency alternating current square wave. Then through high frequency isolation transformer, the square wave is coupled to the transformer vice side. The isolation AC/DC rectifier converts alternating current square wave to low voltage direct current. Finally, the output DC/AC inverter convers direct current to required alternating current. III. THE SYSTEM STRUCTURE OF PET Figure 3 shows the main circuit topology of a three-phase four-wire PET which is suitable for the smart distribution network. This is a typical AC/DC/AC type tertiary structure that contains input, isolation and output. In figure 3, ea,eb,ec are three-phase grid voltage; iga,igb,igc are three-phase grid current; Lg is filter inductance for the grid side; udcH is the high voltage dc bus input voltage; udcL1,udcL2,udcL3 are the isolation low voltage dc bus output Input voltage; iLa,iLb,iLc are the inverter bridge arm output current; Lf is inverter output filter inductance; Cf is inverter output filter capacitor; ioa、iob、ioc are three-phase load current. The input of PET adopts three-phase PWM full controlled rectifier which has mature technology, can run with high power factor and is widely used by medium power capacity PWM rectifier. The isolation single phase full bridge inverter convers high-voltage direct current to high frequency square wave that is access to the primary side of the high-frequency transformer. Transformer's viceside is composed of three single phase full bridge rectifier which convert high frequency square wave to direct current. The output is made up of three two-level single phase full bridge inverter and LC filter, and uses YN connection to form the three-phase four-wire system. This topology is flexible to control, simple and easy to realize three phase independent control. Figure 4 shows the input control block diagram. θ is theoutput power grid voltage vector angle of phase-locked loop, and θ=ωt. ω is the angular frequency of the power grid voltage, in this paper ω=100π rad/s. ed、eq are d and q axis components of power grid voltage. igd、igq are d and q axis components of power grid current. udcH* is the reference value of dc bus voltage udcH. igd*、igq* are the reference values of igd、igq. In order to achieve the unit power factor of input, the reference value of q axis current igq* is set to 0. The isolation module uses open loop control to convert input dc to high frequency square wave whose duty ratio is 50%. Then the square wave is coupled to the vice side of the high-frequency transformer and changes into dc signal through a rectifier. The high frequency transformer mainly Isolation Output iLa Lf udcL1 ea iga Lg eb igb Lg ec igc Lg ioa iLb Lf udcH udcL2 iob b Cf iLc Lf udcL3 a Cf ioc c Cf n Figure.3 A novel Power electronic transformer for distribution system (a) Input (b) Isolation Figure.6 Some photos of the PET Figure.4 control block diagram of input stage has two functions: one is to realize the electrical isolation between input and output, another is voltage grade transformation. The output module whose main function is transform the output direct current of the isolation module to constant-voltage and constant-frequency alternating current is made up of three single phase inverter and LC filter. In order to ensure the output phase voltage of the inverter has constant root mean square (RMS) and sinusoidal waveform, double loop control is used. The inner loop is phase voltage instantaneous value control loop while outer loop is the phase voltage RMS control loop. Inner loop can acquire fast dynamic performance through instantaneous value control to ensure good sinusoidal output waveform. Outer loop ensures the output voltage RMS constant and has high precision by controlling RMS. The control block diagram of output module is shown in figure 5. Firstly, the RMS of output voltage uo is compared with a given reference urms* to get error signal. Then after the PI regulator the signal is multiplied by unit sine wave to be the reference instantaneous value of inner loop. Finally, the value is compared with output voltage instantaneous value to get a new error signal which is sent to SPWM generator through a PI regulator. Restricted by experimental conditions, the load power tested in the lab is only 15KW which means system is at light load condition. Figure 7 shows the PET’s experiment waveforms. Figure (a) shows three-phase input line voltage waveform measured by Fluke434 power quality analyzer. uab is 597.1V, ubc is 595.6 V, uca is 593.5V. Figure (b) shows system operation parameters. The load power is 14.83kW and system power factor is 0.95 which shows that the input runs in high power factor. Figure (c) shows the input dc bus voltage 1050V which achieve the value expected, so input can realize the floating control. Figure (d) shows the dc bus voltage characteristic curve when the load changes suddenly and the voltage fluctuation is small. After 300ms system is stable and the robustness is good. The C phase voltage is almost the same which proves the topological structure has good resistance to the load imbalance. Figure (e) and (f) shows the primary side and vice side voltage of high frequency isolation transformer. The sharp peak is caused by the leakage inductance of the transformer. Figure (g) shows the low voltage dc bus voltage. The single phase full bridge rectifier converts ac square wave into 350V direct current. Figure (h) shows the output three-phase ac voltage whose waveforms are symmetrical and sinusoidal and the RMS is 221V. Compared with the reference 220V, the error is only 0.4%. Figure (I) shows that the output A phase voltage THD is only 2.5% which conforms to the national standard that the impedance load THD must be within 3%. Figure.5 control block diagram of output stage (a) Three phase line voltage input (b) Power and energy udcH Udcl(100V/grid) us(400V/grid) IV. EXPERIMENT To validate the performance of the PET, a 100KW 600V/220V three-phase four-wire PET experiment prototype is built based on the DSP TMS320F2812. Main parameters of the system are as follows: (1) input: line voltage 600V, frequency 50Hz, filter inductance 1.5mH, high voltage dc side capacitor 2160uF, switching frequency 4.8kHz; (2) isolation: working frequency 2kHz, transformer change ratio 3:1:1:1, low voltage dc side capacitor 3000uF; (3) output: filter inductance 0.4mH, filter capacitor 50uF, switching frequency 10kHz; three-phase resistance load of low voltage side 10.67Ω.Figure 6 shows the 100 KW PET prototype’s photos of input and isolation module. t(200μs/grid) uc t(200μs/grid) (c) High voltage dc bus voltage(d) high voltage dc bus and c phase voltage with load change us(400V/grid) up(200V/grid) network is set up. The experimental results prove that the PET topology and its control strategy are feasible. REFERENCE t(200μs/grid) t(200μs/grid) (e) primary side voltage (f) vice side voltage ub uc u/(100V/grid) Udcl(100V/grid) ua t(200μs/grid) (g) Low voltage dc bus voltage . J. Wang, A. Huang, W. Sung, Y. Liu, and B. Baliga, “Smart Grid . . t/(4ms/grid) . (h) Three-phase ac output voltage . . (i)Output A phase voltage THD Figure.7 Experimental waveforms V. CONCLUSION This paper proposes a novel power electronic transformer for distribution system named flexible power distribution unit. Its topology structure, working principle and control strategies are presented. The isolation module adopts open loop control to reduce the control complexity of system. Output module adopts three single-phase inverters to ensure the independence. A 600V/ 220V PET prototype which is suitable for the smart distribution View publication stats . . . Technologies,” IEEE Industrial Electronics Magazine, 2009, 3(2): 16–23. J. Zhao, “Simulation Study of a Power electronic transformer with Constant Output Voltage,” Automation of Electric Power Systems, 2003, 27(18):30-34, 46. D. Wang, C. Mao, J. Li, “Auto Balancing Electronic Power Transformer,” Proceedings of the CSEE, 2007, 27(6):77-83. G. Ortiz, J. Biela, and J. W. 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