- Email: [email protected]
DC Hybrid Distributed Energy System with Power Electronic Transformer
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ScienceDirect ScienceDirect
Energyonline Procedia 00 (2018) 000–000 Available onlineatat www.sciencedirect.com Available www.sciencedirect.com Energy Procedia 00 (2018) 000–000
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www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia 158 Energy Procedia 00(2019) (2017)6687–6692 000–000 www.elsevier.com/locate/procedia
10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Optimization of AC / DC Hybrid Distributed Energy System with Optimization of AC / DC Hybrid Distributed Energy System with The 15th International Symposium on District Heating and Cooling Power Electronic Transformer Power Electronic Transformer Assessing the feasibility of using the heat demand-outdoor a Shiqi Guoa*, Yunfei Muaa, Hongjie Jiaaa, Naishi Chenbb, Tianjiao Pubb, Xiaodong Yuancc Shiqi Guo *, Yunfei Mu , Hongjie Jia , Naishi Chen , Tianjiao , Xiaodong Yuan temperature function for a long-term district heatPudemand forecast Key Laboratory of Smart Grid of Ministry of Education (Tianjin University), Tianjin 300072, China a
b China Research Institute, Beijing 100192, China; Key Laboratory of SmartElectric Grid ofPower Ministry of Education (Tianjin University), Tianjin 300072, China a,b,c aElectric Power Research a b 211103, China c c cb Jiangsu Institute,Beijing Nanjing100192, China Electric Power Research Institute, China; c Jiangsu Electric Power Research Institute, Nanjing 211103, China a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Abstract This paper presents a new method of optimizing the distributed energy of AC/DC hybrid microgrids with power electronic This paper presents a new state method of optimizing the distributed energyinofnormal AC/DC hybrid microgrids with power electronic transformer. Firstly, the steady model of power electronic transformer operation mode is proposed. Subsequently, Abstract transformer. Firstly, thesystem steady with state power model of power electronic transformer in normal operationmodel modeinis order proposed. Subsequently, for the AC/DC hybrid electronic transformer, optimizing the scheduling to achieve the full for the AC/DC hybrid system with transformer, optimizing the scheduling model in order achieve the full disposition of distributed energy in power AC/DCelectronic hybrid system and economic optimization. Ultimately, with the to case to verify the Districtpower heating networks arewith commonly addressed in theand literature as one of thethat: most solutions for decreasing the disposition of distributed energy in power AC/DCelectronic hybrid system economic optimization. Ultimately, with theofcase to verify the AC/DC system model transformer, the results show aseffective the application power electronic greenhouse emissions from thepower building sector. These systems require high investments which areis returned through the heat AC/DC power system model with electronic transformer, the results show application of power electronic transformer ingas AC/DC hybrid system, the power transfer between the AC network andthat: the as DCthe network controlled by the power sales. Duefunction the changed climate conditions and building renovation policies, heat demand in disposition thecontrolled future could transformer intoAC/DC system,electronic the powertransformer, transfer between thecan AC network andrealize the DC network is thedecrease, power regulation ofhybrid the power which effectively the full ofbydistributed prolonging the investment return period. regulation of the powertheelectronic transformer,power whichgeneration can effectively realizethetheoperating full disposition generation. function In addition, through rational distribution application, costs of of the distributed system is The main scope of achieve this paper tothe assess the feasibility of using the heat demand – outdoorthe temperature demand generation. In addition, through rational distribution power generation application, operating function costs of for theheat system is reduced, which can theiseconomic operation of hybrid AC/DC system. forecast.which The can district of Alvalade, located in Lisbon (Portugal), used as a case study. The district is consisted of 665 reduced, achieve the economic operation of hybrid AC/DCwas system. buildings ©that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Copyright 2018 Elsevier Ltd. All rights reserved. ©renovation 2019 The Authors. Published by Elsevier Ltd. intermediate, deep). To estimate the error, scenarios were developed (shallow, obtained heat demand on values were Copyright © 2018 Elsevier Ltd. Allresponsibility rights reserved. Conference Applied Selection and peer-review under of the scientific committee of the 10th International This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) th International compared with results from a dynamic heat demand model, previously developed and validated by the authors. Selection and peer-review under responsibility of the scientific committee of the 10 Conference on Applied Energy (ICAE2018). Peer-review under responsibility of the scientific committee of ICAE2018 – The of 10th International Conferencefor onsome Applied Energy. The results showed that when only weather change is considered, the margin error could be acceptable applications Energy (ICAE2018). (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: Power electronic transformer (PET); AC/DC hybrid systems; Distributed energy resources; Power system optimization dispatch scenarios,Power the error value increased(PET); up toAC/DC 59.5%hybrid (depending the weather andresources; renovation scenarios combination considered). Keywords: electronic transformer systems;onDistributed energy Power system optimization dispatch The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.decrease Introduction scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the 1.renovation Introduction coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and In recent years, with the continuous development of microgrid and distributed generation technology, more and improve the accuracy of heat demand estimations.
In clean recentenergy years, has withbeen the applied continuous development microgrid and distributed generationdevices technology, more and more in power systems.ofBecause distributed power generation are constrained more clean energy has been applied in power systems. Because distributed power generation devices are constrained © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection peer-review under responsibility the scientific 1876-6102and Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.021
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Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
by various conditions of solar energy and wind energy, their power generation is intermittent, and a large amount of access will cause disturbance to the power grid [1]. However, the flexible regulation and interconnection capabilities of the current power grid are not enough to solve the above problems. The problem therefore hinders the full consumption and economic use of distributed renewable energy[2]. In order to solve the above problems, this paper applies power electronic transformer to the AC/DC hybrid system with distributed renewable energy. Since PET has both AC interface and DC interface, it has the functions of voltage transformation, isolation and energy transmission to achieve coordinated management of energy at its port [3]. For the research of power electronic transformers, it is mainly focused on its topology, physical model and control strategy. Most of the research applied to power systems only involves distribution networks. In [4], the combination of power electronic transformers and existing power grids is conceived and prospected. The paper [5] proposes the topology structure and control strategy when applying power electronic transformers to distribution networks. The paper [6] proposed a unified reduced-order modeling method and single-stage control strategy for power electronic transformers for medium and high voltage power grids. For the distributed energy optimization scheduling of AC/DC hybrid system, a local information-based energy management and coordinated control method is proposed in [7]. The paper [8] combines economic costs with environmental costs and the optimal operation model of AC/DC hybrid system is established. The paper [9] uses Monte Carlo and scene reduction technology to predict the load of uncontrollable load and distributed energy in AC/DC hybrid system. The above-mentioned papers have beneficially explored and analyzed the distributed energy optimization scheduling of power electronic transformers and AC/DC hybrid systems respectively, but did not combine power electronic transformers with AC/DC hybrid systems, and failed to give full play to the flexibility of power electronic transformers. Therefore, this paper applies the power electronic transformer to the distributed energy optimization scheduling of AC/DC hybrid system, and establishes a system level optimized operation model, which uses the power adjustment function of the power electronic transformer to connect between the AC network and the DC network. The power transmission is controlled to achieve full consumption of distributed generation and economic operation of the system. 2. PET steady state model Power electronic transformers, also known as electronic power transformers (EPTs), are the combination of power electronic circuit and traditional high frequency transformers. Through power electronics technology, new transformers for energy transfer and power conversion are realized [10-15]. For the constraint problem of power electronic transformers, this paper does not consider its specific internal model, but instead equates it to a lossy three-port power electronic device. Its topology is shown in Fig.1. Main Grid PMo
PMi
PACi
AC System
PDCi
PET PACo
PDCo
DC System
Fig.1 PET three port equivalent model
Power electronic transformers serve as an intermediate hub for energy transmission, and there is a certain amount of power loss inside. Here define the constant k PET as the power loss factor of a power electronic
transformer[16]. On this basis, PET can be reduced to one node. The product of the active power injected into the node and the power loss factor is equal to the active power output from this node. So the following formula is established:
Shiqi name Guo et/ Energy al. / Energy Procedia 158 (2019) 6687–6692 Author Procedia 00 (2018) 000–000
PACot +PDCot +PMot =kPET（PACit +PDCit）
66893
(1)
Due to limitations in the capacity of power electronic transformers, the following constraints are established:
PMot 2 QMot 2 Sm
(2)
Similarly, the following constraints are established at the AC and DC ports of a power electronic transformer:
PACot 2 QACot 2 Sacm PDCot Pdcm
(3) (4)
3. Case study 3.1. Objective function of optimization problem The 24-hour optimization problem of the AC/DC hybrid system can be expressed mathematically as a mixed integer nonlinear optimization (MINLP) problem[17]. The power system network in this paper includes main grid, AC networks, DC networks, and power electronic transformers that connect them [18]. The optimization goal is to minimize the total operating cost of the entire AC/DC system with power electronic transformers. It is expressed by a mathematical expression as shown in equation (5):
Min COST f ( PMTt , PBt , PMot , EVt , LAC ,t , LDC ,t )
[Cost Cost Cost [( P ) Cost ( MTt
t
t
MT
Bt
MTt
Mot
Bt
+CostEVt CostLAC ,t CostLDC ,t ]
Mo
(5)
PMot )+( EV EVt ) ( L LAC ,t ) ( L LDC ,t )] t
3.2. Micro-turbine Model The micro-turbine is used as a control variable during the optimization process, and its power generation constraints are as follows: max 0 PMTt PMT
(6)
RDMT PMTt1 PMTt RU MT
(7)
Formula (6) limits the power generation of the micro-turbine, while equations (7) control the ramp-down and rate of rise of the micro-turbine, 3.3. Energy Storage Battery Model As a device in the system that can store energy, the energy storage battery can balance the asynchronous phenomenon between the load peak value and the power generation peak value. The operating constraints of battery are as follows: max max PBdch PBcht PBch
PBt PBt PBcht 1 PBt Bcap
(8) (9) (10)
Constraints (8) limit the charge and discharge capacity of the battery, The constraint (9) defines the relationship between stored energy in the battery at time t and time t 1 .The constraint (10) limits the available battery capacity, and Bcap is the battery's maximum energy storage.
Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
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3.4. Load Model The load in the system is divided into uncontrollable load and controllable load. For uncontrollable load, the demand power is a fixed value and does not belong to the control variable. For controllable load, reduction is an important load management measure. The constraints of the load reduction are as follows:
0 LAC ,t max L
(11)
0 LDC ,t max L
(12)
Constraints (11) and (12) ensure that the reduced load is always within the allowable range. 3.5. Electric Station Model The range of load reduction for electric vehicles is as follows:
0 EVt max EVt
(13)
The constraint (13) limits the amount of electric vehicle charging that can be reduced in one hour. 3.6. Constraints of optimization problems The constraints of power electronic transformers, micro-turbines, energy storage cells, and loads are already mentioned in equations (1)-(4),(6)-(13) above. In addition, the system should also meet the power balance constraints of AC systems and DC systems: (14) PMT PWP PACi PL PLcri +PACo t
t
t
AC ,t
AC ,t
t
cri LDC ,t
PPVt PPV WPt +PDCit PBt PEVt PLDC ,t P
+PDCot
(15)
Equation (14) is the power balance constraint of the AC system, equation (15) is the power balance constraint of the DC system. 4. Case study 4.1. Case description In this paper, the AC/DC hybrid system with power electronic transformer is calculated and compared with the AC/DC system without power electronic transformer. The system structure diagram of a power electronic transformer is shown in Fig.3. The system consists of an AC network and a DC network. WP
Main Grid
PET
AC Lcri
MT
AC Lctrl
AC system DC system
PV
EV
ES
DC Lcri
DC Lctrl
Fig.2 Structure AC / DC hybrid micro-grid with PET
The AC network voltage class is 380V and the DC network voltage class is 400V. The AC system includes micro gas turbines (MT), wind power plants (WP), controllable AC loads (AC Lctrl), and non-controllable ("critical") AC loads (AC Lcri), where the power generation of the micro-turbine at each moment and the amount of AC controllable load reduction are control variables, and the output of the wind generation output and AC
Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
6691 5
uncontrollable load is a given value. The DC system includes photovoltaic power generations (PV), unidirectional EV charging stations (EV), battery storages (BES), DC controllable loads (DC Lctrl) and DC non-controllable ("critical") DC loads, (DC Lcri). Among them, the load reduction of the electric vehicle charging station, the energy storage capacity of the battery, and the amount of DC controllable load reduction at each moment are control variables. In this paper, the output of photovoltaic power generations within 24 hours is a given value, and the load of the DC uncontrollable load is also a given value. In this case, to ensure that the AC/DC hybrid system can achieve self-sufficiency as much as possible, it also takes the safety and stability of the AC/DC hybrid system into account, PMot is limited to a maximum of 60 kW for the injection power of the main grid. For the active power injection of the AC system PACot and the DC system side
PDCot , the maximum power is set to 150 kW. 4.2. Result analysis Fig.4 shows the output power diagram of the PET DC port and AC port within 24 hours. A positive value indicates that the power flows from the AC/DC system. A negative value indicates that the power is injected into the AC/DC system. It can be seen from the figure that in 1-7h, since the photovoltaic power generation has been 0, the active power in the DC system is in short supply, and the power flows from the AC system into the DC system. Within 10-16 hours, the DC system starts to have an energy surplus due to the photovoltaic power generation device starting to work. It began to inject the AC system from the DC system again. Since the PV generation capacity was again 0 in 20-24 hours, the power flowed into the DC system from the AC system again.
Fig.3 Output power of PET AC/DC port
Fig.4 Power of EV load
Fig.5, Fig.6, and Fig.7 show the load of the electric vehicle station, DC controllable load, AC controllable load in the initial conditions, no introduction of PET, and introduction of PET in AC/DC hybrid system. It can be seen from the figure that the AC/DC hybrid system containing PET has a reduced load reduction compared to the system before the introduction of PET, which improves the safety of the system.
Fig.5 Power of controllable DC
Fig.6 Power of controllable AC
The 24-hour operation cost of the AC/DC hybrid system containing PET is 4667.84 yuan, the cost of the 24 hours operation before the introduction of PET is 6333.64 yuan. The port power regulation capability of the power electronic transformer makes the AC system and the DC system exchange power flexibly which greatly reduces the operating cost of the system and improves the economic efficiency of the system.
Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
6692 6
5. Conclusion This paper starts from the basic operating characteristics of power electronic transformers, and then proposes the optimization dispatch method of the AC/DC hybrid distributed energy system with electronic power transformer. On the basis of in-depth understanding of the operating characteristics of power electronic transformers and the application scenarios of AC/DC hybrid renewable energy systems, the current day-to-day optimization scheduling strategies aiming at the full consumption and efficient use of renewable energy are studied based on the Maltab platform. Through the day-to-day optimal dispatching of AC/DC hybrid systems containing PET, the following conclusions can be drawn: Power electronic transformers can control the power at their ports, enabling the power to be flexibly transmitted in main grids, AC systems and DC systems, thereby enhancing the ability to absorb more new energy sources and improve the economic of the system. At the same time, through the introduction of PET, the load reduction of AC and DC controllable loads in AC/DC hybrid systems and electric vehicle stations has been reduced, which can improve the safety and reliability of power system. Acknowledgements The authors gratefully acknowledge the support of the National Key Research and Development Foundation of China (2017YFB0903300). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Han Shangqing. Research on key technologies for operation control of hybrid AC/DC micro-grid[D]. Beijing Jiaotong University, 2016. Ma Jian. Study on multiple complementary optimal operation of AC/DC hybrid system[D]. North China Electric Power University, 2016. Lan Zheng, Tu Chunming, Xiao Fan, et al. Study on Power Control of AC/DC Hybrid System Based on Power Electronic Transformer[J]. Transactions of China Electro-technical Society, 2015,30(23): 50-57. Chen Lei, Ou Jiaxiang, Zhang Qiuyan, et al. A review of research on power electronic transformers[J]. Power System and Clean Energy, 2015, 31(12):36-42. Liao Guohu, Yuan Xufeng, Qiu Guoyue. Simulation Study of Power Electronic Transformer in Distribution Network[J]. Electric Measurement & Instrumentation, 2014, 51(17):35-41. Liu Haijun, Li Gang, Wang Zhikai, et al. Research on modeling methods and control strategies of power electronic transformers for middle and high voltage smart distribution network[J]. Power System Protection and Control, 2017,45(02): 85-93. Zhang Xue, Pei Wei, Deng Wei, et al. Energy management and coordinated control method for multi-source/multi-load DC micro-grid[J]. Proceedings of the CSEE, 2014, (31): 5553-5562. Di Kaili, Li Xinming, Li Peng, et al. Optimal Operation of AC/DC Hybrid System Based on Improved Black Hole Algorithm [J]. Electric Power Construction, 2016, (10):1-8. Battistelli C, Agalgaonkar Y P, Pal B C. Probabilistic Dispatch of Remote Hybrid Micro-grids Including Battery Storage and Load Management[J]. IEEE Transactions on Smart Grid, 2016, PP(99): 1-1. Li Shuangchao, Ling Yuesheng, Pang Tianyu, et al. Research on power electronic transformer based on AC/DC and DC/AC two level structure [J]. Electrical & Energy Management Technology,2018(06):7-10. Liu Ye, Zhiyue Mei. Topology structure and control strategy of hybrid cascading power electronic transformer [J]. Power System Technology, 2017, 41 (02): 596-603. Zhao C, Dong S, Gu C, et al. New Problem Formulation for Optimal Demand Side Response in Hybrid AC/DC Systems[J]. IEEE Transactions on Smart Grid, 2016, PP(99): 1-1. Jakka VNSR, Shukla A. Integration of AC and DC sources using multi-source fed power transformer (MSF-PET) for modern power distribution system applications [C]// European Conference on Power Electronics and Applications. IEEE, 2016. Yan J, Zhu X, Lu N. Smart hybrid house test systems in a solid-state transformer supplied micro-grid[J]. 2015:1-5. Majumder R.A Hybrid Microgrid with DC Connection at Back to Back Converters[J].IEEE Transactions on Smart Grid,2014,5(1):251259. Zhang Xianglong, Zhou Hui, Xiao Zhihong, Zhang Wei, Wu Di. Application of power electronic transformer in reactive power optimization of active distribution network [J]. Power System Protection and Control, 2017, 45(04):80-85. Li Peng, Hua Haorui, Chen Anwei, et al. Optimization of economic dispatching based on two-tiered programming model for source-load and storage coordinated partitioning of AC-DC hybrid system[J].Proceedings of the CSEE,2016,36(24):6769-6779. Jayawardena AV, Meegahapola LG, Perera S, et al. Dynamic characteristics of a hybrid microgrid with inverter and non-inverter interfaced renewable energy sources: A case study[C]// IEEE International Conference on Power System Technology. IEEE, 2017:1-6.
ScienceDirect ScienceDirect
Energyonline Procedia 00 (2018) 000–000 Available onlineatat www.sciencedirect.com Available www.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia 158 Energy Procedia 00(2019) (2017)6687–6692 000–000 www.elsevier.com/locate/procedia
10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Optimization of AC / DC Hybrid Distributed Energy System with Optimization of AC / DC Hybrid Distributed Energy System with The 15th International Symposium on District Heating and Cooling Power Electronic Transformer Power Electronic Transformer Assessing the feasibility of using the heat demand-outdoor a Shiqi Guoa*, Yunfei Muaa, Hongjie Jiaaa, Naishi Chenbb, Tianjiao Pubb, Xiaodong Yuancc Shiqi Guo *, Yunfei Mu , Hongjie Jia , Naishi Chen , Tianjiao , Xiaodong Yuan temperature function for a long-term district heatPudemand forecast Key Laboratory of Smart Grid of Ministry of Education (Tianjin University), Tianjin 300072, China a
b China Research Institute, Beijing 100192, China; Key Laboratory of SmartElectric Grid ofPower Ministry of Education (Tianjin University), Tianjin 300072, China a,b,c aElectric Power Research a b 211103, China c c cb Jiangsu Institute,Beijing Nanjing100192, China Electric Power Research Institute, China; c Jiangsu Electric Power Research Institute, Nanjing 211103, China a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Abstract This paper presents a new method of optimizing the distributed energy of AC/DC hybrid microgrids with power electronic This paper presents a new state method of optimizing the distributed energyinofnormal AC/DC hybrid microgrids with power electronic transformer. Firstly, the steady model of power electronic transformer operation mode is proposed. Subsequently, Abstract transformer. Firstly, thesystem steady with state power model of power electronic transformer in normal operationmodel modeinis order proposed. Subsequently, for the AC/DC hybrid electronic transformer, optimizing the scheduling to achieve the full for the AC/DC hybrid system with transformer, optimizing the scheduling model in order achieve the full disposition of distributed energy in power AC/DCelectronic hybrid system and economic optimization. Ultimately, with the to case to verify the Districtpower heating networks arewith commonly addressed in theand literature as one of thethat: most solutions for decreasing the disposition of distributed energy in power AC/DCelectronic hybrid system economic optimization. Ultimately, with theofcase to verify the AC/DC system model transformer, the results show aseffective the application power electronic greenhouse emissions from thepower building sector. These systems require high investments which areis returned through the heat AC/DC power system model with electronic transformer, the results show application of power electronic transformer ingas AC/DC hybrid system, the power transfer between the AC network andthat: the as DCthe network controlled by the power sales. Duefunction the changed climate conditions and building renovation policies, heat demand in disposition thecontrolled future could transformer intoAC/DC system,electronic the powertransformer, transfer between thecan AC network andrealize the DC network is thedecrease, power regulation ofhybrid the power which effectively the full ofbydistributed prolonging the investment return period. regulation of the powertheelectronic transformer,power whichgeneration can effectively realizethetheoperating full disposition generation. function In addition, through rational distribution application, costs of of the distributed system is The main scope of achieve this paper tothe assess the feasibility of using the heat demand – outdoorthe temperature demand generation. In addition, through rational distribution power generation application, operating function costs of for theheat system is reduced, which can theiseconomic operation of hybrid AC/DC system. forecast.which The can district of Alvalade, located in Lisbon (Portugal), used as a case study. The district is consisted of 665 reduced, achieve the economic operation of hybrid AC/DCwas system. buildings ©that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Copyright 2018 Elsevier Ltd. All rights reserved. ©renovation 2019 The Authors. Published by Elsevier Ltd. intermediate, deep). To estimate the error, scenarios were developed (shallow, obtained heat demand on values were Copyright © 2018 Elsevier Ltd. Allresponsibility rights reserved. Conference Applied Selection and peer-review under of the scientific committee of the 10th International This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) th International compared with results from a dynamic heat demand model, previously developed and validated by the authors. Selection and peer-review under responsibility of the scientific committee of the 10 Conference on Applied Energy (ICAE2018). Peer-review under responsibility of the scientific committee of ICAE2018 – The of 10th International Conferencefor onsome Applied Energy. The results showed that when only weather change is considered, the margin error could be acceptable applications Energy (ICAE2018). (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: Power electronic transformer (PET); AC/DC hybrid systems; Distributed energy resources; Power system optimization dispatch scenarios,Power the error value increased(PET); up toAC/DC 59.5%hybrid (depending the weather andresources; renovation scenarios combination considered). Keywords: electronic transformer systems;onDistributed energy Power system optimization dispatch The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.decrease Introduction scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the 1.renovation Introduction coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and In recent years, with the continuous development of microgrid and distributed generation technology, more and improve the accuracy of heat demand estimations.
In clean recentenergy years, has withbeen the applied continuous development microgrid and distributed generationdevices technology, more and more in power systems.ofBecause distributed power generation are constrained more clean energy has been applied in power systems. Because distributed power generation devices are constrained © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection peer-review under responsibility the scientific 1876-6102and Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.021
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Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
by various conditions of solar energy and wind energy, their power generation is intermittent, and a large amount of access will cause disturbance to the power grid [1]. However, the flexible regulation and interconnection capabilities of the current power grid are not enough to solve the above problems. The problem therefore hinders the full consumption and economic use of distributed renewable energy[2]. In order to solve the above problems, this paper applies power electronic transformer to the AC/DC hybrid system with distributed renewable energy. Since PET has both AC interface and DC interface, it has the functions of voltage transformation, isolation and energy transmission to achieve coordinated management of energy at its port [3]. For the research of power electronic transformers, it is mainly focused on its topology, physical model and control strategy. Most of the research applied to power systems only involves distribution networks. In [4], the combination of power electronic transformers and existing power grids is conceived and prospected. The paper [5] proposes the topology structure and control strategy when applying power electronic transformers to distribution networks. The paper [6] proposed a unified reduced-order modeling method and single-stage control strategy for power electronic transformers for medium and high voltage power grids. For the distributed energy optimization scheduling of AC/DC hybrid system, a local information-based energy management and coordinated control method is proposed in [7]. The paper [8] combines economic costs with environmental costs and the optimal operation model of AC/DC hybrid system is established. The paper [9] uses Monte Carlo and scene reduction technology to predict the load of uncontrollable load and distributed energy in AC/DC hybrid system. The above-mentioned papers have beneficially explored and analyzed the distributed energy optimization scheduling of power electronic transformers and AC/DC hybrid systems respectively, but did not combine power electronic transformers with AC/DC hybrid systems, and failed to give full play to the flexibility of power electronic transformers. Therefore, this paper applies the power electronic transformer to the distributed energy optimization scheduling of AC/DC hybrid system, and establishes a system level optimized operation model, which uses the power adjustment function of the power electronic transformer to connect between the AC network and the DC network. The power transmission is controlled to achieve full consumption of distributed generation and economic operation of the system. 2. PET steady state model Power electronic transformers, also known as electronic power transformers (EPTs), are the combination of power electronic circuit and traditional high frequency transformers. Through power electronics technology, new transformers for energy transfer and power conversion are realized [10-15]. For the constraint problem of power electronic transformers, this paper does not consider its specific internal model, but instead equates it to a lossy three-port power electronic device. Its topology is shown in Fig.1. Main Grid PMo
PMi
PACi
AC System
PDCi
PET PACo
PDCo
DC System
Fig.1 PET three port equivalent model
Power electronic transformers serve as an intermediate hub for energy transmission, and there is a certain amount of power loss inside. Here define the constant k PET as the power loss factor of a power electronic
transformer[16]. On this basis, PET can be reduced to one node. The product of the active power injected into the node and the power loss factor is equal to the active power output from this node. So the following formula is established:
Shiqi name Guo et/ Energy al. / Energy Procedia 158 (2019) 6687–6692 Author Procedia 00 (2018) 000–000
PACot +PDCot +PMot =kPET（PACit +PDCit）
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(1)
Due to limitations in the capacity of power electronic transformers, the following constraints are established:
PMot 2 QMot 2 Sm
(2)
Similarly, the following constraints are established at the AC and DC ports of a power electronic transformer:
PACot 2 QACot 2 Sacm PDCot Pdcm
(3) (4)
3. Case study 3.1. Objective function of optimization problem The 24-hour optimization problem of the AC/DC hybrid system can be expressed mathematically as a mixed integer nonlinear optimization (MINLP) problem[17]. The power system network in this paper includes main grid, AC networks, DC networks, and power electronic transformers that connect them [18]. The optimization goal is to minimize the total operating cost of the entire AC/DC system with power electronic transformers. It is expressed by a mathematical expression as shown in equation (5):
Min COST f ( PMTt , PBt , PMot , EVt , LAC ,t , LDC ,t )
[Cost Cost Cost [( P ) Cost ( MTt
t
t
MT
Bt
MTt
Mot
Bt
+CostEVt CostLAC ,t CostLDC ,t ]
Mo
(5)
PMot )+( EV EVt ) ( L LAC ,t ) ( L LDC ,t )] t
3.2. Micro-turbine Model The micro-turbine is used as a control variable during the optimization process, and its power generation constraints are as follows: max 0 PMTt PMT
(6)
RDMT PMTt1 PMTt RU MT
(7)
Formula (6) limits the power generation of the micro-turbine, while equations (7) control the ramp-down and rate of rise of the micro-turbine, 3.3. Energy Storage Battery Model As a device in the system that can store energy, the energy storage battery can balance the asynchronous phenomenon between the load peak value and the power generation peak value. The operating constraints of battery are as follows: max max PBdch PBcht PBch
PBt PBt PBcht 1 PBt Bcap
(8) (9) (10)
Constraints (8) limit the charge and discharge capacity of the battery, The constraint (9) defines the relationship between stored energy in the battery at time t and time t 1 .The constraint (10) limits the available battery capacity, and Bcap is the battery's maximum energy storage.
Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
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3.4. Load Model The load in the system is divided into uncontrollable load and controllable load. For uncontrollable load, the demand power is a fixed value and does not belong to the control variable. For controllable load, reduction is an important load management measure. The constraints of the load reduction are as follows:
0 LAC ,t max L
(11)
0 LDC ,t max L
(12)
Constraints (11) and (12) ensure that the reduced load is always within the allowable range. 3.5. Electric Station Model The range of load reduction for electric vehicles is as follows:
0 EVt max EVt
(13)
The constraint (13) limits the amount of electric vehicle charging that can be reduced in one hour. 3.6. Constraints of optimization problems The constraints of power electronic transformers, micro-turbines, energy storage cells, and loads are already mentioned in equations (1)-(4),(6)-(13) above. In addition, the system should also meet the power balance constraints of AC systems and DC systems: (14) PMT PWP PACi PL PLcri +PACo t
t
t
AC ,t
AC ,t
t
cri LDC ,t
PPVt PPV WPt +PDCit PBt PEVt PLDC ,t P
+PDCot
(15)
Equation (14) is the power balance constraint of the AC system, equation (15) is the power balance constraint of the DC system. 4. Case study 4.1. Case description In this paper, the AC/DC hybrid system with power electronic transformer is calculated and compared with the AC/DC system without power electronic transformer. The system structure diagram of a power electronic transformer is shown in Fig.3. The system consists of an AC network and a DC network. WP
Main Grid
PET
AC Lcri
MT
AC Lctrl
AC system DC system
PV
EV
ES
DC Lcri
DC Lctrl
Fig.2 Structure AC / DC hybrid micro-grid with PET
The AC network voltage class is 380V and the DC network voltage class is 400V. The AC system includes micro gas turbines (MT), wind power plants (WP), controllable AC loads (AC Lctrl), and non-controllable ("critical") AC loads (AC Lcri), where the power generation of the micro-turbine at each moment and the amount of AC controllable load reduction are control variables, and the output of the wind generation output and AC
Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
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uncontrollable load is a given value. The DC system includes photovoltaic power generations (PV), unidirectional EV charging stations (EV), battery storages (BES), DC controllable loads (DC Lctrl) and DC non-controllable ("critical") DC loads, (DC Lcri). Among them, the load reduction of the electric vehicle charging station, the energy storage capacity of the battery, and the amount of DC controllable load reduction at each moment are control variables. In this paper, the output of photovoltaic power generations within 24 hours is a given value, and the load of the DC uncontrollable load is also a given value. In this case, to ensure that the AC/DC hybrid system can achieve self-sufficiency as much as possible, it also takes the safety and stability of the AC/DC hybrid system into account, PMot is limited to a maximum of 60 kW for the injection power of the main grid. For the active power injection of the AC system PACot and the DC system side
PDCot , the maximum power is set to 150 kW. 4.2. Result analysis Fig.4 shows the output power diagram of the PET DC port and AC port within 24 hours. A positive value indicates that the power flows from the AC/DC system. A negative value indicates that the power is injected into the AC/DC system. It can be seen from the figure that in 1-7h, since the photovoltaic power generation has been 0, the active power in the DC system is in short supply, and the power flows from the AC system into the DC system. Within 10-16 hours, the DC system starts to have an energy surplus due to the photovoltaic power generation device starting to work. It began to inject the AC system from the DC system again. Since the PV generation capacity was again 0 in 20-24 hours, the power flowed into the DC system from the AC system again.
Fig.3 Output power of PET AC/DC port
Fig.4 Power of EV load
Fig.5, Fig.6, and Fig.7 show the load of the electric vehicle station, DC controllable load, AC controllable load in the initial conditions, no introduction of PET, and introduction of PET in AC/DC hybrid system. It can be seen from the figure that the AC/DC hybrid system containing PET has a reduced load reduction compared to the system before the introduction of PET, which improves the safety of the system.
Fig.5 Power of controllable DC
Fig.6 Power of controllable AC
The 24-hour operation cost of the AC/DC hybrid system containing PET is 4667.84 yuan, the cost of the 24 hours operation before the introduction of PET is 6333.64 yuan. The port power regulation capability of the power electronic transformer makes the AC system and the DC system exchange power flexibly which greatly reduces the operating cost of the system and improves the economic efficiency of the system.
Shiqi Guo et al. / Energy Procedia 158 (2019) 6687–6692 Author name / Energy Procedia 00 (2018) 000–000
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5. Conclusion This paper starts from the basic operating characteristics of power electronic transformers, and then proposes the optimization dispatch method of the AC/DC hybrid distributed energy system with electronic power transformer. On the basis of in-depth understanding of the operating characteristics of power electronic transformers and the application scenarios of AC/DC hybrid renewable energy systems, the current day-to-day optimization scheduling strategies aiming at the full consumption and efficient use of renewable energy are studied based on the Maltab platform. Through the day-to-day optimal dispatching of AC/DC hybrid systems containing PET, the following conclusions can be drawn: Power electronic transformers can control the power at their ports, enabling the power to be flexibly transmitted in main grids, AC systems and DC systems, thereby enhancing the ability to absorb more new energy sources and improve the economic of the system. At the same time, through the introduction of PET, the load reduction of AC and DC controllable loads in AC/DC hybrid systems and electric vehicle stations has been reduced, which can improve the safety and reliability of power system. Acknowledgements The authors gratefully acknowledge the support of the National Key Research and Development Foundation of China (2017YFB0903300). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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