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Hierarchical Coordinated Control of Multi bus-based DC Microgrid considering fault occurred in buses Zhongtian Zhao, Jian Hu, Jianxun Liu School of Electrical and Electronic Engineering, Shandong University of Technology, Zibo, P.R. china, 255000 Abstract—Multi bus-based dc microgrid has serious merits such as the higher reliab ility of power supply and flexib ility of d istributed energy and loads connecting into the system. Its stable operation would be destroyed by the power fluctuation and bus faults. Therefore, the coordinated control strategy, including five h ierarch ical levels, is proposed to ensure the stable operation of dc microgrid. The stable operation in case of bus non -fault is guaranteed by the 1st to 4th control levels and the reliable operation is safeguarded by the 5th control level after fault occurred in one of the buses. The bus voltage acts as a monitor of supply–demand balance, and it’s controlled in allo wable fluctuation range, which is proved by a dc microgrid model in MATLAB/Simu link. Meanwhile, the reliab ility and effectiveness of the proposed control strategy for the DC M G operating in both islanding and grid-connected modes is verified. Index Terms—bus fault, dc microgrid, energy management, hierarchical control, multi-bus. I. INTRODUCTION The microgrid, as defined by the U.S. Depart ment of Energy, consists of distributed generations (DGs), such as photovoltaic modules (PV), fuel cells and wind turbines, energy storage systems (ESS), loads and power electronics, wh ich can operate in either grid-connected or islanding modes. Microgrid deploy ments have the advantage of increasing energy efficiency, enhancing the power supply reliability, and reducing the network power losses and emissions [1] [2]. Microgrid can be classified into three types based on the currents and voltages employed in the system, which are alternating current microgrid (AC M G), direct current microgrid (DC M G) and hybrid microgrid (HB MG). The DC M G has superiority over AC DG, which are summarized in general as follows [3]. a) The reduction of conversion losses of inverters between dc output sources and dc loads. b) No reactive power. c) No need of synchronization. d) No frequency aspect. Due to above significant advantages, DC M G has attracted more attention in recent years [4]. There are three strategies, listed as centralized, decentralized, and distributed, in the aspect of power balance for microgrid [5] [6]. The distributed method is paid more attention as a result of no need of communicat ion and maintaining system reliability. Three hierarchical levels are proposed in [7], which can coordinate the centralized and distributed control. In [8], the hierarchical coordinated control method for each power electronic converter is expatiated under different control levels based on the bus voltage degree. It is valid that the control strategy, based on the dc bus signaling, is adopted to balance the power flow [5] [9]. Depended on the power fluctuation between the sources and loads in hybrid renewable DC M G, the regulat ion is proposed to guarantee the bus voltage in different voltage levels [10]. Multi-bus-based DC M G has some priorities such as the flexib ility fo r DGs and loads interfacing grid and enhancing the reliability for needs of electricity consumers. In the future, for the development of microgrid, Mu lti-bus-based DC M G is a main d irect ion. However, the existing study mostly concentrates on the operation in single bus structure microgrid. Hence, this paper provides the coordinated control strategy including five h ierarch ical levels based on the voltage variation range in Multi-bus-based DC M G system consisted of Multi-source and Multi-loads. The 1st to 4th control levels are given in normal operation condition and the last control level in fault occurred in buses. Finally, the feasibility of control strategy has been proved via MATLAB/Simulink. II. DC MICROGRID SYSTEM A The microgrid structure The structure of DC M G is shown Fig.1. In this system, PV is connected by DC/DC converter to the dc bus. ESS is made up of battery and Bi-d irectional DC/DC converter that can achieve Bi-directional power flo w. The local dc loads are connected to the dc bus via DC/DC converter. The Bi-d irectional AC/ DC converter is adopted to attach grid and DC M G. In this picture, Ppv is the output power of PV; PB is the charging and discharging power of ESS. PL is the consumption of dc loads. PG is the power exchange of DC MG and grid. Ppv DC/DC Photovoltaic cells Grid Bidirectional AC/DC PG Isolation Transformer PB Bidirectional DC/DC Battery DC/DC Load PL DC Bus Fig.1 The DC MG structure The reliability and stability of operation will be impacted in the DC M G that only contains a co mmon dc bus in the case that the corresponding bus occurs fault. Hence, the structure of Multi bus-based DC M G shown in Fig.2 is proposed by scholar. DGs, ESS and local loads can be connected to each segment dc bus. Multi-bus-based microgrid is provided with flexibility for DGs and loads interfacing grid and can enhance the reliability needs of electricity consumers . Grid Grid BUS 1 Bi-directional AC/DC BUS DGs ESS DGs LOAD BUS 2 BUS DGs ESS i ESS LOAD ESS LOAD j DGs LOAD Bi-directional AC/DC Fig.2 DC MG with multiple buses B Equivalent modeling of DC MG with multiple buses It is very difficult in the real time dynamic simulat ion of DC M G that contains mult iple buses, random source and constant power loads (CPL). As a concrete system, we d iscuss a DC M G that includes two buses and the corresponding equivalent modeling is shown in Fig.3. The grid-connected converter is represented by a current source and a smoothing capacitor. The PV and fuel cells are considered as a dc source, and the Li-ion Batteries is applied in the energy storage systems . The line resistances (R) and line inductances (L) are calculated based on the specific cable used in the concrete microgrid. What’s more, the power balance is destroyed if the fau lt occurs in buses so that all electrical equip ment connected with it is forced to cut. Hence, this paper proposed control method based on the voltage variation range is divided into different control level. A Hierarchical Coordinated Control The power management is the key to maintain the bus voltage constant so that the coordinated operation of each converter is significant, wh ich can coordinate each DGs and ESS to deliver power. The h ierarch ical coordinated control strategy is shown in following. 1. The First Control Level 0.95Udcn Udc 1.05Udcn , Islanding operation. The dc bus voltage is monitored by batteries, wh ile the PV operates in maximu m power point tracking (MPPT) mode that can efficiently extract the maximu m renewable energy. The fuel cells are standby at the same time. The power is balanced between sources and loads are formulated in the formula (1), (2). When 0.95Udcn Udc Udcn ,the output power of DGs is less than the loads needs. Therefore, the batteries in discharging mode guarantee the voltage stabilization and power balance. The power equation is shown as (1). In case that the state of charge (SOC) of batteries is under 35%, the control switches to the third control level. Ppv PESS PL (1) When Udcn Udc 1.05Udcn ,the dc bus voltage is higher than rated value. Hence, the batteries in charging mode absorb surplus power. The power equation is formulated as (2). In case that the SOC of batteries is greater than 85%, the control switches to the second control level. Ppv PL+PESS (2) 2. The Second Control Level CPL L C R L L L L R R R R DC BUS 1 DC circuit breaker DC BUS 2 DC circuit breaker R R L L R R R L L L CPL Fig.3 2-bus microgrid equivalent modeling III. HIERARCHICAL COORDINATED CONTROL In the DC M G, the power fluctuation is only reflected by the bus voltage. Therefore, it is the key to balance power flow in order to keep the dc bus voltage constant. The randomness of DGs, the needs variation of loads and the fluctuation of exchange power between DC MG and grid all cause the unbalance of system power. 1.05Udcn Udc 1.10Udcn , Islanding operation. The output power of PV continuous exceeds the needs of load, while the SOC is greater than 85% resulting fro m continuing charging. In order to prevent batteries excessive charging, the batteries are off-line. The dc bus voltage is preserved by PV operated in voltage constant mode and fuel cells are out of work. The power equation is shown as (3) Ppv PL (3) 3. The third Control Level 0.90Udcn Udc 0.95Udcn ,Islanding operation. The power of needs is larger than the DGs output and the SOC of batteries is below 35% resulting fro m continuing discharging in 1st control level. The dc bus voltage is continuous decline, while the batteries are standby to prevent the batteries excessive discharging which results in reducing the batteries service life. Therefore, the fuel cells are devotion that maintain the dc bus voltage stabilization and the PV operate in MPPT mode. The power equation is shown as (4). PF Ppv PL+PESS (4) 4. The fourth Control Level 1.10Udcn Udc or Udc 0.90Udcn , and DC M G is connected to the grid. The power unbalance between sources and loads is represented by the larger fluctuation range of dc bus voltage. In this case, the power flow in DC M G doesn’t balance by itself, so the grid-connected converter is plunge, which can impede the dc voltage over fluctuation. In this control, the PV operate in MPPT mode, the batteries operate in charging mode in case that the grid-connected converter operates in rectification mode and the SOC of batteries is less than 35%, and the fuel cells are standby. This control level contains two sub-control levels, which are 4-1 and 4-2. 4-1 control level: 1.10Udcn Udc . The output power of the DGs is more than the loads demand, which causes the bus voltage rise. The grid-connected converter operates in the inverter state and controls voltage stabilization. 4-2 control level: Udc 0.90Udcn . The grid-connected converter operates in the rectifier state and injects the power to DC M G. The power equation is shown as (5). PG Ppv PL+PESS (5) 5. The fifth Control Level in case of fault To safeguard the reliab ility of the Multi bus-based DC M G, the dc circuit breaker disconnect the bus in case of fau lt occurred in the one, which should cut all electrical device connected it. In this situation, the power balance may be seriously damaged. Based on the strong support of power grid, the grid-connected converter is quickly placed in service after the fault removed and monitors the dc bus voltage. In this control level, the PV operates in M PPT mode that can efficiently extract the maximu m renewable energy, fuel cells are standby, and the battery operates in charging mode in case that the SOC of batteries are less than 85%. Th is control level contains two sub-control levels, which are 5-1 and 5-2. 5-1 control level: Udcn Udc . The dc bus voltage is greater than the rated voltage resulting from the power surplus. The grid-connected converter operates in the inverter state, controls voltage stabilization and balances the power. 5-2 control level: Udc Udcn .The grid-connected converter operates in the rectifier state, which supplies the power shortage and keep voltage stabilization. The power equation is shown as (5). B Converter control 1. The ESS converter control The ESS plays a more impo rtant role in s moothing the power flow in DC M G. In order to preclude the reducing service life of the batteries caused by charging and discharging frequently, the batteries is standby in case of the dc bus voltage variation range in 0.98Udcn ~ 1.02 Udcn . The batteries absorb or deliver the power to balance the power flow between sources and loads while the voltage variation is beyond the allowed range 0.98Udcn ~ 1.02Udcn . The self-adaption linear droop control is adopted by the converter of batteries and its characteristic curves are shown in the Fig.4. Ibmax and Ibmin are the maximu m discharging and min imu m charging current of batteries, respectively. UH2 , UH1 , UL1 and UL2 are the threshold voltage of batteries operation modes conversion, which are 1.05Udcn , 1.02Udcn , 0.98Udcn , 0.95Udcn , respectively. Udc m1 Combined doop characteristic m2 UH2 Standby Charging DisCharging UH1 Udcn UL1 UL2 Combined doop characteristic m1 m2 Ibmax I b I b m in Fig.4 Battery droop characteristic Based on droop characteristic curves, the relationship between charging/discharging current of battery and bus voltage is formulated as (6). UH1 miIb , Ib 0 Udc (6) UL1 miIb , Ib 0 The self-adaption charging/discharging droop coefficient mi of batteries is adjusting by itself based on the real-time SOC of batteries. The droop coefficient mi is defined as (7). µmb , Ib 0 mi (7) mb / µ , Ib 0 Where mb is initial droop coefficient of batteries. An index µ, which is the ratio of SOCi of the i th battery to the average SOC of all batteries, is defined as µ SOCi (8) 1 n SOCj n j 1 The batteries converter designed control is shown in Fig.5. Udc Droop Control Ibref PI PWM Ib Fig. 5 Control diagram of BESS converter 2. The PV converter control In this system, the PV is connected by a DC/DC converter to the bus. The converter can operate in MPPT and bus voltage monitoring mode (BVM). The MPPT mode is adopted and the PV can output the maximu m power corresponding. To maintain stable bus voltage, bus voltage monitoring mode is also employed widely. The converter control diagram is shown in the Fig.6. For M PPT mode, the incremental conductance method is emp loyed to track the maximu m power. Ipv and Upv ,respectively, are the output current and voltage of PV. For BVM mode, the PV can keep stable bus voltage by means of reducing generation power rather than tracking the maximu m power. The U I droop characteristics are adopted for determining the voltage reference Upv-ref . IL is the output current of converter. The proportional-integral (PI) controller is used in both current and voltage control loops. output current of grid-connected converter. The control diagram is shown in Fig .9. When the condition Udcn Udc is satisfied and converter is online, the power flo ws fro m ac grid to dc grid. Otherwise, the power flow reverses. U ref MPPT CONTROL Incremental conductance method Ipv PI Udc Droop Control PWM IL PI Upv-ref PI BVM CONTROL Fig.6 control diagram of PV converter 3. The fuel cells converter control The fuel cells can operate in constant voltage mode and idle mode. It is only online in case that all PV and ESS cannot balance the power flo w. The control diagram is shown in Fig.7. The voltage reference of the fuel cells converter is Uref . IL is the output current of fuel cells converter. The proportional-integral (PI) controller is used in both current and voltage control loops. The proposed controller is only operated in islanding mode of DC MG. To verify the proposed hierarchical coordinated control strategy, the concrete system is modeled by MATLAB/Simulink based on the equivalent modeling of DC M G shown in Fig.3. The simulat ion waveforms of hierarchical coordinated control are shown in Fig.11. 500 U dc Udc Standby U upper 400 350 0 .9 0U d c n (a) IG IG-max (b) IG-max Fig.8 Different droop characteristics (a) Normal (b) One of bus fault Uupper and Ulower are the dc bus allowed maximu m and minimu m voltage, respectively. KG1 , KG2 are the droop coefficient of the grid-connected converter and the corresponding value are shown in formula(9) and (10). Uupper 1.10Udcn (9) IG-min 1.05Udcn 0.95Udcn KG2 (10) IG-min IG-max Where IG-max and IG-min are the maximu m and minimu m KG1 8 10 12 14 2 4 6 8 10 12 14 2 4 6 8 10 12 14 2 4 6 8 10 12 14 2 4 6 8 10 12 14 2 4 6 8 10 12 14 (b) 0 (c) 500 0 -500 0 1500 (d) KG2 IG IG-min 6 500 1000 500 0 -500 KG1 U lower 4 1000 -1000 P-gri d/W U dcn 0.95U dcn 2 1500 1000 0 1500 (e) 1000 500 0 -500 0 P-l oad/W U dcn 0 0 1.05U dcn 1 .1 0U d cn IG-min P-pv/W Fig. 7 Control diagram of FC converter 4. The grid-connected converter control The power grid is connected via AC/DC converter to dc bus and the corresponding converter controls the dc bus voltage constant in case the power fluctuate in wide range. The different U I droop characteristics shown in Fig.8 are adopted in the grid-connected converter control method contained both normal and one bus fault modes. P - ess / W PWM PWM (a) 450 2000 P - FC / W PI PI PWM IV. SIMULATION AND ANALYSIS 300 PI PI PI Fig.9 Control diagram of FC converter C The flow chart of hierarchical coordinated control Based on the above depiction on the hierarchical coordinated control strategy, the flow chart of this strategy is concluded in the Fig.10. IL Udc Uref Droop Control PI U-dc/W Upv I pv IL U dc 2000 (f) 1500 1000 500 Time: 2s / div 0 0 Fig.11 Simulation waveforms of Hierarchical Coordinated Control The rated bus voltage of DC M G discussed by this paper is 400V and the corresponding simu lation waveform of bus voltage is described in Fig.11 (a). Start Control Level Option 1st Level 2nd Level 3rd Level 4th Level 5th Level Grid-connected converter is standby Grid-connected converter is standby Grid-connected converter is standby Grid-connected converter controls voltage Grid-connected converter controls voltage Battery controls voltage Battery is standby Battery is Charging Battery is charging only at 4-2 control level Battery is charging only in case of SOC <85% PV operates at MPPT PV controls voltage PV operates at MPPT PV operates at MPPT PV operates at MPPT Fuel cells is standby Fuel cells is standby Fuel cells control voltage Fuel cells is standby Fuel cells is standby Y Y bus fault ? N Ib>Ibmax Y 5th Level 3rd Level Y bus fault ? N N N Y 2nd Level N 1.10Udcn Udc Y Ib<Ibmin Y bus fault ? N U dc 0.90U dcn N N Udc 1.05Udcn Y bus fault ? Udc 1.10Udcn N Udc 0.95Udcn bus fault ? Y N Control Level Option Udc 0.90Udcn Y 5th Level 4th Level 1st Level 5th Level 4th Level 1st Level 5th Level 2nd Level 3rd Level return Fig.10 Flow chart of Hierarchical Coordinated Control Fig.11 (b) is the output power of PV. Fig.11 (c) displays the power of ESS, and charging is the positive and discharging is the negative and zero is means of off-line. The fuel cells power is shown in Fig.11 (d). The power of grid is depicted in Fig.11 (e). Fig.11 (f) is the consumed power of the local loads. The specific analysis is listed as follows. 0 t 2s : The degree of bus voltage stays at the 1st level, and the bus voltage is controlled by the batteries. The local loads are light, and the corresponding power is 1000W. The PV operates in MPPT, and its output power is 1500W, which is greater than the consumed power of loads. The batteries operate on the charging mode, and the charging power is 500W. The bus voltage is controlled at 400V. 2s t 4s : When t=2s, the SOC of batteries is 85%, and the batteries are standby. The operation of PV switches to voltage constant mode. The output power of PV is confined to 1000W which is equal to the loads power. The bus voltage is controlled at 430V. 4s t 6s : When t=4s, the reduction of loads cause bus voltage increase. The DC M G doesn’t balance the power flo w between DGs and loads. The grid-connected converter is plunged, which regulates the bus voltage. The operation of PV turns to MPPT mode and its output power reaches to 1500W. In this control level, the surplus power 700W flows into grid. The bus voltage is controlled at 450V. 6s t 8s : When t=6s, the output power of PV is reduced to 300W and the loads power is still 800W. The batteries start to discharge regulating the bus voltage, and the grid-connected converter is punched out. The discharging power is 500W, and the bus voltage is controlled at 400V. 8s t 10s : When t=8s,the SOC of batteries is less than 35%, so the batteries is outage in order to reduce the damages resulting from deep discharging. The output power of PV is still 300W and less than the local loads needs 800W. Therefore, the fuel cells start to emit 1000W power. The charging power of batteries is 500W with the purpose of storing energy so as to operation stabilization. The bus voltage is controlled at 370V. 10s t 12s : When t=10s, the power of loads increase to 1500W, and the output power of PV is still 300W. The batteries can’t balance the power shortage, so the grid-connected converter is plunged, which regulates the bus voltage and outputs 1200W power. T he batteries and the fuel cells is outwork. The bus voltage is controlled at 350V. 12s t 14s : When t=12s, the fault occurs in one bus. The fault bus was cut off by dc current breaker. The output power of PV is reduced to 100W and loads power is 400W. In order to safeguard the reliability of operation for DC MG. the grid-connected converter is plunged quickly and monitors the dc non-fault bus voltage and balances the power. The output power of grid is 300W, and fuel cells and batteries are standby. The bus voltage is controlled at 400V. V CONCLUSION The hierarch ical coordinated control is presented for effective and reliable operation of DC M G. To utilize the renewable energy efficiently, the operation of DC M G is firstly regulated by DGs and ESS in case that power can be balanced by itself. The grid only plays a supporting role so that it can guarantee the reliab le operation in case of fault and serious fluctuation of voltage. The stable operation depends on the dc bus voltage detection because it reflects the supply–demand balance. The dc bus voltage is limited in allo wable range, which is proved via this simulation . At the same time, the effectiveness and feasibility of the proposed control strategies have been tested by a dc microgrid model in MATLAB REFERENCES [1] Amin Khodaei, ―Provisional Microgrids,‖ IEEE Transactions on Smart Grid, Vol. 6, No.3 , May 2015, pp. 1107 – 1115. [2] Tomislav Dragicevic, Xiaonan Lu, Juan C. Vasquez, Josep M. Guerrero, ―Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues,‖ IEEE transactions on power electronics, Vol. 31, No. 5, May 2016, pp., 3528 – 3549. [3] Hiroaki Kakigano, Yushi Miura, and Toshifumi Ise, ―Low-Voltage Bipolar-Type DC Microgrid for Super High Quality Distribution,‖ IEEE transactions on power electronics, Vol. 25, No. 12, Dec. 2010.pp.3066 –3075. [4] LI Xialin, GUO Li, WANG Chengshan, LI Yunwei, ―Key Technologies of DC Microgrids: An Overview,‖ Proceedings of the CSEE, Vol.36 No.1 Jan.5, 2016, pp.2 –17. [5] J. Schonberger, R. Duke, and S. D. 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[10] Xiaofeng Sun, Zhizhen Lian, Baocheng Wang and Xin Li, ― A Hybrid Renewable DC Microgrid Voltage Control,‖ IEEE 6th IPEMC, 17-20 May 2009 pp.725 –729. First A, Zhongtian Zhao, male, Linyi city, Shandong province, 1990. Master degree in Shandong University of technology. The study direction is the control of stable operation for microgrid. [email protected]. Jian Hu (corresponding author), male, Associate Professor, Master T utor, The research direction is intelligent optimization theory and its application in power system, power market and power economy. [email protected]. Jianxun Liu, male, Master degree candidate. The study direction is the economical operation for microgrid.