Accurate Current Sharing and PCC Voltage Restoration in LVDC Microgrid without Communication Network (14/05/2021)

Khanh Duc Hoang - School of Electrical Engineering, University of Ulsan, Ulsan, Korea.  E-mail: duckhanhht13989@gmail.com 
Dong-Choon Lee - Department of Electrical Engineering, Yeungnam University, Gyeongsan, Korea.  E-mail: dclee@yu.ac.kr 
Hong-Hee Lee - School of Electrical Engineering, University of Ulsan, Ulsan, Korea. E-mail: hhlee@mail.ulsan.ac.kr 

 Abstract
In  this  paper,  a  control  strategy  is  proposed  for both  accurate  current  sharing  among  distributed  generators  (DGs)  and  the  point  of  common  coupling  (PCC)  voltage restoration  in  low  voltage  direct  current  (LVDC)  microgrid  by injecting a small AC signal to the output voltage of the DG. The small AC signal frequency changes proportionally to the per unit  (pu) output current of the corresponding DG, and the DG output voltage  is  regulated  to  share  every  DG  with  the  equivalent frequency in the steady state. Especially, for easy implementation, the  reactive  power  of  the  AC  signal  is  controlled  to  indirectly regulate  the  AC  signal  frequency.  Moreover,  the  equivalent  pu output current is used to determine the voltage shift-up in order to restore the PCC voltage. Because the proposed control scheme is developed with the fully distributed manner at local controller for  each  DG,  it  can  be  implemented  economically  without external  communication  network.  The  effectiveness  of  the proposed control method is proved by the simulation.  Index Terms—DC  microgrid, droop control, current sharing, voltage improvement.  
I.  INTRODUCTION
With  the  substantial  penetration  of  the  renewable  energy source  that  has  a  dc  output  power  (e.g.,  photovoltaics (PVs),  fuel  cells)  into  power  system  and  the  growing  of  DC load  applications,  DC  microgrid  has  been  attracting  many interests.  The  main  motivations  to  develop  DC  microgrid  are its  highly  efficient  energy  conversion  and  simple  control scheme [1]–[3]. In order to effectively manage power systems in DC microgrids, it is important to ensure the stable operation and cooperation of different types of energy sources.
   In  [4]  and  [5],  the  master-slave  control  methods  were presented to cooperate among the distributed generators (DGs) in  DC  microgrids  where  a  high  bandwidth  communication were used to share the information among the DGs. However, these  approaches  raise  the  risk  of  the  single  point  failure which damages the system operation. Another approach is the voltage  droop  control  to  properly  control  the  load  sharing among the DGs [6], [7]. In this approach, the line resistances are neglected to make the output voltages stable. However, the accurate  current  sharing  is  deteriorated  due  to  the  actual  line resistances.  In  addition,  the  voltage  drop  due  to  the  inherent characteristic of voltage droop control reduces the voltage on the point of common coupling (PCC) in DC microgrid [8]. In order to improve the current sharing accuracy and the voltage regulation  in  DC  microgrid,  the  secondary  control  layers relying  on  communication  network  were  proposed  [9]–[12]. However,  the  communication  delay  affects  the  stability  and the  reliability  of  the  system  especially  when  the  sources  are separated with long feeders [13].
   With the attempt to be independent on the communication network,  authors  in  [14]  proposed  “Power  Talk”  concept  to communicate among the DGs. In this method, high frequency communication signal is modulated to the output power of the corresponding  DG,  and  the  electric  lines  are  used  as communication  links.   However,  this  approach  leans  on  the states  of  the  system  such  as  the  line  resistance  and  the  load condition  which  are  unpredictable  in  practical  application. Meanwhile, the frequency-based power sharing technique was firstly developed for AC power systems [15], and then, it was also  applied  to  the  DC  microgrid  [16].  However,  the  method are  not  suitable  for  the  low  voltage  DC  (LVDC)  microgrids with  the  resistive  line  characteristics  [17].  Based  on  the method proposed in [15] and [16], the frequency-based current sharing technique applied to LVDC microgrids  was proposed in [17]–[19]. However, in [17] and [19], only accurate current sharing  is  achieved,  while  the  PCC  voltage  drop  is  not considered. In [18], the PCC voltage restoration at distributed secondary layer was taken into account along with an accurate current  sharing.  Nonetheless,  the  control  scheme  is  very complicated  due  to  the  secondary  control  was  not  decoupled with the primary control.
   In  this  paper,  we  propose  an  accurate  current  sharing  and the PCC voltage restoration in LVDC microgrid based on the frequency-based  current  sharing  technique.  In  the  proposed control  method,  a  small  AC  signal  with  the  frequency generated  according  to  the  per  unit  (pu)  output  current  is injected to the output voltage of the corresponding DG. In the steady  state,  the  accurate  current  sharing  is  achieved  by regulating  the  injected  AC  signal  frequency  to  be  equivalent frequency.  And also, the equivalent pu output current is  used to  restore  the  PCC  voltage.  Furthermore,  the  controller  only uses  the  local  information  about  the  output  voltages  and current  of  DGs  without  external  communication  network, which  enables  the  system  economic  and  easy  to  implement. The proposed control method is analyzed theoretically, and its effectiveness is also verified by simulation. 

Fig.  1.  DC  microgrid:  a)  Two  source  DC  microgrid  model,  b) Equivalent circuit                                                                Fig. 2. Proposed control scheme

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