Battery State of Charge Balancing with Accurate Power Sharing in DC Microgrid (14/05/2021)

Khanh Duc Hoang - School of Electrical Engineering, University of Ulsan, Ulsan, Korea. E-mail: duckhanhht13989@gmail.com
Hong-Hee Lee - School of Electrical Engineering, University of Ulsan, Ulsan, Korea. E-mail: hhlee@mail.ulsan.ac.kr

 Abstract
 This paper proposes a distributed control method to achieve  state  of  charge  (SoC)  balancing  among  the  distributed battery energy units (BEUs) in a DC microgrid based on accurate power  sharing.  The  distributed  control  algorithm estimates the average  SoC  of  all BEUs by  means  of  consensus  algorithm,  and voltage regulation is determined by comparing the per unit power of  BEU  with  the  those  of  the  neighbors.  At  the  same  time,  the estimated average  SoC  and  the  voltage  regulation  are  used  to implement accurate power sharing according to the SoC levels of BEUs to achieve the SoC balancing regardless of the line resistance difference. Moreover, the control algorithm also restores the DC voltage  level to  the  nominal  value for  stable  operation.  The effectiveness of the proposed method is proved by simulation. 
 Index  Terms—DC  microgrid,  droop  control,  state  of  charge, consensus algorithm. 
 I.  INTRODUCTION
The integration of DC renewable energy sources (RESs) and modern DC loads [e.g., photovoltaics (PVs), fuel cells and LEDs,  etc.,]  motivates  the  development  of  DC  microgrid. Compared with the AC microgrid, the DC microgrid has been received more attention due to its higher efficiency for absence of intermediate AC power conversion stages, and it avoids the problems  related  to  reactive  power,  harmonics  and synchronization [1]–[3]. In DC microgrid, in order to guarantee the reliable operation, battery energy units (BEUs) are usually used to mitigate the power fluctuation caused by RESs such as wind power and PVs. Because BEUs are commonly distributed in microgrid system, their charging or discharging powers are different  because  of  different line  resistance  [4].  As  a  result, their  state  of  charge  (SoC)  levels  become  unbalanced,  which can lead to over-charge or over-discharge of a certain BEU.
  In order to solve this problem, a centralized control strategy was  presented  to  provide  SoC  balancing  among  BEUs  [5]. Nevertheless, the central controller may suffer from the risk of the  single  point  failure, and  it  requires  high  computational burden which  is  not appropriate  for the  microgrid  with  many distributed BEUs [6]. In [7], [8], a droop scheme was proposed without  using  additional  communication  network,  and  the droop coefficient was modified to be inversely proportional to the exponential function of SoC (SoCn), where ‘n’ is identified as the convergence factor. However, it needs stability analysis and computing complicate exponential inequalities to select the suitable convergence factor.

 Fig. 1. DC microgrid 
  In the  previous  studies,  it  is  hard  to  achieve  the  accurate power sharing with the balanced SoC among BEUs due to the line resistance difference, which is inevitable in the distributed DC microgrid [9]. To solve the problem, a decentralized control strategy with  virtual  power  rating  concept was  proposed  to realize  balanced  SoC  with  accurate  power  sharing  in  DC microgrid  [10].  Nonetheless,  this  control  strategy  requires a certain BEU to share its information with all other BEUs in the microgrid  through  full  communication  network,  which  raises the limitation to implement and the scalability in case of large number of BEUs [11].
  In  this  paper,  we  propose  a  consensus-based  distributed control  to  achieve  SoC  balancing  among BEUs  with  the accurate  power  sharing  in  DC  microgrid.  In  the  proposed control method, the consensus algorithm is used to utilize the distributed secondary control at local controller of each BEU in order  to estimate  the  average  SoC  of  BEUs  and  the  per  unit output  power  comparation  with those  of the  neighbors  via neighbor-to-neighbor  communication  network.  The  estimated average  SoC  and  the  per  unit power  comparation  are  further used to regulate accurate power sharing according to SoC levels among BEUs to achieve the accurate SoC balancing regardless of  the  line  resistance  difference.  Moreover,  the  voltage restoration is also considered to guarantee the suitable voltage level  of  the  microgrid.  The  proposed  method  is  analyzed theoretically, and its effectiveness is verified by simulation. 

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