Download Hierarchical Coordinated Control of Multi bus

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
m1
Combined doop
characteristic
m2 UH2
Standby
Charging
DisCharging
UH1
Udcn
UL1
UL2
Combined doop
characteristic
m1
m2
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  miIb , Ib  0
Udc  
(6)
UL1  miIb , 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
Uref

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. Round, ―DC-bus signaling: A
distributed control strategy for a hybrid renewable nanogrid,‖ IEEE
T rans. Ind. Electron., vol. 53, no. 5, Oct. 2006, pp. 1453–1460.
[6] Manoj Lonkar, Srinivas Ponnaluri, ―An overview of dc microgrid
operation and control,‖ IEEE 6th IREC, 24-26 March 2015, pp1–6.
[7] Chi Jin, Peng Wang, Jianfang Xiao, Yi Tang, Fook Hoong Choo,
―Implementation of Hierarchical Control in DC Microgrids‖, IEEE
transactions on industrial electronics, Vol. 61,No. 8, Aug 2014, pp.
4032–4042.
[8] Lirong Zhang, Yi Wang, Heming Li, and Pin Sun, ―Hierarchical
coordinated control of DC Microgrid with wind turbines,‖ in Proc.
IECON, Oct. 25-28, 2012, pp. 3547 – 3552.
[9] C. N. Papadimitriou, V.A. Kleftakis, A. Rigas and N. D.
Hatziargyriou, ― A dc-microgrid control strategy using dc-bus
signaling,‖ IET Conference, 2-5 Nov. 2014, pp.1 –8.
[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.