Voltage and Power Management in a Microgrid System with Diesel Generator and Energy Storage
M. Negnevitsky and O. HaruniUniversity of Tasmania, Australia
Paper No: GM0551
P. Milbourne, J. O’Flaherty, D. Capece and T. NguyenAurora Networks, Australia
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Residential electricity demand:Australian perspective
• Australia’s residential consumption accounts for 12.5% of total energy demand.
• Between 1990 and 2020 the number of occupied residential households is forecast to increase from 6 million to almost 10 million.
• Per capita residential electricity consumption is increasing due to greater use of large appliances and declining number of occupants per dwelling.
• Most householders do not have sufficient technical knowledge to manage their energy use to reduce GHGs.
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Residential electricity demand:Comparing with other demand sectors
Residential energy consumption ranks third, and is growing at near the average for all sectors
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Residential electricity demand:Peak demand
By dealing with residential customers, we can reduce the peak demand, and thus price spikes
Daily peaks are mostly caused by residential and small business customers.
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Microgrid systems• A microgrid system can either be autonomous or
connected to the main grid, depending on the status of the main grid.
• Microgrids may experience overloads at peak conditions for short periods of time due to gradual load growth.
• Local generation using conventional diesel generators can be used to support the short-time system peaks.
• Integration of the energy storage system can also be considered as an alternative solution.
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Microgrid systems (cont.)• In most cases, a microgrid network structure is a
radial system with a high resistance to reactance ratio.
• Significant voltage drops can occur due to power losses in distribution lines.
• Roof-top photovoltaic (PV) cells and small wind turbines can cause significant voltage fluctuations.
• Although, the low voltage problem can often be solved using on-line tap-changing (OLTC). transformers, the OLTC has several operating limitations including slow response time.
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• The system under study is located on the island connected to the main grid via two submarine cables.
• The island, located in southern Tasmania, is a popular tourist destination where peak loads occur during Easter and Christmas periods.
• In order to support the system during peak load conditions, the use of local generation comprising a diesel generator and energy storage system is proposed.
• This paper proposes a robust voltage support and power management strategy for the implementation on the island.
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The Microgrid ModelA B C
Main grid
A1
F1 F
D
E
GK
H
L
I
J
M
PO
N
Q
R
ST
U
V
V1W
X
Y
Z
BusRecloser – Normally Closed
Recloser – Normally Open
Legend
Proposed future Connection
DG
ESS
Voltages on busses from N to Z can drop below 0.94 p.u. during peak load conditions (on remote buses low voltages were recorded even during off-peak conditions). This violates the Tasmanian Electricity Code, and thus the situation has to be rectified.
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The Diesel Generator Model
1se τ− ε1C
2
2
s1Kτ+
∑2C
∑s
K1−
R1
∑
∑DsH2
1+
3C
Bm Bm ip
fp
kp
-
+
maxDmT
minDmT
Oωω
O
ref
ωω-
+Oωω∆
ENGINE
GOVERNOR
+
-
DeT
Block diagram of the diesel engine and governor system
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Block diagram of the diesel engine excitation system
_CV
∑
REFV
UELV
B
C
sT1sT1
++ HV
GATEA
A
sT1K+
SVIMAXV
RMINV
+
+
IMINV
1V
FDCRMAX IKV −
FDE
The Energy Storage System Model
DC- ACConverterVbat
DC- DCConverter
Va1
Vb1
Vc1
Va
Vb
Vc
bus V1
a) Battery storage system
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The Energy Storage System Model (cont.)
*tV
+ _PI PI+
_*P
P
*di
di
+ _ +
*qi
qitV
PI_
tV
_
+
+
+
+
dq
abc
PWM
dsv
qsv
*dv
*qv
ωLsiq
ωLsid
PI
+ _PI +
_*dcV *
didi
PWMPINOT
dcV
1Q
2Q
b) dc-voltage regulator
C) Output voltage and power regulator
The OLTC Model for Voltage Regulation
RL+jXL
Regulated point
VLCV2
V2
RL+jXL
+
--
+ Vref
Bandwidth
Time delay
Tap Changer
DOWNUP
CT
VT
V1
Veff
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Voltage and Power Management Scheme
• The power delivery constraints of the diesel generator (defined by its efficient operation conditions):
wheremax_min_ DGDGDG PPP <<
,p.u.3.0min_ =DGP p.u.0.1max_ =DGP
• The maximum power constraints of the storage system:
max_ESSESS PP <
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Voltage and Power Management Scheme (cont.)
• The SOC constraints of the energy storage system:
where ,
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• The maximum and minimum voltage limits at any bus as per the Tasmanian Electricity Code:
maxmin SOCSOCSOC <<
%40min =SOC %95max =SOC
maxmin VVV <<
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Voltage and Power Management Scheme (cont.)
• The thermal constraints of the system limit:
where is the maximum thermal capacity of each sub-marine cable.
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• The operational limits of the OLTC:
kVA1500max =SmaxSS <
max_min_ OLTCOLTCOLTC VVV <<
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Voltage Control Algorithm • Check current voltages at bus V1 and bus G.• If the voltage level at bus V1 is below Vmin (0.94 p.u.),
the energy storage system is discharged to bring the voltage at bus V1 to the reference voltage (1.0 p.u.), provided that the SOC of the energy storage system is higher than 40%.
• If the voltage level at bus V1 is above Vmax (1.06 p.u.), the energy storage system is charged to bring the voltage at bus V1 to the reference voltage, provided that the SOC of the energy storage system is less than 95%.
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Voltage Control Algorithm (cont.) • The OLTC monitors the SOC of the storage system
and voltage at bus G. Based on the SOC information of the storage system, the OLTC operates only if the reference voltage is outside a voltage bandwidth (VBW). If the voltage deviation of the secondary side of the OLTC (at bus G) is outside the VBW, the tap position is changed to correct the voltage.
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Power Control Algorithm• Check the current load demand (PL).• If the load demand is higher than the
maximum capacity of the submarine cables, the diesel generator and energy storage system will provide the power deficit (Pd).
• Calculate the power deficit as (Pd= PL –PCable2), where Pd, PL and PCable are the deficit power, load demand and power from cable 2.
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Power Control Algorithm (cont.)• If Pd > 0.3 PDG (where PDG is the rated power of
the diesel generator), the diesel generator provides power.
• If Pd < 0.3 PDG, the energy storage provides power given that the SOC > 40%. However, if the SOC < 40%, the diesel generator provides the required power.
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Simulation Results• Simulation studies have been conducted using
MATLAB Simpower to evaluate the performance of the proposed voltage and power management strategy.
• The initial setting of the OLTC is -7, and the diesel generator is connected to the system at bus S at 0.28 s.
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A B C
Main grid
A1
F1 F
D
E
GK
H
L
I
J
M
PO
N
Q
R
S
T
U
V
V1W
X
Y
Z
DG
ESS
Simulation Results (cont.)Voltage at bus V1
0 4 8 12 16 200.8
0.85
0.9
0.95
1
1.05
1.1
Time (sec)
Vol
tage
(pu)
DieselGeneratorconnectedat 0.282second
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Simulation Results (cont.)The OLTC voltage response
0 4 8 12 16 200.8
0.9
1
1.1
Time(sec)
Vol
tage
(pu)
Primary voltage Secondary voltage
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Simulation Results (cont.)Real and reactive power response of the diesel generator
0 4 8 12 16 200
110
220
330
440
550
660
Time (sec)
kW
Real power Reactive power
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Simulation Results (cont.)Bus voltages of the microgrid
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Node Voltage, p.u. Node Voltage, p.u.A1 0.9884 N 0.9898A 0.9835 O 0.9897B 0.9831 P 0.9887C 0.9813 Q 0.9985D 0.9825 R 0.9979E 0.9436 S 1.000F 0.9526 T 1.003
F1 0.9851 U 1.013G 1.011 V 1.027H 1.011 V1 1.028I 1.000 W 1.020J 1.000 X 1.020K 1.000 Y 1.019L 1.000 Z 1.019
M 1.001
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Conclusion• The voltage and power management strategy for
a microgrid system with the diesel generator and energy storage system is proposed.
• The presented results demonstrate that the proposed control strategy can achieve the desired objectives.
• The coordinated approach to both voltage control and power management demonstrates that the system voltages can be maintained within the standards of the Tasmanian Electricity Code.
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