1
A Robust Control Approach to Smooth Output
Power of Wind Units Using Hybrid Storage
Systems Considering State of Charge
Arash Jamehbozorg
California State University, Los Angeles
Los Angeles, CA
[email protected]
Yanzhu Ye, Ratnesh Sharma
NEC Labs America, Inc.
Cupertino, CA
yanzhuye, [email protected]
2
Integration of Wind Units
• Main Challenges:
– Intermittent nature of wind speed
– Output power fluctuation of wind units
– Sound pollution
– Aesthetic issue
– Etc.
Output power smoothing of wind units using storage systems
3
Hybrid Storage Systems
• Types of storage units
– High power, low energy: Supercapacitor (EDLC), SMES,
Flywheel
– High Energy, low power: Batteries
• One of the decisive factors affecting the lifetime of storage
units is State of Charge (SoC).
• It has been shown that when a storage unit operates at SoC
between 30%-90%, its lifetime increases.
4
Modeling
• A SCIG wind turbine parallel to a hybrid
storage unit is connected to infinite bus
5
Modeling
• Modeling of the battery
𝑣𝑏𝑒
𝑄
= 𝑉𝑜 − 𝑅𝑜 𝑖𝑏𝑒 − 𝐾
+ 𝐴. exp −𝐵
𝑄 − 𝑖𝑏𝑒 𝑑𝑡
𝑆𝑜𝐶𝑏𝑒 = 100 1 −
𝑖𝑏𝑒 𝑑𝑡
𝑄
%
𝑖𝑏𝑒 𝑑𝑡
6
Modeling
• Modeling of EDLC
𝑆𝑜𝐶𝑠𝑐
𝑉𝑠𝑐
= 100
𝑉𝑠𝑐,𝑛
2
%
7
Modeling
• DC/AC Converter
8
Proposed Control Strategy
Mode I (Normal Mode)
• When SoC of the battery and EDLC are both inside the acceptable range,
this mode is used. In this mode, the control strategy of each converter is
as follows:
1. DC/DC converter of the EDLC regulates the DC-link voltage of HSS (𝑉𝑑𝑐 ).
2. DC/DC converter of the battery continuously keeps SoC of the EDLC
(𝑆𝑜𝐶𝑠𝑐 ) in range.
3. DC/AC converter controls the output active power of HSS (𝑃ℎ𝑠𝑠 ) to
smooth the output power of wind unit and reactive power of HSS (𝑄ℎ𝑠𝑠 ).
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Proposed Control Strategy
Mode I (Normal Mode)
0.25
0.2
0.15
0.1
Ib e ,re f [pu]
0.05
0
-0.05
-0.1
-0.15
-0.2
-0.25
10
20
30
40
50
60
SOC
sc
𝐼𝑞,𝑟𝑒𝑓
2 𝑃ℎ𝑠𝑠,𝑟𝑒𝑓
= .
3 𝑉𝑞𝑔
𝐼𝑑,𝑟𝑒𝑓
70
[%]
2 𝑄ℎ𝑠𝑠,𝑟𝑒𝑓
=− .
3 𝑉𝑞𝑔
80
90
100
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Proposed Control Strategy
Mode II (𝑆𝑜𝐶𝑠𝑐 is out of range)
• When SoC of EDLC is out of range, this mode is activated. In this mode the
control strategies are as follows:
1. DC/DC converter of EDLC charges/discharges the EDLC based on the SoC
of EDLC in order to bring SoC of EDLC back to normal range. A constant
current equal to 60% of nominal current is used as EDLC reference
current (Isc,ref) .
2. DC/DC converter of battery regulates DC-link voltage.
3. DC/AC converter controls the output active power of HSS to smooth the
output power of wind unit and reactive power of HSS (same as Mode I).
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Proposed Control Strategy
Mode III (𝑺𝒐𝑪𝒃𝒆 is out of range)
• When SoC of battery is out of range Mode III is activated. In this mode
control strategies are as follows:
1. DC/DC converter of the EDLC regulates the DC-link voltage of HSS (𝑉𝑑𝑐 )
(Same as Mode I).
2. DC/DC converter of battery charges/discharges the battery based on the
SoC of battery in order to bring the SoC of battery back to normal range.
A constant current is used as battery reference current (𝐼𝑏𝑒,𝑟𝑒𝑓 ).
3.
DC/AC converter controls the output active power of HSS to smooth the
output power of wind unit and reactive power of HSS (same as Mode I).
12
Proposed Control Strategy
Mode IV (𝑆𝑜𝐶𝑠𝑐 and 𝑆𝑜𝐶𝑏𝑒 are out of range)
• When SoC of both EDLC and battery are out of range this mode is
activated. This mode is the only one in which HSS cannot smooth the
output power of the wind unit. Control strategies of HSS in this mode are:
1. DC/DC converter of EDLC charges/discharges the EDLC based on the SoC
of EDLC to bring the SoC of EDLC back to normal range. A constant
current is used as EDLC reference current (Isc,ref).
2. DC/DC converter of battery charges/discharges the battery based on the
SoC of battery to bring the SoC of battery back to normal range. A
constant current is used as battery reference current (𝐼𝑏𝑒,𝑟𝑒𝑓 ).
3.
DC/AC converter regulates the DC-link voltage for safe operation of HSS,
and controls the output reactive power of HSS.
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Proposed Control Strategy
• State diagram of control system of HSS
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Simulation Results
• Acceptable range of SoC is defined to be 35% < 𝑆𝑜𝐶 < 95%
for both EDLC and battery.
13
12
V w [m/s]
11
10
9
8
7
0
20
40
60
t [sec]
80
100
15
Simulation Results
1
P S C [pu]
0.5
2.4
P
2.2
P
0
-0.5
wind
-1
tot
0
2
20
40
60
80
100
t [sec]
1.8
1
P B a t [pu]
1.4
0.5
0
1.2
-0.5
1
0
10
20
30
40
50
60
70
80
90
100
60
70
80
90
100
t [sec]
0.8
4
0.6
0.4
0
20
40
60
t [sec]
80
100
Control Mode
P [pu]
1.6
3
2
1
0
10
20
30
40
50
t [sec]
16
Simulation Results
SoC
SC
(%)
100
80
60
40
20
0
20
40
60
80
100
t [sec]
80
SOC
B at
(%)
100
60
40
0
10
20
30
40
50
t [sec]
60
70
80
90
100
17
Simulation Results
Control Mode
Activation Time (%)
Mode I
96.20
Mode II
2.40
Mode III
0.79
Mode IV
0.61
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Conclusions
• A new algorithm is presented to control hybrid storage system
• State of charges of both battery and supercapacitor are
considered
• Results show that the proposed algorithm can successfully
smooth the output power of wind unit in more than 99% of
the times
• Using this algorithm, a smaller size of hybrid storage system
can be used