Smart Solar Energy for the Smart Grid - IEEEBrad Lehman, Northeastern University, Boston, MA...
Transcript of Smart Solar Energy for the Smart Grid - IEEEBrad Lehman, Northeastern University, Boston, MA...
Brad Lehman,
Northeastern University, Boston, MA [email protected]
Seminar at IEEE COMPEL; June 26, 2018
Smart Solar Energy for the Smart Grid
Outline 1. PV Basics and Problem Statement
2. Advances in Smart Solar PV’s: a) Smart Fault Detection
b) Dynamic Reconfigurability
c) Lego Solar Panels
d) Modular Differential Power Processing
3. Conclusions
• Presenting research, with gratitude from many students and colleagues: D.
Nguyen, Y. Zhao, P. Li, Y. Li, J. Huang, F. Khan, R. Balog,, S. Lu, S. Shu, L. Yuan, G. Spagnuolo, G. Petrone, C. Ramos Paja, Y. Zheng, B. Scandrett, Y. Zheng, C. Liu, D. Li, J.-F. De Palma, K. Kim, P. Krein, R. Pilawa-Podgurski, B. Scandrett, M. Gutierrez, and others…
• Funding from NSF, PowerFilm, Mersen, DOE (SunShot), US Army Natick, DOD, Arpa-e
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What do you call a bear that uses renewable
energy?
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1. PV Basics and Problem Statement
Central Inverter: PV, power conversion unit, protection devices and utility grid.
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Different approaches grid-connected inverters
a. Centralized technology.
b. String technology.
c. Multi-string technology.
d. AC-module
Optional
(DC optimizer technique not shown) Figure: S. B. Kjaer, J. K. Pedersen and F.
Blaabjerg, IEEE Transactions on Industry
Applications, Sept.-Oct. 2005
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PV1
PV2
PV3
PVn
DC/DC
DC/DC
Voltage Bus
DC/DC
DC/DC
DPP converter
Newer Approach: Differential Power Processing
Work by K. Kim, R. Pilawa-
Podgurski, P. Krein, R. Balog,
S. Qin, P. Shanoyand, D.
Maksimovic, and others…
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Do you agree? • Many of the solar MPPT methods were designed 40
years ago and have not taken advantage of modern technological advances: • Computer/microprocessor revolution • Machine learning • Wireless communication
• PV installation hardwired and left alone until failure. What about expansion of the system?
How can we take advantage of the advances we have seen to create new approaches in PV systems??
2. Advances in Smart Solar Energy
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Fault detection
Fire hazards in a 1,208kW PV array, in Mount Holly,
North Carolina, in 2011.
Traditional Overcurrent Protection Devices (fuses) have “Blind Spots” in PV Systems with Centralized Inverters!!
2. A) PROBLEM - FAULTS
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Sometimes the MPPT makes it worse: moves operating point to lower current before the fuses melt
vf
Utility
grid
Centralized inverter
IPV
VPV
PV module
+
_
OCPD (Series fuse for
overcurrent protection)
String 1 String 2 String 6 String 12
Pos.
Neg.
I1 I6 I12
Line-line faults
created by a switch
LL-m
LL-2
LL-1
String 7
I7
PV
monitoring
board#1
I2
+
_PV
monitoring
board#2
Line-line fault with
m number of
modules mismatch
Current sensor
Switches to
create faults
PV module #1
PV module #2
PV module #3
PV module #4
PV module #9
PV module #10
PV module #11
PV module #12
...
More LL
faults
Problem: Some Line-Line faults or night time faults cannot be cleared.
Zhao, Y. de Palma, J.-F, Mosesian, J., Lyons, R, and Lehman, B., IEEE
Transactions on Industrial Electronics,September 2013.
(See also work by F. Khan, J. Johnson,
X. Wang, A. Etemadi, and others)
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Normal
String PV
Current
Fault at t= T1 may be current
limited in PV (low irradiance,
line-line)
Oh No! MPPT reduced fault current so fast that fuses cannot clear fault!
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Machine Learning in PV Strings
Compare string currents to find faults
Fault detection using outlier rules at PC (Matlab GUI)
Real-time operation
Combiner box (Mersen): DSP fuses, Machine Learning
Statistic outlier detection rules
• Univariate variable {xi}, i = 1….n, x = string current
• xi is an outlier if
• Reference value x0, a measure of variation and a threshold
• Upper bound , lower bound
𝛼
Reference: R. K. Pearson, Mining imperfect data: Dealing with contamination and incomplete records, Society for Industrial and Applied
Mathematics, 2005
xi
p(x)
x0
Outlier?
An outlier
x1 x2
True distribution
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Zhao Y, Ball R, Mosesian J, de Palma JF, Lehman B., IEEE Transactions on Power Electronics, May 2015.
Statistic outlier detection rules
• Three sigma rule
– Might be the best-known criterion, but poor performance in practice
– Breaks down if CL >10%
• Hampel identifier
– Breaks down if CL >50%
• Boxplot rule
– Breaks down if CL >25%
• Outlier rules are updated at each sampling time.
xL is lower quartile (25th percentile)
xU is the upper quartile (75th percentile)
ˆ3ˆ| |ix
Sample mean
Sample variance
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Photo of the experimental setup > Fault algorithm is running in a GUI at PC
Matlab GUI
DC electronic load
DC power supply
GreenString in
the combiner box
Power resistors
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Video Demo
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2. b) Dynamic Reconfiguration
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Why keep all the wiring connections the same between the PV panels all the time?
SMART PV PANEL
• Smart Solar Arrays
– Merge electronics to be inside the solar
panel instead of in a separate room
– Automatically adjusts to shadows and
the environment by changing its
interconnections
– Senses, diagnoses, and alerts user of
problems with the array (cracks,
malfunctions, etc.)
– Heals itself when solar cells degrade
– Smart grid interaction
– Optimum power output at all times
Lab Prototype: Smart Solar Array
The solar
fixed part
PV1 PVA1
V1
IOUT
IA1
VO
UT
Temp, IOUT
PV2
IF2
IA2
PVA2
IA(m-1)IOUT
PVA(m-1)
IAm
PVAm
PVm-1
PVm
The adaptive
bank
A/D CONVERTER
INTERFACE
R1
R2
R(m-1)
C1
C2
Cm
The
Switching
Matrix
Rm
C(m-1)
Vbatt.
V2
Vm-1
Vm
VOCA1
VOCA2
VOCA(m-1)
VOCAm
{VOCA1,VOCA2,…,VOCAi,…,VOCAm}{V1,V2,…, VAi,…,VAm}
r
D. Nguyen and B. Lehman, IEEE Trans. on Industrial Electronics, 2008
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• Place switches between solar PV cells, strings, or arrays
• Switch between series and parallel connections in
response to clouds or shadows
• Increase the power output of the solar PV array
D. Nguyen and B. Lehman, (2008), Review paper G. Spagnuolo, IEEE Industrial Electronics Magazine, 2015
• Fixed solar cells: m x n
When PV cells in fixed part
are shaded or blocked, the
PV cells in reconfigurable
part will rearrange and
connect to shaded rows.
Shaded cell
• Add m x 1 reconfigurable
solar cells
Slow Reconfigurable Panels
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Even Without Faults, Are There Benefits to Changing PV Connections?
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2. c) Lego Solar Panels
“Solar Blanket”-Modularity
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Y. Zheng, Y. Li, S. Sheng, B. Scandrett and B. Lehman (APEC 2016, 2017),
Introduction – modular solar panels
New challenges:
• Need to carefully design control to Shed power if producing too much power for load.
Turn off MPPT if user connects to another MPPT
Submodule 1
DC-DC
DC-DC
Power management/ charger
Battery
Loads
Subpanel
PV Bus
Submodule n
P
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System Operating Mode
Scenario I: Resistive load,
PMPPT Pload_max
IO
VH
VL
Rload
PPV = PMPPT Pload_max
OL
OH
O
O H
O L
VPVBus
Scenario II: Resistive load,
PMPPT > Pload_max
I
VPVBus
O
VH
VL
Rload
Shed Power
PPV = P1oad_maxPMPPT
OH
OL
Scenario III: Constant power
load, PMPPT Pload_max
IO
VH
VL
OL
OH
VPVBus
PPV = PMPPTPload_max
Scenario IV: Constant power
load, PMPPT > Pload_max
IO
VH
VL
Shed Power
OH
OL
VPVBus
PPV = P1oad_max
PMPPT
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Y. Li, et al APEC 2016
System Operating Mode
Scenario I: Resistive load, PMPPT Pload_max
IO
VH
VL
Rload
PPV = PMPPT Pload_max
OL
OH
O
O H
O L
VPVBus
• All PV subpanels are in individual MPPT mode
• The operating point would be the intersection (O) of load curve
Rload and power contour PMPPT
• The operating point moves between OL and OH along the red
straight line (solid), or between O’L and O’H along the blue curve
with source/load changes
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System Operating Mode
Scenario II: Resistive load, PMPPT > Pload_max
I
VPVBus
O
VH
VL
Rload
Shed Power
PPV = P1oad_maxPMPPT
OH
OL
• Controller must shed power
• Option 1: Do NOT have all modules in MPPT mode
• Option 2: turn off power from individual PV module.
• The area shaded in the Figure represents the amount of shed power
When PPV is too high, the voltage might rise above threshold VH
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PV1
PV2
PV3
PVn
DC/DC
DC/DC
Voltage Bus
DC/DC
DC/DCDPP converter
One Last Idea: What happens when we combine Lego (Modular) solar panels
with Differential Power Processing
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2. d) Modular Differential Power Processing (mDPP)
Voltage Bus
PVm,1
PVm,2
PVm,3
PVm,n
DC/DC
DC/DC
DC/DC
DC/DC
Controllable current source
PV1,1
PV1,2
PV1,3
PV1,n
DC/DC
DC/DC
DC/DC
DC/DC
Iout is current difference between PV panels
Virtual PV panel
Vcap is voltage difference between the string and the bus
PVVmpp=?VImpp=0A
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C. Liu, D. Li, Y. Zheng and B. Lehman (2017, COMPEL) and (2018, APEC)
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Advantage of modularity
7V mDPP
Plug out
Advantage of modularity
8V
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7V mDPP
Plug in
8V
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Advantage of modularity
Simulation results
3 PV panel in series
2 PV string in parallel
PV13 - voltage mismatch
PV 22 - current mismatch
PV#11 PV#12 PV#13 PV#21 PV#22 PV#23 Total
Ideal MPP (P) 13.23 12.3 10 12.59 6.13 12.81 67.06
Simulated Power with mDPP 13.23 12.3 10 12.58 6.13 12.81 67.05
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Simulation results • Comparison between Different Methods
TABLE II. SYSTEM EFFICIENCY STUDY OF DIFFERENT DPP STRUCTURES
cMPPT dMPPT sDPP sDPPcc mDPP(ours)
Output Power (Pout - Watts) 53.56 62.36 42.60 61.11 65.86
System Efficiency (ηsys - %) 79.86% 92.99% 63.51% 91.11% 98.21%
cMPPT: centralized MPPT converter- one converter for all PV panels dMPPT: distributed MPPT converter- one converter for each PV panel [1] sDPP: series DPP method [2] sDPPcc: series DPP converter with centralized converter [3]
[1] Y. Zheng, Y. Li, S. Sheng, B. Scandrett, and B. Lehman, "Distributed control for modular plug-and-play subpanel photovoltaic converter system,"
in 2017 IEEE Applied Power Electronics Conference and Exposition (APEC)
[2] S. Qin, S. T. Cady, A. D. Domínguez-García, and R. C. N. PilawaPodgurski, "A Distributed Approach to Maximum Power Point Tracking for
Photovoltaic Submodule Differential Power Processing," IEEE Transactions on Power Electronics
[3] S. Qin, C. B. Barth, and R. C. N. Pilawa-Podgurski, "Enhancing Microinverter Energy Capture With Submodule Differential Power Processing,"
IEEE Transactions on Power Electronics
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,max,
1
out outsys
ideali mpp
i
P P
PP
Harvest most power 98.21%
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Experimental results
Up to 97.6% efficiency with 90% efficiency converter
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CONCLUSIONS
1. Legacy approaches to operating PV arrays?
2. Hidden Faults in PV can now be discovered
3. Dynamic Reconfiguration is possible
4. Modular solar panels due to miniaturization
and control in power electronics
5. Differential Power Processing might
improve performance of Lego PVs
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