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Evolution of PV inverter technologies
for addressing high penetration utility issues
Soonwook Hong
Solectria Renewables
[email protected]
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Alamosa NM 30MVA PV plant: PVI 82kW x 504EA
World Largest CPV Site: Commissioned in early 2012
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Solectria Renewables’ Products Evolution in 3 years
Residential (1.8 ~ 7.6kW)
Commercial (10 ~ 100kW)
Commercial (14 ~ 28kW)
Utility Scale (225 ~ 500kW)
Utility Scale (500 ~ 750kW)
What are the driving factors for this evolution?
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Topics
What happened during the last three years?
What are opportunities for the PV inverter?
What are the challenges for the PV inverters?
A Case Study with high penetration
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DoE Sunshot initiative
Goals
…reduce the total installed cost of solar energy systems to $.06 per kilowatt-hour
(kWh) by 2020. Since SunShot's launch in 2011, price has dropped from about $0.21
to $0.11. (60% achieved in 3 years)
It is equivalent to $1.00 per watt installation cost for utility scale solar project (50%
for panel, 40% for BOS and 10% for inverter)
…Solar energy could meet 14% of U.S. electricity needs by 2030 and 27% by 2050.
http://energy.gov/eere/sunshot/mission
Cost Reduction and High Penetration!
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State RPS Policies
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PV Installations and System Price Reduction
Source: GTM Research/SEIA March 2013
PV Installation exceeded wind Installation in 2013!
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Challenges and Opportunities with High Penetration
(aside from all benefits)
Utility Side
- Intermittency
- Dispatchability
- Over-voltage issue
- Coordinating with the existing utility assets
Manufacturers Consideration
- Cost reduction
- Efficiency/Reliability enhancement
- Energy density increase
- New features to resolve utility side challenges
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Concept changes with High Penetration
Independent resources
Resources to be integrated
Intermittent resources
Dispatchable resources with ESS
Distribution resources
Power System resources
Resources with ‘credit’
Resources to compete with
Invited guest with privileges team member sharing responsibilities
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PV Inverter Evolution
External
Transformer
High Voltage
String
String
Inverters
Innovative
Topology
New
Devices
Cost reduction
√
√
√
√
√
Performance increase
√
√
√
Energy density increase
√
√
Reliability increase
√
√
Efficiency increase
√
√
Component Size reduction
√
√
Modularity
√
Harmonics reduction
√
√
√
√
√
√
√
√
√
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PV Inverter Communication Capabilities
- Remote Monitoring & Control
- More processor HP at very little additional cost allows for multiple
COM interfaces:
Protocols:
Medium:
IEC 61850
• Allowing different sets of settings when feeder reconfigures
• Allowing different modes of operation for different times of the day
Closing the loop for the system integration
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Possibilities as a dispatchable grid asset
Irradiation
+
Real power Generation
Power curtailment
Reactive Power Control
Voltage Regulation
Flicker mitigation
Power Factor Correction
Fault Ride Through
+
Power Dispatch
Emergency Backup Operation
Micro Grid Operation
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Several Grid Supporting Features in PV inverters
Volt var
Fault Ride Through
Frequency Watt
DRCS
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Several Grid Supporting Function Characteristics
Fast Response
Fault Ride Through
Voltage Regulation
Slew Rate Control
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Grid Supporting Feature influencing PV Operation
Fewer String Sizing Choices:
Minimum DC voltage rises:
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Vdc
Vmppt-max
800 V
Current shifts from IGBTs to diodes.
565 V
535 V
Vmppt-min
inductive
0.8
0.9
1.0
0.9
0.8
capacitive
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20
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Grid supporting features affecting the inverter operation
Example: 500kVA inverter, rated current
Unity PF
0.8 capacitive PF
0.8 inductive PF
98.3% CEC
97.9% CEC
97.4% CEC
• Current shifts from IGBTs to diodes.
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Grid Supporting Features influencing inverter reliability
Automobile best warranty - 100,000 miles
2 hours per day with 40 mph average speed
Net operating time:= 2,500 hours
PV Inverter standard warranty - 5 years
8 hours per day on an average
Net operating time:= 14,600 hours
PV Inverter warranty is about 6 times as that of an automobile
The PV inverter warranty model is designed to work for the increased hours and/or
higher operation density when the grid supporting functions are provided.
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High Penetration Scenario Example:
1.7 MW site in Cedarville NJ
4.7 miles from substation 12kV feeder, 6MW mid-day load
Concerns of local overvoltage
Utility has closed circuit for more PV
0.5 MW
1.7 MW
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Overvoltage Concerns
3.0% of points
exceed +5% limit
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Mitigating overvoltage issue with Power Factor control
• Feeder could be reopened for PV after PF adjustment to 0.97
• “Flicker” Mitigation (cloud induced voltage transients)
• Generation reduction due to PF control < 0.4% or less
< 0.1% of points
exceed 5% limit
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How power factor control helps the voltage regulation?
•
If the X/R ratio of the feeder is
known, the grid voltage can be
regulated by adjusting power
factor.
•
The controlled power factor will
be usually higher than 0.95
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Simulation with IEEE 34 Node Test Feeder
848
822
846
820
844
864
818
802
806
808
812
814
850
824
826
842
834
860
836
858
840
816
832
862
800
888
Regulator
810
PV
852
Load
828
830
890
854
856
838
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XR Compensation (Power Factor Control)
Sunny Day
Cloudy Day
1.05
1.05
1.045
1.045
1.04
1.04
1.035
1.035
1.03
1.03
1.025
1.025
With No PV
1.02
Generation mode
1.015
PF Control
1.01
1.01
8:19
Generation mode
1.015
PF Control
5:27
With No PV
1.02
11:12
14:05
16:58
19:51
5:27
8:19
11:12
14:05
16:58
Number of operations of the regulator at 850
40
40
30
30
20
20
10
10
0
0
No PV
Generation Mode
PF Control
No PV
Generation Mode
PF Control
19:51
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Power Factor Control Advantages
Regulates the self-induced voltage change issue
Steady state as well as dynamic voltage regulation
Autonomous function without need for communication
0.4% generation reduction when pf = 0.97 on sunny days
Open up the generation to load ratio, thereby reduce high penetration
issues with a minimum investment.
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Evolution possible with Teamwork!
with leaders from different organizations in the industries
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Thank you!
Soonwook Hong
[email protected]