1) SolarCity analyzed high resolution output data from hundreds of distributed solar PV inverters covering radii of 1.6 and 4 miles. The analysis showed that geographic diversity reduces maximum output swings compared to considering individual inverters, with swings over 5% occurring only 1.5% of the time for the 1.6 mile radius area. 2) The smoothing effect of geographic diversity could help reduce voltage flicker concerns and lower the likelihood of tap operations on voltage regulators compared to considering only single inverters. 3) Distribution system planning has traditionally relied on load diversity; geographic diversity of distributed energy resources provides similar benefits and can also be relied upon.

1. PV Geographic Smoothing Effect
Michael McCarty, Andu Nguyen, and Sarah Walinga
Grid Engineering Solutions
May 10th, 2016
EPRI-Sandia PV Systems Symposium

2. SolarCity Confidential Slide 2
2
SolarCity’s national scope and breadth of experience
can help you save on energy costs.
2,000+ Commercial projects
1,600+ MW installed
400+ School installations
26 States with installations
9 Years of experience
14,000+ Employees
$9 Billion+ Of solar projects
The national leader in solar, SolarCity
has:
SolarCity has experience addressing
major challenges.
We reduce your energy expenses.
Our customers buy energy from us at a lower rate
than they currently pay for electricity from utilities,
with no up front cost.
We offer predictable energy prices.
You’ll be able to accurately predict and manage
your energy costs for 20 or more years.
We help you switch to solar seamlessly.
Our experienced professionals take care of every
step of the process, making the transition to solar
as smooth as possible for our customers.
80+ Operation centers
Source: National leader according to the Q1 2015 GTM Research U.S. PV Leaderboard.
“States currently served” data as of 8/11/15; “Facilities,” “Employees” and “MW Deployed” data as of 6/30/15; “School installations” data as of 9/24/14;
“Commercial projects” data as of 4/22/15; “Funds raised to finance solar projects” data as of 5/28/15.

3. SolarCity Confidential Slide 3
3
SolarCity Collaboration Projects
Transient Overvoltage
and Hosting Capacity
DER Aggregation
for Grid Support
Smart Inverter
Field Demonstration
Multi-Inverter Anti-Islanding
with Grid Support Functions

4. SolarCity Confidential Slide 4
4
• While cloud cover impacts PV generation output,
cloud movement is not instantaneous and the way in
which light reflects off of clouds results in a
geospatial diversity at the ground level.
• Recent high resolution inverter output data from
SolarCity validates this concept of geographic
smoothing as it relates to distributed PV.
• Distribution planning typically relies on a diversity of
load. Now it can rely on a geospatial diversity of
distributed energy resources.
Executive Summary

5. SolarCity Confidential Slide 5
5
Agenda
• Previous Research
• SolarCity’s Distributed PV Generation Data
• Practical Implications

6. SolarCity Confidential Slide 6
6
EPRI Solar Calendar
Image Source: “Is the Grid Ready: Preparing for Solar.” Tom Key. Electric Power Research Institute. (2015)

7. SolarCity Confidential Slide 7
7
Sandia’s Ota City Study
2 MW over 0.15 mi2 and 553 Homes
Image Source: “Ota City: Characterizing Output Variability from 553 Homes with Residential PV Systems on a Distribution Feeder.” Lave et al. Sandia National Labs. (2011)

8. SolarCity Confidential Slide 8
8
Agenda
• Previous Research
• SolarCity’s Distributed PV Generation Data
• Practical Implications

9. SolarCity Confidential Slide 9
9
SolarCity’s Distributed PV Generation Data
• Data was collected in San Jose from 4/01 to 4/22 at one
to five second intervals.
Representing Radius (miles) Number of
Inverters
AC Nameplate
(kW)
Line Regulator 1.6 100 400
Load Tap Changer 4 300 1300

10. SolarCity Confidential Slide 10
10
SolarCity’s Distributed PV Generation Data
:
⋯
⋯

11. SolarCity Confidential Slide 11
11
SolarCity’s Distributed PV Generation Data

12. SolarCity Confidential Slide 12
12
SolarCity’s Distributed PV Generation Data

13. SolarCity Confidential Slide 13
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SolarCity’s Distributed PV Generation Data
• Per unit max output swing in any one minute decreases
as the area covered increases.

14. SolarCity Confidential Slide 14
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SolarCity’s Distributed PV Generation Data
• For the 1.6 mile radius in any one minute interval, a max
output swing greater than 5% was seen only 234 times
(1.5% of time studied).
• A max output swing greater than 10% was seen only 5
times (0.03% of the time studied).
Count of One Minute Intervals with a Maximum
Output Swing > X%
> 5% > 10% > 15%
1.6 Mile Radius 234 5 1
4 Mile Radius 7 0 0

15. SolarCity Confidential Slide 15
15
Agenda
• Previous Research
• SolarCity’s Distributed PV Generation Data
• Practical Implications

16. SolarCity Confidential Slide 16
16
Voltage Flicker
• Flicker, fluctuation, and deviation often used
interchangeably.
• GE curve has been superseded, but flicker
intensity is challenging to proactively determine.
Image Source: IEEE

17. SolarCity Confidential Slide 17
17
• When calculating voltage deviation or flicker, use
a ΔP (kW) of less than 100% of nameplate.
• This is a worst case scenario. The change in
voltage in modeling software will be less.
Voltage Flicker
100% ⇒ ∆
3
400
3 ∗ 1000 ∗ 12
1.9%
30% ⇒ ∆
30%
3
30% ∗ 400
3 ∗ 1000 ∗ 12
0.6%

18. SolarCity Confidential Slide 18
18
• Tap operations can be similarly smoothed.
• Due to the smoothing effect, regulator is more
likely to remain at new tap position or reset.
Tap Operations
; 6
10% ∆ ⇒ 10% ∗ 0.5 ∗ 6 0.3 0.75
50% ⇒ 0.5

19. SolarCity Confidential Slide 19
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Michael McCarty, PE
Principal Engineer, Grid Engineering Solutions
SolarCity
mmccarty@solarcity.com
solarcity.com/gridx
Questions?