1. Nathan Kassees, P.E. Sr. Engineer Oncor Electric Delivery
Flicker Issues Originating from Solar Installed at the Transmission Level
Nathan Kassees, P.E.
Sr. Engineer
Oncor Electric Delivery

2. Introduction
On May 4th, 2017, an electric Co-op served off a POI from a 138 kV Oncor line made a complaint about flicker occurring on sunny days
The Co-op claimed the issue was being caused by a large solar plant that had recently come online nearby
On May 5th, power quality recorders were set at the POI to the solar plant and on a nearby Oncor distribution feeder

3. Voltage Flicker and Human eye sensitivity
The human eye can detect as little as 0.25% change in RMS Voltage through a light source
Changes occurring periodically between 5 and 50 Hz can be detected depending on the person
12 Hz
60 Hz

4. Flicker and Voltage Sags
Customers report flicker when they notice a fluctuation in voltage through the change of light bulb brightness
Most complaints of flicker are related to voltage sags caused by starting equipment, usually motors
Continuous flicker is a consistent, periodic change in the magnitude of RMS voltage
Continuous flicker is generally caused by arc furnaces or electric welders
Further information about flicker can be found in IEEE 1453 or IEC
Ocor Confidential - For Internal Use Only

5. Types of Flicker Temporary Flicker Most commonly reported
Characterized by frequent, non-continuous voltage sags
Dips in minimum voltage on interval recording
Continuous Flicker
Characterized as a frequency (5-50 Hz) of continuous voltage sags
Large difference between min & max voltage on interval recording

6. Voltage Recording on Oncor Feeder 1 minute interval recording
Consistent 3 V gap between min and max voltage
Occurs during the middle of the day

7. Instantaneous Flicker (IFL) and Percentile Short Term (Pst) Flicker
Instantaneous Flicker measures how noticeable the flicker is in a given instant
Best used for root cause analysis of the flicker issue
Percentile Short Term Flicker is a statistical average of the flicker depth and frequencies over a 10 minute period
Best used for determining human sensitivity to flicker. Pst > 1 means, on average, 50% of people should notice the flicker

8. Instantaneous Flicker (IFL) Recording on Oncor Feeder
Continuous Flicker
Temporary Voltage Sag

9. Percentile Short Term Flicker (Pst) Recording on Oncor Feeder
Threshhold of concern

10. Solar Plant recording, 1 minute interval Correlation of Flicker, kW, kVAR, and Voltage
Flicker Threshold of concern
Oscillation of 15 MW
Oscillation of 26 MVAR
Oscillation of 2 V on 114 basis

11. Solar Plant Investigation Results
The recording showed a clear correlation between the flicker and significant power fluctuations at the solar plant
Significant MW and MVAR oscillations were fluctuating the voltage
The power fluctuations appeared to be occurring during peak output each day

12. Volt and Amp recorded at PTs and CTs of POI show clear 15 Hz variation in magnitude

13. Fluctuating current and phase angle at 15 Hz
t = 1 cycle
t = 2 cycles
17°
372 A 7°
390 A
13°
414
t = 3 cycles
t = 4 cycles
23°
408 A
17°
372 A

14. Solar Plant follow-up
Oncor presented the evidence to the solar plant and requested a root cause be determined for the fluctuating power output, and that the plant be limited to 96 MVA output until RCA was completed
The plant complied

15. Follow-up Recording
Flicker Threshold of concern

16. Results
Follow-up showed the flicker was prevented with the power limit
The issue was thought to be resolved until the flicker RCA was completed
A few days later Oncor received additional flicker complaints from the Co-op

17. Solar Plant Off-Peak Flicker
Flicker Threshold of concern
Flicker and power fluctuations produced during off-peak periods

18. RCA from Inverter Manufacturer
Oncor presented the evidence to the solar plant and stated that if the problem was not resolved quickly the plant would need to go offline until the issue was fixed
The inverter manufacturer followed up with a root cause analysis determining that the power fluctuations were caused by an Automatic Phase Shift algorithm
The Automatic Phase Shift algorithm was being triggered through the Anti-Islanding function

19. Automatic Phase Shift and Anti-Islanding
The automatic phase shift is intended to test if the system needs to anti-island. It is not intended to affect system voltage if there is no island condition
Different manufacturers use different Automatic phase shift algorithms
Automatic phase shift is one of multiple types of islanding detection methods

20. Automatic Phase Shift
In this case, the high output of the plant caused a significant voltage shift when Automatic Phase Shift was activated
The Automatic Phase Shift failed to turn off due to the continuous voltage shift
Recordings showed that the Automatic Phase Shift would activate at about Hz
Why does anti-islanding initiaite a phase shift algorithm instead of shutting off the plant?

21. Resolution
The final solution was to turn the anti-islanding function off to prevent further activation of the automatic phase shift function
Anti-islanding is a necessary feature if not using using transfer trip. The relaying scheme at the substation allowed for anti-islanding to be turned off
To this date, Oncor is not aware of any other flicker issues occurring near distributed generation sites on transmission or distribution circuits
New updates to IEEE 1547 set the limit of flicker for inverters to Pst < 0.35
Why is disabling anti-islanding ok on the transmission side?

22. Partial Eclipse on 8/21/17
Partial eclipse reached 72% coverage in solar plant area
Solar plant shows 72% reduction of 100 MW output (28 MVA) during eclipse peak
Partial eclipse peak at 12:58
Voltage variation due to cloud cover

23. Questions? 23