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

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

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

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 61000-4-15 Ocor Confidential - For Internal Use Only

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

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

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

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

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

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

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

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

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

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

Follow-up Recording Flicker Threshold of concern

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

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

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

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

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 59.955 Hz Why does anti-islanding initiaite a phase shift algorithm instead of shutting off the plant?

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?

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

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