IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 33, NO. 2, APRIL 2018
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Power Engineering Letters
Unstable Operation of Photovoltaic Inverter From Field Experiences
Chun Li
, Senior Member, IEEE
Abstract—This letter presents records of unstable operations in
grid-connected photovoltaic generation plants. The instabilities involve a wide range of frequencies from tens to thousands of Hertz.
Possible causes of the instabilities are discussed based on the literature survey. This letter suggests new industry standards or grid
codes for photovoltaic generation integration should consider such
practical challenges.
Index Terms—Photovoltaic inverter, harmonics, instability,
power quality, distributed energy resources.
I. INTRODUCTION
S PHOTOVOLTAIC (PV) generations are penetrating into
power grid rapidly, power quality issues, especially “harmonics” from PV inverters, attract wide interests. Model development, computer simulation and laboratory tests on inverter
“harmonic” instability are available in literatures [1]–[7]. However, unstable operations from real grid-connected PV plants are
rarely reported. Field experiences from real PV operations are
duly needed for research validations, design improvement and
code establishment. This letter presents original records of unstable PV operations covering a wide range of frequencies from
tens to thousands of Hertz. Possible causes for the instabilities
are discussed based on literature reviews due to unavailability of
PV facility models (even black-box model) and internal operating record. The letter suggests future PV connection standards
or grid codes should consider these practical challenges.
A
Fig. 1.
Current and voltage waveforms under normal and unstable operations.
Fig. 2.
Current and voltage waveforms under inverters restart.
II. RECORDS OF UNSTABLE PV OPERATIONS
This section presents three types of unstable operation records
from grid-connected PV plants.
A. Unstable PV Operation at Low-Order Harmonics
The first instability example is from a PV plant under 500 kW.
The plant is connected to utility’s 44 kV feeder via a 600 V/44 kV
step-up transformer. Fault level at 44 kV Point of Connection
(PoC) is close to 200 MVA. Approximately 30 inverters are paralleled at the 600 V bus where no local load is supplied. The
Manuscript received September 12, 2016; revised January 4, 2017; accepted
January 9, 2017. Date of publication January 19, 2017; date of current version
March 22, 2018. Paper no. TPWRD-00161-2016.
The author is with Hydro One Networks, Inc., Toronto, ON M5G 1P5,
Canada (e-mail: chester.li@hydroone.com).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPWRD.2017.2656020
inverter specifications guarantee total current harmonic distortion below 2% at rated power. The left half of Fig. 1 shows a
snapshot of PV current and voltage waveforms at 90% of rated
power output, where harmonic is not a concern. However, the
PV output currents could become highly distorted as shown in
the right half of Fig. 1. FFT analysis from 12-cycle window
shows that the distortion concentrated at 420 Hz. The magnitude of the 7th harmonic current approaches 20% of the PV rated
current. The PV inverters typically restart upon the instability
as shown in Fig. 2, probably by internal protective control. The
distortions drop to normal level once inverters restart.
Harmonic instabilities are also recorded at other frequencies.
Fig. 3 shows current and voltage waveforms of a 28 kV/600 V
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 33, NO. 2, APRIL 2018
Fig. 3.
Current and voltage waveforms under partial inverter shutdown.
Fig. 4.
PV current waveforms with high-frequency oscillations.
solar farm partially shutting down its inverters under oscillations at 780 Hz. The farm has 50 inverters in parallel. In both
events there was no any utility switching, outage or fault event
could be correlated to the harmonic outburst. The causes of the
instabilities are very likely inside the solar plants but in-yard
operating records are not available. Some simulation and laboratory studies show that interactions among multi-paralleled
PV inverters and the grid impedance characteristics could cause
similar instabilities [1]–[4].
B. Unstable Operation at High Frequency
Inverter instabilities at high frequencies (HF) are reported in
simulation or laboratory studies [1], [3], however field records
are not available in literature. Figs. 4 and 5 show currents and
voltages of a PV plant under 500 kW when inverters restart
upon HF oscillations. Fault level at PV 600 V bus is 7 MVA.
FFT analysis from two-cycle data in Fig. 5 shows the oscillations
focus at 2370 Hz. The meter samples 128 points per cycle. The
HF voltage ripples vanish once the inverters restart (current
recovery similar as Fig. 2 is omitted in order to illustrate the
HF ripples). Inverter control system interactions with external
grid characteristics might contribute to the unstable operation
[1], [6].
C. Unstable Operation at Interharmonics
The last example involves sub-synchronous oscillations in
grid-connected PV operations. Three solar farms, 10 MVA each,
Fig. 5.
PV voltage waveforms with high-frequency oscillations.
Fig. 6.
PV current and voltage upon utility capacitor switching.
are connected to utility substation via a 44 kV feeder of 30 km,
all operating at constant power factor. Fault level at the 44 kV
PoC is approximately 120 MVA. Weakly damped oscillations
near 20 Hz are recorded when a 30 Mvar/44 kV capacitor is
energized at the substation bus. Fig. 6 shows current waveforms
and voltage RMS trends (updated every half cycle) measured
at one solar farm’s 44 kV terminal in response to a capacitor
switching. The voltage fluctuations are result of simultaneous
oscillations from the three solar farms. FFT analysis with 6cycle data in Fig. 6 indicates that the output current contain a 80
Hz component with magnitude close to 75% of the fundamental
current. The magnitude modulation at 20 Hz in Fig. 6 could
be explained by the inverter interharmonic injection (80–60 =
20 Hz). The sub-synchronous oscillations triggered by capacitor switching transients could be result of Phase-Locked Loop
dynamics in PV inverters [1], [2], [5], [7].
Due to lack of established standards to regulate PV inverter performance, utilities may face new voltage quality
management challenges. It is further troublesome that the PV
oscillations occur in wide spectra, which makes predicting
and mitigating such “harmonics” with conventional filters
difficult. Active damping technique is proposed in some
researches [4], [6], however operating verification has not been
reported. Future industry standards or grid codes may consider
requiring PV inverters to demonstrate sufficient damping across
reasonable frequency band and operation levels.
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III. CONCLUSION
This letter presents field records of unstable operations from
grid-connected PV plants. Causes of the instabilities may involve interactions among PV inverter control systems and grid
impedance characteristics. The frequencies of the instabilities
vary in large scale therefore conventional filters may not mitigate the problems effectively and solutions from inverter control
systems should be examined. Industry standards or grid codes
to regulate PV inverter frequency characteristics should be considered to manage the new type of power quality disturbances.
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