Variable Generation Interconnection Lessons
Learned and Best Practices in the Western
Interconnection
WECC Variable Generation Interconnection Task Force
February 15, 2016
155 North 400 West, Suite 200
Salt Lake City, Utah 84103-1114
White Paper Title
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Disclaimer
WECC receives data used in its analyses from a wide variety of sources. WECC strives to source its data
from reliable entities and undertakes reasonable efforts to validate the accuracy of the data used.
WECC believes the data contained herein and used in its analyses is accurate and reliable. However,
WECC disclaims any and all representations, guarantees, warranties, and liability for the information
contained herein and any use thereof. Persons who use and rely on the information contained herein
do so at their own risk.
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Table of Contents
Disclaimer ......................................................................................................................................... 2
1 Executive Summary ..................................................................................................................... 5
2 Introduction ................................................................................................................................ 7
3 Variable Generation Characteristics ............................................................................................. 9
4 Reactive Power and Voltage Control ............................................................................................ 9
4.1 Lessons Learned ........................................................................................................................... 9
4.1.1 System Planning Study Issues ........................................................................................ 10
4.1.2 Plant Modeling ............................................................................................................... 10
4.1.3 Voltage Control Mode.................................................................................................... 12
4.1.4 Reactive Power Coordination ........................................................................................ 12
4.1.5 Dynamic Voltage Control ............................................................................................... 13
4.2 Best Practices ............................................................................................................................. 14
4.2.1 Reacive Power Delivery to POI....................................................................................... 14
4.2.2 Static and Dynamic Reactive Power Sources ................................................................. 15
4.2.3 Voltage Control Operation ............................................................................................. 16
4.2.4 Plant Response to Voltage Control Commands ............................................................. 16
5 Active Power and Frequency Control ......................................................................................... 17
5.1 Lessons Learned ......................................................................................................................... 17
5.2 Best Practices ............................................................................................................................. 17
6 System Studies .......................................................................................................................... 18
6.1 Lessons Learned ......................................................................................................................... 18
6.1.1 Modeling ........................................................................................................................ 18
6.1.2 Study Scenarios .............................................................................................................. 19
6.2 Best Practices ............................................................................................................................. 19
6.2.1 Modeling ........................................................................................................................ 19
6.2.2 General Studies .............................................................................................................. 19
6.2.3 Voltage control and reactive power requirement studies ............................................ 20
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6.2.4 Operating flexibility studies ........................................................................................... 20
6.2.5 Generation ramp studies ............................................................................................... 21
6.2.6 Other Studies ................................................................................................................. 21
7 Commercial Operations ............................................................................................................. 21
7.1 Lessons Learned ......................................................................................................................... 21
7.2 Best Practices ............................................................................................................................. 22
7.2.1 Test reactive power capabilities .................................................................................... 22
7.2.2 Dynamic Response Tests and Performance Monitoring ............................................... 23
7.2.3 Harmonics Test .............................................................................................................. 24
8 Other Issues .............................................................................................................................. 24
8.1 Harmonics .................................................................................................................................. 24
8.1.1 Harmonics Study ............................................................................................................ 24
8.1.2 Harmonics Test .............................................................................................................. 24
8.1.3 Harmonics Requirements .............................................................................................. 25
8.2 Subsynchronous Resonance and Interactions ........................................................................... 26
9 Conclusions ............................................................................................................................... 27
10 References ................................................................................................................................ 27
Report Contributors ........................................................................................................................ 28
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Executive Summary
Over the last ten years, variable generation penetration in the West Interconnection has increased
steadily. According to Western Electricity Coordinating Council (WECC) 2015 State of the
Interconnection Report published in June 2015, wind capacity has increased by more than 150% to
24.3 GW and solar capacity by 14 times to 8.4 GW. The focus of this white paper will be on wind and
solar generation interconnected to the bulk transmission system. Wind and solar generation
interconnected to the distribution system or other sources of variable generation are beyond the
scope of this paper.
This white paper is intended to serve as a technical reference for WECC members when they plan and
operate their transmission systems with variable generation connected. The lessons learned and best
practices described in this document may pertain to the authors’ systems, not necessarily applicable to
other entities’ systems. It is not intended to be a standard or policy to follow, rather a reference for
establishing company standards or policies.
The main topics covered in this paper include reactive power and voltage control, active power and
frequency control, system studies, and commercial operations.
Reactive Power and Voltage Control
Lessons Learned
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System planning studies did not identify some operational issues that have since been observed
with the variable generation power plants that did not have dynamic voltage control.
Good variable generation power plant models, both power flow and dynamic, are needed to
determine if appropriate transmission reinforcements or additions are needed and to represent
the actual performance during observed system events.
Having variable generation power plants operating in voltage control can increase the amount
of generation allowed at an interconnection.
Reactive power coordination can be challenging between plants that have different voltage
controls.
Dynamic voltage control with an appropriate reactive power range is beneficial for variable
generation power plant owners.
Best Practices
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A variable generation power plant must be designed to deliver the reactive power to the point
of interconnection (POI) over the voltage range.
Reactive power capabilities can include dynamic and static reactive power sources.
A power plant shall always operate in voltage control mode unless otherwise instructed by the
system operator.
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A power plant shall be able to respond to the system operator commands to raise / lower its
voltage schedule.
Active Power and Frequency Control
Lessons Learned
One of the issues was lack of frequency response capability from variable generators. This was
particularly important during over-generation period normally at light load conditions.
Best Practices
WECC Balancing Authorities are at various stages regarding the requirements on frequency response
capability. BPA requires new wind projects of greater than 50 MW to have the capability to respond to
overfrequency and underfrequency (governor type) control and separate ramp rate control. CAISO is
seeking stakeholder input on how best to meet the new requirements.
In general, we think the following recommendations [1] are good for WECC.
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Require curtailment capability, but avoid requirements for excessively fast response. A
10%/second for rate of response to a step command to reduce power output is reasonable.
Require capability to limit rate of increase of power output.
Encourage or mandate reduction of active power in response to high frequencies.
Consider requiring the capability to provide increase of active power for low frequencies. This
presumably would be a rare occurrence, as the economic penalty associated with enabling
these controls is high.
Consider requiring inertial response in the near future.
System Studies
Lessons Learned
In the early days of system studies, most dynamic models for wind and solar interconnection projects
were user-defined models. These models, normally proprietary and distributed as object files,
presented several challenges such as difficulty in model maintenance. Therefore, WECC has led a
comprehensive effort to develop generic dynamic models for wind turbine generators (WTG) and
Inverter based photovoltaic (PV) systems suitable for system analysis.
Best Practices
Wind/Solar power plant owners shall provide their plant models in accordance with WECC Wind/Solar
Power Plant Power Flow Modeling Guidelines, WECC Wind/Solar Power Plant Dynamic Modeling
Guidelines and WECC Data Preparation Manual. The dynamic models shall be WECC-approved models.
Power flow, voltage stability, and transient stability studies shall be performed to evaluate the impact
of wind/solar integration on system performance as required by NERC TPL-001-4 Reliability Standards.
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Other studies such as voltage/reactive power control requirements, operating flexibility, generation
ramping, and electromagnetic transients may be required under certain circumstances.
Commercial Operations
Lessons Learned
Several incidences involving solar/wind generating plant tripping or oscillations occurred due to
inappropriate control schemes.
Best Practices
Plant owners shall perform tests to validate the collective capability and performance for the entire
solar/wind farm. These tests include reactive power capability tests, dynamic response tests, and
harmonics tests
Other Issues
Harmonics and subsynchronous resonance/interactions shall be considered in certain system
conditions.
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Introduction
Twenty-nine states and the District of Columbia have Renewable Portfolio Standards (RPS) policies, and
an additional eight states have nonbinding renewable portfolio goals.1 These state RPS targets range
from 10% to 100% renewable energy integrated into the power grid from 2015 to 2045. Wind and solar
generation are the most common types of variable renewable generation so far.
According to the WECC 2015 State of the Interconnection Report, published in June 2015,2 over the
last decade, wind generation capacity has increased steadily in the Western Interconnection. A similar
trend in solar capacity has occurred over the last five years. In 2014, utility scale solar capacity in the
Western Interconnection was 14 times that in 2010. Figure 2-1 shows the installed variable generation
capacity by year from 2010 to 2014.
1
http://www.eia.gov
2
https://www.wecc.biz/Reliability/2015%20SOTI%20Final.pdf
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Figure 2-1: WECC Renewable Resources Nameplate Capacity by Year
The higher level of penetration of variable renewable generation has brought many challenges in
planning and operations. This has resulted in various efforts from academia to industry. In 2012, North
America Electric Reliability Corporation (NERC) published a report titled “2012 Special Assessment
Interconnection Requirements for Variable Generation” [1], where a number of recommendations
were made regarding NERC interconnection procedures and standards.
This white paper was developed at the request of the Western Electricity Coordinating Council (WECC)
Planning Coordination Committee (PCC) with consideration of the NERC recommendations in the
above report. The purposes of the white paper are to:
1) Identify lessons learned from areas that have high penetration of variable generation; and
2) identify best practices from the owners, planners, and operators of the interconnection,
transmission, and variable generation facilities
The focus of the white paper is on wind and solar generation interconnected to the bulk transmission
system. Other sources of variable generation such as wind and solar generation interconnected to the
distribution system or distributed generation are beyond the scope of this paper.
This white paper is intended to serve as a technical reference for WECC members when they plan and
operate their transmission systems with variable generation connected. The best practices and lessons
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learned in this document may pertain to the authors’ systems, not necessarily applicable to other
entities’ systems. So, use of the language “must” or “shall”, for example, may not be appropriate for all
entities. It is not intended to be a standard or policy to follow. Rather, it is a reference for establishing
company standards or policies.
3
Variable Generation Characteristics
Table 3-1 is a summary of wind and solar PV generation characteristics with respect to system
operating requirements.
Table 3-1: Wind and Solar Generation Characteristics
Resource
Name
Type
Dispatchability
Frequency
Response
Inertia
Voltage
Support
Blackstart
Capability
Wind
1 and 2
Limited
Limited
Some
No
No
3 and 4
Limited
Yes
No
Some
No
PV
Some
Some
No
Some
No
Thermal
Yes
Yes
Yes
Yes
Yes
Solar
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Reactive Power and Voltage Control
4.1 Lessons Learned
Prior to FERC Order 661-A, there was no process for determining dynamic voltage control and
reactive power requirements for variable generation power plants. Early plants were only required
to operate at the unity power factor at the point of interconnection. FERC issued Order 661-A in
2005, requiring a transmission planner to determine the power plant reactive power requirements
on a case-by-case basis. In 2005, the amount of variable generation was small, and most of these
power plants were interconnected at voltages of 115 kV or lower. The reliability implications of the
power plants were mostly limited to a local area. At the time, the majority of wind generation
technologies were induction generators. Induction generators do not have voltage control
capabilities and needed additional investments in dynamic reactive devices such as STATCOMs to
enable dynamic voltage control. FERC Order 661-A was appropriate at the time, allowing
transmission planners to require voltage-control devices based on the system studies.
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4.1.1 System Planning Study Issues
System planning study did not identify some operational issues that have since been observed
with the variable generation power plants that did not have dynamic voltage control.
Some companies have experienced several operational challenges with early wind power plants,
although the system studies did not identify such problems initially. Typical power flow studies
cannot capture the full diversity of the operating conditions. For example; there were occurrences
when wind generation controls did not respond as planned to wind ramps or system voltage
changes. In other instances, wind power plant models were inaccurate in determining wind power
plant reactive needs.
For example, BPA experienced operational issues at a 150-MW wind power plant connected to its
115-kV transmission system. This wind power plant has variable rotor resistance induction
generators, known as type 2 machines. The plant was required to operate at unity power factor at
the POI and was allowed to use switched shunt capacitors for reactive power control. Several
events of dynamic voltage instability were observed at the plant. As system voltage was declining,
the wind power plant was absorbing more reactive power from the transmission system, thereby
further accelerating voltage decline. Such behavior is the exact opposite of voltage control. The
power plant output was eventually curtailed to maintain stable grid voltages. The investigation
concluded that the phenomenon was related to the controls used by type 2 wind turbine
generators and that the dynamic reactive capability was needed at the plant to solve the voltage
stability issues.
In this particular situation, the plant owner installed STATCOMs at the plant to provide dynamic
voltage control. The plant operation has since been stable with all lines in service. However,
another voltage instability event occurred during a line outage condition in June 2011 because the
STATCOMs were sized only for operating conditions with all lines in service. In addition, BPA has
also experienced several operational challenges at its Jones Canyon 230-kV wind generation hub.
The hub included four 100-MW wind power plants, three of which had type 2 wind generators with
no dynamic voltage control, and one had doubly-fed asynchronous generators (DFAG), also known
as type 3 machines. Although type 3 machines are capable of providing dynamic voltage control,
this particular plant was operating in the power factor mode. A voltage instability event occurred
primarily due to lack of dynamic voltage controls.
4.1.2 Plant Modeling
Good variable generation power plant models, both power flow and dynamic, are needed to
determine if appropriate transmission reinforcements or additions are needed and to represent
the actual performance during observed system events.
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BPA conducted “lessons learned” studies for the events described above. Model validation studies
were performed first, and then “what if” scenarios were studied to determine the causes and
solutions to the observed problems. In most cases, the models were unable to reproduce the
observed phenomenon initially, and the wind power plant (WPP) model and parameters needed to
be tuned after the fact.
The development of WPP models, particularly dynamic models, was lagging the deployment of
wind generation in the Western Interconnection. The WECC Modeling and Validation Work Group
approved “Wind Power Plant Power flow Modeling Guidelines” in 2008, making a very compelling
argument for modeling the collector system equivalent and the wind generator pad-mounted
transformer. The representation of the reactive power losses in the collector system is necessary to
correctly size reactive compensation requirements during normal conditions and grid
contingencies. BPA was one of the first adopters of the WECC guidelines by making appropriate
modifications to power flow base cases and requesting wind power plant owners to provide asbuilt plant data. Because of these efforts, the WPPs connected to the BPA transmission system
have adequate power flow models today.
Dynamic models are still under development. While several wind turbine manufacturers have been
actively engaged from the beginning, other manufactures are only now starting to contribute to the
development of generic wind generation models. Unfortunately, many of the operational issues
occurred at the plants with inadequate models. Our attempts to validate plant responses had
mixed results. In our opinion, the generic dynamic models used previously were deficient in
representing WPP responses to frequency deviations in the grid, as well as power oscillations. Since
then, the models have improved.
BPA participated in the project led by DOE’s National Renewable Energy Lab (NREL) and Utility
Wind Integration Group (UWIG) on WPP dynamic model validation. BPA is in the process of
deploying Phasor Measurement Units (PMUs) at WPPs. The PMU data will be used for model
validation and performance monitoring.
Conventional transmission planning studies are done with wind generators at full power output.
The studies generally assume that dispatchers have adequate time to switch reactive devices
(shunt capacitors and reactors) to optimize the voltage profile in the power system. However,
dispatchers may not be able to follow fast wind generation ramps, and the system can end up in a
sub-optimal position. BPA worked with software vendors to develop time-sequence powerflow
capabilities to address this modeling need. Time sequence power flow allows second-by-second
simulation of wind ramps (over periods of tens of minutes), and has been used by BPA for analysis
of wind events and for reactive power coordination at its Jones Canyon wind hub.
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4.1.3 Voltage Control Mode
Having variable generation power plants operating in voltage control can increase the amount of
generation allowed at an interconnection.
A power versus voltage “nose” curve is often used for assessment of voltage stability margins. With
four Jones Canyon plants operating in power factor mode, the voltage stability limit was reached
with 400 MW of wind generation with no dynamic voltage control. BPA increased the amount of
interconnected wind generation to 600 MW in 2011 by integrating two more 100 MW WPPs. The
two new plants had dynamic reactive capabilities and operated in voltage control mode. One plant
with type 2 generators used a STATCOM to provide dynamic reactive capabilities, and another
project had type 3 generators providing dynamic voltage controls. Both projects also used switched
shunt capacitors to extend their reactive capability ranges. The Jones Canyon hub has operated at
600 MW with no voltage stability issues because of the dynamic voltage controls provided by the
new projects.
4.1.4 Reactive Power Coordination
Reactive power coordination can be challenging between plants that have different voltage
controls.
Having different voltage controls represents a technical challenge for the reactive power
coordination among multiple variable generation power plants within a solar or wind hub, primarily
at the sites where there is a mix of projects with and without dynamic voltage controls. This could
result in reactive power circulating among the plants and therefore increase active power losses.
From the voltage stability standpoint, one plant may have already used its dynamic resources to
compensate for reactive losses in another plant. Should a disturbance occur, the former plant will
not be able to provide voltage support. The solution from BPA was to have the variable generation
power plants operate with reactive power output just sufficient to compensate the reactive losses
through the collector system and transformers during normal system conditions, so that the
dynamic reactive reserves can be used during a contingency. Using switchable capacitors is a good
approach for supplying steady state reactive losses, maximizing dynamic reactive capabilities to
respond to transient events. We also recommend that new plants control high side voltage with a
reactive power droop to enable stable reactive power sharing among multiple projects in a site.
Here is an example from PacifiCorp. Prior to 2012, PacifiCorp had been experiencing difficulties
coordinating the voltage control of several closely connected wind plants in eastern Wyoming
power grid. In some cases, these plants shared a common POI or were regulating close-by plants.
Before proper voltage control coordination was implemented, the plant regulators were fighting
among themselves or “hunting”. This situation was a concern when an unacceptable local voltage
performance was observed. In 2012, PacifiCorp also experienced a separate voltage performance
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related problem in the eastern Wyoming wind corridor. PacifiCorp operators witnessed fluctuating
voltages and high voltages within this high wind area when the wind farms were operated in radial
conditions.
The problem was diagnosed as lack of reactive capacity and poor coordination of voltage control.
The solution was 1) to determine appropriate voltage droop settings for the wind plants; 2) to
determine appropriate line drop compensation for the synchronous condenser; and 3) to validate
and refine shunt device passive control logic. The appropriate tuning allowed for reliable and
robust regulation of the 230kV transmission system.
In summary, PacifiCorp
• Installed a 60MVA synchronous condenser at a 230kV substation which was electrically close to
the neighboring wind plants
•
Added 31.7 MVAr shunt reactor and 25 MVAr capacitor in two of those wind farms
•
Retained wind plant voltage droops
•
Applied line drop compensation of one 230kV POI transformer
•
Reset the voltage control set points of the local switched shunt devices to accommodate
the recommended passive coordinated control scheme
•
Implemented condenser switched device coordination during starting between capacitors
and reactors
•
Implemented reactive reserve coordination scheme using capacitors and reactors for
dynamic headroom of the condenser
In the end, the planned transmission upgrades and fine-tuned control strategies improved system
performance and provided cohesive system benefits.
4.1.5 Dynamic Voltage Control
Dynamic voltage control with an appropriate reactive power range is beneficial for variable
generation power plant owners.
We believe that it is also in the best interest of a wind power plant operator to operate under
continuous voltage control. When Automatic Voltage Regulators were first developed for
conventional generators, the generator owners wanted to have them primarily to prevent
generator over-voltages that result from sudden load rejection. The same argument applies to the
wind power plant generators today.
An event of high over-voltages was observed at Jones Canyon wind hub in February 2012. The risk
of high voltages was known and the wind plant output was limited for loss of a line terminal. Taking
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a line out of service for maintenance had left the WPP’s connected by a long transmission line with
a lot of capacitive charging. One plant was not operating in voltage control mode, and other plants
had their reactors undersized. As a result the substation voltages increased to near 260 kV, as the
plant reactive power remained nearly the same. Had the plant been in voltage control mode, as
planned, the over-voltages would have been avoided.
4.2 Best Practices
To properly determine the most appropriate voltage control requirements, the following questions
need to be addressed:
•
•
•
•
•
What reactive power capabilities are needed?
o What is the relationship between the power plant reactive capabilities and system voltage?
o What portion of the reactive capabilities needs to be dynamic and what portion can be
static?
o How to coordinate dynamic and static resources within the plant?
What voltage is regulated – Point of Interconnection, lower voltage buses, or wind/solar
generator terminals – and over what time frame?
How fast the reactive power needs to be deployed?
o How to cycle the reactive resources
How long does the reactive power need to be sustained?
How to verify the voltage control and reactive power capabilities using staged tests and from
performance observation?
4.2.1 Reacive Power Delivery to POI
A variable generation power plant must be designed to deliver the reactive power to the POI over
the voltage range.
The maximum reactive power boost shall be 33% (approximately 0.95 power factor lagging) of plant
maximum active power capability (Pmax), measured at plant POI, being fully delivered at the minimum
operating voltage.
The maximum reactive power buck is 33% (approximately 0.95 power factor leading) of plant
maximum active power capability (Pmax), measured at plant POI, being fully absorbed at the maximum
operating voltage.
The operating voltage range varies from company to company. For instance, one company operates its
system between 530 kV and 550 kV for the 500-kV system. One company operates its system between
236 kV and 242 kV for the 230-kV system. Another company uses 117 – 120 kV as the operating range
for its 115-kV system.
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Reactive power flows from higher voltage to lower voltage (on per unit basis, adjusted for transformer
tap ratios). Because of distributed nature of wind and solar power plants, there is a concern of voltage
rise or drop through the collector system as the wind turbine generators deliver or absorb reactive
power. Transformer taps need to be selected to ensure acceptable voltage profile and deliverability of
reactive power for design conditions described above.
These requirements are consistent with FERC LGIA-2003 Section 9.6.1.
4.2.2 Static and Dynamic Reactive Power Sources
Reactive power capabilities can include dynamic and static reactive power sources.
•
•
•
•
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If dynamic reactive power is provided by the wind/solar generator converters (vast majority
of cases today):
o the converter capabilities must be sized to provide at least 0.95 power factor, or +/–
33% of generator active power capability at the POI
o additional shunt capacitors may be required to meet the design reactive power range
requirements at the POI defined in section 2.2.1
If dynamic reactive power is provided by supplementary reactive devices, such as STATCOM
or SVC:
o STATCOM / SVC must be sized to provide reactive power of +/– 33% of the plant
maximum active power capability at the POI
o short-term overload capabilities of STATCOMs can be counted if fast switching of shunt
capacitors is used
o mechanically switched shunts must be sized to compensate for reactive power losses in
the generator terminal step-up transformers, collector system and the main step up
transformer at full power output
Switched reactive devices must be sized in steps less than 33% of the dynamic reactive
capabilities in either boost or buck directions
o switched capacitors need to include fast discharge capabilities, so they can be reinserted
in seconds
A power plant shall have controls to optimize coordination of dynamic and static reactive
resources to maximize the availability of dynamic reactive capabilities
If a power plant is limited in providing reactive power at a rate of change needed to the POI
(similar to synchronous machines), and the POI system has characteristics of fast voltage
collapse for contingencies or needs dynamic reactive power to help dampen
transients/oscillations, the developer shall add one or more dynamic reactive power control
devices such as STATCOM or SVC to supplement voltage control.
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4.2.3 Voltage Control Operation
A power plant shall always operate in voltage control mode unless otherwise instructed by the
system operator.
•
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•
•
Continuous voltage control must be enabled when a wind/solar power plant active power
output is between 15%-100% of the total plant capacity, and can be turned off or operated
in power factor control mode when the power plant output drops below 10%. The plant can
be set to either voltage control mode or power factor mode when its output is between
10% and 15% of the total plant capacity.
Reactive power droop shall be in 5% to 12% range
o Reactive power droop is measured as change in POI voltage over change in plant
reactive power delivered to the point of interconnection. Change in voltage is in per unit
on nominal voltage. Change in reactive power is in per unit on power plant maximum
active power of the plant.
o The requirement can be met by either:
- Regulating POI voltage with reactive power droop.
- Regulating the lower class voltage with reactive line drop.
Voltage control must be continuous, use of voltage control dead-band is not allowed
Voltage control must be sustained, use of slow reactive power reset function is not allowed
Voltage control response time needs to be optimized given the capabilities of wind/solar
generation technologies and coordination with other wind/solar power plants in the area.
While fast voltage response is desirable, we need to make sure that such fast response does
not result in oscillations or hunting within a power plant or between power plants within a
solar/wind hub.
4.2.4 Plant Response to Voltage Control Commands
A power plant shall be able to respond to the system operator commands to raise / lower its voltage
schedule.
•
•
W
The system operator commands are sent as pulses, one second voltage pulse corresponding
to voltage step of 0.1%.
The full response to voltage raise / lower command shall be achieved within 30 seconds or
less. Many power plants with current technologies can provide full response within 10
seconds.
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Active Power and Frequency Control
5.1 Lessons Learned
On April 12, 2014 in HE 13, there were over-generation conditions due to excessive wind and solar
generation in California Independent System Operator’s (CAISO) Balancing Authority Area (BAA).
CAISO issued an Over-generation Notice at 12:00 pm on that day. Over-generation occurs when
there is more internal generation and imports into a Balancing Area than load and exports. This
normally occurs at light load conditions with high variable generation production. Before an overgeneration event occurs, the system operator would exhaust all efforts to send dispatchable
resources to their minimum operating levels and will have used all the decremental energy (DEC)
bids available in the imbalance energy market. Consequently, regulation down could not be
procured in one fifteen-minute interval because it would require generators to move up to have
headroom first to provide the regulation down and this additional generation would exacerbate
over-generation conditions.
CAISO included an assessment of frequency response during over-generation conditions in the
annual transmission planning process. The purpose of the study was to evaluate potential overgeneration within the CAISO BAA and its potential consequences. The study results of the 20142015 Transmission Planning Process indicated acceptable frequency response within WECC.
Frequency response from WECC was above WECC Frequency Response Obligation, and the
frequency nadir and settling frequency were acceptable. However, with high amount of renewable
generation in the CAISO that doesn’t respond to frequency disturbances, response from CAISO was
below its Frequency Response Obligation. Compared to the actual system performance during
disturbances, the study results were optimistic since the actual frequency responses for the same
disturbances were lower than the study indicated. It was found that this was caused by large
headroom of the generation and inaccuracy of the dynamic models. Consequently, a few next steps
have been identified such as conducting further model validation and exploring governor response
from other sources.
One of the means of improving the system frequency response is to require new inverter-based
generation to provide frequency response. The CAISO studies showed that if inverter-based units
are providing frequency response, it may solve the deficiency observed in the studies. Modern
inverters are capable of providing frequency response; however, inverters need to be oversized
and there should be some margin in how inverter-based generation is dispatched.
5.2 Best Practices
For primary frequency response, the CAISO currently does not have a procurement target. With NERC
Reliability Standard BAL-003-1 effective on December 1, 2016, the CAISO is seeking stakeholder input
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on how best to meet the new requirement and on the frequency response capabilities of emerging
technologies such as wind, solar, and energy storage devices. Other WECC BAs are also at various
stages regarding the requirements on active power and frequency control.
BPA requires new wind projects (starting construction after 6/1/2011) greater than 50 MW to have the
capability to respond to overfrequency and underfrequency (governor type) control and separate ramp
rate control. BPA will develop policies to address older projects when the need is determined.
Outside of WECC, ERCOT requires wind or solar generators to provide primary frequency response.
In general, we think the following NERC recommendations [1] are still valid for WECC.
1) Require curtailment capability, but avoid requirements for excessively fast response. A
10%/second for rate of response to a step command to reduce power output is reasonable.
2) Require capability to limit rate of increase of power output.
3) Encourage or mandate reduction of active power in response to high frequencies.
4) Consider requiring the capability to provide increase of active power for low frequencies. This
presumably would be a rare occurrence, as the economic penalty associated with enabling
these controls is high.
5) Consider requiring inertial response in the near future.
6
System Studies
6.1 Lessons Learned
6.1.1 Modeling
In the early days of system studies, most dynamic models for wind and solar interconnection projects
were user defined models developed by manufactures and software vendors. These models were
proprietary and generally distributed as object files. Use of these models in system studies in large
interconnected systems presented several challenges [4]:
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Difficult to certify quality assurance: With models developed by individual users and companies,
it is difficult to investigate whether the model development and resulting code meets a
satisfactory level of quality assurance.
Difficult to gain insight on model behavior: Beyond analyzing model behavior through
simulation results, it is sometimes necessary to follow the source code. Object files are the
outcome of the original source code being compiled, and are not viewable by users —
becoming a “black box” with contents hidden from the user. Thus, it is a challenge for model
users to better understand the model behavior.
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Difficult to maintain: To comply with multiple versions of different software, it is necessary to
maintain numerous versions of the same model, and model maintenance procedures become
demanding.
To resolve these issues, WECC has led a comprehensive effort to develop generic dynamic models for
WTGs and Inverter-based PV systems suitable for system analysis.
The generic models satisfy the following requirements:
•
•
•
•
•
•
Generic models are tunable to parameters to represent different WTGs or solar PV generators.
They are nonproprietary (model and data can be shared).
They form a standard library (not user-written).
They are cross-platform compatible.
They are fully documented block diagrams and descriptions, and default parameter sets.
They are validated against field tests, factory tests, or against electromagnetic transient models
that have themselves been validated against test data.
6.1.2 Study Scenarios
For any wind/solar power plant to be interconnected to the system, if it is anticipated that the
interconnection will have regional impact, major generators and transmission paths shall be evaluated
in the study for the most stressed system operating conditions.
6.2 Best Practices
The following sets of studies shall be performed to determine the plan of service for a wind/solar
power plant interconnection.
6.2.1 Modeling
Wind/Solar power plant owners shall provide wind/solar power plant models in accordance with WECC
Wind/Solar Power Plant Power Flow Modeling Guidelines, WECC Wind/Solar Power Plant Dynamic
Modeling Guidelines and WECC Data Preparation Manual. During the generation interconnection
stage, wind/solar power plant owners shall coordinate with their vendors to ensure that the dynamic
models are WECC-approved models, not user-defined models.
6.2.2 General Studies
Power flow, voltage stability, and transient stability studies shall be performed to evaluate the impact
of wind/solar integration on system performance as required by NERC TPL-001-4 Reliability Standards
[7].
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With large amounts of solar/wind generation, Interconnection-wide system impact studies are
necessary to evaluate their impact on Interconnection-wide stability issues, such as:



Interconnection frequency response, a study completed by General Electric and NREL [8]
Frequency, damping, mode shapes of the inter-area modes of power oscillations
Impact on System Operating Limits of voltage-limited and transient stability-limited paths
The most stressed system conditions should be covered by taking load and generation dispatch of the
study area into consideration. For example, one company uses the following scenarios:
•
•
•
On-peak summer case
Off-peak spring case with high-wind high hydro scenario
On-peak winter case
6.2.3 Voltage control and reactive power requirement studies
Usually voltage and reactive power controls at the POI are important interconnection requirements.
Therefore, studies shall be conducted to evaluate if the wind/solar power plant is effective for voltage
and reactive power control. For instance, some entities require that the wind/solar power plant have a
reactive power compensation scheme, 1) sized to provide/control between a net 0.98 power factor
bucking or leading and a net 0.95 power factor boosting or lagging at maximum generation output at
the POI; and 2) the switching/control of the reactive power done in small enough increments to limit
the change in reactive power production or absorption in steady state to steps of no more than 10% of
the generated power. In this case, time series power flow can be used to verify the effectiveness of the
voltage control scheme and reactive power device installation in the wind power plant.
Transient stability studies shall be done to verify if the dynamic reactive power control devices are
effective enough to eliminate fast transient voltage excursions. The simulation time shall be long
enough to verify the effectiveness of static reactive power control devices as well.
6.2.4 Operating flexibility studies
Assessment shall be performed for wind/solar power plant performance and voltage control
coordination under outage conditions. The studies will be done for a scenario when multiple lines can
be taken out of service for maintenance. Studies will identify whether any operational restrictions need
to be applied to the plant and corresponding system conditions and outages. Examples of operational
restrictions include MW curtailment and MVAr control change. The studies shall be able to identify the
required control change and its effectiveness.
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6.2.5 Generation ramp studies
As applicable, wind/solar generation ramps shall be simulated using time-sequence power flow to
assess the impact of wind/solar generation variability on the load customers in the area, specifically
voltage fluctuations and Load Tap Changer operations. Some utilities require that the wind/solar
power plant must be capable of controlling power generation to be compliant with the system
conditions. For instance, the production ramp-up limit, determined as a one-minute average value, or
specified in terms of megawatts-per-minute, must not at any time exceed five percent (5%) per minute
of the maximum power of the wind power plant and production control must be capable of reducing
output by at least fifty percent (50%) of then-current power production in less than two (2) minutes.
These rules should be considered in setting up the time-sequence power flow for ramp studies.
6.2.6 Other Studies
A wind/solar power plant may be located in a remote area and connected to system through a long
radial line, or there may be some possibilities for a wind/solar power plant to connect and operate in
this mode due to outages. Under these scenarios, in addition to conventional TPL studies, additional
studies shall be conducted to examine potential overvoltage when the line is energized to connect the
power plant. This phenomenon is called Ferranti Effect. Due to low short circuit capacity and weak
system nature of this scenario, the inrush current resulting from energizing transformers among the
radial line could be troublesome for local customers and devices. Transient studies via ElectroMagnetic
Transients Program (EMTP)-type tools can be used to evaluate potential hazards and mitigation actions
required.
Contingencies may cause the wind/solar plant to be islanded with potential transient over-voltages.
Fast and coordinated direct transfer trips may be required to mitigate the impact of subject
contingencies.
7
Commercial Operations
7.1 Lessons Learned
In 2014, SCE noticed a 7 Hz oscillation during the noon period. The investigation indicated that one PV
solar output was oscillating at the peak generating time of roughly 11 am to 3 pm, causing this 7 Hz
oscillation. As a result, a modification was made to the inverter control and the oscillation went away.
Another event in 2014, a three-phase fault at PG&E Midway substation with normal clearing caused
approximately 261 MW of solar generation to be tripped in the SCE system. The solar generators
seemed to not have the low-voltage ride through capabilities as required by the Interconnection
Agreement. Further investigation of one of the plants discovered that the frequency calculation
scheme of the inverter was not accurate, resulting in a false trip.
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BPA observed a high frequency of 14 Hz power oscillations at one of its wind power plants when the
plant output exceeded 90% of its capacity. Reactive power oscillations were 80 MVAr peak-to-peak at
the 425-MW plant. The oscillation was detected by real-time applications that utilize PMU technology.
The plant operator subsequently corrected controls to avoid future oscillations.
Wind farms equipped with a single voltage / var controller to regulate the POI voltage may be at risk of
causing unacceptable voltages should the controller fail, particularly if the wind farm is connected to a
weak system and supported with mechanically switched shunt var devices under such controls.
Wind farm designs and control schemes vary depending on the type of turbines and the
interconnecting system. Well prepared commissioning test procedures and documentation would
demonstrate the wind generating facility meets the intended capability and performance requirement
such as var capability (dynamic and static) and the plant control schemes.
7.2 Best Practices
Wind/solar plant operators shall perform the following tests no later than 180 days from the beginning
of the commercial operation to certify the WPP voltage control and reactive power capabilities.
Wind/solar plant operators shall provide “as-build” model for the power plant, according to WECC
Wind Power Plant Powerflow Modeling Guidelines and WECC Wind Power Plant Dynamic Modeling
Guidelines. The model is the statement of the power plant dynamic performance and voltage control
capabilities.
Wind farm plant and voltage control should take the impact of a single element failure (N-1
contingency) into consideration.
Plant owners are to have well prepared commissioning test procedures for review and acceptance by
the interconnected utility to validate the collective capability and performance for the entire wind
farm.
7.2.1 Test reactive power capabilities
This test can be used for compliance with NERC MOD-025 Reactive Capability Verification Reliability
Standard.
Demonstrate that the plant can sustain its reactive power boost and buck capabilities for more than 15
minutes at a power output greater than 90%.
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The test can be done by raising the WPP voltage controller reference until reactive capability
limits are reached. The test is suspended should another operational limitation reached first
(e.g., due to high voltages), the limitation must be documented. The test needs to be
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coordinated with the grid operator to ensure that there are no major outages in the area and to
provide a voltage profile favorable for the test.
The test shall record active and reactive power during the test, voltages at point of
interconnection and the lower voltage class bus, status of shunt reactors and capacitors, and
reactive power output from STATCOM if applicable
When an interconnection customer’s plant is required to meet certain power factor criteria at the POI
such as between 95% bucking or leading to 95% boosting or lagging, start-up tests must be conducted
beginning at 25% (if possible), 50%, 75%, and 100% of rated generator megawatt or real power load.
This is to demonstrate that the switching and control of the reactive power can be done in small
enough increments to limit the change in reactive power production or absorption in steady state of no
more than 10% of the generated power. The capacity tests at the leading mode may be limited
because of operational limitations due to manufacturer’s design criteria or stator end iron heating
concerns.
7.2.2 Dynamic Response Tests and Performance Monitoring
This test can be used for compliance with NERC MOD-026 Generator Excitation Verification Reliability
Standard.
Provide evidence that the response time is within the required range and is consistent with the
provided model. The response can be validated for:
•
•
•
changes in system voltages
steps in the WPP voltage reference
shunt capacitor switching within a plant
The tests shall provide the time recordings sampled at 20 times per second or faster for the following
quantities:
•
•
•
•
•
•
POI voltage
Medium voltage bus voltage
Controller voltage reference
Plant active and reactive power at the POI or voltage control point
Total dynamic reactive power provided by the plant including that by the installed devices such
as STATCOMs
Status of mechanically switched shunt capacitors and reactors
Dynamic test recordings shall be compared with those simulated via wind/solar power plant models.
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PMUs or other monitoring devices with similar capabilities are to be used for dynamic performance
monitoring. Details of PMU installation are described in [9]. PMU-based model validation can be used
for compliance with NERC MOD-05, -026 and -027 Reliability Standards.
7.2.3 Harmonics Test
Harmonics test shall be done during a wind/solar power plant start-up. Refer to Section 6.1 Harmonics
for details.
8
Other Issues
8.1 Harmonics
Harmonics is usually due to non-linear devices such as power electronic-based converters, which are
used in wind/solar power plants. Harmonics can cause various problems including equipment
overheating and interference with sensitive load operation and communication systems. The amount
of harmonics in a system is defined by the total harmonic distortion (THD).
8.1.1 Harmonics Study
It is common that a wind/solar farm connect wind/solar generators through underground medium
voltage power cables. Therefore, the cable charging current and the capacitors existing at the terminal
of a wind/solar generator result in certain amount of charging reactive power at a wind/solar power
plant. When system or plant operating conditions change, such as shutdown of some wind generators,
the charging reactive power could potentially match inductive reactive power and cause paralleled
harmonic resonance, which could damage the devices in the wind/solar farm. When this risk exists,
harmonics scan study using EMTP type tools shall be done under various operating conditions such as
with different numbers of online/offline generators and/or energized/de-energized power cable
collectors.
8.1.2 Harmonics Test
Harmonics test shall be done during a wind/solar power plant start-up.
The start-up wind/solar power plant harmonics test shall be done before the wind/solar power plant
officially operates. Voltage and current harmonics from the generator shall also be measured and must
fall within the required ranges. A power quality analyzer (provided by the Interconnection Customer)
shall be used to monitor all three-phase currents, three-bus voltages, neutral current or generator
neutral current, and an auxiliary contact from the Interconnection Customer’s generator breaker and
also line breaker(s). The analyzer will have a minimum sample rate of 167 microseconds (128 points
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per cycle). The analyzer shall monitor the pre-breaker close conditions, the breaker closing, and the
post-close conditions of the system.
8.1.3 Harmonics Requirements
IEEE Standard 519 shall be the minimum requirement for Interconnection Customer’s Generating
Facility plant to interconnect with the system. A Harmonics test should ensure voltage distortion and
current distortion will meet the limits recommended in IEEE Standard 519.
The Reference of Voltage Distortion Limits and Current Distortion Limits from IEEE Standard 519:
Table 8-1: Voltage distortion limits
Bus voltage V at PCC
V ≤ 1.0 kV
1 kV < V ≤ 69 kV
Individual
harmonic (%)
5.0
3.0
Total harmonic
distortion (THD) (%)
8.0
5.0
1.5
1.0
2.5
1.5a
69 kV < V ≤ 161 kV
161 kV < V
High-voltage systems can have up to 2.0% THD where the cause is an HVDC terminal whose
effects will have attenuated at points in the network where future users may be connected.
Table 8-2: Current distortion limits for systems rated 120 V through 69 kV
Maximum harmonic current distortion in percent of IL
Individual harmonic order (odd harmonics)a, b
ISC/IL 3 ≤ h
4.0
< 20c <11
20 < 50
7.0
50 < 10
10.0
0
100 < 10 12.0
00> 1000
15.0
11≤ h <
172.0
3.5
4.5
5.5
7.0
17 ≤ h <
23 1.5
2.5
4.0
5.0
6.0
23 ≤ h <
35 0.6
1.0
1.5
2.0
2.5
35 ≤ h ≤ 500.3
0.5
0.7
1.0
1.4
TDD
5.0
8.0
12.0
15.0
20.0
Common footnotes for Tables 8-2, 8-3, and 8-4:
a Even harmonics are limited to 25% of the odd harmonic limits above.
b Current distortions that result in a DC offset, e.g., half-wave converters, are not allowed.
c All power generation equipment is limited to these values of current distortion, regardless of
actual Isc/IL.where
Isc = maximum short-circuit current at PCC
IL = maximum demand load current (fundamental frequency component) at the PCC under
normal load operating conditions
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Table 8-3: Current distortion limits for systems rated above 69 kV through 161 kV
Maximum harmonic current distortion in percent of IL
Individual harmonic order (odd harmonics) a, b
Isc/IL 3≤ h
11≤ h < 17≤ h < 23 ≤ h < 35≤ h ≤
TDD
<11
17
23
35
50
2.0
1.0
0.75
0.3
0.15
2.5
< 20c
0.3
20 < 50
3.5
1.7
1.25
0.5
0.25
4.0
5
50 < 10
5.0
2.25
2.
0.7
0.35
6.0
0.5
0
0
5
100 < 10
6.0
2.7
2.5
1.0
0.5
7.5
0.75
00> 1000
5
7.5
3.5
3.
1.2
0.7
10.0
1.0
0
5
1.25
Table 8-4: Current distortion limits for systems rated > 161 kV
Maximum harmonic current distortion in percent of IL
Isc/IL
< 25c
25 < 50
≥ 50
Individual harmonic order (odd harmonics)a, b
3 ≤ h
11 ≤ h <
17 ≤ h <
23 ≤ h <
35 ≤ h ≤ < 11
17
23
35
50
1.0
0.5
0.38
0.15
0.1
2.0
3.0
1.0
1.5
0.75
1.15
0.3
0.45
0.15
0.22
TDD
1.5
2.5
3.75
8.2 Subsynchronous Resonance and Interactions
Subsynchronous Resonance (SSR) [5] is an electric power system condition where the electric network
exchanges energy with a turbine generator at one or more of the natural frequencies of the combined
system below the synchronous frequency of the system. A widely known SSR incident occurred at
Mohave Power Plant in the 1970s. SSR involves interaction between mechanical/torsional masses of a
generator and a series capacitor. It is a well-understood phenomenon.
Subsynchronous Interactions (SSI) include Subsynchronous Torsional Interactions (SSTI) and
Subsynchronous Control Interaction (SSCI). SSTI involves interactions between mechanical/torsional
masses of a generator and a power electronic device such as a wind turbine. SSCI refers to interactions
between a power electronic device such as a wind turbine and a series compensated system.
Although we have not experienced SSI issues in WECC, the incidence of SSI in 2009 at ERCOT also
brought our attentions to this subject. Since then, a number of WECC members have done SSI studies
for wind/solar power plants. So far, we have not found any issues in those studies.
To minimize the risks for SSI, we recommend the following best practices [1]:
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Modify controls of wind/solar turbine converter. This approach has already been demonstrated
and proven at some wind plants.
Avoid known grid configurations that cause subsynchronous interactions. This could involve
transfer-tripping a wind/solar plant or bypassing a series capacitor if certain grid events occur.
Add some damping in network for subsynchronous currents. This is most effective if installed at
the series capacitor, but it could also be installed at a wind/solar plant.
Conclusions
We have summarized lessons learned and best practices based on experience from the WECC
members regarding variable generation interconnection to the Bulk-Transmission System. Note that
many of the issues are evolving as more variable generation is integrated into the existing generation
mix, and new technologies and methodologies emerge. Our focus has been on planning and operations
that reflect the latest developments and trends in the industry, particularly WECC.
10 References
[1] North America Electric Reliability Council, 2012 Special Assessment Interconnection Requirements
for Variable Generation, September 2012
[2] WECC Wind Power Plant Power Flow Modeling Guidelines
[3] WECC Wind Power Plant Dynamic Modeling Guidelines
[4] WECC Variable Generation Planning Reference Book, May 14, 2013
[5] IEEE SSR Working Group, Proposed Terms and Definitions for Subsynchronous Resonance, IEEE
Symposium on Countermeasures for Subsynchronous Resonance, IEEE Pub. 81TH0086-9-PWR,
1981, pp92-97.
[6] CAISO Frequency Response Issue Paper, August 7, 2015.
[7] NERC TPL-001-4 Reliability Standard, http://www.nerc.com/files/TPL-001-4.pdf
[8] Western Wind and Solar Integration Study Phase 3 – Frequency Response and Transient Stability,
NREL and GE, http://www.nrel.gov/docs/fy15osti/62906-ES.pdf
[9] Technical Requirements for Interconnection to the BPA Transmission Grid, page 32,
https://www.bpa.gov/transmission/Doing%20Business/Interconnection/Documents/tech_require
ments_interconnection.pdf
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Report Contributors
WECC Variable Generation Interconnection Task Force (VGITF) Chair
George Zhou
WECC Support Staff
Nathan Powell
Contributors
Dmitry Kosterev, Ann Finley, Shengli Huang, Steven Pai, Song Wang, David Tovar, Irina Green, Jun Wen,
Aldridge Madeleine, David Wang, Robert Easton, Eric Heredia, John Anasis, Jonathan Trejo, Roberto
Favela, George Zhou
Reviewers
This report was reviewed by the WECC VGITF members and the WECC Planning Coordination
Committee (PCC) members.
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