4/18/2021
SHERMAN CHAN
ASPEN
C op yr i g h t © 2 02 1 Ad va n ced
syst em s
for
P ower
E n g in eer ing, I n c.
Sherman Chan, PhD, IEEE Fellow
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MODELING WIND/SOLAR PLANTS
A HISTORY
The current-limited generator model was implemented in
2010, based on an IEEE PSRC report entitled “Fault
Contribution from Wind Farms”.
• We learned that the current is limited to 1.5 times fullload current.
• There is no zero-sequence fault current.
• That was about it.
CURRENT LIMITED GENERATOR MODEL
CIRCA 2010
This model was built “on top” of the synchronous machine
model.
• A limit on the max phase current is set by the user.
• For 3-phase faults OneLiner limits the current by
manipulating the voltage source in the positive
sequence.
• For an unbalanced faults, OneLiner, in addition, inserts a
voltage source in the negative sequence to limit current.
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MODELS FOR VOLTAGE-SOURCE CONVERTER
(VSC) DC-LINE TERMINALS -- CIRCA 2016
Two utilities asked us to model the terminals of a VSC dc line in
OneLiner.
• They connected us with an engineer at ABB office in Austria
who teach us how to do it.
• The engineer suggested that we model each terminal of the dc
line as a perfect current source.
• He gave us a table of current magnitudes and power-factor
angles at different terminal voltages.
• From that we developed the voltage-controlled current source
(VCCS) model in OneLiner.
IEEE PSRC WORKING GROUP C24
STARTED IN 2017
IEEE PSRC convened a working group (C24) to explore
methods for simulating solar, type-4 wind plants and
battery systems in commercial short circuit programs.
Commercial short-circuit program vendors were well
represented in this working group.
EPRI was represented by Dr. Evangelos Farantotas. By
2017, EPRI had already done extensive research in the
modeling of these devices.
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VCCS MODEL FOR CONVERTER-INTERFACED
DEVICES
From Working Group C24, I learned that all converterinterfaced devices are built using VSC technology!
ASPEN took EPRI’s control logic and populate the VCCS
table and see if that can be used in OneLiner to simulate
type-4 wind plants and solar plants, and it did.
We were able to benchmark the output of VCCS against
the EPRI’s results for a sample network.
CONFIRMATION OF EPRI’S CONTROL STRATEGIES
Members of C24 working group concluded that EPRI’
control strategies were in line with details provide to the
working group by Siemens and SMA, two of the world’s
largest converter manufacturers.
Also, with EPRI’s control strategies, we at ASPEN were able
to reproduced the current magnitudes and power-factor
angles similar to those provided to us by ABB earlier for
the VSC dc terminals.
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FAST FORWARD TO THE SPRING OF 2021
Since 2018, we have been recommending OneLiner users
to:
1) Use the current limited generator model to model type3 wind generators.
2) Use the voltage-controlled current source (VCCS) to
model type-4 wind plants, solar plants and battery
systems.
NEW MODELS IN ONELINER V15, TO BE RELEASED IN
MAY 2021
• A new model for type-3 wind plants.
• A new model for converter-interface resources.
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DIALOG BOXES OF TYPE-3 WIND PLANT MODEL
NEW TYPE-3 WIND PLANT MODEL
The model is based on research at EPRI and at EPRI-funded
research at Polytechnique Montreal. This model has:
1) induction machine’s electrical parameters,
2) the logic and limiters of the back-to-back converters that
feed power back from the machine terminal to the rotor
of the induction machine, and
3) the torque characteristics of the wind-turbine blade as a
function of the slip.
Note: EPRI has benchmarked this phasor-based model
against results of time-domain simulations.
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TYPE-3 WIND PLANT MODEL
CROWBARRED STATE
Crowbarred is a transient state that cannot be initiated in
a short circuit program. But for substation design, it
has to be simulated.
To crowbar a wind generator, simply mark the
“crowbarred” checkbox in the Type-3 wind generator’s
info dialog box. Thereafter the generator is modeled
as a conventional induction machine.
DIALOG BOXES OF CONVERTER-INTERFACED
RESOURCE MODEL
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NEW CONVERTER-INTERFACED RESOURCE MODEL
FOR TYPE-4 WIND PLANTS, SOLAR PLANTS AND
BATTERY SYSTEMS
This model uses the same EPRI equations with which we
tested the VCCS model a few years ago in C24.
In addition, we added the logic to inject negative-sequence
reactive current to conform to the latest German grid
code.
You no longer have to enter the current and the powerfactor angles into a table. OneLiner computes those
values internally.
FOR BOTH MODELS, THE PREFAULT VOLTAGE MUST
BE EITHER ‘START FROM NETWORK SOLUTION’ OR
‘FROM POWER FLOW SOLUTION’
You cannot use the ‘Assumed Flat’ prefault voltage option
any more if you want to simulate these new devices.
For a short time we let users of OneLiner v14 to use the
“Assumed Flat” option for simulating VCCS. That did
not work out well.
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MIGRATION PATHS FOR WIND PLANTS
If you are using the current-limited generator model to
simulate wind plants, we suggest you switch to the new
Type-3 Wind Plant model.
MIGRATION PATH FOR TYPE-4 WIND PLANTS,
SOLAR PLANTS AND BATTERY SYSTEMS
Regardless how you model these plants now, we suggest
you change them to the new converter-interfaced
resource model in v15.
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AGGREGATE, AGGREGATE, AGGREGATE
Whenever possible, aggregate units in a “farm” or units
that are within close proximity to each other.
The new models ask you to enter the ‘Number of units’,
‘Unit MVA rating’, and the ‘Unit MW output’, and so on.
Aggregation will result in speedier convergence, faster
simulation speed, and more realistic results.
IS THE VOLTAGE-CONTROLLED CURRENT SOURCE
MODEL GOOD FOR ANYTHING ANY MORE?
The VCCS model is still in v15. In the new Appendix
J in the v15 user’s manual we show how VCCS
can be used to model:
1) STATCOMs and
2) Old solar plants that inject the same current
regardless of the terminal voltage.
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IS THE CURRENT-LIMITED GENERATOR MODEL GOOD
FOR ANYTHING ANY MORE?
NO. Please do not use it any more.
We are keeping this model around for backward
compatibility only.
COMMONLY ASKED QUESTIONS
AND ANSWERS
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QUESTIONS AND ANSWERS
Q: After adding a VCCS to model a new solar plant, I notice
that the fault currents in the vicinity of the new plant
went down, instead of up. Am I doing something
wrong?
A: No. This is a common occurance. (The same thing
would happen if you model your new plant with the new
converter-interfaced resource model.) There are several
reasons why:
REASONS
1) VCCS and Converter-Interfaced Resources (CIR) are
both ideal current sources with infinite internal
impedance. As a result the new device you introduced
has no effect on the Thevenin impedance.
2) The current injected by VCCS and CIR is on the order of
one time the full-load current.
3) The current injected is all reactive (VARS) when the
terminal voltage is 0.5 pu or lower. In other words,
your device is acting like a STATCOM.
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QUESTIONS AND ANSWERS
Q: How do I know in v15 whether the new wind/solar and
type-3 wind plants converge or not?
A: You can find this out by looking at the TTY window after
OneLiner solves a short circuit.
QUESTIONS AND ANSWERS
Q: What are the common causes of convergence problems
for Converter-Interfaced Devices?
A: Here are the most common causes:
•
The transformer that connects the CIR to the utility has
an MVA rating that is too low.
Note: Convergence problems with type-3 wind plants are rare.
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OTHER CAUSES OF NON-CONVERGENCE
• Multiple CIRs are connected to each other through
switches and/or very-low-impedance lines. You can
avoid this problem by aggregating them.
• A CIR is connected directly to the network without a
wye-delta or wye-wye-delta transformer. Please make
sure a wye winding is on the side of the CIR.
QUESTIONS AND ANSWERS
Q: There are no loads in the network model. If all of the solar
and wind generators are generating full load currents in
prefault, where are those currents going to?
A: The currents are going to ground through any positivesequence loads and shunts. If there are no loads or shunts,
they will go through the synchronous machines’ internal
impedances.
To avoid this issue, we suggest you set the Unit MW
Generation to zero for both solar and wind models.
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SET MW GENERATION TO ZERO
Setting the MW generation to zero is consistent with the way
conventional synchronous machines are modeled in a
short circuit program.
If you must model the prefault current of wind and solar
generators, we suggest you add the loads and use the
‘start from power-flow solution’ prefault-voltage option.
Note: To do this you will need to have a license for the ASPEN Power
Flow Program, plus you will have deal with the issue of power-flow
convergence.
THE END
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