CURRENT-LIMITED GENERATORS

The current-limited generator was the first model we implemented around 2010 for modeling type-3 and type-4 wind turbines and solar generators in short circuit studies. 

At that time, megawatt-size wind and solar plants were new in the utility industry. Our best information came from a report entitled “Fault Contributions from Wind Farms” from a working group in the IEEE Power System Relaying & Control Committee.  We learned that the controlled output of these plants was limited to no more 1.5 times the full load current.  This limited information was the basis for the current-limited generator model in OneLiner.  We did not have much to go on.

For current-limited generators, we added two edit boxes for ‘current limit A’ and ‘current limit B’ in the dialog box for generators. Before running the short circuit program, the user has to tell OneLiner which of these two limits is in effect.  (A generator with a current limit of zero means its current is not limited.)

The conventional generator model consists of a voltage source behind an impedance in the positive sequence, and an impedance in the negative sequence.  (The zero-sequence impedance of wind and solar plants, battery systems is infinite.)  The iterative solution scheme for current-limited generators was built “on top of” this model. For a 3-phase faults, the current-limited-generator logic adjusts the magnitude of the internal positive-sequence voltage source until all the phase currents fall within the limit. The solution logic is more complicated for unbalanced faults that involve both the positive sequence and the negative sequence.  In addition to the variable voltage source in the positive sequence, OneLiner inserts a variable voltage source behind the machine’s negative-sequence impedance to adjust the negative-sequence current.

Since that time, we have developed more realistic models for wind and solar generators, battery system and STATCOM devices.  Please read on.

VOLTAGE CONTROLLED CURRENT SOURCES AND BEYOND

We developed the voltage-controlled current source model (VCCS) in 2016 at the request of two large electric utilities that were contemplating the installation of a dc line that utilizes a technology called voltage source converter (VSC) . As per guidance of a major European converter manufacturer, the VSC inverter and rectifier were modeled as perfect current sources during a fault.  The output was made to follows a table of current magnitudes and power-factor angles at various terminal voltage. With that information, we created the Voltage-Controller Current Source model.

A year later, we came to the realization that VSC converter model we developed can be used as the major building block of a model for converter-interfaced resources such as type-4 wind plants, solar plants and battery systems (BESS).  Using equations developed by EPRI, we were able to populate the table for the voltage controlled current source.

From 2018 through the spring of 2021, our recommendations to OneLiner users were to:

1.  Use the VCCS object to model converter-interfaced resources. 

2.  Use the Current-Limited Generator model to model type-3 wind plants.

In the year 2020, a grid code from Germany mandated the injection of negative-sequence reactive current from converter-interfaced resources for unbalance faults (in addition to the positive-sequence reactive current).  Many relay engineers in North America applauded this idea because negative-sequence relays are widely used in today’s high voltage networks.  We believe that utility regulators in the US and Canada are likely to impose the same requirement.

This grid code prompted us to update our model for converter-interfaced resources.  Again EPRI came to our aid by providing us crucial details on how to model the negative-sequence reactive current injection from converter-interfaced resources.  The result is a new Converter-Interfaced Resource model in OneLiner that is capable of injecting both positive- and negative-sequence reactive currents for unbalanced faults.  This model is making its debut in OneLiner v15. 

There are two important differences in the user interface between the VCCS model and the new Converter Interfaced Resource model:

1.  The new model no longer asks the user to enter a table of current magnitudes and power-factor angles at various terminal voltages.  The new model generates those numbers internally based on the control objective and current limits entered by the user.

2.  A new “negative-sequence slope” parameter is added.  A slope of 2.0, for example, means that the device will inject a negative-sequence reactive current with a magnitude equal to 2.0 times the magnitude of the negative-sequence voltage phasor, at an angle 90 degrees ahead of the negative-sequence voltage phasor.

A Type-3 Wind Plant model is also new in OneLiner v15.  This model and its core logic came from research at EPRI and at Polytechnique Montreal (under EPRI funding) for a phasor-based type-3 wind generator model that can be used in short circuit programs.  Published papers showed that EPRI’s model give results that are similar to time-domain simulations. 

At the time of this writing our recommendation to users who have a need to model wind plants and converter-interfaced resources are the following:

•      If you are currently modeling your type-3 wind plants using the current limited generator model, we urge you to switch to the new type-3 wind plant model as soon as possible.  As you are doing this, we urge you to aggregate your units whenever possible.

•      If you are using the VCCS to model your converter-interfaced devices, we urge you to switch to the new Converter-Interfaced Resource model if: you want the model to have the option to inject both positive-sequence reactive current and negative-sequence reactive current.  As you are making this change, we recommend that you aggregate your units whenever possible.

We have made an important discovery in the spring of 2021 that is worth mentioning.  We came to the realization that we at ASPEN -- as well engineers at other software vendors -- made a mistake in simulating the real-power output of wind, solar and battery systems in prefault when there are no loads in the network model.  This incongruity caused little to no problems many years ago when wind and solar plants are small and there are only a few of them in the network.

Wind, solar and battery installation are getting bigger and a lot of them are being built.  Suddenly we are faced with complaints in our program’s ability to solve the short circuit.  After much soul searching and experimentation, we concluded that unless you have loads in the network model and you are solving the short circuit with the “Start from power flow solution” option, you must set the prefault MW generation to zero.  For more details, please refer to Appendix K. 

Finally, we ask you to NOT create any more current-limited generators.  We are keeping the current-limited generator models in v15 only for backward compatibility.

The VCCS model still has limited use for modeling devices that inject no negative-sequence reactive current.  In Appendix J of this manual we show one such application for STATCOMs.  If you are using VCCS to model wind, solar and battery systems, also read Appendix K to find out how to change your VCCS data to avoid outing real power when you have no loads in the network.

For more details on the above topics, we suggest you consult the following:

IEEE Power & Energy Society Technical Report PES-TR78 “Modification of Commercial Fault Calculation Programs for Wind Generators” June 2020

A one-hour talk given by Sherman Chan of ASPEN on the new models in v15.  You can find a recording of this talk in the ASPEN Channel within youtube.com.