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PSCAD PPC Modeling & Requirements for Utility Scale Renewable Plants



Although the PPC is a small part of an overall utility-scale renewable energy plant, the importance of it’s role in grid stability and management over the terms of interconnection energy exchange agreements cannot be understated. As such, the number of ISOs and utilities’ requiring PPC models for renewable power plant interconnections have increased rapidly in the last few years and it is expected to soon be a standard requirement. We sat down with Nayak Corporation President, Om Nayak, to discuss the basics of PSCAD PPC modeling, their recent HIL validation of Nor-Cal Controls (Nor-Cal) PSCAD model, who is driving modeling requirements and where ISO requirements for PPC modeling are heading.


First and foremost, who will PSCAD/PPC modeling most benefit?

PSCAD/PPC modeling is primarily intended for utilities and independent system operators (ISOs), who use it to simulate their plant network, validating their renewable plant performance when connected to the utility grid. Typically, plant controllers are an aggregate control for large plants that have multiple converters, so it is most common for utility-scale plants. Distributed generation systems might also use it if they are 20MW or higher, or need to use a plant controller to aggregate multiple sources and make the plant look like a single point of contact for controls. They also perform other aggregation and coordination tasks. The PPC is the place that interfaces the communication between the utility and the plant converters.


What makes PSCAD/PPC modeling beneficial for renewable plants?

There are different models used for different types of simulation programs. Utilities use two main categories of programs for planning and analysis of power system network: electromagnetic transient programs and load flow and transient stability programs, which are complimentary. Within the electromagnetic transient category, PSCAD is the most popular, most recommended and most required simulation program used within the US and, for that matter, worldwide. The main advantages of PSCAD over other competitors are that it’s easy to use, very robust, well supported, and has been adopted by most ISOs and utilities in the US. One requirement that most of utilities/ISOs in US have in common is that all renewable energy developers must supply their plant models in PSCAD, including the converter model and the PPC model as part of their document submittals during their grid integration application approval process. The converters are manufactured by different suppliers and the PPC can be supplied by the same converter manufacturer, or it can be manufactured by a systems integrator like Nor-Cal. Either way, utilities require the PSCAD model for whatever PPC is going to be installed in the plant.


What is driving the need for PSCAD/PPC modeling? Is it an Independent System Operator (ISO) driven requirement?

Utilities require that PSCAD models of the PPCs are submitted for their approval. They specify the level of detail that is needed and the validation process that is required for these models to be acceptable. When we developed the PPC model for Nor-Cal, we took all that into consideration and provided a fully compliant PSCAD model with thorough documentation (a user manual), that is designed to pass the submittal at the first attempt to minimize delays to the project approval.


Does it vary from utility to utility, or state to state?

Right now, the requirements are specified by ISOs independently, but the ISOs communicate with each other so there are some commonalities. Some of the requirements can vary, particularly in terms of the validation process, or what kind of validation needs to be documented and submitted. There is currently an effort to create a standardized specification led by NERC/FERC, which is the national level regulating body. Once that is finalized, it is likely to be adopted by everyone but may have local additions. For right now, the modeling requirements are provided by each ISO.


Do the Developers or Asset Owners have any say in driving these requirements?

Typically, the PPA does not have much control over the requirements because they are required by the ISO, so you must meet them to be integrated onto the grid. The specs are written with ongoing input from developers and manufacturers, so they are not unreasonable. These requirements keep evolving but once they are part of the specification, the ISOs expect the models to meet those requirements. So far, we have not had any problems meeting any of the requirements we’ve seen.


What is the difference between a generic model and a User Defined Model (UDM)? When is one needed over the other?

There are two types of requirements usually needed from ISOs, one for a PSCAD model and the other for a PSSE/PSLF model. These are two types of programs as we mentioned before. UDM model stands for “user defined model”, so all PSCAD models are UDMs, although they usually aren’t called UDMs and are just referred as PSCAD models. When it comes to PSSE/PSLF types of programs, they can be either a generic model or a UDM.


PSSE models are steady state representation of the actual equipment whereas in PSCAD, it uses the exact same code and equations as that which are being used in the actual device. The PSSE contains a standard library of models for different type of controls. For example, they have a generic model for a PPC (REPCA) and it meets most of the PPC requirements in terms of PSSE studies. Sometimes, if a manufacturer’s controls change greatly and the generic model representation is not enough, you must create your own model by using coding, compiling the code and adding it to the program, which is what makes it a UDM.


Many utilities require both generic models and UDMs. Some only require the generic PSSE, and some will only accept a UDM model. So, it is kind of a mixed bag because the generic model is just a standard model, which makes it easier to maintain. You just upgrade the program when necessary and everything stays compatible. With a UDM, you are creating the compiled code of the model with the version of the program that exists at that time. So, if you need to upgrade it later after a few years, the model you first submitted may no longer work and you will need to recompile the existing code if it is available or create an entirely new model that will be compatible with the new version of the software. This becomes a maintenance issue, which is a downside to UDMs. In order to combat this, many utilities will require you to submit both the PSSE generic model and a UDM so you’re covered either way. Every PPC manufacturer, including systems integrators such as Nor-Cal who supply open source PPCs, must complete and submit their own PSSE (or PSLF) model, which is what we did for Nor-Cal. We have both a generic and UDM equivalent for Nor-Cal’s PPC.


Can you give a brief overview of how Nor-Cal’s PSCAD/PPC modeling works?

The PSCAD PPC model is an exact replica of Nor-Cal’s PPC, so it has the same number of inputs and outputs as the PPC, and it basically controls the PV or battery converter (which can be provided by any manufacturer). The input/output interface of the PPC model is also standardized so that it is very easy to interface with any inverter manufacturers’ model. Essentially, it is a plug and play model. The parameters are standardized and well documented.


Particularly in PSCAD, this model ensures compatibility for working with other manufacturers’ converters. In a power plant, there are two main components: the converter that produces the energy and the PPC which controls the converter and tells it how much to produce at any given point in time. The PPC gets its inputs or guidance from the utilities’ or ISOs’ control room operators, telling it how much power it should convert and then the PPC decides how to communicate that command to each converter. Typically, in a power plant there can be tens to hundreds of converters. The PPC knows this and communicates to the converters appropriately.


When we use the PPC model in the context of PSCAD, for most of the work that we do, we assume certain commands that would come from the utility and test those commands to make sure the converters perform correctly. Then we look to see if the PPC responded the way we expected it to when we asked those specific commands. We don’t model how the specific utility would command the plant, we simply create certain scenarios as part of a standard set of test parameters such as step changes to real and reactive power, step changes to positive and negative power factors, and voltage and frequency regulation by providing step changes to grid voltage and frequency. We also test if the plant follows the commands at the speed and level that we expect it to. We collect all the data and make sure everything is working fine.


Then utilities take this model and apply it to a larger context with other sites in play, because this one plant is just one node of a larger network that utilities have. For instance, they can create a model for one plant in San Francisco and then expand the model results for the rest of their plants within the entire state or area. The idea is that all those plants will work and plug into the larger network that the utility already has. Even though the utilities do their own evaluation of the submitted models, they do depend on the tests and validations that are provided by the developer. The end goal is to make sure that when they plug it all into the larger network it will all work as modeled and provide a true picture of what is going on. This is why a robust and accurate model is so important.


How does a SCADA provider that supplies the PPC, like Nor-Cal Controls, fit into the picture?

The modeling requirement is more commonplace for developers now because of the utilities’ interconnection requirements, but SCADA providers like Nor-Cal have been in business since before these requirements existed. When it comes to the modeling requirement we are discussing here, the utilities ignore most of the other details of the SCADA equipment and focus solely on the PPC, which helps to minimize the scale of the model. Essentially, they just go for the heart or the brain of the plant’s SCADA system which is the autonomous PPC. Everything else is assumed to be working the way it is supposed to be working and it is the developer’s responsibility to demonstrate that during commissioning tests. The utilities’ modeling requirements just care about the dynamics of the PPC. These cannot be fully tested during field commissioning tests, hence why testing is done using accurate models.


PPCs are typically a small part of a SCADA project’s overall budget, but they provide the total, supervisory brainpower for the plant. Converter manufacturers will often supply their own PPCs as a total package, so why does Nor-Cal offer their own? I think the reason for this is very unique and very interesting in that in spite of having a PPC and converter option from the same manufacturer, many clients want Nor-Cal’s PPCs when the developers need flexibility to make changes and need fast turnarounds. Many projects have unique utility, PPA and equipment compatibility requirements. The PPCs provided by converter manufacturers might not be able to communicate or work properly with those requirements or pieces of equipment specified. In those cases, the converter manufacturer’s PPC may not be a good candidate. Like I mentioned, the cost of the PPC is so small compared to the rest of the project, it is not worthwhile for manufacturers to spend time (very often on a short notice) on customizing the PPC if someone else can meet that need. Nor-Cal has the expertise and is a very agile company that also uses open-source development so everything is transparent and well documented. They can make any changes that the developers want, so it is much easier for the developers to work with a company like Nor-Cal to do the PPC. Due to this flexibility, the increase in requirements for the PPC modeling from utilities, and their ability to meet customer needs quickly, business has grown exponentially for Nor-Cal, and more developers are turning to your PPC and SCADA offering. This is based on my observation from several projects I have worked with you on. You have a great service philosophy and great team to execute it.


Do you see converter manufacturers adopting more open-source development for their PPCS?

I don’t really see converter manufacturers going into open source, customizable PPCs because they would rather focus on selling their big-ticket item: the converter. So, if someone else like Nor-Cal wants to do the work of creating custom PPCs, I think they would and should be happy to let them do it. I see more and more PPC requirements coming to Nor-Cal in the future, mainly because of your reputation in helping-out developers that are in a bind, or if some feature was needed and Nor-Cal stepped in and was able to provide the solution quickly, satisfactorily, and get the project commissioned on time. People in the industry talk, and so hearing something like that will lead others to say, “Let’s go with Nor-Cal because they are very competent, and the team is much easier to work with.” You can never go wrong with that combination!


Where do you see PSCAD/PPC modeling going in terms of use cases and necessity over the next 5-10 years?

When it comes to testing, even in the last few years, the requirements for modeling and testing have increased and will continue to increase in the coming years. Right now, we have developed a model for the PPC, validated it, created a document, and we’ve said that the model is good. The utilities are ok with this and accept it. I believe that soon utilities are going to ask for more testing and will want each physical PPC device modeled per each site prior to commissioning, to compare the commissioning results to the PSCAD model. This has already happened for converters to some extent and I can see it also being required for PPCs in the coming years.


Nor-Cal will be one of the first companies to have a setup where they can test the hardware PPC in their own facility, end-to-end, and compare this before the project is commissioned. Nor-Cal recently purchased RTDS (Real Time Digital Simulator) which allows them to perform hardware in loop (HIL) simulations. This allows you to connect the physical PPC box in your factory to a simulated plant in full detail and test all the functions of the hardware PPC for that specific plant. This kind of an approach for controller validation factory or system acceptance tests (FAT/SAT) is going to significantly improve Nor-Cal SCADA systems quality control. I think from that point of view, Nor-Cal is being very proactive. In the next couple of years, that might become a standard requirement asked for by utilities. As far as I know, Nor-Cal will be the first facility that has testing capabilities of this nature.


We have already completed the first HIL validation of the Nor-Cal PSCAD model with a standard set of parameters, where we have plotted the PSCAD simulation results against the RTDS simulation results. That document is available now to anyone who wants to use Nor-Cal’s PPC to demonstrate that the PSCAD PPC model is accurate. This document can be submitted with the model when developers are applying for projects as an added quality check confirmation although it is not site specific.


The HIL test was done at our Nayak office in Princeton, New Jersey with Nor-Cal’s PPC, but now that you’ll have the same setup available in your own factory, Nor-Cal will be able to repeat the same set of tests and many more for every future project. The advantage of this isn’t necessarily for validating the PSCAD model but that your FAT process will be more flexible in it’s validation because now the entire plant dynamics of each specific will be included in the FAT. The RTDS lets us model each plant you come up upon based on each customer’s project, so you can run the whole FAT in the warehouse before it is in the field. Once this is up and running, I believe Nor-Cal would be the first to provide SCADA integration for renewable plants with HIL, end-to-end dynamic feedback testing capabilities for the entire system in your facility.


Fortunately, your CEO, Bob Lopez, is hands-on, technical guy and fully sees the value in this. We really look forward to helping you guys integrate the RTDS into your process in whatever way we can so that every SCADA system that is developed and delivered gets tested on the RTDS. We wish you great success. We enjoy working with you!

Rachel Markus

Written by Rachel Markus