RTDS Simulation of PV Inverters in a Small Power System

The goal of the project is to develop a model of a small power system with multiple photovoltaic (PV) inverters connected. We will then implement external controls to coordinate real and reactive power output from the inverters.​ The purpose of the project is to set up a platform that can be used by future Power Engineering students to more fully understand the effects of inverter based energy sources on the power grid.

=Problem Definition= As renewable energy sources become more and more prevalent in our society's power infrastructure there is a growing need for engineers with an understanding of how inverter based renewable energy sources affect the power grid. This project gives students a foundation to begin understanding these effects.

Background
Appropriate power system models are a necessity for effective engineering in the power field. With the rise of inverter based renewable resources, the need for accurate working models is greater than ever, both for working professionals in the field as well as the future power engineers in the classroom. The RTDS (Real Time Digital Simulator) is a robust and flexible simulation system that can closely model power systems. With a variety of different I/O and communication options available, it can also facilitate the use of other hardware in the simulations. This is where the term Hardware in the Loop comes from.

Deliverables
Our goal is to create a basic distribution model that incorporates photovoltaic inverters and provides a hands on look at how this technology affects larger power system stability. We will also incorporate an automation scheme that can, in real time, regulate power flow to maintain stability for our small system. This is done through the use of separate hardware in the form of an SEL-487B bus protection relay.

Below is a list of files that represent the deliverables we have turned in up to this point in the project.



Our team presentations can be found in the Additional Documentation section.

Specifications
The model we created is based on the IEEE 13 bus system using a Real Time Digital Simulator (RTDS) and implemented with RSCAD software. It incorporates three separate PV inverters with controls for each. We also created a control scheme using an SEL 487B bus protection relay to control real and reactive power output from the PV inverters based on specific grid parameters. A human-machine interface (HMI) was developed for real-time manipulation of the systems controls.

Answering the question "What will the final design look like":

Safety:
 * No real safety standards are necessary due to the project being mostly software based.

Electrical Design:
 * A one line diagram is provided which illustrates the three PV arrays integrated into the IEEE 13 bus system. The diagram of the 13 bus system can be found in the Additional Documentation section under Reference Materials.

HMI Design:
 * The HMI that was used for this project is a customized RSCAD Runtime HMI. It includes switches to control the breakers that bring the PV inverters onto the grid. It also includes bus voltage plots to monitor the health of the system as these PV inverters are switched in and out of the system

=Design Considerations= One of the major considerations for our project design was which model to base it on. We had a few different choices. We could have used a model built from scratch that had any number of busses, loads, sources, etc. There were also two pre-made model options, one was the IEEE 13 bus system and the other was the IEEE 34 bus system. Ultimately we decided that since all of the team members were new to RSCAD and working with the RTDS we probably shouldn't try to build a model from scratch as it would be too time consuming. We also decided that a 34 bus system would be too complex considering the goals of the project. The IEEE 13 bus system was the model we chose to base our project on.

We also had to decide how many inverters we wanted to integrate into the system. Dr. Johnson, our client, recommended two or three. We decided to include three PV inverters. This would make the results of the simulation results variable enough to give a good idea as to how the grid behaves based on different power outputs from the inverters. It was also recommended to connect the inverters at the weakest busses. This meant finding which busses had the lowest power factor, or ratio of real power to total apparent power. We could find this by running the IEEE 13 bus system on the RTDS and finding the power factor of each bus.

For weeks the team was roadblocked while working with the PV array model from RTDS. There were some protection elements that weren't allowing the model to function as it should. We decided to go back to a base model instead of pulling the array model from a file that had it integrated into a different system. This made it easier to deconstruct the model and remove the parts that were unnecessary and causing issues. We are now close to being able to integrate three of the PV arrays on our IEEE 13 bus system. Once this is completed and communications are established between the hardware devices, an HMI can be designed to control our virtual system.

=Project Learning= Our main need for learning at the beginning of this project was to learn how to use RSCAD to build models to run on the RTDS. There is a fairly steep learning curve for this software. After finding a PV inverter model that would be suitable for integration in the IEEE 13 bus system, we found it difficult to find how to make the system register the effects of the inverter.

With help from our mentor from SEL, Asad Mohammad, we found that there were issues with the protection logic of the PV array model. We haven't found the root cause for this issue, but we have found that by bypassing the protection value "blockx8" we can get acceptable results from the PV inverter.

Now that the PV inverters are working properly we need to integrate them into the IEEE 13 Bus model. We decided to use a phased approach to this goal. First we made sure that we connect a single PV array to a single bus. Then we added two arrays to a bus. Since we will be using three arrays on the 13 bus system we will then add a third array to the bus. Once we have all three of these arrays working properly we will then integrate the three arrays onto the three weakest buses on the IEEE 13 bus system.

The next major step in our project will be to establish a connection to the SEL RTAC and create an HMI for manipulation of the inverter controls. We initially had issues connecting to the available RTAC but with some help from Asad we were able to establish a connection after changing some IP addresses. We then were able to connect the RTAC to the RTDS.

Currently the team is focusing on establishing communication channels between the RTAC the RTDS and an SEL-751 relay that we will be using to indicate breaker status changes in the system. We decided to use GOOSE messaging between the RTAC and RTDS and the RTAC and 751. We quikly found that there is a steep learning curve for setting up GOOSE messaging between these devices, especially between the RTAC and RTDS. Dr. Johnson recommended talking to Hari Challah, a PhD student working in the lab, for advice and guidance on communication between devices. Hari suggested using DNP protocol instead of GOOSE messaging because it is much easier to set up. Although DNP is not as powerful as GOOSE messaging, it will be adequate for the communication we plan on using for this project.

=Validation= The Design Validation Plan can be found in the image to the right. It shows the major project requirements and how the will be tested to make sure they meet specifications. As the project progresses and certain requirements are met and tested, the table will be updated to reflect the status of the project.

As of 3/5/2021 most of the design validation dates had to be pushed back because of unforeseeable circumstances. New dates have been chosen for design validation.

=Team Members=

=Additional Documentation=

Project Schedule



Presentations



Reference Materials