Distribution Simulation with the RTDS Simulator
Distributed Energy Resources (DERs) are becoming more prevalent in distribution systems. An increasing number of people are adding solar panels to their houses, solar and wind farms are increasing in number, and electric vehicles and battery storage are increasing in popularity.
Distribution systems were not originally built with these things in mind. Distribution systems were designed with power generated further away from the load. More sophisticated study tools are needed to study the bi-directional power flow of DERs in distribution feeders. The RTDS Simulator is a real time Electromagnetic Transient (EMT) simulator, and can be used to model and study distribution feeders.
In RTDS, when a case has reached the node limit of the hardware, the network has to be split using a travelling wave transmission line or a cross rack transformer. In transmission systems this is no problem, since the travel time of the transmission line is often longer than the timestep. In distribution systems, where the whole case is tightly coupled, a transmission line needs to be lengthened in order to split the system into multiple networks. Introducing artificial subsystem splitting in a case can add error to the system, such as bus voltages becoming unusually high.
Looking at the IEEE123 Node Test Feeder case, the longest transmission line is 825 feet which is approximately 0.25 km. Looking at the table below, the transmission line length would have to be increased significantly to split the system.
RTDS Technologies has developed an operating mode in the RSCAD software called Distribution Mode. Distribution Mode works in a substantially similar way to normal simulation mode, but a few key differences allow the user to model significantly more power system nodes in one tightly coupled area using this mode. In Distribution Mode, feeders of hundreds of single phase nodes can be simulated on one RTDS Simulator rack or chassis!
Distribution Mode relies on the feeder being radial in structure, which reduces the computational burden of solving the network in real time due to the highly sparse nature of the network’s admittance matrix. Also, the simulation timestep for Distribution Mode is slightly larger – in the range of 100-200 microseconds – to provide more time for calculating the network solution. These factors allow a vastly larger number of power system nodes to be modelled in one tightly coupled subsystem.
The IEEE123 Node Test Feeder case has been successfully simulated in one subsystem in Distribution Mode with DERs added in 4 different locations as seen in Figure 1. It can run on both PB5 and NovaCor systems, and has a total of 288 nodes.
The small timestep bridge box cannot be used in Distribution Mode due to the increased timestep, therefore the DERs were modeled with average model components rather than fully switched models in the small timestep. Two average models have been developed. There is the dynamic PQ source, and the DC/AC average model shown in Figure 2.
The dynamic PQ source assumes a constant, large DC source. This model is sufficient for applications where the focus is on how the system responds to real and reactive power flow from a large number of DERs. This model was used in the IEEE123 Node Test Feeder case to model the DERs.
The DC/AC average model allows a DC component, such as a PV or battery model, to be interfaced with the AC system. It neglects the effects of the switching devices but is sufficient for providing the accurate real and reactive power profile of the converter. The average models require much less computational resources than the fully switched small timestep models, so they are much more efficient in terms of RTDS Simulator hardware required for modelling.
CYME to RSCAD Conversion Program
A CYME to RSCAD conversion program has also been developed. It allows a distribution case modeled in CYME to be converted into Distribution Mode in RSCAD.
Due to the difference in graphics in CYME, new graphics were created in RSCAD for the components used in the conversion program. The components can stretch in any direction to connect between any 2 points in draft. Most of the components are 1-3 phases, and all 1-3 phase connections are at one point on the draft canvas. A small sample circuit using these new component graphics is shown in Figure 4. A library tab in Distribution Mode contains these components.
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Author: Melanie Dyck, October 2017
- Dyck and O. Nzimako, 2017, “Real Time Simulation of Large Distribution Networks with Distributed Energy Resources,” in CIRED, Scotland.