Power System Relay Modeling in Electromagnetic Transient Program
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- Institutt for elkraftteknikk 
The simulation program Alternative Transient Program is widely used in time-domain analysis of electrical systems. The program itself and its graphical pre-processor ATPDraw is under constant development. Implementation of complete protective relay models in ATPDraw is an important part of this development. Some work have been done on this already; models for transformer differential relays, overcurrent relays and distance relays have been implemented, as have digital signal processing (DSP) models based on RMS calculations and Fourier transforms.This report presents the results of a literature study on relay protection, corresponding digital signal processing and state-of-the art software modeling of protective relays. The features of currently implemented relay models in ATPDraw have been compared to relay models in similar software simulation programs and conventional relays. Furthermore, new DSP- and relay models have been implemented using the multi-purpose programming language MODELS. Finally, the performance of the relay models have been verified with a Siemens 7SA613 distance relay.The new ATPDraw relay models operate according to theory. When verified with Siemens 7SA613, the ATPDraw models have tripping times that are shorter than those of the relay. This may be caused by signal processing in the conventional relays that have not been incorporated in the software models. Inclusion of a low pass filter on the input signals somewhat increases the tripping time of the models, and hence decreases the error.Accurate software modeling of protective relays is requested by relay engineers due to the complexity of future power system planning. Several such models, e. g. the ones implemented in ATPDraw, are purely generic, and are best used in early state planning and for educational purposes. Some programs, on the other hand, offer models that imitate the behavior of conventional relays more accurately. ASPEN OneLiner provides several relay models categorized by manufacturer and relay design.The accuracy of software relay models may be questioned due to the fact that little is known about internal processing and algorithms of the relays, and the physical features of the relays which are hard to simulate. Many software models of varying complexity exist today. Common to these models is the need for caution with regards to their limitations. The models are best used for simple analysis and early phase planning. Both generic and complex relay models give good insight in protective relaying principles, and are well suited for educational purposes.