The following master’s thesis presents two interface algorithms for real-time digital simulations. The system is calculated so fast using real-time digital simulations, that output variables represents the real situation in the system. This allows the exchange of signals with external devices and, consequently, the testing of various modules of power devices in hardware in the loop simulations. Hardware-in-the-loop simulations are an increasingly common tool for solving engineering problems. They provide time-saving, affordable, and particularly safer, non-destructive testing of devices in power systems. Tested devices are low-voltage modules, such as regulators, protectors and others. Testing is also easier with regard to the need for high-voltage equipment. Simulations are also used to test the operational states, that are difficult to perform in real systems.
The main problem that led to the following analysis was testing of one or more parallel active filters in a large power system. At first, simulations were made on one real-time simulator, however a combination of two simulators turned out to be a better solution. Combining the two simulators required an interface algorithm, which caused stability problems. This resulted in precise analysis of interface algorithms, since selecting an appropriate interface algorithm results in better stability.
Initially, the thesis describes the use and the significance of real-time digital simulations. Then, modelling and implementation of two interface algorithms are presented. The first interface algorithm presented is the Ideal Transformer Model (ITM) algorithm, whose stability criterion is described with the use of the Nyquist and Bode-Nyquist stability criterion, followed by the Transmission Line Model (TLM).
Later on, the focus of the thesis is shifted to simulation models of both algorithms. Simulation references were determined using Matlab/Simulink. Two different cases were the subject of the study, simulations were made for an LC filter that was used as load and for three parallel LC filters, which represented the load. Further, the models on simulators RTDS and Typhoon HIL were made. Both cases were simulated in a laboratory experiment.
In addition, the process of choosing an appropriate interface algorithm is given. The choice of an interface algorithm is shown on the practical example of a network with a two-wire diode rectifier on the load side.
Finally, the results and conclusions are presented. As expected, the ITM algorithm showed high accuracy and poor stability, while TLM showed high stability but poor accuracy. An improvement of the ITM algorithm would be possible by changing the impedance characteristic with the use of additional filters. This would shift unstable operating points in the frequency range, where there is no impact on simulation stability. With the right optimization and improvements, the TLM algorithm could be used in real power system studies. Lastly, the modelling of algorithms is evidently the easiest part in building HIL simulations, due to many other factors like noise, signal scaling, non-constant time-delay and others that cause anomalies, which in turn affect simulation results.
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