FREQUENCY-DOMAIN MODELING OF EXTERNAL SYSTEMS IN AN ELECTROMAGNETIC TRANSIENTS PROGRAM

The standard EMTP (Electro-Magnetic Transients Program) is based on time domain calculations as these are eminently appropriate for simulation purposes. This paper presents an approach for the direct frequency domain representation of an external system of any size or complexity. The requirements for the external system are that it must be linear and time invariant, and it should include a transmission line connecting it to the study zone and providing a travel delay tau. It is represented in the phase domain and does not have to be symmetrical. At the connecting bus between the study zone and the external system, Fourier transforms provide the conversion from time domain to frequency domain and vice versa. The FFTs are however performed only at time intervals 2tau so that the approach is computationally efficient especially in the case of large external systems. One important advantage of the method is that the accuracy and simplicity of direct frequency domain modeling is preserved since no approximations or fitting are necessary, even for complex, three-phase frequency dependent components.