Load flow and short circuit calculations
DC power system analysis in terms of load flow and short circuit calculations is investigated in [26]. Numerous source models (i.e. battery charger, rectifier and generator models) are given. Also connector models for load flow and short circuit calculations are investigated. Loads and branches are modeled. According to [26], the present standards (in the year 1996) are not complete and need further work.
In [3], the methods of the IEC draft standard "Calculation of short circuitcurrents in dc auxiliary installations in power plants and substations" are investigated. The calculations are intended to determine appropriate fusing for the system. Four different loads are investigated in [3]. These are the (thyristor) converter in 50 or 60 Hz AC systems, stationary leadacid batteries, smoothing capacitors and DC motors. The straightforward methods given in the standard are compared with EMTP simulations. The standard gives conservative estimates of the short circuit currents (25% overestimated). Also, the time constants estimated according to the standard are longer than the simulated. The standard was not applicable (in the year 1996) to large DC systems, such as railway traction systems, HVDC transmission systems and photovoltaic systems. According to [3], the reason is that the models developed for the standard do not cover the set of electrical parameters appearing for these systems.
A detailed model of an RLfed bridge converter for use in a NewtonRaphson power flow program for industrial AC/DC power systems is derived in [68]. This is done since bridge models in simulation packages are generally only valid in, for example, HVDC applications where the commutation resistance is negligible. Two systems are simulated to demonstrate the accuracy, a coal mine electrical power system and a railway power system. It is found that the developed model yields a considerably higher accuracy for systems with high commutation resistance.
Due to the differences between loadflow studies of AC and DC power systems, a new algorithm and a DC simulation package are developed in [79]. The load flow algorithm is based on the Newtonâ€”Raphson method. DC load flow simulation models are developed for constant DC voltage power sources, batteries, AC/DC rectifiers, constant power load, constant current load, constant resistance load and timevarying load. Several case studies are simulated, and their voltage profiles are shown.
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