Protection coordination in microgrids

U radu je opisana problematika vezana uz pojavu mikromreža unutar elektroenergetskog sustava koja se posebno manifestira na selektivnom postavljanju zaštite budući da je mreža dvostrano napajana. Različiti kvarovi su simulirani na više mjesta u mreži, s ciljem ostvarivanja selektivnosti za svako od mjesta. Dakle osnovni zadatak je da za svaki kvar odreagira zaštitni element koji je najbliži mjestu kvara te odvoji samo element koji je u kvaru. Pokušaj ostvarivanja koordinacije samo nadstrujnim relejima i to trenutnim i vremenski ovisnim članom nije bio uspješan. Zadaća trenutnog člana je bila isklop na minimalne struje kratkog spoja dok je zadaća vremenski ovisnog bila isklop na preopterećenja u mreži. Da bi koordinacija bila uspješna dodan je na dva mjesta i usmjereni relej, s trenutnim i vremenski ovisnim članom koji reagiraju kada se iznos i smjer struje kvara poklope sa zadanim. Napravljene su i simulacije valnih oblika struje i napona, prije, za vrijeme kvara, te nakon prorade zaštite. The paper describes the problems related to the emergence of the microgrids within the power system which is particularly manifested in the protection selectivity since the network is powered from two sides. Various faults have been simulated on multiple pints in the microgrid, in order to achieve selectivity for each of the points. The main task is that for every fault reacts protective device that is closest to the fault location and disconnect only the element that is in fault. Attempting to achieve co-ordination only with overcurrent relays with the short-time and longtime tripping delay was not successful. Short-time tripping delay is used for clearing shortcircuits while long-time tripping delay is used for preventing overload in microgrid. That is when it was necessary to add directional relay on the points in microgrid. Directional relays will trp only when the value of short circuit current equals or past value that is set in relay and short circuit current must flow in the same direction as it's been set in relay. Simulation of waveforms of current and voltage were also made for the states before, during the failure, and after the protection reacts.