Voraussetzungen des Überstromzeitschutzes in wechselrichterbasierten gewollten Inselnetzen unter Berücksichtigung von Strombegrenzung und transienter Stabilität

摘要

Intentional islanding of electrical distribution systems raises potentials for customer backup power supply in future energy systems. An uncertain amount of additional expenses is needed to ensure an adequate and reliable protection system for the islanded mode of operation. Nowadays, customer installations are mainly equipped with low-cost overcurrent protection devices. Their reliable fault tripping is assumed but is not verified in intentionally islanded grids built by multiple inverter coupled distributed energy resources. It is unknown, whether critical influences and requirements need to be taken into consideration in addition to a sufficient dimensioning of the sources. Potentially influencing factors are investigated by means of time domain simulations of an islanded low voltage distribution system containing two grid building inverters. One and three phased faults are investigated in main and final circuits of customer installations. A sensitivity analysis varies up to 32 factors simultaneously from the areas grid structure and parameters, dimensioning of inverters, inverter neutral current injection capability as well as structure and parameters of LCL-filters, control circuits, current limiting mechanisms and anti-windup. For the first time, these variation studies consider an increasing degree of inhomogeneity of structure and parameter settings and exploit a significant bandwidth of values per setting. A rule based procedure is created as a precondition for parameterizing the overall model and the control circuits. Adequate variants of inverter current reference limiting mechanisms are identified by means of a developed test procedure. For the first time, according mechanisms are systematically derived for grid building inverters with neutral current injection capability.Reliable tripping of overcurrent protection devices is achievable for single wire final circuits with tripping times lower than 400ms, when a successful fault ride through can be ensured for an adequate quantity of grid building inverters. The required installed rated inverter currents are in the range from 0.92 to 1.18 times the overcurrent protection device’s tripping current. Influencing factors are the current limiting mechanisms used, the type of overcurrent protection installed and the inhomogeneity of inverter control structures present.Faults in main circuits experience a critical influence by a loss of transient stability during the fault (desynchronization) due to the resulting longer prospective tripping times. A reliable protection tripping is then impeded by the resulting gradual or cyclic reduction of the total fault current available. In case of three phased faults, the development over time can be estimated by a small number of factors. For one phased faults, an increase of the number of desynchronizing cases is observed with increasing degree of structural inhomogeneity. Dependable tripping is potentially achievable, when mechanisms for reliably avoiding desynchronization are utilized. In that case, an adequate number of inverters must ride through the fault.A fault ride through capability of the grid building inverters requires those to supply and tolerate phase voltages in the range of zero to 1.08*sqrt(3) p.u. and line-line voltages up to 1.175 p.u. Partial loading and reverse power capabilities are required from the inverter coupled distributed energy resources. Cyclic overloading of single phases or complete inverters can occur especially in case of a desynchronization.