Evaluation of New Grid Codes for Converter-Based DERs From the Perspective of AC Microgrid Protection

Abstract

The ever-increasing penetration levels of converter-based distributed energy resources (DERs) in medium and low voltage (MV/LV) distribution networks necessitates new and revised standards or connection requirements of generating units. These standardized connection requirements to be met by DERs are called grid codes. This paper reviews new European and IEEE grid codes of MV/LV converter-based DERs and presents an evaluation of selected grid codes applicable for AC microgrid protection. The selected grid codes are evaluated for both grid-connected and islanded modes of AC microgrid operation. The standardized settings of different protection schemes including symmetrical components-based protection schemes are evaluated for quick fault detection and isolation during the most challenging unbalanced faults in grid-connected and islanded modes. The extent of dynamic reactive power (Q) injection by DERs according to European grid codes is evaluated and the effect of Q-injection on fault detection and protection coordination is observed. The generic grid-forming DER model with coupled dq-control has been enabled to provide enough reactive current during the unbalanced faults with the help of an additional Q-source. It is concluded that the grid code requirement of dynamic Q-injection can be met by converter-based DERs with an additional Q-source of minimum capacity equal to twice the apparent power of individual DER in the grid-connected mode. The same extent of Q-injection is also useful for fault detection and reliable directional element design in the islanded mode. A new five-cycle overvoltage ride-through (OVRT) curve is also suggested for smooth transition to the islanded mode.