There are several utility codes in NA that require ANY distributed generator (DG) whether a rotating shaft generator or a grid-tied inverter based generator (e.g., PV, Wind) to have a "solidly" or "effectively" grounded neutral (EGN), per IEEE definition. Most medium sized (up to 500kW) inverter based DGs for PV systems have a 3-PH/delta connection (usually 208V or 480V), and therefore no accessible neutral point to ground.
The utilities maintain that even these devices need an EGN and to accomplish this requires an additional delta/wye isolation transformer, possibly a zig-zag transformer ? or changing the internal transformer output winding (requires safety agency recertification) in order to comply.
However, inverters are not voltage based machines - they are current sources that monitor the connected grid voltage and inject current to the grid based on the power available from the PV array. The terminal voltage developed by these devices will always follow that of the connected grid. If there are any disturbances in the grid voltage outside of the requirements of UL1741 the inverter is automatically disconnected. The units are smart in that they measure both magnitude and phase of the line- to-line voltages, and via calculations determine the equivalent neutral voltage, and switching at frequencies up to 18kHz have more than sufficient capability to guarantee no variation from the appropriate center point.
The symmetrical component model of this class of generator has no zero or negative sequence components - it is capable only of positive sequence operation. The EGN requirement is based on rotating machinery and the definitions of "effective grounding" are based on existence of zero and negative sequence currents - and they do not in this situation.
I had intended to run simulations using the models we have (PSSE or PSLF) to show reactions of the connection of figure 2 (see attachment) under various fault conditions, but not sure these packages will provide this kind of info - any suggestions here? Matlab?
It is my contention that the extra transformer to accommodate an EGN is not necessary for this kind of DG. Do you have any comments or experience with this topic - from your experience does the argument given above hold water?
The utilities maintain that even these devices need an EGN and to accomplish this requires an additional delta/wye isolation transformer, possibly a zig-zag transformer ? or changing the internal transformer output winding (requires safety agency recertification) in order to comply.
However, inverters are not voltage based machines - they are current sources that monitor the connected grid voltage and inject current to the grid based on the power available from the PV array. The terminal voltage developed by these devices will always follow that of the connected grid. If there are any disturbances in the grid voltage outside of the requirements of UL1741 the inverter is automatically disconnected. The units are smart in that they measure both magnitude and phase of the line- to-line voltages, and via calculations determine the equivalent neutral voltage, and switching at frequencies up to 18kHz have more than sufficient capability to guarantee no variation from the appropriate center point.
The symmetrical component model of this class of generator has no zero or negative sequence components - it is capable only of positive sequence operation. The EGN requirement is based on rotating machinery and the definitions of "effective grounding" are based on existence of zero and negative sequence currents - and they do not in this situation.
I had intended to run simulations using the models we have (PSSE or PSLF) to show reactions of the connection of figure 2 (see attachment) under various fault conditions, but not sure these packages will provide this kind of info - any suggestions here? Matlab?
It is my contention that the extra transformer to accommodate an EGN is not necessary for this kind of DG. Do you have any comments or experience with this topic - from your experience does the argument given above hold water?