Req:inverter dyn

Overview
These represent the requirements for the inverter_dyn object, which is to represent grid-forming and grid-following inverters.

Application Concept
The purpose of developing the new inverter object is to further improve the dynamic models of grid-forming/grid-following inverter, and to better support the dynamic simulations of GridLAB-D. Specifically, inverter models will be classified as two types: grid-following and grid-forming. In grid-following mode, commonly used control strategies such as constant PQ control, frequency-watt control, and volt/var control will be implemented. In grid-forming mode, control strategies such as droop controls, overload mitigation control, and isochronous control will be implemented.

Use Case
The new inverter object aims to better support the dynamic simulations of resilient microgrids and distribution systems. As the penetration of inverter-based DERs continuously increases in distribution systems, their impacts on the stability of microgrids and islanded distribution systems need to be further investigated.

General Requirements
Inverter models should be classified as two types: grid-following and grid-forming.

For a grid-following inverter, it should have the following capabilities:
 * The inverter should be able to inject the specified P and Q into the grid. If the inverter is three-phased, P and Q should refer to the summation of output power of three phases. If the inverter is single-phased, P and Q should refer to the output power of the specified phase.
 * The inverter could be either three-phased or single-phased
 * For single-phase connections, the inverter will support standard mono-phase or split-phase (triplex) connections
 * The inverter should be able to change its output P and Q based on external control functions, such as frequency-watt and volt-var control, etc.

For a grid-forming inverter, it should have the following capabilities:
 * The inverter should be able to work in islanded mode, which means the inverter should be able to generate a constant voltage and frequency and supply the load by itself.
 * The inverter should be able to work in parallel with other grid-forming inverters.
 * The inverter should require droop control to support parallel operation
 * The inverter should be able to avoid being overloaded,
 * The inverter should require overload mitigation control to mitigate overload
 * The inverter could be either three-phased or single-phased
 * The grid-forming inverter will not support split-phase (triplex) connections. We assume most grid-forming inverters have relatively large capacities and connected at the primary side.