= Islanded microgrid operation
<Islanded microgrid operation> supplies a bounded set of interconnected <electrical loads> from its local <distributed energy resources> while the external <electric power system> connection is open. The <microgrid>'s own equipment must then maintain a usable supply and balance demand. The DOE overview explicitly distinguishes this disconnected mode from grid-connected operation and assigns coordination and connection management to the controller.<Microgrid Overview by the U.S. Department of Energy>[Printed p.1, Microgrid Overview and key components; printed p.3, connection to the main grid]{ref}
For an original steady <active power> balance, local generation supplies $25\,\mathrm{kW}$ and storage can supply at most $20\,\mathrm{kW}$. Connected <electrical loads> request $50\,\mathrm{kW}$ and network losses are stipulated as $2\,\mathrm{kW}$. The shortfall is
$$
P_{\rm short}=50+2-25-20=7\,\mathrm{kW}.
$$
At least $7\,\mathrm{kW}$ of the requested demand must be removed, or more usable supply obtained, for this simplified operating point. A full <electric battery> does not remove the $20\,\mathrm{kW}$ output limit. <Electrical load shedding> can balance the remaining demand, but it changes which service is delivered.
Passing the <active power> calculation is necessary but insufficient. An <alternating current> island must also sustain appropriate <electric potential differences> and <frequency>, accommodate <reactive power>, and respond to load changes. A <power inverter> can connect storage to the AC network, but the control capabilities and operating limits of the actual equipment determine whether it can establish or support that supply. The presence of a converter by itself proves none of those capabilities.
A proposed test records the external switch state, individual source <power>, storage state, load connections, <electric potential differences> and <frequency> through the transition into island operation. A load step then tests the local response described by <electric power balance during a demand step>. Surviving the transition and sustaining the island for a required duration are separate observations; <power-rating limit versus energy autonomy in a microgrid> explains why both are needed. Reconnection is another controlled operating transition, rather than simply assuming that two energized AC systems already agree in phase.
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