Hierarchical Control

Operates on the millisecond to second timescale. This layer provides immediate, decentralized stabilization of voltage and frequency without requiring communication links. It relies on droop control characteristics embedded in inverters and governors to autonomously share real and reactive power imbalances. In inverter-dominated microgrids, virtual inertia emulates the physical rotating mass of synchronous generators to slow the rate of change of frequency (RoCoF) during disturbances.

Operates on the seconds to minutes timescale. This centralized or distributed layer corrects the steady-state errors in frequency and voltage introduced by primary droop control. It restores the system frequency to its nominal value (e.g., 50 Hz or 60 Hz) and manages the State of Charge (SoC) of battery energy storage systems to ensure sustained reserve capacity for future islanding events.

Operates on the minutes to hours timescale. This optimization layer manages power flow between the microgrid and the main utility grid based on economic signals. It solves the Optimal Power Flow (OPF) problem to minimize operational costs, maximize renewable self-consumption, or participate in wholesale energy markets. It sets the power references for secondary controllers.

A fundamental design principle where control bandwidths are strictly separated to prevent instability. Primary control acts instantly, secondary control acts deliberately, and tertiary control acts economically. This prevents a slow economic optimizer from interfering with a fast frequency response. IEC 61850 GOOSE messaging is often used for the fast, peer-to-peer communication required at the secondary level.

Hierarchical control logic differs fundamentally based on inverter type. Grid-forming inverters establish the voltage and frequency reference, acting as the master in islanded mode. Grid-following inverters act as slaves, injecting current in sync with the established reference. The secondary controller coordinates the transition between these modes during seamless reconnection to the main grid.

Hierarchical control enables intentional islanding and black start capability. Upon detecting a grid outage, the controller disconnects via a static transfer switch, sheds non-critical load, and re-establishes the local grid using a grid-forming resource. It then sequentially reconnects loads and synchronizes with the main grid once stable utility power is restored.