# Microgrid underfrequency shedding interface

Canonical URL: https://eloragrid.ai/symbols/microgrid-underfrequency-shedding-interface
Publisher: Elora Grid
Updated: 2026-11-03
Lookup label: MUSI
Drawing use: Schematic
Reference families: Manufacturer

Representation: Functional block
Installation focus: microgrids
Input: Qualified underfrequency state
Output: Assigned shedding outputs

Connects the selected frequency-based load reduction function to its assigned load groups.

## In plain English

### What this block does

Connects the selected frequency-based load reduction function to its assigned load groups.

### A simple example

In a microgrid island, the controller can disconnect lower-priority loads when a source is lost and later request restoration when the documented reserve and stability conditions permit it. For this reference, the drawing identifies “Qualified underfrequency state” at the input and “Assigned shedding outputs” at the output. Frequency thresholds require a protection study.

### What the drawings help you do

Start at the named input: Qualified underfrequency state. Identify its source device or document and the state represented on the drawing. Follow the named output: Assigned shedding outputs. Cross-reference its destination and separate the command from confirmation that the equipment responded.

### Terms

- Input: Qualified underfrequency state
- Output: Assigned shedding outputs
- Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections.

Concept diagram: Qualified underfrequency state → MUSI → Assigned shedding outputs
A reading guide. The entry's original illustration and input/output facts describe this application.

- [Avista Shared Energy Economy microgrid protection and control case study](https://selinc.com/api/download/135486/)


## What it represents

Connects the selected frequency-based load reduction function to its assigned load groups. Load management balances the available electrical supply against the loads that remain connected. Shedding, restoration, priority and proof of disconnection are separate control functions and should remain traceable to individual load groups.
In a microgrid island, the controller can disconnect lower-priority loads when a source is lost and later request restoration when the documented reserve and stability conditions permit it. For this reference, the drawing identifies “Qualified underfrequency state” at the input and “Assigned shedding outputs” at the output. Frequency thresholds require a protection study.
Identify each load group, priority, measured contribution and feedback. Shedding amounts, operating times and restoration conditions require the approved power-management design and the actual connected-load data. The functional block is an independent explanation of this named application. Equipment-specific ratings, terminal connections and operating conditions must be established in the project documentation.

## How to read it

- Start at the named input: Qualified underfrequency state. Identify its source device or document and the state represented on the drawing.
- Follow the named output: Assigned shedding outputs. Cross-reference its destination and separate the command from confirmation that the equipment responded.
- Frequency thresholds require a protection study.
- Identify each load group, priority, measured contribution and feedback. Shedding amounts, operating times and restoration conditions require the approved power-management design and the actual connected-load data.

## Information to show or reference

- Label the source and connection reference for qualified underfrequency state.
- Identify the destination and defined state of assigned shedding outputs.
- Record the equipment or control-document revision supporting this functional block.
- Check this application limit in the project design: Frequency thresholds require a protection study.

## Important distinction

Frequency thresholds require a protection study. This reference describes the named application; it does not establish a new IEC graphical symbol, an ANSI device-function number or an exact manufacturer connection diagram.

## Facts

- Representation: Functional block
- Installation focus: Microgrids
- Input / boundary: Qualified underfrequency state
- Output / connection: Assigned shedding outputs
- Lookup label: MUSI is a library lookup label, not a standard device designation.

## Public references

- [Avista Shared Energy Economy microgrid protection and control case study](https://selinc.com/api/download/135486/) — SEL. Public technical context for microgrid load shedding and restoration. Storage, protection, black start, load restoration and coordinated island operation. The application text and educational graphic are independently authored; the publisher does not certify this representation.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.