Introduction
Background:

Load shedding is the term used to describe the deliberate switching off of electrical supply to parts of the electricity network, and hence to the customers in those areas.
This practice is a core part of the emergency management of all electricity networks.
Load shedding can be required when there is an imbalance between electricity demand (customers’ usage) and electricity supply
(the ability of the electricity network to generate and transport the required amount of electricity to meet this demand). When there is a shortfall in the electricity supply, there can be a need to reduce demand very quickly to an acceptable level, or risk the entire electricity network becoming unstable and shutting down completely.
This is known as a “cascade”. Event and can end in a total or wide spread network shutdown affecting very large areas of country.

Load shedding normally happens in two ways:

Automatic Load Shedding:
This is a result of concurrent failures of major element(s) in the national grid(e.g. co-incidental generator or key transmission line failures), resulting in protection schemes initiating the automatic isolation of additional parts of the national grid, to protect the entire grid from cascading to a total blackout.
Automatic load shedding always occurs on the transmission system level, with the result being large amounts of electricity and large blocks of customers taken off supply in a very short time. Typical load reduction amounts can be in the order of 1000MW 2000MW, affecting hundred of thousands of customers.

Manual (Selective) Load Shedding:

Typical load reduction amounts can be in the order of 50MW – 100MW
This occurs where time is available (typically up to 60mins) to make selective choice on what customers are shed. Selective load shedding often occurs on the distribution system