Following instabilities in the grid in the 1980s, the transfer level in the transmission corridor between Scotland and England was severely constrained.
To relieve this constraint, power system stabilisers were installed to improve to improve damping of a 0.5Hz mode. After the installation of the stabilisers, however, there was still uncertainty over the secure transfer capability.
The subsequent deployment of a Psymetrix system for on-line damping monitoring enabled the client to achieve a capacity increase of 300MW, given that an acceptable damping margin was maintained. If damping of the inter-area mode crossed the margin of acceptable damping, transfer was temporarily constrained.