Concept of power system security

Power system security is the ability of the system to withstand sudden disturbances (contingencies), such as loss of a generator, line or transformer, without interruption of supply to customers and without violating operating limits.

Main ideas

• Contingency: an unplanned outage of a component. The common rule is the N-1 criterion: the system must stay within limits after the loss of any single element.
• Reliability has two parts: adequacy (enough generation and transmission to meet demand, a planning issue) and security (ability to survive disturbances, an operating issue).

Operating states (Dy Liacco)

   Normal (secure) ---> Alert (insecure)
       ^                      |
       |                      v
   Restorative <--- Emergency ---> In extremis

• Normal: all loads served, limits met, reserve adequate.
• Alert: limits still met but reserve is low; one more contingency can cause violation; preventive action needed.
• Emergency: limits violated; corrective control (generation shift, load shedding).
• In extremis: cascading outage, islanding or blackout.
• Restorative: reconnecting load and generation (black start).

Functions of security

1. System monitoring (SCADA, state estimation).
2. Contingency analysis (what-if studies for N-1 outages).
3. Security-constrained optimal dispatch and corrective actions.

Adequate spinning reserve, interconnections, protection and under-frequency load shedding all improve security.