How long is the duration that applies to normal post-contingency voltage limits?

The correct response is based on the nature of post-contingency voltage limits in transmission systems. Typically, post-contingency voltage limits are designed to be assessed immediately following a contingency event, such as a fault or an outage. Therefore, the term "Immediate" signifies the timeframe within which operators are expected to respond and address any voltage issues that arise after a disturbance to the system.

In many regulatory frameworks and utility operation standards, the post-contingency conditions are expected to be managed as quickly as possible to maintain system reliability and to avoid further complications or cascading failures. Thus, the concept of having a specific duration such as 15 or 30 minutes doesn't typically apply to the immediate response needed for voltage limits. Instead, the limits are meant to be monitored continuously after a contingency has occurred, reinforcing the understanding that their assessment is critical and requires prompt attention right after the event.

This context clarifies why "N/A" is the chosen answer, as it reflects that there is no applicable duration beyond the immediate assessment requirements for post-contingency voltage limits.