In a power system, ensuring the reliability and stability of the electrical network is paramount. Protection systems are crucial in identifying and isolating faults to maintain system reliability. The main objective of backup protection is an important aspect of these systems. Let's examine the provided options and determine the correct answer:

1. To detect minor disturbances in voltage: Backup protection is not designed to detect minor disturbances in voltage; it focuses on system faults that can affect stability, not voltage disturbances that are usually handled by different types of equipment or control systems.
2. To increase the complexity of the system: This option is incorrect because the objective of backup protection is to ensure the system remains as simple and effective as possible. Complexity can hinder the protection system's efficiency and reliability.
3. To isolate faults if primary protection fails: This is the correct answer. The primary objective of backup protection is to safeguard the power system by isolating faults if the primary protection mechanism fails. It acts as a contingency measure to ensure that faults are cleared, thus maintaining system stability and preventing damage.
4. To operate faster than primary protection: Backup protection is generally designed to operate after the primary protection has failed, not faster. The primary protection is expected to act first due to its designated priority in fault isolation.

Therefore, the correct answer is indeed: To isolate faults if primary protection fails. Backup protection serves as a secondary line of defense, coming into action when the primary protection does not operate as expected.

In conclusion, backup protection is critical for maintaining the integrity and reliability of the power system by ensuring faults are effectively isolated to prevent extensive damage and outages.