Voltage Control & System Reliability Flashcards

A power system is operating near its voltage stability limit. An operator increases generation at a remote plant to serve load. This action will most likely:

Answer: Worsen voltage stability by increasing reactive losses on the transmission path

Increasing active power flow on a heavily loaded path also increases reactive losses (I²X), consuming reactive power and pushing the system closer to voltage collapse.

Which protection relay function is specifically designed to detect loss of excitation on a synchronous generator?

Answer: 40 (Loss of field relay)

ANSI function 40, the loss-of-field (LOF) relay, detects when a generator loses its excitation, preventing sustained under-excited operation that risks system voltage depression.

Why is the Q-V curve used in voltage stability analysis in addition to the P-V curve?

Answer: The Q-V curve shows the reactive power margin available at a specific bus under fixed active power loading

The Q-V curve shows how much additional reactive power (MVAR margin) a specific bus can absorb before voltage collapse at a given active power level.

An operator observes bus voltages dropping progressively across multiple substations despite reactive support being applied. This pattern most closely indicates:

Answer: A progressive voltage collapse propagating through the system

Progressive voltage decline across multiple buses despite reactive support is a hallmark of an evolving voltage collapse, requiring emergency intervention.

Under NERC standard VAR-002, generators are required to:

Answer: Operate within the reactive capability limits specified by their plant operator and follow Transmission Operator voltage schedules

VAR-002 requires generators to operate within their reactive capability and follow voltage schedules set by the Transmission Operator to support bulk electric system voltage.

What is the primary reliability risk of operating a generator continuously at its under-excitation limit?

Answer: Loss of synchronism due to insufficient synchronizing torque

Operating at the under-excitation limit weakens the synchronizing torque, making the generator vulnerable to pulling out of synchronism with a small disturbance.

A voltage schedule requires Bus A to be maintained at 1.02 pu. The operator observes Bus A at 0.98 pu. After exhausting local reactive resources, the NEXT appropriate action is to:

Answer: Coordinate with adjacent Transmission Operators for reactive support and notify reliability coordinator if limits are violated

When local reactive resources are exhausted, operators must coordinate with neighboring entities and notify the Reliability Coordinator if voltage limits cannot be maintained.