Paralleling and Switchgear Controls Flashcards

In generator paralleling switchgear, which ANSI standard device number corresponds to the synchronism-check relay, whose primary function is to prevent the paralleling breaker from closing until voltage, frequency, and phase angle are within acceptable limits?

Answer: 25

ANSI Device 25 is the designation for a synchronism-check relay. This relay verifies that the two sources (e.g., an incoming generator and the bus) have closely matched voltage, frequency, and phase angle before allowing the paralleling breaker to close, thus preventing damage from out-of-phase closure. The other numbers correspond to different protective functions: 51 is an AC Time Overcurrent Relay, 27 is an Undervoltage Relay, and 81 is a Frequency Relay.

A technician is attempting to parallel a generator to a live bus. The synchroscope is rotating slowly in the 'fast' direction, and voltages are matched. What is the MOST likely adjustment required to achieve synchronization?

Answer: Decrease the incoming generator's speed slightly.

A synchroscope rotating in the 'fast' direction indicates the incoming generator's frequency is higher than the bus frequency. To synchronize, the prime mover's speed must be slightly decreased to reduce the generator's frequency, which will slow the synchroscope's rotation. The goal is to have the pointer rotating very slowly and to close the breaker just as the pointer passes the 12 o'clock position.

Which of the following best describes the primary purpose of the 'soft loading' or 'ramping' feature in an automatic paralleling control system?

Answer: To gradually transfer the real power (kW) load to or from the generator to prevent system disturbances.

Soft loading, or ramping, is a control function that gradually increases the governor setting after paralleling to smoothly transfer the real (kW) load onto the incoming generator. This prevents sudden mechanical stress on the prime mover and avoids electrical disturbances like frequency or voltage dips on the bus that would occur if a large block load was applied instantly.

What hazardous condition is a reverse power relay (ANSI Device 32) designed to protect a generator against in a paralleling application?

Answer: The generator absorbing real power from the bus, causing it to be motored.

A reverse power relay (ANSI 32) detects when real power (kW) flows from the bus into the generator. This condition, known as 'motoring,' occurs if the prime mover loses its driving force (e.g., fuel is cut off). The generator then acts as a motor, which can cause severe mechanical damage to the prime mover. The relay trips the generator's breaker to prevent this.

A facility with a multi-generator paralleling system experiences a complete power outage. Which control logic feature is essential for ensuring that the first generator to reach rated speed and voltage can close its breaker to the de-energized bus to begin restoration?

Answer: Dead bus closing logic

Dead bus closing logic is used to restore power after a blackout. Since the bus is de-energized ('dead'), there is no source to synchronize with. This logic bypasses the normal synchronization checks and allows the first generator that is ready to close its breaker, energizing the bus. Subsequent generators will then synchronize to this newly energized bus.

When commissioning a new paralleling system, which of the following is NOT an absolute electrical requirement for safely synchronizing two AC generators?

Answer: Identical prime mover horsepower ratings.

While generators of different sizes can be paralleled, their load sharing will be managed proportionally to their capacity. However, the electrical parameters must be matched. The terminal voltages, phase sequence, frequency, and phase angle must all be aligned before the paralleling breaker is closed. Failure to match these electrical characteristics will result in high circulating currents, severe mechanical shock, and potential equipment damage.