NETA Battery and UPS Systems Testing 2 — Questions and Answers
Question 1: Which UPS topology provides zero transfer time to battery during a utility power outage because the load is always powered by the inverter?
- Standby (offline) UPS
- Line-interactive UPS
- Double-conversion (online) UPS (Correct answer)
- Delta-conversion UPS
Correct answer: Double-conversion (online) UPS
In a double-conversion online UPS, the load is continuously powered through the rectifier and inverter, so there is no transfer time when utility power fails.
Question 2: During a UPS acceptance test, which specific test verifies that the inverter can independently support the full connected load without utility input?
- Static bypass transfer test
- Battery runtime (discharge) test (Correct answer)
- Transformer turns ratio test
- Output THD measurement
Correct answer: Battery runtime (discharge) test
The battery runtime test disconnects utility input and verifies the UPS inverter can sustain the rated load for the specified backup duration using only stored battery energy.
Question 3: What output power quality parameter should be measured at the UPS output terminals during load acceptance testing to assess the quality of power delivered to critical loads?
- Battery specific gravity
- Charger ripple current
- Output voltage regulation and total harmonic distortion (THD) (Correct answer)
- Rectifier input current
Correct answer: Output voltage regulation and total harmonic distortion (THD)
Output voltage regulation and THD characterize the quality of power delivered to critical loads, ensuring the UPS meets the clean power specifications required by sensitive equipment.
Question 4: When the static bypass switch in a UPS system transfers the load, where does the load power come from?
- The battery bank directly
- The inverter operating in boost mode
- Utility (bypass) power, bypassing the rectifier and inverter (Correct answer)
- A motor-generator set
Correct answer: Utility (bypass) power, bypassing the rectifier and inverter
The static bypass switch transfers the load directly to utility power, bypassing the rectifier, battery, and inverter to allow UPS maintenance or to handle overload conditions.
Question 5: Failure of which component in the UPS rectifier/charger section would most likely cause excessive AC ripple voltage on the DC bus?
- Static bypass switch
- DC filter capacitors (Correct answer)
- Output isolation transformer
- Battery monitoring board
Correct answer: DC filter capacitors
DC filter capacitors smooth the pulsating DC output of the rectifier; when they degrade or fail, AC ripple passes through to the DC bus and battery.
Question 6: What is the purpose of applying an equalization charge to a flooded lead-acid battery string?
- To reduce the battery to 80% state of charge for capacity testing
- To apply a controlled overcharge that equalizes cell voltages and specific gravity across all cells (Correct answer)
- To measure each cell's internal resistance under load
- To verify proper calibration of the battery charger's output
Correct answer: To apply a controlled overcharge that equalizes cell voltages and specific gravity across all cells
Equalization applies a higher-than-normal controlled charge voltage to drive stratified electrolyte and sulfated cells back to full, uniform charge across all cells in the string.
Question 7: During UPS output testing, what does proper automatic voltage regulation (AVR) function demonstrate?
- The inverter switches to bypass within 4 milliseconds
- Battery charging current remains constant during input fluctuations
- Output voltage remains within specification despite input voltage variations (Correct answer)
- THD decreases when input voltage rises above nominal
Correct answer: Output voltage remains within specification despite input voltage variations
Proper AVR maintains stable output voltage within specified tolerances even when utility input voltage varies, protecting connected equipment from voltage sags and surges.
NETA Battery and UPS Systems Testing 2
Which UPS topology provides zero transfer time to battery during a utility power outage because the load is always powered by the inverter?