REP Storage and Grids 2 — Questions and Answers
Question 1: Which grid service involves an energy storage system automatically responding within seconds to stabilize grid frequency deviations?
- Frequency regulation (Correct answer)
- Peak shaving
- Volt/VAR optimization
- Load following
Correct answer: Frequency regulation
Frequency regulation requires fast-responding assets (often within 4 seconds) to correct imbalances between generation and load, maintaining grid frequency near 60 Hz.
Question 2: A battery energy storage system (BESS) has a round-trip efficiency of 85%. If 100 MWh is stored, how many MWh are available for discharge?
- 100 MWh
- 85 MWh (Correct answer)
- 15 MWh
- 115 MWh
Correct answer: 85 MWh
Round-trip efficiency represents energy out divided by energy in, so 100 MWh × 0.85 = 85 MWh delivered to the grid.
Question 3: What term describes the practice of charging storage systems during low-price periods and discharging during high-price periods to capture wholesale electricity price spreads?
- Demand response
- Energy arbitrage (Correct answer)
- Capacity firming
- Ramp rate control
Correct answer: Energy arbitrage
Energy arbitrage exploits time-of-use price differentials by storing cheap off-peak energy and selling it back during expensive peak hours.
Question 4: Which interconnection standard governs the technical requirements for connecting distributed energy resources, including storage, to the utility grid in the US?
- NERC CIP-014
- IEEE 1547-2018 (Correct answer)
- ANSI C84.1
- NFPA 855
Correct answer: IEEE 1547-2018
IEEE 1547-2018 sets the technical requirements and testing procedures for interconnecting distributed energy resources (DERs) to the electric power system.
Question 5: In a pumped hydro storage facility, which factor most directly determines the amount of energy that can be stored?
- Turbine blade diameter
- Volume of upper reservoir and head height (Correct answer)
- Penstock pipe diameter
- Generator rotor speed
Correct answer: Volume of upper reservoir and head height
Stored energy in pumped hydro equals mass × gravity × head height, so both reservoir volume (mass of water) and elevation difference (head) are the primary determinants.
Question 6: What grid condition is described when voltage levels at distribution nodes fall below acceptable thresholds, often during peak demand?
- Overvoltage
- Undervoltage (voltage sag) (Correct answer)
- Frequency droop
- Phase imbalance
Correct answer: Undervoltage (voltage sag)
Undervoltage or voltage sag occurs when demand exceeds local generation or reactive power support, causing voltage to drop below the standard range (typically ±5% of nominal).
Question 7: Which storage technology is best characterized by very high cycle life, fast response, and suitability for short-duration, high-power applications like UPS and frequency regulation?
- Lead-acid batteries
- Compressed air energy storage
- Supercapacitors (ultracapacitors) (Correct answer)
- Vanadium redox flow batteries
Correct answer: Supercapacitors (ultracapacitors)
Supercapacitors can deliver very high power for short durations with hundreds of thousands of charge/discharge cycles, making them ideal for rapid-response grid stabilization.
REP Storage and Grids 2
Which grid service involves an energy storage system automatically responding within seconds to stabilize grid frequency deviations?