REP Storage and Grids 3 — Questions and Answers
Question 1: What is the primary function of a Battery Management System (BMS) in a grid-scale BESS?
- Converting DC to AC power for grid injection
- Monitoring cell-level voltage, temperature, and state of charge to protect battery health (Correct answer)
- Scheduling dispatch based on electricity market prices
- Transforming grid voltage to match battery pack voltage
Correct answer: Monitoring cell-level voltage, temperature, and state of charge to protect battery health
The BMS monitors and balances individual cell parameters (voltage, temperature, SOC, SOH) to prevent overcharge, over-discharge, and thermal runaway.
Question 2: Which metric describes the ratio of actual energy delivered by a storage system over its lifetime to the total energy input over the same period?
- State of charge (SOC)
- Depth of discharge (DOD)
- Round-trip efficiency (Correct answer)
- Calendar life
Correct answer: Round-trip efficiency
Round-trip efficiency (RTE) is cumulative energy out divided by cumulative energy in over the system's lifetime, capturing all conversion and parasitic losses.
Question 3: A utility operates a grid with high solar PV penetration. Which grid challenge most commonly emerges during the transition from midday high-solar to evening high-demand periods?
- Voltage flicker from rapid cloud transients
- Steep ramping requirement known as the duck curve (Correct answer)
- Excess baseload capacity overnight
- Transmission thermal overloads at dawn
Correct answer: Steep ramping requirement known as the duck curve
The duck curve describes the steep net-load ramp that occurs in the late afternoon as solar generation drops while evening demand peaks, requiring fast-ramping resources.
Question 4: In FERC Order 841, what action did the Federal Energy Regulatory Commission take regarding energy storage?
- Mandated utilities to procure a minimum percentage of storage capacity
- Required grid operators to allow energy storage to participate in all capacity, energy, and ancillary services markets (Correct answer)
- Set maximum size limits for behind-the-meter storage systems
- Prohibited storage from providing both generation and load services simultaneously
Correct answer: Required grid operators to allow energy storage to participate in all capacity, energy, and ancillary services markets
FERC Order 841 (2018) directed RTOs/ISOs to remove barriers and establish rules enabling electric storage resources to participate fully in wholesale electricity markets.
Question 5: What is the purpose of reactive power compensation provided by grid-tied inverters in renewable energy systems?
- Increasing real power output during curtailment events
- Maintaining voltage stability by supplying or absorbing reactive power (VARs) (Correct answer)
- Reducing inverter switching losses at high DC input voltages
- Synchronizing rotating generators to grid frequency
Correct answer: Maintaining voltage stability by supplying or absorbing reactive power (VARs)
Reactive power (VARs) support from smart inverters helps maintain voltage within acceptable limits, especially on long distribution feeders with high renewable penetration.
Question 6: Which type of compressed air energy storage (CAES) plant uses the heat generated during compression to improve round-trip efficiency?
- Adiabatic CAES (Correct answer)
- Diabatic CAES
- Isothermal CAES
- Aquifer CAES
Correct answer: Adiabatic CAES
Adiabatic CAES captures and stores compression heat in a thermal energy storage medium and reuses it during expansion, significantly improving efficiency over diabatic designs that vent the heat.
Question 7: What grid topology connects multiple microgrids and distributed energy resources through a shared medium-voltage backbone, enabling both islanded and grid-connected operation?
- Radial distribution feeder
- Networked microgrid (Correct answer)
- High-voltage direct current (HVDC) link
- Zonal transmission grid
Correct answer: Networked microgrid
A networked microgrid architecture interconnects individual microgrids at a shared bus, allowing power sharing among nodes and collective islanding from the utility grid.
REP Storage and Grids 3
What is the primary function of a Battery Management System (BMS) in a grid-scale BESS?