Power System Planning, Load Forecasting, and Resource Adequacy
This domain tests how planners convert a service obligation into a defensible capacity plan: forecast methods and their error characteristics, planning criteria such as N-1 and the planning reserve margin, and the probabilistic metrics used to demonstrate resource adequacy to a regulator.
Load forecasting
- Separate peak (kW) from energy (kWh) forecasts. Peak drives capacity; energy drives fuel, revenue, and emissions.
- Horizons: operational (minutes to day-ahead), short-term (1-5 years), long-range (5-25 years).
- Methods: econometric regression on weather, price, income, and population; end-use/load research; time series (ARIMA, exponential smoothing); scenarios.
- Normalize with heating/cooling degree days; apply diversity (after-diversity maximum demand) to convert connected load into coincident demand.
Planning criteria and adequacy
- NERC TPL-001 requires the bulk system to withstand N-1 contingencies without cascading outages or uncontrolled loss of firm load.
- Planning reserve margin = (installed capacity minus forecast peak) divided by forecast peak. It says nothing about deliverability.
- ELCC converts a variable or energy-limited resource into the equivalent amount of perfect-capacity resource that preserves the same reliability. Solar in a summer-peaking system commonly accredits at roughly 10-40% of nameplate; paired storage accredits higher.
- Adequacy metrics: LOLE (a common target is 0.1 day/year, one day in ten years), LOLP, and expected unserved energy.
Worked example
A utility forecasts a 10,000 MW winter peak and must meet a 15% reserve margin: 10,000 x 1.15 = 11,500 MW of accredited capacity. It owns 9,500 MW of thermal capacity and 3,000 MW nameplate of solar accrediting at 25% ELCC, or 750 MW. Total accredited = 10,250 MW, a 1,250 MW shortfall. Counting solar at nameplate would have shown a 1,450 MW surplus, the classic error that produces a shortfall on the first cold snap.
Traps that cost marks
- Summing nameplate MW instead of accredited MW.
- Using an arithmetic sum of feeder peaks as the system coincident peak.
- Treating LOLE and LOLP as interchangeable definitions.
- Ignoring transmission import limits and fuel supply when crediting capacity.
- Presenting a forecast as a single number rather than a weather-normalized distribution with an error band.