Power Grid Cruise Control
Power Grid Cruise Control
This is a brief explanation of the relationship between power grid frequency and power grid supply and demand.  The cost of power imbalance between supply and demand is also discussed.
Terminology
 F Frequency in Hz (Hertz) F0 Nominal Frequency: 60 Hz or 50 Hz ΔF Deviation in Frequency from “Nominal”:   F minus F0 P Power in MW (megawatt) S Apparent power in MVA (megavoltamp) P0 Size of the Power Grid in MW S0 Size of the Power Grid in MVA ΔP Power Imbalance  Between Supply and Demand:  Supply MW minus Demand MW G Grid Frequency Response Characteristic: Depends on Supply Governor Responses and Demand (load) Responses MCP Market Clearing Price in \$/MWh (dollars per megawatt-hour): or other currency depending on the country C Cost in \$/hour, or Other Currency Depending on the Country
Grid Frequency Response Characteristic
Grid frequency response, in this document, is given for steady-state conditions.  We look at grid frequency a few seconds after a disturbance.  The industry standard relationship is given as follows:

ΔP / P0 = G * ΔF / F0
G” is a grid parameter that depends on power supply Governor Response and how demand (load) changes with grid frequency.   Typically, G = 15.  Although this parameter varies for each power grid, the industry tunes this value for Cruise Control purposes.  Too high a value for G will cause control over-correction, while too low a value for G will cause sluggish control.
Example 1
F0 = 60 Hz
P0 = 200,000 MW
ΔP = -1000 MW  (loss of a large generator)
Calculate the Grid Frequency Response:
ΔF = (ΔP / P0) * (F0 / G) = (-1000 / 200000) * (60 / 15) = -0.02 Hz = -20 mHz
Example 2
F0 = 60 Hz
P0 = 200,000 MW
ΔF = 0.01 Hz
MCP = 50 \$/MWh
Calculate the Power Imbalance and the Cost to Bring Frequency Back to Nominal:
ΔP = G * (ΔF / F0) * P0 = (15) *  (0.01 / 60) *  (200000) = 500 MW (excess supply)
Cost = MCP * ΔP = (50) * (500) = 25,000 \$/h
Example 3
NERC considers the minimum setting for “G” for control purposes in USA to be:
A (1%) change in power causes a (0.1 Hz) change in frequency.
Calculate G:
ΔP / P0 = G * ΔF / F0
1% / 100% = 0.01 = G * 0.1 / 60
G = 6

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 Session # 220995 Copyright 2008-2010, GridTrak Updated: July 22, 2009