| Power Grid Cruise Control |
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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. |
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Terminology
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| F |
Frequency in Hz (Hertz)
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| 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 |
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| Grid Frequency Response Characteristic |
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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:
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ΔP / P0 = G * ΔF / F0
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“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.
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| Example 1 |
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F0 = 60 Hz |
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P0 = 200,000 MW |
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ΔP = -1000 MW (loss of a large generator) |
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Calculate the Grid Frequency Response: |
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ΔF = (ΔP / P0) * (F0 / G) = (-1000 / 200000)
* (60 / 15) = -0.02 Hz = -20 mHz |
| Example 2 |
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F0 = 60 Hz
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P0 = 200,000 MW |
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ΔF = 0.01 Hz |
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MCP = 50 $/MWh |
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Calculate the Power Imbalance and the Cost to Bring Frequency Back to Nominal: |
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ΔP = G * (ΔF / F0)
* P0 = (15)
* (0.01 / 60) * (200000) = 500 MW (excess supply) |
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Cost = MCP * ΔP = (50) * (500) = 25,000 $/h |
| Example 3 |
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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.
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Calculate G: |
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ΔP / P0 = G * ΔF / F0
1% / 100% = 0.01 = G * 0.1 / 60
G = 6 |