FR from Governors
Reliability
1.0 Governors
Generating units, with the exception of Solar generators and Wind generators, are equipped with Governors. These devices are set to raise generatos output as the Frequency decreases, and lower generators output as the Frequency increases. With the vertical axis being generation output, and the horizontal axis being frequency, stable governor control requires that the Generation-Frequency locus be a straight line with negative slope. The slope of the line is, in effect, referred to as the "Droop". This is illustrated in the diagram below which illustrates gevernor set points of full load, 70% load, and zero load, for a Droop of 5%.
1.1 Definition of Droop
Droop is defined as the frequency change required to have the governor make the output of the generator go from zero to full output, and it is expressed as a percentage of reference frequency, which in North America is 60 Hz. The graph above shows the generator output vs frequency for a 5% governor droop setting. The frequency change in Hz corresponding to a 5% Droop is calculated as (5/100)*60 = 3 Hz .
1.2 Slope of Droop Line
The slope m of the Droop lines above is given by:
m = Δ P/Δ f pu Mw/pu Hz Eq (1)
However:
Δ P=1, and Δ f=0.95-1 = -0.05 = -Dr where Dr is the Droop in pu.
Therefore :
m = -1/Dr
Using the result above, the pu change in the output of the generator for a Δ f change in the pu frequency, is givem by:
Δ P = m*Δ f = [-1/Dr]*Δ f Eq. (2)
Where Δ P is positive for frequencies below 1 pu
For a 5% droop Eq (2)this gives:
Δ P = 20*|Δ f| pu Eq.(3)
As the above equation is in per unit, this equation also applies to the entire interconnection with the power base set to the total generation remaining in service.
For the 5% droop illustrated, it is noted that :
i. “Full Load” line : No response if Δ f is negative. Full response if Δ f is positive
ii. “No Load” line : Full response if Δ f is negative. No response if Δ f is positive
iii. “70% Load” line : 30% response if Δ f is negative. 70% response if Δ f is positive
For the “Full Load” and the “No Load”, full response is achieved if Δ f gets to +3 Hz or -3Hz respectively. For the “70% Load”, if Δ f is negative, response is present only down to 59.1 Hz.