Droop speed control From Wikipedia, the free encyclopedia
In electrical power generation, Droop Speed Control is a speed control mode of a prime mover driving driving a synchronous generator connected connected to an electrical grid. grid. This mode allows synchronous generators to run in parallel, so that loads are shared among generators in proportion to their power rating. The frequency of a synchronous generator is given by
where F = Frequency (in Hz), P = number of poles, N = Speed of generaor (in !P")
The frequency (F of a synchronous generator is directly proportional to its speed (!. When multiple synchronous generators are connected in parallel to electrical grid, the frequency is fi"ed by the grid, since individual power output of each generator will be small compared to the load on a large grid, and thus all the synchronous generators run at the same speed (!. # speed reference as percentage of actual speed is set in this mode. #s the generator is loaded from no load to base load, the actual speed of the prime mover tend to decrease. In order to increase the power output in this mode, the prime mover speed reference is increased. $ecause the actual prime mover speed is fi"ed by the grid, this difference in speed reference and actual speed of the prime mover is used to increase the flow of working fluid (fuel, steam, etc. to the prime mover, and hence power output is increased. The reverse will be true for decreasing power output. The prime mover speed reference is always greater than actual speed of the prime mover. The actual speed of the prime mover is allowed to %droop% or decrease with respect to the reference, and so the name. For e"ample, if the turbine is rated at &''' rpm, and the machine speed reduces from &''' rpm to ))' rpm when it is loaded from no load to base load, then the droop * is given by
+(&''' &-' &''' + /* + (&''' ))' &''' + /* In this case, speed reference will be -'/* and actual speed will be -''*. For every -* change in the turbine speed reference, the power output of the turbine will change by 0* of rated for a unit with a /* droop setting. 1roop is therefore e"pressed as the percentage change in (design speed required for -''* governor action. For e"ample, how fuel flow is increased or decreased in a 234design heavy duty gas turbine can be given by the formula, FS!N = (FS#!N$ % (&N!'&NH))
FS#!N
Where, FS!N &N! &NH FS#!N$ FS#!N
= = = = =
Fuel Sro*e !eference (Fuel supplied o +as &urbine) for droop mode &urbine Speed !eference cual &urbine Speed -onsan -onsan
#s frequency is fi"ed on the grid, and so actual turbine speed is also fi"ed, the increase in turbine speed reference will increase the error between reference and actual speed. #s the difference increases, fuel flow is increased to increase power output, and vice versa. This type of control is referred to as %straight proportional% control. If the entire grid tends to be overloaded, the grid frequency and hence actual speed of generator will decrease. #ll units will see an increase in the speed error, and so increase fuel flow to their prime movers and power output. In this way droop speed control mode also helps to hold a stable grid frequency. The amount of power produced is strictly proportional to the error between the actual turbine speed and speed reference. The above formula is nothing but the equation of a straight line (y = mx + b). 5ultiple synchronous generators having equal * droop setting connected to a grid will share the change in grid load in proportion of their base load. For stable operation of the electrical grid of !orth #merica, power plants typically operate with a four or five percent speed droop. 6-7 With 0* droop the full4load speed is -''* and the no4load speed is -'0*. This is required for the stable operation of the
net without hunting and dropouts of power plants. !ormally the changes in speed are minor due to inertia of the total rotating mass of all generators and motors running in the net.67 #d8ustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor or by an engine control unit ad8ustment. 2enerally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication. 9oltage control of several power sources is not practical because there would not be any independent feedback, resulting in the total load being put on one power plant. 6&7
:ontiguous ;nited
It can be mathematically shown that if all machines synchroni=ed to a system have the same droop speed control, they will share load proportionate to the machine ratings. 6/7 The thousands of #: generators are running synchronously with the power grid which acts like an infinite sink. !e"t to the inertia given by the parallel operation of synchronous generators,607 the frequency speed droop is the primary instantaneous parameter in control of an individual power plant>s power output (kW.6?7
S is the ratio of frequency deviation when comparing the load versus the nominal frequency.
Notes -. . &.
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See also
3lectric power transmission Wide area synchronous grid 1ynamic demand (electric power
Further reading
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#lfred 3ngler@ Applicability of droops in low voltage grids. International Cournal of 1istributed 3nergy Aesources, 9ol -, !o -, ''0.
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