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Energy 101: HydropowerHistory of HydropowerWhat is Droop?

Energy 101: Hydropower

About 7% of the United States’ electricity comes from hydropower, making it the country’s largest source of renewable power. Of the 80,000 or so dams in the US, fewer than 3% are powered: there is huge potential to use existing structures to reduce our carbon footprint by adding generating stations to these existing structures.


History of Hydropower

Humans have been harnessing the power of water for thousands of years and for more than a century, the technology for using falling water to create hydroelectricity has existed.

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B.C. Hydropower was utilized by the ancient Greeks to turn water wheels that ground wheat into flour.
1802 The U.S. Army Corps of Engineers (USACE) was founded and tasked with the creation and operation of the US Military Academy at West Point. They would, in the 20th Century, come to lead federal flood control agency and become a major provider of the country’s hydroelectricity.
1870 Amos Woodward invents the non-compensating mechanical waterwheel governor and Woodward Governor Company was founded.
1881 The first hydroelectric generating station was built on the Niagara River powering local mills and village streetlamps when it went into operation as the Niagara Falls Power Co in 1895.
1882 The world’s first hydroelectric power plant began operation on Fox River in Appleton, WI.
1896 The first long-distance transmission of electricity began flowing from Niagara Falls to Buffalo, NY, over 25 miles away, thanks to the power generated by hydroelectricity.
1899 The River and Harbors Act of 1899 requires Congressional consent and USACE plan approval before new construction can begin on dams in any navigable US waterways.
1902 The U.S. Reclamation Service (USRS) is established to promote irrigation projects in the arid West to “reclaim” land for human use and settlement.
1906 USACE completes construction of Lock & Dam No. 1 in Minneapolis, MN.
1908 Congress authorizes a height increase for Lock & Dam No. 1 to allow for hydroelectric generation.
The earliest U.S. Reclamation Service hydroelectric plants go into operation.
1920 Congress establishes the Federal Power Commission to coordinate federally-controlled hydroelectric projects.
1923 USRS, now an independent bureau, is renamed the Bureau of Reclamation (USBR).
1928 Congress authorizes the Boulder Canyon Project, ratifying the Boulder Canyon Act and authorizing construction of the Hoover Dam.
1933 Congress establishes the Tennessee Valley Authority (TVA) to address a wide range of environmental, economic, and technological issues. TVA built dams to harness the region’s rivers controlling floods, improving navigation and generating electricity.
1940s USBR dams ramped up power output to support America’s efforts in World War II, producing enough electricity to make 69,000 airplanes and 5,000 ships and tanks during a five year period.
1986 The Water Resources Development Act detailed that non-federal interests should shoulder more of the burden and expense in maintaining aging water resources infrastructure. An update to this act passed in the Senate in May 2013 and requires passage by the House of Representatives before potentially being signed into law by the President.
2000 Woodward Governor is acquired by GE Power Systems. GE discontinues support to Woodward’s legacy hydroelectric product line.
American Governor Company is founded and continues providing top quality parts and services to the world’s hydroelectric power plants abandoned by the Woodward acquisition.
2009 U.S. Dept. of Energy Secretary Steven Chu announces up to $32M in Recovery Act funding to modernize existing hydropower infrastructure in the U.S. to increase efficiency and reduce environmental impact.
2013 President Barack Obama signs the Hydropower Regulatory Efficiency Act and the Bureau of Reclamation Small Conduit Hydropower Development and Rural Jobs Act into law.
Present & Future A vast expansion of hydropower’s potential is possible through new technologies for conventional, pumped storage and marine and hydrokinetic projects, modernizing existing hydropower facilities and adding generation to existing non-powered dams.

Sources: The U.S. Army Corps of Engineers: A Brief HistoryNiagara Power Project, The Library of Congress, Tesla Memorial Society of New YorkRivers and Harbors ActsUpper Mississippi Locks & Dams, History of FERCBrief History of the Bureau of Reclamation, From the New Deal to a New Century, Woodward Governor Company History, Hydropower Milestones

What Is Droop?

Permanent Speed Droop allows generators to be paralleled to a common grid. In the following discussion all references to “Speed Droop” are referring to Permanent Speed Droop.

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Permanent Speed Droop
With no permanent speed droop on the governor, a unit that is paralleled to the grid would respond in one of two ways:

  1. The unit would eventually go to 100% gate or wherever the governor gate limit was adjusted.
  2. The unit would eventually go to zero gate position.

Utilities would find this type of governor control unacceptable, because they want to be able to predict what load each generating unit carries. It must be remembered that whenever the governor opens the wicket gates, load (Generation) will be added to the grid. The permanent speed droop characteristic of the governor allows the utilities to be able to predictably set the load level of each generating unit connected to the grid.

The other characteristic that Speed Droop provides when it is adjusted to any value other than zero is controlling the amount of the governor participation to frequency excursions that occur on the grid.

Speed Adjustment
Before discussing more about Speed Droop another governor characteristic must be defined and that is the governor Speed Adjustment control. The governor speed adjustment controls on most mechanical governors allow the turbine/generator speed to be controlled from 85% normal speed to 105% normal speed. The governor Speed Adjustment indicating dial indicates 100% unit speed when both the red and black hands are pointing in the 12 O’clock position (on cabinet actuators). Although the dial on the Speed Adjustment indicator reads zero at this position it really is the setting for 100 % normal unit speed. When the generator is connected to the grid and the governor has been adjusted for 5% Speed Droop, it merely indicates that it takes a 5% change in the Speed Adjustment control or the system frequency to make the gates move 100%.

Permanent Speed Droop Defined
The magnitude of wicket gate movement, in response to system frequency changes or speed changer adjustments by the operator to adjust load, depends on the droop setting on the governor. Droop can practically be thought of as the inverse of gain between the speed changer position and the wicket gate position.

  • At 5 percent droop the gain is 20:1.
  • A 1 percent speed reference change would then cause: 1 percent x 20 (Gain) = 20 percent gate change.
  • At 2 percent droop the gain is 50:1.
  • A 1 percent speed reference change would then cause: 1 percent x 50 (Gain) = 50 percent gate change.


Figure 1

The normal range of the Speed Droop settings on most mechanical governors is from 0 to 5 %. There are some special cases where 6 % or even 10 % have been used on Impulse Turbine Governors. When Speed Droop is set on the governor, the effective range of the Speed Adjustment is reduced to that of the setting of the Speed Droop control. For example if 5% Speed Droop is adjusted on the governor the effective range of the Speed Adjustment is 5%. If for example 2 % Speed Droop is adjusted on the governor, then the effective range of the Speed Adjustment is 2%. Refer to Figure 1 above.


Figure 2

From the Droop curve (above), it becomes apparent that the Speed Droop Curve pivots at 50% gate position when the speed adjustment is at 100%. Therefore, a unit with a governor set on Speed Droop and 100% speed adjustment will run above normal speed. In order to synchronize the generator to the grid, the speed of the generator must be reduced. In order to do this the unit Speed Adjustment must be used. As shown in Figure 2 the unit Speed Adjustment is able to control the actual unit speed from 102.5% to 97.5%. If speed no load were to be 10% gate position the unit Speed Adjustment would have to be adjusted to 98% in order to synchronize the generator to the grid.

Once the unit has been synchronized to the grid, the unit Speed Adjustment can be used to increase generation by having the gates open as a function of the unit Speed Adjustment setting. From the Droop curve it can be seen that by increasing the Speed Adjustment from 98% to 100% the gates will open to 50% gate. This is a change of 20% gate for a change of 1% Speed Adjustment.

As long as the grid frequency remains at 60 Hz (100% normal speed) the Speed Adjustment will have the relationship as shown in the Speed Adjustment curve shown in Figure 2. Assume a unit is connected to the grid, with 5% Speed Droop and the Speed Adjustment is set for 100% and the gates are at 50% position. If the grid frequency increases 1% or to 60.6 Hz the gates will go closed 20% or from 50% gate position to 30% gate position or open 20% to 50% gate on a frequency drop to 59.4 Hz.

The above description of droop operation is based on setting up the unit off-line with the field breaker closed, droop at zero, speed changer at 100% and running at 60 Hz.

Checking Speed Droop
Droop is set with the dial on the front of the governor cabinet, and calibration of the dial is through either a turnbuckle link (Woodward Governors) or a slotted lever (Pelton Governors) behind the front of the cabinet. Most Woodward Gateshaft and “A” Actuators have adjustable droop cams which when fully open provide 5% droop. Usually percent of droop indication was not provided.

An easy check of droop calibration is to:

  1. Set droop to 5%.
  2. Place the unit on-line tied to large power system and make one percent adjustments with the speed changer. Each one percent speed changer adjustment should produce approximately a 20 percent gate change. An acceptable range for a typical mechanical governor is between 18% and 22%.

On some mechanical governors droop calibration and adjustment will change the speed-no-load position on the speed changer. With the unit off-line the speed-no-load setting should be established by setting droop to zero and the speed changer to100 %, and adjusting the floating lever connecting rod on Woodward Governors, or the pilot valve setscrew on Pelton Governors, to produce 60 hertz generator frequency.

Download our Speed Droop Calculator.