The modifications to the turbine and its governor shown on this page were installed on an identical turbine and generator set to the one shown in the next photograph.
The next photo shows a larger view of the turbine's governor valve and main linkage bar for a similar turbine. The governor is missing but would be connected to the top of the angled bar at the fore of the picture.
This turbine throttle retarding system was installed because, when the generator started automatically during a power failure or for a test, the turbine's speed had sometimes accelerated so quickly that it would overshoot its running and trip speeds by a considerable amount before the governor or the overspeed trip could control the admission of steam to the turbine. The turbine tripping during start tests which made it likely to be unreliable during an actual power failure.
On one occasion the generator shed its coils on start up due to centrifugal force resulting in long and expensive repairs.
The supply to the turbine was superheated - 650 PSI steam which was controlled by a valve that could not be slowed down enough to regulate the steam to the turbine and still maintain a reasonable start-up time. The turbine's exhaust pressure was 40 PSI.
The purpose of the retarder system is to limit the acceleration rate of the turbine by holding the turbine's throttle valve in a partially closed position until the governor can take control. This meant that the turbine would overshoot its normal operating speed by only a small percentage before the governor had taken full control.
As the turbine was still accelerating when the governor tried to take over, the governor will move the throttle valve closed (toward the turbine), allowing the retarder arm to release and drop out of the way of the main governor linkage's extension bar.
Once the retarding bar is clear of the extension bar, the turbine is under the control of the governor and the full range of throttle valve travel is available to control the turbine's steam supply.
The diagrams on this page are not to scale and are not correctly proportioned. They are meant only to illustrate how the system functions.
12 August, 2010