A hydro utility with more than 1,000 MW of capacity bought an 80-year-old hydro plant with a capacity of 8 MW. An inspection of its acquisition revealed the steel surge tank was severely rusted. The utility hired a local consultant with small hydro experience to design a new tank. The consultant proposed to replace the Johnson-type differential tank with a restricted-orifice surge tank with a diameter about 0.5 meter smaller, which would be less expensive.
During commissioning of the new tank, Paul, the plant operator, started testing of load rejections at 25 percent generator load. At the second test, for a 50 percent generator load rejection, the surge tank overflowed, to everyone’s surprise. Paul called George, another consultant, who obtained the drawings for the new tank, as well as copies of the plans and profiles for the old pipeline and penstock. George determined that the orifice on the new surge tank was too large, at 64 percent of the pipeline diameter. For a restricted-orifice tank, the rule of thumb is that the orifice diameter should be 45 to 55 percent of the pipeline diameter.
Paul asked the consultant to review the hydraulic design of the new tank. At the same time, George warned Paul that with the smaller orifice diameter, the penstock pressure rise on load rejection would be larger. This meant the penstock waterhammer should be determined with the new surge tank because it could affect the design of the penstock.
The consultant had written an Excel program for the hydraulic design of the new surge tank, and he had obtained the discharge coefficient for the orifice from a chart. Upon reviewing the chart, George noticed it was for multi-orifice pressure-reducing plates used at industrial plants, not for a single-orifice condition. Based on this feedback, the consultant increased the discharge coefficient to 0.62 from 0.25 and ran the program for various orifice sizes, in the region of 40 percent to 60 percent of pipeline diameter. Based on advice from the consultant, the utility installed an orifice with a diameter of 46 percent of the pipeline diameter. Once this change was made, commissioning of the surge tank was completed successfully.
George then tried to determine the effect of the smaller orifice on the penstock waterhammer and turbine speed rise. Paul informed him that governor time at this facility was about 10 seconds to close from full load opening. With this long interval, George determined the turbine would reach runaway (maximum turbine rotational speed) on any shutdown greater than about 60 percent generator load. Even with a faster governor time calculated to produce a 45 percent waterhammer (the normal maximum for a penstock), the speed rise would be to runaway.
George now focused on the generator inertia because a higher inertia would reduce speed rise. The inertia value George initially found was from a table for modern generators. However, generator rotors built before about 1940 are large and heavy due to the extra spacing between poles required to accommodate the thick insulation. This meant data for modern generators is not applicable to this older unit.
Finally, in looking through his library of hydro books for an inertia value for the unit, George found a table listing the inertias of Westinghouse vertical waterwheel generators. The book, Hydro-electric Handbook, was dated 1927. From this table, George was able to produce a formula for inertia of this particular unit as a function of kilovolt-amperes (kVa) and speed. This calculation resulted in a higher inertia than those supplied for modern units. Using this higher inertia, George determined that speed rise on full load rejection would be less than 50 percent with a governor time of 2.8 seconds. In addition, waterhammer would be less than 50 percent, an acceptable design for an isolated power plant.
However, the current governor close time was about 10 seconds. Should the utility shorten it to about 3 seconds?
After deliberation, Paul decided to retain the slow closing time. His intention was to keep penstock waterhammer as low as possible, in view of the fact that the steel penstock was quite deteriorated. Because this plant was connected to a large grid, fast governing times were not a necessity. Also, the well-built old Westinghouse generator had demonstrated that runaway speeds were not a problem.
First, always test a new computer surge tank design program on an existing tank, to determine whether it reproduces the range of water surge levels observed within the tank.
Second, if tank diameter is being decreased, it is important to assess the effect on the penstock, as the range of surge pressures will change.
Third, retaining information for old units often is difficult. Such esoteric data as generator inertia often is lost. In this case, resort must be made to old references. So don’t throw out that old book!
– By James L. Gordon, B.Sc., hydropower consultant