Pump-turbine

Details Pump-turbine Pump-turbine

1999-08-31

2001-04-03

Look, Edward K. (Department: 3745)

Rotary kinetic fluid motors or pumps

With control means responsive to non-cyclic condition...

C415S015000, C415S016000, C415S017000, C415S036000, C415S042000, C415S050000, C415S910000

Reexamination Certificate

active

06210100

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plurality of pump-turbines wherein an upper reservoir-side conduit or a lower reservoir-side conduit is branched and the conduit portions located farther than the branch points are shared among the pump-turbines, Particularly, the invention is concerned with a plurality of pump-turbines capable of diminishing water hammer.
2. Description of the Related Art
Generally, in conventional pump-turbines, different guide vane closing speeds are set for pumping operation and generating operation, respectively. For the closing speed at each operation mode there is adopted a closing method involving a so-called gooseneck in which the closing speed is changed over to a slow closing speed automatically when guide vanes have been closed to a predetermined opening or less. The reason for this will be described below while referring to a generating operation mode shown in FIG.
7
.
During a generating operation at a guide vane opening corresponding to a full load or a load close thereto, if the load of a generator connected directly to a pump-turbine is disconnected suddenly, that is, if the load is rejected, the rotational speed of the pump-turbine will rise temporarily. In an ordinary type of a pump-turbine, however, it is necessary to also fully satisfy the required characteristics in pumping operation in which the rotation is reverse, so that the diameter of each runner vane is set relatively large and some consideration is given to the runner shape so that a sufficient centrifugal force acts on water. Therefore, also in the generating operation, the influence of the centrifugal force acting on the water flowing down through the runner is serious. With an increase of the rotational speed, the amount of water entering the runner chamber decreases at a gradient which is far steeper than that in an ordinary type of a turbine, eventually leading to a reverse flow, namely the pumping flow. Thus, even without closing of the guide vanes, a mere increase in rotational speed of the turbine results in a steep gradient decrease of the turbine discharge. The higher the head of pump, the more marked this tendency. Particularly, in the case of a pump-turbine having an S-characteristic (dQ
1
/dN
1
>0, where Q
1
=Q/{square root over (H)}, N
1
=N/{square root over (H)}, Q: turbine discharge, N: rotational speed, H: effective head), the rotational speed increases, reaches a peak, then decreases, and just after this decrease there occurs a phenomenon such that the flow in the turbine direction suddenly shifts to the flow in the pump direction as if it were pulled in. This is due to a positive feedback phenomenon such that the increase of an effective head H caused by water hammer decreases the flow rate Q, giving rise to a further water hammer and causing a further increase of the effective head H, as is fully explained in Japanese Patent Laid Open No. 40946/79. As a result, the water pressure in the upper reservoir-side conduit of the pump-turbine increases abruptly. In contrast therewith, the water pressure in the lower reservoir-side conduit of the pump-turbine drops largely. If the rapid closing of guide vanes continues even at this time, this state indicates an overlapped form of the pumping action, especially a natural flow decreasing action based on the S-characteristic, with a rapid throttling effect induced by the guide vanes. This state is dangerous because the increase of water pressure in an iron penstock expands to an abnormal extent. The water pressure in the upper reservoir-side conduit rises to an abnormal extent and may exceed a designed level, while the water pressure on the lower reservoir-side drops to an abnormal extent and may cause a water column separation. The reason for this is explained in Japanese Patent Laid Open No. 143841/78, but is apparent from a complete characteristic graph of a pump-turbine shown in FIG.
8
. As shown therein, closing the guide vanes while following the S-characteristic in a flow decreasing direction means that an operation point N
1
=N/{square root over (H)} shifts in a direction in which it becomes smaller. This is because a rise of H is inevitable on the assumption that N is constant. The broken line in
FIG. 7
shows an example, in which an increasing range of water pressure Hp in the upper reservoir-side conduit increases although an increasing range of rotational speed becomes a little smaller.
According to the prior art, therefore, at a guide vane opening smaller than a predetermined opening, say 80%, in the generating operation mode, the upper limit of the guide vane closing speed is set smaller than the upper limit of the closing speed at a guide vane opening of 80% or more, allowing the closing speed to shift to a slower speed and thereby allowing the guide vane closing pattern to be goosenecked as shown by line
40
in FIG.
7
. For example, if there occurs a load rejection (time t
0
) at a guide vane opening close to 100%, the guide vanes close in a relatively rapid manner at the beginning, and at a time point ta corresponding to the arrival of the guide vanes at a preset opening Ya, the closing speed limit is changed over to a smaller value. Therefore, during the entry of an operation point into the S-characteristic and descent in the flow decreasing direction which operation point begins at the time when the rotational speed of the pump-turbine exceeds its maximum value and turns to decrease, the guide vane closing speed is limited to a relatively slow speed, whereby the foregoing abnormal increase of water pressure can be prevented. The solid line in
FIG. 7
represents the state of the water pressure Hp in the iron penstock and that of the rotational speed N in the case where the guide vane closing speed is goosenecked. As means for changing the guide vane closing speed according to the guide vane opening there is used a guide vane closing speed selector. For example, such a pump-turbine protecting device as disclosed in Japanese Patent Publication No. 38559/85 is known.
In the device disclosed in the Japanese patent publication, no consideration is given to a countermeasure to be taken in the event of failure of the guide vane closing speed selector itself or of a guide vane opening detecting means. In view of this point there also has been proposed a pump-turbine in which even in the event of such failure a guide vane gooseneck equal to that responsive to the guide vane opening mentioned above is ensured to ensure the safety of the pump-turbine in the state of load rejection. This known example is disclosed in Japanese Patent Laid Open No. 42441/96, in which it is intended to ensure a guide vane gooseneck even in the event the gooseneck responsive to the guide vane opening fails to operate, thereby ensuring the pump-turbine concerned, insofar as the rotational speed of the pump-turbine is above a predetermined value in the state of load rejection.
It is known that the relation between a guide vane closing pattern in the state of load rejection and water hammer, especially the rise of the water pressure Hp in the upper reservoir-side conduit is such as that shown in FIG.
6
. More specifically, if the guide vane opening Ya, which serves as a condition in changing over the guide vane closing speed from a rapid closing to a slow closing, is increased, the first peak value Hpx of the water pressure Hp in the upper reservoir-side conduit drops into Hpx
1
, while the second peak value Hpy rises into Hpy
1
. Although the waveform of the water pressure Hd in the lower reservoir-side conduit is not shown, it is such a waveform as is obtained by turning the Hp waveform upside down. The second peak value Hdy
1
is lower than Hdy. The Hp waveform changes also upon change of the speed limit on the rapid guide vane closing portion. That is, if the limit is made to a gentler gradient, the first peak value Hpx drops and the second peak value Hpy rises. The most typical example is the case where the rapid closing speed is equal to a slow clos

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