REVERSIBLE PUMP-TURBINE SYSTEM

Abstract: The present invention relates to a reversible pump-turbine system having a discharge adjustment device such as guide vanes, and particularly, makes it possible to safely stop a pump turbine at the time of emergency shutdown in generating mode by performing a close operation using an inlet valve even if abnormality has occurred with the guide vanes, without causing abnormal water hammer. Also, at the time of pump mode emergency shutdown, reverse rotation can be prevented even if abnormality occurs in the guide vanes, and it is possible to achieve a safe stop while reducing the loads /stresses to the reversible pump-turbine.

Description: The invention relates to a method for executing a shut down at the time of emergency shut down of a reversible pump-turbine system provided with an inlet valve and a discharge adjustment device for controlling adjustment of a discharge of a reversible pump-turbine connected to a generator-motor. In a conventional reversible pump-turbine, guide vanes are caused to close down with a pattern as shown by the dotted lines in Fig, 11 at the time of generation mode emergency shutdown and pump mode emergency shutdown. Description will first be given for generation mode emergency shutdown. In this situation, the guide vanes are suddenly closed at the same time as a circuit breaker of an electrical generator is opened. When the initial open amount is Yt0, the guide vanes are closed to open amount Yt1 at a first restricted rate (rapid closure rate) previously set for use in the generation mode that has been, as shown by the dotted lines in Fig. 11 and from that point it is automatically switched to a second previously set restricted rate (closure rate after change) for use in the generation mode, and after that the guide vanes are closed at that rate. These first and second restricted rates are rendered by restricting an amount of passing oil in a control valve for a guide vane servo motor driving the guide vanes. If the opening amount becomes less than Yt2, a fully closed end cushion of the guide vane servo motor is actuated, which causes the closure rate to be lowered even further, as shown by the dotted lines in Fig. 11. The reason for automatically switching from the first restricted rate (rapid closure rate) to the second restricted rate (closure rate after change) in this way with the opening amount at Yt1 is described in detail in Japanese Patent Laid-open Publication No. Sho. 61-47981, etc., by the inventor of this invention. That is, as a result of raising and lowering the unit speed N1 after the circuit breaker is opened, the operating point trajectory of the pump turbine breaks into S0 to S1 and S1 to S3, the latter moving in a S0-called S shaped characteristic(s) of the pump turbine, as shown by the dotted line in Fig. 7, and because tracing of the S-shaped characteristics in a direction of reduced discharge is commenced there is a need to switch the guide vanes to a slow closure rate before the commencement. What is meant by the S-shaped characteristic here is the portion of dQ1/dN1 > 0 as shown in Fig. 5. Next, emergency shutdown in pumping mode will be described. The guide vanes are also closed rapidly at the same time as the circuitbreaker is opened. One criteria for determining the guide vane closure pattern in this case will be described using Fig. 3. In the case where the guide vanes are closed rapidly, as shown by the solid lines in Fig. 3, the rotation speed is Na when the guide vanes become fully closed, while the rotation speed falls to Na' at the time of fully closed when the guide vanes is closed slowly, as shown by dotted lines in Fig. 3. Fig. 4 shows the relationship between time required for guide vanes to be fully closed and the rotation speed when the guide vanes are fully closed. As will be understood better from Fig. 4, if the time required to fully close the guide vanes becomes a long time, that is if the closure rate of the guide vanes is slow, Na' no longer has a positive value but has a minus value. What this actually means is that reverse rotation may arise in the pump mode. This reverse rotation is extremely damaging due to wear on a thrust bearing. Another criterion determining a guide vane closure pattern at the time of emergency shutdown in the pump mode will be described using Fig. 8. This description will be given with X0 and Xa as guide vane opening amounts, X0 > Xa. A trajectory of the operating point when the guide vanes are closed rapidly are shown by the dotted line in Fig. 8(a). By way of comparison, the case when the guide vanes are closed slowly is shown by the dotted line in Fig. 8(b). Reverse discharge, that is, reverse flow, arises when rotation speed is positive, namely during pump rotation. Even though a reversible pump-turbine has pump rotation, the operating condition becomes to allow turbine flow as accompanied with oscillations and noises. The problem is here that if the guide vane opening amount is too large when reverse flow appears, the operating conditions become bad. Depending on the situation, the reversible pump-turbine can also become damaged.Even further worse condition than in Fig8 (b) is possible. If the guide vanes are closed more slowly reverse rotation will occur too even in the pump mode. The guide vanes will be fully closed before long no matter how slowly the guide vanes are closed, and so the operating points finally converge at (0, 0). The guide vane closure pattern for pumping mode emergency shutdown as described above is determined not to allow reverse rotation, and so determined that reverse flow is suppressed to the minimum. The above describes conventional technology relating to guide vane closure patterns, and on the other hand conventional technology relating to inlet valve closure patterns is shown in Fig. 9. That is, with respect to an inlet valve the maximum closure rate is determined as a rate close to the second control rate and lower than the first control rate for the guide vanes at the time of emergency shutdown in the generating mode, and this rate is used for both the generating mode and the pumping mode. That is, the rate at the time of generating mode emergency shutdown and the rate at the time of pumping mode emergency shutdown are made the same. Strictly speaking, the load torque acting on the inlet valve is different in the generating mode emergency shutdown and pumping mode emergency shutdown, which means that, even if the inlet valve closing rate is set to the same rate at the time of a dry test, there are slight differences when the inlet valve is operated in water, but these can be ignored. A reversible pump-turbine that is the subject of the present invention generally carries out flow amount adjustment using the guide vanes, and so does not cover the case where usual discharge adjustment at the time of start-up etc. is carried out using an inlet valve. An inlet valve is considered whichis to be opened prior to opening the guide vanes and to be closed after closing the guide vanes, and used in maintenance and inspection operations of the reversible pump-turbine. However, in adopting the closure patterns of the above described conventional technology as closely as possible, it will be understood that the inlet valve can not achieve the desired back-up function at the time of pumping mode emergency shutdown. That is, consider the case of Fig. 2 where the guide vanes have failed and can not be closed at all. This can be understood on the basis of an extent to which the inlet valve, can carry out the water flow reducing function in place of the guide vanes. The inlet valve is closed in the pump mode at the same slow closure rate as in the generating mode, and naturally reverse rotation can not be avoided more or less. A typical example of the relationship between the time required to fully close the inlet valve and the rotation speed at the time when the inlet valve is fully closed will be described using Fig. 4. It is assumed that the guide vane closure pattern is as shown by the solid line in Fig. 3 and the inlet valve closure pattern is as shown by the dotted line in Fig. 3. In such a case, if it is assumed that the guide vanes have failed , the rotation speed at the time the inlet valve is fully closed is lowered to Na' and does not lead to reverse rotation. However, As shown in Fig. 4, if the time required to fully close the inlet valve is further prolonged the rotation speed at the time the inlet valve is fully closed is lowered rapidly, and if the time required to fully close the inlet valve becomes longer than Tax, reverse rotation becomes a possibility. In the above described conventional art, for example, even if reverse rotation has been avoided, a large amount of reverse flow is possible transiently during positive rotation, as shown in Fig. 8(b), and clearly the reversible pump-turbine is exposed to a dangerous condition. On the other hand, If the closure rate of the inlet valve is increased, the inlet valve is allowed to play a part in substantially throttling discharge at the time of generating mode emergency shutdown, and depending on the situation when the operation point of the pump turbine enters the S-shaped characteristics there is a possibility of abnormal water hammer. In US 4 624 622 A a reversible pump-turbine system is disclosed which is provided with an inlet valve and a discharge adjustment device for controlling adjustment of a discharge of a reversible pump-turbine connected to a generator-motor. This system comprises a fail-safe system which is used for opening and closing guide vanes of the pump-turbine in both of pump operation mode and turbine operation mode. The fail-safe system has a wire carried by a rotable sheave adapted for transmitting an information concerning an opening degree of the guide vanes, a weight suspended from one end of the wire, an operation rod diposed below and in alignment with the weight in the direction of action of the force of gravity, and a change-over valve adapted to be switched by the operation rod. When the wire is cut off accidentally, the weight drops onto the operation rod to depress the latter to switch the change-over valve so that a servo motor is switched to make the guide vanes close at predetermined rates of change of the opening degree in respective operation modes. From US 4 474 528 A a turbine system is known, wherein a back-up function for the discharge adjustment device at the time of emergency shutdown is provided by closing an inlet valve when a fail-to-close of the discharge adjustment device is detected. The problem underlying the invention is the provision of a method for executing a shut down at the time of emergency shut down of a reversible pump-turbine system which guarantees a back-up function for the dicscharge adjustment device at the time of emergency shutdown, functional in both operating modes of said reversible pump-turbine system, namely in the generating mode and the pump mode. This problem is solved by a method for executing a shut down at the time of emergency shut down of a reversible pump-turbine system according to claim 1. Preferred realizations of the method of the present invention are subject matter of claims 2 to 9. According to the present invention, by switching the closure rate of an inlet valve according to the operating mode, even if an abnormality arises in a discharge adjustment device, it is possible to achieve failsafe operation on the inlet valve side according to the operating mode at the time of the abnormality. Further, according to the present invention, by slowing the inlet valve closure rate to less than that of a guide vane second restricted rate in the generating mode, abnormal water hammer is prevented even when operation point of the pump turbine is tracing down the S-shaped characteristic, while a very fast closure rate enabling avoidance or reduction of reverse rotation and large reverse flow with positive rotation can be selected in the pumping mode. Also, if the inlet valve is closed at the time of pumping emergency shutdown in a time period that is longer than the time the guide vanes take to close under normal closure, or if the inlet valve is closed so that inlet valve opening area is kept larger than guide vane opening area all the way during normal closure, it becomes possible for the inlet valve to play a part in the failsafe operation while reducing or preventing the possibility of the inlet valve being used in substantial adjustment of discharge during normal operation of the guide vanes. According to the present invention, even in the case of pumping emergency shutdown, if the inlet valve is closed at a sufficiently fast rate so that reverse rotation does not occur when the inlet valve is fully closed, it is possible to prevent reverse rotation after that. Further, according to the present invention, at the time of pumping emergency shutdown, it is possible to limit actual damage to the machine by limiting opening of the inlet valve at the instant of rotation reversal smaller than opening of the guide vanes required for starting in the generating mode, even if reverse operation is unavoidable due to a failure of the discharge adjustment device. For reduction of the discharge at the time of pumping mode emergency shutdown, there are three governing factors, that is, closing of the discharge adjustment device, closing of the inlet valve, and lowering of the rotation speed of the reversible pump-turbine itself, but of these factors, lowering of the rotation speed of the reversible pump-turbine itself is an extremely significant. That is, even if the inlet valve is closed at a rapid closure rate, the extent to which it participates in throttling of discharge is not significant. Specifically, in this case, the object of the present invention can be achieved without exerting a large differential pressure on the inlet valve, i.e. without difficulty. In other words it would be reasonable to carry out switching of inlet valve for rapid closure only in the case where lowering of the rotation speed of the reversible pump-turbine itself predominantly affects throttling of discharge. Also, a low speed detector can be used in place of direct detection of power failure conditions, where the motor input power is lost or disconnected during the pumping operation, since the speed immediately drops. Further, if a failure detector is provided to detect a closing failure of the discharge adjustment device not in accordance with the desired rate, it is possible to carry out inlet valve rapid closure only in the case where an AND condition is satisfied with respect to operation of the low speed detector and operation of the failure detector. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a drawing showing an inlet valve closure method of the present invention. Fig. 2 is a drawing showing an inlet valve closure method of the present invention in the case where guide vane closure becomes impossible. Fig. 3 is a drawing showing the relationship between guide vane/inlet valve closure rate and rotation speed lowering amount in the case of pumping mode emergency shutdown. Fig. 4 is a drawing showing the relationship between time required for guide vane/inlet valve closure and rotation speed lowering amount in the case of pumping mode emergency shutdown. Fig. 5 is a drawing showing a four quadrant N1 - Q1 characteristic of a reversible pump-turbine. Fig. 6 is a drawing showing the relationship between inlet valve closure rate and rotation speed lowering amount in the case where guide vane closure has become impossible at the time of pumping mode emergency shutdown in the related art. Fig. 7 is a drawing showing a trajectory of operation point of a pump turbine at the time of generating mode emergency shutdown. Fig. 8 is a drawing showing a trajectory of operation point at the time of pumping mode emergency shutdown. Fig. 9 is a drawing showing an inlet valve closure method of the related art. Fig. 10 shows an example of the specific circuit structure for carrying out the inlet valve closure method of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the attached drawings. Fig. 1 shows an embodiment of the present invention. This drawing shows the emergency shutdown conditions for generating mode and pumping mode, and in the drawing a dotted line represents guide vane opening amount while a solid line represents inlet valve opening amount. At the time of generating mode emergency shutdown, the inlet valve is closed at a rate that is slightly lower than a second restricted rate for the guide vanes. If the inlet valve is closed in this manner, it will be safe even if, guide vane closure becomes impossible. This is because it is possible to avoid abnormal water hammer even if operating point of the reversible pump-turbine is tracing down the S-shaped characteristic. Since closure of the inlet valve is slow compared to the desired guide vane closure, the time period for which the rotation speed is high inevitably becomes longer, but it does not lead to damage of the reversible pump-turbine. Since, in the case of a reversible pump-turbine, raising of rotational speed is substantially limited by the existence of the S-shaped characteristic, it does not lead to an abnormal speed increase. Therefore, in generating mode the back-up function of the inlet valve takes effect to realize failsafe operation. In the example of Fig. 1, a cushioning is provided to reduce closing speed of the inlet valve at the closed end below Xt1 but it is not necessary for the purpose of the present invention. On the other hand, according to the reversible pump-turbine system of the present invention, at the time of pumping mode emergency shutdown the inlet valve is closed at a faster rate than a second limit rate for the guide vanes in the above described generating mode up to the point where the opening amount is Xp1, as shown in Fig. 1. From opening amount Xp1, closure is automatically switched to a lower rate. This change in the closure rate is to prevent water hammer caused by sudden throttling of water already flowing through the pump turbine in turbine direction as a result of flow reversal. With the slower closure rate of the related art, this change is not necessary. In the present invention, by carrying out control in this way it becomes possible to maintain the rotation speed at the time of inlet valve fully closed at a positive value including a desired margin, as shown by Na' in Fig. 4, or to prevent at least reverse rotation. As another embodiment of the present invention, for example, when an abnormality arises in pumping operation mode, it is possible to prevent an undesirable condition that the pump turbine comes in a reverse flow operation before the inlet valve has been closed to a safe opening much smaller than Yb in Fig. 5. It is also possible to have completed closure of the inlet valve down to an opening amount equivalent to a guide vane opening setting for generating mode starting, before the rotation reversal. If reverse rotation occurs with an opening amount larger than the above, operating conditions of the pump turbine becomes rough with large vibrations and noises. As a result, in the present invention, mechanical safety is ensured even when undesirable transfer to a substantial generating mode can not be avoided, as described above. Fig. 10 shows a hardware example for realizing the inlet valve closure method of the present invention. In this drawing, reference numeral 1 is an open or close signal to be supplied to a distributing valve for use with an inlet valve drive servo motor, and is + β(p.u.) for an open operation and - β(p.u.) for a close operation. Reference numeral 3 is an element for judging whether it is a close operation or an open operation, i.e. if a plunger of the distributing valve is moved up then an open side port is connected, while if the plunger is moved down a close side port is connected. Reference numeral 9 is an inlet valve opening speed setting element. Wa 〈 β. Specifically, when an open operation command is inputted, the seed of movement of the inlet valve servo motor is limited to Wa.K(pu/sec). In other words, upward movement of the plunger of the distributing valve is mechanically limited, and/or an amount of oils passing through the open side port of the distributing valve is throttled. Reference numeral 6 is a judging element for carrying out inlet valve rapid closure of the present invention limited to the case when input power during pumping operation is lost or disconnected. Normally, it is connected to the upper side and the closure rate is limited to Wb.K(p.u./sec) through a first closure rate limiting element 10. In the event that input power during pumping operation is lost or disconnected the judging element 6 is connected to the lower side and the closure rate is limited to Wc.K(p.u./sec) through the second closure rate limiting element 11. What the above actually means is that downward movement of the distributing valve plunger is mechanically restricted, and/or an amount of oil passing through the close side port of the distributing valve is throttled in response to the previously described operating mode. Here, |Wb| < |β|, |Wc| < |β|. Reference numeral 18 is the sum of outputs from three control elements 12, 13 and 14, but a selected control element is normally one from these three, and output from an element that is OFF is zero so no interference occurs among the three. INDUSTRIAL APPLICABILITY According to the present invention, an inlet valve performs a backup function for the guide vanes at the time of emergency shutdown, in both operating modes, namely in generating and in pumping. Specifically, at the time of generating mode emergency shutdown, abnormal water hammer arising due to a S-Shaped characteristic does not occur, even if closure of the guide vanes is not possible. Also, at the time of pumping mode emergency shutdown, even if closure of the guide vanes is not possible reverse rotation is not permitted or at least a large reverse discharge with positive rotation or reverse rotation under an excessively large opening are prevented, damage to the reversible pump-turbine and related devices is prevented and it is possible to maintain safety with minimum restriction.

Claims: A method for executing a shut down at the time of emergency shut down of a reversible pump-turbine system provided with an inlet valve and a discharge adjustment device for controlling adjustment of a discharge of a reversible pump-turbine connected to a generator-motor, characterized in that the inlet valve is closed by inlet valve closure means at a rate slower than a normal closure rate of the discharge adjustment device at the time of emergency shut down, wherein the closure rate of the inlet valve is switched according to an operating mode of the reversible pump-turbine. The method according to claim 1, characterized in that the valve is closed by automatic closure means at a higher rate than generating mode emergency shut down when a power supply to the generator-motor is disconnected or at the time of pumping mode emergency shut down for carrying out a stop operation in a condition where the generator-motor power supply is lost. The method according to claim 2, characterized in that at the time of pumping emergency shut down for executing a stop operation accompanying disconnection of a power supply of a drive power supply or in a power failure situation, inlet valve closure is carried out in a period of time longer than the time normally needed for the discharge adjustment device to substantially complete closure, and when closure of the discharge adjustment device is impossible, inlet valve closure is carried out in a time period in which rotation speed of the reversible pump-turbine at the time of closure of the inlet valve has not been reversed. The method according to claim 2, characterized in that at the time of pumping emergency shut down for executing a stop operation accompanying disconnection of a power supply of generator-motor or in a generator-motor power supply failure situation, inlet valve closure is substantially completed in a period of time longer a time needed for the discharge adjustment device in normal condition to substantially complete closure, and when closure of the discharge adjustment device is impossible, the inlet valve is closed at a speed sufficient to lower the opening area of the inlet valve at the time when reverse rotation occurs to a set opening amount, or less, of the discharge adjustment device at the time of generation mode starting. The method according to claim 3, characterized in that at the time of generating mode emergency shut down, inlet valve closure is carried out in a time period longer than a time period required for the discharge adjustment device in normal condition to substantially complete closure. The method according to claim 2, characterised in that at the time of pumping emergency shut down inlet valve closure is carried out so as to keep the opening area of the inlet valve larger than the opening area of the discharge adjustment device closing in normal condition, and if closure of the discharge adjustment device becomes impossible, inlet valve closure is carried out at such a rate that does not lead to reverse rotation of the reversible pump turbine before completion of closure. The method according to claim 3, characterized in that at the time of generating mode emergency shut down accompanying disconnection of output of the generator-motor, inlet valve closure is executed so as to keep opening area of the inlet valve larger than opening area of the discharge adjustment device closing in normal condition. The method acording to claim 1, characterized in that a low speed detector detects whether the rotation speed during pumping operation has fallen below a rated value; and closure means close the inlet valve at a high rate faster than the closure rate at the time of generating mode emergency shut down, in response to a signal from the low speed detector. The method according to claim 1, characterized in that a low speed detector detects whether the rotation speed during pumping operation has fallen below a rated value, a failure detection detects whether a closing operation of the discharge adjustment device has not ben carried out at a specified rate; and closure means close the inlet valve at a fast rate faster than the rate at the time of generating mode emergency shut down in response to operation of the low speed detector and the failure detector.

Inventors: HARADA MASATAKA, KUWABARA TAKAO, HARADA, MASATAKA, KUWABARA, TAKAO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: F03B15
IPC codes: F03B15