Dump load arrangement with submersible load sets and hydropower unit

Abstract

A dump load arrangement (30) comprises at least two dump load sets (31), a dump load connection network (37) and a switch control (36) The at least two dump load sets are configured to be submerged in water (40). Each dump load set comprises at least one phase dump load (32). The dump load connection network has an input connector (33), and a separate output connector (34) for each dump load set. The dump load connection network has a controllable electrical switch (35) for each of the output connectors, operated by the switch control. The electrical switch for one dump load set is operated independently from the electrical switch for other dump load sets for controlling a total electrical resistance of connected dump load sets. A hydropower unit (1) comprising a dump load arrangement and a method for discharging water from a hydropower unit dam (41) are also disclosed.

Description

[0001] DUMP LOAD FOR FREQUENCY CONTROL

[0002] TECHNICAL FIELD

[0003] The present technology concerns in general to arrangements and methods for frequency control in electrical networks and in particular to frequency control in connection with generators operating in electrical islanding mode.

[0004] BACKGROUND

[0005] Hydropower generator facilities are typically provided in direct connection to a water dam. Rain and / or melt water are provided intermittently, and in order provide a continuous operation of the hydropower generator facility, the water is collected in a dam, from which a more continuous flow of water can be extracted. The dam has a certain storage capacity, which preferably is adapted to the expected variations in water supply over the year. If extreme amounts of water are supplied by the flow into the dam, there are always also emergency arrangements, such as spillgates, spillways etc., to enable the discharge of water besides the dam in order to save the dam against breaking. However, such arrangements are typically expensive to build and maintain in operational conditions despite not being used for longer times or protected against e.g. freezing of vital parts. Also, the ongoing climate changes will result in higher variations in e.g. rainfall, which probably will lead to increased demands for emergency discharge arrangements in areas with expected increased rainfall.

[0006] The water that during operation is flowing through the turbines can be seen as a part of the capacity of discharging water from the dam. However, extreme weather conditions that typically cause flooding of the dam are also associated with a very high probability that the electrical grid is damaged. Operating the turbines when any load of an external grid is lacking will lead to runaway of the turbine, potentially causing damages thereto. When the contact to an external grid is removed, the turbines have to be stopped, which also makes it impossible to use the ordinary waterways through the turbines as emergency discharge paths. When calculating the available emergency discharge capacity, the capacity of the turbine cannot be included.

[0007] In the published international patent application WO 2014 / 027951 Al, an arrangement for overspeed reduction in water power plants was disclosed. An electrical load is connected when the rotational speed exceeds a predetermined threshold. Shutdown of water power plants can then be performed in a reliable and lenient manner, allowing e.g. for an increase in closing time.

[0008] In the article “Adding discharge capacity and redundancy using existing turbines” by U. Lundin et al, in Hydropower & Dams, issue five, 2021, pp. 78- 82, it was proposed to use an electrical load in the form of a boiler to act as a local dump-load connected directly to the generator terminals to dissipate energy from the generator. Electrodes put directly into flowing water connected to internal waterways along the side of the draft tube were possible examples of such dump-loads.

[0009] Even if the prior art provides techniques for operating a water power plant for some time without external grid connections, the methods are not reliable enough to ensure that the dump-load receives the intended power associated with the intended flow of water through the turbine water ways.

[0010] SUMMARY

[0011] A general object of the present invention is to improve the possibilities to run a hydropower generator even during electric island conditions.

[0012] The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims. In general words, in a first aspect, a dump load arrangement comprises at least two dump load sets, a dump load connection network and a switch control. The at least two dump load sets are configured to be submerged in water. Each dump load set comprises at least one phase dump load. The dump load connection network has an input connector, and a separate output connector for each dump load set. The dump load connection network has a controllable electrical switch for each of the output connectors. The switch control is configured for operating the electrical switches. The electrical switch for one dump load set is operated independently from the electrical switch for other dump load sets for controlling a lumped electrical resistance of connected dump load sets. The operating of the electrical switches is dependent on a conductivity of the water or on a quantity that is dependent on the conductivity of the water.

[0013] In a second aspect, a hydropower unit comprises a hydropower generator and a dump load arrangement according to the first aspect connected to the hydropower generator.

[0014] In a third aspect, a method for discharging water from a hydropower unit dam comprises establishing that a hydropower generator of the hydropower unit is running while lacking any external grid presently providing access to an outer load for electrical power produced by the hydropower generator. Connection of one or more dump load sets of at least two dump load sets to the hydropower generator is controlled. The phase dump loads of the at least two dump load sets are submerged in water. Power produced by the hydropower generator becomes dissipated as heat into the water while allowing a hydropower unit discharge to be used as a complementary spillway of the dam.

[0015] One advantage with the proposed technology is that a reliable operation of the hydropower generator without connection to an external electrical grid can be ensured. Other advantages will be appreciated when reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

[0017] FIG. 1 is a schematic drawing of an embodiment of a hydropower unit having a dump load arrangement;

[0018] FIG. 2 illustrates schematically an embodiment of a dump load arrangement;

[0019] FIG. 3 illustrates schematically another embodiment of a dump load arrangement;

[0020] FIG. 4 is a flow diagram of step of an embodiment of a method for discharging water from a hydropower unit dam;

[0021] FIG. 5 is a diagram illustrating one embodiment of controlling a total power capacity having access to three load dump sets; and

[0022] FIG. 6 is a diagram illustrating one embodiment of controlling a total power capacity having access to four load dump sets.

[0023] DETAILED DESCRIPTION

[0024] Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

[0025] For a better understanding of the proposed technology, it may be useful to begin with a brief discussion of the drawbacks of prior art. The solution presented in the published international patent application WO 2014 / 027951 A 1 provides indeed a possibility to avoid turbine runaway if connections to an external load disappears, i.e. in an islanding mode, or for quick stops of the operation. The solution is typically based on a load in for of an electrode submerged in an electrical boiler, where the electrical energy is transferred into heat dissipated into the fluid of the boiler. The operation was intended for short-term situations and the electrical boiler did not have capacity enough for taking care of the heat dissipation for any longer electric island operation times.

[0026] The capacity problem could be solved by arranging for an electrode submerged in a flow of water. However, flow of water just for this purpose requires a separate system of pipes and valves that for most of the time is not used. Such seldom used systems often have problems with corroded valves or clogged pipes and in order to ensure a safe operation of such systems, frequent service check have to be performed. Since these systems indeed are intended to be used very seldom, such service schedules are not wanted.

[0027] In the article of U. Lundin, the idea of using the water downstream of the turbine or in internal waterways was launched. However, since the temperature of this water typically follows the temperatures of the different seasons and the electrical conductivity of water is highly dependent on temperature, the load capacity of an electrode submerged in such water will vary considerably over the seasons, which prevents it from assuring a proper operation at all times. Also here, the problem of service of pipes and valves during inactivity periods is present.

[0028] There is now an idea of controlling a dump load to be submerged into water so that the magnitude of the dump load can be adapted to the actual water conductivity. Seldom used valves, pipes or pumps are not very appropriate to use. Instead changes in electrical connections are utilized.

[0029] Figure 1 illustrates schematically an embodiment of a hydropower unit 1. The hydropower unit 1 comprises a hydropower generator 10 connected to a turbine 20. The hydropower generator 10 is during normal operation connected to an external electrical grid 90, for delivering electrical power thereto. The hydropower unit 1 further comprises a dump load arrangement 30 connected to the hydropower generator 10. The operation of the hydropower generator 10 is controlled by a hydropower generator controller 11. The turbine 20 connected to the hydropower generator 10 is provided in an internal waterway 21 transporting water 40 stored by a dam construction 41 from the dam reservoir 42 to a water outlet 43. The potential energy of the water in the dam reservoir 42 is transferred into kinetic energy of the water flowing through the internal waterway 21 and pressure energy therein. The water flowing through the internal waterway 21 causes the turbine 20 to rotate and create a torque on the shaft, that together with the rotational motion is transferred into electrical energy by turning a shaft of the hydropower generator 10.

[0030] The dump load arrangement 30 comprises at least two dump load sets 31. The dump load arrangement 30 of present embodiment has three dump load sets 31. The dump load sets 31 are configured to be submerged in water 40. Each dump load set 31 comprises at least one phase dump load 32. In the present embodiment, one dump load set 31 comprises two phase dump loads 32, while the two other dump load sets 31 comprise one phase dump load 32A each, dump load connection network 37 has an input connector 33, connected to the output of the generator 10. The dump load connection network 37 has further a separate output connector 34 for each dump load set 31. The dump load connection network 37 also has a controllable electrical switch 35 for each of the output connectors 34. A switch control 36 is configured for operating the controllable electrical switches 35.

[0031] In a preferred embodiment, each phase dump load comprises an electrode arranged to be in contact with water when the dump load set is submerged in water.

[0032] In order to be able to adapt the magnitude of the used dump load to the actual temperature conditions as well as chemical conditions, the electrical switch 35 for one dump load set 31 is operated independently from the electrical switch 35 for other dump load sets 31 for controlling a lumped electrical resistance of connected dump load sets 31. By having this possibility to adapt the total electrical resistance of connected dump load sets 31 , there will always be a selection of dump load sets that is appropriate to take care of the desired electrical power from a generator at any water temperature, which will in turn correspond to a desired flow. This in turn ensures that the turbine and generator can be driven constantly over relatively long periods for e.g. assisting in providing water discharge from a dam without damaging the turbine or generator.

[0033] At the same time, the same set-up can additionally be used for providing slow shut-down of a hydropower plant without water surging in the internal waterways or in the dam. It may also be used together with a low power request from a connected power grid to avoid operation at too low turbine set point. For turbines with slow control due to slow hydraulic equipment, the equipment may also be utilized to achieve a fast regulation speed. The dump load arrangement 30 may also be used to avoid rotational speed rushes upon disconnections from external loads.

[0034] A preferred way to perform the adaptation to actual environmental conditions is to let the electrical switch operation be dependent on a conductivity of the water 40 or on a quantity being dependent on the conductivity of the water 40. A measure of the conductivity can for instance be provided from any external means and entered into the switch control 36. Alternatively, different kinds of measurement solutions can be used.

[0035] One embodiment of adaptation utilizes sensors. To this end the dump load arrangement 30 further comprising a water conductivity sensor 50 configured for measuring the conductivity of the water 40. The switch control 36 is communicationally connected to the water conductivity sensor 50 and configured for operating the electrical switches 35 dependent on a water conductivity reading from the water conductivity sensor 50. Figure 2 illustrates schematically one embodiment of a switch control 36. The dump load arrangement 30 here comprises a power meter 52 configured for measuring distributed power through the input connector 33. The power distributed through the input connector 33 together with knowledge of how many and which dump load sets that presently are used makes it possible to estimate a present water conductivity. The switch control 36 is communicationally connected to the power meter 52 and configured for operating the electrical switches 35 dependent on a power reading from the power meter 52. A too low distributed power calls for connection of additional dump load sets or connecting another selection of dump load sets offering a higher power capacity, while a too high distribute power calls for disconnecting dump load sets presently in use or connecting another selection of dump load sets offering a lower power capacity.

[0036] Alternatively, or in combination, the dump load arrangement 30 may comprise a power meter 51 configured for measuring distributed power through all output connectors 34. The switch control 36 is then communicationally connected to the power meter 51 and configured for operating the electrical switches 35 dependent on a power reading from the power meter 51.

[0037] When the input connector 33 of the dump load arrangement 30 is connected to an electrical output 12 of a hydropower generator 10 as is schematically illustrated in Figure 3, readings from the hydropower generator controller 11 can be utilized. The switch control 36 is communicationally connected to a hydropower generator controller 11 for obtaining a measure of distributed power from the hydropower generator 10. This gives knowledge about the needed power dissipation through the dump load arrangement 30. If the hydropower generator 10 is disconnected from any external power grid, the entire power from the hydropower generator 10 has to be dissipated through the dump load arrangement 30. If there still are minor external loads, e.g. during an electric island operation, a part of the generated electrical power has to be wasted by means of the dump load arrangement 30. The switch control 36 is then configured for operating the electrical switches 35 dependent on the measure of distributed power from the hydropower generator 10.

[0038] If the hydropower generator 10 is a three-phase generator, the input connector 33 is preferably a three-phase connector, the output connectors 35 are preferably three-phase connectors and the controllable electrical switches are preferably three-phase switches.

[0039] When dump load arrangement 30 is being used for ensuring an additional water discharge from a dam, the dump load arrangement 30 is used primarily when there is no other load than the dump load arrangement connected to the hydropower turbine. This may e.g. be the case when an external electrical grid is inoperable, e.g. due to weather conditions. In one embodiment, the switch control is configured to enable connection of the dump load sets only if no other load than the dump load arrangement is connected to the hydropower turbine.

[0040] Figure 4 is a flow diagram of steps of an embodiment of a method for discharging water from a hydropower unit dam. In step S 10, it is established whether or not a hydropower generator of the hydropower unit is running while lacking any external grid presently providing access to an outer load for electrical power produced by the hydropower generator. If it is established that there is no access to outer load, the procedure continues to at least step S30. Otherwise the process returns for further controls at a later time. In step S30, connection of one or more dump load sets of at least two dump load sets to the hydropower generator is controlled. Phase dump loads of the at least two dump load sets are submerged in water. Power produced by the hydropower generator becomes dissipated as heat into the water while allowing a hydropower unit discharge to be used as a complementary spillway of the dam. The control is continued until it in step S40 is determined that the hydropower unit no longer is running without external electrical grid. In a preferred embodiment, in step S20, a measure of a conductivity of the water or a measure of a quantity being dependent on the conductivity of the water is obtained. The step S30 of controlling the connection of one or more dump load sets is thereby depending on the measure of a conductivity of the water or the measure of a quantity being dependent on the conductivity of the water.

[0041] In theory, a most even power dissipation in water can be achieved by using a large amount of dump load sets, since every minor change in water conductivity can be met by connecting / disconnecting a small dump load set. However, the connection equipment as well as the control equipment rapidly becomes expensive and complex upon increasing the number of possible dump load sets to connect.

[0042] One way to increase the controllability accuracy without increasing the number of used dump load sets is to use dump load sets of different power capacity. In other words, in one embodiment, a power capacity of one of the at least two dump load sets is different from a power capacity of another of the at least two dump load sets, at corresponding water conditions.

[0043] Figure 5 illustrates an experiment using three dump load sets of differing power capacity at different water conductivity conditions. Note that the numbers given here below are just non-limiting examples used only for demonstrating the principle of the control. The lumped power capacity, illustrated as curve 200, was intended to be kept close to 6.4 kW. At a water conductivity of 20 pS / cm, the first dump load set, represented by curve 201, presented a power capacity of around 3.6 kW, the second dump load set, represented by curve 202, presented a power capacity of around 3.0 kW and the third dump load set presented a power capacity of around 1.3 kW. By connecting the first and second dump load sets, a lumped power capacity of around 6.6 kW was obtained. At a water conductivity of 22 pS / cm, the power capacity has increased considerably, and a combination of the first and third dump load sets, i.e. curves 201 and 203, instead gave a lumped power capacity of around 5.5 kW. By combining the three dump load sets in different combinations, the lumped power capacity 200 was possible to keep within about ±15% around the target value of 6.4 kW for the whole range of water conductivities from of 20 pS / cm to 45 pS / cm. In one embodiment, the dump load arrangement comprises three dump load sets of differing power capacity.

[0044] Figure 6 illustrates an experiment with four dump load sets of differing power capacity, represented by the curves 204, 205, 206 and 207, respectively. In this example, by combining the four dump load sets in different combinations, the lumped power capacity was possible to keep within about ±7% around the target value of 6.4 kW for the whole range of water conductivities from of 20 pS / cm to 45 pS / cm. In one embodiment, the dump load arrangement comprises four dump load sets of differing power capacity.

[0045] It is easily understood that further improvements may be obtained by including further dump load sets, however, a practical limit is presently considered to be at the most five or six dump load sets.

[0046] According to the ideas of the present technology, excess produced electric power is dissipated as heat in water. This water can be of any kind, but preferably of such a volume that the dissipated heat does not change the overall water temperature too much. When the dump load arrangement is applied in a hydropower unit, there is access to large amounts of water, typically both in the dam above the hydropower unit as well as in water that has passed the hydropower unit. The dump load sets can advantageously be placed in either of these water volumes, and the individual arrangement is preferably decided on in dependence of the actual surroundings of the hydropower unit. In other words, the dump load sets can be positioned upstream or downstream of the hydropower unit. It is also possible to use a separate stream for the dump loads, but this may require additional valve constructions. The electrodes of the dump load sets are preferably positioned along a streaming direction of the water, in order to maximize the heat exchange between the electrodes and the water without changing the water temperature too much. Also this is preferably adapted to the individual site of installation.

[0047] The actual installation of the dump load sets may also give the different installations slightly different power capacity. Approximate relations may typically be available from earlier operations or from other sites. The power capacity may also differ between installation sites due to e.g. the composition and amount of chemical substances dissolved in the water. However, in a preferred embodiment, a calibration of the dump load sets may be performed when installed and may even be performed in the beginning of an actual operation.

[0048] When the operation is started, one of the dump load sets is connected and the power capacity of its power dissipation is measured. The dump load set that is believed to best correspond to the required power capacity is preferably selected-. This can be performed by measuring the power to the dump load arrangement or to achieve information from the control unit of the hydropower generator itself. The power information is then used to calculate a corresponding water conductivity. Corresponding values for the other dump load sets can then be calculated and a proper selection of dump load sets can be performed based on the deduced water conductivity. If the temperature of the water changes, a corresponding expected change in water conductivity can be used. Alternatively, if the temperature changes considerably, a new calibration can be made.

[0049] If the operation dependency of the water conductivity and temperature has been calibrated, the operation can be simplified by just using measured values of temperature and / or water conductivity to decide which selection of dump load sets that is the optimum at each occasion. The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

CLAIMS1. A dump load arrangement (30), comprising:- at least two dump load sets (31) configured to be submerged in water (40), wherein each dump load set (31) comprises at least one phase dump load (32);- a dump load connection network (37) having an input connector (33), and a separate output connector (34) for each dump load set (31); said dump load connection network (37) having a controllable electrical switch (35) for each of said output connectors (34); and- a switch control (36) configured for operating said electrical switches (35); wherein said electrical switch (35) for one dump load set (31) is operated independently from said electrical switch (35) for other dump load sets (31) for controlling a lumped electrical resistance of connected said dump load sets (31); wherein said operating of said electrical switches (36) being dependent on a conductivity of said water or on a quantity being dependent on said conductivity of said water (40) .

2. The dump load arrangement according to claim 1, characterized by further comprising a power meter (52) configured for measuring distributed power through said input connector (33) and / or all output connectors (34), whereby said switch control (36) is communicationally connected to said power meter (52) and configured for operating said electrical switches (35) dependent on a power reading from said power meter (52).

3. The dump load arrangement according to claim 1, characterized by further comprising a water conductivity sensor (50) configured for measuring said conductivity of said water (40), whereby said switch control (36) is communicationally connected to said water conductivity sensor (50) and configured for operating said electrical switches (35) dependent on a water conductivity reading from said water conductivity sensor (50) .

4. The dump load arrangement according to claim 1, characterized in that said input connector (33) is connected to an electrical output of a hydropower generator (10), wherein said switch control (36) is communicationally connected to a hydropower generator controller (11) for obtaining a measure of distributed power from said hydropower generator (10), whereby said switch control (36) is configured for operating said electrical switches (35) dependent on said measure of distributed power from said hydropower generator (10).

5. The dump load arrangement according to claim 4, characterized in that said hydropower generator (10) is a three-phase generator, whereby said input connector (33) is a three-phase connector, said output connectors (34) are three-phase connectors and said controllable electrical switches (35) are three-phase switches.

6. The dump load arrangement according to any of the claims 1 to 5, characterized in that a power capacity of one of said at least two dump load sets (31) is different from a power capacity of another of said at least two dump load sets (31), at corresponding water (40) conditions.

7. The dump load arrangement according to any of the claims 1 to 6, characterized by comprising three dump load sets (31) of differing power capacity.

8. The dump load arrangement according to any of the claims 1 to 6, characterized by comprising four dump load sets (31) of differing power capacity.

9. The dump load arrangement according to any of the claims 1 to 8, characterized in that each said phase dump load (32) comprises an electrode arranged to be in contact with water (40) when said dump load set (31) is submerged in water (40).

10. The dump load arrangement according to any of the claims 1 to 9, characterized in that said switch control (36) is configured to enable connection of said dump load sets (31) only if no other load than said dump load arrangement (30) is connected to said hydropower turbine (20).

11. A hydropower unit (1), comprising:- a hydropower generator (10); and- a dump load arrangement (30) according to any of the claims 1 to 10 connected to said hydropower generator (10).

12. A method for discharging water (40) from a hydropower unit dam (41), comprising the steps of:- establishing (S10) that a hydropower generator (10) of said hydropower unit (1) is running while lacking any external grid presently providing access to an outer load for electrical power produced by said hydropower generator (10); and- controlling (S30) of connection of one or more dump load sets (31) of at least two dump load sets (31) to said hydropower generator (10); wherein phase dump loads (32) of said at least two dump load sets (31) are submerged in water (40); wherein power produced by said hydropower generator (10) becomes dissipated as heat into said water (40) while allowing a hydropower unit (1) discharge to be used as a complementary spillway of said dam (41).

13. The method according to claim 12, characterized by the further step of obtaining (S20) a measure of a conductivity of said water (40) or a measure of a quantity being dependent on said conductivity of said water (40), wherein said step of controlling said connection of one or more dump load sets (31) is depending on said measure of a conductivity of said water (40) or said measure of a quantity being dependent on said conductivity of said water (40) .

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