Power transfer system between an AC network and a reversible hydraulic turbine

Abstract

The invention relates to a power transfer system (1), comprising a first branch (4) having a controlled switch (41) and a second branch (8) having a variable frequency converter (5), in parallel between an alternating current network (2) and a reversible pump-turbine (3), the variable frequency converter (5) comprising: - a first alternating current / direct current converter (11) having a first direct current interface, and - a second alternating current / direct current converter (12) having a second direct current interface, the first and second direct current interfaces being connected by a direct current link (13), - a control circuit (7) having a first mode in which it simultaneously opens the switch (4) and transfers electrical energy until it reaches the same frequency on two alternating current interfaces (61, 62),and having a second mode in which it closes the switch (4) of the first branch (41); - an energy storage system (14); - a switching system (15) for selectively connecting the energy storage system (14) to the direct current link (13) Figure to be published with the abbreviation: Fig. 1,

Description

Title of the invention: Power transfer system between an alternating current network and a reversible hydraulic turbine

[0001] The invention relates to the transfer of power between an alternating current network and a reversible hydraulic turbine. The invention relates in particular to the use of a system allowing the turbine to be started in pump mode.

[0002] Many hydroelectric dams include a reversible pump-turbine used either in pump mode to move water over the dam or in turbine mode to generate electrical energy transferred to an AC grid. A known power transfer system includes a first branch and a second branch connected in parallel to form an electrical connection between the AC grid and the reversible pump-turbine. The first branch includes a controlled switch. When the controlled switch is closed, a nominal electrical power can be exchanged between the AC grid and the pump-turbine, in order to reduce electrical losses in the electrical connection. To start the pump-turbine in pump mode, this pump-turbine initially has zero speed and must be accelerated until it reaches the synchronization speed of the AC grid.Therefore, the second branch comprises a variable frequency converter comprising a first and a second AC / DC converter. The AC interfaces of these AC / DC converters are connected to the AC grid and to the pump-turbine respectively. The controlled switch is initially open. The variable frequency converter transfers energy to the pump-turbine until it reaches the synchronization speed of the AC grid. The controlled switch is then closed once this synchronization speed is reached.

[0003] Furthermore, this power transfer system may also include a hybridization system that includes an AC / DC converter connected to the electrical link and an energy storage system connected to the DC interface of the AC / DC converter. This energy storage system is a reversible DC source that is capable of storing electrical energy from the AC grid and returning it to the AC grid via the converter. This hybridization system provides more flexibility to the power transfer system, for example by storing the energy produced by the pump-turbine if the AC grid transiently demands less power. Such a hybridization system allows for better dynamic adaptation to energy needs.

[0004] However, the use of a hybridization system is expensive, which limits its commercial development. The invention aims to solve one or more of these drawbacks. The invention therefore relates to a power transfer system, comprising a first branch and a second branch connected in parallel between a first connection interface to be connected to an alternating current network and a second interface to be connected to a reversible pump-turbine, the first branch comprising a controlled switch, the second branch comprising a variable frequency converter, the variable frequency converter comprising - a first AC / DC converter having a first AC interface connected to the first connection interface and a first DC interface, and - a second AC / DC converter having a second AC interface connected to the second connection interface and a second DC interface, the first and second DC interfaces being connected by a DC link, - a control circuit having a first mode in which it simultaneously opens the switch of the first branch and transfers electrical energy from the first alternating current interface to the second alternating current interface with increasing frequency on the second alternating current interface until it reaches the frequency of the first interface, and having a second mode in which it closes the switch of the first branch; - the system further comprises an energy storage system connected to the direct current link; - a switching system controlled by the control circuit to selectively connect the energy storage system to the direct current link.

[0005] The invention also relates to the following variants. Those skilled in the art will understand that each of the features of the following variants can be independently combined with the above features, but without constituting an intermediate generalization.

[0006] According to one variant, the energy storage system comprises a direct current electrical transducer and a direct current / direct current converter configured to modify the voltage level between the direct current electrical transducer and the direct current connection.

[0007] According to another variant, the direct current electrical transducer is chosen from the group comprising a supercapacitor, a fuel cell, an electrochemical battery, an electric motor driving a flywheel, an air compressor and electromagnetic storage.

[0008] According to another variant, the first alternating current / direct current converter and the second alternating current / direct current converter are reversible in current.

[0009] According to a variant, the first and second alternating current / direct current converters are inverters with two voltage sources and the direct current / direct current converter is reversible in power.

[0010] According to another variant, the first and second AC / DC converters are current source inverters, and the DC / DC converter is a four-quadrant DC / DC converter.

[0011] According to a variant, the direct current connection comprises a first winding connected between the first direct current interface and the switching system and a second winding connected between the second direct current interface and the switching system.

[0012] According to another variant, the nominal powers of the first alternating current / direct current converter and of the second alternating current / direct current converter are less than 20% of the nominal power of the first switch.

[0013] According to another variant, the energy storage system is configured to provide a maximum power greater than the maximum power of the first alternating current / direct current converter and greater than the maximum power of the second alternating current / direct current converter, and less than the sum of the powers of the first and second alternating current / direct current converters.

[0014] The invention also relates to a hydraulic system, comprising: - a power transfer system as defined above, - an alternating current network connected to the first connection interface of the power transfer system, - a reversible pump-turbine connected to the second connection interface of the power transfer system.

[0015] Other characteristics and advantages of the invention will become clearer from the description given below, for information purposes only and in a non-limiting manner, with reference to the appended drawings, in which:

[0016] [fig.l] is a schematic representation of a power transfer system according to a first embodiment of the invention;

[0017] [Fig.2] is a schematic representation of a power transfer system according to a second embodiment of the invention;

[0018] [Fig. 3] illustrates a mode of operation of a power transfer system according to the first embodiment, during a start-up phase of the reversible pump-turbine;

[0019] [Fig.4] illustrates another mode of operation of a power transfer system according to the first embodiment, in which the electrical energy supplied by the reversible pump-turbine bypasses a main switch;

[0020] [Fig.5] illustrates another mode of operation of a power transfer system according to the first embodiment, in which the energy storage system restores energy to both the alternating current network and the reversible pump-turbine in a different starting mode;

[0021] [Fig.6] illustrates another mode of operation of a power transfer system according to the first embodiment, during a phase where the reversible pump-turbine and the energy storage system both send current to the alternating current network;

[0022] [Fig.7] is a schematic representation of a power transfer system according to an independent aspect of the invention.

[0023] [Fig.l] is a schematic representation of a power transfer system 1 according to a first embodiment of the invention; The power transfer system 1 comprises a controlled switch 41, a variable frequency converter 5, a connection interface 61 and another connection interface 62. An electrical connection comprises a main power branch 4 and a secondary power branch 8. The power branches 4 and 8 are connected in parallel between the connection interfaces 61 and 62. Branch 4 comprises the controlled switch 4L. The switch 41 may be an HVAC circuit breaker. Branch 8 comprises the variable frequency converter 5. The connection interface 61 is connected to an alternating current network 2, generally a three-phase alternating current network. The connection interface 62 is connected to a pump-turbine 3, generally comprising a synchronous motor / generator.The pump-turbine 3 generally comprises a hydraulic unit having a hydraulic wheel coupled to a generator rotor by a shaft.

[0024] The variable frequency converter 5 comprises an AC / DC converter 11, a DC link 13, a controlled switch 15, an energy storage system (commonly referred to by the acronym ESS) 14 and another AC / DC converter 12.

[0025] The AC interface of the AC / DC converter 11 is connected to the connection interface 61 and the AC interface of the AC / DC converter 12 is connected to the connection interface 62. The AC / DC converters 11 and 12 are current reversible. The DC link 13 electrically connects the respective DC interfaces of the AC / DC converters 11 and 12, to allow a power transfer between these DC interfaces.

[0026] The power transfer system 1 comprises a control circuit 7 configured to control the switch 41 and the switch 15 of the variable frequency converter 5. The control circuit 7 therefore controls the power transfer via the main branch 4, and the power transfer from / to the energy storage system 14. The main switch 41 makes it possible to selectively connect / disconnect the alternating current interfaces 61 and 62 of the alternating current / direct current converters 11 and 12, or the power interface of the pump-turbine motor 3 and the alternating current network 2.

[0027] The control circuit 7 has in particular a control mode in which it simultaneously opens the switch 41 of the main branch 4 and it transfers the electrical energy from the connection interface 61 to the connection interface 62 via the secondary branch 8, with an increasing frequency on the connection interface 62, until it reaches the frequency on the interface 61. The control circuit 7 also has another control mode in which it closes the switch 41 of the main branch 4 to allow a transfer of power between the alternating current network 2 and the pump-turbine 3.

[0028] The invention allows for integration of the energy storage system 14 at a reduced cost, in order to benefit from efficient power transfer with the electrical connection. Indeed, the energy storage system 14 connected to the direct current connection 13 shares the alternating current / direct current converters 11 and 12 with the variable frequency converter 5, instead of having a dedicated alternating current / direct current converter connected to the electrical connection. Thus, the variable frequency converter 5 can be used to transfer energy to the electrical connection even outside of pump start-up periods. Thus, the power transfer system 1 can offer improved characteristics without significantly increasing its costs.

[0029] The invention is particularly advantageous with a starting variable frequency converter 5 for which the AC / DC converters 11 and 12 have nominal powers typically less than 20% of the nominal power of the switch 41, preferably less than 15% of this nominal power, and more preferably less than 10% of this nominal power. Such a variable frequency converter 5 can thus remain cost-effective and maintain a reduced size. The nominal power of the switch 41 is generally determined on the basis of the current nominal power of this switch.

[0030] The direct current connection 13 may include a filtering circuit. The filtering circuit may include small capacitors, not shown in the figures, capable of storing energy for the purpose of stabilizing the current or voltage of the direct current connection 13. The controlled switch 15 connects in a manner selectively the energy storage system 14 and the direct current link 13. The variable frequency converter 5 also comprises a control circuit 16 and a control circuit 17. The control circuit 16 is configured to control the AC / DC converter 11, in particular when the latter comprises active components such as thyristors belonging to a rectifier. The control circuit 17 is also configured to control the AC / DC converter 12. The control circuits 16 and 17 can be used to control the power flows between the energy storage system 14, the AC network 2 and the reversible pump-turbine 3. The control circuits 7, 16 and 17 can be based on circuits which contain intelligent components (microcontroller, DSP, ASIC, etc.).These control circuits can retrieve information about the production mode of the hydraulic unit, or related states such as hydraulic head, output power or guide vane opening. These control circuits can use digital models of the hydraulic unit.

[0031] The energy storage system 14 of this embodiment is advantageously a reversible direct current voltage source. The energy storage system 14 of this embodiment may comprise an energy storage device selected from the group comprising a supercapacitor, a fuel cell, an electrochemical battery, an electric motor driving a flywheel, an air compressor and an electromagnetic storage. The AC / DC converters 11 and 12 may include a rectifier for converting between an AC voltage of a three-phase network and a DC voltage.

[0032] The AC / DC converter 11 and the AC / DC converter 12 are configured to be current reversible. The converters 11 and 12 may be voltage source inverters (VSIs). For one mode of the control circuit 16, power is transferred from the AC interface of the AC / DC converter 11 to the DC interface of the AC / DC converter 11 and in another mode of the control circuit 16, power is transferred from the DC interface of the AC / DC converter 11 to the AC interface of the AC / DC converter 11.For one mode of the control circuit 17, power is transferred from the DC interface of the AC / DC converter 12 to the AC interface of the AC / DC converter 12 and in another mode of the control circuit 17, power is transferred from the AC interface of the AC / DC converter 12 to the DC interface of the AC / DC converter 12.

[0033] [Fig. 2] is a schematic representation of a power transfer system 1 according to a second embodiment of the invention. This embodiment differs from that of [Fig. 1] only in the structure of the energy storage system 14. In this embodiment, the converters 11 and 12 may be voltage source inverters. The energy storage system 14 comprises a DC / DC converter 141 and an energy storage device 142 used as an electrical transducer to another form of energy. The DC / DC converter 141 electrically connects the storage device 142 and the controlled switch 15. The DC / DC converter 141 is configured to adapt the voltage level between the storage device 142 and the DC link 13.The energy storage system 14 comprises a control circuit 143 configured to control the DC / DC converter 141. The control circuit 143 can, for example, control the switches belonging to the DC / DC converter 141, in particular when the latter is a buck / boost converter, for example. The control circuit 143 can receive information relating to the DC link 13, such as the current and the voltage. The control circuit 143 can also receive information relating to the storage device 142, such as its output voltage or its state of charge. The energy storage device 142 of this embodiment can be selected from the group comprising a supercapacitor, a fuel cell, an electrochemical battery, an electric motor driving a flywheel, an air compressor, and electromagnetic storage.

[0034] The control circuits 16 and 17 can advantageously receive information relating to the state of the hydraulic unit and the energy storage system 14, such as the production mode, the state of charge and the health of the energy storage system 14, the water height in the tanks or the opening of the gates of the hydraulic unit. They can also receive information relating to the direct current connection 13, such as the current or the voltage. The control circuits 16 and 17 can advantageously send or receive information between them. They can be based on a model predictive control approach.

[0035] In the example of [fig.2], the converters 11 and 12 may also be current source inverters (CSI). The voltage of the direct current link 13 may then fluctuate. In this version, the direct current / direct current converter 141 is configured to convert a voltage source into a current source. The direct current / direct current converter 141 is reversible. Advantageously, the direct current / direct current converter (141) is a four-quadrant converter. The direct current link 13 may comprise two windings, the first being located between the converter 11 and the switch 15, the second being located between converter 12 and switch 15.

[0036] [Fig. 3] illustrates an operating mode of the power transfer system 1 according to the first embodiment, during a start-up phase of the reversible pump-turbine 3 in pumping mode. In this operating mode, the control circuit 7 simultaneously opens the switch 41 of the main branch 4 in order to transfer the electrical energy from the alternating current interface 61 to the alternating current interface 62 with increasing frequency on the alternating current interface 62, until it reaches the frequency on the interface 61. The control circuit 7 also opens the switch 15 of the secondary branch 8, so that the energy storage system 14 is not used during the power transfer.

[0037] [Fig. 4] illustrates another mode of operation of the power transfer system 1 according to the first embodiment. The pump-turbine 3 is used in turbine mode to transfer energy to the AC network 2. The pump-turbine 3 is here driven at a rotational speed which differs from the frequency of the AC network 2. In this mode of operation, the control circuit 7 simultaneously opens the switch 41 of the main branch 4 in order to transfer the electrical energy generated by the reversible pump-turbine 3 from the AC interface 62 to the AC interface 61 when the frequencies of the AC interface 62 and the AC interface 61 are different. The control circuit 7 also opens the switch 15 of the secondary branch 8, so that the energy storage system 14 is not used during the power transfer.

[0038] [Fig. 5] illustrates another mode of operation of a power transfer system 1 according to the first embodiment. In this mode of operation, the control circuit 7 simultaneously opens the switch 41 of the main branch 4 and closes the switch 15 of the secondary branch 8. The control circuit 7 causes a transfer of power from the energy storage system 14 to the alternating current network 2 via the converter 11 in order to provide transient power to this alternating current network 2. The control circuit 7 also causes a transfer of power from the energy storage system 14 to the pump-turbine 3, with a variable frequency (here increasing). Once the frequencies on the alternating current interfaces 61 and 62 are equal, the control circuit 7 can close the switch 41 to optimize the transfer of power between the alternating current network 2 and the pump-turbine 3.

[0039] To reduce the duration of the start-up phase, the control circuit 7 can also control a transfer of power from the alternating current network 2 to the pump-turbine 3 via the converters 11 and 12 and a transfer of power from the energy storage system 14 to the pump-turbine 3 via the converters 11 and 12. through converter 12, with increasing frequency, in order to start pump-turbine 3 in pump mode.

[0040] [Fig. 6] illustrates another mode of operation of a power transfer system 1 according to the first embodiment. In this mode of operation, the control circuit 7 simultaneously closes the switch 41 of the main branch 4 and closes the switch 15 of the secondary branch 8. The energy storage system 14 transfers the energy to the alternating current network 2 via the alternating current interface 61. The energy storage system 14 can also transfer the energy to the alternating current network 2 via the alternating current interface 62. The reversible pump-turbine 3 transfers the energy to the alternating current network 2 via the switch 41 of the main branch 4.The energy storage system 14 can thus transiently supply energy to the alternating current network 2 in addition to the energy produced by the pump-turbine 3, in order to respond to a transient increase in energy production on the alternating current network 2. The energy storage system 14 allows more dynamic power adaptation on the alternating current network 2 than would be possible with the pump-turbine 3.

[0041] If the energy consumption on the AC network 2 is transiently reduced, the energy flows can be reversed in this operating mode where the switches 41 and 15 are both closed. The energy storage system 14 transfers energy from the pump-turbine 3 via the AC interface 62. The energy storage system 14 can also receive energy from the pump-turbine 3 via the AC interface 61. The reversible pump-turbine 3 also transfers energy to the AC network 2 via the switch 41 of the main branch 4.

[0042] In one embodiment, the energy storage system 14 has a maximum power between the maximum power of the converter 11 or the converter 12 and the sum of the powers of the converters 11 and 12. The power transfer from the energy storage system 14 to the network 2 is greater than the power transfer allowed by the converter 11 or the converter 12. With this architecture, the power transfer system 1 can allow a more powerful energy storage system 14 than with the use of a single AC / DC converter connected between the connection interface 61 or 62 and the energy storage system 14.

[0043] [Fig.7] is a schematic representation of a power transfer system according to an independent aspect of the invention. The power transfer system 1 comprises a controlled switch 41, a variable frequency converter 5, a connection interface 61 and another connection interface 62. A link electrical system comprises a main power supply branch 4 and a secondary power supply branch 8. The power supply branches 4 and 8 are connected in parallel between the connection interfaces 61 and 62. Branch 4 comprises the controlled switch 4L. The switch 41 may be an HVAC circuit breaker. Branch 8 comprises the variable frequency converter 5. The connection interface 61 is here connected to an alternating current network 2, generally a three-phase alternating current network. The connection interface 62 is connected to a pump-turbine 3, generally comprising a synchronous motor / generator.

[0044] The variable frequency converter 5 comprises an AC / DC converter 11, a DC link 13 and another AC / DC converter 12. The AC interface of the AC / DC converter 11 is connected to the connection interface 61 by a transformer 81. The AC interface of the AC / DC converter 12 is connected to the connection interface 62 by a transformer 82. The transformers 81 and 82 are configured to adapt the voltage on the connection interfaces 61 and 62 to the AC interfaces of the converters 11 and 12. For example, the transformers 81 and 82 can perform a voltage conversion from 15kV to 3kV.

[0045] The power transfer system 1 comprises an energy storage system 14 and an AC / DC converter 18. The energy storage system 14 is connected to the DC interface of the converter 18. The AC interface of the converter 18 is connected between the transformer 81 and the converter 11. The AC interface of the converter 18 is also connected between the transformer 82 and the converter 12. In the example illustrated here, a controlled switch selectively connects the converter 18 and the energy storage system 14.

[0046] A control circuit 7 can control the transfer of power between the energy storage system 14 and the connection interfaces 61 and 62. The control circuit 7 can also control the switching of the switch 4L

[0047] The control circuit 7 therefore controls the transfer of power via the main branch 4, and the transfer of power from / to the energy storage system 14. The main switch 41 makes it possible to selectively connect / disconnect the alternating current interfaces 61 and 62 of the alternating current / direct current converters 11 and 12, or the power interface of the pump-turbine motor 3 and the alternating current network 2.

[0048] The control circuit 7 has in particular a control mode in which it simultaneously opens the switch 41 of the main branch 4 and it transfers the electrical energy from the alternating current interface 61 to the alternating current interface 62 by the secondary branch 8, with increasing frequency on the alternating current interface 62, until it reaches the frequency on the interface 61. The control circuit 7 also has another control mode in which it closes the switch 41 of the main branch 4 to allow a transfer of power between the alternating current network 2 and the pump-turbine 3.

[0049] With this architecture, the power transfer system 1 can also allow a reduction in the costs of the energy storage system 14. Indeed, given that the converter 18 is connected to a voltage lower than that of the connection interfaces 61 and 62, its cost is reduced.

[0050] The energy storage system 14 of this embodiment is advantageously a reversible direct current voltage source. The energy storage system 14 of this embodiment may comprise an energy storage device selected from the group comprising a supercapacitor, a fuel cell, an electrochemical battery, an electric motor driving a flywheel, an air compressor and an electromagnetic storage. The AC / DC converters 11 and 12 may include a rectifier for converting between an AC voltage of a three-phase network and a DC voltage.

[0051] The AC / DC converters 11 and 12 are here reversible in current. The DC link 13 electrically connects the respective DC interfaces of the AC / DC converters 11 and 12, to allow a transfer of power between these DC interfaces.

[0052] The AC / DC converters 11 and 12 are current reversible. The DC link 13 electrically connects the respective DC interfaces of the AC / DC converters 11 and 12, to allow power transfer between these DC interfaces.

Claims

Claims

1. Power transfer system (1), comprising a first branch (4) and a second branch (8) connected in parallel between a first connection interface (61) to be connected to an alternating current network (2) and a second interface (62) to be connected to a reversible pump-turbine (3), the first branch (4) comprising a controlled switch (41), the second branch (8) comprising a variable frequency converter (5), the variable frequency converter (5) comprising: - a first alternating current / direct current converter (11) having a first alternating current interface connected to the first connection interface (61) and a first direct current interface, and - a second alternating current / direct current converter (12) having a second alternating current interface connected to the second connection interface (62) and a second direct current interface,the first and second direct current interfaces being connected by a direct current link (13), - a control circuit (7) having a first mode in which it simultaneously opens the switch (4) of the first branch (41) and it transfers the electrical energy from the first alternating current interface (61) to the second alternating current interface (62) with an increasing frequency on the second alternating current interface (62) until it reaches the frequency on the first interface (61), and having a second mode in which it closes the switch (4) of the first branch (41); characterized in that: - the system (1) further comprises an energy storage system (14) connected to the direct current link (13); - a switching system (15) controlled by the control circuit (7) for selectively connecting the energy storage system (14) to the direct current link (13).,

2. A power transfer system (1) according to claim 1, wherein the energy storage system (14) comprises a direct current electrical transducer (142) and a direct current / direct current converter (141) configured to change the voltage level between the direct current electrical transducer (142) and the direct current link (14).

3. A power transfer system (1) according to claim 2, wherein the direct current electrical transducer (142) is selected from the group comprising a supercapacitor, a fuel cell, an electrochemical battery, an electric motor driving a flywheel, an air compressor and electromagnetic storage.

4. A power transfer system (1) according to one of the above claims, wherein the first AC / DC converter (11) and the second AC / DC converter (12) are current reversible.

5. Power transfer system (1) according to one of the preceding claims, wherein the first and second AC / DC converters (11, 12) are two voltage source inverters and wherein the DC / DC converter (141) is power reversible.

6. A power transfer system (1) according to claims 2 and 4, wherein the first and second AC / DC converters (11, 12) are current source inverters, and wherein the DC / DC converter (141) is a four-quadrant DC / DC converter.

7. A power transfer system (1) according to claim 6, wherein the direct current link (13) comprises a first winding connected between the first direct current interface and the switching system (15) and a second winding connected between the second direct current interface and the switching system (15).

8. Power transfer system (1) according to one of the above claims, wherein the nominal powers of the first AC / DC converter (11) and the second AC / DC converter (12) are less than 20% of the nominal power of the first switch (41).

9. Power transfer system (1) according to one of the above claims, wherein the energy storage system (14) is configured to provide a maximum power greater than the maximum power of the first AC / DC converter (11) and greater than the maximum power of the second AC / DC converter (12), and less than the sum of the powers of the first and second AC / DC converters (11, 12).

10. Hydraulic system, comprising: - a power transfer system (1) according to one of the above claims, - an alternating current network (2) connected to the first connection interface (61) of the power transfer system (1), - a reversible pump-turbine (3) connected to the second connection interface (62) of the power transfer system (1).

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