Gas expansion plant and method for operating such a power plant

Abstract

The invention relates to a gas expansion plant comprising an expander (1) and to a generator (2) which is connected to the expander (1), is driven by the expander (1), and can feed the electric current into a current transmission network (3). A frequency converter (4) with a bypass circuit (5) is arranged between the generator (2) and the feed of the electric current into the power transmission network (3), wherein in a first setting of the bypass circuit (5), the gas expansion plant is operated in a normal mode in which the frequency converter (4) is not engaged, and in a second setting of the bypass circuit (5), the gas expansion plant is operated in a converter mode in which the frequency converter (4) is engaged. At least one first transformer (6) is arranged upstream of the frequency converter (4) in series, and at least one second transformer (7) is arranged downstream of the frequency converter (4) in series. The invention additionally relates to a method for operating such a gas expansion plant.

Description

Description TITLE Expansion power plant and method for operating such a power plant TECHNICAL FIELD

[0001] The invention relates to an expansion power plant according to the preamble of independent patent claim 1 and to methods for operating such a power plant according to independent patent claims 6 and 7. BACKGROUND D

[0002] Due to the fluctuating feed-in of electricity from renewable energies into the power grid, the demands on conventional power plants and storage power plants have changed significantly. The faster the power plants can feed electricity into the grid, the more lucrative the feed-in tariffs become. Conventional expanders, such as steam turbines, are sometimes unable to provide control power in the short term after extended downtime. The startup of such power plants and the necessary gentle warm-up of the components takes longer than required by the secondary reserve (the power grid).

[0003] To be able to provide control power very quickly, the power plants must already be synchronized with the power grid and operate at low partial load. Due to the ventilation of the blades and the associated temperature increase, the partial load cannot be reduced arbitrarily. Instead, there is a plant-specific minimum power (minimum load), which is typically 5% to 10% of maximum power for steam turbines and approximately 20% for air turbines.

[0004] The high minimum power outputs make standby operation of the plant at minimum power unprofitable with current power plants / regulations. A significant reduction in minimum power (ideally island load) by reducing ventilation power would make permanent standby operation of a power plant economically viable at low mass flow (steam / medium mass flow). Starting from this standby operation, it would be possible to provide primary and secondary control power at short notice. The minimum power can only be significantly reduced below the previous limit by reducing the speed and thus the ventilation power of the power plant (train). This is currently not possible, as the plant is synchronized with the power grid at a constant speed, and the aforementioned minimum power is required to maintain the speed. The current limitation of the minimum power makes permanent "standby operation" uneconomical. The applicant's unpublished DE102023207228 discloses an expansion power plant in which a frequency converter with a bypass circuit is arranged between the generator and the electrical power feed into the power transmission grid. In a first position of the bypass circuit, the expansion power plant can be operated in converter mode with the frequency converter engaged, and in a second position of the bypass circuit, the expansion power plant can be operated in normal mode with the frequency converter disengaged. Furthermore, the application discloses a method for operating such an expansion power plant. However, conventional frequency converters are not designed for the voltage generated by the generator. Frequency converters capable of handling such high voltages are very expensive, if available at all. Based on the problem described above, it is the object of the present invention to provide an improved expansion power plant and an improved method for operating such an expansion power plant. SUMMARY OF THE INVENTION The problem is solved with regard to the expansion power plant by the features of independent patent claim 1 and with regard to the methods by the features of independent patent claims 6 and 7.

[0008] Advantageous embodiments of the invention, which can be used individually or in combination with one another, are the subject of the subclaims and the description. The expansion power plant according to the invention, comprising at least one expander and a generator connected to the expander and driven by the expander, which generator can feed the electrical power generated by the generator into a power transmission network, wherein a frequency converter with a bypass circuit is arranged and designed between the generator and the feed of the electrical power into the power transmission network, such that in a first position of the bypass circuit, the expansion power plant can be operated in a normal operation in which the frequency converter is disengaged, and in a second position of the bypass circuit, the expansion power plant can be operated in a converter operation in which the frequency converter is engaged, is characterized in thatthat at least one first transformer is arranged in series before the frequency converter and at least one second transformer is arranged in series after the frequency converter.,

[0009] The invention enables grid-connected operation of the power plant, independent of the expander speed and load. The expander is initially operated in minimum load mode at a reduced speed, where the blading ventilation remains within the permissible limits. The generator is connected to the grid via the frequency converter. A demand for increased power is met by increasing the media flow through the expander, which leads to an increase in the expander speed and, at the same time, an increase in the output power. Once grid synchronization is achieved, the bypass switch can be closed and the frequency converter bypassed. The system is now in normal operation.

[0010] The first transformer, arranged in series upstream of the frequency converter, transforms the electrical voltage generated by the generator to any cost-effective voltage level. The second transformer, arranged in series downstream of the frequency converter, adapts the grid voltage to the converter voltage.

[0011] One embodiment of the invention provides that the bypass circuit can be moved to the first or second position, i.e., to start-up mode (= converter mode) or bypass mode (= normal mode), depending on one or more predeterminable parameters. Particularly suitable predeterminable parameters are the power / voltage output by the power plant and / or the electrical frequency of the generated electrical current, and / or the turbine speed.

[0012] A further embodiment of the invention provides that the bypass circuit can be switched to the second position (converter operation) when the power falls below a minimum partial load that can be achieved and / or specified during normal operation, and this power continues to be fed into the grid. The minimum partial load can be selected accordingly depending on the type of power plant and determined directly or indirectly from various parameters. However, the minimum partial load is always significantly lower than the previous values, which are 5% to 10%, or approximately 20% of maximum power for air turbines.

[0013] A further embodiment of the invention provides that a further partial load, which may be below the minimum partial load, is selected in converter operation such that the expansion power plant is operated in "standby mode" and the generated electrical power is at least sufficient to supply the expansion power plant itself. The medium consumption (air, steam, ...) is thereby reduced to a minimum. In this case, the speed of the expander is lower than the rated speed.

[0014] Standby mode keeps all plant components operational. This allows the plant to respond quickly to power demands. This makes it possible to provide secondary and primary control power at short notice.

[0015] Of particular note is the fact that the turbine generator is kept warm in "standby mode," which significantly reduces the thermomechanical stress on the components when power is required, thus increasing their service life. The reduction in start-stop cycles compared to conventional operation also contributes to this. Due to its permanent grid-synchronous operation, the train also provides the transmission grid with a certain amount of inertia due to its rotating mass.

[0016] A further embodiment of the invention provides that the bypass circuit can be switched directly or indirectly via a control device. Such a control device comprises a synchronization device and, if necessary, additional control components and enables automated and rapid switching between the two operating modes: normal operation and converter operation.

[0017] A first method according to the invention for operating an expansion power plant according to one of the preceding claims is characterized by the following method steps: - Operation of the expansion power plant in “standby mode” at speeds below the rated speed - Rapid power and speed increase on demand up to the rated speed and up to a specified partial load - If the specified partial load is exceeded, the grid connection is carried out using the bypass circuit, thus bypassing the frequency converter and establishing normal operation, which can be maintained for any length of time. - Reduction of the power output of the expansion power plant by reducing the mass flow of the expansion medium - engaging the frequency converter arranged between the generator and the power transmission network when the partial load falls below a predetermined level by opening the bypass circuit, whereby the first transformer reduces the electrical voltage generated by the generator to a maximum electrical voltage that the frequency converter can tolerate and the second transformer increases the electrical voltage of the frequency converter to at least the normal operating voltage of the network.

[0018] By switching to converter operation, the power plant can remain connected to the grid while reducing the expander speed. By reducing the expander speed, the ventilation of the blades and the associated temperature increase can be significantly reduced, enabling operation even at very low partial loads.

[0019] The first transformer, arranged in series upstream of the frequency converter, reduces the electrical voltage generated by the generator to a lower voltage. This prevents damage to the frequency converter due to overvoltage. At the same time, it allows for a smaller frequency converter to be used, thus reducing the cost of the frequency converter. The second transformer, arranged in series downstream of the frequency converter, increases the electrical voltage to the nominal voltage generated by the generator, which corresponds to the grid voltage.

[0020] The expansion power plant is preferably operated in “standby mode” at speeds below the rated speed and with minimal generated electrical power to cover the power plant’s own needs and minimal media consumption when the generator is connected via the frequency converter path.

[0021] A second method according to the invention for operating a Expansion power plant according to one of claims 1 to 5, is characterized by the following process steps:

[0022] - Operation of the expansion power plant below a predetermined partial load at an operating speed which is below the normal operating speed, in which the frequency converter (4) is connected between the generator (2) and the power transmission network (3) (converter operation), wherein the first transformer (6) reduces the electrical voltage generated by the generator (2) to a maximum electrical voltage that can be tolerated by the frequency converter (4) and the second transformer (7) increases the electrical voltage at least to the normal operating voltage of the generator (2), - Increasing the power output of the expansion power plant by increasing the mass flow of the expansion medium, - when a predetermined frequency is reached, disengage the frequency converter (4) arranged between the generator (2) and the power transmission network (3) by closing the bypass circuit (5).

[0023] One embodiment of the method according to the invention provides that the frequency converter is coupled to a control device that engages or disengages the frequency converter depending on the power and / or frequency. The control device enables automated and rapid switching between the two operating modes: normal operation and converter operation.

[0024] In summary, it can be stated that the expansion power plant according to the invention and the method according to the invention for operating such a power plant enable grid-connected operation at low speeds and low power output, and can immediately increase the active power output with a high gradient when power is requested from the grid. Ideally, the plant in "standby mode" provides just the power required for the power plant's own power supply. Self-sufficiency in standby mode is thus possible. The media consumption (mass flow) can be reduced to a minimum. Due to the possible reduction in speed, the ventilation is significantly reduced and the temperature load on the components is thus kept within the permissible limits. Due to the permanent grid-synchronous During operation, the train also provides a certain amount of inertia to the transmission grid due to its rotating mass. The use and arrangement of two transformers, one in series before the frequency converter and a second in series after the frequency converter, enables the use of inexpensive frequency converters.

[0025] Further advantages of the invention are explained below using an exemplary embodiment. Figure 1 is not a scaled representation of the expansion power plant according to the invention; rather, it is a schematic representation that essentially shows only the components necessary for the invention. DESCRIPTION OF THE EMBODIMENTS

[0026] Fig. 1 shows an embodiment of an expansion power plant according to the invention, based on which the inventive method for operating such a power plant will also be explained below. The expansion power plant comprises an expander 1 and a generator 2 connected to and driven by the expander 1, which generator can feed the electrical current into a power transmission network 3. A frequency converter 4 is arranged between the generator 2 and the electrical current feed into the power transmission network 3. The frequency converter 4 is provided with or connected to a bypass circuit 5.The bypass circuit 5 allows at least two switch positions, wherein in a first position of the bypass circuit 5 the expansion power plant is operated in a normal mode in which the frequency converter 4 is disengaged, and in a second position of the bypass circuit 5 the expansion power plant is operated in a converter mode in which the frequency converter 4 is engaged.

[0027] A first transformer 6 is arranged in front of the frequency converter 4, which reduces the electrical voltage generated by the generator 2 to a lower electrical voltage. This makes it possible to Frequency converter 4 is optimized for cost. A second transformer 7 is arranged behind frequency converter 4, which increases the electrical voltage to the nominal voltage generated by generator 2, which corresponds to the mains voltage. The bypass circuit 5 is switched by a control device 8. The control device 8 includes a synchronization device and, if necessary, other control components. The control device 8 sends a corresponding switching signal to electrical switches 9 depending on one or more predeterminable parameters. Suitable parameters include, for example, power or frequency.

[0029] If the expansion power plant is operated above a predetermined partial load, the generator 2 is directly coupled to the power transmission network 3 by means of the bypass circuit 5, bypassing the frequency converter 4, and the power plant is in so-called normal operation. [033Cj When the load falls below a predetermined partial load, the frequency converter 4 is activated by opening the bypass circuit 5. The power plant is then operated in converter mode and is connected to the power grid via the frequency converter. With the help of the frequency converter 4, it is possible to operate the power plant at very low partial loads, since the frequency converter 4 enables the reduction of the speed and thus limits the temperature increase due to ventilation.

[0031] The partial load can be reduced to such an extent that only a "standby mode" occurs, in which just the power required for the power plant's own supply is provided. The mass flow (media consumption) is thereby reduced to a minimum, but the power plant nevertheless provides a mass inertia for the Electricity transmission network 3 and thus contributes to increasing grid stability.

[0032] If the power plant is to be restarted from standby mode, the power increase and the mass flow of the expansion medium are initially achieved in converter mode. When a specified frequency is reached (at lower speeds, the phase angle shifts, which must be synchronized with the power grid), the frequency converter 4 is deactivated by closing the bypass circuit 5, thereby reconnecting the generator 2 directly to the power transmission grid 3. Synchronization with the power grid is achieved by means of the control device 6, which includes a synchronization device and, if necessary, other control components.

[0033] The expansion power plant according to the invention is suitable for compressible Expansion media such as air, water / steam or organic media.

Claims

Claims 1. Expansion power plant, comprising at least one expander (1), and a generator (2) connected to the expander (1) and driven by the expander (1), which generator can feed the electrical power generated by the generator (2) into a power transmission network (3), wherein a frequency converter (4) with a bypass circuit (5) is arranged and designed between the generator (2) and the feed of the electrical power into the power transmission network (3), such that in a first position of the bypass circuit (5), the expansion power plant can be operated in normal operation, in which the frequency converter (4) is disengaged, and in a second position of the bypass circuit (5), the expansion power plant can be operated in converter operation, in which the frequency converter (4) is engaged, characterized in thatthat at least one first transformer (6) is arranged in series upstream of the frequency converter (4) and at least one second transformer (7) is arranged in series downstream of the frequency converter (4).

2. Expansion power plant according to claim 1, characterized in that the bypass circuit (5) can be moved into the first or second position depending on one or more predeterminable parameters, in particular power, voltage or frequency.

3. Expansion power plant according to claim 1 or 2, characterized in that the bypass circuit (5) can be moved to the second position (converter operation) when the load falls below a minimum partial load that can be achieved and / or specified during normal operation.

4. Expansion power plant according to claim 3, characterized in that a further partial load, which may be below the minimum partial load, is selected in converter operation such that the expansion power plant is operated in standby mode and the generated electrical power is sufficient at least to supply the expansion power plant itself.

5. Expansion power plant according to one of the preceding claims, characterized in that the bypass circuit (5) can be switched via a control device (8).

6. A method for operating an expansion power plant according to one of the preceding claims, characterized by the following method steps: - Operation of the expansion power plant in “standby mode” at speeds below the rated speed - Power and speed increase on demand up to the rated speed and up to a specified partial load - If the specified partial load is exceeded, the grid connection is carried out by means of the bypass circuit (5) and the frequency converter (4) is thus bypassed and normal operation is established, which can be maintained for any length of time. - Reduction of the power output of the expansion power plant by reducing the mass flow of the expansion medium - engaging the frequency converter (4) arranged between the generator (2) and the power transmission network (3) when the load falls below a predetermined partial load by opening the bypass circuit (5), wherein the first transformer (6) reduces the electrical voltage generated by the generator (2) to a maximum electrical voltage that can be tolerated by the frequency converter (4) and the second transformer (7) increases the electrical voltage of the frequency converter (4) at least to the normal operating voltage of the network.

7. A method for operating an expansion power plant according to one of claims 1 to 5, characterized by the following method steps: - Operation of the expansion power plant below a predetermined partial load at an operating speed which is below the normal operating speed, in which the frequency converter (4) is connected between the generator (2) and the power transmission network (3) (converter operation), wherein the first transformer (6) reduces the electrical voltage generated by the generator (2) to a maximum electrical voltage that can be tolerated by the frequency converter (4) and the second transformer (7) increases the electrical voltage at least to the normal operating voltage of the generator (2), - Increasing the power output of the expansion power plant by increasing the mass flow of the expansion medium, - when a predetermined frequency is reached, disengage the frequency converter (4) arranged between the generator (2) and the power transmission network (3) by closing the bypass circuit (5).

8. Method for operating an expansion power plant according to claim 6 or 7, characterized in that the frequency converter (4) is coupled to a control device (6) which engages or disengages the frequency converter (4) depending on the power and / or frequency of the generator (2).

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