System and method for supplying electric power to a power grid and for supporting a power grid

Abstract

A system for supplying electrical power to an electrical grid and for supporting the electrical grid is described. The system comprises: (a) a power generator comprising a main converter coupled to the electrical grid; (b) an energy buffer comprising an energy storage device and an auxiliary converter coupled to the electrical grid in parallel with the main converter; (c) a grid stability monitor configured to provide a grid stability indication representative of a level of stability of the electrical grid; and (d) a controller configured to control the main converter and the auxiliary converter in dependence on the grid stability indication such that (d1) when the level of stability is at least equal to a predetermined threshold, the main converter is controlled to operate as a virtual synchronous machine and the auxiliary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage device, and (d2) when the level of stability is below the predetermined threshold, the auxiliary converter is operated to provide a predetermined response in order to support the electrical grid stability.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of grid-connected power electronic converters used in the generation or consumption of electric power, in particular systems and methods for supplying or consuming electric power to a power transmission or distribution grid, which electric power supports the frequency and voltage stability of the distribution grid. BACKGROUND

[0002] With the increasing proportion of wind turbine generators and other renewable energy sources using grid-connected power electronic converters in the mix of power production plants, modern generators, e.g. wind turbine generators, can at least to some extent be operated to emulate the inherent characteristics provided by synchronous machines in order to assist in stabilizing the grid, in particular the grid frequency and voltage.

[0003] However, in the case of severe frequency disturbances, as occur in sudden loss of a major generator or other faults, even this way of operating wind turbine generators can not be sufficient to provide the required grid stabilization response, at least without additional hardware, which would come with a significant increase in costs and power loss.

[0004] Therefore, there can be a need for a renewable energy based system for supplying electric power to a grid, which system is also able to support the impact of severe faults and disturbances on the grid without an extended cost and / or power loss. SUMMARY

[0005] The subject matter of the independent claims can meet this need. The dependent claims describe advantageous embodiments of the invention.

[0006] According to a first aspect of the application, there is provided a system for supplying electrical power to an electrical grid (transmission grid or distribution grid) and for supporting the electrical grid. The described system comprises: (a) a power generator (or load) comprising a primary converter coupled to the electrical grid; (b) an energy buffer comprising an energy storage device and an auxiliary converter coupled to the electrical grid in parallel with the primary converter; (c) a grid stability monitor configured to provide a grid stability indication representative of a level of stability of the electrical grid; and (d) a controller configured to control the primary converter and the auxiliary converter as a function of the grid stability indication, such that (d1) when the level of stability is at least equal to a predetermined threshold, the primary converter is controlled to operate as a virtual synchronous machine (with a defined inertia H) and the auxiliary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage device, and (d2) when the level of stability is below the predetermined threshold, the auxiliary converter is operated (as a virtual synchronous machine with inertia H) to provide a predetermined response in order to support the electrical grid stability. In this context, the term virtual synchronous machine VSM can be used to describe that the control action of the converter mimics to some extent the stabilizing characteristics of a grid-connected synchronous machine with shaft-connected inertia with respect to its electrical response to changes in grid voltage magnitude, phase and frequency.

[0007] This aspect of the application is based on the idea that the energy buffer is operated in parallel with the power generator, the energy buffer having its own (auxiliary) converter, the power generator having its primary converter coupled to the electrical grid. This is done in such a way that the auxiliary converter is active only when the energy buffer is needed to provide a sufficient response to electrical grid stability problems, or only when the charging of the energy buffer is needed. Thus, the power losses are kept to a minimum, while the ability to provide a sufficient response to significant disturbances is significantly improved.

[0008] The grid stability indication can be a Boolean value indicating whether the energy buffer is needed or not. Alternatively, any other form of control function can dynamically change the distribution of the stabilizing function provided between the primary converter and the auxiliary converter in response to the monitored level of stability of the electrical grid. This also includes the possibility of a smooth handover from the primary converter to the energy buffer (i.e. where the response is gradually taken over by the energy buffer during a certain time period).

[0009] According to an embodiment of the application, when the auxiliary converter is not operated to provide grid stabilization functionality, the controller is configured to operate the auxiliary converter to maintain a predetermined amount of energy in the energy storage device by operating the auxiliary converter to supply energy to the energy storage device when the amount of energy in the energy storage device falls below a predetermined minimum energy level and until the amount of energy in the energy storage device reaches a predetermined maximum energy level.

[0010] In other words, once the amount of energy in the energy storage device falls below a predetermined minimum energy level, a charging or fill up of the energy storage device is initiated and continues until the predetermined maximum energy level is reached. The latter can in particular correspond to the total storage capacity of the energy storage device, while the minimum energy level can in particular equal 90% or 95% of the total storage capacity.

[0011] The fall below the minimum energy level can in particular occur due to losses from the energy storage device.

[0012] Alternatively, a separate so-called trickle charger can be used to maintain the required energy level.

[0013] According to a further embodiment of the application, the energy storage device comprises a capacitor.

[0014] The capacitor can comprise a plurality of capacitor units arranged in parallel.

[0015] According to a further embodiment of the application, the energy storage device comprises a bank of supercapacitors or ultracapacitors.

[0016] The total capacity of the energy storage device can be chosen such that the energy buffer is capable of delivering between 10% and 30% of the rated power of the power generator for a period of up to 10 seconds.

[0017] According to a further embodiment of the application, the grid stability monitor is configured to monitor one or more of: (a) a deviation of the grid frequency from a nominal grid frequency, (b) a grid frequency gradient, (c) a deviation of the grid voltage from a nominal grid voltage, d) a grid voltage gradient, (e) a DC link voltage, and (f) a main converter current.

[0018] These parameters can be considered individually or in combination when determining the grid stability indication. Furthermore, the grid stability monitor can compare the various parameter values to one or more threshold values and count the time period during which a particular threshold value is exceeded.

[0019] For example, if the grid frequency drops by more than 0.1 Hz below the prevailing grid frequency within 10 ms, the grid stability monitor can output a grid stability indication that activates the energy buffer to take over the provision of the virtual synchronous machine functionality of the main converter.

[0020] According to a further embodiment of the present application, the predetermined threshold comprises one or more of: (a) a predetermined deviation of the grid frequency from a nominal grid frequency, (b) a predetermined grid frequency gradient, (c) a predetermined deviation of the grid voltage from a nominal grid voltage, (d) a predetermined grid voltage gradient, (e) a predetermined DC link voltage value, and (f) a predetermined main converter current value.

[0021] According to a further embodiment of the present application, the main converter is controlled to supply electrical power to the grid without supporting grid stability when the stability level is below the predetermined threshold.

[0022] In other words, during the response to the grid instability, the main converter does not operate as a virtual synchronous machine. Thus, during this period, the power generator does not attempt to support the grid, but rather, this task is completely handed over to the energy buffer.

[0023] According to a further embodiment of the present application, the power generator comprises a wind turbine generator, and the main converter comprises a rectifier, a DC link, and an inverter.

[0024] The rectifier converts the AC output from the wind turbine generator. The corresponding DC voltage is supplied via the DC link to the inverter. The inverter converts the received DC voltage into a desired AC output, e.g. using pulse width modulation (PWM).

[0025] It is to be expressly noted that the main converter can in principle be any topology of power converter with any type of switching method that allows controlling the power (and optionally, the reactive power) to the grid (or another connected entity). Furthermore, the main converter can be connected to any form of power generation, load, or storage system.

[0026] According to a further embodiment of the present application, the auxiliary converter comprises an inverter with semiconductor switches.

[0027] The inverter of the auxiliary converter can utilize PWM to generate a desired AC output (active and / or reactive power). When supplying energy to the energy storage device, the semiconductor switches will be operated to allow current to flow to the energy storage device.

[0028] Likewise, the auxiliary converter can in principle be any topology of power converter with any type of switching method that allows for control of the power (and reactive power) to the grid (or another connected entity). Furthermore, the auxiliary converter can be connected to any form of power generation, load or storage system that allows for fast changes in power.

[0029] According to a further embodiment of the application, the controller comprises a primary controller for controlling the primary converter and an auxiliary controller for controlling the auxiliary converter.

[0030] The primary controller and the auxiliary controller can be implemented as separate hardware units, or as functional units running on the same hardware with coordination between the two. Furthermore, the controller can be implemented using a single integrated "Multiple Input Multiple Output", MIMO, control algorithm that uses inputs / outputs from / to the primary and auxiliary converters.

[0031] According to a second aspect of the application, there is provided a wind farm comprising a plurality of systems according to the first aspect of the application, where the power generator of each system is a wind turbine generator.

[0032] This aspect of the application utilizes the idea according to the first aspect on each wind turbine in the wind farm.

[0033] According to a further embodiment of the application, the energy buffers of all systems are formed as a single integrated wind farm energy buffer.

[0034] In other words, not every wind turbine generator has its own energy buffer, but the wind farm has a central energy storage. In the case of an off-shore wind farm, the energy buffer can be located on shore, e.g. at the grid connection point.

[0035] According to a third aspect of the application, there is provided a method for supplying electrical power to an electrical grid and for supporting the electrical grid. The described method comprises: (a) providing a power generator comprising a primary converter coupled to the electrical grid; (b) providing an energy buffer comprising an energy storage device and an auxiliary converter coupled to the electrical grid in parallel with the primary converter; (c) operating a grid stability monitor to provide a grid stability indication representative of a level of stability of the electrical grid; and (d) operating a controller to control the primary converter and the auxiliary converter in dependence on the grid stability indication, such that (d1) when the level of stability is at least equal to a predetermined threshold, the primary converter is controlled to operate as a virtual synchronous machine and the auxiliary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage device, and (d2) when the level of stability is below the predetermined threshold, the auxiliary converter is operated to provide a predetermined response in order to support the grid stability.

[0036] This aspect of the application is essentially based on the same idea as the above-described first aspect.

[0037] It is noted that embodiments of the application have been described with reference to different subjects. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to device type claims. However, a person of skill in the art will gather from the above and the following description that, unless other notified, any combination of features described in relation to a subject of one type, is also contemplated to be a subject of the document even if it is not explicitly described in relation to that subject type. In particular, a combination of features described in relation to a subject of method type and a subject of device type is also contemplated to be a subject of the document.

[0038] The above and further aspects of the application are evident from the examples of embodiments to be described hereinafter and are explained with reference to examples of embodiments. The application will be described in more detail with reference to examples of embodiments. It is however explicitly stated that the application is not limited to the described exemplary embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0039] Fig. 1 shows a system according to an embodiment of the application. DETAILED DESCRIPTION

[0040] The illustrations in the drawings are schematic. It is noted that in different drawings similar or identical elements are provided with the same reference numerals or with reference numerals differing only in the first digit.

[0041] Fig. 1 shows a system for supplying electrical power to a power grid and for supporting the power grid according to an embodiment of the present application. More specifically, the system comprises a wind turbine power generator comprising a wind rotor 1, a generator 2, and a main converter 10 coupled to a power grid 8 via an inductance 6 and a transformer 7. The main converter 10 comprises a rectifier 3, a DC link 4 and an inverter 5. The main converter 10 is connected to a main controller 20 providing control signals to the semiconductor switches in the rectifier 3 and the inverter 5, respectively.

[0042] The system further comprises an energy buffer comprising an energy storage device 30 and an auxiliary converter 34 coupled to the power grid 8 in parallel with the main converter 10 via an inductance 36 and the transformer 7. The energy storage device 30 comprises a plurality of capacitors 32, in particular super capacitors, coupled in parallel to form a capacitor bank. The auxiliary converter 34 is coupled to an auxiliary controller 40 providing control signals to the semiconductor switches in the auxiliary converter 34. The main controller 20 and the auxiliary controller 40 are interconnected via a cable 42 forming a controller for the entire system. Although the controllers 20 and 40 are shown as separate interconnected units, they can also be formed as individual functional blocks within an integrated controller.

[0043] The system further comprises a grid stability monitor (not shown) configured to provide a grid stability indication representative of a stability level of the power grid 8. The grid stability monitor can be a separate unit or integrated in one of the controllers 20, 40. The controllers 20, 40 are configured to control the main converter 10 and the auxiliary converter 34 in dependence on the grid stability indication such that when the stability level is at least equal to a predetermined threshold, the main converter 10 is controlled to operate as a virtual synchronous machine and the auxiliary converter 34 is controlled to operate to maintain a predetermined amount of energy in the energy storage device 30. On the other hand, when the stability level is below the predetermined threshold, the auxiliary converter 34 is operated to provide a predetermined response (active and / or reactive power) in order to support the grid stability. During this response period, the main controller 20 can change the control of the main converter 10 to temporarily stop acting as a virtual synchronous machine and only produce the (active and reactive) power output currently required.

[0044] As described above, the auxiliary converter 40 remains inactive during normal operation except when the voltage across the bank of capacitors 32 falls to a predetermined minimum level, such as 90% of the voltage corresponding to a fully charged state, at which point any additional grid support is not required in addition to the support provided by the main converter 20 acting as a virtual synchronous machine. When this occurs, the semiconductor switches of the auxiliary converter 34 are closed so that charging current is branched from the output current from the main converter 10 and supplied to the capacitors 32 until these are fully charged. Thereby, electrical losses are significantly reduced compared to a situation in which the auxiliary converter 34 would also continuously switch during normal (undisturbed) operation.

[0045] The grid stability monitor is in particular configured to monitor one or more of:

[0046] - a deviation of the grid frequency from a nominal grid frequency,

[0047] - a grid frequency gradient,

[0048] - a deviation of the grid voltage from a nominal grid voltage,

[0049] - a grid voltage gradient,

[0050] - a DC link voltage, and

[0051] - a main converter current.

[0052] Further, the predetermined threshold values can comprise one or more of:

[0053] - a predetermined deviation of the grid frequency from a nominal grid frequency,

[0054] - a predetermined grid frequency gradient,

[0055] - a predetermined deviation of the grid voltage from a nominal grid voltage,

[0056] - a predetermined grid voltage gradient,

[0057] - a predetermined DC link voltage value, and

[0058] - a predetermined main converter current value.

[0059] The combination of the wind turbine main converter 10 and the parallel energy buffer, both of which use Virtual Synchronous Machine (VSM) type controllers, are expected to supply power and energy to the power system 8 in response to changes in frequency and or phase about their nominal values. Small changes in power system frequency occur very frequently due to the difference between the power generated and the power consumed by different loads, and only require small changes in the power / energy output of the combined wind turbine and energy buffer. The small nature of these continuous frequency changes (typically < + / - 0.2 Hz over extended periods) means that the wind turbine converter 10 can respond to them without any action by the energy buffer, as the required dynamic energy exchange between the AC system and the power converter 10 is very small.

[0060] However, occasionally (for example, twice a month) the power system frequency will deviate from its normal operating point by a much larger amount; requiring the combined wind turbine and energy buffer to provide a much larger power / energy change in response. In this case, the individual wind turbine main converter is not able to provide the required dynamic energy response (due to its limited energy storage), and so the energy buffer must assist.

[0061] A second requirement for the energy buffer is to assist the wind turbine main converter 10 in responding to grid faults; where the voltage at the terminals of the wind turbine falls towards zero. In this situation, the combination of the energy buffer and the wind turbine main converter 10 is required to feed a minimum amount of fault current (possibly 150% of the future main converter rated current) into the grid; in particularly severe fault situations, the magnitude of this fault current is large enough that the energy buffer must output the maximum current to assist the main converter 10. In less severe fault situations, the main converter 10 will be able to provide the required fault current without additional assistance.

[0062] The ability of the wind turbine to respond to continuous small power system frequency changes and less severe grid fault events will mean that the parallel energy buffer will spend most of the time exchanging very little energy with the power system. The continuous switching of the power electronics, when not exchanging energy with the grid, will also consume energy and dissipate it as heat losses, reducing the efficiency of the energy buffer and the associated wind turbine. The only energy exchanged by the energy buffer with the power system during normal operation will be the energy required to cover the losses of its converter 34.

[0063] The present invention overcomes this problem, the present invention reduces the energy losses associated with the continuous switching of the power electronics of the energy buffer, and so limits the change in overall wind turbine powertrain efficiency when the energy buffer is introduced.

[0064] In order for the energy buffer to provide assistance during large grid frequency and low voltage fault events, its converter 34 must be charged and its controller 40 active. However, as it does not need to exchange power / energy or fault current continuously with the power system, its power electronics can be kept in an off state until the controller detects a grid frequency change or low voltage fault event significant enough to require the energy buffer to assist. At this point, the electronics are allowed to switch, if necessary, to provide the power / energy to provide the required response.

[0065] Keeping the switches in an off position when the buffer is not required will mean that there will be no energy losses experienced when each device is switched, and therefore the continuous energy losses of the energy buffer will be minimised and efficiency improved.

[0066] The activation of the energy buffer switching will be coordinated with the controller of the wind turbine network bridge converter 10, so that the wind turbine effectively hands over its response to a large power system frequency or low voltage fault event to the energy buffer, so that it can respect its power, energy and current limits. Once the energy buffer has completed the required response to the power system frequency or low voltage fault event, it will return itself to its pre-event charged state for a period of time, and then return its power electronics to an off state.

[0067] In not switching its devices continuously, the energy buffer must maintain its charged state, which will decay over time due to charge leakage from the capacitor 32.

[0068] The energy buffer must therefore also intermittently engage in switching of its power electronics to allow the necessary energy to be input to maintain sufficient charge in its capacitor 32. A hysteresis band around the converter DC link voltage will be used to activate and deactivate this process; the converter will switch its devices to charge the capacitor 32 to a maximum level, and then stop switching until the voltage drops to a lower level. The time between charging periods will be in the order of tens of seconds / minutes, and the charging time will be relatively short, so this process will not have a significant impact on the efficiency of the converter 34.

[0069] The advantage of the present invention is that the energy buffer power electronics 34 switching is activated by the coordinated control of the primary converter 10 and the energy buffer only when it is required for either responding to a large power system frequency change, low voltage fault event, or for maintaining the charged state of the capacitor 32.

[0070] Thus, when the energy buffer is not required to exchange power / energy with the grid, the continuous energy loss that occurs when each device is switched is prevented. The inactivity of the switches will significantly reduce the operational energy loss of the energy buffer and thus limit the reduction in wind turbine drivetrain efficiency that would normally be associated with the addition of additional parallel power converters.

[0071] To achieve the reduction in energy buffer losses, a coordinated control system 20, 40 is required between the wind turbine power converter 10 and the energy buffer, which allows the wind turbine to respond to continuous small changes in power system frequency and less severe low voltage fault events, but then "hands over" the response to larger events to the energy buffer.

[0072] The present invention will provide the benefit that it will minimise the losses in the wind turbine drivetrain caused by the introduction of a parallel energy buffer and thus the power transfer efficiency of the wind turbine will not be significantly affected.

[0073] An alternative solution would be to use a converter arrangement where additional energy storage is added by increasing the capacitance of the main wind turbine converter DC link 4 and increasing the rated current / power of the grid bridge. This would mean that the wind turbine generator network side converter 5 would have sufficient capacity (current capacity and stored energy) to provide the required power / energy response to large power system frequency changes without the assistance of a parallel connected energy buffer. Thus, the increased switching losses associated with the additional parallel energy buffer would not be present (assuming that the increased rated current of the grid bridge does not increase its switching losses).

[0074] It is to be noticed that the term "comprising", used in the description, does not exclude other elements or steps. Furthermore, it is to be understood that elements or steps that are described as following or chronologically subsequent to each other need not be necessarily that way. Moreover, the use of the term "a" or "an" does not exclude a plurality. Furthermore, it is to be understood that features described with respect to different embodiments can be combined. It is further noted that the reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A system for supplying electrical power to an electrical grid and for supporting the electrical grid, the system comprising: a power generator comprising a main converter coupled to the electrical grid, an energy buffer comprising an energy storage device and an auxiliary converter coupled to the electrical grid in parallel with the main converter, a grid stability monitor configured to provide a grid stability indication representative of a level of stability of the electrical grid, and a controller configured to control the main converter and the auxiliary converter in dependence on the grid stability indication such that when the level of stability is at least equal to a predetermined threshold, the main converter is controlled to operate as a virtual synchronous machine and the auxiliary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage device while its power electronics are kept in an off state, and when the level of stability is below the predetermined threshold, the auxiliary converter is controlled to operate as a virtual synchronous machine and to provide a predetermined response in order to support the electrical grid stability, and the main converter is controlled to stop operating as a virtual synchronous machine and to supply electrical power to the electrical grid without supporting the electrical grid stability, wherein the auxiliary converter is configured to support the electrical grid stability only when the energy buffer is needed to provide a sufficient response to a grid stability problem or only when a charging of the energy buffer is needed.

2. The system according to claim 1, wherein the controller is configured to operate the auxiliary converter to maintain the predetermined amount of energy in the energy storage device by operating the auxiliary converter to supply energy to the energy storage device when the amount of energy in the energy storage device falls below a predetermined minimum energy level and until the amount of energy in the energy storage device reaches a predetermined maximum energy level.

3. The system according to claim 1 or 2, wherein the energy storage device comprises a capacitor.

4. The system according to claim 1 or 2, wherein the energy storage device comprises a bank of supercapacitors.

5. The system according to claim 1 or 2, wherein the grid stability monitor is configured to monitor one or more of: a deviation of the grid frequency from a nominal grid frequency, a grid frequency gradient, a deviation of the grid voltage from a nominal grid voltage, a grid voltage gradient, a DC link voltage, and a main converter current.

6. The system according to claim 5, wherein the predetermined threshold comprises one or more of: a predetermined deviation of the grid frequency from a nominal grid frequency, a predetermined grid frequency gradient, a predetermined deviation of the grid voltage from a nominal grid voltage, a predetermined grid voltage gradient, a predetermined DC link voltage value, and a predetermined main converter current value.

7. The system according to claim 1 or 2, wherein the power generator comprises a wind turbine generator, and wherein the main converter comprises a rectifier, a DC link and an inverter.

8. The system according to claim 1 or 2, wherein the auxiliary converter comprises an inverter with semiconductor switches.

9. The system according to claim 1 or 2, wherein the controller comprises a main controller for controlling the main converter and an auxiliary controller for controlling the auxiliary converter.

10. A wind farm comprising a plurality of systems according to any of the preceding claims, wherein the power generator of each system is a wind turbine generator.

11. The wind farm according to claim 10, wherein the energy buffer of all systems is formed as a single integrated wind farm energy buffer.

12. A method for supplying electrical power to an electrical grid and for supporting the electrical grid, the method comprising: providing a power generator, the power generator comprising a main converter coupled to the electrical grid, providing an energy buffer, the energy buffer comprising an energy storage device and an auxiliary converter coupled to the electrical grid in parallel to the main converter, operating a grid stability monitor to provide a grid stability indication representative of a level of stability of the electrical grid, and operating a controller to control the main converter and the auxiliary converter in dependence on the grid stability indication, such that when the level of stability is at least equal to a predetermined threshold, the main converter is controlled to operate as a virtual synchronous machine and the auxiliary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage device while its power electronics are kept in an off state, and when the level of stability is below the predetermined threshold, the auxiliary converter is controlled to operate as a virtual synchronous machine and to provide a predetermined response in order to support the grid stability, and the main converter is controlled to stop operating as a virtual synchronous machine and to supply electrical power to the electrical grid without supporting the grid stability, wherein the auxiliary converter is configured to support the grid stability only when the energy buffer is needed to provide a sufficient response to a grid stability problem, or only when charging of the energy buffer is needed.

13. A wind turbine generator comprising a power generator according to any of claims 1 to 9.

14. A wind turbine comprising a wind turbine generator according to claim 13.

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