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

Abstract

A system for supplying electric power to a grid and for supporting the grid is provided. The system includes (a) a power generator including a main converter, (b) an energy buffer including an energy storage and a secondary converter, (c) a grid stability monitor-configured to provide a grid stability, and (d) a controller configured to control the main converter and the secondary converter in dependency on the grid stability indication such that when the stability level is at least equal to a predetermined threshold value, the main converter is controlled to operate as a virtual synchronous machine and the secondary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage, and when the stability level is below the predetermined threshold value, the secondary converter is operated to provide a predetermined response in order to support the grid stability.

Claims

1. A system for supplying electric power to a grid and for supporting the grid, the system comprising a power generator having a main converter coupled to the grid, an energy buffer having an energy storage and a secondary converter coupled to the grid in parallel with the main converter, a grid stability monitor configured to provide a grid stability indication representative of a stability level of the grid, and a controller configured to control the main converter and the secondary converter in dependency on the grid stability indication such that when the stability level is at least equal to a predetermined threshold value, the main converter is controlled to operate as a virtual synchronous machine and the secondary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage, and when the stability level is below the predetermined threshold value, the secondary converter is operated to provide a predetermined response in order to support the grid stability.

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

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

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

5. The system according to claim 1, 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 value comprises one or more of: a predetermined deviation of the grid frequency from the nominal grid frequency, a predetermined grid frequency gradient, a predetermined deviation of the grid voltage from the 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, wherein when the level of stability is below the predetermined threshold value, the main converter is controlled to supply electric power to the grid without supporting the grid stability.

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

9. The system according to claim 1, wherein the secondary converter comprises an inverter with semiconductor switches.

10. The system according to claim 1, wherein the controller comprises a main controller for controlling the main converter and a secondary controller for controlling the secondary converter.

11. A wind farm comprising a plurality of systems according to claim 1, wherein the power generator of each system is a wind turbine generator.

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

13. A method of supplying electric power to a grid and for supporting the grid, the method comprising providing a power generator comprising a main converter coupled to the grid, providing an energy buffer comprising an energy storage and a secondary converter coupled to the grid in parallel with the main converter, operating a grid stability monitor to provide a grid stability indication representative of a stability level of the grid, and operating a controller to control the main converter and the secondary converter in dependency on the grid stability indication such that when the stability level is at least equal to a predetermined threshold value, the main converter is controlled to operate as a virtual synchronous machine and the secondary converter is controlled to operate to maintain a predetermined amount of energy in the energy storage, and when the stability level is below the predetermined threshold value, the secondary converter is operated to provide a predetermined response in order to support the grid stability.

Description

BRIEF DESCRIPTION

(1) Some of the embodiments will be described in detail, with reference to the following FIGURES, wherein like designations denote like members, wherein:

(2) FIG. 1 shows a system according to an embodiment of the present invention.

DETAILED DESCRIPTION

(3) The illustration in the drawing is schematic. It is noted that in different FIGURES, similar or identical elements are provided with the same reference numerals or with reference numerals which differ only within the first digit.

(4) FIG. 1 shows a system for supplying electric power to a grid and for supporting the grid in accordance with an embodiment of the invention. More specifically, the system comprises a wind turbine power generator comprising wind rotor 1, generator 2 and a main converter 10 coupled to the grid 8 via inductances 6 and 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 which provides control signals to semiconductor switches in the rectifier 3 and the inverter 5, respectively.

(5) The system further comprises an energy buffer comprising an energy storage 30 and a secondary converter 34 coupled to the grid 8 in parallel with the main converter 10 via inductances 36 and the transformer 7. The energy storage 30 comprises a plurality of capacitors 32, in particular supercapacitors, coupled in parallel to form a capacitor bank. The secondary converter 34 is coupled to a secondary controller 40 which provides control signals to semiconductor switches in the secondary converter 34. The main controller 20 and the secondary controller 40 are interconnected by cable 42, thereby forming a controller for the entire system. Although the controllers 20 and 40 are shown as separate interconnected units, they may just as well be formed as individual functional blocks within an integrated controller.

(6) The system further comprises a grid stability monitor (not shown) configured to provide a grid stability indication representative of a stability level of the grid 8. The grid stability monitor may 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 secondary converter 34 in dependency on the grid stability indication such that when the stability level is at least equal to a predetermined threshold value, the main converter 10 is controlled to operate as a virtual synchronous machine and the secondary converter 34 is controlled to operate to maintain a predetermined amount of energy in the energy storage 30. On the other hand, when the stability level is below the predetermined threshold value, the secondary 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 may change the control of the main converter 10 to temporarily cease acting as a virtual synchronous machine and just producing the currently required (active and reactive) power output.

(7) As described above, the secondary converter 40 remains inactive during normal operation (where no additional grid support is needed beyond that provided by the main converter 20 acting as a virtual synchronous machine) except when the voltage across the bank of capacitor 32 drops to a predetermined minimum level, such as 90% of the voltage corresponding to the fully charged state. When this happens, the semiconductor switches of the secondary converter 34 are closed such that a charging current is branched off 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 in comparison to a situation where the secondary converter 34 would be switching continuously also during normal (undisturbed) operation.

(8) The grid stability monitor is in particular 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.

(9) Furthermore, the predetermined threshold value may comprise one or more of a predetermined deviation of the grid frequency from the nominal grid frequency, a predetermined grid frequency gradient, a predetermined deviation of the grid voltage from the nominal grid voltage, a predetermined grid voltage gradient, a predetermined DC link voltage value, and a predetermined main converter current value.

(10) The combination of the wind turbine main converter 10 and parallel energy buffer, where both are using a virtual synchronous machine (VSM) type controller, is expected to supply power and energy to the power system 8 in response to variations of frequency, and or phase, about its nominal value. Small variations of the power system frequency occur very frequently due to the variance between the generated power and the power consumed by the different loads and require only small changes to the power/energy output of the combined wind turbine and energy buffer. The small nature of these continual frequency changes (typically <+/−0.2 Hz over extended times) are such 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.

(11) However, occasionally (for example twice per month) the power system frequency deviates from its normal operating point by a much larger amount; requiring the combined wind turbine and energy buffer to provide a much larger change of power/energy in response. In this case the main wind turbine converter alone is not capable of providing the required dynamic energy response (due to its limited energy storage) and therefore the energy buffer must assist.

(12) A second requirement on 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 are required to feed a minimum amount of fault current into the grid (potentially 150% of the main converter rated current in future); the magnitude of which, in particularly severe fault situations, is large enough that the energy buffer must output is maximum current to assist the main converter 10. In less severe fault situations, the main converter 10 will be capable of providing the required fault current without extra assistance.

(13) The ability of the wind turbine to respond to the continual small power system frequency variations and less severe grid fault events will mean that the parallel energy buffer will spend the majority of the time exchanging very little energy with the power system. While not exchanging energy with the grid, continual switching of the power electronic devices would consume energy and dissipate it as heat loss, lowering the efficiency of the energy buffer and associated wind turbine. The only energy exchanged by the energy buffer with the power system during normal operation would be energy required to cover the losses of its converter 34.

(14) This problem is overcome by embodiments of the present invention which reduces the energy losses associated with the continual switching of the energy buffer's power electronic devices and therefore limits the change in the overall wind turbine power train efficiency when the energy buffer is introduced.

(15) For the energy buffer to provide assistance during large grid frequency and low voltage fault events, its converter 34 must be charged and its control 40 active. However, since it does not need to continuously exchange power/energy or fault current with the power system its power electronic devices can be held in the off state until the controller detects a variation of the grid frequency or a low voltage fault event significant enough to require the assistance of the energy buffer. At which point the devices are allowed to switch as necessary to provide the power/energy to provide the required response.

(16) Holding the switches in the off position when the buffer is not required will mean that the loss of energy that occurs when each device is switched will not be experienced and therefore the continual energy loss of the energy buffer will be minimized, and the efficiency improved.

(17) 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 large power system frequency or low voltage fault events to the energy buffer, so that it can respect its power, energy and current limits. Once the required response to the power system frequency or low voltage fault event has been completed by the energy buffer, it will return itself to its pre-event state of charge over a period of time and then return its power electronic devices to the off state.

(18) While not continuously switching its devices the energy buffer must maintain its charged state, which will decay over time due to charge leakage from the capacitors 32.

(19) Therefore, the energy buffer must also engage the switching of its power electronic devices intermittently to allow the import of the necessary energy to maintain adequate charge in its capacitors 32. This process will be activated and deactivated using a hysteresis band around the converter DC link voltage; the converter will switch its devices to charge the capacitors 32 to a maximum level and then cease to switch until the voltage falls to a lower level. The time between charge cycles will be of the order of several tens of seconds/minutes and charge time will be relatively short, so this process will not have a significant impact on the efficiency of the converter 34.

(20) An advantage of embodiments of the present invention is the activation, by coordinated control of the main converter 10 and the energy buffer, of the energy buffer power electronic device 34 switching only when it is required for either responding to a large power system frequency variation, a low voltage fault event or for maintaining the charged state of the capacitors 32.

(21) Therefore, preventing the continual loss of energy that occurs when each device is switched when there is no requirement for the energy buffer to exchange power/energy with the grid. The inactivity of the switches will significantly reduce the operational energy losses of the energy buffer and therefore limit the reduction of the wind turbine power train efficiency that would normally be associated with adding an additional parallel power converter.

(22) To achieve the reduction of the energy buffer losses a coordinated control system 20, 40 is required between the wind turbine power converter 10 and the energy buffer that allows the wind turbine to respond to the continual small variations of the power system frequency and less severe low voltage fault events, but then ‘hand over’ the response to larger events to the energy buffer.

(23) Embodiments of this invention will provide benefits it will minimize the losses in the wind turbine power train, that result from introducing the parallel energy buffer, and therefore the power transfer efficiency of the wind turbine will not be significantly impacted.

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

(25) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(26) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.