Method for connecting a power transformer to an electrical grid

Abstract

A method for connecting a power transformer, located between an inverter of a wind turbine and an electrical grid, to the electrical grid; the method comprises the steps gradually increasing a voltage at a primary side of the transformer from a low starting voltage to a target voltage equal or close to a nominal voltage of the transformer, by means of the inverter of the wind turbine or by means of an auxiliary inverter, thereby increasing the voltage at a secondary side of the transformer, wherein the gradually increasing of the voltage uses energy of an internal energy storage device, connecting the secondary side of the transformer to the electrical grid after predefined target conditions have been reached.

Claims

1. A method for connecting a power transformer to an electrical grid, the power transformer being located between an inverter of a wind turbine and the electrical grid, the method comprising: gradually increasing a first voltage at a primary side of the power transformer from a starting voltage to a target voltage, the starting voltage being less than the target voltage, the target voltage being equal to or close to a nominal voltage of the power transformer, wherein the gradually increasing the first voltage comprises using the inverter of the wind turbine, thereby increasing a second voltage at a secondary side of the power transformer, wherein the inverter supplies electrical power generated by the wind turbine to the electrical grid, wherein the gradually increasing of the first voltage uses energy of an internal energy storage device, and wherein the internal energy storage device is pre-charged from the wind turbine during normal operation of the wind turbine, and connecting the secondary side of the power transformer to the electrical grid in response to predefined target conditions being reached.

2. The method according to claim 1, wherein: gradually increasing the first voltage of the primary side of the transformer avoids or prevents an overload current of the inverter, and the first voltage of the primary side is increased gradually to the target voltage over a predefined rising time.

3. The method according to claim 2, wherein gradually increasing the first voltage of the primary side of the power transformer avoids or reduces an inrush-current in the power transformer.

4. The method according to claim 2, wherein the predefined rising time is in a range of 10 milliseconds (ms) to 300 seconds.

5. The method according to claim 4, wherein the predefined rising time is in a range of 100 ms to 30 seconds.

6. The method according to claim 2, wherein the starting voltage is in a range of 0 to 70% of the nominal voltage of the power transformer.

7. The method according to claim 2, wherein the target voltage is in a range of 75% to 110% of the nominal voltage of the power transformer.

8. The method according to claim 7, wherein the target voltage is the nominal voltage of the power transformer.

9. The method according to claim 1, wherein: the wind turbine is operated in a self-sustain mode once the target voltage has been reached and at least until the secondary side of the power transformer has been connected to the electrical grid.

10. The method according to claim 1, wherein: if the electrical grid is in operation when connecting the secondary side of the power transformer to the electrical grid: active power is readily consumed from the electrical grid or injected into the electrical grid once the secondary side of the power transformer is connected to the electrical grid, and/or reactive power is readily consumed from the electrical grid or injected into the electrical grid once the secondary side of the power transformer is connected to the electrical grid, or the wind turbine is connected to the electrical grid without consuming or injecting any power.

11. The method according to claim 1, wherein if the electrical grid is not in operation, the wind turbine operates the electrical grid or part of the electrical grid in an island mode after connecting the power transformer to the electrical grid.

12. The method according to claim 1, wherein: if the electrical grid is separated into a first part and a second part and after connecting the secondary side of the power transformer to the electrical grid, the power transformer is connected to the first part, the wind turbine operates the first part in an island mode, the wind turbine synchronizes the first part of the electrical grid to the second part of the electrical grid to prepare a reconnection, and a reconnection of the first part and the second part of the electrical grid is initiated after the first part and second part of the electrical grid are synchronized to rebuild the electrical grid or a part of the electrical grid.

13. The method according to claim 1, wherein the inverter is a voltage source inverter.

14. The method according to claim 1, wherein the energy storage device is not buffered from the electrical grid after a recovery of the electrical grid.

15. A wind turbine system, comprising: a wind turbine having: an inverter for supplying electrical power generated by the wind turbine to an electrical grid; and an internal energy storage device for storing energy to supply to the inverter or an auxiliary inverter; a power transformer located between the inverter of the wind turbine and the electrical grid, the power transformer having: a primary side for connecting to the inverter; and a secondary side for connecting to the electrical grid, wherein the wind turbine system comprises a controller configured to: gradually increase a voltage at the primary side of the power transformer from a starting voltage to a target voltage equal to or close to a nominal voltage of the power transformer by the inverter of the wind turbine or by the auxiliary inverter, thereby increasing the voltage at the secondary side of the power transformer, wherein the gradually increasing of the voltage uses energy from the internal energy storage device, wherein the starting voltage is less than the target voltage, and wherein the internal energy storage device is pre-charged from the wind turbine during normal operation of the wind turbine, and connect the secondary side of the power transformer to the electrical grid after predefined target conditions being reached.

16. The wind turbine system according to claim 15, wherein at least one of: the inverter or the auxiliary inverter is provided as a voltage source inverter.

17. The wind turbine system according to claim 15, wherein the wind turbine comprises the controller.

18. A wind turbine, comprising: an inverter for supplying electrical power generated by the wind turbine to an electrical grid; and an internal energy storage device for storing energy to supply the inverter or an auxiliary inverter, being configured to be connected to: a power transformer located between the inverter of the wind turbine and the electrical grid, the power transformer having: a primary side for connecting to the inverter; and a secondary side for connecting to the electrical grid, wherein the wind turbine comprises controller configured to: gradually increase a voltage at the primary side of the power transformer from a starting voltage to a target voltage equal to or close to a nominal voltage of the power transformer by the inverter of the wind turbine or by the auxiliary inverter, thereby increasing the voltage at the secondary side of the power transformer, wherein the starting voltage is less than the target voltage, wherein the gradually increasing of the voltage uses energy of the internal energy storage device, and wherein the internal energy storage device is pre-charged from the wind turbine during normal operation of the wind turbine, and connect the secondary side of the power transformer to the electrical grid after predefined target conditions being reached.

19. The wind turbine according to claim 18, wherein at least one of: the inverter or the auxiliary inverter is a voltage source inverter.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention will now be explained by way of example using at least one embodiment and the enclosed figures.

(2) FIG. 1 shows a wind turbine system having a wind turbine connected to an electrical grid in a schematic view and partially in a perspective view.

(3) FIG. 2 shows a flow chart of a method for connecting a power transformer according to one embodiment.

(4) FIG. 3 shows a diagram of a voltage value over time as part of a method for connecting a power transformer according to one embodiment.

DETAILED DESCRIPTION

(5) FIG. 1 shows a wind turbine 100 having a tower 102 and a nacelle 104. At the nacelle 104 there is a rotor 106 having three rotor blades 108 and a spinner 110. The rotor 106 is rotated during operation by means of the wind and thereby drives a generator which is located inside the nacelle 104.

(6) This way electrical energy is generated and provided as a DC current to the inverter 150. The inverter 150 having an inverter output 152 being connected to a primary side 161 of a transformer 160. The inverter 150 is capable of adjusting its output voltage V.sub.1 at its inverter output 152. This way the output voltage V.sub.1 of the inverter output 152 which is supplied to the primary side 161 of the transformer 160 can gradually be increased from 0% or 5% according to another embodiment, to 100% of a nominal value or close to the nominal value of such output voltage.

(7) For operating such method the output voltage V.sub.1 is measured by means of a voltage sensor 154 and provided as a sensor signal to a control unit 156. The control unit 156 thus controls the inverter 150. For increasing the output voltage V.sub.1 of the inverter 150 the control unit 156 provides a set value V.sub.S for the output voltage V.sub.1. This set point voltage V.sub.S can also gradually be increased from 0 to 100% with respect to a nominal voltage V.sub.N. The control unit 156 can also provide further control signals as known by the person skilled in the art. A storage device 158 is provided, being connected to the inverter 150 at a DC-input 159. The storage device 158 delivers energy to the inverter 150 for increasing the output voltage V.sub.1.

(8) However, the output voltage U.sub.1 and thus the set point voltage V.sub.S can also be increased stepwise. If such steps are small enough, that can be similar to gradually increasing the voltage.

(9) By gradually increasing the output voltage V.sub.1, the voltage at the primary side 161 of the transformer 160 is increased, as these are the same voltages. Accordingly, the output voltage V.sub.1 at the inverter output 152 can also be depicted as the primary voltage V.sub.1 of the primary side 161 of the transformer 160. By increasing the primary voltage V.sub.1 at the primary side 161 of the transformer 160, there is also an increase of a secondary voltage V.sub.2 at the secondary side 162 of the transformer 160. Accordingly, the inverter 150 can gradually or in a different way increase the secondary voltage V.sub.2 which is supplied to the electrical grid 170. In the embodiment shown in FIG. 1, the electrical grid 170 comprises at least a first part 171 and a second part 172. This first and second part 171, 172 can be separated or connected by means of a second switch S.sub.2. The transformer 160 can be connected to the first part 171 by means of a first switch S.sub.1.

(10) The wind turbine 100 including the inverter 150 and the control unit 156 forms a wind turbine system 180, including the transformer 160.

(11) FIG. 2 shows a flow chart describing a method for connecting a power transformer to the electrical grid according to one embodiment. According to this flow chart 200 the method starts at a start block 202. The method is started when the power transformer, such as the power transformer 160 according to FIG. 1, is not connected to the electrical grid and is not powered by any voltage.

(12) As part of the start block 202 it could be checked whether the transformer is disconnected from the grid. This could in particular mean that it is checked whether according to FIG. 1 transformer 160 is separated from the first part 171 of the grid 170 by means of the first switch S.sub.1. If such starting conditions are met the voltage can be increased according to the increase block 204. Accordingly, the increase block 204 is illustrated by a time dependent increase of the voltage. Such increase can be done by increasing the voltage von 0% to 100% of a nominal voltage using a corresponding increase ramp which defines a slope linearly and continuously increasing the voltage over time. However, other kinds increasing the voltage can also be implemented. One possibility is to start at a low voltage value such as 5% of the nominal voltage. In addition or alternatively it might be suggested that the voltage does not exactly reach 100% of a nominal value, but a slightly smaller or slightly larger value. Many other strategies could be use such as increasing the voltage by small steps.

(13) Once the increase process according to the increase block 204 is fulfilled, i.e., once the final voltage, in particular the nominal voltage, is reached, the process goes on with examining whether the grid, in particular the first part 171 of the grid 170 according to FIG. 1 is available or not. This is done according to the examination block 206. Whether the grid is available or not particularly means whether there is a stable voltage in said grid or if there is no grid available. If there is no grid available this includes a situation according to FIG. 1 when the second switch S.sub.2 is open and the first part 171 of the grid 170 is not available, i.e., having a voltage level of zero volts. If such second switch S.sub.2 is open the examination block 206 does not need to examine whether the second part 172 of the grid 170 is available.

(14) Depending on the result of the examination according to the examination block 206 different predefined target conditions will be checked or prepared.

(15) Accordingly, if the grid is available the examination block 206 will branched to the synchronization block 208. According to the synchronization block 208 voltage frequency and phase of the voltage in the grid, in particular the voltage in the first part 171 of the grid 170 according to FIG. 1, will be measured continuously. Based on such measurement the wind turbine, in particular the inverter which has increased the voltage according to the increased block 204, will adapt its voltage output at the inverter output 152 of the inverter 150 according to FIG. 1. According to that the output voltage V.sub.1 will be adjusted such that the secondary voltage V.sub.2 at the secondary side 162 of the transformer 160 matches to a voltage in the first part 171 of the grid 170 with respect to voltage level, frequency of the voltage and phase of the voltage.

(16) If all the synchronization has been done successfully the method goes on from the synchronization block 208 to the connection block 210. According to the connection block 210 there will basically a switch being closed, such as the first switch S.sub.1 according to the embodiment explained in FIG. 1.

(17) After connection according to the connection block 210 the method proceeds to grid support according to the support block 212. Such support particularly includes feeding active and reactive power particularly depending on the state of the grid and/or a requirement by the grid operator operating the corresponding grid. In particular, the support block 212 provides for feeding active power depending on the frequency in the grid and feeding reactive power depending on the voltage level in the grid.

(18) Finally, the method ends in the end block 214 if the grid and the feeding into the grid by means of the inverter operates in a basically regular and stable way. Thus, the end of the proposed method means, that after that the wind turbine continuous feeding power into the grid in a regular way.

(19) If in the examination block 206 it was realized that the grid is not available the examination block 206 branches to the black start block 216. According to the black start block 216 the voltage level is controlled to a nominal value. This may in particular mean that either the output voltage V.sub.1 according to the embodiment of FIG. 1 is controlled to a nominal value of the output voltage V.sub.1 and thus of the voltage at the primary side 161 of the transformer 160. It may also mean that the secondary voltage V.sub.2 at the secondary side 162 of the transformer 160 is controlled to a value similar to a nominal value of the voltage of the grid, in particular of a nominal value of the voltage in the first part 171 of the grid 170.

(20) In addition in the black start block 216 the frequency of the voltage is controlled to a nominal frequency. This is in particular 50 Hz or 60 Hz depending on the particular grid.

(21) Once these values of the voltage amplitude and the frequency and phase of the voltage have been reached, the transformer can be connected to the grid or at least the separated part of the grid. In particular, according to the embodiment of FIG. 1 the first switch S.sub.1 can be closed, this way connecting the transformer 160 and thus the inverter 150 and thus the wind turbine 100 to the first part 171 of the grid 170. It may however also be checked before that whether this first part 171 of the grid 170 is ready to receive a voltage, i.e., a nominal voltage when connecting the first switch S.sub.1. This may include to check whether consumers are disconnected or at least only few consumers are connected to this first grid part 171. It may also include to check that there is no grid failure such as a short circuit. All this is done according to the first step connection block 218.

(22) After the transformer 160 and thus wind turbine 100 are connected to the first grid part 171 and thereby providing a nominal voltage value to this first grid part 171 the grid part 171 is thus basically started or restarted. Further steps can then be fulfilled such as connecting first or further consumers to the first grid part 171 and concurrently increasing the power fed into the this first grid part 171 by means of the wind turbine 100. This may include that the wind turbine 100 further connects or powers further inverters which may particularly be connected in parallel to the inverter output 152 according to the first embodiment illustrated in FIG. 1.

(23) If the first grid part 171 is thus further brought into a basically normal operating condition, the wind turbine and thus the inverter 150 may as well support the first grid part 171 similar to the options described with respect to the support block 212. However, such support of the first grid part 171 will be different as this first grid part 171 is just a small part of a grid 170 and thus may react differently when compared to a large grid. Steps similar to the support block 212 maybe adapt to the particular characteristics of such first grid part, in particular of the first grid part 171 according to the embodiment shown in FIG. 1.

(24) Once this first grid part operates basically properly, it may be reconnected to a further part of the grid or to the rest of grid. According to the embodiment shown in FIG. 1 it may be reconnected to the second grid part 172 by closing the second switch S.sub.2. This is done according to the second step connection block 220.

(25) Of course, before the second switch S.sub.2 is closed it must also be taken care of that the first and second grid parts 171 and 172 are synchronized. Such synchronization is basically similar to the synchronization explained with respect to the synchronization block 208. However, there are of course differences as for reconnecting the first and second grid part 171, 172 the inverter 150 needs to synchronize the complete first grid part 171. A further difference may be that after closing the first switch S.sub.1 but before closing the second switch S.sub.2 further inverters could be involved in addition to the inverter 150. Accordingly, all these inverters then need to take care for the synchronization. It may also be that in the meantime further energy producers have been connected to the first grid part 171 and thus all these generating units together need to take care for the synchronization. However, the inverter 150 may lead such synchronization. It may also be that alternatively or in addition the second grid part 172 is synchronized to the first grid part 171.

(26) After such reconnection according to the second step connection block 220 the whole grid, in particular the whole grid 170, may be supported as explained with respect to support block 212. As support may in general be similar, this is illustrated by the flow chart 200 continuing from the second step connection block 220 to the support block 212.

(27) FIG. 3 shows a diagram of the output voltage V.sub.1 which can be the output voltage V.sub.1 according to the embodiment shown in FIG. 1. Accordingly FIG. 3 shows a diagram for the primary voltage V.sub.1 at the primary side 161 of the transformer 160. However, instead the secondary voltage V.sub.2 at the secondary side 162 of the transformer 160 could be used as well but will of course have a higher voltage level.

(28) Accordingly, the diagram shows some steps of time t. At the beginning it is assumed that a transformer, in particular the transformer 160, is not powered and thus the primary voltage V.sub.1 is 0. Then a start process starts, such as according to the start block 202 shown in FIG. 2. At that time the voltage may be kept at the level of 0 volts but according to another embodiment the voltage may be increased to a small value such as 5% of a nominal value. This embodiment is shown in FIG. 3. This small value of 5% is generalized by the variable a, representing a low start voltage.

(29) After the starting process is basically fulfilled the method can continue with increasing the voltage gradually starting at the first point in time t.sub.1. Beginning from there with a small value of a, i.e., 5%, the voltage is increased linearly to the final voltage level of 100%. This final voltage value is generalized with the variable b, representing the target voltage and this value could thus also be slightly different to 100%, e.g., 105% or 95%. However, this 100% of the nominal voltage value V.sub.N is preferred and the linear increase reaches this final value at the second point in time t.sub.2.

(30) The way of increasing the voltage from the first point of time t.sub.1 to the second point of time t.sub.2 with a linear function can be defined according to the formula given below the diagram in FIG. 3, namely the following formula:

(31) $V_1=.Math.\left(a+\frac{b-a}{t_2-t_1}\left(t-t_1\right)\right).Math.t=\left[t_1{\mathrm{.Math.t}}_2\right]$

(32) Accordingly, this formula is only for the time section between the first point of time t.sub.1 and the second point of time t.sub.2. The value a represents a smallest value as a percentage of the nominal voltage and the variable b represents the largest value with respect to nominal voltage. V.sub.1 represents the nominal voltage. The variable V.sub.1 represents the primary voltage at the primary side of the transformer. In the same way the secondary voltage V.sub.2 of the secondary side of the transformer can be defined by the same formula by exchanging V.sub.1 by V.sub.2, whereas in that case V.sub.N represents the nominal voltage with respect to the voltage in the grid.

(33) Finally, at the point of time t.sub.2 the voltage V.sub.1 shall remain at this preferred level of 100% while the method examines the grid in a way explained with respect to the examination block 206 of FIG. 2 and also performs the following blocks of the flow chart 200 of FIG. 2, until the end block 214 is reached. This is identified in the diagram of FIG. 3 by the point of time t.sub.E.