Field of the Invention

The present invention relates generally to a wind turbine, and in particular, to a wind turbine which is capable of increasing a current supplied to a power grid when there is a voltage drop at the power grid.

Background of the Invention

A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind is applied to wind turbine blades of the wind turbine to rotate a rotor. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. Power grids require a constant frequency electrical power to be provided by the wind turbine. Because wind speed fluctuates, the force applied to the wind blades and hence the rotational speed of the rotor can vary. This results in the frequency of the electrical power generated by the generator to be varying.

A variable speed wind turbine normally includes a power converter to convert the varying frequency electrical power from the generator into fixed frequency required by the power grids. The power converter usually includes a generator side converter coupled to a grid side converter via a direct current (DC) link. The generator side converter regulates the power of the generator. This power passes through the DC-link, and is eventually fed to the grid through the grid side converter. The same is true for the Doubly Fed Generator (DFG) systems where only a portion of the power from the generator passes through the power converter.

When there is a grid fault, for example a low voltage event, there is a sudden drop in demand for active power from the grid. Previously, wind turbines or wind farms are allowed to disconnect during such grid fault event. With increasing penetration of wind power generation, the disconnection of wind turbines or wind farms from the grid during grid fault events is no longer acceptable by grid operators. This is because the disconnection of a wind turbine/farm under grid fault condition leads to instability of the power grid. Grid operators in many countries now require wind farm operators to comply with certain grid requirements specified in grid codes before they are allowed to connect to the grid.

Grid requirements vary in different countries, but they have a common aim of permitting the development, maintenance and operation of a coordinated, reliable and economical transmission or distribution system. For example, grid codes typically require that wind turbines should be able to ride-through a fault causing the voltage at a Point of Common Coupling (PCC) at a wind farm to decrease substantially. Grid codes may also require wind turbines or farms to provide reactive current to support the grid when there is substantial grid voltage drop at the PCC.

Summary of the Invention

According to a first aspect of the invention, a wind turbine includes a power converter and a detection unit. The power converter is connected between a generator in the wind turbine and a power grid, and includes a direct-current (DC) link. The power converter is adapted to supply a current to the power grid. The detection unit is adapted to detect a voltage of the power grid. When the detection unit detects that the voltage of the power grid drops to a predetermined level, a voltage at the DC link is decreased thereby increasing the current supplied to the power grid.

The predetermined level may be a level which corresponds to a level where a low voltage event is said to occur. This level may vary according to various grid codes and can be set differently for each turbine. When the grid voltage drops to or below the predetermined level, the DC link voltage is decreased.

The detection unit may detect the grid voltage by measuring the voltage directly at a connection point between the wind turbine and the power grid. Such a connection point is known as the point of common coupling (PCC). The detection unit may also detect the grid voltage indirectly, for example, by detecting if there is any increase in the DC link voltage. When it is detected that there is a drop in the grid voltage, either directly or indirectly, the DC link voltage is decreased. The decrease in the DC link voltage allows the current supplied to the grid to be increased. Therefore, the wind turbine according to the invention provides a simple yet effective way of providing current to the grid when there is a substantial drop in the grid voltage, thus fulfill grid requirements.

According to an embodiment, the power converter includes a generator- side converter and a grid-side converter, and the DC link is arranged between the generator- side converter and the grid-side converter.

According to an embodiment, the wind turbine further includes a power converter controller. The power converter is adapted to define a reference DC link voltage. In other words, the power converter controller defines the voltage level at the DC link.

According to an embodiment, the wind turbine further includes a DC link controller. The DC link controller is adapted to control the voltage at the DC link to be at least substantially close to the reference DC link voltage. The DCC may be integrated within the power converter controller, or may be located as a separate unit outside the power converter controller.

According to an embodiment, the power converter controller is adapted to decrease the reference DC link voltage when the detection unit detects that the voltage of the power grid drops to the predetermined level, thereby decreasing the voltage at the DC link.

According to an embodiment, the voltage at the DC link is decreased in proportion to the voltage drop in the power grid.

According to an embodiment, the voltage at the DC link is decreased based on the voltage of the power grid and an output power of a generator of the wind turbine.

According to an embodiment, the current supplied to the power grid includes active current or reactive current or a combination of active and reactive current. Thus, this allows the power factor of the supplied current to be controlled between 0 and 1, in accordance to the grid requirements in different countries. According to an embodiment, the wind turbine further includes a dissipating unit connected to the DC link. The dissipating unit is adapted to dissipate power from the DC link when the detection unit detects that the voltage of the power grid drops to the predetermined level. The use of the dissipating unit helps in achieving a fast rate of decrease of the DC link voltage.

According to a second aspect of the invention, a method for operating a wind turbine connected to a power grid is provided. The method includes detecting a voltage of the power grid and decreasing a voltage at a DC link of a power converter when it has been detected that the voltage of the power grid has dropped to a predetermined level, thereby increasing a current supplied to the power grid. The DC link is connected between a generator in the wind turbine and the power grid.

According to an embodiment, the voltage at the DC link is controlled to be at least substantially close to a reference DC link voltage defined by a power converter controller.

According to an embodiment, the method includes decreasing the reference DC link voltage when it has been detected that the voltage of the power grid has dropped to the predetermined level, thereby decreasing the voltage at the DC link.

According to an embodiment, the voltage at the DC link is decreased in proportion to the voltage drop in the power grid.

According to an embodiment, the voltage at the DC link is decreased based on the voltage of the power grid and an output power of a generator of the wind turbine.

According to an embodiment, the current supplied to the power grid includes active current or reactive current or a combination of active and reactive current.

According to an embodiment, the method further includes dissipating power from the DC link when it has been detected that the voltage of the power grid has dropped to the predetermined level.

It should be noted that a person skilled in the art would readily recognize that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa. According to a third aspect of the invention, a wind farm including at least one wind turbine connected to a power grid is provided. The wind turbine includes a power converter and a detection unit. The power converter is connected between a generator in the wind turbine and a power grid and includes a DC link. The power converter is adapted to supply a current to the power grid. The detection unit is adapted to detect a voltage of the power grid. When the detection unit has detected that the voltage of the power grid has dropped to a predetermined level, a voltage at the DC link is decreased thereby increasing the current supplied to the power grid. Brief Description of the Drawings

The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings.

Figure 1 shows a general structure of a wind turbine.

Figure 2 shows an electrical system of the wind turbine according to an embodiment.

Figure 3 shows a flow-chart of a method for operating the wind turbine according to an embodiment.

Figure 4 shows a graph illustrating the relationship between the DC link voltage and the output current of the power converter according to an embodiment.

Figure 5 shows part of the electrical system where a dissipating unit is connected to the DC link according to an embodiment.

Detailed Description of the Invention

Fig.l shows a general setup of a wind turbine 1. The wind turbine 1 includes a tower 2 having a number of tower sections, a nacelle 3 positioned on top of the tower 2, and a rotor 4 extending from the nacelle 3. The tower 2 is erected on a foundation 7 built in the ground. The rotor 4 is rotatable with respect to the nacelle 3, and includes a hub 5 and one or more blades 6. Wind incident on the blades 6 causes the rotor 4 to rotate with respect to the nacelle 3. The mechanical energy from the rotation of the rotor 4 is converted into electrical energy by a generator (not shown) in the nacelle 3. The electrical energy is subsequently converted into a fixed frequency electrical power by a power converter to be supplied to a power grid.

Although the wind turbine 1 shown in Figure 1 has three blades 6, it should be noted that a wind turbine may have different number of blades. It is possible to find wind turbines having two to four blades. The wind turbine 1 shown in Figure 1 is a Horizontal Axis Wind Turbine (HAWT) as the rotor 4 rotates about a horizontal axis. It should be noted that the rotor 4 may rotate about a vertical axis. Such a wind turbine having its rotor rotates about the vertical axis is known as a Vertical Axis Wind Turbine (VAWT). The embodiments described henceforth are not limited to HAWT having 3 blades. They may be implemented in both HAWT and VAWT, and having any number of blades 6 in the rotor 4.

Fig.2 shows an electrical system of the wind turbine according to an embodiment of the invention. The electrical system includes a generator 20, a power converter 22, a turbine transformer 24, a power converter controller 26 and a DC link controller (DCC) 28. The power converter 22 is arranged between the generator 20 and the primary windings of the turbine transformer 24. The secondary windings of the transformer 24 are connected to the power grid 30. The DCC 28 is arranged inside the power converter controller 26. As mentioned earlier, it is possible that the DCC 28 is a separate unit outside the power converter controller 26.

A detection unit 50 is also included to sense the parameters of the grid 30, such as the grid voltage. The detection unit 50 senses the grid parameters at the primary windings of the transformer 24 in this embodiment. In other embodiments, the grid parameters may be determined in other manner, such as by monitoring the DC link voltage or sensing the stator windings of the generator if a Doubly-Fed Generator is used.

The power converter 22 includes a generator- side converter 32, a grid-side converter 24 and a DC link 36. The DC link 36 is arranged between the generator- side converter 32 and the grid-side converter 34. The DC link 36 includes a DC link capacitor 38. The electrical system of the wind turbine may include grid filter 40 between the power converter 22 and the turbine transformer 30. The grid filter 40 includes grid inductors 42 and grid capacitors 44.

The generator 20 converts mechanical energy to electrical energy having AC (alternating current) voltage and current (collectively referred to as "AC signals"), and provides the generated AC signals to the generator- side converter 32. The AC signals from the generator have a variable frequency, due to varying wind. The generator- side converter 32 converts or rectifies the AC signals to a DC (direct current) voltage and a DC current (collectively know as "DC signals") which is placed on the DC link 36. The DC link capacitor 38 smoothens the DC signals. The grid- side converter 34 converts the DC signals on the DC link 36 into fixed frequency AC signals for the power grid 30. The power comprising the fixed frequency AC signals (I _G , U _v ) at the output of the grid- side converter 34 is stepped up by the turbine transformer 24 into a level suitable for to be received and transmitted by the power grid 30.

The power converter controller 26 controls the power produced by the generator

20 (by controlling the generator- side converter 32) and the power supplied to the power grid 30 (by controlling the grid-side converter 34). The power supplied to the power grid 30 is controlled by maintaining a constant voltage at the DC link 36 under normal conditions,. In an embodiment, the power converter controller 26 includes a DC link reference voltage module (not shown) for defining a DC link reference voltage. This DC link reference voltage can be selected, for example from a look-up table based on a given operating point. The DCC 28 then controls the DC link voltage to be as close to the DC link reference voltage defined by the DC link reference voltage module as possible.

In one embodiment, the DCC 28 controls the DC link voltage by first monitoring or measuring a current value of the DC link voltage. When the measured value of the DC link voltage is higher than the DC link reference voltage, more power is delivered to the power grid 30. When the measured DC link voltage is lower than the DC link reference voltage, less power is delivered to the power grid 30. In an embodiment, the DC link reference voltage is defined such that it is higher than the highest peak value of the grid voltage. Additionally, it is defined such that it is higher than the sum of the highest peak value of the grid voltage and the voltage across the grid filter 40. Furthermore, the DC link voltage should be high enough to maintain full control of the generator 20.

In an embodiment, the generator-side converter and/or grid-side converter includes insulated-gate bipolar transistors (IGBT). It should be noted that the converters may include other types of semiconductor switches such as bipolar junction transistors (BJT) in other embodiments. Each converter may include 2 IGBTs for each phase, i.e. a total of 6 IGBTs for 3-phase power. The 2 IGBTs for each phase operate in a complementary manner, and the switching on/off of the IGBTs is controlled using PWM signals generated by the power converter controller 26. The output current I _G from the grid-side converter 34 is thus controlled by controlling the switching of the IGBTs.

There are losses in the IGBTs during switching as there is current flowing through each IGBT when it is switched off. The loss in each IGBT can be estimated by multiplying the voltage across the IGBT and the current flowing through it when it is switched off. The voltage across the IGBT is the DC link voltage, and the current flowing through the IGBT is the phase/output current I _G . Therefore when the DC link voltage is reduced when it has detected that the voltage of the power grid has dropped to a predetermined level (e.g. during a grid fault) according to the embodiment of the invention, the output current is increased accordingly due to reduced switching losses in the IGBTs.

It should be noted that the electrical system described with reference to Fig.2 is only an example of the electrical system of the wind turbine where the embodiment of the present invention can be implemented, and only the main components are shown to illustrate the embodiments. The present invention should not be limited to the exact electrical system configuration shown in Fig.2. Other electrical configurations are possible. Also, many components in the electrical system of the wind turbine are not shown in Fig.2. For example, the electrical system may include filters between the generator 20 and the power converter 22. Also, there may be switches arranged at various locations for connecting or disconnecting certain components of the turbine.

The embodiment of the invention can also be implemented in a doubly-fed electrical system where the stator windings of a doubly-fed generator are connected to the power grid via a turbine transformer. The rotor windings of the generator are connected to the transformer via a power converter.

Fig.3 shows a flow-chart of a method for increasing the current supplied to the power grid when there is a voltage drop in the power grid according to an embodiment. Step 60 includes detecting the voltage of the power grid. This can be done by the detection unit 50 as described in Fig.2. Step 62 includes determining whether the detected grid voltage has dropped to a predetermined level. If the grid voltage has dropped to the predetermined level, it may mean that there is a fault in the grid or a low voltage event has taken place. The predetermined level corresponding to a low voltage event may differ depending on the grid code requirements. When it is determined that the sensed grid voltage has dropped to the predetermined level, the DC link voltage is decreased in step 64. Otherwise, the detection unit 50 continues to detect the grid voltage.

As described earlier, the DC link voltage may be decreased by decreasing the reference DC link voltage provided by the power converter controller 26. As the DCC 28 controls the DC link voltage to be substantially close to the reference DC link voltage, the DC link voltage is decreased accordingly. As a result of the decreased DC link voltage, the output current I _G from the grid- side converter 34 is increased accordingly.

According to an embodiment, the DC link voltage is decreased proportionally to the grid voltage drop. For example, if the grid voltage drops to half of its nominal voltage, the DC link voltage is decreased to half. As a result, the output current I _G from the grid-side converter is doubled. In other words, the output current I _G is increased proportionally with the decrease of the DC link voltage and hence the grid voltage drop.

Fig.4 shows the relationship between the DC link voltage and the output current I _G - The solid line 70 represents the DC link voltage U _dc or the reference DC link voltage Udc _r e _f . The dotted line 72 represents the output current I _G . When the low voltage event at the grid occurs at point A, the reference DC link voltage, and hence the DC link voltage, is decreased. As a result of the decrease of the DC link voltage, the output current I _G is increased accordingly. When the grid voltage recovers at point B, the reference DC link voltage is increased back to its original voltage level, causing the DC link voltage to be increased. As a result, the output current I _G is also decreased accordingly.

In an embodiment, a dissipating unit 80 is connected to the DC link 36 of the power converter 22 as shown in Fig.5, and can be activated by closing a switch SW1. The dissipating unit 80 may be used to aid in the decrease of the DC link voltage, and also to increase the rate of decrease of the DC link voltage. When it is detected that the grid voltage has dropped to the predetermined level, the dissipating unit 80 is activated by closing switch SW1 to dissipate power from the DC link 36. The closing of the switch SW1, and hence the activation of the dissipating unit 80, may be controlled by the power converter controller 26. The dissipating unit 80 may include a resistor or a resistor bank in other embodiments.

In another embodiment, the DC link voltage is decreased based on the voltage of the power grid and the generator power. In other words, the amount of decrease of the DC link voltage is dependent on the power output of the generator and the grid voltage. During a grid fault, the generator power may be transferred to the power grid and/or be directed to be dissipated via the dissipating unit at the DC link of the power converter. Therefore in this embodiment, both the current needed to be supplied from the power converter to support the grid during a grid fault and requirement to dissipate power from the generator are taken into account when determining the amount of decrease of the DC link voltage. This ensures an optimal operation of the dissipating unit of the DC link.

One of the advantages of lowering the DC link voltage according to the embodiments besides lowering the switch losses is that the effect of over-voltage across each IGBT each time the IGBT turns off is reduced. The over-voltage across the IGBT happens due to commutation loop made of copper busbars between the IGBT and the DC link capacitor, and the internal inductance in the IGBT and the DC link capacitor. These busbars have stray inductance which creates the over-voltage across the IGBT each time it turns off. If this overvoltage is too high, the IGBT may be destroyed. This overvoltage is more critical when the DC link voltage is high. As mentioned, this over-voltage effect is reduced when the DC link voltage is decreased according to the embodiments of the invention.

It should be emphasized that the embodiments described above are possible examples of implementations which are merely set forth for a clear understanding of the principles of the invention. The person skilled in the art may make many variations and modifications to the embodiment(s) described above, said variations and modifications are intended to be included herein within the scope of the following claims.