DOUBLE-FED WIND POWER PLANT WITH A MEDIUM-VOLTAGE TRANSFORMER

Description

[0001] The present invention relates to a wind turbine with a doubly fed asynchronous generator comprising a stator path and a rotor path, the latter including a rotor-side and a grid-side converter. The rotor path and stator path terminate at a coupling point and are connected to an electrical supply network via this point. The wind turbine is connected via a transformer, with the stator path and the rotor path feeding together on the low-voltage side of the transformer's low-voltage winding.

[0002] The typical implementation of such wind turbines involves a complex and expensive isolation concept consisting of several components. According to current technology, at least one circuit breaker is provided on the low-voltage side of the transformer for the entire electrical power path. For current power classes of 5 MW and above, typical for such wind turbines, a qualified electrician would design the circuit breaker for a current of at least 6300 A. Furthermore, a medium-voltage switchgear (MVSG) is provided on the high-voltage side of the transformer. This allows the entire wind turbine to be de-energized and protects it from overload and / or excessive short-circuit currents by opening the circuit.

[0003] The low-voltage circuit breaker serves as primary protection against overload and short circuit for the entire electrical power path. This breaker is also designed as a safe disconnect device, allowing safe work to be carried out on the de-energized side when maintenance work on the wind turbine is required, even with the breaker open. The circuit breaker for the entire electrical power path can safely disconnect many severe and medium-severity faults.The wind turbine's own power supply is usually tapped before the circuit breaker, so that even after serious faults the wind turbine is still accessible for fault analysis and systems that are powered by the own power supply, such as electric drives for adjusting rotor blade angles or aligning the nacelle, are not affected by a fault and remain functional even after the low-voltage side circuit breaker is opened.

[0004] State-of-the-art wind turbines, in addition to the low-voltage and high-voltage circuit breakers, also have further protective devices. For example, the power path of the grid-side converter has a grid-side circuit breaker, and the power path of the stator has a stator contactor, by means of which the two power paths can be connected to or disconnected from the grid independently as needed.

[0005] After the circuit breaker for the entire electrical power path is switched on, the line-side circuit breaker is used to connect or disconnect the line-side inverter from the grid. Since the power path of the line-side inverter in doubly fed asynchronous machines is designed for a significantly lower rated current than the entire electrical power path, and the line-side circuit breaker is considerably smaller than the low-voltage-side circuit breaker, the power path is equipped with additional protection against overload and short circuits.

[0006] When the stator contactor is closed, the generator stator is connected to the grid. The contactor opens and closes at low currents to minimize wear. Over- and short-circuit currents are interrupted by the circuit breaker for the entire electrical power path, after which the stator contactor is de-energized.

[0007] For the exemplary 5 MW wind turbine described, a specialist would, according to the state of the art, provide a stator contactor for 4000 A and a grid-side circuit breaker for 2000 A, in addition to the low-voltage circuit breaker for currents of 6300 A. From an economic perspective, such a wind turbine would require a total of 12300 A of disconnectors, which entails corresponding costs.

[0008] From EP 1 914 877 A2, a wind turbine with a doubly fed asynchronous generator capable of fault ride-through is known. The asynchronous machine feeds into a power supply network via a transformer. The wind turbine has a switching system consisting of a stator switch, a rotor switch, a low-voltage main switch, and a high-voltage network switch. The same system is found unchanged in numerous recent patent applications, such as WO 2018 / 208649 A1.

[0009] US patent 2013 / 0234434 A1 discloses a surge protection device for a wind turbine in which the current mains voltage applied to a transformer is measured and the transformer is short-circuited in the event of an overvoltage.

[0010] A wind turbine with a doubly fed asynchronous generator designed for fault ride-through is disclosed in US patent 2016 / 033856 A1. For this purpose, the impedance can be variably switched, with series-connected residual current circuit breakers (RCCBs) protecting the system from overcurrent. Wind turbines with inverter systems and surge protection circuits are also disclosed in the following documents: EP2169222A2 and EP2434607A2.

[0011] Based on the prior art, the invention aims to provide a doubly fed wind turbine that has a safe and cost-effective separation concept which meets all conventional requirements for the operational safety of wind turbines and personal protection.

[0012] According to the invention, the problem is solved by a doubly fed wind turbine with the features of claim 1. Advantageous embodiments are the subject of the dependent claims.

[0013] The double-fed wind turbine designed according to the invention has a stator path and a rotor path, the latter comprising a rotor-side and a grid-side converter. The rotor path and the stator path terminate at a coupling point. Via the coupling point, the rotor path and stator path are jointly connected to an electrical supply network through a medium-voltage transformer; that is, the rotor path and stator path preferably feed into the same winding of the medium-voltage transformer on the low-voltage side. According to the invention, a stator contactor is provided in the stator path and a circuit breaker in the rotor path, with a first overvoltage protection device being provided between the medium-voltage transformer and the stator contactor. The overvoltage protection device protects the stator contactor from overvoltages from the medium-voltage network.The first surge protection device protects the stator contactor from overvoltages originating in the medium-voltage network. This integrates the stator contactor into the wind turbine's protection concept. Its performance with regard to conducting and switching short-circuit currents is tested and guaranteed by the surge protection device. The particular advantage of the doubly fed wind turbine according to the invention is that the main circuit breaker on the low-voltage side is no longer required, thus avoiding associated costs. A stator contactor in the stator path and a circuit breaker in the rotor path are sufficient.

[0014] According to the invention, a second overvoltage protection device is provided in the stator path between the stator contactor and the stator. This second overvoltage protection device also serves to protect the stator contactor from overvoltage, this time from overvoltage originating from the generator.

[0015] In a preferred embodiment, a third overvoltage protection device is provided in the rotor path. This third overvoltage protection device is preferably located between the generator rotor and the rotor-side converter. The third overvoltage protection device protects the rotor-side converter from overvoltage from the generator.

[0016] In a preferred embodiment, at least one of the surge protection devices, preferably each of the surge protection devices, is designed for a protection level of U p ≤ 5 kV.

[0017] In the wind turbine according to the invention, the first surge protection device is provided between the connection point and the medium-voltage transformer. The first surge protection device is sensitive to overvoltages present in the medium-voltage network and can divert current peaks caused by the overvoltage when the protection level is exceeded. Thus, the stator contactor and the circuit breaker in the rotor path are protected from current peaks. In a preferred embodiment, an auxiliary power supply for the wind turbine is provided between the first surge protection device and the medium-voltage transformer. This auxiliary power supply ensures the continued operation of important functions of the wind turbine in the event of a grid failure.

[0018] Furthermore, a medium-voltage switch is preferably provided on the high-voltage side of the medium-voltage transformer. This allows the wind turbine to be de-energized and disconnected from the grid at the medium-voltage level.

[0019] A preferred embodiment of the invention is described in more detail below. Figures shown are: Fig. 1 a wind turbine in a side view and Fig. 2 a schematic circuit diagram of a doubly fed asynchronous machine with its connection to the medium voltage network.

[0020] Fig. 1 Figure 1 shows a wind turbine 100 with a tower 110 and a nacelle 120. The nacelle 120 is mounted on an azimuth system (not shown) so that it can be rotated around a longitudinal axis of the tower and thus aligned with the wind direction. The nacelle 120 houses a drive train (see Figure 1). Fig. 2 ), which is connected to a rotor 130, which is equipped with a rotor hub 140 and three rotor blades 150. Kinetic energy is absorbed from the wind via the rotor 130 and converted into electrical energy via a generator.

[0021] Fig. 2 shows a schematic view of the wind energy plant 100 with regard to its feed-in into a medium voltage network 210. Fig. 2For clarity, the diagram shows the feed-in to the medium-voltage grid for a single wind turbine. However, the feed-in can also occur via multiple wind turbines, for example, grouped together in a wind farm. The drive train 201 of the wind turbine is connected to the rotor 130 via the rotor hub 140 and is configured to transfer the mechanical energy of the rotor 130 to the generator rotor 205 of a generator 203 via a rotary motion. This transmission is achieved by means of a gearbox 202. The drive train 201 has a shaft that is connected to the generator rotor 205 and rotates it to convert the mechanical energy into electrical energy. In this embodiment, a doubly fed asynchronous generator 203 is provided, whose generator stator 204 is electrically connected to the grid via a stator path and whose generator rotor 205 is electrically connected to the grid via a rotor path, and which feeds electrical energy or power into the grid.A converter 206 is provided in the rotor path, comprising a rotor-side converter 207 and a grid-side converter 208. A DC link is provided between converters 207 and 208. The rotor path of the generator rotor 205 and the stator path of the generator stator 204 are connected to the medium-voltage grid 210 via a coupling point 218, together with the low-voltage winding of a medium-voltage transformer 209. An auxiliary power supply network 211 is provided between coupling point 218 and medium-voltage transformer 209. Important components, such as the plant control system, pitch and azimuth adjustment drives, and the aircraft lighting system, can be operated via the auxiliary power supply network 211, even when the rotor and stator paths are open and the wind turbine is not feeding into the grid 210.A medium-voltage switchgear 212 is provided between medium-voltage transformer 209 and medium-voltage network 210, with which the entire wind power plant including its medium-voltage transformer can be disconnected from the medium-voltage network.

[0022] A first surge protection device 215 is located between the medium-voltage transformer 209 and the coupling point 218, where the stator path and rotor path converge. The stator path can be disconnected by a stator contactor 213. A second surge protection device 216 is provided in the stator path between the stator contactor 213 and the generator stator 204. A circuit breaker 214 is provided in the rotor path between the grid-side converter 208 and the coupling point 218. A third surge protection device 217 is also provided in the rotor path and is located between the generator rotor 205 and the rotor-side converter 207. The stator contactor 213 of the exemplary wind turbine is designed for currents of 4000 A, and the circuit breaker 214 for currents of 2000 A. From an economic point of view, switching devices for a total of 6000 A are therefore provided on the undervoltage side of the transformer.

[0023] The above design results in a significant reduction in costs and space requirements within the converter. The total current of the circuit breakers on the low-voltage side is considerably reduced to 6000 A compared to the prior art (12300 A). In the solution according to the invention, a circuit breaker for the entire electrical power path on the low-voltage side (6300 A) is eliminated. Only a stator contactor 213 in the stator path and a circuit breaker 214 in the rotor path are provided. This circuit breaker 214 reliably disconnects the rotor path from the grid. As a safe disconnecting device, it allows for safer work downstream of the circuit breaker 214. The circuit breaker 214 is typically only switched off a few times a year for maintenance and service purposes.Since there are usually only a few trips of the circuit breaker 214 due to faults or disturbances as well as overcurrents, a low number of switching cycles of a few thousand is sufficient for the required service life, which also saves costs.

[0024] The stator contactor 213 switches on and off with low currents due to operational requirements. To avoid having to disconnect the load for every moderately severe fault, the stator contactor 213 is designed to handle the majority of potential faults. Alternatively, in the event of numerous faults, disconnection would have to be performed by the medium-voltage switch 212. However, disconnection by the medium-voltage switch 212 leads to long downtimes of the wind turbine and thus to yield losses. Furthermore, due to the current implementation of the low-voltage tap for the turbine's own power supply, there is no longer any power available for functions such as pitch control, yaw control, or communication.The medium-voltage switch 212 therefore only switches in the event of a prolonged short circuit or an earth fault in the medium-voltage transformer 209, if the stator contactor 213 or the circuit breaker 214 does not disconnect from the network on all poles upon request, and if insulation faults occur between the medium-voltage transformer 209 and the circuit breaker 214 or the stator contactor 213.

[0025] If a potential earth fault occurs between the stator contactor 213 and the generator stator 204, the earth fault currents are low and are safely and quickly interrupted by the stator contactor. The medium-voltage circuit breaker 212 remains open in this case. The probability of a simultaneous, multi-phase earth fault is very low. Through design measures, primarily in the cable connection area of ​​the converter and the generator, a direct short circuit between two or three stator phases can be prevented. If a short circuit were to occur on the stator side, increased short-circuit currents would result. This short-circuit current is supplied by both the grid and the generator, with the magnitude of the short-circuit current depending on the impedance of the medium-voltage transformer and the generator. This short-circuit current would thus be supplied from two sides. These would have to be separated by the medium-voltage circuit breaker 212.The stator contactors 213 could be overloaded and destroyed by the resulting short-circuit currents, even if the short-circuit current is only conducted and not switched. Therefore, a short circuit on the stator side must be avoided.

[0026] Unlike on the stator side, a short circuit on the rotor side of the inverter or generator cannot be ruled out. In doubly fed generators with slip-ring rotors, for example, conductive brush dust can cause ground faults or short circuits via arcing. Short circuits in the inverter can also occur due to defective power modules. For instance, alloyed power modules may prevent the inverter from switching off the short-circuit current using the pulse-stop function of the IGBTs, causing it to persist for an extended period. The short circuit on the rotor side can be fed via the generator, which can also lead to increased short-circuit currents on the stator side. The magnitude of the short-circuit currents on the stator side is caused by a short circuit on the rotor side and also depends on the impedances of the generator and the medium-voltage transformer.However, the impedances are in series here and are therefore significantly higher than if the short circuit occurred directly on the stator side. This results in reduced short-circuit currents on the stator side, which must be switched off by the stator contactors without the medium-voltage circuit breaker 212 completely disconnecting the wind turbine from the grid. Reference symbol list

[0027] 100 Wind turbine 110 Tower 120 Nacelle 130 Rotor 140 Rotor hub 150 Rotor blade 201 Drive train 202 Gearbox 203 Doubly fed asynchronous generator 204 Generator stator 205 Generator rotor 206 Converter 207 Rotor-side converter 208 Grid-side converter 209 Medium-voltage transformer 210 Medium-voltage grid 211 Auxiliary power grid 212 Medium-voltage switchgear 213 Stator contactor 214 Circuit breaker 215 First surge protection device 216 Second surge protection device 217 Third surge protection device 218 Coupling point

Claims

1. Wind turbine with a doubly-fed induction generator, having a stator path and a rotor path, the rotor path having a rotor-side and a grid-side converter (207, 208), wherein the rotor path and the stator path end in a connection point (218) and are jointly connected to a medium-voltage grid (210) via a medium-voltage transformer (209), characterized in that a stator contactor (213) is provided in the stator path and a circuit breaker (214) is provided in the rotor path, respectively, wherein a first overvoltage protection device (215) is provided between the medium-voltage transformer (209) and the stator contactor (213) to protect the stator contactor (213) from overvoltages from the medium-voltage grid (210), and a second overvoltage protection device (216) is provided in the stator path between the stator contactor (213) and the stator (204).

2. Wind turbine according to claim 1, characterized in that a third overvoltage protection device (217) is provided in the rotor path between the generator rotor (205) and the rotor-side converter (207).

3. Wind turbine according to claim 1 or 2, characterized in that at least one of the overvoltage protection devices has a protection level of Up ≤ 5 kV.

4. Wind turbine according to any one of claims 1 to 3, characterized in that the first overvoltage protection device (215) is provided between the connection point (218) and the medium-voltage transformer (209).

5. Wind turbine according to any one of claims 1 to 4, characterized in that a medium-voltage circuit breaker (212) is provided on the grid side of the medium-voltage transformer (209).

6. Wind turbine according to any one of claims 1 to 5, characterized in that an auxiliary power system (211) is provided between the first overvoltage protection device (225) and the medium-voltage transformer (209).