DK201771037A1

DK201771037A1 - Protection of a dc motor in a wind turbine

Info

Publication number: DK201771037A1
Application number: DKPA201771037A
Authority: DK
Inventors: Mølhave Christiansen Per; Linding Frederiksen Kristian; Schifter Thiel Høgholt Kasper; Zenner Torp Dorrit
Original assignee: Kk Wind Solutions A/S
Priority date: 2017-12-29
Filing date: 2017-12-29
Publication date: 2019-07-15
Also published as: WO2019129327A1

Abstract

The present invention relates to an electric system of a wind turbine facilitating control of a DC motor of the wind turbine, the electric system comprise a frequency converter connected to an AC voltage supply and the DC motor connected to the output of the frequency converter. The frequency converter is an active rectifier, facilitating conversion of the AC voltage to a determined DC voltage by means of Pulse Width Modulation. the DC motor is designed with at least one series field and / or at least oneshunt field. A low pass filter is placed in the electric path between the frequency converter and the series field and / or shunt field of the DC motor, and the low pass filter is designed to attenuate DC voltage peaks.

Description

Protection of a dc motor in a wind turbine

Field of the invention

The present invention relates to an electric system of a wind turbine facilitating control of a DC motor of the wind turbine.

Background of the invention [0001] Prior art document EP1707807 disclose an example of rectifying and smoothening the DC voltage to a DC pitch motor to obtain an efficient control of the pitch motor. The rectifying of the voltage is done by a bridge circuit and a DC link capacitor is used for smoothening the DC link voltage and act as energy sink and 10 source for the pitch motor. EP1707807 is silent in relation to problems that might occur as consequence of the rectified DC voltage.

DK 2017 71037 A1

Summary of the invention [0002] It is an object of the present invention to provide an electric system of a wind turbine facilitating control of a DC motor of the wind turbine, the electric system comprise a frequency converter connected to an AC voltage supply and the DC motor connected to the output of the frequency converter. The frequency converter is an active rectifier, facilitating conversion of the AC voltage to a determined DC voltage by means of Pulse Width Modulation. The DC motor is designed with at least one series field and / or at least one shunt field. A low pass filter is placed in the electric path between the frequency converter and the series field and / or shunt field of the DC motor, and the low pass filter is designed to attenuate DC voltage peaks.

[0003] The PWM (PWM; Pulse Width Modulation) control of the frequency converter is advantages in that it has the effect, that it increases flexibility in the DC output voltage from the frequency converter and thereby the input voltage to the DC motor.

[0004] The low pass filter is advantageous in that it has the effect, that the DC voltage to the DC motor is smoothed thereby it is ensured that the DC voltage to the DC motor does not exceed a determined level. It is further advantages in that the windings of the DC motor and / or electronics after the lowpass filter in the electric path is protected from high voltage peaks and associated wear of isolation of windings of the DC motor. This is leading to an increased lifetime of the windings and thereby of the DC motor.

[0005] The low pass filter is advantageous even though it introduces latency in the control of the DC voltage supply to the DC motor i.e. when voltage out of the frequency converter is changed, the change is received later at the DC motor compared to an electric system without such low pass filter.

[0006] According to an embodiment of the invention, the electric system is an electric pitch drive and the DC motor is a pitch motor facilitating turning a wind turbine blade around its longitudinal axis. This is advantages in that it has the effect, that the blades can be energized by batteries and thereby controlled even if the AC supply fails.

DK 2017 71037 A1

Alternatively, or in addition electric systems could be systems for yaw, cooling, etc. including a DC motor.

[0007] According to an embodiment of the invention, the DC motor is a DC compound motor having both a series field and a shunt field. The DC compound motor is advantages in that it has the effect of benefitting from speed / torque properties of both the series and parallel (shunt) windings of the DC motor. This is further advantages in that it leads to a more flexible control of the actuator e.g. a pitch motor and thereby better adaption to given ambient conditions such as wind speed, wind direction and temperature.

[0008] According to an embodiment of the invention, the low pass filter is placed in the electric path between the frequency converter and the shunt field of the DC motor. This is advantages in that it has the effect of protecting specifically the shunt windings of the DC motor.

[0009] According to an embodiment of the invention, the low pass filter is placed in the electric path between the frequency converter and the series field of the DC motor. This is advantages in that it has the effect of protecting specifically the series windings of the DC motor.

[0010] According to an embodiment of the invention, a first low pass filter is placed in the electric path between the frequency converter and the shunt field of the DC motor and a second low pass filter is placed in the electric path between the frequency converter and the series field of the DC motor. This is advantages in that it has the effect of protecting both the shunt and series windings of the DC motor.

[0011] According to an embodiment of the invention, the supply voltage to the frequency converter in case of absence of the AC supply voltage is received from a battery bank. A battery bank is advantageous over a capacitor bank in that it has the effect that the energy density is higher, price is lower, self-discharge is high, and the charge / discharge profile is advantageous. Further, this is advantages in that during fault in the AC supply, the frequency converter still facilitates operating the DC motor.

DK 2017 71037 A1 [0012] According to an embodiment of the invention, the DC voltage peaks are damped by the low-pass filter below 250V, preferably below 275V, most preferably below 330V.

[0013] The cutoff frequency of the low-pass filter is designed based on desired protection of the electronics located in the electric path from the filter to the DC motor. This is advantages in that it has the effect, that sensitive electronics can be protected independent of the AC supply to the frequency converter forming basis for the PWM modulated DC voltage. Thereby, flexibility is added to the level of the AC voltage supplying the frequency converter in that no matter the level hereof, the DC motor (or other sensitive electronics located downstream of the filter) will never be exposed to frequencies above the cutoff frequency.

[0014] According to an embodiment of the invention, the DC voltage at the output of the frequency converter is between 0 and 565V.

[0015] According to an embodiment of the invention, the low-pass filter is implemented as a stand-alone unit connectable to the electric system in the electric path between the frequency converter and the DC motor. The stand-alone implementation of the filter is advantages in that it enables retrofitting existing electric systems of wind turbines with a low-pass filter according to the present invention. Further, by appropriate control, it is possible to couple in and out the filter if desired.

DK 2017 71037 A1

The drawings [0016] For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts:

Figure 1 illustrates an electric system in an embodiment of the invention,

Figure 2 illustrates an example of the filter in an embodiment of the invention, and

Figure 3 illustrates a standalone filter connected to an electric system in an embodiment of the invention.

Detailed description [0017] The invention will now be explained with reference to an electric system 1 in the form of a pitch drive facilitating the control of a pitch angle of a blade of a wind turbine. This example however should not limit the invention to pitch systems, all electric systems in a wind turbine including a DC motor could benefit from the present invention. Examples of such system could be yaw and cooling systems.

[0018] The pitch drive is illustrated in figure 1 includes an AC voltage source 2 and a DC voltage source 3 supplying a frequency converter 4. The frequency converter 4 is controlling a DC compound motor 5 i.e. a DC motor 5 having a series field 5A and/or a shunt field 5B. In the electric path between the frequency converter 4 and the DC motor 5 a filter 6 is located. The filter 6 preferably comprises a low-pass filter part 6A and a RC damping part 6b. Between the filter 6 and the DC motor 5 electronics 7 sensitive high voltage peaks may be located.

[0019] As mentioned, the main purpose of the filter 6 is to protect the sensitive electronics 7 (if present) and the windings of the DC motor 5. In the embodiment illustrated in figure 1, the filter 6 is illustrated in front of the shunt field 6B, however

DK 2017 71037 A1 it should be mentioned, that it can be substituted with or in addition to it, a similar filter 6 can be located in front of the series field 5A of the DC motor 5.

[0020] The frequency converter 4 is preferably connected to both a DC and an AC voltage source. Preferably, the DC voltage source is a battery bank which is preferably used as a backup if the AC voltages source should fail. The DC battery pack preferably comprises a plurality of series connected 12V batteries. The AC voltage source is preferably the electric grid to which the wind turbine is supplying energy.

[0021] The battery bank comprises a plurality of batteries which are charged under the control of the frequency converter 4. Preferably, a service switch (not illustrated) is used to disconnect the battery bank from the remaining system during service. Further, preferably the battery bank enclosure is equipped with ventilation holes to facilitate removal of gasses produced by the batteries.

[0022] To dual voltage supply to the frequency converter 4 is advantages in that it allows flexibility in the control of the DC motor 5. A first operation mode is referred to as grid operation. Here the motor is operated by the converter which is preferably supplied from a 400VAC grid. The motor is in this mode preferably operated in all four quadrants. This is advantageous in that if e.g. a wind gust is pushing the blade, the DC motor 5 may deliver energy back to the frequency converter 4. Such momentary feedback energy (e.g. during braking of the DC motor 5) is not fed back to the grid but instead absorbed in an intermediate circuit of the converter (e.g. a capacitor) or, if a maximum voltage is exceeded, dissipated using a brake chopper (not illustrated).

[0023] As mentioned, the frequency converter can be supplied from the battery bank 5 in case, the AC (which is the preferred voltage source) fails. This second operation mode is referred to as battery operation and is connected to the DC link of the frequency converter 4. Thereby, control of the DC motor 5 can be continued by the frequency converter. Momentary feedback energy (e.g. during braking of the DC motor 5) is not fed back to the battery but instead absorbed in an intermediate circuit

DK 2017 71037 A1 of the frequency converter 4 or, if a maximum voltage is exceeded, dissipated using a brake chopper (not illustrated).

[0024] Finally, the motor can be controlled directly from the battery bank 3 bypassing the frequency converter 4, this mode of control would be referred to as emergency shut down of the wind turbine.

[0025] The DC motor 5 is preferably a compound motors having a field connected in series with the armature and a separately excited shunt field. The series field provides better starting torque and the shunt field provides better speed regulation. Hence for pitch control, the DC compound motor is advantageous in that during startup, the series field 5A of the DC motor 5 is used and gradually as speed is increased of the pitch motor, the shunt field is taking over, thereby a gently and more flexible pitch control can be performed.

[0026] The frequency converter 4 preferably comprise a converter unit, a temperature regulating means such as ventilator and or heater, various contactors / switches, means for monitoring voltage (e.g. AC and DC supply voltage), etc.

[0027] The frequency converter 4 may comprise but is preferably connected to a controller which in this embodiment would be a pitch controller 8. A connection between the frequency converter 4 and pitch controller 8 could be a CAN communication and/or hardwired ±10V signal.

[0028] In the preferred embodiment, the converter is supplied with 400VAC from the grid and outputs a PWM voltage to the DC motor's 5 armature and shunt field. The shunt field supply is as mentioned filtered. The converter also control charge / discharge of voltage of the batteries and measure the speed for the motor preferably by an encoder (not illustrated).

[0029] According to an embodiment, the frequency converter 4 produces a PWM voltage, also known as cycled voltage, with an amplitude corresponding to the DClink voltage (V3*400VAC ~ 565VDC). A DC voltage having such high amplitude is degrading the insulation of the motor windings and experiments has found that the

DK 2017 71037 A1 degradation happens faster than expected. As mentioned, electronics sensitive to such high voltage may be used in front of the DC motor 5 e.g. for filtering (e.g. active) peaks, measuring or the like.

[0030] Accordingly, protection is required and one simple way is to passively rectify the AC voltage by a bridge circuit leading to a DC voltage of (230V*^2) 325V. This is a simple and cheap way of obtaining protection, however it provides no flexibility in the protection i.e. e.g. sensitive electronics must be able to comply with 325V.

[0031] Figure 1 illustrates a controller 8, which in this example is referred to as a pitch controller. The pitch controller may monitor motor speed and current. It may control the PWM voltage out of the frequency converter 4 or at least give a pitch reference to the frequency converter 4. The controller 8 may also control the operation mode of the pitch system and thereby the voltage supplies to the frequency converter

4.

[0032] With the above said in relation to the controller 8, then it should be mentioned, that the frequency converter 4 also may comprise a controller (not illustrated). Such internal controller is preferably controlling the DC voltage out of the frequency converter 4 i.e. the PWM (active rectifying) of the input voltage (also if both the voltage to the series and shunt field has to be actively rectified), switches to connect / disconnect the voltage supplies, charging of the batteries, monitoring for performing the above.

[0033] Returning to the example above, leading to a controlled PWM voltage changing between 0-565V (see signal B in figure 1). By introducing a filter 6, a smoothened voltage as illustrated in signal C in figure 1 is obtained. In the example in figure 1, the desired voltage i.e. the remaining of the filtered 565V is 330VDC (with a negligible ripple) which is then what is supplied to the sensitive electronics 7 / DC motor 5. The maximum modulated voltage (PWM voltage) is controlled by the frequency inverter and should in the case described above not be higher than the desired voltage (smoothened voltage), In the case described above, the desired voltage

DK 2017 71037 A1 should not be higher than 58,4% of the modulated voltage (330VDC/565VDC*100%=58,4%).

[0034] As described, the filter 6 comprises a filter part 6A and a damping part 6B. In figure 2, an example hereof is provided. The example provided results in a damping of the DC voltage to 330VDC on the output of the filter 6. The limit of 330V is determined by the design of the low-pass filter 6A whereas the damping part 6B is determined by the design of the RC-part 6B. It should be noted, that the circuit illustrated on figure 2 only serves as an example of a low pass filter 6 and that by adjusting the size of the components, the properties of the filter is also adjusted. Further, the damping part 6B do not necessarily has to be include in the filter 6 for it to achieve the purpose of the filter 6 and thereby the present invention.

[0035] With reference to the embodiment of figure 2 an example of the switching frequency could be 4KHz. The cutoff frequency would then e.g. be around 400Hz leading to the desired damping of the PWM voltage peaks resulting in a DC voltage (with a ripple) around 325VDC.

[0036] In relation to figure 2, it should be mentioned, that as a non-limiting example, the current to the shunt field from the frequency converter 4 may be 1A whereas the current to the series field may be 33A at least one time during operation of the pitch drive. Due to the size of the inductance of the DC motor 5, the filter 6 will have no or negligible effect on the current.

[0037] Figure 3 illustrates an advantage of the invention, namely the implementation of the filter 6 as a standalone device. Today, typically the filter is part of the design of the frequency converter, however this leads to a less flexible design of (as an example) the pitch drive in that it is very complicated to change the properties of the filter if e.g. additional sensitive electronic is added to the pitch drive. Further, by designing the filter as a standalone device, it becomes possible to install the filter 6 after commission of the wind turbine i.e. as a retrofit update of the wind turbine.

[0038] The standalone device, is preferably an electric panel with the parts comprised by the filter 6 and interfaces enabling mounting of the electric panel in the electric path

DK 2017 71037 A1 between frequency converter 4 and DC motor 5. Naturally, the electric path of the pitch drive has to be adapted to be connected to the standalone device, however this is considered as trivial to the skilled person and is therefore not described in further details.

[0039] What is not trivial is the calculations which have to made on the standalone device in relation to short circuit currents, power / heating distribution, distances between components to avoid flash overs, etc. this is of course something that must be considered during the design of the standalone device. Therefore, inserting a filter standalone device is not a trivial task for a person skilled in the art.

[0040] It should be mentioned, that according to the invention the filter (standalone or built in) may be by-passed in case that the frequency converter 4 / DC motor 5 is supplied directly from the battery bank 3.

[0041] It should be mentioned, that the filter 6 can include or be used together with a common mode filter. However, with or without such common mode filter, the filter 6 is advantages in that it is damping common mode currents from the frequency converter 4 and thereby protecting the DC motor from high dv/dt edges.

[0042] It should be mentioned, that even though throughout this description only uses DC motors as an example, the filter 6 may also be used in relation to AC motors with the same advantages as described above.

[0043] From the above it is hereby clear that the present invention relates to an electric system such as a DC pitch drive for a wind turbine. The pitch drive between the frequency converter 4 and the DC motor 5 is equipped with a filter 6. The filter 6 being part of the frequency converter 4 or a standalone device for installation in the electric path between the frequency converter 4 and the DC motor 5. The DC motor 5 may have one of a series field 5A and shunt field 5B and a filter in front of one or both fields. Thereby protecting the motor windings and sensitive electronics (if any) from the DC voltage peaks of the PWM voltage established by the frequency converter 4.

DK 2017 71037 A1

List

1. Electric system (e.g. a pitch drive)

2. AC voltage supply (e.g. grid)

3. DC voltage supply (e.g. a plurality of batteries)

4. Frequency converter (an active rectifier)

5. DC motor (e.g. a compound motor)

a. Series field (of the DC motor)

b. Shunt field (of the DC motor)

6. Filter

a. Filter part (e.g. Low-pass filter)

b. Damping part (e.g. an RC part)

7. Sensitive electronic

8. External controller (e.g. a pitch controller)

Claims

Patent claims

1. An electric system of a wind turbine facilitating control of a DC motor of the wind turbine, the electric system comprise a frequency converter connected to an AC voltage supply and the DC motor connected to the output of the frequency converter, wherein the frequency converter is an active rectifier, facilitating conversion of the AC voltage to a determined DC voltage by means of Pulse Width Modulation, wherein the DC motor is designed with at least one series field and / or at least one shunt field, wherein a low pass filter is placed in the electric path between the frequency converter and the series field and / or shunt field of the DC motor, and wherein the low pass filter is designed to attenuate DC voltage peaks.
2. An electric system according to claim 1, wherein the electric system is an electric pitch drive and the DC motor is a pitch motor facilitating turning a wind turbine blade around its longitudinal axis.
3. An electric system according to claim 1 or 2, wherein the DC motor is a DC compound motor having both a series field and a shunt field.
4. An electric system according to any of the preceding claims, wherein the low pass filter is placed in the electric path between the frequency converter and the shunt field of the DC motor.
5. An electric system according to any of the claims 1-3, wherein the low pass filter is placed in the electric path between the frequency converter and the series field of the DC motor.
6. An electric system according to any of the preceding claims, wherein a first low pass filter is placed in the electric path between the frequency converter and the shunt field of the DC motor and a second low pass filter is placed in the electric path between the frequency converter and the series field of the DC motor.

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7. An electric system according to any of the preceding claims, wherein the supply voltage to the frequency converter in case of absence of the AC supply voltage is received from a battery bank.
8. An electric system according to any of the preceding claims, wherein the DC voltage

5 peaks are damped by the low-pass filter below 250V, preferably below 275V, most preferably below 330V.
9. An electric system according to any of the preceding claims, wherein the DC voltage at the output of the frequency converter is between 0 and 565V.
10. An electric system according to any of the preceding claims, wherein the low-pass

10 filter is implemented as a stand-alone unit connectable to the electric system in the electric path between the frequency converter and the DC motor.