Patent Description:
The stator insulation system in an electric generator is a sensitive area, which degrades due to thermal, electrical, mechanical as well as environmental stress over time and eventually can have short circuit failure between windings and stator lamination or ground. Due to errors in workmanship or bad material quality also premature failures may occur. Most of the time such failures need extensive onsite repair or the exchange of a stator element. In large electrical machines, a circumferential segmentation of the stator may be required to ease manufacturing and transportation. Each stator segment may be configured to cover for example an arc of <NUM>, <NUM>, <NUM>, <NUM> degrees (or any other angle) along the circumferential direction of the stator. The stator segments are circumferentially joined to form the stator (for example a stator may comprise six stator segments, each covering an arc of <NUM> degrees). Each stator segment comprises respective windings. Failures in the insultation system of the segment windings may require the exchange of the entire respective stator segment.

If such failures could be detected in an early stage of the failure inception, turbine outage time period could be minimized. In most cases secondary damages, which are experienced due to the incapacity to detect primary damages (main failure mode), could be avoided.

<CIT> discloses an insulation detection and compensation circuit for a rotating machine.

It is desirable that the detection of overall insulation degradation as well as of the location of the insulation damage is performed before short circuit faults occur and develop. At the same time, it is desirable that this is performed with as low as possible hardware requirements, in order to limit costs of the installation. It is further desirable that a continuous monitoring could be established, which provides the above described valuable information.

It may be an object of the present invention to provide an apparatus and a method, in particular to be used for an electric generator of a wind turbine, which permits to monitor the health status of the insulation of the stator windings of the electric generator. It may be a further object of the present invention to detect a failure in an electric generator that is due to defects (which may be caused by degradation or material defects or workmanship errors) of the insulation of the stator windings at an early stage, in order to be able to plan repair or replacement of defective elements in advance.

This objective may be solved by an apparatus and a method for determining a status of the insulation of an electrical circuit in an electric generator according to the subject matter of the independent claim.

The invention relates to an electric generator for a wind turbine including a stator having one or more segments circumferentially extending about a longitudinal axis of the stator, an electrical circuit associated with said segments, the electrical circuit including at least a coil winding wound on one of said segments the coil winding including an insulation. The electrical circuit comprises at least one current sensor for measuring a current flowing in the electrical circuit. The electric generator includes a controller for receiving a current measurement from said current sensor, calculating parameters depending on an impedance of at least a portion of said electrical circuit, determining a status of the insulation of a portion of the electrical circuit.

The invention further relates to a method for determining a status of the insulation of an electrical circuit in the above described electric generator. The method includes the steps of:.

According to embodiments of the electric generator of the present invention, the electrical circuit is a <NUM>-phase circuit including at least a star connection of three coils wound on one of said segments, a sensor for measuring a current flowing in the coils being provided at the common neutral point of the three coils of said star connection, which can be considered as summation of <NUM>-phase currents. According to other embodiments of the electric generator of the present invention, delta connections of three coils wound on one of said segments may be provided.

According to other embodiments of the electric generator of the present invention, a sensor for measuring a current flowing in the cable system or busbar system is provided in at least one cable or busbar of a cable system or busbar system of the electrical circuit.

The electric generator of the present invention may include a stator having a plurality of segments, each segment being associated with a respective current sensor of the electrical circuit for measuring a current flowing in the coil windings. Alternatively, the electrical circuit may include a plurality of pairs of coil windings connected in series, each pair comprising a respective current sensor of the electrical circuit for measuring a current flowing in the coil windings.

The current signals derived with the sensors positioned as above described are used in the controller of the electric generator and according to the method of the present invention for determining a status of the insulation of a portion of the electrical circuit, in particular a portion including the coil windings or the cable or busbar system.

The present invention provides a solution for online condition monitoring and protection of electric generators employed in wind turbines. This solution requires current measurements only, providing at the same time an improved sensitivity and a simpler, more effective and robust fault detection. The sensitivity may be particularly improved in embodiments using both amplitude and phase of the current measurements.

According to one embodiment of the method, the present invention includes monitoring partial discharge occurrence in the electrical circuit and comparing it with healthy partial discharge and patterns.

According to the method of the invention, the present invention includes measuring a plurality input current signals and deriving one or more diagnostic signals based on at least a difference of the input current signals. In such embodiments, the diagnostic signals may be derived from input current signals by extracting a plurality of harmonics from the input current signals or from at least a difference of the input current signals.

The method further includes monitoring a variation of a phase of the input current signals with respect to a reference angle signal. This allows determining a degradation of the complete insulation system if both current and phase remain equal between segments, but the phase varies over time with respect to a reference angle signal.

<FIG> shows a wind turbine <NUM> according to the invention. The wind turbine <NUM> comprises a tower <NUM>, which is mounted on a non-depicted foundation. A nacelle <NUM> is arranged on top of the tower <NUM>. The wind turbine <NUM> further comprises at least a wind rotor <NUM> having a hub and at least one blade <NUM> (in the embodiment of <FIG>, the wind rotor comprises three blades <NUM>, of which only two blades <NUM> are visible). The wind rotor <NUM> is rotatable around a rotational longitudinal axis Y. The blades <NUM> extend substantially radially with respect to the longitudinal axis Y. In general, when not differently specified, the terms axial, radial and circumferential in the following are made with reference to the rotational longitudinal axis Y. The wind turbine <NUM> comprises at least one electric generator <NUM>, including a stator <NUM> and a rotor <NUM>. According to embodiments of the present inventio, the electric generator <NUM> may be a permanent magnet electric generator and have an external or internal rotor <NUM>. The rotor <NUM> may include a plurality of permanent magnets, while the stator <NUM> may include a teeth-slots structure, where a plurality of coils is arranged. The stator may have a segmented structure including a plurality of circumferential segments circumferentially extending about a longitudinal axis Y about an arc measuring <NUM>°/N, where N is the number of the segments. The rotor <NUM> is rotatable with respect to the stator <NUM> about the rotational longitudinal axis Y. The wind rotor <NUM> is coupled with the rotor <NUM> and both are connected to an outer ring of a main bearing assembly <NUM>. The outer ring of the main bearing assembly <NUM> rotates about the rotational longitudinal axis Y with respect to a static inner ring, which is coupled with a main shaft <NUM> extending along the longitudinal axis Y. On a rear side of the nacelle <NUM>, opposite to the wind rotor <NUM>, a converter <NUM> is provided. The converter <NUM> electrically connected to the electrical generator <NUM> to transform the electrical output of the generator <NUM> to a certain predetermined power output at a predetermined voltage level to be provided to an electrical network, which is electrically connected to the transformer <NUM>.

<FIG> shows an electrical circuit <NUM> associated with a stator <NUM> having a plurality of N segments S1,. , SN circumferentially extending about a longitudinal axis Y of the stator <NUM> (in <FIG> only the two segments S1, S2 are represented, even if the stator <NUM> may in general have more than two segments). The electrical circuit <NUM> includes a plurality of N coil windings <NUM> respectively wound on the segments S1,. The coil windings <NUM> may be of the concentrated or distributed type and includes a plurality of cable conductors including an insulation. The electrical circuit <NUM> is threephase circuit, each coil winding <NUM> comprising three respective coils 31a, 31b, 31c, each coil 31a, 31b, 31c being connected to a respective phase U, V, W. Each coil 31a, 31b, 31c may be represented through one or more impedances Za, Zb, Zc (three respective impedances are shown in <FIG> for each coil 31a, 31b, 31c). In each coil winding <NUM> the coils 31a, 31b, 31c are connected according to a star connection at a common neutral point N. The electrical circuit <NUM> further comprises a plurality of cable systems <NUM>, each cable system comprising three cables 32a, 32b, 32c, respectively associated with the phases U, V, W. Alternatively, according to other possible embodiments of the present invention, instead of cable systems, the electrical circuit <NUM> comprises a plurality of busbar systems <NUM>, each busbar system comprising three busbar 32a, 32b, 32c, respectively associated with the phases U, V, W. The electrical circuit <NUM> comprises at least one sensor <NUM> for measuring a current flowing in the electrical circuit <NUM>. In the embodiment of <FIG>, a current sensor <NUM> is provided on each of the three cable 32a, 32b, 32c (six sensors <NUM> are therefore shown in <FIG>, in general being 3N the number of sensors comprised in the electrical circuit <NUM>). Alternatively, according to other possible embodiments of the present invention (not shown), a sensor <NUM> may be provided in only one of the three cable 32a, 32b, 32c of each cable system <NUM> (therefore, N current sensor <NUM> in total). The current sensors <NUM> may be current transformer. According to possible embodiments of the present invention, the current transformers may be operating at a frequency comprised between <NUM> and <NUM>. Alternatively, according to other possible embodiments of the present invention, current sensors <NUM> may be common mode current sensors operating at a frequency comprised between <NUM> to <NUM>. The "common mode current" is the in-phase current flowing in the same direction on the multiple conductors which are comprised in each segment S1,. , SN, for example included in the coils 31a, 31b, 31c. The electric generator <NUM> include a controller (not shown) for receiving a current measurement from the current sensors <NUM>, parameters depending on an impedance of the electrical circuit <NUM> and determining a status of the insulation of the coil windings <NUM> based on such impedance calculations. The procedure for determining status of the insulation of the coil windings <NUM> is better detailed in the following.

<FIG> shows another embodiment of the electrical circuit <NUM>. Differently from the embodiment of <FIG>, the electrical circuit <NUM> comprises a current sensor <NUM> for measuring a current flowing in the coils 31a, 31b, 31c provided at each of the neutral point N (therefore, N current sensor <NUM> in total).

<FIG> shows an electric generator <NUM> including a stator having a plurality of N segments S1,. To each segment S1,. , SN it is associated a respective coil winding <NUM> having one respective current sensor <NUM>. The coil windings <NUM> are connected in parallel to each other, each being connected to the converter <NUM> through a branch <NUM> of the electrical circuit <NUM>. The branch <NUM> includes a circuit breaker <NUM> and a further current sensor <NUM> between the circuit breaker <NUM> and the converter <NUM>. Each sensor <NUM> may be used to measure the common mode current in each segment S1,.

<FIG> shows an electric generator <NUM> including a stator having a plurality of N segments S1,. , SN, N being an even integer (N=<NUM> in the embodiment represented in <FIG>). To each segment S1,. , SN it is associated a respective coil winding <NUM>. The electrical circuit <NUM> comprises N/<NUM> pairs (three pairs in the embodiment represented in <FIG>) of coil windings <NUM>, in each pair being the respective two coil windings <NUM> connected in series. The pairs of coil windings <NUM> are connected in parallel to each other, each pair being connected to the converter <NUM> through a circuit breaker cabinet <NUM>. The circuit breaker cabinet <NUM> comprises N/<NUM> current sensor <NUM> (each current sensor <NUM> being connected to a respective pair of coil windings <NUM>) and a circuit breaker <NUM>. The one-line connections in <FIG> represent threephase cables. Each sensor <NUM> may be used to measure the common mode current in each pair of segments S1,S2; S3,S4 and S5,S6.

<FIG> shows an electric scheme of the equivalent circuit <NUM> representing the electrical insulation of the coils 31a, 31b, 31c and the cable system <NUM>. The electrical insulation is represented as an equivalent capacitance Ceq and an equivalent resistance Req, disposed in parallel between the conductor of the coils 31a, 31b, 31c or the cable system <NUM> and the ground. The total leakage current Itot flows in the equivalent circuit <NUM> and comprises a first portion Ic flowing in the equivalent capacitance Ceq and a second portion Ir in the equivalent resistance Req. The total leakage current It is given by: <MAT>.

Where V is the common mode voltage in the coils 31a, 31b, 31c or the cable system <NUM>. The above expression makes it evident that a reduction of the insulation resistance leads to an increase of the leakage current amplitude, whereas a reduction of insulation capacitance leads to a decrease. Thus, when assuming V as constant, the variation of amplitude and/or phase of I over time is a result of the equivalent impedance variation. The diagnostic signals derived from the above expression of the leakage current It are well suited for fault detection, operating point independent and robust against false alarms. Alternatively, fault detection may include monitoring partial discharge occurrence in the coils 31a, 31b, 31c or the cable system <NUM> and comparing partial discharge occurrence with healthy partial discharge and patterns. Characterization for healthy partial discharge level and partial discharge pattern of the electric generator <NUM> in the field may be based on a previous monitoring or measurement experience.

<FIG> shows a functional block diagram of a controller <NUM> of the electric generator. The controller <NUM> comprises an extraction block <NUM> for receiving a plurality raw input current signals <NUM>, <NUM>, <NUM> (three input signals are represented in <FIG>, a different number of input signals being possible according to other embodiments of the invention) and deriving according to signal processing techniques one or more diagnostic signals <NUM>. The feature extraction is carried out at relatively high sampling frequencies (tens to hundreds of kHz). The current signals <NUM>, <NUM>, <NUM> may be for example measurement signals produced by the three current sensors <NUM> shown in <FIG>. The diagnostic signals <NUM> are provided to a fault detection block <NUM> which generates an alarm output signal <NUM> to enable a fast stop of the converter <NUM> in the presence of fast developing faults that exceed predefine critical threshold values derived from the raw input current signals <NUM>, <NUM>, <NUM> or a difference thereof, as better detailed in the following. The reaching of such threshold values corresponds to an unacceptable deterioration status of the insulation of the coil winding <NUM> or of the cable system <NUM>. The fault detection may be at the same sampling rate of the feature extraction in the extraction block <NUM> (tens to hundreds of kHz) or lower (still in the range of kHz). The diagnostic signals <NUM> are also provided in parallel to a detection and prognostic block <NUM>, normally realized as a SCADA (Supervisory Control and Data Acquisition) system which generates a diagnostic output <NUM> representing a health status and remaining useful life of the electrical insulation of the coils 31a, 31b, 31c. The fault detection block <NUM> and the detection and prognostic block <NUM> operates in parallel. The novelty detection and prognostic are done at much lower rates (sampling periods from seconds to minutes) with the aim to support predictive maintenance. The detection and prognostic block <NUM> may employ a fitted model and an outlier analysis as an example of novelty detection or alternatively more advanced machine learning techniques, in addition to a model-based estimation of the remaining useful life. In block <NUM> the diagnostic signals <NUM> may be combined with various other turbine/generator signals (e.g. speed, power, current, voltage, temperatures, etc.) for building a comprehensive model of the condition of the electric generator <NUM>. Blocks <NUM>, <NUM>, <NUM> may be implemented as hardware or software components.

<FIG> shows more in detail the extraction block <NUM>. Having measured the raw input common current signals <NUM>, <NUM>, <NUM>, the differences between currents are calculated in the difference calculation block <NUM>. Taking as an example three current signals coming from the three current sensor <NUM> of <FIG>, two differences are calculated in the difference calculation block <NUM> as follows: <MAT> <MAT>.

As opposed as to I<NUM>, I<NUM> I<NUM> amplitudes, the amplitude of the differences ΔI<NUM>, ΔI<NUM> are more operating point independent. Moreover, the differences are less sensitive to transients. Then, the harmonics of interest are selected by means of digital band-pass filters <NUM> (other techniques like DFT may be employed too). The band-pass filters <NUM> receives as input the differences ΔI<NUM>, ΔI<NUM> calculated in the difference calculation block <NUM> and the raw input common current signals <NUM>, <NUM>, <NUM>. Examples are harmonics of the switching frequency fc of the converter <NUM> or their side bands (±fundamental frequency fg of the electric generator <NUM>), which are provided to the band-pass filters <NUM> as further inputs <NUM>, <NUM>, respectively. The harmonics calculated by the band-pass filters <NUM> may be k*fc, for odd multiples of fc (k=<NUM>,<NUM>,. ) and k*fc + h*fg for even multiples of fc(k=<NUM>,<NUM>,. ), where h*fg are integer harmonics of the generator fundamental electrical frequency (h=±<NUM>, ±<NUM>, ±<NUM>,. Odd multiples of fc may be preferred due to their independence on the generator fundamental electrical frequency. Finally, it is desirable to provide the fault detection block <NUM> and the detection and prognostic block <NUM> with diagnostic signals <NUM> that are mainly DC signals. This is achieved by calculating amplitude and phase of the harmonic signals in the first calculation block <NUM> and in the second calculation block <NUM>, respectively receiving as input the harmonics of the current differences <NUM> and the harmonics of the currents signals <NUM> calculated by the band-pass filters <NUM>. The first calculation block <NUM> generates diagnostic signals <NUM> corresponding to amplitudes of the harmonics of the current differences. The second calculation block <NUM> generates diagnostic signals <NUM> corresponding to amplitudes and phases of the current harmonics. The amplitude values of the measured currents and their differences are the primary diagnostic signals, whereas the phase is optional. To calculate the phase from the harmonic the amplitude and phase calculation block <NUM> receives as further a phase signal <NUM>, representing a phase of the electric generator <NUM>.

The variation over time of the diagnostic signals <NUM>, which is identified in the fault detection block <NUM> and the detection and prognostic block <NUM>, provides an indication of the degradation of the insulation of the coils 31a, 31b, 31c. Fault detection may be done by evaluating the error between the actual values of the diagnostic signals and their initial values considered as a reference for healthy conditions. The fact that signals from multiple segments S1, S2,. , SN are available eases location of an identified fault. Voltage measurements are not required, when all segments are excited by the same common mode voltage. Faults are localized by analyzing variation of the current differences. Considering the embodiment of <FIG>, ΔI<NUM> permits to analyze information from a first current signal <NUM> and a second current signal <NUM>, while the ΔI<NUM> permits to analyze information from the first current signal <NUM> and a third current signal <NUM>. Ageing of the electrical insulation may result in little variation of the amplitude of the current differences (ΔI), thus being required to have at least one diagnostic signal <NUM> corresponding to the amplitude of at least one current signal <NUM>, <NUM>, <NUM>.

Concerning the phase information in the leakage current It, the ratio between IR and IC is typically used to derive the so-called dissipation factor: <MAT>.

Thus, even though the amplitude of the leakage current remains constant, a change in δ indicates a change in the insulation equivalent impedance. For example, at insulation degradation with both or either of the resistance Req and capacitance Ceq being decreased, the dissipation factor will be increased, and the excitation frequency may provide a freedom of design for improvement of discrimination sensitivity. The angle δ may also be given as: <MAT>.

The relative change of the angle δ may be additionally or alternatively used for indication of an insulation degradation. Firstly, it is assumed that the current amplitude (|Itot|)remains unchanged for two pairs of segments (for example the pair of segment S1, S2 and the pair of segments S3, S4 shown in <FIG>), whereas δ varies in a faulty segment pair, this resulting in different phase angles δ1 and δ2. The corresponding currents I<NUM> and I<NUM> may be represented as: <MAT> <MAT>.

And the amplitude of the error ΔI<NUM> between I<NUM> and I<NUM> increases with the increase of |δ2 - δ1|, as evident from: <MAT>.

Claim 1:
An electric generator (<NUM>) for a wind turbine (<NUM>) including:
a stator (<NUM>) having a plurality of segments (S1, ..., SN) circumferentially extending about a longitudinal axis (Y) of the stator (<NUM>),
an electrical circuit (<NUM>) associated with said segments (S1, ..., SN), the electrical circuit (<NUM>) including a plurality of coil windings (<NUM>) respectively wound on said segments (S1, ..., SN), each coil winding (<NUM>) including an electrical insulation,
wherein the electrical circuit (<NUM>) comprises a plurality of current sensors (<NUM>) for measuring currents flowing in the electrical circuit (<NUM>),
wherein the electric generator (<NUM>) includes a controller (<NUM>) for receiving current signals (<NUM>, <NUM>, <NUM>) from said current sensors (<NUM>), calculating parameters depending on an impedance of at least a portion of said electrical circuit (<NUM>), determining a status of the insulation of a portion of the electrical circuit (<NUM>),
wherein
each segment (S1, ..., SN) is associated with a respective current sensor (<NUM>) of the electrical circuit (<NUM>) for measuring a current flowing in the coil windings (<NUM>) and determining a status of the insulation of the electrical circuit (<NUM>); characterised in that
the controller (<NUM>) comprises an extraction block (<NUM>), the extraction block (<NUM>) comprising a difference calculation block (<NUM>) configured to calculate a difference (Δ) of the input current signals (<NUM>, <NUM>, <NUM>), a band-pass filter (<NUM>) configured to calculate harmonics (<NUM>) of the difference (Δ) of the input current signals (<NUM>, <NUM>, <NUM>), and a first calculation block (<NUM>) configured to generate a diagnostic signal (<NUM>) based on amplitudes of
the harmonics (<NUM>); and/or
the controller (<NUM>) comprises an extraction block (<NUM>), the extraction block (<NUM>) comprising a band-pass filter (<NUM>) configured to calculate harmonics (<NUM>) of the input current signals (<NUM>, <NUM>, <NUM>), and a second calculation block (<NUM>) configured to calculate a variation of a phase (δ) of the harmonics (<NUM>) with respect to a reference angle signal (<NUM>) and to generate a diagnostic signal (<NUM>) based on the variation.