Bearing Currents in Doubly-Fed Induction Generators

Dipl.-Ing. Johann Zitzelsberger, Prof. Dr.-Ing. Wilfried Hofmann, Dipl.-Ing. Andreas WieseChemnitz University of Technology

09107 Chemnitz, GermanyPhone +49 (0) 371 5 31 33 18 / Fax +49 (0) 371 5 31 33 24

johann.zitzelsberger@e-technik.tu-chemnitz.dewww.tu-chemnitz.de/etit/ema/index.php

Dr.-Ing. Peter StupinVEM Sachsenwerk GmbH

Pirnaer Landstraße 17601257 Dresden, Germany

Phone +49 (0) 351 2 08 23 82 / Fax + 49 (0) 351 2 08 35 05stupin@vem-group.comwww.vem-group.com

Keywords

Wind Energy, Wind Generator Systems, Doubly Fed Induction Generator, Modelling, Reliability

AbstractAn increasing demand on the quality of wind power systems affects also more and more the fields oflifetime and reliability. But, problems like bearing damages because of bearing currents counteractthese desired demands. Especially doubly fed induction generators, which are wide spread in use forwind turbines up to the MW-class can be limited in reliability because of the mentioned problem. He-reby, the rotor-side feeding by IGBT voltage source inverters causes an uprise of bearing currents.Unfortunately, this parasitic effect, well known from stator-side inverter-fed machines, is differentand maybe even worse since the capacitive voltage divider, responsible for the transfer of the commonmode voltage to the bearings, differs to a great extend. The analysis of bearing current circuits givenin this paper will help to explain these differences. Moreover, some fundamental modelcharacteristicsof doubly-fed induction generators will be mentioned in order to give hints for measures against be-aring currents. Measured effects gained from a 1.5 MW generator support these investigations andevaluate the efficiency or the problems of measures for decreasing bearing currents.

Introduction to the common mode circuit characteristicsIn the so-called MW-class the doubly fed induction generator (DFIG) and the gearless synchronousgenerator are the most widely used machines in wind generator systems. Concerning this, some appro-ximated calculations based on the potential yearly energy production of the year 2000 [1] estimatedthat meanwhile over 40% of the energy are already generated by doubly-fed machines and their rate isstill increasing. This is because gearless synchronous generators have to be of high numbers of poleswhich leads to a big frame size and therefore to problems in handling. Moreover, by using synchro-nous generators the inverter has to be designed for carrying the full rated power.

In contrast, as it can be seen in Fig. 1 the stator of the doubly-fed generator is directly connected tothe mains, while the rotor is fed by a voltage-source inverter. Hereby, a voltage based DC link con-nects a rotor-side converter to a net-side converter. Both converters are fully controlled in order to al-low both over- and undersynchronous operation. The inverters themselves are controlled in current by

using a space vector modulation in a synchronous frame, oriented to the net voltage space vector. Theadvantage of this doubly-fed operation is well known and lays in the fact that the inverter in the rotorcircuit has to be designed only for about one third of the rated power. The final connection to the gridis accomplished through a LC-filter that is mainly used to damp the higher order harmonics generatedby the switching of the semiconductors. But, this filter has also the function of furnishing reactivepower to enable power factor correction on the net within a desired range.

Fig. 1: Electric circuit diagram of a doubly-fed-induction-generator in wind generator systems

Now it is known, that induction machines which are fed by a voltage source inverter tends to loosetheir normal MTBF because of bearing damages occurring much earlier than in the case of a sinusoi-dal voltage supply and same load. This phenomenon is well known from stator-fed machines and dee-ply investigated [2,3,4,5]. Therefore, concerning doubly-fed induction machines it can be summari-sed, that these kinds of bearing damages result from bearing currents which have their origins in ahigh-frequency common mode voltage V0 in the rotor circuit. This voltage is inherent to the switchingstrategy of the (rotor-side) inverter and depends on both the amplitude of the dc link voltage and theelectric system behaviour of the common mode circuit determined by R0, L0 and C0. Hereby, the com-mon mode voltage can be calculated due to

V 0 =V an V bn V cn

3 (1)

where Van, Vbn and Vcn are the phase-to-ground-voltages of the (rotor-side) inverter output. Unfortuna-tely, the common mode circuit is normally weak damped so that the real time behaviour of the com-mon mode voltage can contain tremendous overshoots, like it is shown in Fig. 2.

Regarding bearing damages because of bearing currents the severeness of the always existing com-mon mode voltage depends mainly on the so-called bearing voltage ratio (BVR) [6]. This ratio descri-bes the part of the common mode voltage which is finally transferred to the bearings in terms of a be-aring voltage Vbg. Since this transfer is done via a capacitive voltage divider, the BVR depends mainlyon the capacities of the common mode circuit which are situated between the rotor terminals and thebearings. These capacities can be located generally like it is shown in Fig. 3. But physically these ca-pacities differs in location and in value if the induction machine is stator-fed or rotor-fed. Therefore itis important to analyse the linking capacities in order to be able to estimate the worst BVR or to findalternatives in the built-up of the generator aiming a reduction of the bearing voltage ratio.

trans-former

LC filter

rotor-sideconverter

net-sideconverterdc link

DFIG

wind turbine + DFIG

Fig. 2: Time behaviour of the phase-to-ground & Fig. 3: General electric circuit diagram of the com- common mode voltage of a 1.5MW wind mon mode capacities incl. the simplified generator, dc link voltage equ. to 1000V model of the isolated motor bearings [2]

Due to Fig. 3 especially the value of the linking capacity C2 determines the altitude of the bearing vol-tage, finally. In case of a stator-fed induction machine this capacity is built by the stator-to-rotor-capa-city, like it is defined in [6]. Thus, determined by a relatively small area across the air gap, the valueof the capacity C2 is rather small. But, in case of a doubly-fed induction machine the linking capacityC2 is defined by the isolation between the rotor windings and the rotor core. Thus, this capacity is de-termined by a big surface and, moreover, by a permittivity greater than one; see Table I.

linking capacity C2

stator-fed rotor-fed9.21 nF 121 nF

Table I: Comparison of the linking capacity C2 when the induction machine is stator- or rotor-fed (measured at a 1.5MW doubly-fed wind generator)

As it can be seen in Table I, while operating a doubly-fed induction generator the value of the linkingcapacity C2 can be more than ten times higher than in the case of a stator-fed machine. This makes itobviously, that under this operation a higher bearing voltage occurs than it is known from standard in-dustrial drive applications. This means that the problem of bearing damages because of bearing cur-rents increase significantly if no measures are done against it. Hereby, the reason for this intensificati-on can be seen finally in the energy input into the bearing due to

Ebg = C bg ∫V bg t d V bg t dt

dt K (2)

Especially the high amplitude of the bearing voltage can cause high bearing currents in terms of so-called electric discharge machining currents (EDM) and high frequency circulating currents [5,7].These kinds of currents are assumed to damage the bearings directly by fuzing the roller balls. Butalso the contribution of fast dv/dt-currents increases because their energy leads to a higher temperatu-re of the grease. Thus, the grease can become fluid and looses its lubricating characteristics. Therefo-re, the question is which measures can be made against the intensification of bearing currents in dou-bly-fed induction generators and which characteristics have to be taken into account by using thesemeasures. Both points of view will be figured out in the following section.

C1 C3 Cbg

C2 Ciso

Rbg

Zbg

V0 Vshaft Vbg

V bgV 0

=C2C iso

C2C3Cbg C2C 3CbgC iso

Rbg 0 ; Z bg ∞

Measures against bearing currents in wind generator systemsIsolation of bearingsDue to Fig. 3, beside the linking capacity C2, also the capacity of the bearing isolation Ciso affects thevalue of the resulting bearing voltage ratio. Such an isolation is widely spread in wind generator sys-tems and suppresses the occurring of high frequency circulating currents through the bearings [7].But, what's about the influence on EDM and dv/dt-currents?

Well, in case of no bearing isolation, the EDM current can be calculated due to

I EDM = I Z bg = I C 2 I C 3 I C bg (3)

In contrast, in case of installing a bearing isolation (but without grounded shaft), the circuit diagramof Fig. 3 can be transformed into the one shown in Fig. 4. That means, capacities which are located infront of the shaft don't contribute to the EDM currents anymore (if the shaft voltage is constant whilethe electrical breakdown of course). By this assumption, the EDM current becomes

I EDMI EDM0

= 21 C bg / C iso

(4)

with

I EDM0 = C bg ,0V shaft

t (5)

Fig. 4: Reduced common mode circuit in case of Fig. 5: Percentage amount of the EDM current by the electrical breakdown of the bearing installing a bearing isolation (Vshaft is assumed to be constant)

Fig. 5 shows an interesting point concerning the effectiveness of isolating bearings. So it can be seen,that, also an isolation is installed, the EDM current can become twice as high as the bearing can causeby itself. This is the case when the capacity of the bearing isolation is much higher than the capacityof the bearing. Only in cases where the capacity of the isolation is lower than the bearing capacity, asignificantly reduction of the EDM current can be reached. This is an important point in designing theisolation. But, it is obviously, that to know about the bearing capacity itself cause many problems, be-

Cbg

Ciso

ZbgVbg

Viso

Vshaft

ICiso

ICbg

IEDM

cause this capacity is very non-linear and surely not constant, at all. So designing an optimal bearingisolation would need a detailed investigation of the bearing conditions, too. Since this is rather diffi-cult, mostly a compromise between mechanical demands and acceptable values of EDM currents aremade finally, which results mainly from practical experience. So this might be an interesting field formore detailed investigations in future. However, installing an isolation of the bearing will decreasethe bearing currents to a great extend, compared to the one, given by equation (3); see Table II.

Xiso = 0

I EDM0 ≈ C 3V 0

t>>

Xiso > 0

I EDM0 = C bg ,0V shaft

t

Table II: General effectiveness of the bearing isolation

The same situation concerning the ratio of the isolation and bearing capacity can be observed in thefield of dv/dt-currents. But while the EDM currents always have an upper limit in value (comp. Fig.5), the maximum value of dv/dt-currents depends on whether a closed lubricant film in the bearing ispresent or not. In the first case, also the dv/dt-currents have an upper limit due to:

I dv /dtI dv /dt ,0

= 11 C bg / C iso

(6)

where Idv/dt,0 is equal to IEDM0. But, if there is no closed lubricant film in the bearing, that means the be-aring reacts like a resistor, the dv/dt-currents are mainly determined by the capacity of the isolation.So, the higher this capacity is the higher are the dv/dt-currents. This fact can be seen in Fig. 6. Fig. 7shows measured dv/dt-currents through the bearing while an isolation of the bearing was installed.

Fig. 6: Percentage amount of the dv/dt-currents Fig. 7: Measured dv/dt-currents through the bea- at different bearing behaviour (remark: in rings of a 1.5MW generator while an isola- case of Xbg = 0, Ciso is related to Cbg,0) tion of the bearing was installed

Because of these results it can be concluded:

• an isolation of the bearings reduces EDM currents to a great extend but• dv/dt-currents can increase very much if the ratio between the capacity of the isolation and the

bearing is not optimal, that means if Ciso >> Cbg

Shaft groundingLike in any other power system, it is also the aim of doubly-fed wind power systems to operate withmaximum efficiency. To reach this, optimized design principles for both the wind turbine and thewind generator (incl. the inverter) have been developed over the last few years, mainly based on clas-sical design rules for high voltage induction machines. Moreover, manufacturers of wind generatorsystems combined these physical design principles with economic points of view in order to get besi-de a high energy effectiveness also a high cost effectiveness. Unfortunately, the optimization of theBVR which is as mentioned mainly responsible for the occurrence of bearing voltages and bearingcurrents in inverter-fed wind generator systems is not a part of these classical design rules and the ma-nufacturers are still at the beginning to concern this new aim of optimization as a part of their compa-ny intern design principles. Thus, a certain value of the BVR resp. the existence of a shaft/bearingvoltage is unavoidable up to now. But, due to equation (2) this existing voltage is responsible for be-aring damages, finally. Therefore, it must be the goal to limit this voltage down to an uncritical value.Beside reducing the common mode voltage itself, e.g. by using special modulation techniques, shaftgrounding is one possibility to fulfil this demand [7], see Fig. 8 and Fig. 9. However, there are somepoints which have to be taken into account by using this instrument of bearing current reduction.

Fig. 8: Shaft voltage of a 1.5MW wind generator Fig. 9: Shaft voltage of a 1.5MW wind generator with ungrounded shaft with grounded shaft (peak value: 531V) (peak value: 26V)

As it can be seen in Fig. 8 and Fig. 9 shaft grounding has an unnegligible effect on the amplitude ofthe existing shaft/bearing voltage. However, there could be still a relatively huge shaft voltage even inthe case of a grounded shaft. Hereby, especially the inductive behaviour of the complex grounding im-pedance can be a main reason for this effect; see Fig. 10. Moreover, the time behaviour of the restedshaft voltage depends much on the contact characteristics of the grounding brushes and their reliabili-ty. That means, the time behaviour of the shaft voltage will be better concerning the generation of be-aring currents the more number of brushes will be used for grounding, because the statistical contactreliability is higher. This conclusion results in using grounding brushes both on the n-side and on thed-side of the shaft.In case of such an optimized, double-sided grounding technique the amplitude of the resting shaft vol-tage can be lowered to a great extend, see Fig. 11. But, on the other hand by using such a groundingtopology an oscillating current through the shaft will occur. Of course, this current doesn't flowthrough the bearings and recent investigations on this problem have shown that the fundamental fre-quency of this current, which is driven by flux asymmetries, is dominant in case of doubly-fed in-duction generators but also manageable by the grounding brushes concerning its amplitude. Moreover,

this primary undesired circulating current can improve the contact reliability of the grounding brushesbecause of the so-called current lubrication.

Fig. 10: Measured and inductive value of the Fig. 11: Shaft voltages at the n- and d-side of a voltage coupled in by using a „high“ 1.5MW wind generator, bearings isolated, inductive earth wire (L = 0.8µH) n-side grounded

Like it is obviously that shaft grounding hasvery positive effects on the value of the shaftvoltage, under optimal conditions of course, itcan be concluded that also an influence on thedifferent kinds of bearing currents exists. Oneexample is shown in Fig. 12 in which the dv/dt-currents are compared between an ungroundedand grounded shaft. By calculating the RMSvalue of the bearing currents, given in Table III,it can be seen, that the energy input into the be-aring in case of a grounded shaft is decreaseddown to 16% compared to the one when theshaft is ungrounded.

dv/dt-RMSshaft ungrounded shaft grounded

0.251A 0.032A

Table III: Comparison of the dv/dt-RMS value Fig. 12: Comparison of the dv/dt currents in case in case of a 1.5MW wind generator of a 1.5MW wind generator with groun- with grounded/ungrounded shaft ded/ungrounded shaft

The conclusion is, that grounding the shaft can help to reduce the problem of bearing currents in dou-bly-fed induction wind generators. But, for maximum effectiveness some points of view have to be ta-ken into account. One of them is the contact reliability of the brushes. Here the investigations haveshown that the more brushes are used the better the shaft voltage can be reduced. But, also importantis the realisation and application of the grounding equipment (one sided, both sided, inductive beha-viour). So, using shaft grounding requires rather the same exact investigations concerning the optimalway of conversion than it was mentioned referably the isolation of bearings.

Other measures against bearing currents in doubly-fed wind generatorsIn the following section two other possibilities for reducing bearing currents in doubly-fed wind gene-rator systems will be introduced. Hereby the passive measure „filtering“ and the active measure „vol-tage modulation“ will be generally investigated.

FilteringSince it is known that the rectangle shape of the output voltages of the inverter leads to a high fre-quency common mode voltage (comp. equ. (1)), it can be assumed that reducing the dv/dt of the out-put voltages will also reduce the occurrence of bearing currents. This reduction could be reached byusing special dv/dt filters or common mode current filters. Concerning the investigations for this pa-per common mode current filters in terms of specially adapted nano-crystalline toroidal cores havebeen used. But, since these filters reduce both the amplitude and the dv/dt of the responsible commonmode voltage relatively poor, the effects on reducing EDM and dv/dt-currents in doubly-fed wind ge-nerators are marginal; see Fig. 13 and Fig. 14. So, this validates the statements in [8] and enhancethem to the field of wind generator systems.

Fig. 13: Comparison of the common mode volta- Fig. 14: Comparison of the dv/dt of the common ges with and without nano-crystalline mode voltages with and without nano- toroidal cores crystalline toroidal cores

A better solution would be to install a sinusoidal filter which is back-drafted to the dc-link. Becauseof its active controllability a pure sinusoidal output voltage could be supplied to the rotor terminalswhich is equivalent to a common mode free voltage system. The problem here is, that this filter coulddecrease the over all efficiency of the wind generator system significantly. Moreover, if the inductan-ce of the rotor is not enough to be used within the filter, an additional series inductance has to be in-stalled which increase either the size of the inverter or the weight of the turbine gondola.

Optimized modulation strategiesAnother measure for decreasing bearing currents in doubly-fed wind generator systems would be theusage of optimized modulation strategies. Hereby, two principles would be possible. First, only therotor-side inverter is controlled via an optimized modulation method [10] or second, both the rotor-side and the net-side inverter is controlled in this way [9]. Which method should be preferred dependson the operation range of the generator and is not investigated in detail up to now. But, by using thisoptimized modulation strategies, the common mode voltage itself would be significantly reducible.

ConclusionLike in standard industrial drive applications, also in doubly-fed wind generator systems an additionalstress of the bearing can be observed which has its origin in a common mode voltage, generated by thevoltage source inverter. Because of the capacitive voltage divider within the machine, this commonmode voltage is transferred to the bearings and leads to different kinds of bearing currents, finally.

In order to reduce these bearing currents the responsible bearing voltage has to be avoided or, at least,reduced. To do this, several possibilities are available, which reach from a simple isolation of the be-aring over shaft grounding techniques to controlled sinusoidal filters at the rotor terminals. Beside ofthese primary measures, which are (apart from the usage of filters) mainly effective for reducing thebearing voltage, secondary measures like optimized modulation strategies can help to decrease theover all responsible common mode voltage itself.

However, it could be shown that every solution for reducing bearing currents in doubly-fed wind ge-nerator systems has its special characteristics which have to be taken into account to get the most ef-fective bearing current reduction. Here, especially the ratio between the capacity of the isolation andthe bearing, the contact reliability of grounding brushes and the characteristics of the groundingequipment and its impedance are playing major rolls. Unfortunately, some of these points are ratheruncertain in determination so that their optimal usage needs very exact investigations combined withappropriate practical experiences. Concerning this some fundamental dependencies have been mentio-ned in this paper, supported by practical measurements.

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