T 2463/16 31-05-2021

I. The appeal is against the decision of the opposition division rejecting the opposition against the European patent EP 2 485 358 B1. In the contested decision the opposition division concluded that the document combinations cited by the opponent, i.e. document D1 with any of documents D10/D3/D4, "did not challenge the inventiveness" of the subject-matter of the independent claims 1 and 8 of the patent, Article 56 EPC.

II. The document references used are as follows:

D1: US 2010/264666 A1;

D3: "Voltage Disturbances Introduction to Unbalance" in "Power quality Application guide", Dr Johan Driesen & Dr Thierry van Craenenbroeck, Copper Development Association, May 2002;

D4: GB 1 198 460 A;

D5: "Design of D-STATCOM for Voltage Regulation in Microgrids", Tzung-Lin Lee et al, Energy Conversion Congress and Exposition, 12 September 2010, pages 3456-3463;

D6: "Power Quality And Transient Stability Improvement of Wind Farm with Fixed-Speed Induction Generators Using a STATCOM", Guizhen Tian et al, 2010 International Conference On Power System Technology, 24 October 2010, pages 1-6;

D7: US 2010/332040 A1;

D10: "Merkmale der Spannung in öffentlichen Elektrizitätsversorgungsnetzen; Deutsche Fassung EN 50160:2007", DIN Deutsches Institut für Normung e.V., Beuth Verlag GmbH, April 2008.

III. In the statement setting out the grounds of appeal the appellant (opponent) maintained that the subject-matter of claims 1 and 8 of the patent as granted lacked an inventive step in view of a combination of document D1 with D10, D3 or D4.

In respect of the dependent claims the appellant referred to section 4 of their notice of opposition dated 23 December 2014 and maintained the lack of inventive step arguments outlined therein. There it was argued inter alia in section 4.2 that the subject-matter of dependent claim 3 lacked an inventive step in view of document D1 or in view of document D1 in combination with D7.

IV. With the reply to the appeal the respondent (patent proprietor) filed sets of amended claims according to auxiliary requests 1, 2 and 3.

The respondent argued that the subject-matter of independent claims 1 and 8 of the patent as granted (main request) involved an inventive step, countering the appellant's objections based on a combination of document D1 with D10, D3 or D4.

The respondent submitted that auxiliary request 1 had already been presented in the proceedings before the opposition division and made reference to item 7.1 of their first-instance submission dated 19 May 2015 for their supporting arguments.

V. The Board summoned the parties to oral proceedings, setting out their preliminary observations in a communication pursuant to Article 15(1) RPBA 2020. The Board noted it to be uncontested that it was known from D10, D3 and D4 for the voltage unbalance factor (hereinafter: VUF) to be used "a posteriori" to evaluate voltage unbalance to decide whether a voltage signal fulfilled predefined engineering standards or norms. The critical question remained, whether it would be obvious to use this known evaluation criterion to in some way improve the effectiveness, reliability and flexibility of the unbalance mitigation control system of D1. The Board was not convinced by the appellant's submissions that it would be. In particular, it seemed that the thresholds defined in D10, D3 and D4 defined the boundaries of the ranges of VUF at which it became intolerable. These boundaries did not seem to be suitable for making judgements about when or how to mitigate an unbalance.

VI. With a letter dated 27 April 2021 the appellant submitted observations in response to the communication pursuant to Article 15(1) RPBA 2020 and the respondent's reply to the appeal.

VII. Oral proceedings were held on 31 May 2021. The respondent presented amended description pages 2 to 7.

The appellant (opponent) requested that the decision under appeal be set aside and the patent be revoked.

The respondent (patent proprietor) requested that the appeal be dismissed (main request), auxiliarily that the decision under appeal be set aside and the patent be maintained in amended form in the following version:

- Claims no. 1 to 8 of auxiliary request 1 filed with letter of 5 May 2017;

- Description pages 2 to 7 as filed during the oral proceedings of 31 May 2021;

- Figures no. 1 and 2 of the patent specification.

VIII. Independent claim 1 of the patent as granted reads as follows (feature references added by the Board):

1.0 "1. A system for mitigating an electric unbalance of a three-phase current at a Point of Common Coupling (120, 220) being located between a wind farm (100, 200) comprising a plurality of wind turbines (110) and a power grid, the system comprising

1.1a a measurement device (122, 222) for measuring the electric unbalance at the Point of Common Coupling (120, 220) and for providing a measurement signal (122a, 222a) being indicative for the measured electric unbalance,

1.1b wherein the measurement signal (122a, 222a) is a Voltage Unbalance Factor which is given by the ratio between the amplitude of a negative sequence and the amplitude of a positive sequence of the voltage signal of the three-phase current at the Point of Common Coupling (120, 220), and

1.2 a central wind farm controller (130, 230) for controlling the operation of the plurality of wind turbines (110), wherein the central wind farm controller (130, 230) is coupled to the measurement device (122, 222),

1.3 wherein in response to the measurement signal (122a, 222a) the central wind farm controller (130, 230) is configured for providing a control signal (130a, 230a) for mitigating the electric unbalance at the Point of Common Coupling (120, 220), and wherein

1.4 the central wind farm controller (130, 230) is configured for determining a difference value between the measured Voltage Unbalance Factor and a predefined reference Voltage Unbalance Factor, and wherein

1.5 the central wind farm controller (130,230) comprises a control unit, which is configured

(a) for processing the determined difference value and

(b) for outputting a negative sequence voltage compensation signal which is used for mitigating the electric unbalance at the Point of Common Coupling (120, 220)."

Independent claim 8 of the patent as granted sets out a method which corresponds to the system as set out in claim 1 of the patent as granted. As this has not been disputed, the wording of claim 8 of the patent as granted does not need to be cited.

Claims 2 to 7 and 9 of the patent as granted are dependent on claims 1 and 8 respectively.

Independent claim 1 of auxiliary request 1 differs from claim 1 of the patent as granted by the deletion of the words "and wherein" at the end of feature 1.4 and by the addition of the following feature at the end of the claim (reference added):

1.6 ", and the control signal being provided by the central wind farm controller (130) is a compensation signal (l30a) for power converters of at least some of the plurality of wind turbines (110)".

Independent claim 7 of auxiliary request 1 differs from claim 8 of the patent as granted by the addition of a feature which corresponds to that added to claim 1.

Claims 2 to 6 and 8 of auxiliary request 1 are dependent on claims 1 and 7 respectively.

IX. The opposition division's findings in the contested decision may be summarised as follows:

The opposition division held it to be uncontested that the subject-matter of independent claim 1 of the patent as granted differed from the closest prior art document D1 by the features 1.1b, 1.4 and 1.5.

According to the opposition division the technical effect of these features was the possibility to output a compensation signal which was directly representative of the negative sequence voltage of the system, since it was based on the comparison between the measured unbalance factor VUF and a predefined threshold VUF.

The opposition division held that document D1 disclosed a scalar compensation method based on a simple comparison of the phase voltage amplitudes with their nominal values, and considered that whilst it might be obvious, from D1 itself or in the light of D10, D3 or D4, to use the VUF, derived from the positive and negative sequence components, as a measure by which to assess the effectiveness of the scalar compensation in mitigating the unbalance, it would not be obvious to use these as "primary regulation quantities" or "leading parameters" in the regulation process.

X. The appellant's submissions may be summarised as follows:

Main request

The appellant set out in the grounds of appeal that the subject-matter of claims 1 and 8 of the patent as granted lacked an inventive step in view of a combination of document D1 with D10, D3 or D4.

The appellant conceded that independent claim 1 of the patent as granted differed from document D1 in that the measurement signal being a voltage unbalance factor in accordance with feature 1.1b was not shown, and in that the central wind farm controller known from document D1 was not configured for determining a difference value between the measured voltage unbalance factor and a predefined reference voltage unbalance factor in accordance with feature 1.4, and processing it by a control unit comprised of the central wind farm controller and outputting a negative sequence voltage compensation signal for mitigating electric unbalance at the PCC in accordance with feature 1.5 was not shown (grounds for appeal, paragraph spanning pages 13 and 14).

The appellant submitted that the opposition division's assessment that the output negative sequence voltage compensation signal was "directly representative of the negative sequence voltage of the system" was incorrect, and that as a result of this erroneous assessment the opposition division made a mistake in defining the problem solved. The opposition division's characterisation of positive and negative sequence components as "primary regulation quantities" in the patent was also incorrect. The appellant argued that it was agreed on page 7 of the decision that claim 1 was silent about the nature of the negative sequence voltage compensation signal. The appellant saw this as an acknowledgement in the decision that it was the use of the VUF as the measurement parameter that was critical for inventive step, not the choice of the negative sequence voltage compensation signal output by the control unit of the central wind farm controller.

The appellant submitted that D1 disclosed in paragraph [0056] to detect voltage imbalance by determining if the voltages at the Point of Common Coupling (PCC) form a negative voltage sequence or a [zero] voltage sequence, and to do so would require also a determination of the positive voltage sequence. The patent additionally provided for taking the ratio between the negative and positive voltage sequences (i.e. VUF) as the measurement parameter and that this ratio enabled a convenient threshold criterion to be formulated for determining when it was required to mitigate an unbalance. Based on this technical difference the appellant formulated the objective problem in the second paragraph on page 10 of the grounds for appeal as "selecting a suitable parameter for conveniently handling voltage unbalance mitigation in the central controller".

According to the appellant, the feature 1.4 could be interpreted as having the effect of indicating when to start unbalance mitigation control on the basis of a predetermined threshold (reference VUF) determining a tolerance level until which unbalances were considered as tolerable. There was a gap in the teaching of D1 as to when to perform unbalance compensation. It would be part of the common general knowledge of the person skilled in the art from section 4.1 of the technical standard D10 that VUF was the ratio of negative and positive voltage sequence components and that under normal conditions the negative voltage sequence component must be smaller than a threshold of 2% of the positive voltage sequence component. This filled the gap in D1 as to when unbalance mitigation was to take place in order to comply with the norm.

Document D3 referred on page 3 to the requirement in the international standards limiting the VUF to smaller than 2% and thus also provided a clear motivation as to when and how to perform mitigation in the set-up of D1.

Document D4, in the sentence spanning pages 2 and 3, also gave a motivation to confine an unbalance factor to smaller than 0.3.

In the communication dated 27 April 2021 the appellant emphasised that feature 1.5 of claim 1 left open whether there was any relationship between the two functions (a) and (b) for which the control unit was configured and argued that it could not be concluded that the negative sequence voltage compensation signal was based on the determined difference between measured VUF and reference VUF. The appellant noted that the patent did not specify any direct relationship between the VUF and the negative sequence voltage compensation signal and paragraph [0057] of the patent taught otherwise.

In the oral proceedings the appellant maintained that feature 1.5 did not specify any connection between the processing of the determined VUF difference value and the outputting of the negative sequence voltage compensation signal used for mitigating the electric unbalance at the Point of Common Coupling. According to the appellant these could be entirely separate processes. Hence, it could be that the VUF difference value was only used for deciding when to initiate negative sequence voltage compensation, and that the compensation was then controlled not on the basis of the VUF, but on the basis of the magnitude of the negative sequence voltage, as in D1. This was consistent with the disclosure in paragraphs [0052] and [0057] of the patent, according to which the control signal either was the amplitude of the negative sequence of the unbalanced three-phase voltage signal at the PCC 120, or was proportional to it. According to the appellant, the only difference between claim 1 and the disclosure of D1 was the use of the VUF. This solved the problem of selecting a suitable parameter for deciding when to initiate negative sequence voltage compensation. In the light of D10 it was obvious to choose VUF for this, as the VUF allowed a decision to be taken whether or not the voltage unbalance was within an acceptable range.

Auxiliary request 1

Claim 1 of auxiliary request 1 is a combination of granted claims 1 and 3. In the grounds of appeal the appellant submitted that the granted dependent claims lacked an inventive step, making reference to the arguments in section 4 on pages 17 to 20 of the grounds of opposition dated 23 December 2014. There it was argued that D7 disclosed that the active and reactive currents may be generated by components in the wind turbine by a power or frequency converter (c.f. [0019]) and it was submitted that the subject matter of claim 3 lacked an inventive step in view of document D1 or in view of document D1 in combination with document D7. This objection was confirmed in respect of auxiliary request 1 in the appellant's letter of 21 April 2021.

In the oral proceedings before the Board the appellant maintained the objections based on D1 and D7, arguing that converters were a typical feature of a wind turbine, that if there was a negative sequence voltage at the PCC there must also be reactive power present, and that it would be a straightforward matter for the skilled person to control the converter using PWM phase control to mitigate the imbalance.

The appellant also sought in the oral proceedings to introduce new objections based on documents D5 and D6, arguing that it was legitimate to refer to these documents as they had been referred to in the Extended European Search Report (with other references), had been mentioned in the grounds of opposition and had been discussed in the respondent's reply to the appeal. Furthermore, D6 had been referred to in respect of auxiliary request 2 in their submission of 21 April 2021. An exceptional circumstance was that the respondent was presenting arguments in the oral proceedings regarding the viability of controlling the power converter of the wind turbine using phase locked loop (PLL) control.

XI. The respondent's submissions may be summarised as follows:

The respondent in essence concurred with the opposition division's findings, submitting that features 1.1b, 1.4 and 1.5 were not disclosed in D1 and that the objective technical problem as formulated in the decision was correct.

The respondent maintained that the "negative sequence voltage compensation signal" output by the control unit, and used for mitigating the unbalance at the PCC, was directly representative of the negative sequence voltage in the system, because a change in the negative sequence amplitude would directly result in a change of the negative sequence voltage compensation signal.

The respondent accepted that it was known from D10, D3 and D4 for VUF to be used "a posteriori" to evaluate voltage unbalance to decide whether a voltage signal fulfills predefined engineering standards or norms, but emphasised that the invention by contrast used the VUF "a priori" as a closed-loop regulation quantity for unbalance compensation. This was not known or obvious from the prior art.

In the oral proceedings the respondent maintained the above, arguing furthermore that when feature 1.5 was read with a mind willing to understand it, it was clear that the features (a) and (b) were related to one another, i.e. that the output negative sequence voltage compensation signal was related to the difference determined between measured and predetermined VUF. Hence, there was closed-loop control of unbalance mitigation using the VUF. Paragraphs [0052] and [0057] of the patent described arrangements which did not fall within the scope of the claims as granted, but it had been omitted to amend them for conformity with the claims.

Auxiliary request 1

The respondent argued that it was not obvious in view of D7 to mitigate an unbalance using a converter of the wind turbine. Whilst converters as such were well known, D7 only taught their use to control active and reactive power. This was a single-phase consideration determined by the phase difference between voltage and current. It had nothing to do with an unbalance in the voltages of the three phases.

In respect of the admittance into the proceedings of the arguments based on documents D5 and D6, the respondent submitted that the appellant had not at any stage relied on these documents when arguing against the features added with the independent claims of auxiliary request 1. There were no exceptional circumstances which justified their being admitted in the oral proceedings under Article 13(2) RPBA 2020.