T 0532/20 - Optional but essential

Key points

•  The issue in this decision is Art. 123(2). In particular, whether the feature added from the claim and taken from the description is an impermissible intermediate generalisation because another feature is not specified.
• The feature, added before the grant, is: "that the generator remains connected to and synchronised with the grid during a low voltage event of less than 50%". Quite possibly, this feature specifies a (desirable) result to be achieved.
• The PCT application, para. [0003], mentions the objective as follows: "In order to reliably supply power to the power grid, wind turbine generators (as well as other types of generators) must conform to power grid interconnection standards that define requirements imposed on power suppliers and large power consumers. In particular, a "low voltage ride through" (LVRT) requirement typically requires that a power generation unit must remain connected and synchronized to the grid when the voltage at the terminals of the generation unit fall to prescribed levels"; the prescribed level is defined as less than 50% elsewhere. This paragraph is part of the discussion of the background art.
• In para. [0026] discussing the invention,  the application states: "Thus, crowbar circuit 440 and converter controller 430 are part of a system that allows a wind turbine generator to ride through low voltage events and remain synchronized to the power grid."
• This para. [0026] is part of the discussion of Fig.4 which starts as follows: "[0024] Figure 4 is a block diagram of one embodiment of a power converter having functionality to respond to a low voltage event. In one embodiment, power converter 400 includes inverters 410 and 420, converter controller 430 and crowbar circuit 440. Other components can also be included in power converter 400."
• The patentee would like to specify the "remain connected and synchronized to" feature without the feature of the crowbar circuit. 
• The Board does not accept the arguments of the proprietor: "A skilled person does not read an application as a linguist would, but rather on the basis of their technical understanding. The application clearly teaches the skilled person that a severe voltage drop at the generator terminals causes high currents in the generator windings, which can damage the semiconductor switches in the inverter. If this happens, the generator cannot remain connected and synchronised. The original application does not contain a single technically useful piece of information as to how a severe voltage drop could be accommodated otherwise than with a protective circuit. It also does not contain a single piece of credible disclosure of conditions under which a low-voltage event did not cause excess currents in the generator. "
• "In view of this, a skilled person sees beyond the [applicant's] strategic choice to represent most parts of the disclosure as optional [in the application as filed]. In this context, it is simply irrelevant how often the words "in one embodiment" are used correctly or not, contrary to what the [proprietor] argued."
• "This view applies in particular since the description has remained unamended despite the amendments made to claim 1 [during the grant procedure]. Claim 1 as originally filed was so broad as to encompass prior art turbines with battery (viz. UPS) powered blade pitch control that merely tripped offline during a low-voltage event according to original paragraph [0005]. In this original context, a skilled person easily understands that a protective circuit is not essential. However, this is not in contradiction to the fact that a protective circuit becomes essential once the generator is required to remain connected and synchronised with the grid during a low voltage event of a certain severity as is required by claim 1 as amended."

• EPO 

The link to the decision is provided after the jump, as well as (an extract of) the text of the decision.

https://www.epo.org/law-practice/case-law-appeals/recent/t200532eu1.html

3. Main Request - Added Subject-Matter

3.1 The ground for opposition pursuant to Article 100(c) EPC prejudices the maintenance of the patent as granted. Claim 1 as granted (main request) defines subject-matter that extends beyond the content of the application documents as originally filed. This concerns in essence the feature that the generator remains connected to and synchronised with the grid during a low voltage event of less than 50%.

3.2 Originally filed claim 1 specified a wind turbine with a blade pitch control system and an uninterruptible power supply. The wind turbine generator possesses two modes of operation. In the first (normal) mode the generator provides power. In the second mode, which is a low voltage event, the blade pitch control system is powered by the uninterruptible power supply, and the pitch of the blades is changed in an undefined way.

3.3 In the following section, paragraph numbers refer to the description as originally filed, if not otherwise indicated.

According to paragraph [0005], prior art wind turbines were allowed to trip offline in the case of a low voltage event, which entailed putting the blades in a feathered position by means of a battery powered blade pitch control system. This did not fulfil low voltage ride through requirements because the generator can not remain synchronised and connected to the grid to support it.

According to paragraph [0007], low voltage events at 70% of rated voltage can be accommodated - i.e. the generator can remain connected to the grid and synchronised during them - through, for example, increased capacity in various components (motors, generators, converters, etc.) and by use of uninterruptible power supplies (UPSs) for sensitive control circuits. However, more severe voltage fluctuations, for example, voltages at 15% of rated voltage cannot be accommodated using these techniques.

According to paragraph [0015] and figure 4, use of a converter controller that monitors the current in one or both of the inverters to selectively enable a current limiting circuit can protect against damage that can be caused by high currents during a low voltage event. In one embodiment, a crowbar circuit is selectively enabled to shunt current away from the inverters and/or other components that can be damaged by excessive currents.

Paragraphs [0020] and [0021] describe the problem that power converters and generators are particularly susceptible to voltage fluctuations. Generators can store magnetic energy that can be converted to high currents when the generator terminal voltage decreases quickly, viz. during a low voltage event. Those currents can cause failure of the semiconductor devices of power converters coupled with the generators. The board notes that this information is technically consistent with the disclosure of E9/O2 (Akhmatov, V.: "Variable-speed Wind Turbines with Doubly-fed Induction Generators Part 11: Power System Stability", Wind Engineering Volume 26, No. 3, 2002 pages 171 to 188), page 173, first paragraph, which was cited by appellant 1 during the first instance proceedings and in appeal.

Paragraph [0022] discloses that during low voltage events the generator torque does not balance the mechanical rotor torque, which leads to the rotor speeding up, and which can eventually lead to overspeed tripping of the rotor.

Paragraph [0023] starts with the words "As described in greater detail below" and summarises three measures for accommodating a low voltage situation. As a first measure the power converter is powered by an uninterruptible power supply and includes a protective circuit that maintains currents within an allowable range. The converter controller selectively activates and deactivates the protective circuit to maintain current flow within an acceptable range. As a second measure, the turbine controller is also powered by an uninterruptible power supply and operates to prevent overspeed trips. As a third measure, one or more non-vital loads are de-energised during the low voltage event if necessary to protect those components from potential damage.

The following paragraphs [0024] to [0027] then elaborate the above measures. These paragraphs specify that the protective circuit keeps current levels in the converter, and more particularly in the inverters of the converter, within acceptable ranges in order to avoid damage of the semiconductor components. This description passage is contradictory. On the one hand, paragraph [0024] appears to describe inverters as part of the converter to be optional ("In one embodiment, power converter 400 includes inverters 410 and 420, converter controller 430 and crowbar circuit 440."). On the other hand, the entire three paragraphs deal exclusively with converters having inverters, and the problem of overcurrent in inverters. However, these paragraphs do not contain any disclosure of other elements in which currents should be maintained at acceptable levels in order for a successful low-voltage ride-through, as the introductory paragraph [0023] states. Paragraph [0033] reaffirms that the converter controller is powered by a UPS to guard against excessive currents in the inverters.

Paragraphs [0034] to [0038] repeat in essence the previous information in the context of a process rather than a device. Paragraph [0036] deals with the behaviour of the blade pitch control system during a low voltage event. Paragraph [0037] reaffirms that certain non-essential components, such as the yaw-system, can be disabled if they are prone to damage from low voltage and high current situations. Paragraph [0038] finally reaffirms that inverters of a converter need to be guarded against excessive currents using a protective circuit, such as a crowbar circuit, in order to protect the semiconductor elements from damage.

3.4 Summarising the original disclosure, in the prior art, wind turbines were allowed to trip offline by changing the blade pitch to counter a speeding of the rotor if the torque of the generator ceases to balance the mechanical torque. Increasing robustness of certain components and powering certain loads can accommodate low voltage events down to about 70% of rated generator voltage.

If more severe voltage drops are to be accommodated, i.e. if the generator is not allowed to trip offline or disconnect, but rather is required to remain connected to the grid and synchronised with it, a bundle of measures is necessary. Any fast drop in generator terminal voltage will induce excess current in the generator windings which in turn might damage the semiconductor elements in the inverters, which make up the converter. The skilled person realises that this is a severe problem with very high probability of occurrence. Furthermore, the patent very briefly discloses the possibility that blade pitch control can be used as a measure for low-voltage ride-through without this necessarily implying that the generator trips offline. Thirdly, the opposed patent discloses that non-vital loads, such as a yaw system can (also) be powered by a UPS for protection against damage due to low voltages and high currents.

It is not necessary for the patent to explicitly state this, but a skilled person clearly understands, that in order for a generator to remain connected to and synchronised with the grid during voltage drops to below 70%, it can neither be tolerated that excess currents damage the converter nor that the wind turbine blade pitch controller is allowed to trip offline. Contrary to that, the skilled person would understand that non-vital component damage might be tolerated. This is despite the - in the board's view - excessive use of expressions indicating optionality. This is also in spite of four different problems being formulated in original paragraph [0008]. Any partial solution to only a subset of these partial problems is simply not sufficient for the claimed objective of continued connection and synchronisation.

3.5 Turning now from the analysis of the original disclosure to a closer examination of the amendments, in the course of the examination proceedings, claim 1 was amended to specify that the predetermined level defining a low voltage event is less than 50% of the rated voltage for the generator and that the generator remains connected to and synchronized with a power grid during the low voltage event.

No elements of the originally disclosed solutions, in particular no protective circuit for the inverters was added to claim 1 of the main request. A skilled person is therefore clearly faced with new information that a continuing connection and synchronisation of the generator with the power grid even at voltage drops to less than 50% of the rated voltage, could be achieved by a UPS powered blade pitch control system and turbine controller alone, as argued by the appellants and the intervener.

3.6 The respondent's counter-arguments did not convince the board.

3.6.1 The main underlying assertion on which most of the respondent's argument is built is that, due to the entirety of the original disclosure, a skilled person would have understood claims 1 and 3 as originally filed to be limited to wind turbines that were capable of a low-voltage ride-through. Moreover, the skilled person would have understood that a low-voltage ride-through capability implied the capability to remain connected and synchronised, because the application as originally filed contained a corresponding definition in paragraphs [0003] and [0009]. Furthermore, the voltage range 50% or less of rated voltage would in reality be understood by a skilled person to be 15% to 50%.

This premise is putting the cart before the horse. Rather than identifying a direct and unambiguous original disclosure, the respondent argues that precisely those amendments that were added to the subject-matter of original claims 1 and 3 had been implicitly part of these claims from the beginning. It is apparent that any amendment could be justified if that logic were to be accepted. Rather, on any objective reading neither claim 1 nor claim 3 as originally filed specified that the claimed generators were capable of low-voltage ride through or that they remained connected and synchronised during a low-voltage event of any size. Claim 1 as originally filed encompassed prior art wind turbines with a battery powered blade pitch control system that were allowed to trip offline by placing the blades in a feathered position, exactly as described in original paragraph [0005]. The repeated assertion that it was only important how a skilled person would have understood these claims turns the assessment of added subject-matter from an exercise of objective assessment into subjective, and to a large degree arbitrary, claim construction.

3.6.2 The respondent argued furthermore that the appellants and the board in reality raised an objection of missing essential features against claims 1 and 3 of the main request in the guise of an objection of added subject-matter. This is not correct. Claim 1 was amended so as to be limited - only after the amendment - to a generator that is capable of remaining connected and synchronised during low voltages of under 50% of rated voltage. However, the measures that were originally disclosed to be necessary to achieve that result, in particular a protective circuit for the inverters and a blade pitch system controlled so as not to trip the generator, were not added to the amended claim. The fact that the claimed generator could achieve this technical effect without the originally disclosed corresponding measures clearly represents new information. Likewise, the board's analysis does not imply that claims 1 and 3 as originally filed already contained added subject-matter. This argument, like the previous one, is contingent on the same false premise that the originally filed claims were more limited than their actual wording.

3.6.3 The respondent also argued that a protective circuit, in particular in the form of a crowbar circuit, was originally disclosed only as an embodiment or an optional feature, referring to various passages, such as paragraphs [0024] to [0026] and claim 5 as originally filed, reasoning that if an applicant drafted an application by representing a protective circuit as optional, then this represented an original disclosure to the skilled person.

This argument does not convince the board. A skilled person does not read an application as a linguist would, but rather on the basis of their technical understanding. The application clearly teaches the skilled person that a severe voltage drop at the generator terminals causes high currents in the generator windings, which can damage the semiconductor switches in the inverter. If this happens, the generator cannot remain connected and synchronised. The original application does not contain a single technically useful piece of information as to how a severe voltage drop could be accommodated otherwise than with a protective circuit. It also does not contain a single piece of credible disclosure of conditions under which a low-voltage event did not cause excess currents in the generator. In view of this, a skilled person sees beyond the respondent's strategic choice to represent most parts of the disclosure as optional. In this context, it is simply irrelevant how often the words "in one embodiment" are used correctly or not, contrary to what the respondent argued. This view applies in particular since the description has remained unamended despite the amendments made to claim 1. Claim 1 as originally filed was so broad as to encompass prior art turbines with battery (viz. UPS) powered blade pitch control that merely tripped offline during a low-voltage event according to original paragraph [0005]. In this original context, a skilled person easily understands that a protective circuit is not essential. However, this is not in contradiction to the fact that a protective circuit becomes essential once the generator is required to remain connected and synchronised with the grid during a low voltage event of a certain severity as is required by claim 1 as amended.

3.6.4 The respondent also adopted the opposition division's argument from the decision under appeal that generator synchronisation with the power grid and the damage of components were two different problems which were not inextricably linked to each other. A low voltage event could also be addressed by increasing the capacity in various components such as the motor, generator and/or converter.

The board is not convinced by this argument. The opposition division's stance concerning continued synchronisation and avoiding inverter damage by excess current being two different technical problems is very difficult to accept. The variable frequency current produced in the generator is transformed in the converters to the stable grid frequency before being fed to the grid. Quite obviously, if the inverter is destroyed, the generator cannot remain connected and synchronised to the grid. Hence protecting the inverter is a prerequisite for, and hence inextricably linked with, the claimed technical effect expressed in claim 1 after the amendment.

Moreover, the logic of the argument of the respondent and opposition division is not correct. They appear to argue that the fact that there might be more than one solution to dealing with excess currents justified the conclusion that the respondent did not have to specify any solution in the amended claim. Regardless of the fact that the original disclosure does not contain a single further credible solution against excess currents, what is to be examined is whether the amended subject-matter was originally disclosed. The board fails to see how the existence of particular further solutions should justify not specifying any in the amended claim.

3.6.5 The respondent further argued in the context of the question of a protective circuit being presented as optional that presenting many examples in an application was helpful to the public and should not be held against them. This argument is largely irrelevant for the question as to what was originally disclosed. However, the board also disagrees with the respondent's characterisation of their own patent. In the board's view it contains exactly one example of how to deal with excess currents, and that is presented as optional, despite being obviously essential. Such a way of disclosing the invention is not helpful in understanding it.

3.6.6 The respondent also asserted that paragraphs [0007] and [0008] disclosed that a generator had to remain connected and synchronised with the grid without any mention of a protective circuit.

The board notes that original paragraph [0007] concerns prior art wind turbines. Original paragraph [0008] contains essentially assertions of achievements that do not specify the prescribed voltage levels to which claim 1 was amended and which cannot be viewed in isolation. The respondent's reference to paragraph [0036] is an attempt to read this paragraph in isolation from paragraph [0038], which refers to the same process. Moreover, the fact that parts of this process are again presented as optional is unpersuasive for the same reasons as mentioned above in the context of the device. By the same token, the argument that some low voltage situations might be conceivable in which a protective circuit did not have to be activated is not a replacement for a direct and unambiguous disclosure as to the details of those allegedly existing situations and a corresponding limitation in the claim to such situations.

Therefore, as argued by the appellants and the infringer, the above passages cannot serve as a basis for the amendments, and the ground for opposition pursuant to Article 100(c) EPC prejudices the maintenance of the opposed patent.

4. Auxiliary Request 1 to 3 - Added Subject-Matter

4.1 Claim 1 of the main request is present in unamended form in auxiliary request 1. Therefore, the conclusion for the main request concerning added subject-matter applies correspondingly under Article 123(2) EPC.

4.2 The amendments in auxiliary requests 2 and 3 concern only the introduction of a lower boundary of 15% for the prescribed level of the low voltage event. Thus they do not have any effect on the conclusions of the board concerning the main request.

4.3 Auxiliary requests 1 to 3 therefore do not meet the requirements of Article 123(2) EPC.