The present disclosure relates to a method for voltage balancing between phase-legs of a Modular Multilevel Converter (MMC). BACKGROUND

An MMC, also known as Chain-Link Converter (CLC), comprises a plurality of converter cells (also known as modules or submodules), serially connected in converter branches (also known as arms) that in turn may be arranged in a star (also known as Y or wye), double-star (double-Y/wye) or delta, direct or indirect converter topology. Each converter cell comprises, in the form of a half-bridge (also known as monopolar) or full-bridge (also known as H- bridge or bipolar) circuit, a capacitor for storing energy and power

semiconductor switches such as Insulated Gate Bipolar Transistor (IGBT) devices, Gate-Turn-Off Thyristor (GTO) devices, Integrated Gate

Commutated Thyristor (IGCT) devices or Metal-Oxide-Semiconductor

Field-Effect Transistor (MOSFET) devices for connecting the capacitor to the converter branch with one or two polarities. The MMC can for instance be used in a three-to-single-phase railway intertie.

A dedicated part of the closed-loop control (MMC inner control) is used to keep the individual branches of the MMC balanced. "Balanced" means that the voltages across the individual branches do not deviate but instead converge asymptotically to their mean value. The balancing is achieved through two different mechanisms: a) active power transfer between the three converter phase-legs (also referred to as horizontal balancing) and b) active power transfer between the upper and lower branches of the same phase-leg (also referred to as vertical balancing).

The horizontal balancing control loop compares the three UDc_sum values (the sum of the cells' capacitor voltages (∑U _ce ii, I. N) in one phase-leg) and, in case of an unbalance between the three voltages, creates a circulating current to equalise the UDc_sum values. For a railway intertie, this circulating current is of the railway supply frequency, e.g. 16.7 Hz, to interact with the respective single-phase side converter voltage and achieve active power transfer.

The vertical balancing control loop compares the voltages of the upper branches with the voltages of their corresponding lower branches and, in case of an unbalance between those voltage pairs, creates a circulating current to equalise the upper and lower branch voltages. This circulating current is of the three-phase utility grid frequency, e.g. 50 Hz, to interact with the respective three-phase side converter voltage UDc_sum and achieve active power transfer. This concept works well when the converter pulses are released and the MMC is connected to both the three-phase and single-phase grids.

In certain conditions however, the converter pulses are released but the MMC is disconnected from one or both of the three-phase and single-phase grids. These conditions include, but are not limited to: starting up of the converter (in particular during magnetising of the grid transformer(s)), reactive power compensation mode (Volt-Ampere Reactive (VAR)-compensation mode). In the aforementioned cases, there is an insufficient number of degrees of freedom to achieve full converter controllability, caused by the missing single-phase voltage. This leads to a failure of the horizontal balancing mechanism. A failure of the horizontal balancing will lead to the divergence of the three UDc_sum values, which ultimately will lead to a shutdown of the converter. During starting up of the converter, the converter is typically disconnected from the single-phase grid, and may also be disconnected from the three-phase grid if a pre-charge module is used on three-phase side of the MMC.

SUMMARY

Embodiments of the present invention relate to a method of controlling a an MMC configured to be connected between a first power grid and a second power grid, wherein the first power grid is a three-phase AC grid.

Embodiments also describe a voltage balancing controller, comprised in a control arrangement of the MMC, to implement said method, in order to keep all branches of an MMC at the same voltage level. The voltage balancing controller may not be fully operational under certain operating conditions in accordance with the prior art, especially when the converter is disconnected from the second grid (as discussed in the background section above). To make the voltage balancing controller operational under all operating conditions, embodiments of the method of the present disclosure may be used. As soon as the MMC is modulating (i.e. pulses are released to drive the valves, semiconductor switches, of the MMC), a non-zero voltage reference for the second side of the MMC is sent from the controller to the MMC. This non-zero voltage reference ensures the proper active power transfer between the converter phase-legs of the MMC for balancing purposes.

According to an aspect of the present invention, there is provided a method for voltage balancing between phase-legs of a Modular Multilevel Converter (MMC). Each phase-leg comprises a plurality of series connected converter cells. Each cell comprises an energy storage and a plurality of semiconductor switches. The MMC is configured to be connected between a first power grid, which is a three-phase AC grid, via a first circuit breaker of a first side of the MMC, and a second power grid, having a nominal voltage UR, _n0 m, via a second circuit breaker of a second side of the MMC. The MMC is disconnected from the second grid by means of the second circuit breaker. The method comprises setting a non-zero voltage reference for the second side voltage, sending firing pulses for controlling the semiconductor switches of the converter cells, and balancing the phase-leg voltages of the phase-legs by means of the voltage resulting from the set voltage reference on the second side of the MMC.

According to another aspect of the present invention, there is provided a computer program product comprising computer-executable components for causing a control arrangement to perform the method of any preceding claim when the computer-executable components are run on processing circuitry comprised in the control arrangement. According to another aspect of the present invention, there is provided a control arrangement for voltage balancing between phase-legs of an MMC in which each phase-leg comprises a plurality of series connected converter cells, each cell comprising an energy storage and a plurality of semiconductor switches, the MMC being configured to be connected between a first power grid, which is a three-phase AC grid, via a first circuit breaker of a first side of the MMC, and a second power grid, having a nominal voltage, via a second circuit breaker of a second side of the MMC, the MMC being disconnected from the second grid by means of the second circuit breaker. The control arrangement comprises processing circuitry, and data storage storing instructions executable by said processing circuitry whereby said control arrangement is operative to set a non-zero voltage reference for the second side voltage, send firing pulses for controlling the semiconductor switches of the converter cells, and balance the phase-leg voltages of the phase-legs by means of a voltage resulting from the set voltage reference on the second side of the MMC.

The non-zero voltage reference results in a voltage being produced at the second side of the MMC, allowing balancing of the voltages of the phase-legs.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig l is a schematic circuit diagram of an embodiment of an MMC having a double-star topology, in accordance with the present invention. Fig 2 is a schematic circuit diagram of an embodiment of a full-bridge converter cell, in accordance with the present invention.

Fig 3 is a schematic block diagram of an embodiment of how an MMC is connected between a three-phase grid and a single-phase grid, in accordance with the present invention. Fig 4 is a schematic flow chart of an embodiment of a method of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. Embodiments of the present invention are below discussed primarily with reference to the example of a railway intertie, where the MMC is a three-to- single-phase AC converter, e.g. in a double-star configuration as in figure 1, having a (first) three-phase side connected to a (first) three-phase grid and a (second) single-phase side connected to a (second) single-phase grid.

However, embodiments of the present invention may also be useful for other types of MMC, e.g. a AC-to-DC converter where the second side is a DC side connected to a DC grid, or a three-phase-to-three-phase converter where both the first and second sides are three-phase sides connected to first and second three-phase grids. It should be noted that when this three-to-single phase converter example is discussed herein, the discussion is

correspondingly also relevant to other topologies, if applicable.

With reference to figure l, the MMC ι is in a double-star (also called double-Y or -wye) topology and connected between a three-phase AC grid 5a, having the three Alternating Current (AC) phases a, b and c, and a single-phase AC grid 5b, e.g. a railway grid, formed between two phases (outputs) of the MMC. For the three-phase grid 5a, the line current of phase a is given as iLi, of phase b as 1L2 and of phase c as 1L3. For the single-phase grid 5b, the current is given as iR and the voltage as UR. The double-star topology comprises one phase leg 2 for each phase a, b and c of the three-phase grid 5a, each phase leg 2 comprising two branches 3 (also called arms), a first (also called upper) branch 3a and a second (also called lower) branch 3b, of series connected (also called chain-linked or cascaded) converter cells 4. Of the three-phase grid 5a, the first phase a connects a first phase leg 2a at a point Li between the first and second branches 3a and 3b of said first phase leg 2a, the second phase b connects a second phase leg 2b at a point L2 between the first and second branches 3a and 3b of said second phase leg 2b, and the third phase c connects a third phase leg 2c at a point L3 between the first and second branches 3a and 3b of said third phase leg 2c. A control arrangement 10, comprising the voltage balancing controller, of the converter 1 controls the converter, e.g. voltage balances the branches 3 horizontally and vertically and sends firing pulses to the semiconductor switches S of the converter cells 4. The control arrangement 10 may comprise a central part, e.g. comprising the voltage balancing controller, and distributed parts, e.g. cell controllers associated with each converter cell. The voltage and current over the first branch 3a of the first leg 2a is given as uia and iia, respectively. The voltage and current over the second branch 3b of the first leg 2a is given as uib and iib, respectively. The voltage and current over the first branch 3a of the second leg 2b is given as u2a and 12a, respectively. The voltage and current over the second branch 3b of the second leg 2b is given as u2b and 12b, respectively. The voltage and current over the first branch 3a of the third leg 2c is given as u3a and 13a, respectively. The voltage and current over the second branch 3b of the third leg 2c is given as u3b and 13b, respectively. Figure 2 illustrates an MMC cell 4, e.g. of a converter 1 as illustrated in figure 1. The cell 4 comprises an energy storage 21, typically a capacitor arrangement comprising at least one capacitor, over which a DC voltage of the cell is formed. A plurality of semiconductor switches S form a half-bridge (also called monopolar) or full-bridge (also called H-bridge or bipolar) topology, herein (as is preferred in some embodiments of the invention) four semiconductor switches S1-S4 form a full-bridge topology.

Figure 3 illustrates an embodiment of how an MMC 1 in accordance with the present invention may be connected between a first power grid 5a, here again exemplified as a three-phase AC grid 5a, in this case of 50 Hz, and a second power grid, here again exemplified as a single phase AC grid 5b, in this case of 16.7 Hz e.g. a railway grid. The MMC may e.g. be as shown in figure 1.

On the single-phase side of the MMC 1, the MMC is connectable to the single- phase grid 5b via a second circuit breaker 31b, able to disconnect the MMC from the single-phase grid (in which case embodiments of the present invention are especially useful). The MMC is typically also connected via a second grid transformer 32b (e.g. a rail grid transformer) or, alternatively, via a line inductor, typically positioned in series between the MMC and the second circuit breaker. In case of a grid transformer 32b being used, the nonzero voltage reference UR, _re f preferably has a non-zero frequency in order to not saturate said grid transformer. If a line inductor is used instead of a grid transformer, the non-zero voltage reference UR, _re f may be allowed to have a frequency of zero, i.e. to substantially be direct current (DC). That the reference voltage UR, _re f is non-zero thus refers to the amplitude (or DC voltage, in case of zero frequency) not being zero for enabling horizontal voltage balancing between the phase legs 2 of the MMC 1. On the three-phase side of the MMC 1, the MMC is connectable to the three- phase grid 5a via a first circuit breaker 31a. In some embodiments of the invention, the first circuit breaker 31a is ON (conducting/closed) and thus connecting the MMC to the three-phase grid, e.g. when acting as a Static Synchronous Compensator (STATCOM) for the three-phase grid 5a, and in some other embodiments of the invention the first circuit breaker 31a is OFF (not conducting/open) and thus disconnecting the MMC from the three- phase grid, e.g. during start-up of the MMC by means of a pre-charge module 34. The MMC is typically also connected via a first grid transformer 32a (e.g. a utility grid transformer), typically positioned in series between the MMC and the first circuit breaker. In some embodiments, an additional circuit breaker in the form of a transformer disconnector 34 is positioned in series between the MMC and the first grid transformer 32a, able to disconnect the MMC from said first grid transformer. Other standard devices may also be connected on the three-phase side of the MMC, depending on the application, such as a grid filter 33, e.g. connected between MMC and the first grid transformer.

As discussed herein, some embodiments of the present invention may be especially useful during start-up of the MMC, when the MMC is disconnected from the single-phase grid but being charged from its three-phase side. In some embodiments, the MMC is connected to the three-phase grid during such a start-up. Alternatively, a pre-charge module 35 may be used during start-up for charging from the three-phase side of the MMC, in which case the MMC is typically disconnected from the three-phase grid 5a, and may also be disconnected from the first grid transformer 32a by means of the transformer disconnector 34, during start-up. The pre-charge module 35 is typically connected between the MMC and the first grid transformer 32a or the transformer disconnector 34 (if present). Embodiments of the invention relate to a method of controlling an MMC l. Embodiments of the invention also relate to a controller 10, connected to the MMC, to implement said method.

As soon as the MMC 1 is modulating (i.e. pulses are released), a non-zero voltage reference UR, _re f for the single-phase side voltage UR is provided by the controller 10 to the MMC l. This non-zero voltage reference UR, _re f ensures the proper active power transfer between the three converter phase-legs 2a-c (also known as horizontal balancing), which is described above. The voltage reference UR, _re f adds the missing degree of freedom for achieving full MMC controllability. To add this degree of freedom, the frequency of the voltage reference UR, _re f can be set to any value and the amplitude of the voltage reference UR, _re f has to be set to above zero, e.g. to between zero and UR.

However, there may be limitations to these values (frequency and amplitude of the voltage reference) given by system components. For example, if a transformer 32b is connected to the single-phase side of the MMC 1, the frequency is not allowed to be zero in order to avoid saturation of the transformer. Obviously, frequency and amplitude of UR, _re f may only be chosen freely if the single-phase side circuit breaker 31b is open. Otherwise, these values are dictated by the connected grid 5b. Applying the voltage reference UR, _re f while the second circuit breaker 31b is open gives full converter controllability. But this applied voltage resulting from the non-zero voltage reference also causes additional losses in the system, e.g. semiconductor S switching losses and transformer 32b

magnetising losses. To keep these losses at a minimum, the voltage reference UR,ref is preferably kept below the nominal voltage UR, _n0 m, optimally as close to zero as possible while still allowing for the desired horizontal voltage balancing.

Below follows a discussion of embodiments of the present invention with reference to the situation of start-up of the MMC. State of the art is to magnetize the first grid transformer 32a and charge the cells 4 of the converter 1 through the three-phase grid 5a. This happens either by closing the first circuit breaker 31a or, to limit the inrush current, by closing a by-pass switch which is connected in series with a resistor in parallel with the first circuit breaker 31a. The resulting inrush current may cause disturbances in the three-phase grid 5a and puts strain on the converter components.

To avoid these negative effects, the converter cells 4 may be charged using a separate pre-charging module 35, while the utility grid transformer 32a is disconnected from the converter by means of the transformer disconnector 34. After the cells 4 are completely charged, the pre-charging module 35 is disconnected from the converter and the transformer 32a is connected to the converter 1 by closing the transformer disconnector 34, while still being disconnected from the grid 5a by means of the circuit breaker 31a. The stored energy of the cell 4 is then used to magnetize the transformer 32a by releasing pulses in a controlled manner, e.g. as described in EP 3 010 104. If the three-phase side as well as the single-phase side transformers 32a and 32b are magnetized simultaneously, the balancing of the branches 3 works as expected. There are however scenarios, whereby only the three-phase side transformer 32a is magnetized. These scenarios include one or more of the following:

The stored energy in the cells 4 is not sufficient to charge two

transformers and the possibly connected grid filters 33 simultaneously to their nominal voltage value. · To keep energy consumption in the system to a minimum, the MMC 1 is brought to a standby state, where only the three-phase grid 5a is connected to the MMC to keep the cells 4 charged. This standby state allows a fast transition into active power transfer mode (ON state), while keeping energy consumption to a minimum. • The MMC l will only be operated in VAR-compensation mode on the three-phase grid 5a, making it unnecessary to connect the MMC to the single- phase grid 5b.

For these scenarios, the three UDc_sum values diverge during the magnetising of the transformer if the voltage reference UR, _re f of the single-phase side is kept at zero. This divergence may ultimately lead to a shutdown of the converter.

By using an embodiment of the invention, a non-zero voltage reference UR, _re f is applied to the single-phase side as soon as the firing pulses are released, i.e. the MMC is modulating. The resulting voltage adds the missing degree of freedom for achieving full MMC controllability. In other words, horizontal balancing is enabled and therefore, the three UDc_sum values of the three phase-legs 2 will remain balanced.

To keep energy consumption and hence system losses to a minimum, the single-phase side voltage reference UR, _re f is kept to a value of between zero volts and UR, _n0 m, where UR, _n0 m is the nominal single-phase grid voltage. The closer the voltage reference is to zero, the smaller the created losses are.

Below follows a discussion about embodiments of the invention with reference to the situation of VAR-compensation. State of the art is to use converters 1 in rail network interties mainly as devices to transfer power from the three-phase grid 5a to the single-phase grid 5b. Sometimes, the converters 1 are used to compensate reactive power in the single-phase grid 5b without transferring power from the three-phase grid 5a. For this reason, a VAR-compensation mode is implemented. As a means of stabilising electricity grids, utilities are tending away from the traditional passive reactive power compensation (i.e. capacitor banks) to static reactive power compensation (i.e. STATCOM devices). Because converters for rail network interties already have the capability of working as a STATCOM, utility companies may benefit from this capability not only for the single-phase side but also for the three-phase side. It may therefore be desirable to provide a VAR-compensation mode for the three-phase side as well as the single-phase side of the MMC 1. Thus, some embodiments of the present invention may be especially useful when the MMC is functioning as a STATCOM.

In order to operate an MMC in VAR-compensation mode on the three-phase side, it may not be necessary to connect the converter ι to the single-phase grid 5b. However, in the absence of a reference voltage UR, _re f on the single- phase side, the three UDc_sum values diverge, as previously discussed, which ultimately may lead to a shutdown of the converter.

By using an embodiment of the method of the invention, a non-zero voltage reference UR, _re f is applied to the single-phase side as soon as the pulses are released, i.e. the MMC is modulating. The resulting voltage enables the horizontal balancing between the phase-legs 2. With horizontal balancing enabled, the three UDc_sum values may remain balanced.

To keep energy consumption and hence system losses to a minimum, the single-phase side voltage reference UR, _re f is kept to a value of between zero volts and UR, _n0 m , where UR, _n0 m is the nominal single-phase grid voltage. The closer the voltage reference is to zero, the smaller the created losses are. Below are some currently preferred embodiments of the present invention mentioned.

With reference to figure 4, an embodiment of the method of the present invention is disclosed. The method is for voltage balancing between the phase-legs 2 of an MMC 1. Each phase-leg 2 comprising a plurality of series connected converter cells 4. Each cell 4 comprises an energy storage 21 and a plurality of semiconductor switches S. The MMC is configured to be connected (connectable) between a three-phase AC power grid 5a (herein called the first power grid) via a first circuit breaker 31a of a first (three- phase) side of the MMC, and a second power grid 5b, e.g. a single-phase AC grid, a three-phase AC grid or a DC grid, having a nominal voltage UR ,nom, Via a second circuit breaker 31b of a second side of the MMC. However, the MMC is disconnected from the second grid 5b by means of the second circuit breaker 31b. The method comprises, setting Ml a non-zero voltage reference UR, _re f for the second side voltage UR, which results in a voltage on the second side of the MMC even when not connected to the second grid 5b. The method also comprises, before or after the setting Mi (as indicated by the double- headed arrow), sending M2 firing pulses for controlling the semiconductor switches S of the converter cells 4, i.e. the MMC is modulating. The method also comprises balancing M3 the phase-leg voltages UDc_sum of the phase-legs 2 by means of the voltage resulting from the set Mi voltage reference on the second side of the MMC.

In some embodiments, the method is performed during start-up of the MMC 1, e.g. as discussed as an example above. In some embodiments, the MMC 1 is disconnected from the first grid 5a by means of the first circuit breaker 31a and a pre-charging module 35 is connected to the first side of the MMC for charging the MMC during start-up.

In some other embodiments, the method is performed as part of a VAR- compensation mode of the MMC 1 in which the MMC acts as a STATCOM for compensating reactive power in the first power grid 5a, being a three-phase grid, as discussed as another example embodiment above.

In some embodiments, the non-zero voltage reference UR, _re f has an amplitude which is less than the nominal voltage UR, _n0 m of the second grid 5b. A second side voltage of the MMC which is reduced compared with the nominal voltage of the second grid 5b reduces the losses in the MMC and transformers.

Preferably, the voltage reference is close to zero, but still not zero.

In some embodiments, the non-zero voltage reference has a non-zero frequency. This is preferred especially when a second grid transformer 32b is used, since such a transformer risks being overcharged in case of a DC voltage on the second side of the MMC. In case a second grid transformer is not used, e.g. if a line inductor is used instead, the non-zero voltage reference may have a zero frequency.

In some embodiments, the second grid 5b is a railway grid. Embodiments of the present invention may be especially useful for a railway intertie, but other types of single-phase or three-phase grids 5b are also contemplated. As mentioned herein the second grid 5b may preferably be a single-phase AC grid. However, in other embodiments of the present invention, the second grid may be e.g. a three-phase AC grid or a DC grid.

In some embodiments, the second side, e.g. a single-phase side, of the MMC has a frequency of 16.7 Hz, e.g. 50/3 Hz, or 25 Hz, preferably 16.7 Hz. These are typical examples of railway grid frequencies used in different countries.

In some embodiments, the first side of the MMC, being a three-phase AC side has a frequency of 50 Hz or 60 Hz, preferably 50 Hz. These are typical examples of distribution grid frequencies used in different countries. In some embodiments, the MMC 1 has a double-star topology, e.g. as illustrated in figure 1. However, the invention is not limited to this topology, but may be used with any MMC topology, e.g. for AC-to-DC MMC or three- phase-to-three-phase MMC.

In some embodiments, the plurality of semiconductor switches S1-S4 form a full-bridge topology. This is preferred to provide bidirectional functionality.

In some embodiments, each of the plurality of semiconductor switches S comprises an IGCT. Although, IGCTs are currently preferred semiconductor devices for the switches S, any other semiconductor switch device may alternatively be used with embodiments of the invention, e.g. IGBT, GTO or MOSFET.

Embodiments of the method of the present invention may be performed by a control arrangement 10 of the converter 1, which control arrangement comprises processing circuitry associated with data storage. The processing circuitry may be equipped with one or more processing units CPU in the form of microprocessor(s) executing appropriate software stored in associated memory for procuring required functionality. However, other suitable devices with computing capabilities could be comprised in the processor, e.g. an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a complex programmable logic device (CPLD), etc., in order to control the converter 1 and perform embodiments of the method of the present disclosure, while executing appropriate software, e.g. stored in a suitable data storage, such as a RAM, a Flash memory or a hard disk, or in the processing circuitry itself ( as e.g. in case of an FPGA). Embodiments of the present invention may be conveniently implemented using one or more conventional general purpose or specialized digital computer, computing device, machine, or microprocessor, including one or more processors, memory and/or computer readable storage media programmed according to the teachings of the present disclosure.

Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art.

In some embodiments, the present invention includes a computer program product which is a non-transitory storage medium or computer readable medium (media) having instructions stored thereon/in which can be used to program a computer to perform any of the methods/processes of the present invention. Examples of the storage medium can include, but is not limited to, any type of disk including floppy disks, optical discs, DVD, CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, flash memory devices, magnetic or optical cards,

nanosystems (including molecular memory ICs), FPGA or any type of media or device suitable for storing instructions and/or data. In some embodiments, the data storage of the control arrangement 10 may be a computer program product as discussed herein. The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.