Direct power and stator flux vector control of a generator for wind energy conversion system

Abstract

A method for controlling a variable speed wind turbine generator is disclosed. The generator is connected to a power converter comprising switches. The generator comprises a stator and a set of terminals connected to the stator and to the switches of the power converter. The method comprises: determining a stator flux reference value corresponding to a generator power of a desired magnitude, determining an estimated stator flux value corresponding to an actual generator power, determining a difference between the determined stator flux reference value and the estimated stator flux value, and operating said switches in correspondence to the determined stator flux reference value and the estimated stator flux value to adapt at least one stator electrical quantity to obtain said desired generator power magnitude.

Claims

1. A method for controlling a variable speed wind turbine generator connected to a power converter comprising switches, said generator comprising a stator and a set of terminals connected to said stator and to said switches, said method comprising: determining an estimated stator flux value corresponding to an actual generator power; calculating a stator flux difference value between a stator flux reference value and the estimated stator flux value; calculating a stator flux difference vector with a magnitude and direction, wherein the stator flux difference vector is a difference between a stator flux reference value and an estimated stator flux vector value; and operating said switches of said power converter with switching times based at least on the stator flux reference value, the estimated stator flux value, the calculated stator flux difference value, the stator flux difference vector, and a DC-link voltage of the power converter to adapt at least one stator electrical quantity to obtain a desired generator power magnitude, wherein said switches are operated according to a space vector modulation scheme for controlling a switching pattern of said switches, and wherein said switching pattern is formed by applying one or more vectors during one or more switching times, and said switching times for the switching pattern is determined from the magnitude and direction of the stator flux difference vector.

2. The method according to claim 1, wherein said switches are operated according to a pulse width modulation scheme in order to generate a synthesized voltage waveform at the stator terminals.

3. The method according to claim 1, wherein the switches comprise a first and a second set of switches and first set of the switches are operated to an on-state during a first time interval, τa, and the second set of the switches to an on-state during a second time interval, τb.

4. The method according to claim 3, wherein the first and second time intervals are determined according to ${\tau }_a=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.\sin \left(\frac{\pi }{3}-\gamma \right)}{\frac{\pi }{3}\sin \left(\frac{\pi }{3}\right)}$ ${\tau }_b=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.\sin \left(\gamma \right)}{\frac{\pi }{3}\sin \left(\frac{\pi }{3}\right)}.$

5. An apparatus for controlling a variable speed wind turbine generator connected to a power converter comprising switches, said generator comprising a stator and a set of terminals connected to said stator and to said switches, said apparatus comprising: a power controller adapted to: determine a stator flux reference value corresponding to a generator power of a desired magnitude; calculate a stator flux difference value between the stator flux reference value and an estimated stator flux value; and calculate a stator flux difference vector with a magnitude and direction, wherein the stator flux difference vector is a difference between a stator flux reference value and an estimated stator flux vector value; and a switch control unit adapted to operate said switches of said power converter with switching times based at least on the determined stator flux reference value, the estimated stator flux value, the calculated stator flux difference value, the stator flux difference vector, and a DC-link voltage of the power converter to adapt at least one stator electrical quantity to obtain a desired generator power magnitude, wherein said switch control unit is adapted to operate the switches according to a space vector modulation scheme for controlling a switching pattern of said switches, and wherein said switch control unit is adapted to determine the switching times for the switching pattern from the magnitude and direction of the stator flux difference vector.

6. The apparatus according to claim 5, wherein said switch control unit is adapted to operate the switches according to a pulse width modulation scheme in order to generate a synthesized voltage waveform at the stator terminals.

7. The apparatus according to claim 5, wherein the switch control unit is adapted to establish a switching pattern by providing control signals which operates a first set of the switches to an on-state during a first time interval, τa, and a second set of the switches to an on-state during a second time interval, τb.

8. The apparatus according to claim 7, wherein the switch control unit is adapted to determine the first and second time intervals according to ${\tau }_b=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.\sin \left(\gamma \right)}{\frac{\pi }{3}\sin \left(\frac{\pi }{3}\right)}$ ${\tau }_a=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.\sin \left(\frac{\pi }{3}-\gamma \right)}{\frac{\pi }{3}\sin \left(\frac{\pi }{3}\right)}.$

9. The method of claim 3, wherein the first time interval and the second time interval are determined based on the DC-link voltage of the power converter.

10. The apparatus of claim 7, wherein the first time interval and the second time interval are determined based on the DC-link voltage of the power converter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

(2) FIG. 1 illustrates generator converter system according to a preferred embodiment of the present invention.

(3) FIG. 2 illustrates a vector diagram for a synchronous generator represented in a stationary reference frame.

(4) FIG. 3 illustrates a vector representation of the voltages present at the stator terminals of a generator.

(5) FIG. 4a is a more detailed illustration of the generator side converter illustrated in FIG. 1.

(6) FIG. 4b illustrates eight switching states which determine a space vector hexagon.

(7) FIG. 5a illustrates one sector of the space vector hexagon illustrated in FIG. 4b.

(8) FIG. 5b illustrates a normalized voltage vector.

(9) FIG. 6 illustrates a control system for controlling the power of a wind turbine generator according to an embodiment of the present invention.

(10) FIGS. 7a-c illustrate a signal flow graph of a generator power and stator flux vector controller according to an embodiment of the invention.

(11) FIG. 8 illustrates a graph for achieving predictive control to mitigate a phase error in the stator flux vector.

(12) FIG. 9 illustrates a principle of current limiting in reference stator flux vector generation.

(13) FIG. 10 illustrates a flow diagram of field weakening decission.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(14) FIG. 1 illustrates an example of a generator converter system according to a preferred embodiment of the present invention.

(15) A shaft 10 transfers mechanical energy from an energy source, preferably a set of wind turbine blades (not shown), to a rotor of a variable speed generator 11. The shaft is preferably connected to the wind turbine blades, and to the rotor via a gearbox in order to adapt the rotational speed of the shaft 10 (i.e. speed of the wind turbine blades) to a speed range suitable for the generator 11. The generator 11 then converts the mechanical energy provided via the shaft 10 into electrical energy and delivers the electrical energy at a set of stator terminals 12a, 12b, 12c. For optimum performance in respect of converting the wind energy into electrical energy the shaft 10 will vary its speed as a function of the wind speed. Since the rotational speed of the rotor of the generator 11 is proportional to the rotational speed of the shaft 10, the amplitude and frequency of the voltage signal provided by the generator 11 at the stator terminals 12a, 12b, 12c will vary according to the rotational speed of the shaft 10. The generator may be a singly- or doubly-fed synchronous generator, e.g. a permanent magnet (PM) generator, an induction generator or any other type of generator comprising a stator winding.

(16) The terminals 12a, 12b, 12c of the generator 11 are connected to a generator side power converter 13. The converter 13 is preferably a three phase bridge converter 13 which includes six switches illustrated for the sake of clarity by the single switch and diode in FIG. 1. As will be disclosed in more detail below, the switches are arranged in a set of upper and lower switches which preferably are in the form of solid state devices, such as MOSFETs, GTOs or IGBTs. Other kind of switches, such as BJTs, however are equally possible depending on design considerations of the converter 13. The converter 13 will under normal operation function as an active rectifier converting the variable frequency AC voltage provided by the generator 11 into a DC voltage. The conversion is controlled using a pulse width modulation scheme, wherein control signals are applied to the switches in the converter 13 in order to provide the desired conversion functionality. In a preferred embodiment the switches are controlled by employing space vector modulation scheme, as will be disclosed below.

(17) The output from the converter 13 is provided to a DC link 14, which comprises a link capacitor for reducing the voltage ripple on the DC link.

(18) The DC link 14 is connected to a grid side power converter 15. The topology of the grid side power converter 15 is similar to the generator side power converter 13 disclosed above. The grid side power converter 15 normally operates as an inverter for converting the DC voltage on the DC link 14 into a regulated AC voltage for feeding active and reactive power to the grid 18. The switches of the grid side power converter 15 are provided with suitable control voltages in order to provide the desired voltage and power to a grid 18.

(19) The output from the grid side power converter 15 is filtered by means of inductors 16a, 16b, 16c in order to e.g. remove higher order harmonics from the output power signal. The output power signal is then provided to the grid 18 via a transformer 19. The output power signal may, if needed, be filtered by a second filter 17 in order to keep the interference or harmonic distortion at a low value.

(20) FIG. 2 illustrates a vector diagram for a synchronous generator represented in a stationary reference frame. The diagram comprises two stationary axes denoted α and β. The transformation from the stationary three phase abc reference frame to the αβ reference frame may be performed as disclosed above.

(21) A first vector represents the magnetizing flux, denoted Ψ.sub.mag in the figure. In the example shown in FIG. 2, which refers to a synchronous generator, the magnetizing flux corresponds to the rotor flux. The rotor flux may be generated by means of a permanent magnet, as in a PM generator, or by excitation of a field coil in the rotor. The arc at the tip of the rotor flux vector illustrates that the vector rotates about the origin of coordinates in the figure. The angular displacement of the rotor flux vector from the α axis is denoted θ.sub.r in the figure.

(22) In a corresponding manner is the stator flux vector, denoted Ψ.sub.s in the figure, represented by a vector which rotates about the origin of coordinates. In steady state operation the stator flux vector rotates in the stationary reference frame with an angular speed equal to the rotor flux vector. The angular displacement of the stator flux vector from the rotor flux vector is denoted by δ in the figure.

(23) The electromagnetic power of a synchronous generator can be expressed as:
P.sub.EM=v.sub.ai.sub.a+v.sub.bi.sub.b+v.sub.ci.sub.c∝ωψ.sub.s×ψ.sub.r
which gives
P.sub.EM=f(|ψ.sub.s|,|ψ.sub.r|,δ)

(24) It is seen from the power equation above that for a given speed of operation, the electromagnetic power depends on the magnitude of the stator flux vector and it's location with respect to the rotor flux vector. If the position of the rotor flux vector is known, it is possible to apply a voltage that will position the stator flux vector to give the desired magnitude of the power at a given speed. Hence, by controlling the stator flux vector, the electromagnetic (EM) power, which corresponds to the load power, can be obtained as commanded.

(25) Since the control is carried out in the stationary reference frame, it may be necessary to compensate for the phase delay created. This is achieved by a linear prediction carried out in polar co-ordinates.

(26) FIG. 3 illustrates a vector representation of the voltages present at the stator terminals of a generator. In order to control the power created by the generator it is necessary to control the signals that are applied to the stator terminals. In this respect is space vector modulation (SVM) is an effective averaging algorithm for providing an AC output signal from a DC voltage. SVM also minimizes the harmonic contents which determines the copper losses in the generator. SVM is also effective in that it minimizes switching losses in the switches of the generator side power converter 13.

(27) For a three phase generator, the voltages in the stationary abc reference frame may be represented as three 120° phase-shifted vectors (directions u.sub.a, u.sub.b and u.sub.b) in space, as shown in FIG. 3. For a balanced three phase system, these vectors sum to zero. This implies that the three vectors may be represented by a single space reference vector (u.sub.s). The idea behind SVM is to control the amplitude and the frequency of Vs, which implies that voltage amplitude, phase and frequency at the stator terminals 12a, b, c and hence the flux in the stator can be controlled.

(28) Referring to FIG. 4a, which is a more detailed illustration of the generator side converter 13 shown in FIG. 1. The switches 52ab b, c and 53a, b, c in the figure are illustrated as BJTs. It is, however, equally possible to use MOSFETs, GTOs, IGBTs etc as switching devices. Irrespective of the technology used for manufacturing the switches 52a, b, c, and 53 a, b, c, the switching sequence, or switching pattern, of the devices must follow certain rules. More specifically, whenever one of the upper switches 52a, b, c is conducting (i.e. in an on-state) the corresponding lower switch 53a, b, c should be off and vice versa. Moreover must three of the switches always be on and three switches always be off. These rules gives rise to eight distinct combinations for the switching states of the devices 52a, b, c and 53a, b, c. These combinations are denoted (abc) where e.g. a=1, b=0 and c=0 indicates that the upper switch 52a is on (thereby turning switch 53a off) while switches 52b and c are off. Six of the states are active states producing a voltage vector in a predefined direction while two of the states are inactive states, i.e. all upper switches 52a, b, c are off and all lower switches 53a, b, c are on, or vice versa.

(29) The eight switching states defined above determine eight phase voltage configurations as illustrated in FIG. 4b. As seen in the figure, the vectors define a hexagon with six equally sized sectors spaced by 60°. Each sector is bounded by two active vectors. The inactive states are represented by the vectors (000) and (111) which are zero and are located at the hexagon origin. Two adjacent voltage vectors are chosen depending on the sector in which the vector u.sub.s is located (100 and 110 in FIG. 4b). From FIG. 4b it is clear that only one of the upper and lower switches are changing state when switching pattern moves from one sector to the adjacent sector, wherein the switching losses are kept at a minimum.

(30) Normally, the switches are operated at a frequency F which is substantially higher than the grid frequency. The switching frequency F defines the sample period τ.sub.s via the relationship τ.sub.s=1/F. The sample period τ.sub.s is used when generating the vector V.sub.s from the various voltage vectors (100, 110, etc). More specifically is the vector u.sub.s formed by time weighting the vectors during one sample period τ.sub.s. Mathematically may the vector u.sub.s be expressed as

(31) $u_s=\frac{{\tau }_0}{{\tau }_s}{u}_0+\frac{{\tau }_1}{{\tau }_s}{u}_1+\mathrm{.Math.}+\frac{{\tau }_7}{{\tau }_s}{u}_7$

(32) where τ.sub.0, τ.sub.1 . . . τ.sub.7 is the time each vector u.sub.0, u.sub.1 . . . u.sub.7 is applied, respectively. The vectors u.sub.0 and u.sub.7 are the zero vectors (000, 111) which are applied in order to output a zero voltage.

(33) When u.sub.s and τ.sub.s are known it is possible to determine the on time for each vector, respectively, from the equations

(34) $u_s=\frac{{\tau }_1}{{\tau }_s}{u}_1+\frac{{\tau }_2}{{\tau }_s}{u}_2+\frac{{\tau }_{07}}{{\tau }_s}{u}_{07}$

(35) and
τ.sub.s=τ.sub.1+τ.sub.2+τ.sub.07

(36) A problem resides in how to determine the desired vector u.sub.s in order to provide efficient control of the electrical power provided by the generator.

(37) FIG. 5a illustrates one sector of the space vector hexagon shown in FIG. 4b. The desired stator flux vector Ψ.sub.s at two subsequent time instances is illustrated as the vectors Ψ.sub.s(k) and Ψ.sub.s(k+1). The reference flux Ψ.sub.s* is represented by a circular arc in the figure. The difference between the desired stator flux vector and the reference flux creates a flux error vector ΔΨ.sub.s*(k) with a direction that is perpendicular to the direction of the desired flux. The flux in the stator relates to the generator EMF by the equation (Faraday's law)

(38) $e=\frac{\partial {\Psi }_s}{\partial t}$

(39) This implies that the flux error vector ΔΨ.sub.s*(k) is proportional to a voltage vector that can be obtained as an average in a sample using adjacent vectors and is displaced by an angle γ with respect to the voltage vector u.sub.a (i.e. the active vectors u.sub.0, u.sub.1 etc) in any sector of operation. Hence γ varies from 0 to 60 degrees in a sector. The time each active vector, e.g. u.sub.1 and u.sub.2 in FIG. 5 is applied is denoted by τ.sub.a and τ.sub.b in the figure

(40) FIG. 5b illustrates an example where a normalized voltage vector is used to generate the switching times. The base for normalization is taken to be the peak value of the fundamental component of the phase voltage during six-step operation

(41) $u_{\mathrm{peak}}=\frac{2}{\pi }{U}_{\mathrm{DC}}$

(42) where U.sub.DC is the DC-link voltage of a two level inverter disclosed above. In the space vector modulation scheme it can be shown that the length of each of the six vectors (u.sub.1-u.sub.6) is

(43) $u_u=\frac{2}{3}{U}_{\mathrm{DC}}$

(44) wherein the normalization of the voltage becomes

(45) $\mathrm{Normalization}=\frac{\pi }{3}$

(46) From the figure it can be seen that

(47) 0$\frac{\frac{\pi }{3}{\tau }_a}{\sin \left(\frac{\pi }{3}-\gamma \right)}=\frac{\frac{\pi }{3}{\tau }_b}{\sin \left(\gamma \right)}=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.}{\sin \left(\frac{\pi }{3}\right)}$

(48) from which the switching times for each of the active vectors may be derived, such that they define the control signals that are applied to the switches in the generator side power converter 13, according to

(49) ${\tau }_a=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.\sin \left(\frac{\pi }{3}-\gamma \right)}{\frac{\pi }{3}\sin \left(\frac{\pi }{3}\right)}$${\tau }_b=\frac{.Math.\Delta {\Psi }_s^{*}\left(k\right).Math.\sin \left(\gamma \right)}{\frac{\pi }{3}\sin \left(\frac{\pi }{3}\right)}$${\tau }_0={\tau }_s-\left({\tau }_a+{\tau }_b\right)$

(50) where τ.sub.a is the time the first vector is applied (e.g. vector u1 in FIG. 4b) and τ.sub.b is the time the second vector is applied (e.g. vector u2 in FIG. 4b).

(51) Referring briefly to FIGS. 1, 4a and 7, the determined switching times are used as control signals by a switch control unit indicated by the PWM block 82. The PWM block 82 use the control signal for controlling the switches 52, 53 in the generator side power converter 13. By switching the states of the switches in the generator side power converter 13 it is possible to established stator electrical quantities such that a desired generator power level is achieved. More specifically, the control signals causes switches 52, 53 of the generator side power converter 13 to adjust the phase and voltage magnitude of its AC terminal voltage with respect to the EMF of the generator 11 in order to provide the desired electrical power.

(52) Voltage generated by the generator side power converter 13 is defined by the requirement of the flux controller. So switching has to be carried out to mitigate the error in the stator flux vector ΔΨ.sub.s*(k). This approach of flux vector control can be extended to any modulation index. During the normal space vector modulation range, the error can be compensated through switching in one sample.

(53) FIG. 6 illustrates a control system for controlling the power of a wind turbine generator according to an embodiment of the present invention.

(54) The power command to the converter control is compared with the estimated power provided by the generator 71. The mechanical dynamics of the system being slower allows the power controller 79 to be used directly to give the stator flux vector reference.

(55) The generator 71 does not require reactive power unless at very high speeds when field weakening is needed. The EM design caters to this aspect of the generator. Hence it is the active power requirement that drives the power controller at slower generator dynamics. The stator flux vector is still controlled in a similar manner as explained above. The switching carried out using the stator flux vector error is same as has been described earlier.

(56) The generator controller will be described with the aid of FIGS. 7a-c.

(57) The basic operation of the controller is as follows: The generator side inverter receives the power command Pref based on either a difference of a desired generator speed and an actual generator speed developed in partial load mode or a constant value based on environmental conditions in the full load mode. The controller operates in a fixed coordinate system of the generator stator.

(58) The controller operates on the basic principle that the electrical power output of a generator is
P.sub.e=ω*|ψ.sub.r|*|ψ.sub.s*|sin γ

(59) where: ω is the rotational speed of the generator; |ψ.sub.r| is the magnitude of flux provided by the permanent magnet of the rotor |ψ.sub.s| is the magnitude of the stator flux controlled by the PWM of the generator side inverter sin γ is the sine of a desired angle γ between flux generated by the permanent magnet of the rotor and stator flux controlled by the PWM of the generator side inverter

(60) There are no operations to set either a rotor current component or stator current component in quadrature with a flux vector.

(61) A power command, Pref, from load controllers (not shown) is fed to error detector 501 and compared to the system output power provided to the point of common connection. The output of detector 501 is passed through switch 502 to PI controller 504. Switch 502 is a protective device to remove the Pref command if, during a flux control process, stator currents were to exceed a predetermined value as sensed at block 512.

(62) The output of block 504 is fed through two gain blocks 505 that adjust the power command as a function of generator rotor speed and stator current and converts the power error signal into a required flux ref ψ.sub.PEM.sub._.sub.REF.

(63) ψ*.sub.PEM.sub._.sub.REF is then compared, at block 509, with an output flux value ψ.sub.PEM, that is proportional to real output power, to form a ψ.sub.PEM.sub._.sub.REF.sub._.sub.ERROR which after processing by PI controller 510 becomes ψ*.sub.PEM. Likewise ψ*.sub.MAG.sub._.sub.REF (from field weakening block 506) is compared to ψ.sub.MAG from block 520 to form a ψ.sub.MAG.sub._.sub.ERROR which after processing by PI controller 508 and the addition of ψ.sub.r becomes ψ*.sub.MAG. The logic contained in field weakening block 506 is shown in FIG. 10.

(64) The values of ψ.sub.PEM and ψ.sub.MAG are calculated in blocks 520 and 522 using a total stator flux ψ.sub.s (determined at block 528) using load angle delta (δ), the angle between a desired magnetization flux and desired flux responsible for power production. δ*.sub.load.sub._.sub.angle is determined at block 514 using the arctan(ψ*.sub.PEM/ψ*.sub.MAG) while the magnitude of the total stator flux |ψ*.sub.s| is also determined at block 514 as the square root of the sum of the squares of the flux components into block 514.

(65) Up to this point all signals have been DC values, there have been no transformations into rotating reference systems, nor have there been placement of torque producing current vectors in quadrature with identified rotor flux vectors.

(66) The stator flux vector, having a magnitude and a position, is developed in block 516 and corrected in block 518 for delays caused by the rotational speed of the generator and a need to sample at discrete time intervals. More specifically, and as shown in FIG. 8 the flux reference is adjusted by adjusting the time interval between k's using the delta theta signal fed to block 518.

(67) Field weakening is needed when there is a wind gust and speed of the rotor exceeds the nominal speed. The generated EMF increases leaving small voltage margin to for Generator power and stator flux control. It becomes essential to weaken the resultant air-gap field, so that the effective generated EMF is maintained to a desired value even if there is an increase in speed. FIG. 10 illustrates a flow diagram of field weakening method. The set maximum modulation index is used to compute the resultant magnetizing reference magnitude. Under non-field weakening or low speed conditions, the computed magnetizing reference is greater or equal to the rotor flux magnitude. At high speed, the computation result will be applied. The method also has the possibility to “strengthen” the field in case the magnets are weakened due to high temperatures.

(68) Predictive control to mitigate the phase error of the stator flux vector is achieved as shown in FIG. 8. The prediction is carried out in polar co-ordinates and generates the stator flux vector ψ.sub.ps*. The estimated stator flux vector ψ.sub.s, as shown is compared with the predicted reference stator flux vector and the error vector Δψ.sub.s defines the switching states for control of active power and stator flux vector in the stationary frame of reference.

(69) Principle of current limiting reference stator flux vector generation is shown in FIG. 9 The fact that no magnetization is needed for a rotor magnetized machine like surface mounted PM machine or a rotor fed synchronous machine, can be exploited to define the desired reference flux vector magnitude. FIG. 9 illustrates this. The current vector needed in such control is just to cater to the active power demand and not to set up any flux in the machine. Hence, the minimum current vector magnitude that can achieve this requirement should lie along a direction perpendicular to the rotor flux vector.

(70) If the machine has to be used as motor, the current vector should lead the rotor flux vector otherwise it should lag the rotor flux vector as shown in the figure. Hence, the component of the reference flux vector that contributes to the torque or active power can be derived directly with the information of this current vector location. This involves the input of the rotor flux vector location, which is available from the position and/or incremental encoder attached to the shaft of the machine. For generators with saliencies in the rotor structure, sensorless operation can be incorporated by measuring the voltage and currents thereby removing the need for a speed/position sensor. The advantage is the possibility for the controller to limit the current in the stationary reference frame. At very high operational speeds, it is possible to have a de-magnetizing component of the stator flux vector. Such component may also be needed when an interior PM machine is employed for power generation.

(71) The invention 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 invention, as defined by the appended patent claims.