Patent ID: 12244244

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Aspects of the present invention are particularly useful for pulse width modulation (PWM) of ANCP power converter of a wind turbine DFIG in fault mode. Accordingly, general concepts and operation of a wind turbine power system are described herein for an appreciation of this particular working embodiment of the invention. It should be understood, however, that the present disclosure is not limited to implementation with a power converter in a DFIG (or in a wind turbine power system in general).

Referring now to the drawings,FIG.1illustrates a perspective view of one embodiment of a wind turbine10. As shown, the wind turbine10generally includes a tower12extending from a support surface14, a nacelle16mounted on the tower12, and a rotor18coupled to the nacelle16. The rotor18includes a rotatable hub20and at least one rotor blade22coupled to and extending outwardly from the hub20. For example, in the illustrated embodiment, the rotor18includes three rotor blades22. However, in an alternative embodiment, the rotor18may include more or less than three rotor blades22. Each rotor blade22may be spaced about the hub20to facilitate rotating the rotor18to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, as will be described below, the rotor18may be rotatably coupled to an electric generator120(FIG.2) for production of electrical energy. One or more wind conditions, such as wind speed and/or wind direction may also be monitored via a wind sensor24, such as an anemometer, located on the nacelle16or any other suitable location near the wind turbine10.

Referring now toFIG.2, a schematic diagram of one embodiment of a wind turbine power system100(“wind turbine system”) is illustrated in accordance with aspects of the present disclosure. Although the present disclosure will generally be described herein with reference to the system100shown inFIG.2, those of ordinary skill in the art, using the disclosures provided herein, should understand that aspects of the present disclosure may also be applicable in other power generation systems, and, as mentioned above, that the invention is not limited to wind turbine systems.

In the embodiment ofFIG.2, the rotor18of the wind turbine10(FIG.1) may, optionally, be coupled to a gearbox118, which is, in turn, coupled to a generator120, which may be a doubly fed induction generator (DFIG). As shown, the DFIG120may be connected to a stator bus154. Further, as shown, a power converter162may be connected to the DFIG120via a rotor bus156, and to the stator bus154via a line side bus188. As such, the stator bus154may provide an output multiphase power (e.g., three-phase power) from a stator of the DFIG120, and the rotor bus156may provide an output multiphase power (e.g., three-phase power) from a rotor of the DFIG120. The power converter162may also include a rotor side converter (RSC)166and a line side converter (LSC)168. The DFIG120is coupled via the rotor bus156to the rotor side converter166. Additionally, the RSC166is coupled to the LSC168via a DC link136across which is at least one DC link capacitor138,140. As shown inFIG.3, the DC link136includes an upper capacitor138and a lower capacitor140. The LSC168is, in turn, coupled to a line side bus188.

The RSC166and the LSC168may be configured for normal operating mode in a three-phase, pulse width modulation (PWM) arrangement using insulated gate bipolar transistor (IGBT) switching elements, as will be discussed in more detail with respect toFIG.3.

In addition, the power converter162may be coupled to a converter controller174in order to control the operation of the rotor side converter166and/or the line side converter168as described herein. It should be noted that the converter controller174may be configured as an interface between the power converter162and a local wind turbine control system176and may include any number of control devices. In one embodiment, the controller174may include a processing device (e.g., microprocessor, microcontroller, etc.) executing computer-readable instructions stored in a computer-readable medium. The instructions when executed by the processing device may cause the processing device to perform operations, including providing control commands (e.g., switching frequency commands) to the switching elements of the power converter162. For an individual DFIG wind turbine power system100, the reactive power may be supplied primarily by the RSC166, via the generator120and the LSC168.

In typical configurations, various line contactors and circuit breakers including, for example, a grid breaker182may also be included for isolating the various components as necessary for normal operation of the DFIG120during connection to and disconnection from a load, such as the electrical grid184. For example, a system circuit breaker178may couple the system bus160to a transformer180, which may be coupled to the electrical grid184via the grid breaker182. In alternative embodiments, fuses may replace some or all of the circuit breakers.

In operation, alternating current power generated at the DFIG120by rotating the rotor18is provided to the electrical grid184via dual paths defined by the stator bus154and the rotor bus156. On the rotor bus side156, sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power is provided to the power converter162. The rotor side power converter166converts the AC power provided from the rotor bus156into direct current (DC) power and provides the DC power to the DC link136. As is generally understood, switching elements (e.g., IGBTs) used in the bridge circuits of the rotor side power converter166may be modulated to convert the AC power provided from the rotor bus156into DC power suitable for the DC link136.

In addition, the line side converter168converts the DC power on the DC link136into AC output power suitable for the electrical grid184. In particular, switching elements (e.g., IGBTs) used in bridge circuits of the line side power converter168can be modulated to convert the DC power on the DC link136into AC power on the line side bus188. The AC power from the power converter162can be combined with the power from the stator of DFIG120to provide multi-phase power (e.g., three-phase power) having a frequency maintained substantially at the frequency of the electrical grid184(e.g., 50 Hz or 60 Hz).

Additionally, various circuit breakers and switches, such as grid breaker182, system breaker178, stator sync switch158, converter breaker186, and line contactor172may be included in the wind turbine power system100to connect or disconnect corresponding buses, for example, when current flow is excessive and may damage components of the wind turbine power system100or for other operational considerations. Additional protection components may also be included in the wind turbine power system100.

Moreover, the power converter162may receive control signals from, for instance, the local control system176via the converter controller174. The control signals may be based, among other things, on sensed states or operating characteristics of the wind turbine power system100. Typically, the control signals provide for control of the operation of the power converter162. For example, feedback in the form of a sensed speed of the DFIG120may be used to control the conversion of the output power from the rotor bus156to maintain a proper and balanced multi-phase (e.g., three-phase) power supply. Other feedback from other sensors may also be used by the controller174or control system176to control the power converter162, including, for example, stator and rotor bus voltages and current feedbacks. Using the various forms of feedback information, switching control signals (e.g., gate timing commands for IGBTs), stator synchronizing control signals, and circuit breaker signals may be generated.

The power converter162also compensates or adjusts the frequency of the three-phase power from the rotor for changes, for example, in the wind speed at the hub20and the blades22. Therefore, mechanical and electrical rotor frequencies are decoupled, and the electrical stator and rotor frequency matching is facilitated substantially independently of the mechanical rotor speed.

Under some states, the bi-directional characteristics of the power converter162, and specifically, the bi-directional characteristics of the LSC168and RSC166, facilitate feeding back at least some of the generated electrical power into generator rotor. More specifically, electrical power may be transmitted from the stator bus154to the line side bus188and subsequently through the line contactor172and into the power converter162, specifically the LSC168which acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into the DC link136. The capacitor138facilitates mitigating DC link voltage amplitude variations by facilitating mitigation of a DC ripple sometimes associated with three-phase AC rectification.

The DC power is subsequently transmitted to the RSC166that converts the DC electrical power to a three-phase, sinusoidal AC electrical power by adjusting voltages, currents, and frequencies. This conversion is monitored and controlled via the converter controller174. The converted AC power is transmitted from the RSC166via the rotor bus156to the generator rotor. In this manner, generator reactive power control is facilitated by controlling rotor current and voltage.

Referring now toFIG.3, a detailed, schematic diagram of one embodiment of the three-level neutral point clamped (NPC) voltage source power converter (i.e., a converter where output voltage has three possible values) shown inFIG.2is illustrated for sake of discussion and an understanding of the operating principles of the present disclosure.

As shown, the RSC166includes a plurality of bridge circuits and at least one clamping diode155with each phase of the rotor bus156input to the rotor side converter166being coupled to a single bridge circuit. In addition, the LSC168may also include a plurality of bridge circuits and at least one clamping diode155. Similar to the rotor side converter166, the line side converter168also includes a single bridge circuit for each output phase of the line side converter168. In other embodiments, the line side converter168, the rotor side converter166, or both the line side converter168and the rotor side converter166may include parallel bridge circuits without deviating from the scope of the present disclosure.

Moreover, as shown, each bridge circuit may generally include a plurality of switching elements (e.g., IGBTs) coupled in series with one another. For instance, as shown inFIG.3, the plurality of switching devices of each bridge circuit may be arranged in a neutral point clamped (NPC) topology. As described herein, an NPC topology generally refers to a topology containing two series-connected high-side switches and two series-connected low-side switches. Often, IGBTs with anti-parallel diodes are used as the switches for an NPC converter, but other two-quadrant switch configurations can also be employed. In particular, as shown inFIG.3, the RSC166may include a plurality of first IGBTs212and the LSC168may include a plurality of second IGBTs214. In addition, as shown, a diode216may be coupled in parallel with each of the IGBTs212,214. As is generally understood, the LSC168and the RSC166may be controlled, for instance, by providing control commands, using a suitable driver circuit, to the gates of the IGBTs. For example, the converter controller174may provide suitable gate timing commands to the gates of the IGBTs of the bridge circuits. The control commands may control gate timing commands of the IGBTs to provide a desired output. It should be appreciated by those of ordinary skill in the art that, as an alternative to IGBTs, the power convertor162may include any other suitable switching elements.

Three-level neutral point clamped (NPC) converters must take less voltage stress because of the series connection arrangement compared to two-level converters. However, due to large commutation loops, inner switching devices of the power converter experience higher voltage spikes and demands to have lossy snubbers in place. To avoid snubbers, active neutral point clamped (ANPC) converters with active clamped switches offer shorter commutation loops compared to NPC. Further, double dead time PWM techniques applied to ANPC offer lesser voltage stress on the inner switching devices. In line with ANPC converters, it is important to detect device failure quickly and assign a failure response mode instantly to protect the DC link and avoid chain reaction failure on other devices, and to provide for continued “fault tolerant” operation of the power converter.

Referring now toFIG.4, a schematic diagram of a single-phase main circuit210of a switching branch of an ANPC power converter is illustrated in accordance with aspects of the present subject matter. The switching branch may be part of the rotor side converter166or the line side converter168inFIG.3. Furthermore, as mentioned, the main circuit210has an active neutral point clamped (ANPC) topology, which generally refers to a topology having six diodes (e.g., D1to D6) and six controllable semiconductor switches (e.g., S1to S6). A converter comprising one or more switching branches, like that ofFIG.4, may operate as a rectifier or as an inverter. In addition, it should be understood that each phase of the rotor bus156or the stator bus188may be coupled to a circuit with the same topology as the main circuit210.

Still referring toFIG.4, the plurality of the switching devices S1-S6(e.g., IGBTs) are coupled together with a diode D1-D6coupled in parallel with each of the IGBT switches. As shown, each leg of the converter includes two outer switching devices S1, S4and four inner switching devices S2, S3, S5, and S6. The line side converter168and the rotor side converter166are controlled, for instance, by providing control commands (“gate drive signals”), using a suitable driver circuit, to the gates of the IGBTs S1-S6. For example, in an embodiment, the controller174can provide suitable gate timing commands to the gates of the IGBTs to control the pulse width modulation of the IGBTs in order to produce a desired output. In one embodiment, the main circuit210may be controlled by the controller174according to a substantially non-interleaved switching pattern such that the switching elements of the main circuit210are switched in phase with one another. In other embodiments, the main circuit210may be controlled according to any other suitable switching pattern. It will be appreciated by those of ordinary skill in the art that other suitable switching elements can be used in place of IGBTs.

Input voltages V1and V2are controlled to each have a voltage equal to Vdc/2, where Vdc is the total DC link voltage. Voltage V3P is the phase A output voltage measured with respect to a center point of DC link. The potential difference between VP and V3is the pole voltage of the controller. Switching device S1is complementary to switching device S3so that, when switching device S1is gated on, switching device S3is gated off and vice versa. Similarly, switching devices S2and S4are complementary.

In operation, each leg of the ANPC three-level converter has three switching stages. In the first switching stage, switching devices S1, S2are turned on, S5and S6are turned off, and S3and S4are turned off. Assuming a stable operation, V1=V2=Vdc/2, and V3becomes Vdc/2. In the second switching stage, switching devices S2, S3are turned, S1and S4are turned off, and S5and S6are turned on. In this stage, V3is equal to zero. In the third switching stage, switching devices S1, S2are turned off, S5and S6are turned off, and S3and S4are turned on. This results in V3becoming −Vdc/2. Thus, it can be seen that the phase voltage V3has three levels Vdc/2, −Vdc/2 and 0. When all three legs of the ANPC three-phase converter are combined, then the resulting line to line voltages have five levels namely Vdc, Vdc/2, 0, −Vdc/2 and −Vdc. The three-level converter ofFIG.3may be increased to any level depending on the circuit topology and number of switching devices and diodes in the circuit. As the number of levels in the converter increases, the output waveform of the converter approaches a pure sine wave, resulting in lower harmonics in the output voltage. In general, the number of switching stages can be higher than three as switching devices may not be gated on if the corresponding free-wheeling diode is going to conduct current. This operation mode does not affect the number of levels of the output phase voltage.

In a three level ANPC converter, a key failure mode exists when an inner switching device (e.g., S2, S3, S5, S6) fails short. Under this condition, one half of the DC link, V1or V2inFIG.4, is charged to the peak line-to-line voltage of the machine or grid side voltage. When S1fails, the inner switching device S2is susceptible to full DC link voltage. These values are typically higher than the maximum allowed blocking voltage of the switching devices and the capacitors. In this case, other switching devices or clamping diodes may be stressed in terms of voltage or current beyond their capability. Hence, this will cause additional switching devices, in particular switching devices in phase legs connected to the same DC bus, to fail after the failure of the initial switching device.

Suitable logic circuits are known in the art to detect failures in the switching device and prevent secondary damage to the multi-level power converter. An example of a suitable failure-detection method is described, for example, in U.S. patent application Ser. No. 17/534,507 filed on Nov. 24, 2021.

It should be appreciated that the present methods and systems are not dependent upon actual detection of a failed switching device. As explained below, the on/off scheme for the switching devices may be implemented regardless of whether or not a switching device has actually failed in the converter. However, it is also within the scope of the present methods and systems that the on/off scheme is for the converter is not implemented until a switching device has actually failed.

FIG.6presents a block diagram of aspects of the current methodology300for providing continuous fault-tolerant operation of the converter ofFIG.4when one of the outer switches S1or S4has failed in a shorted condition.

At step310, the method includes determining an on/off scheme for the switching devices in the first converter that provides for continuous fault-tolerant operation of the first converter. This scheme may be predetermined and stored in the controller for subsequent retrieval by the controller as the operational scheme for the controller regardless of whether or not a switch has failed. Alternatively, the scheme may be determined or selected at the time of detecting that the specific switch has failed. The on/off scheme is implemented (via gate drive signals from the controller) upon change of the phase current direction through the first converter in the zero voltage (pole voltage) state of the first converter. However, basing this transition on directly sensing the phase current via a current sensor may not be reliable since the current is already near its zero crossing and it is difficult to detect the exact instance in time where the current crosses into the other polarity.

Accordingly, at step3200, the method detects the pole voltage of the first converter via, for example, a voltage sensor (sensor175inFIG.4) that is configured to directly sense the pole voltage across the converter. Alternatively, pole voltage information can also come from the DESAT circuitry of the switching devices. The pole voltage switches essentially instantaneously from zero to +−vdc/2 when current polarity changes in a constant zero voltage state. Since the pole voltage change is relatively large in magnitude, it is easier and more reliable to detect with a voltage sensor and is an accurate indirect indication of the current polarity reversal.

At step330, based on the sensed voltage change, the method implements the switch scheme that protects the converter from a cascading failure and enables continued fault-tolerant operation of the converter.

FIGS.5athrough5cdepict aspects of the present method and apparatus related to a single bridge circuit210(similar to the bridge circuit ofFIG.4) of a multi-level ANCP converter. It should be appreciated that the discussion pertains to each circuit or level in a multi-level converter, such as the three-level rotor-side and line-side converters of the power converter depicted inFIG.3.

InFIG.5A, the circuit includes six switches S1-S6with associated parallel diodes D1-D6. All of the switches are functional. Phase current has switched direction through the circuit and is indicated by the heavy directional line in the zero-voltage state of the circuit, wherein all four inner switches S2, S3, S5, S6are all “on” or closed and the outer switches S1, S4are “off” or open.

InFIG.5B, the outer switch S1has failed in a shorted state. Upon closure of the inner switches S2, S3, S5, S6for phase current reversal in the zero-voltage state of the circuit, an instantaneous direct current path is established from the positive voltage side of the circuit through the DC link that couples the converter to a second converter, for example in the type of power converter depicted inFIG.3. This is referred to as a “shoot-through” event. Such events can lead to a cascading failure of the converter components.

Referring toFIG.5c, the shoot-through event can be avoided by selecting only the switching devices in the circuit that are required for the direction of phase current at that particular time. This strategy or “on/off scheme” for the switches is termed in the art as skip-fire PWM, which selects the necessary switching vector based on the direction phase current. When the current switches phase direction, the switching vector needs to be changed accordingly to implement the skip-fire PWM scheme wherein switches S5, S3are “off” or open and switches S2, S6are “on” or closed.

Although not depicted inFIGS.5a-5c, the failed switch may be the opposite outer switch S4. In this case, the skip-fire PWM scheme would have switches S5, S3“on” or closed and switches S2, S6“off” or opened.

Further aspects of the disclosure are provided by the subject matter of the following clauses:

Clause 1: A controller-implemented method for operating a multi-level bridge power converter of an electrical power system in a fault-tolerant operational mode, the electrical power system connected to a power grid and the multi-level power converter comprising a first converter coupled to a DC link, the first converter comprising a plurality of switching devices, the method comprising: determining an on/off scheme for the switching devices in the first converter that provides for continuous fault-tolerant operation of the first converter, wherein the on/off scheme is dependent upon phase current direction through the first converter; and indirectly determining the phase current direction by sensing a pole voltage of the first converter and implementing the on/off scheme in the fault-tolerant mode when the pole voltage changes from zero to a +/−value indicating that the phase current through the first converter has switched direction.

Clause 2: The method according to clause 1, wherein the plurality of switching devices in the first converter includes at least four inner switching devices and at least two outer switching devices in an active neutral point clamped topology, wherein the first converter connects to the DC-link by turning on the inner switching devices, and wherein in the fault-tolerant operational mode one of the outer switching devices is failed in the shorted state and the on/off scheme prevents a current shoot-through across the DC-link when connecting the first converter to the DC-link.

Clause 3: The method according to any one of clauses 2-3, wherein the switching devices are insulated gate bipolar transistors (IGBTs) and designated as a first IGBT through a sixth IGBT, wherein the second IGBT, the third IGBT, the fifth IGBT, and the sixth IGBT are the inner switching devices, and the first IGBT and the fourth IGBT are the two outer switching devices, wherein in the on/off scheme:the first IGBT is failed in the shorted state, the second IGBT and the sixth IGBT are on; and the fifth IGBT and the third IGBT are off; orthe fourth IGBT is failed in the shorted state, the third IGBT and the fifth IGBT are on; and the second and the sixth IGBT are off.

Clause 4: The method according to any one of clauses 1-3, wherein the pole voltage is directly sensed by a voltage sensor.

Clause 5: The method according to any one of clauses 1-4, wherein the first converter is coupled to a second converter via the DC-link, the second converter comprising a plurality of the switching devices, wherein the method functions to provide operation in the fault tolerant mode of the second converter when one of the switching devices in the second converter has failed in a shorted state.

Clause 6: The method according to any one of clauses 1-5, wherein the electrical power system is a wind turbine power system and includes a generator connected to the power grid.

Clause 7: A controller-implemented method for operating a multi-level bridge power converter of an electrical power system in a fault-tolerant operational mode, the electrical power system connected to a power grid and the multi-level power converter comprising a first converter and a second converter coupled together via a DC link, each of the first and second converters comprising a plurality of switching devices, wherein one of the switching devices in the first or second converter is failed in a shorted state, the method comprising:determining an on/off scheme for the switching devices in the first and second converters that provides for continuous fault-tolerant operation of the respective converter when one or more of the switching devices in the respective converter has failed in a shorted state, wherein the on/off scheme is dependent upon phase current direction through the respective converter; andindirectly determining the phase current direction through the respective converter by sensing a pole voltage of the respective converter and implementing the on/off scheme when the pole voltage changes from zero to a +/−value indicating that the phase current through the respective converter has switched direction.

Clause 8: The method according to clause 7, wherein the plurality of switching devices in each of the first and second converters includes at least four inner switching devices and at least two outer switching devices in an active neutral point clamped topology, wherein the first and second converters connect to the DC-link by turning on the inner switching devices, and wherein in the fault-tolerant operational mode one of the outer switching devices is failed in the shorted state and the on/off scheme prevents a current shoot-through across the DC-link when connecting the first or second converter to the DC-link.

Clause 9: The method according to any one of clauses 7-8, wherein the switching devices are insulated gate bipolar transistors (IGBTs) and designated as a first IGBT through a sixth IGBT, wherein the second IGBT, the third IGBT, the fifth IGBT, and the sixth IGBT are the inner switching devices, and the first IGBT and the fourth IGBT are the two outer switching devices, wherein in the on/off scheme:the first IGBT is failed in the shorted state, the second IGBT and the sixth IGBT are on; and the fifth IGBT and the third IGBT are off; orthe fourth IGBT is failed in the shorted state, the third IGBT and the fifth IGBT are on; and the second and the sixth IGBT are off.

Clause 10: The method according to any one of clauses 7-9, wherein the pole voltage is sensed directly by a voltage sensor.

Clause 11: A multi-level bridge power converter with a plurality of phase legs, comprising:a first converter and a second converter coupled together via a DC link, each of the first and second converters comprising a plurality of switching devices;a controller configured to operate the first and second converters in a fault-tolerant mode when one of the switching devices in the first or second converter is failed in a shorted state, the controller configured to:implement an on/off scheme for the switching devices in the first and second converters that provides for continuous fault-tolerant operation of the respective converter when one or more of the switching devices in the respective converter has failed in a shorted state, wherein the on/off scheme is dependent upon phase current direction through the respective converter;indirectly determine the phase current direction through the respective converter by sensing a pole voltage of the respective converter; andimplement the on/off scheme when the pole voltage changes from zero to a +/−value indicating that the phase current through the respective converter has switched direction.

Clause 12: The multi-level bridge power converter according to clause 11, wherein the plurality of switching devices in each of the first and second converters comprises at least four inner switching devices and at least two outer switching devices in an active neutral point clamped topology, wherein the first and second converters connect to the DC-link by the controller turning on the inner switching devices, and wherein in the fault-tolerant operational mode one of the outer switching devices is failed in the shorted state and the on/off scheme implemented by the controller prevents a current shoot-through across the DC-link when connecting the first or second converter to the DC-link.

Clause 13: The multi-level bridge power converter according to any one of clauses 11-12, wherein the switching devices are insulated gate bipolar transistors (IGBTs) and designated as a first IGBT through a sixth IGBT, wherein the second IGBT, the third IGBT, the fifth IGBT, and the sixth IGBT are the inner switching devices, and the first IGBT and the fourth IGBT are the two outer switching devices, wherein in the on/off scheme of the fault-toleration operation:the first IGBT is failed in the shorted state, the second IGBT and the sixth IGBT are on; and the fifth IGBT and the third IGBT are off; orthe fourth IGBT is failed in the shorted state, the third IGBT and the fifth IGBT are on; and the second and the sixth IGBT are off.

Clause 14: The multi-level bridge power converter according to clauses 11-13, further comprising a voltage sensor configured to detect the pole voltage of the respective converter, the voltage sensor in communication with the controller.

Clause 15: An electrical power system configured to supply electrical power to a power grid, the electrical power system comprising the multi-level power converter according to clauses 11-14.

Clause 16: The electrical power system according to clause 15, wherein the electrical power system comprises a wind turbine generator configured with the multi-level power converter.

This written description uses examples to disclose the disclosure, including the best mode, and to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.