Turboexpander system with low-voltage ride-through control architecture

Abstract

The generator includes a turbine wheel configured to receive process gas and rotate in response to expansion of the process gas entering through an inlet and exiting through an outlet, a rotor coupled to the turbine wheel and configured to rotate therewith, and a stationary stator, the generator generating alternating current upon rotation of the rotor within the stator. The generator is capable of supplying power to a power grid. During a low voltage event, current from the generator can be shunted to a brake resistor assembly. The brake resistor assembly can include a brake resistor designed to operate the generator during a low voltage event.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 815,865, filed July 28, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a turboexpander system with a low voltage ride-through control architecture. [Background technology]

[0003] Natural gas is one of the primary energy sources for many of our daily needs and activities. Its abundance and relative cleanliness make it an attractive fossil fuel. Hydrogen is a rapidly expanding global energy storage market. Hydrogen is used in many manufacturing processes, from oil refining to food processing. Hydrogen is also used as a fuel source for gas turbines and in a wide range of fuel cells for power generation in the industrial and commercial transportation sectors.

[0004] Large, complex transmission systems are required to efficiently and effectively move this gas from production areas to consumption areas. Natural gas transported through pipelines moves under high pressure within the pipelines. Summary of the Invention

[0005] Aspects of the embodiments include an apparatus including a generator including a turbine wheel configured to receive a process gas and rotate in response to expansion of the process gas entering through an inlet and exiting through an outlet, a rotor coupled to the turbine wheel and configured to rotate therewith, and a stationary stator, the generator generating alternating current upon rotation of the rotor within the stator. The apparatus also includes a brake resistor assembly including a brake resistor, a switching component that causes current generated by the generator to flow through the brake resistor assembly upon detection of a low voltage event, and a low voltage detection circuit that detects voltage and controls the switching component upon detection of a low voltage event.

[0006] Aspects of an embodiment include a method including: flowing gas through a turbine wheel of a generator downstream of a flow control valve; generating an electric current by the generator based on rotation of the turbine wheel by the gas flow; directing the electric current generated by the generator to a power grid; detecting a low voltage event on the power grid; and based on the detection of the low voltage event, directing the electric current generated by the generator to a brake resistor assembly.

[0007] An aspect of the embodiment includes a system including a generator. The generator includes a turbine wheel configured to receive a process gas and rotate in response to expansion of the process gas flowing through an inlet and an outlet, a rotor coupled to the turbine wheel and configured to rotate with the turbine wheel, and a stationary stator, the generator generating alternating current through rotation of the rotor within the stator. The system includes a brake resistor assembly including a brake resistor, a switching component configured to cause current generated by the generator to flow through the brake resistor assembly upon detection of a low-voltage event, and a low-voltage detection circuit configured to detect voltage and control the switching component upon detection of the low-voltage event. A bidirectional inverter circuit receives alternating current (AC) from the generator and converts the AC to direct current (DC), and an active front-end (AFE) rectifier receives DC from the bidirectional inverter circuit and converts the DC to AC with a frequency and amplitude compatible with a power grid.

[0008] In one embodiment, the switching component comprises a static switch that connects the brake resistor assembly to the electrical output of the generator when the low voltage event is detected and that disconnects the brake resistor assembly from the electrical output of the generator when completion of the low voltage event is detected.

[0009] In one embodiment, the brake resistor comprises a peak kilowatt rated resistor.

[0010] In one embodiment, the brake resistor assembly is co-located with the generator.

[0011] In one embodiment, the switching component includes a speed-controlled, current-regulated alternating current / direct current (AC / DC) rectifier that derives current from the electrical output of the turboexpander when the low-voltage event is detected and electrically isolates the brake resistor assembly when the low-voltage event is completed.

[0012] In one embodiment, the brake resistor comprises a continuous rated brake resistor.

[0013] In one embodiment, the switching component includes an AC isolation contactor for electrically isolating the brake resistor assembly from the electrical output of the generator.

[0014] In one embodiment, the power electronics include a bidirectional inverter circuit that receives alternating current (AC) from the electrical output of the generator and converts the AC to direct current (DC), and an active front-end (AFE) rectifier that receives DC from the bidirectional inverter circuit on a DC bus and converts the DC to AC having a frequency and amplitude compatible with a power grid.

[0015] In one embodiment, the power electronics includes the switching component and the brake resistor assembly, the switching component connected to an output of the bidirectional inverter circuit on the DC bus.

[0016] In one embodiment, the bidirectional inverter circuit includes a speed-controlled, current-regulated alternating current / direct current (AC / DC) rectifier that directs current to the brake resistor assembly when the low-voltage event is detected and to the AFE rectifier after the low-voltage event ends.

[0017] In one embodiment, a DC isolation contactor is between the bidirectional inverter circuit and the brake resistor assembly.

[0018] In one embodiment, a static switch is present between the bidirectional inverter circuit and the brake resistor assembly, the static switch electrically connecting the bidirectional inverter circuit to the brake resistor assembly when the low voltage event is detected and disconnecting the bidirectional inverter circuit from the brake resistor assembly when the low voltage event is no longer detected.

[0019] An embodiment includes detecting that the low voltage event has ended; and detecting that the low voltage event has ended and directing the current generated by the generator to the power grid.

[0020] Some embodiments include closing a circuit between the electrical output of the generator and the brake resistor assembly based on the detection of the low voltage event.

[0021] Some embodiments include maintaining gas flow through the turbine wheel during a low voltage event.

[0022] Some embodiments include converting the electrical current produced by the generator to an electrical current having a frequency and amplitude compatible with the power grid.

[0023] Some embodiments further include a speed-controlled, current-regulated alternating current / direct current (AC / DC) rectifier electrically connected between the generator and the brake resistor assembly.

[0024] In one embodiment, the switching component comprises a static switch that is activated when the low voltage event is detected and deactivated when completion of the low voltage event is detected. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a schematic diagram of a power generation system connected to a power grid in accordance with an embodiment of the present disclosure. [Figure 2]FIG. 1 is a schematic diagram of an exemplary turboexpander system including a low-voltage ride-through control architecture according to an embodiment of the present disclosure. [Figure 3] FIG. 1 is a schematic diagram of another example turboexpander system including a low-voltage ride-through control architecture according to an embodiment of the present disclosure. [Figure 4] FIG. 1 is a schematic diagram of another example turboexpander system including a low-voltage ride-through control architecture according to an embodiment of the present disclosure. [Figure 5] FIG. 1 is a process flow diagram for operating a turboexpander generator during a low voltage ride-through event according to an embodiment of the present disclosure. [Figure 6] FIG. 1 is a process flow diagram for initiating a low voltage ride-through procedure according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0026] Like reference symbols in different drawings indicate like elements. The drawings are not to scale.

[0027] Natural gas, hydrogen, and other process gases are pressurized to facilitate efficient transportation through pipelines that can be many miles long. Pipelines transport gas, for example, from production sites (e.g., wells) to processing facilities, from processing facilities to local distribution networks (such as regional, city, or district networks), or to on-site industrial plant networks. To safely distribute and use the gas through the local distribution network, the process gas is reduced to a lower level (often using pressure regulators). The pressure is reduced at pressure letdown (PLD) stations and distributed to industrial, commercial, and residential end users. PLD stations use control valves to achieve the required pressure reduction, but the process generates a significant amount of waste energy. Additional control valves may be used elsewhere for pressure control, such as in the pipeline between the production facility and the processing facility, within the processing facility subprocess, and within the end-user process or piping. Turboexpander generators can be installed in parallel with the control valves to recover the waste energy from pressure reduction and generate electricity. The electricity can be channeled to the power grid or elsewhere. Similarly, a turboexpander generator can be installed at a gas storage facility upstream of one or more storage tanks to recover high-pressure gas discharged from a tank truck via the turboexpander generator into the storage tank. Turboexpander generators are also relevant in other applications, such as hydrogen liquefaction processes, in which cooled, pressurized gaseous hydrogen is expanded to a liquid state. The expansion can be performed through the turboexpander generator, and waste energy from the expansion can be recovered to generate electricity. As noted above, the electricity can be routed to a power grid or elsewhere, for example, to power compressors or other components of the liquefaction process. In either example, by recovering energy lost from applying reduced pressure to the natural gas and hydrogen, the turboexpander can simultaneously generate electricity and reduce CO2 emissions, increase overall plant efficiency, offset electricity bills, and generate additional revenue.

[0028] The power grid that the turboexpander can supply power to (and draw power from) can be a national or regional power grid, a city or district local power grid, or a small-scale or microgrid such as an on-site grid (e.g., at or near an industrial plant).

[0029] A fault condition on the power grid can lead to a low-voltage event. The turboexpander 102 can include circuitry that facilitates operation of the turboexpander 102 during a low-voltage event. The ability of the turboexpander 102 to operate during a low-voltage event can be referred to as low-voltage ride-through (LVRT). LVRT provides the ability to maintain connection and operate during momentary or prolonged low-voltage conditions or power grid outages. One cause of a low-voltage event can be a temporary loss of grid input. LVRT is also referred to as fault ride-through (FRT) or undervoltage ride-through (UVRT). This disclosure describes a turboexpander system that includes electrical components that allow the turboexpander to continue operating during a low-voltage event while simultaneously protecting the turboexpander from damage due to rotor overspeed. Overspeed can lead to a shutdown of the turboexpander. By preventing rotor overspeed, the turboexpander avoids shutdown and allows the rotor to continue rotating, allowing it to quickly supply power to the grid after a low-voltage event. Power from the turboexpander can be routed to a brake resistor assembly or other power sink to make up for load losses on the grid.

[0030] The active front end (AFE) temporarily disconnects the turboexpander from the grid during an undervoltage event. To maintain operation during an undervoltage event, the turboexpander's electrical output is connected to a brake resistor assembly, preventing overspeed and shutdown during an undervoltage event. By maintaining rotation of the rotor within the stator during an undervoltage event, the turboexpander can quickly reconnect to the network with minimal downtime after the undervoltage event has ended.

[0031] FIG. 1 is a schematic diagram of a power generation system 100 connected to a power grid 140 in accordance with an embodiment of the present disclosure. The power generation system 100 may be added to a PLD station to capture energy from gas expansion from the PLD process, or to any of the other applications described above. The power generation system 100 includes a turboexpander 102 in parallel with a pressure control valve 130. The turboexpander 102 is axially positioned such that the turboexpander 102 is mounted in-line with piping. The turboexpander 102 functions as a generator by generating electrical energy from rotational kinetic energy obtained from the expansion of gas through a turbine wheel 104. For example, rotation of the turbine wheel 104 can be used to rotate a rotor 108 within a stator 110 to generate electrical energy.

[0032] FIG. 1 is a schematic diagram of a power generation system 100 connected to a power grid 140 in accordance with an embodiment of the present disclosure. The power generation system 100 can be added to a PLD station to capture energy from the expansion of gas from the PLD process. The power generation system 100 includes a turboexpander 102 in parallel with a pressure control valve 130. The turboexpander 102 is axially positioned such that the turboexpander 102 is mounted in-line with piping. The turboexpander 102 functions as a generator by generating electrical energy from the rotational kinetic energy obtained from the expansion of gas through a turbine wheel 104. For example, the rotation of the turbine wheel 104 can be used to rotate a rotor 108 within a stator 110, generating electrical energy.

[0033] The turboexpander 102 includes a high-performance, high-speed permanent magnet generator that integrates a radial expansion turbine wheel 104 with low-loss active magnetic bearings (AMBs) 116a,b. The rotor assembly consists of a turbine wheel 104 and permanent magnet sections mounted directly to a rotor hub. The rotor 108 is levitated by the magnetic bearing system, which provides a frictionless (or nearly frictionless) interface between the dynamic and static components. The AMBs 116a,b facilitate lossless (or nearly lossless) rotation of the rotor 108.

[0034] The turboexpander 102 includes a high-performance, high-speed permanent magnet generator that integrates a radial expansion turbine wheel 104 with low-loss active magnetic bearings (AMBs) 116a,b. The rotor assembly consists of a turbine wheel 104 and permanent magnet sections mounted directly to a rotor hub. The rotor 108 is levitated by the magnetic bearing system, which provides a frictionless (or nearly frictionless) interface between the dynamic and static components. The AMBs 116a,b facilitate lossless (or nearly lossless) rotation of the rotor 108.

[0035] The turboexpander 102 is designed to allow process gas 120 to flow through the system, cooling the generator section and eliminating the need for auxiliary cooling equipment. The turboexpander's power electronics 118, in one embodiment, integrates a variable speed drive (VSD) 166 and a magnetic bearing controller (MBC) 168 into a single cabinet. The VSD enables consistent, clean transport of generated power from the turboexpander 102 to the power grid 140. The VSD 166 adjusts the frequency and amplitude of the generated current to match the local grid. After expansion, the gas exits the turboexpander 102 along the same axial path for downstream processing.

[0036] The turboexpander 102 includes a flow-through configuration. The flow-through configuration allows process gas to flow from an inlet side of the turboexpander 102 to an outlet side of the turboexpander 102. The gas enters through a radial gas inlet 154 to the turbine wheel 104 and a gas outlet 156 from the turbine wheel 104. The gas then passes through a generator and exits through outlet 156, where the gas rejoins the gas pipeline 170. Generally, the high-pressure process gas 120 is directed into the turboexpander 102 through a flow control system 126. The flow control system 126 includes flow or mass control valves and emergency shut-off valves. In an embodiment, the turboexpander housing 112 is sealed.

[0037] The high-pressure process gas 120 is expanded by passing through the turbine wheel 104, resulting in a pressure reduction of the process gas 120. Low-pressure process gas 128 exits the turboexpander. The expansion of the high-pressure process gas 120 through the turbine wheel 104 causes the turbine wheel 104 to rotate, which in turn rotates the rotor 108. The rotation of the rotor 108 within the stator 110 generates electrical energy. The turboexpander 102 achieves the desired pressure reduction and extracts energy from the reduced pressure to generate electricity. A pressure control valve 130, such as a conventional pressure regulator, may be installed in parallel with the turboexpander 102. The pressure control valve 130 may be used to control the pressure of the high-pressure process gas 120 passing through the turboexpander. Excess high-pressure process gas not routed to the turboexpander may be routed through the pressure control valve 130.

[0038] In one embodiment, a heater 122 can heat the high-pressure process gas 120 before passing the gas to the turboexpander 102. For example, if the expansion of the gas through the turbine wheel 104 reduces the temperature of the process gas and moisture in the gas may freeze at the turbine wheel or at other downstream locations in the pipeline, the pressurized process gas 120 may be heated by the heater 122. The heated high-pressure process gas 124 may then be directed to the turboexpander 102. Heating the process gas can prevent moisture from freezing as the gas expands and reduces its temperature.

[0039] The turboexpander 102 includes a turbine wheel 104. The turbine wheel 104 is shown as a radial-flow turbine wheel, although other configurations, such as an axial-flow turbine wheel, are within the scope of this disclosure. In this example, heated high-pressure process gas 124 is received from an inlet conduit 150 in the housing 112 and enters a radially oriented inlet 154 of the turbine wheel 104. In certain embodiments, the fluid flows through the inlet conduit 150 and is diverted by a flow diverter to the radial inlet 154, which directs the flow to the radial inlet of the turbine wheel 104. After expansion, the low-pressure process gas exits the turbine wheel 104 through an axially oriented outlet 156 and passes to an outlet conduit 152 in the housing 112.

[0040] The turbine wheel 104 may be directly secured to the rotor 108 or an intermediate common shaft, for example, by fasteners, a rigid drive shaft, welding, or other methods. For example, the turbine wheel 104 may be received at one end of the rotor 108 and held to the rotor 108 by a shaft. The shaft is threaded into the rotor 108 at one end and captures the turbine wheel 104 at the other end between one end of the rotor 108 and a nut threaded onto the shaft. The turbine wheel 104 and rotor 108 are connected without a gearbox and rotate at the same speed. In other examples, the turbine wheel 104 may be indirectly connected to the rotor 108, for example, by a gear train, a clutch mechanism, or other methods.

[0041] The turbine wheel 104 includes a plurality of turbine wheel blades 106 that extend outward from a hub and react with the expanding process gases to rotate the turbine wheel 104. Figure 1 illustrates an unshrouded turbine wheel in which the turbine blades 106 each have an exposed, generally radially oriented blade tip that extends between a radial inlet 154 and an axial outlet 156. As described in more detail below, the blade tips are substantially sealed against a shroud 114 inside a housing 112. In certain cases, the turbine wheel 104 is a shrouded turbine wheel.

[0042] In a configuration with an unshrouded turbine wheel 104, the housing 112 includes an inwardly facing shroud 114 that is in close proximity to the turbine wheel blades 106 but generally does not contact the turbine wheel blades 106 during operation. The close proximity of the turbine wheel blades 106 and the shroud 114 substantially blocks the passage of process gas between the turbine wheel blades 106 and the shroud 114 as the process gas flows through the turbine wheel 104. While some amount of process gas may leak or pass between the turbine wheel blades 106 and the shroud 114, this leakage is not significant to the operation of the turbine wheel 104. In certain cases, leakage may be comparable to other similar unshrouded turbine / shroud surface interfaces using conventional tolerances between the turbine wheel blades 106 and the shroud 114. The amount of acceptable leakage may be predetermined. Optimizing the operating parameters of the turbine generator can reduce leakage. In an embodiment, the housing 112 is sealed to prevent process gases from leaking out of the radial inlet 154 of the turbine wheel 104 .

[0043] The shroud 114 may be spaced a predetermined distance from the turbine wheel blades 106 and is maintained at that distance from the turbine wheel blades 106 during operation of the turboexpander 102 using a magnetic positioning device including active magnetic bearings and position sensors.

[0044] The bearings 116a and 116b are positioned to rotatably support the rotor 108 and turbine wheel 104 relative to the stator 110 and shroud 114. The turbine wheel 104 is supported in a cantilevered manner by the bearings 116a and 116b. In embodiments, the turbine wheel 104 may be supported in a non-cantilevered manner, and the bearings 116a and 116b may be positioned on the outlet side of the turbine wheel 104. In certain cases, one or more of the bearings 116a or 116b may include a ball bearing, a needle bearing, a magnetic bearing, a foil bearing, a journal bearing, or the like.

[0045] Bearings 116a and 116b may be a combination radial and thrust bearing that provides radial and axial support for rotor 108. Other configurations are also possible. Bearings 116a and 116b do not have to be the same type of bearing.

[0046] In embodiments in which the bearings 116a and 116b are magnetic bearings, a magnetic bearing controller (MBC) 168 is used to control the magnetic bearings 116a and 116b. The position sensors 117a and 117b may be used to detect the position or change in position of the turbine wheel 104 and / or rotor 108 relative to the housing 112 or other reference point (e.g., a predetermined value). The position sensors 117a and 117b may detect axial and / or radial displacement. The magnetic bearings 116a and / or 116b may respond to information from the position sensors 117a and 117b and adjust the detected displacement as needed. The MBC 168 may receive information from the position sensors 117a and 117b, process the information, and provide control signals to the magnetic bearings 116a and 116b. The MBC 168 may communicate with various components of the turboexpander 102 via the communication channel 162.

[0047] The turboexpander 102 can operate without the need for seals (e.g., without the need for dynamic seals) by using the magnetic bearings 116a, 116b and position sensors 117a, 117b to maintain and / or adjust the position of the turbine wheel blades 106 so that they are in close proximity to the shroud 114. The use of active magnetic bearings 116a, b in the turboexpander 102 eliminates physical contact between rotating and stationary components and also eliminates the need for lubrication, lubrication systems, and seals.

[0048] The turboexpander 102 may include one or more backup bearings. For example, the bearings may be used to rotatably support the turbine wheel 104 during startup and shutdown, or during a power outage that affects the operation of the magnetic bearings 116a and 116b. The backup bearings may include ball bearings, needle bearings, journal bearings, etc.

[0049] As previously described, the turboexpander 102 is configured to generate electricity in response to the rotation of the rotor 108. In certain cases, the rotor 108 may include one or more permanent magnets. The stator 110 includes multiple conductive coils. Rotation of the magnets within the coils of the stator 110 generates current. The rotor 108 and stator 110 may be configured as a synchronous permanent magnet multi-phase alternating current (AC) generator. The electrical interface 160 may, for example, have a three-phase output. In certain cases, the stator 110 may include multiple coils (e.g., three or six coils for a three-phase AC output). As the rotor 108 rotates, a voltage is induced in the stator coils. In either case, the magnitude of the voltage induced in the coils is proportional to the rate at which the magnetic field surrounding the coil changes over time (i.e., the rate at which the magnetic field passes on both sides of the coil). When the rotor 108 is connected to rotate at the same speed as the turbine wheel 104, the turboexpander 102 is configured to generate electricity at that speed. Such a turboexpander 102 is referred to as a "high speed" turbogenerator. For example, in an embodiment, the turboexpander 102 can generate up to 280 kW at a continuous rotational speed of 30,000 rpm. In an embodiment, the turboexpander can generate in the order of 350 kW at higher rotational speeds (e.g., in the order of 35,000 rpm).

[0050] The electrical interface 160 may be bidirectional for receiving electrical power from the grid or elsewhere to the turboexpander 102. For example, electrical power from the grid 140 or elsewhere may be received at the electrical interface 160 to power the rotor and other electrical components of the turboexpander.

[0051] In certain embodiments, the design of the turbine wheel 104, rotor 108, and / or stator 110 may be based on desired parameters of the output gas from the turboexpander 102. For example, the design of the rotor and stator may be based on the desired temperature of the gas 128.

[0052] The turboexpander 102 may be connected to power electronics 118. The power electronics 118 may include a variable speed drive (VSD) 166 (or variable frequency drive) and a magnetic bearing controller (MBC) 168 (discussed above).

[0053] The turboexpander 102's electrical interface 160 is connected to a VSD 166, which can be programmed for specific power requirements. The VSD 166 may include an insulated-gate bipolar transistor (IGBT) bidirectional inverter 204 that converts the variable frequency high voltage output from the turboexpander 102 to direct current (DC). The bidirectional inverter 204 may be a three-phase rectifier for three-phase AC input current. An active front-end rectifier (AFE) 206 then converts the rectified AC from 204 to DC for supplying power to the grid 140. The AFE 206 can convert the DC at 50-60 Hz to a current of 380 VAC to 480 VAC for supplying to the power grid. The specific output of the VSD 166 depends on the power grid and application. Other conversion values are within the scope of this disclosure. The VSD 166 matches its output to the power grid 140 by sampling the grid voltage and frequency and changing the output voltage and frequency of the AFE 206 to match the sampled power grid voltage and frequency.

[0054] The turboexpander 102 is also connected to an MBC 168 within the power electronics 118. The MBC 168 constantly monitors position, current, temperature, and other parameters to ensure the turboexpander 102 and active magnetic bearings 116a and 116b are operating as desired. For example, the MBC 168 is connected to position sensors 117a and 117b to monitor the radial and axial positions of the turbine wheel 104 and rotor 108. The MBC 168 can control the magnetic bearings 116a and 116b to selectively vary the stiffness and damping characteristics of the magnetic bearings 116a and 116b as a function of spin speed. The MBC 168 can also control desynchronization, including auto-balancing control, adaptive vibration control, adaptive vibration cancellation, and unbalanced force cancellation control.

[0055] A low voltage event on the power grid 140 can occur, for example, due to a fault condition or other problem on the grid. If the turboexpander process gas flow loop is not immediately terminated (e.g., through the flow control system 126), the rotor can overspeed to destructive levels. The process gas flow loop, controlled by a mechanical valve system (the flow control system 126), has a latency of several seconds. Based on the rotor's inertia, the rotor's rotational speed can accelerate to several thousand RPM in less than one second. FIG. 2 illustrates an exemplary turboexpander system 200 that can provide ride-through during a low voltage event, allowing the turboexpander rotor to continue rotating within its operating speed range without accelerating or shutting down.

[0056] 2 is a schematic diagram of an exemplary turboexpander system 200 including a low-voltage ride-through control architecture according to an embodiment of the present disclosure. The turboexpander system 200 includes a turboexpander 102 that can generate power by recovering energy lost during a pressure reduction sequence, as described above. During operation, power generated by the turboexpander 102 is channeled to a power grid 140. For example, the bidirectional power electronics 202 may include a bidirectional inverter 204 and an active front-end (AFE) rectifier 206. The bidirectional power electronics 202 can channel power to the power grid 140 and / or critical loads. The bidirectional power electronics 202 can also provide power from the power grid 140 to the turboexpander 102 for start-up purposes, powering specific components, or other reasons. The bidirectional power electronics 202 may include components the same as or similar to the power electronics 118 described above.

[0057] The electrical interface 160 may be electrically connected to the bidirectional inverter 204. The alternating current from the turboexpander 102 may be routed to the bidirectional inverter 204 using the electrical interface 160. The bidirectional inverter 204 may convert the alternating current to direct current for crossing the DC bus 214. The active front end 206 may include one or more rectifiers and may convert the direct current on the DC bus 214 to alternating current with a frequency and amplitude compatible with the power grid or other electrical loads. The AFE 206 may output power to the power grid 140 or other electrical loads.

[0058] The AFE 206 can also receive AC from the power grid 140 and convert the AC to DC for crossing the DC bus 214. The DC can be converted to AC by the bidirectional inverter 204 to power the turboexpander 102.

[0059] The turboexpander system 200 may include an LVRT detection circuit 212 that may detect a low voltage event. Upon detecting a low voltage event, the LVRT detection circuit 212 may disconnect the AFE rectifier 206 from the power grid 140. Turning off or disconnecting the AFE rectifier 206 may interrupt the flow of power from the turboexpander 102 to the power grid 140. The LVRT detection circuit 212 may also close the contactor 210, thereby directing power from the turboexpander 102 to the brake resistor assembly 208. In an embodiment, the LVRT detection circuit 212 may include a pulse width modulation circuit for regulating the DC bus voltage.

[0060] The turboexpander system 200 includes a brake resistor assembly 208 connected via terminals of the turboexpander electrical interface 160. The brake resistor assembly 208 is designed for three-phase AC output from the turboexpander 102. The brake resistor assembly 208 can include a resistor in series with each phase of the turboexpander electrical interface 160. In some embodiments, the brake resistor assembly 208 can include a capacitor in series with each resistor on either the up-current or down-current side of the resistor. Temporarily connecting the brake resistor assembly during a low-voltage event provides an alternate path for the power generated by the turboexpander to maintain a load on the turboexpander. This load prevents an overspeed condition during a low-voltage event.

[0061] The brake resistor assembly 208 may include a short-term peak kilowatt rated brake resistor. The maximum rating of the brake resistor is determined by the maximum rated kilowatt of the turboexpander's electrical output. The brake resistor assembly 208 may be temporarily connected during low-voltage event detection. For example, a contactor 210 or other switch circuit or switching component may be controlled (e.g., based on input from the LVRT detection circuit 212) to connect the brake resistor assembly 208 to the turboexpander's 102 electrical interface 160 upon detection of a low-voltage event trigger. Switching the turboexpander's 102 electrical interface 160 to the brake resistor assembly 208 disconnects the electrical interface 160 from the power electronics 202. When the low-voltage event is resolved, the LVRT detection circuit 212 may cause the contactor 210 to disconnect the brake resistor assembly 208 from the turboexpander's 102 electrical interface 160. The electrical interface 160 transmits power from the turboexpander to the power electronics 118. In an embodiment, the contactor 210 may be a contactor or other type of switch that can handle a rated power and operate and deactivate based on the presence or absence of an input or trigger. The contactor 210 may include an AC isolation contactor, which may facilitate maintenance of the brake resistor assembly without interrupting operation of the turboexpander.

[0062] LVRT detection circuit 212 may include circuitry to measure voltage or current output to the grid to detect a low-voltage event. LVRT detection circuit 212 may also include circuitry or other logic to control AFE 206 and contactor 210 to direct current from electrical interface 160 to brake resistor assembly 208 during a low-voltage event. For example, LVRT detection circuit 212 may include circuitry or other logic to turn off or disconnect AFE 206 during a low-voltage event and turn on or reconnect AFE 206 after the low-voltage event has cleared. LVRT detection circuit 212 may include circuitry or other logic to close contactor 210 to connect electrical interface 160 to brake resistor assembly 208 during a low-voltage event.

[0063] 3 is a schematic diagram of another example turboexpander system 300 including a low-voltage ride-through control architecture according to an embodiment of the present disclosure. The turboexpander system 300 includes a brake resistor assembly 302. The brake resistor assembly 302 can include a brake resistor in series with each phase of the turboexpander electrical interface 160. In some embodiments, the brake resistor assembly 302 can include a capacitor in series with each resistor on either the rising current side or the falling current side of the resistor. The resistors in the brake resistor assembly 302 can be continuous-rated resistors capable of carrying power continuously for a period of time. Thus, the resistors can be selected based on the maximum power output expected from the turboexpander during a low-voltage event.

[0064] The brake resistor assembly 302 can be temporarily connected to the electrical interface 160 of the turboexpander 102 when a low-voltage event is detected. A speed-controlled, current-regulated AC / DC active rectifier 304 (active rectifier 304 for short) can be connected between the electrical interface 160 and the brake resistor assembly 302. The LVRT detection circuit 212 can monitor the voltage of the power grid to determine whether or not a low-voltage event exists. The LVRT detection circuit 212 can control the AFE 206 and the active rectifier 304 to direct current from the turboexpander 102 to the brake resistor assembly 302 during a low-voltage event.

[0065] The active rectifier 304 may be activated by the LVRT detection circuit 212 when a low-voltage event is detected. The active rectifier 304 may be deactivated by the LVRT detection circuit 212 when the low-voltage event ends (and operation returns to normal). The active rectifier 304 may monitor the current to the brake resistor assembly 302 using a current monitor 306. In an embodiment, the active rectifier 304 is a fully rated AC / DC converter capable of handling the power output from the turboexpander 102. This architecture allows the turboexpander 102 to maintain its operating speed during a low-voltage event, with the added benefit of reducing potential restart delays and associated power consumption. Additionally, the decompression sequence of the process gas flow does not need to be interrupted, allowing the process gas to be directed to its next destination.

[0066] 3, the use of a speed-controlled, current-regulated AC / DC active rectifier 304 reduces the impact of the inductance of the turboexpander 102 and its connected cables on the power consumption of the brake resistor assembly 302. Therefore, the brake resistor assembly 302 does not need to be co-located with the turboexpander 102. The use of a speed-controlled, current-regulated AC / DC active rectifier 304 also provides latitude in the design of the brake resistor assembly to accommodate various turboexpander speeds and power outputs. The brake resistor assembly 302 can also be a DC circuit, thereby reducing the cost and design complexity of the brake resistor assembly 302.

[0067] The LVRT detection circuit 212 can turn on or reconnect the AFE 206 after the low voltage event is cleared.

[0068] FIG. 4 is a schematic diagram of another example turboexpander system 400 including a low-voltage ride-through control architecture according to an embodiment of the present disclosure. The architecture of the brake resistor assembly 302 of the turboexpander system 400 is similar to that shown in FIG. 3. In this embodiment, the architecture of the brake resistor assembly 302 is connected via a DC bus 410 of the power electronics 402. The power electronics 402 is similar to the power electronics 118 and power electronics 202 described above in that the power electronics 402 includes a bidirectional inverter 204 that can convert AC from the electrical interface 160 of the turboexpander 102 to DC and direct the DC to the AFE 206. The AFE 206 can convert the DC to AC with a frequency and amplitude compatible with the power grid 140. The AFE 206 can also receive current from the power grid 140 and convert the current to DC. The bidirectional inverter 204 can convert the DC to AC with a frequency and amplitude to initiate rotation of the rotor in the stator for start-up operation of the turboexpander.

[0069] In the exemplary embodiment of FIG. 4 , the bidirectional inverter 204 can direct current flow from the turboexpander 102 to the brake resistor assembly 302. The brake resistor assembly 302 is connected to the contactor 404 and the DC chopper 406 via a DC bus 410. The DC chopper 406 can monitor the current to the brake resistor assembly 302 using a current monitor 408. If a low-voltage event causes a rapid overspeed current flow from the turboexpander, feedback from the DC chopper 406 and the current sensor 408 facilitates pre-charge control current flow. In the embodiment shown in FIG. 4 , the existing bidirectional inverter 204 can be used to direct current from the turboexpander 102 to the brake resistor assembly 302, saving space, cost, and complexity. The bidirectional inverter 204 can include a speed-controlled, current-regulated AC / DC active rectifier, similar to that described in FIG. 3 .

[0070] The brake resistor assembly 302 can be isolated from the circuit by a contactor 404. The contactor 404 can be a static switch controlled by a signal indicating a low-voltage event (e.g., a signal from the LVRT detection circuit 212). The contactor 404 can disconnect the brake resistor assembly 302 when the low-voltage event ends. This circuit can include a pre-charge function to facilitate current inrush as well as increase the switching speed of the DC chopper 406. The DC chopper 406 can also receive a signal from the LVRT detection circuit 212 to operate the DC chopper 406. In some embodiments, the DC isolation contactor 404 can be included to isolate the brake resistor assembly 302 for maintenance without interrupting turboexpander operation.

[0071] The turboexpander system 400 also includes an LVRT detection circuit 212. The LVRT detection circuit may monitor the voltage of the power grid 140 to detect the presence or absence of a low-voltage event. Upon detecting a low-voltage event, the LVRT detection circuit 212 may close the contactor 404 and turn off or disconnect the AFE 206 from the DC bus 410, causing current from the turboexpander 102 to flow through the brake resistor assembly 302. Upon detecting that the low-voltage event has ended, the LVRT detection circuit may open the contactor 404 and turn on or reconnect the AFE 206 to the DC bus 410, causing current to flow through the AFE 206 and providing restored power to the power grid 140.

[0072] In some embodiments, instead of or in addition to the brake resistor assembly, a heat sink may be used to bleed power away from the turboexpander to prevent rotor overspeed during a fault condition.

[0073] 5 is a process flow diagram for operating a turboexpander generator during a low-voltage ride-through event according to an embodiment of the present disclosure. In one embodiment, electrical power is received by a turboexpander operating as a generator, initiating rotation of a rotor within a stator (502). In one embodiment, process gas flows through a turbine wheel of the turboexpander as part of a pressure reduction sequence. Energy in the gas flow is used to rotate a turbine wheel connected to the rotor. Rotation of the turbine wheel causes the rotor to rotate within the stator, generating electrical current (504).

[0074] The current from the turboexpander is sent to the power grid (506). The current from the turboexpander is alternating current (AC). The AC from the turboexpander is converted to direct current (DC) by power electronics. The DC is then converted to AC with a frequency and amplitude compatible with the power grid. The compatible AC is sent to the power grid (506).

[0075] In some circumstances, a fault condition may cause an undervoltage event. For example, a power grid failure or loss of input current to the power grid may cause an undervoltage event. An undervoltage event may be determined by the grid voltage detected by the LVRT detection circuit dropping below an LVRT threshold (508). Upon detecting an undervoltage event, current from the turboexpander may be redirected to the brake resistor assembly (510). In an embodiment, AC from the turboexpander is converted to DC by a rectifier and directed to the brake resistor assembly.

[0076] The completion of the low voltage event may be detected (512). The completion of the low voltage event may be determined by the grid voltage detected by the LVRT detection circuit being equal to or greater than the LVRT threshold voltage. Once the completion of the low voltage event is detected, the LVRT detection circuit may redirect AC from the turboexpander through the power electronics to the power grid (514).

[0077] 6 is a process flow diagram 600 for initiating a low voltage ride-through procedure according to an embodiment of the present disclosure. During operation of the turboexpander, the grid voltage may be monitored at regular intervals or as needed (602). The grid voltage is constantly monitored to ensure that a low voltage event is detected and that the turboexpander can resume supplying power to the power grid when the low voltage event is resolved. The grid voltage may be monitored using an LVRT detection circuit.

[0078] If the grid voltage is less than the LVRT threshold voltage (604), an LVRT timer may be started (606). The LVRT threshold voltage is implementation dependent. In some embodiments, the LVRT threshold voltage may be based at least in part on the power output characteristics of the turboexpander. The LVRT timer may be initialized to zero and begin counting [timer count]. The timer count may provide a maximum time for LVRT before the turboexpander, power electronics, and other components are shut down. This maximum time varies by implementation and is referred to herein as the LVRT allotted time. If the timer count is less than the LVRT allotted time 608 and the timer count is greater than zero 610, an LVRT process may be initiated (612). The LVRT process may include disconnecting or deactivating the AFE and connecting the turboexpander's electrical interface to the brake resistor assembly, thereby drawing current from the turboexpander to the brake resistor assembly.

[0079] If the timer count is equal to or greater than the LVRT allotted time (608), the turboexpander and power electronics may be shut down (616). If the LVRT process has started, the occurrence of the turboexpander and power electronics shutdown also terminates the LVRT process (618).

[0080] If the grid voltage is greater than or equal to the LVRT threshold voltage 604 and the LVRT process has not yet begun, the LVRT detection circuitry can continue to monitor the grid voltage 602 .

[0081] However, if the LVRT process has begun, when the grid voltage returns to a value above the LVRT threshold voltage 604, the LVRT detection circuitry can terminate the LVRT process (614). The timer can be reset to zero (606). The LVRT process is terminated only if the grid voltage is above the LVRT threshold and the timer count is less than or equal to the LVRT allotted time. If the timer count exceeds the LVRT allotted time, the turboexpander and power electronics are shut down and the LVRT process is terminated (618).

[0082] Accordingly, the specification and drawings should be regarded in an illustrative rather than a restrictive sense. Moreover, the use of the aforementioned embodiment and other exemplary language does not necessarily refer to the same embodiment or identical example, but may refer to different, separate, and potentially identical embodiments. The foregoing specification has described in detail with reference to certain exemplary embodiments. However, it will be apparent that various modifications and changes can be made thereto without departing from the broad spirit and scope of the present disclosure as set forth in the appended claims.

Claims

1. It is a generator, A turbine wheel configured to receive process gas and rotate in accordance with the expansion of the process gas flowing in from the inlet and flowing out from the outlet, A rotor connected to the turbine wheel and configured to rotate together with the turbine wheel, A fixed stator, wherein the generator generates alternating current by the rotation of the rotor within the stator, A generator including, A brake resistor assembly including a brake resistor, A switching component that, based on the detection of a low-voltage event, causes the current generated from the generator to flow to the brake resistor assembly, A bidirectional inverter including a speed-controlled, current-regulated AC / DC rectifier, wherein the speed-controlled, current-regulated AC / DC rectifier directs current from the turbo expander's electrical output to the brake resistor assembly via a DC bus when the low-voltage event is detected, and electrically isolates the brake resistor assembly when the low-voltage event is complete. A low voltage detection circuit that detects voltage and controls the switching component based on the detection of a low voltage event, A device that includes this.

2. The apparatus according to claim 1, wherein the switching component includes a static switch that connects the brake resistor assembly to the electrical output of the generator when the low voltage event is detected, and disconnects the brake resistor assembly from the electrical output of the generator when the completion of the low voltage event is detected.

3. The apparatus according to claim 2, wherein the brake resistor includes a peak kilowatt rated resistor.

4. The apparatus according to claim 1, wherein the brake resistor assembly is installed together with the generator.

5. The apparatus according to claim 1, wherein the brake resistor includes a continuous rated brake resistor.

6. The apparatus according to claim 1, wherein the switching component includes an AC isolation contactor for electrically isolating the brake resistor assembly from the electrical output of the generator.

7. Includes power electronics, said power electronics The aforementioned bidirectional inverter, An active front-end (AFE) rectifier receives direct current from the bidirectional inverter circuit via the DC bus and converts the direct current into alternating current with a frequency and amplitude suitable for the power grid, The apparatus according to claim 1, including the following:

8. The apparatus according to claim 7, wherein the power electronics include the switching component and the brake resistor assembly, the switching component being connected to the output of the bidirectional inverter circuit on the DC bus.

9. The apparatus according to claim 8, further comprising a DC isolation contactor between the bidirectional inverter circuit and the brake resistor assembly.

10. The apparatus according to claim 8, further comprising a static switch between the bidirectional inverter circuit and the brake resistor assembly, wherein the static switch electrically connects the bidirectional inverter circuit to the brake resistor assembly when the low voltage event is detected, and disconnects the bidirectional inverter circuit from the brake resistor assembly when the detection of the low voltage event ceases.

11. A generator, A turbine wheel configured to receive process gas and rotate in accordance with the expansion of the process gas flowing in from its inlet and out from its outlet, A rotor connected to the turbine wheel and configured to rotate together with the turbine wheel, A fixed stator, wherein the generator generates alternating current by the rotation of the rotor within the stator, A generator including, A brake resistor assembly including a brake resistor, A switching component that, based on the detection of a low-voltage event, causes the current generated from the generator to flow to the brake resistor assembly, A bidirectional inverter including a speed-controlled, current-regulated AC / DC rectifier, wherein the speed-controlled, current-regulated AC / DC rectifier directs current from the turbo expander's electrical output to the brake resistor assembly via a DC bus when the low-voltage event is detected, and electrically isolates the brake resistor assembly when the low-voltage event is complete. A low voltage detection circuit that detects voltage and controls the switching component based on the detection of a low voltage event, A bidirectional inverter circuit that receives alternating current (AC) from the generator and converts the AC to direct current (DC), An active front-end (AFE) rectifier that receives DC from the aforementioned bidirectional inverter circuit and converts the DC into AC having a frequency and amplitude suitable for the power grid, A system that includes this.

12. The system according to claim 11, wherein the switching component includes a static switch that operates when the low voltage event is detected and deactivates when the completion of the low voltage event is detected.

13. A method, To allow gas to flow through the turbine wheel of the generator located downstream of the flow control valve, Based on the rotation of the turbine wheel by the gas flow, the generator generates alternating current, The alternating current generated by the generator is guided to a bidirectional inverter circuit that receives the alternating current from the generator and converts the alternating current into direct current. The low-voltage detection circuit detects low-voltage events in the power grid, Based on the detection of the low-voltage event, a switching component directs the current generated by the generator to the brake resistor assembly, wherein the bidirectional inverter includes a speed-controlled, current-regulated AC / DC rectifier, which, upon detection of the low-voltage event, directs current from the turbo expander's electrical output via a DC bus to the brake resistor assembly, and electrically isolates the brake resistor assembly when the low-voltage event is complete. A method that includes this.

14. The low voltage detection circuit detects that the low voltage event has ended, Upon detecting that the low-voltage event has ended, the switching component directs the current generated by the generator to the power grid. The method according to claim 13, including the method described in claim 13.

15. The method according to claim 14, comprising converting the current generated by the generator into a current having a frequency and amplitude that is compatible with the power grid.

16. The method according to claim 13, comprising closing the circuit between the electrical output of the generator and the brake resistor assembly based on the detection of the low voltage event.

17. The method according to claim 13, comprising maintaining the gas flow through the turbine wheel during the low voltage event.

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