Brake resistor design to control rotor speed during generator shutdown sequence

Abstract

The generator receives the process gas and includes a turbine wheel configured to rotate in response to expansion of the process gas entering an inlet of the turbine wheel. The rotor is coupled to the turbine wheel and is rotatable with the turbine wheel within the stator. The generator generates an electric current upon rotation of the rotor within the stator. The power electronics system is electrically connected to an electrical output of the generator and receives alternating current from the generator. The generator includes a brake resistor assembly having an impedance matching the impedance of the generator. The contactor is electrically connected to the electrical output of the generator and configured to connect the generator to the brake resistor assembly based on a fault condition.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 823,842, filed August 31, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to the design of a braking resistor for controlling rotor speed during a generator shutdown sequence. [Background technology]

[0003] The efficient and effective movement of gas from production areas to consumption areas utilizes an extensive and sophisticated transportation system. Gas transported via pipelines can be pressurized and can travel long distances through the pipeline at high pressure. For example, natural gas transported through pipelines travels at high pressure within the pipeline. Natural gas is one of the primary energy sources for many of our daily needs and activities. Natural gas is an attractive fossil fuel due to its abundance and relative cleanliness. As another example, hydrogen can also be transported at high pressure using pipelines. 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 to generate electricity in the industrial and consumer transportation sectors. Other gases can also be transported through pipelines at high pressure, including propane, oxygen, and carbon dioxide. [Brief explanation of the drawings]

[0004] [Figure 1] FIG. 1 is a schematic diagram of a power generation system coupled to a power grid, according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a schematic diagram of an exemplary turboexpander system including a brake resistor assembly according to an embodiment of the present disclosure. [Figure 3A] FIG. 3A is a schematic diagram of an exemplary power circuit including an electrical output of a turboexpander selectively coupled to a power grid or a brake resistor assembly according to an embodiment of the present disclosure. [Figure 3B] FIG. 3B is a schematic diagram of another exemplary power circuit including an electrical output of a turboexpander selectively coupled to a power grid or a brake resistor assembly in accordance with an embodiment of the present disclosure. [Figure 3C] FIG. 3C is a schematic diagram of another exemplary power circuit including an electrical output of a generator selectively coupled to a variable speed drive or a brake resistor assembly according to an embodiment of the present disclosure. [Figure 4A] FIG. 4A is a schematic diagram of an exemplary single-phase brake resistor load circuit according to an embodiment of the present disclosure. [Figure 4B] FIG. 4B is a schematic diagram of another exemplary single-phase brake resistor load circuit according to an embodiment of the present disclosure. [Figure 5] FIG. 5 is a process flow diagram for controlling the rotational speed of a rotor during a shutdown sequence according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0005] The same reference symbols in the various drawings indicate the same elements. The drawings are not to scale.

[0006] Natural gas, hydrogen, and other process gases are pressurized to facilitate efficient transportation in pipelines that are sometimes several miles long. Pipelines transport gas, for example, from production sites (e.g., wells) to processing facilities, and from processing facilities to local distribution networks, such as regional, city, or district networks or on-site industrial plant networks. To safely deliver the gas through the local distribution network and for use, the process gas is reduced in pressure (often using pressure regulators) to a lower level. The pressure is then stepped down at pressure drop (PLD) stations for delivery to industrial, commercial, and residential end users. PLD stations use regulating valves to achieve the required pressure drop, but waste a significant amount of energy in the process. Additional regulating valves can be used elsewhere for pressure control, such as in pipelines between production and processing facilities, within subprocesses at processing facilities, and within end-user processes and piping. Turboexpander generators can be installed in parallel with the regulating valves to recover wasted energy from the pressure reduction and generate electricity. The electricity can be directed to the power grid or elsewhere. Along the same lines, turboexpander generators can be installed at gas storage facilities upstream of one or more storage tanks, allowing high-pressure gas dispensed from tank trucks to be returned to the storage tanks through the turboexpander generator. Turboexpander generators are also relevant in other applications, such as hydrogen liquefaction processes, where cooled and pressurized gaseous hydrogen is expanded to a liquid state. The expansion can be performed via the turboexpander generator to recover waste energy from the expansion and generate electricity. As noted above, the electricity can be directed to the power grid or elsewhere, such as to power compressors or other components of the liquefaction process. In each application, by recovering lost energy from natural gas and hydrogen pressure reduction applications, turboexpanders can generate electricity while reducing CO2 emissions, increasing overall plant efficiency, offsetting electricity costs, and generating additional revenue.

[0007] The power grid that the turboexpander may supply (and draw power from) may be a national or regional power grid, a local power grid for a city or district, or a smaller grid, local grid, or microgrid, such as an on-site grid that supplies power to a building, campus, industrial manufacturing or processing plant, or neighborhood.

[0008] FIG. 1 is a schematic diagram of a power generation system 100 coupled to a power grid 140 according to an embodiment of the present disclosure. The power generation system 100 can be added at a PLD station to capture energy from gas expansion from the PLD process or in 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 so that the turboexpander 102 can be mounted in series with a pipe. The turboexpander 102 acts as a generator by generating electrical energy from the rotational kinetic energy derived 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, which then generates electrical energy.

[0009] FIG. 1 is a schematic diagram of a power generation system 100 coupled to a power grid 140 according to an embodiment of the present disclosure. The power generation system 100 can be added at a PLD station to capture energy from gas expansion 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 arranged so that the turboexpander 102 can be mounted in series with a pipe. The turboexpander 102 acts as a generator by generating electrical energy from the rotational kinetic energy derived 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, which then generates electrical energy.

[0010] The turboexpander 102 may include a high-performance, high-speed permanent magnet generator. In one embodiment, the turboexpander 102 includes a radial in-flow expansion turbine wheel 104. The turboexpander 102 may also include low-loss active magnetic bearings (AMBs) 116a,b. The rotor assembly may include a permanent magnet section of the turbine wheel 104 mounted directly to the rotor hub. The rotor 108 may be levitated by a magnetic bearing system that creates a frictionless (or near-frictionless) interface between dynamic and static components. The AMBs 116a,b facilitate lossless (or near-lossless) rotation of the rotor 108.

[0011] The turboexpander 102 is shown with a process gas flow through the system, which cools the generator section and eliminates the need for auxiliary cooling equipment. In some embodiments, a non-flow-through overhung system can also be implemented. The power electronics 118 for the turboexpander, in some implementations, combines a variable speed drive (VSD) 166 and a magnetic bearing controller (MBC) 168 into a single cabinet. The VSD enables a consistent and clean delivery 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.

[0012] The turboexpander 102 is shown as having a flow-through configuration. The flow-through configuration allows process gas to flow from the inlet side of the turboexpander 102 to the outlet side of the turboexpander 102. The gas flows to a radial gas inlet 154 to the turbine wheel 104 and to an axial gas outlet 156 from the turbine wheel 104. The gas then flows through a generator and out the outlet 156, where it rejoins the gas pipeline 170. Generally, 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 an emergency shut-off valve. The flow control system 126 can be electrically controlled from the power electronics 118 by control line 164. In an embodiment, the turboexpander housing 112 is hermetically sealed. As mentioned above, the turboexpander can be non-flow-through and overhung without departing from the scope of this disclosure. The high-pressure process gas 120 is expanded by flowing through the turbine wheel 104, resulting in a pressure drop of the process gas. The low-pressure process gas 128 exits the turboexpander and recombines into a pipeline for downstream transportation and distribution. The expansion of the high-pressure process gas 120 through the turbine wheel 104 rotates the turbine wheel 104, 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 drop and captures energy from the pressure drop to generate electricity. A pressure control valve 130, such as a conventional pressure regulator, can be installed in parallel with the turboexpander 102. The pressure control valve 130 can be used to control the pressure of the high-pressure process gas 120 flowing through the turboexpander. Any excess high-pressure process gas not routed to the turboexpander can be routed through the pressure control valve 130.

[0013] In some embodiments, a heater 122 can heat the high-pressure process gas 120 before allowing the gas to enter the turboexpander 102. For example, if the expansion of the gas through the turbine wheel 104 reduces the temperature of the process gas to a point where moisture in the gas would freeze at the turbine wheel or other downstream locations in the pipeline, the compressed process gas 120 can be heated by the heater 122. The heated high-pressure process gas 124 can then be directed to the turboexpander 102. Heating the process gas can prevent moisture from freezing as the gas expands and its temperature drops.

[0014] The turboexpander 102 includes a turbine wheel 104. While the turbine wheel 104 is shown as a radial-inflow turbine wheel, 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 toward 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 to an outlet conduit 152 in the housing 112.

[0015] The turbine wheel 104 may be attached directly 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 on the end of the rotor 108 and held to the rotor 108 by a shaft. The shaft threads into the rotor 108 at one end and captures the turbine wheel 104 at the other end between the end of the rotor 108 and a nut threaded onto the shaft. The turbine wheel 104 and rotor 108 may be coupled without a gearbox and rotate at the same speed. In other examples, the turbine wheel 104 may be indirectly coupled to the rotor 108, for example, by a gear train, a clutch mechanism, or other methods.

[0016] The turbine wheel 104 extends outward from a hub and includes a plurality of turbine wheel blades 106 that react with the expanding process gases to rotate the turbine wheel 104. FIG. 1 illustrates a non-penetrating 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 substantially seal against a shroud 114 inside a housing 112. In certain examples, the turbine wheel 104 is a shrouded turbine wheel.

[0017] In a configuration with an unshrouded turbine wheel 104, the housing 112 includes an inwardly extending shroud 114 that closely abuts the turbine wheel blades 106 and, at most, does not contact them during operation. The close proximity of the turbine wheel blades 106 and the shroud 114 substantially seals against the passage of process gas therebetween 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 insignificant in the operation of the turbine wheel 104. In certain instances, the leakage can be balanced with other similar unshrouded turbine / shroud surface interfaces using conventional tolerances between the turbine wheel blades 106 and the shroud 114. The amount of leakage considered acceptable may be predetermined. The operating parameters of the turbine generator can be optimized to reduce leakage. In an embodiment, the housing 112 is hermetically sealed to prevent process gas from escaping from the radial inlet 154 of the turbine wheel 104.

[0018] The shroud 114 can be a specified distance away from the turbine wheel blades 106 and is maintained at that distance from the turbine wheel blades 106 during operation of the turboexpander 102 by using a magnetic positioning device that includes active magnetic bearings and position sensors.

[0019] The bearings 116a and 116b are positioned to rotatably support the rotor 108 and the turbine wheel 104 relative to the stator 110 and the shroud 114. The turbine wheel 104 is supported in a non-cantilevered manner by the bearings 116a and 116b. In an embodiment, the turbine wheel 104 may be supported in a cantilevered manner, and the bearings 116a and 116b may be positioned on an outlet side of the turbine wheel 104. In certain examples, 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.

[0020] Bearings 116a and 116b may be a combination radial and thrust bearing that radially and axially supports rotor 108. Other configurations may also be utilized. Bearings 116a and 116b do not have to be the same type of bearing.

[0021] 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. Position sensors 117a, 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, 117b may detect axial and / or radial displacement. The magnetic bearings 116a and / or 116b may respond to information from the position sensors 117a, 117b and adjust the detected displacement, if necessary. The MBC 168 may receive information from the position sensors 117a, 117b, process the information, and provide control signals to the magnetic bearings 116a, 116b. The MBC 168 may communicate with various components of the turboexpander 102 via a communication channel 162.

[0022] The use of magnetic bearings 116a, 116b and position sensors 117a, 117b to maintain and / or adjust the position of the turbine wheel blades 106 allows the turboexpander 102 to operate at high efficiency as the turbine wheel blades 106 remain 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 eliminates the lubrication system. In some embodiments, brush seals can be used to prevent gas leakage. The magnetic bearings 116a, b and position sensors 117a, b allow the rotor to remain in close proximity to the brush seals.

[0023] The turboexpander 102 may include one or more backup bearings. For example, during startup and shutdown, or in the event of a power outage affecting the operation of the magnetic bearings 116a and 116b, the bearings may be used to rotatably support the turbine wheel 104 during such periods. The backup bearings may include ball bearings, needle bearings, journal bearings, etc. As previously described, the turboexpander 102 is configured to generate electricity in response to rotation of the rotor 108. In certain examples, the rotor 108 may include one or more permanent magnets. The stator 110 includes multiple conductive coils. Electric current is generated by the rotation of the magnets within the coils of the stator 110. The rotor 108 and the stator 110 may be configured as a synchronous permanent magnet polyphase alternating current (AC) generator. The bidirectional electrical connection 160 may include, for example, a three-phase output. The bidirectional electrical connection 160 facilitates the transfer of (e.g., three-phase) electrical power output from the generator and can also power the generator for starting (e.g., to rotate the rotor while increasing process gas or other working fluid pressure, thereby rotating the turbine wheel 104). In a particular example, the stator 110 can 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. At any instant, the magnitude of the voltage induced in the coils is proportional to the rate at which the magnetic field surrounded by the coil changes with time (i.e., the rate at which the magnetic field passes through the two sides of the coil). When the rotor 108 is coupled 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 speed of 30,000 rpm. In an embodiment, the turboexpander may produce approximately 350 kW at a higher rotational speed (eg, approximately 35,000 rpm).

[0024] In some 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.

[0025] The turboexpander 102 may be coupled 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).

[0026] The bidirectional electrical connection 160 of the turboexpander 102 is connected to a VSD 166 that can be programmed to specific power requirements. The VSD 166 can include an insulated gate bipolar transistor (IGBT) rectifier 208 that converts the variable frequency high voltage output from the turboexpander 102 to direct current (DC). The rectifier 208 can be a three-phase rectifier for three-phase AC input current. An inverter 210 then converts the DC from the rectified AC to supply to the power grid 140. The inverter 210 can convert the DC to 380 VAC to 480 VAC at 50-60 Hz for supply to the power grid 140. 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 samples the grid voltage and frequency and then matches its output to the power grid 140 by changing the inverter's output voltage and frequency to match the sampled power grid voltage and frequency. In an embodiment, rectifier 208 and inverter 210 are bidirectional so that power from grid 140 can be supplied to turboexpander 102. Power from grid 140 can be used to initiate rotation of rotor 108 within stator 110 during start-up.

[0027] 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 that the turboexpander 102 and active magnetic bearings 116a and 116b are operating as needed. For example, the MBC 168 is coupled to position sensors 117a, 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, 116b to selectively vary the stiffness and damping characteristics of the magnetic bearings 116a, 116b as a function of spin speed. The MBC 168 can also control synchronous cancellation, including auto-balancing control, adaptive vibration control, adaptive vibration cancellation, and unbalanced force cancellation control.

[0028] The turboexpander 102 described above includes exemplary implementation-specific features. Some features may be modified, added, removed, or redesigned without departing from the scope of this disclosure. For example, other types of bearings, such as ball bearings, fluid film bearings, etc., may be used instead of or in addition to the AMB. Different rotor and stator designs may be used, such as brushless DC, induction, etc. Other types of stator cooling architectures may be used, such as non-flow-through and overhanging architectures.

[0029] 2 is a schematic diagram of an exemplary turboexpander system 200 including a brake resistor assembly 202 according to an embodiment of the present disclosure. The turboexpander system 200 includes a turboexpander 102 and power electronics 118. The turboexpander 102 receives heated, high-pressure process gas 124, which rotates a turbine wheel 104. The rotation of the turbine wheel 104 rotates a rotor 108, which supports multiple permanent magnets. The rotation of the permanent magnets on the rotor 108 induces current through coils or windings on a stator 110.

[0030] The generator system acts as a brake for the rotor 108. This braking torque converts shaft power generated by the process gas flow into electrical power that can be applied, for example, to the power grid. In the event of a grid fault, inverter failure, or other fault condition, the braking torque is lost, and the rotor 108 may spin up toward an undesirable overspeed. To prevent overspeed, power can be diverted to a brake resistor assembly 202, which can temporarily absorb rotor power until the process gas flow is reduced or eliminated (e.g., by the flow control system 126) or until the fault condition is resolved. The flow control system 126 can include one or a combination of flow control valves, mass control valves, or emergency shut-off valves. The flow control system 126 can be controlled by the power electronics 118 or other electrical, mechanical, or electromagnetic signals. For example, a fault condition can signal the flow control system 126 to close or partially close, thereby eliminating or restricting the gas supply to the turboexpander 102. Restricting or eliminating gas flow to the turboexpander slows the rotation of the turbine wheel, which in turn slows the rotor. In the example shown in Figures 1 and 2, signal channel 164 from power electronics 118 can be used to open or close flow control system 126.

[0031] The flow control system 126 can control the pressure of the high-pressure process gas flow entering the turboexpander 102. The flow rate of the high-pressure process gas can be controlled by the flow control system 126 based on the size of the turbine wheel 104 and the desired output of the power electronics 118, and the rotational speed of the rotor 108 can be determined based on the desired rotordynamic performance of the rotor 108. The flow control system 126 can be manually or electronically controlled. For example, the flow control system 126 can be electronically controlled based on information about the power output in the power electronics 118. If the power output is too low, the flow control system 126 can adjust to increase the flow rate of the high-pressure process gas input to the turboexpander 102 to increase the torque of the rotor 108. Conversely, if the power output is too high, the flow control system 126 can adjust to decrease the flow rate of the high-pressure process gas input to the turboexpander 102 to decrease the torque of the rotor 108.

[0032] The fault condition may include a grid fault, a VSD fault, an inverter fault, or other fault condition. The fault condition may include any condition that removes or reduces the braking torque of the rotor 108.

[0033] The brake resistor assembly 202 is electrically connected to the bidirectional electrical connection 160 (e.g., the output of the generator) of the turboexpander 102. The brake resistor assembly 202 can have a tailored impedance to allow efficient transfer of power from the turboexpander 102 to the brake resistor assembly 202. The design of the brake resistor assembly is further described in the text accompanying Figures 3A, 3B, 3C, 4A, and 4B.

[0034] In an embodiment, a contactor 204 can connect the output current of the turboexpander 102 to the brake resistor assembly 202 when there is a fault condition in the VSD 166 or the power grid 140. The contactor 204 is an electrically controlled switch for switching in a power circuit. The contactor 204 can accommodate three-phase current output from a generator to pass direct current to the brake resistor assembly 202. In an embodiment, the contactor can be controlled using a master controller that can detect the fault condition and provide the necessary signals to the contactor to open or close the circuit between the brake resistor assembly 202 and the turboexpander 102. For example, the fault condition can be detected by the master controller by a change in electrical activity, a circuit breaker trip, or other indication. The master controller can then open or close the contactor to connect the brake resistor assembly 202 and the turboexpander 102.

[0035] In some embodiments, the contactor 204 is directly connected to the turboexpander 102 (three-phase) bidirectional electrical connection 160. In some embodiments, the brake resistor assembly 202 and / or the contactor 204 are not part of the power electronics, but are connected to the turboexpander 102 electrical output 160 outside of the power electronics 118.

[0036] The VSD 166 provides an energization signal 220 to the coil of the contactor 204, causing the contactor 204 to connect the turboexpander bidirectional electrical connection 160 to the brake resistor assembly 202. Depending on the implementation choice, the contactor 204 may be a normally closed (NC) contactor or a normally open (NO) contactor.

[0037] For example, in an example implementation using NO contactors, during normal operating conditions, the bidirectional electrical connection 160 of the turboexpander 102 is connected to the VSD 166 and supplies three-phase AC current to the VSD 166. Under a fault condition, the VSD can apply a voltage to the contactor, connecting the contactor to the bidirectional electrical connection 160 of the turboexpander 102. In some implementations, the energization signal 220 to the contactor can be provided by another source that can respond to the fault condition (e.g., another component in the power electronics 118 or another component external to the power electronics 118). In this implementation, if a fault in the VSD 166 is the cause of the fault condition, the contactor can operate independently of the VSD 166.

[0038] If an NC contactor is used, the VSD 166 (or other source) provides an energization signal 220 to the contactor 204 to keep the contactor switch open during normal operating conditions. A fault condition can result in the removal of the energization signal 220 to the contactor, causing the contactor switch to close and complete the circuit between the bidirectional electrical connection 160 of the turboexpander 102 and the brake resistor assembly 202.

[0039] In some embodiments, the brake resistor 202 may be disconnected from the bidirectional electrical connection 160 of the turboexpander 102 when the operating condition returns to normal.

[0040] FIG. 3A is a schematic diagram of an example power circuit 300 including a bidirectional electrical connection 160 for a generator 302 selectively coupled to either the variable speed drive 206 or the brake resistor assembly 202a, in accordance with an embodiment of the present disclosure. FIG. 3B is a schematic diagram of another example power circuit 320 including a bidirectional electrical connection 160 for a generator 302 selectively coupled to either the variable speed drive 206 or the brake resistor assembly 202b, in accordance with an embodiment of the present disclosure. FIG. 3C is a schematic diagram of another example power circuit 340 including a bidirectional electrical connection 160 for a generator 302 selectively coupled to either the variable speed drive 206 or the brake resistor assembly 202c, in accordance with an embodiment of the present disclosure. These figures can be discussed together. In FIGS. 3A-3B and 3C, the turboexpander 102 is simplified to a three-phase permanent magnet synchronous AC generator 302. Although not shown, a current monitor can be coupled in parallel to the brake resistor assembly to monitor the current through the brake resistor.

[0041] The brake resistor assembly (either 202a, 202b, or 202c) is designed for a three-phase AC generator 302. In FIG. 3A, the brake resistor assembly 202a includes a resistor in series with each phase of the generator 302. R1 is the first phase, R2 is the second phase, and R3 is the third phase. All three resistors (R1, R2, R3) or resistor assemblies are connected to the neutral in a Wye configuration (a three-phase wye circuit). Other configurations, such as a Delta configuration, are within the scope of this disclosure.

[0042] Capacitors can be added to the brake resistor assembly to compensate for generator and cable inductance. Capacitors C1, C2, and C3, which are placed in series with each phase, can be placed either on the neutral side (FIG. 3B) or the non-neutral side (FIG. 3C) of the resistors. In brake resistor assembly 202b of FIG. 3B, capacitors C1, C2, and C3 are located on the neutral side of resistors R1, R2, and R3, respectively. In brake resistor assembly 202c of FIG. 3C, capacitors C1, C2, and C3 are located on the non-neutral side of resistors R1, R2, and R3, respectively. The capacitor values ​​can be adjusted according to the generator's operating speed and frequency and the corresponding impedance from the system inductance. Calculation of various brake resistor assembly parameters is discussed in more detail in FIGS. 4A-4B.

[0043] 4A is a schematic diagram of an example of a single-phase brake resistor load circuit 400 according to an embodiment of the present disclosure. FIG. 4A shows a simplified structural diagram of the brake resistor load circuit for one phase, where Vbemf is the line-neutral back electromotive force (EMF), Lm is the motor inductance per phase, Lc is the cable inductance, Lr is the brake resistor inductance, and R is the brake resistor.

[0044] The total inductance is L = Lm + Lc + Lr. The total impedance is Z = Z L +R, where Z L is the impedance of the inductance and is equal to jωL=jω(Lm+Lc+Lr), where ω is the angular frequency.

[0045] The phase currents can be expressed as follows:

number

[0046] The power of the brake resistor for a three-phase system is

number

[0047] When the electrical frequency of the generator is low, the impedance of the inductance is low compared to the impedance of the resistive load (abs(ZL) << R) and can be ignored. Therefore, the braking resistor load power can be expressed by the following equation.

Equation

[0048] When the electrical frequency of the generator is high, the impedance of the inductance cannot be ignored, and the phase current and the load current / power of the braking resistor decrease significantly. To overcome this problem, a capacitor C can be added in series to compensate for the impedance of the inductance shown in FIG. 4B.

[0049] FIG. 4B is a schematic diagram of another exemplary single-phase braking resistor load circuit 402 according to an embodiment of the present disclosure. As shown, a capacitor C is added in series to the neutral side of the resistor R. The impedance of the capacitor C is expressed by the following equation.

Equation

[0050] Therefore, the total impedance of the line can be expressed as follows.

Equation

[0051] By selecting the capacitor (C) value to

Equation

Equation

[0052] FIG. 5 illustrates a process flow diagram for controlling the rotor rotational speed during a pressure drop sequence according to an embodiment of the present disclosure. During a fault condition, the heated high-pressure gas supplied to the turboexpander should be turned off to prevent the turboexpander from continuing to operate. During a fault condition, a shutdown sequence can be followed to prevent damage to turboexpander components, including the rotor, and to protect the electronics. For example, during a fault condition in which braking torque to the rotor is lost, the rotor may experience overspeed from the process gas rotating the turbine wheel. To prevent rotor overspeed during a fault condition, a contactor can connect the turboexpander to a brake resistor assembly. The brake resistor assembly acts as a load on the turboexpander, providing a braking torque to the rotor, thereby preventing the turboexpander rotor from overspeeding. Controlling the rotor speed using the brake resistor assembly allows the time required to close a flow control valve (or mass control valve or emergency shutoff valve) on the inlet side of the turboexpander.

[0053] Initially, under normal operating conditions, for example, at a pressure reduction station, a flow control valve present on the inlet side of the turboexpander opens to allow heated, high-pressure process gas to enter the turboexpander. The heated, high-pressure process gas enters the inlet of a turbine wheel, which rotates, expanding the heated, high-pressure process gas and exiting the turbine wheel outlet. The rotation of the turbine wheel rotates a rotor, which may include multiple permanent magnets. Rotation of the rotor within the stator induces current in the stator coils. In some embodiments, the output of the turboexpander is three-phase alternating current. The expanded process gas is directed to the turboexpander outlet, where it can continue to other points within the pressure reduction station. (502)

[0054] The three-phase AC current is directed to power electronics. The power electronics can include a variable speed drive (VSD). The VSD can include a rectifier circuit that converts the three-phase AC current to DC current. The DC current can be directed to an inverter circuit that converts the DC current to AC current having an amplitude and frequency suitable for supplying AC current to a power grid. The VSD can supply AC current to the power grid. (504)

[0055] When a fault condition occurs, power from the generator is sent to the brake resistor assembly by a contactor circuit. The brake resistor assembly is sized to limit rotor acceleration. The contactor can connect the brake resistor assembly directly to the generator output. The contactor can be a normally open or normally closed contactor. An energization signal sent to (or taken from) the contactor can cause the contactor to connect the turboexpander's (three-phase) electrical output to the brake rotor brake resistor assembly. (506)

[0056] A fault condition can also trigger the closure or partial closure of a flow control system (e.g., a flow control valve, mass control valve, or emergency shutoff valve) to stop or reduce gas flow to the turboexpander. The flow control system is expected to act slower than the contactor to prevent the rotor from overspeeding, and therefore the brake resistor can act faster to prevent overspeed and potential damage to various components of the turboexpander and / or power electronics. (508)

[0057] Accordingly, the specification and drawings should be regarded in an illustrative rather than a restrictive sense. Moreover, the foregoing use of embodiment and other exemplary language does not necessarily refer to the same embodiment or the same example, but may refer to different, separate, and potentially the same embodiment. In the foregoing specification, a detailed description has been given with reference to certain exemplary embodiments. However, it will be apparent that various modifications and changes can be made without departing from the broader spirit and scope of the present disclosure as set forth in the claims.

Claims

1. A generator, a turbine wheel configured to receive a process gas and rotate in response to expansion of the process gas entering an inlet of the turbine wheel and exiting an outlet of the turbine wheel; a rotor coupled to the turbine wheel and configured to rotate with the turbine wheel; a static stator; a generator that generates alternating current upon rotation of the rotor within the stator; a power electronics system electrically connected to an electrical output of the generator to receive alternating current from the generator; a brake resistor assembly including an impedance matching the impedance of the generator; a contactor electrically connected to the electrical output of the generator and configured to connect the generator to the brake resistor assembly based on a fault condition; An apparatus comprising:

2. The apparatus of claim 1 , wherein the contactor connects the brake resistor assembly to the generator in response to the presence or absence of an energization signal.

3. The apparatus of claim 2 , wherein the presence or absence of the energization signal indicates the fault condition.

4. the generator having a three-phase electrical output; the brake resistor assembly comprising a brake resistor in series with each phase of the electrical output; 10. The apparatus of claim 1.

5. The apparatus of claim 4 , wherein each brake resistor of the brake resistor assembly is coupled to a neutral or ground plane in a Wye configuration.

6. 5. The apparatus of claim 4, wherein each brake resistor comprises a resistance based on matching an impedance of the brake resistor assembly to an impedance of the generator.

7. 7. The apparatus of claim 6, wherein the brake resistor assembly comprises a capacitor in series with each brake resistor.

8. 8. The apparatus of claim 7, wherein each capacitor has a capacitance based on matching an impedance of the brake resistor assembly to an impedance of the generator.

9. 7. The apparatus of claim 6, wherein a capacitor is present between the brake resistor and a neutral or ground plane.

10. 10. The apparatus of claim 1, wherein the power electronics comprises a variable speed drive connected to the electrical output of the generator, the variable speed drive converting the AC received from the generator to AC compatible with the power grid.

11. The variable speed drive a rectifier that receives AC current from the generator and converts the AC current into DC current; an inverter for receiving the direct current from the rectifier and converting the direct current into an alternating current having an amplitude and frequency compatible with the power grid; The apparatus of claim 10, comprising:

12. The apparatus of claim 1 , wherein the generator comprises a three-phase permanent magnet synchronous generator.

13. flowing the gas through a turbine wheel of a generator downstream of the flow control valve; generating an electric current with the generator; directing the electrical current generated by the generator to power electronics; activating a contactor present between the generator and the power electronics based on the presence of a fault condition; directing current generated by the generator through the contactor to a brake resistor assembly; closing a flow control valve to restrict gas flow to the turbine wheel; A method comprising:

14. directing the current generated by the generator through the power electronics to a power grid; The method of claim 13 , wherein the fault condition comprises a power grid fault condition or a power grid failure.

15. the power electronics include an inverter; The method of claim 13 , wherein the fault condition comprises a fault condition associated with the inverter.

16. directing an energization signal to the contactor; closing one or more switches of the contactor based on the energization signal; 14. The method of claim 13, comprising:

17. The method of claim 13, comprising applying a braking torque to the generator with the brake resistor assembly.

18. a flow control valve for controlling the flow of the process gas; A generator, a process gas inlet downstream of the flow control valve for receiving the process gas into the electric machine; a turbine wheel configured to receive the process gas and rotate in response to expansion of the process gas flowing into an inlet of the turbine wheel and out an outlet of the turbine wheel; a rotor coupled to the turbine wheel and configured to rotate with the turbine wheel; a static stator, the generator generating an alternating current upon rotation of the rotor within the stator; a process gas outlet that directs the process gas to a pressure drop station. A generator and a power electronics system electrically connected to an electrical output of the generator to receive alternating current from the generator, the power electronics system connecting the alternating current from the generator to a power grid; a brake resistor assembly having an impedance that matches the impedance of the generator; a contactor configured to electrically connect the electrical output of the generator and to connect the generator to the brake resistor assembly based on a fault condition; Including, the system.

19. the generator having a three-phase electrical output; The brake resistor assembly comprises: a brake resistor in series with each phase of the electrical output, each brake resistor of the brake resistor assembly being coupled to a neutral or ground plane in a Wye configuration; each brake resistor having a resistance based on matching the impedance of the brake resistor assembly to the impedance of a generator; 20. The system of claim 18.

20. the brake resistor assembly including a capacitor in series with each brake resistor, each capacitor having a capacitance based on matching an impedance of the brake resistor assembly to an impedance of the generator; 20. The system of claim 18.

Images