Systems and methods for protecting electrical loads of a drive system

Abstract

The present invention relates to a drive system (500) comprising a power converter (510) and a central control system (512), the power converter having a power unit (312) that supplies power to one or more output phases (A, B, C), the central control system communicating with the power converter (510) and controlling the operation of the power unit (312), wherein the central control system (512) includes an advanced protection module (APM 514) that is configured, by executable instructions: to receive input data from an electrical load (520) operatively coupled to one or more output phases (A, B, C), using power converter feedback from the electrical load (520); to determine one or more operating conditions of the electrical load (520) based on the input data; and to output one or more protection parameters based on the determined operating conditions of the electrical load (520) to protect the electrical load (520).

Description

Technical Field

[0001] Aspects of the present disclosure generally relate to a drive system, and more particularly to a system and method for protecting an electrical load of a drive system. Such a drive system can be, for example, a medium voltage variable frequency drive. Throughout the specification, the terms "drive", "drive system", "multilevel power converter", "converter", "power supply", and "variable frequency drive (VFD)" may be used interchangeably. Background Art

[0002] Medium voltage (MV) variable frequency drives, such as multilevel power converters, are used in applications such as medium voltage alternating current (AC) drives, flexible AC transmission systems (FACTS), and high voltage direct current (HVDC) transmission systems because a single power semiconductor device cannot handle high voltages. Multilevel power converters typically include multiple power units per phase, each power unit including an inverter circuit having semiconductor switches that can vary the voltage output of the respective unit. An example of a multilevel power converter is a cascaded H-bridge converter system having multiple H-bridge units, such as that described in U.S. Patent No. 5,625,545 to Hammond, the content of which is hereby incorporated by reference in its entirety. Another example of a multilevel power converter is a modular multilevel converter system having multiple M2C or M2LC subsystems.

[0003] A power converter receives three-phase power from an AC source and delivers the output power to a load, such as a three-phase AC motor. A motor protection relay (MPR) is designed to protect a motor, such as a three-phase AC motor, from faults. The MPR provides overcurrent, overload, thermal protection, and many other functions. The MPR is designed to be applied between the load (AC motor) and the grid power supply, rather than between the AC motor and the power converter. However, it is desirable to apply the MPR to the output of the power converter, such as a VFD. Summary of the Invention

[0004] Briefly, the content of the present disclosure relates to a drive system, such as embodied as a medium voltage variable frequency drive, and more particularly to a system and method for protecting an electrical output load of a drive system.

[0005] A first aspect of the present disclosure provides a drive system, the drive system including a power converter and a central control system, the power converter including power units that supply power to one or more output phases, each power unit including a plurality of switching devices, the central control system communicating with the power converter and controlling the operation of the power units, wherein the central control system includes an advanced protection module and at least one processor, the processor being configured to, by executable instructions: receive input data from an electrical load operably coupled to one or more output phases using power converter feedback from the electrical load; determine one or more operating conditions of the electrical load based on the input data; and output one or more protection parameters based on the determined operating conditions of the electrical load to protect the electrical load.

[0006] A second aspect of the present disclosure provides a method for protecting an electrical load of a drive system, including by operating at least one processor: receiving input data from an electrical load coupled to one or more output phases of a power converter using power converter feedback from the electrical load; determining one or more operating conditions of the electrical load based on the input data; and outputting one or more protection parameters based on the determined operating conditions of the electrical load to protect the electrical load.

[0007] A third aspect of the present disclosure provides a non-transitory computer-readable medium having encoded thereon processor-executable instructions that, when executed by at least one processor, cause the at least one processor to perform the method for protecting an electrical load of a drive system described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A schematic diagram illustrating a known basic configuration of a cascaded H-bridge converter system according to an exemplary embodiment disclosed herein.

[0009] Figure 2 A schematic diagram illustrating a known additional basic configuration of a cascaded H-bridge converter system according to an exemplary embodiment disclosed herein.

[0010] Figure 3 A schematic diagram illustrating a drive system according to an exemplary embodiment disclosed herein.

[0011] Figure 4 A schematic diagram illustrating a drive system having a conventional motor protection relay (MPR) according to an exemplary embodiment disclosed herein.

[0012] Figure 5 A schematic diagram illustrating a drive system having an advanced protection module (APM) according to an exemplary embodiment disclosed herein.

[0013] Figure 6A hardware block diagram of a resistance temperature detector (RTD) interface associated with an advanced protection module according to an exemplary embodiment disclosed herein is illustrated.

[0014] Figure 7 A flowchart of a method for protecting an electrical load of a drive system according to an exemplary embodiment disclosed herein is illustrated. Detailed Description

[0015] To facilitate understanding of the embodiments, principles, and features of the present disclosure, they will be explained below with reference to the implementations in the illustrative embodiments. In particular, they are described in the context of a drive system, specifically a medium-voltage (MV) variable frequency drive, which includes a multi-cell power supply such as a modular multilevel converter system and a cascaded H-bridge converter system. However, the embodiments of the present disclosure are not limited to use in the described apparatus or method.

[0016] As used herein, "medium voltage" is a voltage greater than about 690V and less than about 69kV, while "low voltage" is a voltage less than about 690V. Those of ordinary skill in the art will understand that other voltage levels may be designated as "medium voltage" and "low voltage". For example, in some embodiments, "medium voltage" may be a voltage between about 3kV and about 69kV, while "low voltage" may be a voltage less than about 3kV.

[0017] The components and materials constituting the various embodiments described below are illustrative and not restrictive. Many suitable components and materials will perform the same or similar functions as the materials described herein, and they are also within the scope of the embodiments of the present disclosure.

[0018] Figure 1 and Figure 2 respectively illustrate schematic diagrams of a known multi-cell power supply 10, specifically a cascaded H-bridge converter system, which receives three-phase power from an alternating current (AC) source and delivers power to a load 12, such as a three-phase AC motor.

[0019] Referring to Figure 1, the multi - unit power supply 10 includes a transformer 14, a power supply circuit 16, and a central control system 18, also referred to herein as a controller. The transformer 14 includes a primary winding that excites nine secondary windings, and the power supply circuit 16 includes a plurality of printed circuit board (PCB) power units 26, herein simply referred to as power units 26 or power modules, which are respectively operatively coupled to the secondary windings of the transformer 14. Since the power supply 10 includes nine secondary windings and the power units 26 are operatively coupled to each secondary winding, the power supply 10 includes nine power units 26. Of course, the power supply 10 can include more or fewer than nine power units 26 and / or more or fewer than nine secondary windings, depending on the type of the power supply 10 and / or the type of the load 12 coupled to the power supply 10.

[0020] The power unit 26 is configured to provide a medium - voltage output to the load 12. Each output phase A, B, C of the power supply circuit 16 is fed by a set of serially - connected power units 26. The outputs of the power units 26 are serially coupled in a first phase group 30, a second phase group 32, and a third phase group 34. The output voltage of each phase is the sum of the output voltages of the power units 26 in the respective phase groups 30, 32, and 34. For example, the first phase group 30 includes power units 26 labeled A1, A2, and A3, where the phase output voltage of output phase A is the sum of the output voltages of power units A1, A2, and A3. This also applies to output phase B and power units B1, B2, B3, and output phase C and power units C1, C2, C3. In this regard, the power supply circuit 16 uses power units 26 rated for low voltage to provide a medium - voltage output to the output load 12, and these power units include components rated for low - voltage standards. Each power unit 26 is coupled to the central control system 18, for example, via an optical fiber communication link, and the central control system can use current feedback and voltage feedback to control the operation of the power unit 26.

[0021] As Figure 2 shown, the multi - unit power supply 10 includes a three - phase AC power supply 20, a power supply circuit 16, and a central control system 18. The three - phase AC power supply 20 includes two diode bridges 22, which are respectively connected to the secondary windings of the power converter transformer 24 on the AC voltage side and are serially electrically connected on the direct - current (DC) voltage side. Positive and negative DC voltage buses are provided for the parallel connection of these phase groups. The power supply circuit 16 includes power units 28 coupled to the DC voltage bus generated by the power supply 20. The power units 28 are rated for low voltage, for example, and are configured to provide a medium - voltage output to the load 12. Although the load 12 can be illustrated as being within the multi - unit power supply 10, the load 12 is not part of the multi - unit power supply 10. Instead, the load 12 is separated from the multi - unit power supply 10 and is connected to the multi - unit power supply 10, as Figure 1 more clearly shown in

[0022] Each output phase A, B, C of the power supply circuit 16 is fed by a series-connected set of power units 28, i.e., these power units 28 are labeled A1 - A4, B1 - B4, and C1 - C4 with reference to the output phases A, B, C. These power units 28 are serially coupled in a first phase group 30, a second phase group 32, and a third phase group 34. The output voltage of each phase is the sum of the output voltages of the power units 28 in the phase groups 30, 32, and 34, as previously described with reference to Figure 1 The power supply circuit 16 uses power units 28 rated for low voltage to provide a medium voltage output to the load 12, and these power units 28 include components rated to low voltage standards. Each power unit 28 is coupled to the controller 18, for example, via an optical fiber communication link, and the controller 18 can use current feedback and voltage feedback to control the operation of the power unit 28.

[0023] It should be noted that in Figure 1 and Figure 2 the number of power units 26, 28 in each phase group 30, 32, 34 can be between 2 and 12 to provide different (medium voltage) outputs according to the requirements of the load 12. As described in the embodiment of Figure 1 the number of secondary windings of the transformer 14 matches the number of power units 26. In the embodiment of Figure 2 the number of diode bridges and secondary windings of the transformer can vary from 1 to 6 for harmonic elimination on the primary side of the transformer 24. Those of ordinary skill in the art will understand that other unit numbers and diode bridge numbers can be used depending on the application, and the configurations shown and described herein are intended to be exemplary in nature.

[0024] Figure 3 is a schematic diagram of a drive system 300, which includes a cascaded H - bridge multilevel converter 310 having a seven - level topology, including three phases, each phase having three power units. According to one aspect of the present disclosure, the system includes a control system 400. An example of the cascaded H - bridge multilevel converter 310 is the Perfect Harmony driver manufactured by Siemens Industry, Inc.

[0025] In the example of Figure 3 the system 300 is a medium - voltage driver, which includes a three - phase power supply that provides a power input 302 via lines L1, L2, and L3. The multilevel converter 310 is connected to the AC power input 302 and generates a three - phase AC power as an output 303 via the output phases A, B, and C. The AC output 303 can be connected to a load 320, which in this example includes an AC induction motor. The motor 320 can be operated by controlling the frequency and / or amplitude of the output voltage generated by the multilevel converter 310.

[0026] Each phase of the multilevel converter 310 includes a corresponding phase leg, which is formed by a plurality of power units 312 arranged in a cascaded manner. In Figure 1 the example of, phase legs Leg A and Leg B are each formed by the same number of, i.e., three, power units 312, which are connected in series. Each power unit 312 of a phase is connected to the power supply input 302 through corresponding input lines L1, L2, and L3. The power to the input lines L1, L2, and L3 can be provided, for example, through a polyphase winding transformer.

[0027] The power units 312 of the three phases are respectively labeled as Unit A 1 to Unit A 3 Unit B 1 to Unit B 3 and Unit C 1 to Unit C 3 . Each power unit 312 responds to control signals from the control system 400, which include, for example, pulse width modulation (PWM) signals to change the voltage level and / or frequency output, resulting in a multilevel voltage waveform for each phase. The power unit 312 generally includes a power semiconductor switching device, passive components (inductor, capacitor), a control circuit, a processor, an interface, and other components for communicating with the control system 400, i.e., operating the power unit 312 according to signals from the control system 400.

[0028] Each power unit 312 includes a single-phase inverter circuit, which is connected to a separate direct current (DC) source generated by rectifying the AC power input to each power unit 312 through the input lines L1, L2, and L3. In this example, the rectification is performed by diode rectifiers 313a-f arranged in a bridge rectifier configuration. This example also uses a filter circuit, which includes, for example, a capacitor 314, to smooth the voltage ripple of the rectified direct current.

[0029] The inverter circuit of each unit 312 includes power semiconductor switching devices 315a-d arranged in an H-bridge (also known as a full bridge). The switching devices 315a-d can include, for example, power transistors such as insulated gate bipolar transistors (IGBTs), but are not limited thereto. The switching devices 315a, 315b are connected to the unit output line 316a, and the switching devices 315c, 315d are connected to the unit output line 316b. The transistors 315a-d receive pulse width modulation signals, which are controlled by the control system 400 according to pulse width modulation, for example, in the form of gate input signals 318. The control system 400 selects either one of the transistors 315a or 315b to be turned on (ON) through the first switch leg 317a, and selects either one of the transistors 315c or 315d to be turned on (ON) through the second switch leg 317b, which will allow power to be transferred to the load 320 through the lines 316a or 316b, respectively. In other words, the switching event of the switch leg 317a triggered by the controller causes one of the transistors 315a, 315b to be in the on (ON) state and the other to be in the off (OFF) state. Similarly, the switching event of the switch leg 317b triggered by the controller causes one of the transistors 315c, 315d to be in the on (ON) state and the other to be in the off (OFF) state. In the illustrated embodiment, the switch legs 317a, 317b of a single unit 312 are simply referred to as switch leg A and switch leg B of the single unit 312.

[0030] Each power unit 312 can be constructed internally according to low voltage standards, although it is included in the medium voltage equipment driver 300. For example, each power unit 312 can have a rated voltage of 600V. Therefore, the maximum voltage level that each power unit 312 can output is approximately 600V of direct current. Depending on which transistor is turned on (ON), the output voltage on the unit output lines 316a, 316b of each power unit 312 can be either polarity or zero. Therefore, each power unit 312 can have three output states: +600V direct current voltage, -600V direct current voltage, or 0V direct current voltage. Due to the series connection between the three power units 312 in each phase output line, for example, between unit A 1 、A 2 、A 3 connected in series between, it is possible to generate a maximum output voltage amplitude of approximately 1800V direct current voltage for the corresponding phase output line. Each power unit 312 can operate independently of the other power units. Therefore, it is possible to provide at least seven voltage levels per phase for the motor 320. The approximations of these line-neutral voltage states include + / -1800V direct current voltage, + / -1200V direct current voltage, + / -600V direct current voltage, and 0V direct current voltage.

[0031] The electric machine 320 can include any type of alternating current (AC) machine, such as synchronous, asynchronous, permanent magnet, and can be rated for low voltage, medium voltage, or high voltage. For example, medium voltage AC machines (such as those used in industrial process control) can operate in the range of 4.16 kV to 13.8 kV. Larger or smaller voltages can be used. More than one electric machine 320 can be connected. Other loads can be used in place of or additionally to the electric machine 320. The electric machine 320 responds to the voltages applied by the multilevel converter on three phases, such as increasing, decreasing, or maintaining speed or position.

[0032] Figure 4 FIG. illustrates a schematic diagram of a drive system 400 having a conventional motor protection relay (MPR) in accordance with an exemplary embodiment disclosed herein. The drive system 400 includes a VFD 410, which can be configured as described, for example, with reference to Figure 1 , Figure 2 or Figure 3 . The VFD 410 is operatively coupled to an electrical output load 420, which can be, for example, a three-phase AC induction motor. As previously described, the VFD 410 receives three-phase power from an alternating current (AC) source and provides three-phase power (voltage) to the output load 420 via three-phase electrical conductors 430.

[0033] The conventional drive system 400 further includes a motor protection relay (MPR) 440, which is designed to protect the load 420, such as a three-phase AC motor, against faults. The MPR 440 provides overcurrent protection, overload protection, thermal protection, and many other protection functions. The MPR 440 can be microprocessor-based and receives voltage and current information of the electrical load 420 via voltage and current sensors 450. Such sensors 450 can include, for example, current transformers (CTs) and potential transformers (PTs). Based on the received voltage and current information, for example, via current transformers (CTs) and potential transformers (PTs), the MPR 440 determines whether the load 420 is operating under normal conditions or abnormal (fault) conditions. The load 420 operating under abnormal conditions, such as thermal overload, overcurrent, etc., may lead to a fault of the load 420. When an abnormal operating condition occurs or exists, the MPR 440 provides a corresponding fault input signal to the VFD 410, and the VFD 410 in turn controls the VFD 410 to protect the load 420, such as isolating the load 420 from the input power supply.

[0034] In drive system 400, MPR 440 is arranged between load 420 and the grid power supply (electrical conductor 430). However, it is desirable to apply the MPR to the output of VFD 410. The range of frequency and voltage variation is much smaller on the grid than on VFD 410. Currently, protection levels, such as thermal model parameters, under-speed / over-speed, under-voltage / over-voltage, under-frequency / over-frequency, etc., must be selected as single values and cannot be input as a function of the speed of the AC motor (load 42).

[0035] Figure 5 FIG. illustrates a schematic diagram of a drive system 500 with an advanced protection module (APM) in accordance with an exemplary embodiment disclosed herein. Drive system 500 includes a power converter 510, which is configured, for example, as a VFD as described with reference to Figure 1 , Figure 2 or Figure 3 . The VFD 510 is operatively coupled to an electrical output load 520, which may be, for example, a three-phase AC induction motor. As previously described, the VFD 510 receives three-phase power from an alternating current (AC) source and delivers three-phase power (voltage) to the output load 520 via three-phase electrical conductors 530.

[0036] The VFD 510 includes a central control system 512, which is configured to control the operation of the VFD 510, such as controlling the operation of a plurality of power units of the VFD 510. The central control system 512 uses, for example, current feedback and voltage feedback for control purposes. In one example, the central control system 512 is a purpose-specific digital control system that divides the commands and states of the control loop, power unit control information, and tasks of the external communication interface into three separate components. These three separate main components are a control processor / host central processing unit for control loop commands, states, and non-drive interfaces; a field programmable gate array (FPGA) for power unit control and communication; and an electronically programmable logic device (EPLD) for external communication. The main components require dedicated data buses on a printed circuit board (PCB) such that information can be exchanged between the main components to successfully operate the drive system.

[0037] In an exemplary embodiment of the present disclosure, the central control system 512 includes an advanced protection module (APM) 514. The APM 514 can be embodied as software, hardware, or a combination of software and hardware. The APM 514 is fully integrated into the VFD 510 itself and thus does not require separate installation or assembly.

[0038] In the example of a medium voltage AC motor as load 520, the load runs directly online (DOL) or via a VFD 510. Therefore, the motor needs protection against input line events, high temperatures, insulation and bearing failures, and conditions resulting from load changes, including but not limited to overload, underload, imbalance, and disturbances. The APM 514 is configured to ensure protection of the load (such as an AC motor) and process protection, and the protection settings are defined to prevent load and process damage, which may occur in various ways and conditions. Today, industrial equipment is designed to operate for 20 years or more. Loads, such as medium voltage AC motors, are exposed to environmental and mechanical stresses and may therefore degrade and fail over time. Monitoring and protection of such medium voltage motors are important factors in overall industrial process protection. These protection schemes are needed to avoid economic losses due to unexpected processes.

[0039] In one embodiment, the APM 514 is configured as a combination of hardware and software and includes algorithms for protecting the system 500, particularly a load 520 such as a motor, from such events. Additionally, some functions can also be configured to detect and protect against unwanted process conditions.

[0040] Figure 6 A hardware block diagram of a resistance temperature detector (RTD) interface 600 associated with an advanced protection module, in accordance with an exemplary embodiment disclosed herein, is illustrated. The hardware of the RTD interface 600 is used to measure temperature in order to provide detection to a load, such as load 520.

[0041] The RTD interface 600 includes a controller, which is configured, for example, as a programmable logic controller (PLC) 610. The PLC 610 is communicatively coupled to a detector module A 620 and a detector module B 630. The detector module A is, for example, a 4-channel resistance temperature detector (RTD) module 620, and the detector module B is, for example, an 8-channel resistance temperature detector (RTD) module 630. The PLC 610 and the detector modules 620, 630 receive power to operate from a power supply 650, which may be, for example, a +24V power supply. Additionally, the PLC 610 and the detector modules 620, 630 are communicatively coupled to a network switch 640, such as an Ethernet switch, in order to be able to communicate with other control components of the central control system 512, see Figure 5 . For example, the PLC 610 converts the analog signals of the detector modules 620, 630 into digital signals and transmits the digital signals to the central control system 512 via the network switch 640. Specifically, the PLC 610 converts the analog temperature signal into a digital temperature signal / digital temperature value.

[0042] In an exemplary embodiment, the RTD interface 600 is operably coupled to the central control system 512 and provides input data to the APM 514, e.g., via a network switch 640. The APM 514 runs on and is included within the central control system 512 of the VFD 510. The central control system 512 includes at least one processor 513 configured to receive input data from an electrical load (e.g., electrical load 520) operably coupled to a power converter (e.g., VFD 510) via executable instructions; determine one or more operating conditions (states) of the electrical load 520 based on the input data; and output one or more protection parameters based on the determined operating conditions (states) of the electrical load 520 to protect the electrical load 520.

[0043] The drive system 500, particularly the central control system 512 and the APM 514, uses an integral closed-loop Hall effect current sensor and an output attenuator to obtain accurate input load (motor) data and integrate the data into the algorithms of the APM 514. The closed-loop Hall effect current sensor and the output attenuator may already be installed within the drive system 500, where the data of these components can now be used by the APM 514. This data is provided as input data to the APM 514, e.g., via the network switch 640. As previously mentioned, the network switch 640 can be an Ethernet switch for communicating with other devices or components of the central control system 512. In addition to the current sensor and the output attenuator, the APM 514 also receives temperature data related to the motor (load 520) from detectors A and B via the RTD interface 600, where detectors A and B are resistance temperature detectors 625, 635. Specifically, the resistance temperature detector (RTD) 625 feeds information to the detector module 620, and the resistance temperature detector 635 feeds information to the detector module 630. The temperature-related information, along with the current sensor data and the output attenuator data, is utilized and processed by the APM 514 within the central control system 512 for load and process protection functions.

[0044] In one embodiment, one or more protection parameters include a trip level and an enable level, which are functions of the speed of the electrical load 520 (based on a speed curve) and / or can be based on a process curve of the drive system 500. For example, the trip level and the enable level can be selected at different points on the speed curve of the electrical load 520, e.g., at different points corresponding to normal operating conditions, alarm setting conditions, and fault setting conditions on the speed curve.

[0045] The protection parameters include fixed levels and variable levels. These fixed levels and variable levels (parameters) include, for example:

[0046] - fixed overspeed, - variable overspeed,

[0047] - Fixed under-speed, - Variable under-speed,

[0048] - Fixed under-current, - Variable under-current,

[0049] - Fixed under-power,

[0050] - Fixed torque ripple,

[0051] - Fixed negative-sequence over-current,

[0052] - Maximum start time,

[0053] - Longest stop time,

[0054] - Fixed thermal overload, - Variable thermal overload,

[0055] - Fixed RTD protection,

[0056] - Fixed instantaneous over-current,

[0057] - Fixed zero-sequence over-voltage,

[0058] - Fixed inverse-time over-current,

[0059] - Fixed instantaneous zero-sequence over-voltage,

[0060] - Fixed maximum power factor,

[0061] - Fixed minimum power factor,

[0062] - Intermittent or jogging, starts per hour,

[0063] - Intermittent or jogging, cold starts per hour,

[0064] - Intermittent or jogging, hot starts per hour,

[0065] - Intermittent or jogging, maximum thermal capacity for starting,

[0066] - Fixed over-frequency, - Variable over-frequency,

[0067] - Fixed under-frequency, - Variable under-frequency,

[0068] - Fixed high frequency change rate.

[0069] The fixed levels and variable levels mentioned above are described as follows.

[0070] Fixed (Enabled) Overspeed is used to protect the motor and the connected load 520 against overspeed. The Fixed Enabled Overspeed function provides a single speed point setting such that a trip or alarm condition is generated when the speed is exceeded. Once the motor is started, the function can be activated after a programmable time period.

[0071] Variable (Enabled) Overspeed is used to protect the motor and the connected load 520 against overspeed or to detect conditions under which the motor speed rises above the desired setpoint. The Variable Enabled Overspeed function provides a curve of overspeed points based on the commanded motor speed. A trip or alarm condition occurs when the speed is exceeded. Once the motor is started, the function can be activated after a programmable time period.

[0072] Fixed (Enabled) Underspeed is used to protect the motor and the connected load 520 from operating at speeds below the desired speed. The Fixed Enabled Underspeed function provides a single underspeed point setting such that a trip or alarm condition is generated when the speed is below this value. The function provides minimum speed enablement and allows activation of the function only after a programmable minimum speed is reached. Once the minimum speed is reached, the function remains activated regardless of the speed until the drive is stopped.

[0073] Variable (Enabled) Underspeed is used to protect the motor and the connected load 520 from operating at speeds below the desired speed or to detect conditions under which the motor speed drops below the desired setpoint due to excessive load torque problems or torque generation difficulties in the machine. The Variable Enabled Underspeed function provides a curve of speed points based on the commanded motor speed. A trip or alarm condition occurs when the speed drops below this curve at a given speed setting. The function provides minimum speed enablement and allows activation of the function only after a programmable minimum speed is reached. Once the minimum speed is reached, the function remains activated regardless of the speed until the drive is stopped or the demand is set below the minimum speed reset value. The minimum speed reset is used to define the range of the demand setting below which the function will remain in the reset condition.

[0074] Fixed (Enabled) Undercurrent is used to protect the motor from operating with the RMS (Root Mean Square) phase current below the desired level. The Fixed Enabled Undercurrent function provides an RMS phase current setting such that a trip or alarm condition is generated when the current is below this value. The function can be programmed to generate a trip or alarm when any one, any two, or all three phase currents (phase A, phase B, phase C RMS currents) are below the setpoint. The function provides the following enablement, which locks once a programmable minimum speed is reached. Once the motor is started, the function can be activated after a programmable time period.

[0075] Variable (enabled) undercurrent is used to protect the load 520 / system 500 from operating in a manner where the RMS phase current is below the desired level, and sensitivity to speed demand settings at the desired level is important. The variable enabled undercurrent function provides a curve of undercurrent setpoints based on the commanded motor speed. When the current is below this curve at a given speed setting, a trip or alarm condition occurs. This function can be programmed to trip or alarm when any one, any two, or all three phase currents are below the setpoint. This function provides the following enablement, which locks once the programmable minimum speed is reached. The function can be started when the speed reference is above the programmable level. Once the motor is started, the function can be started after a programmable time period.

[0076] Fixed (enabled) underpower is used to protect the motor and the connected load 520 from operating at a power level below the desired power level. The fixed enabled underpower function provides a single power point setting, and a trip or alarm condition occurs when the power is below this value. This function provides the following enablement, which locks once the programmable minimum speed is reached. Once the motor is started, the function can be started after a programmable time period.

[0077] Fixed (enabled) torque ripple is used to protect the motor and the connected load 520 from operating under conditions of high torque ripple. The fixed enabled torque ripple function provides a single RMS torque ripple point setting, and a trip or alarm condition occurs when the RMS torque ripple rises above this value. The function can be started when above the minimum speed. Once the motor is started, the function can be started after a programmable time period.

[0078] The torque-producing current and motor flux associated with motor operation allow the motor torque to be calculated as the product of the torque-producing current and the machine flux. The torque-producing current is the component of the machine current that is in phase with the machine voltage. A given machine has a maximum rating of torque-producing current, which combines with any flux-producing current to form the overall rated stator current. Torque can be decomposed into two components, one component being the average torque and the other being a small amplitude variation (or periodic component), which can be added together to obtain the total torque. Torque ripple protection is mainly concerned with the RMS value of the time-varying part, particularly the pulsating part of the torque. This protection calculates the RMS torque ripple by sampling and recording the torque over a time window of a specific length. The time window is selectable and should be chosen long enough to contain several cycles of torque variation.

[0079] Fixed (enabled) negative sequence overcurrent is used to protect the motor and the connected load 520 against operation under conditions of high negative sequence current or phase current imbalance. The fixed enabled negative sequence overcurrent function provides a single negative sequence overcurrent setting at which a trip or alarm condition is generated when the negative sequence current rises above this value. The function provides the following enabling, which is locked once the programmable minimum speed is reached. Once the motor is started, the function can be started after a programmable time period.

[0080] The maximum start time can protect the motor against an excessive time between start-up and reaching the desired speed. When the machine fails to reach the adjustable speed threshold within the adjustable time period after start-up, this function generates a trip or alarm condition.

[0081] The maximum stop time can protect the motor against an excessive time between the stop command and dropping to the desired speed. When the machine fails to reach the adjustable speed threshold within the adjustable time period after stop, this function generates a trip or alarm condition.

[0082] The fixed (parameter) thermal overload function uses a first-order differential equation to track the amount of thermal capacity used in the machine, as described in IEC 60255-149. The thermal capacity is exhausted when the machine temperature approaches the maximum rated or permitted conditions. The equivalent heating current is calculated, which takes into account the RMS phase current of the machine as well as the amount of negative sequence current. The thermal input to the machine is determined based on the square of the equivalent heating current divided by the adjustable rated current. The thermal capacity is adjusted according to a first-order differential equation that takes into account the thermal input and thermal output in the machine, based on the thermal time constant for heating, cooling, or stop conditions in the machine. The fixed parameter function uses single values for the rated current, heating time constant, and cooling time constant. An adjustable threshold can be set to limit the maximum use of the thermal capacity. When the thermal capacity used exceeds the programmed value, this function reports a trip or alarm condition, or can prevent the motor from starting. The thermal model of the machine can be biased by RTD measurements of the ambient temperature and / or stator temperature, such as provided by the RTD interface 600. The ambient RTD reading is used to compensate for the effect of non-rated ambient temperature, and the stator RTD reading is used to set the minimum thermal capacity usage value based on the stator temperature.

[0083] The variable (parameter) thermal overload function uses a first-order differential equation to track the thermal capacity used in the machine, as described in IEC60255-149. When the machine temperature approaches the maximum rated or permitted conditions, the thermal capacity is exhausted. The equivalent heating current is calculated, which takes into account the RMS phase current of the machine as well as the amount of negative-sequence current. The thermal input to the machine is determined based on the square of the equivalent heating current divided by the adjustable rated current. The thermal capacity is regulated according to a first-order differential equation that takes into account the thermal input and thermal output in the machine, based on the thermal time constant for heating, cooling, or stopping conditions in the machine. The variable parameter function uses the values of the rated current, heating time constant, and cooling time constant, which are functions of the required speed. Adjustable thresholds can be set to limit the maximum use of the thermal capacity. When the thermal capacity used exceeds the programmed value, the function reports a trip or alarm condition, or can prevent the motor from starting. The thermal model of the machine can be biased by RTD measurements of the ambient temperature and / or stator temperature, provided, for example, by the RTD interface 600. The ambient RTD reading is used to compensate for the effect of non-rated ambient temperatures, and the stator RTD reading is used to set the minimum thermal capacity usage value based on the stator temperature.

[0084] The fixed (enabled) RTD function allows up to 12 RTD temperature sensors to be used to provide general overtemperature protection, provided by the RTD interface 600, see Figure 6 . The fixed temperature enable levels can be assigned individually to each RTD. The RTDs can also be assigned to stator or ambient groups for use in fixed or variable enabled thermal models. Alarm or trip responses to RTD open or short circuits can be selected.

[0085] The fixed (enabled) instantaneous overcurrent is used to protect the motor and the connected load 520 very quickly against operation under high-current conditions. The fixed enabled instantaneous overcurrent function provides a single instantaneous overcurrent setting that generates a trip or alarm condition when the current rises above this value. The function can be programmed to generate a trip or alarm when any one, any two, or all three phase currents exceed the setpoint. The function provides the following enabling, which locks once the programmable minimum speed is reached. Once the motor has started, the function can be activated after a programmable period of time.

[0086] Fixed (enabled) inverse time overcurrent is used to protect the motor and the connected load 520 from operating under high current conditions. It has a trip time that is inversely proportional to the magnitude of the current. The fixed enabled inverse time overcurrent function provides a single instantaneous overcurrent setting that generates a trip or alarm condition when the selected inverse time characteristic is met. This function can be programmed to generate a trip or alarm when any one, any two, or all three phase currents reach their inverse time curves. Various IEEE, ANSI, IEC, and IAC inverse time curves, as well as user-defined curve functions, can be selected. This function provides the following enabling: it will be locked once the programmable minimum speed is reached. Once the motor starts, this function can be activated after a programmable time period.

[0087] Fixed (enabled) maximum power factor is used to protect the motor from operating under high power factor conditions, where a high power factor indicates an abnormal condition in the machine. The fixed enabled maximum power factor function provides a single maximum power factor setting that generates a trip or alarm condition when that power factor is reached. This function can be activated when the speed demand is higher than a programmable level. Once the motor starts, this function can be activated after a programmable time period.

[0088] Fixed (enabled) minimum power factor is used to protect the motor from operating under low power factor conditions, where a low power factor indicates an abnormal condition in the machine. The fixed enabled minimum power factor function provides a single minimum power factor setting that generates a trip or alarm condition when that power factor is reached. This function can be activated when the speed demand is higher than a programmable level. Once the motor starts, this function can be activated after a programmable time period.

[0089] Fixed (enabled) instantaneous zero-sequence overvoltage is used to very quickly protect the motor under high zero-sequence voltage conditions, which may be caused by high phase-to-ground leakage or a ground fault. The fixed enabled zero-sequence overvoltage function provides a single zero-sequence overvoltage setting that generates a trip or alarm condition when the zero-sequence voltage rises above this value. This function provides the following enabling: it will be locked once the programmable minimum speed is reached. Once the motor starts, this function can be activated after a programmable time period.

[0090] Fixed (enabled) determined minimum time zero-sequence overvoltage is used to protect the motor from continuous operation under high zero-sequence voltage conditions, which may be caused by high phase-to-ground leakage or a ground fault. The fixed enabled zero-sequence overvoltage function provides a single zero-sequence overvoltage setting that generates a trip or alarm condition when the zero-sequence voltage rises above this value. This function provides the following enabling: it will be locked once the programmable minimum speed is reached. Once the motor starts, this function can be activated after a programmable time period.

[0091] The hourly start (intermittent or jog) function is used to enforce a minimum time between machine starts. A programmable minimum time since the last start can be set. Start attempts before the minimum time has expired can be programmed to trip, alarm, or prevent a start.

[0092] The hourly cold start (intermittent or jog) function is used to enforce a maximum number of cold starts of the machine within an adjustable time period. Attempts to cold start the machine beyond the allowed number can be programmed to trip, alarm, or prevent a start. A cold start is defined as a start that occurs when the heat capacity used is below an adjustable value.

[0093] The hourly hot start (intermittent or jog) function is used to enforce a maximum number of hot starts of the machine within an adjustable time period. Attempts to hot start the machine beyond the allowed number can be programmed to trip, alarm, or prevent a start. A hot start can be defined as any start or a start that occurs when the heat capacity used is above the adjustable value used by the hourly cold start function.

[0094] The maximum heat capacity for start (intermittent or jog) function is used to ensure that the machine has sufficient heat capacity available to allow a start. Attempts to start the machine without sufficient heat capacity can be programmed to trip, alarm, or prevent a start. The maximum amount of heat capacity at which starting is no longer allowed is an adjustable parameter.

[0095] Fixed (enabled) overfrequency is used to protect the motor and connected load 520 from continuous operation at frequencies above the desired frequency. The fixed enabled overfrequency function provides a single overfrequency setting at which a trip or alarm condition is generated when the frequency rises above this value. This function is enabled as follows and locks once the programmable minimum speed is reached. Once the motor has started, the function can be activated after a programmable time period.

[0096] Variable (enabled) overfrequency is used to protect the motor and connected load 520 from operation at frequencies above the desired frequency or to detect conditions under which the motor frequency rises above the desired setpoint due to load regeneration issues or other difficulties in the machine or load. The variable enabled overfrequency function provides a curve of overfrequency points based on the commanded motor speed. A trip or alarm condition occurs when the frequency rises above this curve at a given speed setting. This function is enabled as follows and locks once the programmable minimum speed is reached. The function can be activated when the speed reference is above a programmable level. Once the motor has started, the function can be activated after a programmable time period.

[0097] Fixed (enabled) underfrequency is used to protect the motor and the connected load 520 from continuous operation at a frequency below the desired frequency. The fixed enabled underfrequency function provides a single underfrequency setting at which a trip or alarm condition is generated when the frequency drops below this value. This function is enabled as follows and is locked once the programmable minimum speed is reached. Once the motor is started, this function can be started after a programmable time period.

[0098] Variable (enabled) underfrequency is used to protect the motor and the connected load 520 from operation at a frequency below the desired frequency or to detect conditions under which the motor frequency drops below the desired setpoint due to excessive load torque problems or other difficulties in the machine or load. The variable enabled underfrequency function provides a curve of underfrequency points based on the commanded motor speed. A trip or alarm condition occurs when the frequency drops below this curve at a given speed setting. This function is enabled as follows and is locked once the programmable minimum speed is reached. This function can be started when the speed reference is above the programmable level. Once the motor is started, this function can be started after a programmable time period.

[0099] Fixed (enabled) high frequency rate of change is used to protect the motor and the connected load 520 from rapidly changing frequencies or high acceleration rates. The fixed enabled high frequency rate of change function provides a single frequency rate of change setting at which a trip or alarm condition is generated when the rate of change of frequency rises above this value. This function is enabled as follows and is locked once the programmable minimum speed is reached. Once the motor is started, this function can be started after a programmable time period.

[0100] In an exemplary embodiment of the present disclosure, the status of the protection parameters (fixed and variable levels) and the temperatures of the RTDs 625, 635 can be displayed on a display or screen such as a keyboard, a control system, or a human-machine interface (HMI). In one example, the central control system 512 can be connected to the display, which is used to display different information and data such as the status of the protection parameters, etc.

[0101] In another exemplary embodiment, the central control system 512 is configured to store faults or alerts related to the operating conditions and associated protection parameters of the electrical load 520, such as in a drive event log. The faults and alerts can then be viewed through the drive event log. As previously described, the operating conditions of the motor (load 520) include normal operating conditions, alert-set operating conditions, and fault-set operating conditions. In one example, when the APM 514 has determined that the motor (load 520) is operating under alert-set operating conditions, the APM 514 outputs corresponding protection parameters, such as a fixed overspeed or variable thermal overload. Thus, the drive system 500, and in particular the central control system 512, controls the VFD 510 and / or the load 520 such that the load 520 is protected and operates, for example, within a predetermined fixed and / or variable level. In the case of a fixed overspeed, a trip or alarm condition is generated when a predetermined speed is exceeded, and the VFD 510 can reduce its output power to slow down the motor and reduce the speed such that the speed is below the fixed overspeed value. Additionally, as previously described, the corresponding protection parameters can be stored and displayed on a display, for example, using a Boolean value, such as "fixed overspeed: 1". When the motor is operating under normal conditions, the drive system 500 can display "fixed overspeed: 0".

[0102] Figure 7 FIG. 700 is a flow chart of a method 700 for protecting an electrical load of a drive system, according to an exemplary embodiment disclosed herein. The method 700 facilitates control and / or protection functions. Although the method is described as a series of actions performed in sequence, it should be understood that the method may not be limited by the sequence order. For example, unless otherwise stated, some actions may occur in a different order than described herein. Additionally, in some cases, one action may occur simultaneously with another action. Furthermore, in some cases, not all actions are required for implementing the method described herein.

[0103] The method may begin at 710 and may include, by operation of at least one processor 513, actions 720: receiving input data from an electrical load 520 coupled to one or more output phases (A, B, C). The method 700 may further include action 730: determining one or more operating states of the electrical load 520 based on the input data, and action 740: outputting one or more protection parameters based on the determined operating states of the electrical load 520 to protect the electrical load 520. At 750, the method may end.

[0104] The method 700 pertains to a drive system that includes a power converter, such as a VFD, and an electrical load, such as an AC induction motor, as previously referenced Figure 5 and Figure 6described, wherein the power converter can be configured to refer to Figure 1 , Figure 2 or Figure 3 as described.

[0105] In another embodiment, method 700 can be performed by at least one processor 513 to include the following actions: controlling an electrical load 520 based on one or more output protection parameters. In another embodiment, the method 700 can include displaying the determined operating conditions and / or associated protection parameters on a display. The determined operating conditions can be normal operating conditions, alarm set operating conditions, or fault set operating conditions of the electrical load 520.

[0106] As previously referred to Figure 5 and Figure 6 described, one or more protection parameters include a predetermined fixed level and a predetermined variable level, which include levels that are functions of the speed of the electrical load 520 and / or levels based on the process curve of the drive system 500. The predetermined fixed levels are selected from the following set: fixed overspeed, fixed underspeed, fixed undercurrent, fixed underpower, fixed torque ripple, fixed negative sequence overcurrent, fixed thermal overload, fixed resistance temperature detector (RTD) protection, fixed instantaneous overcurrent, fixed zero sequence overvoltage, fixed inverse time overcurrent, fixed instantaneous zero sequence overvoltage, fixed maximum power factor, fixed minimum power factor, fixed overfrequency, fixed underfrequency, fixed high frequency rate of change, and combinations thereof. The predetermined variable levels are selected from the following set: variable overspeed, variable underspeed, variable undercurrent, variable thermal overload, variable overfrequency, variable underfrequency, and combinations thereof.

[0107] The systems 500, 600 and method 700 described allow for a protection level that varies with the speed of the motor (load 520). The protection level can match the way the motor (load) parameters themselves change. Since inaccurate protection levels do not allow for the full utilization of the motor, a more accurate protection level is generated, and the motor can be utilized to a greater extent. The fixed and variable protection levels provide comprehensive motor and load monitoring and protection. The drive integrated sensors provide reliable motor feedback for protection across the entire speed range of the motor. All faults or alarms are stored in the drive event log for reference. By integrating the APM 514 into the drive (VFD 510), the protection scheme is simplified, and space and engineering overhead are saved.

[0108] In another exemplary embodiment, a non-transitory computer-readable medium is encoded with processor-executable instructions that, when executed by at least one processor, cause the at least one processor to perform the method for protecting an electrical load 520 coupled to a drive system 500 as described herein, such as the reference method 700.

[0109] It should be understood that actions associated with the method 700, features, and functions (other than any manual actions) may be performed by one or more data processing systems, such as a central control system 512, by running at least one processor 513. As used herein, a processor corresponds to any electronic device configured to process data through hardware circuitry, software, and / or firmware configuration. For example, the processors described herein may correspond to one or more (or a combination) of a microprocessor, a CPU, or any other integrated circuit (IC) or other type of circuitry capable of processing data in a data processing system. As previously mentioned, a processor configured to perform a particular described / claimed process or function may correspond to a CPU that executes computer / processor-executable instructions stored in a memory in the form of software and / or firmware to perform such a described / claimed process or function. However, it should also be understood that such a processor may correspond to an integrated circuit (e.g., an FPGA or ASIC integrated circuit) hardwired to processing circuitry to perform such a described / claimed process or function.

[0110] Furthermore, it should be understood that a processor configured to perform a particular described / claimed process or function may correspond to a combination of a processor and executable instructions (e.g., a software / firmware application) loaded / installed into a memory (volatile and / or non-volatile) that are currently being executed and / or are executable by the processor to cause the processor to perform the described / claimed process or function. Thus, a processor with power off or executing other software but having the described software may also correspond to a processor configured to perform the particular processes and functions described / claimed herein, the described software being installed on a data memory operatively connected to its operation (e.g., on a hard disk or SSD) in a manner set to be executed by the processor (when initiated by a user, hardware, and / or other software).

[0111] In addition, it should be understood that references to "processor" may include multiple physical processors or cores configured to perform the functions described herein. Additionally, it should be understood that a data processing system may also be referred to as a controller that operably controls at least one operation.

[0112] It should also be noted that, although the present disclosure includes descriptions in the context of a full - fledged system and / or a series of actions, those skilled in the art will understand that at least a portion of the mechanisms and / or the described actions of the present disclosure can be distributed in the form of computer / processor - executable instructions (e.g., software and / or firmware instructions) that are contained in a data memory corresponding to any of a variety of forms of non - transitory machine - usable, computer - usable, or computer - readable media. The computer / processor - executable instructions can include routines, sub - routines, programs, applications, modules, libraries, and / or the like. Additionally, it should be understood that the computer / processor - executable instructions can correspond to source code, bytecode, runtime code, machine code, assembly language, Java, JavaScript, Python, Julia, C, C#, C++, or any other form of code and / or can be generated from these codes, which can be programmed / configured to cause at least one processor to perform the actions and features described herein. Further still, the results of the described / claimed processes or functions can be stored in a computer - readable medium; displayed on a display device; and / or the like.

Claims

1. A drive system (500), comprising a power converter (510), the power converter comprising power units supplying power to one or more output phases (A, B, C), each power unit (312) comprising a plurality of switching devices (315a - d), a central control system (512), the central control system communicating with the power converter (510) and controlling the operation of the power units (312), wherein, the central control system (512) comprises an advanced protection module (APM 514) and at least one processor (513), the processor being configured by executable instructions, to receive input data from an electrical load (520) operatively coupled to one or more output phases (A, B, C) using power converter feedback from the electrical load (520); to determine one or more operating conditions of the electrical load (520) based on the input data; and to output one or more protection parameters based on the determined operating conditions of the electrical load (520) to protect the electrical load (520), wherein the advanced protection module (APM 514) interfaces with a resistance temperature detector (RTD) interface (600) for receiving temperature input data associated with the electrical load (520), and wherein the advanced protection module is further configured to obtain accurate input data using a closed - loop Hall effect current sensor and an output attenuator and integrate the input data into the algorithms of the advanced protection module.

2. The drive system (500) according to claim 1, wherein, the resistance temperature detector (RTD) interface (600) comprises a programmable logic controller (610).

3. The drive system (500) according to claim 2, wherein, the programmable logic controller (610) is operatively coupled to at least one temperature detector module (620, 630) and operatively coupled to a network switch (640) for communicating with the central control system (512) and the advanced protection module (APM 514), the resistance temperature detector interface (600) being integrated in the central control system (512) of the drive system (500).

4. The drive system (500) according to claim 1, 2 or 3, wherein, the one or more protection parameters comprise a predetermined fixed level and a predetermined variable level.

5. The drive system (500) according to claim 4, wherein, the one or more protection parameters comprise a level that is a function of the speed of the electrical load (520) and / or a level based on the process curve of the drive system (500).

6. The drive system (500) according to any one of claims 1 to 3, wherein, the one or more operating conditions comprise normal operating conditions, alarm - set operating conditions and fault - set operating conditions of the electrical load (520).

7. The drive system (500) according to claim 4, wherein, The central control system (512) is further configured to control the operation of the power converter (510) and / or the electrical load (520) within a predetermined fixed level and / or a variable level.

8. The drive system (500) according to claim 6, wherein, the central control system (512) is further configured to display on a display one or more operating conditions and associated protection parameters of the electrical load (520).

9. The drive system (500) according to any one of claims 1 to 3, the drive system (500) being configured as a medium voltage variable frequency drive, and the electrical load (520) being configured as a medium voltage alternating current induction motor.

10. A method (700) for protecting an electrical load (520) of a drive system (500), comprising by operating at least one processor (513): Receiving (720) input data from the electrical load (520) coupled to one or more output phases (A, B, C) of the power converter (510) using power converter feedback from the electrical load (520), wherein the input data includes temperature data associated with the electrical load (520) received through a resistance temperature detector (RTD) interface (600); Determining (730) one or more operating conditions of the electrical load (520) based on the input data; and Outputting (740) one or more protection parameters based on the determined operating conditions of the electrical load (520) to protect the electrical load (520), and Using a closed-loop Hall effect current sensor and an output attenuator to obtain accurate input data of the electrical load.

11. The method (700) according to claim 10, further comprising by operating at least one processor (513): Controlling the power converter (510) and / or the electrical load (520) based on the one or more output protection parameters.

12. The method (700) according to claim 10 or 11, further comprising: Displaying on a display the determined operating conditions and / or associated protection parameters.

13. The method (700) according to claim 10 or 11, wherein, the one or more protection parameters include a predetermined fixed level and a predetermined variable level.

14. The method (700) according to claim 13, wherein, the one or more protection parameters include a level as a function of the speed of the electrical load (520) and / or a level based on a process curve of the drive system (500).

15. The method (700) according to claim 13, wherein, The predetermined fixed levels are selected from the group consisting of: fixed overspeed, fixed underspeed, fixed undercurrent, fixed underpower, fixed torque pulsation, fixed negative sequence overcurrent, fixed thermal overload, fixed resistance temperature detector (RTD) protection, fixed instantaneous overcurrent, fixed zero sequence overvoltage, fixed inverse time overcurrent, fixed instantaneous zero sequence overvoltage, fixed maximum power factor, fixed minimum power factor, fixed overfrequency, fixed underfrequency, fixed high frequency rate of change, and combinations thereof.

16. The method (700) according to claim 13, wherein, the predetermined variable levels depend on the speed demand setting or other settings of the drive system (500).

17. The method (700) according to claim 16, wherein, the predetermined variable levels are selected from the group consisting of: variable overspeed, variable underspeed, variable undercurrent, variable thermal overload, variable overfrequency, variable underfrequency, and combinations thereof.

18. The method (700) according to claim 16, wherein, the predetermined variable thermal overload level includes one or more thermal time constants and / or maximum steady state operating current to improve the accuracy of thermal capacity usage prediction.

19. A non-transitory computer-readable medium encoded with processor-executable instructions that, when executed by at least one processor, cause the at least one processor to perform the method for protecting an electrical load (520) coupled to a drive system (500) according to any one of claims 12 to 18.

Images