Overvoltage protection for drives with a variable frequency

Abstract

Overvoltage protection devices, systems, and methods for variable-frequency motor drives are disclosed. In one embodiment, a variable-frequency motor drive system is provided with a rectifier, a DC bus electrically coupled to the rectifier, and an inverter electrically coupled to the DC bus. A protection device is provided that is electrically coupled to the rectifier and configured to interrupt the system's power supply based on fault conditions.A controller is provided and configured to receive information indicating a voltage of the DC bus, to process the received information to model a state of at least one component of the variable frequency motor drive, and to control the protection device to interrupt the power supply to the variable frequency motor drive when the modeled state meets a disturbance criterion.

Description

BACKGROUND

[0001] Variable frequency motor drives offer several potential advantages, including improved efficiency, control, and performance. However, they also present several design challenges, including the need to protect power electronics circuits and components from voltage surges that can lead to catastrophic failures. Various approaches have been taken to address these issues, including the placement of chokes, fuses, and / or circuit breakers between a power supply and an input stage of the drive; the use of metal oxide varistors, interference suppression circuits, and / or oversized DC bus capacitors within the drive; and the use of transistor-based switches or active transistor-based voltage limiting or circuitry.While these approaches offer some advantages, they all also have limitations and shortcomings. These disadvantages can be particularly significant for high-load drive applications, such as compressor drives for heating, ventilation, air conditioning, or refrigeration (HVACR) systems, including refrigeration systems. There remains a considerable need for the unique and inventive overvoltage protection for variable-frequency drives, and for the devices, methods, and systems disclosed herein. REVELATION

[0002] For the purpose of a clear, concise, and precise description of embodiments of the invention, its nature, manufacturing process, and use, and to facilitate its exercise, manufacture, and use, reference is now made to specific embodiments, including those shown in the figures, and special formulations are used to describe them. It is understood that this does not limit the scope of protection of the invention and that the invention encompasses and protects such modifications, alterations, and further applications of the embodiments that would occur to a person skilled in the art in the field of the invention. SUMMARY

[0003] Unique devices, methods, and systems for overvoltage protection of variable frequency drives are disclosed. In one embodiment, a variable frequency motor drive system is provided, comprising a rectifier, a DC bus electrically coupled to the rectifier, and an inverter electrically coupled to the DC bus. A protective device is provided, electrically coupled to the rectifier and configured to interrupt the system's power supply based on fault conditions.A controller is provided and configured to receive information indicating a voltage on the DC bus, to process the received information to model the state of at least one variable-frequency motor drive component, and to control the protection device to interrupt the power supply to the variable-frequency motor drive when the modeled state meets a fault criterion. Further embodiments, forms, functions, features, advantages, aspects, and benefits will become apparent from the following description and drawings. BRIEF DESCRIPTION OF THE FIGURES

[0004] Fig. Figure 1 is a schematic representation of an exemplary HVACR system.

[0005] Fig. Figure 2 is a schematic representation of an exemplary motor drive system with a variable frequency.

[0006] Fig. Figure 3 is a flowchart illustrating an exemplary diagnostic and control procedure.

[0007] Fig. Figure 4 is a diagram illustrating characteristics of an example operating current trip function.

[0008] Fig. Figure 5 is a diagram illustrating characteristics of an exemplary operating current release function along with other drive characteristics. DETAILED DESCRIPTION

[0009] It will now be on Fig. 1. Reference is made to an exemplary HVACR system. 100 This illustrates a refrigerant circuit with a compressor. 110 , a capacitor 120 and an evaporator 130 encompasses. Through the system 100 Refrigerant flows in a closed circuit from the compressor. 110 to the capacitor 120 , to the evaporator 130 and back to the compressor 110. Various embodiments may also include further refrigerant circuit elements, such as valves for controlling the refrigerant flow, refrigerant filters, preheaters, oil separators and / or cooling components and flow paths between different system components.

[0010] The compressor 110 is powered by a drive unit 150 powered by a permanent magnet electric motor 170 includes a drive with a variable frequency 155 is driven. In the illustrated embodiment, the drive operates at a variable frequency. 155 set up to output a three-phase PWM drive signal, and the motor 170This is a motor whose permanent magnet has a magnetic surface. The use of other types and configurations of variable speed drives or variable frequency drives, and other types of electric motors such as motors whose permanent magnet is an embedded magnet, and induction motors, is also considered.

[0011] The capacitor 120 is designed to use the compressed refrigerant that it receives from the compressor 110 receives heat to transfer. In the illustrated embodiment, the capacitor 120 a water-cooled condenser, which is attached to an inlet 121 receives cooling water, transfers heat from the refrigerant to the cooling water, and at an outlet 122It releases cooling water. It is also considered that other types of capacitors may be used, for example, air-cooled capacitors or evaporative capacitors. Furthermore, it is understood that references to water here include aqueous solutions and may contain additional components, unless otherwise specified.

[0012] The evaporator 130 is set up to be from the capacitor 120 To receive refrigerant, to expand the refrigerant to lower its temperature, and to transfer heat from a cooled medium to the refrigerant. In the illustrated embodiment, the evaporator 130 designed as a water cooler connected to an inlet 131 receives the supplied water, transfers heat from the water to the refrigerant, and discharges it at an outlet. 132It discharges chilled water. Consideration is being given to the use of several special types of evaporators and refrigeration systems, including, but not limited to, dry expansion evaporators, flood-type evaporators, smooth-tube evaporators, plate surface evaporators, and finned-tube evaporators.

[0013] Furthermore, the HVACR system includes 100 a controller 160 , which sends control signals to the drive at a variable frequency 155 spends to operate the engine 170 and the compressor 110 to control the controller 160 It also receives information about the operation of the drive unit. 150 In exemplary implementations, the controller receives 160This includes information related to the motor current, the motor cut-off voltage, and / or other operating characteristics of the motor, as well as information about the DC bus voltage, as described below. It is understood that the controllers, control routines, and control modules described herein can be implemented using hardware, software, firmware, and various combinations thereof, and can use executable instructions that may be stored on one or more non-volatile, computer-readable media. Furthermore, it is understood that the controller 160 It can be provided in various forms and can include a number of hardware and software modules and components.

[0014] It will now be on Fig. 2 Reference is made to a circuit diagram of an exemplary electric motor drive system with a variable speed 200The system is shown. 200 is connected to a power source 210 connected, such as a power supply network or an emergency power generator that supplies alternating current to a protective device 220 emits. The protective device 220 This could be a shunt trip circuit breaker, a contactor, or another type of device designed to protect the power source. 210 to disconnect based on a received control signal from downstream components. Normally, the protective device provides 220 Three-phase current to a mains filter 230 However, it can be controlled to interrupt the power supply upon receiving a command or control signal. The protective device 220It is capable of disconnecting the supply voltage from the assembly within a specified time period when it receives a trip signal or command, preferably within a reactive power cycle. The components comprising the variable-speed drive assembly, including rectifiers, capacitors, MOVs, and the inverter, which are described in more detail below, are selected to maintain normal operation at a normal supply voltage with a tolerance, and the protective device 220 It is controlled in such a way that operation only takes place within the tolerance.

[0015] The mains filter 230 includes a choke designed to dampen harmonics in order to reduce losses resulting from harmonic feedback from drive components to the power source 210 can occur. The mains filter 230three-phase alternating current is supplied to a rectifier 290 from which converts the alternating current into direct current and the direct current to a DC intermediate circuit 285 delivers. In the illustrated embodiment, the rectifier 290 set up as a 6-pulse diode bridge, however, it is understood that 12-pulse, 18-pulse, 24-pulse or 30-pulse rectifiers can be used together with phase-shifting transformers that provide suitable phase-shifted inputs for 12-pulse, 18-pulse, 24-pulse or 30-pulse operation, and that other types of rectifiers or AC-DC converters than those comprising active components can be used.

[0016] The DC link 285 includes a metal oxide varistor (MOV) 286 and a capacitor 287 The MOV 286It is designed to limit the DC bus voltage and provides protection against excessively high transient voltages by diverting current caused by the increased voltage from other system components until a failure point is reached. In further embodiments, the DC link can be further modified. 285 , which also serves to limit the voltage, may contain various components, for example, components with a short settling time, semiconductor components, diode-based limiting circuits, and other types of voltage limiting. The capacitor 287 is set up to handle the rectifier output 280 to smooth out and attenuate harmonic feedback from the inverter switching. In the illustrated embodiment, the capacitor 287A film capacitor with a relatively low capacitance compared to electrolytic capacitors. Other embodiments can use capacitors with a higher capacitance, such as electrolytic capacitors. The output of the DC link 285 is connected to the inverter input 290 coupled.

[0017] The inverter 290 includes switches connected to the positive and negative leads of the DC intermediate circuit 285 are connected. In the illustrated embodiment, the inverter's switches are 290 Designed as IGBT- and diode-based switches, other types of power electronic switching elements such as power MOSFETs or other electrical switching devices can also be used. The inverter 290 It is controlled in such a way that it sends a pulse-width modulated (PWM) voltage signal to the terminals. 271 ,272 and 273 of the engine 270 spends.

[0018] The engine 270 It has a stator, a rotor, and a gap between the rotor and the stator. The motor connections 271 , 272 and 273 are in the stator 271 The provided windings are connected. In exemplary embodiments, the motor is a permanent magnet motor that has several permanent magnets provided on the surface or inside the rotor.

[0019] The system 200 It also includes a control circuit. 240 , which includes both a microprocessor-based digital signal processor (DSP) 241 and a gate drive module 242 as well as other components that ensure the ability to communicate internally and externally. The DSP 241It is designed to receive control commands specifying a desired system operation, to process the received commands in order to send switch control signals to the drive module. 240 to supply, which in turn sends control signals to the switching elements of the inverter. 290 outputs. In the illustrated embodiment, the switch control signals are IGBT gate control voltages, which are amplified to a final control voltage by intermediate amplifier units. However, it is considered that, depending on the inverter design, 290 A variety of different designs could be used.

[0020] The DSP 241 receives information from a number of other components of the system 200 From one perspective, the DSP 241 set up to receive information indicating the voltage of the DC bus 285This received information can be used to provide control functionality for a variety of other components, including the protective device. 220 to be provided. In exemplary implementations, the DSP 241 designed to implement a fail-safe trip function that activates the protective device 220 It triggers to create a power-free state in order to prevent catastrophic system failures. In certain forms, the protective device 220 A shunt trip circuit breaker is used; however, it is understood that contactors, solid-state switches, or other devices designed to disconnect or interrupt a circuit may be used. Therefore, it is understood that although certain embodiments are described with reference to a shunt trip circuit breaker, this description also applies to other protective devices unless otherwise specified.

[0021] During transient processes or faulty / full power supply, the system may experience problems. 200Voltages exceeding the normal values ​​and usual tolerances of the power supply may be present. In typical cost / performance optimized systems, many components cannot tolerate these overvoltages for the time it takes for the voltage to be detected and the protective device to trip due to an overcurrent condition. Successful operation and protection of these components can be achieved by selecting component technologies, values, and voltage ratings such that the component is tolerant of operation at an input voltage above the intended normal tolerance range and, in accordance with the limiting impedance, tolerates atypical voltage fluctuations caused by malfunctions / failures in the power supply up to a detected value and for a time sufficient for the protective device to trip.The techniques disclosed here can be implemented to provide proactive protection for drive components by limiting their operation to the nominal values ​​for which they are designed and completely avoiding harmful overcurrent conditions.

[0022] In certain embodiments, the DSP is used 241A shunt trip function is implemented, preferably configured to energize a shunt trip relay of a circuit breaker located between a power supply and a variable-frequency drive, in order to prevent catastrophic drive failures. In exemplary embodiments, the shunt trip function is designed to detect the DC bus voltage, identify failure mechanisms, and prevent failures where possible. The shunt trip function is preferably configured to interrupt the current to the drive before the failure causes the circuit breaker to trip via its overcurrent protection function.

[0023] For example, an analysis of an exemplary drive system with a variable frequency, in conjunction with the above, revealed Fig. Based on the two described features, various criteria can be identified that an exemplary short-circuit trip function must fulfill, including MOV energy absorption curves, a DC bus overvoltage, and practical limits to the response time of the protection device and detection circuitry. In the present example, the MOV was identified as the system component most sensitive to overvoltage faults, and the MOV connection curves and energy absorption curves were used to determine failure criteria. However, it is understood that in other embodiments, other components or devices with different minimum conduction curves can be used as the basis for modeling failure modes.In some embodiments, the minimum conduction curve can be a function of the characteristics of several components or elements as a whole, for example, several MOVs in parallel or series connection, MOVs connected in parallel with other protective devices to extend their minimum conduction curve, or other combinations of protective devices.

[0024] During commissioning, a DC bus overvoltage occurs, and it is most severe when the bus MOV happens to follow the maximum line curve. Practical limitations, such as a delay time when the coil is energized and the magnitude limit of the bus voltage sensing circuits, restrict the upper left voltage-time limit before a feasible protection function can be implemented. Based on these considerations, an exemplary operating current trip function can be defined as a sixth-order differential voltage accumulator with respect to time, for example, as described by Equation 1: (VDC – STminV) 6 ·t = (STmaxV – StminV) 6 ·STtime (1)

[0025] In Equation 1, VDC is the voltage of the DC bus, STminV is the minimum voltage that could theoretically trigger a shunt trip, STmaxV is a threshold above which a shunt trip should always be triggered, STtime is the time required to trigger a shunt trip when the DC bus voltage equals STmaxV, and t is the time. For simplicity, the terms (STmaxV – StminV) 6 STtime can be considered a constant STK, and a substitution can be made which yields a working current trip function according to equation 2: (VDC – STminV) 6 ·t = STK (2)

[0026] It is understood that the operating current trigger functions according to equation 1 or 2 can be provided in a variety of implementations.

[0027] In certain embodiments, the behavior of the MOV can be modeled in more detail by adding a derivative constant to the accumulator. For example, if an MOV has a maximum power dissipation of 2.5 W and a maximum energy input of 2325 J, and a conservative assumption is made that the maximum power dissipation is a constant rate, the behavior of the MOV can be modeled based on these parameters. If the MOV receives a relatively large current surge at 2325 J, it will require 2325 J / 2.5 W, or 930 seconds, to cool down. A short-circuit current function can be provided that accounts for this behavior. With four time constants, the function will decay to 0.67 percent of its original value, so 930 / 4 yields a time constant of 232.5 seconds. At 12 kHz, 2.325 seconds multiplied by 1200 samples per second yields 2,790,000 samples.Consequently, adding a multiplier constant of 2790000 / 2790001 or 0.99999964 to the operating current trip function will allow it to decay at approximately the required time constant to simulate the derivative by the MOV.

[0028] It will now be on Fig. 3 Reference is made where a flow chart according to an exemplary working current release control process 300 The process is illustrated. 300 This is an exemplary implementation of the techniques described above, which models the physical system of one or more drive components and is designed to proactively disconnect the drive from the power source at a variable frequency in the event of an impending failure. This protects a drive from catastrophic failures that might otherwise require replacing the entire drive.

[0029] In the process 300 The operator receives 310Calibratable sizes STmaxV 301 , STminV 302 and STtime 303 as inputs. The operator also receives 310 voltage sampling VDC(k) 304 The calibratable sizes 301 , 302 and 303 These can be pre-defined values ​​stored in a computer-readable memory of a controller, which are then executed by an executable routine that controls the process. 300 They can be implemented, retrieved, and used. The values ​​of the calibratable quantities. 301 , 302 and 303 can be determined empirically and can vary depending on the specific components used in the variable frequency drive and the size of the drive to which the process is applied.

[0030] The calibratable size STmaxV 301This serves as a threshold. Any voltage greater than the STmaxV value can be used to trigger a short circuit. Furthermore, the calibratable value STmaxV 301 in conjunction with the operating current trip time variable STtime 303 the “knee” of the operating current tripping curve is fixed, for example as in connection with Fig. 4 described. The calibratable quantity STminV 302 STtime is the minimum voltage that could theoretically trigger a short circuit breaker. Since the short circuit breaker trip is a power function and therefore asymptotic, it would take an infinitely long time to reach this minimum; nevertheless, the variable is used advantageously. The calibratable quantity STtime 303 is the time required to trigger a short circuit trip function when the voltage is equal to, greater than, or equal to the maximum voltage of the short circuit trip. 301of the operating current release. The variable 304 VDC(k) is a sample value of the DC bus voltage, which can be detected, measured, or determined using a voltage sensing circuit.

[0031] The operator 310 It processes the inputs it receives to determine an accumulated operating current trip value according to the following equation 3: STaccum(k) = (VDC(k) – STminV) 6 ·Ts + STaccum(k – 1) (3) where k is a sample number, STaccum is the accumulated operating current trip value for a given sample number, VDC is a DC link voltage for a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the time. Furthermore, embodiments using the above terms together with a derivative constant Kb can determine an accumulated operating current trip value according to the following equation 4: STaccum(k) = (VDC(k) – STminV) 6 ·Ts + STaccum(k – 1) ·Kb (4)

[0032] From the operation 310 The process continues 300 to the operator 320 next. The operator 320 Determines whether the accumulated operating current trip value STaccum(k) is greater than the operating current trip constant STK. If STaccum(k) is not greater than STK, the process restarts. 300 for the operation 310back. If STaccum(k) is greater than STK, the process goes back. 300 for the operation 330 about, which activates a shunt trip by sending a shunt trip command. It is understood that the shunt trip command can be used with a variety of devices, including, but not limited to, a shunt trip circuit breaker, a contactor, a solid-state switch, or any other device capable of producing a de-energized state in response to a signal or command.

[0033] It will now be on Fig. Reference is made to Figure 4, which shows a diagram with the DC voltage on the vertical axis versus the logarithmic time on the horizontal axis, in units of seconds. The diagram in Fig. Figure 4 illustrates the characteristics of an example of a short-circuit current trip function. A voltage 410is defined as the maximum voltage of the operating current release. A voltage 420 is defined as the minimum voltage of the short-circuit release. And the short-circuit release time extends over a duration 430 .

[0034] It will be on Fig. Reference is made to Figure 5, which shows a diagram of DC voltage versus logarithmic time in units of seconds. Fig. Figure 5 illustrates a maximum conduction curve. 501 and a minimal conduction curve 502 , which define the maximum and minimum time-voltage curves for a MOV failure. MOVs exhibit statistical fluctuations in these time-voltage conduction curves. The maximum conduction curve 501 This represents the upper statistical limit above which all MOVs in a statistical sample group fail under stationary conditions. The minimum conduction curve 502represents the lower statistical limit below which no MOV will occur in the sample group under stationary conditions. The area between the curves 501 and 502 Contains all failure points for MOVs in the sample. Depending on the desired confidence level, an attempt can be made to approximate a curve using a work-stream trigger function, to follow a curve, or to maintain a specific distance from a curve, with this curve lying between or on the curves 501 and 502 lies. In certain exemplary embodiments, a working current release function will tend to follow the curve. 502 to follow. The operating current release function 505 is shown as an example of such a function, which generally tends to follow the curve 502The intended behavior is to follow the given curve, but as a compromise to keep the complexity of the calculations low, a certain degree of deviation is permitted. It is understood that other operating current tripping function curves can also be used, following different paths than the one shown in the example.

[0035] Fig. 5 further illustrates a time 503 of a shunt trip circuit breaker, which represents the minimum response time of a circuit breaker or contactor, and a maximum DC bus detection voltage 506 at the point that represents the maximum DC bus sensing input level. The time 503 and the tension 506These parameters effectively define the range within which the shunt trip function can be expected to activate, as they define the limits for the operation of system components. These parameters are useful for setting the value of a shunt trip function in such a way as to allow for the delay in the circuit breaker's response time. For example, the shunt trip function 505 until the time 503 a constant value and does not attempt to follow the curve. 502 to follow. This creates some leeway to adapt to a system component delay. Furthermore, it shows Fig. 5 a maximum conduction curve 504 for a 480 V pre-charge, which represents the theoretical worst-case scenario for transient response during startup. The curve 504 This represents a worst-case scenario for a settling-in period during commissioning. The curve 504is usable to define a limit condition for an unwanted shutdown. A work current trip function that applies to a track above the curve. 504 This will prevent unnecessary, unwanted shutdowns during commissioning events.

[0036] It is understood that the exemplary embodiments summarized and described in detail above and shown in the figures serve only for illustration and are not to be interpreted as limiting or restrictive. Only the currently preferred embodiments have been presented and described, and all modifications and variations that fall within the scope of the invention are to be protected. It is understood that the embodiments and forms described below can be combined in certain cases and may be mutually exclusive in other cases. It is also understood that the embodiments and forms described below can be combined with other aspects and features disclosed elsewhere herein.It is understood that various features and aspects of the embodiments described above may not be necessary, and embodiments lacking these features and aspects are also protected. When reading the claims, the use of words such as "a," "a," "at least a," "at least one," or "at least a part" should not be interpreted as limiting the claim to only one element, unless expressly stated otherwise in the claim. When the terms "at least a part" and / or "a part" are used, the element may comprise a part and / or the entire element, unless expressly stated otherwise.

Claims

[1] Procedure, encompassing: Operating a variable frequency frequency motor drive comprising an AC-DC converter electrically coupled to an AC source, a DC link electrically coupled to the AC-DC converter, and a DC-AC converter electrically coupled to the DC link; Receiving information indicating the voltage of the DC link at a microprocessor-based controller; Processing the received information to model a state of at least one power electronics component of the motor drive with a variable frequency; and Disconnecting the AC-DC converter from the AC-DC power source when the modeled state meets a protection criterion. [2] Method according to claim 1, wherein the at least one power electronics component comprises a varistor which is electrically coupled to the positive and negative rails of the DC intermediate circuit. [3] Method according to claim 2, wherein the processing models a heat capacity and a power dissipation of the varistor. [4] Method according to claim 1, wherein the processing utilizes a function that includes a sixth-order stress differential. [5] Method according to claim 4, wherein the processing involves calculating an accumulated value according to the equation ACCUM(k) = (VDC(k) – MINV) 6 ·Ts + ACCUM(k – 1) includes, where k is a sample number, ACCUM is the accumulated value for a given sample number, VDC is a DC link voltage for a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the sampling time. [6] Method according to claim 5, wherein the protection criterion comprises determining whether the value of ACCUM is greater than a predetermined value. [7] Method according to claim 1, wherein the disconnection comprises supplying a control signal to a shunt trip circuit breaker which is electrically coupled between the AC power source and the AC-DC converter. [8] Method according to claim 7, wherein the rectifier comprises a 6-pulse diode bridge and the inverter comprises an IGBT-based three-phase circuit. [9] Method according to claim 1, wherein the AC-DC converter comprises a rectifier and the DC-AC converter comprises an inverter. [10] Method according to claim 4, wherein the processing involves calculating an accumulated value according to the equation ACCUM(k) = (VDC(k) – MINV) 6 ·Ts + ACCUM(k – 1)·Kb includes, where k is a sample number, ACCUM is the accumulated value for a given sample number, VDC is a DC link voltage for a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, Ts is the sampling time, and Kb is a derivative constant. [11] System, encompassing: a variable frequency motor drive comprising a rectifier, a DC bus electrically coupled to the rectifier, and an inverter electrically coupled to the DC bus; a protective device that is electrically coupled to the rectifier and is designed to interrupt the power supply to the motor drive at a variable frequency based on a received trigger signal; a controller configured to receive information indicating a voltage of the DC bus, to process the received information to model a state of at least one component of the variable frequency motor drive, and to send a trip signal to the protection device to interrupt the power supply to the variable frequency motor drive when the modeled state meets a disturbance criterion. [12] System according to claim 11, wherein the at least one component comprises a metal oxide varistor. [13] System according to claim 11, wherein the controller is configured to determine a sixth-order differential voltage and to use the determined differential in connection with controlling the protective device. [14] System according to claim 11, wherein the controller is configured to store an accumulated value according to the equation ACCUM(k) = (VDC(k) – MINV) 6 ·Ts + ACCUM(k – 1) to determine where k is a sample number, ACCUM is the accumulated value for a given sample number, VDC is a DC link voltage for a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the sampling time. [15] System according to claim 14, wherein the controller is configured to determine whether the value of ACCUM is greater than a disturbance threshold and, on the basis of reaching or exceeding the disturbance threshold, to send the trigger signal to the protective device. [16] System according to claim 11, wherein the controller is configured to store an accumulated value according to the equation ACCUM(k) = (VDC(k) – MINV) 6 ·Ts + ACCUM(k – 1)·K to determine where k is a sample number, ACCUM is the accumulated value for a given sample number, VDC is a DC link voltage for a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, Ts is the time, and K is a constant. [17] Device comprising: a non-volatile, computer-readable medium configured with instructions that can be executed by a computer to: to determine a model state of at least one component of a variable-frequency drive system comprising an AC-DC converter, a protective device electrically coupled between the AC-DC converter and the AC power source, and a DC link electrically coupled to the AC-DC converter, based on sampled DC link voltage information; and to control the protection device in order to disconnect the AC-DC converter from the AC power source based on the specified model state. [18] Device according to claim 17, wherein the model state is an accumulated differential voltage value and the at least one component comprises a varistor. [19] Device according to claim 17, wherein the model state is an accumulated value determined according to the equation ACCUM(k) = (VDC(k) – MINV)6 ·Ts + ACCUM(k – 1) is determined where k is a sample number, ACCUM is an accumulated value at a given sample number, VDC is a DC link voltage at a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the time. [20] Device according to claim 19, wherein the accumulated value is compared with a threshold and the comparison is used to control the protective device. [21] Device according to claim 17, wherein the model state is an accumulated sixth-order differential stress value. [22] Device according to claim 21, wherein the model state according to the equation ACCUM(k) = (VDC(k) – MINV) 6 ·Ts + ACCUM(k – 1)·Kb is determined where k is a sample number, ACCUM is an accumulated value at a given sample number, VDC is a DC link voltage at a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, Ts is the time, and Kb is a constant that takes into account the properties of a varistor.