Overvoltage protection for drives with a variable frequency

Abstract

Procedure, comprehensive: Operating a variable frequency frequency motor drive (155, 200) comprising an AC-DC converter (280) electrically coupled to an AC source (210), a DC link (285) electrically coupled to the AC-DC converter (280), and a DC-AC converter (290) electrically coupled to the DC link (285); Receiving information indicating a DC intermediate circuit voltage (285) at a microprocessor-based controller (160); Processing the received information to model a state of at least one power electronics component of the motor drive (155, 200) with a variable frequency; and Disconnecting the AC-DC converter (280) from the AC-DC power source (210) when the modeled state meets a protection criterion, and where the processing uses a function that contains a voltage difference of the sixth power, and a calculation of an accumulated value according to the equation: STaccum (k) = (VDC (k) − MINV) 6 * Ts + STaccum (k − 1) includes, where k is a sample number, STaccum is the accumulated value at a given sample number, VDC is a DC link voltage (285) at a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the sampling time.

Description

BACKGROUND

[0001] Variable-frequency motor drives offer several potential advantages, including opportunities for improved efficiency, control, and performance. However, they also present several design challenges, including the need to protect power electronics circuits and components from overvoltages that can lead to catastrophic failures. Various attempts have been made to address these issues, including providing chokes, fuses, and / or circuit breakers between a power supply and a drive input stage; using metal-oxide varistors, suppressor circuits, and / or oversized DC bus capacitors in the drive; and using transistor-based switches or active transistor-based voltage limiting or snubbering.While these approaches provide some advantages, they all 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, such as refrigeration systems. There remains a significant need for the unique and inventive surge protection for variable frequency drives, devices, methods, and systems disclosed herein.

[0002] JP H08 - 79 963 A D1 discloses a system with a DC bus with two capacitors connected in series. To protect the lower of the two capacitors, the voltage across the lower capacitor is monitored. If this voltage VRB2 falls below a minimum limit or rises above a maximum limit, the controller deactivates a relay switch, which in turn activates another switch to shut off the power supply. JP H06 - 105 456 A discloses a system that disconnects its drive from a power source in the event of an overvoltage condition. A simple, constant threshold value (overvoltage detection value) is used to determine whether an overvoltage condition exists. JP 2001- 241 732 A relates to a system that detects an abnormally high voltage in a power supply.If an unusually high voltage of the power supply is detected relative to a predetermined limit, the operation of an inverter is first stopped and a relay is opened to disconnect the power supply. WO 2011 / 026250 A1 discloses a generator system with a voltage limiter arranged on a secondary side of the generator. Upon detection of a change in the amplitude of a grid voltage, the voltage limiter is activated, whereby the voltage level at the inlet of the converter is kept within the operating voltage range of the voltage limiter 5 so that no current flows into the converter. Several limit values, each of which is constant over time, are used to define different operating ranges. US 2011 / 0210553 A1 describes a system with a generator that is intended to supply electrical energy to the power grid.In this process, an intermediate circuit DC voltage is measured and compared with limit values ​​in order to start a DC breaker or to activate overvoltage protection. DE 10 2004 026 950 A1 describes a method for regulating the electrical system of a hybrid vehicle, which provides that, during an engine operating phase at a predetermined speed, a desired operating voltage is determined for a DC bus integrated into the electrical system. The desired operating voltage lies between a minimum operating voltage at the predetermined speed and a maximum operating voltage at the predetermined speed, wherein the minimum operating voltage is determined by a parameter of the energy storage device. It is an object of the invention to propose a method for overvoltage protection of a variable-frequency motor drive, a system, and a device.This object is achieved by a method according to the invention according to claim 1, a system according to the invention according to claim 9 and a device according to the invention according to claim 13. REVELATION

[0003] For the purpose of clearly, concisely, and accurately describing embodiments of the invention, the nature and method of making and using the same, and to facilitate the practice, manufacture, and use of the same, reference will now be made to specific embodiments, including those shown in the figures, and specific language will be used to describe them. It is to be understood that this does not limit the scope of the invention, and that the invention is intended to cover and protect such variations, modifications, and other applications of the embodiments as would occur to one skilled in the art to which the invention pertains. SUMMARY

[0004] 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, including a rectifier, a DC bus electrically coupled to the rectifier, and an inverter electrically coupled to the DC bus. A protection device is provided, electrically coupled to the rectifier and configured to interrupt power to the system based on fault conditions.A controller is provided and configured to receive information indicative of a DC bus voltage, process the received information to model a condition of at least one component of the variable frequency motor drive, and control the protection device to interrupt the power supply to the variable frequency motor drive when the modeled condition meets a fault criterion. Further embodiments, forms, objects, features, advantages, aspects, and benefits will become apparent from the following description and drawings. BRIEF DESCRIPTION OF THE CHARACTERS Fig. Figure 1 is a schematic diagram of an example HVACR system. Fig. Figure 2 is a schematic diagram of an exemplary variable frequency motor drive system. Fig. 3 is a flowchart illustrating an example diagnostic and control procedure. Fig. Figure 4 is a diagram illustrating characteristics of an exemplary shunt trip function. Fig. Figure 5 is a diagram illustrating characteristics of an exemplary shunt trip function along with other drive characteristics. DETAILED DESCRIPTION

[0005] It will now Fig. 1, which illustrates an exemplary HVACR system 100 including a refrigerant circuit with a compressor 110, a condenser 120, and an evaporator 130. Refrigerant flows through the system 100 in a closed loop from the compressor 110 to the condenser 120, to the evaporator 130, and back to the compressor 110. Various embodiments may also include additional refrigerant circuit elements, such as valves for controlling refrigerant flow, refrigerant filters, preheaters, oil separators, and / or cooling components, and flow paths between various system components.

[0006] The compressor 110 is driven by a drive unit 150 comprising a permanent magnet electric motor 170 driven by a variable frequency drive 155. In the illustrated embodiment, the variable frequency drive 155 is configured to output a three-phase PWM drive signal, and the motor 170 is a motor whose permanent magnet has a magnetic surface. The use of other types and configurations of variable speed 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 contemplated.

[0007] Condenser 120 is configured to transfer heat from the compressed refrigerant it receives from compressor 110. In the illustrated embodiment, condenser 120 is a water-cooled condenser that receives cooling water at an inlet 121, transfers heat from the refrigerant to the cooling water, and discharges cooling water at an outlet 122. It is also contemplated that other types of condensers may be used, such as air-cooled condensers or evaporative condensers. Furthermore, it is understood that references to water herein include aqueous solutions and may contain additional components unless otherwise limited.

[0008] Evaporator 130 is configured to receive refrigerant from condenser 120, expand the refrigerant to reduce its temperature, and transfer heat from a cooled medium to the refrigerant. In the illustrated embodiment, evaporator 130 is configured as a water chiller that receives water supplied to an inlet 131, transfers heat from the water to the refrigerant, and discharges cooled water at an outlet 132. It is contemplated that a variety of specific types of evaporators and refrigeration systems may be used, including, but not limited to, dry expansion evaporators, flooded evaporators, smooth tube evaporators, plate surface evaporators, and finned tube evaporators.

[0009] The HVACR system 100 further includes a controller 160 that outputs control signals to the variable frequency drive 155 to control the operation of the motor 170 and the compressor 110. The controller 160 also receives information about the operation of the drive unit 150. In exemplary embodiments, the controller 160 receives 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 may be implemented using hardware, software, firmware, and various combinations thereof, and may utilize executable instructions stored in one or more non-transitory computer-readable media.Further, it is understood that the controller 160 may be provided in various forms and may include a number of hardware and software modules and components.

[0010] It will now Fig. 2, a circuit diagram of an exemplary variable-speed electric motor drive system 200 is shown. The system 200 is connected to a power source 210, such as a utility grid or a standby generator, which supplies alternating current to a protection device 220. The protection device 220 may be a shunt trip circuit breaker, a contactor, or another type of device configured to disconnect the power source 210 from downstream components based on a received control signal. Typically, the protection device 220 supplies three-phase power to a line filter 230, but may be controlled to interrupt power upon receipt of a command or control signal.The protection device 220 is capable of disconnecting the supply voltage from the assembly within a specified time period when provided with a trip signal or command, preferably within one reactive power cycle. The components comprising the variable speed drive assembly, including the rectifier, capacitors, MOVs, and the inverter, described in more detail below, are selected to maintain normal operation at a normal supply voltage applied within a tolerance, and the protection device 220 is controlled to operate only within the tolerance.

[0011] The line filter 230 includes a choke configured to attenuate harmonics to mitigate losses that may arise from harmonic feedback from drive components to the power source 210. The line filter 230 outputs three-phase alternating current to a rectifier 290, which converts the alternating current to direct current and supplies the direct current to a DC link 285. In the illustrated embodiment, the rectifier 290 is configured as a 6-pulse diode bridge, but it should be understood that 12-pulse, 18-pulse, 24-pulse, or 30-pulse rectifiers may be used in conjunction with phase-shifting transformers that provide phase-shifted inputs suitable for 12-pulse, 18-pulse, 24-pulse, or 30-pulse operation, and that other types of rectifiers or AC-to-DC converters than those including active devices may be used.

[0012] The DC link 285 includes a metal oxide varistor (MOV) 286 and a capacitor 287. The MOV 286 is configured to limit the DC bus voltage and provides protection against excessive transient voltages by shunting current caused by the elevated voltage away from other system components until a failure point is reached. In further embodiments, the DC link 285, which also serves to limit the voltage, may include various devices, such as fast settling time devices, solid-state devices, diode-based clamping circuits, and other types of voltage clamps. The capacitor 287 is configured to smooth the output of the rectifier 280 and mitigate harmonic feedback from inverter switching.In the illustrated embodiment, capacitor 287 is a film capacitor with a relatively low capacitance compared to electrolytic capacitors. Other embodiments may use capacitors with higher capacitance, such as electrolytic capacitors. The output of DC link 285 is coupled to the input of inverter 290.

[0013] Inverter 290 includes switches connected to the positive and negative lines of DC link 285. In the illustrated embodiment, the switches of inverter 290 are configured as IGBT and diode-based switches, but other types of power electronic switching elements, such as power MOSFETs or other electrical switching devices, may also be used. Inverter 290 is controlled to output a pulse-width modulated (PWM) voltage signal to terminals 271, 272, and 273 of motor 270.

[0014] The motor 270 includes a stator, a rotor, and a gap between the rotor and the stator. The motor terminals 271, 272, and 273 are connected to windings provided in the stator 271. In embodiments, the motor is a permanent magnet motor having a plurality of permanent magnets provided on the surface or inside the rotor.

[0015] The system 200 further includes a control circuit 240, which includes a microprocessor-based digital signal processor (DSP) 241 and a gate drive module 242, as well as other components that provide internal and external communication capability. The DSP 241 is configured to receive control commands indicating a desired system operation, process the received commands, and provide switch control signals to the drive module 240, which in turn outputs control signals to the switching elements of the inverter 290. In the illustrated embodiment, the switch control signals are IGBT gate control voltages that are amplified to a final control voltage by intermediate amplifier units. However, it is contemplated that a variety of different configurations could be used depending on the configuration of the inverter 290.

[0016] The DSP 241 receives information from a number of other components of the system 200. In one aspect, the DSP 241 is configured to receive information indicative of the voltage of the DC bus 285. This received information can be used to provide control functionality for a variety of other components, including the protection device 220. In embodiments, the DSP 241 is configured to implement a shunt trip function that trips the protection device 220 to create a de-energized state to prevent catastrophic system failures. In certain forms, the protection device 220 is a shunt trip circuit breaker, but it is understood that contactors, solid-state switches, or other devices configured to disconnect or interrupt a circuit may be used.Therefore, it is to be 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.

[0017] During transient or faulty / full power conditions, System 200 may experience voltages that exceed the normal values ​​and typical tolerances of the supply. In exemplary cost / performance-optimized systems, many components cannot tolerate these overvoltages beyond the time it takes for the voltage to be sensed and the protective device to trip due to an overcurrent condition. Successful operation and protection of these components can be achieved by selecting device technologies, values, and voltage ratings so that the device is tolerant of operation at an input voltage above the intended normal tolerance range and, in accordance with the limiting impedance, tolerates atypical voltage excursions caused by power supply malfunctions / failures up to a sensed value and for a time sufficient for the protective device to trip.The techniques disclosed herein can be implemented to provide proactive protection of drive components by limiting their operation to the ratings for which they are designed and avoiding harmful overcurrent conditions altogether.

[0018] In certain embodiments, the DSP 241 implements a shunt trip function, preferably configured to energize a shunt trip relay of a circuit breaker provided between a power supply and a variable frequency drive to prevent catastrophic failures of the drive. In embodiments, the shunt trip function is configured to sense the DC bus voltage, detect failure mechanisms, and prevent failures whenever possible. The shunt trip function is preferably configured to interrupt power to the drive before the failure causes the circuit breaker to trip via its overcurrent protection function.

[0019] For example, an analysis of an exemplary drive system with a variable frequency with the above in connection with Fig. 2, various criteria that an exemplary shunt trip function must meet, including MOV energy absorption curves, a DC bus overvoltage, and practical limits on the operating time of the protection device and sensing circuits. In the present example, the MOV was identified as the system component most susceptible to overvoltage faults, and the MOV connection curves and energy absorption curves were used to determine failure criteria. However, it should be understood that in further embodiments, other components or devices with different minimum conduction curves may be used as a basis for modeling failure modes.In some embodiments, the minimum conduction curve may be a function of the characteristics of multiple components or devices as a whole, for example, multiple MOVs in parallel or series, MOVs connected in parallel with other protection devices to extend their minimum conduction curve, or other combinations of protection devices.

[0020] A DC bus overvoltage occurs during commissioning and is most severe when the bus MOV happens to follow the maximum conduction curve. Practical limitations, such as a delay time in coil energization and the magnitude limit of the bus voltage sensing circuits, restrict the upper left voltage-time boundary before a feasible protection function can be implemented. Based on these considerations, an exemplary shunt trip function can be defined as an accumulator of a sixth-power voltage difference with respect to time, for example, as described by Equation 1: (VDC−STminV)6*t=(STmaxV−StminV)6*STtime

[0021] In Equation 1, VDC is the DC bus voltage, 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 is equal to 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 to yield a shunt trip function according to Equation 2: (VDC−STminV)6*t=STK

[0022] It is understood that the shunt tripping functions according to equation 1 or 2 can be provided in a variety of implementations.

[0023] 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 consumption of 2325 J, and a conservative assumption is made that 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 shunt trip function can be provided to account for this behavior. With four time constants, the function will decay to 0.67 percent of its original value, so 930 / 4 gives a time constant of 232.5 seconds. At 12 kHz, 2.325 seconds times 1200 samples / second gives 2790000 samples.Consequently, adding a multiplier constant of 2790000 / 2790001 or 0.99999964 to the shunt trip function will allow it to decay at approximately the time constant needed to simulate dissipation by the MOV.

[0024] It will now Fig. 3, which illustrates a flowchart according to an exemplary shunt trip control process 300. Process 300 is an exemplary implementation of the techniques described above that models the physical system of one or more drive components and is configured to preemptively disconnect the drive from the drive source at a variable frequency in the event of an impending failure. This may protect a drive from catastrophic failures that may require replacement of the entire drive.

[0025] In process 300, operator 310 receives calibratable quantities STmaxV 301, STminV 302, and STtime 303 as inputs. Operator 310 also receives voltage sample VDC(k) 304. Calibratable quantities 301, 302, and 303 may be predetermined values ​​stored in a computer-readable memory of a controller that may be retrieved and used by an executable routine implementing process 300. The values ​​of calibratable quantities 301, 302, and 303 may be determined empirically and may vary depending on the specific components used in the variable frequency drive and the size of the drive to which the process is applied.

[0026] The calibratable quantity STmaxV 301 serves as a threshold. Any voltage greater than the STmaxV value can be used to activate a shunt trip. Furthermore, the calibratable quantity STmaxV 301, in conjunction with the shunt trip time variable STtime 303, defines the "knee" of the shunt trip curve, for example, as in connection with Fig. 4. The calibratable quantity STminV 302 is the minimum voltage that could theoretically trigger a shunt trip. Since shunt tripping is a power function and therefore asymptotic, it would require 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 shunt trip function when the voltage is equal to the maximum voltage of the shunt trip, greater than or equal to the maximum voltage STmaxV 301 of the shunt trip. The variable 304 VDC(k) is a sample of the DC bus voltage, which can be captured, measured, or determined using a voltage sensing circuit.

[0027] The operator 310 processes the inputs it receives to determine an accumulated shunt trip value according to the following equation 3: STaccum(k)=(VDC(k)−STminV)6*Ts+STaccum(k−1) where k is a sample count, STaccum is the accumulated shunt trip value at a given sample count, VDC is the DC link voltage at a given sample count, MINV is the minimum voltage at which a power interruption should occur, and Ts is the time. Further, using the above terms along with a derivative constant Kb, embodiments may determine an accumulated shunt trip value according to the following equation 4: STaccum(k)=(VDC(k)−STminV)6*Ts+STaccum(k−1)*Kb

[0028] From operation 310, process 300 proceeds to operator 320. Operator 320 determines whether the accumulated shunt trip value STaccum(k) is greater than the shunt trip constant STK. If STaccum(k) is not greater than STK, process 300 returns to operation 310. If STaccum(k) is greater than STK, process 300 proceeds to operation 330, which activates a shunt trip by sending a shunt trip command. It should be understood that the shunt trip command may be used in conjunction 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 providing a de-energized condition in response to a signal or command.

[0029] It will now Fig. 4, which shows a graph with DC voltage on the vertical axis versus logarithmic time on the horizontal axis in units of seconds. The graph in Fig. Figure 4 illustrates characteristics of an exemplary shunt trip function. A voltage 410 is defined as the maximum voltage of the shunt trip. A voltage 420 is defined as the minimum voltage of the shunt trip. And the shunt trip time extends over a duration 430.

[0030] It will be Fig. 5, where a graph of DC voltage versus logarithmic time in units of seconds is shown. Fig. Figure 5 illustrates a maximum performance curve 501 and a minimum performance curve 502, which define the maximum and minimum time-voltage curves for MOV failure. MOVs exhibit statistical fluctuations for these time-voltage performance curves. The maximum performance curve 501 represents the upper statistical limit above which all MOVs in a statistical sample group will fail under steady-state conditions. The minimum performance curve 502 represents the lower statistical limit below which no MOV in the sample group will fail under steady-state conditions. The region between curves 501 and 502 contains all failure points for MOVs in the sample.Depending on the desired level of confidence, a shunt trip function may attempt to approximate, follow, or maintain a certain distance from a curve, with this curve lying between or on curves 501 and 502. In certain exemplary embodiments, a shunt trip function will strive to follow curve 502. Shunt trip function 505 is shown as an example of such a function, which generally strives to follow curve 502, but allows for some deviation from it as a compromise to keep computational complexity low. It is understood that other shunt trip function curves may also be used, following different trajectories than the example shown.

[0031] Fig. Figure 5 further illustrates a shunt trip circuit breaker time 503, which represents the minimum response time of a circuit breaker or contactor, and a maximum DC bus sense voltage 506, which represents the maximum DC bus sense input level. Time 503 and voltage 506 effectively establish the range within which the shunt trip function can be expected to operate, as they define the limits for the operation of system components. These parameters are useful for setting the value of a shunt trip function so that some margin is left to account for the delay in the circuit breaker response time. For example, the shunt trip function 505 is a constant value until time 503 and does not attempt to follow curve 502. This provides some margin to accommodate a system component delay. It also shows Fig.5 shows a maximum conduction curve 504 for a 480 V precharge, representing the theoretical worst-case startup transient. Curve 504 represents a worst-case startup transient scenario. Curve 504 can be used to define a boundary condition for unwanted tripping. A shunt trip function following a trajectory above curve 504 will avoid unnecessary, unwanted trips during startup events.

[0032] It is to be understood that the embodiments summarized and described in detail above and shown in the figures are for illustrative purposes and are not to be construed as restrictive or limiting. Only the presently preferred embodiments have been shown and described, and all changes and modifications that fall within the scope of the invention are intended to be protected. It is to be understood that the embodiments and forms described below may be combined in certain cases and may be mutually exclusive in other cases. Likewise, it is to be understood that the embodiments and forms described below may 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 them are also protected. When reading the claims, if words such as "a," "an," "at least one," "at least one," or "at least one part" are used, this should not be construed as limiting the claim to only one element, unless the claim expressly states the contrary. When the terms "at least one part" and / or "a part" are used, the element may include part and / or all of the element, unless expressly stated the contrary.

Claims

[1] Method comprising: Operating a variable frequency frequency converter motor drive (155, 200) comprising an AC-to-DC converter (280) electrically coupled to an AC power source (210), a DC intermediate circuit (285) electrically coupled to the AC-to-DC converter (280), and a DC-to-AC converter (290) electrically coupled to the DC intermediate circuit (285); Receiving information indicative of a voltage of the DC link (285) at a microprocessor-based controller (160); Processing the received information to model a state of at least one power electronics component of the motor drive (155, 200) with a variable frequency; and Disconnecting the AC-DC converter (280) from the AC-DC power source (210) if the modeled condition meets a protection criterion, and wherein the processing uses a function containing a sixth power voltage difference and calculating an accumulated value according to the equation: STaccum(k)=(VDC(k)−MINV)6*Ts+STaccum(k−1) where k is a sample number, STaccum is the accumulated value at a given sample number, VDC is a voltage of the DC link (285) at a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the sampling time. [2] The method of claim 1, wherein the at least one power electronics component comprises a varistor (286) electrically coupled to positive and negative rails of the DC link (285). [3] The method of claim 2, wherein the processing models a thermal capacity and a power dissipation of the varistor (286). [4] The method of claim 1, wherein the protection criterion comprises determining whether the value of STaccum is greater than a predetermined value. [5] The method of claim 1, wherein said isolating comprises providing a control signal to a shunt trip circuit breaker (220) electrically coupled between said AC power source (210) and said AC-to-DC converter (280). [6] The method of claim 5, wherein the rectifier (280) comprises a 6-pulse diode bridge and the inverter (290) comprises an IGBT-based three-phase switching circuit. [7] The method of claim 1, wherein the AC-DC converter (280) comprises a rectifier (280) and the DC-AC converter (290) comprises an inverter (290). [8] The method of claim 1, wherein the processing comprises calculating an accumulated value according to the equation: STaccum(k)=(VDC(k)−MINV)6*Ts+STaccum(k−1)*Kb where k is a sample number, STaccum is the accumulated value at a given sample number, VDC is a voltage of the DC link (285) at 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. [9] System comprising: a variable frequency motor drive (155, 200) comprising a rectifier (280), a DC bus (285) electrically coupled to the rectifier (280), and an inverter (290) electrically coupled to the DC bus (285); a protective device (220) electrically coupled to the rectifier (280) and configured to interrupt the power supply to the motor drive (155, 200) at a variable frequency based on a received trigger signal; a controller (160) configured to receive information indicative of a voltage of the DC bus (285), process the received information to model a state of at least one component of the variable frequency motor drive (155, 200), and output a trip signal to the protection device (220) to interrupt the power supply to the variable frequency motor drive (155, 200) when the modeled state satisfies a fault criterion, and wherein the controller (160) is configured to determine a sixth power voltage difference, use the determined difference in connection with a control of the protective device, and calculate an accumulated value according to the equation: STaccum(k)=(VDC(k)−MINV)6*Ts+STaccum(k−1) where k is a sample number, STaccum is the accumulated value at a given sample number, VDC is a voltage of the DC bus (285) at a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the sampling time. [10] The system of claim 9, wherein the at least one component comprises a metal oxide varistor (286). [11] The system of claim 9, wherein the controller (160) is configured to determine whether the value of STaccum is greater than a fault threshold and to output the trigger signal to the protection device (220) upon reaching or exceeding the fault threshold. [12] The system of claim 9, wherein the controller (160) is configured to calculate an accumulated value according to the equation STaccum(k)=(VDC(k)−MINV)6*Ts+STaccum(k−1)*Kb where k is a sample number, STaccum is the accumulated value at a given sample number, VDC is a voltage of the DC bus (285) 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. [13] Device comprising: a non-transitory computer-readable medium configured with instructions executable by a computer to: to determine a model state of at least one component of a variable frequency drive system (155, 200) comprising an AC-to-DC converter (280), a protection device (220) electrically coupled between the AC-to-DC converter (280) and the AC power source (210), and a DC link (285) electrically coupled to the AC-to-DC converter (280), based on sampled DC link voltage information; and to control the protection device (220) to disconnect the AC-DC converter (280) from the AC power source (210) based on the determined model state, where the model state is an accumulated value of a voltage difference to the sixth power, which is calculated according to the equation: STaccum(k)=(VDC(k)−MINV)6Ts+STaccum(k−1) where k is a sample number, STaccum is an accumulated value at a given sample number, VDC is a voltage of the DC link (285) at a given sample number, MINV is the minimum voltage at which a power supply interruption should occur, and Ts is the time. [14] The apparatus of claim 13, wherein the at least one component comprises a varistor (286). [15] The apparatus of claim 13, wherein the accumulated value is compared with a threshold, and the comparison is used to control the protection device (220). [16] Apparatus according to claim 13, wherein the model state is determined according to the equation STaccum(k)=(VDC(k)−MINV)6*Ts+STaccum(k−1)*Kb where k is a sample number, STaccum is an accumulated value at a given sample number, VDC is a voltage of the DC link (285) 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 properties of a varistor (286).

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