Systems and methods of overvoltage limiting for series connected inverters

Abstract

A series-connected inverter (SCI) system for driving an alternating current (AC) multiphase machine is provided. The SCI system includes a plurality of inverters each including an inverter unit, the plurality of inverters electrically coupled in series with one another. The SCI system also includes a controller assembly configured to control inverter units of the plurality of inverters. The controller assembly is further configured to limit direct current (DC)-link voltages of the plurality of inverters while the plurality of inverters are in an idle state by measuring a voltage across each of the plurality of inverters, detecting an overvoltage inverter unit by comparing each measured voltage with an upper voltage threshold, injecting a DC current to the multiphase machine at least by the overvoltage inverter unit, and repeating injecting the DC current until a voltage across the overvoltage inverter unit is less than a bottom voltage threshold.

Description

SYSTEMS AND METHODS OF OVERVOLTAGE LIMITINGFOR SERIES CONNECTED INVERTERSBACKGROUND

[0001] The field of the disclosure relates generally to alternate current (AC) motor drives, and more particularly, to series connected inverter systems for driving multiphase AC motors.

[0002] When series connected inverters (SCIs) are used to drive a multiphase AC motor, one of the requirements is the voltage balancing of the direct current (DC) link capacitors of the inverters. During the operation period of the inverters, the DC link voltages may be balanced via controlling the distribution of the active power supplied to the motor among the inverters. This scheme, however, is inapplicable when the inverters are in an idle state (or stand-by), because the inverters do not supply AC power to the motors at an idle state. Known systems and methods are disadvantaged in some aspects and improvements are desired.BRIEF DESCRIPTION

[0003] In one aspect, a series-connected inverter (SCI) system for driving an alternating current (AC) multiphase machine is provided. The SCI system includes a plurality7of inverters each including an inverter unit, the plurality of inverters electrically coupled in series with one another. The SCI system also includes a controller assembly configured to control inverter units of the plurality of inverters. The controller assembly is further configured to limit direct current (DC)-link voltages of the plurality of inverters while the plurality of inverters are in an idle state by measuring a voltage across each of the plurality of inverters, detecting an overvoltage inverter unit by comparing each measured voltage with an upper voltage threshold, injecting a DC current to the multiphase machine by at least the overvoltage inverter unit, and repeating injecting the DC current until a voltage across the overvoltage inverter unit is less than a bottom voltage threshold.

[0004] In another aspect, a method of limiting DC-link voltages in an SCI system for driving an AC multiphase machine is provided. The SCI system includes a plurality of inverters each including an inverter unit. The SCI system also includes a controller assembly- configured to control inverter units of the plurality- of inverters, the plurality of inverters electrically coupled in series with one another. The method includes while the plurality of inverters are in an idle state, measuring a voltage across each of the plurality of inverters, detecting an overvoltage inverter unit by comparing each measured voltage with an upper voltage threshold, injecting a DC current to the multiphase machine by at least the overvoltage inverter unit, and repeating injecting the DC current until a voltage across the overvoltage inverter unit is less than a bottom voltage threshold.BRIEF DESCRIPTION OF DRAWINGS

[0005] These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings.

[0006] FIG. 1A is a schematic diagram of an example series connected inverter (SCI) system.

[0007] FIG. IB is a schematic diagram of an example embodiment of the SCI system shown in FIG. 1A, where the SCI system includes two inverter units, and each inverter unit includes three phase legs.

[0008] FIG. 1C show s waveforms of the SCI system illustrated in FIG. 1A.

[0009] FIG. ID is a flow chart of an example method of limiting overvoltage when the inverters of the SCI system are in an idle state.

[0010] FIG. 2A is a flow- chart of an example embodiment of the method shown in FIG. ID.

[0011] FIG. 2B show s waveforms of the first inverter in the SCI system illustrated in FIG. IB, when the method shown in FIG. 2A is applied and switches of only one phase leg are switched.

[0012] FIG. 2C shows waveforms of the second inverter in the SCI system illustrated in FIG. IB, when the method shown in FIG. 2 A is applied and switches of only one phase leg are switched.

[0013] FIG. 2D show-s waveforms of the first inverter in the SCI system illustrated in FIG. IB, when the method shown in FIG. 2A is applied and switches of two phase legs are switched.

[0014] FIG. 2E shows waveforms of the second inverter in the SCI system illustrated in FIG. IB, when the method shown in FIG. 2 A is applied and switches of two phase legs are switched.

[0015] FIG. 3 A is a flow chart of another example embodiment of the method shown in FIG. ID.

[0016] FIG. 3B shows waveforms of the first inverter in the SCI system illustrated in FIG. IB, when the method shown in FIG. 3 A is applied.

[0017] FIG. 3C shows waveforms of the second inverter in the SCI system illustrated in FIG. IB, when the method shown in FIG. 3A is applied.

[0018] FIG. 4A is a schematic diagram of the SCI system illustrated in FIG. 1A, when the machine shaft is rotating without a load being coupled with the machine shaft while the inverters are in an idle state.

[0019] FIG. 4B is a schematic diagram of the SCI system illustrated in FIG. 1A, when the machine shaft is rotating with a load being coupled with the machine shaft while the inverters are in an idle state.

[0020] FIG. 4C show s waveforms of the SCI system illustrated in FIG. 4A when the method illustrated in FIG. 2A is applied.

[0021] FIG. 4D shows w aveforms of the SCI system illustrated in FIG. 4B when the method illustrated in FIG. 2A is applied.

[0022] FIG. 4E shows waveforms of the SCI system illustrated in FIG. 4A when the method illustrated in FIG. 3A is applied.

[0023] FIG. 4F shows waveforms of the SCI system illustrated in FIG. 4B when the method illustrated in FIG. 3A is applied.DETAILED DESCRIPTION

[0024] The disclosure includes systems and methods of limiting overvoltage in series connected inverter (SCI) systems. As used herein, the voltage across an inverter in an SCI system is also referred to as the direct current (DC)-link voltage of the inverter. Method aspects will be in part apparent and in part explicitly discussed in the following description.

[0025] In an alternating current (AC) multiphase machine driven by series connected inverters, one of the biggest concerns is high voltage imbalance of DC-link capacitors. Therefore, voltage balancing among the inverters is required. During the operation of the inverters, the DC-link voltage balancing depends on the active power distribution among different inverters and the winding set, where if the active power is equal, voltages among inverters are balanced, and if active power is different, voltages are unbalanced. However, during an idle state of the inverters, where the SCI is not in an operation state and the SCI does not supply AC power to the machine, such as when the machine is in charging, stand-by (or idle), or discharging periods, the SCI system is unstable due to minor differences in the active power consumption by auxiliary circuits, such as power supplies, controllers, sensors, or cooling fans, among different inverters. The unstableness may lead to voltage differences to such a level that renders the system inoperable, and may cause the capacitors to explode and pose as a safety risk.

[0026] FIGs. 1 A-1C show an example SCI system 100. FIG. 1 A is a block diagram of the SCI system 100. FIG. IB is an example circuit diagram of the SCI system 100. FIG. 1C shows waveforms of the SCI system 100.

[0027] In the example embodiment, the SCI system 100 includes a plurality of inverters 102. The inverters 102 are electrically connected in series with one another. An inverter 102 includes an inverter unit 104 configured to convert DC to AC. The converted AC may be in three phases. The inverter unit 104 may include a plurality7of phase legs 106.Each phase leg 106 includes a pair of switches 108. The inverter 102 may include a local controller 110 configured to control the inverter unit 104 by controlling the switching of switches 108 in the inverter unit 104. The inverter 102 may further include one or more gate drivers 112. The local controller 110 may control the inverter unit 104 by controlling the gate drivers to control switching of the switches 108 in the inverter unit 104. The inverter 102 may further include one or more sensors 114 and / or one or more cooling fans 116. The local controller 110 may be configured to control the sensors 114 and / or the cooling fans 116. The inverter 102 may further include a power supply 118. The power supply 118 is a DC to DC converter that converts the DC from the main power supply between terminals 120-p, 120-n of the SCI system 100 to a desired voltage for the cooling fans 116, the local controller 1 10, sensors 114, and gate drivers 112. The inverter 102 further includes a capacitor 122 electrically couple in parallel with the inverter unit 104. The inverter 102 may include a balancing resistor 124.

[0028] In the example embodiment, the SCI system 100 further includes a machine 126. The machine 126 may be a multiphase machine, such as a multiphase motor used to drive a load 406 (e.g.. a fan) (see FIG. 4B described later). The inverters 102 supply AC power to and drive the machine 126.

[0029] In the example embodiment, the SCI system 100 includes a controller assembly 128 configured to control the inverter units 104. The controller assembly 128 may include the local controller 110, such as a first local controller 110-1 configured to control the first inverter unit 104-1 of the first inverter 102-1 and a second local controller 110-2 configured to control the second inverter unit 104-2 of the second inverter 102-2. The controller assembly 128 may further include a master controller 130. In the depicted embodiment, the master controller 130 is connected with local controllers 110. Control signals are sent from the master controller 130 to the local controllers 110 to control operation of the inverter units 104. In some embodiments, master controller 130 may be directly connected to gate drivers 112 of the inverter unit 104 and the control signals are sent from the master controller 130 directly to the inverter 102 to control the inverter unit 104.

[0030] The SCI system 100 has four periods of operation, which are a pre-charge period 132-p, a stand-by (or idle) period 132-s, a machine operation period 132-o, and a discharge period (not shown) (see FIG. 1C). As to the inverters 102. the inverters 102 maybe in two states. The first state is an operational state. At the operational state, the inverters convert DC power to AC power and output the AC power to drive the machine 126. During the machine operation period 132-o, the inverters 102 are in an operational state. The other state of the inverters 102 is an idle state, where the inverters 102 do not output AC power to the machine 126. During other periods of the SCI system 100, the inverters 102 are in an idle sate. At the idle state, the machine 126 may or may not be in motion, such as rotation of the shaft 404 (see FIGs. 4A and 4B described later) of the machine 126, and the SCI system 100 may or may not include the load 406.

[0031] During the machine operation period, active power distribution is controlled via a main control algorithm. However, in the remaining periods when the inverters do not operate and AC power does not flow to the machine 126, the SCI system is unstable. During periods when the machine 126 does not operate, the SCI system 100 includes two types of power loads. The first type includes constant power loads, such as auxiliary electronics 138 like local controllers 110, sensors 114, gate drivers 112, or cooling fans 116. The other type of power loads are non-constant power loads, such as balancing resistors 124 with static resistance. Due to inherent differences among constant power loads among different inverters 102, when the consumption by the constant power loads reaches or exceeds a level such that the inherent differences surpass the power handling capacity of balancing resistors, voltage differences between the SCI DC-link voltages occur. Among the auxiliary power loads, cooling fans 116 are one of largest contributors to the imbalance of inverters. Different inverters have different operation points due to slight differences in inverter temperatures or measurement tolerances, which results in different speed references of the cooling fans 116 and, thus, different power loads from the cooling fans 116. Power supplies 118 are another one of the largest contributors to the imbalance. Due to slight differences in quiescent currents during standby because of component tolerances, the active power of the power supplies 118 is different among different inverters 102. Over time, if the voltage imbalance is not managed, voltage over more loaded capacitor may collapse to 0 V, while the voltage over other capacitors may reach full DC-link voltage of the system, potentially damaging the capacitors 122 and causing the system inoperable.

[0032] Increasing power handling capacity of balancing resistors to balance DC- link voltages is undesirable. The approach results in overall reduction of efficiency. Other approaches such as voltage balancing units and / or voltage limiting units require additional hardware and are energy inefficient, rendering the system bulky and costly. For example, in one known method, voltage balancing units that transfer energy between DC-link capacitors 122 may be used. This approach is impractical if the system includes more than two series- connected inverters. In another known method, voltage limiting units are used to discharge the excess energy’ via braking resistors, producing high losses in the system.

[0033] In contrast, the systems and methods disclosed herein balance the DC-link voltages without additional hardware. DC current is injected to the inverter unit to balance the voltages between the inverter units. Systems and methods disclosed herein are cost- effective and reliable. Further, systems and methods disclosed herein are scalable to systems having any number of inverters.

[0034] FIG. ID is a flow chart of an example method 150 of balancing DC-link voltage. In the example embodiment, the method 150 includes checking 152 whether the plurality of inverters are in an idle state. If the inverters 102 are in an idle state, the method 150 proceeds. If the inverters 102 are not in an idle state, the method 150 is not performed. The method 150 includes measuring 154 a voltage across each of the plurality of inverters. The method 150 also includes detecting 156 an overvoltage inverter unit by comparing each measured voltage with an upper voltage threshold. If the measured voltage of an inverter unit 104 is greater than the upper voltage threshold, the inverter unit 104 is an overvoltage inverter unit. The upper voltage threshold may be determined based on the rated voltages of the capacitors 122 and / or the operating voltage of the SCI system 100. The upper voltage threshold does not exceed the rated voltages of the capacitors 122. The upper voltage threshold may be set as the minimum among the rated voltages of the capacitors 122.

[0035] In the example embodiment, the method 150 further includes injecting 158 a DC current at least to the overvoltage inverter unit. In addition, the method 150 includes repeating injecting 158 the DC current until the voltage across the overvoltage inverter unit is less than a bottom voltage threshold. The bottom voltage threshold is determined based on the nominal voltage of the capacitors 122 or the system operating voltage. For example, the bottom voltage threshold may be Vdc.syS / n, where Vdc.sys is the operating voltage of the SCIsystem 100 (see FIG. 1 A, the voltage between the positive terminal 120-p and the negative terminal 120-n) and n is the number of inverters 102 in the SCI system 100. Once the voltage across the overvoltage inverter unit is less than a bottom voltage threshold, the method 150 goes back checking 152 whether the plurality of inverters are in an idle state.

[0036] The method 150 prevents voltage deviation among inverters 102 by injecting DC-current to the inverters and the machine 126, which introduces active losses in the machine and inverters. The active power is high enough to overcome the differences in active powers among inverters and to limit and balance the voltages. Injecting DC current is advantageous because the DC current does not induce torque to the multiphase machine.

[0037] FIGs. 2A-2E show an embodiment of the method 150-i that controls individual inverters in voltage limiting. The machine shaft 404 of the machine 126 is not rotating. FIG. 2A is a flow chart of the method 150-i. FIGs. 2B-2E are waveforms of the inverter units 104, when the method 150-i is applied.

[0038] In the example embodiment, inverters 102 are individually controlled by the method 150-i to suppress voltage deviation. The control algorithm is implemented in the local controller 110 for the overvoltage inverter unit 104. To create DC current in the machine with inverters 102, switches 108 of phase legs 106 (106-a, 106-b. or 106-c) in the overvoltage inverter unit 104 are switched at a duty cycle. The duty cycle is relatively small, such as 10% or less, 5%, or 2%, to prevent high phase DC currents in the inverter units 104 due to absence of counter electromotive force (back-EMF). Current will rise according to the resistances and inductances in the machine and / or the inverter and the applied duty cycle.

[0039] In the example embodiment, when the SCI system 100 is in an nonoperation period, the DC-link voltage across an inverter is measured 202. The measured voltage is compared 204 with an upper voltage threshold. If the measured voltage is less than the upper voltage threshold, the method 150-i goes back to measuring 202 the voltage across the inverter to monitor the voltage. If the measured voltage is greater than or equal to the upper voltage threshold, the inverter unit 104 is an overvoltage inverter unit 104. In the depicted embodiment in FIGs. 2B-2E, the first inverter unit 104-1 is the overvoltage inverter unit. A pulse- width modulation (PWM) sequence for DC current injection is generated 206. Because of the time scale, the PWM sequence appears as a solid block. Instead, the PWMsequence includes square waves between 0 and 1 at a duty cycle. The duty cycle of the PWM is relatively small, such as less than 5%, to prevent high DC currents in the injected phase leg(s) due to the absence of counter electromotive force or back EMF. The PWM sequence is forwarded 208 to the overvoltage inverter unit 104 to control the switching of the switches 108 in the overvoltage inverter unit 104 according to the PWM sequence. Once the PWM is enabled in the overvoltage inverter unit 104, the DC current will rise and thus the active power will rise, which will cause the DC-link voltage to drop. The DC-link voltage of the overvoltage inverter unit will continue dropping due to the higher active power of the overvoltage inverter unit compared to other inverters, while the PWM sequence is applied. The voltages across inverters conform to the Kirchoff voltage law (KVL), where the sum of the voltages across individual inverters 102 is equal to the system operating voltage (Edcl+ ic2 + "• dc,sys)-

[0040] In the example embodiment, after the overvoltage inverter unit 104 operates according to the PWM sequence, voltage across the overvoltage inverter unit 104 is measured 210 and compared 211 with a bottom voltage threshold. If the measured voltage is greater than or equal to the bottom voltage threshold, the method 150 goes back to generating 206 a PWM sequence and repeats generating 206 a PWM sequence, forwarding 208 the PWM sequence, measuring 210 the voltage across the overvoltage inverter unit 104, and comparing 211 with the bottom voltage threshold. If the measured voltage is less than the bottom voltage threshold, the generation of a PWM sequence is stopped 212. Whether the SCI system 100 is in an operation mode is checked 214. If the SCI system 100 is not in an operation mode, the method 150 goes back to initial measuring 202 the voltage of the inverter 102. If the SCI system 100 is in an operation mode, the method 150-i is stopped.

[0041] In the example embodiment, the PWM signals control switches of the voltage inverter. In the depicted example, the first inverter 102-1 is detected to be the overvoltage inverter unit, which has a voltage greater than the upper voltage threshold. The first inverter 102-1 includes three phase legs 106. Each phase leg 106 includes two switches 108-t, 108-b, where the top switch 108-t and the bottom switch 108-b are connected in series with the other end of the top switch 108 connected to the positive terminal of the inverter 102-1, and the other end of the bottom switch 108 connected to the negative terminal of the inverter 102-1. The PWM signals includes signals Si l (for phase leg 106-a). S2_l (forphase leg 106-b), S3_l (for phase leg 106-c) supplied to individual phase legs and indicating the switching of the switches in the corresponding phase legs. For example, if S 1 1 , S2_l, or S3_l is 1, the top switches 108-t are on and the bottom switches 108-b are off.

[0042] FIGs. 2B and 2C show waveforms of inverters 102-1 and 102-2. FIG. 2B shows waveforms of the first inverter unit 104-1, on which the PWM signals are applied. FIG. 2C shows waveforms of the second inverter unit 104-2, on which the PWM signals are not applied. In the depicted example, the first inverter 102-1 is shown to have a lower auxiliary power consumption (e.g., 180 W) than that of the second inverter 102-2 (e.g., 220 W). The voltage across the first inverter 102-1 rises while the voltage across the second inverter 102-2 decreases. When the voltage of the first inverter 102-1 reaches the upper voltage threshold, which is set as for example 1020 V, the PWM signals Si l for the first phase leg 106-a starts. The DC-current in the first phase leg 106-a of the first inverter 102-1 starts to increase while the current in the other two phase legs follows the Kirchoff current law (KCL) for three-wire three phase system (Iai+Ibi+Li=0). The injected DC-current in the first inverter 102-1 causes active power losses in the machine and the inverter 102, for example, in an amount of approximately 1600 W. The active power losses cause the DC-link voltage to drop. When the DC-link voltage drops below the bottom voltage threshold, for example 1000 V, the PWM signals are disabled. In the meantime, switches 108 of the second inverter 102-2 are not switched because the voltage across the second inverter 102-2 is lower than the upper voltage threshold. Although the PWM signals are not applied to the second inverter 102-2, the voltage across the second inverter 102-2 is restricted from reducing further due to the switching of switches 108 in the first inverter 102-1, based on the KVL.

[0043] Referring to FIGs. 2D and 2E, in some embodiments, two of the three phase legs are switched. FIG. 2D shows waveforms of the first inverter 102-1. FIG. 2E shows waveforms of the second inverter 102-2. In other embodiments, the switching of the phase legs 106 are cycled through the phase legs 106. For example, switches 108 of phase legs 106- a and 106-b are switched when the PWM signals are applied al the first time, switches 108 of phase legs 106-b and 106-c are switched when the PWM signals are applied at the next time, the switches of phase legs 106-c and 106-a are switched at the time after, the switches of phase legs 106-a and 106-b are switched at the time further after, and so on. Additionally, cycling among phase legs may be applied for the method 150-i described in FIGs. 2B and2C. For example, switches 108 of the first phase leg 106-a are switched when the PWM signals are applied at the first time, switches 108 of the second phase leg 106-b are switched when the PWM signals are applied at the next time, the switches of the third phase leg 106- c are switched at the time after, the switches of the first phase leg 106-a are switched at the time further after, and so on. Cycling among phase legs is advantageous in reducing stresses on and overheating of the switches of a particular inverter 102, especially w hen the duration of the inverters 102 being in an idle state is relatively long.

[0044] FIGs. 3A-3C show another embodiment of the method 150-c, where all inverters will receive the same PWM signals from a master controller 130 to suppress voltage deviation among the inverters 102. The machine shaft 404 of the machine 126 is not rotating. FIG. 3 A is a flow chart of the method 150-c. FIG. 3B shows the waveforms of the first inverter 102-1. FIG. 3C shows the waveforms of the second inverter 102-2.

[0045] In the example embodiment, voltage across each inverter 102 is measured 302. The voltages are each compared 304 with the upper voltage threshold. The upper voltage threshold in the method 150-c may be the same as the upper voltage threshold in the method 150-i. If all voltages are less than the upper voltage threshold, the method 150-c goes back to measuring 302 the voltages.

[0046] In the example embodiment, if the voltage of any inverter 102 is greater than or equal to the upper voltage threshold, the SCI system 100 has an overvoltage inverter unit 104. In the depicted example shown in FIGs. 3B and 3C, the first inverter unit 104 is the overvoltage inverter unit. Instead of only switching the overvoltage inverter unit as shown in FIGs. 2A-2E, all inverters 102 in the SCI system 100 are switched. A PWM sequence for DC current injection is generated 306. The PWM sequence is forwarded 308 to all inverters. The same PWM signals may be sent to all inverters in series or at the same time. In the depicted embodiment (see FIGs. 3B and 3C), switches of only one phase leg 106 in the inverter 102 are switched in response to the PWM signals. The duty cycle of the PWM signals is relatively small, such as less than 5%, to prevent excessive high DC currents due to the absence of back EMF. Once the PWM signals are enabled, the DC currents of all inverters 102 will increase, and the active power consumption of the inverters will also increase. Due to similar active power consumption, the DC-link voltages of the inverters 102 will converge.

[0047] In the example embodiment, after the PWM signals are applied, the voltage of each inverter is measured 310. The voltages are compared 312 with the bottom voltage threshold. The bottom voltage threshold in the method 150-c may be the same as the bottom voltage threshold in the method 150-i. If the voltage of any inverter is greater than or equal to the bottom voltage, DC current injection is continued by repeating generating 306 a PWM sequence, forwarding 308 the PWM sequence, measuring 310 the voltages, and comparing 312 with the bottom voltage threshold. If all voltages are less than the bottom voltage threshold, the generation of the PWM sequence is stopped 313. Whether the SCI system is in an operation mode is checked 314. If the SCI system 100 is not in an operation mode, the method 150-c goes back to the initial measuring 302 of the voltages. If the SCI system 100 is in an operation mode, the method 150-c is stopped.

[0048] FIGs. 3B and 3C show waveforms of the inverters 102. FIG. 3B shows waveforms of the first inverter 102- 1. FIG. 3C shows waveforms of the second inverter 102- 2. In the depicted embodiment, the first inverter 102-1 has a lower auxiliary power consumption (e.g., 180 W) than that of the second inverter 102-2 (e g., 220 W). The voltage of the first inverter 102-1 increases while the voltage of the second inverter 102-2 decreases. When the voltage of the first inverter 102-1 reaches above the upper voltage threshold, the PWM signal Sl_l for the first inverter 102-1 and the PWM signal Sl_2 for the second inverter 102-2 start. In the depicted example, the PWM signals are applied to phase legs a of both inverters 102-1, 102-2. The duty cycle may be 2%. The DC current in phase leg a increases. Currents in other two phase legs b, c follow the KCL for a 3-wire 3-phase system (Iai+Ibi+Ici=0 and Ia2+Ib2+IC2=0). The injected DC currents cause active power losses (e.g., about 2600 W) in the machine 126 and the inverter 102 in both three-phase winding sets of the machine 126. The active power losses cause the voltages of the inverters to converge. After a period of DC current injection, the overvoltage will drop to the bottom voltage threshold. The bottom voltage threshold may be set as 1010 V. Once the voltage drops below the bottom voltage threshold, the PWM signals are disabled.

[0049] In some embodiments, switching of two of three phase legs 106 may be implemented in applying the method 150-c. In other embodiments, cycling through phase legs 106 may be implemented in applying the method 150-c. Cycling phase leges isadvantageous in limiting overheating of the switches, especially when the duration of the inverters 102 being in an idle state is relatively long.

[0050] Compared to the method 150-i, where only switches of the overvoltage inverter unit are switched, in the method 150-c, switches of all inverter units are switched. As a result, the system losses in the method 150-c are higher than those in the method 150- i. Further, in the method 150-c. it takes longer to reach the bottom voltage threshold, causing more losses within each inverter than in the method 150-i.

[0051] FIGs. 4A-4F show that the methods 150-i, 150-c effectively limit overvoltage when the SCI system 100 is at other periods of the inverters being in the idle state, such as the machine 126 is rotating without (FIGs. 4A, 4C, and 4E) or with (FIGs. 4B, 4D, and 4F) a load 406. The machine shaft 404 rotates due to inertia after the inverters 102 cease to be in the operational mode, where the inverters 102 cease to output AC power to the machine 126. FIGs. 4A and 4B are schematic diagrams of the SCI system 100 when the machine shaft 404 rotates without being coupled with a load 406 (FIG. 4A) or with a load 406 (FIG. 4B). FIGs. 4C and 4D are waveforms of the SCI system 100 when the method 150-i is applied, while being without a load (FIG. 4C) or with a load (FIG. 4D). FIGs. 4E and 4F are w aveforms of the SCI system 100 when the method 150-c is applied, while being without a load (FIG. 4E) or with a load (FIG. 4F). In the depicted embodiments shown in FIGs. 4C-4F. the first inverter unit 104-1 is the overvoltage inverter unit.

[0052] In the example embodiments, when the method 150-i is applied, control to the overvoltage inverter unit 104 is applied to suppress the voltage deviation (FIGs. 4C and 4D). In the depicted example, the rotating machine shaft 404 has an inertia (e.g., moment of inertia J = 4 Kg m2) and a nominal load 406, if coupled with the machine 126, is Tmax=1400 Nm at the speed of 2000 rpm.

[0053] Referring to FIG. 4C, in the example embodiment, the method 150-i is effective in limiting overvoltage in the SCI system 100 when the machine 126 is rotating without a load. The speed of the machine reduces due to friction. The injected current and the power are greater than those for the situations where the machine shaft is not rotating (see FIGs. 2A-2E). The effects of the increased injected current and power on the inverters 102, however, are negligible because the DC-current pulses are short (see waveform 402).

[0054] Referring to FIG. 4D, in the example embodiment, the method 150-i is effective in limiting overvoltage in the SCI system 100 when the machine 126 is rotating with a load 406. The speed of the machine reduces due to the mechanical torques on the shaft 404 from the load 406. The injected DC currents affects the speed due to the development of negative electrical torque, but the effects are at a negligible level. The application of the method 150-i does not noticeably interfere with machine dynamics or negatively affect the inverters.

[0055] In the example embodiments, when the method 150-c is applied, the PWM signals are applied to all inverters 102 to suppress voltage deviation (see FIGs. 4E and 4F). In the depicted example, the machine shaft 404 rotates due to inertia (e.g., moment of inertia J = 4 Kg m2) and an example nominal load, if applied, is Tmax=1400 Nm at the speed of 2000 rpm.

[0056] Referring to FIG. 4E, in the example embodiment, the method 150-c is effective in limiting overvoltage in the SCI system 100 when the machine 126 is rotating without a load. The negative electrical torque is greater than that for the method 150-i. The speed of the machine 126 reduces due to friction and engagement of the inverters in contributing to the DC -current braking effect on the machine 126. The injected current and power are greater than those in the situation when the machine shaft is not rotating (see FIGs. 3A-3C). The effects of the increased injected current and power on the inverters 102, however, are within the handling capacity of the inverters 102.

[0057] Referring to FIG. 4F, in the example embodiment, the method 150-c is effective in limiting overvoltage in the SCI system 100 when the machine 126 is rotating with the load 406. The negative electrical torque is greater than that for the method 150-i. The speed of the machine 126 reduces to zero faster than previous situations (see FIGs. 4C- 4E) due to friction, the load, and engagement of the inverters in contributing to the DC- current braking effect on the machine 126. The injected current and power are greater than those in the situation when the machine shaft is not rotating (see FIGs. 3A-3C). The effects of the increased injected current and power on the inverters 102, however, are within the handling capacity of the inverters 102.

[0058] The application of the method 150-c when the machine shaft 404 is rotating with a load 406 noticeably interfere with the machine dynamics. In operation, if little or no effect on machine dynamics is desired during the idle state of the inverters, the method 150- i may be selected. If effects on the machine dynamics are not a consideration, either method 150-i, 150-c may be used.

[0059] An SCI system 100 including two inverters are described herein as an example for illustration purposes only. The systems and methods disclosed herein may be applied to an SCI system having any number of inverters, such as 4 or 8, connected in series. For example, the number of inverters is n. In the method 150-i, where only switches of the overvoltage inverter unit are switched, the overvoltage inverter unit is determined among the n inverters 102. In the method 150-c, where all inverters are switched, the generated PWM signals are forwarded to all n inverters.

[0060] The systems and methods disclosed herein are advantageous in limiting overvoltage in an SCI system without requiring any additional hardware components and any alteration in the control of the system during the operation mode of the machine. Further, the control to limit overvoltage does not require sophisticated control mechanisms, such as proportional integral control, thereby facilitating a relatively stable, quick control.

[0061] At least one technical effect of the systems and methods described herein includes (a) limiting overvoltage in an idle state of the inverters by injecting DC current; (b) switching only switches in the overvoltage inverter unit; (c) the DC current being injected to all inverter units; and (d) being scalable to any number of inverters in the system.

[0062] Example embodiments of systems and methods of limiting DC-link voltages are described above in detail. The systems and methods are not limited to the specific embodiments described herein but, rather, components of the systems and / or operations of the methods may be utilized independently and separately from other components and / or operations described herein. Further, the described components and / or operations may also be defined in, or used in combination with, other systems, methods, and / or devices, and are not limited to practice with only the systems described herein.

[0063] Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and / or claimed in combination with any feature of any other drawing.

[0064] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

WHAT IS CLAIMED IS:

1. A series-connected inverter (SCI) system for driving an alternating current (AC) multiphase machine, the SCI system comprising: a plurality7of inverters each comprising an inverter unit, the plurality of inverters electrically coupled in series with one another; and a controller assembly configured to control inverter units of the plurality’ of inverters, the controller assembly further configured to limit direct current (DC)-link voltages of the plurality of inverters while the plurality of inverters are in an idle state by: measuring a voltage across each of the plurality of inverters; detecting an overvoltage inverter unit by comparing each measured voltage with an upper voltage threshold; injecting a DC current to the multiphase machine by at least the overvoltage inverter unit; and repeating injecting the DC current until a voltage across the overvoltage inverter unit is less than a bottom voltage threshold.

2. The SCI system of claim 1, wherein injecting the DC cunent further comprises: generating a pulse-width modulation (PWM) sequence for a DC current injection; and forwarding the PWM sequence at least to the overvoltage inverter unit.

3. The SCI system of claim 2, wherein the overvoltage inverter unit comprises a plurality of phase legs, forwarding the PWM sequence further comprising forwarding the PWM sequence to switches in one of the plurality of phase legs.

4. The SCI system of claim 2, wherein the overvoltage inverter unit comprises a plurality of phase legs, forwarding the PWM sequence further comprising cycling through the plurality of phase legs in forwarding the PWM sequence.

5. The SCI system of claim 2, wherein a duty cycle of the PWM sequence is 10% or less.

6. The SCI system of claim 1, wherein the controller assembly comprises a plurality of local controller each configured to control one of the inverter units, injecting the DC current further comprising: controlling, by a corresponding local controller of the overvoltage inverter unit, the overvoltage inverter unit to inject the DC current to the multiphase machine by the overvoltage inverter unit.

7. The SCI system of claim 1. wherein the controller assembly comprises a master controller configured to control the inverter units, injecting the DC current further comprising: controlling, by the master controller, the inverter units to inject the DC current to the multiphase machine by the inverter units.

8. The SCI system of claim 1, wherein the plurality of inverters each further comprises a capacitor electrically coupled in parallel with the inverter unit, and the upper voltage threshold is determined based on rated voltages of capacitors of the plurality of inverters.

9. The SCI system of claim 1. wherein the bottom voltage threshold is determined based on an operating voltage of the SCI system.

10. The SCI system of claim 1 comprising at least three inverters.

11. A method of limiting direct current (DC)-link voltages in a series-connected inverter (SCI) system for driving an alternating current (AC) multiphase machine, the SCI system including a plurality of inverters each including an inverter unit and a controller assembly configured to control inverter units of the plurality of inverters, the plurality of inverters electrically coupled in series with one another, the method comprising: while the plurality of inverters are in an idle state,measuring a voltage across each of the plurality of inverters; detecting an overvoltage inverter unit by comparing each measured voltage with an upper voltage threshold; injecting a DC current to the multiphase machine at least by the overvoltage inverter unit; and repeating injecting the DC current until a voltage across the overvoltage inverter unit is less than a bottom voltage threshold.

12. The method of claim 11, wherein injecting the DC current further comprises: generating a pulse-width modulation (PWM) sequence for a DC current injection; and forwarding the PWM sequence at least to the overvoltage inverter unit.

13. The method of claim 12, wherein the overvoltage inverter unit comprises a plurality of phase legs, forwarding the PWM sequence further comprising forwarding the PWM sequence to switches in one of the plurality of phase legs.

14. The method of claim 12, wherein the overvoltage inverter unit comprises a plurality of phase legs, forwarding the PWM sequence further comprising cycling through the plurality of phase legs in forwarding the PWM sequence.

15. The method of claim 12, wherein a duty cycle of the PWM sequence is 10% or less.

16. The method of claim 11 , wherein the controller assembly comprises a plurality of local controller each configured to control one of the inverter units, injecting the DC cunent further comprising: controlling, by a corresponding local controller of the overv oltage inverter unit, the overvoltage inverter unit to inject the DC current to the multiphase machine by the overvoltage inverter unit.

17. The method of claim 1 1 , wherein the controller assembly comprises a master controller configured to control the inverter units, injecting the DC current further comprising: controlling, by the master controller, the inverter units to inject the DC current to the multiphase machine by the inverter units.

18. The method of claim 11, wherein the plurality of inverters each further comprises a capacitor electrically coupled in parallel with the inverter unit, and the upper voltage threshold is determined based on rated voltages of capacitors of the plurality of inverters.

19. The method of claim 11, wherein the bottom voltage threshold is determined based on an operating voltage of the SCI system.

20. The method of claim 11 comprising at least three inverters.

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