DC bus voltage balancing and control in multilevel inverters

A multilevel inverter system comprising:

• (a) three DC bus capacitors connected in series at nodes between the capacitors and with the series connected capacitors connected across high and low DC bus lines;
• (b) an inverter connected to the high and low DC bus lines and to DC bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom, the inverter comprised of a plurality of controlled switching devices connected in a configuration having redundant switching states to provide three-phase power on three output load lines and at four voltage levels on each output line;
• and
• (c) control means receiving signals indicative of the voltages across the DC bus capacitors and load currents on the output lines and connected to the switching devices of the inverter to control the switching of the same, the control means using the signals indicative of the voltages and currents at each switching event for controlling the switching of the switching devices to provide a selected output voltage waveform at the output lines utilizing redundant switching states when such states are available to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein the control means determines which switching state to utilize of the available switching states by evaluating the quantity Q=.DELTA.V.sub.C1 I.sub.1 +.DELTA.V.sub.C2 I.sub.2 +.DELTA.V.sub.C3 I.sub.3 for each available switching state and selecting one of the available redundant switching states for which the quantity Q is .gtoreq.0, wherein .DELTA.V.sub.C1, .DELTA.V.sub.C2, and .DELTA.V.sub.C3 are the respective differences between the measured voltage across each capacitor and the nominal voltage across each capacitor, and I.sub.1, I.sub.2, and I.sub.3 are currents flowing through DC bus lines to the inverter determined for the load currents and the available switching states.

A method of balancing the voltages on the DC bus capacitors connected to a multilevel inverter that provides multilevel three-phase power on output lines, wherein at least three DC bus capacitors are connected in series at nodes between the capacitors with the three series capacitors connected across high and low DC bus lines, the inverter connected to the high and low DC bus lines and to intermediate bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom and wherein the inverter is comprised of a plurality of controlled switching devices and diodes connected in a configuration having redundant switching states to provide three phase power on three output lines and at at least four voltage levels on each output line, comprising the steps of:

• (a) monitoring signals indicative of the voltages across the DC bus capacitors and the load currents on the output lines;
• and
• (b) switching the switching devices of the inverter to provide a desired output voltage waveform at the inverter output lines utilizing redundant switching states when such states are available on each voltage transition by using the signals indicative of the voltages and currents to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein there are at least two outer DC bus capacitors and an inner DC bus capacitor, and wherein at modulation depths at which sufficient redundant states are not available to effectively equalize all of the capacitor voltages, the switching devices are switched utilizing one of the available redundant states that transfers charge from the outer capacitors to the inner capacitor.

A multilevel inverter system comprising:

• (a) three DC bus capacitors connected in series at nodes between the capacitors and with the series connected capacitors connected across high and low DC bus lines;
• (b) an inverter connected to the high and low DC bus lines and to DC bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom, the inverter comprised of a plurality of controlled switching devices connected in a configuration having redundant switching states to provide three-phase power on three output load lines and at four voltage levels on each output line;
• and
• (c) control means receiving signals indicative of the voltages across the DC bus capacitors and load currents on the output lines and connected to the switching devices of the inverter to control the switching of the same, the control means using the signals indicative of the voltages and currents at each switching event for controlling the switching of the switching devices to provide a selected output voltage waveform at the output lines utilizing redundant switching states when such states are available to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein the control means determines which switching state to utilize of the available switching states by evaluating the quantity Q=max {.DELTA.V.sub.c1 i.sub.c1, .DELTA.V.sub.c2 i.sub.c2, .DELTA.V.sub.c3 i.sub.c3 } for each available switching state and selecting one of the available redundant switching states for which the quantity Q is .ltoreq.0, wherein .DELTA.V.sub.c1, .DELTA.V.sub.c2, and .DELTA.V.sub.c3 are the respective differences between the measured voltage across each capacitor and the nominal voltage across each capacitor, and i.sub.c1, i.sub.c2, and i.sub.c3 are the currents flowing through the respective capacitors determined for the load currents and the available switching states.

A multilevel inverter system comprising:

• (a) at least three DC bus capacitors connected in series at nodes between the capacitors and with the series connected capacitors connected across high and low DC bus lines;
• (b) an inverter connected to the high and low DC bus lines and to DC bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom, the inverter comprised of a plurality of controlled switching devices connected in a configuration having redundant switching states to provide three-phase power on three output load lines and at least four voltage levels on each output line;
• and
• (c) control means receiving signals indicative of the voltages across the DC bus capacitors and load currents on the output lines and connected to the switching devices of the inverter to control the switching of the same, the control means using the signals indicative of the voltages and currents at each switching event for controlling the switching of the switching devices to provide a selected output voltage waveform at the output lines utilizing redundant switching states when such states are available to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein the control means controls the inverter to switch the switching devices under sine-triangle pulse-width modulation when redundant states are available and to switch the switching devices in accordance with space vector modulation when redundant switching states are not available.

A multilevel inverter system comprising:

• (a) at least three DC bus capacitors connected in series at nodes between the capacitors and with the series connected capacitors connected across high and low DC bus lines;
• (b) an inverter connected to the high and low DC bus lines and to DC bus lines connected to the nodes a between the capacitors to be supplied with the voltages therefrom, the inverter comprised of a plurality of controlled switching devices and diodes connected in a diode clamped multilevel inverter configuration to provide three-phase power on three output lines and at at least four voltage levels on each output line;
• and
• (c) control means connected to the switching devices of the inverter for controlling the switching of the switching devices to provide a selected output voltage waveform at the output lines, wherein the control means controls the switching devices to switch under pulse-width modulation for modulation depths m.sub.a less than a selected value and under space vector modulation for modulation depths m.sub.a greater than the selected value where ##EQU12## and V.sub.ph, pk is the load phase peak voltage and V.sub.DC is the voltage across the high and low DC bus lines.

A method of balancing the voltages on the DC bus capacitors connected to a multilevel inverter that provides multilevel three-phase power on output lines, wherein at least three DC bus capacitors are connected in series at nodes between the capacitors with the three series capacitors connected across high and low DC bus lines, the inverter connected to the high and low DC bus lines and to intermediate bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom and wherein the inverter is comprised of a plurality of controlled switching devices and diodes connected in a configuration having redundant switching states to provide three phase power on three output lines and at least four voltage levels on each output line, comprising the steps of:

• (a) monitoring signals indicative of the voltages across the DC bus capacitors and the load currents on the output lines;
• and
• (b) switching the switching devices of the inverter to provide a desired output voltage waveform at the inverter output lines utilizing redundant switching states when such states are available on each voltage transition by using the signals indicative of the voltages and currents to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein the switching devices are switched under sine-triangle pulse-width modulation when redundant states are available and switched in accordance with space vector modulation when redundant switching states are not available.

A method of controlling an n level inverter system of the type having (n-1) DC bus capacitors connected in series across high and low DC bus lines, wherein n is at least four, and an inverter connected to the high and low DC bus lines and to DC bus lines connected to the nodes between the capacitors, the inverter comprised of controlled switching devices and diodes connected in a diode-clamped multilevel inverter configuration to provide three-phase power on three output lines at n voltage levels on each output line, comprising the steps of:

• (a) monitoring signals indicative of the voltages across the DC bus capacitors and the load currents on the output lines;
• and
• (b) controlling the switching of the switching devices to provide a selected output voltage waveform at the output lines using redundant switching states when such states are available on each voltage transition, and determining which state to utilize by evaluating the quantity Q=max {.DELTA.V.sub.c, i.sub.c, .DELTA.V.sub.c2 i.sub.c2, . . . , .DELTA.V.sub.c(n-1) i.sub.c(n-1) } for each available switching state and selecting one of the available redundant switching states for which the quantity Q is .ltoreq.0, wherein .DELTA.V.sub.c1, .DELTA.V.sub.c2, . . . , .DELTA.V.sub.c(n-1) are the respective differences between the measured voltage across each capacitor and the nominal voltage across each capacitor, and i.sub.C1, i.sub.C2, . . . , i.sub.C(n-1) are the currents flowing through the respective capacitors for the load currents and the available switching states.

A method of balancing the voltages on the DC bus capacitors connected to a multilevel inverter that provides multilevel three-phase power on output lines, wherein three DC bus capacitors are connected in series at nodes between the capacitors with the three series capacitors connected across high and low DC bus lines, the inverter connected to the high and low DC bus lines and to intermediate bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom and wherein the inverter is comprised of a plurality of controlled switching devices and diodes connected in a configuration having redundant switching states to provide three phase power on three output lines and at four voltage levels on each output line, comprising the steps of:

• (a) monitoring signals indicative of the voltages across the DC bus capacitors and the load currents on the output lines;
• and
• (b) switching the switching devices of the inverter to provide a desired output voltage waveform at the inverter output lines utilizing redundant switching states when such states are available on each voltage transition by using the signals indicative of the voltages and currents to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein the switching state to be utilized of the available switching states is determined by evaluating the quantity Q=.DELTA.V.sub.C1 I.sub.1 +.DELTA.V.sub.C2 I.sub.2 +.DELTA.V.sub.C3 I.sub.3 for each available switching state and selecting one of the available redundant switching states for which the quantity Q is .gtoreq.0, wherein .DELTA.V.sub.C1, .DELTA.V.sub.C2, and .DELTA.V.sub.C3 are the respective differences between the measured voltage across each capacitor and the nominal voltage across each capacitor, and I.sub.1, I.sub.2, and I.sub.3 are currents flowing through the DC bus lines to the inverter determined for the load currents and the available switching states.

A multilevel invert system comprising:

• (a) n-1 DC bus capacitors connected in series at nodes between the capacitors and with the series connected capacitors connected across high and low DC bus lines wherein n is a selected number of output voltage levels for the inverter and is at least four;
• (b) an inverter connected to the high and low DC bus lines and to DC bus lines connected to the nodes between the capacitors to be supplied with the voltages therefrom, the inverter comprised of a plurality of controlled switching devices and diodes connected in a diode clamped multilevel inverter configuration to provide three-phase power on three output lines and at n voltage levels on each output line;
• and
• (c) control means receiving signals indicative of the voltages across the DC bus capacitors and load currents on the output lines and connected to the switching devices of the inverter to control the switching of the same, the control means using the signals indicative of the voltages and currents at each switching event for controlling the switching of the switching devices to provide a selected output voltage waveform at the output lines utilizing redundant switching states when such states are available to select one of the available redundant switching states that serves to provide the desired phase-to-phase voltages and to tend to equalize the voltages across the DC bus capacitors, wherein the control means determines which switching state to utilize of the available switching states by evaluating the quantity Q=max {.DELTA.V.sub.c1 i.sub.c1, .DELTA.V.sub.c2,i.sub.c2, . . . , .DELTA.V.sub.c(n-1) i.sub.c(n-1) } for each available switching state and selecting one of the available redundant switching states for which the quantity Q is .ltoreq.0, wherein .DELTA.V.sub.c1, .DELTA.V.sub.c2, . . . , .DELTA.V.sub.c(n-1) are the respective differences between the measured voltage across each capacitor and the nominal voltage across each capacitor, and i.sub.c1, i.sub.c2, . . . , i.sub.c(n-1) are the currents flowing through the respective capacitors determined for the load currents and the available switching states.