Method and apparatus for monitoring an electric power circuit

A method for monitoring an electric power circuit configured to transfer a power signal to a torque module that is electrically isolated from a chassis ground includes injecting a common voltage component into a common mode voltage of electrical phases during operation and monitoring a negative-ground voltage and a positive-negative voltage of the electric power circuit. An AC line resistance is determined based upon the negative-ground voltage and the positive-negative voltage. Faults in electrical isolation between the electric power circuit and a chassis ground are detected based upon the AC line resistance.

TECHNICAL FIELD

This disclosure is related to electric isolation of AC power circuits.

BACKGROUND

Multi-phase, multi-pole electric motors can be employed on hybrid and electric vehicles to provide torque for propulsion and to meet other mechanical power needs. Such electric motors conduct some form of alternating current electric energy through wound electric cable to induce a magnetic field that acts upon a rotor and causes rotation thereof. The wound electric cable is assembled from insulated wire. Degradation in the insulation wire can reduce torque capacity of the electric motor, and measurement of wire insulation resistance can be employed to evaluate the condition of electrical insulation.

SUMMARY

A method for monitoring an electric power circuit configured to transfer a power signal to a torque module that is electrically isolated from a chassis ground includes injecting a common voltage component into a common mode voltage of electrical phases during operation and monitoring a negative-ground voltage and a positive-negative voltage of the electric power circuit. An AC line resistance is determined based upon the negative-ground voltage and the positive-negative voltage. Faults in electrical isolation between the electric power circuit and a chassis ground are detected based upon the AC line resistance.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,FIG. 1schematically shows an electric power circuit20and controller10for controlling operation of an electric module, which includes a multiphase electric motor/generator unit (torque module)90in one embodiment. The electric power circuit20has high-impedance isolation from a chassis ground12associated with a mechanical system in which the electric power circuit20and torque module90are mounted. The electric power circuit20includes a high-voltage DC electric power source (battery)22, a multi-phase power inverter module30and a gate drive module40that are signally and operatively connected to controller10. The battery22electrically connects to positive and negative sides of a high-voltage DC power bus (power bus)24and26, respectively, which electrically connects to the inverter module30.

The torque module90is a multi-phase, multi-pole electric machine including a rotor and stator that can operate to either transform electric power to mechanical torque or operate to transform mechanical torque to electric power. The torque module90is a three-phase device in one embodiment and as shown, and includes electrical components that form a first phase91, a second phase92, and a third phase93. Other multi-phase configurations may be employed without limitation. Electrical power elements of the torque module90are electrically isolated from the chassis ground12. Some mechanical elements of the torque module90electrically connect to the chassis ground12.

The inverter30includes a plurality of complementary paired switch devices32,34electrically connected in series between the positive and negative sides of the power bus24,26with each of the paired switch devices32,34associated with one of the phases of the torque module90. Each of the paired switch devices32,34is a suitable high-voltage switch, e.g., a semi-conductor device having low-on impedance that is preferably an order of magnitude of milli-ohms. In one embodiment, the paired switch devices32,34are insulated gate bipolar transistors (IGBT). In one embodiment, the paired switch devices32,34are field-effect transistor (FET) devices. In one embodiment, the FET devices may be MOSFET devices. The paired switch devices32,34are configured as pairs to control electric power flow between the positive side of the power bus24and one of the electric cables connected to and associated with one of the phases of torque module90and the negative side of the power bus26.

The inverter30also includes a pair of high-voltage DC link capacitors (capacitors)36,38that electrically connect in series between the positive and negative sides of the power bus24,26. The junction37between the capacitors36,38electrically connects to the chassis ground12. The capacitors36,38preferably have the same capacitance, which is a capacitance of 3000 μF in one embodiment. The capacitors36,38are suitable to maintain electrical potential across the positive and negative sides of the power bus24,26, but may lack capacity to fully substitute for the battery22. Resistors39electrically connect in parallel with the capacitors36,38, including electrically connecting between the positive and negative sides of the power bus24,26and at the junction37. The inverter30may also include other circuit elements, including by way of example, an active DC bus discharge circuit including a resistor and a switch that electrically connects in series between the positive and negative sides of the power bus24,26.

The gate drive module40includes a plurality of paired gate drive circuits42, each which signally individually connects to one of the paired switch devices32,34of one of the phases to control operation thereof. Thus there are three paired gate drive circuits42or a total of six gate drive circuits42when the torque module90is a three-phase device. The gate drive module40receives operating commands from the controller10and controls activation and deactivation of each of the switch devices32,34via the gate drive circuits42to provide motor drive or electric power generation functionality that is responsive to the operating commands. During operation, each gate drive circuit42generates a pulse in response to a control signal originating from the control module10, which activates one of the switch devices32,34and permits current flow through a half-phase of the torque module90.

A series junction33of each of the paired switch devices32,34electrically connects to the corresponding phase of the torque module90to transfer electric power. As shown, AC output line61electrically connects first series junction33-1to the first phase91of the torque module90, AC output line62electrically connects a second series junction33-2to the second phase92of the torque module90, and AC output line63electrically connects third series junction33-3to the third phase93of the torque module90to transfer electric power.

The controller10provides operating commands for the paired gate drive circuits42of the gate drive module40, and monitors operation of the electric power circuit20and torque module90, including monitoring a negative-ground voltage VGN54and a positive-negative voltage VPN55. The operating commands for the paired gate drive circuits42of the gate drive module40control the electric power circuit20, which generates power signals that are conveyed to the first, second, and third phases of the torque module90. The power signals can be in the form any one of a sine PWM (pulsewidth-modulation) signal, a space vector PWM signal, a third harmonics insertion PWM signal and discontinuous PWM signals, e.g., DPWM1, DPWM2, and DPWM3.

Electrical potentials of interest include phase A voltage (VAN)51, which is the electrical potential between the first series junction33-1electrically connected to the first phase91of the torque module90and the negative power bus26, phase B voltage (VBN)52, which is the electrical potential between the second series junction33-2electrically connected to the second phase92of the torque module90and the negative power bus26, and phase C voltage (VCN)53, which is the electrical potential between the third series junction33-3electrically connected to the third phase93of the torque module90and the negative power bus26, the negative-ground voltage VGN54, which is the electrical potential between the junction37between the capacitors36,38electrically connected to the chassis ground12and the negative power bus26, and the positive-negative voltage VPN55, which is the electrical potential between the positive power bus24and the negative power bus26. A pseudo-resistance RAC-line56is shown for purposes of illustration, and represents an electric resistance or isolation between the electric power circuit20and the chassis ground12, but does not represent an actual device. The pseudo-resistance RAC-line56indicates electrical isolation between the electric power circuit20and the chassis ground12. An equivalent isolation resistance Rsysindicates an equivalent isolation resistance of the electrical system, and incorporates resistances associated with the battery22, inverter22, gate drive module40and other electrical components and systems taken in parallel.

Control module, module, control, controller, control unit, processor and similar terms mean any one or various combinations of one or more of Application Specific Integrated Circuit(s) (ASIC), electronic circuit(s), central processing unit(s) (preferably microprocessor(s)) and associated memory and storage (read only, programmable read only, random access, hard drive, etc.) executing one or more software or firmware programs or routines, combinational logic circuit(s), input/output circuit(s) and devices, appropriate signal conditioning and buffer circuitry, and other components to provide the described functionality. Software, firmware, programs, instructions, routines, code, algorithms and similar terms mean any instruction sets including calibrations and look-up tables. The control module has a set of control routines executed to provide the desired functions. Routines are executed, such as by a central processing unit, and are operable to monitor inputs from sensing devices and other networked control modules, and execute control and diagnostic routines to control operation of actuators. Routines may be executed at regular intervals, for example each 100 microseconds, 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing operation. Alternatively, routines may be executed in response to occurrence of an event.

FIG. 2schematically shows portions of the electric power circuit20to provide support for an analytical framework for determining a magnitude of electrical isolation between the electric power circuit20and the chassis ground12, indicated herein by the pseudo-resistance RAC-line56. The illustrated portion of the electric power circuit20includes battery22, positive and negative sides of power bus24and26, capacitors36,38, junction37between the capacitors36,38electrically connected to chassis ground12and the resistors39. Impedances include Phase A impedance ZA71, Phase B impedance ZB72, and Phase C impedances ZC173and ZC274. Voltage VFN57indicates the electrical potential occurring between phase C impedances ZC173and ZC274, and can be used to determine the pseudo-resistance RAC-line56and thus indicate the electrical isolation between the electric power circuit20and the chassis ground12. The phase C impedances ZC173and ZC274are shown to depict analysis of the isolation of phase C of the electric power circuit20. It is appreciated that an analogous analysis as described herein is performed on each of the phases A, B, and C. Under ideal operating conditions with no degradation in the isolation of the circuit, Phase C impedance ZC173is equal to zero and the voltage VFN57is equal to phase C voltage (VCN)53.

A process for monitoring an electric power circuit configured to transfer a power signal to a torque module that is electrically isolated from a chassis ground to determine a magnitude of AC electrical isolation during operation with the system under stress of power signals includes injecting a common voltage component into the common mode voltage of each of the electric phases of the electric power circuit during operation during a period of time when the AC lines are energized with the PWM voltage from the inverter. Monitoring includes monitoring a negative-ground voltage and a positive-negative voltage of the electric power circuit. An AC line resistance is determined based upon the negative-ground voltage and the positive-negative voltage. A fault can be detected in electrical isolation between the electric power circuit and a chassis ground based upon the AC line resistance.

The process of injecting a common voltage component into the common mode voltages of all the phases during a period of time when the AC lines are energized with the PWM voltage from the inverter includes injecting a voltage component that is between a half and a third of the potential across the high-voltage DC power bus at a low operating speed for a period of time that is sufficient to exclude momentary faults, preferably between three and five seconds. Preferably the load, i.e., the torque module90sees or is exposed to line-to-line voltages (i.e. Vab=Va−Vb, Vbc=Vb−Vc, Vca=Vc−Va) to produce the current. Phase voltages without the injected common voltage component have the same effect upon the torque module90as phase voltages with the injected common voltage component. Thus, by way of example, a first set of phase voltages of Va=10, Vb=50, Vc=−60 have the same effect as a second set of phase voltages of Va=10+37, Vb=50+37, Vc=−60+37, with each of the second set of phase voltages having 37 volts of common mode voltage. In one embodiment, the common voltage component injected into each of the common mode voltages of all the phases is a DC voltage component. Alternatively, the common voltage component injected into each of the common mode voltages of all the phases is a waveform, e.g., a 60 Hz triangle wave, with a detection scheme seeking to identify the waveform to determine if loss of isolation has occurred. Alternatively, the common voltage component injected into each of the common mode voltages of all the phases is a plurality of DC voltages. Other common voltage components may be employed without limitation.

As described herein, monitored electrical potentials include only negative-ground voltage VGN54and positive-negative voltage VPN55, which are measured for other purposes related to system control. An insulation resistance between the AC output lines of the inverter and the torque module, i.e., lines61,62and63and chassis12can be determined, thus permitting detection of AC loss of isolation at any point on the AC lines including the neutral line. The described process overcomes any electro-endosmosis effect by employing a charge-negative injected common voltage component. Electro-endosmosis is a phenomenon wherein different insulation resistance values may be obtained when the polarity of the tester leads are reversed due to the presence of moisture, e.g., due to intrusion of rainwater into the system.

Line-to-line electrical potentials, i.e., VAB, VBC, and VACremain unchanged after injecting a common voltage component in the common mode voltage of each of the three phases during a normal operation, since the inverter30is high-impedance isolated from the chassis12. This can be demonstrated in accordance with the following set of relationships:
VA′N=VAN+VdcCMode
VB′N=VBN+VdcCMode
VC′N=VCN+VdcCMode[1]
whereinVANis the electrical potential between the first phase91of the torque module90and the negative power bus24,VBNis the electrical potential between the second phase92of the torque module90and the negative power bus24,VANis the electrical potential between the third phase93of the torque module90and the negative power bus24,VdcCModeis the injected common voltage component in the common mode voltage,VA′Nis the electrical potential between the first phase91of the torque module90and the negative power bus24with addition of the injected common voltage component in the common mode voltage,VB′Nis the electrical potential between the second phase92of the torque module90and the negative power bus24with addition of the injected common voltage component in the common mode voltage, andVC′Nis the electrical potential between the third phase93of the torque module90and the negative power bus24with addition of the injected common voltage component in the common mode voltage.

The line-to-line electrical potentials can be calculated in relation to other common electrical potentials, e.g., the negative power bus24in accordance with the following set of relationships.
VAB=VAN−VBN
VAC=VAN−VCN
VBC=VBN−VCN[2]

As is clear, when VdcCMode, i.e., the injected common voltage component in the common mode voltage is the same for each of the lines the following set of relationships are true.
VAN−VBN=VA′N−VB′N
VAN−VCN=VA′N−VC′N
VBN−VCN=VB′N−VC′N[3]

Therefore, from the preceding sets of relationships, the following set of relationships are derived.
VAB=VAN−VBN=VA′N−VB′N=VA′B,
VAC=VAN−VCN=VA′N=VC′N=VA′C, and
VBC=VBN−VCN=VB′N−VC′N=VB′C.  [4]

Thus, injecting the common voltage component in the common mode voltage of the three phases VdcCModedoes not affect the normal operation.

When AC line isolation degrades, injecting the common voltage component in the common mode voltage VdcCModecauses the negative-ground voltage VGN54to shift toward the voltage on the negative power bus26(HV−) through RAC-line56when the common mode voltage VdcCModeis chosen to be negative. This voltage shifting behavior can be employed to measure pseudo-resistance RAC-line56in accordance with the following set of relationships:
(VFN−VGN)/RAC-line+(VPN−VGN)/Rsys=VGN/Rsys[5]
whereinVFNindicates the electrical potential occurring between phase C impedances ZC173and ZC274,VGNindicates the negative-ground voltage, andVPNindicates the positive-negative voltage.

The process is described and detailed for determining the electrical potential occurring between phase C impedances ZC173and ZC274. It is appreciated that an analogous process is employed to monitor and evaluate each of the phases, e.g., phases A and B in this embodiment.

Thus the electrical potential occurring between phase C impedances ZC173and ZC274can be determined, in accordance with the following relationship.

VFN=(ZB+ZC⁢⁢2)*ZC⁢⁢1*ZA′⁢N+ZC⁢⁢1*(ZA+ZC⁢⁢2)*VB′⁢N+(ZA+ZC⁢⁢2)*(ZB+ZC⁢⁢2)*VC′⁢N(ZA+ZC⁢⁢2)*(ZB+ZC⁢⁢2)+(ZB+ZC⁢⁢2)*ZC⁢⁢1+ZC⁢⁢1⁡(ZA+ZC⁢⁢2)[6]wherein the Z terms are depicted and described herein with reference toFIG. 2.

A known system operating in response to the power signal without presence of a system fault yields results in accordance with the following relationship.
VdcAN=VdcBN=VdcCN=VPN/2  [7]

When a common voltage component in the common mode voltage VdcCModeis injected into the power signal, electrical potentials result in accordance with the following relationship:
VdcA′N=VdcB′N=VdcC′N=VPN/2+VdcCMode[8]
Therefore, combining EQ. 6 and EQ. 8, results in the following relationship.
VdcFN=VPN/2+VdcCMode[9]

From EQ. 5 and EQ. 9, the isolation resistance of AC lines RAC-linecan be obtained when only electrical potentials of VPNand VGNare monitored in accordance with the following relationship.
RAC-line=(VGN−VPN/2−VdcCMode)*Rsys/(VPN−2*VGN)  [10]

The result of this analysis indicates that, for an inverter electric system with high-impedance isolation from a chassis, isolation resistance between the AC lines and the chassis can be measured in real-time by injecting a common voltage component in the common mode voltage of all three phases, while the AC lines are energized with the PWM voltage. Furthermore, the injected common voltage component is not limited to a DC component to effect measurement of the AC isolation resistance. It can include multiple DC voltage levels or a repeated waveform as an input. The associated signals of high-voltage voltage sensing can be employed to characterize the AC isolation resistance. However, a DC voltage component is preferred since it is the simplest to implement.

FIG. 3schematically shows an isolation fault detection process300for monitoring and detecting presence of a fault associated with isolation resistance between an electric power circuit and a chassis ground, e.g., the electric power circuit20including the inverter30and torque module90ofFIG. 1. The isolation fault detection process300operates to monitor insulation resistance between AC lines and chassis while the AC lines are PWM-energized. Table 1 is provided as a key toFIG. 3wherein the numerically labeled blocks and the corresponding functions are set forth as follows.

The isolation fault detection process300operates by injecting a common voltage component in the common mode voltage VdcCModeof a power signal while the AC lines are energized with the power signal from the inverter (302). Preferably, the injected common voltage component in the common mode voltage VdcCModeis charge-negative to overcome any risk of an electro-endosmosis effect. Electrical potentials including VGNand VPNare monitored (304). The term VGNindicates the negative-ground voltage and the term VPNindicates the positive-negative voltage, as previously described. The isolation resistance RAC-lineis determined using the monitored values of VPNand VGN, preferably as described with reference to EQ. 10 (306). The isolation resistance RAC-lineis compared to a threshold resistance (308), and when the threshold resistance is exceeded, a fault associated with the isolation resistance RAC-lineof the selected system is detected (310). Upon detecting a fault, the system executes remediation, which can include derating the motor capacity (312).

FIG. 4schematically shows a multi-function electric power circuit420including a battery422that electrically connects to positive and negative sides of a high-voltage DC power bus (power bus)424and426, respectively, which electrically connect in parallel to a plurality of modules. The modules include a multi-phase power inverter module430that electrically connects to an electric torque module490, an electric pump power module440, and an inverter module and an electric AC compressor torque module450. A control module410is configured to monitor and control operation of each of the aforementioned elements. Each of the aforementioned elements is high-impedance isolated from a chassis ground12associated with a mechanical system in which they are mounted. In operation, the system can identify which set of AC lines has low isolation resistance by individually injecting a common voltage component in the common mode voltage VdcCModeof the power signals and executing the isolation fault detection process300one-by-one for each of the aforementioned modules.

Thus a system and method are provided that measure the insulation resistance between AC lines and chassis while the AC lines are energized with the PWM voltage from the inverter based upon measurements of electrical potentials. The process includes measuring the insulation resistance in real-time and identifying which set of AC lines has low isolation resistance (if any), and overcoming the electroendosmosis effect, without additional hardware such as a micro-ammeter to measure current resistance. Instead, the insulation resistance of inverter AC outputs based on only voltage readings. This AC isolation resistance can classify the severity of the AC-to-chassis fault and determine the necessary response.