BUS LEAKAGE RESISTANCE ESTIMATION FOR ELECTRICAL ISOLATION TESTING AND DIAGNOSTICS

[0001] Not Applicable.

[0002] Not Applicable.

[0003] The present invention relates in general to electrified vehicles using a high voltage bus, and, more specifically, to accurate estimation of the effective isolation resistance present between each high-power bus and a chassis ground.

[0004] Electrified vehicles such as for electric vehicles and hybrid electric vehicles typically utilize a high voltage power bus driven by a DC power source which may include storage and/or conversion devices such as a multi-cell battery pack or a fuel cell. The presence of high-voltage buses creates a need for monitoring of the electrical isolation of each bus with respect to the electrically conductive components of the vehicle chassis (ground).

[0005] Any leakage resistance present between a DC bus and chassis ground must be sufficiently large. In typical leakage resistance detection systems, there is an assumption that leakage resistance will be between one of the positive or negative DC buses and chassis ground. A typical leakage detector circuit operates by periodically connecting one bus at a time to chassis ground through a current-limiting resistance, and using the resulting current flow to calculate the leakage resistance between the opposite bus and ground. The battery voltage divided by the calculated leakage resistance characterizes the electrical isolation.

[0006] The invention is based, in part, on a determination that conventional leakage resistance detection systems based on supplying a current from one bus through a known resistance to the leakage resistance between the ground and the other bus may ignore a potential balanced component of leakage resistance from both buses to ground that can sometimes result in the mischaracterization of the electrical isolation because of a potential discrepancy in the derived leakage resistance values. More specifically, a resistance may exist between the positive bus and chassis ground as well as a resistance of equal value between the negative bus and chassis ground. These resistances, both being equal in value, are hereafter referred to as symmetrical or balanced leakage resistance. A resistance on one bus to chassis without a matching value on the other bus to chassis is hereafter referred to as non-symmetrical or unbalanced leakage resistance. The additional current flow through the balanced leakage resistance may cause the prior art detection system to overestimate the composite balanced and unbalanced resistance which exists between one bus and chassis ground. Estimating this latter composite resistance is desirable in order to more accurately determine the isolation.

[0007] One typical source of a balanced leakage resistance would be a hydrogen fuel cell vehicle, wherein a deionizer intended to remove ions from water being used as a coolant fails to maintain proper deionization. As ions build up, the conductance of the cooling water increases and the electrical isolation between both positive and negative fuel cell electrodes and ground is reduced. Another possible source of a balanced leakage resistance includes a symmetrical breakdown in cable insulation.

[0008] The invention recognizes the presence of both balanced and unbalanced leakage resistance that exist between the positive and negative power buses and chassis ground. By differentiating between the leakage resistances that are the same for the two buses versus leakage resistance that is not the same, the invention more accurately determines the electrical isolation. Moreover, by separately identifying the balanced and unbalanced leakage resistances, detection and/or prediction of electrical or other vehicle conditions are enabled.

[0009] In one aspect of the invention, an electrified vehicle comprises a positive bus connectable to a positive output of a DC power source and a negative bus connectable to a negative output of the DC power source. A chassis ground is distributed within the vehicle. A first detector circuit is selectably activated to i) connect a first fixed resistance between the positive bus and chassis ground and ii) sense a resulting first current. A second detector circuit is selectably activated to i) connect a second fixed resistance between the negative bus and chassis ground and ii) sense a resulting second current. A control circuit identifies the positive and negative bus leakage resistances in response to a nonlinear function (e.g., ratio) of both the first and second currents. In one preferred embodiment, the current ratios provide a correction factor that can be applied to conventionally derived resistance values.

[0017] The term “electrified vehicle” as used herein includes vehicles having an electric motor for vehicle propulsion, such as battery electric vehicles (BEV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV). A BEV includes an electric motor, wherein the energy source for the motor is a battery that is re-chargeable from an external electric grid. In a BEV, the battery or other DC source supplies energy for vehicle propulsion. A HEV includes an internal combustion engine and an electric motor, wherein the energy source for the engine is fuel and the energy source for the motor is a DC storage unit such as a battery. In a HEV, the engine is the main source of energy for vehicle propulsion with the battery providing supplemental energy for vehicle propulsion (e.g., the battery buffers fuel energy and recovers kinematic energy in electric form). A PHEV is like a HEV, but the PHEV may have a larger capacity battery that is rechargeable from the external electric grid. In a PHEV, the battery may be the main source of energy for vehicle propulsion until the battery depletes to a low energy level, at which time the PHEV operates like a HEV for vehicle propulsion.

[0018] By way of example, FIG. 1 depicts a vehicle 10 as a battery electric vehicle (BEV) propelled by an electric motor 11 without assistance from an internal combustion engine. Motor 11 receives electrical power and provides drive torque for vehicle propulsion. Motor 11 also functions as a generator for converting mechanical power into electrical power through regenerative braking. Motor 11 is part of a powertrain 12 in which a gearbox 13 couples motor 11 to driven wheels 14. Gearbox 13 adjusts the drive torque and speed of motor 11 by a predetermined gear ratio.

[0019] Vehicle 10 includes a battery system 15 including a main battery pack 16 and a battery energy controller module (BECM) 17. An output of battery pack 16 is connected to an inverter 18 which converts the direct current (DC) power supplied by the battery to alternating current (AC) power for operating motor 11 in accordance with commands from a traction control module (TCM) 20. TCM 20 monitors, among other things, the position, speed, and power consumption of motor 11 and provides output signals corresponding to this information to other vehicle systems including a main vehicle controller 21 (which may be a powertrain control module, or PCM, for example).

[0020] FIG. 2 shows a typical bus architecture wherein a DC power source 30 is selectively coupled to a positive bus 31 and a negative bus 32 via contactor switches 33. Buses 31 and 32 may be further coupled to a DC-to-DC converter 34, a link capacitor 35, and an inverter 36 which drives a traction motor 37. A chassis ground 40 represents conductive parts of the vehicle whose electrical potential is taken as a reference and which are conductively linked together.

[0021] Electrical isolation of buses 31 and 32 is determined by the electrical leakage resistance between each bus and chassis 40. A leakage resistance 41 represents the level of isolation between positive bus 31 and chassis 40. Leakage resistance 42 represents the isolation between negative bus 32 and chassis 40. Leakage resistances 41 and 42 are the unbalanced or nonsymmetrical leakage resistances. While resistances 41 or 42 may occur only one can be present for the unbalanced resistance. In addition, a balanced leakage resistance, 43a and 43b having the same resistance, may be present between buses 31 and 32 having a junction between them connected to chassis 20. In addition, a balanced resistance may be introduced across buses 31 and 32 within DC source 30, shown as leakage resistances 44a and 44b having their junction coupled to chassis 40.

[0022] FIG. 3 shows apparatus for detecting leakage resistance wherein a first detector circuit 45 is arranged between positive bus 31 and chassis ground 40 and a second detector circuit 46 is arranged between negative bus 32 and chassis ground 40. First detector circuit 45 includes a current-limiting resistor 50 in series with a sampling switch 51 and a current-sensing resistor 52. A controller circuit 47 is connected to switch 51 for selectively activating switch 51 so that a resulting first current flowing through detector circuit 45 creates a voltage across current-sensing resistor 52 proportional to the current passing through resistor 52 which is provided to controller circuit 47. Likewise, second detector circuit 46 includes a series connection of a current-limiting resistor 53, sampling switch 54, and current-sensing resistor 55 similarly connected to controller circuit 47. Controller circuit 47 may include a microcontroller such as in a battery energy controller module.

[0023] In the leakage resistance detecting system based on conventional assumptions shown in FIG. 4, a current flowing through a current limiting resistor R_d and current sensing resistor R_s passes through leakage resistance R_lm, which denotes leakage resistance to the minus bus. Using a measured battery voltage V_B, the first measured current I_rsm, and known resistance values for R_d and R_s, leakage resistance R_lm is determined using the following formula:

$\begin{array}{cc}{R}_{\mathrm{lm}}=\frac{V_B}{I_{\mathrm{rsm}}}-\left(R_d+R_S\right).& \left(\mathrm{Eq}.1\right)\end{array}$

As previously explained, however, the resulting value for the leakage resistance does not take into account the balanced resistance which instead creates the more complex equivalent circuit shown in FIG. 5. In this case, the sense current is not entirely due to the parallel negative bus leakage resistances R60 and R61a. In addition, a current flows through R61b from the positive bus, providing an opposing component of the balanced leakage current i_bal. Hence, the total sensed current under-represents the negative bus leakage current.

[0024] As seen from FIGS. 3 and 5, the total current through R_lm (the leakage resistance associated with negative bus 32) includes balanced part from resistance 61 and a sense current I_RS (i.e., including balanced and unbalanced portions of resistances 44, 41, and 42 of FIG. 3). When the sampling switch is closed, the current I_RS is

$\begin{array}{cc}{I}_{\mathrm{RSM}}=\frac{V_BR_{\mathrm{lp}}}{R_CR_{\mathrm{lp}}+R_{\mathrm{lm}}\left(R_C+R_{\mathrm{lp}}\right)}& \left(\mathrm{Eq}.2\right)\end{array}$

where R_lp is the leakage resistance 41 and balanced portion of 44 associated with positive bus 31 (FIG. 3), R_lm is the leakage resistance 42 and balanced portion of 44 associated with negative bus 32 (FIG. 3), I_RSM is the sense current I_RS as shown in FIG. 4 when switch 51 (FIG. 3) is closed, and resistance R_C is a combined resistance (e.g., a fixed 519 k-ohms) which includes the sum of the current-limiting resistance R_D and the current-sensing resistance R_S. While the following text assumes the values of R_D and R_S in detector circuits 45 and 46 are equal to each other, for those familiar with the art these values may be changed while retaining the intent of this description. Thus, when the other sampling switch is closed, the current I_dp can be determined using the formula:

$\begin{array}{cc}{I}_{\mathrm{RSP}}=\frac{V_BR_{\mathrm{lm}}}{R_CR_{\mathrm{lm}}+R_{\mathrm{lp}}\left(R_C+R_{\mathrm{lm}}\right)}.& \left(\mathrm{Eq}.3\right)\end{array}$

Equations 2 and 3 are solved simultaneously for leakage resistances R_lp and R_lm as follows:

$\begin{array}{cc}{R}_{\mathrm{lm}}=\frac{V_B}{I_{\mathrm{RSM}}}-R_C\left(1+\frac{I_{\mathrm{RSP}}}{I_{\mathrm{RSM}}}\right)& \left(\mathrm{Eq}.4\right)\\ R_{\mathrm{lp}}=\frac{V_B}{I_{\mathrm{RSP}}}-R_C\left(1+\frac{I_{\mathrm{RSM}}}{I_{\mathrm{RSP}}}\right)& \left(\mathrm{Eq}.5\right)\end{array}$

Thus, the ratios of the two measured currents provides respective correction factors to the conventional determination of the respective leakage resistances. The resulting values for the positive and negative leakage resistances, R_lp and R_lm, based on the current ratios provide a more accurate assessment of the leakage resistances, which consequently enables more accurate determination of a corresponding electrical isolation.

[0025] In a preferred embodiment, a bus having a lower isolation is used to calculate an isolation value. Thus, the smaller of the calculated leakage resistances is selected and then divided by a predetermined voltage in order to calculate the isolation value. The predetermined voltage can be comprised of the measured voltage (V_B) of the DC source or a predetermined constant voltage (e.g., the nominal system voltage or a value specified by regulations). The isolation value can be calculated as follows:

$\begin{array}{cc}\frac{\mathrm{Min}\left(R_{\mathrm{lp}},R_{\mathrm{lm}}\right)}{V_B}& \left(\mathrm{Eq}.6\right)\end{array}$

The resulting isolation value is compared with an isolation threshold (e.g., 500 ohms/volt), and if it is less than the threshold then the invention signals that an atypical condition has been detected. The signaling may be comprised of informing a driver of the condition and/or automatically disconnecting the DC power source from the power buses (e.g., opening the contactor switches).

[0026] The calculated values for leakage resistances R_lp and R_lm can further be used to separate the balanced and unbalanced components of the resistances so that the components can be monitored over time in order to detect or predict certain potential failures in the electrical system. Examples of balanced leakage resistances that can change over time in a manner that identifies an impending fault include a) insulation breakdowns, and b) loss of effectiveness of a cooling-water deionizer in a fuel cell system. A plurality of leakage resistance measurements over time can be separated into balanced/unbalanced components and stored in a database. Both the magnitude and slope within the stored data (e.g., either the balanced or unbalanced components) is used to predict potential failures. Examples of unbalanced leakage resistances that may change over time include contact of battery terminals to chassis, wire contact to the chassis, and other forms of contact.

[0027] The calculated values for leakage resistances R_lp and R_lm can be separated as follows. The larger one of leakage resistances R_lp and R_lm will correspond to the balanced resistance, i.e., R_bal=max(R_lp, R_lm). This is because, by definition, the value of R_bal must be the same from chassis ground to both the positive and negative buses. An unbalance resistance exists in parallel with the value of R_bal, from either the positive or negative bus to ground. Since parallel resistances always result in a total resistance lower that each of the parallel resistances, the maximum of R_lp and R_lm corresponds to the balanced resistance.

[0028] The unbalanced resistance component can be calculated using R_lp and R_lm as follows:

$\begin{array}{cc}{R}_{\mathrm{unbal}}=\frac{R_{\mathrm{lp}}·R_{\mathrm{lm}}}{R_{\mathrm{lp}}-R_{\mathrm{lm}}}& \left(\mathrm{Eq}.7\right)\end{array}$

[0029] A method of the invention will be summarized in connection with the flowcharts of FIGS. 6 and 7. In step 70, the voltage V_B of the DC power source is measured. In step 71, the positive bus is connected to the chassis ground and a resulting current is measured. The negative bus is then sequentially connected to chassis ground and a resulting current is measured in step 72. In step 73, leakage resistances R_LP and R_LM are calculated. The smaller of the two resistances is chosen in step 74 in order to identify a worst-case bus leakage resistance. In step 75, an isolation value is calculated using the chosen worst-case resistance.

[0030] In step 76, the isolation value is compared to an isolation threshold. If less ii) than the threshold, then the invention signals an atypical condition in step 77 (e.g., by notifying the driver or disconnecting power to the high-voltage buses). If the isolation value is not less than the threshold, then the method is completed at step 78. Optionally, an additional method can be executed wherein the balanced and unbalanced resistance values are determined in order to provide prediction of potential failures as shown in FIG. 7.

[0031] In FIG. 7, the larger resistance value is assigned as the balanced leakage resistance in step 80. In step 81, an unbalanced leakage resistance is calculated (e.g., using Equation 7). The calculated values for the balanced and/or unbalanced resistances are compiled over time in step 82. In step 83, predetermined potential failures can be predicted in response to predetermined decreases in the balanced or unbalanced resistances. More particularly, a potential failure can be predicted in response to a magnitude of a resistance value falling below a threshold or in response to data points accumulated over time exhibiting a negative slope that points toward an impending drop in the resistance value below a threshold. For example, a predetermined decrease in the balanced resistance component may correspond to a certain loss in effectiveness in deionizing the coolant water for a fuel-cell (leading to decreased power generation). Empirical testing may identify the magnitude or slope values of interest that can be used to detected a need for corrective action to repair or replace the deionizer. Whenever a potential failure is detected or predicted, step 84 is executed to provide notification to other vehicle systems (e.g., a controller for deactivating the main contactors between the DC source and the buses), the driver (e.g., via a malfunction indicator light), or a remote monitoring system (e.g., wirelessly to a manufacturer's database via an electronic cloud).