Abstract:
A fuel cell system includes a fuel cell having conductive coolant flowing there through and a high voltage direct current (HVDC) bus interconnected with the fuel cell. An active isolation circuit includes coolant fault current sensors that detect a fault current (also called residual current) in the coolant and generates a fault signal when the fault current is detected. A switching circuit compensates and redirects the fault current based on the fault signal, providing active fault current limitation thereby.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cells or other automotive HVDC sources and more particularly to voltage and/or current isolation systems for conductive (i.e., non-isolating) fluid cooled fuel cell stacks. 
     BACKGROUND OF THE INVENTION 
     Fuel cell stacks operate at relatively high voltage levels and higher temperatures. Liquid flowing through a coolant loop is typically used to control the temperature of the fuel cell stack. The coolant loop typically includes radiators, pumps, tubes and/or other components. To improve safety, steps are typically taken to isolate the high voltage levels of the fuel cell stack from the coolant flowing in the coolant loops. In other words, to provide electrical isolation, the coolant loops should be either electrically isolated or non-conductive coolant should be used. 
     Current approaches employ very low-conductivity or isolating coolant and long/thin isolating coolant tubes. For example, the low-conductivity coolant can be de-ionized (DI) water or isolating coolant could be oil. The low-conductivity or isolating coolants typically have significant performance disadvantages when compared to higher conductivity coolants, such as automotive (i.e. water and glycol-based) coolants. For example, the isolating coolants typically have low heat capacity, low heat conductivity and high viscosity (e.g. oil). The isolating coolants therefore adversely impact system power density, radiator size, radiator fan size, and/or coolant pump power. The low-conductivity coolants may also pose various environmental constraints. The low-conductivity coolants lack anti-freeze characteristics and/or may cause corrosion (e.g., de-ionized water). Contaminations in the coolant system also tend to increase the conductivity of a low-conductivity coolant over time and hence isolation gets worse. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a fuel cell system including a fuel cell having coolant flowing therethrough and a high voltage direct current (HVDC) bus interconnected with the fuel cell. An active isolation circuit includes a first/second or multiple current sensors that connects to the coolant on the coolant exit/entrance paths to the fuel cell and detects a ground fault current in the coolant and generates a fault signal when the fault current is detected. A switching circuit redirects and compensates the fault current based on the fault signal. 
     In one feature, the first/second current sensor is immersed in the coolant. 
     In another feature, the switching circuit monitors the fault signal. 
     In another feature, the switching circuit includes an operational amplifier that receives the fault signal and that generates an output signal. A switching device selectively enables an alternate path to ground for the fault current based on the output signal. The switch includes MOSFET—transistors that enable the alternate path when in a conductive mode. 
     In another feature, the active isolation circuit further comprises a third or more fault sensors that detect the fault current in all coolant paths (exits/entrance/vents etc.) and contribute to the fault signal when the fault current is detected. The switching circuit includes an operational amplifier that generates an output signal and a first switch that selectively enables an alternate path from HVDC minus to ground for the fault current based on the output signal polarity. A second switch selectively enables an alternate path from HVDC plus to ground for the fault current based on the polarity of the output signal. When any of the fault sensors detects a positive fault current, the op-amp signals the first switch to enable the alternate path. When any of the fault sensors detects a negative fault current, the op-amp signals the second switch to enable the alternate path. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is an electrical schematic of a conductive liquid cooled fuel cell system high voltage (HV) bus including Y-capacitors; 
         FIG. 2  is an electrical schematic of the HV bus incorporating an active isolation circuit according to the present invention; 
         FIG. 3  is a graph illustrating fault discharge currents according to the present invention; and 
         FIG. 4  is an electrical schematic of the HV bus incorporating a Y-capacitor (Y-cap) discharge compensation circuit and active isolation circuit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference designations will be used in the drawings to identify similar elements. 
     Referring now to  FIG. 1 , a fuel cell system  10  includes a high voltage direct current (HVDC) power bus  12  and a fuel cell stack  14 . The fuel cell stack  14  is represented as two voltage sources V 1  and V 2 . Exemplary values for V 1  and V 2  are 200V, although other values may be used. Assuming 200V for V 1  and V 2 , the total voltage across the fuel cell stack  14  is 400V. The fuel cell stack  14  includes conductive coolant flowing through manifolds. The coolant entering/exiting the fuel cell is indicated as parallel resistors R c . Exemplary values for the resistors R c  are 20 kΩ each or 10 kΩ total. As the coolant may enter (exit) the fuel cell stack through manifolds at any defined points of the fuel cell, the resistors Rc may connect to the fuel cell voltage at any intermediate voltage and are shown in a balanced configuration (=entering/exiting the fuel cell in the middle) here for clarity. 
     The HVDC power bus  12  includes positive and negative nodes (HV+ and HV−, respectively) and a capacitor circuit  16 . Given the exemplary values of V 1  and V 2  and assuming the voltage balance is symmetrical, HV+ is at +200V and HV− is at −200V. The cap circuit includes capacitors C 1 , C 2  and C 3 . Exemplary values for C 1 , C 2  and C 3  are 3000 μF, 5 μF and 5 μF, respectively. The cap circuit  16  shields the HVDC power bus  12  from electromagnetic interference (EMI). The Y-capacitors C 2 ,C 3  bridges the HVDC power bus  12  to a vehicle chassis (not shown) or safety ground. The capacitors C 1 ,C 2 ,C 3  may be distributed across multiple components of a real fuel cell system connected to the HVDC bus but are represented as lumped components here. 
     A typical fault contact, for example a human body, is indicated as a fault resistance R FAULT . Although the fault contact is shown at HV+, the fault contact can also occur at HV− or at any intermediate voltage. An exemplary value for R FAULT  is 1 kΩ. As a result of the fault contact, a discharge current causes the Y-cap circuit  16  to discharge through R FAULT  to ground. The energy in the Y-cap circuit that is dissipated during the fault contact is equal to ½CV 2 . As shown in  FIG. 3 , which is discussed in further detail below, the typical discharge current immediately peaks upon fault contact and then gradually decreases to under 25 mA, given the exemplary values provided herein. The area beneath the typical discharge current curve indicates the energy that is dissipated through R FAULT  (e.g., human body). 
     Referring now to  FIG. 2 , a fuel cell system  20  includes a high voltage direct current (HVDC) power bus  22  and a fuel cell stack  24 . The fuel cell stack  24  is represented as two voltage sources V 1  and V 2 . Exemplary values for V 1  and V 2  are 200V, although other values may be used. Assuming 200V for V 1  and V 2 , the total voltage across the fuel cell stack  24  is 400V. The fuel cell stack  24  includes conductive coolant flowing through manifolds, which is indicated as parallel resistors R 1  and R 4 . Exemplary values for R 1  and R 4  are 22 kΩ and 18 kΩ, respectively. The coolant is provided by a coolant system  26  as indicated by parallel resistors R 9  and R 8 . Exemplary values for R 9  and R 8  are 10 kΩ each. R 9  and R 8  are in respective series connection with R 1  and R 4 . 
     The HVDC power bus  22  includes positive and negative nodes (HV+ and HV−, respectively) and a capacitor (cap) circuit  28 . Given the exemplary values of V 1  and V 2  and assuming that the voltage balance is symmetrical, HV+ is at +200V and HV− is at −200V. The cap circuit  28  includes capacitors C 8 , C 1  and C 2 . Exemplary values for C 8 , C 1  and C 2  are 3000 μF, 5 μF and 5 μF, respectively. The cap circuit  28  protects the HVDC power bus from electromagnetic interference (EMI). The Y-capacitors C 1 ,C 2  bridges the HVDC power bus to a vehicle chassis (not shown) or safety ground. 
     The fuel cell system  20  includes an active isolation circuit  29 . The active isolation system consists of a monitoring circuit  60  and a switching circuit  32 . The monitoring circuit  60  includes fault sensors  62 ,  64  that are associated with the coolant, and resistors RS 1  and RS 2 . The fault sensors  62 ,  64  collect net fault current flowing through all coolant resistant paths of the fuel cell system  20  to ground through RS 1 ,RS 2 . RS 1 ,RS 2  (which could be a single combined resistor) convert the fault currents coming from sensors  62 , 64  to a fault signal voltage. The fault signal voltage is connected to the inverting input  40  of the op-amp  34 . The switching circuit  32  includes an operational amplifier (op-amp)  34 , a first MOSFET transistor S 1  and a second MOSFET transistor S 2 . The op-amp  34  includes a positive input  36  that is connected to ground. An output  38  is connected to Si and S 2 . A negative input  40  is connected to the monitoring circuit and the output through a capacitor C 7  and a resistor R 7 . S 1  includes a gate input  42  that is connected to the op-amp output  38 . An input  46  (drain) is connected to HV− through a resistor R 17  and an output  48  (source) is connected to ground through a resistor R INJ . S 2  includes a gate input  50  that is connected to the op-amp output  38 . An input  54  (drain) is connected to HV+ through a resistor R 16  and an output  56  (source) is connected to ground through the resistor R INJ . Exemplary values for R 16  and R 17  include 50Ω each and an exemplary value for R INJ  includes 10Ω. S 1  and S 2  function as switches. When in a conductive state, S 1  or S 2  provide a current path from the HVDC bus positive or negative to ground through R INJ  and R 16  or R 17 . C 11  and R 19  provide low pass filtering for the injected current signal coming from R inj , R 18  feeds the filtered signal back to the Op-Amp input  40 . Exemplary values for C 11 , R 18 , R 19  include 1 μF, 5 kOhms and 5 kOhms. 
     In operation, in the event of a sufficient fault current through the coolant resistance paths, the active isolation circuit  60  signals the switching circuit  32  to provide an alternate current path to ground. For example, when a sufficient positive fault current is detected by the fault sensor  64  or  62 , the op-amp output closes S 2  to create an alternate current path to ground through R 16 ,R INJ . As a result, the fault current is forced towards 0 mA. Similarly, when a sufficient negative fault current is detected by the fault sensor  62  or  64 , the op-amp output closes S 1  to create an alternate current path to ground through R 17 ,R INJ , again resulting in the fault current being forced towards 0 mA. 
     The active isolation circuit  29  enables a fuel cell stack coolant scheme that includes a conductive coolant entering and exiting the fuel cell stack  24  at a common voltage potential plate or at any fuel cell voltage location. The voltage potential plate can include an end plate or a center tap plate in the case of multiple stack arrangements. The active isolation circuit  29  further provides an additional safety ground for all conductive components of the coolant loop that are in contact with the coolant. Furthermore, implementation of the active isolation circuit  29  requires the use of isolated or non-conductive coolant manifolds or non-conductive coolant entrance and exit areas to form a defined coolant resistance path upstream and downstream of the fault sensors  62 ,  64 . 
     Referring now to  FIG. 4 , an active isolation circuit with additional Y-cap discharge compensation circuit  29  bridges the HVDC power bus  22  and includes a monitoring circuit  30  and  60  and a switching circuit  32 . The monitoring circuit  30  includes capacitors C 12  and C 13  and resistors R Y-CAP , R 21  and R 22 . Exemplary values for C 12  and C 13  include 1 μF each. An exemplary value for R Y-CAP  includes 100Ω and exemplary values for R 21  and R 22  include 5 kΩ each. The monitoring circuit  60  also includes the fault sensors  62 ,  64  which are connected to the inverting input  40  of the op-amp  34  and ground through resistors R S1  and R S2 , respectively. The fault sensors  62 ,  64  measure net fault current flowing through all coolant resistant paths of the fuel cell system  20  to ground through RS 1 ,RS 2 . 
     The switching circuit  32  includes an operational amplifier (op-amp)  34 , a first MOSFET transistor S 1  and a second MOSFET transistor S 2 . The op-amp  34  includes a positive input  36  that is connected to ground. An output  38  is connected to S 1  and S 2 . A negative input  40  is connected to the monitoring circuit and the output through a capacitor C 7  and a resistor R 7 . S 1  includes a gate input  42  that is connected to the op-amp output  38 . An input  46  (drain) is connected to HV− through a resistor R 17  and an output  48  (source) is connected to ground through a resistor R INJ . S 2  includes a gate input  50  that is connected to the op-amp output  38 . An input  54  (drain) is connected to HV+ through a resistor R 16  and an output  56  (source) is connected to ground through the resistor R INJ . Exemplary values for R 16  and R 17  include 50Ω each and an exemplary value for R INJ  includes 10Ω. S 1  and S 2  function as switches. When in a conductive state, S 1  or S 2  provide a current path from the HVDC bus positive or negative to ground through R INJ  and R 16  or R 17 . 
     In operation, the monitoring circuit  30  provides current to the switching circuit  32  indicating a discharge current of the Y-cap circuit  28 . More particularly, the monitoring circuit  30  monitors the rate of change of voltage (dV/dt) of the cap circuit  28  with respect to ground. If dV/dt of the cap circuit  28  is greater than a threshold level, an external discharge current situation is indicated. That is to say, the Y-capacitors C 2 ,C 1  is being caused to discharge by a fault contact such as a person touching either HV+, HV− or any intermediate voltage point. 
     The op-amp  34  receives the current signal from the monitoring circuit  30  when dV/dt of the Y-cap circuit  28  exceeds a predetermined threshold. More particularly, the dV/dt signal is generated by the differentiating capacitor-resistor network that includes R Y-CAP  and C 12 . The dV/dt signal is filtered and smoothed by R 21  and C 13 . The filtered signal causes the output  38  of the Op-Amp to change to positive or negative depending on the sign of dV/dt, which depends on the fault location being on the positive or negative HVDC bus terminal. If the OpAmp output exceeds the turn on gate threshold voltage of the MOSFET switches S 1  (e.g. −5V) or S 2  (e.g. +5V), it causes S 1  or S 2  to turn on, which redirects the main fault discharge current path. For example, in the event of a fault at HV+, as illustrated in  FIG. 4 , the op-amp output closes S 2  to create a discharge path to ground through R 16  and R INJ . As a result, the energy of the Y-cap circuit  28  is dissipated mainly through R 16  and R INJ  instead of through R FAULT . Similarly, in the event of a fault at HV−, the op-amp output closes S 1  to create a discharge path to ground through R 17  and R INJ . Although the fuel cell system  20  of  FIG. 4  is shown to include both the Y-cap discharge compensation circuit  29  and the active isolation circuit  60  together, the function of the active isolation circuit  60  can be achieved using the active isolation circuit  60  and the switching circuit  32  alone. 
     Referring now to  FIG. 3 , a graph illustrates Y-cap fault discharge currents according to the present invention. Typical discharge currents for conventional circuits are illustrated by the highest dashed line. The discharge current for the active isolation circuit  29  of the present invention is illustrated by the middle dashed line. The discharge current drops to a much lower and safe steady state value, equivalent to a high isolation. The discharge current for the discharge compensation circuit  29  of the present invention is illustrated by the solid line. The discharge current drops more rapidly. Additionally, the area under each of the curves indicates the amount of energy dissipated through R FAULT . A significantly decreased amount of energy is dissipated through R FAULT  using the discharge compensation circuit  29 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the current invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.