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Original question:

TIDA-010232: DC Injection Method for High Voltage Insulation Monitoring in addition to method used in TIDA - 010232

TIDA-010232: Clarification of transfer function for Insulation Resistance

Mathew Maher49
Expert 2125 points

Part Number: TIDA-010232
Other Parts Discussed in Thread: TLV6001, REF2033, AMC3330

Tool/software:

ok, its been a long day, so I apologise if I'm asking stupid questions, but...

Following the implementation of "electric bridge method" of insulation monitoring on DC systems, can anyone clarify what the transfer function is between Line resistance and input to the AMC3300? 

Specifically Page 5 does a brilliant job of proving a formula from base principles but no matter how many times I read it and play with the numbers, I'm loosing confience in my calculations - and this is one area I can't afford to!!! Specifically around the terms Vdc/Vn/Vp/VinP/VinN which appear and disappear at will. 

1. For a given input voltage range, how do I correctly size the Rst/RinAMC resistor pack? 

2. How do I then calculate a transfer function to correlate either AMC Input Voltage to specific line insulation resistance?  (or the whole chain, but I get the rest of it...) 

For reference I'm using the same AMC3300/REF2033/TLV6001 chain

Thanks!

Mat

  • 0
    TI__Expert 4985 points

    Hello Mat,

    Thanks for reaching out.
    1. For the ratio between Rst and RinAMC we just used the worst case scenario. According to the equivalent circuits shown in  Figure 1-5 and Figure 1-6 the maximum voltage applied to Rst + RinAMC divider is the full bus voltage if RisoP or RisoN would be 0. So we designed the divider ratio in a way that the maximum bus voltage is divided down to the maximum input voltage of the AMC3330 with some margin.
    For the 400V example we used Rst = 68.1k and RinAMC = 120 Ohm, which leads to 400V * 120/(68100+120) = 0.7V.
    Now the absolute value of Rst should be selected in the same range as the critical value for the isolation resistance you are measuring. This offers best accuracy. In the 400V case the critical values are 100 Ohm/V = 40kOhm (Fault) and 500 Ohms/V = 200kOhm (Warning). We selected rst in betwwen these values with 68.1 kOhm.

    2. Equations (1) to (12) explain how VP and VN are measured. If the positive switch SP is closed we measure VP and if the negative switch is closed we measure VN.
    Now we can use equations (4) and (11) and solve for RisoP and RisoN. (2 equations 2 unknowns). The resulting equations for RisoP and RisoN are shown in (13) and (14).

    I hope this helps.

    Best regards,
    Andreas
     

  • 0 in reply to Andreas Lechner
    Expert 2125 points

    ok, thanks, that helps. 

    With regards to the 400V example, can this circuit actually produce an IEC Fault indicator with the current resistor setup?

    Running the calculations, if Iiso = Vp/Riso_p = 400/68100 = 5.9mA which doesn't equate to the 10mA fault condition as per ISO? 

  • 0 in reply to Mathew Maher49
    TI__Expert 4985 points

    Hi,

    No this setup does not produce a fault. We stay below the 10mA threshold on purpose.
    We are calculating the insulation resistance without causing a fault.

    Best regards,

    Andreas