Several important concepts in the insulation detection bridge method
Latest update time：2020-05-11
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The May Day holiday went by quite quickly. I also finished all the big things. The process was full of twists and turns, and then I continued to work hard.
Recently, I have searched for many domestic patents related to insulation testing, which I have compiled and shared with you before. Generally speaking, the mainstream solution for insulation testing in China is still the electric bridge method, which can be seen on many products of competitors.
I have written more or less two articles about the electric bridge method before. Today I will continue to introduce several key concepts of the electric bridge method.
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Balanced bridge and unbalanced bridge
These two concepts are often encountered, but it is difficult to find official sources. Here we will probably clarify them.
A balanced bridge
means that the resistance values of the resistors artificially incorporated into the upper and lower bridge arms are equal. In the figure below, that is, R1=R2; the changes it introduces into the circuit are balanced. When the insulation resistance is not equal between the upper and lower arms, it causes upper and lower divisions. If the voltage is inconsistent, the insulation resistance can be qualitatively judged by detecting this inconsistency; of course, this method has limitations and cannot accurately measure the resistance of the insulation resistance. In addition, it cannot accurately identify situations such as resistance changes in proportion and grounding. However, the balanced bridge method can be used for rapid insulation testing.
Unbalanced bridge
is a method we often encounter. The resistance incorporated into the bridge arm contains unequal conditions; for example, the solution in the national standard is a typical example, and this method will not be expanded on.
One more thing to add here is that after the upper and lower bridge arm voltages are detected for the first time in the national standard, the magnitude of the two voltages is judged to determine the position of the resistor for the second time, that is, which position has the highest voltage, and it will be at this position for the second time. Incorporate the known resistance R0; the reason may be that this
will not continue to reduce the small insulation resistance, and it also has advantages in calculation
.
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Bridge arm resistance value restrictions
Is there a limit to the value of the resistor artificially incorporated in the bridge method?
Of course the answer is yes, there are no rules without rules; the value of R0 in the above figure, from the perspective of resisting the influence of Y capacitance, the smaller R0, the shorter the voltage convergence time, and the shorter the detection time; but R0 The value has a lower limit.
In ISO 6469-3, there are the following provisions:
This is stipulated in GB/T 18384.1:
There are still some differences between the two standards, but the lower limit of the resistance value is unified. The values of the solutions seen in practice are generally above 500KΩ. From here we can also understand the problem mentioned before, that is, when testing the insulation and withstand voltage of the battery pack, the insulation detection circuit of the BMS needs to be turned off, otherwise there will be misjudgments.
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Y capacitor and X capacitor
In the battery system, as shown in the figure below, C1 is generally called the X capacitor, and C2 and C3 are generally called Y capacitors.
There are several sources of Y capacitance: the equivalent capacitance composed of the battery and tray, the filter capacitor on the BMS, etc.; the capacitance is about a few nanofarads to a few microfarads. The entire vehicle has requirements for the size of the Y capacitance, which will not be discussed here. discuss.
The X capacitor mainly comes from the filter capacitor on the DC bus of the motor controller, with a capacitance of several hundred to thousands of microfarads, depending on the needs of the vehicle.
Insulation detection is mainly affected by Y capacitance, and generally X capacitance will not have an impact.
The mechanism of influence of Y capacitance is that, for example, in the bridge method, it delays the establishment of steady state. If unconverged voltage values are used for calculation, the results obtained are also inaccurate; Y capacitance is an insulation testing solution for engineers. The number one enemy.
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100Ω/V and 500Ω/V
Are these two values familiar?
The national standard requires that the minimum insulation resistance of DC circuits must be greater than 100Ω/V, and that of AC circuits must be greater than 500Ω/V
. So where does this requirement come from?
In the standard
IEC/TS 60479-1, the impact of current on humans and livestock
, there are the following requirements: In the DC-2 and AC-2 areas, it is a safe current range for the human body; then we put DC-100Ω/V and AC-500Ω/V are converted into current values of DC-10mA and AC-2mA. You can see that these two values are within the safe area, which means that the insulation resistance is guaranteed to be between DC-100Ω/V and AC-500Ω. Above /V, it is harmless to humans.
Summarize:
Regarding Tesla, I specifically searched for their insulation testing patents. They were all applied for many years ago, and the solutions were all based on the bridge method, not the AC injection method, etc. I don’t know if there are any omissions in my search. If you know If so, I hope you can send me a private message; all the above are for reference only.
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