In battery storage systems (BESS), leakage capacitance and parasitic voltages (stray capacitance and common mode voltage generated in ungrounded systems, etc.) directly affect the accuracy and speed of insulation resistance measurement by the insulation monitoring device (IMD).

 

Effect of leakage capacitance

• As leakage capacitance increases, the overall insulation resistance value of the battery system may be measured lower than the actual value.

• The IMD's measurement response time increases in proportion to the capacitance value. This means that the greater the stray capacitance, the longer it takes to measure insulation resistance. Furthermore, leakage capacitance varies depending on the battery's State of Charge (SOC), increasing measurement variability.

• For large ESS, as leakage capacitance and leakage current increase, common mode voltage (CMV) increases, and the resulting potential imbalance can reduce IMD measurement accuracy.

• Occasionally, if the capacitance value is too large, the IMD may generate errors or false detections.

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Parasitic voltage and current effects

• Parasitic voltages (Vfloating, CMV, etc.) can cause errors in the insulation resistance measurement circuit, causing the insulation condition to be displayed as worse or better than the actual insulation condition.

• As the insulation leakage current increases, the IMD alarm threshold may be reached sooner, causing unnecessary alarms.

 

Countermeasures and Design Recommendations

• If the leakage capacitance is large, you should adjust the IMD settings (measurement interval, alarm threshold, etc.), secure the insulation distance during the PCB design stage, and minimize the leakage path within the system.

• Response speed and accuracy can be improved by operating multiple IMDs in parallel or by applying a distributed (DIMS) insulation monitoring strategy.

• It is important to review the actual measured insulation resistance and system operating environment through capacitance and parasitic voltage analysis, and insulation level reinforcement and measurement value correction are required.

 

summation

In ESS, a comprehensive battery and measurement system design is required because increased leakage capacitance and parasitic voltage can slow down local or overall insulation resistance measurements by insulation monitoring devices, cause errors in measured values, and even trigger unnecessary alarms.

 

As the leakage capacitance increases, the performance of the IMD (insulation monitoring device) does not improve, but rather has a negative impact on measurement accuracy and reliability .

 

The impact of increased leakage capacitance on IMD

• As the leakage capacitance increases, the capacitive leakage current increases, which may cause the actual insulation resistance to be underestimated or the measured value to become unstable.

• IMD sets a certain limit (maximum allowable value) for the leakage capacitance of the system, and if this limit is exceeded, problems such as malfunction, false alarm, or distortion of insulation resistance calculation values ​​may occur.

• If the IMD fails to properly compensate for the effects of capacitance, frequent malfunctions may occur in large storage devices such as ESS, making accurate monitoring difficult.

 

Leakage capacitance allowed by IMD

• Some high-performance IMDs can monitor leakage capacitances of hundreds to thousands of μF in various operating modes, but this is an allowable amount and higher capacitance does not necessarily mean better performance.

• In fact, the larger the leakage capacitance, the slower the IMD measurement speed and the lower the precision, which may make it difficult to detect insulation abnormalities (lower insulation resistance) early.

 

conclusion

A large leakage capacitance does not improve the performance of the IMD, but rather causes a deterioration in measurement performance (delay, error, malfunction). Therefore, during design, the leakage capacitance should be kept as small as possible and managed within the allowable value of the IMD.