Inverter Insulation Monitoring via Differential Voltage
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Solution Overview
Problem
Conventional insulation monitoring methods for inverter applications in IT-networks are prone to interference from capacitive fault currents, leading to misleading error messages and increased costs due to difficulty in setting and resetting warning and fault limits, as well as the inability to accurately locate insulation defects.
Innovation Solution
A method that utilizes the existing measurement circuit of the inverter to determine the operating condition and measure differential voltages between the positive and negative branches of the intermediate circuit, allowing for precise detection of insulation defects without additional measurement devices, and adapts threshold values based on the current operating conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional insulation monitoring methods are used in inverter applications, then insulation monitoring is performed, but capacitive fault currents interfere with the measurement leading to misleading error messages
Solution Approach 1:
The patent applies preliminary action by determining the operating condition of the inverter before performing insulation monitoring. The method checks whether the inverter is in a state suitable for measurement (e.g., whether switches are in specific positions, whether the intermediate circuit is charged) and only performs the measurement when conditions are appropriate. This prevents false readings caused by capacitive fault currents that occur during normal inverter operation, thereby resolving the interference problem while maintaining monitoring capability.
2Reliability
If additional measurement devices are incorporated for self-monitoring of insulation monitoring apparatus, then monitoring capability is enhanced, but the failure rate increases due to additional points of insulation loss
Solution Approach 1:
The patent applies universality by making the existing inverter measurement circuit perform dual functions: both normal inverter control measurements and insulation monitoring measurements. The same voltage sensors and processing circuits used for controlling the inverter are also used to measure the differential voltage for insulation assessment. This eliminates the need for separate dedicated measurement devices, reducing the number of insulation boundaries while maintaining comprehensive monitoring capability.
3Reliability
If insulation monitoring is performed in complex installations, then fault detection is achieved, but fault localization becomes difficult leading to high correction costs
Solution Approach 1:
The patent applies segmentation by dividing the insulation monitoring into separate measurements for the positive branch and negative branch of the intermediate circuit. By independently measuring the voltage on each branch relative to earth and comparing them, the system can identify which specific branch or side of the inverter has the insulation defect. This segmented approach transforms a single undifferentiated fault detection into localized diagnosis, enabling precise identification of the problematic area in complex installations.
4Reliability
If warning limits and fault limits are set for insulation monitoring, then fault detection is enabled, but the limits must be frequently reset due to changing capacitive fault currents
Solution Approach 1:
The patent applies dynamics by making the monitoring approach adaptive to changing operating conditions. Instead of using fixed threshold limits that require frequent resetting, the method dynamically determines whether an insulation defect exists by evaluating the operating condition of the inverter and the relationship between positive and negative branch voltages. The system can distinguish between temporary voltage imbalances caused by normal operation (such as switching transients or capacitive effects) and actual insulation defects, eliminating the need for frequent limit adjustments.
Data Source
AI summary
An inverter application comprises at least one apparatus fed by an inverter (10). The determining of insulation defects is effected by way of a differential voltage of part voltages (U, U′) of an intermediate circuit (7) or the ratio of one of these part voltages (U, U′) to the total voltage of an intermediate circuit, and by way of an operating condition of the inverter (10). The monitoring of the insulation is carried out only with the means (1a, 1b, 1c, 1a′, 1b′, 1c′, 5, 5′) which are present in the inverter (10) in any case. The operating condition is set as a particular switch position in the inverter (10) by an insulation monitoring system. Alternatively, the operating condition is, while the inverter is on-line or in operation, transferred from the inverter control (9) to an insulation monitoring system for improving the quality of the monitoring.