Ground Fault Site Detection in Power Conversion Devices
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Solution Overview
Problem
Existing power conversion devices cannot accurately determine whether a short circuit or ground fault occurs in a motor or a cable, as the methods used to estimate fault sites are inadequate for ground faults due to differences in current causes and inductance values involved.
Innovation Solution
A power conversion device with a control circuit that includes a current ratio calculation unit and a ground fault site determination unit, which calculates the current ratio and inductance values to differentiate between motor and cable ground faults, and notifies the user or external systems of the fault site through display or wireless transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the total two switching elements of upper arm and lower arm are turned ON in a short time after short circuit occurs to estimate the site by viewing a slope of the current, then the short-circuit site can be estimated on the basis of the inductance L, but the configuration cannot cope with specifying a ground fault site due to different current causes and indefinite inductance values
Solution Approach 1:
The patent changes the detection parameter from current slope (which gives indefinite inductance in ground faults) to current ratio, which provides a consistent measurement basis for both short-circuit and ground fault detection. By measuring the ratio of currents in different phases rather than the absolute slope, the system achieves adaptability across different fault types while maintaining measurement precision.
2Measurement precision
If the current value is used to determine the fault site, then the inductance can be obtained from the slope of the current, but the current value significantly varies by the influence of input power line length and input AC voltage in ground faults
Solution Approach 1:
The patent uses current ratio as an intermediary parameter that eliminates the influence of external variables such as input power line length and AC voltage. Instead of directly measuring absolute current values that vary with these factors, the system measures the ratio between phase currents, which cancels out the common-mode variations and provides reliable, consistent measurements for fault detection.
3Quantity of substance
If the inductance value is obtained from the slope of the current in ground fault, then the inductance value contains all inductance values including earth, power source line, and transformer, but it is hard to estimate the necessary inductance values of the cable and motor
Solution Approach 1:
The patent segments the fault detection by analyzing current ratios in different phases separately. By comparing the current ratios and identifying which phase has the abnormal ratio, the system can determine whether the fault is in the cable or motor without needing to measure individual inductance values. This segmentation approach isolates the fault location from the total inductance of all components.
Data Source
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AI summary
Provided is a power conversion device which determines a ground fault site on a motor or a cable, and notifies a user, an external device, or a system of a determination result. A power conversion device includes a plurality of semiconductor switch elements and drives a three-phase motor connected by three-phase cables by controlling the switch elements on or off. When a ground fault site occurred in the cable or the motor is inspected, the plurality of switch elements of any one of upper arms and lower arms of the three half bridge circuits are simultaneously turned on to generate a current to the plurality of connection cables. Output current values of a plurality of phases are measured in a period when the switch elements are simultaneously turned on or immediately after the period. A ground fault site is determined on the basis of a current ratio of a maximum value and a minimum value of the output current values of the plurality of phases. Notification of a result of the determination is provided to an outer side.