Fault Loop Impedance Measurement in Circuits Fed by UPS and Principle of Safety Protection

Jacek Katarzyński,Marek Olesz

doi:10.3390/su122310126

Jacek Katarzyński, Marek Olesz

Open Access

https://doi.org/10.3390/su122310126

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Journal: Sustainability	Publication Date: Dec 4, 2020
Citations: 2	License type: CC BY 4.0

Affiliation: Gdańsk University of Technology

Abstract

This paper indicates a significant problem of uncertainty of fault loop impedance (FLI) measurement in circuits powered from uninterruptible power supply (UPS) (double-conversion AC-DC-AC). The correctly determined value of this impedance, related to the short-circuit current disconnection time and to the reference value, is one of the most important elements that determines the approval of an electrical installation and its receivers for operation. To define the principles of FLI measurement, several hundred measurements of the short-circuit loop impedance in the circuits fed by the UPS, in various UPS operation modes and with various FLI instruments, were made, which allowed for the definition of measurement rules that reduce the error in assessing the effectiveness of protection against electric shock by automatic disconnection of supply. Based on the analysis of voltage and current waveforms recorded during the real short-circuit tests in the circuit fed by the UPS, a proprietary algorithm for determining the short-circuit loop impedance has been proposed.

Highlights

Uninterruptible power supply (UPS) units are currently the most popular means of counteracting disturbances in the grid and protecting the load against the effects of disturbances [1]
A distant short-circuit condition occurs when the impedance value of the circuit fed by the uninterruptible power supply (UPS) is higher than the limit value Zlim and the forced fault current is less than the maximum inverter current for which UPS still stabilizes the output voltage with an accuracy of 1% (UPS overload condition)
A near short-circuit (Zs < Zlim) in the line fed by UPS, during double conversion operation, always causes the UPS to switch to electronic bypass mode

Summary

Introduction

Uninterruptible power supply (UPS) units are currently the most popular means of counteracting disturbances in the grid and protecting the load against the effects of disturbances [1]. The discrepancy in the results of the FLI measurements showed that it is necessary to research this area and explain the reasons for these discrepancies Based on the analysis of the FLI measurement results and the behavior of the online UPS during the faults in the UPS output circuits [9], Section 4 presents the method of proper FLI measurement in the circuits fed by UPS in the form of an algorithm, which at the same time enables the correct assessment of the effectiveness of protection against electric shock by automatic disconnection of supply. Recommendations are given by an inspector and they can be wrong, because of the lack of awareness of the problem presented in the article

Short-Circuit in a Line Fed by Online UPS

Findings

Conclusions