Case Study: Preventing Switchgear Failure at South African Mine

Introduction

Medium and low voltage switchgear is becoming more prevalent in modern substations and industrial power systems. Many of the older installations are beginning to experience failure modes typical of equipment reaching the end of serviceable life. Therefore, the need for cost-effective monitoring is becoming critical to mitigating the cost associated with offline maintenance and in-service failures.

Mine Photo

The Challenge

A South African mining company that relies of smelting furnaces to run some of their operation experienced a failure on the cable terminations. This resulted in costly downtime and complete disruption of their production. While performing PD diagnostic analysis of the online monitoring data, it was discovered that a very high amount of partial discharge (PD) activity was occurring on the furnace cable terminations. Potential defects were monitored until an emergency shut down was arranged for further inspection and investigation.

The Solution

As part of the investigation of the 33kV cable terminations, high voltage (50Hz) was applied to the termination. The termination earth strap was folded over the black heat shrink tube and secured with roll springs. Tracking discharges were identified on the black heat shrink tube. The screen of the terminations were found to be incorrectly installed. Voids were identified between the screen and XLPE insulation. Discharges could be identified at phase voltage.

The two terminations with the highest discharges were replaced, and an investigation and tests were conducted on the two terminations to determine the root cause of the discharges.A PD monitoring system, including Dynamic Ratings Switchgear and Cable Monitor (SCM), was installed on the 33kV switchgear to measure electrical partial discharges within the switchgear components.

Result: SCM Identifies Source of PD Activity in Switchgear

Through performing diagnostic of PD phenominas that could turn into unwanted premature failures and down time, the source of the PD activity was identified and repaired.

90% of cable failures occur at splices and terminations. As these accessories breakdown, they generally produce partial discharges prior to failure. Therefore to monitor cables, the SCM takes data from radio frequency current transformers (RFCTs) that are placed around the cable termination shields. If the shields are ungrounded then a GPCS sensor is used.

The Switchgear and Cable Monitor (SCM) continually monitors partial discharge (PD), and stores and correlates operating dynamics. The SCM provides information on the health of the medium voltage insulation systems of switchgear, cables, bus duct and unit substations.

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