After a series of ACB tripping at Main Switchboard, Chemical plant project in Pasir Gudang, Malaysia , client commissioned PowerQ to investigate cause by monitoring the electrical system. PowerQ performed the monitoring of the Main Control Centre Board and spot measurement of the MSB supplying power to MCC.
Monitoring at LV SBB SAP/CO
The inrush current was recorded at individual motors was performed at using Fluke recorder. The data collected has been analyzed and checked against IEEE 519 standards.
Following recommendation implemented to prevent he tripping problem:
- Reset setting ABB SPAJ relay characteristic of ACB characteristic. Previously the relay tripping ACB at SSB SAP/CO incomer is set lower that MCC board incomer, which was suspected to have contributed to the occurrence of nuisance tripping at incomer due to lack of discrimination.
- Resetting and programming the Soft Starter to prevent High Inrush Current. The hooked-up power monitor had captured short-duration inrush as high as 600% (e.g. Main Acid Pump 90kW has approximately full load current of 160 A but start current of 971 A while for BL 402 the full load current is 317 A while start inrush is as high as 974A), If plant is already running and a large motor such as P408 or BL 402 is restarted possibility of tripping will be there.
To avoid this scenario the soft starter was recommended be set to limit current to 300% and/or ramp time of the soft starter should be increased. Generally a ramp time of 8 second should be sufficient to limit inrush below 300%. This was be done in confirmation with process requirements. It is recommended that the correct setting of soft starter shall be implemented so as to reduce chances of tripping due to starting current.
Existing and Recommended Settings for Overcurrent Relay