How HT Motor Protection should be designed when running in parallel with Capacitor Bank

All of our Case studies are shared with the clear aim to ensure that the problems that have arisen in one plant can be avoided in other plants, which would lead to a reduction in downtime and increase the productivity and profit margins. This case study is about a frequent Motor Failure problem in the processes industry. You can also check out my other case studies on Electrical maintenance, to read about the different problems that we face in the industry and how we solve it.

In one process plant, the switchboard was designed by some consultant who showed ignorance in the Design & Engineering of protection schemes. This has led to frequent failure of 6.6 kV HT motors without any warnings and abnormality reflected.

We as the Electrical Solution Provider were called upon to identify and rectify the root cause. We were told that few motors have burnt due to overheating/overloading in just 9 -10 months. Initially, we never suspected the Protection Schemes, so we started analyzing the data like motor ratings, CT sizing, transformer ratings, relay settings, load patterns, connected loads, etc.

When no checkpoints left, we finally had to examine the Protection Scheme and SLD. This led us to an instant conclusion that it was the wrong protection scheme for Motors that was causing frequent failures and loss of revenue with significant repairing cost and downtime. We were now sure that the common CT and Relays for both motors and capacitor banks were the main cause of frequent failure of motors.

Here is the SLD Representation of the existing/old scheme and corrected Protection Scheme for HT motors with the capacitor bank.

Hard to Detect Faults

Whenever relay trips due to fault, it takes longer time for Engineers to identify the fault/location since common protection for motor and capacitor bank has been used, and hence it increases the downtime as Engineers have to check motors, associated rotor equipment's (if any) in the field and ca

Hard to Detect Faults

Thus, it can be summed up that the plant authorities should modify the protection scheme by implementing separate protection for motors and capacitor banks. It is also suggested that the motor protection relay settings must be reduced to 88% approximately until the modification in the protection scheme is made.

pacitor bank in the substations.