Increasing Motor Life and Process Continuity

Author: Wayne Hartmann, Beckwith Electric, USA

Example causes of utility interruptions are:

  • Storms
  • Animal contact
  • Conductor failures
  • Tower failure
  • Switch failure
  • Substation failure, and
  • Planned outages

Example causes of in-plant interruptions are:

  • Generating unit faults
  • Transformer faults
  • Cable failure
  • Switchgear failure
  • Conductor dig-in
  • Maintenance
  • Inadvertent breaker tripping

When challenged with power interruption, the ability to rapidly and safely transfer motors to another power source is paramount to maintain operational continuity and not violate motor and driven load torque limits on reconnection and reacceleration. Individual motors and buses of motors may be transferred.
This process is known as motor bus transfer (MBT).  While motor protection clears faults and abnormal conditions that damage motors, the proper performance of a MBT serves to mitigate the opportunity for motor damage. 
Although improper MBTs may not cause immediate catastrophic damage, the damaging effects are cumulative and may lead to premature motor failure and costly unplanned outages. 

Process continuity cannot be taken lightly.  In power plants, it can cause a boiler or turbine shutdown requiring hours of sequencing to restart with revenue and reserve loss.  In industrial facilities, motor failure can cause production batch losses that can incur huge costs, and may cause safety and environmental issues. (see Figure 1).

We can divide this exploration into six parts:

  • Types of MBTs by method and mode
  • General setting guidance
  • Initiate signals and elements
  • Blocking/Not Ready signals
  • Special Situation Logic
  • Legacy limits and the future

Types of Transfers by Method
There are five methods of MBT.  We use a basic two breaker supplied bus for our discussions.  We refer to an “Old Source” and a “New Source.”  The Old Source is the source the motor bus transferring from.  The New Source is the source where the motor bus is transferring to.

 Closed Transition
Hot Parallel: This is where the New Source is connected to the motor bus before the Old Source is tripped.  There is a period where the sources are paralleled.  If conditions permit, the use of a hot parallel transfer makes seamless transition without any interruption of process or increased torque on the motor/driven load.  Other considerations:

  • Voltages and phase angle between the Old Source and the New Source must be evaluated prior to the transfer to assure that:
    • Motor bus connected to Old Source and the New Source are in synchronism (no slip frequency and low magnitude phase angle difference)
    • New Source voltage is within acceptable limits
  • Can be used only under non-faulted conditions, such as a planned transfer without faults or other abnormal events
  • During the closed transition period, the bus is subject to double infeed fault current should a fault on or downline of the bus occur.  Equipment downline needs to be rated accordingly to meet this additional fault capability at a higher cost.
  • If a Hot Parallel MBT is initiated and the New Source breaker is closed, but the Old Source breaker remains closed, the transfer system must immediately trip the New Source breaker

BeijingSifang June 2016