Motor protection relays--part 3 [Industrial Electrical Power Systems]

General

The bi-metal thermal overload relay has proved itself an effective and economical solution for small to medium-sized motors up to about 22 kW. On larger, more expensive motors or when maximum motor utilization is required under varying operational conditions more sophisticated flexible and accurate microprocessor protection relays should be considered.

These relays typically include:

1. • Thermal overload protection, monitoring all three-phases with thermal replicas for direct and frequency convertor-controlled drives
2. • Short-circuit protection
3. • Start-up and running stall protection
4. • Phase unbalanced protection
5. • Single-phasing protection
6. • Ground fault protection
7. • Undercurrent protection
8. • Digital read-out of set values, actual measured values and memorized values
9. • Self, supervision system
10. • Outstanding accuracy
11. • Optimum philosophy.

The present day concept is use of microprocessor-based numerical relays for both HV and LV motors (say beyond 50 kW), as the relays come with lot of features which allow them to be interchangeable, ensures site settings and give valuable feedback on the load details whether a trip occurs or not.

Typical protective settings for motors

(a) Long time pick-up:

• 1.15 times motor FLA times motor service factor for applications encountering 90% voltage dip on motor starting

• 1.25 times motor FLA times motor service factor for applications encountering 80% voltage dip on motor starting.

(b) Long-time delay:

• Greater than motor starting time at 100% voltage and the minimum system voltage

• Less than locked rotor damage time at 100% voltage and the minimum system voltage

• On high-inertia drives, it’s common for the start time to be greater than the locked rotor withstand time. Under these circumstances, set the time to permit the motor to start. Supplemental protection should be added for locked rotor protection. One example of this is a speed switch set at 25% of rated speed tripping through a timer to trip if the desired speed has not been reached in a pre-determined time.

(c) Instantaneous pick-up:

• Not less than 1.7 times motor LRA for medium-voltage motors

• Not less than 2.0 times motor LRA for low-voltage motors.

(d) Ground-fault protection:

• Minimum pick-up and minimum time delay for static trip units

• Core-balance CT and 50 relays set at minimum for medium-voltage, low-resistance grounded systems

• Residually connected CT, and 50/51 for medium voltage, solidly grounded systems. Minimum tap and time dial equals 1 for 51 relay

• Minimum tap (not less than 5 A) for 50 relay.