Motor Protection Relays [50/51]:

Motor protection relays are used to provide various protections for the motors. These differ from the over current relays in terms of protections they offer.

To understand the working of motor protection relay an example relay shown below is taken, which is configured with the following protection functions.

Motor Protection Relays Functions:

Protection Functions used in this relay are

1. 50/51 Phase Over Current Protection
   Definite time/Time over Current IEC
2. 50N/51N Earth over Current Protection
   Definite time/Time over Current IEC
3. Ground Fault Protection
   Definite time/Time over Current IEC
   Sensitive Earth Fault protection is provided by using CBCT
4. Thermal Overload Protection
   Without ambient temperature measurement
5. Load Jam Protection
6. Motor Starting Protection 48
7. Negative Sequence/Unbalanced Load
8. Under Voltage Protection 27
9. Trip circuit Supervision 74
   With 2 Binary inputs/With 1 Binary input
10. Breaker Failure

In the following figure the motor protection relay is configured and LEDs are assigned as follows.

LED Configuration:

1.General Trip
This indicates when any protection tripping occur causes breaker trips.

2.Thermal Over Load
The purpose of motor thermal protection is to allow the motor to start and run within the defined thermal Capacity limit but trip if the motor heat energy exceeds those ratings because of overloads, negative-sequence current,or locked rotor starting. Under transient and overload conditions the Thermal Capacity of the motor rises, but within the corresponding limits. When the overload persists for a considerable amount of time, the motor temperature and thermal capacity will rise. A trip occurs when the thermal capacity used by the motor reaches its 100 %.

3.Load Jam/Start up
The Load Jam protection prevents overheating and mechanical damage due to a locked rotor. Load jam in motor running condition is taken care by Locked rotor protection available in the relay. In locked rotor condition the rotor gets locked due to presence of the excessive load. As a result, the motor draws higher current to drive the excessive load. The high current flow in the motor heats up the rotor quickly due to skin effect.

4.No. of Starts
This function protects the motor against prolonged startup procedures and the consequent thermal overload of the rotor. Motors can quickly heated up above their thermal limits when multiple starting attempts occur in a short time. If the duration of these starting attempts are lengthened e.g. by excessive voltage surges during motor starting, by excessive load moments, or by blocked rotor conditions, a trip signal will be initiated by the protective relay.

5.NPS/UV/EPB
This indicates tripping due to either of Negative Phase Sequence, Under Voltage or Emergency Push Button at motor side Pressed.
The same LED is used to show indication for any of above three tripping. The tripping can be known by view the Trip Log.
• Negative sequence protection detects unbalanced loads on the system.The major cause of unbalance is due to Single Phasing of motor .
• The under voltage protection function detects voltage collapses on transmission lines and electrical machines and prevents inadmissible operating states and a possible loss of stability.
• In case of any Emergency condition the EPB is pressed by operator.

6.Over Current trip
This indicates for any fault causes over current in the phase element of the relay that exceeds the setting value.

7.Earth Fault trip
This indicates for any fault causes over current in the earth element of the relay that exceeds the setting value. Earth fault current can be calculated in two ways.
1.Derived Earth fault: Earth Fault Current is equal to the vector sum of three line current values.
2. CBCT (Core Balance Current Transformer):
CBCT is used for earth leakage and sensitive Earth Fault conditions.The 3-phases supply to the motor passes through the CBCT which senses the Earth Fault current under abnormal conditions. This is used as an input by the Relay to measure Earth Fault current.