Protection methods of three-phase motors with the use of the MICOM P211 relay

Laboratory test of Protection Methods of three-phase motors with the use of the MICOM P211 relay

The subject of the exercise was the analysis of the operation of the propulsion system consisting of 230 kW – 400 V three-phase low voltage motor powered with 2.2 kW power self-excited single-phase generator with regulated load in increments from 0 to 3 kW.

Figure 1 shows the diagram of the measurement station, including elements used during the exercise.

In the tested system, similarly to the diagram shown in Figure 1 to relay P211 no executive elements were connected, it acted only as an indicator of occurence and a recorder of their parameters. Measurement of phase currents in the system can be realized directly through the relay (in accordance with its instructions), the zero current component measurement must be implemented through an external DC blocker.

Short-circuit currents that may appear in the system, switch off the overcurrent breaker, Thermal protection is connected with the auxiliary contact to the contactor of the main switch, again switching on the motor after the thermal relay is activated is possible only after cooling down a bimetallic element.

The breaker located in the L3 phase can be interrupted in the L3 phase only during normal engine operation, it is not possible to start the engine and try to start it at open breaker.

The generator load can be regulated in steps method by switching its selected ones elements.

Figure 2 shows a schematic of a star-delta switch that allows manulas direct start of the electric cage motor. Connection of motor windings to a starit allows to lower the starting current of the motor but also reduces its starting torque. After switching on supply voltage and acceleration of the motor to a speed close to synchronous speed there is a manual switchover of the windings to a delta configuration. The engine then accelerates to nominal speed and goes from starting to nominal operation.

In the period of the exercise for a 1.6 kW generator load, the analysis was carried out direct motor starting at connection of stator windings in a star, direct start-up at delta connection, and direct start using the star-delta switch, the work of a star-connected engine was also tested in the absence of one of the phases.

Starting the engine in star connection method

When connected in a star, the voltage on individual motor windings is reduced to 230V. With this connection method, the maximum starting current of the motor is approximately 3x smaller than when connected in a triangle.

The graph 1 shows the recorded currents of the three phases of the star-connected motor while start-up. You can see the moment of voltage supply (t = 1.895 s), after which the instantaneous current value is the highest:

for phase L1 at t = 1,900, the instantaneous current is -2.175A

for phase L2 at t = 1,905, the instantaneous current is 2,058A

for phase L3 at t = 2.025, the instantaneous current is -1.996A

Then you can see the engine starting from speed 0 up to the speed exceeding the speed synchronous, which results in lowering the value of the current amplitude so that after a while t = 2,6s motor starts to brake. At time t = 2.7 sec the motor speed drops below synchronous speed and again the current amplitude begins to rise, the motor accelerates so that after a while t = 3,0s reaches a stable speed nominal and goes from starting to normal operation, working at a current amplitude equal to approximately 0.8A.

The normal state of the engine connected to the star is reached after 1.1s from its moment inclusion.

Speed oscillation is practically non-existent due to the fact that very large damping is in the mechanical system.

Starting the engine in triangle connection method

When connected in a triangle, the voltage at each motor winding is 400V. With this connection method the maximum starting current of the motor is about 3x greater than when connected motor windings in a star.

Diagram 2 shows the recorded currents of the three phases of the delta connected motor while start-up. You can see the moment of voltage supply (t = 0.670 s), after which the instantaneous current value is the highest:

for phase L1 at t = 0.675 the instantaneous current is 7.0514A

for phase L2 at t = 0.740 the instantaneous current is -6,282A

for phase L3 at t = 0.715 the instantaneous current is -6.609A

It can be seen that the currents connected with the start-up of a delta-connected motor are more than three times greater than that when connecting the motor windings in star method.

Then you can see the engine starting from speed 0 up to the speed exceeding the speed synchronous, which is manifested by a decrease in the value of the current amplitude so that after a while t = 0.8s motor it starts to brake. At time t = 1.0s, the engine speed drops below synchronous speed and again the current amplitude begins to rise, the motor accelerates so that after a moment t = 1.2s it reaches the nominal speed and works at a current amplitude of approximately 1.1A.

When connected in a triangle, the starting torque is three times greater than when connected in star, you can see that the time of starting the engine connected in a triangle is definitely shorter – the normal working condition is reached 0.6 seconds after switching on.

Similar to the start-up of a star-connected motor, the velocity oscillation is practically non-existent occur because there is very high attenuation in the mechanical system.

Starting the engine in star-delta switch-over

For engines which are not loaded at the moment of starting the rated torque (eg fans, conveyor) a start-up with a star-delta switch is used to reduce the duration shock currents. When connecting the motor windings to a star, the motor starting current is 3 times smaller, but also the starting torque is three times smaller. After switching on the voltage supply and acceleration of the motor to the speed close to synchronous speed is switched windings for a delta configuration. The motor accelerates to nominal speed and passes from start-up to nominal operation.

The diagram 3 shows the recorded currents of the three phases of the motor connected initially in a star and then a triangle. You can see the moment of voltage supply for a star connection, start-up the same engine as in point 3 of the exercise. Then, at time t = 1.8s, the voltage is turned off, The motor winding has been reconfigured for delta connection and at time t = 2.41s again, there is tension. After applying the voltage, the instantaneous current value is the highest:

for phase L1 at t = 2.420, the instantaneous current value is -5,325A

for phase L2 at t = 2.415, the instantaneous current value is -5,605A

for phase L3 at t = 2.420, the instantaneous current value is 6.858A

You can see that the currents associated with the engine starting after switching its coils into a triangle, are again about three times higher than when connecting the motor windings to a star, however, the operating time shock currents of the start-up is much shorter (about twice) than when starting from zero speed. After a while t = 2.7 s, the engine reaches nominal speed and works at a current amplitude of approximately 1.1A.

Similarly to points 3 and 4, the oscillations of the motor speed are practically non-existent.

Operation of the engine connected in a star connection with a break in one of the phases

For the motor operating at the connection of the stator windings to the star at time t = 1.360 was turned off one of the phases (L3), from now on the engine works under the influence of the oval rotating field formed from two phases at an angle of 120 degrees.

Diagram 4 shows the recorded currents of the three phases of the star-connected motor working from time t = 1.360 in the absence of the L3 phase. You can see that until t = 1.360 the engine worked normally with an amplitude of currents around 0.8A. You can see the moment of switching off one of the power phases, growing the amplitude of the currents of the other two phases. From t = 1.6s, the engine runs stably under the influence of the field oval formed by two phases at an angle of 120 degrees at a current of about 1.5 A. Electricity is higher than in normal operation in a star-connected system (0.8A) and in a system connected in a triangle (1.1A).

Work under the influence of an oval field leads to mechanical overloads in the engine, and elevated current leads to excessive overheating of the windings, both of which can result in engine damage. In order to protect against the effects of power asymmetry in the system there is an overload protection in the form of a RT series thermal relay General Electric.

Thermal protection worked after 53 seconds.