The two-phase operation of three-phase diesel generator sets is a common cause of damage to diesel generating units. Why is the diesel generator set equipped with fuse protection, magnetic starter additional thermal relay protection, circuit breaker over-current protection, can not be a good two-phase operation of the generator set effective protection?
First of all, according to the principle of motor, if connected to the two-phase power supply, the stator winding can not produce a rotating magnetic field, the torque of rotation is zero, and the generator only vibrates but does not rotate. The generator set is actually in a short circuit state when entering the two phase power supply, and its short circuit current is 0.866 times that of the three phase starting current. However, the start-up current of a typical asynchronous generator unit is 4 ~ 7 times of the rated current, so the starting current of the generator set when entering the two-phase power supply is equivalent to 3.464 ~ 6.062 times the rated current when the generator enters the two-phase short circuit. Therefore, the above current, that is, less than the start-up current, is much larger than the rated current of the generator.
When the generating set is started in two phases, the generator does not turn, and the operator will find it immediately, and the fuse will also fuse. Because the fuse current of the fuse is generally selected according to the following two principles: for the generator set with less startup times and shorter start-up time, select by IH=IZ/2.5; For recurrent starting and slow acceleration generator sets, select by IH=IZ/ (1.6 / 2). In the above two formulas, IH is the rated current of the fuse and IZ is the three-phase start-up current of the generator. When the generator is in operation, the hysteresis speed can be rotated under some special conditions under the action of mechanical inertia after the one phase power supply is suddenly cut off.
The overcurrent ratio of the generator unit is related to the actual load of the generator unit and the maximum torque multiple K of the generator unit itself. When the maximum torque multiple is greater than 2, the generator will maintain two-phase operation, but the speed will be greatly reduced, and the larger the K generator is, the greater the overload multiple will be. When the maximum torque factor K is equal to 2, the overcurrent of the generator unit is about 3.5 times the rated current when the generator has rated load and occurs two-phase operation. In this case, if the generator unit is protected by the specified fuse, Fuses can fuse and protect. However, when the generator has only 50% rated load, the two-phase operating current is roughly equal to the rated current. When the load of generator set is above 50% rated load and two phase operation below rated load, the fuse can not play a protective role reliably. The starting current of the normal generator is 4 ~ 7 times of the rated current of the generator. From this we can see that the fuse can not reliably protect the two-phase operation of the generator set.
In the third case, if the maximum torque factor K is less than 2, the generator will slow down and stop until the fuse is broken. In addition to fuse protection, thermal relay is also used in three-phase low-voltage generator protection. Generally, 1.1 times rated current is selected as the operating current, and reserve margin is considered to prevent the generator set from being mistakenly cut off by changing the voltage and ambient temperature. Generally, the thermal element is selected according to the rated current of 1.2n 1.3 times. Rely on thermal protection of thermal inertia produced by the delay, avoid starting current. Therefore, it is impossible to reliably protect the two-phase operation of generator set by overload protection composed of heat element. Also for circuit breaker overcurrent protection, generally according to evasive generator start-up current setting, it is obvious that such a set value can not correctly protect the two-phase operation of the generator set.
In recent years, many schemes have been put forward for the protection of diesel-generator sets in two-phase operation, which can basically be classified into two categories: one is the signal indication of the installation of the one-phase fuse of the motor, The other is negative sequence protection using transistors. These methods also have some effect, but they are still not perfect, so the popularization and application are not universal. For this reason, two-group fuses can be used to form a simple and reliable two-phase motor protection. The method is to use 6 fuses, each two in parallel to form a three-phase fuses protection, one of the two fuses in each phase is set according to the rated current of the motor 1.2n 1.3 times, the other is selected according to the formula of the rated current of the fuses mentioned above. The motor starts normally by closing the latter three fuses when starting, and after starting, the first three fuses are closed, and the latter fuses are opened to make the motor run normally.
A large number of practices have proved that in order to prevent the two-phase operation of the motor, we should only strengthen the monitoring, sum up the experience, pay attention to the abnormal phenomenon of the non-phase operation, cut off the two-phase running motor in time, and ensure the safe and reliable operation of the motor.