The two-phase operation of a three-phase asynchronous generator set is a common cause of damage to the generator set. Why do the fuse protection installed on the generator set, the thermal relay protection fixed on the magnetic starter and the overcurrent protection of the circuit breaker fail to effectively protect the two-phase operation of the generator set?
First of all, according to the principle of electromechanics, if it is connected to a two-phase power supply, its stator winding cannot generate a rotating magnetic field, the rotation torque is zero, and the generator only vibrates but does not turn. When the generator set enters the two-phase power supply to start, it is actually in a short circuit state. The short circuit current is 0.866 times the starting current of three-phase starting. The asynchronous generator set is 4 to 7 times the rated current. ParkConsequently, the generator is in a short circuit state. When starting with a two-phase power supply, the current equivalent to a two-phase short circuit is 3.464 to 6.062 times. the rated current, so the current above is less than the starting current and much larger than the rated current of the generator set.
When the generator set starts in two phases, if the generator set does not work, the operator will find it immediately, and the fuse will also blow, because the fuse blowing current is usually selected according to the following two principles: for the number of starts. For generator sets with few starts and short start-up times, select IH=IZ/2.5; for generator sets with repeated starts and slow acceleration, select IH=IZ/(1.6~2).
In the two formulas above, IH is the rated current of the fuse; IZ is the three-phase starting current of the groupgenerator. For a running generator set, after one phase of the power supply is suddenly cut off, under the action of mechanical inertia, it can still rotate at an offset speed under some specific conditions. Because the overcurrent multiple of the generator set is related to the actual load of the generator set and the maximum torque multiple K of the generator set itself.
When the maximum torque multiple is greater than 2, the generator set will maintain two-phase operation, but the speed will be significantly reduced. The larger K is, the greater the overload multiple of the generator during operation. in two phases. When the maximum torque multiple K is 2 and the generator set has a rated load and two-phase operation occurs, the overcurrent of the generator set is about 3.5 times the rated current at that time , if the fuse is selected inur the. If the generator is protected according to regulations, the fuse will blow. The device can be protected by a fuse.
However, when the generator set carries only 50% of the rated load, the two-phase operating current is approximately equal to the rated current. When the generator set load is more than 50% of the rated load and the two-phase operation is less than the rated load, the fuse cannot provide reliable protection.
The starting current of a normal generator set is 4 to 7 times the rated current of the generator set. It appears that the fuse cannot reliably protect the two-phase operation of the generator. The third situation is when the maximum torque multiple K of the generator set is less than 2, the generator set decelerates and stops until the fuse blows.
In addition to fuse protection, relays thermics are also used in the protection of three-phase low voltage generator sets for generator set overload protection. The operating current is generally 1.1 times the rated current, taking into account the reserve margin to avoid erroneous switching due to generator set voltage changes and ambient temperature changes.Generator sets generally select thermal elements based on 1.2-1.3 times the rated current, relying on thermal power to protect the delay caused by thermal inertia and avoid starting current. Therefore, the overload protection composed of thermal components cannot reliably protect the two-phase operation of the generator set. Similarly, for circuit breaker overcurrent protection, it is usually set to avoid generator set starting current. EObviously, this setting value cannot properly protect the two-phase operation of the generator set.
Concerning the protection of the two-phase operation of the motor, many projects have been proposed in recent years in various places, which can basically be summarized into two categories: one is to install a signal indicating that the one The motor phase is fused, and the other is to use transistor reverse protection.
The use of these methods also has certain effects, but they are not yet perfect, so their promotion and application are not yet common. To this end, a double set of fuses can be used to form a relatively simple and reliable two-phase motor protection. The method is to use 6 fuses, two of which are connected in parallel to form three-phase fuse protection. One of the two fuses in each phase is set according to 1.2 to 1.3 timesthe rated current of the motor, and the other. is selected according to the fuse rated current formula mentioned above. When starting the engine, close the last three fuses to allow the engine to start normally. After starting, close the first three fuses and open the last fuse to allow the engine to operate normally.
Finally, it should be emphasized that a large amount of practice has proven that to prevent the motor from running in two phases, the only way is to strengthen monitoring, summarize experience, pay attention to anomalies. phase loss operation phenomena and quickly shut down the motor running in two phases to ensure the safety of reliable operation of the motor.