Missing running diesel generators consequence analysis

Diesel generating sets two-phase operation, is the common cause of damage of diesel generator set. Why diesel generators installed in the over-current protection fuse protection, magnetic starter additional thermal relay protection, circuit breaker, are not good for generator two-phase operation effective protective effect?

Firstly, according to the principle of motor learning, such as access to the two-phase power, the stator winding is not likely to produce a rotating magnetic field, the rotating torque to zero, generating units only shock without rotation. Generating units in into the two-phase power starting, in fact is in short circuit state, the short circuit current is 0.866 times of the starting current of three-phase startup, and general asynchronous generator starting current is 4 ~ 7 times of the rated current, the generator in the two-phase power when entering the starting phase, when the current in the two phase short circuit is 3.464 ~ the rated current of 6.062 times, so the current, namely the ratio of small starting current, rated current than the much larger generating units. Generator two-phase starting, generating units is not running, the operating personnel can immediately find, and fuse will fuse, fuse fuse because generally by the following two principles: selected for starting the generator set less number and a short start-up time, press IH=IZ/2.5 to select; for the generator repeatedly starting and accelerating the slow, according to IH=IZ/ (1.6 ~ 2) selected. The two type, IH is the rated current of the fuse; IZ start-up current for three phase generator set. For generating units in operation, suddenly cut off one phase power supply, in under the action of inertia of rotating machinery, in certain conditions can delay rate.

Because the generator set current ratio and power generation load and generator maximum torque is related to K itself. When the maximum torque is larger than 2, generating units will maintain the two-phase operation, but the speed is reduced greatly, the greater the K generator two-phase run-time overload multiples larger. When the maximum torque is K equals 2, power unit with a rated load and running under two-phase, over current generator is approximately 3.5 times the rated current of the generator, at this time if the fuse is provided for the selection of the protection, the fuse can fuse, and play a protective role. However, when the rated load generator with only 50%, two-phase operation current and rated current of approximately equal. But when the power unit load in more than 50% of the rated load, and the following two phases running at rated load, the fuse cannot be reliably protect to the. Starting current normal turbine generating units for 4 ~ 7 times of rated current. From this we can see that the two phase protection unit operation fuse may not be reliable.

The third case is generating set maximum torque is K less than 2, generating units will be slowed to a stop, until the fuse fuse. In addition to fuse protection in three phase low voltage generator protection, also uses the thermal relay, load protection for generator. Use of the current action is generally 1.1 times the rated current, considering reserve margin to voltage fluctuation and the change of environmental temperature to prevent the generator and the miscut of generating units, usually by 1.2 ~ 1.3 times of rated current thermal element selected, depending on the thermal protection thermal inertia generated delay, to avoid the starting current. So is composed of a thermal element over load protection, also cannot reliably protect generator two-phase operation. Likewise for the over-current protection circuit breakers, general according to reservoir development turbine starting current tuning, obviously, in such a setting value is not correct to protect two-phase power generating unit operation.

On the protection of two-phase operation of diesel generator sets, in recent years put forward many solutions, basically can be divided into two categories: one category is the signal to mount the motor single phase fusing indication, another kind is the negative sequence protection using transistor constitute. Using these methods, also have certain effect, but still not perfect, so the application is not universal. This can be adopted double fuse, motor phase protection is relatively simple and reliable. The method is to use the 6 fuses, each of the two are connected in parallel to form a three phase fuse protection, two fuses each phase, a motor in accordance with 1.2 ~ 1.3 times of rated current setting, another according to the fuse rated current formula selection. When the motor starts to close after three fuses, normal start the motor, after startup, then close three fuses, re opened after a fuse, so that the normal operation of the motor.

A large number of practice has proved that to prevent motor two-phase operation, only strengthen the surveillance, sum up experience, pay attention to find abnormal phenomenon of open phase operation, timely removal motor two-phase operation, ensure the safe and reliable operation of the motor.

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