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 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.
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| Cummins Generator Set |
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