Parallel Operation of Diesel Generator Set - Starlight Power
by Starlight Generator dieselgeneratortechStatic
stability is only a requirement for parallel operation of the generator, and it
does not indicates the ability of the system to maintain synchronous operation
in the event of a sharp disturbance. Take the short circuit in the grid as an
example to illustrate this problem. When the power grid is short circuited, the
power angle characteristics of each generator and its output power will
suddenly change. For example, a short circuit occurs near the busbar, the bus
voltage is close to zero, and the generators are completely lost in contact and
influence.
Since
the diesel governor has a certain inertia, the prime mover does not change
immediately. The generator rotors are accelerated (and possibly decelerated)
under the residual power of the diesel engine. The acceleration of each
generator set depends on the mechanical inertia of the genset, electrical
characteristics, operating parameters before short circuit and short circuit
impedance and short circuit duration. Therefore, after the short circuit is cut
off, not only the relative position of each generator rotor changes, but also
the rotational speed is not consistent. Whether the generator rotor can resume
synchronous operation after a certain relative oscillation depends on the
kinetic energy of each genset. And the size and direction of the residual
torque of the diesel engine. In order to qualitatively explain the problem,
only the switching process of one generator rotor is relatively seen,
regardless of the influence of other generator sets dynamic processes.
It
is assumed that after the short circuit is cut off, a generator has the power
angle characteristic shown in figure, and the power angle of the generator is
θa at this time, and it is assumed that the voltage vector of the generator
rotor and the grid has the same rotational speed at this moment. The speed V is
zero. Then, under the action of the residual power of the diesel engine, the
generator rotor is accelerated by θa, and the θ angle is gradually increased.
Before reaching point b, the diesel engine is always greater than the generator
torque, and the acceleration process continues until point b. when θ=θb, the
generator output power and the diesel engine power are balanced, the residual
torque is zero, and the relative rotational speed V is no longer increased to
reach a maximum value.
Since
the speed of the generator rotor is greater than the speed of the voltage
vector at this time, the generator rotor will continue to increase the angle θ
by inertia over point b, but the residual power changes the sign, and the
generator rotor begins to decelerate until point c. The relative velocity V is
zero and the angle θ reaches a maximum value. Under the action of the residual
torque, the rotor starts the opposite motion process. The kinetic energy
obtained by the generator during the acceleration process can be expressed by
the acceleration area a'ab. The kinetic energy consumed during the deceleration
can be expressed by the deceleration area bcc’. according to the principle of
energy conservation, the acceleration area and the deceleration area should be
equal. Obviously, the maximum value of the deceleration area is bcb’b. If the
acceleration area is smaller than this maximum value (as shown in figure), the
generator rotor can be stabilized after a number of oscillations under the
action of the damping factor. 6 points, indicating that the system has a
stability after being disturbed by a short circuit, if the acceleration area is
greater than bcb'b, the motion of the generator rotor will exceed the 6 point,
and the residual torque of the diesel engine changes the sign again to make the
generator. The rotor is further accelerated until it loses synchronization, so
the system does not have dynamic stability.
Similar situations and problems can occur when the system is subjected to other large disturbances. For example, if the frequency difference of the input is too high or the deceleration swinging process will occur under the residual power of the diesel engine, and the deceleration area cannot be offset by the acceleration area. In the case of the parallel unit, the dynamic stability will also be lost. When the high power asynchronous motor starts, the parallel genset is suddenly disassembled, etc., the dynamic stability problem of the parallel diesel generator set is also generated.
For
diesel power stations, because the capacity of the power station is small, and
the power of a single generator and the power of the power station are
relatively close, the bus voltage also needs to change drastically during the
dynamic process. Therefore, dynamic stability analysis usually requires
calculation of each generator set. The movement of the rotor of the genset and
the relative motion between the rotors. In addition, most diesel power plants
have asynchronous motor loads comparable to the capacity of the power plant.
Since the torque of the asynchronous motor is proportional to the square of the
voltage, when the bus voltage decreases, the torque drops sharply, and the
asynchronous motor is rapidly decelerated or even stopped. Turning, this in turn
greatly increase the power drawn by the motor from the grid, which in turn
affects the stability of the generator.
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Created on Jan 25th 2019 20:51. Viewed 411 times.