3848 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
current of 820A. The energization of these transformers
results in an inrush current which is a phenomenon caused
by the saturation of the transformer when it is energized.
These inrush currents can reach values 10 to 50 times larger
than the rated transformer current and can flow for several
seconds. The calculated inrush current per mill can be seen
in Figure 3.
During normal operation of the mine, it is very com-
mon to have the other mills in operation when a set of
converter transformers are energized. High inrush current
combined with the weak inertia of the network result in
a voltage distortion of the supply network that very often
would lead to nuisance tripping of the other operating
mills. The network asymmetry, network undervoltage or
transformer saturation that is normally associated with a
transient event result in the inability of the drive to provide
proper current control.
The network voltage during such an event (Figure 2)
was characterized by a drop in the network voltage followed
by a subsequent rapid recovery (within 40ms). This would
subsequently affect the intermediate DC link of the voltage
source inverter variable frequency drive (Figure 4) resulting
in an overvoltage condition or, in overcurrent condition in
certain cases.
A similar reaction of the drive system was also
observed during network transients due to external fac-
tors. Additionally, the frequent overvoltage and overcurrent
situations resulted in additional weakening of the elec-
tronic components on the drive resulting in more frequent
replacement of power components (IGCT’s and diodes).
Figure 2. Measured network voltage during a network disturbance. Each sample is 0.1ms. The data span is 102.4ms
current of 820A. The energization of these transformers
results in an inrush current which is a phenomenon caused
by the saturation of the transformer when it is energized.
These inrush currents can reach values 10 to 50 times larger
than the rated transformer current and can flow for several
seconds. The calculated inrush current per mill can be seen
in Figure 3.
During normal operation of the mine, it is very com-
mon to have the other mills in operation when a set of
converter transformers are energized. High inrush current
combined with the weak inertia of the network result in
a voltage distortion of the supply network that very often
would lead to nuisance tripping of the other operating
mills. The network asymmetry, network undervoltage or
transformer saturation that is normally associated with a
transient event result in the inability of the drive to provide
proper current control.
The network voltage during such an event (Figure 2)
was characterized by a drop in the network voltage followed
by a subsequent rapid recovery (within 40ms). This would
subsequently affect the intermediate DC link of the voltage
source inverter variable frequency drive (Figure 4) resulting
in an overvoltage condition or, in overcurrent condition in
certain cases.
A similar reaction of the drive system was also
observed during network transients due to external fac-
tors. Additionally, the frequent overvoltage and overcurrent
situations resulted in additional weakening of the elec-
tronic components on the drive resulting in more frequent
replacement of power components (IGCT’s and diodes).
Figure 2. Measured network voltage during a network disturbance. Each sample is 0.1ms. The data span is 102.4ms