XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3847
Each mill drive has two 7.5MW synchronous ABB low
speed motors, with a rated speed of 180 rpm, directly cou-
pled to the mill, with a torque limiting coupling in place of
an air clutch. A single variable speed drive (ABB ACS6000)
is used to control both synchronous machines, with one
acting as the master and the other as the follower. Figure 1
shows a basic diagram of the mill drive system arrange-
ment. Load sharing is done internally within the ABB drive
with a torque limiting device installed as a safeguard in case
of a short circuit on the motor terminals. In case the torque
exceeds the release torque of the torque limiting device, the
coupling releases and decouples the motor from the mill
and protects the pinions, bearings, and the mill. The direct
coupling eliminates the need for a gearbox or air clutch
assembly.
EARLY CHALLENGES
Since grinding is an initial stage in the mineral recovery
process, for which there is no redundancy or ability to
bypass, unplanned trips of these drives translate into plant
downtime and production loss. Additionally, due to the
fine particle size present in the ball mills during operation,
unplanned trips result in material settling in a very short
time. This settling is referred to as frozen charge. Due to
high throughput and heavy loading at Detour Lake Mine,
ball mill frozen charges are difficult to release, resulting in
additional downtime and production loss.
Attributes of the Electrical Grid
A 230kV overhead power line connects the mine site to
the Ontario electrical grid at the Pinard Transformer
Station. The utility provides a maximum short circuit
power of 5,232MVA. After taking into consideration the
177km, 230kV overhead power line, the maximum short
circuit power at the Detour Lake Mine site is 542MVA.
2 × 230 – 13.8kV, 42MVA power transformers operating
in parallel supply power to the four mill drives. Therefore,
the theoretical maximum short circuit power for each drive
is 470MVA resulting in a short circuit power ratio of 5.9.
(Bouillon, M. 2016) The short circuit power ratio is the
ratio of the network short circuit power to the power of
the load connected on a medium voltage bus. Additionally,
since Detour Lake Mine is connected to the utility at a
remote substation location, which often has local genera-
tion or connection to the utility grid off-line, the actual
short circuit power ratio is usually lower than the theoreti-
cal maximum. The electrical network at Detour Lake Mine
is considered to be a weak network since the short circuit
power ratio (Ksc) is less than 20.
A network with low short circuit power has a higher
risk of voltage stability issues. This along with the changes
in the power generation result in frequent transient voltage
stability issues. These transient voltages can be caused due
to sudden and rapid changes on the grid, such as switching
events, lighting strikes or short circuit events. An example
of the fluctuation in the network voltages during a transient
event can be seen in Figure 2.
Early Behavior of the Mills During Transient Voltages
As mentioned in the previous section, a switching event
can result in a transient voltage. One example is the initial
energization of the mill transformers. Each mill at Detour
Lake Mine is equipped with 3 transformers connected in
series, with a total rated power of 19.6MVA and a nominal
Figure 1. Low speed ring geared Mill Drive
Each mill drive has two 7.5MW synchronous ABB low
speed motors, with a rated speed of 180 rpm, directly cou-
pled to the mill, with a torque limiting coupling in place of
an air clutch. A single variable speed drive (ABB ACS6000)
is used to control both synchronous machines, with one
acting as the master and the other as the follower. Figure 1
shows a basic diagram of the mill drive system arrange-
ment. Load sharing is done internally within the ABB drive
with a torque limiting device installed as a safeguard in case
of a short circuit on the motor terminals. In case the torque
exceeds the release torque of the torque limiting device, the
coupling releases and decouples the motor from the mill
and protects the pinions, bearings, and the mill. The direct
coupling eliminates the need for a gearbox or air clutch
assembly.
EARLY CHALLENGES
Since grinding is an initial stage in the mineral recovery
process, for which there is no redundancy or ability to
bypass, unplanned trips of these drives translate into plant
downtime and production loss. Additionally, due to the
fine particle size present in the ball mills during operation,
unplanned trips result in material settling in a very short
time. This settling is referred to as frozen charge. Due to
high throughput and heavy loading at Detour Lake Mine,
ball mill frozen charges are difficult to release, resulting in
additional downtime and production loss.
Attributes of the Electrical Grid
A 230kV overhead power line connects the mine site to
the Ontario electrical grid at the Pinard Transformer
Station. The utility provides a maximum short circuit
power of 5,232MVA. After taking into consideration the
177km, 230kV overhead power line, the maximum short
circuit power at the Detour Lake Mine site is 542MVA.
2 × 230 – 13.8kV, 42MVA power transformers operating
in parallel supply power to the four mill drives. Therefore,
the theoretical maximum short circuit power for each drive
is 470MVA resulting in a short circuit power ratio of 5.9.
(Bouillon, M. 2016) The short circuit power ratio is the
ratio of the network short circuit power to the power of
the load connected on a medium voltage bus. Additionally,
since Detour Lake Mine is connected to the utility at a
remote substation location, which often has local genera-
tion or connection to the utility grid off-line, the actual
short circuit power ratio is usually lower than the theoreti-
cal maximum. The electrical network at Detour Lake Mine
is considered to be a weak network since the short circuit
power ratio (Ksc) is less than 20.
A network with low short circuit power has a higher
risk of voltage stability issues. This along with the changes
in the power generation result in frequent transient voltage
stability issues. These transient voltages can be caused due
to sudden and rapid changes on the grid, such as switching
events, lighting strikes or short circuit events. An example
of the fluctuation in the network voltages during a transient
event can be seen in Figure 2.
Early Behavior of the Mills During Transient Voltages
As mentioned in the previous section, a switching event
can result in a transient voltage. One example is the initial
energization of the mill transformers. Each mill at Detour
Lake Mine is equipped with 3 transformers connected in
series, with a total rated power of 19.6MVA and a nominal
Figure 1. Low speed ring geared Mill Drive