By Mike Ross, Director of FACTS Product Line at AMSC
Intro: Mining and Its Impact on the Grid
Australia’s decade-long resource boom has increased mining industry value by more than 150% which significantly boosted domestic economy and raised local income as well as helped Australia to become a global leader in mining equipment and technology. However, large motors and drives used in mining can shift the power factor of the utility grid system away from the desired unity level, thereby decreasing the efficiency and security of the power system, not to mention potentially degrading equipment. Therefore, the influx of large inductive loads connected to the utility power system can wreak havoc on the electricity grid.
At large industrial sites, power quality – with voltage stability being a key criterion – is essential both for reliable operation of large 3-phase motors as well as avoidance of adverse impacts, such as brown-outs, at adjacent customers. Technically, the “quality” of power at a large industrial site is a function of how precisely reactive power is managed. Power factor, phase angle and VARS are all typical metrics, but VAR management is the paramount measure. By selecting an appropriate STATCOM system, such as AMSC’s D-VAR system, line voltage, power factor, and VAR (volt amperes reactive) power can be maintained within precise parameters. AMSC’s D-VAR system is capable of precise management of the VARs needed to regulate voltage and quickly drive voltages to acceptable levels.
Field Application of D-VAR Systems
AMSC has deployed D-VAR systems worldwide. In Australia, where mining, liquefied natural gas facilities and wind power parks are also seeing substantial growth, interest in D-VAR systems have grown, too. For instance, the OZ Minerals Prominent Hill mine, opened in South Australia in 2009, has become a showcase for high power quality by virtue of their installation of AMSC’s D-VAR system.
The OZ Minerals copper concentrator uses high powered crushing, grinding and flotation equipment. The plant’s peak demand can be as high as 35 MW. As a consequence of the large individual motor loads at the plant and the characteristics of the local grid, OZ Minerals early on sought electrical engineering studies to identify grid reinforcement and power quality needs. Their search ultimately resulted in the installation of a state-of-the-art D-VAR system, and ancillaries, designed and sold by AMSC.
In anticipation of the new 35 MW load at the mine, AMSC carried out system planning studies of the local area grid. These studies substantiated the need for additional steady-state reactive support at the Prominent Hill 11 kV bus serving the mine over and above that for unity power factor correction of the large motor loads. Specifically, design for the voltage control and power factor correction.
Load flow studies showed the likely effects of uncompensated normal mill motor starts. The mine features two sets of motors for the ball mill and SAG mill. Each mill startup includes two 6 MW motors that draw a maximum of 130% of rated full load current for up to 20 seconds. The two mills are typically started with about a 30-minute interval.
Start-up of such large motors could cause voltage at the Prominent Hill 11 kV bus to drop to 84% of nominal, low enough to cause loss of load on under-voltage tripping, as well as unacceptable voltage dips on the high voltage transmission backbone in the area, namely the Olympic Dam 132 kV line. The resulting degradation in power and torque of the mill motors clearly confirmed the need for dynamic reactive support to mitigate the voltage depression during mill start-up.
The AMSC D-VAR system installed at the mine was sized to maintain the 11 kV load power factor at around unity and limit the step voltage change during start-up of each set of two 6 MW motors to a maximum of 5%. In addition, all this had to be done at the least possible cost. Once the planning decision was made to install the D-VAR system, the turn-around time for installation was minimal. Within a few months of OZ Minerals’ decision, the D-VAR system was up and running prior to the formal start of operations of the plant in May 2009.
The D-VAR system employed by OZ Minerals delivers continuous reactive power, controls each phase individually and has overload capability. Using control and monitoring software, OZ Minerals’ D-VAR solution is well documented now in its capacity to detect and compensate for voltage disturbances by injecting leading or lagging reactive power. The solution has attenuated rapid voltage variations and provided post-fault voltage support to mitigate any tendency for voltage collapse, in addition to acting as a fast transient voltage support device. By integrating the dynamic VAR output of the reactive power compensation device with a mechanically switched capacitor bank, this system is a very economical alternative to SVC's and is equally effective at solving common transmission grid problems such as voltage instability. In addition, the D-VAR system does not produce substantial harmonics that a traditional SVC system would, further simplifying the installation. When not performing the motor start assistance, OZ Minerals’ D-VAR system regulates the power factor of the 11 kV mine load to unity by utilizing a portion of the D-VAR and by switching the 11 kV substation capacitor banks.
AMSC’s D-VAR solution represents the state-of-the-art in reactive power management. The product features a modular design, excellent short-term overload capability, coordination with mechanically or electronically switched shunt capacitor or reactor banks, and a sophisticated, flexible and adaptable control system. The availability of the D-VAR solution means that voltage instability problems caused by the start-up of large drives, motors, metal shredders and crushers or other similar high power processes can be readily and economically corrected. When placed at the point of load connection, a D-VAR system will support and stabilize the voltage, ensuring premium power quality.