By Dr. Joy Mazumdar

Advances in electric-drive haul truck technology over the past 20 years have been remarkable. The drive systems for these trucks consist of two electric drive-motors that are integrated through gears into the rear wheels of the trucks, an electric generator and a powerful diesel engine. The characteristics of the truck’s drive system allow it to harness electrical power directly. In some parts of the world, electrical power can be generated inexpensively and with less emissions, depending on the fuel source. Truck trolley systems, which simply substitute electricity for diesel fuel, may offer another avenue for further advancement.

Instead of generating electricity from the diesel engine and the electric generator on the truck, it’s sourced from a dedicated substation and transmitted via an overhead catenary to the drive motors on the truck.

Past experience has shown the best system to fulfill this function should be based on designs used in conventional catenary systems, such as those used for traction drives on light railway infrastructure. A similar system has been optimized for mining operations, which have similar demands such as mechanical stability, operating reliability and low maintenance. The infrastructure for a complete trolley system includes a catenary system, traction substations, a trolley mast-mounted high voltage supply, a trolley box mounted on the truck deck, and illumination of the catenary system.

The overhead wires are fed from a transportable rectifier station via high speed DC circuit breakers. Masts are placed along the haul routes carrying two catenaries on a single cantilever. Each catenary comprises copper messenger wires and copper silver alloy contact wires. The messenger and contact wires are each tensioned via weight tensioning equipment. The mast and foundation poles are painted, while all other fabricated steel work on the line is galvanized. The line equipment is assembled using galvanized copper alloys or stainless steel hardware.

A robust transportable traction substation has been designed to cope with rough environmental and operational conditions, including the repeated load that is so typical in traction applications, along with dust, corrosive elements, high temperatures and 24 hour operation. The substation equipment is modularly designed and mounted on skids that allow it to be easily moved from site to site as mining progresses.

Benefits of Trolley
Longer hauls and steeper grades present an opportunity for trolley assisted haulage. Typical truck payload requires a huge power source (around 2,200 kW) that has to be efficiently used. Keeping the gross vehicle weight for haul trucks as low as possible is also important, so power consumption is limited to just ore transport. Every ton added to the truck itself is one less ton of ore the truck can haul. Most mines are located in remote areas (deserts, high mountains, etc.) and the trucks have to contend with extreme environmental conditions. These parameters make electronic packaging a fundamental component in haul truck design because of the small space left to place adequate power source drivers.

The energy crisis in the 1980s led to the development of a trolley system for mines. Today, Siemens is a leading supplier of trolley technology and infrastructure and the company has seen a lot of renewed interest, primarily due to diesel fuel consumption. Aside from the obvious reduction in fuel cost, further advantages have been realized with modern systems, including:
•    Increased production capacity of the mine and a reduced number of trucks due to higher speed of the trucks on a trolley system (better fleet utilization);
•    Greater accessibility in the deeper parts of the mine. The trucks under trolley power are able to achieve high-er gradients and operate at full load for longer periods;
•    Reduction of maintenance costs on the trucks, particularly on the diesel engine, which would normally suffer the greatest wear while operating at full load only while on the ramp;
•    Increased availability and decreased life cycle costs for the diesel engine (less operating hours);
•    Ability to handle a wide range of line voltages;
•    Ability to run on the line at any speed and payload; and
•    Environmental improvements (loweremissions and less noise).

In terms of installed base, Africa is the leader in trolley assist. However, as fuel costs continue to climb, mines globally are taking a proactive approach.

The Trolley Concept
The truck trolley system is most cost-effective on the ramps, where the most energy is required. The remaining haul cycle uses conventional diesel power. However, as will be shown later, it is extremely beneficial to put trolley lines on both uphill and downhill hauls to recover the braking energy. Some mines are now considering trolley assist for flat hauls just to reduce diesel consumption and extend the life of the truck engine.

Instead of feeding power to the truck from the diesel engine, power is drawn from overhead lines (See Figure 1). The overhead lines are connected directly to the DC link.

At the engine’s top speed, the fuel rate is 450 liters/hr (See Figure 2), but with trolley assist, the engine idles and the fuel rate drops to 40 liters/hr. This reduction leads to considerable fuel savings and is a significant step toward mines becoming more energy efficient.

Trolley systems are advantageous for mines where there is a big difference between diesel and electricity costs. For example, a power plant may have its own mine, or a mine may have its own power plant. In such cases, the mines want to run the trucks on trolley as power is much cheaper for them.

Mining haul trucks, like any other mobile application, have constant motoring and braking modes. The full regeneration of the braking energy is one of the most promising sources of energy savings to a mine. Trolley systems for mining are derivatives of traction overhead systems. Solutions in the traction overhead lines exist for 750 VDC or 1,500 VDC. Mining trucks operate at the higher voltages.

Trolley systems for most mining haul trucks are installed for uphill hauls. On the downhill haul, the energy is wasted in the grid resistors. However, the trolley assist system could be designed in such a way that it improves the overall line receptivity of the DC power system by transferring the braking energy to the AC side, regenerating it, via the transformer, to the AC medium voltage distribution network. This energy could be used by other loads in the mine or used by the utility grid.

This system can be easily achieved by replacing the uncontrolled diode bridge rectifier with an active rectifier. This transforms the traditional unidirectional substation into a reversible one. The key benefits expected from reversible substations are:

•    Regeneration of the braking energy at all times, while maintaining priority to natural exchange of energy between trucks.
•    Reduction/elimination of the braking resistors, and thus reduction of the truck mass and heat release.
•    Regulation of the output DC voltage to make the DC overhead line voltage in-dependent of the AC line fluctuations.
•    Reductions in the levels of harmonics and improvement of the power factor on the AC side.

The term Active Front End (AFE) is normally used to describe the line-side converter with active switches such as IGBTs. A typical Siemens AFE-based rectifier system topology consists of AFE converter(s) on the line-side, a DC-link capacitor and boost inductors. Depending on the power requirements, the rectifier system could have multiple AFE converters all connected to a common DC-link. The AFE converter normally functions as a rectifier. But, during regeneration it can also be operated as an inverter, feeding power back to the line. The AFE inverter is also popularly referred to as a PWM rectifier. This is due to the fact that, with active switches, the rectifier can be switched using a suitable pulse width modulation technique.

The AFE inverter basically operates as a boost chopper with AC voltage at the input, but DC voltage at the output. The intermediate DC-link voltage should be higher than the peak of the supply voltage. This is required to avoid saturation of the PWM controller due to insufficient DC link voltage, resulting in line side harmonics. The required DC-link voltage needs be maintained constant during rectifier as well as inverter operation of the line side converter. The ripple in DC-link voltage can be reduced using an appropriately sized capacitor bank (See Figure 3).

Replacing the diode bridge rectifier in a unidirectional substation with IGBTs transforms it into a bidirectional substation (See Figure 4).

The key features of using AFE inverters include:

•    Regenerative Capabilities—In normal motoring mode of the drive, power flows from supply-side to the motor. The line-side converter operates as a rectifier, whereas the load-side con-verter operates as an inverter. During regenerative braking mode, the con-verters’ respective roles are reversed. The system can seamlessly regenerate power whenever needed;
•    Unity Power Factor Operation—With the line currents in phase with the line voltages, the unwanted reactive currents are eliminated. Since regen-eration is also possible at unity power factor, the overall power quality is improved significantly;
•    Reactive Power Compensation—Alter-natively, the kVA ratings saved due to the unity power factor operation can be used to provide reactive power compensation to the utility system. The double-sided power converter acts as VAR compensator while sup-plying the load;
•    Harmonic Cancellation and Improved In-terface with the Utility—Harmonics in-troduced by line-commutated rectifiers do not exist in AFE inverters. The har-monics introduced by switching active devices (IGBTs) are reduced by stagger-ing the AFE inverters. Overall total har-monic distortion (THD) in line currents and line voltage is much less and com-ply with the utility regulations; and
•    Satisfactory Operation During Line Voltage Dips—The line reactor and transformer secondary impedance allows AFE inverters to push DC-link voltage higher than peak line voltage. During line voltage dips of up to 30% a constant DC-link voltage can be maintained and satisfactory operation of the drive is possible.

Potential Savings
Based on mine-specific system parameters such as haul cycle distances, grade, production requirements and prices of diesel fuel and electricity, it is possible to predict investment costs, energy costs and maintenance costs as well as production values and payback time (return on investment). Ultimately, the cost savings from trolley assist occur when hauling the same amount of material while using less trucks or by hauling more payload using the same number of trucks.

Calculations have shown a 300-ton loaded truck going down the ramp will regenerate approximately 3 MW into the grid. An empty truck going downhill on a similar profile will regenerate approximately 1.3 MW into the grid. Calculations as well as feasibility studies have shown that a trolley system achieves payback in two to four years.

Based on analysis that was performed in South Africa, it was observed that a loaded truck going 1-km downhill generated approximately 7,900 kWh of energy per day (20 hr operation, 5 minutes loading and spotting time per cycle). At an energy cost of 0.60 ZAR ($0.077) per kWh, the cost of regeneration energy per day per truck will be 4,750 ZAR ($608). For an empty truck on a similar profile, the cost of regeneration energy per day per truck will be 2,050 ZAR ($262). The impact and savings will be higher as the ramp length and the truck fleet size increase.

Haul truck technology has shown enormous development in recent years. It has produced important performance improvements with increasing payload capacity. Now, electric-drive haul trucks outfitted with pantographs can pull power from an overhead trolley line. The trucks run on diesel power in the pit, around the crusher and on level segments. The trolley line provides power on the grade resulting in increased truck speed, extending intervals between engine overhauls and reducing energy costs. To date, Siemens is the only company that provides trolley solutions for AC haul trucks in the ultra class range.

References
1.    J. Mazumdar and W. Koellner; “System and Method for Reinjection of Retard Energy in a Trolley Based IGBT Electric Truck,” U.S. patent application filed 2009.
2.    J. Mazumdar and W. Koellner; “Peak Demand Reduction in Mining Haul Trucks Utilizing an On-Board Energy Storage System,” U.S. patent application filed 2009.

Mazumdar is the product marketing manager, haul trucks for Siemens Industry, based in Atlanta, Georgia, USA. He can be reached at T: 770-740-3707 (E-mail: [email protected]). This article was adapted from a presentation he delivered at the Haulage & Loading conference, which took place during May 2011, in Phoenix, Arizona, USA.

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