Increased Productivity with AC Drives for Mining Excavators and Haul Trucks

Gerald M. Brown, Ph.D. Bemard J. Ebacher, MSEE Siemens Energy & Automation, Inc.

100 Technology Dr. Alpharetta, GA 30005, USA

Walter G. Koellner, BSEE

Abstract - This paper reviews how the effective implementation of AC drive systems on mining excavators (shovels and drag- lines) and haul trucks has led to dramatic increases in produc- tivity. The review focuses on the major factors that influence productivity: Production rate, reliability, maintenance cost, power grid compatibility, system intelligence, and efficiency.

Key features of the haul truck drive are powerful GTO tech- nology and the ability to operate on overhead trolley lines at twice the power of existing haulers. Depending upon the truck’s gross vehicle weight (GVW), the on-grade speed can increase by up to 80%. The system is designed for operation on 2600 V lines, but can also operate on existing 1600 V lines using a novel diesel boost mode. The drive system also features software for enhanced drivability, optimum engine control, and an electronic dash display.

The key feature of the excavator drive is its active front end (AFE) technology, using 3.3 kV IGBT’s, that achieves powerful dynamic performance, while providing exceptional compatibil- ity with the line in terms of power factor and total harmonic distortion (THD). The shovel achieves current THD of 3.5% and the dragline 0.5%. The drive system also features a contex- tual maintenance system.

Key Words: AC Drive, Haul Truck, Excavator, High Voltage IGBT, Active Front End, AFE, Interactive Maintenance System, Trolley, GTO, Evaporation Bath, Series Connection, Productivity, Lifecycle Support, Maintenance and Repair Contract, MARC

1. INTRODUCTION There is a continual economic push to increase the productiv- ity of mobile material handling equipment used in open pit mines. This includes the largest electrical machines: excava- tors (draglines and shovels) and haul trucks (Fig. 1). Existing DC drive systems used on this type of equipment are a ma- ture technology. They have no further economic potential to accommodate larger equipment because of the high mainte- nance costs and the trade-off between power, speed and space limitations of their DC traction motors. In recent years the large-scale introduction of AC drives in the rail transportation sector [ l ] , and the availability of powerful GTO and IGBT modules have led to the development of compact, powerful, and rugged AC inverter drives specifically adapted to the mining market [2], [3].

The major advantage of AC drives stems from the squirrel cage induction motor which eliminates the DC commutator. This leads directly to higher speed, increased power density, higher reliability, greater efficiency and lower maintenance of the traction motors. In the context of excavators and haul trucks, additional benefits driving the move to AC are (i) in- creased production rate due to higher machine speed, and power density, (ii) increased reliability and lower cost invert-

Fig. 1 . 85 ton Shovel with AC Drive 3-Pass Loading a 3 10 ton Hauler

ers using 3.3 kV IGBT’s, (iii) active front-end (AFE) tech- nology that improves compatibility with the mine power grid, and improved dynamic response, (iv) intelligent converter design that provides increased fault tolerance, (v) contextual maintenance software that facilitates troubleshooting, and (vi) remote diagnostic capabilities. This paper describes how these and other features of AC drives lead to higher produc- tivity in mining shovels, draglines, and haul trucks.

11. MINIMIZE THE COST PER TON OF MATERIAL MOVED

The overall goal that drives design and sales of excavators and haul trucks is the constant requirement to reduce the cost per ton of material moved. Whether this involves blasting; loading trucks, or hauling material to the crusher or waste dump, equipment that reduces the cost per ton of material moved contributes to a more profitable operation.

An accurate determination of the true cost per ton to move material must consider every aspect of the equipment and the circumstances under which it is used. Procurement and capi- tal costs, he l , operating, and maintenance costs over the use- ful life, and eventually the cost of scrapping should all be taken into account to determine the real cost per ton of mov- ing material. We will focus on two broad areas that relate to the equipment itself; factors influencing the amount of mate- rial moved (Table 1) and factors influencing the cost of mate- rial moved (Table 3). Emphasis will be given to those ele- ments in the tables that most influence the productivity of excavators and haul trucks due to the application of AC drive technology.

0-7803-6401-5/00/$10.00 0 2000 IEEE P-28

III. MAXIMIZE THE AMOUNT OF MATERIAL MOVED A. Maximize Production Rate

The production rate refers to the quantity of material that can be loaded or moved per hour. This generally involves larger, faster, and more powerful equipment that operates more reliably to minimize downtime.

1) AC Motors: Fig. 2 compares the hoist motor speed- torque response of two 77 t (85 short ton) shovels, one AC the other DC. The absence of commutation limits on the AC motors enables the AC shovel to operate with a greater area under the speed-torque curve, resulting in faster lowering speeds and reduced times for the swing to return to the pit. This is especially important for short swing angles where the operator may have to wait for the dipper to lower. Reducing the cycle time by 6 to 8 seconds at the shovel translates into an increase in shovel production of about 20%.

2 ) Larger Equipment: The bucket capacity of excavators has increased approximately 100 fold over the last century. Fig. 3 illustrates the trend over the last 50 years toward con- tinually larger excavators and haul trucks. Larger equipment brings economies of scale associated with higher production rates in comparison to maintenance and operating costs. But efficient operation requires a good match between shovel dipper size and haul truck payload capacity. The generally accepted benchmark of three-pass loading requires that in- creases in shovel size remain in step with increases in haul truck size to minimize the loading time. At present, the larg-

FACTI

Production Factors

A. Maximize Production Rate

B. Maximize Reliability (Increase MTBF)

TABLE 1 :S INFLUENCING THE AMOUNT OF MATEUAL MOVED

Drive System Influence on Feature Productivity

speed, torque, power)

Larger equipment I Increased payloads

Innovative design, Increased net-to-tare and high power density power to weight ratio

Trolley operation Greatly increased power and speed, reduced fuel con- ----I-- sumption. noise reduction

~~

Mine production ‘Dispatch’ type systems management I reduce vehicle idle time

AC motors I NO commutator failures

3.3 kV IGBT No fuses, fewer component technology failures, higher peak current

caoabilitv

Evaporation bath High power density, her- cooling technology metically sealed modules

quency IGBT’s interface, robust

I - -L- I 20

0 ! I~

1

0 500 1000 1500 2000 2500 Motor Speed (rpm)

Fig 2 Comparison of AC and DC Hoist Motor Speed-Torque Curves

est AC hauler has a capacity of 327 t (360 short tons), com- pared to the largest DC machine with 2 18 t (240 short tons).

3) Innovative Design - Increased Drive Power Density Almost all off-highway haul trucks with payloads larger than 150 tons are built around a two axle, six tire configuration. Power is transferred to the rear wheels via either a mechani- cal transmission or a DC electric drive system. Trucks of this design typically have a net-to-tare (payload to empty vehicle weight, EVW) ratio between 1.3 and 1.5. Despite the huge size of these vehicles, there is surprisingly little space avail- able for the drive system components.

An innovative new truck design [4] departs from the nor- mal configuration in two significant aspects, (i) the placement of the traction motors and rear end drive layout, and (ii) the dumpbody support and dumping geometry. The new design has the traction motors mounted parallel to the centerline of the truck and each drives two wheels via a separate differen- tial gearbox that can float over uneven ground (Fig. 4). This effectively reduces typical tire overloads from 200% to around 105%, thereby increasing the allowable GVW.

The dumpbody on the new truck is suspended between rear pivots, located over the differentials at the rear of the truck, and the dump cylinders, which are located under the canopy

700

600 - E

500

f 400

p 300

0 - X

t 200

a a I 100

0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Fig. 3. Coordinated Growth in Payload Capacity Increases Production Rate

P-29

I Y

Fig 4 This Innovative Haul Truck Design Raises the Net-to-Tare Ratio and Utilizes AC Drive Technology with a Very High Power Density

in front of the dumpbody. This configuration integrates structural features of the frame and body to improve struc- tural efficiency and decrease EVW. The frame alone is 9000 kg (19,800 Ib) lighter.

These two features yield a net-to-tare ratio of 1.9 or 2.0, making this the most efficient vehicle on the market. None- theless, the vehicle has no more space than a standard DC haul truck for the drive components. In this case, the use of evaporation bath cooling for the AC drive provides high enough power density to fit the available space. Since the AC drive is also designed for trolley operation the new truck will easily outperform any DC vehicle in the world.

4) Trolley Operation Increases Motor Power Utilization: The ability to operate electric haul trucks with high power AC drive systems from overhead trolley lines increases the available power by a factor of two, compared to the largest currently available diesel engine. Lines are generally only installed along the most energy intensive portions of the haul route - uphill segments that are relatively permanent over the life of the mine [ 5 ] , [6].

Drawing power from overhead lines brings a double pay- back. Production rate increases dramatically since on-grade speed is proportional to the available power and the traction motors can deliver more power for a given frame size. The increased power utilization of the motors is determined by the thermal balance between the heat generated in the motors and the ability of the cooling system to remove the heat over the haul cycle. The motors can deliver peak power levels well in excess of their continuous rating as long as the rms power remains below their continuous rating.

Within the speed range under consideration for trolley op- eration, the motors operate at essentially constant torque, which depends on gross vehicle weight (GVW) and grade - but not on vehicle speed. The thermal losses in the motors in this case can be approximated by the relation:

where rpm2 and rpm, are the motor speeds while operating on trolley and under diesel power, respectively. The effect of (1) is that the increased losses at higher speeds are more than compensated for by the reduced travel time on grade. For a traction motor operating at 1750 kW with heating losses of 70

kW, a typical breakdown between stator, rotor, core and stray losses is 27%, 18%, 23% and 32%, respectively. From (l) , it can be seen that a 62% speed increase on trolley actually re- sults in a 13% reduction in energy dissipated in the motors.

An example from the Palabora mine in South Africa, in 1981 [SI involved putting 154 t (170 short ton) electric haul- ers on trolley. In this case, the speed on grade jumped by 84% to 22 km/h (13.8 mph) and the average interval between armature overhauls on the traction motors doubled from 10 to 20 weeks. This postponed the need to purchase larger wheel motors for seven years (as the mine got deeper). The tractive power, measured at the ground, increased from 865 kW (1 160 HP) on diesel to 1590 kW (2 130 HP) on trolley.

5 ) Increased Production with AC Drives on Trolley The largest currently available diesel engine, 2750 HP, yields about 1650 kW (2210 HP) of tractive power at the ground. This will soon be supplanted by a 3500 HP engine yielding about 2160 kW (2890 HP). The largest DC traction motors, when operating on trolley, are currently achieving about 2300 kW (3100 HP). In comparison, the present GTO system can reach at least 3500 kW (4700 HP) at the ground. This is 100% more power than under standard diesel operation, 62% more than the new large diesel engine and 50% more power than the best DC trolley system.

The higher power level possible using AC drives on trolley is best utilized at 2600 V, compared to existing DC drive trolley voltages of 1200 V to 1600 V. This reduces i2r losses in the distribution system and keeps the line current within the limits of existing brush technology.

Fig. 5 shows the tractive effort diagram, or rimpull curve, for a 327 t (360 short ton) haul truck with AC drive. The curves are characterized by very high starting torque and a constant power region for diesel propel operation that begins at very low speed, 6 km/h (4 mph). The dynamic brake (re: tard) curve is approximately double the propel curve at high speeds. The trolley curve lies in between.

The diagonal load line illustrates the relationship between grade, GVW, power, and speed. For example, a truck with a GVW of 558,000 kg (1,230,000 Ibs) and rolling resistance of 2% will require a tractive effort of 547 kN (143,000 Ibf) to ascend an 8% grade. The intersection with the tractive effort curve indicates the maximum speed possible for that power level. The curves show a 62% increase from 14.3 to 23.1 k m h (8.9 to 14.3 mph).

6 ) Direct Trolley Operation: Fig. 6 shows the major com- ponents of the AC drive. A diesel driven alternator con- nected to a diode bridge(s) feeds a DC link. Two inverters operate in parallel from the DC link, each one driving one traction motor. Regenerated power during retard operation is diverted from the DC link into braking resistors by two pow- er fd GTO choppers.

To connect to a 2600 VDC trolley line the operator drives the truck (on diesel power) under the line and initiates the

P-30

GVW (kg'1000)

I100 , . .. ~ . _ ~ ~ * __.__ ~ 4

0 100 200 300 400 500 600 700

I '

Trolley Propel (center curve) I Rcwrd (right CUNC) I

. . . . . . .~ IO00

I

-

Inverter 1

U d

- I d / 2 Pd/2

DC Link

-

- a 2 600

400

200

I I I

i- Motors

1 0 io 20 30 40 50 60 70

Vehicle Speed ( k d h )

Fig 5 Tractive Effort Curve for a 327 t (360 short ton) Haul Truck

Brake Resistors

Brake Choppers

1

External Power Source, PL

I Inverter 2

Ud I dl2 Pd/2 I U2

7 l P 2 12

I -

(Trolley Line) / - & I 2 - +UL12 3

Operating Diesel Boost Trolley Direct Trolley Mode Diesel Only

I I DC Link Voltage U d = URI f U R ~

Pd = PD = PI + P2

SI , S2, Si, and S4 in Position 1

U d = UL + URI = UL + u R 2

Id = 1L = IR1 + I R 2

pd = Ud I[ = Po + PL = PI + P2 SI , S2, and S 3 in Position 2

S4 in Position 1

U d = UL Id = I L , IRI = IR2 = 0 Pd = U1 II = PI = PI + P2

SI, S2, S3 in Position 1 (Rectifiers inactive) S4 in Position 2

DC Link Current I d = I R I = I R 2

DC Link Power

Switch Configuration (Rectifiers in series) (Rectifiers in parallel)

1 Pantograph

Notation

U =Voltage I =Line 1 =Current R =Rectifier p = Power = DC Value

,, = Diesel I = Motor I

= Motor2 * Losses neglected

r

connect sequence with a push button. The drive control computer raises the pantograph, adjusts the DC link voltage via the alternator field regulator to match the line voltage and closes the line contactor. The computer raises the tractive effort setpoint to match the higher trolley power limit and drops the engine speed to idle. The sequence takes about 5 seconds and the truck smoothly accelerates. The vehicle can connect to the trolley line at any speed without risk of the controller breaking off the connection due to overcurrents (A common problem that often prevents slow moving over- loaded DC trucks from successfully connecting to trolley).

7) Diesel Boost Trolley Operation - A New Flexibility. Mines that presently operate DC trucks on trolley typically have line voltages between 1200 V and 1600 V. The present AC drive system operates at a nominal DC link voltage of 2400 V to 2600 V. In order to run the AC drive on existing trolley lines an innovative circuit, referred to as diesel boost operation, was proposed by the first author (Fig. 6).

In diesel boost mode, the diesel engine, alternator, and rec- tifier combination are used to boost the line voltage up to the

--

Rectifier I

In I

I Line Current, 11 I Q l

' I ----A Line Contactor L,-,--------------------------J

Fig 6 AC Drive Haul Truck with Diesel, Direct Trolley and Diesel Boost Trolley Operating Mode Power is supplied from the trolley line (PI ) and or the diesel engine (PD) Two voltage source inverters are fed from a common DC link Each Inverter is connected to an AC traction motor Braking

energy is dissipated in the brake resistors via the brake choppers The inverters are supplied with power Pd at voltage U d and total current Id

P-3 1

Fig. 7. 254 t (280 short ton) Haul Truck on Trolley. The System Automati- cally Switches from Diesel to Diesel Boost or Direct Trolley Operation

optimum DC link voltage. The circuit makes it possible to smoothly transition from diesel to diesel boost operation while operating under full diesel power. The controller senses the line voltage and automatically positions S4 for op- eration on either high voltage or low voltage lines. All con- tactors (S, to s,) open and close with minimal or no current and the truck doesn’t slow down during the transition from diesel to trolley. Fig. 7 shows a 254 t (280 short ton) hauler operating on trolley at Iscor’s Grootegeluk mine in South Africa. The trolley capability of this truck’s AC drive system makes it the fastest and most powerful hauler in the world.

During direct trolley operation, the trolley line supplies 100% of the propulsion power and the diesel only covers engine parasitic losses (including hydraulic loads). During diesel boost operation the altemator/rectifier output is in se- ries with the trolley line and the proportion of power supplied by the line PL is determined by the ratio of line voltage UL to the DC link voltage u d :

(2) PL - UL - Ud-UR Pd Ud Ud

It is therefore most economical to operate with the trolley line at the highest possible line voltage UL during diesel boost mode to minimize the power delivered by the engine. B. Maximize Reliability (Increase MTBF)

Reliability is commonly assessed in terms of the Mean Time Between Failures, or MTBF. Failures are defined as unscheduled shutdowns, requiring immediate maintenance before the equipment can be returned to normal service.

1) AC Motors: The absence of commutators on AC motors is their main advantage, allowing higher speed and currents to be reached while improving the MTBF.

2) 3.3 kV IGBT Technology: The availability of 3.3 kV IGBT inverter technology makes it possible to achieve the required power levels for excavator drives without having to parallel devices. In addition, IGBT technology does not re- quire extensive snubber circuits and expensive gate drivers

common to GTO technology. This results in a much simpler power circuit and reduced parts count, increasing reliability. In contrast to GTO’s, IGBT’s can be safely tumed off in the presence of fault currents without being destroyed. This eliminates the need for expensive fuses and the associated risk of premature fuse failure, thereby increasing MTBF.

3) Evaporation Bath Cooling - Reliable and Compact: The AC haul truck drive utilizes GTO technology to meet the high power requirements of each truck inverter. The inverters and choppers are built using 4500 V, 3000 A GTO devices in- stalled in sealed modules (Fig. 8). The addition of Fluorinert FC72 coolant to the modules yields a system known as evaporation bath cooling. The process facilitates heat trans- fer from the power devices via a low-temperature evapora- tion-condensation cycle to the inner walls of the module. Unfiltered dirty air drawn across the module’s fins effectively removes the waste heat and cools the electronic components.

Evaporation bath technology is very compact and espe- cially suited for dirty environments because the modules are totally sealed. The resulting power density for the AC drive cabinet is approximately 1 MW/m3 (40 HP/ft3). Over 23,000 of these modules are currently in service throughout the world. AC locomotives using the technology consistently achieve mean time between failure (MTBF) rates for the en- tire locomotive electrical system of over 900 days.

4) Traditional Interface with the Power Grid: Static DC and older AC shovel drives use standard phase-controlled thyristor (SCR) technology for the interface with the line. The input circuit consists of forward and regenerative phase controlled bridges. The drive typically operates at up to 200% current and is at or near its maximum power much of the time. The line-side rectifier control must provide a stable DC link voltage under all line and load conditions or drive system performance may suffer.

Maintaining a stable DC link voltage in the presence of wide power swings is made more difficult because the rectifi- ers present a poor power factor to the line. The rectifier is always phased back a fair degree to give the controller mar- gin to regulate the DC link voltage. This results in lagging power factor that never approaches unity. On weaker than expected distribution systems, the voltage available for recti- fication is reduced. In this case, phase-controlled rectifiers are not very tolerant of dips in the network voltage, especially while regenerating. Whenever line conditions deviate outside the predefined range, the controller cannot maintain the bus

I -

s I t f ,,:::%,*

Fig. 8. Evaporation Bath Phase Module used in the AC Haul Truck Drive with 1 MW/m3 Power Density and Extremely High MTBF

P-32

voltage, and if this happens during regeneration, a fault may occur.

A second limitation of controlled rectifiers is that during rapid power reversals, the short delay introduced during the switchover from one rectifier to the other creates a distur- bance on the DC link. The poor power factor and limited dy- namic response (300 - 360 Hz) of phase-controlled rectifiers require a large amount of capacitance in the DC link to minimize the voltage fluctuations seen by the inverters.

5 ) Improved Power Grid Interface with AFE Technology: The new AC excavator drives use high power IGBT’s to eliminate the shortcomings of traditional rectifier front ends. The IGBT’s do not rely on passive line commutation. In- stead, they are actively switched at a much higher frequency to precondition the incoming power before it reaches the DC link. Hence the term ‘Active Front End’.

The AFE functions by first boosting the incoming line voltage to a DC voltage higher than that normally produced with a diode bridge. The AFE takes advantage of the net- work’s inherent reactance, a disadvantage in a controlled rec- tifier, to pump current into the DC link by rapidly switching the IGBT’s on and off. The system is designed to operate with enough reserve in the control range that the desired DC link voltage can always be maintained, even in the presence of large dips in the incoming line voltage.

The current flow between the line and the AFE is directly dependant on the voltage difference applied to the network reactance by the AFE. Adjusting the magnitude and phase of this voltage gives the AFE continuous control of the line cur- rent amplitude and phase in all four power quadrants. The controller regulates the DC link voltage by maintaining the balance of active power through the converter and is free to independently control the reactive power.

The ability to control reactive power on the network opens the door to many possibilities. For example, unity power factor can be maintained at the primary of the transformer that feeds the AFE, or at any other given point in the network,

I---- 1 - ... -~ ” T

D ~ I 10% Lme Voltage 0 - 0 9 m P F

I I I I I -I 5 -I 0 -05 0 0 0 5 I O 15

Regenuahve Watts - M O ~ O M ~ Wam (p)

Fig 9 Dragline AFE Power Capability Curve Unity PF at Full Load ‘F’

to improve the voltage regulation and overall efficiency. Fig. 9 shows the power capability of the swing drive for a

363 t (400 short ton) dragline using AFE technology. Peak power is used to establish the dragline’s per unit base power. The voltage ratio between the line and the DC link imposes a limit on the amount of reactive power that can be transferred to or from the power grid. Additionally, the AFE’s current rating imposes constraints on both active and reactive power.

The AFE is designed to maintain a power factor (PF) of 0.9 leading at the primary of the swing transformer, under any variation of line voltage between 90% and 110%. The point F in Fig. 9 shows that the AFE can maintain rated production when the line voltage drops as low as 8 1 % and still hold unity power factor at the primary of the swing transformer.

6) AFE Implementation on Excavators: Fig. 10 shows the block diagram for a typical large mining shovel using 3.3 kV IGBT modules and AFE topology to provide unmatched per- formance under all line conditions.

It can be seen from the figure that three-phase power flows from the line through the drive transformer, the input reac- tors, and into four AFE modules. The AFE modules operate similar to an inverter, converting the three-phase, constant frequency, variable amplitude AC voltage into DC current at constant voltage to maintain the DC link at its nominal level. The reactors behave much as the windings of a motor load on an inverter; they serve to limit harmonic currents, limit short circuit currents and provide some of the reactance required for step up converter operation.

The DC link provides motoring power to the inverters for each motion (hoist, crowd, swing, and propel) and accepts regenerative power from each inverter. The DC link capaci- tance facilitates power flow between the inverters for each motion, independent of the AFE’s, and helps smooth and stabilize the DC voltage. The choppers provide effective overvoltage protection to the IGBT’s. The bus is precharged to eliminate inrush currents through the AFE’s when the

3 P W E LINE

COLLECTOR RINGS

OMRVOLTAGE ~ PROTECTION

WE I

P=

I P n o p E L a

Fig. 10. Block Diagram: AC Shovel Drive using 3.3 kV IGBT with Active Front End ( M E ) for Power Factor Control and Very Low THD

P-33

drive transformer is energized. The AFE is fully regenerative and can provide leading or

lagging power factor in both motoring and regenerative modes. The magnitude of Vars the AFE can deliver varies greatly with the prevaling operating conditions. To facilitate correct dimensioning of the equipment during the design stage, detailed and accurate simulation programs have been developed that predict the thermal losses for 2 and 3-level inverter topologies as a function of the terminal power factor imposed by the AFE. This enables the designer to forecast the AFE’s usable operating range early in the design stage.7) The Impact of AFE Operation on MTBF: The use of IGBT’s in the AFE circuit and inverters allows the normal hses and snubbers circuits to be eliminated. The high switching fre- quency of the GTO’s gives the AFE high dynamic perform- ance that ensures continuous control over the high operating currents. This improves the reliability of the AFE as well as other equipment on the same feeder. The high switching fre- quency also permits the DC capacitance to be reduced, elimi- nating some of the capacitors.

The very high short-term overload capability of IGBT’s coupled with the AFE’s ability to compensate for wide varia- tions in line power factor make the drive system more robust and reliable than other technologies.

Iv. MINIMIZE THE COST OF MATERIAL MOVED Table 3 summarizes the factors that most influence the cost

of material moved. A . Power Grid Compatibility

Shovels and draglines are proportionally large consumers of power from the mine’s electrical grid and influence the quality of power throughout the mine. Their performance, in terms of power factor and harmonics generated, is often linked to the rate at which power is billed to the mine by the utility. If the mine operates its own power generation facility, the cost of poor power factor and high harmonic content re- mains. Distribution transformers and power cables through- out the mine may have to be oversized to handle higher reac- tive power, if these loads operate at low power factor.

1) AFE Improves Total Harmonic Distortion (THD): In comparison to traditional 6 or 12-pulse front ends, IGBT front ends have very high effective pulse counts. The dra- gline, with two sets of seven AFE’s in parallel (each switched at 450 Hz), has an effective pulse count of 3150 Hz. The shovel has four AFE’s in parallel (each switched at 900 Hz), for an effective pulse count of 3600 Hz. This high frequency operation means the AFE inherently generates very low THD.

The AFE controller in the standard shovel configuration actively regulates to maintain unity power factor at the pri- mary of the transformer when motoring or regenerating and to limit current THD to 3.5% (Fig. 1 1). The dragline operates to provide 0.9 leading power factor and to limit current THD to 0.5% (Fig.12). The spectrums shown represent the har- monic percentage with respect to the power demand, aver- aged over a complete cycle of each machine, when operating

TABLE 3 FACTORS INFLUENCING THE COST OF MATERIAL MOVED

Drive System Factor Feature Cost I

equipment sizing

B. Increase I Unit PF (with AFE) System Efficiency

I Independent blower

C. Minimize AC motors, extended Maintenance regrease interval

(Reduce MTTR)

Modular design, with knife connectors

Separation of clean and dirty cooling air

Maintenance station (computer)

D. Increase Self-protecting System circuitry and control Intelligence strategy

Contextual maintenance station

I l l

Influence on Productivity

IJ1 I High tolerance to line fluc- tuations

IJ l 1 Reduced capital outlay

Increased efficiency

Max. power from engine, highest speed on grade, minimum fuel consumption

Higher power, fuel savings I I J l No repair or regular mainte- nance

Reduced engine mainte- nance, fuel consumption, and noise

I Faster repair, reduced cost of parts

I JI No filters, reduced cleaning

Reduced diagnostic time, self-teaching

Crowbar replaces fuses, multi-level protection minimizes/prevents damage

Self-teaching diagnostics and repair procedures

at peak load and rated network voltage. The performance improvement brought about by the AFE

is dramatic and greatly enhances the compatibility of the AC excavator drive system with the electrical network. Table 4 compares the differences in power grid compatibility using static DC (SCR), GTO, and IGBT (AFE) technologies, re- spectively.

2) Optimized Power Grid Equipment Sizing: All feeder equipment and every load between the point of common con- nection with the utility (the PCC) must be adequately dimen- sioned. The equipment must be sized according to the total apparent power required and the total harmonic distortion (THD) generated, taking into account the individual duty cycles of each excavator. The sizing of circuit-breakers, switchgear, contactors, transformers, reactors, filters, cables, reactive power compensators, voltage regulators (for sensir tive equipment), etc, must take into account the worst case scenario of the combined duty cycles. Defining the worst case scenario, even when supported by statistical studies, can be difficult at best, but it is clear, the feeder equipment must always be oversized compared to the active power require- ments and this significantly increases the capital cost.

P-34

A correctly sized AFE operating at unity power factor al- lows the feeder equipment to be sized for active power only. If the AFE operates at 0.9 leading power factor, or as a net- work voltage regulator, the AFE system can even unload part of the network by providing Vars to surrounding loads. This also reduces the sizing of the surrounding equipment, com- pared to an installation with uncorrected power factor.

The high switching frequency of the AFE system generates such low THD on the line that it doesn't require oversizing of the feeder equipment. This is also true on weak networks, where it is more important to keep the harmonic voltage low.

The impact on the reduction in capital cost is obvious. The need for reactive power compensators, voltage regulators and filtering is eliminated, and the reduced current rating of all the related feeder equipment contribute to reducing the over- all capital costs of installing and operating these excavators. B. Increase System EfJiciency

AC drives present several ways to reduce the cost per ton by im- proving the drive system efficiency.

1 ) AFE Provides Unity Power Factor and Reduces THD: Operation at leading or unity power factor reduces the reac- tive power that must be carried by the mine power grid. This reduces the current flowing through transformers, switchgear and cables, etc. If the electrical load of the excavator is a significant portion of the mine's overall electrical load, the power factor improvement can result in lower rates and de- mand charges for electricity.

Another factor affecting efficiency is the degree of total harmonic distortion (THD). Harmonic currents produce eddy current losses in ferrous materials. Therefore, the low THD produced by the AFE contributes to overall efficiency (Fig. 1 1 and 12).

2) Optimized Engine Control: On haul trucks, the drive controller implements three governors for optimized engine

Efficiency has an ongoing effect on energy cost.

0 30% -

0 0%

I

o ~ ~ m ~ o ~ ~ m x f o ~ ~ m ~ o ~ ~ ~ v o ~ ~ m ~ ~ n n x f x f ~ ~ w ~ c m h m o o = ~ ~ 2 2 ~

Harmonic Order h

Fig. 11. Harmonic Spectrum ' AC Shovel with AFE. Current THD < 3.5%

TABLE 4

POWER GRID COMPATIBILITY - DIFFERENT TECHNOLOGIES

Parameter

Power Factor

Total Harmonic Distortion (THD) Voltage Tolerance

Efficiency

Static DC

0.40 lagging. (uncompensated) 1 .o (with RPC)

11% with RPC 5'h, 7'h filters

+IO% - 10% @ Full power

0.98 Rectifier 0.98 RPC

DC motor has field losses at idle

GTO AC

0.93 Lagging (at peak power) 0.82 Lagging (at idle), no RPC

29% for 6 pulse

13% for 12 pulse

+lo% - 10% @ Full power

(shovel)

Reduced power (shovel)

-IO% - 30% @

0.98 Rectifier 0.98 Inverter

No motor field losses at idle

IGBT AC (AFE)

1 .O PF (shovel) 0.9 Leading PF

(drag1 ine) No RPC

3.5% (shovel) 0.5% (dragline)

+ I O % -19% @ Full power, PF = 1 (dragline)

:ull power (shovel)

Reduced power (shovel)

0.98 AFE 0.98 Inverter

No motor field losses at idle

+ lo% - 10% @

-10% - 30% @

o a " a o W " x f 0 , , , * , ~ a ~ ~ ~ a x x ~ ~ ~ q ~ y ~ ~ Harmonic Order h

Fig. 12. Harmonic Spectrum: AC Dragline with AFE. Current THD < 0.5%

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C. Minimize Maintenance Cost (Reduce A4TTR) Equipment maintenance is costly due to lost production,

the cost of parts, and the labor and overhead involved. Of these, loss of production often far exceeds the other costs combined. The true cost of maintenance, therefore, must in- clude an assessment of the Mean Time To Repair (MTTR).

The time required to repair equipment depends on the availability of components, the time required to make the repair, and the time required to accurately diagnose the fault. The latter is often a hidden cost of maintenance because wrongly diagnosed equipment often continues to run poorly or fails again if the root problem has not been addressed.

1 ) AC Motors with Extended Bearing Life: The major fac- tor affecting AC motor life and maintenance cost is the safe allowable interval before the bearings must be replaced. Bearing temperature monitoring allows early detection so that a machine can be taken out of service before the bearing fails. A machine out of service in the pit incurs more indirect loss than a machine in the shop for pre-emptive repair.

2 ) Trolley Operation Reduces Engine Power Demand: Trolley operation significantly reduces the percentage of the haulage cycle that the engine is required to work at full power. Fuel consumption and engine wear directly follow the amount of work the engine performs. Fuel consumption on grade typically runs as high as 80% of the total fuel con- sumption. The consequential reduction in fuel consumption and engine wear with trolley represents a tremendous cost saving over the life of the vehicle. The operating hours be- tween engine overhauls at Palabora, for example, went from 6000 to 15,000 hours after trolley was introduced [7].

Engine emissions (smoke), noise, and oil consumption are all proportional to fuel consumption. Air quality issues can be greatly mitigated by the use of trolley. An efficient radial blower design and the use of non-magnetic stainless steel for the braking resistors means the AC truck makes almost no noise during retard or operation on direct trolley.

3 ) Modular Design of AC Power Modules: The AC drive systems utilize interchangeable modules to the greatest extent possible to minimize the MTTR and reduce inventory costs. The AC shovel and dragline systems use the same IGBT power module throughout the entire power section (Fig. 13).

Fig. 13. IGBT Power Module with Stab Type Connector Facilitates Rapid Removal and Installation to Minimize MTTR

The module plugs onto bus bars that are fixed in the cabinet using precision stab type connectors that automatically make the power connections when the module is inserted. There are no power cables to connect. D. Increase System Intelligence

As designers strive to continually improve drive system productivity, they often find themselves pushing the equip- ment to its physical limits. To maximize reliability under these conditions, supervisory and protective functions are added to ensure the equipment remains within its safe operat- ing region. As the equipment becomes more complex prob- lems that occur can be very difficult to properly diagnose and repair, which negatively impacts MTTR.

Building more intelligence into the equipment and into how it is maintained can minimize the likelihood of serious damage and reduce the time it takes to get the equipment back up and running.

1) Self-protecting Circuitry and Control Strategy: Careful analysis of the most likely failure modes often leads to inno: vative ways to minimize the adverse consequences of such failures, leading to a more intelligent design.

Fuses, for example, work well to limit damage to the equipment but the cost and inconvenience of replacing blown fuses can run into tens of thousands of dollars per year and hundreds of hours of down time.

Newer AC drives employ protective functions more so- phisticated than fuses. The systems are designed to actively detect faults and immediately shut the equipment down in a controlled manner before damage occurs. If possible, the equipment attempts to automatically restart. In many cases, the fault, protective reaction, and automatic reset happen so quickly the operator is unaware an incident has occurred.

The strategy used on new AC shovels and draglines makes use of the square SOA (Safe Operating Area) of IGBT’s. This characteristic of IGBT’s allows both devices in a faulty phase leg to be turned off immediately, before the overcurrent has a chance to cause damage to either device. This is much better than replacing large fuses.

2) Contextual Maintenance Station: Modem AC drives have extensive built-in self-diagnostic capabilities. In the past, these systems often provided detailed information at a basic level that was difficult to interpret. The new systems utilize integrated diagnostics that provide fault information in its full context. The troubleshooter pushes buttons on the touch screen to pinpoint the faulty component using graphics, text, and schematics of the drive system. Once the fault is located, the “Troubleshoot” button links it to a series of screens which outline the steps required to repair the fault. Links via key words also provide detailed descriptions of the system components.

The advantage of this type of contextual maintenance sys- tem is its ability to be self-teaching. It recognizes that most service personnel do not understand the detailed operation of

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the drive and need assistance in troubleshooting problems. Further, when equipment is reliable, even trained personnel often forget how to use the system’s diagnostic features. Therefore, the system is designed to be self-teaching.

The AC haul truck drive has a display system comprised of a full color LCD display (Fig. 14) integrated into an industrial PC with a standard Windows operating system and flash drive for cold weather reliability. The PC receives a telegram from the drive system computer and creates the dashboard display complete with instruments and gauges. A touch pad on the dash is used to enter commands and scroll through the screens. Wamings and fault indicators are displayed along with text messages that tell the operator the system status and the appropriate response to take.

3 ) Remote Diagnostics: A third way to minimize the MTTR is to draw upon the knowledge of people familiar with the machine and its performance history. This can be done via radio, telephone, and satellite links to experts who are on call. A single 24-hour service center can monitor and provide immediate response to inquiries from around the world. The service center personnel have access to an extensive database containing customer specific parts lists, inventory status, shipping information, maintenance procedures, and service history for the particular vehicle in question. The system can be Internet based for universal access, including data entry by commissioning engineers and mine personnel.

Once the infrastructure is in place for this type of remote diagnostic system, the system can be expanded to provide ongoing monitoring of the equipment’s condition and to automatically prescribe preventative maintenance procedures before failure occurs. This requires a continual feed of status information from the truck or shovel’s control computer to the radio link computer. Such systems are currently in devel- opment and undergoing field trials and refinements.

SUMMARY AND FUTURE DEVELOPMENTS We have looked closely at factors influencing productivity

of excavators and off-highway haul trucks. The application of AC drives on this equipment has facilitated dramatic im- provements in production rate, reliability, power grid com- patibility, efficiency, maintenance cost, and system intelli- gence. Of these areas, we would like to suggest four ongoing

developments that will perhaps have the greatest influence on surface haulage operations over the next 7 years:

Innovations in truck design that raise the net-to-tare ratio by up to 50%, along with compact AC drives, will lead pro- ductivity gains in truck operations throughout the world.

Widespread implementation of powerful GTO-based AC drives for operating haulers on overhead trolley lines will increase on-grade power by 100% and raise on-grade speeds by up to 80%. This will revolutionize truck production and significantly reduce engine maintenance and fuel costs.

IGBT shovels and draglines using active front end (AFE) technology will set new standards for dynamic performance, control of power factor, and minimal total harmonic distor- tion. This will reduce cycle times, increase reliability and lower capital costs associated with power grid infrastructure.

Maintenance stations that provide contextual, online trou- bleshooting will enhance on-site diagnosis and facilitate re- mote fault diagnosis to minimize the mean time to repair (MTTR) and maximize machine availability.

REFERENCES

[ l ]F . Bilz, and H. Segerer, “A Decade of Three-phase AC Traction Technology for Diesel-Electric Locomotives in North America - Market Developments and Technical In- novations,” Siemens Aktiengesellschaft, Publication No. A1 9 100-V600-B 179-X-7600. Translated from an article in ZEV + DET Glasers Annalen 12 1, No. 94997.

[ 2 ] G . M. Brown and W. Koellner, “A GTO Powered AC Drive System Increases the Performance of Off-Highway Haul Trucks,” IEEE-IAS 1999 Annual Meeting, Phoenix, AZ, USA, October 3-7, 1999.

[3] R. Mickleborough and W. Koellner, “Review of Liebherr Model T 282 and Ti 272 Mining Truck Development and Performance,” CIM 1999 Annual General Meeting, Cal- gary, Canada, May 2-5, 1999.

141 D. Miller, “The Outlook for Ultra-Class Haul Trucks: ILMT Developments”, Liebherr Mining Truck, Inc., Haulage 2000 Conference, Tucson, AZ, USA, Sept. 1997.

[5]“The Truck Trolley Dump System. Energy for Dump Trucks,” Siemens Aktiengesellschaft, Publication No.

[6] David P. Hutnyak, “Trolley Assist at Barrick Goldstrike”, A19100-V300-B4 14-X-7600.

Hutnyak Consulting, Haulage 2000 Conference, Tucson, Arizona, September 8-9, 1997 David M. Lake, “Truck Haulage using Overhead Electri- cal Power to conserve Diesel Fuel and improve Haulage Economics”, 110” AIME Annual Meeting, Chicago, I l l i - nois, February 22-26, 198 1.

H. B. Sumner, “Economic Benefits of the Trolley Assist Programme at Palabora, South Africa”, Presented at RTZ Mining and Metallurgy Conference, 1985.

Fig. 14. Haul Truck Dash Display Provides the Operator with Full Instrumentation and an Interactive Diagnosis System

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