INVESTIGATION INTO THE DESIGN OF A 6600V LONGWALL MINING ...

INVESTIGATION INTO THE DESIGN OF A 6600V LONGWALL MINING SYSTEM

PRELIMINARY REPORT

29th of May, 2004

Author: Adrian Trevor Supervisor: Bruce Penfold Industry Supervisor: Peter Henderson, Beltana Highwall Mining

__________________________________________________________________________________ Adrian Trevor – Elec4800 Interim Report II

ABSTRACT

As the installed power on Underground Longwall Mining systems increases, so does the

amount of current drawn from the supply at the industry standard voltage of 3300V. These

high currents cause voltage drops in the conductors, which in turn causes a squared reduction

in torque to all machinery, especially during motor starts. This voltage drop can only be

overcome by increasing the conductor size. This is not a viable option as these cables are

already physically difficult to manoeuvre and space is at a premium in many of the

flameproof enclosures. To overcome this problem it is proposed to increase the voltage to

6600V, which will in turn reduce the current drawn from the supply and hence reduce the

voltage drop problem.

This project involves the investigation of a design of a 6600V longwall electrical system.

There has not previously been a 6600V longwall in Australia with all existing Longwalls

being either 3300V or 1000V. All electrical equipment on the Longwall must be explosion

protected as it is in a hazardous zone (as defined by the Coal Mines Regulation Act (1982)).

This increase in voltage will have a drastic effect on all electrical items from flameproof

enclosures, to intrinsically safe circuits.

Research carried out so far has mainly centred on the future electrical power requirements of

a Longwall at Beltana in the next 7-10 years. The future increases in installed power to

achieve the required coal output that is desired have been approximated after detailed

conversation with the Beltana Mine Electrical Engineer and by looking at what the leading

coal mines in Australia are presently using. This has lead to an approximate power budget

on which calculations can be based.

From this power budget cable sizes have been determined and voltage calculations

performed. A load study simulation has been performed to confirm these values.

Literature research has also been instigated to obtain knowledge in the approval process

required for equipment to be used in an underground coal mine in NSW with several reports

obtained.

__________________________________________________________________________________ Adrian Trevor – Elec4800 Interim Report III

CONTENTS

CHAPTER 1 Background Information………………………………………………….....1

1.1 Abbreviations………………………………………………....…………1

1.2 Coal Mining History in the Hunter Valley…………………....…………2

1.3 The Longwall Mining Technique……………………….…….…………3

1.4 Overview of Beltana Highwall Mining……………………….…………4

CHAPTER 2 Future Power Requirements…………………………………………………5

2.1 The Voltage – Torque Relationship……………………………………..5

2.2 Shearer……………………………………………….…………………..7

2.3 Armored Face Conveyor (AFC)……………………….………………...8

2.4 Beam Stage Loader (BSL) and Crusher…………………………………9

2.5 Hydraulic and Water Pumps………………………………….…………9

2.6 Summary…………………………………………………….…………10

CHAPTER 3 Cable Selection………………………………………………….………….11

3.1 Overview…………………………………………………….…………11

3.2 Full Operational Load Current at 6600V………………………………11

3.3 Cable Sizing………………………………………………….………...13

3.3.1 Monorail Cable……………………………………………...14

3.3.2 BSL Cable…………………………………………………...16

3.3.3 Crusher Cable………………………………………………..17

3.3.4 Shearer Cable………………………………………………..18

3.3.5 AFC Cable…………………………………………………...19

CHAPTER 4 Load Flow Simulations………………………………………….…………20

4.1 Scenario 1 – Full Operational Load Current…………………………...21

4.2 Scenario 2 – Full Operational Load Current and 1 Shearer

Motor Starting……………………………………………….…………22

4.3 Scenario 3 – Full Operational Load Current and one TG

AFC Motor Starting................................................................…………23

CHAPTER 5 Future Work………………………………………………………………..24

__________________________________________________________________________________ Adrian Trevor – Elec4800 Interim Report IV

TABLES AND FIGURES

Table 1. Present and Future Shearer Power Ratings…………………………………7

Table 2. Present and Future Longwall Power Ratings……………………………...10

Table 3. Motor Currents at Full Operational Load………………………………….12

Table 4. Cable Currents at 6600V…………………………………………………..12

Fig 1. Example of a Longwall Panel…………………………………………………3

Fig 2. Overview of the Longwall Machine……………………………………….….4

Fig 3. Punch Longwall Mining Method……………………………………………...4

Fig 4. Motor Torque Vs Terminal Voltage…………………………………………..5

Fig 5. Longwall Substation Bus Voltage…………………………………………….6

Fig 6. Monorail Feeder Current over 3 Hours with Average……………………….15

Fig 7. BSL Current over 3 Hours with Average……………………………………16

Fig 8. Crusher Current over 3 Hours with Average………………………………...17

Fig 9. Shearer Current over 3 Hours with Average…………………………………18

Fig 10. TG AFC Current over 3 Hours with Average………………………………19

Fig 11. Single Line Diagram Showing Voltage Drop at Full Operational

Load Current….…………………………………………………………….21


Load Current with 1 x 1000kW Shearer Cutter motor starting…………….22


Load Current with 1 x 1000kW TG AFC Motor Starting………………….23

REFERENCES

• www.australiancoal.com.au

• Coal Services Pty Ltd – Mine Statistics

• Olex Mining Cable Catalogue

• Pirelli Mining Cable Catalogue

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Adrian Trevor – Elec 4800 Interim Report Pg 1

INTRODUCTION 1.1 ABBREVIATIONS

AFC - Armoured Face Conveyor

BSL - Beam Stage Loader

C&M - Control and Monitoring

DCB - District Control Box

DOL - Direct On Line

HV - High Voltage

SCADA - Supervisory Control And Data Acquistion

TG - Tailgate

UG - Underground

MG - Maingate

VVVF - Variable Voltage Variable Frequency

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1.2 COAL MINING HISTORY IN THE HUNTER

Coal was first discovered in Australia in the mouth of the Hunter River in 1791 by a convict,

and the first mining settlement was settled there in 1801. The coal was primarily used for

domestic cooking and heating. Since these early days the coal mining industry has become

Australia’s number 1 exporter with the Australian coal industry producing in excess of 288

million tonnes. The main use for this coal is energy production, with 83.8% of Australian

coal consumption in 2001 used by power stations, followed by 8.2% used by the steel

industry (source: Coal Services Pty Ltd).

The method used to extract this coal has changed vastly over this period. Long gone is the

day’s of the pick along with the horse and cart. Today coal mining is a highly mechanised

industry that is becoming more and more automated. The methods of extraction can be

clearly divided into two main categories. The first of these is open cut coal mines, where the

overburden (dirt and rock above the coal seam) is removed by machinery such as a dragline

to gain access to the coal seam, which is then removed and the overburden replaced and

rehabilitated. Presently this is the most cost effective form of coal mining when the seam is

within a fixed distance of the surface. Once the seam drops to a depth below this limit it

becomes more economical to mine via underground methods.

Underground coal mining usually takes the form of one of two methods. Bord and pillar

mining is an older technique using continuous miners. It initially involves cutting panels into

the coal and pillars behind to support the roof. This method has a recovery of around 50-60

percent, however the cutting rate is very slow, with yearly production totals below 1 million

tonnes.

The most efficient technique to extract coal underground is via the Longwall mining

technique. This technique allows high seam recovery rates and yearly production totals of up

to approx 7 million tonnes have been achieved.

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1.3 THE LONGWALL MINING TECHNIQUE

The longwall mining technique is the most common method of underground coal mining

presently in use in Australia. This technique involves the development of a large rectangular

blocks of coal, typically 3km long by 250m wide, which is then extracted in a single

continuous operation. Generally each block of coal, referred to as a panel, is created by

driving tunnels parallel to each other which are then connected at the far end.

Fig 1. Example of a Longwall Panel

The Longwall machinery is then installed and a mechanised “Shearer” traverses the face

cutting the coal and loading it onto an armoured face conveyor (AFC), which then removes

the coal to the surface. Self advancing hydraulic roof supports support the roof above the

shearer as it advances, and then move forward allowing the roof to cave in behind them

(called the “goaf”). The other main components of a longwall system are the Beam Stage

Loader (BSL) and Crusher. The BSL conveys the coal from the AFC to the conveyor belt

which exits the mine, while the crusher breaks up large chunks of coal into smaller pieces

more suitable for the conveyor belt system.

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Fig 2. Overview of the Longwall Machine

1.4 OVERVIEW OF BELTANA HIGHWALL MINING

Beltana is a new underground longwall mine situated near Singleton in the New South Wales

Hunter valley. Longwall coal extraction commenced in 2003, with Beltana being a punch

longwall mine. This is where the longwall panel is developed directly in from an open cut

Highwall, which has a main advantage of removing all mains development. An example of

this is shown in the figure below.

Fig 3. Punch Longwall Mining Method

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2. FUTURE POWER REQUIREMENTS 2.1 THE VOLTAGE - TORQUE RELATIONSHIP

Torque is proportional to voltage squared. This is because the torque of a 3 phase motor is

proportional to the internal air gap flux, which is generated by two components. These are

the stator winding voltage and the generated rotor voltage. A reduction in stator voltage leads

to an equal reduction in rotor voltage, which combined causes a squared reduction in air gap

flux and hence a squared reduction in torque. This is shown graphically below.

Motor Torque Vs Terminal Voltage

0%

20%

40%

60%

80%

100%

0%20%40%60%80%100%

Voltage (% rated)To

rque

(% R

ated

)

Fig 4. Motor Torque Vs Terminal Voltage

The voltage torque relationship is one of the main reasons why a move to 6600V is being

investigated. This is because the higher currents drawn from the 3300V supply at an

increased installed power cause large voltage drops in the supply cables. If the supply is

doubled to 6600V the currents are halved. Also any voltage dropped in the cables is a lower

percentage of 6600V then they would have been of 3300V.

The graph below shows the level of the 3300V bus at the longwall substation. As it can be

seen the voltage levels vary greatly, with typical values of 3100V when loaded to 3400V

with no load. This is due mainly to the voltage drop in the 11kV reticulation to the Longwall

__________________________________________________________________________________


transformer, as there are significant voltage drops in the 11kV aerials that supply the mine.

For the purposes of this project the voltage drops in the 11kV reticulation are to be ignored

with a constant 11kV supply assumed hence giving a constant 3300V bus at the longwall

Substation.

Supply Voltage

2500

2700

2900

3100

3300

3500

Time (Approx 16 Hours)

Vol

tage

(Vol

ts)

Fig 5. Longwall Substation Bus Voltage

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2.2 SHEARER

The present shearer at Beltana has the most installed power of any machine currently

operating in Australia. A shearer is typically made up of 5 main electrical drives. These are

listed in table 1 below along with the present power rating and proposed future power rating

of the current shearer at Beltana. Both the cutter motors start DOL, while the traction motors

are a reduced voltage VVVF drive.

SHEARER POWER RATINGS

Drive Present Rating (kW)Future Rating

(kW) Left Cutter Motor 850kW 1000kW Right Cutter Motor 850kW 1000kW Left Traction Unit 150kW 165kW Right Traction Unit 150kW 165kW Hydraulic Pump Motor 50kW 55kW Total 2050kW 2385kW

Table 1. Present and Future Shearer Power Ratings

This Shearer is a significant increase in power over the rest of the industry. It is not

envisaged that the power requirement of the shearer will increase dramatically over the next

7-10 years. The main increase will come from the cutter motors. At present the ranging arms

(the arms that raise and lower the cutting drums) have a mechanical rating of 1000kW. It is a

logical progression to increase the cutter motors to 1000kW each. It must be noted that this

value is a continuous rating. The motor will actually run at 1200kW while doing the leading

cut and then at 500kW while doing the trailing cut.

For calculations used in this project the future power rating of the shearer has been assumed

to be 2385kW. This is derived from increasing the cutter motors to 1000kW, and a 10%

increase on all other drives. It is shown in fig 6 above.

__________________________________________________________________________________


2.3 ARMOURED FACE CONVEYOR (AFC)

The AFC that is presently in use at Beltana consists of 3 x 750kW motors. These are

arranged with two motors at the MG and one at the TG. These are mechanically coupled via

a water filled coupling, so that the motors start DOL with no load.

The future power requirement of the AFC is very dependant on the mining technique used,

and specifically the width of the face. In recent years the trend in longwall face widths is to

increase them. This brings several gains, the most important being reduction in required gate

road development. As Longwalls continue to increase output and advance at an increasing

pace, it applies pressure for higher development rates. By increasing the width of the mining

face the amount of development that is required is reduced. This however places a higher

electrical and mechanical load on the AFC due to the increased amounts of coal that is

placed on it at any one time.

Beltana’s current face width is 262m. This is fairly typical of the industry at present.

However several mines have recently been investigating increasing face width to

approximately 400m. For the purpose of this report it will be assumed that Beltana will

follow industry trends of increasing face width, and a face width of 400m will be used. A

report has been commissioned by another mine to investigate this option and it was

calculated that it will require 4 x 1000kW motors.

For calculations performed in this report the future AFC power requirements will set at

4000kW.

__________________________________________________________________________________


2.4 BEAM STAGE LOADER (BSL) AND CRUSHER

At present the BSL and Crusher motors are rated at 300kW. They are both mechanically

coupled via fluid couplings and start DOL. To handle an increased throughput in tonnes per

hour the power rating will have to be increased.

For calculations performed in this report the future BSL power requirements will be set at

600kW. This allows for the connection of a second 300kW drive. The crusher motor power

requirements will be set at 400kW.

2.5 HYDRAULIC AND WATER PUMPS

The longwall face hydraulics at Beltana is presently serviced by 3 x 200kW motors driving

pumps. These pumps have an unloader valve so that the motors are not starting under load.

There is also a single shearer water pump that is rated at 200kW.

The hydraulic requirements for a 400m wide face are significantly greater than that of the

present 262m wide face. This is compounded when consideration is taken of an increase in

the speed of the shearer, which will require the hydraulic roof supports to move and reset

faster.

There are two main options that can be considered when increasing the hydraulic system.

One is to keep the present amount of pumps fixed but increase their power (from 200kW to

350kW), and the other is to increase the number of pumps (from 3 to 5).

In either case it is estimated that approximately 1000kW of electrical power will be required

for the hydraulic system, and this is the figure that will be used for calculations in this report.

The present shearer water pump, at 200kW should be sufficient for future use.

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2.6 SUMMARY

The following table summarises the present and future power requirements. It can be seen

that the future installed power rating is an increase of 50% on the present values.

LONGWALL POWER RATINGS Equipment Present Rating (kW) Future Rating (kW) Shearer 2050kW 2385kW AFC 2250kW 4000kW BSL 300kW 600kW Crusher 300kW 400kW Hydraulic Pumps 600kW 1000kW Shearer Water Pump 200kW 200kW Total 5700kW 8585kW

Table 2. Present and Future Longwall Power Ratings

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3. CABLE SELECTION _ 3.1 OVERVIEW

All electrical power cables used in an underground coal mine must comply with Australian

Standards AS1802 and AS1972. The selection of cable sizes is based on two constraints.

These are the current carrying capacity of the cable and the voltage drop that occurs in the

cable. The answer to both of these issues is to increase the size of the cable, however from a

practical perspective the cable sizes must remain within a reasonable limit.

The current carrying capacity of a cable is limited due to the heating effect that current has in

a conductor and the limited ability that a cable has of dissipating this heat. When calculating

cable size the average value of current over time is used, as it is this value that continuously

heats the cable.

The voltage drop in a cable is dependant upon the cable size, current and length of cable. The

voltage drop is calculated using the starting current of the motor, which is taken as being 6

times the full load current, as it as these moments that full voltage is required. This is

because the torque capability of the motor must be maintained above the required torque of

the load, otherwise the motor will stall.

A maximum limit of 5% voltage drop has been set from the Longwall Substation to any

motor so that a minimum torque of 90% is maintained at start-up.

3.2 FULL OPERATIONAL LOAD CURRENT AT 6600V

The following table shows the calculated full load current of each motor. The following

formula was applied to calculate the motor currents.

ληVPI FL 3

=

where P = mechanical rated power

V = Supply voltage

λ = Power Factor

η = Motor Efficiency

The power factor has been set to 0.85 while the motor efficiency has been set to 0.9

__________________________________________________________________________________


CURRENT AT FULL OPERATIONAL LOAD

Drive Rated kW Load Factor LF kW I @ 6600V

Shr Cutter 1 1000 1 1000 112 Shr Cutter 2 1000 0.5 500 56 Shr Traction 1 165 1 165 18 Shr Traction 2 165 1 165 18 AFC MG1 1000 1 1000 112 AFC MG2 1000 1 1000 112 AFC TG1 1000 1 1000 112 AFC TG2 1000 1 1000 112 BSL 600 0.5 300 34 Crusher 400 0.5 200 22 Hyd Pumps 1000 0.7 700 78 SWP 200 0.7 140 16

Table 3. Motor Currents at Full Operational Load

It must be noted that the full load current in this application is NOT simply the full load

current of each motor. The longwall operation dictates that not all motors are fully loaded at

the same time. Each motor is given a load factor which specifies its loading when the

longwall is fully loaded.

The full operational load current on each cable is given in the following table. The monorail

current is made up of the shearer, AFC, BSL and crusher currents.

CABLE CURRENT Cable Current Monorail (x2) 354 AFC (x4) 112 BSL 34 Crusher 22 Shearer 205

Table 4. Cable Currents at 6600V

Note: Monorail cables are a dual supply. The figure given above is the current in each cable.

__________________________________________________________________________________


3.3 CABLE SIZING

The values for the future average current are determined by correlating data obtained from

the Beltana SCADA system. The present average values for the current in each cable were

calculated over several continuous shears, and then turned into a ratio of that cables current

under full load conditions. This ratio was then kept constant and applied to the future cables

predicted full load current, and hence an average current for each cable is obtained. This is

then used to calculate the minimum cable size.

Average Current @6600V = FLC@6600V x (Avg@3300V/FLC@3300V)

The voltage drop calculation is then performed, and a cable is selected that satisfies both

voltage drop and thermal requirements.

__________________________________________________________________________________


3.3.1 Monorail Cable

The graph below shows the typical average current in the monorail cables over a 3 hour

period. The average value over this period was calculated to be 562A. As the monorail

supply is a dual supply the average current in each cable is 281A. The present full load

monorail current is 450A per cable. The full operational load current at 6600V is calculated

to be 354A in each cable.

The average current in the monorail cables is calculated to be 220A Using this value a cable

size of 120 sq mm is selected. It has a current carrying capacity of 339A when a rating factor

is applied for an ambient temperature of 25 deg.

The voltage drop in this cable is calculated using the mV/A.m value provided in the Pirelli

cable catalogue. The starting current for the monorail cable is set to the full load operation

current with 1 x 1000kW motor starting. This equates to 1268A or 634A in each cable. Most

of the motors on the longwall are interlocked so that they can not start simultaneously,

however it is possible for the shearer cutter motors to be started at the same time as one of

the AFC motors. It may be necessary to interlock these motors in the control system so that

this is not possible.

A 120 sq mm cable has a 3 phase voltage drop of 0.392 mV/A.m.

The calculated voltage drop for the 570m monorail cable is;

VD = 570 metres x 634Amps x 0.392mV/a.m

= 142V

= 2.1%

The Cable recommended for the monorail supply is a 120 sq mm type 240.6

__________________________________________________________________________________


MONORAIL FEEDER CURRENT

0100200300400500600700800900

1000

Time (Approx 3 hours)

Curr

ent (

Am

ps

Fig 6. Monorail Feeder Current over 3 Hours with Average.

__________________________________________________________________________________


3.3.2 BSL Cables

The average BSL current as displayed in the graph below is 48A. Its full load current is 67A.

The full load current of a 300kW drive at 6.6kV is 34A. This will give an average BSL

Current at 6600V of 24A.

The minimum cables size made in type 241.6 is 16 mm sq. This has a current carrying

capacity at 25 deg of 103.5A.

Voltage drop is not considered in this cable as it is only several meters long.

BSL CURRENT

0102030405060708090

100


Cur

rent

(Am

ps

Fig 7. BSL Current over 3 Hours with Average.

__________________________________________________________________________________


3.3.3 Crusher Cable

The average Crusher current as calculated and displayed in the graph below is 25.9A. It’s

full load current is 67A. The full load current of a 400kW 6600V drive is 44A. This will give

an average Crusher current of 17A.

As per the BSL cable the minimum cable size made is 16mm sq and this more than adequate.

Voltage drop is not considered as the cable is only 13m and lightly loaded.

CRUSHER CURRENT

0

10

20

30

40

50

60


Curr

ent (

Amps

Fig 8. Crusher Current over 3 hours with Average

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3.3.4 Shearer Cable

The average Shearer current as calculated and shown in the graph below is 212A. It’s full

operational load current is 329A. The full operational load current of a 2385kW 6600V

Shearer is 205A. This will give an average Shearer cable current of 132A.

A 50 sq mm cable has a current carrying capacity of 201A at an ambient temperature of 25

deg.

The voltage drop in the 400m shearer cable is calculated using a voltage drop value of 0.921

mV/A.m. and a starting current of 672A

VD = 400 metres x 672 amps x 0.921 mV/A.m

= 247V

= 3.75%

This voltage drop is unacceptably high as it results in a voltage drop of 3.75 + 2.1 =

5.85% from the longwall substation.

Increasing the cable size to 120 sq mm gives a voltage drop of 1.6% in the Shearer

cable and a total voltage drop of 3.7%

SHEARER CURRENT

0

50

100

150

200

250

300

350

Time (Approx 3 hrs)

Curr

ent (

Amps

Fig 9. Shearer Current over 3 Hours with Average.

__________________________________________________________________________________


3.3.5 AFC Motor Cables

The average AFC motor current as calculated and displayed below is 98A. Its full load

current is 168A. The full load current of a 1000kW 6600V drive is 112A. This will give an

average current of 65.3A.

A 16 sq mm cable has a current carrying capacity of 103A at an ambient temperature 25 deg.

The voltage drop in the 400m TG AFC cable is calculated using a voltage drop value of 2.75

mV/A.m and a starting current of 672A.

VD = 400 metres x 672 amps x 2.75mV/A.m

= 739V

= 11.2%

This gives a total voltage drop from substation to TG AFC motor of 13.3%, which is

unacceptably high.

Increasing the cable size to 70 sq mm gives a voltage drop of 2.52% which equates to a total

voltage drop of 4.62%.

TG AFC MOTOR CURRENT

020406080

100120140160180

0 1000 2000 3000 4000

Time (Approx 3 Hrs)

Curr

ent (

Amps

Fig 10 TG AFC Current over 3 Hours with Average.

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4. LOAD FLOW SIMULATION _

The longwall system was simulated at 6600V with the future power requirements to confirm

that the calculations performed previously were correct. This was done using the

“EasyPower” power system analysis software, which is used onsite at Beltana. Three main

scenarios were looked at. These were;

• Scenario 1 - full operational load current,

• Scenario 2 - full operational load current and one shearer cutter motor starting,

• Scenario 3 - full operational load current and one TG AFC motor starting.

Scenario 1 is with the longwall running and cutting coal with a maximum load. Scenario 2

and 3 were both simulated because they are both worse case scenarios. They are 1000kW

motors with over 900m of cable between them and the substation.

__________________________________________________________________________________


4.1 Scenario 1 – Full Operational Load Current

The proposed system was simulated at full operational load current. The only potential

weakness noted in the system is that that the monorail is operating to within 5% of it’s rated

current value. This is acceptable, but if any more power is installed on the longwall face a

larger cable will need to be used for the monorail supply. The Voltage drop to the Shearer

and TG AFC drives is excellent being at a value of less than 2%. This confirms the

calculations performed.

Fig 11. Single Line Diagram Showing Voltage Drop at Full Operational Load Current

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4.2 Scenario 2 – Full Operational Load Current and One Shearer Motor Starting

The simulation was run with full operational load current and 1 x 1000kW motor on the

Shearer starting. At this point the total voltage drop to the shearer is 3.8% compared with a

calculated value of 3.7%. This is well below the 5% maximum voltage drop limit. There are

no thermal issues with the current being well below the rated value of the cable.

Fig 12. Single Line Diagram Showing Voltage Drop at Full Operational Load Current with

1 x 1000kW Shearer Motor Starting

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4.3 Scenario 3 – Full Operational Load Current with One TG AFC Motor Starting

The simulation was run with full operational load current and the 1000kW TG1 AFC motor

starting. At this point the voltage drop at the TG1 AFC motor was 4%, compared to a

calculated value of 4.6%. This is well below the 5% voltage drop limit. There are no thermal

issues with the current being well below the rated value of the cable.

Fig 12. Single Line Diagram Showing Voltage Drop at Full Operational Load Current with

1 x 1000kW TG AFC Motor Starting

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5. FUTURE WORK _

The next stage of this project involves investigating the ability to source the components

required for the increase in voltage to 6600V. The process for gaining approval also needs to

be more completely investigated and contact will be made with the department of mineral

resources about this process.

Other issues that have been identified and will need to be addressed include;

• Step potential/touch potential limiting

• Earth fault spark ignition

• Fault current energy

• Size of equipment to cater for additional creepage/clearance

• Corona discharge

• Effect of EMI on control systems

INVESTIGATION INTO THE DESIGN OF A 6600V LONGWALL MINING ...

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