Mining Power Systems – characteristics and issues
Zoran Božić
PhD, MIEAust, CPEng
Western Power
EAust Auditorium Perth
7 March 2012
Approved 10/11/2011,
effective 23/12/2011
1 December 2012
DM# 8864392
DM # 9130034
The Picture - Super Pit in Kalgoorlie, Western Australia (WA)
The purpose of the presentation today is to talk about mining power
systems.
We will see what makes them so special and what we need to know about
them.
That body of knowledge often includes a mix of mechanical and electrical
engineering issues, as reflected in the name of the society.
This presentation is largely based on the author’s experience.
1
Table of content
Mining operations:
–
Characteristics
–
Need to know
–
Large machines
–
Effects on power system
Will it work ?
Power system studies
required.
2
This is how the presentation is organised.
2
Power systems of the world
3
The Picture - Google Earth night shot Concentrated in Eastern USA,
Western Europe and Japan, but watch China, India and Brasil. Australia is a
small player, especially WA.
Those few lights in the desert are mining power systems.
It all started 120 years ago from a small mine in Colorado…
The start.
In late 1800s, mines at high altitudes were using steam for their operations.
Those above the timber line, after using up all their local timber, had to bring
coal by mule train.
As the surface seams became exhausted, the miners had to dig deeper and
needed even more power for pumping, winching and working the deeper
seams.
In 1890, one of these was the King Gold Mine in Telluride, Colorado.
The cost of shipping coal by mules ($50 per tonne) was pushing them
towards bankruptcy.
Nearby San Miguel river was a ‘free energy source’. The problem was that
the river was 4km away from the mine, too far for the Edison’s DC system.
3
Power systems of the world
4
Continued ..
They were lucky.
Brother of the company’s lawyer was an engineer who attended Tesla’s 1888
AIEE (now IEEE) presentation in New York on AC power systems.
So they went to Tesla and Westinghouse to ask to supply them with the first
mining Tesla Electricity Supply System.
The contract was for a 1.8m water-wheel generator driving a 100 horsepower
generator that made electricity at 3,000 Volts AC and at 133Hz.
One 4km power line transmitted electricity to the mine, driving a 100
horsepower motor in the King Gold Mine.
In April 1891, the system was successfully commissioned and commenced
commercial operation.
Two years later, in 1893 Tesla’s polyphase alternating current was chosen
for Niagara Falls power plant on the basis of its undeniable merits – birth to
the great power systems of the world – that system stretched for just 35km
and supplied the nearby town of Bufallo by one 11kV line.
Since then, the world has embraced rapid electrification and electricity has
reached the furthest corners of our planet. Heavy power lines became a
synonym for progress of our civilisation…
4
First mining power system 1891 Telluride, Colorado
•
Ames hydro power plant, San Miguel river
•
100hp (75kW), 133Hz generator driven by a 1.8 m water-wheel turbine
•
4km of 3kV line supplied electricity to King Gold Mine, driving one
100hp (75kW) motor.
5
You will see that key characteristics of this power system are also typical
for modern mining power systems …
…major loads and generators of comparable size.
5
Mining operations – overview (1)
– Characterized by large machines
– Largest machines for excavating,
overburden removal, ore transport and
crushing: draglines, shovels, winders,
conveyers and mills
– Local ore processing may require pumps
and compressors of relatively large
ratings
6
These could be summarized as follows …
6
Mining operations – overview (2)
•
Providing a supply of electricity to large mining
machines is always challenging
•
These challenges are mainly due to their load cycle and
are compounded by the relatively small, geographically
separate electricity supply systems
•
Dynamic interaction between generators and motors
requires knowledge of the limits of generators’ capability
•
Remote location of mining operations is challenging,
even if the supply can be provided from the grid via a
high voltage interconnection
7
And that …
7
Mining operations - characteristics (1)
1. Relatively small number of very large loads –
mining machines
2. Largest machines generally electrically powered
3. Very small % of general loads (camp, canteen,
administration, township) in the total load
4. No ‘natural’ load growth
5. Step load changes due to mine expansions
6. Remote locations, impact of climate and wildlife
8
Mining operations can be characterized by …
Impact of climate - Cyclone proof, flood proof and earthquake proof
Impact of wildlife - Termite proof cables vermin proof switchgear and bird
proof overhead lines...
For example, termites (white ants), are of key concern in the Pilbara
region of WA as they can eat through ordinary cable insulation. It is
interesting that their small teeth cannot eat through brand new smooth and
shiny cable sheath, however as soon as the smallest scratch appears,
either during laying or cable operation in harsh mine environment, they
can penetrate inside...
Of particular concern are trailing cables (‘electric ropes’) which power
mobile equipment and are regularly exposed to damage due to movement
over rough surfaces or by heavy vehicles passing over them ...
There is a need to specify a brass layer in the cable sheath design,
because termites cannot eat through the brass…
A similar specification for termite proofing may be required for concrete
cable conduits, trenches and other concrete works...
In cold climates, control rooms are sometimes snake proofed, as heat
dissipated by power cables attracts them.
8
Mining operations - characteristics (2)
7. 24 hours operation – ‘flat’ load profile
8. Load swings of largest machines
(shovels, draglines, winders, …) important
9. Machines often regenerate in one part of their
duty cycle
10.Large % of motor loads
11.Motor fault current contribution significant
12.Large % of motor loads connected via power
converters (VV & VVVF drives)
9
VV - Variable Voltage
VVVF - Variable Voltage and Variable Frequency
9
Mining operations - characteristics (3)
13.Harmonics current injection significant
14.Load swings of active (MW) and reactive
(MVAr) power during one duty cycle or motor
starting are large relative to the capacity of the
supply power system
15.Additional stress on local generators if they are:
– located in isolated power systems
– of different sizes and characteristics
– there is no or little intentional spinning reserve
10
10
Mining operations - knowledge (1)
1. Knowledge of the largest mining machines
(driven by more than one motor):
– Technical data sheet
– Duty cycle of operation – individual motions and total
MW & MVAr consumption versus time
– On-board reactive power compensation
– Any regeneration capability
– Impact of the operator’s experience (if any)
– Alarms for individual motions (if any)
– Protection trip settings (thermal)
11
11
Mining operations - knowledge (2)
2. Knowledge of the fleet of largest machines
3. Knowledge of large mining machines driven by
a single motor:
– Technical data sheet
– Driven load
– Motor starting method
– Any regeneration capability
– Any harmonics injection
12
12
Mining operations - knowledge (3)
4. Any synchronous motors
5. Any induction motors with wound rotor
6. Knowledge of motors fed via power converters
– Technical data sheet
– Pulse number (6, 12, 18 or 24) and harmonics
current injection
– Any regeneration capability
7. Knowledge of the largest motor that starts DOL
13
8. Dynamic models of motor loads often required
DOL - Direct On Line
13
Mining operations - knowledge (4)
8. Impedance single line diagram & load data
9. Knowledge of the power system protection
10.Knowledge of any local generation and/or
power supply system
11.Knowledge of the limits of operation for all local
generators
14
14
Mining machines – dragline (1)
Bucket size 30-60m3, maximum 168m3
Boom size 45-100m
Four motions:
hoist, drag, swing & propel
In 1970s, the fleet of seven
at Peak Downs Mine, QLD
could black the North
Queensland power system
(anecdotal evidence)
Similar anecdotal evidence for a fleet of
15
shovels at Newman mine in WA in 1980s
Draglines are most efficient for excavation below the base.
15
Mining machines – dragline (2)
99m3 dragline at Moura, QLD had a potential for
load swing of 40 – 45 MW
16
Electrical signature of the dragline is shown on the right.
One can see the amount of power regenerated and fed back into the grid
equals peak load consumption, though the duration is much shorter.
16
Mining machines – shovel (1)
Bucket size 8-80m3,
maximum 138m3
Four motions – hoist,
crowd, swing & propel
30s typical duty cycle
Typical P&H4100 data:
Running load (1+j0.5)MVA
Peak load (3.1+j1.3)MVA
or
(2.8+j0.75)MVA
17
Shovel is the next largest mining machine.
There is a motor for each motion …
There is on-board reactive power compensation – typically a few thyristor
controlled capacitor banks.
These load models are based on the manufacturer’s data.
The peak load consumption can exceed 4.6MW for less than a second.
It occurs during digging when crowd, hoist and swing motors are simultaneously
used.
The actual power drawn depends on the experience of operators and how ‘softly’
he or she operates the machine.
Visual and sound alarm is activated when the motor (for the particular motion)
stalls for about 30 seconds, before the thermal protection cuts power…
17
Mining machines – shovel (2)
Recorded data for
operation of a fleet of
six shovels
‘Sinusoidal’ swings:
9MW, 25s period
5MVAr, 10s period
Note that the fleet has a
potential for much larger
load swings –
importance of skilled
operators
18
Recorded electrical signature for the fleet during normal operation.
18
Mining machines – shovel (3)
Typical shovel motor
starting profile:
2MW for 8s
3MVAr for 8s
19
Recorded electrical signature during starting of a shovel.
Typically occurs at the beginning of each shift.
19
Mining machines – winder (1)
Winder load characteristic – MW, MVAr & MVA vs time
20
Winders in underground mines...
Calculated duty cycle for a winder.
We see the periods of rest, rapid acceleration and reduction in demand
once the maximum speed is reached.
The small consumption spike in the middle (between 5 and 6) is due to the
air-pressure surge when two cages meet half-way in the shaft.
Sudden application of electric breaking (between 6 to 7) results in the
short interval of regeneration before the cages stop....
20
Mining machines – crushers & mills
A typical circuit in a mine may consist of:
• Primary crusher
• SAG mill
• Ball mill
• Rod mill
• …
Conveyers often used for transport …
21
Primary crushing reduces large pieces to a size which can be handled by
downstream machinery.
Individual electrical signatures are different for each machine.
Motor starting is the largest disturbance these machines present to the
power system.
The starting current depends on the motor starting method…
Unloading motor during starting will not reduce the starting current.
During normal operation, their electrical consumption is relatively constant …
This is quite important,
… except that too big a rock may stall the primary crusher motor or stop the
conveyer belt ..
21
Effects on power system (1)
Effects on power system voltages due to:
1. Cyclic fluctuations (flickers) due to the load cycle of
draglines, shovels or winders (a fleet of)
2. Power oscillations between synchronous machines
(generators and motors)
3. Motor starting transients
4. Resonance due to harmonics current injection & PQ
5. Overvoltages due to partial load rejection
6. Voltage collapse due to massive motor stalling
7. Motor stalling in one operation upon other operations
22
22
Effects on power system (2)
Effects on power stations due to:
1. Variation in boiler drum water level due to rapid swings of
the MW load
23
Fluctuating mining loads cause variations in boiler drum water level.
Opening of steam valves, in response to increased MW demand, lowers
the steam pressure and boiler water level rises.
When the electrical demand falls, steam valves close and water level falls.
Steam generators designed for base load applications typically have small
boilers which may not accommodate these variations, resulting in
generator trips. That was reported in Queensland in 1970s.
A quick-fix solution was to limit the MW swings by limiting power draw of
draglines to 70% of the maximum (hoist and swing motors).
That had effect of marginally slower operation of draglines (longer duty
cycle).
A longer-term solution is to inform the steam turbine manufacturer of the
expected load duty cycle, so that they can design the boiler of sufficient
size (marginally larger than the standard size) to accommodate the
fluctuating load (boiler water level variations) without tripping.
23
Effects on power system (3)
Effects on power stations due to:
2. Sharing of MW swing loads between sets
(identical & different makes of machines)
3. De-rating of MW unit output due to active governing
4. Sharing of MVAr swing loads between sets
24
Generators differ in how quickly they pick-up the load. The slowest are
hydro generators, whereas gas turbines and diesel sets are the fastest.
Even when the generators are of the same type, they will not equally share
load changes between them, because of different wear and tear of the
governing system and fuel racks. Some units will pick-up faster than others.
So, apart from regularly maintaining and adjusting turbine governors, some
form of supplementary control must be implemented to ensure proportional
loading of individual generators over time – a power station controller or its
power system equivalent - AGC (automatic generation control)
The rate of governor response was overlooked in the mine expansion of the
Kununurra power system in the North of WA, resulting in about 150kW of
load transfer from hydro generators to gas turbines during each load cycle.
This quickly de-loaded hydro power station…
A SCADA solution was implemented to overcome the problem, by sending a
signal to the gas turbine at the beginning of each load cycle to reduce its
output by 150kW.
Generators which are supplying fluctuating load, cannot be fully loaded to
their maximum output, hence the de-rating.
Similar considerations apply to sharing of reactive (MVAr) load swings,
because some excitation systems are faster than others. This is also known
as the quadrature droop (Q-droop).
24
Effects on power system (4)
Effects on power stations due to:
5. System frequency variations
6. No or limited intentional spinning reserve
7. Pro-active under frequency load shedding
If supplied via a transmission line:
1. Voltage stability
2. Flickers & other PQ issues
3. Generators trip if load loss exceeds the spinning reserve
25
Any mismatch between the electricity consumed and generated results in
frequency variations.
Large load fluctuations during normal operation of draglines and shovels may
create relatively large regular frequency variations. This occurs because some
generators cannot match their rapid load fluctuations.
Generally, there is no or limited intentional spinning reserve in predominantly
mining power systems.
A consequence is the need for rapid load shedding following sudden trip of any
generator.
A rapid pro-active under frequency load shedding (UFLS) scheme is
implemented in the Pilbara region in the North-West of WA, part of the NWIS
(North West Interconnected System). It monitors generator circuit breakers for trip
and sends the UFLS trip signal to designated loads (before the frequency
changes). The conventional (reactive) UFLS waits for the frequency to fall first
however, it was found to be too slow for the NWIS.
Large mining operations can also impact relatively large power systems from
which they are supplied, typically via a transmission line.
Typical issues are voltage stability, PQ, flickers and trips of generators.
Voltage stability and PQ typically require STATCOMS at the mine end.
Generators can also trip if load loss exceeds the spinning reserve. That
situation occurred in the SWIS (South West Interconnected System) in WA, which
is the largest power system in the state. It was proposed to supply few new mining
operations in the Mid West of WA via a 330kV transmission line. The aggregate
line load exceeded the overnight spinning reserve for load rejection, so the loss of
a line would trip generators, which is not acceptable. One solution is to increase
the spinning reserve. Another is to split the load and supply each half via a
separate 330kV line. Both solutions are costly…
25
Electric versus diesel-hydraulic
Draglines and shovels can be electric or dieselhydraulic powered.
1. Electric powering:
-
The most cost effective in long term
-
Large up-front investments
2. Drivers for diesel hydraulic powering
-
Shorter project pay-back periods
-
-
Impact of taxation/depreciation rules for mining
Rationing use of available electricity
26
This slide is about long-term changes in mining power systems.
Electric powering is the most cost effective in the long term.
However, large up-front capital investments required to construct power
stations, transmission lines and substations for electrical powering, generally
require long payback periods.
There are two principal drivers for diesel-hydraulic powering of the largest
mining machines: shorter project pay-back periods and need to ration use
of the available electricity (to defer capital expenditure in generation).
Taxation/depreciation rules can also be a driver for diesel-hydraulic powering
of draglines and shovels …
The need for rationing arises when the limited available electric power
constrains intended mine expansion.
One way of rationing use of electricity in the mining environment is
conversion from electric to diesel-hydraulic powering …
That effectively removes large fluctuating loads, which releases available
electricity to supply other loads.
The remaining electrical loads have relatively constant consumption during
normal operation, resulting in a much ‘flatter’ overall load diagram …
For example, in the past decade, a similar transformation was experienced in
the Pilbara region of WA, where only one electrical shovel remained in
operation, whereas the aggregate mining output of the region considerably
increased ….
26
Key issue
Will it work when put together ?
27
To answer that question, we do power system studies.
27
Power system studies
Typical power system studies for assessment:
1. Load flow
2. Fault studies
3. Harmonics studies
4. Dynamic studies
5. Special studies (for example, voltage level
selection for the supply line) (not discussed today)
6. Protection studies (not discussed today)
28
28
Load flow studies
Typical purpose of load flow studies:
1. Investigate whether the voltage profile of the
system is satisfactory under the most adverse
operating conditions (steady-state & flickers)
2. Assist in transformer specification
3. Investigate system performance during
emergencies
4. To propose an appropriate voltage profile for
system operation
29
Tapping range, tap step and the number of steps.
Is the standard tap changer adequate or a non-standard is required.
For example, during line outages …
29
Fault studies
Typical purpose of fault studies:
1. Selection of equipment ratings
(making and breaking duties and maximum earth
fault current)
2. Relay protection settings – the prospective
minimum and maximum fault currents
Regeneration & motor fault contributions considered
30
30
Harmonics studies
Typical purpose of harmonics studies:
1. Investigate whether the harmonics voltage
distortion is within the acceptable limits
2. Check for any harmonics resonances
31
The harmonics current injection data largely depends on the pulse number
of the equipment.
The pulse number is generally proportional to the equipment rating.
The actual harmonics injection is generally much higher than the
theoretical spectra, particularly for low cost equipment.
For example, a typical 6 pulse drive typically injects 25% (of its rating) as
the 5th harmonics.
31
Dynamic studies
In mining power systems, the purpose of dynamic
studies is to investigate dynamic performance of
the system under the following disturbances:
1. Large motor starting
2. Symmetrical short circuits
3. Partial loss of load or generation
4. Shovel/dragline load swings
5. Shovel/dragline stalling
6. Large deficiencies of generating capacity & UFLS design
32
It is essential to represent motor loads fully, as constant impedance
representation can lead to optimistic results.
Induction motors should be modelled with equivalent circuits taking into
consideration stator and rotor impedances at standstill and at running.
Torque versus speed characteristic of the driven load, as well as the motor
inertia are important components of the dynamic model...
32
Summary
•
First mining power system in 1891, Telluride, Colorado
•
Mining operations:
–
characteristics
–
knowledge/data required
–
largest mining machines
–
effects on power system
•
Key issue – will it work ?
•
Power system studies
33
For validity of many of these studies, it is essential to represent motor
loads correctly.
33
Any questions?
34
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