2. 1
CONTENTS
1) PRODUCTION AT THE NCHANGA SMELTER………………….2
2) POWER DISTRIBUTION…………………………………………...3
THE 11kV SYSTEM………………………………………………....3
THE 33kV SYSTEM………………………………………………....6
3) PREVENTIVE MAINTENANCE…………………………………....7
MONTHLY PM……………………………………………………....7
QUARTERLY PM……………………………………………………8
4) PREVENTIVE MAINTENANCE ON EARTH PITS………………..8
5) TYPES OF STARTING CONNECTIONS AT THE SMELTER…...10
STAR CONNECTION OF THE MOTOR WINDINGS…………....11
DELTA CONNECTION OF THE MOTOR WINDINGS………….11
6) ELECTRICAL IMPROVEMENTS AT THE SMELTER…………..13
7) TROUBLESHOOTING …………………………………………….15
REFERENCES
3. 2
PRODUCTION AT THE NCHANGA SMELTER
This is a very complex process in which copper, acid and cobalt alloy is
produced, the concentrate comes from a lot of different places namely the
underground and other sources given the current situation where the
underground section has been put on care and maintenance meaning no
mining is going there.
The process starts by sending the concentrate via a very efficient
conveyer system to the steam dryer where the concentrate is dried by the
use of steam, after the drying process the concentrate is later transferred
to the flash smelting furnace (FSF) where the process of smelting starts
from by the use of oxygen from the oxygen plant together with other
fuels necessary for flash smelting. Three main resultants come from the
FSF namely blister, sulphur dioxide and FSF slag. The blister is later sent
to the anode furnaces for the production of copper anodes which are later
sent to the refinery for the production of copper cathodes generally
known as copper. The sulphur dioxide is transferred to the acid plant for
the production of sulphuric acid and the slag is later processed for the
extraction of colbalt alloy in the cobalt recovery furnaces and the
remaining blister is transferred to the anode furnances while the waste is
later passed on as granulated slag. The flow sheet below illustrates the
full process at the nchanga smelter
4. 3
POWER DISTRIBUTION
An electric power distribution system is the final stage in the delivery of
electric power; it carries electricity from the transmission system to
individual consumers. In this situation the final stage of transmission is
when the electricity reaches the smelter where it is later stepped down
and distributed to other substations for use. The smelter has 5 main
substations namely substation 1(SS1), (SS2), (SS3),(SS4) and (SS6). The
lack of SS5 was due to planning issues when the smelter was being
constructed given the smelter is a modern day plant that was completed
and commissioned in 2008. All the power comes from ZESCO the
biggest energy company in Zambia, later the power is regulated by the
Copperbelt Energy Cooperation (CEC) inorder to give a more stable form
of electricity to KCM. CEC have 2 substations around KCM namely
Avenue and Stadium. The Avenue Substation steps down 66/11kV and
distributes it to SS1 where it is later distributed to other substations
THE 11kV SYSTEM
11kV incomers from Avenue to SS1
The Avenue substation steps down 66/11kV using two 20MVA and
three 15MVA thus feeding SS1 through 4 incomers namely I/C1, I/C2,
5. 4
I/C4 and I/C5. I/C3 has been rerouted to TLP, thus leaving SS1 with 4
incomers . SS1 has BUS A,B,C,D,E and F with BUS E and F having been
equipped with all the emergency loads incase of a power failure. Each
substation has two transformers, a main transformer and a back up
transformer that kicks in whenever there is a power failure. The back up
transformer gets its power from the Diesel Generators (DG) which
becomes operational 3-4 secs when the power from CEC has a fault.
Basically when power is limited in I/C4 and I/C5 a signal is sent to the
main control system that authorizes the opening of B/C 4 and the closing
of bus section B/C5 that’s after 2 or more DGs are operational thus
supplying power to the loads that are on bus E and F which mainly are
used for cooling and lighting. This is because the DGs can only produce
up to 8MW when running at full capacity thus can not sustain the full
load of the smelter in turn can only manage to run a few loads, the critical
loads
Switching on and off of equipment in the 11kV is done by using a
vacuum circuit breaker which can be seen in the picture below
VACUUM CIRUIT BREAKER
6. 5
11kV BUS E
The above figure illustrates emergency bus E with bus section B/C4 and
B/C5. It also shows incomer 4 (IC4) and the loads on bus E
The figure below shows emergency bus F where a capacitor bank, loads
on bus F and all the emergency transformers namely 12,14,17,19,21 and
23 are located. All these emergency transformers get their power from the
diesel generators (DGs) when there is a power failure. Power failures are
categorized in numbers, the worst of them is known has category 3 (cat
3). A category 3 power failure is when there is a nation wide blackout.
That’s when the DGs kick in automatically like explained earlier. When
there is insufficient power in I/C4 and I/C5 a signal is relayed to the main
control panel which authorizes the DGs to start running inorder to
provide power to the loads that need power at all times. The main control
panel is always on thanks to an effective UPS system
8. 7
The 33kV system gets its power from the CEC Substation called
stadium where power is stepped down 66/33kV with a current rating of
1250A and fed to substation 2 SS2 at the smelter via I/C1 and I/C2 which
is equipped with sulfur hexafluoride circuit breakers. There onwards
power is fed to the electric furnaces transformers namely Slag cleaning
furnaces (SCF) and Cobalt recovery furnace (CRF) transformers. The
CRF furnaces are on the 33kV bus A system and the SCF on bus B. The
electric furnaces use transformers that step down the voltage from
33kV/0.32kV using two 15 MVA transformers for the Cobalt recovery
furnace and one 20MVA transformer for the slag cleaning furnace. The
transformers step down 33kV/0.32kV producing up to 46kAmps. The
current increases substantially in-turn using the current heating effect for
slag cleaning and cobalt recovering. The 33kV system is also used to
supply power to some auxiliary equipment, mainly pumps, cooling fans
etc around the electric furnaces. Their motor control centre (mcc) and
(pmcc) power motor control centre for bigger equipment are located in
respectable substation
PREVENTIVE MAINTENANCE
Preventive maintenance in KCM is a procedure carried regularly to
ensure equipment around the plant stays in tiptop condition. Maintenance
at the smelter electrical department can be broken down in 2 parts,
namely quarterly pm and monthly pm. Monthly pm is carried out once a
month and quarterly pm is carried out every after a 3 month period hence
the name quarterly.
MONTHLY PM
Mainly there is checklist with procedures that need to be followed for
example, the characteristics of a monthly pm on a motor rated 185kWatts,
RPM 1475 on the motor side there is need check motor and local
pushbutton station cleanliness, check for slip ring cleanliness, check for
carbon brush worn out, check for power & control cables cerawool
covering, check for motor TB & JB sealing properly, check for bed bolt
tightness and earthing cable tightness, observe for smooth run/no noise.
On the panel side not much work is done, mainly check for the
feeder/module cleanliness, check for spare gland holes sealed properly,
observe for any burnt out/Terminal colour change, check for SFU handle
operation and check for indication lamps (ON, OFF, TRIP, etc)
9. 8
QUARTERLY PM
The quarterly checklist has everything that can be found on the monthly
pm checklist and more tasking procedures that cant be done everytime
you carry out preventive maintenance. Example of a quarterly pm
checklist on a motor air compressor (MAC) B filter fan motor in the
oxygen plant is as follows, the first thing that can be done on the motor
side is checking for power cable tightness, earthing cable tightness, check
for power cable tightness at jbs , check for motor space heater cable
tightness. So basically what needs to be done is to check for tightness on
all cables on the compressor. Also check motor and local pushbutton
station cleanliness, check for slip ring cleanliness, check for carbon brush
worn out, check for power & control cables cerawool covering, check for
motor TB & JB sealing properly, check for bed bolt tightness
On the panel side check for power cable tightness, control cable
tightness, check for main contactor fixed contactor tightness, check for
moving contact spring tension, check for main & moving contacts
cleanliness. There is also need to check for feeder/module cleanliness and
observe for any burnt / terminal colour change, check for SFU handle
operation and also check for indication lamps (ON,OFF TRIP etc)
These are the tests that need to be carried out during the quarterly pm
schedule. The most vital of all the procedures is checking for Insulation
Resistance Test that is carried out using an insulation resistance tester
commonly known as the megger. The first step is to carry out an IR test
with the power cable and motor together Red phase-Earth, Yellow phase-
Earth and Blue phase-Earth. If the insulation is ok the megger will show
give an infinite reading. The same procedure is repeated with the control
cable and the motor without the control cable.
These are examples of the procedures set up by KCM to ensure all the
equipment is tip top condition at all time
PREVENTIVE MAINTENANCE ON EARTH PITS
The main reason for doing earthing in electrical networks is for safety.
When all metallic parts in electrical equipment are grounded then if the
insulation inside the equipment fails there are no dangerous voltages
present in the equipment case. What makes earthing work are earth pits
below shows an illustration of an earth pit
10. 9
EARTH PIT
Preventive maintenance is carried out on earth pits by the help of an
instrument known as an earth tester that checks the resistance of the soil.
The earth resistance test is done by using a 3 point system test In this
method earth tester terminal P1 and P2 are shorted to each other and
connected to the earth electrode (pipe) under test. Terminals E1 and E2
are connected to the two separate spikes driven in earth. These two
spikes are kept in same line at the distance of about 25 meters due to
which there will not be mutual interference in the field of individual
spikes. After the tests are carried out the resistivity should be at its
minimum so that the soil should be able to conduct any over current for
safety purposes, it is the resistance of soil to the passage of electric
current. The earth resistance value (ohmic value) of an earth pit depends
on soil resistivity. It is the resistance of the soil to the passage of electric
current. To reduce the resistivity, it is necessary to dissolve in the
moisture particle in the Soil. Some substance like Salt/Charcoal is highly
conductive in water solution but the additive substance would reduce the
resistivity of the soil, only when it is dissolved in the moisture in the soil
11. 10
after that additional quantity does not serve the Purpose. And make sure
all Oxidation is removed from joints and joints should be tightened
EARTH RESISTANCE TESTER
TYPES OF STARTING CONNECTIONS AT THE SMELTER
The objective of star-delta connection is this starting method is used
with three-phase induction motors, is to reduce the starting current. In
starting position, current supply to the stator windings is connected in star
(Y) for starting. In the running position, current supply is reconnected to
the windings in delta (∆) once the motor has gained speed.
As the name suggests, direct-on-line starting means that the motor is
started by connecting it directly to the supply at rated voltage. Direct-on-
line starting, (DOL), is suitable for stable supplies and mechanically stiff
and well dimensioned shaft systems – and pumps qualify as examples of
such systems. Direct-on-line (DOL) starting motors only require adequate
12. 11
motor protection, whereas star-delta starting implies a more complex
circuitry composed of several relays and contactors.
The following two images demonstrate the different possibilities for
electrical connection:
The following images show the electrical circuitry of the three motor
windings and their phase voltages
STAR CONNECTION OF THE MOTOR WINDINGS
13. 12
DELTA CONNECTION OF THE MOTOR WINDINGS
As shown in the above illustrations, the difference in connections is not
just limited to the different circuitries, but also in the resulting voltages of
the windings. Where as in the DOL connection the windings are supplied
by the voltage they are designed for permanently, the STAR connection
is operated at a voltage reduced by the factor √3. Following principles
result from this, which have to be respected by all means for a continuous
operation of a STAR-DELTA motor
The motor-current (Amps) of a STAR-DELTA motor during start (IҮ) is
reduced roughly by factor 0,58; a motor with a nominal starting current of
550A will consume only 317 A in STAR – connection. This fulfills the
requirement of a current reduced start. The available mechanical
performance on the shaft to drive the pump or whatever equipment
is being run, is also reduced to 1/3 of the nominal performance. To
avoid an overloading of the motor, it is necessary to observe the
power demand of the equipment during startup in comparison with
the start current of the motor.
The available shaft torque (MҮ) also reduces while start to 1/3 of the
nominal torque. This also raises the necessity, to compare the demanded
starting torque of the equipment to the available torque of the motor, and
to do the right selection.
14. 13
THE ADVANTAGES OF STAR DELTA CONNECTION
Normally, low-voltage motors over 3 kW will be dimensioned to run at
either 400 V in delta (∆) connection or at 690 V in star (Y) connection.
The flexibility provided by this design can also be used to start the motor
with a lower voltage. Star-delta connections give a low starting current of
only about one third of that found with direct-on-line starting.
Star-delta starters are particularly suited for high inertias, where the load
are initiated after full load speed.
ADVANTAGES OF DOL
DOL starting is the simplest, cheapest and most common starting
method. Furthermore it actually gives the lowest temperature rise within
the motor during start up of all the starting methods.
ELECTRICAL IMPROVEMENTS AT THE SMELTER
At the smelter the electrical department is slowly moving over to a
more complex and efficient way of starting equipment by the introduction
of a soft start device. This device ensures smooth starting by torque
control for gradual acceleration of the drive system thus preventing jerks
and extending the life of mechanical components. It also helps in the
reduction of the starting current to achieve break-away, and to hold back
the current during acceleration, to prevent mechanical, electrical, thermal
weakening of the electrical equipment such as motors, cables,
transformers & switch gear.
A motor soft starter is a device used with AC electrical motors to
temporarily reduce the load and torque in the power train and electric
current surge of the motor during start-up. This reduces the mechanical
stress on the motor and shaft, as well as the electro dynamic stresses on
the attached power cables and electrical distribution network, extending
the lifespan of the system.
It can consist of mechanical or electrical devices, or a combination of
both. Mechanical soft starters include clutches and several types of
couplings using a fluid, magnetic forces, or steel shot to transmit torque,
similar to other forms of torque limiter. Electrical soft starters can be any
control system that reduces the torque by temporarily reducing the
voltage or current input, or a device that temporarily alters how the motor
is connected in the electric circuit.
Soft starters can be set up to the requirements of the individual
application. In pump applications, a soft start can avoid pressure surges.
Conveyor belt systems can be smoothly started, avoiding jerk and stress
on drive components. Fans or other systems with belt drives can be
started slowly to avoid belt slipping.. In all systems, a soft start limits the
15. 14
inrush current and so improves stability of the power supply and reduces
transient voltage drops that may affect other loads. Like seen at the
smelter acid plant, a blower of rating 1850kW using a soft starter system
to enable the blower starts smoothly avoiding any pressure surges. The
pictures below show examples of a soft starter for the 4650kW blower
found at the acid plant. The soft starter is located inside of Substation 6
(SS6)
SOFT STARTER FOR 4650KW BLOWER
16. 15
4650 kW BLOWER AT THE ACID PLANT
TROUBLESHOOTING
Troubleshooting in electrical engineering means tracing, finding and
rectifying a fault in an electrical system. In a complex electrical system at
the smelter it is inevitable not to have problems. We face enough
challenges but we have enough competent people to tackle any problem
that can come alone the way. One of the most common troubleshooting
techniques taught to technicians is the so-called “divide and conquer”
method, whereby the system or signal path is divided into halves with
each measurement, until the location of the fault is pinpointed. However,
there are some situations where it might actually save time to perform
measurements in a linear progression (from one end to the other, until the
power or signal is lost). Efficient troubleshooters never limit themselves
to a rigid method if other methods are more efficient. Here is an example
of troubleshooting encountered at the smelter by the lighting crew. An
electrician is troubleshooting a faulty light circuit, where the power
source and light bulb are far removed from one another
17. 16
As you can see in the diagram, there are several terminal blocks (“TB”)
through which electrical power is routed to the light bulb. These terminal
blocks provide convenient connection points to join wires together,
enabling sections of wire to be removed and replaced if necessary,
without removing and replacing all the wiring.
The electrician is using a voltmeter to check for the presence of voltage
between pairs of terminals in the circuit. The terminal blocks are located
too far apart to allow for voltage checks between blocks (say, between
one connection in TB2 and another connection in TB3). The voltmeter’s
test leads are only long enough to check for voltage between pairs of
connections at each terminal block.
In the next diagram, you can see the electrician’s voltage checks, in the
sequence that they were taken
18. 17
Based on the voltage indications shown, you can determine the location
of the circuit fault. The fault is located somewhere between TB3 and
TB4. Whether or not the electrician’s sequence was the most efficient
depends on two factors not given in the problem. The distance between
terminal blocks and the time required to gain access for a voltage check,
upon reaching the terminal block location.
Another of problem encountered at the smelter that needed
troubleshooting was with SCF bottom cooling fans. The problem was that
the earth leakage relay kept showing signals of an earth leakage in turn
resulting in the bottom cooling fan going off. So we had to troubleshoot
by carrying out a continuity test. A continuity test is a test carried out to
determine weather they are any cables touching thus a complete pathway
for electric current. So we had to start from the power source to the VFD
valuable frequency drive. The circuit showed continuity, we later tried to
carry out continuity from the VFD to the motor, the circuit showed
continuity. The diagnosis was that the earth leakage relay malfunctioned
and that it had to be replaced. The image below shows an example of
valuable frequency drive that can be found at the smelter
VALUABLE FREQUENCY DRIVE
