SWITCHGEAR

ZZIMULA MARVIN U2206106

26TH FEB – 19TH MARCH 2023

DECLARATION

I, ZZIMULA MARVIN, hereby solemnly and sincerely declare that the information presented
in this report is an original work done by me as a result of training I underwent in Switchgear
department. However, several sources have been referred to ensure the correctness of the
information contained herein.

ZZIMULA MARVIN
U2206106
Graduate Trainee Engineer
Signature:………………………….. Date: ………………………………

MR. WANDIRA FRED

Principal Engineer Substations
Signature: ……………………………... Date: ……………………………….

i
ACKNOWLEDGEMENT

I would like to express my deepest appreciation to Mr. Wandira Fred and Mr. Magembe
Charles for their wonderful collaboration and guidance throughout the training period. Their
guidance in the substation department enhanced my knowledge about the various substation
assets on the network.

In the same spirit, I recognize the efforts of Mr. Kalule Edward, Mr. Kaweesi Derrick, Mr.
Ndawula Joseph, Mr. Sebagala Titus, Mr. Odeke Nathan, and Mr. Lutaaya Ronald for the
support and guidance during my training.

I would like to profusely thank the entire substation team who had a hand in ensuring that the
training was successful.

ii
CONTENTS

1 INTRODUCTION TO SWITCHGEAR ........................................................................ 1

1.1 Categories of maintenance practices ........................................................................... 1

1.2 Uses of Switchgear equipment .................................................................................... 1
1.3 Specifications of switchgears ...................................................................................... 2
1.4 Classification of Switchgear........................................................................................ 2
1.5 The different switching devices and their functionality .............................................. 2
1.6 Types of switchgear .................................................................................................... 3
1.7 Switchgear makes on the Umeme network. ................................................................ 4
1.8 Operation of a switchgear ........................................................................................... 4
1.9 Activities carried out during the maintenance of switchgear. ..................................... 7
1.10 Power flow in a switchgear at a distribution substation .............................................. 8
2 PRACTICAL WORKDONE IN SWITCHGEAR...................................................... 10

2.1 Maintenance of RMUs for Kitante 1 11kV feeder from Lugogo substation ............ 11
2.2 Routine Switchgear maintenance of the 33kV and 11kV buses at Portbell 33/11kV
Substation ............................................................................................................................. 13
2.3 Routine Switchgear maintenance of the 33kV bus at Mutundwe UETCL 132/33/11kV
substation ............................................................................................................................. 16
2.4 Routine Switchgear maintenance on the 11kV bus at Kawanda Umeme 33/11kV
substation ............................................................................................................................. 18
2.5 Replacement of vacuum interrupters for Queensway Bay circuit breaker at Kisugu
33/11kV substation .............................................................................................................. 19
3 CHALLENGES AND RECOMMENDATIONS ........................................................ 25

3.1 Challenges ................................................................................................................. 25

3.2 Recommendations ..................................................................................................... 25

iii
ACRONYMS

CT Current Transformer

LV Low Voltage

MCB Miniature Circuit Breaker

MV Medium Voltage

ORHVS Operating Regulations for High Voltage Systems

RMU Ring Main Unit

RMU Ring Main Unit

UETCL Uganda Electricity Transmission Company Limited

VT Voltage Transformer

iv
List of figures

Figure 1:1 Siemens, LUCY RMU, Fuji ..................................................................................... 4

Figure 1:2 Vacuum circuit breaker ............................................................................................ 5
Figure 1:3 Left: Current transformers Right: Voltage transformers. ......................................... 5
Figure 1:4 Operating an RMU during maintenance. ................................................................. 6
Figure 1:5 Layout of Kisugu 33/11kV Substation ..................................................................... 9
Figure 2:1 Left: Worker's register Right: Work permit. .......................................................... 12
Figure 2:2 Left: Cleaning outside of the RMU Right: Striker pin fuses. ................................. 12
Figure 2:3 Left: Checking oil level Right: Operating the LUCY type RMU .......................... 12
Figure 2:4 Ring Main Unit preventive maintenance checklist ................................................ 13
Figure 2:5 Prohibitory signages on switchgears under maintenance ....................................... 15
Figure 2:6 Left: Workers Register Right: Work Permit .......................................................... 15
Figure 2:7 Right: 33kV Circuit Breaker Left: 11kV Circuit Breaker ...................................... 16
Figure 2:8 Cleaning the 11kV switchgear HV compartment................................................... 16
Figure 2:9 Refilling of SF6 gas of the red bus bar, Checking for gas leakage. ....................... 18
Figure 2:10 Blowing dust from the CBs, Cleaning the switchgear HV compartment. ........... 19
Figure 2:11 Burnt Vacuum interrupters. .................................................................................. 21
Figure 2:12 Replacement of burnt vacuum interrupters. ......................................................... 21
Figure 2:13 Insulation resistance test. ...................................................................................... 22
Figure 2:14 Contact Resistance test ......................................................................................... 24

List of tables

Table 2:1 Tools used during routine maintenance. .................................................................. 10

Table 2:2 Tools used during replacement. ............................................................................... 19
Table 2:3 Insulation Resistance test across open contacts ....................................................... 22
Table 2:4 Contact resistance results across closed contacts .................................................... 23
Table 2:5 Contact resistance results across open contacts. ...................................................... 24

v
1 INTRODUCTION TO SWITCHGEAR
Switchgear section plays an important role towards the business model of the company by
reducing Energy Not Served to customers through the various maintenance activities and
emergencies attended to. Most of the maintenance activities carried out by this section are
planned save for a few emergencies that occur on the network. They are responsible for the
following works on Medium Voltage Switchgear on the network.

• Routine maintenance activities.

• Emergency repairs.
• Supervision of contractors on Installation and upgrade of new Medium Voltage (MV)
switch gear.
• Ensure Safety of operators, public and equipment.

1.1 Categories of maintenance practices

Maintenance practices may be categorized into 3 types, and they include:

• Preventative maintenance: These are activities that are regularly done to equipment
to reduce their likelihood of failing. They are performed to the equipment while still
working so as it doesn’t break down unexpectedly.
• Periodic maintenance: These refer to activities that are carried out on a regular or
predetermined scheduled basis to maintain the condition of equipment. This type of
maintenance usually consists of data collection, visual inspection, cleaning, lubrication,
etc.
• Corrective maintenance: These are the activities that are carried out to identify, isolate
and rectify a fault or defect on failed equipment so to restore it to an operational
condition within acceptable limits.

1.2 Uses of Switchgear equipment

• Carry, make and break electrical current under normal and abnormal operating
conditions.
• To distribute and carry load including permissible overloads with minimum
interruptions.
• To de-energize equipment to allow work to be done.
• Allows remote and local operation in switching.
• Ensure quick and safe operation during fault operation to prevent extensive damage to
the equipment.

1
 • Isolate the damaged equipment on the network so as to ensure reliability of supply.
• Enhance system availability by allowing more than one source to feed one load.

1.3 Specifications of switchgears

Switching devices are used to close (make) and open (Break) electrical circuits. The following
are the common switchgear specifications and are paramount for switchgear selection.

a) The rated breaking capacity (rms value) (Icu): Is the maximum fault current a circuit
breaker can successfully interrupt without being damaged. They are high currents of extremely
low probability. In normal circumstances, the fault currents are considerably less than (Icu) of
the circuit breaker.

b) Making capacity (peak value): Is the maximum peak value of fault current that the breaker
can interrupt without any damage if the breaker is closed at fault.

c) Rated voltage (Ur): Is the maximum voltage that a device can operate at safely.

d) Lightning impulse withstand voltage (Up): Is the maximum abnormal voltage that the
circuit breaker can withstand when the voltage surges momentarily due to lightning.

e) Power frequency withstand voltage: It is the maximum rms value of voltage that the
equipment can withstand permanently. It defines the level of rms over-voltages that the
equipment may withstand for 1 minute.

1.4 Classification of Switchgear

Several different classifications of switchgear can be made according to the following:

• By the current rating.

• By voltage class i.e. low voltage, medium voltage, high and very high voltage.
• By insulating/arc quenching medium.
• By construction type i.e. indoor, outdoor, industrial, marine and utility.
• By operating method e.g. manually operated, stored energy operated etc.
• By type of current i.e. direct current and alternating current.
• By application i.e. distribution and transmission.
• By purpose.

1.5 The different switching devices and their functionality

a) Circuit breakers: Make and break all currents within the scope of their ratings, from small
inductive and capacitive load currents up to the full short circuit current, and this under all fault
conditions in the power supply system, such as earth faults, phase opposition and so on.

2
b) Switches: Switch currents up to their rated normal current and make on existing short
circuits (up to their rated short circuit making current).

c) Disconnectors(isolators): Used for no-load closing and opening operations. Their function
is to “isolate” downstream devices so they can be worked on.

d) Three position Disconnectors: Combine the functions of disconnecting/isolating and

earthing in one device. Three-position disconnectors are typical for gas insulated switchgear.
e) Switch-disconnectors (Load-break switches): The combination of a switch and a
disconnector, or a switch with isolating distance.

f) Contactors: Load breaking devices with a limited short-circuit making or breaking capacity.
They are used for high switching rates.

g) Earthing switches: To earth isolated circuits.

h) Make-proof earthing switches (earthing switches with making capacity): Are used for the
safe earthing of circuits, even if voltage is present, that is, also in the event that the circuit to
be earthed was accidentally not isolated.

i) Fuses: Consist of a fuse-base and a fuse-link. With the fuse-base, an isolating distance can
be established when the fuse-link is pulled out in the de-energized condition (link in a
disconnector). The fuse-link is used for one single breaking of a short-circuit current.

j) Surge arresters: To discharge voltage surges caused by lightning strikes (external over
voltages) or switching operations and earth faults (internal overvoltages). They protect the
connected equipment against impermissibly high voltages.

1.6 Types of switchgear

• Air Insulated Switchgear (AIS)–these simply create a separation (gap) between contacts
and ensure that no conduction takes place. They range from open air isolators to air
circuit breakers.
• Gas insulated switchgear (GIS) – These stretch the arc and rely on the high dielectric
strength and high cooling effect of the gas to quench the arc. The best gas used sulphur
hexafluoride (SF6).
• Vacuum insulated switch gear – these ensure minimal arc is generated since there is no
air between the contacts to ionize other than the contact material so at or near current
zero the arc is not hot enough to maintain conduction and current ceases.

3
1.7 Switchgear makes on the Umeme network.
These include Siemens, ABB, Fuji, Schneider, Cook and Fergusson, Gec Alstom, Joslyn,
Nulec, GVR/Whip & Bourne, Lucy RMUs among others.

Figure 1:1 Siemens, LUCY RMU, Fuji

1.8 Operation of a switchgear

The operation of switchgear may be grouped into 2 modes i.e. abnormal conditions and
intentional switching operations. All the modes are briefly highlighted in this section.

Current transformers (CTs), Potential transformers (PTs), and protective relays as were earlier
mentioned, form part of the switchgear and their functions in the operation of switchgear are
briefly explained below.

The CT is connected to the current carrying component for purposes of measuring the current.
The primary side of the CT is connected in series with the current carrying component while
the secondary side of the CT is connected to protective relay or/and to a meter. The current is
monitored to ensure that it is in the allowable/acceptable limits by the protective relay, which
is also connected to the circuit breaker.

The Potential transformers (PT) are used to reduce the high voltages to small voltages such the
system voltage may be monitored and measured easily. PTs are usually installed in parallel
with transformer feeders or/and at busbar coupler (busbar section) feeders so as to ensure that
the monitored system voltage is within the allowable and acceptable range.

Additionally, the CTs and PTs allow for currents and voltages respectively to be reduced to
values that can be handled safely by responsible personnel who may want to access the
switchgear.

4
 Figure 1:2 Vacuum circuit breaker

Figure 1:3 Left: Current transformers Right: Voltage transformers.

1.8.1 Operation of the switchgear during switching operations

Switching operations are usually carried out for purposes of carrying out maintenance of
different network installations. Switching allows for apparatus to be de-energized and earthed
to carry out work safely, while reverse-switching is done to allow for power to be restored after
maintenance is completed.

During switching, trip commands are sent to enable opening of contacts of the interrupting
device by pressing the ‘OPEN’ control button. The command may be initiated remotely by
control or it may be initiated locally by the authorized person.

5
Once the interrupting device (breaker) is opened, safety testing to ensure that the circuit breaker
is not live is carried out. Thereafter, isolation is done. Isolation is done to create a visible gap.
The isolation may be done using several methods, e.g. by racking out of a breaker and locking
it to its’ isolated position, opening of air break switches, opening of jumpers, etc.

After isolation, the earth switch is activated and put ‘ON’. This allows for the earth to be
applied on the cable side of the breaker to allow for any residual charges to be safely dissipated
to the ground. A close command is sent to enable closing of contacts of the breaker so as to
allow for complete earthing. This command is initiated by pressing the ‘CLOSE’ control
button.

After works, reversing is carried out, where the earth switch is first put ‘OFF’. The breaker is
checked to ensure that it is in the ‘OPEN’ position. The breaker is there after removed from
its’ isolated position and placed in its’ service position. This may be done by racking in the
breaker (interrupting device). This stage may be referred to as closing the isolator.

Figure 1:4 Operating an RMU during maintenance.

1.8.2 Operation of the switchgear during abnormal conditions

During abnormal conditions (e.g. short circuit & open circuit faults), the monitored current
doesn’t lie within the acceptable limits. Therefore, the relay sends ‘TRIP’/ ‘OPEN’ commands
to the circuit breaker.

6
When the circuit breaker (interrupting device) trips, its’ contacts are forced to open creating an
open circuit such that an arc is formed between the contacts. This arc is extinguished by an arc-
quenching medium in which the contacts are immersed.

When the fault has been resolved, a ‘CLOSE’ command is sent to the circuit breaker to enable
the action of closing of its’ contacts. The close command maybe sent by the protective relay
like in the case of auto-reclosers alternatively, it may be initiated by the authorized person. The
‘OPEN-CLOSE’ cycles are recorded via a counter reading which is attached to the relay.

1.9 Activities carried out during the maintenance of switchgear.

Switchgear maintenance works are usually carried out following a planned shutdown schedule.
Before works commence, a risk assessment is carried out to identify any risks that may be
interfaced during the required maintenance work, as well as determine preventative ways to
combat such risks.

A workers’ register is thereafter filled to show which workers shall be involved in the
maintenance works.

The equipment is open, isolated, earthed and safety tested to ensure that it is not energized as
a safety precaution before works commence, and therefore a permit is issued to declare that the
equipment can be worked upon safely.

Some of the maintenance activities that are carried out on the switchgear follow the procedure
briefly indicated include.

• Visual inspection of the equipment to understand the status in which the equipment is
operating in. This may include identifying the status in which the switchgear was found
in before switching was carried out i.e. whether it is open, or isolated or earthed before
maintenance works can commence. Additionally, the visual inspection of the
equipment may show any anomalies such as oil leakages for oil insulated switchgear.
• Data collection regarding the operational status of the equipment, e.g. counter reading
of the interrupting device, level of gas (SF6) insulated switchgear.
• Opening, isolating, earthing and safety testing of equipment to ensure equipment is
dead before it can be worked upon.
• Discharging of charged springs for motor-operated circuit breakers. This is done to
ensure that no harm is done to personnel who will be involved in the maintenance of
the breaker.

7
 • Cleaning of equipment (i.e. dusting equipment with cotton waste to unsettle
accumulated dust on equipment, blowing of the equipment using a blower to further
unsettle and removed dust from the equipment and use of dump cloth with soapy water
to clean the remaining dust)
• Spraying WD-40 on to equipment to stop squeaks, remove / loosen rusty parts on the
equipment, freeing sticky mechanisms (lubricating) and protecting equipment from
moisture.
• Lubrication of movable parts of the equipment using grease to reduce likelihood of
friction occurring within the equipment and thereby wear and tear, at times
malfunctioning in the operation of the equipment.
• Carrying out functional testing of switchgear operation to ensure operation indicators
and counter readings work effectively depending on the command operation issued to
the equipment and the ‘open-close’ cycles carried out.
• Noting any remarks or recommendations that should be carried out on the equipment
after satisfactory maintenance has been carried out.
• Returning equipment back to the operational status that it was found in, before
maintenance works were carried out.
• Reverse switching is done after maintenance works have been completed to restore the
equipment back into operation on the network.

1.10 Power flow in a switchgear at a distribution substation

Overhead lines that carry power into a substation are terminated via cable terminations, from
which cable connections leading to the switchgear. This is usually termed as the incomer. This
switchgear configuration comprises of a circuit breaker (which is connected between the cable
and the isolator), followed by an isolator which is placed between the breaker and the busbar.
In order to allow for the power-flow to reach the busbar, the circuit breaker and isolator should
be in the closed position. Once the power reaches the busbar, switchgear connections are one
end of an isolator is connected to the busbar while the other end is connected a circuit breaker
which has cable connections leading to the high voltage terminals of the power transformer at
the substation. For power to reach the transformer, the isolator and circuit breaker are in the
closed position. After power is stepped down by the transformer, cable connections are made
between the terminals on the low voltage side of the transformer, and a switchgear. This
switchgear configuration comprises of a circuit breaker which is connected between the cable
and the isolator, while the isolator is connected to the low voltage busbar. This is the busbar to

8
which outgoing feeders are connected. Feeder connections are made from the busbar through
switchgear. This switchgear configuration comprises of an isolator which is connected to the
busbar and a circuit breaker which is connected between the isolator and the outgoing feeder
cable. The outgoing feeder cable may be terminated at cable terminations from which jumper
connections are made to overhead lines. The flow of power in a switchgear at a substation is
shown in the figure below.

Figure 1:5 Layout of Kisugu 33/11kV Substation

9
2 PRACTICAL WORKDONE IN SWITCHGEAR
I was involved in a couple of routine maintenance activities which followed a planned
shutdown schedule that is always shared at the beginning of every month. Below are some of
the routine maintenance activities I witnessed while in switchgear department.

Table 2:1 Tools used during routine maintenance.

Tool Use
Flat spanners For tightening and loosening
of bolts and nuts

Screw drivers For tightening and loosening

of bolts and nuts

Cotton waste For dusting off RMU body

WD-40 For lubricating movable

parts and removing rust

Blower For removing dust

Grease For lubricating moving parts

Buckets For holding and carrying

water

10
 Towels For wiping and scrubbing
RMU outer surface

Detergent For removing dirt from the

RMU surface

2.1 Maintenance of RMUs for Kitante 1 11kV feeder from Lugogo substation
2.1.1 Procedure followed
• The switching operation was done by the district operations authorized person
according to ORHVS safety operating regulation to make RMU dead by opening and
isolating all incoming and outgoing feeders.
• Before proceeding with any work, the authorized person assigned a work permit to the
responsible person Mr. Odeke Nathan and it was signed by both parties. We filled and
signed in the worker’s register, did a risk assessment and work commenced.
• Checked the circuit breaker and isolators i.e. closing, tripping operation with respect to
the status indicator before earthing was done.
• Earthed the incoming, outgoing and transformer circuit breaker feeders at the RMU in
order to commence work.
• Opened circuit breaker chamber and pulled out the striker pin fuse carrier, ensured that
proper fuses are in place by replacing with proper fuse rating.
• Checked for oil level and oil status if carbonized.
• Opened cable box chamber and checked condition of cable terminations, checked if
cable earths were terminated on the RMU body.
• Cleaned off the dirt from terminations and tightened loose connections.
• Cleaned the RMU tank with detergent and water on the soil-stained parts all over the
tank.
• Checked and corrected positioning of the feeder labels, those missing were recorded
and planned on how to change them.
• After the works, all the workers were withdrawn, cancelled permit and workers register
and finally handover to authorized person to reverse the switching.

11
 Figure 2:1 Left: Worker's register Right: Work permit.

Figure 2:2 Left: Cleaning outside of the RMU Right: Striker pin fuses.

Figure 2:3 Left: Checking oil level Right: Operating the LUCY type RMU

12
 Figure 2:4 Ring Main Unit preventive maintenance checklist

2.2 Routine Switchgear maintenance of the 33kV and 11kV buses at Portbell 33/11kV
Substation
2.2.1 33kV switchgear details
• Type – Gas insulated Switchgear with non-rackable circuit breaker
• Make - Siemens
• Insulation medium – SF6 gas

2.2.2 11kV switchgear details

• Type – Air insulated Switchgear with rackable circuit breaker
• Make - Siemens
• Insulation medium - Air insulated

13
2.2.3 Procedure followed.
• The switching operation was done by the district operations authorized person with
support from Control officer on duty according to ORHVS safety operating regulation
to make the 33kV and 11kV buses dead for maintenance.
• Before proceeding with any work, the authorized person assigned a work permit to the
responsible person Mr. Odeke Nathan, and it was signed by both parties. We filled and
signed in the workers register, did a risk assessment, attached prohibitory signages and
work commenced.
• We switched off the control MCBs of the circuit breaker from the LV compartment.
• Springs were discharged manually by closing and opening the circuit breaker.
• We then removed the covers of the panel, cleaned the circuit breaker operating
mechanism, three position switch operating mechanism first with a dry cloth then a
damp cloth.
• We checked the operating mechanism i.e. motor, tripping coil and closing coil to ensure
that they are in good working conditions.
• We confirmed the tightness of all terminations.
• We sprayed a degreaser (WD40) on all bearings, moving parts, plunger of both coils
and all terminations.
• We lubricated the operating mechanism with grease, put back the cover of the panel
and switched on the control MCBs.
• With the Bus bar isolator and earth switch open. We checked the operation of the circuit
breaker (close/trip) against status indicator and counter reading.
• Lastly, we recorded the counter reading, pressure reading, name of the feeder and serial
number of the circuit breaker.

14
Figure 2:5 Prohibitory signages on switchgears under maintenance

Figure 2:6 Left: Workers Register Right: Work Permit

15
 Figure 2:7 Right: 33kV Circuit Breaker Left: 11kV Circuit Breaker

Figure 2:8 Cleaning the 11kV switchgear HV compartment.

2.3 Routine Switchgear maintenance of the 33kV bus at Mutundwe UETCL

132/33/11kV substation
2.3.1 Switchgear Details
• Type – Gas insulated Switchgear with non-rackable circuit breaker
• Make – Siemens
• Insulation medium – SF6

16
2.3.2 Procedure followed.
• The switching operation was done by the area authorized person with support from
Control officer on duty according to ORHVS safety operating regulation to make the
33kV bus safe for maintenance.
• Before proceeding with any work, the authorized person assigned a work permit to the
responsible person, and it was signed by both parties. We filled and signed in the
workers register, did a risk assessment and work commenced.
• We switched off the control MCBs of the circuit breaker from the LV compartment.
• Springs were discharged manually by closing and opening the circuit breaker.
• We then removed the covers of the panel, cleaned the circuit breaker operating
mechanism, three position switch operating mechanism first with a dry cloth then a
damp cloth.
• We checked the operating mechanism i.e. motor, tripping coil and closing coil to ensure
that they are in good working conditions.
• We confirmed the tightness of all terminations.
• We sprayed a degreaser (WD40) on all bearings, moving parts, plunger of both coils
and all terminations.
• We lubricated the operating mechanism with grease, put back the cover of the panel
and switched on the control MCBs.
• With the Bus bar isolator and earth switch open. We checked the operation of the circuit
breaker (close/trip) against status indicator and counter reading.
• Lastly, we checked for gas level from gas indicators for all bus bars. It was observed
that the gas level for red bus bar was below the minimum level and it was therefore
refilled.

17
 Figure 2:9 Refilling of SF6 gas of the red bus bar, Checking for gas leakage.

2.4 Routine Switchgear maintenance on the 11kV bus at Kawanda Umeme 33/11kV
substation
2.4.1 11kV Switchgear Details
• Type – Air insulated Switchgear with rackable circuit breaker
• Make – Fuji
• Insulation medium – Air

2.4.2 Procedures followed.

• The switching operation was done by the district operations authorized person
according to ORHVS safety operating regulation to make the 11kV bus safe for
maintenance by opening and isolating all incoming and outgoing feeders.
• Before proceeding with any work, the authorized person assigned a work permit to the
responsible person Mr. Odeke Nathan and it was signed by both parties. We filled and
signed in the worker’s register, did a risk assessment and work commenced.
• We racked out and withdrew all the circuit breakers from the bays of 11kV switchgear
and rolled them on level ground ready for maintenance.
• We discharged the circuit breaker springs and we ensured there was no further
movement from any moving parts of the circuit breaker.
• We blowed off dust from the empty switchgear bays and circuit breakers with the help
of a blower.
• Using a damp towel, we cleaned the dust-stained circuit breaker surface and similarly
the compartments of the switchgear.

18
 • With the help of WD-40, we lubricated all the movable parts of the circuit breaker.
• We also applied grease on the male connections of the circuit breaker and female
connections of the switchgear bays.

Figure 2:10 Blowing dust from the CBs, Cleaning the switchgear HV compartment.

2.5 Replacement of vacuum interrupters for Queensway Bay circuit breaker at Kisugu
33/11kV substation
Table 2:2 Tools used during replacement.

Tool Use
Flat spanners For tightening and loosening
of bolts and nuts

Screw drivers For tightening and loosening

of bolts and nuts

Allen key For tightening and loosening

of bolts and nuts

CPC tester For measuring contact

resistance

19
 Insulation resistance megger For measuring insulation
resistance

Cotton waste For cleaning vacuum

interrupter chambers

Thinner For removing soot and

cleaning vacuum interrupter
chambers

a) Circuit Breaker details

• Make: Fuji Electric

• Arc interrupter: Vacuum
• Isolation: Withdrawable type (Racking in/out)

2.5.1 Procedure followed.

• Troubleshooting was done by the switchgear team under supervision of Mr. Kalule
Edward. The vacuum interrupters had been burnt on account of a fault.
• The affected feeder bay had been opened, isolated and earthed by the authorized person.
• The vacuum circuit breaker was withdrawn from Queensway Bay 33kV line feeder.
• It was then rolled to a level ground and prepared for maintenance.
• The shuttered vacuum interrupters were withdrawn from the circuit breaker with the
help of necessary tools such as spanners and screw drivers.
• The sooty coated chambers for vacuum interrupters were cleaned with cotton waste that
was soaked in thinner.
• Using a manual guideline, we replaced all the vacuum interrupters, and the following
tests were carried out.

20
 Figure 2:11 Burnt Vacuum interrupters.

Figure 2:12 Replacement of burnt vacuum interrupters.

2.5.2 Tests carried out.

2.5.3 Insulation resistance test
• Insulation resistance test was done across open contacts of the circuit breaker.
• One of the leads was connected to the lower limb of the red phase while the other lead
was connected to the upper limb of the same phase.
• A test voltage of 5kV was injected across the open contact for a duration of 30 seconds
using an insulation resistance tester.
• A resistance of >3GΩ was registered by the megger and therefore it implied that the
upper contact was not making with the lower contact within the vacuum interrupter for
that phase.

21
 • Using a multimeter, a continuity test was also carried out to ascertain if the contact was
actually open.
• The procedure was repeated across open contacts for yellow and blue phases.

Table 2:3 Insulation Resistance test across open contacts

Phase Resistance (GΩ)

R >3GΩ
Y >3GΩ
B >3GΩ

Figure 2:13 Insulation resistance test.

2.5.4 Contact resistance test

This test was carried out to ascertain the status of contacts under open and close status in terms
of resistance recorded.

Procedure followed across closed contacts of the circuit breaker

• The circuit breaker was charged manually on site since it was out of the bay.
• When the spring was charged, we pressed the close button in order to close the breaker
contacts within the vacuum interrupter.

22
 • The CPC tester was then powered, and the program menu set up.
• Two current probes from the CPC tester were connected to the upper contact and lower
contact of the red phase respectively of the circuit breaker.
• Similarly, two voltage probes from the CPC tester were connected to the upper and
lower contacts on the same phase under test of the circuit breaker.
• A test current of 50A for resistance range of R(4µΩ - 100µΩ) was sent through the
current probes to the red phase under test.
• The procedure was repeated for blue and yellow phases under the close status of the
circuit breaker.
• We obtained the following test results for all the phases tested.

Table 2:4 Contact resistance results across closed contacts

Phase Resistance (µΩ)

R 335.4
Y 430.5
B 229.0

Procedure followed across open contacts of the circuit breaker.

• Since the circuit breaker was in a closed status, we pressed the open button to open the
breaker contacts within the vacuum interrupter.
• The CPC tester was then powered, and the program menu set up.
• Two current probes from the CPC tester were connected to the upper contact and lower
contact of the red phase respectively of the circuit breaker.
• Similarly, two voltage probes from the CPC tester were connected to the upper and
lower contacts on the same phase under test of the circuit breaker.
• A test current of 50A for resistance range of R(4µΩ - 100mΩ) was sent through the
current probes to the red phase under test.
• The procedure was repeated for blue and yellow phases under the open status of the
circuit breaker.
• We obtained the following test results for all the phases tested.

23
 Table 2:5 Contact resistance results across open contacts.

Phase Resistance (Ω)

R 196.67
Y 163.86
B 245.71

Figure 2:14 Contact Resistance test

Conclusion
It was observed that the contact resistance test results across closed contacts were tending to
zero (micro-ohms) whereas the contact resistance test results across open contacts were
significant results in ohms. Therefore, the vacuum interrupters were ready to serve their
purpose.

24
3 CHALLENGES AND RECOMMENDATIONS
3.1 Challenges
• There are no spare switchgear equipment specially allocated for training purposes,
always it required me to move with maintenance team when troubleshooting faults
and/or during routine maintenance and repairs. This limited me from fully utilizing my
time to learn as much as possible since many aspects couldn’t be exhausted due to fear
to cause unnecessary power interruptions and limited working time.
• The department has inadequate fleet to move all the resource to the place of work, this
affected me whereby sometimes I could use my own money to move to designated place
of work.
• Due to the many travels the switchgear maintenance team makes throughout the country
for routine maintenance activities, I was financially constrained considering the fact
that subsistence allowance wouldn’t come in time and bookings were done after making
the trip.

3.2 Recommendations
I suggest that spare equipment including switchgear bays, ring main units should be secured
and assembled in a training workshop where trainees can go and fully acquaint themselves with
different configuration, operations, maintenance and diagnostic troubleshooting of simulated
faults.

25