United_Mavuso_Substation_Detailed_Report_20151102 - 20160131-Rev 1

Designed to order. Built to last© 1

Substation Report Training Report

Mavuso United M.

Swaziland Electricity Company

[email protected] Substation Engineer Signature:

02nd November – 31st January 2016 Trainee Signature:

Energy For The Future

mailto:[email protected]


Abstract

The purpose of this report is to reveal the scope of work covered by Substation department as part of

my Engineering Training programme at SEC. The goal of this report is to show a clear understanding

of the work done by the substation department in SEC. It also shows my involvement to the work done

in every day routine to meet its goals. A preamble of the Substation department in general is first

presented followed by a summary of the SEC Substation department. The full details of industrial

exposure and acquired knowledge is then shared under the tasks involved in section. Finally

recommendations are made which may make SEC substation department a better company in the

future.


Table of Contents

Abstract ................................................................................................................................................... 2

Abbreviations ......................................................................................................................................... 4

Table of Figures ....................................................................................................................................... 5

1. Preamble ......................................................................................................................................... 6

2. Why We Need Electrical Power Substations ................................................................................. 7

2.1 Practical Considerations .............................................................................................................. 7

2.2 Technical Considerations ............................................................................................................ 7

3. Equipment Used ............................................................................................................................. 8

3.1 Voltage Transformer (VT)............................................................................................................ 8

3.2 Current Transformer (CT) ............................................................................................................ 8

3.3 Power Transformer ..................................................................................................................... 9

3.3.1 Main Tank ............................................................................................................................. 10

3.3.2 The Conservator .................................................................................................................... 10

3.3.3 Tap Changer .......................................................................................................................... 11

3.3.4 Guard Unit ............................................................................................................................. 12

3.4 Circuit Breakers ......................................................................................................................... 12

3.4.1 Oil Circuit Breaker ................................................................................................................. 13

3.4.2 SF6 Gas Circuit Breakers ........................................................................................................ 13

3.4.3 Vacuum Circuit Breaker ........................................................................................................ 14

3.5 Protection Panel ........................................................................................................................ 15

4. Substations Activities ................................................................................................................... 16

4.1 Oil Sampling and Testing ........................................................................................................... 16

4.2 Infra-red Scanning ..................................................................................................................... 18

4.3 Transformer Movement............................................................................................................ 19

4.4 Tap Changer Maintenance .............................................................................................................. 20

4.5 Power Transformer Maintenance ............................................................................................. 21

5. Tasks Involved In .......................................................................................................................... 22

6. Recommendations ....................................................................................................................... 24

7. Conclusion .................................................................................................................................... 26


Abbreviations

SEC Swaziland Electricity Company

NCC National Control Centre

ISO International Standards Organisation

SCADA Supervisory Control and Data Acquisition

AC Alternating Current

DC Direct Current

HV High Voltage

VT Voltage Transformer

CT Current Transformer

CB Circuit Breaker

REF Restricted Earth Fault

NCT Neutral Current Transformer

IDMT Inverse Definite Minimum Time

O/C Overcurrent

IED Intelligence Electronic Device

E/F Earth Fault

OLTC On Load Tap Changer

LV Low Voltage

MV Medium Voltage

HV High Voltage

SFRA Sweep Frequency Response Analysis

FAT Factory Acceptance Test

SAT Site Acceptance Test

HIRA Hazard Identification and Risk Assessment


Table of Figures

Figure 1: Substation Diagram Mimic ....................................................................................................... 6

Figure 2: Voltage Transformer (VT) ........................................................................................................ 8

Figure 3: Current Transformer (CT) ......................................................................................................... 9

Figure 4: Atmoseal Conservator ........................................................................................................... 11

Figure 5: Infrared Camera ..................................................................................................................... 18

Figure 6: Mpaka Power Transformer Movement ................................................................................. 20

Figure 7: Mpaka Transformer Transportation ...................................................................................... 22

Figure 8: Stonehenge Cable Laying ....................................................................................................... 23

Figure 9: Leaking Lobamba Transformer .............................................................................................. 24

Figure 10: Positioning of Cable Rollers ................................................................................................. 25


1. Preamble

Substation is a part of an electrical generation, transmission, and distribution system. The main

objective of substation is to transform voltage from high to low, or the reverse. Between the

generating station and consumer, electrical power may flow through several substations at different

voltage levels.

Substations are left unattended, relying on SCADA for remote supervision and control. Substations

generally have switching (switchgear), protection, control equipment, and transformers. Other

equipment found in a substation could be capacitor banks, and voltage regulators. Substations do not

usually have generators, although a power plant may have a substation nearby.

The Swaziland Electricity Company Substation department operates as depicted on figure 1.

Substations is a department which is mainly supported by the linked departments. All this

departments have their equipment which they operate in substations. The critical equipment in a

substation is the transformer, everything that happens in the substation is benchmarked by the

transformer limit levels.

Figure 1: Substation Diagram Mimic


2. Why We Need Electrical Power Substations

We build substations for the following reasons:

2.1 Practical Considerations

To satisfy load growth; when people move to a new location that has no power

supply infrastructure, it might make a case for building a substation close to that

location. Supplying this new load from distant substations is inefficient because most

power will be lost in the transmission lines as heat (Ploss = I2R) and on distribution

lines.

To accommodate new generation power plants; either you want to build a solar or

wind farm, you will need a collector substation to tie all generators and connect

them to the power grid.

To maintain reliability requirements; new transmission lines are constructed by

utilities to address any deficiency in the power grid. When building new transmission

lines, effort is made to connect to an existing substation. When no existing power

substation, a new substation is considered.

2.2 Technical Considerations

To transform voltage levels; since higher voltages mean lower currents. Lower

currents yield to lower I2R (copper) losses. Thus more energy can be delivered to do

useful work. Voltages at the distribution end need to be stepped down for customer

utilisation. Substations provide the necessary real – estate to install a transformer

for power transmission and distribution.

To break power flow; quite often a fault requires complete isolation of the line until

the fault is removed. To safely interrupt hundreds of amperes you will need circuit

breakers that can handle high current magnitudes. All substations contain circuit

breakers which they trip and isolate transmission lines connected to it.

Provide support to the power flow; unlike DC power flow, AC power flow needs to

overcome not only the resistive impedance but also the impedance offered by the

inductive nature of various equipment (like motor loads, transmission lines,

reactors) connected to the system. For this reason, substations have capacitor banks

connected to all three phases of the lines to ease the power flow. This also improves

the power factor of the electrical system.


3. Equipment Used

3.1 Voltage Transformer (VT)

Voltage in power systems is too high to connect measuring instruments or relays directly, coupling

is made through VT. This is a device which is used for stepping down the system voltage (AC) to a

safe value which can be fed to relays. The VT has lower turns winding at its secondary when

compared to its primary windings.

A VT is insulated to withstand over voltages, including impulse voltages, of a level equal to the

withstand value of the high voltage system. Voltage transformers are designed to maintain the

specified accuracy in voltage output at their secondary. Primary of VT is connected across the

phase and ground. If long secondary leads are required, a distribution box is fitted close to the VT

to supply relay and metering instruments over separate leads. Usually the secondary voltage of

voltage transformer is generally 110 V (AC).

Figure 2: Voltage Transformer (VT)

3.2 Current Transformer (CT)

Same as the voltage transformer, current in power systems is high to connect directly to

measuring instruments, CT produce a reduced current accurately proportional to the current in

the circuit as coupling. The CT has the same principle as VT but here is the difference. For a VT or

general transformer, primary current varies with load (secondary current). With CT, primary

current is the system (primary) current transforms to the CT secondary, hence load (burden

current) depends upon primary current of the CT.


Again, protection CT has the same principle with metering CT but here is the difference. For

metering CT, accuracy ratio, between primary and secondary currents are essential within normal

working range. The normal accuracy up to 125% of rated current as allowable system current must

be below 125% of rated current. Due to faults the secondary of the CT could be saturated but

saturation is prevented to reach the metering instrument connected to secondary of the CT as

secondary current does not go above desired limit even when primary current rises to a very high

value.

But in the case of protection, the CT is desired not to be saturated during high fault current passing

through primary. So core of protection CT is designed that it would be not saturated for long range

of currents even on secondary of CT.

Figure 3: Current Transformer (CT)

3.3 Power Transformer

This is the main equipment in a substation, rather a critical and expensive component of the power

system. Its working principle is exactly the same as voltage transformer (VT), except of the

transformation voltage ratio level. SEC has 66/11kV, 132/66/11kV, and 11/66kV substation

transformers which are mainly protected against fault currents, over voltages, over loads. The


type of protection used should minimize the time of disconnection for faults within the

transformer. Any extended operation of the transformer under abnormal condition comprises the

life of the transformer, which means adequate protection should be provided for quicker isolation

of the transformer.

A transformer is made up of many components, that is, main tank, tap changer, guard unit,

conservator, bushings, breathers, cooling system (radiators(ONAN, ONAF)), and the relays.

3.3.1 Main Tank

This is the main component of the transformer where voltage transformation takes place. On its

own, it has different parts which form part of the transformation process. It consists of insulating

oil, laminated core, and paper insulated windings.

Laminated core; this is done to reduce eddy currents as low as possible since they

interfere with the efficient transfer of energy. The eddy currents cause energy to be lost

from the transformer as they heat up the core and energy getting lost as heat.

Paper insulated windings; between windings turns is a small voltage difference with the

neighbouring winding turns. The voltage difference between the windings and

neighbouring turns could be excessive and equals the voltage drop across a large number

of turns in one winding layer. Therefore an extra insulation layer using paper strip is used

between windings layers. This is not just an ordinary paper but cellulose paper with

outstanding electrical properties making it appropriate for insulation.

Insulating oil (SEC uses power oil); it serves mainly as liquid insulation in the transformer,

it also dissipates heat of the transformer, that is, it acts as a coolant. It further preserves

the core and the winding as they are fully immersed inside oil and again the oil prevents

direct contact of atmospheric oxygen (air) with cellulose made paper insulation of

windings, which is susceptible to oxidation.

3.3.2 The Conservator

This is a cylindrical tank mounted on supporting structure on the roof of the transformer main

tank. Its main function is to provide adequate for expansion of oil inside the transformer. When a

transformer is loaded (due to load increase) and when ambient temperature rises, the volume of

oil inside the transformer increases. The conservator provides adequate space to this expanded

transformer oil. When the load decreases and when ambient temperature decrease, the


transformer oil contracts due decrease in temperature. This then causes outside air to enter in

the conservator tank of transformer through silica gel breather. The conservator also acts as a

reservoir for transformer insulating oil.

An example of a conservator is shown in figure 4. This is an atmoseal type of a conservator. This

kind of conservator, an air cell is fitted inside the conservator reservoir. The silica gel breather is

connected at the top of this air cell. The oil level in the main tank rises and falls with respect to

the air cell deflation and inflation. When air cell deflates the air inside cell comes out via breather

and if cell inflates, the outside air comes in through breather. This arrangement prevents direct

contact of oil with air, reducing ageing of insulating oil.

Figure 4: Atmoseal Conservator

3.3.3 Tap Changer

In larger electrical power transformers, for proper voltage regulation of transformer, a tap

changer is required. Depending on which type is used, there are two kinds of tap changers which

are on load tap changer (OLTC) and no load tap changer (NLTC). The tapping arrangement is placed

in a separate tank attached to the transformer tank. On load tap changers are recommended since

as the name suggests, permit tap changing and hence voltage regulation with the transformer on-

load. Tap changing is done on HV winding for two reasons:

Because the currents are lower, the tap changer contacts or leads could be smaller


The HV winding is wound outside the LV winding, it is easier to get the tapping

connections out to the tap changer.

3.3.4 Guard Unit

This is a unit which monitors winding and oil temperatures inside the transformer main tank. This

is generally a precision unit. It is a temperature indicator of power transformer specially designed

for protection of transformer in addition to its temperature indication and cooling control

features. It performs mainly the following:

i. It indicates instantaneous temperature of oil and windings of transformer.

ii. Also records the maximum temperature rise of oil and windings.

iii. The unit operates high temperature alarm at a predetermined value of allowable

temperature limit.

iv. The indicators can further trip the circuit breakers associated with power

transformer when the temperature of oil or winding reaches a predetermined

limit.

v. The unit also controls the cooling system of the power transformer. Switches on

the cooling equipment (cooling fans) when the winding or oil temperature

reaches a pre-set temperature and switch off when temperature decreases.

According to power transformer maintenance manual provided by ABB, the maximum

temperature permitted for oil is of 90°C and the maximum temperature of the hottest point is

110°C. The guard unit will monitor the temperatures as reflected above.

3.4 Circuit Breakers

This is a switching device which can be operated manually or automatically for controlling and

protection of electrical power system. Its primary function is to provide opening and closing

current carrying contacts. It remains at its closed position for maximum period of its life span.

With SEC, we have some which are normally opened for the network stability. So these remain

opened for maximum period of their lives unless there is a fault which will require system

controllers to optimize the network through opening and closing the normally opened points.

The operation of a circuit breaker is then required to be reliable without any delay or sluggishness.

Many parameters come into play in the operation of a circuit breaker, which is, closing and

opening distance of contacts, velocity of moving contacts, and stroke between contacts.


At the moment SEC has different kinds of circuit breakers enrolled in the network, that is, oil circuit

breaker, SF6 circuit breaker, and vacuum circuit breakers which are remotely operated (mostly)

for the HV transmission network.

There are different kinds of circuit breakers SEC has in the network, these are:

a. Oil circuit breakers (OCB)

b. SF6 gas circuit breakers (Sulphur Hexafluoride)

c. Vacuum Circuit Breakers (VCB)

3.4.1 Oil Circuit Breaker

The current carrying contacts are immerged inside the insulating oil. Whenever there is a

separation of current carrying contacts in the oil, the arc in circuit breaker is initialised at the

moment of separation of contacts, and due to this arc the oil is decomposed in mostly hydrogen

gas and create hydrogen bubble around the arc. The highly compressed bubble around the arc

prevents re-striking of the arc after current reaches the zero crossing of the cycle.

Advantages of Oil Circuit Breakers

It is cheaper

Disadvantages of Oil Circuit Breaker

Oil is flammable and may cause fire hazards. When a defective circuit breaker fails under

pressure, it may cause an explosion.

The hydrogen generated during arcing, when combined with air, may form an explosive

mixture.

During arcing, oil decomposes and becomes polluted by carbon particles, which reduces

its dielectric strength. Hence it requires periodic maintenance and replacement.

Oldest type of circuit breaker.

3.4.2 SF6 Gas Circuit Breakers

This is a circuit breaker in which the current carrying contacts operate in sulphur hexafluoride (SF6)

gas. It has high affinity of absorbing free electron. When a free electron collides with the gas

molecule, it is absorbed by that gas molecule. We know that mobility of charged particle is majorly

responsible for conducting current through a gas. Over all mobility of the charged particle in the

SF6 gas is much less as compared other common gases.


For heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength.

SF6 gas has a good dielectric strength and a good property of fast recombination after the source

energizing the spark is removed.

Advantages of SF6 Gas Circuit Breakers

High dielectric strength

It insulates 2.5 times better than air

Good heat transfer property

Fast recombination after the source energizing the spark is removed

SF6 gas is identified as one of the greenhouse gases which can reradiate back to earth

some of the earth’s natural infrared radiation. It is environmentally friendly, that is, does

not deplete the ozone layer.

It is not a health hazard.

So due to the SF6 gas having high dielectric strength and high cooling effect, the gas is

approximately 100 times more effective arc quenching media than air. Due to this unique

properties of this gas is used for voltages ranging from 33kV to 800kV and even more.

3.4.3 Vacuum Circuit Breaker

This is a breaker where the arc quenching takes place in a vacuum. The operation of opening and

closing of current carrying contacts and associated arc interruption take place in a vacuum

chamber called vacuum interrupter. It mainly used for medium voltage switchgear.

Advantages of VCB

No chance of fire hazard compared to OCB

High insulation strength

No gas/oil filling required

No emission of gases, hence pollution free.

Rapid recovery of very high dielectric strength on current interruption.

Disadvantages of VCB

Loss of vacuum due to transit damage or failure makes the entire interruption useless and

cannot be repaired on site.


3.5 Protection Panel

This is a box where all information provided by either VT’s, CT’s, or power transformers is

interpreted as to be safe enough for the equipment operation. The panel could either be for the

bus-zone or power transformer or transmission lines (feeders), which simply means that panel has

relays which interprets information with respect to the mentioned equipment.

Different protection relays are used for different purposes in SEC with respect to the kind of

protection scheme enrolled. A protection relay is an automatic device which senses an abnormal

condition of electrical circuit and closes its contact so to isolate the abnormal condition.

The basic characteristics which ensures high performance of protection equipment are as follows:

1. Reliability – this is an assurance that protection will perform correctly when required.

2. Selectivity – this is an assurance that protection will trip only those circuit breakers whose

operation is required to isolate fault condition.

3. Speed – this is to ensure isolation of faulty conditions on the power system as rapidly as

possible before fault leads to collapse of entire system.

4. Sensitivity – protection equipment should be high sensitive, that is, should have minimal

operating time during abnormal conditions.

In wiring a protection panel, different codes (ferruling) are used depending whether it is an AC or

DC signal being wired. Furthermore, the ferruling depends on the kind of connection you are

doing, could be SCADA controls, metering, bus bar protection, inter alia. Below is a summarised

table for codes of practice for power system protection.

CODE Signal type Used for

A AC CT secondary for main protection

B Bus bar protection

C Protection circuits (O/C and Earth fault)

D Metering circuit

E Voltage transformer secondary

H Auxiliary transformer secondary

P Bus bar controls

J DC DC incoming from charger

K DC voltage controls – Closing, tripping

L DC voltage indications – Alarms, indications, annunciations

M DC voltage for motor spring circuit


N DC voltage for OLTC circuit

X Scada controls and indications

1 Positive signal

2 Negative signal

Table 1: Codes of Practice for Power System Protection

There are further rules that apply with regards to panel wiring under protection. Which are:

Entire wiring of circuitry should be of permanent wiring

Leads should be marked and identified near terminals (ferruling).

Every lead should end at a terminal point and no junction by twisting is allowed.

DC and AC supplies should not be taken through different cores of the same cable. Totally

different cables should be used for DC and AC supplies.

The cable entries in protection panels should be through appropriate size of cable glands. No

other means.

Wherever CT cores are not used “shorting loops” should be provided in CT secondary

terminals and not at panels.

The mentioned are some of the many rules that apply under power system protection.

Oil sampling, infra-red scanning, substation inspection, transformer/substations maintenance,

projects, faults,

4. Substations Activities

Substation department is equipped in doing some activities which may inform better the status of her

assets on the ground, that is, in terms of asset maintenance, substation yard maintenance, inter alia.

4.1 Oil Sampling and Testing

The department engages itself to do oil sampling and testing in all substation transformers (54

functional substations) once every year. This is done since ABB (1ZCL000002EG-EN – rev. 1) manual has

mentioned that we should do it at least once a year. This activity is outsourced to South African companies (the likes

of TSS, wear-check, inter alia).

After the sampling, samples are tested for the following:

1) Oil Condition


a. Karl Fischer (Moisture content determination, should be less than 20 ppm

according to the manual (ABB))

b. Acidity (should be less than 0.3 mg KOH/gram of oil)

c. Dielectric Strength (kV, minimum of 30kV)

d. PCB (Polychlorinated Biphenyls)

e. Oil temperature (maximum of 90°C)

f. Dissolved Gas Analysis (each gas is a prediction of a certain kind of fault as

depicted in Table 2)

Table 2: Power Transformer Gases Interpretation

Gas Normal

Condition

Abnormal

Condition

Temperature

(°C)

Interpretation

H2

(Hydrogen)

< 150 ppm > 1000 ppm > 900 Arcing Corona, Partial

discharge, present in all

types of faults but mainly

corona and arcing

CH4

(Methane)

< 25 ppm > 80 ppm 150 > Temp >

300

Sparking, Core bolts

faults, Under designed

core.

C2H6 (Ethane) < 10 ppm > 35 ppm 250 > Temp >

500

Local Overheating, tap

selector contacts, core

bolt faults

C2H4

(Ethylene)

< 20 ppm > 100 ppm 500 > Temp >

900

Severe Overheating,

bushing connections

C2H2

(Acetylene)

< 15 ppm > 70 ppm > 900 Arcing

CO (Carbon

Monoxide)

< 500 ppm > 1000 ppm > 900 Severe Overloading

CO2 (Carbon

Dioxide)

< 10 000 ppm > 15 000 ppm > 900 Severe Overloading

Table 2 shows is called the California State University Guidelines type of analysis.

2) Transformer Condition


a. Paint condition

b. Silica Gel Condition

c. Oil Level

3) Additional Tests

a. Sludge condition

b. Flash point

c. Furanic value

d. Degree of polarization (for a healthy transformer should be 700 – 1200. It

basically reflects the insulation paper condition)

After the tests, the contractor will mention its recommendations as action plans to be considered by

substation department.

4.2 Infra-red Scanning

Increased resistance and heat are the primary reasons most electrical equipment fail. Before any

failure, the equipment will heat up. This activity (Infrared scanning) is used to inspect electrical

equipment because excess heat is usually the first sign of trouble. Loose connections imbalanced or

overloaded circuits, defective breakers, damaged switches, faulty fuses, and material defects all lead

to equipment failure. Using infrared scanning as an early warning tool for potential electrical

equipment/system failure is an important part of maintaining mission critical uptime.

SEC substation department has committed herself in performing the activity for every substation

within the SEC.

Figure 5: Infrared Camera

Figure 5 shows an infrared camera which is used for the activity. The great thing about this camera, it

produces its own report on completion of job.


The early detection will allow maintenance personnel to take corrective action before equipment

failure, minimizing damage to the components and reducing repair costs.

4.3 Transformer Movement

This is mainly a relocation of power transformers with their associated equipment in a safe and

controlled movement; ensuring the integrity of the transformer. The activity include uplifting and

movement of old/new power transformers to new location either for installation or repairs or

refurbishments. In transformer movement there are key considerations to be taken with their

activities. These are:

Planning activity; transport routes, lifting plans, tools to be used (hydraulic jacks, turfors,

winch truck, rail slippers, rails, pulleys, slings)

Mechanical dismantle of auxiliary equipment, that is, bushings, conservator, buchholz relay,

inter alia.

Uplifting, loading, transport, offloading, and final positioning at new location.

Depending on where you transporting the unit, for new installation you should consider

testing (SFRA) after offloading.

Figure 6 shows a 3 MVA Mpaka transformer being transported by a winch truck. The transformer had

to go for refurbishment since it had a high leakage between main tank and tap changer which had an

impact to the environment.


Figure 6: Mpaka Power Transformer Movement

4.4 Tap Changer Maintenance

The tap changer operation has been discussed in 3.3.3 but SEC has a commitment to maintain power

transformer tap changers. This maintenance is done so to reduce arcing times and extend contact life.

The switching contacts are made of copper/tungsten, or in the case of tap changers for lower currents,

the contacts are made of copper. After a certain number of operations the tap changer is expected to

be maintained to see if it still has good oil, good contacts. It is worth noting that a tap changer gives a

service life that equals to power transformer service life. The major parts which need attention during

maintenance are:

The contacts which may need replacement

The motor drive mechanism

Prior to the maintenance is an annual inspection which has to be performed to read counting devices.

The readings are used to determine when overhaul is due, which is done every seven years. During

overhaul, the following is done:

Checking dielectric strength of the oil

Filtering the oil


Checking motor drive mechanism

Checking the pressure relay

And checking the contact wear.

SEC substation department usually outsource this activity to the company which designed the tap

changers. The maintenance is mainly done just because during tapping (switching), arcs are generated

on the main switching and transition contacts of the diverter switch. The arcs cause carbonisation of

the switching oil and lead to wear.

4.5 Power Transformer Maintenance

A power transformer is the most costly and essential equipment found in a substation. For getting

high performance and long functional life of the transformer, it is desired to perform various

maintenance actions including measurement and testing of different parameters of the transformer.

This maintenance is done mainly for the objectives stated in 2.1 and 2.2. Loss of a transformer in a

substation will result in many customers being affected.

Two types of transformer maintenances are available: for getting smooth performance from a

transformer we have to perform maintenance in regular basis, and the other type is performed as in

when it is required. If regular maintenance (condition maintenance) is performed properly, one may

not have any provision of performing emergency maintenance. This could avoid emergency and

breakdown maintenance.

Before any maintenance within SEC substations, there is a substation inspection which is done which

informs the power transformer maintenance for that substation. Mainly for power transformer

maintenance the following is considered:

The oil level in oil cap under silica gel breather, if high than expected level oil should be topped

up.

Colour of silica gel in breather depending on what kind of silica gel is used.

Oil leakage from any point of the transformer.

Reading of oil level gauge of main tank and tap changer. If oil is less, should be topped up.

Transformer bushings be cleaned and inspected for any cracks

Mechanical inspection of buchholz relay

Insulation resistance of transformer must be checked

Transformer oil dielectric strength be done


Amongst others, the above should be done for every transformer maintenance performed within SEC.

Other activities in SEC substation are H.V yard maintenance, Circuit breaker maintenance, H.V cable

termination, H.V isolator installation and commissioning, and FAT’s and SAT’s.

5. Tasks Involved In

In my attachment with SEC substation department, listed below are the tasks I was involved in:

Transformer movement from Mpaka substation. The 3 MVA developed a leak while in service

and was taken out of service and the oil was drained.

o 3MVA, 66/11 kV, S/N: Y16113, Yz11, Yorkshire transformer, year of manufacture:

1966, 2678 gallons of oil.

o Tools used: winch (low belt), slings, pulleys, 2 hydraulic jacks, two railroads, 2 turfors

(5 & 3.5 tonnes), rail slippers. It was the first time to experience such, moving such a

massive, with all the weight and height, it was pretty impressive. Unfortunately there

was no procedure to move the transformer. Transformer was loaded safely but the

challenge was that tap changer was too big such that it overlapped on to the road.

Yes we used an abnormal truck to move the transformer but it was not good enough.

We were a hazard to the other traffic going opposite us.

Figure 7: Mpaka Transformer Transportation

3MVA Transformer

Trainee & Technician

Railroad

Low belt truck


Cable laying at Stonehenge (4 x 120 mm2 XLPE cables). These cables were from low voltage

side (11kV bushings) of 10 MVA transformer 3 and 4 to incomer breakers. The job took 2 full

days to complete it.

Figure 8: Stonehenge Cable Laying

High voltage cable termination at Mkinkomo substation. After conducting HIRA and obtaining

permit to work from the NCC we laid and terminated a 120 mm2 x 3 core cable which

connected a PFC panel. Cable termination has its own procedure which when not followed

properly it might lead to sever faults which could be electrical treeing, water treeing, inter

alia.

Maloma substation maintenance; this involves transformer maintenance, circuit breaker

maintenance, and H.V yard maintenance.

Refilling of SF6 gas into Sikhuphe substation breaker. The breaker due to decrease in gas level

inside, tends to lock out and require refill and resetting for it to operate again.

At Nhlangano I substation, a project was ongoing, that is, the removal of outdoor circuit

breakers into indoor circuit breakers. I was involved in the assembling of the indoor 11 kV

panels inside the control room.

At Lobamba substation: transformer 2 had a leak around the core bushing. We had to re-

gasket the bushing. This activity required us to drain about 10 drums of oil from the main tank.

We missed the fact that we were supposed to close the conservator valve which leads into

the main tank. This then delayed the whole process, which led the team spending most of the

day on site. Figure 9 shows where the leak had occurred.


Figure 9: Leaking Lobamba Transformer

6. Recommendations

The substations department in SEC is responsible for 54 substations and a number of switchyards

country wide, this then makes the management of the substations difficult. The management part of

the substations involves a lot of responsibility on substation engineer. For improvement, the

substation department requires to have maintenance engineer which will ensure maintenance is

executed effectively. The importance of maintenance is very effective such that for 100% condition

maintenance you will expect 0% breakdown of any equipment.

Faults amongst the department occur now and then repeatedly but with the workload on the team, it

is difficult to find time to find the root cause of every fault. As a transmission department in whole, I

believe with a fault analysis team (amongst the existing resources) in place we could avoid having the

same faults occurring repeatedly. This will further improve the substation (or even SEC at large) team

competency in understanding the departments’ equipment operation.

Infrared scanning being done on substations yard only, it is worth noting that even the panels require

scanning as there are lots of connection done. I believe that every time an electrical panel is accessed,

it should be infrared scanned. Anyone authorized to work in an electrical panel or high voltage cabling

needs to be trained on infrared scanning.

Core Bushing


Substation department needs to make sure that for every meter or indicator used in a substation is

calibrated or rather verified for its accuracy and operation. Mainly for oil temperature and winding

temperature indicators (OTI and WTI) at least once in two years.

The cable laying process is a tiring job without cable rollers. Substations need to have cable rollers as

shown in Figure 10. The rollers are necessary to avoid abrasion of the cable by keeping it clear of the

ground and to reduce friction during pulling.

Figure 10: Positioning of Cable Rollers

The ISO standards should be communicated in every monthly meetings and further explained its

existence and implementation.

In my time in the department, we have lost track of keeping engineering standards. These include the

following;


Yard stone cover; a 100mm thick layer of clean, washed, hard granite crusher stone of

minimum sieve grid size of between 25mm and 36mm spread the entire substation yard and

extend 1000mm beyond the substation fence. The lack of this motivates vegetation growth

within the substation.

7. Conclusion

I believe for future purposes substation department could do much better if the recommendations

mentioned could be considered.

Analysing insulating oil taken from transformers is a unique way of identifying problems occurring

within a transformer. By identifying and quantifying the gases found in transformer oil, the condition

of the transformer can be monitored.

The interpretation of transformer oil gas analysis is still an art and not an exact science (the facts

around DP tests). The interpretation should be left to a specialist and his advice and recommendations

should be followed. Samples should be taken regularly and records kept.

This is a huge department under transmission but I believe that if we could follow the management

systems in place for every work executed we could achieve much better performance.

United_Mavuso_Substation_Detailed_Report_20151102 - 20160131-Rev 1

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