substation dc systems

8/10/2019 Substation DC systems

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State Training Services

Install and maintain substation DCs stems UETTRDSB03ACertificate IV in ESI Substation Resources (UET40206)Learner Guide


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Install and maintain substation DC systems Learner Guide -3 - NSW DET 2009

TABLE OF CONTENTS

Using this Learner Resource ..........................................................................................5

Use as Refresher Training .......................................................................................5

Mapping to Training Package ........................................................................................5

Essential Knowledge and Associated Skills ..........................................................6Learning Outcomes....................................................................................................6

Pre-Requisite Knowledge ......................................................................................6

Assessment.................................................................................................................7

Recognition of Prior Learning/Current Competence.................................................7

Introduction....................................................................................................................8

Hazards Associated with DC Systems...........................................................................8

Environmental Considerations.......................................................................................9

Performance Characteristics of DC Systems .................................................................9

Storage Battery Principles..............................................................................................9

Primary Cells .............................................................................................................9

Secondary Cells .......................................................................................................10

Lead Acid Batteries..................................................................................................10

Sulphation ............................................................................................................11

Terminal Corrosion..............................................................................................11

Sealed Lead Acid (SLA) Batteries...........................................................................12

Nickel Iron/ Nickel Alkaline....................................................................................12

Nickel Cadmium Cells.............................................................................................12

Identification of Battery Type..................................................................................13

Student Activity: Comparing Cell Type, Physical Size and Capacity.................13

Internal Resistance ...................................................................................................13

Battery Capacity.......................................................................................................14Voltage Curves.........................................................................................................15

Lead Acid Discharge Curve.................................................................................15

Nickel Cadmium Discharge Curve......................................................................15

Specific Gravity of Lead Acid Batteries..................................................................16

Specific Gravity Testing ..........................................................................................17

Student Activity: Using a Hydrometer ................................................................17

Batteries and Cells ...................................................................................................18

Student Activity: Constructing a Battery Bank ...................................................19

Battery Maintenance ................................................................................................20

Lead Acid Battery Maintenance ..........................................................................20

Alkaline (Nickel Cadmium / Nickel Iron) Batteries ............................................20Student Activity: Battery Maintenance................................................................20

Replacing Defective Cells........................................................................................21

Student Activity: Replacing Defective Cells .......................................................21

Battery Charging......................................................................................................21

Lead Acid Charging.............................................................................................21

Nickel Cadmium / Nickel Iron Charging.............................................................22

Stratification of battery cells ................................................................................22

Discharge Testing ....................................................................................................23

Quick Discharge Test...............................................................................................23

High Current Discharge Test ...................................................................................24

Capacity Testing ......................................................................................................24Impedance Testing ...................................................................................................25


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Principle of Impedance Testing ...........................................................................25

Student Activity: Discharge and Impedance Testing...........................................26

Recycling Secondary Battery Cells .............................................................................26

DC Testing ...............................................................................................................27

Student Activity: DC Testing...............................................................................27

DC Lighting Systems...................................................................................................27First Aid ...................................................................................................................28

DRABC................................................................................................................28

Electric Shock ......................................................................................................29

Acid and Alkali Spills ..........................................................................................29

Muscle Strain .......................................................................................................30

Learning Tasks and Practical Exercises.......................................................................31


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Introduction

DC systems are installed in substations to supply power for control, protection,

alarms, communications, and other critical auxiliary circuits where maximum

reliability of supply is essential.

AC supplies can be unreliable, whether it is obtained from the local supply or from

on-site alternator sets. In the event of AC supply failure, DC electricity is stored in

batteries with sufficient capacity to provide enough power until the AC supply

becomes available again.

Different DC voltages are used within substations depending upon equipment

requirements. Common voltages are 50, 120 and 400.

The storage batteries may be of a few main types: lead-acid, alkaline, and nickel-

cadmium; each type with its own characteristics.

Substation staff need to have an understanding of how batteries are maintained, the

principles of charging and discharging of batteries, how to recognise and diagnosebattery faults, and how to diagnose faults which may occur in the DC distribution

network. Installation in the context of this Learning Module refers to replacement

of defective units. (Installation and commissioning of battery banks will be generally

performed by contractors from the supplier.)

The principles contained within this module are also appropriate to other electrical

and electronic fields that use DC storage systems, including telecommunications,

security, computer and renewable energy.

Hazards Associated with DC Systems

There are a number of hazards that may be present when working with DC systems inelectrical substations. These include:

Electrical shock DC voltages and large currents may be high enough to causesevere burns or electrocution.

Acids and alkalis Can burn skin and eyes.

Large mass batteries and cells are very heavy and can cause injury if not liftedand transferred using appropriate techniques.

Confined spaces gases from battery cells can build up and require ventilationbefore battery rooms can be entered.

This list is not definitive. A risk assessment should always be performed before

commencing any activity. The work method statement for your organisation can also

provide guidance about how to work safely.

Treatment of these injuries is covered in the section on First Aid later in this

Learning Guide.

For more detail on working safely in electrical substations refer to Learning Module:

UETTDRIS22A - Implement and monitor the organisational OHS policies,

procedures and programs.


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Environmental Considerations

As you will learn, battery cells may be constructed using the heavy metals of lead or

cadmium. Both of these metals are known to be detrimental to the environment, and

if absorbed by the human body they can be very detrimental to health. If nickel

cadmium cells are carelessly disposed of in landfills the cadmium eventuallydissolves and the toxic substance can seep into the water supply, causing serious

health problems.

Cells which have reached the end of their life or are faulty are returned to the

manufacturer on an exchange basis for replacement new cells, or are sent to specialist

recycling facilities where the metals are recovered and reused.

Battery rooms must be kept clean. Liquid spills or leaking electrolyte must be

cleaned up.

Battery rooms should be bunded to prevent harmful chemicals entering the

environment. Bunding is a method of sealing the flooring and walls so that liquids

cannot escape into the environment. Any signs of damage to the proofing membrane(cracking, flaking etc) should be reported.

All waste associated with DC Systems should be disposed of correctly, in accordance

with your organisational guidelines.

Further detail on environmental management can be found in the Learning Module:

UETTDRIS23A - Implement and monitor environmental and sustainable energy

management policies and procedures.

Performance Characteristics of DC Systems

The battery is required to supply the electrical requirements of the system substation

when there is no output from the battery charger. This may be due to a loss of the

A.C. supply to the substation or a fault in the battery charger or its supply. Under

these conditions the battery is required to supply the loads it is connected to for a

period of 10 hours.

The battery should be able to be recharged from its design end-of-discharge voltage to

full charge in 5 hours.

Storage Battery Principles

Primary Cells

The simplest form of battery is non-rechargeable. These are known as primary

cells.

Without getting overly complicated, a battery is formed when two different metals

have an electrolyte (a solution that an electrical current can pass through) placed

between them. A potential difference (voltage) is developed between the two metals.

If the circuit is closed by placing a wire between the two metals then a chemical

reaction begins as electrons and ions circulate. In a primary cell a non-reversible

reaction occurs whereby the two metals are permanently changed. (This ustechnically called a redox reaction, which means reduction-oxidisation, of which


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common metal rusting is a type.) The common zinc-carbon dry cell and alkaline dry

cell is an example of this type of battery. A typical voltage of such a primary cell is

1.5 volts. Lithium dry cells may have voltages higher to 3 volts due to the higher

electrochemical potential of this metal and its compounds. The name dry cell is

given because the electrolyte is in a paste-form rather than liquid form.

Primary cells must not be recharged as they may explode.

Secondary Cells

Also called storage or accumulator cells, these battery cells can be recharged because

the chemical reaction that occurs during discharge can be reversed by applying a

reverse current into the cell. The cell can be discharged and recharged many times

(often many thousand times) before it is degraded to the point where it can no longer

provide reliable service.

Lead Acid Batteries

This is perhaps the most common type of rechargeable battery,

especially in substation environments. It is also called a wet-cell

or flooded-cell battery because the electrolyte is in a liquid form.

They are vented batteries because the charging process can

produce gasses of hydrogen and oxygen which needs to be able to

escape from the confines of the battery case.

In its charged state the cathode (positive plates) are lead peroxide,

the anode (negative plates) are lead, and the electrolyte is dilute

sulphuric acid. As the cell is discharged the plates are converted to

lead sulphate and the electrolyte becomes water. The chemical

reaction looks like this:

Cell charged Cell discharged

+ve plate -ve plate +ve plate -ve plate

PbO2 + Pb + 2H2SO4 PbSO4 + PbSO4 + 2H2O

(leadperoxide)

(lead) (sulphuricacid)

(leadsulphate)

(leadsulphate)

(Waterperoxide)

The open circuit voltage of a fully charged lead acid cell is between 2.3 volts and 2.4

volts. Under load the voltage will typically be between 2.0 volts and 2.2 volts.

Lead acid batteries have reduced life expectancy if they are left in a discharged

condition. Ordinarily they do not deal well with deep discharge cycles, although

recent advances in design have produced lead acid batteries more suitable to such

tasks.

Figure 1 A lead acid rechargeable

cell.


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Often a number of cells are packaged together in one case to give a battery of higher

terminal voltage. Other common voltages for lead acid batteries are 6 volts and 12

volts.

Sulphation

Sulphation is a natural occurrence in all lead/acid batteries including sealed and gel-

cel batteries. It the prime cause of early battery failure and is when the sulphur in the

sulphuric acid forms sulphur crystals attach to the lead plates and then act as an

"insulation" keeping the battery from accepting a charge.

Terminal Corros ion

Lead acid batteries suffer from terminal corrosion because of the corrosive

atmosphere created by the misting of sulphuric acid which is vented from thebattery. Most people are familiar with the corrosion that forms around the terminals

of a cars lead acid battery. The crystals that form are often yellow in colour

(sulphur) and bluish-green (copper salts). To minimise this corrosion it is common

practise to use petroleum jelly to create a barrier between the sulphuric acid and the

metal terminals and connectors.

Figure 2 A number of cells

can be combined to provide a

battery of greater voltage and

energy capacity.

Figure 3 Terminal corrosion on a lead acid battery.


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Sealed Lead Acid (SLA) Batteries

This is a particular type of lead acid battery which is becoming more common

because of the reduced maintenance requirements. More correctly, they should be

called a valve regulated lead-acid battery, because they do have a valve to release

internal gas build up which can result from overcharging. The electrolyte has been

jellified making the battery more resistant to

extreme temperatures, vibration and shock.

This is also why they are sometimes called

Gel Cells. They also have calcium included

in the plate construction which reduces the

gassing effects, minimising loss of electrolyte.

Nickel Iron/ Nickel Alkaline

Also abbreviated to NiFe cell (or simply written as Nife). This type of battery cell is

becoming far less common, however can still be found in some older substations.

Manufacturing of this type of battery has almost ceased.

It uses a nickel oxide (Ni2O3) cathode, an iron anode, and an electrolyte of potassium

hydroxide, which is alkaline.NiFe cells have a nominal voltage of 1.2 volts (1.4 volts open circuit).

They have advantages of being very robust, lifetimes in excess of 30 years are

possible, and can be deep cycled.

Disadvantages include excessive weight, steep voltage drop off with state of charge,

high self-discharge rates, can only be charged slowly, and are only able to be

discharged slowly.

Nickel Cadmium Cells

Most of us are aware of the round sealed nickel cadmium rechargeable batteries used

in many of todays consumer items. Although they appear very physically differentthey use a similar chemical reaction to the

vented stationery batteries used in

substations and other standby power

arrangements. Sometimes referred to by

the abbreviation NiCad, although strictly

speaking this is a copyrighted name to one

particular manufacturer.

Nickel cadmium cells cost as much as five

times more than lead acid batteries,

however they have the advantage of largecapacities and discharge rates. Vented

Figure 4 A valve-regulated lead acid

(VRLA) battery.

Figure 5 Vented wet cell nickel

cadmium cells.


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nickel cadmium cells are not normally damaged by excessive rates of overcharge,

discharge or even reverse charging. Oxygen and hydrogen are released through the

vent, and this explains the need to top up the water levels of vented nickel cadmium

cells. (Note: Sealed nicads are damaged by such adverse actions.)

The nominal voltage of a nicad cell is 1.2 volts.

Can deliver very high currents.

They can be left for long periods in a discharged state without damage (unlikelead acid cells).

The electrolyte in a nicad is potassium hydroxide (alkaline - similar to the nickeliron cell), and cross contamination from sulphuric acid must be avoided. It does

not change

(Like nickel iron cells) the specific gravity of nickel cadmium cells is unchanged by

the charging-discharging process.

Identification of Battery TypeThe external appearance of lead acid and alkaline (nickel cadmium or nickel iron)

batteries can be very similar. However the electrolyte is not interchangeable, and it is

important that when doing any testing or servicing that correct identification of

battery type is undertaken. Generally a label will be placed on the battery container to

indicate its type.

Student Activity: Comparing Cell Type, Physical Size and Capacity

Develop a table which has three columns: type of cell, physical volume (its

dimensions) and AH capacity. Access data on batteries by looking a examples in the

field, samples provided by your trainer, and by accessing battery cell data onmanufacturers websites.

Internal Resistance

Electrically a battery cell appears as a voltage source with a resistor in series. The

actual output voltage at the battery terminals will be less than the voltage source

depending upon the value of the internal resistance and the current being drawn. As

the internal resistance of the battery increases, then Ohms Law tells us that as current

(load) increases then the available terminal voltage will drop, eventually making the

battery useless for the intended purpose. However, with little to no load, the voltageat the terminals will not be significantly different from the internal voltage source. It

can be deceptive to decide the serviceability of a battery simply by measuring its

terminal voltage it is only under load conditions that we can truly know if its

internal resistance is satisfactorily low.

The principle of internal resistance

applies to both non-rechargeable and

rechargeable batteries. It is an

important concept and measurement

in determining whether a battery is

still serviceable or not.

Figure 6 A battery cell has internal

resistance.


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Battery Capacity

The capacity of a cell or bank of cells is quoted in amp.hours (Ah). This is the

theoretical capacity of the battery to deliver a certain current for a particular length of

time. For example, a 50Ah battery could theoretically deliver 1 amp current for 50

hours, 50 amps for 1 hour, 5 amps for 10 hours, etc. In practice, this is not the case

because of internal battery losses.

The C Rate is the charging or discharging rate of a cell or battery, expressed in terms

of its total storage capacity in Ah. So a rate of 1C means transfer of all of the stored

energy in one hour; 0.1C means 10% transfer in one hour, or full transfer in 10 hours;

5C means full transfer in 12 minutes, and so on.

The lead acid cell does not perform well at a 1C discharge rate. To obtain a

reasonably good capacity reading, manufacturers commonly rate these batteries at

0.1C or less (10 hour discharge). For example, a 40Ah battery is reckoned to be able

to provide 4 amps for 10 hours. If, however, the current is doubled to 8 amps, the

time to discharge would be less than half somewhere around 4 hours. The effect

is more dramatic as the discharge current increases ten times the current (40 amps)

would reduce the capacity such that it would only supply current for 30 minutes. The

capacity would in effect have been reduced to the equivalent of 20Ah.

Referring to the Capacity vs Load Chart for a lead acid cell we can see how the

capacity is reduced as the current drawn from the cell is increased.

This derating of lead acid battery capacity needs to be taken into consideration when

designing DC power supply systems. This is to ensure that the required capacity is

available for the expected load and duration that the cell must be able to providepower.

Capacity vs Load - Lead/Acid Cell

0

20

40

60

80

100

120

0.1 1 10 100 1000

Multiple of discharg e current

Percentage

ofavailable

capacity

Figure 7 In the case of a lead acid cell, as the discharge rate is increased

its available capacity decreases.


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Install and maintain substation DC

s stems UETTRDSB03A

Certificate IV in ESI Substation Resources (UET40206)Trainer Guide


12/22


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Install and maintain substation DC systems Trainer Guide - 9 -

Timing Instructional Content Notes to Train

Introduction

DC systems are installed in substations to supply power for control, protection,

alarms, communications, emergency lighting, and other critical auxiliary circuitswhere maximum reliability of supply is essential.

AC supplies can be unreliable, whether it is obtained from the local supply or from

on-site alternator sets. In the event of AC supply failure DC electricity is stored in

batteries with sufficient capacity to provide enough power until the AC supply

becomes available again.

While the battery provides the reserve of stored energy, this is only normally used

in an emergency, or for supplying the short time heavy current drain of circuit

breaker closing solenoids. Under normal conditions the station load and the small

current required to maintain the battery in a fully charged state is supplied by the

battery charger.Different DC voltages are used within substations depending upon equipment

requirements. Common voltages are 50, 120 and 400.

The storage batteries may be of a few main types: lead-acid, alkaline, and nickel-

cadmium; each type with its own characteristics.

Substation staff need to have an understanding of how batteries are maintained, the

principles of charging and discharging of batteries, how to recognise and diagnose

battery faults, and how to diagnose faults which may occur in the DC distribution

network. Installation in the context of this Learning Module refers to

replacement of defective units. (Installation and commissioning of battery banks

will be generally performed by contractors from the supplier.)

The principles contained within this module are also appropriate to other electrical

and electronic fields that use DC storage systems, including telecommunications,

security, computer and renewable energy.

Display Sli

Some ESI ovoltages su

Furthermortolerance adescriptor; 48VDC, 120110VDC deESI organis

400VDC is Uninterruptsystems: in

critical maincomputers.


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Learning Outcomes

(As per those listed on Page 5 of this Trainer Guide.)

Display Sli

Topics Covered in this Learning Module

DC Equipment in Substations

Primary and secondary cells

Lead-acid cells

Alkaline cells

Cell capacity

Battery maintenance

Battery testing

Earth fault detection

Display Sli


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Hazards Associated with DC Systems

There are a number of hazards that may be present when working with DC

systems in electrical substations. These include:

Electrical shock DC voltages and large currents may be high enough to causesevere burns or electrocution.

Acids and alkalis Can burn skin and eyes.

Large mass batteries and cells are very heavy and can cause injury if notlifted and transferred using appropriate techniques.

Confined spaces gases from battery cells can build up and require ventilationbefore battery rooms can be entered.

This list is not definitive. A risk assessment should always be performed before

commencing any activity. The work method statement for your organisation canalso provide guidance about how to work safely.

Treatment of these injuries is covered in the section on First Aid later in this

Learning Guide.

For more detail on working safely in electrical substations refer to Learning

Module: UETTDRIS22A - Implement and monitor the organisational OHS

policies, procedures and programs.

Display Sli

Environmental Considerations

As you will learn, battery cells may be constructed using the heavy metals of lead

or cadmium. Both of these metals are known to be detrimental to the environment,and if absorbed by the human body they can be very detrimental to health. If

nickel cadmium cells are carelessly disposed of in landfills the cadmium

eventually dissolves and the toxic substance can seep into the water supply,

causing serious health problems.

Cells which have reached the end of their life or are faulty are returned to the

Display Sli


16/22


manufacturer on an exchange basis for replacement new cells, or are sent to

specialist recycling facilities where the metals are recovered and reused.

Battery rooms must be kept clean. Liquid spills or leaking electrolyte must be

cleaned up.

Battery rooms should be bunded to prevent harmful chemicals entering theenvironment. Bunding is a method of sealing the flooring and walls so that liquids

cannot escape into the environment. Any signs of damage to the proofing

membrane (cracking, flaking etc) should be reported.

All waste associated with DC Systems should be disposed of correctly, in

accordance with your organisational guidelines.

Further detail on environmental management can be found in the Learning

Module: UETTDRIS23A - Implement and monitor environmental and sustainable

energy management policies and procedures.


17/22


Performance Characteristics of DC Systems

The battery is required to supply the electrical requirements of the system

substation when there is no output from the battery charger. This may be due to a

loss of the A.C. supply to the substation or a fault in the battery charger or itssupply. Under these conditions the battery is required to supply the loads it is

connected to for a period of 10 hours.

The battery should be able to be recharged from its design end-of-discharge

voltage to full charge in 5 hours.

The period supply DC period, mayorganisatiotreated as organisatio

Storage Battery Principles

Primary Cells

The simplest form of battery is non-rechargeable. These are known as primary

cells.

Without getting overly complicated, a battery is formed when two different metals

have an electrolyte (a solution that an electrical current can pass through) placed

between them. A potential difference (voltage) is developed between the two

metals. If the circuit is closed by placing a wire between the two metals then a

chemical reaction begins as electrons and ions circulate. In a primary cell a non-

reversible reaction occurs whereby the two metals are permanently changed. (This

is technically called a redox reaction, which means reduction-oxidisation, ofwhich common metal rusting is a type.) The common zinc-carbon dry cell and

alkaline dry cell is an example of this type of battery. A typical voltage of such a

primary cell is 1.5 volts. The name dry cell is given because the electrolyte is in

a paste-form rather than liquid form.

Primary cells must not be recharged as they may explode.

Display Sli

Lithium dry higher to 3 electrochemits compoun


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Secondary Cells

Also called storage or accumulator cells, these battery cells can be recharged

because the chemical reaction that occurs during discharge can be reversed by

applying a reverse current into the cell. The cell can be discharged and recharged

many times (often many thousand times) before it is degraded to the point where itcan no longer provide reliable service.

Display Sli

Lead Acid Batteries

This is perhaps the most common type of rechargeable battery, especially in

substation environments. It is also called a wet-cell or flooded-cell battery

because the electrolyte is in a liquid form. They are vented batteries because the

charging process can produce gasses of hydrogen and oxygen which needs to be

able to escape from the confines of the battery case.

In its charged state the cathode (positive plates) are lead peroxide, the anode

(negative plates) are lead, and the electrolyte is dilute sulphuric acid. As the cell is

discharged the plates are converted to lead sulphate and the electrolyte becomes

water. The chemical reaction looks like this:

Cell charged Cell discharged

+ve plate -ve plate +ve plate -ve plate

PbO2 + Pb + 2H2SO4 PbSO4 + PbSO4 + 2H2O

(leadperoxide)

(lead) (sulphuricacid)

(leadsulphate)

(leadsulphate)

(Waterperoxide)

The open circuit voltage of a fully charged lead acid cell is between 2.3 volts and

2.4 volts. Under load the voltage will typically be between 2.0 volts and 2.2 volts.

Lead acid batteries have reduced life expectancy if they are left in a discharged

Most peoplebatteries us

Display Sli

Display Sli

Display Sli


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condition. Ordinarily they do not deal well with deep discharge cycles, although

recent advances in design have produced lead acid batteries more suitable to such

tasks.

Often a number of cells are packaged together in one case to give a battery of

higher terminal voltage. Other common voltages for lead acid batteries are 6 volts

and 12 volts.

Sulphation

Sulphation is a natural occurrence in all lead/acid batteries, including sealed and

gel-cell type batteries. It the prime cause of early battery failure. It occurs when

the sulphur in the sulphuric acid forms hard sulphate crystals attach to the lead

plates. These crystals then act as an insulator, keeping the battery from accepting

a full charge.

Terminal Corrosion

Lead acid batteries suffer from terminal corrosion because of the corrosive

atmosphere created by the misting of sulphuric acid which is vented from the

battery. Most people are familiar with the corrosion that forms around the

terminals of a cars lead acid battery. The crystals that form are often yellow in

colour (sulphur) and bluish-green (copper salts). To minimise this corrosion it is

common practise to use petroleum jelly to create a barrier between the sulphuric

acid and the metal terminals and connectors.

Automobileconnected voltage of 113.8 volts u

Display Sli

Display Sli

Sealed Lead Acid (SLA) Batteries

This is a particular type of lead acid battery which is becoming more common

because of the reduced maintenance requirements. More correctly, they should be

called a valve regulated lead-acid battery, because they do have a valve to releaseinternal gas build up which can result from overcharging. The electrolyte has been

jellified making the battery more resistant to extreme temperatures, vibration and

shock. This is also why they are sometimes called Gel Cells. They also have

calcium included in the plate construction which reduces the gassing effects,

minimising loss of electrolyte.

Display Sli


20/22


Nickel Iron/ Nickel Alkaline

Also abbreviated to NiFe cell (or simply written as Nife). This type of battery cell

is becoming far less common, however can still be found in some older

substations. Manufacturing of this type of battery has almost ceased.

It uses a nickel oxide (Ni2O3) cathode, an iron anode, and an electrolyte of

potassium hydroxide, which is alkaline.

NiFe cells have a nominal voltage of 1.2 volts (1.4 volts open circuit).

They have advantages of being very robust, lifetimes in excess of 30 years are

possible, and can be deep cycled.

Disadvantages include excessive weight, steep voltage drop off with state of

charge, high self-discharge rates, can only be charged slowly, and are only able to

be discharged slowly.

Display Sli

The electrocompatible Contaminatelectrolyte m

Unlike a leanot changedischargingpotassium hspecific grastate of bat

Nickel Cadmium Cells

Most of us are aware of the round sealed nickel cadmium rechargeable batteries

used in many of todays consumer items. Although they appear very physically

different they use a similar chemical reaction to the vented stationery batteries

used in substations and other standby power arrangements. Sometimes referred to

by the abbreviation NiCad, although strictly speaking this is a copyrighted name

to one particular manufacturer.

Nickel cadmium cells cost as much as five times more than lead acid batteries,

however they have the advantage of large capacities and discharge rates. Vented

nickel cadmium cells are not normally damaged by excessive rates of overcharge,

discharge or even reverse charging. Oxygen and hydrogen are released through

the vent, and this explains the need to top up the water levels of vented nickel

cadmium cells. (Note: Sealed nicads are damaged by such adverse actions.)

The nominal voltage of a nicad cell is 1.2 volts.

Can deliver very high currents.

Display Sli


21/22


Instructions to Assessors

This Assessment Guide is part of a suite of resources that have been developed to

support 8 core units of competency from the Certificate IV in ESI Substation

(UET40206) as follows:

UETTDRIS05A Perform substation switching operation to a given schedule

UETTDRIS22A Implement and monitor the organisational OHS policies,

procedures and programs

UETTDRIS23A Implement and monitor environmental and sustainable energy

management policies and procedures

UETTDRSB01A Diagnose and rectify faults in power systems substation

environment

UETTDRSB02A Carry out substation inspections

UETTDRIS03A Install and maintain substation DC systems

UETTDRIS04A Maintain HV power system circuit breakers

UETTDRIS05A Maintain HV power system transformers and instruments

This Assessment Guide together with a Trainer Guide and a Learner Guide are

designed for UETTDRSB03A Install and maintain substation DC systems. This

guide is intended to provide some direction to assessors who are determining

competence of students who have completed the theoretical and practical instruction

in this learning module. Assessors are expected to use their own judgement in

designing appropriate assessment questions and tasks and putting them into context

for the assessment candidate. At all times the evidence requirements as set out in the

unit and the principles of assessment, that is, validity, reliability, flexibility andfairness must be complied with.

Use these guidelines to assist in preparing your own assessment instruments and tools.

The checklist should be treated as a starting point. You may choose to add more

checkpoints to highlight particular aspects of knowledge and skill that you want to see

evidence of. This could be through practical tasks or problem-based questions.

Evidence Required

Evidence for competence in this unit shall be considered holistically. Each element

and associated Performance Criteria shall be demonstrated on at least twooccasions in

accordance with the Assessment Guidelines UET06. Evidence must also reflectthe critical aspects of evidence which includes the following:

Install and maintain substation DCsystems (UETTRDSB03A)Certificate IV in ESI Substation Resources (UET40206)Assessment Guide


22/22

A representative body of performance criteria demonstrated within the timeframes

typically expected of the discipline, work function and industrial environment. In

particular this must incorporate evidence that shows a candidate is able to:

Implement Occupational Health and Safety workplace procedures and practices

including the use of risk control measures as specified in the Performance Criteriaand Range Statement.

Apply sustainable energy principles and practices as specified in the PerformanceCriteria and Range Statement

Demonstrate an understanding of the essential knowledge and associated skills asdescribed in this unit to such an extent that the learners performance outcome is

reported in accordance with the preferred approach; namely a percentile graded

result, where required by the regulated environment.

Demonstrate an appropriate level of skills enabling employment.

Conduct work observing the relevant Anti Discrimination legislation, regulations,polices and workplace procedures.

To be deemed as competent in this Unit, the candidate must provide sufficient

evidence of being able to maintain and repair the range of DC systems within the

electrical substation.

Where summative (or final) assessment is used it is to include the application of the

competency in the normal work environment or, at a minimum, the application of the

competency in a realistically simulated work environment. In some circumstances,

assessment in part or full can occur outside the workplace. However, it must be in

accordance with industry and regulatory policy. (For more detail on assessment

practices you are advised to refer to the Training Package and the Evidence Guide

for this Unit of Competence, especially where longitudinal competency developmentand Profiling has been used).

This assessment guide covers all tasks and equipment included in the section of the

Unit: Critical aspects for assessment and evidence required to demonstrate

competency in this unit, as shown in the table below.

The minimum number of items

on which skill is to be

demonstrated.

Item List

At least one of the

following:

Nickel cadmium batteries

Sealed/unsealed lead acid batteriesAt least one of thefollowing:

Main batteries

Communication batteries

Pilot isolation batteries

All of the following: Battery chargers

DC control circuits

At least two of the

following: Cell voltage test

Hydrometer/specific gravity test

Battery discharge and capacity tests

Impedance testing

substation dc systems

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