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State Training Services

Install and maintain substation DC systems (UETTRDSB03A)Certificate 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 ..........................................................6 Learning 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.......................................................................................................14 Voltage 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 ............................................20 Student 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 ......................................................................................................24 Impedance 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...................................................................................................27

First 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 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.

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 cause severe 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 lifted

and transferred using appropriate techniques.

Confined spaces gases from battery cells can build up and require ventilation before 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. SA

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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 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 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 us technically 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

(lead peroxide)

(lead) (sulphuric acid)

(lead sulphate)

(lead sulphate)

(Water peroxide)

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 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.

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. SA

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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 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

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 (unlike

lead acid cells).

The electrolyte in a nicad is potassium hydroxide (alkaline - similar to the nickel iron 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 Type The 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 on manufacturers 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 voltage at 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 provide power.

Capacity vs Load - Lead/Acid Cell

0

20

40

60

80

100

120

0.1 1 10 100 1000

Multiple of discharge current

Perc

enta

ge o

f ava

ilabl

e ca

paci

ty

Figure 7 In the case of a lead acid cell, as the discharge rate is increased its available capacity decreases. SA

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Install and maintain substation DC

systems (UETTRDSB03A)Certificate IV in ESI Substation Resources (UET40206) Trainer Guide

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

TABLE OF CONTENTS Using this Trainer Resource...........................................................................................4

Modifying the Training Resources ............................................................................4 Training Learners in Specific Tasks ..........................................................................5 Use as Refresher Training .......................................................................................5 Mapping to Training Package....................................................................................5 Learning Outcomes....................................................................................................6 Further Reference Materials ......................................................................................6

Introduction....................................................................................................................9 Learning Outcomes......................................................................................................10 Topics Covered in this Learning Module ....................................................................10 Hazards Associated with DC Systems.........................................................................11 Environmental Considerations.....................................................................................11 Performance Characteristics of DC Systems ...............................................................13 Storage Battery Principles............................................................................................13

Primary Cells ...........................................................................................................13 Secondary Cells .......................................................................................................14 Lead Acid Batteries..................................................................................................14 Sealed Lead Acid (SLA) Batteries...........................................................................15 Nickel Iron/ Nickel Alkaline....................................................................................16 Nickel Cadmium Cells.............................................................................................16 Identification of Battery Type..................................................................................17 Internal Resistance ...................................................................................................18 Battery Capacity.......................................................................................................19 Voltage Curves.........................................................................................................21 Specific Gravity of Lead Acid Batteries..................................................................22 Specific Gravity Testing ..........................................................................................24 Batteries and Cells ...................................................................................................25 Battery Maintenance ................................................................................................27 Replacing Defective Cells........................................................................................28 Battery Charging......................................................................................................30 Discharge Testing ....................................................................................................33 Quick Discharge Test...............................................................................................33 High Current Discharge Test ...................................................................................34 Capacity Testing ......................................................................................................35 Impedance Testing ...................................................................................................37

Recycling Secondary Battery Cells .............................................................................38 DC Testing ...................................................................................................................39 DC Lighting Systems...................................................................................................40 First Aid .......................................................................................................................41 SA

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Timing Instructional Content Notes to Trainer

Introduction DC systems are installed in substations to supply power for control, protection, alarms, communications, emergency lighting, 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.

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 Slide 2 & 3 Some ESI organisations may use other voltages such as 20VDC and 30VDC. Furthermore, there may be some tolerance around the actual voltage descriptor; 50VDC may also be known as 48VDC, 120VDC may also be know as 110VDC depending upon the particular ESI organisation. 400VDC is commonly used to supply Uninterruptible Power Supplies (UPS) systems: inverters generating 240VAC for critical mains powered equipment such as computers. SA

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

Display Slide 4

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 Slide 5

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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 cause severe 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

lifted and transferred using appropriate techniques.

Confined spaces gases from battery cells can build up and require ventilation before 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.

Display Slide 6

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 Slide 7

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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.

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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.

The period that the battery is required to supply DC power, and the re-charge period, may vary from organisation to organisation, and this figure should be treated as typical or modified for the organisation that training is occurring in.

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, of which 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 Slide 8 Lithium dry cells may have voltages higher to 3 volts due to the higher electrochemical potential of this metal and its compounds. SA

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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 it can no longer provide reliable service.

Display Slide 9

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

(lead peroxide)

(lead) (sulphuric acid)

(lead sulphate)

(lead sulphate)

(Water peroxide)

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 people would be aware of lead acid batteries used in automobiles. Display Slide 10 Display Slide 11 Display Slide 12 SA

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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.

Automobile batteries have 6 cells connected in series providing an output voltage of 12 volts loaded and closer to 13.8 volts unloaded. Display Slide 13 Display Slide 14

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.

Display Slide 15

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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 Slide 16 The electrolyte of a NiFe cell IS NOT compatible with a lead acid battery. Contamination of the two different types of electrolyte must be avoided. Unlike a lead-acid cell, the electrolyte is not changed during charging and discharging of a NiFe cell. It remains as potassium hydroxide (2KOH). Therefore specific gravity testing will not indicate the state of battery charge.

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 Slide 17 SA

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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 and fairness 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 two occasions in accordance with the Assessment Guidelines UET06. Evidence must also reflect the critical aspects of evidence which includes the following:

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

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Install and maintain substation DC systems Assessment Guide 2 NSW DET 2009

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 Criteria and Range Statement.

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

Demonstrate an understanding of the essential knowledge and associated skills as described 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 development and 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 batteries

At least one of the following:

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

SAMP

LE

325

Documents

battery capacity

nickel cadmium cells

learner resource

lead acid batteries

nickel iron nickel alkaline

identification of battery

training package

refresher training