reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

•Ordinary keyed digits call to handportable.

•Priority override keyed digits call to handportable.

•Conference/station emergency group(s) calls (open channel).

A pilot scheme of the above system is being engineered for use at Ironbridge power station (1988), to determine the performance and acceptability of the system for power station use.

s• wiron PA X and/or PA BX access

•Direct dialling between PAX/PABX telephones and handportable radiotelepliones/vehicle-mounted radios.

S. v%tem and equipment detail

•Frequencies used The channels to be used by a 'stem will be allocated by the JRC from the band

a 24 UHF channels available to them. This is done after consideration of the need (a) to minimise interference to other users of the channels within he mutual reception range of the power station radio system and (b) to minimise the on-site intermodulation products.

17 Additional references

Philips Telecommunications Ltd Engineering Notes:

TSP 1267 Minimising Intermodulation and Blocking Effects in VHF/UHF Radiotelephone Systems.

TSP 377 The Location of Antennas on Motor Vehicles TSP 427 Electrical Noise in Motor Vehicles

TSP 480/1 Intermodulation in VHF and UHF Radio Systems — locating and minimising the effects

TSP 588/1 The use of Circulators/Isolators to Minimise Transmitter Intermodulation

Morton, A. H.: Advanced Electrical Engineering: Pitman: 1966

Electronic Engineers Reference Handbook (fifth edition): Edited by Mazda, F.: Butterworths

1introduction

1.1Introductory statement on batteries

1.2Introductory statement on systems

2Batteries

2.1Explanation of terms

2.2Possible types

2.2.1Heavy duty lead-acid Plante positive plate cells

2.2.2Tubular plate lead-acid cells

2.2.3Pasted flat plate lead-acid cells

2.2.4Nickel-cadmium cells

2.2.5Sealed lead-acid SLA) or recombination cells

2.2.6Summary

2.3Heavy duty lead-acid Plante cell — description and

chemistry

2.3.1General

2.3.2Positive plates

2.3.3Negative plates

2.3.4Separators

2.3.5Plate interconnections or group bars

2.3.6Plastic containers

2.3.7Cell lids

2.3.8Vent plugs

2.3.9Terminal pillars

2.3.10Terminal pillar seals

2.3.11Intercell connectors

2.3.12Polarity identification

2.3.13Electrolyte

2.3.14Battery stands

2.3.15Chemistry

2.4Battery accommodation

2.4.1General requirements

2.4.2Ambient temperature

2.4.3Ventilation

2.4.4Lighting

2.4.5Battery main connections in battery rooms

2.4.6Access to battery rooms

2.5Initial tests, charging, maintenance and site testing

2.5.1Tests in manufacturer's works

2.5.2Tests at site

2.5.3Charging

2.5.4Factors affecting cell life and precautions to be taken

2.5.5Inspection

2.5.6CEGB experience

2.5.7The case for testing

2.5.8End of life

2,5.9 Uncharacteristic behaviour of odd cells

2.5.10 System tests of essential battery-backed DC systems

3 Battery systems

3.1Introduction

3.2Provision of DC systems

3.2.1220 V DC systems for switchgear closing

3.2.2110 V DC systems for switchgear control, protection and interlocks

3.2.348 V DC systems for telecommunications, plant control

and alarms

3.2.4250 V DC systems for emergency lighting and emergency drives

3.3Duplication of battery/charger systems

3.4DC system voltage limits

4Chargers

4.1Introduction

4.2Required characteristics

4.2.1Initial charge

4.2.2Maintaining charge 4.2,3 Charger ratings

4.2.4Boost charging

4.2.5General additional requirements

4.2.6Earthing

4.2.7Protection and monitoring

4.2.8Alarms

4.2.9Nuclear safety

4.3 Description of equipment

4.3.1Introduction

4.3.2Basic principles

4.3.3Main transformer

4.3:4 Thyristor rectifier

4.3.5Control board

4.3.6Reference transformer

4.3.7DC transformers

4.3.8Display

4.3.9Battery float/boost control circuitry

4.4Testing

4.4.1Introduction

4.4.2Type testing in manufacturer's works

4.4.3Routine tests in manufacturer's works

4.4.4Tests at site

5 Diesel generators

5.1 System requirements

5.1.1Purpose of diesel generator installation

5.1.2Starting and loading

5.1.3Rating and number of diesel generators

5.1.4Protection against external hazards

5.2 Engine and auxiliaries

5.2.1Engine types and characteristics

5.2.2Engine design and construction

5.2.3Starting equipment

5.2.4Cooling system

5.2.5Fuel oil systems

5.2.6Inlet and exhaust air pipework, turbochargers and

silencers 5.2.7 Governors

5.3 Generator and electrical equipment

5.3.1Generator design and construction

5.3.2Excitation equipment and automatic voltage regulator ( AVR)

5.3.3Diesel generator control and protection equipment

5.3.4Control of auxiliary systems

5.4Testing

5.4.1Tests in manufacturer's works

5.4.2Tests at site

5,4.3 In-service operational testing

6 Additional references

6.1 British Standards (BSI

Temperature Low battery temperatures temporarily reduce the available ampere-hour capacity and discharge voltage. Capacity and voltage are restored to nominal with a return to normal temperature, even without a charge. An increase in battery temperature results in an increase in capacity, particularly at high rates of discharge. The capacity of lead-acid Plante cells is specified at 20 ° C.

Rating The ampere-hour capacity of a positive plate or cell assigned to it by the manufacturer, under specified conditions of discharge.

Charging The passing of an electric current through a cell to bring it to a chemical condition where it is capable of supplying electricity to an external circuit. The quantity of electricity put in is known as the 'charge' and is usually measured in ampere-hours.

Fully charged The condition of a battery when the voltage and the electrolyte specific gravity of every cell have not varied appreciably during three consecutive hours at the end of the charging period, account being taken of temperature variations.

Discharge The quantity of electricity in Ah taken out of a cell connected to an external circuit when the current flows through the cell in the reverse direction to that of charge.

Plate The unit that, singly or in groups, is submerged in electrolyte of dilute sulphuric acid, or potassium hydroxide in the case of nickel cadmium cells (see Section 2.2.4 of this chapter), so that it forms the whole or part of one of the electrodes of the cell.

Positive plate The plate that forms the anode or part of the anode during the charge. For lead-acid batteries, it can be of three main types: Plante, tubu-

2.2.1 Heavy duty lead-acid Plante positive plate cells

This cell is derived from the conventional long-life lead-acid battery and is designed to provide low to medium currents for, say, 1-3 hours. It has a high cell voltage and is tolerant of temperature changes, although these affect its output capacity. With good maintenance, a life of 25 years is not uncommon in CEGB experience.

Plante positive plates are made of pure lead instead of the pasted-plate of a flat plate battery (Fig 91 (a)). An electrochemical formation process produces a thin layer of lead dioxide on the total active surface area.

The enclosures are generally transparent styrene acrylonitrile (SAN), stress-relieved to give clarity and mechanical stability through life, together with 'at a glance' inspection of electrolyte levels. The SAN enclosures replace the moulded-glass containers and leadlined wood containers used previously. The latter were employed for very large capacity cells. One major advantage of this type of cell is that maintenance personnel can assess life by visual inspection (see Section 2.3 of this chapter for a detailed description of the high performance Plante cell).

2.2.2 Tubular plate lead-acid cells

They are physically some 66 07o of the volume and

80% of the price of the corresponding ampere-hour capacity Plante positive plate battery, but the life expectancy of tubular positive plate batteries is only 10

to 15 years. This was confirmed by experience at Heysham / and Hartlepool nuclear power stations,

where tubular plate lead-acid cells needed replacement after about ten years. They have higher open-circuit

losses and need more frequent inspection.

Tubular plates (Fig 9.1 (b)) are constructed from tubing manufactured from Terylene or a combina - tion of perforated PVC and woven glass fibre, fitted

ov er cast antimonial-lead spines. The tubes are filled ith lead oxide and then undergo a formation process. Their construction does not permit a visual inspection of the plates: their condition is therefore not easily ascertainable and any slow deterioration is not readily

detected.

Again, the enclosures are generally SAN, stress-re- lieved to give clarity and mechanical stability throughout life, together with 'at a glance' inspection of electrolyte levels.

2.2.3 Pasted flat plate lead - acid cells

These suffer from the same disadvantages of life ex-

pectancy and open-circuit losses as tubular positive plate lead-acid cells. Their life expectancy is even lower, about 5-6 years.

They are designed for low performance applications only and are unsuitable for even moderate discharges lasting more than a few minutes.

The flat plates consist of a paste made from lead oxide, sulphuric acid, water, and other additives, applied to a lattice grid made of lead or lead alloy. The plates are dried under controlled conditions and then undergo a formation process (Fig 9.1 (c)).

Again, the enclosures are generally stress-relieved SAN.

2.2.4 Nickel-cadmium cells

When correctly maintained, nickel-cadmium cells can have a life of more than 30 years, during which approximately 10-20% capacity is lost. They employ perforated steel pocket plate construction, using nickel hydroxide as the basis of the active material for the positive electrode and cadmium hydroxide for the riegati% e electrode (Fig 9• I (d)).

The electrolyte is an aqueous solution of potassium hydroxide, with the addition of lithium hydroxide. The purpose of the electrolyte is solely to support the ieactions between the electrodes and there is no significant change in specific gravity from a fully charged to a discharged condition.

The containers can be of nickel-plated steel, thus affording maximum strength and durability for use in rugged environments, also where shock and vibration are present, Alternatively, the containers can be of translucent high-impact polystyrene plastic for ease of maintainance and mechanical strength and stability throughout an extended life. These also enable the electrolyte level to be checked at a glance.

An advantage of the nickel-cadmium battery is its ability to be left idle for long periods in any state of charge and its rapid recovery after neglect by toppingup and recharging.

The following disadvantages outweigh the advantages:

•It is not possible to check the state of charge without carrying out a discharge test.

•The electrolyte in the cells requires replacement, initially after two years and subsequently between five and eight years. Failure to replace reduces the high rate discharge capacity significantly.

•The internal resistance is higher than for a lead-acid cell, hence there is a greater propensity to higher electrical noise levels generated by currents such as charger harmonics, signals, telephone speech, etc. These currents generate volt-drops across the battery internal resistance and are transferred to other circuits connected to the battery.

•The cost of a nickel-cadmium battery is higher than the corresponding lead-acid Plante battery as a larger number of cells is needed for the same ampere-hour capacity because of their lower cell voltage.

Maintenance costs of a nickel-cadmium battery are higher than for a lead-acid Plante battery because of the larger .number of cells and the greater need for cleanliness to prevent tracking between poles due to close spacing.

Its high rate performance is lower than that of a lead-acid Plante cell.

Any attempt to boost charge may result in the 'self destruct syndrome'. These cells have been known to disintegrate somewhat forcefully and resoundingly after such a treatment, due to the release of large volumes of hydrogen and oxygen.

2.2.5 Sealed lead-acid (SLA) or recombination cells

When a traditional lead-acid battery is charged normally, the electrochemical reaction evolves unpleasant and potentially explosive hydrogen and loss of water occurs (for details see Section 2.3,15 of this chapter). Hence the need for ventilation and topping-up. By th e use of 'gas recombination' both these problems lia\,e been solved, whilst retaining the inherent advantages of low cost and long life of the lead-acid battery. The gases usually liberated into the atmosphere during float/recharge operation in conventional lead-acid cells, recombine to form water in a recombination cell. As a consequence, these cells do not lose water during normal operation and therefore topping-up and ventilation are not required.

The recombination principle works when oxygen evolved from the positive plates diffuses through the highly porous glass-microfibre separator to the reactive negative plate and is electrochemically reduced to water. The separator acts like a sponge and holds captive a closely controlled quantity of acid in a stable condition, whilst providing the medium by which as recombination can take place.

Even if the cell container is accidentally damaged, it will not leak acid, unlike a conventional lead-acid cell with its copious quantities of dilute sulphuric acid electrolyte.

Although the cell is virtually sealed, a safety valve together with a flame retardant device is provided to stop damage by inadvertent overcharging. Hence the term 'sealed' battery is really a misnomer.

These cells suffer rapidly from anything other than very carefully controlled voltages. Even under these conditions, their life is claimed to be only ten years and can be as low as three to four years if charging voltage or ambient temperature is increased.

Sealed lead-acid cells use a flat pasted construction for both positive and negative plates. A grid alloy with a high hydrogen overpotential is essential to long term operation and lead-tin-calcium alloys are used.

The separator is crucial in achieving efficient oxygen recombination in SLA cells. Glass microfibre paper is used for the separator because of its inertness and large uniformly porous volume, so that when the separator is not fully saturated there is an electrolytefree path between positive and negative plates along which oxygen diffuses from positive to negative and there reduced to water.

The cells are formed by compressing together the plates and separator into the container with a carefully measured quantity of electrolyte, the lid then being

sealed.

The containers are of flame-retardant ABS plastic which is mechanically robust and abuse-resistant. It withstands stress, thermal shock and vibration.

Like nickel-cadmium cells, the maintenance of recombination cells is essentially limited to monitoring

battery and cell voltages and charging current, in

addition to normal good housekeeping such as clean-

liness and tight joints.

In addition to being maintenence-free, SLA recombination cells do not require separate ventilated accommodation. Their low internal resistance gives them high ,hort-term current capability for duties such as engine

...,arting. They are also inherently more robust, smaller J lighter than Plante cells for a given duty.

an The main advantage they offer is one of civil cost which balance their shorter life and conse-

quently increased lifetime replacement costs.

The condition of a SLA cannot be assessed visually and neither can the electrolyte specific gravity be check- 0:i, In view of present CEGB maintenance practice, ;hese are serious disadvantages. Alternative condition m onitoring techniques are being developed by users of sLA cells; one of these is the comparison of current in parallel strings of cells, a configuration common in SLA batteries. Variations in the string currents i n di c ate problems in one string. Routine discharge testing is also available and special test load sets have been developed. Monitors using ripple or pulsed load techniques are other options that are also becoming available. Finally of course, the CEGB practice of per iodic cell dismantling can be used to assess the residual life of a battery.

2,2.6 Summary

The evidence over many decades of satisfactory performance and long life of heavy duty lead-acid Plante cells has led the CEGB to continue their use to the present time. Although the recombination version of the lead-acid cell now appears to offer an alternative which may well be explored in future power station schemes. Initially, lack of operating experience discouraged detailed consideration of its use but there is now a growing %.olurne of such experience. Within the CEGB, Transmission Division have some four years' experience of [hese batteries for telecommunications systems.

The use of batteries as emergency power supplies means that condition monitoring is an important issue. While this is a problem with a sealed cell, techniques are being developed and generally experience of failure rates with SLA batteries has been good.

It seems likely, therefore, that in the future, leadacid batteries of the recombination type will offer a serious alternative to the Plante cell.

Using heavy duty Plante cells and by careful maintenance, only one complete battery replacement may be necessary in the life of a power station, whereas other types necessitate more than one. This considerably increases the lifetime costs for the station, and there are therefore no overall economic advantages in using types other than Plante.

2.3 Heavy duty lead-acid Plante cell — description and chemistry

In view of the almost exclusive use in CEGB power

Batteries

stations of the lead-acid high performance Plante positive plate battery, further details of the cell construction and operation are given in this section (Fig 9.2).

2.3.1 General

High performance Plante cells designed to BS440, now superseded by BS6290, provide the highest integrity source of standby power with a long and predictable life. They are designed for operation under constantpotential float or trickle charge conditions, not involving regular deep cycles of charge and discharge.

In addition, their high rate performance makes them particularly suitable for circuit-breaker tripping and closing duties together with diesel and gas turbine starting operations. By virtue of the high power/weight ratio and sealed construction, high performance Plante cells are ideal for installations where space is limited. As mentioned already, they have a typical life of 20-25 years.

2.3.2 Positive plates

The positive plates are cast from pure lead and consist of numerous thin vertical laminations, strengthened by a series of horizontal cross-ribs to increase the surface area by as much as 12 times that of a plain lead plate of similar width and length. This ensures that there is no fall-off in capacity throughout their long life.

The positive plates are hung from ledges moulded in the container.

2.3.3 Negative plates

The negative plates are of interlocking design to ensure active material retention and provide balance with the positive plate to give maximum performance and life. The negative group always has one more plate than its matching positive group, so that when the groups are interleaved, each positive plate is located between two negative plates to ensure that both surfaces are worked equally and thus prevent distortion or buckling.

The negative plates are supported on ribs in the bottom of the container.

2.3.4 Separators

Separators are made of microporous PVC, providing a complete diaphragm between the plates and giving maximum electrolyte utilisation, together with high mechanical and electrical strength. Separators are chemically inert and their high porosity ensures minimum internal resistance. This permits more efficient circulation of electrolyte and, combined with maximum physical strength, prevents internal short-circuits by active material deposition.

2.3.5 Plate interconnections or group bars

To obtain the desired ampere-hour capacity, each respective group of positive and negative plates are joined

2.3.14 Battery stands

Battery stands can be constructed of high quality knotfree timber and finished with three coats of acidresisting paint. Alternatively, steel stands with an acidresistant epoxy coating can be provided. Each stand module is fitted with adjustable nylon feet to allow for any variation in floor finish and for insulation.

Batteries can be arranged in single tier or double tier, with single rows for positioning against walls or twin rows where access can be provided from both sides.

Where seismically-qualified batteries are provided for nuclear power stations, they are accommodated on special seismic stands using mild steel. They are constructed in a similar manner to the normal stand but use larger cross-section material and additional tie bars. Each module is bolted to the floor.

2.3.15 Chemistry

The fundamental parts of the lead-acid Plante cell are two dissimilar plates or electrodes immersed in an electrolyte, i.e., positive plate (lead dioxide), negative plate (spongy lead) and dilute sulphuric acid electrolyte.

Cell on discharge

Assuming the cell is fully charged, the sulphate ions from the electrolyte move to the negative plate and give up their negative charge when an external load is connected across the cell terminals. This produces an excess of negative charge at the plate, which is relieved by a flow of electrons via the load to the positive terminal, i.e., from low potential to higher potential, which is opposite to the conventional direction of electric current. This passage of surplus electrons allows more sulphate ions to combine with the lead in the negative plate to form lead sulphate.

At the positive plate, the highly oxidised lead dioxide is short of negative charge, so it readily accepts the electrons from the negative plate via the load. Hydrogen ions from the electrolyte now combine with oxygen ions from the plate to form water. This leaves some lead-free to combine with the sulphuric acid to form lead sulphate and more water.

As the discharge proceeds and current continues to flow, more lead sulphate is formed in both plates by combination of the acid from the electrolyte. Water is also produced, which helps to dilute the electrolyte, and it is this progressive weakening of the electrolyte by formation of water which provides a convenient way of measuring the amount of discharge taking place. The cell is discharged when its voltage falls rapidly: at this stage, most of the active material has been converted to lead sulphate and the plates are almost identical in chemical composition.

Cell on charge

To reverse the chemical changes which take place in the cell during discharge, it is necessary to pass a DC

current into the cell in the opposite direction to that of discharge.

The charging source must therefore have a voltag e greater than that of the cell or battery to be charged. The charging source connected across the cell supplies an excess of negatively charged electrons to the negatRe plate and creates a shortage at the positive plate. Th e result is that positively charged hydrogen ions are attracted to the negative plate, where the hydrogen combines with lead sulphate to form lead and acid.

The shortage of charge produced at the positive plate results in sulphate ions being attracted and combining with the hydrogen of the water to form sulphuric acid. This releases the oxygen ions in the water, some of which combine with the lead of the positive plate to form lead oxide. At the negative plate, the process of recombination of the hydrogen and sulphate continues as long as there is sulphate present. When the process of conversion of lead sulphate to lead is almost complete, hydrogen bubbles form at the negative plate and rise through the electrolyte. This is known as 'gassing'.

Similarly, sulphate ions react with water at the positive plate, forming sulphuric acid and leaving oxygen to react with lead to form lead dioxide. When most of the lead is converted, the oxygen appears as gas at the positive plate. The formation of hydrogen and oxygen gas at the plates is a sign that the cell is reaching the fully charged condition.

As the charge proceeds, acid which is released from the plates passes into the electrolyte and the specific gravity slowly increases. Measurement of the specific gravity of the electrolyte during the course of a charge does not give a true indication of the charged condition of the cell or battery. It is not until gassing commences that the stronger acid, liberated from the plates, is mixed with the weaker acid at the top of the cell. Specific gravity readings can therefore be of value only towards the end of the charge, when constancy of readings indicates that all strong acid has been liberated from the plates and the cell is fully charged.

Although some gassing is necessary to bring into circulation the strong acid released during the charge, excessive and prolonged gassing can in time shorten the life of a battery by scouring the active materials at the surface of the plates.

Hydrolysis of the electrolyte results in loss of water which must be replaced by adding more pure water from time to time. The demands of a battery as regards the amount of topping-up water required, when working at known duty are a good guide to correct charging. Excessive water consumption usually means overcharging, whilst too little means undercharging.

The chemical reaction can be expressed as follows: