CHAPTER XLV
STORAGE BATTERY SYSTEMS
Storage batteries are used for many purposes, such as to supply current for electric vehicles, gas engine ignition, lighting, and in connection with power stations and distribution work.
* * * * *
The latter is an important field, the storage battery being used in connection with the power station for the following purposes:
1. To carry the peak load, during hours of maximum demand; 2. To carry the entire load during hours of minimum demand, or for a short time in case of emergency; 3. To act as an equalizer; 4. For regulation of load and voltage; 5. As compensation for feeder drop; 6. As a preventive against shut downs.
In almost every electric lighting plant there are long periods during the day and late at night when the number of lamps lighted is so small that it may not pay to run the generating machinery. In such cases, storage batteries may usually be used to advantage to aid in carrying the maximum load and to supply the entire current at minimum load as illustrated in fig. 1,179. In other words, batteries are substituted for a certain portion of the machinery plant or are used in place of the latter.
=Ques. What provision must be made in power plants when storage batteries are not used?=
Ans. The capacity of the generating machinery must be sufficient for the heaviest overloads which may occur, and it must be operated continuously for 24 hours a day in the majority of central stations supplying current for lighting and power.
=Ques. What results are obtained with this method of working?=
Ans. The engines working under very variable loads, not only operate at low efficiency, but are continually subjected to severe mechanical strains.
=Ques. How may greater efficiency be secured with steam engines under variable loads?=
Ans. Judicious selection of the number and sizes of the engines enable them to be worked in most cases at a considerable fraction of their full capacity nearly all the time.
=Ques. What further improvement is secured in most cases with the storage battery?=
Ans. The plant is made more flexible, and the economy of the engines is increased by making their loads nearer uniform, and nearer to full capacity while they are running.
=Ques. What is the effect of a battery connected in parallel with a dynamo, as in fig. 1,180?=
Ans. It is not necessary for the dynamo to have a capacity exceeding that which is sufficient for the average daily load, at which it may be worked practically all the time.
When the load is below the average, the dynamo charges the battery, and when the load rises above the average, during the hours of maximum demand, the battery discharges into the line in parallel with the dynamo. During the hours of minimum demand the engines may be shut down and the necessary current supplied from the battery alone, thus not only increasing the efficiency of the plant, but serving to maintain a steadier pressure under fluctuating loads.
=Ques. What is understood by the expression "floating the battery on the line"?=
Ans. A storage battery is said to _float_ on a line when connected across the circuit at some distance from the power station, so that a heavy load on the line, within the range of the battery influence, causes sufficient line drop to allow the battery to discharge, while with a light load on the line, the drop is small and the impressed voltage at the battery high enough to charge the battery. This usage is confined chiefly to electric railway service, where large voltage changes are permissible.
=Ques. When the battery is floated on the line, how may the amount of charge be made to approximately equal the amount of discharge?=
Ans. By properly proportioning the number of cells in series.
=Connections and Circuit Control Apparatus.=—When a storage battery is used in an electric lighting plant, provision must be made for feeding the lamps, etc., from either the dynamo or battery separately, or from the two working in parallel, and it should be possible to charge the battery at the same time the lamps are being supplied. To accomplish these results requires three switches, for the following connections:
1. To connect the lamps to the dynamo; 2. To connect the lamps to the battery; 3. To connect the battery to the dynamo.
In some plants, the first switch is omitted, because the lamps are always fed by the battery alone, the latter being charged during the day, when no lamps are in use.
It is desirable, however, to have all three switches in every plant in order to be able to supply lamps and charge the battery at any time.
In the battery circuit there should be an ammeter having a scale on both sides of zero, to show whether the battery is being charged or discharged, as well as the value of the current. Another similar ammeter is required in the circuit between the dynamo and the battery, to show the direction and amount of current. A third ammeter is desirable in the lamp circuit, to show the total current supplied to the lamps, but it need only indicate on one side of zero, since the current there always flows in the same direction.
A voltmeter is required with a three-way switch to connect it to the dynamo, battery or lamps, and a circuit breaker must be inserted in the battery circuit in order that it may be opened when the current becomes excessive.
A discriminating cut out or reverse current circuit breaker is required between the dynamo and the battery to open the circuit when the charging current falls below a certain value, and thus avoid any danger of the battery discharging through the dynamo, if from any cause the voltage of the latter drop below that of the battery. This completes the ordinary measuring and circuit controlling apparatus employed with storage batteries.
=Methods of Control for Storage Batteries.=—As the external voltage of a storage battery varies with the amount of charge it contains and with the direction of the current, it is necessary to employ some means for compensating this variation in order to maintain a constant voltage on the line supplied by the battery. The various devices used for this purpose are as follows:
1. Variable resistances; 2. End cell switches; 3. Reverse pressure cells; 4. Boosters.
The particular method selected will depend upon the size of the battery, the purpose for which it is used, the allowable limits pf current and voltage variations, the cost of the system, etc.
=Variable Resistance.=—Regulation by variable resistance may be used advantageously only with batteries of small capacity, and in small lighting plants such as those of yachts, where the space available for battery auxiliaries is limited, and where the cost of energy is so low that the loss of power in the resistance is not objectionable.
The connections for one of the simplest methods is shown in fig. 1,185. The battery is divided into two halves, which are connected in series for discharging and in parallel for charging. Since the voltage of each cell at the end of a discharge should not be lower than 1.8 volts, a battery intended for use on a 110 volt lighting circuit will require 110 ÷ 1.8 = 62 cells. The voltage necessary, however, for each cell at the end of a charge is about 2.6 volts, or a total of 2.6 × 62 = 161 volts for the battery, a value which is far above the line voltage. By dividing the battery into two halves and connecting them in parallel only 80.5 volts are necessary for charging. The excess voltage of the line, 29.5 volts is taken up by the resistance, which also controls the output of the battery on discharge.
=End Cell Switches.=—These may be used to advantage in small installations where there is not demand for current during the day, or where the charging is done by means of _boosters_.
=Ques. What is an end cell switch?=
Ans. A form of switch employed in connection with a storage battery in order to control the end cells for regulating the voltage.
=Ques. Describe the construction of an end cell switch.=
Ans. This is shown in fig. 1,187. The switch contact arm is made in two parts, A and B, which are insulated from each other as shown, and connected with each other through the protective resistance R. The end cell contacts are so spaced that when the main current carrying part A of the switch arm is squarely on one end cell contact such as X, the part B, does not touch any other contact such as Y, but when the switch arm is advanced for cutting into circuit another end cell, the part B, reaches the contact Y before the part A, leaves the contact X, thus keeping the battery circuit closed, while the resistance R, limits the current in the short circuited cell at the instant the switch arm passes from one end cell contact to the next.
=Ques. How should the conductors joining the end cells to the end cell switch contacts be proportioned?=
Ans. They must have the same sectional area as the conductors of the main circuit.
The reason for this is that when any end cell is in use, the conductor connecting it to the switch becomes a part of the main circuit. An allowance of 1,000 amperes per sq. in., when the battery is discharging at the two-hour rate, is considered good practice.
=Ques. Describe some of the features of end cell switch construction.=
Ans. Those of small capacity are made circular; the larger sizes are made horizontal in form, and both types may be either operated by hand or motor driven.
=Ques. Where are end cell switches of large capacity located?=
Ans. Generally they are placed as near the battery room as possible to avoid the cost of running the heavy conductors, and when such switches are motor driven, the usual practice is to control their operation from the main switchboard.
In fig. 1,188 is shown the method of regulation with an end switch. The diagram shows the battery being charged with the main switch open, and the voltage of the dynamo raised to the charging pressure. During discharge the cells are connected in series, and as the voltage of each cell at the beginning of discharge is at least 2.1 volts, only 52 or 53 cells are required to give the desired pressure of 110 volts, but as the discharge continues, and the voltage of each cell decreases, the end cells, 1, 2, 3, 4, etc., are cut into circuit successively by means of the end cell switch, thereby adding to and compensating the drop in the total voltage until, at the end of discharge when the voltage of each cell has fallen to 1.8 volts, the entire 62 cells are in series to supply the required line pressure.
For a 110 volt circuit, the number of cells required is 110 ÷ 1.8 = 61, and the number in series when the battery begins to discharge is 110 ÷ 2.1 = 52. Hence, in a 110 volt circuit an arrangement must be provided whereby 61 - 52 = 9 cells may be cut out or switched in, one by one.
The number of end cells for any voltage may be obtained by the following formula:
Number of end cells = (E/1.8) - (E/2.1)
E = voltage of supply circuit; 1.8 = minimum voltage of cell during discharge; 2.1 = voltage of fully charged cell.
=Reverse Pressure Cells.=—These consist of unformed lead plates immersed in the ordinary electrolyte of dilute sulphuric acid. As they have no active material, they possess no capacity, but are capable of setting up an opposing pressure of about 2 volts each to the discharging current flowing through them, thereby cutting down the total voltage of the battery, so that the net voltage across the line depends on the number of reverse current cells in series in the battery circuit. As the voltage of the battery falls during discharge, the reverse pressure cells are cut out, successively, thus keeping the external or line voltage constant.
It is obvious, that as these cells do not possess any capacity, the number of active cells required in the battery will be the same as when end cell control is employed. Therefore, the reverse pressure cells represent an increase in equipment, which entails an additional expense of at least 8 per cent. For this reason, and also on account of the fact that the amount of energy lost in discharging against reverse pressure cells, is the same as when the resistance methods of controlling the discharge are employed, the use of cells for this purpose is now practically obsolete.
=Boosters.=—In general, a booster may be defined as _a dynamo inserted in series in a circuit, to change its voltage_. It may be driven by an electric motor, in which case it is sometimes called a _motor-booster_. The function of a booster is to add to an electric pressure derived from another source.
For instance, if a storage battery be used in conjunction with one or more dynamos to supply current to an electric light installation, the battery cannot be charged from the machines which are feeding the lamps, because it requires a pressure higher than that required for the lamps to complete the charge. A small dynamo is therefore connected in series, with the main machines and the battery, acting in conjunction with the former to provide the necessary pressure.
The power for running such a dynamo is obtained in various ways. The dynamo or charging booster may be belt driven or arranged on an extension of the armature shaft of the main dynamo; again, it may consist of a single armature with a double winding (fig. 1,191), or a motor and dynamo coupled together on one bed plate as in figs. 800 and 805. Boosters may be divided into several classes as follows:
1. Series boosters; 2. Shunt boosters; 3. Compound boosters; 4. Differential boosters; 5. Constant current boosters; 6. Separately excited boosters.
=Series Boosters.=—The series booster acts so as to compound the battery, and tends to maintain a constant voltage on the line, whatever the load may be. Its operation depends on the fact that the dynamo voltage must rise and fall with the load. It can, therefore, be used only with a shunt dynamo or its equivalent as the source of supply.
=Ques. What use is made of the series booster system?=
Ans. It is suited to power, but not to incandescent lighting purposes, being similar in operation to a floating battery. It is not extensively used as the other types give better service, under the same conditions.
=Ques. Describe some characteristics of the series booster.=
Ans. It is automatic and adjusts its voltage to produce the proper ratio of charge or discharge with varying external load, and it also tends to maintain a constant voltage across the line, under all conditions of change in circuit.
=Shunt Boosters.=—This type of machine is simply a shunt dynamo, having its armature circuit in series with the line from the main dynamo to the battery. A rheostat controls the field excitation. Its function is to send charge into the battery. It is used in plants where the battery is not designed to take up load fluctuations, but is in service only to carry the peak of the load, being charged during periods of light loads and discharged in parallel with the dynamo.
The shunt booster acts to increase the voltage applied to the battery so that the charging current will flow into the latter.
=Ques. How is a battery used with a shunt booster proportioned?=
Ans. Usually sufficient battery is provided to carry the entire load during the light load period.
=Ques. Explain the use of the rheostat controlling the field excitation.=
Ans. It is used to vary the booster voltage so as to hasten the charging of the battery if desired.
=Ques. For what service is the shunt booster not suited?=
Ans. It is not adapted to circuits where there are sudden fluctuations that are great compared with the capacity of the dynamo.
=Ques. What is its action in changing from charge to discharge?=
Ans. It is not automatic, the switching must be done by hand.
=Ques. How may it be used reversibly?=
Ans. It will give a pressure to assist the battery to discharge when excited from the bus bars and provided with a reversing rheostat.
In this case it will assist the battery to discharge when the direction of the field magnetization is changed. When so used, no end cells are necessary, but the booster must be run continuously during the entire period of discharge.
=Ques. What should be the battery capacity on a 110 volt circuit with a reversible booster?=
Ans. 56 cells will be sufficient.
The voltage to fully charge is 56 × 2.6 = 146, or 36 volts above dynamo voltage. Minimum voltage of discharge = 1.8 × 56 = 100 volts, or 10 volts less than that of the line. Hence, the booster need give only 36 volts maximum, and is required to add 10 volts to the battery voltage toward the end of battery discharge. In this case, the booster voltage is only 36/49 or about ¾ of that required in the preceding case; five cells less of battery are necessary and the end cell switches and leads are eliminated.
The machine will be larger, however, than it would be if used only for charging, because the discharge current is unusually greater than that of charge, and the current carrying of the armature must be great enough to take care of the heaviest currents.
=Compound Boosters.=—These machines are used on railway and power circuits where there are great fluctuations in load, the battery acting to prevent excessive drop and to assist the generating machinery in carrying the load, relieving it from the strain of sudden rushes of current.
The connections are shown in the diagram fig. 1,197. Under ordinary working conditions, the shunt field of the booster creates an electric pressure in the same direction as that of the battery, tending to discharge it.
When no current is flowing into or out of the battery, the following relation exists:
Dynamo voltage = booster voltage + battery voltage
In this case the dynamo carries the whole external load. If the load increase, the dynamo voltage decreases, so that the booster voltage + battery voltage is greater than the dynamo voltage, and the battery begins to discharge.
In discharging, the current passes through the series field of the booster and produces a proportional pressure acting with the shunt field to raise the voltage of the booster, thus increasing the battery discharge and shifting more of the load from the dynamo, until the system becomes balanced.
If the load on the external circuit be small, the dynamo voltage rises and current flows into the battery. In this case the series field acts against the shunt field and decreases the booster voltage so that the pressure at the dynamo is greater than booster and battery voltage combined, thus increasing the rate of charge of the battery until the load causes the dynamo voltage to drop to normal and the system is again balanced.
The battery and booster can be placed at the power house or where the greatest drop is likely to occur. As this system, like the series booster, depends for its action upon the drop of voltage with increase of load, it is only adapted to shunt wound dynamos.
From the foregoing description it will be seen that the compound booster is automatic within certain limits of battery charge. Any marked change of battery voltage will be followed by a corresponding change in dynamo current, unless the rheostat be manipulated to bring battery voltage + booster voltage back to normal.
While the theoretical dynamo current variation is small for a given change of load, there is always a sudden, momentary, current rush from the dynamo on increase of load, the duration of which is equal to the time lag of magnetization of the booster field.
Lights on a circuit with variable load will "wink" on sudden changes of load. In this respect the compound booster is not so satisfactory as the constant current booster, as in the latter _all_ dynamo current passes through the series fields, which, by reason of their self-induction, oppose and check any sudden current rush, giving the booster field time to change its magnetization to the proper degree.
=Differential Boosters.=—In this type of booster, a series coil energized from the main current, tends to discharge the battery, and a shunt coil, excited from the battery, tends to charge the cells. These two coils are opposed to one another, and the difference in their respective strengths represents the net strength available for boosting. In order to produce quicker reversal, additional compound coils are sometimes added.
=Ques. For what service is the differential booster adapted?=
Ans. It is suited to power and railway circuits where the load fluctuates widely and suddenly.
There are several varieties of this type of booster, and many patents have been issued covering the different methods of varying the voltage of the machine.
=Constant Current Boosters.=—In installations where it is desired to supply both an approximately constant load and a fluctuating load from the same dynamos (as for instance, in office buildings or hotels, where it is necessary to supply lights and elevators from the same source), the fluctuations in the power circuits must not interfere with the lighting circuits and to prevent this, two sets of bus bars are provided.
The dynamos are connected in the usual manner to one set of bus bars, and the lighting circuits are connected across these.
Across the other set of bars are connected the circuits supplying the fluctuating load, and the battery is also connected directly across these power bars.
The power bars are supplied with current from the lighting bars, a non-reversible or so called constant current booster being interposed between the two as shown in fig. 1,202. Since this permits only a constant current to pass from the lighting bus bars, the load on the dynamo does not vary, although the load on the power busses may vary widely.
=Separately Excited Boosters.=—In some forms of booster the field excitation is secured by a small exciting dynamo. An example of this class is shown in fig. 1,203. The exciter is provided with a single series coil, through which the station output or a proportional part thereof passes. The armature of the exciter is connected to the exciting coil on the booster, and thence across the mains as shown.
NOTE.—Reversible boosters should be used where the average total current to the fluctuating load is greater than the battery discharge current, and where the pressure of the power bus bars must not fall off with increase in load. Electric railway and lighting plants having long feeders are examples of the systems to which reversible boosters are suited. Non-reversible boosters should be used where the average total load is less than the battery discharge current, and where a drop in the voltage of the power bus bars is of advantage. Examples of such plants are hotels or apartment houses where electric elevators are operated from the lighting dynamos. Boosters are usually driven by electric motors directly connected to them, though any form of driving power may be used.
With the average current passing through the field coil or the exciter, its armature generates a voltage which is equal to that of the system, and in opposition to it. These two opposing pressures balance, and no current flows in the booster field coils.
With an increase in external load above the average, the tendency is for an increase to take place through the exciter series coil, augmenting its field strength and consequently the exciter armature voltage. This latter now being higher than that of the line, causes current to flow in the booster field coil in such a direction as to produce a pressure in the booster armature which assists the battery to discharge, and is of a magnitude to compensate for the battery drop occasioned thereby.
When the load decreases below the normal, the current in the exciter field is decreased, and its armature voltage falls below that of the system. Current will now flow in an opposite direction in the booster field coil, generating a voltage in the booster armature to assist charge. Since the exciter always generates a voltage in opposition to that of the line, this system is known commercially as the counter pressure system.
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=ELECTRICAL GUIDE, NO. 1=
Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery.
=ELECTRICAL GUIDE, NO. 2=
The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos.
=ELECTRICAL GUIDE, NO. 3=
Electrical Instruments, Testing, Practical Management of Dynamos and Motors.
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Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries.
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[Transcriber's Note:
Where possible, unicode characters are used. Some of the fractions do not have a unicode character which will cause some inconsistencies in the display.
Inconsistent spelling and hyphenation are as in the original.]
End of Project Gutenberg's Hawkins Electrical Guide, Vol 4, by Hawkins