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CENTRAL OFFICE POWER PLANT
The telephone power plant started with the development of the common battery manual switchboard about the year of 1893. Since that time the lead acid storage battery has had a large influence on the design of the central office power plant. More than ordinary consideration is given to reliability and continuity of power. There are many central offices in the Bell System in which power has not been interrupted for years.
A power plant, in a telephone exchange, furnishes electric current in various forms and at different voltages to operate telephone apparatus continuously. The telephone company is in business to give service and can only give service as long as there is a continuous source of electric current in its central offices.
Primary Power Service
Commercial power service is purchased from a power company. In the larger offices this power is nearly always 3 phase, 60 cycle, high voltage alternating current and is transformed to 120 - 240 volts for use in the building. For small offices the source is usually single phase, three wire, 60 cycle power, supplied directly by the power company at a voltage of 120-240 a-c.
In many respects a telephone power plant is similar to a substation in the commercial power field. Power is purchased, transformed to a safe usable alternating current voltage, and converted to the desired direct current or alternating current voltage to operate the exchange equipment. At some point in this cycle the commercial power is metered. The most economical cost of power is a study in itself involving such factors as the purchase of high voltage power, ownership of transformers, cost of vault equipment, and power factor correction.
The power is brought to a distribution board in the building and from there to the telephone power plant, the building power, the ventilating equipment and the lighting. At the distribution board the Telephone Company enters the safety picture for the first time since the equipment up to this point is operated and maintained by the power company. The present practice is to use dead front panels on the distribution boards in large offices and safety switches in smaller offices.
Commercial power is generally continuous and dependable but occasional interruptions occur due to storms, strikes, fires, feeder short circuits and grounds. Since the telephone business is one of furnishing a service, emphasis must always be placed on the importance of duplication in order to restore service in the minimum time interval. For that reason reserve power equipment is provided as an insurance against a service interruption. The reserve usually is in the form of increased storage battery reserve, gasoline or diesel driven generators either permanently installed or portable or combinations of the two. The type of reserve plant provided depends on the cost of accomplishing an objective.
Everyone is familiar with the 60 cycle, 120 volt A-C electric power that is used in the home to light lamps, operate radio sets, run vacuum cleaners and toast bread. In the telephone office requirements are largely for direct current at 24, 48 and 130 volts. D.C. current is usually obtained from storage batteries and associated charging equipment. Telephone service has become big business and although the current for one circuit or one operation may be measured in milliamperes, there are so many operations happening simultaneously that the D-C drain in a large dial office amounts to hundreds and thousands of amperes during the busy hour.
There are various types of storage batteries depending on the application. For instance a battery may be called upon to give a high rate of discharge for a short time, such as engine starting, or another application may require a relatively small rate of discharge for a long time such as a community dial office in an isolated section. In the first case a high gravity electrolyte may better serve the purpose whereas a low gravity type battery may give much longer life for the second application.
When a storage battery is being charged some of the hydrogen and the oxygen atoms do not combine readily and are liberated in the form. of gas bubbles which rise to the surface of the electrolyte and burst, liberating the gas. This gas can cause the atmosphere above the cell to become explosive. That is why open flame should be kept away from batteries under charge and why adequate ventilation should be provided.
Thirty or more years ago storage batteries were made up of open type cells in either glass jars or lead lined wood tanks. On newer installations the cells are enclosed and contained in either glass, hard rubber or plastic. Trials were made of ceramic containers but these were not found to be satisfactory due to the glaze breaking down and leakage developing.
Why are storage batteries used? Originally there were two reasons. First, to provide a reserve of power to maintain continuous telephone service if anything should happen to the commercial power service or to the charging equipment, and second, to smother or filter noise and cross-talk. Today there are better ways to filter noise but there is still need for the reserve power supply.
Years ago storage batteries were operated on a charge-discharge routine. The average life of a battery under that method was five years. The charge period was harmful to the plates, causing the lead to flake off and fall to the bottom of the container in the form of sediment. Today, most batteries are operated on a full float routine and the life of the cells has been raised to fifteen years for lead-antimony and twenty five years for lead-calcium. It has been found that if the charge is held at 2.17 volts per cell it neither charges nor discharges but remains in a fully charged condition. This is not always easy to do and over a period of time the cells will sulphate if the voltage per cell is low, or will disintegrate if the voltage is high.
Counter-E.M.F. cells are used in telephone power plants to reduce voltage for a tap or feeder and also to control the voltage on the battery bus bar while charging. Their function is similar to that of a series resistor except that the voltage drop in a countercell averages 2.to 2-1/2 volts per cell and varies little with changes in current.
The cell container or jar is filled with an alkaline solution over which there is a layer of neutral mineral oil. Plates are suspended in this solution and connected in series with the battery. The voltage drop in the cell is the voltage expended in forcing the current from one group of plates through the solution to the other plates. The plates are all of the same material (nickel or stainless steel which contains nickel) so that the plates can be connected to either polarity. There is practically no storage of charge in the plates so that cells in a working circuit can be shorted with safety. The disadvantage of their use lies in the fact that energy is dissipated in the counter cells, produces heat and the decomposition of water, which results in a large amount of gassing. The gases are hydrogen and oxygen which form an explosive mixture. There has been a number of explosions of counter cells in the Bell System and in a few cases have resulted in physical injuries. Today the use of these cells are not looked upon favorably and their use is decreasing.
The Ni-Cad Battery
The nickel cadmium type storage battery has been recommended for engine starting use in the Bell System. This type battery should give appreciably longer life but it has not been in service long enough to give conclusive results. The manufacturer anticipates a 15 year life on engine starting applications as compared with a 2 year life with the lead-acid type, and the NI-CAD cells on trial in the Bell System for the past few years have given excellent service with a minimum of maintenance.
The following facts about the so called NI-CAD storage batteries are of interest. The positive plates contain nickel hydroxide and the negative plates contain finely divided cadmium metal. The cell containers are of steel. The electrolyte is a solution of potassium hydroxide covered by a layer of oil similar to that used in counter cells. There is no appreciable change in specific gravity on charge or discharge so that hydrometer readings do not indicate state of charge. The electrolyte is corrosive to many materials including aluminum, zinc, clothing, the skin and paint but does not attack iron or steel. The proposed float voltage of 1.40 volts per cell is safely below the 1.47 volts per cell gassing point. With gassing held to a minimum and the electrolyte covered with oil it is not expected that carbonation of the electrolyte will force its replacement in 15 years. Water loss under these conditions is quite low.
The voltage per cell is lower than the lead acid type and as a result 38 cells are needed for a 48 volt battery, 19 cells for 24 volts and 10 cells for a 12 volt battery.
Battery Charging Equipment
Charging equipment machines, consisting of alternating current motors connected to direct current shunt generators, are used to convert commercial power for charging and floating the central office batteries. The early generators were of special design to deliver almost ripple free direct current to keep the noise at a tolerable level. These machines employed a large number of armature coils mounted in relatively shallow slots and terminated on many narrow commutator bars which, together with copper gauze brushes, kept the slot and commutator ripple to a minimum. Where the battery was connected with separate charge and discharge leads and the latter was paired and engineered for low impedances, the noise and crosstalk levels were kept within reasonable limits without filters employing capacity other than that of the storage battery. Machines of that type are still in service but most of them have been equipped with carbon brushes to reduce maintenance.
Wet electrolytic capacitors were developed in the 1920's to a point where it was practical to use them to suppress the ripples of commercial type direct current generators, when combined with suitable series inductors. This permitted the Bell System to take advantage of the economies of mass production and competition available to users of electric machinery. Common filters, for all the talking load, located at the power board were first used. The present practice is to locate the filters at the equipment frames requiring quiet battery thus limiting the size of the inductors. This was made possible by the use of economical, commercially available dry electrolytic condensers. With this scheme, the cost of the filters are not only reduced but a short low impedance path for talking circuits is provided by capacitors and the long discharge leads to the power plant are only subject to voltage drop considerations. To float the varying loads encountered charging machines are used in capacities from 200 to 1200 amperes.
Rectifiers are the other source of charging power in smaller offices. Fifty ampere mercury arc rectifiers were used in the Bell System as early as 1910. Tungar rectifiers were introduced about 1920. Small capacity copper oxide rectifiers came into use about 1925. These early rectifiers, like the charging machines were arranged for manual regulation. It was not until 1935 that work was started on automatically controlling the voltage of rectifiers. Two element rectifying tubes with input control using motor driven continuously tapped transformers were developed for 24 and 48 volt batteries in 30 ampere capacities. For plate supplies, (130 to 150 volts) the three element grid controlled thyratrons were used. These regulated rectifiers have been in extensive use for about 15 years. The 24 and 48 volt rectifiers are the charging means for power plants in the range of 3 to 240 amperes that are in use in PBX'S, community dial and other smaller offices. The plate supplies are used ill toll terminal and repeater stations.
Since 1940, the trend has been toward the use of selenium rectifiers using input saturable reactors controlled by vacuum tube type or magnetic amplifiers. Regulated selenium rectifiers of this type are in extensive use in Bell System power plants in 1, 3 and 9 amperes, 24 volts for the straight magnetic control and 200 amperes, 24 volts and 100 amperes, 48 volts for the type using tube amplifiers. The 100 and 200 ampere rectifiers are used in combination with motor generator charging machines in the larger power plants while the smaller ones are used in recently designed toll and PBX power plants. Development work is now in progress on a 200 ampere, 48 volt rectifier using either selenium or germanium electrodes with forced air cooling.
In addition to the battery charging rectifiers mentioned above, there is a wide field of use for both tube and metallic rectifier as power supplies for teletypewriter, carrier and repeater equipment. For these applications, it has been necessary to provide both unregulated metallic types and series tube types closely regulated to provide a low impedance source.
Many equipment variations of each basic type have been made available since they are usually located in the frames with their associated communication equipment and provided with covers and chassis which harmonize.
In a broad sense, control equipment can be considered as the means of integrating such basic components as batteries and chargers, into a complete power plant for a particular application. Included are means of regulating, switching, protecting and alarming the components. Modern control techniques make possible the automatic power plant, which, in remote microwave stations, operate without attendants for periods up to a month or more. Even in the case of usual central office power plant the necessity of operating during night and week end periods without attention -is requiring more and more features to be automatically controlled.
Load Control of Charging Units
In the discussion on floating the batteries, the automatic voltage regulation problems and the special control developed for this purpose were mentioned. This is illustrative of many control problems which are peculiar to telephone power plants. Another is automatic load control of the charging generators.
The larger central office 48 volt power plants, such as the 302A type, are designed to supply loads from 200 to 10,000 amperes using a regular and' spare voltage regulated machine controlled by instantaneously responsive electronic exciters and enough additional "slave" machines to carry the load. The voltage regulating machine is' normally operated between 1/4 and 3/4 capacity to permit it to follow rapid load changes up to at least 1/4 capacity. To prevent overloading, the regulated exciter provides a steep voltage drop characteristic at the full load point.
At the first load swing to the full load point of the first machine, the second machine is started and assumes the load regulation, unless the load drops back radically, while the first machine operates on a constant current basis. The second voltage regulated machine then operates between 1/4 and 3/4 load by sending "raise" signals from its ammeter relay to the third and subsequent machines' motor driven rheostats until its 1/4 load point is reached. When the load decreases, "lower" signals send the slave machines to reverse current to release them.
To avoid abortive attempts to start machines due to swings through a given load point, overlap in the control is provided by causing the first machine to call in the second at full load, and then back off to 3/4 load when the -second machine comes in. When the third machine is called in at the full load point of the second, the first one is sent to full load.
While the controls described above are applied to charging motor generator sets, they also operate in a similar manner when rectifiers are involved which can be used interchangeably with machines as far as their capacity permits. As with the machines, two types of rectifiers are used, one with automatic voltage regulation and the other arranged for "raise" and "lower" signals.
Twenty-four volt loads up to 100 amperes can be supplied from the 48 volt plant through a series of counter-E.M.F. cells to drop the voltage to 24 volts. An automatic control circuit activated by a voltage relay maintains the 24 volt supply within limits by short circuiting or open circuiting groups of the C.E.M.F. cells. Where larger 24 loads are involved for reasons of efficiency, C.E,M.F. cells are replaced by a 24 volt plant which operates with automatic control similar to the 48 volt plant. It was mentioned earlier that the counter cell method of providing 24 volts is being questioned.
Twenty-four and 48 volt automatically controlled plants for smaller .offices in the range of 10 to 400 ampere (110A Power Plant) and I 'to 30 amperes (105D Power Plant) are available using rectifiers. The 110A plant uses 30 amperes and 100 ampere selenium rectifiers, which have replaced the thyratron type. Floating voltage for the battery of 24 cells and one C.E.M.F. cell is maintained by a common controller passing raise and lower signals to the rectifier motor. The controller uses a bridge circuit balanced at the float voltage. Operation of the motor drive is limited by an ammeter relay in the rectifier.
When the voltage goes beyond the floating limits indicating that the last rectifier connected is operating at either its high or low limit a marginal relay in the controller causes the next rectifier to cut in if a raise signal is operated or to cut out if a lower signal is present. The raise or lower signals are then transferred to a rectifier which can restore the voltage to the float value.
The discharge voltage is automatically maintained during power failure by short circuiting the counter cell.
The smaller 105D plant uses regulated rectifiers which automatically cut in or retire the second or third rectifiers when the first rectifier is at its limits.
Primary power in the form of engine driven alternators has practically taken over the long term reserve from the battery. This tendency has become accelerated as the reliability of the internal combustion engine increased.
The early engines were fueled by illuminating gas. These were followed by gasoline engines. At present, diesel engines are used in sizes from 10 to 1000 kW. Gasoline engines are still used for the smaller capacities although inexpensive small diesels are available.
The first reserve engines were coupled to d-c generators which charged the battery. As the need arose for more than one battery voltage, it was found more practical to use engine driven alternators which would activate the charging equipment normally used.
Portable diesel sets from 20 to 60 kW are shipped on trucks from a central storage point to cover several offices not equipped with stationary sets. These sets can be arranged by parallel operation to cover loads beyond the capacity of one set. Portable gasoline sets in capacities from 1 to 10 kW are also available for the smaller offices. Five kW gasoline sets can also be obtained in small metal enclosures for semi-portable use.
Stationary sets are available for both manual and automatic operation up to 170 kW. Above this size, only manual control has been developed up to 300 kW. Where maintenance forces are available or can be summoned at night or week ends, the manual controls are used. On this basis, the engine is not started until it is ascertained that the power failure is not temporary. The sets are arranged for electric push button starting and in case more than one set is available, for parallel operation. Most sets under 170 kW are cooled with engine driven pusher fans operating radiators adjacent to the engine. The 120 and 170 kW sets can also be obtained with remotely located radiators with motor driven fans. The remote radiators are used to permit less expensive airduct installations. The stationary sets are equipped with high lift type fuel pumps which pull the fuel from tanks buried outside the building for safety reasons.
With the advent of unattended repeater stations, it was necessary to develop automatically controlled engine sets which would operate for long periods without attention. While commercial controls are available which would automatically start the engine and assume the load, they depended largely on having attention within a reasonable period. The automatic control used for the Bell System sets delays the starting until the power has been off about two minutes, and delays shutdown until power has been restored for five minutes. It also delays assuming the load until the set is warmed up. Protective features such as automatic shutdowns for over-speeding, low oil pressure and high water temperature are furnished. The sets can also be started and stopped for routine maintenance over a tie line from a remote control point. The air handling equipment is also automatic, opening the intake shutters at once and the discharge shutters after the cooling water warms up and by passing enough warm air to heat the engine room when it is cold. Such operation is in use on diesel sets up to 170 kW. It has been particularly helpful in the transcontinental microwave relay where several remote stations were snowed in without commercial power for two weeks in the winter of 1951-1952. On the transcontinental radio relay route, there are three stations without any commercial power in which two such automatic diesels alternately carry the load on a continuous basis.
L 3 Carrier and TH Microwave
A 230 volt 60 cycle power supply is required for L 3 carrier and TH microwave radio. It is obtained from single phase self excited alternators. They are normally driven by 3 phase induction motors on the AC service but during emergencies by 130 volt DC motors on the station batteries. The output varies with the slip of the AC motor but is normally Z30 volts at about 58.5 cycles. Obtaining the power from an alternator reduces the effect of AC service fluctuations and insulates it from lightning, power line crosses, etc. on the AC service.
The alternator sets are available in 10, 16, and 21 kVA sizes. An emergency set is run continuously at no load and automatically replaces a regular set if it fails or is removed from service for maintenance.
Ringing and Signaling Equipment
The telephone ringer operates on alternating current. Some three quarters of those in use are now the high impedance type which ring on 1/2 watt. Many years ago, 20 cycles was adopted as the standard ringing frequency. Many different voltages were employed, but since about 1935, the new ringing plants other than low cost models have been voltage regulated, the limits being 86 @ 2 volts.
In order for ringing to stop when a call is answered, a tripping relay is provided at the central office. To operate this relay, the ringing supply also includes a direct current tripping potential. A condenser at the station ringer blocks the d-c until the phone is lifted off the hook. For 4-party selective lines, a cold cathode 3 or 4 element tube replaces the capacitor and the polarity of the d-c supplied controls which ringer will respond.
These d-c voltages are either 38 or 48 volts. Negative 48 volts is taken from the main office battery and the other voltages from either storage or dry batteries. If the load justifies it, these batteries are floated by rectifiers.
The ringing load is divided into parts, usually three, each of which in turn is connected to the a-c plus d-c supply by interrupter contacts which close in sequence. During the silent intervals, the d-c remains connected so that tripping can occur at the instant the call is answered. This plan, called machine ringing, not only enables the supply to handle three times the load but also serves to signal the desired party on non-selective lines by means of different combinations of "rings".
In addition to machine ringing, the ringing power plant must also furnish audio frequency tones for various purposes such as the dial and busy tones and other, that are less familiar; audible tone added to the 20 cycle supply so that the calling party can "hear" the ringing; high tone and howler for those who forget to hang up. The interrupter also supplies timed impulses at various rates such as 30, 60, and 120 per minute. Further d-c sources are required for prepay coin boxes, in this case plus and minus 120 volts for collecting and refunding coins.
In the smaller offices being engineered today, 60 cycle power from the commercial service is converted to 20 cycles by a static type subharmonic generator which also supplies audible ringing tone, 420 cycles modulated by 40 cycles. During power failures, a rotary converter is automatically started, converting battery power to a-c to keep the ringing generator in operation. These static type generators are economical up to 50 watts output, the 5 to 25 watt models being in widespread use in community dial, manual and PBX applications. Tones are also derived from 60 cycle power by other static type generators which use saturable transformers and tuned filters to select 60 cycle harmonics; 540 cycles for high tone and 600 modulated by 120 cycles for low tone.
With static type generators, a separate motor driven cam and spring type interrupter is used for machine ringing and busy signal interruptions. In these smaller offices, all of the ringing equipment such as interrupters, ringing and tone generators, usually runs under start-stop control as the switching equipment calls for it. This results in appreciable power savings and prolongs the useful life of the moving parts in the interrupter.
In older small offices, battery driven 20 cycle rotary converters are used, the commonest size being 1/4 ampere, and about 20 watts. In this case high frequency commutator type tone interrupters and machine ringing interrupters are combined with and driven by the converter in a compact machine. Two are usually provided together with automatic controls to transfer the load from one to the other in case of failure. 'For unattended offices an additional arrangement is widely used whereby the load may be transferred between machines by dialing assigned numbers from any location. These small rotary machines have been extensively used in community dial and PBX installations and their low cost, due in part to economies derived from large production, has limited the use of the newer static sources.
In large offices, motor driven alternators are used in capacities of 1, 2, 4 and 6 amperes. Except for the 1 ampere size, these are table mounted and consist of a battery or line driven motor directly connected to a 20 cycle self excited generator and an inductor type tone generator, and through a gear box are connected to machine ringing and busy back interrupters of the mercury type. These interrupters are hollow steel drums in which mercury is sealed to break the circuits. The ringing machines are furnished in duplicate, one with a line driven motor, and the other battery driven. Automatic controls activate the reserve machine upon abnormally high or low 20 cycle output, tone failure, and interrupter failure. The exciter commutators on these generators are also used to provide plus and minus 120 volts d-c for coin box operation. These sets are in use. in practically all of the larger Bell System offices and have a good record of continuous service.
The 1 ampere machines, produced since late 1953, are a refinement on the compact machines used in small offices. They differ by having line and battery motors, tone alternators, better reduction gearing, and nylon cams in the interrupter. Although giving greater output, they are still small enough to be mounted in the framework of the power board. The gradual conversion to combined telephone sets with their more efficient ringers has reached a point where these new machines may be used in all but the largest metropolitan areas with consequent savings of about half the installed cost and floor space formerly needed.
Cabling and Wiring
Conductors are required to tie the various component parts of a power plant together and to deliver power to the various frames, switchboards and fuse panels. At one time it was thought that all power conductors regardless of the voltage, current or type of power they carried must be enclosed in metal conduit. Early dial exchanges had miles of conduit either buried in the floor or hung from the ceiling. Today only conductors carrying alternating current are run in conduit all others are installed on cable racks similar to switchboard cables.
Power cables conform to standards of the National Board of Fire Underwriters with respect to insulation, covering and fusing. The commonly used sizes range from No. 14 to 800,000 circular mills in area. The sizes of the conductors used are determined by the following considerations:
a. Safe current carrying capacity which is the amount of current a conductor will carry without being overheated.
b. The permissible voltage drop or the amount that can be lost in a pair of conductors and still supply sufficient potential at the end of the loop to insure that the apparatus will operate satisfactorily.
c. Crosstalk limits for talking current required that the conductor be sufficiently low in reactance or effective resistance so that what is being transmitted in one telephone circuit cannot be overheard in others.
In the larger power plants bus-bars are used to carry the current between the strings of storage battery, the power switchboard and the charging motor generator sets. Formerly only copper bus-bars were employed due to the high conductivity. Aluminum bus-bars are used extensively today. The busbar made of aluminum must be one and one half times larger than a copper bus-bar to carry the same current.
Within the past five years bus-duct have found a place in the telephone power plant. It is a prefabricated section of three bus-bars enclosed in a metal casing carefully insulated one 'from the other. These bus-bars carry three phase, 60 cycle alternating current to feed the motors of a bank of charging motor generator sets. Sections of bus-duct may be joined together to obtain the desired length and conduit risers join the motor terminals with the overhead runs at junction boxes. The use of this device eliminates overhead runs of conduit, one to each motor in the bank.
Central Office Ground
The central office ground system is regarded as a part of the power plant cabling and wiring system. Ground leads are usually No. 0 and are run in duplicate via different routes. They may be installed in conduit for protection from mechanical injury or run openly. The ground lead is usually run to a clamp on the water pipe where it enters the building and a bond placed around the water meter. Non-metallic pipe is used in some water distribution systems making it necessary to use ground rods.
The central office ground protects the telephone user and workmen from injury due to lightning or crosses between telephone cable and high potential lines. It is also part of the operating path for ringing circuits, toll line, signaling circuits, etc.