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Willson Carpenter Clark; 

April 17, 1936 


This thesis deals with the history, construction, and 
operation of the Takoma Park Jab-station. After a little more 
than two months of hard work in the winter, the sub-station 
was operated for the first time on March 3, 1933, Its capacity 
Is 60,000 KW, furnished to Washington through three westing- 
house transformers, which step the voltage down from 230,000 
volts on the transmission line from Safe Harbor to 13,000 to 
be sent to other stations in the city. The transformers are 
protected by lightning arresters, circuit breakers, and other 
safety devices, while one attendant is always on duty. In the 
three years of its service, this substation has proved that 
it is a worth while project, and will probably be enlarged in 
the future. All of the above details are taken up 
in the thesis and discussed with more detail. 

singularly • ■ ' 



Since the beginning of the use of electricity in Washington, the 
Potomac Electric Power Company, although not always under that name, has con- 
tinued to expand and increase its output, for many years the power was pro- 
duced wholly in one location, the Bennlngs station, which was adequate for the 
requirements of this city. In the past few years, however, this station has 
proved that it would not be capable of suppling the increasing demand that was 
being placed upon it. For this reason, as well as for the safety obtainable in 
seperated plants, it was decided to supplement the system with another steam 
plant, which was built and named The Buzzard's Point station. This was completed 
in 1933, and has an output of 35,000 kilowatts. Tae other addition to the »ash- 
ington power system was the Takoma Park Sub-station, which is a transformer step- 
down station supplied by hydro -electric power. The remainder of this thesis is a 
brief, but detailed, discussion of this sub-station. 

At the time when the Potomac Electric Power Company decided to in- 
crease its output capacity by building the Buzzard's Point station, a high vol- 
tage transmission line was being constructed from the recent Safe Harbor devel- 
opement on the Susquehanna River to Baltimore City. This line passed through 
Ellicott City, only twenty two miles from Washington. By branching off at this 
point and continuing a line to Washington would not only give a ready outlet to 
the econemic power generated at Safe Hartoor, but also Berve the purpose of tying 
in the P. E, ?, Co, with the great interconnected system of Penhslavania, New 
Jersey, New York, and other states, Baltimore and Washington would likewise be 
connected directly together, which had been planned for a long time to give 
economy and protection to both cities. Another reason for the immediate con- 

struction of this sub-station was due to the agitation at this time over a pro- 
posed hydro -electric dam at Great Falls, an the Potomac River, which would offer 
serious competition to the monopoly enjoyed by the Potomac Electric Power Go. 

Having obtained the location for the substation, which is on the 
crest of a hill about half a mile East of Takoma Park, the contract for the con- 
struction was given out In October, 193£. Not until December of that year, however, 
did the work actually begin. At that time the contracting company began the con- 
struction of the building and transformer foundations. This building is a two 
story brick structure, about 50 by 60 feet in sii.e. On each floor is a large room 
which contains the line disconnects, circuit breakers, etc. On the second floor 
is also the control room, while a loc&er, with a few spare parts, are in a small 
room on the first floor. The foundations for the transformers have a depth of 
eight feet, and around each one is a three foot layer of gravel for the purpose 
of draining any oil which might leak from the transformers, A concrete fire wall, 
one foot thick, was placed between the transformers. The transformers were 
delivered and assembled by the ieetinghouse Manufacturing Company, each one being 
brought in two pieces, the bottom half and the coil together being the heaviest 
and weighing about 55 tons. It was necessary to lay a plank roadway from the main 
highway to the eub station so that the large trailers carrying the transf ormers 
could get over the soft road. They were assembled by means of block and tackle. 
The lightning arresters, line disconnects, spill gaps, and the steel framework to 
support them were built by the Susquehanna Transmission Company, a company organized 
for the purpose of construoting the transmission lines leading from the Safe Harbor 
developement, This company was also building the transmission line from Ellicott 
City at the same time that the sub-station was being constructed. The transmission 
is composed of three aluminum- steel cables, 795,000 CM each, spaced horizontally 
£8 feet apart. They are suspended by 9-l/£ feet insulators from steel towers, 


1000 feet apart, and are 84 feet above tne ground. Twenty five feet above these 
three are two other wires, grounded at each tower, to afford lightening protection. 
The B. 3. P. Co, owns the transmission line and ground for a distance of about ten 
miles, the total cost of which was about #30,000 per mile. The sub -station itself 
cost ^456,000, including the transformers. It was completed and put in service on 
March 3, 1933, and since that time no serious or prolonged inter apt ions have been 
encountered, although the transformers have been taken out of service for minor 
replacements several times. 

The entire substation is enclosed in a £00 foot square. The drawing 
below (not drawn to scale) illustrates the relative position of the various elements. 
As shown, the high voltage line approaches from the north-east side of the enclosure, 
passes through the lightning arresters, line disconnects, and spill gaps to the 
transformers. At this point it is stepped down to 13,000 volts from £30,000 and 
passed to the control building underground. In the west corner is a smaller trane- 

2-30 KV Line 


Li~n. Arresters 




Sp/// Go. fas 


Mate, Trans, (w) 

33 KV Tr&HS 


p£) Shed 



former which steps 10,000 KW from 13,000 baci up to 33,000 volts to be sent to 
Bennings, In the south corner is the repair shed, which is connected to the 
transformers by means of a steel track with a small trolley. 


In a sub-station of this kind the transformers are the most important 
item, and should be, as was done In this case, constructed and tested under the 
most exact specifications. The three main transformers, each a single phase, 
60 cycle unit of £0,000 EVA capacity, were constructed by the Westinghouse Man- 
ufacturing Company, Before being accepted by the P. E. P, Co. they were tested 
for open and short oircuit conditions, a heat run carried out, and the insulation 
strength tested under standard A. I.E. E. conditions. They were designed as 
15,000 KVA units when self cooled, but the addition of electric fans on the air 
cooled radiaters boosted this capacity to 20,000 KVA. The primary winding is 
connected to the £30,000 transmission line through a tap changer, which permits 
a variation in voltage from 192,000 to £36,000 by nine different steps. The 
purpose of this tap changer is to keep the voltage constant and permit Baltimore 
to carry the reactive load, since Washington is the customer, Whenever a meter 
In the control building indicates that the reactive is swinging over to Washing- 
ton, the attendant will, by remote control, change to the tap of next higher 
voltage. If the motor of the taj? changer should fail, the tap could be changed 
by hand. 

The cooling of transformers of this size is always a major problem. 
In each of these transformers there are 9,350 gallons of a thin oil, which cir- 
culates through the main shell and the radiaters on the side of the shell. The 
electric fans that cool the radiaters turn on automatically when the temperature 
of the oil reaches 50 degrees Centigrade. On the outside of each transformer is 
a hot spot temperature recorder, and in the meter room is also an automatic 
recorder that tabulates the temperature every ten minutes. 

As a protection against air getting into to the inside of the trans- 


formers and da;naging the coils, a nitrogen tank is connected to each one which 
keeps the shell filled with nitrogen gas to a pressure of 5 pounds per Sq. In. 
Shis pressure is released by a safety valve if it should become any greater. If 
the valve should stick* a pressure release diaphram will blow out if the pressure 
reaches 7 lbs per Sq.In, If this happens, a relay operates an oil circuit breaker 
and disconnects the load. Then, before the transformer can be put back in service 
the oil must be drained off and filtered. Another protection offered by the 
nitrogen is in case of fire about the transformer due to the transformer breaking, 
when a blanket of the gas would help to smother it. 

Both the high and low aide of the transformers are connected "Y", that 
is, each primary winding is connected between a 230,000 volt line and the ground, 
and each secondary winding between a 13,000 volt bus and ground. In addition there 
is a third winding on each transformer for the purpose of eliminating the higher 
harmonics in the voltage wave. These windings are connected in a closed delta 
between the three transformers. 

Besides the transformers above there is another smaller one made by 
the General Electric Company with a 10,000 KVA capacity for 60 cycle, 3 phase 
current which steps the voltage up from 13,000 to 33,000 volts for transmission 
to Bennings. This transformer is also oil cooled, but without the use of fans. 
Since the load on it is practically constant it requires little attention or 

After the voltage has been stepped down irom £.30,000 volts it passes 
underground into the control building that houses the oil circuit breakers, line 
disconnects, reactors for smoothing out any instantaneous irregularities in the 
current, transformers for the operation of the relays and meters, and the meters 
and controls. The meters are on one large board with the controls. Xhey consist 
of watt hour meters furnished by both the Potomac Electric Power Company and the 


Consolidated Gas and Electric Light and Power Company of Baltimore as a check 
on one another t numerous wattmeters and voltmeters to indicate the load and 
voltage of the high and low side of the transformers at any time, a synchro- 
scope to show whether the sub-station is in phase with the Bennings plant, a 
temperature recorder for the three large transformers, a reactive load meter, 
and an automatic oscillograph, which will take pictures of the current and vol- 
tage instantly if there should be a ten percent increase or decrease in the 
normal voltage. A bank of ISO Exide batteries in this building supply the cur- 
rent for the operation of the oil circuit breakers and the tap changers. These 
batteries are kept floating by a small motor-generator set. 

The relays are for the purpose of detecting any dangerous disturbance 
on the line and operating the necessary safety device, For instance, to operate 
the oil circuit breakers, there are the overload relays and the directional 
ground relays. The former operate with a dangerous rise in current, while the 
latter will trip the circuit breaker if there is a ground on the line which is 
closer to this substation than to the other end of the line. These relays are 
set to operate from 45 to 60 cycles after the disturbance begins, which will 
allow for any instantaneous surge that might not be harmful. Among trie other 
relays are the over voltage and undervoltage relays for the oscillograph, and 
an oil circuit breaker relay for protection against the pressure release dia- 
phram blowing out, 

The circuit breakers are of two types, oil and air, which means that 
the flash over from breaking the circuit is broken in oil or in air. The oil 
breakers are all on the 13,000 volt lines, and have a capacity of 1200 amperes. 
They are gang operated, which means that two or three lines are connected to one 


breaker . If one line should be grounded the circuit breaker will disconnect the 
three lines, but the relay will indicate the grounded line and this one could be 
taken out of service and the others used by closing the breaker, The air circuit 
breaker is on the £20,000 volt line and ia opened by hand after the oil breakers 
are opened. The flash-over caused by opening the circuit in air is often five 
feet long, 


The power for lighting and operating the sub-station is obtained from 
a line directly from the Brightwood station, and is stepped down to 220 volts. 
At night the sub-station is lighted by a number of 500 watt bulbs around the 
grounds, so that it is a blaze of light and can be seen for miles around. This 
power also operates the fans on the transformers, the electric hoist in the 
repair shed, and the motor-generator set. It is also used for heating, so that 
no fires are required on the premises. If this power should fail for any reason 
the batteries could be connected so as to light the building and run any neces- 
sary equipment during the emergency. 


Besides the safety given by the circuit breakers and lightning protec- 
tors there are many other safety measures that must be taken. All the metal parts 
of the sub-station are grounded except, of coarse, the lines, which are well 
shielded from the body. Pilot lights will Indicate whether the line is dead or 
not so that it can be worked on. Although there is little that would burn, fire 
extinguishers are placed at convenient points, A tall wire fence around the sub- 
station prevents some person from breaking in and unknowingly pulling a control 
handle, which might cause untold trouble. 


The repair shed consists of a tall barn like structure, made of steel 
framework and covered with galvanized sheet iron. On the Inside there is an elec- 


tric hoist which is capable of raising any part of a transformer ao that it can 
be worked on. On one side of this shed is a large oil drum into which the oil in 
the transformer can be drained and filtered before replacing* A track and email 
trolley in front of the shed permit the transformers to be wheeled from their 
positions to the shed, 

The entire personnel at the sab -station is composed of one man who is 
actually kept busy only a small portion of the time. Besides being constantly on 
alert for any indication of trouble, it ia his job to tabulate the load, tempera- 
tures of the transformers, and other meter readings every hour. At his desk is a 
telephone that la directly connected at all times to a person of higher authority 
to whom he reports every hour after taking the readings. Before he makes a move. 
such as changing the taps of a transformer, he must obtain permission to do so, 

The accompaning blue print shows tne method of connecting this sub- 
station with the Washington power system, Seven 500,000 CM feeders lead to the 
sub-etation in Brigdtwood, where the power is distributed as needed. The connec- 
to Brightwood is underground, whereas the 33,000 volt. 10,000 kilowatt line to 
Benninga ia an overhead line, This line is very impractical in that it can not 
be used during an electrical storm, when it might be needed the most. It is seen, 
then, that none of the power from this sub-station is sent directly to the custo- 
mer, but passes through some other station first. This gives added protection to 
the customer since the other station could pick up the load in case this sub- 
station went out. In itself this sub-station is a good protective measure in case 
of disability of the steam plants. Such was almost the case during the recent 
flood, which came dangerously near to submerging the steam plants, but had no 
effect upon this transformer station.