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3 Phase Distribution

3 Phase Power Distribution and Transmission

3 phase electricity distribution is the process in the delivery of 3 phase power from the
generation equipment to the business or location for use. This include the transmission over
power lines, possibly through electrical substations and pole-mounted transformers, and the
appropriate distribution 3 phase wiring and sometimes electricity meters.
3 Phase Power Distribution Transformer
After numerous further conversions in the transmission and distribution network the 3 phase
power is finally transformed to the standard mains voltage (the voltage of "house" or
"household" current in American English). The power may already have been split into single
phase at this point or it may still be 3 phase. Where the step-down is 3 phase, the output of
this transformer is usually star connected with the standard mains voltage (120V in North
America and 230V in Europe) being the phase-neutral voltage.
Another system commonly seen in the USA is to have a delta connected secondary on the step
down transformer with a center tap on one of the windings supplying the ground and neutral.
This allows for 240V 3 phase as well as three different single phase voltages (120V between
two of the phases and the neutral, 208V between the third phase (sometimes known as a wild
leg) and neutral and 240V between any two phases) to be made available from the same
Generating 3 Phase Power From Single Phase
When single phase power is readily available but 3-phase power is not already allocated, there
is an easy way to generate 3 phase power with a 3 phase power generating Rotary Phase
Converter or with a modern Motor Generator Set. Today these are a super efficient method to
get 3 phase power anywhere single phase is already available. Read more about super
efficient 3 phase generating Rotary Phase Converters here.
Electric Power Distribution History
In the early days of electricity generation, direct current (DC) generators would be connected
to loads at the same voltage. The generation, transmission and loads all needed to be of the
same voltage because, at the time, there was not a common way of doing DC voltage
conversion (other than motor-generator sets which today have became super efficient). The
voltages usually had to be fairly low with old generation systems due to the difficulty and
danger of distributing high voltages to small loads. The losses in a line transmission cable are
proportional to the square of the current, the length of the cable, and the resistive nature of
the conductor line wire material, and are inversely proportional to cross-sectional area. Early
power transmission networks were already using copper, which is one of the best conductors
that is also very economically feasible for this application. To reduce the current while keeping
power transmission constant requires increasing the voltage which, as previously mentioned,
was, at that time, problematic. This meant in order to keep losses to a reasonable level the
(DC) Edison power transmission system needed thick cables and local power generators.
Alternating Current (AC) Becomes Most Common Standard
Soon, the adoption of alternating current (AC) for electricity generation dramatically changed
the situation. Power transformers, installed at power substations, could be used to raise the
voltage from the generators and reduce it to supply loads. Increasing the voltage reduced the
current in the power transmission and distribution lines. Thus the size of conductors required
and distribution losses incurred were also reduced. This made it more economic to distribute
power over long distances. The ability to transform to extra-high voltages enabled power
generators to be located far from loads with transmission systems to interconnect generating
stations and distribution networks.

Though due to power line losses, it is still often valuable to locate the power generators nearby
the actual power load.
In North America, the early power distribution systems used a voltage of 2200 volts cornergrounded delta. Over time, this was gradually increased to 2400 volts. As cities grew, most
2400 volt systems were upgraded to 2400/4160 Y three-phase systems, which also benefited
from better surge suppression due to the grounded neutral. Some city and suburban power
distribution systems continue to use this range of voltages, but most have been converted to
European systems used higher voltages, generally 3300 volts to ground, in support of the
220/380Y volt power systems used in those countries. In the UK, urban power generation and
transmission systems progressed to 6.6 kV and then upgraded to 11 kV (phase to phase), the
most common power distribution voltage.
North American and European power distribution systems also differ in that North American
power distribution systems tend to have a greater number of low-voltage step-down
transformers located closer to customers' premises. For example, in the US a pole-mounted
transformer in a suburban area may supply only one or a very few houses or small businesses,
whereas in the UK a typical urban or suburban low-voltage substation might be rated at 2MW
of power and supply a whole neighborhood. This is because the higher voltage used in Europe
(230V vs 120V) may be carried over a greater distance without an unacceptable power loss.
An advantage of the North American setup is that failure or maintenance on a single power
transformer will only affect a few customers. Advantages of the UK setup are that fewer
transformers are required; larger and more efficient transformers are used, and due to
diversity there need be less spare capacity in the transformers, reducing power wastage.
Rural power electrification systems, in contrast to urban power systems, tend to use higher
voltages because of the longer distances covered by those power distribution lines. 7200 volts
is commonly used in the United States; 11kV and 33kV are common in the UK, New Zealand
and Australia; 11kV and 22kV are common in South Africa. Other voltages are occasionally
used in unusual situations or where a local utility simply has engineering practices that differ
from the normal practices
Power Distribution Network Layout
Power distribution networks are typically arranged out in one of two types, radial or
interconnected. A radial network leaves the station and passes through the network area with
connections to other points of supply.
opening switches. The benefit of the interconnected model is that in the event of a fault or
supply. The only downside to this design occurs when there is a major power outage that
more customers without power. There are protections in place to keep this from happening
though it still occurs every few years in places where this method of power distribution and
transmission is used.
Characteristics of the supply given to customers are generally mandated by law and by
contract between the electric power supplier and customer. Variables include: AC or DC Virtually all public electricity supplies are AC today. Users of large amounts of DC power such
as some electric railways, telephone exchanges and industrial processes such as aluminum
smelting either operate their own generating equipment or have equipment to derive DC from
the public AC supply).
Phase and Frequency Converters
There are several instances where the equipment may need not only the phase changed from
1-phase, or the rare 2-phase (in the US this is mostly used in Chicago) to 3 phase power, but

also the frequency converted from 50Hz to 60Hz or 400Hz (400Hz is mostly used in ships and
aircraft). Click here to read more about 3 phase frequency converters.