Standards, oh standards!
I will try to explain the clearest way I can how the power distribution
system in Chile is configured. The same or almost the same is used in
most other countries that have 220 to 240V nominal voltages.
The distribution from generating plants to the transforming stations in
the periphery of cities is done in 3 phase circuits without a neutral,
at typical nominal phase-to-phase voltages of 500, 220, 154, 110 and 66
kilovolts, depending on the power level and distance each line has to serve.
In the transforming stations this is transformed down to 23 kilovolt,
three phase, no neutral, using delta-connected three-phase transformers.
These three-wire 23kV lines form a sort of network in the cities,
typically along the main streets.
Distribution transformers in a power class of typically 100 to 500kVA
transform this down to the voltage level that is provided to homes and
businesses. These transformers have a delta-connected primary using 23kV
windings, while the secondary delivers about 230V per winding, and is
connected in star fashion. The center point of the star is earthed at
the transformer location, AND NOWHERE ELSE. Four wires (three phases and
one neutral) start at the transformer, and run along all streets,
feeding homes and businesses. The no-load voltage is about 230V between
any of the phases and the neutral, and close to 400V between any two
phases. These voltages drop somewhat due to the wire resistance, so that
the nominal supply voltage is 220V +/-7.5% between phase and neutral,
and 380V with the same tolerance between phases.
All normal houses get one phase and the neutral. The same number of
houses are connected to each phase of a transformer, to achieve
reasonable load balance. The maximal power that can be connected is
10kW, but most homes have are limited to only 6kW or even just 3kW. The
limit is set by a fuse in the phase line, where the wires depart to a
customer's energy meter.
Businesses and small workshops typically get all three phases and the
neutral, with higher power limits, as needed. Appartment buildings also
get the three phases plus neutral, which each appartment connected to
just one of the phases, but the three phase supply being available for
things like elevator motors and water pumps.
Large industries usually get a direct higher voltage feed, and use their
own internal distribution systems.
In a typical Chilean home, one fused phase and the unfused neutral come
into the property, the energy meter, a main circuit breaker rated lower
than the fuse on the light pole, and then to the home's distribution
panel. At that place there are a few circuit breakers and a ground fault
interrupter. The GFI is common to all circuits that have outlets, but
not mandatory for lighting circuits. The circuit breakers are one per
circuit, and are in the phase. The neutral is connected directly from
the GFI to all circuits, without any further switching, and certainly
without grounding.
There is a safety ground circuit, connecting all ground pins in outlets
throughout a building, and this goes to an earthing system which belongs
to each building.
There are no local GFIs, fuses or circuit breakers in any outlet.
The whole system operates at 50 Hertz.
Each distribution transformer powers many homes, typically 30-200.
In rural areas the 23kV three-phase lines are strung over long
distances, and rural homes are connected either singly or in groups of
two or three, when possible, to small "pole pig" type distribution
transformers, which are single phase, having a 23kV primary connected to
two phases, and a 230V secondary of which one side is earthed at the
transformer site, forming a neutral and a single phase.
For the last several kilometers of such rural lines, the phases are
split up, and each area gets just two wires (one phase-to-phase
circuit). So, three-phase power is not available in the most remote
rural areas.
I understand that in Europe and many other countries much of the same is
true, only that the voltage is nominally about 5% higher, and that the
power level connected to homes is higher too, often requiring
three-phase connections to homes. In Germany, for example, it's common
to have electric kitchen ranges taking 5 to 7kW of max power. In Chile
these are extremely uncommon. People mostly use gas (natural gas or
propane) for cooking and for hot water. For heating, most people in
southern Chile use firewood, which is abundant and cheap.
Now it would be good if someone would give a similar description about
the US system. I know that homes use split-phase systems, with
center-tapped 120+120V windings in the distribution transformers, with
those center taps earthed, so that with a three-phase main distribution
that would lead to a six-phase low voltage system, and that it all runs
at 60 Hz. But the whole earthing arrangement and the upstream details
are not clear to me.
In any case, the power supply of my half-developed amplifier, using
direct line feeding, can run equally well from the 220 to 240 V with one
grounded side used in Europe, Chile, and many other countries, or the
240V with grounded center point available in the US and some other
countries. So don't worry, you can all copy my circuit, if I ever
complete it! ;-)
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|