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"Main grid / local generation load sharing",
(c) 1993 Alternative Technology Association, Inc. All rights reserved.

The following article is from Issue #46 (Summer 1993) of Soft Technology
Magasine:

In issue #45 of Soft Technology we had a look at a range of city-based
renewable energy systems. Now we're taking a look at the way these systems
work in with houses that are wired for mains 240 volts. 
 
Mick Harris

When people start to use a renewable source of electricity while `mains'
power is available, they almost always continue to use grid power in some
form.  In some cases grid power is only used as a back-up energy source to
`top up' the system when the renewable energy source is inadequate. In other
cases part or most of the home continues to be run on grid power, while
renewables supply just part of the power needs.
 
Rules and regulations
If you are going to make any changes to the electrical system of a standard
`mains' connected house, the wiring must be done by a registered `A Grade'
electrician. Mistakes made with  integrating renewable energy into your
wiring could lead to fires or electrocution.  And you can be sure that your
insurance company won't cover you for any problems you have with your wiring
if the wiring in question was done illegally. 
All the wiring changes must be done to the relevant and current Australian
Standard. Switching from the mains to the inverter must maintain complete
isolation so that the output of the inverter cannot be connected with the
mains. 
Once a renewable energy system is integrated into a mains system, it is very
important to clearly label that the inverter, as well as the mains, are to
be turned off before working on the wiring. Otherwise an unsuspecting
visiting electrician may get an unpleasant surprise.
Inverters used in mains connected house systems must use the MEN (Multiple
Earth Neutral) wiring standard. This means the neutral and earth must be
connected back to a common point and connected to ground.
Some  inverters use a floating earth system where the inverter output is not
isolated from the battery.  If one of these inverters is used, and the
battery bank is earthed, current could flow  from the mains neutral through
the inverter to ground, potentially damaging the inverter. Floating earth
systems have been used  for many years and  can be quite safe in properly
wired RAPS systems, but should be avoided in a mains connected house.
 
The low-voltage option
The simplest way to add renewable electricity to your 240 volt,
utility-powered home is to add a completely separate low voltage circuit.
Leave all the existing wiring and add some light and power circuits which
run straight from your solar or wind system. There are three big advantages
to this approach: safety, legality and efficiency.
 
Safety
All the other approaches mentioned in this article involve modification to
240 volt wiring. If this is done incorrectly there can be serious
consequences. Working with a low voltage such as 12 or 24 volts is safe.
There is no danger of electrocution and provided care is taken there should 
be no risk of fire.
 
Legality
You can muck around with low voltage DC quite legally. You don't need any
special qualification.
 
Efficiency 
Low voltage appliances tend to have lower power requirements and higher
efficiencies. They give you a built in limitation on the amount of energy
they use, whereas when you have 240 volts available it is all too easy to be
tempted to keep using those standard 240 volt energy inefficient appliances.
Despite these advantages many people tend to go for an inverter powered 240
volt system which is integrated into the existing house wiring. This
overcomes the need to run special low voltage wiring and obtain what are
often more expensive low voltage appliances.
 
Integrating with 240 volts
The simplest way to integrate the systems is with a plug and socket
arrangement or with a changeover switch. 
The plug and socket approach is simplicity itself. An additional board is
placed next to the existing switchboard. It consists of two sockets; one
connected to the existing mains supply and one connected to the inverter of
your renewable energy system. A heavy cable with a plug on the end allows
you to connect the house to the power supply of your choice.
This approach is foolproof. There is no way you can accidentally connect the
mains and renewable energy supply. The only thing you need to be careful of
is that the current rating of the plug, lead and sockets are adequate for
the current required to run the house. (The diagram above shows what this
system looks like.)
 
 
The changeover switch option
A changeover switch is a neater way of doing the same thing and should be
just as safe if installed correctly. A heavy duty two pole, two throw switch
with an off position in the centre, is fitted next to or into, the existing
switchboard. 
This switch is similar to the changeover switches used to switch electric
hot water units from off-peak to day rate. However in this case it is used
to switch the whole house from drawing on the mains electricity supply to
using the renewable energy system. Because the switch must be turned through
the centre `off' position before a different energy source is connected
there should  be no chance of the renewable and non-renewable energy sources
making a connection.
With this approach you could also wire a battery charger into the mains
supply and use it to charge the batteries of the renewable energy system. So
whenever the batteries get a bit low and you switch to the mains back-up,
the batteries get a  boost from the battery charger as well as from the
renewable energy source. 
 
 Splitting the load
The biggest problem with this kind of system is that it is an all or nothing
approach. If your solar or wind system is not adequate to run the whole
house you may only want to run parts of your home, maybe the lighting
circuits and some power points.  If you want to keep the circuits with say,
the fridge, stove or hot water on the mains while the rest run on
renewables, you will have to be a little more imaginative with the wiring. 
If you choose to do this, you can put your changeover switch on a selected
number of circuits. To achieve this the main switch board would have to
opened. The active and neutral wires for the selected circuits would be
connected to the output of the changeover switch. The changeover switch
would then have the two inputs connected, to the one from the inverter, and
the one from the mains. You could then run the selected circuits from either
source of power while the rest of the circuits remain `on the mains'.
 
The lazy way
Another approach is to use an automatic changeover switch. Some regulators
have an automatic low battery disconnect or alarm. If this is connected to a
relay, which switches the 240 volts supply from the inverter back to the
mains, then you have an automatic system to reconnect to the mains as soon
as your batteries get too low. 
When the batteries have recharged, the relay switches your renewable energy
power system back on. This was the method used by Sandy, whose system was
described in the `Solar in the City' article in the last issue of Soft
Technology.  He used a Pecan Engineering regulator in conjunction with an
Omron 12V + relay to switch his system.   The wiring diagram and component
list is included on the final page of this article.
 
The top of the line
The Rolls Royce way of integrating your renewable energy system to the grid
involves the use of a grid interactive inverter. These inverters are
designed to feed excess power from your renewable energy system back to the
mains grid and with the use of appropriate metering, you could even get paid
by the local electricity supplier for the power you feed back into the
system. In practice Australian energy utilities are reluctant to pay people
for the energy they generate from this kind of system. They tend to argue 
that the value of the electricity you have available to sell to them is so
small that it isn't worth their effort.  
Be aware, though, that you cannot set up a grid interactive system with a
standard, modified square wave inverter.  None of the approaches we describe
in this article allow you to feed back into the grid.JGrid interactive
systems are outside the scope of this article. 
If you would like to try this approach, talk to one of the sine wave
inverter manufacturers listed below and discuss it with,  and get the
necessary approvals from your utility.  The inverter manufacturers that can
do grid interactive inverters are Butler Solar (Vic), Advanced Energy
Systems (WA) and CSA Energy Technique (Vic) . 
In practice, the money most people would make selling power back to the grid
is so small that it would never get close to paying the cost of the grid
interactive inverter and the metering that is needed to set up the system.
In conclusion
A surprising number of people are now engaging in this kind of `uneconomic'
use of renewable energy.   If you like renewables, and want to play with
them in your own home, it is certainly possible and safe, as long as the
wiring is done correctly.

				------

Soft Technology is a quarterly magazine providing practical information on
how to harness renewable energy sources such as the sun, wind and water; how
to build with natural materials and conserve energy; and how to work with
appropriate technology to create a sustainable future.
 
Soft Technology is published by the (Australian) Alternative  Technology
Association, a non-profit community group involved in promoting and using
appropriate technology. ATA has more than two thousand members worldwide who
share information and ideas. ATA also produces  a wide range of publications
including practical booklets on building your own renewable technology.
 
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