AC Power Supplies at Home

Ever wondered how does electricity comes to your house or suppose if the power is off, how you are still getting electricity at home. Actually there can be many ways to get AC power supply without actually having to stay without electricity.

4 Sources of AC Power Supply at Home

AC Mains:  Basically owing to the ease of its transmission and low cost and easy to convert to DC, AC power is preferred over DC power for supply to homes. Ever wondered how this whole power distribution system works? No?

Let me give a brief idea about the whole system

The basic power distribution grid consists of the following subsections:

• Power Plant: The power plant is the place where the 3 phase AC power is generated. The reason for using 3 phases is that all the phase currents tend to cancel each other, maintaining a balanced load and can produce a rotating magnetic field used for electric motors. The power plant generally consists of steam turbine generators that work on steam obtained by burning coal, oil, and natural gas or from nuclear power plants. The AC power generated from the generators is converted to a high voltage at about 155KV using large step-up transformers.
• Transmission substations: The generated power at a high voltage of 155KV enters the transmission substations which consists of a step-down transformer, circuit breakers, and control equipment and converts the high voltage AC power to low voltage AC power of 60kV to be fed to the transmission circuits to the power distribution unit.
• Transmission Unit: The transmission unit consists of each 3-wire towers each carrying a phase and also a fourth wire which acts as a ground to protect from lightning. Normally transmission distance is about 400 km.
• Distribution Grid: It consists of step-down transformers that convert the incoming high voltage AC supply of 60kV to 12kV and distribution buses to transmit the AC power.
• Transmission Units to home: The transmission unit consists of 3 wired towers that carry the AC power in each phase and also consists of regulator banks to prevent transients in voltages and taps to obtain single phase or 2 phase supply from the 3 phase supply.
• AC power unit near homes: The AC power unit consists of a step down transformers on the electric poles, which step down the AC voltage from the transmission lines to normal AC voltage of 240V for home supply. The 240V supply comes with three wires, with two wires carrying 120V each at 180 degrees phase difference and the third wire a neutral or ground wire.

Solar Power: Another source of obtaining power at your home is using solar energy. Owing to its replenishment and ease of availability, solar energy is emerging as one of the main sources of energy. Solar power distribution at homes consists of the following components:

• Solar Panels: An array of solar panels consisting of solar cells is placed on the roof of the homes in such a direction so as to achieve maximum sunlight and convert this sunlight into electric energy.
• Charge Controller: The work of the charge controller is to control the charging of the batteries to ensure excess DC voltage doesn’t flow to the batteries. It also ensures the charging of the battery in case of drained out power for the battery.
• Batteries: A set of almost 12 batteries are used to store the DC electric power from solar cells.
• Inverter: It is used to convert the DC power from batteries to require AC power for running the appliances which need AC power for their operation.

Uninterrupted Power Supply: In the previous point, we came to know about storing solar power and then converting the DC power to AC using inverters. The same can be done for AC power from mains.

In normal mode, the power supply comes from the AC supply mains and is given to the loads after being regulated by the stabilizer. This AC voltage is converted to DC voltage to charge the batteries.

In the backup mode, the stored DC power in batteries is converted to AC power using inverters. A basic Inverter consists of a transformer with center-tapped primary winding along with switches which allow the current to flow back to the battery through the primary windings, thus allowing the creation of AC voltage across the primary windings.

Generators: A backup Generator for homes works on natural gas or diesel. It consists of a controller that monitors the flow of current from mains supply through the automatic transfer switch. In the case of power failure, the automatic transfer switch closes the mains lines and opens the power line from the Generator. Thus after a gap of 10 seconds from the outage of power supply, the Generator starts working and supplies power to the home appliances. When power comes back, the controller senses this and automatically switches off the power supply from the generator and starts monitoring the main supply again. A Generator is cheaper and has less consumption, but is noisy compared to Inverters.

Automatic Selection of Power Supply Source at Homes

We can build a simple Automatic unit to select any one of the power supply sources. What we need is a basic Microcontroller, a relay driver, and 4 relays.

The system consists of 4 push buttons interfaced with the Microcontroller, each representing the condition of the availability of each power source. The Microcontroller accordingly drives the relay driver to select the proper relay connected to the corresponding power source.

In normal operation, the Microcontroller drives the relay driver so as to make the load connected to the Mains supply through the corresponding relay. When the first push-button representing the Mains supply is pressed, it indicates the failure of Mains Supply. In this case, the Microcontroller is programmed so as to give logic high input to one of the input pins of the relay driver (connected to the corresponding alternate power source) and the relay driver accordingly develops a logic low signal at its corresponding output pin. The relay connected to that alternate power source is connected and allows the power supply to the load. When any of the alternate power supply along with the Mains supply fails, the other available supply is chosen. In other words, if both the Mains Supply push button and the adjacent push button are pressed, the alternate power source corresponds to the third push button. An LCD can be used to display the load condition.

Photo Credit

• Power Distribution System by wikimedia
• Solar Power to Homes by cmacpower
• A Practical Generator used at Homes from Flickr