Overcurrent protection

In households circuit breakers or fuses are used to switch off the supply of electricity very quickly if the current is too large, for example a limit of 15 Amperes in a 115 Volt circuit.

In distribution systems automatic protection systems are used for that purpose. There may be two stages:

• A very fast disconnection if the problem causing the overcurrent is nearby, and

• A time-delayed backup operation if the overcurrent originates outside the local area.

Unfortunately in some cases that can have a cascading effect, because a switching-off here can lead to overcurrent(s) in adjacent curcuits that will switch off later. "Blackouts" can be the result.

There is also the problem of the source of power generation getting disconnected from the load, causing big problems with the balance between the amount of power needed and the amount of power available in many parts of, or even in the whole system.

There is a difference between the time it will take to restore that balance later, depending on the kind of generation (from coal, oil, or nuclear), and after a "blackout" it may take many hours to restore that balance.

Single phase electric power

The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120°. The design of the power generators has three sets of coils placed 120 degrees apart rotating in a magnetic field. This creates three separate sine waves of electricity that are displaced from each other in time by 120 degrees of rotation (1/3 of a circle). Standard frequencies of rotation are either 50 Hertz (cycles per second) in Europe or 60 Hertz in North America. The voltage across any pair of these three conductors, or between a single conductor and ground (in a grounded system) is what is known as "single phase" electric power. Single phase power is what is commonly available to residential and light-commercial consumers in most distribution power grids. In North America, the single phase that is supplied is developed across a transformer coil at the utility pole (for aerial drop) or transformer pad (for underground) distribution. This single coil is center tapped and the tap is grounded to develop two waveforms that are 180 degrees out of phase with each other with 1/2 the voltage. This then creates a 120/240 volt system that is delivered to the customer. The voltage from either side of the coil to the center tap (ground) is 120 volts whereas the voltage between the the two conductors on either end of the coil develops the full voltage of 240 volts.

Inverters and Battery Based ac

An inverter is a circuit for converting direct current to alternating current. An inverter can have one or two switched-mode power supplies (SMPS).

Early inverters consisted of an oscillator driving a transistor, that is used to interrupt the incoming direct current to create a square wave. This is then fed through a transformer to smooth the square wave into a sine wave and to produce the required output voltage.

More efficient inverters use various tricks to try to get a reasonable sine wave at the transformer input rather than relying on the transformer to smooth it. Capacitors can be used to smooth the flow of current into and out of the transistor. Also, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the transformer input terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped sinusoid is generated at the transformer input and the current drain on the direct current supply is less choppy.

Modified Sine Wave inverters convert the (usually 12 V DC) battery voltage to high frequency (20 kHz) AC, so that a small transformer can be used. This is then stepped up to a higher voltage (say 160 V) AC. This output is converted to DC at the same voltage, and then inverted again to a quasi sine wave output (about 120 V RMS). Another disadvantage of the modified sine wave inverters is that the output voltage depends on the battery voltage.

It is quite expensive to obtain a good sine wave from an inverter. The quoted accuracy (harmonic distortion) for most is less than 60%, and this will have an effect on the appliances connected to the output of the inverter. This leads to noise in lot of appliances and damages electric motors, as they run significantly hotter.

High end inverters (> $2,000) produce waveforms which are closer to a mathematical sine wave than those produced by the utility.