Electricity + Control October 2017

EARTHING + LIGHTNING PROTECTION

&RPSDULVRQ %HWZHHQ ac and dc LQ 3RZHU 7UDQVPLVVLRQ 1LNKLO 1DLGRR (QVLJKW (QHUJ\ 6ROXWLRQV Both alternating current and direct current present their own strengths and weaknesses, which need to be considered along with the changing technolo- gies in power transmission.

Take Note!

With changing tech- nologies, it is surmised that ac and dc do not have to compete in their traditional manner. Various applications can be characterised as being suited to either ac or dc. Ac and dc can share the market for power distribution and can complement each other in terms of frequency synchronisation.

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P ower transmission can be broadly catego- rised as being based on alternating current (ac) or direct current (dc). These two current types were the subject of what is known as ‘the War of Currents’, in which Thomas Edison and Nikolai Tesla competed in promoting their own in- ventions dc and ac, respectively. Since ac could allow for easy transformation, and that the technology of the late 19 th century rendered dc unable to compete successfully in this regard, Tesla’s invention was adopted in the trans- mission of power. However, whilst ac is still the current type most often used in power transmis- sion infrastructure today, it needs to be understood that both ac and dc present their own strengths and weaknesses, which need to be considered along with the changing technologies in power transmission. How ac and dc work There are two main types of current, namely al- ternating (ac) and direct (dc). The names of these current types are descriptive of the direction of electron flow in each of these arrangements. Ac current works by rotating a magnet near a stationary coil (usually comprised of copper), to in- duce current. In ac arrangements, electrons rapidly change direction within a closed circuit. The num- ber of directional changes that occur per second is referred to as frequency, measured in Hz. In South Africa, the current is supplied by Eskom at a fre- quency of 50 Hz. In dc arrangements, electrons flow from posi- tive to negative terminals, in one uniform direction. A rotating coil positioned between two opposing ends of magnets results in pulses of charge flow. Each revolution of the coil is characterised by two pulses (as the coil is on the same plane as that of the magnets for every 180°). In order to overcome the issue of intermittent current, dc generators are comprised of multiple coils that turn with a shaft.

Transformers and Inductance In comparing ac and dc, it is first important to un- derstand electromagnetic inductance. A changing magnetic flux in the presence of an electrically conducive coil would result in a force across said coil. This electromotive force, regarded as voltage in this case, causes charge to flow in a closed cir- cuit. Electromagnetic inductance is described by two laws, namely Faraday’s Law and Lenz’s Law. Faraday’s Law states that the magnitude of the electromagnetic force is dependent on the rate of magnetic flux change.

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| H | = –– d T dt

(1)

H is electromagnetic force magnitude T is magnetic flux t is time

It is clear that ac and dc each have their strengths and weaknesses.

In terms of the direction of the induced force, Lenz’s Law explains that it is opposite to that of the changing magnetic flux. Thus, the electromag- netic force can be described as follows:

H = – –– d T dt

(2)

The aforementioned physical concept is applied in the use of transformers, which consist of coils each wrapped around opposing ends of a holder. In such an arrangement, there is a primary and secondary coil, the former of which creates a mag- netic flux, which in turn induces a current in the secondary coil. The number of turns of each coil affects the ratio of voltages between the two coils, as described by the following:

–– = –– Va Vb Na Nb

(3)

This theoretically assumes a 100% efficiency Va is the primary voltage Vb is the secondary voltage

36 Electricity + Control

OCTOBER 2017