Friday 11 March 2022

The Electric Arc and DC Solar Systems

If you cut a wire in an electrical circuit, current will stop flowing because the air in the gap is a very good electrical insulator so electrons cannot travel across it.

A electric field is formed between the positive and negative sides of the gap (which are called the electrodes), represented by the red field lines in the diagram below.

If you increase the voltage between the two electrodes, or narrow the distance between them, the electric field across the gap increases in strength.

If the electric field is strong enough, it will pluck electrons away atoms in the gas molecules in the field, causing the gas molecules to split into positive ions and negative electrons.  Both electrons and positive ions are attracted towards the oppositely charged electrode, so they accelerate off in that direction.  A current is now flowing in the circuit again.

But as the ions and electrons move in the field they can collide with gas molecules.  The energy from the collisions is seen as a temperature increase.

More electrons moving between the electrodes means there are more collisions with the uncharged gas molecules.  A chain reaction (or avalanche effect) occurs with more collisions leading to more charged particles and more charged particles leading in turn to more collisions.  We now have a high temperature plasma between the electrodes and this is a far better conductor of electrical current than air because of the greater number of charged particles available to move across the gap.

Temperatures are now so high in the gap that an intense white light is given off and the conductor material  may be vaporised away - an electric arc has formed.  

Even though the loss of electrode surfaces means the gap width increases the plasma's higher conductivity means that the current continues to flow.  A much lower voltage is needed to maintain an arc than is required to start it in the first place, an effect is well know in welding where 'striking an arc' refers to tapping the welding stick on the work piece and pulling it away.  The arc forms when the gap is narrow but is maintained as the stick is moved away to the working distance.

Arcs can be useful in applications such as welding, but undesired arcs are a potential source of fires with  the high temperatures in electric arcs easily capable of setting alight combustible material nearby.  Great attention is paid in electrical installations to avoiding arcs forming in the first place.

DC Solar Systems

There are two reasons why arcing is a particularly relevant consideration for solar systems.  

Firstly the voltages in solar systems can be very high compared to the Alternating Current (AC) supplied from the grid, and as we've seen the higher the voltage, the stronger the electrical field across a gap and the more likely is is that an arc will form.

In European countries AC electricity is supplied at 230-240V for domestic and small commercial buildings.  The most common electrical arrangement for the solar panels in a solar PV installation is to connect the panels (which might be 35V per 340Wp module) in a series string with the voltage increasing with each panel added.  For a 4kWp, 12 panel installation the voltage reaches 420V.  In larger commercial and utility scale installations voltages up to 1,000V or 1,500V are commonplace. 

Secondly, an AC electric arc is more likely to self-extinguish once formed because the voltage goes through zero volts 100-120 times a second (50 or 60Hz supply) and each time this happens the arc needs to re-establish.  Because the field strength required to start the arc is much higher than that needed to keep one going any increase in the arc length due to electrode erosion will mean that the arc will not re-establish.  By contrast the DC voltage in a solar PV wiring system is constant and the gap will need to open up much further before the arc is extinguished (this is why most forms of arc welding use DC current).

These factors mean that greater attention must be paid to arc risks in solar PV systems.  Despite this the safety of solar PV systems is very high, and this will be the topic of my next blog.