Jeremy Lester, Technical Director at Switchtec uncovers the popularity of DC switches and their suitability for use in photovoltaics applications.
In recent times we have seen a number of disasters around the world. Under such extreme conditions, things can happen that in the general scheme of things just would not ‘normally’ happen. For example whole houses getting swept away by floodwater or tsunamis.
Terrible though these (often natural) disasters are, they inevitably provoke designers to think again about technologies and designs that previously they believed to be 'good enough' for purpose. Whilst the designer cannot legislate for every eventuality, it is a trade off; a careful balance between that which is considered normal use or endurance, against the cost and likelihood of an extreme situation occurring.
In the world of photovoltaic switching we have a potentially similar situation. Once seen mostly as an interesting and relatively expensive curiosity, photovoltaic arrays are gaining rapid acceptance as renewable energy sources - particularly in domestic applications.
Analysts predict that by 2013, the global market for photovoltaic production plants will increase by around 80 percent relative to the 2008 level that was then worth some USD 5 billion-and this is despite that year's financial crash that caused the photovoltaic market to stagnate.
Together with continual improvements in process technology, sustained growth in manufacturing investment continues to drive down production costs to help solar power move towards the ‘holy grail’ of price parity with other electricity generating methods.
A typical PV array is configured thus. In simple terms, the PV array is connected to an inverter via a DC isolation switch. The output of the inverter is connected to the main fuse box (and hence back into the mains) by an AC switch (see figure 2). So, within that scheme we have a DC switch being used to switch DC, and an AC switch that is switching AC, both as it should be.
Mechanically, AC switches are not dissimilar in operation to a large relay, whereas a DC switch has a totally different mechanism, a so called knife mechanism where spring assistance is used to definitively and rapidly part the contacts to avoid the arching effect when breaking a large DC current.
AC switches do not have this type of design, so arching when breaking DC is possible. However, there are now switches available on the market, for the purpose of switching this DC current in a photovoltaic application that are based on AC switch designs.
They are tested to achieve the required DC rating, but under extreme conditions, it is clearly much better to have a dedicated DC switch that is inherently better at breaking large DC currents, and remove the risk of bad arching within the switch and any (possibly unseen) resultant damage.
Although one would not expect someone to be so foolish as to try it, it is also possible for some AC isolators to be made to physically sit in a mid position point.
Combine that with the arching issue and it can be seen that this is not a desirable feature of, or a particularly suitable application for the AC style of switch. As mentioned, the DC switch operates with a powerful spring assisted knife action, making it either on or off, and virtually impossible to set half way.
The flip-flop DC design operates at a constant - and fast - speed as a result of the strong, spring assisted mechanism. It therefore begs the question, why use an AC style switch for this application? The issue is that the 'proper' DC switch, compared to an AC design that has been 'DC tested', is more expensive.
The DC switch has a higher-level of engineering, and the spring mechanism obviously requires the manufacture and assembly of more component parts into the final product. But what is the cost of ultimate safety and peace of mind - especially in a domestic setting or application?
An unforeseen natural hazard could result in an extreme overload situation that only the DC design of switch could cope with, avoiding massive damage and interruption to supplies, fire, and worse still, the possibility of injury or even worse to personnel.