In recent times we have seen a number of disasters around the world. Under such extreme conditions, out of the ordinary things can happen. For example, whole houses can get 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 what 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% relative to the 2008 level that was then worth some $5bn – and this despite that year’s financial crash that caused the photovoltaic market to stagnate.
Together with continual improvements in process technology, such 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 1).
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. However, let’s take a closer look at those switches. 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 will have been 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 stupid as to try it, it is also possible for some AC isolators to be made to physically sit in a mid-position. 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. But even if the operator is not being mischievous, ‘user input’ still plays a part in the operating speed of an AC style switch. 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. However, 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.
Switching company Switchtec has been involved with photovoltaic components for some years, and the company is keen to make customers and end users of such equipment aware of this issue. Switchtec offers a dedicated DC style switch from Telergon to disconnect the DC side of the inverter used in photovoltaic schemes and designs (see Figure 2). A range of enclosed and base mounted on/off load break switch disconnectors is available in ratings from 16A to 1,250A. The units can be supplied with lever or rotational style operating handles and importantly, they have been tested and approved by arsenal research, the Austrian company that is a prestigious leader in the field of testing components for PV applications.