When I was asked why the TV image becomes speckled or why the lights sometimes flicker and other times not or why the microwave suddenly stop working and a few minutes later carry on working as if nothing has happened, I did not know how to explain that since the audience is not technically inclined. Why do the freezers, fridges or washing machines not last as it used to? Well, probably the short answer is that, nowadays, the electricity supply is “not as clean” as it is supposed to.
I started by explaining that, in a balanced three-phase network, the magnitudes of the voltages and currents in all three phases are supposed to be the same and that voltages and currents phases are shifted symmetrically by 120-degrees to each other, but I realized that I am about lose my audience. When end-users, are supplied by unbalanced voltages, many appliances suffer from the consequences. The effect of current and voltage unbalances is usually widespread, and it can affect an entire province as I have experienced before. Or it could affect only those connected to one substation. But even in the case of just one step-down substation, the number of end-customers can still be in the hundreds.
In the United States, voltage magnitude variations are limited to ±5% and in most European countries, it is limited to ±10%. For the rest of the world, these limits are not much different. But the question is: what about South Africa? Looking at Eskom’s Quality of Supply document, there is a lot being said about the consequences of unbalanced voltages, but I cannot find the voltage magnitude variations limits.
In NRS 097-2-1:2017 which deals with Grid Interconnection of Embedded Generation, reference is made of Total Harmonic Distortion (THD). In addition, there is a section dealing with flicker and in this piece, reference is made to IEC-61000-3. In one of my blogs, I said that the acceptable current and voltage harmonic levels generated in the microgrid is specified by the IEEE-standards 519 and 1547 and the IEC-61000-3 standard, and this should not exceed a THDu (the u indicates voltage) of more than 8%. Click here also to read more about Negative Phase Sequencing.
This is all well and good, but who “police” this? I recently came across yet another incident where the THD far exceed the 8%. This is in the Gauteng, but who can guarantee that the same is not happening in every other province and on every feeder right now?
Naturally, utility companies are responsible to ensure that power quality issues are monitored and addressed immediately, but in my experience, this is not done. Apart from the “willingness” to implement power quality monitoring exercises, they need knowledgeable and experienced people who know what to do, where to look, and once they have collected the data, how to interpret the results. What is perhaps far more important is fast action once the problem has been identified, and not wait more than a month to initiate further investigations.
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