Energy Metering, Smart Metering, Digital Metering, Energy Management System (EMS), et al., have become the buzz words in the industry today. What will happen with the Energy Conservation Act, 2001, the Energy Conservation Building Code, 2018 and the stipulations of the Bureau of Energy Efficiency? And, it has become a fancy for electrical & energy professionals that they have implemented the best energy management system in their installations. But are all these installations really ‘efficient’?
Selection of the CT/PT Accuracy Class
More often than not, many energy managers & electrical engineers, dutifully specify the most modern smart energy meter, with the best class of accuracy, available in the market today (i.e.) that of Class 0.2S. The advantage of such a class of meter is that it would maintain its accuracy of an error of +/- 0.2%, even at very low loadings of the order of 20% of the rated value.
But such ‘pseudo-smart’ engineers fail to realise that such a smartest energy meter would work accurately, only, and only if the associated Current Transformers (CT) and Potential Transformers (PT) are also of the best accuracy class available for commercial use.
That is, if the meter is of Class 0.2S, then the CT must also be of Class 0.2S and the PT, of Class 0.2. If the CT and/or PT are of a poorer accuracy class, such as Class 0.5 of Class 1.0, then there is no point in having a Class 0.2S energy meter alone.
In many specifications, this practice of specifying a Class 0.2S Energy Meter, but a Class 0.2 or Class 0.5 or Class 1.0 CT and/or a Class 0.5 or Class 1.0 PT is prevalent. This includes specifications of many Central Government Establishments, State Electricity Boards, DISCOMs, private power utilities, leading consultants, leading industries, etc.
Lesson 1
Unless the CT, the PT and the Energy Meter are of the best accuracy class available, there is no big advantage of having only a class 0.2s energy meter.
Selection of the CT/PT Burden: In earlier days, in the era of Electro-mechanical Meters, the burden of the Current Coils and that of the Voltage Coils of the meters were of the order of 5VA to 7.5VA each. So, it was normal practice to specify CTs with a typical burden of 15VA or more, as more than one current operated meters – like Analogue Ammeters, Analogue Power Factor Meters, Analogue Power (kW) Meters, Analogue Energy Meters, etc. – were to be connected in series. Same is the case with PT, wherein the PT Burdens were of the order of 100VA, as more than one Voltage operated meters and equipment – like Analogue Voltmeters, Analogue Frequency Meters, Analogue Power Factor Meters, Analogue Power (kW) Meters, Analogue Energy Meters, Phase Indication Lamps, etc. – were to be connected in parallel.
But now, with the advent of modern digital electronic multifunction meters, the total burden imposed by the meter on the current coil is of the order of 0.2 to 0.25VA and that of the potential coil, is of the order of
0.5VA maximum.
So, there is no point in continuing the age-old practice of specifying CTs with a burden of 15VA or more and PTs with a burden of 100VA or more.
In fact, one is doing a gross injustice to the measuring system, by such over-specification of the CTs & PTs.
Because, as per IS & IEC Standards, the accuracy of a given CT is guaranteed only between 25% to 100% of the rated burden connected to the CT Secondary. If the actual connected burden is less than 25% of the rated burden of the CT, then its accuracy will be greatly affected, and the errors would increase manyfold.
You may have specified & procured a CT with the best accuracy class of Class 0.2S, in the fond hope that the errors of such a CT would only be +/- 0.2%.
But remember: This accuracy of Class 0.2S (error of +/- 0.2%) is guaranteed only if the actual burden connected to the CT Secondary is at least 25% of the rated burden of the CT.
If you specify a Class 0.2S CT with a rated burden of (say) 10VA, and connect only a Digital Multifunction Meter, with a current coil burden of 0.2VA to this CT Secondary, and even considering the CT Secondary Winding Burden and the connecting lead burden, the total burden connected to the CT Secondary would be less than 10% of its rated burden. This is sure to increase the errors and record incorrect energy readings, thus negating the very purpose of having opted for a Class 0.2S CT and a Class 0.2S Energy Meter.
Same is the case with PT too.
Lesson 2
The rated burden of the CT and the PT secondary must be as close as possible, but, not less than, the aggregate of the actual burdens connected to the CT/PT seconadry. Else, even with the best class of CT, PT and Meter, the desired results could not be achieved in the Eneregy Management System.
Selection of the CT Primary Current: As per the Standards, the accuracy of a Class 0.2S/Class 0.5S CT is guaranteed between 20% to 120% of the rated primary current of the CT. If the primary current is below 20%, then the error multiplies, and the given accuracy of the CT is not guaranteed. The advantage of having gone for a Class 0.2S CT/Energy Meter are lost.
It is fancy for many electrical engineers to over-specify the CT Primary Current, thinking that higher is better. Imagine one has specified a Class 0.2S CT with a primary current of 1000A, but the maximum continuous load in the feeder is 100A, then, the CT will no more be Class 0.2S and its error would increase.
So, the primary current rating of the CT must also be as close as possible to the normal anticipated continuous current in the feeder.
If one wants to cater to future expansions too, then one may go for multi-tap CTs, say, 250-500-1000A, so that, when the load is less, prior to expansion, a lower tap can be selected. And, as and when the load increases, appropriate higher taps may be selected. This way, the accuracy of the CT would be guaranteed and at the same time, provision is available for future expansions too.
Lesson 3
The rated primary current of the CT must be as close as possible to the maximum anticipated continuous current in the feeder in which the CT is proposed to be installed. Else, given CT accuracy would not be guaranteed and errors would increase, defeating the very purpose of an efficient energy management system.
Selection of PT Accuracy Class: On more than one occasion, I have come across specifications, even from reputed consultants, specifying the Accuracy Class of the PT Metering Secondary as Class 0.2S / Class 0.5S.
This is ridiculous, to say the least. As per the Standards, there is no Class 0.2S or Class 0.5S for PTs.
The ‘S’ Class is mentioned only in the Standards for CTs, to get the guaranteed accuracy even at 20% rated primary current. This is logical, as the load current in a feeder is a variable phenomenon.
Whereas, whether the load is 0% or it is 100%, the system voltage is almost constant.
So, it is not logical to specify an ‘S’ Class for PTs. That is why the Standards have not specified the same.
But, some pseudo-intellectual electrical professionals, thinking that they are smart, specify Class 0.2S/Class 0.5S PTs too. If any specifications call for a Class 0.2S or a Class 0.5S PT, then it is a gross violation of Standards and such a PT cannot be offered at all as such a PT cannot be manufactured & tested.
Lesson 4
There is no class ‘s’ in PTs. Specifying such PTs, would only highlight the incompetency of the specifying engineer.
Conclusion
As seen in the above paragraphs, it is not just enough to have a partially efficient Energy Metering System. For the Energy Metering System to work effectively, all the constituent components also have to be cautiously chosen, avoiding any of the above mistakes.
Hope that this article serves as an eye opener and concerned energy professionals avoid such mistakes in the implementation of their Electrical Energy Metering System.
K. Sivakumar is a Technical Advisor
at Sriwin Electric, Salem.
