Tariff Metering With Earthed Voltage Transformers

Well, if this contentious issue has large financial considerations, and if the issue has been unresolved since 16 Feb 2009, then a global, anonymous, internet forum is maybe not the correct vehicle for resolving it.

Consider hiring a local electrical engineering consultant instead.

Digital extraction is required.

Dear Instruform,

I read through the extensive post, and it is unclear how this question relates to large financial considerations.

You referred to being locked out from the ability to verify your guesses and estimates of inaccuracy. I must presume you are not a direct party to the financial considerations, or you would have access to a listening ear within a regulatory oversight body that could mandate an unequivocal answer to your question.

I am aware of attempts to induce customers of revenue metering systems with high financial considerations to spend money on attempts to save money, supposedly by changing the metering parameters that, according to guess and estimate, were inaccurate.

I am not aware that any of these schemes have proved of any valid consideration. The foundation issues you raised hinge on capacitive charging currents, core saturation and core losses, all of which tend to reduce the values seen on the secondary side of the instrument transformer, and as power quality has shifted over the past two decades, so have the techniques to ensure highly accurate metering.

Can you share the reasons why you have brought the issue to this forum, and why do you call your assumptions a "contentious issue"? Who has contended with you, and why are you so invested in the discussion?

Regards, CJM

Dear CJM,

Thank you for your valued comment, I submitted the issue to this global forum as it may get to the ear of some regulatory body, I have tried to bring it to the most influential body in our neck of the woods but its the scenario of big fish vs tiny fish and I was told to mind my own business, the issue is not my concern.

We are a small business making Instrument Transformers (since 1969) & we are only trying to assist our clients(much much bigger than ourselves) with application advice on our products, I have personal experience in the field of +50 years. The specific financial considerations will not make us personally richer, in fact slightly poorer as what we advocated is a cheaper voltage transformer. We are not users and the ultimate proof is to compare the EVT to a UVT on a user system, not a easy accomplishment that requires structural changes & that scares the user.

The technical reasons were given before in the discussion, if they are incorrect than we are ready to be proved wrong. There is no global or reputable regulatory tarriff code of practise that I can find and on Earthed VTs(EVT) there is indisputably inversion(EVT primary winding earth oscilation) as per my preamble, this results in normal use secondary overvoltages, which in turn overcharges the user. Core saturation is not considered a factor as the EVT core should not seriously saturate for 200% voltages (1.9RVF). In serious inversion cases the cores will overheat due to overvoltages. The solution is extra load, see Jenkins in the preamble, hard to believe but true as I have found from experience.

In a seach of meter manufacturers you will find that the recommended & typical connections for meters from VTs are from Open Vee 1.2 RVF Uniformly insulated VTs (UVT) which have line to line connected primaries, no neutral connection & therefore no neutral oscillation issues. Unfortunately the manufacturers make a universal 3Ph3W/ 3Ph4W meter for any voltage system. This has prompted the use of EVTs which can provide a 3PH4W output connection onto the 3Ph universal meter and the overvoltages results.

Modern medium voltage switchgear has a form of phase segregation with separated breakers, current & voltage transformers per phase. The voltage transformer in this arrangement must be an EVT and I believe 3Ph4W Tarriff metering is the preferred connection.

Inaccurate Tarriff metering affects all of us, both supplier & user.On high financial considerations, the number of users is large and the overcharge will be considerable for medium voltage users.

I very much appreciate your contribution and hope you will contribute further to this discussion.

Regards,

Instruform

Wow Instruform,

Your post puts a few things in perspective for me. First, I graduated High School in 1969 when you began in your industry. I just returned from my 40th yr HS reunion.

Second, you are of the same dedication to accuracy that I and my colleagues have been invested in for the past 25 years or so in metering. As issues arose that were identified, such as dirty sine wave and increasing harmonic distortion due to the 'alternative ' generation increases, DC components introduced by large numbers of electronic power supplies (CT core saturation issue) - corrective changes to the methods and equipment were undertaken zealously.

Measuring accurately to within 1/10th of a percent (1/1000th accuracy) is a matter of personal pride, even though regulatory realities only demand 1 or 2 % or more, is a core standard for us.

I read your post intently, and there are a few of the concepts that I rely on the 'old timers', the metering engineers of your era, to understand and convey to the rest of us. For this commercial electric contractor turned meter guru, there are still some pieces of the puzzle that I take on faith. I loved to tap the amazing depths of concept detail in the mind of one of our now retired meter engineers; triplen harmonics, cemf, ferroresonance, lightening, capacitive charging currents, pf stuff (the reality with perspective). I never cease to be amazed with how much more there is to learn just when I think I have it down solid.

My instinct is that this issue has been given due consideration, since not much gets past these guys. I can only remember very few 4wire metering installations in the 4kV and above range. Most were definitely the delta configurations, something very solid and balanced, capacitive charging currents w/o harmonic issues, but as I stated, I will get detail that is up to par with the new electronic magic that is metering in this age.

I will be looking into this inversion issue, as I still have the ear of the higher gurus, and I will post as I receive comments or answers in that regard. There are some powerful international bodies that are dedicated to good stewardship of accuracy and practice. IEEE, IEC etc. I wouldn't expect only one overall body for awhile, since there is so much diversity in design and usage depending on your GPS numbers (where you are located.)

I was curious about your mention of overheating due to overvoltage, did you mean IR² or core losses or DC eddys (?), which is what I relate to as the causes of heat. There are some other fine detail that I will qualify by asking as I proceed, probably a terminology issue, but you have piqued my interest. I can't lie, I love this sort of stuff.

Regards, CJM

Dear CJM,

I apologise, I took a short cut on core heating, core loss results in higher currents in the energising coils and these in turn require I² R watts to be dissipated. The core itself is not likely to generate much loss in heat.

On a EVT of RF1.9 it is expected that the following Tesla's apply in ideal operation:The normal M5 GOS core material has a knee point at 1.35 Tesla and the Tesla at RVF1.9 should be about that level. The voltage range of measuring accuracy(120%, 100%, 80%) are at 0.85, 0.71 & 0.57 Tesla respectively. This material can normally reach 1.9Tesla without undue core overheating but the core in saturation at 1.9Tesla has large exciting currents.

The Voltage Error measuring accuracy(say 0.2VE%) will not be adversely affected on overvoltages up to 1.0 Tesla(140%v) as it is on the straight line portion of the core characteristic.

My works are at Lat26DegSouth,Long29DegEast & I reside at Lat41DegSouth, Long175Deg east.

Regards,

Instruform

I

Duly noted, thanks. CJM

For the sake of clarity, here is Instruform's original question:

Garthh has made a valuable contribution in clarifying the issue.

I would clarify further.

Although both the EVTs & the meter are accurate to the measurement classes required for tariff metering, on certain earthed systems and meter connections there is a possibility (even probability) of serious overcharging.

There is a large amount of such bad connections in practice.

That is why a regulating code of practice is most desirable & neccessary.