Power Quality/Revenue Meters on MV Switchgear

[philly]

I've seen several customers that ask for Power Quality/Revenue meters (SEL 735 for Instance) on their MV Switchgear Mains and feeders. To me this has always seemed like a bit of overkill in a lot of cases considering that all of these mains and feeders have protective relay's with metering capabilities (SEL 751 for instance).

I've heard some customers say that they prefer the PQ meter for more accurate monitoring of PQ measurements (harmonics, sub-cycle events,etc..). I've also heard some say that the revenue metering is preferred for metering kWH consumption for feeders that support a particular part of their process to tie electrical cost into process operations.

I know that most protective relays provide both PQ and Power monitoring so have always wondered in most of these cases what the need for a specific meter was other than the fact it looked fancier and had a more user friendly display?

Anyone have strong opinions or experience on when it makes sense to use a meter in conjunction with relay?

 

[dy11]

We usually ask for meters in addition to relays for the reasons you mentioned. I think it's a benefit to be able to compare readings between at least two devices.

There is also the human error reduction element. If meters are provided, the operators have the ability to interact with a read only device that is not capable of causing a nuisance trip. If they are interacting with the relay simply for load data, the likelihood of a mistake is increased. This can be mitigated with settings (disable the relay manual trip button for example) and training but the meter is more of an engineering solution than an administrative control.

Cybersecurity is another reason to do this. The meters could provide data without control capability and the relays would not need to be connected to a network. I know this leaves a lot of capability on the table and there are proven ways to mitigate this but depending on the customer, they may value this more than the relay provided data, especially if they do not have a robust cybersecurity program.

Also, the instrument transformers used for protection may not be accurate enough for billing quality data. This may require seperate PTs, CTs and meters to achieve the desired accuracy.

 

[Hv&Lv]

I've seen several customers that ask for Power Quality/Revenue meters (SEL 735 for Instance) on their MV Switchgear Mains and feeders. To me this has always seemed like a bit of overkill in a lot of cases considering that all of these mains and feeders have protective relay's with metering capabilities (SEL 751 for instance).

I've heard some customers say that they prefer the PQ meter for more accurate monitoring of PQ measurements (harmonics, sub-cycle events,etc..). I've also heard some say that the revenue metering is preferred for metering kWH consumption for feeders that support a particular part of their process to tie electrical cost into process operations.

I know that most protective relays provide both PQ and Power monitoring so have always wondered in most of these cases what the need for a specific meter was other than the fact it looked fancier and had a more user friendly display?

Anyone have strong opinions or experience on when it makes sense to use a meter in conjunction with relay?

Not nearly as accurate with the protection CTs.
Also the 751 doesn’t offer line loss compensation, or instrument transformer compensations.
If your revenue metering you want to be way closer than a 751 will get you.
If your checking against metering points the 751 or even the 787 will be close enough. The 787 reguires the use of (I think, been about 4 years since I programmed the math variables) 16 math variables.

if your using this meter you also want to use revenue class CTs with a different accuracy class than protection CTs

 

[Hv&Lv]

I really like the SEL equipment and the 735 is next gen to 734.
that said I was really disappointed when SEL took a 734 and used it for an expensive cap control.
Called it a 734B...
Only upside there I saw was the ability to individually monitor each phase for cars and switch caps individually.

 

[paulengr]

The CT issue is far less of a problem when the burden is almost nonexistent so the accuracy issue is less of a problem. Most customers are happy with 3-5% accuracy, not trying to do revenue metering. If that is your goal you almost have to have two sets of CTs and two meters/relays.

The 751A does everything you want as far as basic V/A/watts/vars on all 4 phases (ground/neutral either measured or calibrated). It even records both trends and totals although you need to pull the trend data somehow. So it does what power meters have always done. On the power quality side as far as sags/swells you can creatively program event recording to capture those. But in terms of harmonics you are out of luck.

That being said two further points. First is the 751A gives BASIC information. If I am troubleshooting a non-harmonic power quality problem or I just need a load study or sags/swells the 751A does everything I need. And that’s the point.

But if I want detailed data, harmonics...pretty charts, in detail for analysis, the 735 is just so much better at it. But it is best used at the incident site which is where 751As and 710s shine. Having one big meter at the main is not very useful most of the time. It is far more useful in the portable (suit case) version where it can be temporarily located.