Substation Voltage Regulator - question on source and load amps 2

[Manindemand]

Below is an email I sent to engineering after changing out a bad MW Transducer in one of our stations. I got this data off our server (TOIS) that night via VPN. The XFMR is 30 MVA (161kv/13.8kv) and Regulator is 3333 KVA 10% Raise and Lower. Were all kind of scratching our heads thinking... is this right? This is critical because in August the load was showing 28 MVA in SCADA. We're trying to justify installing a 2nd XFMR.

Question: When the regulator is on 16 lower it lowers the voltage but does it raise the amps on the load side of the regulator and lower the amps on the source side?


"I took these values out of TOIS. It looks like the readings are correct now.

01-OCT-2003 19:15:00 (13.8 Bus kv) 7.8 (Feeder Amps) 157.99 + 118.92 + 149.2 + 167.76 + 159.95 + 147.25 + 168.25 + 164.83 + 184.37 = (Total Amps) 1418

(T1 kv) 8.59 (MVA) -12.09 (MVA1) 10.16 (MW) -11.16 (MVAR) -4.63

The regulator is probably on about 13 lower. Keep in mind that the total current flowing through the metering CT's (in the XFMR) is also the total combined current flowing out of the feeders. However, the regulator has dropped the kv from 8.59 down to 7.8. The MW and MVAR xdcrs are getting their voltage from the 8.59 kv source. To calculate MVA using feeder amps (to compare to SCADA), you have to use 8.59 kv instead of 7.8 kv.

1418 x 8590 = (MVA) 12.18 That's pretty close to the reading in TOIS at 19:15 this evening. Keep in mind that the T1 kv reading may come from the highest of the three T1 kvs.

The point I'm making is, it might look like the MVA reading is too high if you use 7.8 kv value - (1418 x 7800 = 11.06 MVA). On Aug. 17 the load was high enough that the T1 and Bus kvs were close so this wasn't as much of a factor."

 

[alehman]

The load MVA = sum of feeders x output voltage. You must calculate the load MVA based on the load voltage.

The transformer MVA = load MVA + regulator losses

Think of the regulator as a transformer. input MVA = output MVA + regulator losses

 

[Manindemand]

alehman, your statement “Think of a regulator as a transformer” is where my question is.

A 161000D/13800/7970Y transformer’s ratio is 11.67/1 .

Example: If I have 473 amps per phase on the low side I would have 40 amps per phase on the high side (473 / 11.67 = 40.5).

So, if a regulator is like a transformer you should be able to calculate the ratio of the regulator when it’s at step 16 lower. The regulator drops voltage 10% so it’s ratio is 1.10/1 . The current ratio would be 1/1.10 . With 473 amps per phase on the load side you should have 430 amps on the source side of the regulator.

To me, this is where it gets confusing. I seems that if I have 473 amps per phase going out of the bus then I should have 473amps per phase (+ regulator losses) coming out of the transformer.

Example: Customer Amps + Regulator Loss = Source Amps.

Thank you for your help and please excuse my confusion, I have a bad head cold.

 

[dpc]

The regulator is an auto-transformer. The amps will not be the same on the input and output because of the effective turns ratio of the transformer. As alehman says, the kVA in and out will be the same, allowing for the regulator losses.

 

[Manindemand]

dpc,

Then would you agree that this statement above is correct?

"So, if a regulator is like a transformer you should be able to calculate the ratio of the regulator when it’s at step 16 lower. The regulator drops voltage 10% so it’s ratio is 1.10/1 . The current ratio would be 1/1.10 . With 473 amps per phase on the load side you should have 430 amps on the source side of the regulator."

Thanks

 

[dpc]

Yes, ignoring the exciting current. But as you mention, the KVA calculations have to based on the appropriate voltage.

It is actually this difference in current that determines the autotransformer loading. This is why a 300 kVA regulator can handle 3000 kVA of load.

 

[Manindemand]

So now I need to figure out why the SCADA reading is 12.09 MVA when calculated MVA = 11.06.

This is what I’ve already done:

1.) I calibrated the MW and MVAR transducers with a Scientific Columbus Transducer Calibrator and I verified their outputs match the SCADA readings (which verify RTU cards are good).

2.) For the feeder breaker amp xdcrs, I used a clamp-on amp meter (with good accuracy) and adjusted the outputs (reading DC volts with a Fluke). I then verified the dispatcher was reading the correct values.

3.) I had our SCADA Host programmer verify that all ranges and scale factors were correct in the Host computer.

So the only explanations I can think of are:

1.) Bad CT accuracy somewhere.

2.) I need to double check my work.

3.) We have 1.03 MVA loss through the regulator.

To prove it is regulator loss what voltage do I use to calculate it?
Example: 103000 / 7800 = 132 amp loss OR 103000 / 8590 = 120 amp loss. So roughly 42 amps loss per phase.

Either way, are these typical losses for regulators operating on steps 13 – 16 lower?

 

[alehman]

You said you transducer is sensing voltage at the regulator input, but your current sensing is on the output (feeder circuits):

your MVA = (input V)*(output A)

This is incorrect. To get accurate MVA numbers, your transducers must sense voltage and current on the same side of the regulator. You must add CT's at the regulator input or PT's at the output, or move your voltage sensing point to the output bus:

actual regulator input MVA = (input V)*(input A)
or
actual regulator output MVA = (output V)*(output A)

regulator loss = (input V)*(input A) - (output V)*(output A)

 

[Manindemand]

I'm sorry for the confusion. The watt and VAR transducers are fed from CTs and PTs on the source side of the regulator. The CTs are on the X1, X2 & X3 bushings in the 161/13.8 XFMR. The MW & MVAR xdcrs get their potential from PTs mounted on a structure next to the low side of the 161/13.8 XFMR. We use one of those PTs to feed our XFMR kv transducer.

Our Bus kv transducer is fed from an internal PT on the load side of the regulator.

The problem is that engineering looks at SCADA and sums up the feeder amps and multiplies it by the Bus kv reading and gets 11.06 MVA but the SCADA MVA reading is 12.09.

I'm trying to understand why SCADA MVA is 1.03 MVA higher than calculated MVA.

By the way, I’ve also verified the Bus kv transducer is correct.

 

[jbartos]

Suggestion: There may a need to verify the situation as-built of SCADA and associated instruments. Also, any SCADA software may have to be checked for accuracy.

 

[busbar]

A couple of stabs in the dark… Having tried everything else, is it possible to reverse the respective watt/var transducers and see what readings reveal?

Watt and var measurements are mentioned, but VA readings are displayed. A very long shot— but when the right-triangle relationship is invalid, that may point to a harmonic problem roughly similar to that described in the (sort of new) IEEE 1459 algorithms. Then, it’s hard to imagine a difference in harmonic content on either side of a step-voltage regulator.

 

[jghrist]

I assume that SCADA measures the feeder amps on the load side of the regulator. If there is a single bus regulator (as opposed to feeder regulators), there is no way to measure feeder amps on the source side. If you want to calculate total MVA, you have to use current and voltage from the same side of the regulator. This means you will have to use load side voltage to match the load side feeder amps. You will get 12.18 MVA as indicated in your first post. This is less than 1% different from the measured 12.09 MVA. What is the problem?

 

[jbartos]

Suggestion: SCADA may need an update.
Visit

http://www.elipse-software.com/Home.htm

http://www.cooperpower.com/library/pdf/95009.pdf

http://www.pcsutilidata.com/White Papers/WEPI 1999 Paper.pdf

http://www.pcsutilidata.com/White Papers/IEEE Rural AVC Paper.pdf

for more info

 

[Manindemand]

jbartos - calculated MVA is 11.06 (1418 x 7.8kv) that’s (combined feeder amps X bus kv). The 12.09 MVA came from my original error using feeder amps x XFMR kv.

The regulator is a single bus regulator.

Our SCADA host system software is good quality -

http://www.esca.com/includes/pop.cfm?content_ID=1361

The RTU is an OLD reliable TRW S9000 - pretty old but amazingly accurate. We're gradually upgrading all RTUs to GE Harris D20.

The SCADA MW & MVAR readings come directly from the xdcrs in the station. The host system just calculates the MVA reading from (MW sq + MVAR sq = MVA sq).

I think I'll go back to the station and verify every transducer reading. I've done this over a period of time but not all readings in one visit.

I can sync my watch with the SCADA Host time and then verify and record each value and time I read it. Then I can verify them through my DSL VPN connection that evening.

If they're all correct then I can rule out human error and SCADA problems as possible causes.

 

[jbartos]

Suggestion: The SCADA as well as its "peripherals" (e.g. static voltage regulator) using software may have to be checked for proper updates (e.g. software drivers). Do you use outside services or do you do everything in-house?

 

[stevenal]

You cannot simply add the feeder amps. Real and reactive components must be added separately to get the combined value. You need phase angle information to do this calculation, something a clamp-on will not provide.

 

[jghrist]

stevenal is right, but adding arithmetically will result in a larger than actual value for the total current. The MVA calculated with the feeder amps is less than the measured total MVA, so this factor just makes things further off than is apparent.

 

[stevenal]

True. This thread is rather confusing. Another error source is the assumption that the three phase voltages on the load side are balanced. Any voltage imbalance on the source side will carry over to the load side of a three phase regulator. If you have per phase values, you could look at just the sensed phase.

Have you double checked all instrument transformer ratios, polarities and phasing?

 

[alehman]

Just to be clear, the feeder amp numbers in your original post were individual phase values? (three 3-ph feeders for a total of 9 values?).

I think stevenal's point about unbalanced voltage may be the answer. Can you tell from the SCADA transducer if the phase voltages are apporximately equal? He is also correct in that you cannot sum scalar current values. You must sum the feeders vectorially (e.g. with a summing current transformer).

 

[Manindemand]

Thank you all for your comments and suggestions.

jbartos - none of the equipment in the station uses software. The Regulator control is a Beckwith M-0067E

http://www.beckwithelectric.com/tapchangers/m0067/m0067e.htm

All of our field equipment is maintained in house. The ESCA host was installed by the vender about 5 years ago and is now maintained in house. I'll ask about updates but this problem has not shown up in other stations (that I'm aware of). My territory has about 100 substations and there are about 300 more around the state.

This is the only station that I know of where our regulator stays around 13 lower.

The reason the regulator is running at 13 lower is because the XFMR is on the lowest tap- "E Tap" -which makes the secondary voltage too high. It ran near neutral until they added another section of transmission line. Now the high line is running a lot higher.

stevenal - you may have hit on something. Is it possible that we really do have 12.09 MVA on the source side and 11.06 MVA on the load side? After all, all were talking about here is Volts x Amps. The amps are probably lower on the source side but the voltage is so high that it makes MVA higher. The phase voltages are pretty balanced. I did hand over hand wire check the meter panel current circuits (Watt and VAR current circuits).

Jghrist - what is the correct way to calculate MVA? I use either of these two formulas:
1418 Amps x 7800 Volts = 11060400 Volt Amps
OR
1418 / 3 = 473 x 3sqrt = 819 x 13510 (that's 7800 x 3sqrt) = 11060400 Volt Amps.
Remember, we’re not trying to calculate MW on the load side.


It just dawned on me that I can actually check the source side MVA by measuring the metering CT secondary amps and meter PT secondary volts. They're both connected wye so I can check each phase individually. - Man why didn't I think of that before?

If source side calculated MVA matches the SCADA MVA then we can all accept that. I’m looking forward to tomorrow.