×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Substation Voltage Regulator - question on source and load amps
2

Substation Voltage Regulator - question on source and load amps

Substation Voltage Regulator - question on source and load amps

(OP)
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."

RE: Substation Voltage Regulator - question on source and load amps

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

RE: Substation Voltage Regulator - question on source and load amps

(OP)
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.

RE: Substation Voltage Regulator - question on source and load amps

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.  

RE: Substation Voltage Regulator - question on source and load amps

(OP)
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

RE: Substation Voltage Regulator - question on source and load amps

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.

RE: Substation Voltage Regulator - question on source and load amps

(OP)
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?

RE: Substation Voltage Regulator - question on source and load amps

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)

RE: Substation Voltage Regulator - question on source and load amps

(OP)
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.

RE: Substation Voltage Regulator - question on source and load amps

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.

RE: Substation Voltage Regulator - question on source and load amps


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.
  

RE: Substation Voltage Regulator - question on source and load amps

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?

RE: Substation Voltage Regulator - question on source and load amps

(OP)
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?con...

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.

RE: Substation Voltage Regulator - question on source and load amps

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?

RE: Substation Voltage Regulator - question on source and load amps

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.

RE: Substation Voltage Regulator - question on source and load amps

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.

RE: Substation Voltage Regulator - question on source and load amps

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?  

RE: Substation Voltage Regulator - question on source and load amps

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).

RE: Substation Voltage Regulator - question on source and load amps

(OP)
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/tapchang... 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.




RE: Substation Voltage Regulator - question on source and load amps

Suggestion to the previous posting: If there were some modifications made as stated below:
""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.""
Is not there a chance that the SCADA is sensing voltage or current or both from incorrect locations or instruments?
12.09/11.06=1.093, which may be just a matter of improper connections. Have instruments been modified or wired during the transmission line addition?

RE: Substation Voltage Regulator - question on source and load amps

(OP)
alehman - yes, three 3 phase feeders.

You gentlemen are saying "... you cannot sum scalar current values. You must sum the feeders vectorially (e.g. with a summing current transformer)."

This is the first time I've heard this.  Thank you.  Can you give any links so I can study this?

What's more confusing is that other technicians and managers use the same calculation that I've been using.  I can connect to our server and check 10 different stations comparing calc MVA with SCADA MVA and they will always be very close.

But like I said, this station is unusual due to the XFMR on lowest tap.

Thanks again.

RE: Substation Voltage Regulator - question on source and load amps

(OP)
jbartos - excuse my thick skull,  good idea! I'll take a Poly Meter with me and check phase angles to verify all my potentials are connected to the right terminals on the Watt and VAR transducers.  Like I said, I verified current circuits but not potentials.

RE: Substation Voltage Regulator - question on source and load amps

Both of your formulas are mathematically identical and correct as long as you use the right current, which should be the magnitude of the vectorial sum of each phase current, and all phase voltages are equal.  If all loads have the same power factor, the vectorial sum will be the same as the arithmetic sum.  I don't know of any links to show that a vectorial sum is needed, but the transformer measurement is of the sum of the feeder currents, and the currents are vectors with different angles so they have to be summed vectorially.

To see the possible difference, assume that the first three feeder currents given in your first post are phase A for three separate feeders, that phase A voltage angle is zero, and the the power factors are 90%, 70%, and 60% for the 157.99A, 118.92A, and 149.2A currents respectively.

Current angles are acos(pf):  -25.84°, -45.57°, -53.13°
Real currents are I·pf: 142.19A, 83.24A, 89.52A
Imaginary currents are I·sin(angle): -68.87A, -84.92, -119.36
Sum of real currents = 314.95A
Sum of imaginary currents = 273.15A
Magnitude of vectorial sum = sqrt(314.95²+273.15²) = 416.9A
Arithmetic sum of current magnitudes = 426.11A


Not a whole lot of difference even for this wide range of power factors.  Actual power factors are undoubtedly much closer, which is why your simplified formulas work pretty closely.

RE: Substation Voltage Regulator - question on source and load amps

"Is it possible that we really do have 12.09 MVA on the source side and 11.06 MVA on the load side?"

I don't think so. Apparent power is a conserved quantity, so you would have a 1.03 MVA loss in your regulator. Sounds rather extreme.

If you're checking individual CTs and PTs, don't forget you have a single phase set in your regulator. The CT will reflect the phasor sum of the feeder quantities on that phase. I'm assuming regulator uses line to ground quantities, sometimes line to line quantities are used. Check the nameplate. Also, instrument transformer ratios used may seem a little strange. 1000/0.2 CT ratio for example.

I agree with jghrist's post following mine, arithmetic addition just makes the discrepancy more confusing. If power factors are similar on the feeders, a simple arithmentic sum may provide a fair approximation. Kinda sorry I muddied the waters with that post.

RE: Substation Voltage Regulator - question on source and load amps

(OP)
The results are in and the case is closed.  All phase angles were correct.  Below is the email of my findings that I sent to engineering.  Comments are welcome and thanks again for the help.

"I think I found the cause for the 1.03 MVA difference between SCADA and calculated.  It turns out there was a combination of small problems.  I didn’t notice them during my original check.  Pardon all the details, I wanted a good record of this to put in my notes for future reference.

The MW and MVAR xdcrs are accurate but the “zero” trim potentiometer on the Analog Control board (in the RTU) was off and made each reading approx .25 higher than actual (.25 mw high & .25 mvar high).  I adjusted it.  

The Bus kv transducer was giving a reading that was 45 volts low ( DOC read 7802 but it was really 7847).  The xdcr was slightly off and I adjusted it.

While troubleshooting, I also noticed the RTU cards and card cage were covered with dust.  The card headers and edge connectors also had corrosion buildup.  I gave all parts of the RTU a thorough cleaning.  This type of problem commonly causes inaccurate analog readings but I’m not sure how much effect it had in this case.

I finished all the repairs about 16:45 on 10-9-03.  Since then, I’ve checked MVA readings a couple of times and they match the calculated value: FEEDER AMPS x BUS KV = SCADA MVA.

I believe I’ve also solved the question of how much MVA drop we have across the regulator.  I read the three voltage inputs and the three amp inputs to the MW xdcr and calculated 9.924 MVA.  I read all the feeder amps (and again compared to DOC - ok) and bus kv and calculated 9.542 MVA.  This was with the regulator on 11 lower.  Below are the values:

Watt xdcr - Amps = 1.66 + 1.64 + 1.58 = 4.88 x 240 = 1171
            Volts = (121.2 + 121.2 + 120.8) / 3 =121 x 70 = 8475
            1171 x 8475 =  9.924 MVA

Feeder amps x Bus kv - 1216 amps x 7847 volts = 9.542 MVA.

So, .382 MVA drop is more realistic (at 11 lower and with these load conditions).  It does show that you can have higher MVA on the source side of the regulator.  I’ll have to wait until load is back up again to find out what the difference is during heavy load.  I suspect stations with regulators running near neutral will have very little difference.


BOTTOM LINE - we need to go by the test results from the bad MW xdcr.  Those results show we were would have read 75 % of the true MW value (when true MWs were between 23 and 30).  The peak MWs reading in TOIS on 8-17-03 was 20.36 but the actual MWs were 27.15.  With MVARs at 9.39 we would have had 28.73 MVA.  I can’t say how much effect the “zero” trim pot and the dirt/corrosion in the RTU had but it does appear we’re getting close to max load on the transformer.  I feel confident that all the analog readings are now accurate.

SIDE NOTE - my original estimate that the actual MVA was 25.3 on 8-17-03 must have been off.  The total feeder amps in SCADA were 4.24 % too high.  I assumed 6 % after bench testing two different model amp transducers.  I dialed in a set 5 % error at 1 amp then plotted the error through 5 amps.  I assumed the xdcrs at ********** would have given similar readings at approx 27 MVA.  It looks like I was off a hair - my bad.

Unless I can be of some help, I'm considering this "case closed"."

RE: Substation Voltage Regulator - question on source and load amps

Thanks for the wrap up. Too many threads simply die away without any of us knowing the final result.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources