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."
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 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
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
RE: Substation Voltage Regulator - question on source and load amps
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
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
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
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
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
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
RE: Substation Voltage Regulator - question on source and load amps
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for more info
RE: Substation Voltage Regulator - question on source and load amps
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
RE: Substation Voltage Regulator - question on source and load amps
RE: Substation Voltage Regulator - question on source and load amps
RE: Substation Voltage Regulator - question on source and load amps
Have you double checked all instrument transformer ratios, polarities and phasing?
RE: Substation Voltage Regulator - question on source and load amps
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
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
""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
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
RE: Substation Voltage Regulator - question on source and load amps
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
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
"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