Confusion around base kV and PU voltage

asked 2019-10-30 08:08:13 -0500

likethevegetable
52 ●12 ●16 ●18

updated 2019-10-31 11:37:27 -0500

Edit:

To further illustrate my point, consider a sequence of buses. 1, 2a, 2b, 3. All lines impedances are negligible. Now, I have my generator set at 1 pu controlling bus 1 on a 1 kV base. So, for my study, I could confidently say that bus 1, 2a, 2b and 3 are all at 1 pu, or 1 kV. But what if the base of bus 2b was 2 kV? The PSS/E solution tells me that it is 1 pu on a 2 kV base, so I would record it as 2 kV. Clearly this is incorrect as the generator is regulating the voltage at 1 kV. Fortunately, this network is simple and I can figure this out. But what if I had a much larger network, and connected two buses of different base kVs. The only way I would be able to tell you the actual kV (and true base kV for that matter) is to trace the lines back to a voltage regulated bus, and take note of the transformer tap ratios along the way. To me, this seems like a needless step as PSS/E should incorporate this already with the base kV. As far as I can tell, base kV is only used for equipment ratings, but other than that it is misleading.

O-|--|--|--|->

..1..2a.2b.3..

Original:

I encountered what I thought would be obvious and intuitive, but has left me confused. I made a simple 3 bus radial system to illustrate my point: a swing generator (1), connected to an intermediate bus (2), serving a load (3). Everything is set to 1: Vswing = 1 pu (controlling bus 1), load is 1 MW, all buses are at a base KV of 1 kV. Branch impedances are negligible.

O-|--|--|->

..1..2..3..

So everything solves without error, and leaves all the voltages at 1 pu. Now, if I change the middle bus's (2) base kV, to let's say 2 kV, and then solve, then shouldn't it's new PU voltage be 0.5 pu? Since the swing is controlling the volage of bus 1 to 1 PU on a 1 kV base, that is 1 kV, and bus 2's voltage should also be the same (negligible impedance between). The way I see it, the base kV for a type 2 bus is just an interface for us users to see the pu voltage. When PSS/E is solving the loadflow in the background, I imagine it uses whatever base it so desires, and then converts it to a PU number on the base we chose.

If I change the base to an arbitrary number, the PU voltage stays the same--shouldn't this change? How do I know what the actual base that PSS/E is using to derive the PU values.

I'm using 33.10.0

Here's the raw data if anyone is interested.

0, 100.00, 33, 0, 1, 60.00 / PSS(R)E-33.10 WED ... (more)

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answered 2019-10-31 12:59:08 -0500

acd1

63 ●3 ●5 ●11

A couple comments that probably don't help you at all:

It's not good modeling practice to connect buses of different nominal voltages without a transformer
As a couple of others have said, this is the nature of the pu system. All calculations are done based on the per unit values, which will all be 1 pu in your case as expected. What you are seeing is expected behavior, and you will need to add a couple 2:1 transformers if you want to see 0.5 pu voltage on your 2 kV bus.

RAW data with transformers to create the 0.5 pu value:

0,   100.00, 33, 0, 1, 60.00     / PSS(R)E-33.11   THU, OCT 31 2019  13:06


     1,'            ',   1.0000,3,   1,   1,   1,1.00000,   0.0000,1.10000,0.90000,1.10000,0.90000
     2,'            ',   2.0000,1,   1,   1,   1,0.50000,  -0.0001,1.10000,0.90000,1.10000,0.90000
     3,'            ',   1.0000,1,   1,   1,   1,1.00000,  -0.0001,1.10000,0.90000,1.10000,0.90000
0 / END OF BUS DATA, BEGIN LOAD DATA
     3,'1 ',1,   1,   1,     1.000,     0.000,     0.000,     0.000,     0.000,     0.000,   1,1,0
0 / END OF LOAD DATA, BEGIN FIXED SHUNT DATA
0 / END OF FIXED SHUNT DATA, BEGIN GENERATOR DATA
     1,'1 ',     1.000,     0.000,  9999.000, -9999.000,1.00000,     0,   100.000, 0.00000E+0, 1.00000E+0, 0.00000E+0, 0.00000E+0,1.00000,1,  100.0,  9999.000, -9999.000,   1,1.0000
0 / END OF GENERATOR DATA, BEGIN BRANCH DATA
0 / END OF BRANCH DATA, BEGIN TRANSFORMER DATA
     2,     1,     0,'1 ',1,1,1, 0.00000E+0, 0.00000E+0,2,'            ',1,   1,1.0000,   0,1.0000,   0,1.0000,   0,1.0000,'            '
 0.00000E+0, 1.00000E-4,   100.00
0.50000,   0.000,   0.000,     0.00,     0.00,     0.00, 0,      0, 1.10000, 0.90000, 1.10000, 0.90000,  33, 0, 0.00000, 0.00000,  0.000
1.00000,   0.000
     2,     3,     0,'1 ',1,1,1, 0.00000E+0, 0.00000E+0,2,'            ',1,   1,1.0000,   0,1.0000,   0,1.0000,   0,1.0000,'            '
 0.00000E+0, 1.00000E-4,   100.00
0.50000,   0.000,   0.000,     0.00,     0.00,     0.00, 0,      0, 1.10000, 0.90000, 1.10000, 0.90000,  33, 0, 0.00000, 0.00000,  0.000
1.00000,   0.000
0 / END OF TRANSFORMER DATA, BEGIN AREA DATA
0 / END OF AREA DATA, BEGIN TWO-TERMINAL DC DATA
0 / END OF TWO-TERMINAL DC DATA, BEGIN VSC DC LINE DATA
0 / END OF VSC DC LINE DATA, BEGIN IMPEDANCE CORRECTION DATA
0 / END OF IMPEDANCE CORRECTION DATA, BEGIN MULTI-TERMINAL DC DATA
0 / END OF MULTI-TERMINAL DC DATA, BEGIN MULTI-SECTION LINE DATA
0 / END OF MULTI-SECTION LINE DATA, BEGIN ZONE DATA
0 / END OF ZONE DATA, BEGIN INTER-AREA TRANSFER DATA
0 / END OF INTER-AREA TRANSFER ...

(more)

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To your first point: For my particular study, I am looking at interconnecting our system to a neighbor with a different base voltage, so this is unavoidable. I definitely would not deliberately connect to different bases.

likethevegetable ( 2019-10-31 15:00:37 -0500 )edit

It's not that I really want to see the voltage as 0.5 pu--it's what I expected to see. In my case, I am stuck with a 110 kV bus connected to a 124 kV bus (by a 0 impedance line) both at 1.05 pu. I do not know what to report the voltage as! 1.05 on a 110 kV base, of 124 kV. It can only be one.

likethevegetable ( 2019-10-31 15:02:33 -0500 )edit

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answered 2019-10-30 12:00:38 -0500

perolofl

3312 ●1 ●8 ●18

PSSE is always using pu in the network calculations. Voltage in kV is only for user interface and it is of course Vpu*Vbase. All buses should have a pu voltage around 1 pu, regardless of the base kV for the bus.

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I understand that, but when I call `psspy.busdat(ibus, 'KV')`, wouldn't it only make sense to return the actual bus kV voltage? I can set bus 2 to an arbitrary base, and it still shows 1 PU. How do I know what the actual kV is? How do I know what the base used in the network calculation is?

likethevegetable ( 2019-10-30 14:53:35 -0500 )edit

Base kV is not used in the network calculation. Everything is in pu!

perolofl ( 2019-10-30 16:31:13 -0500 )edit

I understand that Base kV is not used in the solution.... But it is definitely used when I call for the bus voltage in kV, and it's giving me incorrect results. If I have a base kV of 1 kV and a bus at 1 pu, PSSE should tell me the bus kV is 1 kV. If I change the base to 2 kV, it should tell me 0.5

likethevegetable ( 2019-10-30 21:22:39 -0500 )edit

You don’t understand at all! The voltage will be 2 kV! You have completely misunderstood the whole concept of the pu system.

perolofl ( 2019-10-30 22:15:20 -0500 )edit

I think what @perolofl is trying to say is that if you change the base kV, the whole PU system moves along with it and thus it is gonna be 2 kV and 1 pu, not 0.5. This is actually how the PU system works.

drsgao ( 2019-10-31 04:54:39 -0500 )edit

Can you please look at my example. The generator setpoint is 1 pu on a 1 kV base - so it is 1 kV. Then how will bus 2 be at 2 kV? It's connected by a zero impedance line! I understand that 'internally' PSS/E uses whatever base it wants, but it SHOULD display PU voltages on the base I specify.

likethevegetable ( 2019-10-31 06:54:42 -0500 )edit

I can theoretically connected two buses by a 0 impedance line, one at 200 kV base and one at 1 kV base, yet both of them say 1 pu. I'm a power systems engineer and I want to know the ACTUAL voltage. So what is it? It can't be 200 kV and 1 kV. Now if I used a 110 kV base and 124 kV base, what is it?

likethevegetable ( 2019-10-31 07:00:59 -0500 )edit

You must understand that PSSE only uses the pu voltage. You can choose whatever base kV you want, the kV will always be Vpu*Vbase. This is completely in line with your observations, so what is your problem?

perolofl ( 2019-10-31 09:54:34 -0500 )edit

If you are a power system engineer you should have basic knowledge about PU system.

perolofl ( 2019-10-31 09:56:49 -0500 )edit

No need to be condescending. I know how the PU system works. If I can use whatever base I want, then I can change the base on one bus to 999999 kV, yet the voltage shows 1 pu? How am I supposed to interpret the results? There is a disconnect between PSS/E and physical voltage

likethevegetable ( 2019-10-31 10:58:16 -0500 )edit

I am working on a study where I connect a 110 kV system to a 124 kV system (formerly isolated). I solve the LF and have the pu voltage as 1.05 on the 110 kV bus AND 124 kV bus (small impedance between them). So what I am supposed to report the new voltage as? 1.05 pu on what base kV?

likethevegetable ( 2019-10-31 10:59:59 -0500 )edit

@perolofl please see the edited question and try to understand my dilemma. Several of my colleagues agree with me.

likethevegetable ( 2019-10-31 11:38:41 -0500 )edit

The pu branch impedance and charging for the 124 kV network must be recalculated to 110 kV base and the base voltage for the buses shall be changed to 110 kV! All buses connected with branches should have the same base kV, even if PSSE allows different base voltages.

perolofl ( 2019-10-31 11:42:08 -0500 )edit

Would the converse, i.e. recalculating the 110 kV branch impedance etc., be equivalent? Does this not seem like a shortcoming by PSS/E, and to not even warn us of this behaviour?

likethevegetable ( 2019-10-31 12:18:13 -0500 )edit

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Confusion around base kV and PU voltage

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