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 | 1 | initial version |
Confusion around base kV and PU voltage
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, OCT 30 2019 8:12
TEST
TEST
1,' ', 1.0000,3, 1, 1, 1,1.00000, 0.0000,1.10000,0.90000,1.10000,0.90000
2,' ', 1.0000,1, 1, 1, 1,1.00000, 0.0000,1.10000,0.90000,1.10000,0.90000
3,' ', 1.0000,1, 1, 1, 1,1.00000, 0.0000,1.10000,0.90000,1.10000,0.90000
0 / END OF BUS DATA, BEGIN LOAD DATA
3,'1 ',1, 1, 1, 1.000, 1.000, 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, 1.000, 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
1, 2,'1 ', 0.00000E+0, 1.00000E-4, 0.00000, 0.00, 0.00, 0.00, 0.00000, 0.00000, 0.00000, 0.00000,1,1, 0.00, 1,1.0000
2, 3,'1 ', 0.00000E+0, 1.00000E-4, 0.00000, 0.00, 0.00, 0.00, 0.00000, 0.00000, 0.00000, 0.00000,1,1, 0.00, 1,1.0000
0 / END OF BRANCH DATA, BEGIN TRANSFORMER DATA
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 DATA, BEGIN OWNER DATA
0 / END OF OWNER DATA, BEGIN FACTS DEVICE DATA
0 / END OF FACTS DEVICE DATA, BEGIN SWITCHED SHUNT DATA
0 / END OF SWITCHED SHUNT DATA, BEGIN GNE DATA
0 / END OF GNE DATA, BEGIN INDUCTION MACHINE DATA
0 / END OF INDUCTION MACHINE DATA
Q
| | 2 | No.2 Revision |
Confusion around base kV and PU voltage
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, OCT 30 2019 8:12
TEST
TEST
1,' ', 1.0000,3, 1, 1, 1,1.00000, 0.0000,1.10000,0.90000,1.10000,0.90000
2,' ', 1.0000,1, 1, 1, 1,1.00000, 0.0000,1.10000,0.90000,1.10000,0.90000
3,' ', 1.0000,1, 1, 1, 1,1.00000, 0.0000,1.10000,0.90000,1.10000,0.90000
0 / END OF BUS DATA, BEGIN LOAD DATA
3,'1 ',1, 1, 1, 1.000, 1.000, 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, 1.000, 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
1, 2,'1 ', 0.00000E+0, 1.00000E-4, 0.00000, 0.00, 0.00, 0.00, 0.00000, 0.00000, 0.00000, 0.00000,1,1, 0.00, 1,1.0000
2, 3,'1 ', 0.00000E+0, 1.00000E-4, 0.00000, 0.00, 0.00, 0.00, 0.00000, 0.00000, 0.00000, 0.00000,1,1, 0.00, 1,1.0000
0 / END OF BRANCH DATA, BEGIN TRANSFORMER DATA
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 DATA, BEGIN OWNER DATA
0 / END OF OWNER DATA, BEGIN FACTS DEVICE DATA
0 / END OF FACTS DEVICE DATA, BEGIN SWITCHED SHUNT DATA
0 / END OF SWITCHED SHUNT DATA, BEGIN GNE DATA
0 / END OF GNE DATA, BEGIN INDUCTION MACHINE DATA
0 / END OF INDUCTION MACHINE DATA
Q
