I have started studying for the PE and so I picked up a copy of Stevenson's Elements of Power System Analysis, 4th ed.
I was looking at the section on 3p, 3winding xfmrs and didn't follow why something WASN'T done.
Here's the problem: pg. 154 for those following along.
Primary: Y-15MVA, 66kV
Secondary: Y-10MVA, 13.2kV
Tertiary: D-5MVA, 2.3kV
Zps: 7% on 15MVA, 66kV base
Zpt: 9% on 15MVA, 66kV base
Zst: 8% on 10MVA, 13.2kV base
Now the example states to convert all the impedances to a common15MVA, 66kV base.
Zps and Zpt don't need to be modified (already on the correct base). So, the conversion to the base for Zst, according to the book, is Zst = 8% x 15/10 = 12%
Here's what I don't get. The impedance was measured on a different base for voltage and VA. So why is there not a term for the voltage?
That is, IMO, it should have read: Zst = 8%x(15/10)x(13.2/66)^2 = 0.48%
I'm willing to conceded that I'm mistaken, but I don't understadn why the term is not used. Can someone explain what I'm missing?
Thanks.
I was looking at the section on 3p, 3winding xfmrs and didn't follow why something WASN'T done.
Here's the problem: pg. 154 for those following along.
Primary: Y-15MVA, 66kV
Secondary: Y-10MVA, 13.2kV
Tertiary: D-5MVA, 2.3kV
Zps: 7% on 15MVA, 66kV base
Zpt: 9% on 15MVA, 66kV base
Zst: 8% on 10MVA, 13.2kV base
Now the example states to convert all the impedances to a common15MVA, 66kV base.
Zps and Zpt don't need to be modified (already on the correct base). So, the conversion to the base for Zst, according to the book, is Zst = 8% x 15/10 = 12%
Here's what I don't get. The impedance was measured on a different base for voltage and VA. So why is there not a term for the voltage?
That is, IMO, it should have read: Zst = 8%x(15/10)x(13.2/66)^2 = 0.48%
I'm willing to conceded that I'm mistaken, but I don't understadn why the term is not used. Can someone explain what I'm missing?
Thanks.