Potential Transformer (Bus PT) Concept 1

[Rejeckted]

Hello,

As you can see above is an electrical diagram of Bus PT for 11kV Bus. I have a question that if the primary winding of PT is Star Connected with Neutral point earthed, then why does it not carry extremely high current when connected to 11kV? (Ex. I = (11kV - Vn)/ Zwinding as per KVL)

Is it because the neutral point voltage is zero under balanced condition? If so what happens when one of the phases fuse would blow up?

Thanks

 

[davidbeach]

Just like any transformer. Current in A-N is proportional to current in a1-n1 by the turns ratio. And there's always essentially zero current in a1-n1. One fuse open, say on B and all that happens is that the b1-n1 current goes to zero and the voltage between b1 and n1 via E30 becomes zero. Currents A-N, C-N, a1-n1, and c1-n1 remain unchanged as to the measured voltages at a1 and c1.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Rejeckted]

Thanks for your reply. It makes sense when we do I2/I1 = N1/N2. But when you think about it primary winding resistance is so minimal (in terms of few ohms), then what exactly is stopping that heavy current to flow between Primary Phase A and N (E90). Like what law of magnetism is working to stop that current/ or what is the hidden phenomenon?

 

[davidbeach]

Resistance is only a small part of impedance. Ampere’s law, amp*turn balance and all that. If current can’t flow in one winding it doesn’t flow in the other.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[xnuke]

The "hidden phenomenon" you seek are the loads connected to the secondary of the VT. Since modern power meters and protection relays connected to the VT secondary have high input impedances, barely any current flows in the secondary windings, meaning even less current (by a factor of the turns ratio) flows in the primary windings. Remember, that high secondary load impedance is reflected across the VT into the primary as a function of the turns ratio squared. This is my understanding of why grounding the primary of a VT doesn't create a ground on the primary system the way a grounding transformer would. The only way current could flow from the ground through a primary winding is if a corresponding proportional current were to flow in the linked secondary winding, and that just isn't going to happen.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[che12345]

Dear Mr. Rejeckted (Electrical)(OP)15 Aug 22 04:39
".... It makes sense when we do I2/I1 = N1/N2. But when you think about it primary winding resistance is so minimal (in terms of few ohms), then what exactly is stopping that heavy current to flow between Primary Phase A and N (E90). Like what law of magnetism is working to stop that current/ or what is the hidden phenomenon?..."
1. I noticed all the learned advice do NOT answer your question.
2. I try to answer it as following for your consideration:
Yes, its primary winding resistance is so minimal (in terms of few ohms). This is an AC circuit NOT DC.. In an AC circuit, it comprising of Resistance (R) and reactance XL=(wL). In this case, the winding is wound on an iron core, NOT Air-core. The reactance is very very high. So, in an AC circuit, the impedance Z= R+jXL is very very high. Note: in a DC or winding wound on an air-core, The Z=R , XL=0.
3. As the primary winding is wound on an iron-core, the Impedance is very very high. Therefore, FS fuses are experiencing only a very very low iron-core magnetization current; even Ns are connected to -ET (Earth); when I2=0.
4. Attention: I2/I1 = N1/N2 is correct. So, FS would blow=open circuit when I2 reflected to I1 is exceeded. Note: FS must be sized to handle the initial magnetizing current surge; after that, the secondary current.
Che Kuan Yau (Singapore)