Delta Wye Transformer Zero Sequence Voltage

[Jim0505]

Hello fellas,
Is it correct that for a delta (High) wye (low side grounded )transformer, wye side line to neutral zero sequence voltage is always zero? I used the following equation to get this, I assumed n is equal to (square root of 3 times voltage ratio)
On wye side, Van= 1/n (VAN-VCN) = VAC (Delta side line voltage)
Vbn= 1/n (VBN-VAN) = VBA
Vcn= 1/n (VCN-VBN) = VCB

Then Va0= 1/3(Van+Vbn+Vcn)=0; For delta side, VAC0 (zero sequence line voltage) is also 0. These equations should hold true regardless of the system balance. However, I modeled a simple power system using computer software, I faulted wye side phase a to ground, the result I got is the opposite, the computer shows delta side line to neutral zero sequence voltage is zero (VAN=0) and wye side zero sequence line to neutral voltage Vao is not zero. I am struggling to find out where I was wrong. I would greatly appreciate it if anybody can let me know where the error is!

 

[aap2]

If you subtract two phase voltages that are the same you will get zero. The line voltage is equal to root 3 x phase voltage.

 

[krisys]

Jim0505 (Electrical)(OP),


You are right, but for the balanced condition for the three phase distribution system. When a fault occurs, the zero sequence voltage appears, which drives the ZS current.

So the always zero, when the system is balanced!

 

[HamburgerHelper]

If you look at the sequence networks for a delta-wye transfomer, the delta side is an open connection and the wye side is a ground source. For a SLG fault, there is a zero sequence voltage profile that develops on the system with the highest magnitude being at the disturbance, in your case an SLG fault, and it decays as you get closer to your ground sources or decays due to system capacitance to ground. The zero sequence voltage that develops on the lowside of the grounded wye side will be any grounding impedance plus the zero sequence winding impedance of the wye and delta (for the circulating current) and the return path through the earth times the ground current. This will always not be zero but it should always be low if you don't have high impedance grounding or bad soil or just a fault close to the transformer. If you are applying a fault right next to the transformer, just remember that the zero sequence voltage that develops at the wye transformer windings is going to be high due to the zero sequence impedance from the fault to the transformer being low compared to the transformer to earth zero sequence impedances. You got a zero sequence voltage divider. Move the fault further out and the zero sequence voltage that develops at the wye will drop. Draw out the sequence network for your fault and what I am saying is pretty clear. Zero sequence voltage shouldn't develop on the delta side for a fault on the wye side due to the windings on the delta side being line to line.

You might want to keep in mind that a ground fault on the wye side though will look like a phase to phase fault on the delta side due that side only providing phase shifted negative and postive sequence components.