Y-Delta Scheme for transformer 1

[HamidEle]

Hi,

What is the benifit of having Y-Delta connection(Y on the primary side)? I have seen some people do this. I would use Delta-Y connection., which can eliminate the 5,7,9 harmonics elements. Plus, we can protect the transformer secondary against ground faults by using high resitance grounding system. Can anyone give me some ideas about this?
Thanks in advance.

 

[waross]

Well, if you leave the primary neutral floating, you can expect switching to generate over voltage transients.
If you connect the neutral to the system neutral, you can expect any primary voltage unbalance to cause heavy circulating currents in the delta secondary.
Also loss of two phases feeding the circuit will result in something less than 50% voltage being back fed into the system. If this is followed by a two phase situation, such as blown fuses being replaced one at a time, slowly, the voltage will be less than 100% back fed. This kills refrigerators and freezers like a bullet.
But many systems are in use. Utilities often provide a fourth cut-out for the neutral. To energize a bank the neutral is connected first. Then the three phases are energized. Then the neutral connection is opened and the neutral fuse cartridge is put in the line truck.
I have seen an unbalanced line inadvertently balanced by a wye delta bank with a connected neutral.
In rural areas a main three phase distribution line often has single phase or two phase lines tapped off to feed lightly loaded areas. This results in heavy neutral currents and neutral voltage drops. This results in phase shifts and unbalanced voltages on the three phase line back to the point that the neutral current has been canceled out by single phase loads.
A wye delta bank with the wye point connected to neutral at the point were the single phase or two phase line is connected will tend to balance the voltages and phase angles. Care and judgement must be used in sizing the bank. Expect to lose fuses on the bank if there is a phase lost on the three phase line. If if there are a lot of phase loss events the crews tend to over-size the replacement fuses. Then you get burned out transformers.
I much prefer wye wye connections.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Rodmcm]

The main reason Yd is used in my country is to reduce the cost when using automatic tap changers for high voltage transformers ( typically 110kV and 220kV). The taps are on the HV side. The secondary is fitted with an earthing transformer ( zigzag) often with a tertiary winding to supply local service.

 

[apowerengr]

You might select Y-Delta when there's generation connected to the secondary side of the transformer. That way if the system is operated islanded with just the generation providing supply you still have a ground fault source for the supply line.