Selecting a 220-kV Cable for Connection to Ground

For an unbalanced load, the neutral conductor needs to be the same size as the phase conductors.

Thank you.

Further, the primary conductor selected is 1c 630 Sqmm Cu 220kV XLPE cables.

Do we necessarily maintain the same voltage level for neutral conductor?

That means, can we select same cable for connection with primary neutral also?

These matters are best addressed by referring to the end Client's specifications.

AND local codes.

(Sorry, I thought this was 220V not 220 kV. Local Codes probably are silent. But check with local utility requirements or practice.)

generally we are using a copper strips to connect to the ground.an earthin plate is extended to the transformer neutral point.

Hello "Guest".

It would be a very unusual installation where a 220kV/33kV transformer is required to have both windings running star connected to ground.

It is normal for the Primary winding: in this case the 220kV winding, to be Delta connected.

The reason for this is simple:

If both Primary and Secondary windings are Star connected, it becomes possible for Neutral ground faults on the Secondary, to interfere with protection and fault discrimination relays at the supplying end of the transmission line feeding the 220kV Primary winding.

That is the main reason for never connecting both Primary and Secondary in Star form.

By connecting both neutral points to ground, at the same location, gives problems, because of electrical non-isolation.

Can you please advise the Installation Location/Country where you have seen the YY with both neutrals earthed?

Reply here, with

Kind Regards....

Hello

Thank you for your replies.

Meanwhile, I must correct, the vector group is YNynO(d), the primary neutral is solidly grounded, the secondry neutral is grounded through neutral reactor and the tertiary (stablising) winding is delta solidly grounded.

rgds

Hello again "Guest",

That is a "horse of a very different colour".

Kind Regards....

Actually Spark, a wyw/wye with grounded star points is becoming a preferred method of connecting transformers for many utilities across Canada and I'm sure the US as well.

Yes... its an autotransformer. Yes by the Canadian code it is illegal to use an autotransformer (other then in a reduced voltage starter) inside of a building as a power transformer, but the utilities love them.

They save money on conductor cost as they only have the one neutral and it does not effect their protection systems or coordination. Many utilities Have active conversion programs to change over their distributions.

Though... this is not being done, that I know of, on systems with the voltages mentioned in this thread.

The conductor size shall be calculated based on Earth Fault current of the system.

First of all, Check the earth fault current(? KA) and calculate the grounding conductor size. IEEE80 shows the calculation method.

The "Neutral cable you said" is not neutral cable. It is grounding conductor that is connected to neutral terminal. The grounding points of the TR are 4 points, Primary, Secondary, tertiary and Transformer body and shall be connected to earthing(Grounding) system.

The conductor type will be Copper Tape or Inslated conductor.