Neutral earthing in solar Transformers 3

[prc]

Three winding transformers are usually used for collecting the power from solar power inverters, popular connection being Delta/Star-star. The neutral of secondary stars is never earthed, many times it is not even brought out. In one case, it was inadvertently kept earthed and inverters started failing frequently. Why it is not earthed in service? How earthing of neutral is affecting inverters,leading them to fail?

 

[Power0020]

Having a star-delta step-up connection is a bit uncommon (at least, for me!), usually delta faces the power flow direction.

I think that in the case of solar inverters, the inverter should have an earthing point at origin (next to solar cells), so faults would circulate at this point only...I guess this is to facilitate protection operation instead of including the transformer impedance in case of start point eartthing (to help discriminating the faulty branch/inverter.

Failure due to star point earthing is a bit stragne for me but I think it may be due to common mode noise?....

 

[waross]

Is the power fed into the delta or into the star windings?
A Delta/Star transformer is a common configuration but it is unwise to back-feed a delta star.
When back-fed, this becomes a star/delta. A star delta with the neutral connected to the source only works in a perfect world.
The world is not perfect.
In a less that perfect world, even small unbalances of voltage and or phase angle give rise to heavy circulating currents in the delta.
These currents may lead to fuse failure, transformer burn out and/or overloading the source.


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

 

[prc]

Yes it is a bit strange connection for a step up transformer. But this is the one used for solar projects.LV star is fed from inverter out put (1-2 MVA <1kV) and delta HV is connected to 11or 33kV collector grid. Recently an IEEE standard came out for solar transformers C57.159-2016 where under clause 5.1.4 it is said quote -The inverter operation is not affected by the inverter transformer vector group (Dy1, Dy5, or Dy11 will make no difference). No neutral connection is required on the primary (LV) side of the transformer. If a neutral point of the primary (LV) winding is available, it is recommended that this neutral point is neither grounded nor connected to other ground points. On the secondary (HV) side, the inverter transformer can have an isolated neutral point or resonant grounding or low resistance grounding -unquote
Hope power electronics experts can explain why this connection is used ( usually converter/inverter transformers are with two LVs one delta and other star)and why star neutral should not be grounded.

 

[waross]

Hi prc;
Assume three identical transformers connected in wye:delta.
The issue is with the delta secondary.
The delta locks in the voltage and phase relationships of the three phases.
My favorite explanation is to consider the delta as an open delta plus a single phase transformer.
The open delta forms a virtual transformer across the open side. This virtual transformer has similar characteristics to the two real transformers.
Now when we complete the delta, we may consider a single phase transformer in parallel with a single phase virtual transformer.
What happens when we parallel two transformers with unequal voltages? Circulating currents, limited by the transformer impedances.
Now take a wye:delta bank with the neutral connected on the wye side.
Assume that the impedance of the transformer bank is 3%
Consider the voltage on one phase to be 9% low.
Now we have a virtual transformer with 100% voltage in parallel with a real transformer with only 91% voltage.
This 9% voltage will cause a circulating current limited by three times the transformer impedance or 100% of rated current.
This will cause rated KVA to be drawn from the source, but supplied by only two transformers and by two phases.
If we have three 100 KVA transformer the almost 300 KVA load will now be fed by two 100 KVA transformers or a 50% overload.
In this example a 9% voltage unbalance has caused an almost 50% overload with no power drawn on the output.
Mitigation.
The delta does not have to be an equilateral triangle. The voltages may be unequal if the phase angles are allowed to shift to compensate for the uneven voltages.
If the neutral is connected, the voltages and phase angles are locked in and the trouble starts.
Now consider supply voltages of 100%, 100% and 90%.
Draw a triangle of sides 10, 10, and 9. The angles will not be 60 degrees. Not a problem. These voltages and phase angles will be proportional on the secondary of the transformer bank.
The important thing is the delta vector diagram will be closed. It is when the delta does not close that issues with circulating currents arise.
Leaving the neutral floating allows the delta triangle to shift to accommodate the error in voltages and phase angles.

Note:
We should be referring to a neutral connection not a ground connection. Grounding the wye point makes no difference UNLESS the source wye point is also grounded. Then the connection becomes a neutral connection that is incidentally grounded.
If the transformer wye point is solidly connected to the source neutral or wye point but neither point is grounded the same problems will arise.
Distribution engineers and linemen tend to refer to the neutral as the ground. Well, the neutral is always grounded so it makes little difference most of the time.
Utilization engineers and electricians make a distinction between the ground and the neutral They must as North American codes make a difference and different rules apply to utilization grounds and utilization neutrals.
In this case, the accepted distribution language may be somewhat misleading as the issue here is not grounding, the issue is connecting the neutral from the source to the transformer wye point.
Ground the neutral of the source.
Float the neutral of the wye:delta bank.


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

 

[xnuke]

An excellent explanation, Bill! If I ever write a book, would you mind if I paraphrased it?

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.

 

[waross]

Thank you for the kind words, xnuke. I can always use a good editor. Fell free to use, paraphrase or build on this.
Thank you for asking.
Yours
Bill

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

 

[waross]

A couple of other notes in regards to real world applications.
The issues with circulating currents in deltas is similar to the issue of relatively large currents caused by unbalanced voltages applied to induction motors.
In motors as in delta windings, a small error in line to line voltages or phase angles will cause a large increase in reactive current.
An normally running induction motor is also an induction generator. The induction generator inherently generates equal phase voltages and phase angles. If there is an error in either phase voltages or phase angles, the induction generator will draw extra current from the high phases and transfer it to the lower phases.
The induction generator tends to correct the unbalances and is limited by the motor impedance together with the source impedance.
The effect is sometimes explained as a counter torque developed in the rotor.
While both explanations are valid,I prefer to consider an induction motor driving an induction generator to explain the extra current caused by unbalanced supply voltages and phase angles.
Here again when we go back to our vector sketches, we see that the output of the induction generator will be shown as an equilateral triangle. When the supply phase voltages are not equal, the vector sketch of the supply voltages will not be an equilateral triangle.
If the supply triangle is not similar to the back EMF equilateral triangle then circulating currents will flow.

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

 

[itsmoked]

Fell free to use, paraphrase or build on this

Yes, a good editor indeed!

But, damn, that was a good description! I actually sort of understand now.




Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thank you for the kind words.
I was painfully aware of my need for a good editor even before the hammer dropped. grin

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

 

[prc]

waross, you are at your favorite topic! As I make it simple( or rather as I under stood it), any closed delta will carry circulating current for any induced unbalanced voltages from other earthed star windings. eg Stabilizing tertiary in star/star connected transformers with earthed neutral. How this will change in distribution transformers where power is fed to delta and secondary star (with earthed N) goes to load? In the case we are discussing, power flow is from star to delta with balanced voltages from inverter. I don't know whether there is any neutral in the inverter and whether it is grounded. Will the star neutral grounding cause any circulating currents in inverter so as to damage it? Will the circulating current in delta will affect the inverters?

 

[waross]

Not any delta. The neutral connection on the primary locks in the secondary phase angles and that is where the problem originates.
The issue is specifically with a four wire star connection feeding a delta secondary.
As you know the secondary voltages and phase angles of a transformer will closely follow the primary voltages and phase angles.
Unbalances are acceptable in some delta connections.
Consider a delta:delta transformer or transformer bank.
This is fed from a 480 Volt supply and delivers 240 Volts.
However, the line to line voltage from A to B is only 450 Volts.
Now our phase displacement is no longer 120 degrees. However if the vectors are arranged in tail to head fashion, they will produce a closed triangle.
The triangle produced by the secondary vectors will be similar and will also close on itself.
No problem.
Where the problem arises;
We have a supply of 480/277 Volts feeding a star:delta transformer. The neutral point is connected.
The neutral connection locks in the phase angles at 120 degrees.
With balanced primary voltages of 277 Volts, the secondary voltages will all be equal and there is no problem. The vectors are all equal and at 120 degree phase angles and the triangle closes.

Now suppose that the voltage on one phase drops 10%
When the secondary triangle is plotted, we have 240 Volts, 240 Volts and 216 Volts.
The phase angles are 120 degrees, locked in by the primary neutral connection, and one side of the triangle is 10% short. We now have a gap and the triangle does not close on itself.
That 10% gap causes the circulating current to flow. The current is limited by three times the transformer impedance so in the case of a transformer with 3% impedance voltage, a 10% unbalance will put the transformers into overload with no load on the secondary.
Now if we break the connection to the transformer wye point and allow it to float, the circulating currents go away.
Now we are looking at line to line voltages but the phase angles are no longer locked in.
The secondary voltages and phase angles may follow the primary voltages and phase angles and the secondary vector triangle will close on itself.
No problem (almost)
In a distribution application, energizing a wye;delta bank with an open neutral may result in transient over-voltages.
That may not be an issue with invertors. I don't know.


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

 

[waross]

In the sketch, the green triangle shows a delta:delta or a star delta with a floating neutral. The voltage is low on one phase. The phase angles adjust to compensate for the uneven voltages.
The second sketch shows a star connection with low voltage on one phase.
The neutral is connected and this locks the phase angles in at 120 degrees.
One side is short on the delta secondary.

A couple of effects when a four wire wye:delta is used on a distribution circuit.
If one primary phase is missing, the bank will back feed that phase.
If two primary phases are missing, the two open phases will be back-fed with approximately 50% voltage. On a mixed, industrial and residential circuit, this kills refrigerators and freezers.
A line to line short will be back-fed and the fault current will be twice the expected current.
When I had responsibility for the small system it took me a couple of years to get rid of the star delta transformer banks.
Once the star-deltas were gone, there were no more refrigerator burn outs in the community.

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

 

[Power0020]

Great. just to understand that the circulating current will be pure zero sequence current that will never escape the delta?

Consider the case of a 33/11 kV distribution transformer, Dyn11 (say), with star point earthed through an NER, say to keep the fault current at 500 A, this is pretty much the practice around most of IEC world. If the unbalance appears on 33 kV side due to distribution unequal loading, load rejections...etc,... the delta is closed on itself, but the unbalanced voltages arriving at the 33 kV bus will cause delta circulating currents?

 

[waross]

A delta:wye is no problem. (I somehow read wye:delta.)
A primary delta takes whatever voltages and phase angle presented to it.
The secondary wye line to neutral voltages are reflections of the primary line to line voltages (times the turns ratio).
The secondary line to neutral phase angles are similar to to the primary line to line phase angles. It is what it is. There is nothing to drive circulating currents in the delta.
The four wire wye:delta connection is unique.
The secondary voltages are proportional to the primary voltages.
The neutral connection on the wye primary locks in the phase angles. If the neutral is connected/grounded through a current limiting resistor, the resistor will also limit the circulating current in the delta.
With the angles locked in, often at 120 degrees, the secondary phase angles will be locked in.
With 120 degree phase angles the secondary diagram must be an equilateral triangle.
If the primary voltages are unequal, the secondary voltages will be unequal and an equilateral triangle can not be formed.
This is what causes the circulating currents.


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