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Understanding delta VT ratios 5

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veritas

veritas

Electrical
Oct 30, 2003
467
Hi

I have come across a 11kV VT which has one 33kV primary winding, one three-phase secondary winding and one delta winding. Ratios are given as [11kV/SQRT(3)]:[110V/SQRT(3)]: [110V/3].

I am comfortable with the ratios of the three-phase windings being [11kV/SQRT(3)]:[110V/SQRT(3)] which boils down to 100:1.

But I cannot understand where the 110V/3 comes from for the delta winding. The way I see it the open-delta voltage is the sum of the 3 phase voltages, VA + VB + VC = 3V0 = 0 with a healthy system. With a fully blown EF (say on A phase), VA is missing and so output is VA. This would be 63.5V and the primary side on the A-phase is 33kV/SQRT(3) which leads to 100:1 again? So why 110V/3?

Thanks.
 
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Actually, I've just noticed that one of the most useful parts - the link to the Basler article - is broken now. It is available at Basler's website under the "Technical Resources" tab as Application Note PC-59N01.
 
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Each of the individual delta phase windings is rated at 110 Volts. The delta may be closed. That will allow you to use a floating wye point on the primary winding. If the wye point is grounded but doesn't have a good low impedance connection back to the supply neutral, the delta winding will stabilize the primary VT voltages.

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

Thanks for that. If that be the case that the delta windings are 110V phase-neutral, then the voltage ratio is then [11kV/SQRT(3)]/110 = 173.2, correct?

Is that the value then I would enter for the VN ratio of the delta connected VT in the relay? I have set the phase VTR = 100.

Secondly, this means that the open delta voltage is 110V for an EF?

Thanks.

 
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The open delta winding is normally provided with 110 V secondary for solidly earthed systems and 110/3 V for unearthed, reactance or resistance earthed systems. This will give an open delta output of 110 V during a solid earth fault in both systems.

Regards
Marmite
 
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An open delta may be used to detect unbalance issues on the primary.
A closed delta will stabilize a floating neutral on the primary.
If the delta winding is rated at 110 Volts then it will be three 110 Volt windings connected in delta. (Open or closed).


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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The 110/3V winding is normally labeled as a residual voltage winding under IEC. You can tell for sure by looking at the terminal labeling...the residual voltage winding will/should be labeled as da-dn.

Note that under IEC, these windings are only intended to be connected in a broken-delta configuration for residual voltage sensing. The winding rated for standard metering/protection (a-n winding for example) is only tested with the residual voltage winding (da-dn) with 0 burden up to 100% Vrated. The assumption is that the residual voltage winding will only see an effective burden during transient conditions and thus not impact the steady-state performance of the other winding(s).

If the da-dn winding does not have a listed accuracy class on the nameplate, then it is only intended to be used for ferroresonance damping purposes.



 
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Coming back to my original question then, my voltage ratio is then [11kV/SQRT(3)]:[110V/3] of the broken delta winding, which equates to 6350.85:36.67 = 173.2

I have a temporary unearthed system whilst generator gets synchronised to grid. An EF will then result in a full neutral shift and so the output voltage will be 110/3 = 36.67V.

Marmite - I am curious to know how you got 110V?
 
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First - It's called "broken delta". "Open delta" is something else.

"I am curious to know how you got 110V?"
The voltage on the broken delta is the total zero sequence voltage (3*V0) that appear during an earth fault.
All three windings will contribute to this voltage and since all three voltages are equal in magnitude and have the same angle
they will add to each other;
(110/3)+(110/3)+(110/3)V=110V.

-BAK
 
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BAK

If what you are saying is true, then it implies that all three primary phase voltages are equal in phase and magnitude as well. But how can this be for under an A-phase EF condition the A-phase voltage is 0?
 
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You are confusing real voltages with symmetrical component voltages - these are not the same.

-BAK
 
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Hello BAK. Welcome to the site.
This is an international forum. What is called a broken delta in some parts of the world is called an open delta in other parts of the world.

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

I'm not sure that is completely accurate.

I believe the attached picture accurately describes the naming convention universally. If someone would refer to the broken-delta configuration as "open-delta", then what do they call what is shown as open-delta?

 
 http://files.engineering.com/getfile.aspx?folder=a8b166eb-4222-45fc-8a62-8a87a9e9c468&file=etip.57133fig1A6.jpg
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BAK,

Well, the output of the broken delta VT (or open delta) in the phase domain, is the sum of the three phase voltages, VA + VB + VC. If VA is missing then VN = VB + VC = 3V0. Now we are given that the rated voltage per phase is 110/3. This is a phase value which equals the positive sequence voltage under healthy conditions. For EF's the phase and sequence quantities will be different.

I realise that you are using 3V0 = V0A + V0B + V0C. At the end of the day though this must lead to the same result as a phase domain analysis. In this example a phase domain analysis is not that complicated as there is no fault current and the healthy voltages are not distorted.

However, purely from a phase domain perspective how can the output voltage be 110V? I still believe it is 110/3. If I'm wrong I'd very much like to know how.

Thanks.
 
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Veritas, you brought a very interesting discussion, thank you!
By the way, I call it open corner delta, to me it is more clear on the concept.
Well, missing one healthy phase under unfaulted balanced conditions is different than single phase fault conditions.
If just sum two phase VT secondary vectors, you are correct, you will get Vb+Vc=-Va. Be noticed I mention individual VT's who has no magnetic flux interaction between them.
In the open corner delta, if there is a L-G fault, whether the system is grounded or not, there is negative and zero sequence involved, it is not same as missing a healthy phase. So, positive and negative sequence sum out zero, but zero sequence tripped. What is the magnitude of 3V0, depends, I don't know.
 
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The 110 Volt rating refers to the voltage of each individual phase winding, not the voltage across the broken corner. It is useful to know what voltage your relays may be subject to under fault conditions.

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

I'll check if I have some hand made calculations that explains this, or maybe someone else has an example that they would like to share...

-BAK
 
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In the broken delta the three 110 Volt voltages sum to zero. If one phase is lost, the circuit becomes similar to an open delta (two transformer) connection. The voltage across the open delta will be 110 volts. Either by calculation or by vector drawing.
Commonly, for ground detection on an 11 kV circuit, the primary will be connected in wye. The applied voltage will be 6350 Volts.
The voltage of the delta winding will be 63.5 Volts.
Under ground fault conditions the applied voltage will rise to 11 kV on two phases and the secondary voltage will rise to 110 Volts on two phases. The voltage on the secondary of the faulted phase will drop to zero.
The directed resultant of two equal voltages displaced by 120 degrees will be an equal voltage displaced by 240 degrees from the reference voltage. 110 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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waross - in this case the open corner delta windings are built in, it will not be completely missing phase in this open corner delta even the primary has one phase missing.
 
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