Open Delta for 3-winding VT 1

[power2020]

Dear Folks,

I have seen that one Power transformer has 3 star connected windings and one Tertiary winding (Y/Y/Y/D). Tertiary winding is not loaded.
PTs for power transformer secondary windings have also 3-winding. one primary star winding, one secondary star winding and one open-delta (broken delta) winding. Open delta winding is connected with a Resistor. What is the purpose of this resistor? If anyone knows, please share with me.
Advanced Thanks...

 

[slavag]

Hi Power2020.
Are you IEC land or ANSI land?
Resistor ( I think 100 or 200 Ohm) installed in Broken delta
connection for damping ferroresonance.
Regards.
Slava

 

[slavag]

O.K.
I have now more time for the answer.
Ferroresonance is a non-linear resonance phenomenon that can affect in unearthed and compensated power networks. Ferroresonance is formed by the voltage transformer (VT) non-linear magnetizing inductance and the earth capacitances of the network. For a ferroresonance, it is typical that:
- the oscillation is initiated by some switching action
- the resonance ceases, when further lines are connected
- phase voltages are highly distorted, but not line voltages
- overvoltages arises, which may damage the equipment.

Protection against ferroresonance is gained by coupling a damping resistor in the open delta winding. A typical size of this is 50 to 60 ohms.( in my previous post I wrote 100 or 200Ohm, it's depend on type of protection 59N)
Regards.
Slava

 

[power2020]

Many thanks slavag for your reply,
The secondary winding of the transformers are solidly grounded / earthed. This project belongs to Saudi Arabia (60HZ)
Is this kind of connection with resistance necessary for VTs of Star/Star power transformers only?

 

[slavag]

Hi Power2020.
According to my excperience you don't need additional resistance for VT's of Star/Star xfr with solidly grounding.
We used it only on VT ( broken-delta connection) on Delta
Tertiary winding of xfr.
Regards.
Slava

 

[power2020]

Hi Slavag,

Could you please specify any IEC/IEEE standard dealing this? B'cas we are refurbishing the system. In existing system, resistors are connected across open-delta terminals of VT. All Power transformer windings are solidly grounded.

Thanks a lot

 

[slavag]

Hi Power2020.
I'm not sure, if are some IEC/IEEE standard.
This resistor is used to limit stress on the VT and extend it's life. I check all my papers about this issue, no information.
I basis on standard design of 161kV/400kV GIS of ABB, Areva,
Siemens and GEC Alstom, isn't include this resistor at the broken-delta VT connection.
More from this, our distribution system is compensated system and we don't use resistor.
I learn it from our Finlanad partners, them have a good excperience with compensated and ungrounded system.
Regards.
Slava

 

[davidbeach]

Open delta and broken delta are not the same thing and the terms can not be used interchangeably. Open delta is two windings forming two sides of the delta. Broken delta is three windings with one corner of the delta "broken" open with a resistor inserted to measure 3V0.

 

[slavag]

David.
You are right.
I wrote broken delta and I'm sure Power2020 meant same( please see OP).
I think IEC land and ANSI land agree with your terms ( on this forum ).
Regards.
Slava

 

[power2020]

Dear Davidbeach and Slavag,
Thanks a lot for your information.
Slavag is correct that resistor i.e.27 ohms,370W, is connected across the broken delta terminals and I understood the same.
As the system is refurbishing, what will be the consequences if broken delta winding with resistor is considered in PTs of Star winding of power transformer (Power transformer neutrals (Y/Y/Y/D) are solid grounded)

 

[slavag]

Hi Power2020.
Formulas for this issue is:
P=(3 x Us)^2/R (Us= 100/3 or 110/3).
But power P ( of resistor) must not exceed rated power of VT. for 27Ohm is 450W ( on 110/3).
Conseguences? I don't know, I don't see any problem.
Could you please send data of PT's are connected in broken delta.
Regards.
Slava

 

[RRaghunath]

Purpose of broken delta winding is two fold, one to provide polarisation in cse of directional earth fault protection, the other to detect earth faults in high impedance earthed / isolated systems.
It appears you have solidly earthed star winding secondaries in power transformer. Hence, you may not need broken delta winding in VT for earth fault protection in this case (as earth fault sensing can be current based). I am not sure whether you are using directional relays!!
Further, as part of refurbishment project, if you are going to replace the relays with present day numerical types, the residual voltage is derived within the relay from phase VT inputs and hence, you may not need broken delta secondary in the VT.

 

[power2020]

Thanks for information.
Sorry!In my previous post, I supposed to ask "the consequences if broken delta winding with resistor is NOT considered in PTs of Star winding of power transformer (Power transformer neutrals (Y/Y/Y/D) are solid grounded)"
PT details:
13800:1.73/100:1.73/100:3V
50VA/0.5-50VA/3P

 

[prc]

I am getting little confused.As already mentioned broken delta is used to detect earthing of one phase in an unearthed system.eg stabilising Tertiary formation in a EHV auto transformer bank or tertiary loading from a three phase unit.When one phase of primary is earthed, full voltage of 100 v will appear across the broken delta and during normal occasion zero voltage will be there.If a resistor is put across it, how this purpose of earthing detection can be achieved?

But I have seen maloperation of the earth detection relay connected across open delta due to neutral shifting/ferroresonance ie voltage will appear across the broken delta even without earthing of primary phase.Solution for this is (a) connect resistors across the star secondary of the PT or (b) connect surge absorbers ( capacitors) across the primary to earth. I have seen this solution in India for protection of tertiaries of 400/220/33 Kv Auto transformer banks.

Slavag, calculation of resistor load is not clear.Max current that resistor will see is 100/27 A and wattage will be I2R ie 370 W

 

[power2020]

Hi prc,
Application No.1 :Broken delta connection with resistor is for protection against ferro-resonance. Pl. go through from beginning of this post for more details.
Application No.2: Broken delta connection with E/F protection relay is for detecting earth faults.

 

[slavag]

Hi.
We need separate several things.
First:
PRC.
I calculated P for the 110/3V and for the not solidly grounding system 3U0, for the solidly grounding system
voltages on broken delta will 100(110)/1.732.
Resistor will connected to broken delta VT connection only is protection against ferroresonance. But protection for the VT, nothing do for the grounding protection.
You are right, actually is used for broken delta VT connection installed on the Delta side of xfr.( all types of xfr.).
Second:
Protection issue ( function 59N, 67N).
Yes, Raghun is right, for the solidly grounding distribution system isn't used function 67N, but for radial line only. If you have double-line or ring connection, you must use 67N functionality.I don't now what is a type of new relays Power2020 will use ( maybe are include only one voltages input for the broken delta connection, it's depend).
And last for Power2020, I don't see any reason for used resistor for the 13.8kV solidly grounding system.
If those lines are radial, as Raghun saied, you don't need broken delta connection.
Regards.
Slava

 

[power2020]

Thank you Slavag for Summarizing.
In our system, only resistor is connected across the broken delta terminals for protection against ferro-resonance.
Broken delta connection is not used for ground fault detection in system.

 

[RRaghunath]

power2020,

You say the broken delta output is not connected to any protection relay. If so, I think the purpose of Star/Delta VT in your case is to provide earth reference for your 13.8kV system.
The Y would have been grounded and the resistor in broken delta on secondary controls the magnitude of earth fault current that can flow in 13.8kV system.
Thus, this is to be treated as Earthing transformer than a protection VT.
Any book detailing on System Earthing can provide you more insight into the subject.

 

[power2020]

Hi Raghun,
It is a three winding VT (star/star/delta). One secondary star winding for metering and protection (27) AND another secondary broken delta winding is connected to anti-ferroresonance resistor