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Thread: Transformer Open Delta

  1. #1

    Transformer Open Delta

    what is open delta connection and where and when is it used? Today as i was doing secondary injection for the PT circuits - i had noticed that the PT secondary side had three phase voltages (Va.Vb and Vc)of which Voltage b(Vb) was grounded. Latter i had come to know that this type of PT is known as Open Delta PT and is a 3-Wire Delta, 2½-Element, 2 PT, 3 CT connection. I am a bit confused as to why they go for such a type of connection. Is there any advantages. Is it that they only want a ph-ph voltage value? In that case - the selector switches available at site have Ph-Ph selection and Phase to Neutral selection. So now i guess we will have to change the selector switch so that it will only have Ph-Ph selections.

  2. #2
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    The open-delta used to be common in this area where the customer had a limited 3 phase load as compared to the single phase load. Often a small
    commercial/residential with all single phase except for HVAC or small motors.
    It allowed POCO to use only two transformers, one normally small and sized to the 3 phase load only.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #3
    thanks for the reply ..I had found the answer in another questionare..I will just copy and paste it so that it can be a reference for me latter..

    1. The reason for using 2 pts in an open-delta configuration is usually one of cost. If the loading on the pt circuit is less then 57.7% of what it would be for using 3 pts then you can use the 2 pt set-up. It still provides you with all three phases so you do not sacrafice any metering or relay protection.
    2. Open Delta” VT's are two VT's connected phase-to-phase, usually A-B and B-C with the secondary outputs connected the same way with "B" phase grounded. This arrangement gives you readings for all three phase-to-phase voltages but no phase-neutral or phase to ground readings. It is usually set up for 120V output. Almost all metering and relaying works great, except phase-ground readings.

    A “Broken Delta” VT arrangement uses three VT's connected phase to ground (Wye) on the primary and in delta on the secondary, except one corner of the delta is broken open and a connected to a loading resistor and voltage relay. If the primary voltages are all matched phase-neutral, the secondary delta connections add the voltage to zero and no voltage is on the relay. A phase-ground fault unbalances the delta and puts a voltage on the relay.

    Phase-phase VT's in open delta are very common in ANSI equipment. A big advantage is they are not as susceptible to ferro-resonance as wye connected VT's. They also take less space in the switchgear and are 30% cheaper.

    A disadvantage with Open Delta is there are no phase-ground voltage measurements, which is only a disadvantage if directional relaying or distance relaying is needed. Some directional and voltage based ground fault detection relays don't work without the phase to ground input. Since most industrial systems MV are high resistance grounded and ground fault protection is provided by overcurrent, there isn’t a need for the phase-ground measurements.
    Why will the directional or distance relaying not work in this case?
    5. Most directional and distance relays need a voltage to ground reference to compare with the phase current nfr groudn faults. Some can use a zero sequence polarising current from CT's on the neutral of the transformer, but most need phase-neutral voltages for accurate calculation of fault direction and distance.

    The relay needs to see a zero sequence voltage to compare to the zero sequence current to find out which direction the current is flowing.

    Precise Phase-ground voltages are needed for this type of calculation, whether it is done digitally or with analog circuits or windings, coils and phae shifting circuits.

    The voltages provided by the Open Delta do not have enough zero sequence component.

  4. #4
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    Quote Originally Posted by bejoyeleengineer View Post
    Since most industrial systems MV are high resistance grounded and ground fault protection is provided by overcurrent, there isn’t a need for the phase-ground measurements.
    Do you mean low resistance or solidly grounded ? HRGs are generally not offered above 5KV, due to system capacitance. HRGs generally limit ground current to under 10A total, including the pulsing current.
    Advise is a dangerous gift, even from the wise to the wise.

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    Quote Originally Posted by bejoyeleengineer View Post
    A disadvantage with Open Delta is there are no phase-ground voltage measurements
    Where did you get this info from, that has not be my experience.

  6. #6
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    The answer put me to sleep, why didn't you use Google, Image. Of course some phase are connected it's Delta... I did like the 57%... Good Tell of usage and a Year!
    If your even thirsty, your two quarts low.

  7. #7
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    Quote Originally Posted by cadpoint View Post
    The answer put me to sleep, why didn't you use Google, Image. Of course some phase are connected it's Delta... I did like the 57%... Good Tell of usage and a Year!
    Jude, to be honest with you buddy, I don't understand the above.

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    OK, in #3, second paragraph which started to put me to sleep, but the information of a transformers at 57.7% (and my birth year) was interesting...
    If your even thirsty, your two quarts low.

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    Quote Originally Posted by chris kennedy View Post
    Where did you get this info from, that has not be my experience.
    With an open delta PT connection, B-phase is grounded for safety, at least in medium voltage. That makes the the L-L voltage the same as the L-G voltage for that system. You can only measure three voltages, 1-2, 2-3, & 3-1. The voltmeter switch would have four positions. The fourth being "OFF".
    Advise is a dangerous gift, even from the wise to the wise.

  10. #10
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    Open delta and corner grounded delta are sometimes confused, and some systems are both open delta AND corner grounded delta.

    An open delta system provides 3 phase service from a 3 phase MV supply but requires only 2 transformers, not the 3 transformers of a standard 3 phase delta.
    The merit of such a system is reduced capital cost as 2 transformers need to be purchased, not 3.
    Any desired point on such a transformer bank may be grounded.
    A common arrangment is to ground the mid point of one winding in order to provide a standard 120/240 volt service, but also allow the use of 240 volt 3 phase motors for air conditioning or other purposes.
    Alternatively, one corner can be grounded, as described below.
    The capacity of such a system is 57% of a closed delta, 3 transformer system, and not 66% as one might expect. Sometimes a third transformer is added later when the load grows.

    A corner grounded delta is a 3 phase, 3 wire system with one corner of the delta grounded. It may be derived from either a 2 transformer open delta transformer bank, or from a 3 transformer closed delta bank.
    The merit of such a system is simplified premises wiring, especialy for 3 phase machinery. The grounded neutral in such a system should be white or gray, and no fuses etc should be inserted in it, as with any other neutral.
    This means that 3 phase machines need only 2 pole dissconects and MCB etc.

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