I am just wondering that in distribution lines we have delta configuration at the secondary of distribution tranformers but when we have phase to ground short the current flow, why? and what will be the voltage if we measure between any phase and ground?

 

Check out the information at:
http://www.applied-energy.us

 

Sorry but I have not found anything there my question is that with HV distribution lines(132KV, 220KV etc)we use star to delta transformer means delta on secondary side that is no star point as star then why the current flow from phase to earth in case of phase to earth short cicuit why not it work as ungrounded system and also what voltage between earth and phase multimeter would measure.

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Responding to SNA's Aug 5, 11:38pm query... "ungrounded" delta-systems are actually "grounded" by system capacitance.

If there is no intentional short-to-ground, then, all three terminals should measure (ph-ph) / sqrt(3) to ground. However, when one phase is grounded, then the other two "healhy" phases will measure ph-to-ph between each terminal and ground! For a detailed explanation see thread:

http://www.control.com/984504035/index_html

If a simpler explanation is required, please contact me!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Responding to SNA's Aug 8, 2005, 4:43pm comments... more info please:

1) Does the star-delta transformer have a tertiary winding?

2) Is the star side of the star-delta transformer earthed?

3) Where do you "see" the current flow to earth you mentioned? Primary? Or secondary?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

I think it will flow. Because you will have a PT for Protection and metering purpose whose primary will be grounded. So if there is a ph to earth fault, fault current will take the path through PT.

Rajan

 

Many thanks for your interest i am working in a power plant where we are supplying power to the main grid by 220KV transmission line which have distance/grid protection on it. we have used star delta transformer with star grounded on the primary side when we have a coductor brake and it falls on ground. our distance protection trips the breaker on phase to earth fault. it means ground does sink current which causes P-N fault. otherwise it should always be tripped on phase unbalance as it does sometimes in really hot dry summer when ground is really dry.

 

Responding to Anonymous' Aug 10, 11:48pm reply... are any of the following assumptions correct?:

1) The transformer LT is on the star-side, and the HT on the delta-side?

2) The star mid-point is earthed?

3) Distance/grid protection (essentially measuring line impedance to the fault location) is being done via PTs and CTs on the LT star-side? Or, are there PTs and CTs on the HT delta-side?

4) Line-break (LB) fault-mode detection (without short-circuit magnitudes) is very difficult. That is because the only current available is that flowing in the faulted conductor. It is especially exacerbated by the increase in fault-resistance in the dry season.

Nevertheless, there is a technique one can use for LB detection, but it will depend on the answers to the assumptions above.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Many thanks again
1) ye LT on star and HT on delta

2)its earthed

3)Ye its mesuring line impedance but by CTs & PTs on secondary side/HT side and also one leg of PTS are grounded as normal

The question which one of my junior asked me that why its not behaving like a ungrounded system means no current flow in to ground if we have ungrounded secondary winding and why we consider the voltage between voltage/sqrt3 (same as of star). I did not have some logical answers for these simple looking questions

 

Responding to SNA's Aug 13, 5:37pm comments:

A) Junior's Query About Circuit Behavior.
Neglecting PT's, the three lines are connected to earth through their line-to-earth capacitance. There is also line-to-line capacitance, but it and the subject of ferroresonance will be ignored for purposes of simplifying this discussion! Each PT is connected between it's line and earth. Each measures (ph-ph)/sqrt 3. Current flow thru each PT is its magnetizing or exciting current. For practical purposes

When an earth-fault occurs, the faulted line voltage drops to zero while the voltage of the two healthy lines rise to ph-to-ph voltage in magnitude. This change in the relative magnitude of the three voltages signals a fault condition and corrective action is initiated. Note, current flows are not necessary.

B) Distance Relaying for Earth-Faults.
Although distance relaying enjoys success in earthed systems, such is not the case with many un-earthed systems.

As stated in previous correspondence voltage and current magnitudes essentially establish the "impedance" (I=E/Z) to the fault location.
However, in un-earthed lines the magnitude of current is very small to begin with. Furthermore, wide variation in parameters such as arc-resistance and soil-conditions (earth-return-circuit) exacerbate the problem.

C) Line-Break or Open-Phase Protection.
Distance relaying does not work for this type of fault. Success of these systems should not rely on voltage and current magnitudes, but instead, on their phase displacements.

For example, if a line breaks downstream of the PT, then there isn't a change in voltage. Earth-fault current magnitude will vary based on location of the break, i.e., sending-end (close-in) or receiving-end (far-out!) Nevertheless, protective schemes have been developed to detect such faults!

SNA, I hope this discussion answers your questions. Please be aware that the subject of protective relaying is a science of its own. But, if you have additional questions please contact me.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

There might be some insulation leakage at your distribution line there fore the current is flowing in between phases not ground to phase.
there will be no change in voltage between line voltage.