negative and zero sequence question

[royclh]

would the negative and zero sequence components on the generation side be affected by the grid's negative and zero sequence components when there is a fault, let's say on the common point of coupling? i thought they wont but perhaps i'm wrong. just hope someone could clarify for me.

thanks.

 

[ijl]

Every component has its own negative and zero (and positive) sequence parameters (impedances). These are constants. The negative and zero sequence networks are constructed and solved using these parameters in the same way as the positive sequence network. This means that the currents and voltages in the network depend on all components in the network. So, yes, in this sense the components of the current and voltage of one component depend on other components. But no, the parameters themselves do not depend on other components.

 

[RRaghunath]

royclh,
Presume you are talking of a case where there is YNd connected Generator step-up trafo between generator and the PCC.
If true, the zero sequence currents from faults in the grid cannot be seen by the protective relays at generator terminals. Positive and negative sequence currents ofcourse can be.
Thus, an earth fault in the grid can cause operation of negative sequence protection at generator terminals (earth faults have negative phase sequence currents in addition to zero sequence ones).
Hope the explanation is relevant.

 

[royclh]

thanks for the reply. when the fault is on the common point of coupling, the fault currents will be contributed by both grid and the generator. Viewing from the generator side, doesn't the generator only see up to the fault point and not beyond that? Isn't the fault contribution dependent on the value of impedance in between? If this is the case, I'm still confused that how the zero sequence and negative sequence components on the generator side can be affected by the grid. Hope you can enlighten me from here. When the network impedance changes in the grid, how this will affect the fault contribution from the generator side?

Thanks.

 

[odlanor]

Negative sequence protection of generator are applied to conditions of unbalance load or open circuit.
It is a objective different from short circuits unbalanced!.

 

[dpc]

It might help if you draw out the sequence diagrams showing the appropriate sequence networks for the faults you are interested in.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[RRaghunath]

royclh,
It is like the impedance of the G+GT circuit is in parallel with that of grid at the fault point. Thus, lower the grid impedance, lower is going to be the contribution to the fault from the generator (higher fault contribution from the grid, in other words). This is generally speaking.
If you have a specific query or looking for more specific details, you need to share more information / draw sequence diagrams as dpc suggested.

 

[ProtectionEngineer]

Please see attached document.
Hope this will answer part of your question.

 

[odlanor]

In a dYn transformer with generation at d side:
A fault-to-ground in Yn side of 1pu , will result in contribution current of 1/V3 pu of 2 phases from d side.

 

[royclh]

thanks all for the inputs. sorry for the late response as i was away for two days. i didnt have any diagram for this problem. actually it was a question from a colleague of mine. the question is, if there is a fault at the point of common coupling, will the sequence components (zero and negative) of the wind farm be affected by the sequence components of the grid?

Assume the grid impedance increases when one transformer in parallel is out of service, does the wind farm still contribute the SAME magnitude of zero and negative sequence components to the common coupling point? Sorry, I dont know about the configuration of the transformer on the grid side connected to the common coupling point.

thanks,

 

[odlanor]

This answer is very simple for me.
But, I do not understand nothing abou your system. Youneed to show an unifilar diagram with the information about grounding generators and type of conexion of power transformers

 

[Pikoplatt]

As Raghun said, the Ground(51GS) (zero sequence) relays on the Generator will not respond to the ground faults on the transformer secondary side (PCC), but the current unbalance (46) (negative sequence) and phase overcurrent(51V) (positive sequence) relays on the Generator will respond in accordance with their pick-up and time-delay curve settings.