Distance protection & Cable feeders 2

[RRaghunath]

Distance protection and Pilot wire differential protection are two options available as primary protection for HV power cable feeders. Of the two, only Pilot wire differential protection is employed widely for the cable feeders and the distance protection is popular for over head lines.

Any reasons.

Thanks in anticipation.

 

[busbar]

It seems like it’s a matter of tradeoffs. Shielded cables tend to be shorter, where comm-channel distances are in favor, but where impedances are low—making distance relaying more difficult to apply.

Overhead spans are longer, and line impedances comparatively higher—making distance relaying easier to configure and more readily differentiate in-zone versus out-of-zone conditions.

 

[RRaghunath]

Thanks, Busbar for the response.

Some times you come across a 2km long OHL having distance protection but a 10km long is provided with pilot wire differential protection.

 

[PowerSim]

I think, Feeder Differential with Fibre Optic communication link is the preferred protection For underground and submarine cable circuits, and transmission circuits less than 10 km length.

 

[Aakhrinaam]

Another aspect is the high SIR (source impedance ratio) which comes into play with short cables. It is difficult for distance relays not to over-reach under such conditions.
So a unit protection scheme (Current Differential) is preferred in such cases.

 

[RRaghunath]

Aakhrinaam,

Thanks. You have raised an interesting point. Could you please elaborate more as to how over reaching occurs (I didn't grasp completely).

Thanks in advance.

 

[SidiropoulosM]

In addition to the excellent comments above you may want to consider the following:

1. On a radial cable feeder with intermediate loads, pilot relaying provides better selectivity than distance relaying.

2. Some critical loads require high speed tripping which may be beyond the capability of distance relays.

3. For short cables, the ohms being measured by the distance relays would be of the same order of magnitude as the ohmic errors of the measurement, making distance protection impractical.

 

[piterpol]

The physical length of a line does not suffice to classify it as a long, medium, or short line. SIR is a parameter that plays an important role. For instance, for an SIR=0.1 a 10km line is a medium length one. For an SIR=30 a 20km line can be considered as a short one.

 

[jbartos]

Suggestion: The differential protection covers the line from the transmission one side CTs to the transmission other side CTs.
Distance relays are often set to protect about 80% of the transmission length only.
A three terminal transmission line is also possible to protect by differential relays.

 

[Aakhrinaam]

Most Mho relays use partial cross polarization (adding voltage derived from healthy phases' to faulted phase voltage). This increases the reach along the resistive axis.

Since the comparison is between V-IZ and Vpol, this leads to
expansion of the mho characteristic along the resistive axis for high SIR's.

 

[Kiribanda]

Hi rragunath,

It depends on the S.I.R. as Akhrinaam pointed out. The best reference for S.I.R. and application of Distance protection is IEEE C37.113-1999-IEEE Guide for Protective Relay Application to Transmission Lines. Everything is explained there.

Regards!


Regards! :->

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 242-2001 Buff Book
4.4.1 Distance relay (Device 21)
4.4.15 Differential and pilot wire relays (Device 87)

 

[piterpol]

The possibility of overreach and the impossibility of applying instantaneous overcurrent units or distance protection elements becomes more evident when the SIR's are high and the lines are short since the fault current magnitudes are practically the same for faults along the entire line.

 

[xabproject]

Dear RRN,
May be you treated this thread as closed. But I came across simplified approach to this in Warrington's book, pp no 419, art 15.9 recently. I cannot attach the scanned file since there is no possibility of such a thing in the forum. If only I get your email address!
I have done the detail engineering for atleast 3 projects (230kV and 380kV OHLs!) with 87L as Main1 Protection and 21+67N as Main2 Protection and cannot agree more with the aforementioned reference.
Best regards

 

[RRaghunath]

Thanks xabproject. I would be interested. My mail id is rraghunath@elwatt.com.

 

[Bung]

The earth return path has a much larger bearing on a cable zero phase sequence impedance than it does on overhead line, so getting reliable operation under earth fault conditions could be interesting using a distance relay with ground distance elements.

For instance, a fault just out of the cable will have have a different earth return path (ground plus maybe some in sheath, depending on how sheath is earthed). A fault on the cable may or may not have a return path on the sheath, depending on the nature of the fault. Three different earth fault currents for a fault which could be just at the end of the cable or just beyond the end of the cable, ie at a single distance.

If I've got it wrong, somebody please put me right - you'll make me much happier!


Bung
Life is non-linear...