BUSBAR PROTECTION IN MESH SYSTEM 6

[Tanzeelur]

Hello All,

I am going to design the busbar protection scheme in a mesh system.how should i start and implement the same.i want to know all the details which help regarding the same.

i shall be greatfull to all if you send some feedback soon.

Regards,

Tanzeel

 

[cuky2000]

Tanzeelur,

Will be helpful if you should clarify what you mean by “mesh system”. I am not sure if you are referring to a network interconnected system or other type of installation.

Additional system detail may be required. Below is a partial list of a generic project Design Basis requirement that may worth to review it.

<sub>Partial Project Design Basic Checklist:
1. Electrical parameter/rating (voltage levels, frequency, single or 3 phase, SC, kVA/MVA…etc.).
2. Application (commercial, residential, industrial, utility)
3. Type of installation (underground, overhead, enclose within a building, AIS, GIS),
4. Major equipment (transformer, protective devices, cap-bank, generator, etc)
5. Type of equipment (xfmr: vault, pole or pad mounted. Bkr: SF6, vacuum, etc).
6. Connection: delta/wye, wye/wye, delta/delta, etc.
7. Grounding system (solidly, impedance gnd, ungrounded, high resistance, etc)
8. Prefer protection system (network protector, fuse, bkr, arrester, etc).
9. Code & standards (ANSI/IEEE, IEC, etc).
10. Utility/IPP Interconnection requirement (protection, redundancy, and others)
11. Unusual Environment condition (max/min temperature, seismic risk zone, max. wind speed, max snow pack, isokeraunic level, contamination, etc.)
12. Other relevant details of the propose installation.</sub>

 

[RRaghunath]

Mesh is same as Ring bus. I understand that no separate bus bar differential protection is required for a ring bus. The protections provided for individual circuits connected to the bus include the bus (and also the circuit breakers)as well within their zone of protection (summated CT output is given to the feeder protection relays).

Art and Science of Protective Relaying by C. Russel Mason has brief explanation on this in Page 253/254.

 

[jghrist]

rraghunath,

You are correct. No separate bus protection is required for a ring bus. One caution is that the CTs carry the ring current but the protection may be for a transformer differential for which you might want a much smaller CT ratio to match the secondary side. Modern differential relays can accomodate the mismatch of using a large CT ratio, but you should use a separate restraint for each of the two CTs on the primary to avoid problems with a heavy fault outside the differential zone but fed through the two CTs.

 

[RalphChristie]

In addition to cuky2000's comments:

From Protective relaying Theory and Applications - ABB (Walter A. Elmore)

When local bus prtection is applied, the following information is required for the schemes selection, relay selection, and settings calculations:

Information required for preperation of a bus protective scheme
1.Bus configuration information about the bus configuration is required, such as single bus, double bus, main-and -transfer bus, ring bus, breaker and a half, bus tie-breaker, double-bus-single breaker etc.
2.Maximum and minimum bus fault currents (single-phase-to-ground-fault and three-phase fault)
3.CT information including:
CT location
CT ratios
CT accuracy class
CT saturation curves
4.operating speed requirements

Normal practices on bus protection
1. one set of bus-relays per bus section
2.Use a dedicated CT for bus diff protection. If possible, avoid connection of meters, aux CTs, and other relays in diff-type bus schemes. It add additional burden into main circuit.
3.Lead resistance, as well as CT winding resistance, contribute to CT saturation. The Lenght of secondary lead runs should be held to the minimum.
4. Usually, the full-CT secondary winding tap should be used. It minimizes the burden effect of the cable and leads by minimizing the sec. current and makes use of the full-voltage capability of the CT.
5.Special arrangements should be considered if there are any other apparatus, such as cap-banks, surge arresters, etc. inside the bus-zone.
6.No bus relay required for a ring bus (mentioned by rraghunath and jghrist)

Also check

http://www.areva-td.com/static/largefiles/15-busbar_protection.pdf

pg 240 and 241

Regards
Ralph

 

[Luis1947]

Busbar protection is not applicable to Mesh bus because the protection zone of each circuit connected to the busbar includes part of the bus. In other words, there is no dedicated relay to the Busbar and of you have a fault in the bus, the corresponding circuit will trip the breakers to clear and isolate the fault.
I have worked in several 220 kV substations with Bus protection system and RalphChristie provided detailed information you need, however in mesh busbar there is no relay for this protection.

 

[manmadhan04]

hello,

like the others said, there is no bus protection for mesh stations per se. However you will be required to provide mesh corner protection and this is done with a differntial relay + CT supervision (low or high impedance though these days low impedance is preferred) per corner. The differential relays used for this application are relays such as RED 521 from ABB

rgds

 

[RRaghunath]

Good info Manmadhan.

We also come across corner protection in case of 1-1/2 breaker schemes (typically practiced for 400kV substations in India). It is also referred as "Stub protection".

 

[manmadhan04]

hi

Mesh stations are a little bit tricky and can only be found in UK and probably central Saudi Arabia, these days.

The station uses one Switch or CB to control multiple circuits (line/trafo or even banked trafo bays) and has complex control & protection requirements. there are single switch or multi to full switch mesh type stations. The requirements are decently covered in the Areva PRAG book.

Stub protection is another animal altogether and is at times handled differently within the distance relay using a special overcurrent function that is activated when the line isolator is open with an isolator aux contact...in the past we used diff relays too for this purpose.

rgds

 

[cuky2000]

Manmadhan04,

FYI, mesh stations (ring bus) configuration are very common in the ANSI marketplace in contrast with the IEC regions.

For high voltage applications, the number of mesh (ring bus) substation in the USA far exceeds the number of breaker 1-1/2 and other configuration combined.

 

[Bung]

We have just been through a similar exercise. No specific busbar protection is needed in theory, but you do have to think about the "stub bus" formed if an element (line or transformer) is taken out of service and the ring is kept closed. This could be the "mesh corner" protection referred to by others - maybe sombody could confirm this?

Another thing to look out for is the logic involved in any circuit breaker fail scheme you implement. Any element must be tripped by two CBs. Also, any one CB looks after two elements, one either side. You need to keep a track of this for the CB fail logic to work properly.

One last point is Operator concerns on ring busses with GIS gear. How do you tell if the fault is in the switchgear or on the line? What are the risks to an Operator if a ring has to be closed and no line fault had been found - is the switchgear faulted or not? We are using an arc fault detecting system to get around that one, and is possibly one reason why you can find explicit busbar protection on ring systems. Obviously not a problem with outdoor gear.

In short, the primary side of ring busbars is nice and simple, but the secondary systems do seem quite complex in comparison with some other schemes. Maybe its just because we have never done it before! (BTW, we are looking at up to 8 elements in the ring, at 132kV with fault levels up to about 6GVA).


Bung
Life is non-linear...