Where to use High & Low impedance differential protection 1

[RAgrawal]

I want to know, where should we use high impedance differential protection and where low impedance should be used.

What are the advantages & disadvantages of both schemes?

 

[slavag]

Hi.
Please see attached thread.
thread238-216247
I think, SEL have good document on the issue too.
From my point of view, today with newer numerical protection,
only LI diff protection, but it's just my opinion.
For expand out discussion, please tell us, what is a protection you are intrested:
1. Generator 87G
2. Transformer 87T
3. restricted E/F REF, 87N ( or 87G).
4. BBP (busbar protection) 87B
Best5 Regards.
Slava

 

[RAgrawal]

Thanks Mr Slava,

well in interested to know the application of HI & LI differential protection be it 87G, 87T, 87N, 87B. I mean for which protection HI or LI should be used and why?

 

[Kiribanda]

RAgrawal,

In my opnion it is really difficult to draw a line specially with present day microporocessor relays.

High Impedance:Since it is having greater stability (during CT sat)for out of zone faults than low impedance version. Mainly used in Bus protection and Transformer (resistance grounded) REF protection schemes. Low cost, simple design, easy testing, needs accurate CT and wiring data to set the voltage setting.

Low impedance:Low stability for out of zone faults. Mainly used in Bus Protection, Transformer, Generator and Motor Differential protection schemes.Also with present day microprocessor based relays, Transformer REF schemes can have low impedance version too.

Further more when you select a diff. relay, if it is a microprocessor based one, then it will have other functions which may be useful to cover the entire protection scheme.

Hope this helps.

 

[dpc]

Hi Z is traditionally used where there are numerous CT inputs to deal with, such as bus differential. Low Z was normally used for differential zones with two or three sets of CTs, such as transformers, generators and motors.

 

[slavag]

Hi.
I realy don't from where start.
1. Kiribanda
"High Impedance:Since it is having greater stability (during CT sat)for out of zone faults than low impedance version. Mainly used in Bus protection and Transformer (resistance grounded) REF protection schemes. Low cost, simple design, easy testing, needs accurate CT and wiring data to set the voltage setting."
True ( in additional very sensetive and very fast), but...
1.1. Always need special type of CT, for example class X, all CT's must be with same ratio.
1.2 Need exactly calculation of Vx, current wiring loop, resitor and varistor.
2."Low impedance:Low stability for out of zone faults. Mainly used in Bus Protection, Transformer, Generator and Motor Differential protection schemes.Also with present day microprocessor based relays, Transformer REF schemes can have low impedance version too".

2.1 Newer relay could work with few types of CT's ( I say about 87B), with differnce of ratio up to 20:1.
2.2 No problems with stability:
2.2.1 Saturation current reconstraction
2.2.2 Minimum two creterious for operation.
2.3. For the trafo or generator REF, save CT cores and wiring, on the same current inputs you acn build:
2.3.1. Standard diff 87T/87G protection, 87N (REF) protection, 51G, 51N ( BEF, SEF).
Very sensetive setting for REF too ( minimum 1% of Inom)
no neede special trafo on the star point.
2.4 for the 87B BBP protection in the double BB no need
reconnection of current wiring with external relays,
2.4.1 possible build additional logik for the
End fault protection ( dead zone between CB and CT)
BFP are include
Dinamic disconnectors/breakers replica w/o additional relays, that means
Build in check zone functionality. Eazy commissioning, setting, maintanance.

BTW, TODAY, we finished our first project: upgrade of old HI-Z of 170kV power plant SS to the newer LI-Z BBP+BFP, it's tested on-load and back to services.
Best Regards.
Slava

 

[loganath4880]

I dont think any problem in using Low impedance protection for busbar protection, For multiple bus configuration we must go for LI protection, if we use HI protection, we have to switch the CT circuit, which may lead to mal operation of protection if there is any failure in swtching, IF we use LI, only we have to give isolator input for bus selection, central unit of BB protection will do the rest.

In LI we can use different ratio CT's but not in HI, there everything should be identical starting from turns ratio. knee point voltage, winding resistane and exicting current.

Ct's designed for HI protection will have higher knee point voltage when compared to LI protetion which will increase the cost of CT production.

I dont agree with the ohers who are saying that HI will be stable during external faults but not LI to counter this one we are using biased differential protection, how transformer biased differential protection works, it has to be stable for external faults, same way LI for busbars also.

Tripping time of LI is less when compared to HI, it is an added advantage in protection.

 

[dpc]

High impedance bus differential relaying is far less costly than low impedance relaying when there are more than a few breakers in the system. This applies to new equipment. Retrofits can be problematic since dedicated CTs are required and must be the same ratio, at least.

Hi Z diff works very very well and has for the last 60 years or so. Configuration, setup, and testing is much simpler with the Hi Z bus diff relaying.

I would dispute the idea that there is much real difference in tripping time. Both approaches are very fast.

 

[vic3fan]

A good reference on the principles of busbar protection can also be found at

http://pm.geindustrial.com/FAQ/Documents/B30/GER-3984.pdf

Hope this helps too.

 

[slavag]

Isn't simple say, HI-Z or LI-Z are faster .
Please take in account and it tested, typical time of trip
in lot of manuals..have additionals when and where.
It's very depend, how much feeders, hoe much zones, depend on the dinamic zone, etc.
Please take in account also that B30 is for amall application only and I think for single bus only. B90 it's somthig other. If you have triiple bus with additional com. bus, situation with trip time other. If you have metal trip out of zone, but near to bus and simultaniues trip on the bus, oher too.
Actually HI-Z arenht have all this additionals and becouse it, HI-Z faster and simple, but ..with another problems.
Best Regards.
Slava

 

[SMB1]

Hi everyone,

In my opnion for new substation where CTs are identical High Impedance 87B, is the best solusion. However, in case of expansion or having different CTs low impedance should be use.

High impedance relay use for 87B, 87REF, 87 shunt reactor, etc.

High impedance relays are much cheaper, simple design, and have been in service 10's of years.

All other relays are Low impedance type. Diff low impedance is used for 87T.

thanks & Best Regards