High Impedance bus Differential vs Biased transformer differential 3

[sabap]

Can somebody explain the reason of faster speed for high impedance differential normally used on busbars as compared to Baised Differential used on transformer, motor or generator?

What i have studied from a literature is that high impedance busbar DP operating time is 1 cycle whereas tranformer differential operates in 3-6 cycles atleast.

Can we use high impedance DP on transformer and biased differential on busbar? what will be the pros and cons of each application then?

I know this question is a bit academic but it will help me a lot.

 

[RalphChristie]

Operating times for diff protection, excluding breaker tripping time are generally of the following order:

Transformer diff - 10 cycles
Busbar diff - 4 cycles
Feeder diff - 5 cycles
Mordern relays act even faster

These operating times are practically independent of magnitude of fault current.

Triptime for transformer diff is a little longer to ensure that the relays do not operate incorrectly due to initial transients.

Modern relays are moving in the direction of low impedance diff schemes because CTs are much cheaper (Compare prices of a 10P10 CT and a Class X CT)

Check these papers at the SEL-website:

http://www.selinc.com/techpprs/6025.pdf

http://www.selinc.com/techpprs/6107.pdf

http://www.selinc.com/techpprs/6042.pdf

Regards

Ralph

 

[sabap]

I suppose that high impedance utilizes a series resistor with the overcurrent differential element and low impedance refers to biased / percentage differential relay.

I have been through GE's GER-3984 document as well which supports your point of view in detail. Thanks for the documents you have mentioned. I have already downloaded these and others from Selins site. Its a good informative website.

But i was a bit confused as to how this resistor is utilized in DP. Is it the same referred to as Metrosil.

Thanks in advance.

 

[ScottyUK]

Sabap- the metrosil is there to prevent very high voltages occuring under heavy fault conditions. These voltages would otherwise damage the CT, the relay, and the wiring. The stabilising resistor in principle converts the current operated relay into a voltage operated relay.

The Areva Network Protection & Automation Guide is available as a number of free downloads, or as a hardback book for about 70 GBP. It contains some excellent explanations of how different relay schemes operate. Highly recommended.

http://www.areva-td.com/servlet/Con...id=1056536208199&rid=1018348589697&norid=true

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[RalphChristie]

High impedance system - voltage operated
Low impedance system (bias or unbias) - current operated

Main disadvantage of circulating current protection using low impedance relays is through fault instability due to CT saturation.

In a high impedance system:

A stabelizing resistor is connected in series with the relay coil. The value of the resistor is calculated so that the product of the relay setting current and the sum of relay resistance and stability resistor is equal to, or slightly higher than the stability voltage.

Like Scotty said, a metrosil (non-linear resistor) protect the CTs, secondary wiring and relay from damage due to high voltages during a fault condition. This resistor is of a type which reduces in value as the voltage across it increases, and is selected with caracteristics which limit the voltage to between 1kV and 2kV. It is connected across the relay coil and the stability resistor.






Regards

Ralph

 

[sabap]

Thanks for good explanation.

I would also like to know the current industry/best practices for selection of high and low impedance relays with regards to generator, transformer and busbar protections. I mean where will we use high impedance or low impedance. Or is it purely economics based decision.

 

[RalphChristie]

The degree of protection provided depends to a great extent upon the size and importance of the unit been protected. A futher important factor is the economic aspect or cost aspect. The views of the designer could also be a factor to a lesser extend.

For older type of schemes:

Generators: Low impedance diff (bias or unbias) over the windings.
Transformators: Low impedance diff (bias or unbias) over the windings and high impedance diff (REF-protection) over the windings and the neutral CT. REF stands for restricted earth fault protection.
Busbars: High impedance diff in all the zones which include different incomers and feeders. A typical zone can consist out of an incomer and 4 feeders (15 CTs), which make a low impedance diff circuit difficult.

The link Scotty provided show the most common connections on the different units.

But, like a said in a previous post, the trend in modern relays is to go with low impedance schemes. Where you had to use different relays in the older days for different types of protection, (3 x IDMT relays for overcurrent and earth fault protection, 1 x diff relay, 2 x REF-relays, (REF-LV, REF-HV) and 1 x SEF relay = 7 relays on a transformer feeder) all this functions is packed into one microprocessor relay today.
Relays use different techniques to detect faults today and older methods and connections of relaying are on the way out.



Regards

Ralph

 

[Bung]

The main disadvantage of high-impedance busbar protection is the need for dedicated CTs. Low impedance schemes can share CTS with other protection. Also, modern indoor switchgear just doesn't have the space to fit the extra CTs, nor even the xpace for decently sized CTs.

The downside of packing 7 protections into one relay is taht a single power supply failure leaves you stranded. You have to be vry carefl to ensure that as far as possible the protection schemes in one relay are covering different zones. And testing tends to get interesting. Relay hardware is cheap - a few extra $5k relays in a subsation with $5m worth of other assets is not significant.


Bung
Life is non-linear...

 

[sabap]

Lastly could some body explain how is the value or ratio of bias to operating coil current is selected for biased differential and how is stablizing resistor calculated for high impedance relay.

 

[RalphChristie]

Bung, what you've said is very true. Actually it is good practice to have a back-up system in place for failure of certain relays.

What I actually want to imply is the fact that everything you want to do, or have to do, can be done inside a microprocessor relay. Phase shifting (older days a spesific connection of the CTs), CT mismatch correction (older days an interpose relay), elimination of zero sequence currents (older days a zero sequence shunt) etc. is nowdays done inside the relay. Thus, dedicated CTs are not such a factor these days, and that is why the trend is to go with low impedance systems.

Sabap:

You have to determine the biggest current unbalance under healthy conditions to obtain the bias slope setting. A OLTC is responsible for the biggest current unbalance in a transformer to give an example.

The Stability Resistor size is obtained by:

R = (Vs - Vr)/Ir

where:
R = Stability resistor
Vs = maximum voltage or stability voltage
Vr = Operating voltage of relay element
Ir = Operating current

The maximum voltage is obtained by:

Vs = Is x (Rct+R1)

where:
Is = CT secondary current corresponding to the max steady-state through-fault current
Rct = secondary winding resistance of CT
R1 = Largest value of lead resistance between relay and CT

I think the best would be to find some Protective Relaying books, it is a bit unpractical to help someone through a forum.

Some of the books are:
Protective relaying, principles and applications by Blackburn
Power System Protection by The Electricity Training Association

Some authors of relaying books are:
Walter Elmore
Stanley Horowich
Arun Phadke

You can also search on the websites of IEEE and IEE for good books regarding protection.















Regards

Ralph