High Impedance Sformer Differential

7/27/2019 High Impedance Sformer Differential

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HIGH IMPEDANCE BUS DIFFERENTIAL VS BIASED TRANSFORMER

DIFFERENTIAL

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

Transformer diff - 10 cycles

Busbar diff - 4 cyclesFeeder diff - 5 cycles

Modern relays act even faster

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

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

LOW IMPEDANCE DIFF SCHEMES HIGH IMPEDANCE DIFF SCHEMES

Low impedance system (bias or unbias) -

current operated

High impedance system - voltage operated

CTs are much cheaper (Compare prices ofa 10P10 CT and a Class X CT).

low impedance refers to biased / percentage

differential relay.

high impedance utilizes a series resistor

with the overcurrent differential element

Main disadvantage of circulating current

protection using low impedance relays isthrough fault instability due to CT

saturation.

and the low cost, if used on asimple bus system. However withthe introduction of numerical

relays and their low CT burden aswell as their ability of measuringseveral feeder currents, a lowimpedance bus differentialprincipal could be applied onsimple busses also

High impedance bus differential relays are

used on most of the busses in North

America. The popularity can be explained

by the good performance of this scheme inrelation to CT saturation and the low cost,

if used on a simple bus system

Common advantages of all


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low impedance bus protectionschemes are the ability to be able to use

CT's of different ratios on respectivebranch inputs and the fact that the same CT

used for the bus protection can be shared

with the feeder protection relay

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 voltageoperated relay.

Highly recommended.

http://www.areva-td.com/servlet/ContentServer?page...

For older type of schemes:

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.
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Q: Where to use High & Low impedance differential protection? 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?

Ans: Please see thread 238-216247: High Impedance Differential protection

High Impedance:

Since it is having greater stability (during CT sat)for out of zone faults than lowimpedance 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. , there everything should be identical starting fromturns ratio. knee point voltage, winding resistance and exciting current. Hi Z is

traditionally used where there are numerous CT inputs to deal with, such as bus

differential. Ct's designed for HI protection will have higher knee point voltage whencompared to LI protection which will increase the cost of CT production. In my opinion

for new substation where CTs are identical High Impedance 87B, is the best solution.

However, in case of expansion or having different CTs low impedance should be use.Configuration, setup, and testing is much simpler with the Hi Z bus diff relaying.

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 ofyears.

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

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 schemescan have low impedance version too. In LI we can use different ratio Ct's but not in HI

Low Z was normally used for differential zones with two or three sets of CRTs, such as

transformers, generators and motors.I don't think any problem in using LI protection scheme for bus bar, 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

switching, IF we use LI, only we have to give isolator input for bus selection, central unit


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of BB protection will do the rest.

I don't agree with the others 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 transformerbiased differential protection works, it has to be stable for external faults, same way LI

for bus bars also.

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

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

Creepage Distance-Is it a Reliable Design Parameter?Thomas Johansson, Robert Axelsson, Sven Nord Ceram GroupIntroductionCreepage Distance(CD) has many advantages. It is easy to understand, visualise,

calculate and measure. A disadvantage is that it does not accurately predicting the

electrical performance of Insulators of various diameters. Even following IECstandards, the accuracy of the calculations are limited. The poorer performance,

under polluted conditions, of lager diameter versus smaller diameter Insulators,both designed in accordance with IEC standards, similar profiles and located in the

same position, indicates the need for more accurate design parameters than those

offered by CD.


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3.11. Creepage DistanceThe shortest distance, or the sum of the shortest distances, along the contours of the externalsurfaces of the insulating parts of the insulator between those parts which normally have theoperating voltage between them.3.12. Specific Creepage Distance

The overall creepage distance of an insulator divided by the highest operating voltage acrossthe insulator. It is generally expressed in mm/kV.CE to check3.13. Dry Arcing DistanceThe shortest distance in air external to the insulator between those parts which normally have

the operating voltage between them.


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