SETTING THE BIAS FOR A TRAFO DIFFERENRIAL SCHEME

[PUNGLU]

How do you determine the bias for any trafo differential scheme and what important factors must you consider. Any rules?

 

[RalphChristie]

The bias setting is used to ensure stability during external faults/conditions while the relay is still sensitive for internal faults.

Settings depends on:
Type of relay, (electromechanical, static, electronic etc) If a Tapchanger is present or not. (especially for an on-load tapchanger)
Type of CTs used.

Normally the first bias setting is used to avoid relay operation during energizing (inrush) of the transformer.
A second setting is used to incorporate changes in the tapchanger during tapping and applications where CT-saturation is not expected.
A third setting might be used for applications where moderate CT-saturation might appear during external fault conditions.

Regards
Ralph

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[gbk]

The first setting (pick-up) is used for magnetizing current (since magnetizing current can be present with LV circuit breaker open). Inrush is usually taken care of either with harmonic restraint or harmonic blocking element.

 

[RalphChristie]

gbk: Thanks for the correction.

The first setting is indeed not for inrush but for the magnetizing or energizing current.

Regards

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[slavag]

Hi.
Punglu, I hope you ask about digital/numerical xfr protection.
Ralph and Gbk are right.
I would like add my practical excpirience.
Put your first setting always on the max value, it's depend
on the protection type, for example 50% ( upper level of slope) and 2d harmonic blocking on the minimum value, for example 10%. Put this, provide stability test, connect trafo, provide on-load test and never don't change this first setting. Maybe add cross-blocking option ( it's big Q, we need this option or not).
I know, what will reaction!!! but it's my opinion ( I'm not say, that I'm right), we put in services lot of xfr's with this setting, at size from 15MVA up to 650MVA, it's work and work correct ( was several unwanted trips, was CT problems).
Please pay attention, I told about phase diff protection, not about REF.
New options , as TCH position, very sensitive slope,etc., I don't know, we need it or not.
Regards.
Slava

 

[ELEP]

The bias is to be decided greater than sum of the maximum tap of the transformer and possible CT errors. Modern numerical relays have bias characteristic with three sections. The first section(0%) is set higher than the transformer magnetising current(0.1 to 0.5Id). The second section is set to allow for off-nominal tap settings(which is fixed in some manufacture relays 20%, some manufacture user settable), while the third has a larger bias slope(more than 50%) beginning well above rated current to cater for heavy through-fault conditions.

A harmonic analysis of a typical magnetizinginrush-current have following Harmonic Component Amplitude in Percent of Fundamental.
2nd 63.0
3rd 26.8
4th 5.1
5th 4.1
6th 3.7
7th 2.4
So, set harmonic blocking setting considering above content.

 

[slavag]

Hi.
Several "theoretical tips".
Example of calculation.
Several terms.
X1- first setting
X2- second setting.

X1=0.5*X2+10%

HV CT 5P20
LV CT 5P20
TC 9*1.67%

X2= 5%( LV CT) +5%( HV CT) + 4-6% ( relay accuracy) + 15%
( TC position) + 3-5% (margin)= 32%-36%
First turn point: normal setting 0.5Ib ( that mean half of nominal current)
X1=26%-28%.

And last sector, second turnpoint: normal setting:
1.5-2Ib.
Ib=Ibias=(I1+I2)/2 or sqr (I1*I2*cos.phi) depend on relay.
Regards.
Slava
But : X1=50% and X2=50% and second turnpoint = 1.5Ib
it very practical setting