inrush presentation

8/12/2019 Inrush Presentation

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Fundamentals of TransformerInrush

Suhag Patel, P.E.GE Digital Energy

Placentia, CA

Texas A&M Protective Relay Conference

College Station, TX April 13, 2010


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Objective Understand why transformer inrush

occurs

Understand the characteristics of aninrush waveform

Understand the impact transformer

inrush can have on differential relays Discuss various methods to reliably

restrain differential relay operation


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Basics of Differential Relays Very Simple

Sum of Allcurrents shouldbe zero.

Must Compensatefor Phase Shift andMagnitudeDifference


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Problems with Transformer

Differential Relays


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Inrush Current Impact on

Differential Relay


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What is Inrush Current

All transformers must establish flux in the transformer core This flux causes a current to flow known as the

magnetizing current Magnetizing current appears as differential current


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Steady State Magnetization

Current

Non-Linearity of the core results in a non-linearmagnetizing current waveform

Notice flux lags excitation voltage by 90 degrees Steady State Magnetizing current is in the range of 1-3% of

XFMR FLA


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Magnetizing Current Under

Non-Steady State Conditions

When an abrupt change in excitation voltage occurs, alarge magnetizing current can flow.

The Magnetizing Inrush Current is dependant on severalfactors, which will be discussed on the following slides


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Impact of Switching Point

Highest magnitude inrush occurs when excitation voltage isapplied at zero crossing.

Lowest magnitude inrush occurs when excitation voltage is

applied at 90 degrees.

Time

e

tar t of event

Time

e

tar t of event


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Impact of Remnant Flux

Remnant Flux can be positive or negative

This can lead to increased or decreased magnetizing

inrush current


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Impact of Power System

Impedance

The peak magnitude of the inrush current isdictated by the strength of the power systemsource

The duration of an inrush event is dictated by the

resistive losses in the circuit


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Impact of Transformer Design Electrical steel has remained fairly

constant over the years

Laminated core designs lead to lowerreluctance cores

Lower reluctance cores are more

efficient leading to lower magnetizingcurrent


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Transformer Inrush Waveform

No CT Errors Time Domain


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Transformer Inrush Waveform

No CT Error Freq Domain


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Transformer Inrush Waveformwith CT Sat Time Domain


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Transformer Inrush Waveformwith CT Sat Freq Domain


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When Does Inrush Occur?

During Transformer Energization: Typically the most severe case, because excitation voltage is going

from zero to maximum value

During Post Fault Voltage Recovery: During a fault the system voltage is depressed, and then returns to

full value Not typically as sever as Energization because Flux wont be fully

offset from excitation voltage

Sympathetic Inrush:


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Inrush Restraint Methods

As shown earlier, high levels of inrushcurrent can cause differential relay

misoperation

We need to identify this condition and stopthe differential relay from operating while

inrush condition is present

Many methods exist, all rely on thecharacteristics of the inrush waveform


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Percentage of Total Harmonic

This method utilizes the fact the inrushwaveform is rich in harmonics.

EM relays applied this per phase. Problems

More efficient core designs produce less harmoniccontent

CT Saturation essentially creates a setting floor


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CT Saturation Waveform

Note that a saturated CT waveform ishighly non-linear


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CT Saturation Spectrum

Note the high 2nd

harmonic component


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Typical 2nd Harmonic Ratios

Typical values of 2nd

harmonic tofundamental ratios in

the range of 10%-60% Can be much lower as

shown

Microprocessor relayshave introducedmethods to deal withthis problem


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Percentage of 2nd Harmonic

This method utilizes the fact the inrushwaveform has a dominant second

harmonic component. EM relays applied this per phase. CT Saturation still a problem


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Percentage of 4th Harmonic

This method utilizes the fact the inrushwaveform is not symmetric, leading to evenharmonics

Used in some microprocessor relays CT Saturation still a problem No significant benefit over 2nd harmonic

methods


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Waveshape Based Method

Relies on flat spotsnear zero value

CT saturation cancompromisesecurity anddependability

Were used widelyin solid-staterelays


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Adaptive 2nd Harmonic Method

Method utilizes 2nd

HarmonicMagnitude and

Angle Dynamically

restrains over amaximum of 6cycles

May slowoperation by a fewcycles if 2nd

harmonic current

is present


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How to Apply VariousMethods?

EM relays typically used either % totalharmonic or % 2nd harmonic methods

EM relays applied them on a per-phasebasis Microprocessor relays can apply many

methods on a per-phase, 1 out of 3

(cross blocking), 2 out of 3, or averagebasis Pros and Cons to each


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Considerations When ApplyingHarmonic Restraint

Reliability Ability for the differentialrelay to operate on all internal faults

Security Ability for the differential relayto restrain for all transformer inrushevents

Speed How quickly internal faults are

cleared No method is best, depends on user

requirements


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Considerations When ApplyingHarmonic Restraint

1 out of 3 (Cross Blocking) Very secure,but can reduce reliability or speed: Consider fault during energization

Per Phase Less secure, very reliable: Consider low 2nd harmonic inrush

2 out of 3 More secure then Per Phase,

potentially less reliable Averaging Method More secure then

Per Phase or 2 out of 3, no compromiseon reliability


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Transformer Inrush Impact onGenerator Differential

High DC component of Inrush may saturateGen CTs.

Using harmonic restraint is not a good

solution, adds too much delay

87G


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Flux balanced CT configuration can be used onsmaller Generators


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For problem installations, transformer CB closecan be used to delay 87G

87G

Transformer Close CB Command

Delays 87G Relay


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Importance of GoodWaveform Capture

Depending on specific system conditions and transformerdesign, varying levels of 2nd harmonic content may bepresent

It is in the users best interest to capture inrushwaveforms whenever possible

If a fairly complete library of actual waveform data isavailable, this can be used to fine tune settings andevaluate new methods


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Conclusion

Transformer Inrush will occur anytime a change tothe transformer excitation voltage occurs

Transformer Inrush appears as differential current to

the transformer differential relay 2nd harmonic based methods should not be set lower

then 15% otherwise dependability is put at risk

Many blocking methods exist, however, they pose

various compromises to security, reliability, andspeed.

The right choice of blocking method depends on theindividual user

Generator Differential relays can also be impacted by

transformer inrush


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Thank You

Suhag Patel

[email protected]

562-233-1371

inrush presentation

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