07 sep674 ret670 overexcitation protection

8/17/2019 07 SEP674 RET670 Overexcitation Protection

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1MRG019263

Transformer protection RET670Overexcitation protection

Substation Automation Products


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Chapter 07

© ABB GroupSeptember 10, 2015 | Slide 2

Contents

Application

Operating characteristics

Logical diagram

Settings

Monitored data


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Chapter 07


Application overexcitation protection function

The overexcitation is not an internal transformerfault, although it can lead to one

The peak flux density Bmax is directly proportionalto induced voltage E, and inversely proportional to

frequency f, turns n and area AE = 4.44 × f × n ×B max × A

Overexcitation results from excessive voltage orbelow-normal frequency or a combination of thetwo, such that the volts/Hz exceed ratedvalues

Overexcitation of a transformer can occur wheneverthe ratio of the pu voltage to pu frequency (V/Hz)exceeds its rating of 1.05 pu on transformer base atfull load or 1.1 pu at no load


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Chapter 07


Overexcitation will cause

overheating

increase in magnetizingcurrents

increase in vibration andnoise

IEC 60076-1Continuously 10% above ratedvoltage at no load and ratedfrequency

Overexcitation of power transformers Application


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Chapter 07


Overexcitation protection function Application

The function is measuring the terminal voltage and current. It uses thefundamental frequency components.

Calculates the relative Volts per Hertz ratio.

Two operate characteristics (IEEE / Taylor made)

Operating range:Uph-ph > 70% of rated value (if below, the algorithm is not calculating theoverexcitation) and

33< f


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Chapter 07


Overexcitation protection function Application

OEX PVPH

2 4 U/ f>

YY

∇

OEX PVPH

24 U/ f>

SMAI

SMAI

YY

∇

SMAI Σ

SMAI

Number of instances: < 2

Analog measurements must not be taken from awinding where OLTC is located

Measures input voltage

Single phase-to-phase voltage or threephase voltage (positive sequence)

Calculates the induced voltage E if theleakage reactance (and current) is known

otherwise the terminal voltage is used

The current measurement should have the samecomponent selection as voltage (positivesequence or delta current)

OEX PVPH

24 U/f>

YY

∇

SMAI


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Chapter 07


The relative excitation M (relative V/Hz) isexpressed as:

If M < V/Hz> then the transformer is notoverexcited (< 1.0)

V/Hz>: the maximum continuously allowedno-load voltage at rated frequency, set asa % of UBase/fr (default: 110 %)

The relative overexcitation is thus defined

E max

0 Mmax - V/Hz>

V/Hz>>

V/Hz> E (only if f = fr = const)

tMax

overexcitationunder -excitation

Delay

Overexcitation M-V/Hz>

Excitation MM=V/Hz>

tMin

harmful

Overexcitation protection functionOperating characteristics


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Chapter 07


0 Mmax - V/Hz>

V/Hz>>

V/Hz> Emax E (only if f = fr = const)

tMax

inverse delay


Delay


Excitation MM=V/Hz>

tMin

Operate characteristic

tMax: Maximum cut-off operate time at lowdegree of overexcitation

tMin: Minimum cut-off operate time at high

degree of overexcitationBeyond M = V/Hz>> the operate timewill always be tMin

Overexcitation protection functionOperating characteristics – IEEE curve


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Chapter 07


0

V/Hz>

Mmax - E maxcont

V/Hz>>

tMin

tMax

Delay

Overexcitation M-E maxcont

Excitation M

under-excitation overexcitation

The interval between M = V/Hz> andM = V/Hz>> is automatically divided into5 equal subintervals

6 operating time points has to be defined

Straight line between the points tocalculate operating time

tMax: Maximum cut-off operate time

tMin: Minimum cut-off operate time

Beyond M = V/Hz>> the operate timewill always be tMin

Overexcitation protection functionOperating characteristics – Tailor made curve


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Chapter 07


E max

0 Mmax - V/Hz>

V/Hz>>

V/Hz> E (only if f = fr = const)

tMax


Delay


Excitation MM=V/Hz>

tMin

harmful

Overexcitation causes overheating

An exponential cooling feature isimplemented

i.e. shorter operating time if the transformer

has been exposed for overexcitation beforeand not yet reached normal temperature

tCooling: Time constant (default setting1200 seconds (20 minutes))

AlarmPer cent of operate level (heat content,THERMSTA)

Time delay

Reset input to clear heat content

Cooling and alarmOperating characteristics


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Chapter 07


First instance ofOEX PVPH

Advancedsettings

Overexcitation protection function settingsOverview


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Chapter 07


Leakage reactance of adjacentwinding

Operation On/Off Base values

Alarm in % of operate level

(9000 s ~ 2,5 h)

Voltage and current selection:ph-ph or positive sequence

NB : MeasusedU andMesururedI must be set to thesame value (ILxILy is calculated

internally)

Trip pulse length

Overexcitation protection function settingsGeneral


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Chapter 07


Characteristic IEEE/Tailor made

Trip delay:

Core cooling time constant

Base values

Operate level:- No load and rated frequency- High level where tMin is used

- Minimum- Maximum

Tailor made curve

IEEE curve

Overexcitation protection function settingsCharacteristic


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Chapter 07


Test/Functions status/Voltageprotection/overexcitation(PVPH,24)

Trip

Start

Alarm

TMTOTRIP – Calculated time totrip (s)

VPERHZ – V/Hz (pu)

THERMSTA – Thermal status in% of trip level

Monitored data


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Chapter 07


Monitored data

Time

V/Hz>>

tMax

tMin

THERMSTA=100% and TMTOTRIP=0

THERMSTA 100% (if no trip)

07 sep674 ret670 overexcitation protection

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