07 SEP674 RET670 Overexcitation Protection

Substation Automation Products

Transformer protection RET670 Overexcitation protection

1MRG019263

Chapter 07

Contents

© ABB Group September 10, 2015 | Slide 2



Application



Operating characteristics



Logical diagram



Settings



Monitored data

Application overexcitation protection function

Chapter 07



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



The peak flux density Bmax is directly proportional to induced voltage E, and inversely proportional to frequency f, turns n and area A E = 4.44 × f × n ×Bmax× A




Overexcitation results from excessive voltage or below-normal frequency or a combination of the two, such that the volts/Hz exceed rated values



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

Overexcitation of power transformers Application 



Overexcitation will cause 

overheating



increase in magnetizing currents



increase in vibration and noise

IEC 60076-1 


Chapter 07

Continuously 10% above rated voltage at no load and rated frequency

Overexcitation protection function Application



The function is measuring the terminal voltage and current. It uses the fundamental 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 the overexcitation) and



33< f <60 Hz in 50 Hz system or 42< f <75 Hz in 60 Hz system

Binary inputs to 

empty heat content



block the function

Chapter 07

Overexcitation protection function Application

24

Y



Number of instances: < 2



Analog measurements must not be taken from a winding where OLTC is located



Measures input voltage

U/f>

OEX PVPH Y

SMAI

∇

Σ

SMAI



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



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



otherwise the terminal voltage is used

SMAI

24

U/f>

Y Y

OEX PVPH

∇



SMAI Y Y


Chapter 07

∇

SMAI

24

U/f>

OEX PVPH

The current measurement should have the same component selection as voltage (positive sequence or delta current)

Overexcitation protection function Operating characteristics

Chapter 07

Delay tMax



The relative excitation M (relative V/Hz) is expressed as:



If M < V/Hz> then the transformer is not overexcited (< 1.0)

harmful

under excitation

overexcitation

tMin 0 M=V/Hz> V/Hz>

V/Hz>: the maximum continuously allowed no-load voltage at rated frequency, set as a % of UBase/fr (default: 110 %)

Mmax - V/Hz> Overexcitation M-V/Hz> V/Hz>> Emax

Excitation M E (only if f = fr = const)




The relative overexcitation is thus defined

Overexcitation protection function Operating characteristics – IEEE curve

Delay

Chapter 07



Operate characteristic



tMax: Maximum cut-off operate time at low degree of overexcitation

tMax

inverse delay under excitation

overexcitation



Excitation M



tMin: Minimum cut-off operate time at high degree of overexcitation

E (only if f = fr = const)




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

Overexcitation protection function Operating characteristics – Tailor made curve

Delay



The interval between M = V/Hz> and M = V/Hz>> is automatically divided into 5 equal subintervals



6 operating time points has to be defined



Straight line between the points to calculate operating time



tMax: Maximum cut-off operate time



tMin: Minimum cut-off operate time

tMax

underexcitation

overexcitation

tMin

Overexcitation M-Emaxcont 0

V/Hz>


Mmax - Emaxcont Excitation M V/Hz>>

Chapter 07



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

Chapter 07

Cooling and alarm Operating characteristics Delay tMax



Overexcitation causes overheating



An exponential cooling feature is implemented

harmful

 under excitation

overexcitation




tCooling: Time constant (default setting 1200 seconds (20 minutes))



Alarm


Excitation M E (only if f = fr = const)




i.e. shorter operating time if the transformer has been exposed for overexcitation before and not yet reached normal temperature



Per cent of operate level (heat content, THERMSTA)



Time delay

Reset input to clear heat content

Chapter 07

Overexcitation protection function Simplified logical diagram BLOCK AlarmLevel

t

RESET

Heat content

t V/Hz>

Calculation of internal induced voltage Ei

I3P

Xleak

Ei

M


t>tMin

&

TRIP

M

IEEE

M Tailor-made M>V/Hz>>

V/Hz>>

ALARM

tMin k

M= (Ei / f) (Ur / fr)

&

tAlarm

M>V/Hz>

tCooling

U3P

t>tAlarm

t tMax

≥1

&

START

Chapter 07

Overexcitation protection function settings Overview

First instance of OEX PVPH

Advanced settings


Overexcitation protection function settings General

Chapter 07

Voltage and current selection: ph-ph or positive sequence NB: MeasusedU and MesururedI must be set to the same value (ILxILy is calculated internally)

Operation On/Off Base values Leakage reactance of adjacent winding Trip pulse length Alarm in % of operate level (9000 s ~ 2,5 h)


Overexcitation protection function settings Characteristic

Chapter 07

Base values Operate level: - No load and rated frequency - High level where tMin is used Trip delay: - Minimum - Maximum Core cooling time constant Characteristic IEEE/Tailor made IEEE curve

Tailor made curve


Chapter 07

Monitored data




Test/Functions status/Voltage protection/overexcitation (PVPH,24) 

Trip



Start



Alarm



TMTOTRIP – Calculated time to trip (s)



VPERHZ – V/Hz (pu)



THERMSTA – Thermal status in % of trip level

Chapter 07

Monitored data Time

THERMSTA <100%

tMax TMTOTRIP=tMax s THERMSTA >100% (if no trip)

THERMSTA=100% and TMTOTRIP=0

THERMSTA <100% TMTOTRIP

tMin Under excitation © ABB Group September 10, 2015 | Slide 16

V/Hz> VPERHZ=1.100

Over excitation

V/Hz>>

Chapter 07

© ABB SA-TGroup Training September 10, 2015 | Slide 17

© SA-T Training

07 SEP674 RET670 Overexcitation Protection

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