Equipment Protection.pdf

8/16/2019 Equipment Protection.pdf

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Equipment Protection


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Types of Transformers Generator Transformer

Power Transformer

Distribution Transformer Pole-mounted lighting Transformer

Grounding Transformer

Regulating Transformer

Welding Transformer Converter Transformer

Instrument Transformer (CT & PT)


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Types of connection Types of connection: Y-Y, Y- Δ, Δ-Y, Δ- Δ

Y- Δ and Δ-Y introduce phase shift b/w

primary & secondary side. Since the ends of the transformer are

physically closed together, we use differential

protection


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Phasor Diagram for a 3-phase transformer


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In star side, line currents, I A, IB & IC

In delta side, the phase currents are


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Each line current on delta side is the phasor

sumof two of the phase currents.

There is a phase shift of 30° b/w line currentof two sides of Y- Δ transformer


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Schematic representation of Y- Δ

transformer


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Phasor diagram showing 30° phase

shift b/w line currents on two sides of

transformer


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Schematic diagram- 1 phase transformer


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Equivalent circuit


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Equivalent ckt of transformer Z shunt branch > Z series branch

In SC, series branch decides the SC current.

If Zse= 0.08 pu, then SC current = 1/0.08 = 12.5 pu


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Model for finding SC current


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Types of Faults in transformer

Winding core fault – weakening of insulation

Phase fault inside transformer – rare

Phase fault in transformer terminals (outside) – fall within transformer protection zone

Variation in fault current – depends – type of

connection, method of grounding and current

being refereed to primary or secondary


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Variation in fault current wrt location in

Δ-Y transformer


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For resistance earthed star connected

winding- winding to earth fault - fault current

depends on earthing resistance value and

distance of the fault from neutral

Minimum value of If occurs at 30 – 40% of the

distance end of winding from the neutral end


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•••••••••••

Q)

c

0.4 u 4

oe :::J

•••••••

••••••

••••••

1.0 Jo 10 e;,"

....... 0.8

c ::::> "-

R e = 1 p.u. C:>'f'tj

....... 8

c :::J RE = 0 I "- Q)

a. 0.6

a. 6

....... c

Q) "- "- ::::>

.(

....)

.

Q) "- "- :::J

.......

::::> ro


17/56

Variation of If wrt location for Y-Δ transformer


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For delta, minimum voltage on delta winding

is at the centre of one phase and 50% of

normal phase to earth voltage.

The range of If values are less than star

connected wndg.

The minimum value of If occurs at the centre

of one phase winding.


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Over current Protection of

transformer


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In fig two number of phase-fault over current relaysand one number ground fault over current relay isprovided for either primary protection for small

transformers and back up protection for biggertransformers.

The pickup value is set not to pickup maximumpermissible overload but sensitive to smallest phasefault.

The neutral current is because of load unbalance. The third harmonic currents ends up as zero

sequence current & flows through neutral.


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••

••••

••••• Percentage differential protection of

transformers

Delta-star transformer

with neutral grounded

•

•••••••••

•••••••


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Star point is grounded – turns ratio 1:1


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••

•••

•

••••• Determination of IL on two sides of

transformers Delta-star transformer with neutral grounded

l c- I a

l c

• •••••••••

••• •••

••• •


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Determine instantaneous direction of Ia, Ib, Ic throughsecondary winding

Primary winding currents are then determined I A=Ia,

IB=Ib, IC=Ic ( turns ratio is 1:1) Line currents on star side is determined (same as

phase currents Ia, Ib, Ic )

Line currents on delta side is determined (IC-I A),(I A-IB), (IC-I A) [phasor difference]

If Secondary wndg of CTs are connected in star,spill current is produced (currents will not matchup)

If secondary wndg of CTs on the star side isconnected in delta – currents are same – providedCTs on delta side is connected in star.


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••••••

lA- lc /A Ia

-

/A! Ia Ia

••••••

l'b 0

0

-o

z

i 3 ro

I a -o •••••••••

Q)

••••• lA- lc lc

(_)

c: ro

•••• Is- lA

Is-lA lo- Ialo (ij

.0

(ij c...:. 2 X

Is- lA

w lo

.. 0

3::

Is! =

lo ro

l c lc- Ia ro

E

lc- Ia

lcl ilc

lc - lo 0

lc

r-- 1

lc- Is : lc - /0 I I I

I

I

I

I

I

I

I I

:Zero


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••••••

lA- lc /A Ia

- I

•------------- --

: Ia - lc I I

I

I

I

I


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/A

••••• l(f

• •

I

.,lc •••••

/Al Ia

••••••••

lt:J- Ia lo

•••••••••

Ia-

lA llJ

lo

lsl /lJ lo

lc

lc - lo

r

c-g external fault

•---------------4I

Percentage differential relay

External fault (c-g)


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Due to fault in C, over current in Phase C.

The If is supplied through two lines on delta

side. Due to delta connection, the CTs in star carry

the current.

Hence no spill path.

The scheme is stable in c-g fault.

L


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L l A- l c / A I a

•••••

+-- •••••

l A - l c l c

/Al j I a I a ••••••••

Ia- / A l b- I a l b

•••••••••

+-- Ia - lA

I a- l A

l al i /b

l b • • +--

l b

l c- I a

l b, l c

c -g internal fault l c - lb

l c

l c- I a I

Spill : Unitcurrent : operates = l e- l a•

l b - I a

Unitrestrains

Unitoperates

I

I

•---------------I

Percentage differential relay

Internal fault (c-g)


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Since the fault is internal, no If through the

primaries of CTs on star side.

The fault current flows through spill path,causing the differential units to operate and

tripping the transformer.


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•••••••••

Inrush Phenomenon •••••• ••••••••

••• •

Switch-on Flux

= 1/lR + 1/lm COS 9 - 1/Jm COS (OJ t + 9)

AC supply

""'Flux

Inrush

Open circuit

\ /Full load


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•• •••

• • ••••

•••••••• ••• •••

••• •

Let the flux in the transformer be written as

qy = ¢rn sin w t

The induced voltage can then be written as

- N d¢ v- dt

= N


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Inrush Phenomenon

•


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••••••

•••• ••• ••••

281--------------------- -----=.-:..:-..-::.-::.:-:...-::..;;;...;::..:-:----;---- -- ::

Bm --------

('\J

E ..0

co

Excitation characteristics of core

Steady-statemagnetizing current

l o 8/o

Inrush current

H(AT/m)

•


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••••••

•••••••••

•••••••••

••••••

'+' Operating

torque Restraining

torque

CT secondary

(/1 - /2) currents /1, /2 (/1 + /2)/ 2

_., ....

1 I

Spi!l current

I I Circulating current I

I Filter l Unfiltered

l Fundamental All harmonics

Fundamental +

component all harmonics

I

L

Reiay

•


37/56

••••••

•••••••••

•••••••••

•••••• CT

Transformer

Fundamental + Harmonics

[I] [TI

Passes only thefundamental

Fundamental + Harmonicscirculating current

All harmonicsspill current

Fundamentalspill current


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•


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••••••

••

•••

•

Transformer

•••••••••

••• •••

••• •

L 0

ad

Reach of restrictedearth fault relay

OC relay

•••


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••••••••••

•••••

••••••••••

••••••

Zero

400 kV

J3

It= 5000 A

Inter-tum fault 11 kV

t J3

5V Rt = 1 mn

•••


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••••••••

••••• ••

••••••

•••••

Conservator Alarm

Breather

Buchholzrelay

Trip

Transformer

Oil

Tank

••••


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•

••

•••

• •••• •••••••• ••• ••• •••

•

Alarm

- - - - - - - - - Oil level

To conservator Vane

To tank

Buchholz trip


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••••••


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•••••

••••

-•••••••

-

•

Steam

Wret

governor

••••••••• •

Steam control Actual speed

valve

Induceddraft

mill

Steam

Condenser

,------ •,'151515'.:..

Exciterfieldcontrol

Error

Air

Pulverized

Forced draft Water

Water

Water

Actualvol tage

Water Waterspray

-------------- Pond

••••••


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••••• •••••••••

•••••••••

••••••

Generatortransformer Main CB

EHV bus

t------ "----+----To grid

..... C'O

Q)

c Q)


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••••• •••••••••

•••••••••

••••••

Alternator Phase a

Main CB To generatortransformer

Phase b

Groundingtransformer(Step-downtransformer )

Phase c

Groundingresistance

t Trip

•••••


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••

•••

• • ••••

•••••••• ••• ••• •••

•

Field interrupter

Field current

J

Excitation controlsystem

I

\ I

'- _;;.

Exciter coupled toalternator shaft

t

Fieldsupressor

Field windingof generator

DC system is isolated "7'"rl7r from ac ground

••••••


48/56

•• ••• •

Stator

.,.... Phase f ault

•••••••••

••• •••

••• •

Electr icalfaults

,... Thr ee-phase stator

winding

"'

Rotor

,., Field winding

, Inter -turn f ault on thesame phase

Gr ound f aults

, Short cir cuit to ground

Sta tor

, Electrical .... Three-phase stator _, Unbalanced loadingwinding

Abnormaloperating

conditions

Rotor

.... Field winding .... Loss of excitation

.

.... Mechanical Loss of

,

pnme mover

Over -speedmg

••••••


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•••••••••

•••••••••

••••••

'

Field windingof generator

Slip ring

Exciter

e

Generator shaft

B Alarm

/

/ I

I I

I

I

: T1me setting

: Plug setting I

-----+.-- Trip circuit of main CB of generator

\

\ \ \

\

' ' ''' ... '

••••••••


50/56

•••••••••

•••••••••

••••••

Negative sequence component in statorcurrent

Stator armature MMF due to negativesequence currents rotates at -Ns (rpm)

No

Field winding rotating at Ns (rpm)

Relative speed between rotor and the field due

to negative sequence currents = 2N s

Double frequency currents of frequency 2Nspl 120 induced in rotor core and winding

Additional heat generated

••••••••


51/56

••••••

•• •

Stator currents I a l b l c

Negative sequence f ilter /2 = f (l a, l b. l c)

•••• •••••••

• •

/2 Negative sequence current (rms) '

Over-current relay with inverse

characteristic To p = K/1 Set K for the machine

'If

Trip main CB of generator

••••• •


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••••••••• •••

Tachogenerator

output

Over -frequency

relay

••••••••••

•••••

Yes

Stop steam supply

to turbine

t Start shutdown of

generator

••••••


53/56

••• ••••••••••

••••••

Voltage = V rated Oe

••••••

Mechanical -+-...;.....

input

Generator

Speed = N s

Field current lr

'---------P e

Generator

Voltage = VLoE

Mechanical -.lor--

input

Speed > N s

Generator

Field current 1, = zero (Loss of excitation)

r--------------- Pe

Generator

••••• • •••


54/56

•• ••• •

•••••••••

••• •••

••• •

T II X

Medium initial

Rated initial

output

1

output

2 Low initial output

3

Offset mho relay functioning

as loss of excitation relay

Ill

Time increasing

IV

Locus of apparent

impedance

•••••••


55/56

•••••••••••

t

••

•••••••••••

•

•

Mechanical

input

Generator

Ns

Oe

Field current 11 excitation

Loss of prime +---T----i Running as motor ..,_... ;

mover

iQ· Field current 11 excitation

••

•••••


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•••••••

I

Active power Generator

Loss of primemover

Reactive power -- )o

•• ••••••

• t •

Trip

'-----4------J Directional +

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Equipment Protection.pdf