Instrument Transformer.pdf

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Instrument

Transformers



Introduction

Theory of Current Transformer

Theory and Selection of Voltage Transformer

Conclusion



IntroductionIntroduction



Protection System Analogy

Brain

Relay

Eyes, Ears, Nose & Skin

Hands & Le s

,

Circuit Breakers

4



Protection System Analogy

Sensed by Instrument

Transformers &

communicated to

Relay

Relay Issues Trip

Command To Breaker

Breaker Trips &

Clears Fault

5



Instrument Transformers

• A Vital Part of the Protection and Metering System

• Instrument Transformer transforms the high current orhigh voltage connected to their primary windings to

the standard low values in the secondary within the

required accuracy limits which feed the metering andprotect on apparatus

• Provide insulation against High voltage (isolation)

• Protect personnel and apparatus from high voltages• Provide possibilities of standardizing the relays and

nstruments

6



Classification of Instrument Transformers

• Metering

• Based on use‐

• n oor

• Outdoor• Types o Instrument Trans ormer

• Current Transformer (CT)

• Voltage Transformer (VT)• Electromagnetic Voltage Transformer (EVT)

• Capacitive Voltage Transformer (CVT)

7



Theor of Theor of

Current TransformersCurrent Transformers



What is Current Transformer

• Direct use of high current (in the tune of 100A or more)

is not possible as protective relays and metering

devices are not designed to handle such huge amountof current

• Current Transformer is an instrument transformer

level, such as, 1000A/1A (CT ratio) i.e. transforms

curren rom e eve o n o curren o eve

9



Why Current Transformer is required

• ystem as two as c requ rements

metering of energy sourced or consumed

Protection of the electrical system fromfaults and disturbances

• Types of Current Transformer (CT)

• Measuring CTs

• Protection CTs

• Protection CTs for s ecial a lications

10



Current Transformer Theory

• u w ywinding, that current creates a MMF which results in a

EMF in the primary winding and in any other windingwound on, or linked with, the core

11



How Current Transformer is connected

wound on core of magnetic material

• Metering and Protection devices are connected to the

secondaries of the CT• Primary winding connected in series and transforms the line

P R I M A R Y W I N D I N GO R B U S H IN GP R I M A R Y W I N D I N GO R B U S H IN G

M A G N E T I C

C O R E

M A G N E T I C

C O R E

W I N D I N GW I N D I N G

12



How Current Transformer is connected

• In current operat on or ser es

mode, the primary winding is

connected in series with the

power system whose relativelyhigh impedance determines the

current which is independent of

the secondary winding load• The current transformer has

assigned rated output termed as

burden in VA which are invariabl

small as against the high outputs

in KVA or MVA of power

13




• or a s ort‐c rcu te trans ormer t e o ow ngrelation holds good ‐

Primary Ampere turns (I1N1) = Secondary Ampere Turns (I2N2)

21I N

=

• ‐

12 N I

transformer where the secondary terminal voltage iszero and the magnetizing current is negligible

•minimum when the secondary is short circuited andmaximum when open circuited




• Simplified CT equivalent circuit

Ip Is ≠ Ip

p sIe

E Z ∠

Im Iw

p s




Phasor Diagram

16




• Primary current has two components, first is secondarycurrent which is transformed in inverse of the turnsratio and an excitin current which su lies the edd &hysteresis losses and magnetize the core

• Exciting current is not transformed and causes errors

accuracy that can be achieved with a current

transformer

Ip = Ie + Is , or

Is = Ip ‐ Ie




The error in the reproduction will appear both in

amplitude and phase. The error in amplitude is called

curren or ra o error an e error n p ase s ca e

phase error or phase displacement

18




PrimaryI

Kn

p

Kn.Is

s

Kn =Ip

Is

19




,error δ could be directly read in percent on the axis

( δ = 1% = 1 centiradian = 0.572 dgree = 34.4 minutes)

• the current error is positive if the secondary current is too high,and the phase error is positive if the secondary current is

20



Current Transformer Accuracy

Why at all CTs are inaccurate?

The culprit is core loss and magnetizing current, which

•

• The secondary current which we get is not true

reflection of its primary current. for example, for a CTwith CT ratio of 1000/1 amps, if we get 0.99 amps insecondary leading primary current by 15 minutes (0.25

,has ratio error of (0.99‐1)/1 x 100= ‐ 1% and phaseerror of 15 minutes

21




• T e exc t ng current Ie ntro uces rat o error, w c s e neas the difference in magnitude of the primary and secondary

current expressed as percentage of primary current

100.

)( ×−= psn I I K Error RatioCurrent

Kn= Rated transformation ratio

p

p = ctua pr mary current

Is = Actual secondary current

• The Phase angle error is the phase angle difference betweenthe primary current and the reversed secondary current




• ompos e error • Under steady‐state conditions, the r.m.s. value of the difference between

the instantaneous values of the actual primary current, and the

instantaneous values of the actual secondary current multiplied by the rated

transformation ratio, integrated over one cycle including the effects of phasedisplacement and harmonics of excitation current

• ompos e error s genera y expresse as a percen age o r.m.s. va ue o

primary current according to the formula

T

−= psn

p

c dt iiK T I

0

2ε

Kn is the rated transformation ratioIp is the r.m.s. value of the primary current

ip is the instantaneous value or the primary current

is is the instantaneous value of the secondary current

T is the duration of one cycle23



Knee Point Voltage (KPV)

• ‐

zcharacteristic (plot between secondary applied

at which an increase of 10% in exciting e.m.f.produces an increase of 50% in the exciting current

24



•

Burden of Current Transformer

The external load (e.g. meters, transducers, relays etc)connected to the secondary of a CT is called the burdenThe burden can be ex ressed in volt‐am eres or in ohms

VA = I2 x ZZ = Total CT secondary impedanceI = Secondary current (Generally 1A or 5A)

• Total burden is the sum of . ev ce trans ucer, meter, re ay etc ur en ‐ urn s e y

the manufacturer2. Burden of Interconnecting Leads ‐ can be calculated by usingthe above formula use conductor resistance total to the device

and back) for Z3. Internal Burden of CT Windings ‐ This is so small that it cangenerally be ignored or specified by manufacturer

25



Magnetization Curve

• The excitation of CT

depend on

a) Cross‐sectional

area

b) Length of magnetic path

of core

c) Number of turns in the winding

d) Magnetic characteristics

o t e core

26




Es = 4.44 * Bm * Aeff * f * Ns

m ‐ m eff

Aeff ‐

Core effective

areaf ‐ frequency

‐s

Es ‐ Induced voltage in the secondary

• A component of primary current excites the core to the flux density. . .

secondary current through total impedance of secondary circuit

• Hence core flux density is dependent on the magnitude of primary current

Es is decided by the total burden

Es= Total burden (VA + lead burden + sec. winding burden) * Is

s s ec e y e ra o .e. p, p, s

Ns= NpIp/Is

27




t constant ur en, core ux ens ty var es rect y as t esecondary current, hence, as the primary and consequently

secondary current increases, a point is reached when core

material start saturating and exciting current becomes

excessive, thus resulting in excessive current error

m . m

accuracy.

Lower the Bm lower is the excitation current yielding better accuracy u arger core area

Rewriting the equation

A = E /(4.44 * B * f * N )

Higher core area is required for

• better accuracy (lower Bm , lower Ie‐ Excitation current ),

‐ s ,

• and higher burden (higher Es)

28



Effect of Secondary Open Circuiting

• E.M.F. induced in secondary winding is that required to drive

secondary current through total impedance of secondary

difference between primary and secondary m.m.f. (ampereturns)

• With secondary open circuited, there are no secondary ampere

turns to oppose those due to primary current and whole of

rimar m.m.f. act on the core as an excessive excitin force,which drive core into saturation on each half wave of the

current

• s g ra e o c ange o ux n e reg on o pr mary currenzero induces an e.m.f., Es of high peak value in the secondary

winding

29



Effect of Secondary Open Circuiting

• With rated primary current, peak value may be as low as fewhundred volts in small measuring CT with 5A secondary

winding, but it might reach many kilovolts, in the case of, say

2000/1A protective CT with a large core section

• With system fault currents flowing in primary, even highervoltages would be induced and not only constitute hazard to

nsu a on o se an connec e ns rumen s, re ays an

associated wiring, but also to life30



Selection of Selection of

Current TransformersCurrent Transformers



Current Transformer Secondary Rating

• Choice of CT secondary rating – 5A Secondary

. .

switchgear cubicles with closely located relays)• Preferred where primary current ratings are very high

• Comparatively low peak voltage when secondary gets

open

• Fine turns ratio ad ustment is not ossible when rimarrating is low

– 1A Secondary

• re erre w en s are ou oor an ea ur en are g• Comparatively high peak voltage when secondary is open

• Fine turns ratio adjustment possible

32



Current Transformer Accuracy

• Measuring CTs are required to be accurate overnormal working range of current, while protective CTs

of the rated current• Metering if we want to measure current for metering

purpose, we desire that

whatever current we measure, that should be very

purpose

• Accuracy Class

A designation assigned to a current transformer, theerrors of which remains within specified limits under

33



Classification of Current Transformer

• Metering Class CTs

0.1 class : High precision testing

0.2 class : Laboratory class.

1.0 class : First grade indicating wattmeter

3.0 & 5.0 class : For general use WTI

• Protection Class CTs

– 5P, 10P, 15P

–

34



Measuring Current Transformer

• es gnat on o eter ng sMetering CTs are specified in terms of –

at o, ccuracy c ass, ur en rat ng ,

(Instrument Security Factor) –, . , ,

• –

• The errors are specified between 5‐120% of ratedcurrent and 25‐100% of rated burden connected

• Higher errors are permitted at lower currents

35



Current Transformer Accuracy Limits

• ‐

Metering Cores

Class 5% of 20% of 100% of 120% of

rate rate rate rate

. . . . .

0.5 1.5 0.75 0.5 0.5

36




• IEC60044‐1 has laid down standards on this

37



Instrument Security factor (ISF)

• T e nstruments connecte to t e secon ary o a T s ou eprotected from getting damaged during primary fault condition,

when primary current is many times higher than the rated value,

the core should get saturated

• For this purposes, Instrument Security Factor (ISF) for Metering

• The CT cores should be such that it saturates at its instrument

security factor (ISF) for safeguarding the instrument from gettingamage un er au t current con t on

• ISF is defined as the ratio of rated instrument security

• ISF is expressed as 3,5,7 or 10 (it shall be chosen as

small as possible)

38



Protection Current Transformer

• Protection Class• During fault condition, value of primary current may

be 10 to 20 times the rated primary current

• Here, main requirement is ability of CT to faithfully

trans orm t e pr mary current ur ng au t con t on

• At such high level of primary current, if CT is not

proper y es gne , may sa ura e an re ay w

receive very less current and, therefore, would not

39

P i C T f




• es gnat on o rotect on sProtection CT are specified in terms of –

at o, ccuracy c ass, ur en rat ng , ccuracy

Limit Factor), ,

• – Error Class 5P, 10P, 15P

– ALF 5, 10, 15, 20, 25, 30

– VA rating 5, 10, 15, 30• The errors are specified at rated current and ALF times

40

P t ti C t T f




.

C t T f A Li it




Protection Cores

Accuracy Current Error Phase displacement Composite Error

Primary

Current

Primary Current

limit (ALF)

Primary Current

5 ± ± m n ± .

centiradians

±

± - - ±

42

A Li iti F t (ALF)



Accuracy Limiting Factor (ALF)

• Un e measur ng Ts, w c are requ re to e accurate overthe normal working range of currents, protective CTs are usually

required to maintain their ratio up to several times the rated

primary current

• At some value of primary current above the rated value, core

,

current error

• Protection Class CTs cores should not get saturated below itsccuracy m t ng actor up to w c t e pr mary current

should be faithfully transformed to the secondary side,

maintaining the specified accuracy

• ALF is defined as the ratio of the rated accuracy limitprimary current to the rated primary current

43




•


, , ,connected burden is less than rated then ALF would increase

• Applications of this CT are Over current relay, Inverse relay,

earth fault protection, Phase fault protection etc.

• While selecting 5P10 class CT for IDMT O/C or Earth fault relays

– CT should have optimum ALF/VA rating, so that they do not

saturate up to at least 20 times current rating (either by

appropriate high value)

–

high secondary currents during severe faults (in excess of 20times setting) that may cause thermal stressing of relay

curren co s an even ua a ures

44





• Designation of Protection CTs for special applicationsFor protection like circulating current differential, restricted

.

between associated CTs with close toleranceSpecial class Protection CT of are specified in terms of –at o

2) Accuracy class

3) Knee Point Voltage (Vk)4 CT Secondary winding resistance RCT corrected to75 C

5) Excitation current (Ie) usually at Knee Point Voltage or a stated

percentage thereof

Example ‐

200/1, PS Class, Vk > 200V, RCT < 2.0 ohms, Ie < 30mA at Vk/4• The turn ratio error are limited to +0.25% which helps in

maintaining balance between the protection system during

maximum through fault condition

45

Outdoor Current Transformer




• Outdoor CTs are basically of 3 types of Construction

– Dead Tank with U (Hair Pin) shaped

– Dead Tank with Eye Bolt primary

–

46

Dead Tank Current Transformer




CB

Insulator

Primary winding

Secondary winding

Core

Terminal BoxS

S47





1) Eye Bolt Type 2) Hair Pin Type

48





P2P1

CO RE 5CO RE 1

CO RE 2

CO RE 4CO RE 3

49





50



CT Secondary in Progress

Live Tank Current Transformer




52





53



420 kV Live Tank CT420 V ea Tan CT

(Hair Pin

Design) 54



Theor of Theor of

Volta e TransformersVolta e Transformers

What is Voltage Transformer



g

• Voltage Transformer is an instrumenttransformer which transforms voltage

rom one eve o ano er eve suc as

400KV/ √3:110V/ √3 (VT ratio) i.e.

400KV/ √3 into voltage of 110V/ √3 level

•the tune of 3.3kV or more) is not

possible as devices used for

measurement of voltage are notdesigned to handle such high level of

vo age

56

Why Voltage Transformer is Required



• System has two basic requirements

me er ng o energy

sourced or consumed

electrical system from

disturbances

57

Why Voltage Transformer is Required



• Faults can be of many kinds, some faults such as O/Ccan be detected solely on current measurement, but

curren oes no prov e scre on a ou na ure an

location of the fault• ,

current during faults, we can in a way compute power

or impedance of system along with its direction• Moreover O/V, U/V, O/F, U/F and over fluxing

protections are also configured from VTs

• Voltage signal also used for synchronizing, Disturbancerecorders and event logs

58

How Voltage Transformer is connected



• VT has a primary and one or more secondarywindings

• e er ng an ro ec on ev ces are connec e o

the secondaries of the VT• v u , y

winding is connected in parallel with the power

volts suitable for the meter or relay

59

Voltage Transformer Theory



• For a transformer in no load the following is validVoltage transformation is proportional to the ratio of primary

11E N =

• An ideal voltage transformer is a transformer under no‐

load

22

conditions where the load current is zero and the voltage drop is

only caused by the magnetizing current and is thus negligible




• Simplified VT equivalent circuit

Ip Is ≠ Ip

p sIe

E Z ∠

Im Iw

p s




IpRp

Vp

′

IsRs Es

′p

Ip

IeRp

Vs

θ s

Ie

θ

Phasor diagram with referance to voltage error

0

62




• Ratio error, which is defined as the difference inmagnitude of the primary and secondary voltage

100. ×−= pns V K V

Error RatioVolta e

Kn= Rated transformation ratio

pV

Vp = Actual primary voltage

Vs = Actual secondary voltage

• Phase Angle error is the difference between the

Voltage Factor



• o tage actor eterm nes t e max mum operat ng vo tage or vo tagetransformers expressed in per unit of rated voltage, which in turn

dependent on the system and voltage transformer earthing conditions

• s use n non‐e ec ve y ear e sys em ave g vo age ac or s nce n

the event of an earthed fault in one of the phases, the healthy phase

voltage may rise to phase to phase value

Voltage

Factor VF

Duration Earthing conditions

V.T. primary

winding

System

1.2 Continuous Non‐earthed Effectively or non‐effectively earthed

1.5 30 s Earthed Effectively earthed

1.9 30 s Earthed Non‐effectively earthed with

automatic E/F tripping

1.9 8 h Earthed Isolated neutral or resonant earthed

without automatic E/F tripping

64

Protection of Voltage Transformer



across its secondary terminal is achieved by incorporating fuses

or MCB in secondary circuit located near to transformer as

possible

• Normal secondary current is not more than 5A and short circuit,

• Short circuit on secondary winding gives only a few amperes in

primary winding and is not sufficient to rupture a high voltage

fuse at primary side (HRC fuses on primary side up to 66kV)

• Hence high voltage fuse on primary side do not protect

,

circuit on the primary side• CVT invariably solidly connected to the system so that there is

no primary protection

65

Voltage Transformer Accuracy



• As stated for CT, we need it for

Metering voltage measurement, energy, powermeasurement

Protection for distance protection, O/V, U/V, O/F, , ‐

• For metering VTs we need high accuracy in the voltage

measurement durin stable conditions i.e. 80% to120% of nominal system voltage with burdens from25% to 100% of rated burden at power factor of 0.8

• Combination of magnitude and phase error depends

66




• IEC 60044‐2 and 60044‐5 defines this as

67




• For Protection VTs we need faithfulness of

voltage such as from value as low as 2% of

by rated voltage factors such as 1.2, 1.5, 1.9

at 0.8 pf lagging

68




• IEC 60044‐2 and 60044‐5 defines this as

69

Voltage Transformer Connections



• There are three types of connections – V‐V connection

– Star/Star connection

– Star/Open delta connection

• ‐ connec on

– Used for measurement and for those protections which do

– Primary of VTs is connected in V (one VT primary across R‐Y

phase and other across Y‐B phase) with identical V

connection for the secondary

– In this connection zero sequence voltage can not be

70




• Star/Star connection – Either 3 separate single phase

s or a s ng e p ase, m

is used – Both rimar and secondaries

are connected in star with both

star neutrals solidly grounded

– ac pr mary p ase m s us

connected between phase to

earth of the supply circuit and

replicate similar phase to earth

voltage on the secondary

71




• tar pen De ta connect on – Primary windings are connected in

star with star neutral solidly

grounded and the secondaries are

connected in series to form ano en delta connection

– This type of connection is called

residual connection and require

single 3 phase 5 limb VT

– This residual connection is used for

po ar s ng rect ona eart au t

relays or for earth fault detectionin non‐effectively grounded or

isolated neutral system

72

Types of Voltage Transformer



• Types of Voltage Transformer VT• Electromagnetic Voltage Transformer (EVT)

• apac ve o age rans ormer

P M

P

P

INDUCTIVE VOLTAGE

TRANSFORMER

CAPACITIVE VOLTAGE

TRANSFORMER73

Types of Voltage Transformer



• ec romagne c o age rans ormers s m ar o a sma powertransformer and differs only in details of design that controlratio accuracy over the specified range of output, coolingou pu no more an ‐ , nsu a on es gne or

system impulse voltage level) and mechanical aspects

transformer is high, due to prohibitive cost of insulation,hence, at 132 kV and higher voltages, CVT may be more

capacitors can serve also for carrier current coupling (PLCC),but may be inferior in transient performance

• apac tors a ow t e n ect on o g requency s gna s onto

the power line conductor to provide end‐to‐endcommunications between substations for distance relays,te emetry supervisory an voice communication

74

Capacitive Voltage Transformer



Primary Terminal

Capacitor Part

Electromagnetic Unit

HF Terminal

Sec. Terminal Box

75

CVT Internal Components



Tank

PT

Resistor

Capacitor

F

R

FR Chokek

t.

HV Choke

76

Capacitive Voltage Transformer



Power Line Carrier (PLC) equipment



C1Wave Trap

>500KHZ NOISE PICKUP<30KHZ-HARMONIC

LIGHTENING,CORONA

C3 L3L1

C4

C2

Carrier

oscillator Matching

Coupling capacitor

VTL2

Transmitterand receiver

f a = 30kHz to 500 kHz

78



79

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