Application of Instrument Transformers - CT.ppt

8/14/2019 Application of Instrument Transformers - CT.ppt

1/24

APPLICATION OFINSTRUMENT TRANSFORMERS

Current TransformersVoltage Transformers - Electro-magnetic Type

- Capacitor Type

for

Metering &Protection


2/24

ESSENTIAL PRE-REQUISITES FORINSRTUMENT TRANSFORMERS

1.HIGH RELIABILITY 2.CAPABILITY TO WITHSTAND HIGHEST SYSTEM

LOAD AND FAULT CURRENTS AND TRANSIENTVOLTAGES

3.LOW MAINTENANCE 4.LONG AND TROUBLE-FREE SERVICE


3/24

Current Transformers

Service Conditions:Pollution Levels-- Normal, Heavy Rated Primary Current

Rated Short Time Current B.I.L Number of Ratios

Number of Cores Output ,Accuracy,Accuracy limit factor etc of Individual Cor

Specification Requirements


4/24

Requirement of Metering And ProtectiveCurrent Transformers

MeteringShould maintain accuracywithin

limits over-Entire Current range of 20 to120 % of rated current-Burden Range of 25 to 100 %&Preferably saturate beyond theRated current

ProtectionShould maintain accuracy from

beyond the rated current to the

highest possible fault current inany location

Consequently should notsaturate up to highest fault

current


5/24

Current Transformers

Definition of Accuracy

Ratio Error or Current Error:Kn Is - Ip * 100 percent

Ip

Phase Error or Phase displacement:Difference between the Primary Current and

Secondary Current Vectors


6/24

Vector Diagram of C.T


7/24

Variation of Ratio Error Permitted With 0.5 & 1.0 Accuracies


8/24

VARIATION OF PHASE ERROR WITH BURDEN


9/24

VARIATION OF ERRORS WITH PRIMARY CUAccuracy % Ratio Error at Phase Displaceme Class % of Primary Current % of Primary Curr 5 20 100 120 5 20 100 120

0.2 0.75 0.35 0.2 0.2 30 15 10 10

0.5 1.5 0.75 0.5 0.5 60 45 30 30

1.0 3.0 1.5 1.0 1.0 180 90 60 60


10/24

P1

S1 S2 S3 S4 S1 S2

P1 C2 C4 C6

C3 C5 P2

SECONDARY RECONNECTION PRIMARY RECONNECTION

PRIMARY SECONDARY RATIOTERMINALS TERMINALS P1 P2 S1 S2 100/1 P1 P2 S1 S3 200/1 P1 P2 S1 S4 400/1

PRIMARY CONNECT SECONDARY RATIOTERMINALS TERMINALS P1 P2 C1-C2,C3-C4,C5-C6 S1 S2 100/1 P1 P2 C1-C2,C3-C4,C5-C6,C4-P2 S1 S2 200/1 P1 P2 C1-C3-C5-P1,C2-C4-C6-P2 S1 S2 400/1


11/24

SECONDARY RECONNECTON V/S PRIMARY RECONNECTION

SECONDARYRECONNECTION ADVANTAGES

INDEPENDENT SELECTION OFRATIO ON INDIVIDUAL CORES RATIO CHANGE POSSIBLE

WITHOUT INTERRUPTINGPOWERE SUPPLY

DISADVANTAGE . OUTPUT , ACCURACY ON

LOWER RATIOS LOWER THANWITH HIGHER RATIOS

PRIMARY RECONNECTION ADVANTAGE

OUTPUT , ACCURACY UNIFORMLYSAME ON ALL RATIOSDISADVANTAGES

RATIOS ON INDIVIDUAL CORESCAN NOT BE INDEPENDANTLYCHOSEN

NECESSARY TO INTERRUPTPOWER SUPPLY FOR MAKINGCHANGE IN RATIOS


12/24

Force Of Repulsion =

K I * I d

Force Of repulsion = K N I* NI d


13/24

Normal VA Burden Of Instruments and

Meters Instrument/Meter VA BurdenAmmetersRecording Ammeters

3

Current Coils of -Wattmeters, Power Factor Meters, Kwh, Kvar meters, Recording Wattmeters

5


14/24

Accuracy Classes for Protective Current Transformers

AccuracyClass

Ratio Error atRated Current

Composite Error at RatedA.L.F times Rated Current

5P + or- 1 % + or 5%

10P + or 3 % + or 10%

15P + or 5 % + or 15 %


15/24


16/24

INTERDEPENDANCE OF VA , ACCURACY, A. L. F

VA= OUTPUT OF CT IN VOLTAMPERES A= CROSS SECTIONAL AREA OF CORE OF CT

B normal=FLUX DENSITY IN CORE AT NORMAL OUTPUTB sat =FLUX DENSITY IN CORE AT A.L.FA.L.F- ACCURACY LIMIT FACTOR

TAKING A SECONDARY CURRENT OF 1 AMP , VOLAGE ACROSS SECONDARYWINDING

VAV=--------=VA

1THEREFORE, VA= 4.44*FREQUENCY(f)*B normal* A* No OF TURNS IN SECONDARY(N)

AS FLUX DENSITY AT A.L.F I S RESTRICTED BY SATURATION IN THE COREHIGHER A.L.F MEANS LOWER VA AND VICE-VERSA


17/24

Requirements for Special Purpose

Current Transformers The performance is specified in terms of

_ Minimum Knee Point Voltage

_ Maximum Exciting Current at the Rated KneePoint Voltage(K.P.V )

_Maximum Resistance of Secondary Winding atK.P.V

_ No Turns Correction


18/24


19/24

Dead Tank Type Current Transformer

N 2

Oil

Dead Tank Type Current Transformer(Hair Pin Type) (Eyebolt Type)

N 2

Oil

1opics for Discussion

1.Pry Current2.Cost3.Dynamic withstand

.Porcelain cost


20/24

N 2

Live Tank Current Transformer

Topics for Discussion1.Primary Taping2. E.Q withstand3.Secondary Terminals4. Placement of Insula-ted Secondary

5.Susceptibility forfailure in case of oilleakage


21/24


22/24


23/24

CAUSES OF FAILURE

SEAL FAILURE----Causing CT to breathe atmospheric air.Moresevere in the case of LIVE TANK CTs(NTPC 400kV CTs )

LOOSE H.V CONNECTION-----Causing severe local heating andmelting of soldered joints

FORMATION OF CONDUCTING DRY BAND----due to pre-monsoon showers and un-seasonal rains causing external flashover

LOOSE P.F TERMINAL CONNECTION----Causing local arcing andconsequent weakening of insulation

MECHANICAL FAILURE OF PORCELAIN DUE TO GROUNDVIBRATIONS----More severe in LIVE TANK CTs due to inherentconcentration of weight on top of porcelain


24/24

Schematic Diagram of Zero-flux C.T

Application of Instrument Transformers - CT.ppt

Documents

Transcript of Application of Instrument Transformers - CT.ppt