31 October 2013

Instrument Transformer - Technology & Design Conference on Electrical Power Equipment Diagnostics Bali, Indonesia Thomas Prevost

Instrument Transformers: Link between primary and secondary equipment

Function of an Instrument Transformer

• Measurement – Transform the primary signal into a

secondary signal which can be transmitted and measured to secondary equipment.

• Isolation – Isolation of the secondary circuit and

equipment from the high voltage of the primary circuit.

Measurement Function • Transform the primary signal into a

secondary signal which can be transmitted and measured to secondary equipment – Transform primary current into secondary

current Ip = 1000 A Is = 1 A – Transform primary voltage into secondary

voltage Up = 400/√3 kV Us = 110/√3 V

CT for Measuring and Protection

CT for Measuring: • High accuracy in the range of rated current • Saturation with over current (FS)

CT for Protection : • 1% accuracy at rated current • defined accuracy in high current range (e.g. 5P20 error < 5% for 20 times rated current)

Accuracy for Measuring CT

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 20 40 60 80 100 120 140

cl. 0.2cl. 0.2Scl. 0.5cl. 0.5S

Accuracy classes according to IEC 60044 – 1 class 0.2 - 0.2S and 0.5 - 0.5S

Limits of amplitude error

Limits of phase displacement

Current in % IN Current in % IN

%

min

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140

cl. 0.2cl. 0.2Scl. 0.5cl. 0.5S

Accuracy for Protection CT

• Standard accuracy classes for protection 5P and 10P • Definition according to IEC60044 -1 § 12: • Burden in VA (cos φ = 0.8), Accuracy limit factor (ALF) and

accuracy limits

• Example: 30 VA 5P20 (max. 5% Error at 20 times rated current)

Accuracy for Protection CT

Other accuracy classes for protection cores acc. IEC 60044-1:

PR: - 5PR and 10PR Accuracy defined as 5P and 10P - low remanence factor Kr ≤ 10

PX: - defined with knee point voltage EK and max. excitation current Ie at EK (with increase of voltage EK to EK + 10% max. increase of excitation current 50 %) - max. secondary resistance RCT at 75°C

Accuracy for Protection CT

Transient requirements according to IEC 60044 – 6:

TPS: defined with knee point voltage and max. excitation current TPX: max. error during defined breaker cycle no requirements for remanence TPY: max. error during defined breaker cycle max. remanence 10 % TPZ: max. Error (only a.c. component) Specified secondary time constant Very low remanence

Hysteresis for different CT Definitions

B

H

P, TPS TPX TPY TPZ

Remanence points

ref Trench

Inductance during transient current (TPY cores)

t

t’ 1st fault duration

t’’ 2nd fault duration

tfr dead time

tal permissible time to accuracy limit

with air gap

without air gap B

Saturation threshold

ref Trench

Accuracy Definition for Voltage Transformers

IEC 60044-2

Measuring Voltage Transformers (0.8 …1.2 UN,)

Protection Voltage Transformers (0.05 … fu UN)

Accuracy of Voltage Transformers

Function of an Instrument Transformer

• Measurement – Transform the primary signal into a

secondary signal which can be transmitted and measured to secondary equipment.

• Isolation – Isolation of the secondary circuit and

equipment from the high voltage of the primary circuit.

External Insulation

Requirements: • Withstand voltages: A.C. lightning and switching impulse arcing distance • Withstand voltage during rain arcing distance, shed profile • Pollution requirements Shed form, creepage distance • Mechanical requirements

Material, sizes

Porcelain Insulator Composite Insulator

Internal Insulation

• Insulation between cores and housing • Bushing insulation • Voltage grading inside the bushing

Oil-Paper Insulation: • Paper impregnated with mineral oil • Compact design • Low aging when hermetically sealed against outside air • Maintenance free

SF6 Insulation: • Gas insulation with overpressure of some bar • Some bigger design • No aging • Pressure monitoring

Conventional Instrument Transformers Inductive principle

Current Transformer

Oil-Paper Insulation 72.5 kV … 550 kV

SF6 Insulation 72.5 kV … 800 kV

Principle of Current Transformers

i2 = x i1 n1

n2

n1

n2

Magnetic core

i1

i2

Inductive transformer secondary side short circuited

i1 : primary current n1 :primary turns

i2 : secondary current n2 : secondary turns

Equivalent circuit and phasor diagram of Current Transformers

R‘1 X‘1

RFe

R2 X2 RB

RB

XH

U1 UH U2

I0

I2 I‘1

IXh

I1

IFE

I2

UH U2 I2 ·RB

I2 ·XB

I2 ·R2

I2 ·X2

δ

ΔI

Principle of Current Transformers

• Magnetic core with secondary winding to fulfill the measuring task

• Internal and external insulation to fulfill the isolation task

primary bar

insulation

metallic shield

secondary wires

core

ref Trench

Current Transformer

aluminium cap

terminals

head casing

lifting lug

external insulator

metallic bellow

Cores

primary rod

high voltage

insulation

bushing

filling oil

terminal box

(Oil-paper insulated)

ref Trench

72.5 – 550kV

High voltage terminal

Insulator (Porcelain or composite)

Metallic expansion bellows (hermetically sealed)

Fine grade bushing

active part CT

Ground electrode

CT measuring cores

Oil / paper high voltage insulation design for AIS

Current Transformer

ref Pfiffner

Current Transformer

245 – 550kV High voltage terminal

Composite insulator

grading cylinder

Secondary connection

SF6 gas device incl. monitor

SF6 gas insulation design for AIS Current Transformer

ref Pfiffner

Voltage Transformers

Oil-Paper Insulation 72.5 kV … 550 kV

SF6 Insulation 72.5 kV … 800 kV

Principle Voltage Transformers

u1, n1

u2, n2

u2 = x u1 n2

n1

insulation

Secondary windings

iron core

Primary winding

bushing

high voltage

High voltage terminal

Metallic expansion bellows (hermetically sealed)

Insulator (Porcelain or composite)

Fine graded bushing

HV electrode

Primary winding

Secondary winding

Iron core

Oil / paper high voltage insulation design for AIS

36 – 245kV

Conventional measuring devices – voltage transformer

ref Pfiffner

Equivalent Circuit and phasor diagram of Voltage Transformers

R‘1 X‘1

RFe

R2 X2 RB

XB

XH

U1 UH U2

I0

I2 I‘1 U1

U2

UH

IFe

IXh

I2 I‘1

I2·RB

I2·XB

I2·R2

I2·X2

I‘1·R’1

I‘1·X’1

δ

ΔU

Voltage Transformer

Voltage Transformer Type

aluminium cap

external insulator

lifting lug

terminal box

metallic bellow

filling oil

bushing

high voltage insulation

Windings

iron core

(Oil-paper insulated)

ref Trench

Gas insulated Voltage Transformers

GIF-outdoor voltage transformer

HV-electrode

GIS-voltage transformer

HV-spacer

pressure vessel

rupture disk

LV - winding

ref Trench

Conventional measuring devices – voltage transformer

High voltage terminal

Composite insulator

Grading cylinder

HV connection

SF6 gas insulation design for AIS

Primary winding

Secondary winding

Iron core

Ground electrode

HV electrode

Type EGF 245 – 550kV

SF6 gas device incl. monitor

ref Pfiffner

High voltage terminal

Metallic expansion bellows (hermetically sealed)

Insulator (Porcelain or composite)

High voltage capacitor (C-divider)

Bushing (tap voltage approx. 8 – 12 kV)

MV – VT (EMU)

Secondary winding Primary winding

Compensating coil

72.5 – 550kV

Oil / paper high voltage insulation design for AIS

Conventional measuring devices – CVT’s

ref Pfiffner

primary terminal

lifting lug

head

porcelain insulator

housing

terminal box

Housing

metallic bellow

capacitor

intermediate transformer + compensating coil

Active part

Capacitive Voltage Transformer

ref Trench

Principle of CVT Active Part

Intermediate transformer

Compensating coil

Secondary windings

Secondary windings

N

Damping impedance and varistor

Spark gap NHF

High voltage

Capacitive divider

ref Trench

Combined CT/VT Type EJGF

245 – 550kV

Type EJOF 72.5 – 170kV

Current transformer

Voltage transformer

ref Pfiffner

Combined Transformer Type IVOKT

high voltage

Current transformer active part

Voltage transformer active part

(Oil-paper insulated)

ref Trench

Questions