ct & pt basic

28
Copy Right YMPL 1 Instrument Transformers for revenue metering (Training for metering school ) By N.K. BHATI Copy Right YMPL 2 Course Introduction • Objectives: - what is your expectation? Please spell out.

Upload: santanu-rout

Post on 24-Oct-2014

199 views

Category:

Documents


18 download

TRANSCRIPT

Page 1: CT & PT basic

Copy Right YMPL 1

Instrument Transformersfor revenue metering

(Training for metering school )

By

N.K. BHATI

Copy Right YMPL 2

Course Introduction

• Objectives: - what is your expectation? Please spell out.

Page 2: CT & PT basic

Copy Right YMPL 3

Course objectives

• Understand importance of instrument transformers in revenue metering.

• Basics of design and construction of Voltage and Current transformers.

• Issues arising in field and specification consideration .

• Standards applicable (though as old as 1992)

• Testing methods.

Copy Right YMPL 4

Importance• Metering system accuracy is governed by

instrument transformers, if used in LT CT or HT CT/VT systems.( HT3 or HT 4)

• All errors add algebraically.

Meter errors

CT ERRORS

VT ERRORS

ENERGY METERINGSystem ACCURACY

Page 3: CT & PT basic

Copy Right YMPL 5

Current transformers in LT circuits

• Most common window type ( round or rectangle)

• Ratios 100/5 or 100/1 to 25000/5 are common

Copy Right YMPL 6

Importance

• Meters may be of high accuracy, but are instruments transformers too?,

generally not, may be one class lower.Large old installed base, meters have been replaced but

CT/PT not changed.• Accuracy of instrument transformers do change due to

• Aging, • Environmental effects like ambient temperature,

temperature cycling , humidity etc• line transients and surges leading to insulation leakage,

shorted turns , • fault currents leading to saturation of cores,• Connected burden, lead length or drop across leads.

Page 4: CT & PT basic

Copy Right YMPL 7

Importance

• Ratio error of 0.5 % and phase error of 30 min contribute 1.2 % error at 0.8 pf. These are allowed errors as per standards for CTs of Cl 0.5 at 100 % current

• Similarly for VT of Cl 0.5, as per standards, errors allowed are 0.5 % and 20 min phase error, they may contribute about 1 %.

‘Combined effect may be up to 2.2 %’

• At lower loads, say 20 %, CT errors increase by 50 %, ( 1.8%) at 5 % load , increase to 3 times (3.6 %)

• If CT/VT are class 1, then this may be ….??

• Double of these figures.

Copy Right YMPL 8

Importance

• Higher ratio CTs when used at lower loads, may have large errors as they may operate at lower % load.

• With static meters burden being low, if higher burden CT/PT are used, their performance at lower burden is questionable, as not even specified by standards.

• Drop across leads in VTs, depending on burden connected.

• If a PT or Ct is used for indication only, then ratio errors are important, phase error doesn’t matter.

Page 5: CT & PT basic

Copy Right YMPL 9

Instrument transformers

• Voltage transformers• Current transformers• Combined CT/PT units or metering Units

Copy Right YMPL 10

Page 6: CT & PT basic

Copy Right YMPL 11

Instrument transformers

• A transformer intended to supply input to measuring instruments, meters, relays and other similar apparatus.

• These provide isolation and a measurable (standardized) signal at safe voltage levels.

• Safety of persons and equipments which are interfaced

Copy Right YMPL 12

Voltage Transformers

Page 7: CT & PT basic

Copy Right YMPL 13

Voltage transformer

• An instrument transformer in which, in normal conditions of use and for an appropriate direction of the connections.– Secondary voltage, is substantially

proportional to the primary voltage.– Differs in phase from it by an angle which is

approximately zero

Copy Right YMPL 14

Electromagnetic voltage transformer

• A voltage transformer which transforms the primary voltage to the secondary voltage entirely by electromagnetic phenomena.

Page 8: CT & PT basic

Copy Right YMPL 15

Capacitor voltage transformer

• A voltage transformer comprising a capacitor divider unit and an electromagnetic unit so designed and interconnected that the secondary voltage of the electromagnetic unit is substantially proportional to and in phase with the primary voltage applied to the capacitor divider unit.

Copy Right YMPL 16

Types of Voltage Transformers• Earthed voltage transformer

A single phase voltage transformer which is intended to have one end of its primary winding directly earthed or a three phase voltage transformer which is intended to have the star point of its primary winding directly earthed.

• Un earthed voltage transformerA voltage transformer which has all parts of its primary winding, including terminals, insulated from earth to a level corresponding to its insulation level.

Page 9: CT & PT basic

Copy Right YMPL 17

Definitions• Rated Primary Voltage / Secondary Voltages

The rms values of the voltage upon which the performance of the voltage transformer is based.

• Rated Transformation Ratio The ratio of the rated primary voltage to the rated secondary voltage.

• Normal System Voltage The rms line-to-line voltage by which the system is designated.

Copy Right YMPL 18

Errors in VTs, definitionsVoltage error (ratio error)

• Introduces into the measurement of a voltage and which arises when the actual transformation ratio is not equal to the rated transformation ratio.

• The voltage error, expressed in percent, is given by the formula : kn. Us – Up

VOLTAGE ERROR = --------------- X 100%Up

Wherekn = The rated transformation ratio,Us = The actual secondary voltage, when Up

is applied under the conditions of measurement, and Up = The actual primary voltage

Page 10: CT & PT basic

Copy Right YMPL 19

Errors in VTs, definitionsPhase errors

• The difference in phase between the primary voltage and the secondary voltage vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer.

• This phase displacement is said to be positive when the secondary voltage vector leads the primary voltage vector and negative when it lags. It is usually expressed in minutes.

( definition is strictly correct for sinusoidal voltage only.)

Copy Right YMPL 20

Rated insulation level

That combination of voltage values which characterized the insulation of a transformer with regard to its capability to withstand dielectric stresses .

• Eg.: 12 / 28 / 75 KVp

• Impulse withstand voltage

• power frequency withstand voltage• Highest system voltage

Page 11: CT & PT basic

Copy Right YMPL 21

Voltage Transformer construction

• Basic elements of construction of voltage transformer are

• Magnetic circuit (core)• Electrical circuit i.e. Primary and secondary windings• Insulation, basic and additional• Housing and other mechanical details• Terminals and mounting arrangements• Earthing facility.

Copy Right YMPL 22

Voltage Transformer construction• Potential transformer consists of two separate windings

on a common magnetic steel core. One winding consist of relatively large number of turns of fine wire, called primary winding. The other winding consists of fewer turns of heavier wire and is called secondary winding.

• The core material used is generally, GOSS.• Sec. winding is made of thicker wire, with tight coupling

between primary and secondary windings. These help to keep resistances of coils as well as leakage inductance low. Which in turn help to keep errors low.

• Suitable insulation between primary and secondary and to the body is maintained.

Page 12: CT & PT basic

Copy Right YMPL 23

Voltage transformer design

• Working and design principles are similar to conventional power transformers.

• In ideal transformer, voltage will be in proportional to turns ratio and phase difference will be 180 deg.

• If suitably connected (secondary reversed) then phase difference is zero.

• In practical transformer, due to magnetisation current and watt loss due to cores will cause ratio and phase errors.

• Drop across the primary and secondary winding’s resistance and leakage inductance also adds to these errors.

Copy Right YMPL 24

Voltage transformer design

• The VT normally works on a constant magnetic flux density.

• The power rating of a VT is low, thus the load current and excitation currents are of similar order.

Page 13: CT & PT basic

Copy Right YMPL 25

Precautions in using VTs

• observe correct polarity marking.• Never short circuit the secondary terminals of a

potential transformer. A secondary short circuit will cause the unit to over heat and fail in a very short period of time.

• For protection , there may be fuses in Primary or secondary or both.

• For reasons of tamper, these days fuses are not used.

Copy Right YMPL 26

Testing a VT

• Many methods, like voltmeter, with standard PTs, resistor or capacitor dividers with voltmeters.

• Comparision Method:: This is a modification of the basic ‘potentiometer’ method. Instead of reading the primary and secondary voltages, the secondary voltage of a calibrated ‘standard’ transformer and the transformer undergoing test are compared with each other by nullingmethod using a bridge , or direct error computation. The comparision method , of course requires the use of a ‘Standard ‘ potential transformer with a rating which corresponds to that of each potential transformer involved in calibrating.

• Reference temp specified is 40 deg C

Page 14: CT & PT basic

Copy Right YMPL 27

Copy Right YMPL 28

LIMITS OF VOLTAGE ERRORS AND PHASE DISPLACEMENT AS PERIS 3156 / 1992 & IEC 60044-2 / 1997

Not specified+/- 3.0%3.0

+/- 40+/- 1.0%1.0

+/- 20+/- 0.5%0.5

+/- 10+/- 0.2%0.2

+/- 5+/-0.1%0.1

PHASE DISPLACEMENT (MINUTES)

PERCENTAGE VOLTAGE

(RATIO) ERRORCLASS

Page 15: CT & PT basic

Copy Right YMPL 29

Instruments typical VA burden

• Volt meters 5 VA• Voltage coils of watt meters and power factor

meters 5 VA • Voltage coils of frequency meters 7.5 VA • Voltage coils of kwh, kvar meters 7.5 VA• Recording – voltmeters 5 VA • Voltage coils of recording power factor meters

and watt meters 7.5 VA • Voltage coils of synchroscope 15 VA

Copy Right YMPL 30

Current Transformers

Page 16: CT & PT basic

Copy Right YMPL 31

Current transformer

• An instrument transformer in which the secondary current, in normal conditions of use is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.

Copy Right YMPL 32

Types of CTs………… by application

• Measuring current transformer A current transformer intended to supply indicating instruments, integrating meters and similar apparatus.

• Protective current transformer A current transformer intended to supply protective relays and similar apparatus.

• Dual purpose current transformers A current transformer intended to serve the dual purpose of measuring and protection.

Page 17: CT & PT basic

Copy Right YMPL 33

Types of CTs.. …….by construction

• Window type or ring type current transformer A current transformer having no primary winding as an integral part of the structure. This current transformer has an opening in the center to accommodate a primary conductor through it.

• Wound-primary current transformer A current transformer in which the primary winding consists of asingle turn loop or multi-turn coil integral with the transformer.

• Bar-primary current transformer A current transformer in which the primary winding consists of abar of suitable size and material forming an integral part of the current transformer the primary conductor is the equivalent of asingle primary turn.

• Bushing type current transformerA current transformer having neither primary winding nor insulation for a primary winding. This type of current transformer is for use with a fully-insulated conductor as the primary winding.

Copy Right YMPL 34

Types of CTs.. …….by construction

• Multiple secondary current transformer A current transformer having more than one secondary winding on a common core. One secondary winding is closed, other unused secondary should be left open circuit and not closed or short circuited.

• Multiple-primary current transformer A current transformer having more than one primary winding on a common core.

• Multiple ratio current transformer A current transformer from which more than one transformation ratio can be obtained by the use of taps on the primary or secondary windings or both, or by series / parallel connection of separate primary or secondary windings on a common core, or by other means.

Page 18: CT & PT basic

Copy Right YMPL 35

Types of CTs.. …….by insulation

• Tape wound CTs: these are ring type CTs, with polyester or fiber glass tape .

• Resin cast CTs: these are ring type CTs, with polyester or PVC tape and then cast into resin. Resin works as insulation as well as provides environmental protection. Terminals and supporting clamps are also cast in the resin.

• Oil filled CTS: these are generally HT type of CTS. In these, oil is used as insulation and provides cooling also.

Copy Right YMPL 36

Types of CTs………… by application

• Interposing current transformer A current transformer which is intended to amend the ratio of a main current transformer by having its primary winding connected to secondary winding of the main current transformer and its secondary winding connections to the burden.

• Summation current transformer A current transformer which is intended to summate the currents in a number of feeders in association with the feeder current transformer core balance current transformers. The core balance current transformers are intended for providing earth leakage protection on 3 phase electrical system.

Page 19: CT & PT basic

Copy Right YMPL 37

Some definitions

• Rated primary current:The value of primary current which appear in the designation or rating plate and on which CT performance is based. The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current.

• Rated secondary current:The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current are 1 A or 5 A.

• Rated burden:The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)

Copy Right YMPL 38

Some definitions …. Contd.

• Accuracy class:In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor.

Page 20: CT & PT basic

Copy Right YMPL 39

Some definitions …. Contd.

• Short time rating:The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.

• Instrument security factor (factor of security):This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are thereforeprotected. In the case of double ratio CT's, FS is applicable for the lowest ratio only.

Copy Right YMPL 40

CT errors ( Ratio error )

• The error which a transformer introduces into the measurement ofa current and which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio.

• The current ratio error expressed in percent is given by the formula.

(KnIs – Ip) x 100CURRENT ERROR % = -------------------

IpWhere

Kn = the rated transformation ratio ip = the actual primary current is = the actual secondary current when

Page 21: CT & PT basic

Copy Right YMPL 41

Phase displacement or error

• The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer.

• The phase displacement is said to be positive when the secondary current vector leads the primary current vector.

(this definition is correct for sinusoidal currents)

Copy Right YMPL 42

Design aspects….principleWith the secondary winding connected to the burden, and a primary current (Ip) flowing, an alternating flux is induced in the core. This flux is of just sufficient value to induce in the secondarywinding an emf (Es), again of just sufficient magnitude to pass a secondary current (Is), through the secondary circuit such that there is absolute equality between the primary and secondary ampere turns and precise phase opposition between them.

Np. Ip =Ns. Is

where Np = No. of primary turns.Ns = No. of secondary turns.

• Because of primary exciting current ampere-turns all primary ampere-turns shall not be transferred to secondary ampere-turns which results into ratio and phase angle error.

Page 22: CT & PT basic

Copy Right YMPL 43

Design aspects….principle

• Errors of a current transformer are directly propotional to 1) mean length of magnetic core 2) sum of external and internal burden

and inversly propotional to 1) ampere turns2) number of turns in secondary3) effective area of magnetic core.

Copy Right YMPL 44

Calculation of errors

RATIO ERROR. (Im Sin δ + Iw Cos δ)

% R. E. = - 100----------------------------------------Ip

PHASE ANGLE ERROR

Im Cos δ – Iw Sin δ P. E. = 3438 ------------------------------------ minutes

IpWHERE,

δ = OVERALL SECONDARY POWER FACTOR ANGLE. Im = MAGNETIZING COMPONENT OF EXCITING CURRENT. Iw = WATT-LOSS COMPONENT OF EXCITING CURRENT . Ip = ACTUAL PRIMARY CURRENT.

Page 23: CT & PT basic

Copy Right YMPL 45

Some facts about CTs…

• In CT , primary current is controlled by the load of the circuit in which it is connected in series, not by CT itself.

• CT is a variable flux device, unlike PT , which is a constant flux device.

• This results in variation in ratio and phase errors , over the dynamic range, which is large,

• The dynamic range may be 1 % to 120 % of rated current.

• For a given design, the Im and Ie becomes larger proportion of the current, lower the current , leading to increased ratio and phase errors.

Copy Right YMPL 46

• CTs must be designed with cores which have lower excitation current ( Im and Ie).

• Higher permeability.• Short magnetic path• Larger cross section• Lower flux density

• Generally class 0.5 CTS use GOSS material, class 0.2 uses mumetal.

• Most common type of construction uses tape wound toroids .

Page 24: CT & PT basic

Copy Right YMPL 47

Some facts about CTs…

• Primary and secondary windings need to be tightly coupled.

• Turns compensation is invariably used, actual turns ratio is less than rated ratio.

• Secondary is generally wound with multiple insulated parallel wires,

• CT secondary shall never be open circuited, when primary is energised, it produces very high voltages, which may be fatal, as well cause damage to equipments.

• Generally below 100 A, primary wound type of CTs are used,

Copy Right YMPL 48

Some facts about CTs…

• CTs may saturate due to• Open secondary winding.• Due to DC current in secondary, like for testing

resistance using a multimeter.• Due to short circuit fault

• A saturated core will have lower permeability, thus increased ratio and Phase errors.

• Such CTs shall be demagnetised, • Lower the burden in VA , lower the error.• For practical purposes , variation of errors due to

change of frequency can be ignored.

Page 25: CT & PT basic

Copy Right YMPL 49

Limits of errors.

AS PER IS 2705/1992 AND IEC 60044.1/1996.

6060901801.01.01.53.01.0

303045900.50.50.751.50.5

101015300.20.20.350.750.2

558150.10.10.20.40.1

120100205120100205

PHASE DISPLACEMENT IN MINUTES % OF RATED CURRENT

+ % CURRENT (RATIO) ERROR AT % of RATED CURRENT.

ACCURACY CLASS

Copy Right YMPL 50

Effect of load power factor on phase error

LAGGING POWER FACTOR PHASE ERROR TO CAUSE + 0.1% INCREASE IN INSTRUMENT READING

0.5 2.0 MINUTES LEADING 0.6 2.6 MINUTES LEADING0.7 3.4 MINUTES LEADING0.8 4.5 MINUTES LEADING0.9 7.0 MINUTES LEADING0.95 9.5 MINUTES LEADING

Page 26: CT & PT basic

Copy Right YMPL 51

Testing of current transformers

• Absolute or bridge methods.. Not much used these days.• Comparision Method: or Primary injection method: This is a

modification of the basic bridge methods. Instead of reading theprimary and secondary currents, the secondary current of a calibrated ‘standard’ / reference current transformer and the transformer undergoing test are compared with each other by nulling method using a bridge , or direct error computation. The comparision method , of course requires the use of a ‘Standard ‘current transformer with a rating which corresponds to that ofeach current transformer involved in calibrating.

• We require a source, reference CT, calibrated burden set, and a comparator.

• Reference temp specified is 40 deg C

Copy Right YMPL 52

Page 27: CT & PT basic

Copy Right YMPL 53

Metering Units.• Metering units for 11 and 33 KV are

combined CT /PT units, where three CTs and two or three CTS are built in one casing. Generally oil is used as insulation and are completely sealed units.

Copy Right YMPL 54

Metering unit circuit

Page 28: CT & PT basic

Copy Right YMPL 55

11 KV metering unit… oil filled , out door type