Www2.Schneider-electric.com Documents Technical-publications en Shared Electrical-Engineering Breaking-techniques-switchgear General-knowledge Ect170

n° 170from currenttransformers tohybrid sensors,in HV

E/CT 170 first issued, march 1995

photographiephotographie

Christian Teyssandier

In 1962 he joined Merlin Gerin,going on to graduate from theInstitut National Polytechniquede Grenoble (INPG) in 1967.Working first on magnetismproblems for linear motors until1974, he then held a variety ofpositions of technicalresponsibility in the HV, MV andLV power capacitor field.In 1983 he was appointed projectmanager for the MV instrumenttransformer activity.In 1992 he joined theAnticipation section of MerlinGerin's Medium VoltageDivision.

Cahier Technique Merlin Gerin n° 170 / p.2

glossary

birefringence:materials with a refraction coefficientdepending on propagation direction,polarisation status and light wavefrequency, are said to be anisotropic orbirefringent.

EMC:electromagnetic compatibility: this isthe capacity of a device to operatecorrectly in its electromagneticenvironment without generatingintolerable disturbances for theequipment placed in this environment(see «Cahier Technique» n° 149)

hybrid sensor:current or voltage sensor comprising atleast one element sensitive to the valueto be measured, coupled to an electronicsystem sending a secondary signal(current or voltage), which reflects themodule and phase of the primary value.

RemarkVoltage levels enter a variety of classifications according to decrees, standards andother more specific specifications such as those of certain energy distributors, e.g.as regards AC voltages above 1 000 V:

c French decree of November 14th 1988 defines two voltage ranges:HVA = 1 kV < U i 50 kV,HVB = U > 50 kV.

c the CENELEC (European Electrotechnical Standardisation Committee) states inits circular of July 27th 1992:MV = 1 kV < U i 35 kV,HV = U > 35 kV.

c publication IEC 71 specifies the highest voltage ranges for equipment:range A = 1 kV < U < 52 kV,range B = 52 kV i U < 300 kV,range C = U u 300 kV.A revised edition, which keeps only two ranges, is scheduled:range l = 1 kV < U i 245 kV,range ll = U > 245 kV.

c the French electrical power distributor, EDF, currently uses the classification of thedecree quoted above.


1. Introduction p. 4

2. General Sensor functions p. 4

Sensor evolution p. 5

The values to be measured p. 5

The various types of currentsensors p. 6

Standardisation p. 7

3. Current transformers Use p. 9

Standards p. 9

Specification of a CT p. 10

Special applications p. 12

EMC behaviour p. 13

A special risk p. 13

4. Rogowski coil current sensor Operation p. 14

Standards p. 15

Steady and

transient state operation p. 15

Specification of Rogowski coilcurrent sensors p. 16

EMC behaviour p. 16

5. Hybrid sensors Faraday effect optical sensors p. 17

Hall effect current sensors p. 19

Zero flux current sensors p. 20

6. Comparison table, synthesis p. 21

7. Conclusion and future Present solutions p. 22

Future solutions p. 22

Appendix 1: CT accuracy as in IEC 185 p. 23

Appendix 2: CT classification as in IEC 44-6 p. 24

Appendix 3: bibliography p. 24

Technical and technological changes inthe protection and control/monitoringequipment used in electrical powerdistribution networks must be mirroredby parallel changes in their informationsources, i.e. in current and voltagesensors.This «Cahier Technique» mainly dealswith current sensors for mediumvoltage applications.Following a few reminders oninformation needs and on currenttransformers, this document thenpresents the new hybrid sensors, withparticular emphasis on those based ona Rogowski coil. It points out both theadvantages and disadvantages ofthese solutions according to their fieldof application.

from current transformersto hybrid sensors, in HV

content


c voltage level (low voltage - LV, highvoltage - HVA and HVB-);c transient evolutions of these valueslinked to status changes occurringnaturally or accidentally in operation ofelectrical networks.

A number of physical phenomena canbe used to measure AC currents.These methods result in levels ofperformance which have a varyingdegree of compatibility with those

required by the various protection,operation and safety levels sought.Evaluation of sensor performance isvital to ensure their best possiblespecification for installation on anetwork. This requires knowledge ofhow the different types of sensors work.

networks to which they are connectedby means of the measuring sensors.

Separating the power network fromthe measuring, protection andcontrol/monitoring networkElectrical networks are affected bystrong electrical and electromagneticdisturbances, particularly severe in highvoltage substations. Thesedisturbances are the result ofswitchgear operations (disconnectors,switches, circuit-breakers andcontactors), of the atmosphericdischarges to which overhead lines areexposed and of the appearance anddisappearance of faults on theoperating networks.These disturbances are locally andtemporarily superimposed on ratedcurrent and voltage values, thuscausing disturbances.Transmission of these disturbances tothe sensor secondary must becompatible with the insulation and inputimpedance levels of the measuring,protection and control/monitoringequipment. The level of thistransmission depends on a galvanicinsulation of varying quality between

sensor functionsSensors have three main functions:c providing a correct image, asaccurate as possible, of the electricalvalue to be measured;c isolating the power networks from themeasuring, protection and control/monitoring networks;c ensuring either interchangeabilitybetween the measuring, protection andcontrol/monitoring units or performingone specific function of these units.

Providing a correct and accurateimageBased on the two characteristic valuesof all electrical networks, i.e. currentand voltage, the measuring, protectionand control/monitoring equipmentdefines a certain number of parameterssuch as: cos ϕ, threshold overshooting,instantaneous power,...Thus, there are a number of reasons(financial, safety, operatingdependability) why the signals sent bythe sensors supplying this equipmentmust be correct and accurate:c correctA sensor is correct if it gives, inspecified conditions, a signal x2 which

is identical, in a ratio of a measuringfactor, to the one to be measured.x1 = k x2where k = measuring factor.

c accurateA sensor is accurate if the measuringfactor k is not dependent on time andusage conditions, provided the latterremain within the specified values:if at t1 x1 = k1 x2and at t2 x1 = k2 x2and if k1 ≠ k2,then the measuring sensor is notaccurate.Further on in this «Cahier Technique»,examples will be given of sensorswhich are neither correct nor accuratein certain operating conditions,particularly in transient states, whichdiffer from specified conditions.The winding ratio, or more generally themeasuring factor, is used to adapt thesignal to be measured to theperformances of the device measuring,analysing and processing this signal.The measuring, protection and control/monitoring equipment, which uses lowlevel input values, cannot accept thedisturbances existing on the power

1. introduction

Proper operation and safety ofelectrical power distribution networks,from the electrical power station rightthrough to the point of use, is ensuredby protection and control/monitoringequipment. This equipment requirespermanent knowledge of the twofundamental electrical values, namelycurrent l and voltage U.The knowledge of these values hasmany angles:c type of current (AC or DC);

2. general


In certain cases, however, particularlyfor differential protections (zone,busbar, transformer,....), sensors musthave at least two outputs.These developments have not yet beenfully taken into consideration by sensorstandards, thus meaning that full usecannot be made of all the advantagesoffered by microprocessor technologiesand sensors developed in recent years.However, measuring, protection andcontrol/monitoring units benefiting fromthese developments are available onthe market, particularly in HVA andHVB. This equipment is associated withspecific sensors ensuring the bestmatch possible between the powernetwork and the control and monitoringunit. These sensors can only be usedwith the measuring, protection andcontrol/monitoring equipment for whichthey were designed.

sensor evolutionBesides the fact that modernequipment is less energy consuming,sensor evolution in recent years isabove all related to three requirements:c reliabilityMotivated by permanent search forcontinuity of service and limitation ofexternal effects in the case of incident.c correctness and accuracyEvolution of network equipment,particularly in HVB with the emergenceof gas insulated devices, and search forcontinuity of service, have resulted indevelopment of linear or linearisedsensors. These sensors ensureprotection and control/monitoringsystems can take efficient action intransient states.c costProper network operation to increasecontinuity of service, requiresknowledge of network characteristicvalues in a large number of points. Thisresults in an increasingly large numberof sensors being used. Sensor cost isthus an important factor.

the values to be measuredVoltage levelThis is an important characteristic ofthe network on which the sensors willbe installed. This voltage leveldetermines the dielectric stresses usedto determine sensor size.

Standard IEC 71 defines the highestvoltage for equipment, Um, as themaximum value of phase to phasevoltage that the network can assume.The power frequency short durationwithstand, the impulse voltagewithstand, 1.2/50 and, whereapplicable, switching impulse andbroken wave withstand are associatedwith Um both in this publication and inpublications on measuring sensors.This publication divides standard Umvalues into three ranges:c range A: from 1 kV to under 52 kV,c range B: from 52 kV to under 300 kV,c range C: from 300 kV and over.The remainder of this «CahierTechnique» chiefly concentrates onsensors for range A networks.

Value typesManagement, monitoring, protectionand remote control of all electricalnetwork types require use of the twovalues, voltage and current,characteristic of all electrical circuits.

c voltageThe rated value of an electrical HVAnetwork varies from a few hundredvolts to a few tens of thousand volts.Fault voltages are generally low andoften close to zero.

c currentRated value varies from a few amperesto a few thousand amperes. Faultcurrents can reach several tens ofkiloamperes.Only current sensors will be presentedin this «Cahier Technique», as theyaccount for the majority of sensorsdetecting electrical values in networks.Their economic influence is greatest inHVA... and must therefore beminimised.This requires:c firstly, knowledge of requiredperformances for the application to dealwith and then specify theseperformances as accuratly as possible;c secondly, knowledge of the operationand evaluation of the performances ofthe various current sensor types.

Using the valuesFor network operation, thesecharacteristic values (voltage andcurrent) are used by a variety ofequipment. Knowledge of thisequipment makes it possible to specify

the sensor primary and secondarycircuits. The transmission factor is afunction of:c sensor production technologies;c and the physical phenomena chosen(to perform the measurement),some of which result in a virtually zerofactor.Galvanic insulation therefore plays avital role in EMC (Electromagneticcompatibility) behaviour of the varioussensor types.The EMC behaviour of each type ofsensor presented in this «CahierTechnique» will be examined.Behaviour also depends on theassociated equipment since sensorsare used to adapt the variousmeasuring, protection and control/monitoring equipment to the powernetwork.

Interchangeability and satisfactionof specific functions of the devices«supplied» by the sensorsMeasuring, protection and control/monitoring equipment underwent majorchanges in the nineteen eighties and isstill being developed today. As a result,three different technologies can befound in the same electricalswitchboard.

c electromechanicalThis is the oldest technology, using theelectromagnetic effects of the electricalvalues. This means that sensors mustsupply a very high energy, around15 VA in normal operation andreaching, in certain specific cases,3 400 VA on occurrence of a fault onthe sensor primary circuit.

c analog electronicsThis more recent technology emergedwith the intensive development ofsemiconductors. This equipmentrequires far less energy, around 1 VA innormal operation and 225 VA on afault. A number of protection relays canthen be connected onto the samesensor output.

c digital electronics

This microprocessor based technology,which is the most modern, is still beingdeveloped. The energy required is verylow, around mVA (0.001 VA) in normaloperation and 0.25 VA on a fault.Consequently, sensors may generallyonly have one very low power outputsufficient to supply their associatedprotection and control/monitoring unit.


the characteristics of the current sensorsecondary.

c measuring instrumentsv switchboard indicators: not veryaccurate, they are used to read thevalue of the measured values. Notethat pointer type display is increasinglybeing replaced by digital display builtinto the control and monitoring unit.

These instruments are ammeters,voltmeters, wattmeters, frequencymeters, etc... From class 1.5 to 3, theirassociation with a class 1 sensor isgenerally sufficient for the function tobe performed.v meters and recorders: used either forbilling by energy distributors or forbreakdown of power consumptionbetween workshops belonging to thesame consumer. Their accuracy isgenerally greater than the indicatorsdescribed above. For billing purposes,the associated sensor is generallyclass 0.5. For power consumptionbreakdown, the accuracy required islower, and a class 1 sensor is generallysufficient.

c protection relaysOf modular type, each element has aclearly defined function. Theseelements are often combined to monitora part of the electrical network (seefig. 1). The best known elementaryfunctions are the following protections:v phase overcurrent(overload or short-circuit),v zero sequence overcurrent,v current directional (phase and zerosequence),v zone differential,v active reverse power,v reactive reverse power.

c protection and control/monitoringunitsThese are incorporated, configurablemicroprocessor based units (see fig. 2).These units combine, in the cubicles(level 1 of an electrical networktechnical management system), in avery small space, the various functionsrequired to operate electrical networks,namely:v measurements,v protection devices,v automatic controls,v communications to higher levels 2(substation) or 3 (operating station).

Note that high currents and voltagesmust be avoided on the input circuits tothis equipment. The function of thecurrent and voltage sensors is to adaptsignal level to these input circuits(measurement and/or protection).

the various current sensortypesCurrent sensors fall into three mainfamilies:c transformers,c specific sensors,c hybrid sensors.

Current transformer - CT -This sensor has two electrical circuits, aprimary and a secondary, and amagnetic circuit. It supplies asecondary signal of the same type asthe primary value to be measured: it isa current source. Although it is notlinear and its operating range is limitedby magnetic saturation phenomena, itis at present the most commonly usedsensor type in HVA and HVB.

A CT may contain several secondaries,each used for a precise function,measurement or protection.

c «measurement» secondaryIt has a very narrow range of accuracy,generally limited to currents equal to orless than the rated primary current.

c «protection» secondaryIts range of accuracy is very large,often one to twenty times rated primarycurrent. The design of this secondaryvaries considerably according to the

operating mode, steady state ortransient state, for which it is intended.Note that the operating range of a CT isgenerally far more extensive than itsrange of accuracy, since it must allowfor short-circuit current.

Specific sensor - SS - or Rogowskicoil sensorRogowski defined the principle of thissensor in 1912. It differs from previousdesigns by the fact that it contains noferromagnetic materials, thus ensuringa perfect linearity in a wide currentrange, a linearity unaffected by thevarious frequencies present on HVAand HVB networks. This sensor typecombined with a load impedance Z ofhigh value (≈ 10 kΩ at 50 Hz) is avoltage source (see fig. 3).

Hybrid sensors - HS -The definition of a hybrid sensor givenin the glossary covers several types ofsensors. Only the best known typesand those most commonly used in HVAand HVB are described below.

c optical current sensorIts sensing element is either an opticalfiber or an optical crystal. In both cases,Faraday's principle, discovered in 1845by the physicist of the same name, isused.

c zero flux transformersIn this type of HS, the sensing elementis a CT in which the flux created by theprimary is cancelled for each secondarywinding by an auxilary winding (seefig. 4). This cancels the distortioncaused by saturation but only in a

fig. 2: protection and control/monitoring unit(SEPAM 2000 - Merlin Gerin).

fig. 1: modular type protection relay(Vigirack - Merlin Gerin).


fig. 3: schematic diagram of a SS.

limited operating range (current andfrequency).

c Hall effect current sensorIts sensing element is a Hall cell (seefig. 5), which enables both AC and DCcurrents to be measured. As itgenerally uses a magnetic circuit toincrease its sensing capacity, it isaffected by saturation phenomena, justlike a CT.

standardizationExisting standards, both national andinternational, only cover CTs (currenttransformers). Work is currently inprogress to draw up standards forhybrid sensors (HSs). However, thereare as yet no plans to deal with specificsensors.

National standardsIn the EEC, national standards of thedifferent countries are currently beingharmonised by CENELEC on the basisof the international standards edited bythe IEC.

International standardsThe IEC, via the Technical CommitteeN° 38 draws up the standards forcurrent and voltage sensors.

AccuracyOn the basis of current CT standards, acertain number of generic terms can bedefined for application to all sensortypes. In this standard they are referredto as «accuracy» and are used tospecify and evaluate the performancesand fields of application of currentsensors.

c theoretical winding ratioRatio between the rated rms values ofthe primary and secondary values,generally referred to as Kn. For CTs itis a number without dimension. ForSSs and HSs it is often expressed inAmperes (primary) /Volts (secondary).

c errorAll sensors have imperfections whichintroduce distorsions in the restorationof the secondary signal. There arethree error types in AC current:v ratio error: expressed as apercentage, it is calculated from thedifference between real and theoreticalwinding ratios (see appendix 1);v phase error: normally expressed inangular minutes, it gives, to the

Z

secondary winding (fine wiring)

mean toroid radius

support radius

non-ferromagnetic support

output voltage

I

u = f ( )dI

dt

I1 = current to be measured,I2 = secondary circuit current,CM = magnetic circuit,Z = load impedance,generally low,A = current amplifier,ES = secondary winding,SD = zero flux detectionwinding controllingamplifier A.

I = current to be measured,

Z

I1

A

CM

SD

ES

I2

fig. 4: schematic diagram of a zero flux CT.

I = current to be measured,CM = magnetic circuit,CH = Hall cell,i = cell supply current,Vh = Hall voltage, proportionalto i and I,A = voltage or currentamplifier.

CM

CH

i

I

Vh +

-A

fig. 5: schematic diagram of a Hall effect current sensor.


nearest π (CT) or π/2 (SS), the phasedisplacement existing between thevector of the primary value and thevector of the secondary value (seeappendix 1);v composite error: expressed as apercentage of the rms value of theprimary current, in steady state it is therms value of the difference between:- the instantaneous value of the primarycurrent,

- and the product of the rated windingratio by the instantaneous value of thesecondary current.

c accuracy classThe accuracy class defines for currentsensors the maximum error limits (ratioand phase errors) in specifiedconditions.

c accuracy loadExpressed in ohms, with a specifiedpower factor, it is the value of the

impedance connected to the sensorsecondary terminals on which theaccuracy conditions are based.

c accuracy output powerExpressed in VA, it is the apparentpower that the sensor can supply at itsaccuracy load when the rated primarycurrent flows through it.


standardsSteady stateCTs having to operate in this state mustmeet international, European andnational standards.

c international standardsIEC 185, second edition of 1987currently being revised by CE 38: itconcerns class P protection andinstrument CTs (see appendix 2).

The main purpose of this revision is toremodel clauses on dielectriccharacteristics and to add a certainnumber of measures concerning onlyCTs for HVB such as mechanicalforces on connections, radioelectricinterference.

c European standardsThese standards, edited by CENELEC,are based on IEC documents. In 1993there were not yet any EN documentsfor CTs. Only the harmonisationdocuments (HD) are under discussion.

c National standardsThe different European standards varyconsiderably from each other.However, in the future they should bemore similar since they will be made toconform to the EN standard on CTs forsteady state operation.v FranceStandard NF C 42-502 (February 1974)complies with IEC documents invirtually all aspects, and will complywith EN standards completely exceptfor the ways of marking secondaryterminals.

Note :Standard NF C 42-502 states that thesecondary terminal connected to theearth is always marked S2. It is also theterminal common to all ratios in thecase of CTs with several winding ratiosobtained by connections to thesecondary winding. Moreover, thesame standard states that windingsused for measurement must have anodd number and windings used forprotection an even number.

v Great BritainStandard BS 3938 (February 1973)closely resembles the IEC standardand will virtually comply with theEN document. Moreover, it contains theclass X windings for protection. Thistype allows a more accuratespecification of protection windings.This class may be included in the ENEuropean standards in the near future.v ItalyStandard CEI 1008 (October 1987)(Comitato Electronico Italiano) complieswith IEC 185. It will also comply withthe EN standard when it is published.v USAStandard ANSI/IEEE C57 13 (1978)differs considerably from IEC 185 andEuropean standards:- accuracy classes and powers are notdefined in the same terms,- terminal marking is very different,- and the devices frequently take upmore space.

Transient stateMajor energy distributors have longsince possessed companyspecifications for CT transient stateoperation.

These specifications were and still are,met by special manufacturingprocesses and are the subject of directagreements between manufacturersand users.

c international standardsSpecifications concerning CTs forprotection, for a transient stateresponse, are now stipulated atinternational level by standard IEC 44-6(first edition 1992-03).

c European and national standardsThese standards do not yet exist.The European standard, currently beingdrawn up, will closely resembleIEC 44-6. National standards will beedited by the various organisations onthe basis of the EN document.

3. current transformer

Its principle (summarised in chapter 2)gives it properties which, althoughadvantageous, also present problemsin some cases for network operation.

A technical description of this currenttransformer, including its behaviour, isgiven in detail in «Cahier Technique»N° 164 which deals mainly with theproblems relating to operation ofelectrical installations in steady state.

useIn HVB, emergence of gas insulatedmetalclad equipment and the searchfor permanent dynamic stability ofnetworks containing high powergenerators make it necessary toconsider operation when the networkchanges status (transient state).

Saturation and hysteresis, with nomajor oversizing of CT magneticcores, mean that the transient stateresponse of this sensor type is neithercorrect nor accurate. In general, onlyat the end of the transient state can acorrect response be obtained. Thisdelay, in certain operating and faultcases, is not always compatible withsafety of equipment and persons.Sometimes the fault needs to bedetected during the first period of thetransient state which, in certainnetwork types, can last 200 ms (i.e.10 periods).

Correctness and accuracy are alsonecessary in transient states:c in HVB, for equipment located nearhigh power stations and on busbars ofmajor substations;c in HVA, near sources, when a highpower HVB network is supplied eitherby a transformer with a high windingratio (e.g. 220 kV/20 or 36 kV) or bygenerators with a very high unit power.

Current standards make it possible, forCTs and in both operating cases(steady and transient states), inspecified conditions, to accuratelyevaluate the performances of thisequipment.


For switchboard devices, class 1 isgenerally more than enough. Thissecondary is normally referred to asfollows:

10 VA Cl1 FS < 10

v protectionThe protection windings can bespecified in two ways:

- As in IEC 185 and Europeanstandards: by specifying the ratedoutput (in VA), the accuracy class (5Por 10P) and the accuracy limit factor(ALF).

The accuracy class gives the maximumcomposite error allowed on thesecondary current for a primary currentequal to ALF times the rated primarycurrent (5P = 5%, 10P = 10%). Thecharacteristics and constraintsassociated with the various accuracyclasses are given in the variousstandards.

The windings are then referred to asfollows:

10 VA 5P 10

- As in BS 3938: by specifying the valuein volts of the knee point voltage (Vk),the maximum winding resistance (Rct)and, if necessary, the maximumexciting current (Io) for the voltage Vk.In this case, the windings are referredto as follows.

0,050 150 R 0,50

CT imperfectionsMagnetic imperfections (saturation,remanence, eddy current andhysteresis losses) generateinaccuracies in the CTs: ratio andphase errors, imperfect linearity,

specification of a CTThe various people involvedThe user, the network designer, theprotection system manufacturer and theCT manufacturer all have different rolesto play, at different levels, in CTspecification.

c the network designer, for operatingsafety reasons, tends to increase thesizing factors linked to the CT primary:v short time thermal and dynamiccurrent withstand represented by theroot mean square (rms) and peakvalues of the short-circuit currents to bewithstood for a period generally of1 second.v length of assymetrical (unbalanced)state by giving overestimated X/R ratioand time constants.

c the relay manufacturer who, also forequipment operating safety reasons,tends to specify high secondaryperformances.v accuracy level power byoverestimating the value of the coupledimpedances between relay and CT;v accuracy class by asking the CT notto introduce any additional errors intothe unit. Whereas it would perhaps bemore advantageous to use slightlymore accurate equipment and slightlyless accurate CTs to obtain an identicaloverall accuracy.

Example: a class 3 measuringinstrument with a class 0.5 CT gives anoverall accuracy of class 3.5. In certaincases, it is more advantageous(financially) to take a class 2 instrumentand a class 1 CT which give an overallaccuracy of class 3. This is particularlytrue in the case of small primarycurrents and a high short-circuitcurrent.

c the CT manufacturer who tries toreconcile the various requirements aswell as his own, in order to meet thedemand. The I th / In ratio (short timethermal withstand 1s / rated primary I)gives a good idea of CT feasibility,irrespective of the secondaryperformances required. For example:v I th / In i 100: the CT obtained can beconsidered standard with normalsecondary performances.v 100 < I th / In i 400: the CT meeting

this specification is a CT whosefeasibility is studied individually: itssecondary performances are reduced.v I th / In > 400: this CT is not alwaysfeasible. When it is, its secondaryperformances are very poor.

The values to be specifiedA number of values must be specifiedto make a CT. Some of these valuesare standardised (refer to the standardsquoted in the «standards» paragraph).For CTs needing a specified accuracyin the transient state, the reference iseither standard IEC 44-6 or companyspecifications.

The following list only concerns CTsoperating in steady state.

c primaryv insulation level defined by threevoltages, the highest network voltage(Um), the rated time power frequencywithstand voltage and the lightningimpulse withstand voltage;v the rated short time thermal current(I th) and its its duration if it differs from1 s;v the rated dynamic current (Idy) if itspeak value differs from 2.5 I th;v the rated primary current.Rules for the profession stipulate thatthe rated current of the network onwhich a CT is installed, be between 40and 100% the rated primary current ofthe CT.

c secondaryThe function, measurement orprotection, of the secondary must bespecified and leads to varyingconstraints and specifications. In bothcases, the rated secondary currentmust be specified (1 or 5 A).v measurementThe rated output power (in VA), theaccuracy class and the maximumsafety factor (SF), normally between 5and 10 and very exceptionally lessthan 5, must be specified.

Note: The safety factor is the ratiobetween the primary current for whichthe winding ratio error is greater than orequal to 10%, and the rated primarycurrent.

The various accuracy classes and theresulting constraints are given in thestandards.

resistance Rct in Ohmsvoltage Vk in Voltscurrent Io in Amperes

accuracy class = 1safety factor < 10

rated output = 10 VA

accuracy limit factor = 10

accuracy class = 5Paccuracy level power = 10 VA


response depending on previoussituations.... Other imperfections arelinked to the electromagnetic andelectrical environment of the CT.

c magnetic phenomenaSaturation and hysteresis are the twomain causes of disturbance: the outputsignal of a saturated CT. Output sendsa signal which is no longer sinusoidal,and its accuracy can no longer beguaranteed (highly amplified ratio andphase errors).

These phenomena appear:v in transient state, for example closingof a circuit on a fault with or without DCcomponent: the state of saturationreached depends on the initialmagnetic state of the magnetic circuit(degree of residual induction present);v in short-circuit steady state if thevalue of this circuit is greater than ALFtimes the rated primary current;v when the value of the load, to whichthe CT is connected, is greater than itsrated burden, as is the case for verylong connections or in case of additionof equipment to the load circuit of asecondary winding;v if network frequency is less than ratedfrequency: use in 50 Hz of CTs with arated frequency of 60 Hz causes a 20%induction increase; on the other hand,use in 60 Hz of CTs with a ratedfrequency of 50 Hz presents no risks.

Operation in saturated state must notbe allowed to continue, as saturationcauses abnormal overheating the CTcomponents:v in the magnetic circuit, since eddycurrent and hysteresis losses increase;v in the secondary winding, since thecurrents, although highly deformed, arealso very high.

c external phenomenav positions of the primary conductorand of the adjacent conductorsTheir respective geometries andpositions may have a considerableeffect on the accuracy of instrumenttransformers as a result of thenonlinearity of the ferromagneticmaterials. A typical case is of currenttransformers installed in a loop (seefig. 6) or installed in staggered form in abusbar (see fig. 7). These twoassemblies cause a localised increase

in induction, thus introducing an error.v reclosing cycleAfter a primary short-circuit current hasbeen broken, return to the remanencevalue of the induction in the CTmagnetic core is not instantaneous.

This induction decreases according toan exponential law with a time constantT2. Depending on the secondary circuit,this constant is generally between oneand three seconds. When rapidreclosing occurs, a residual induction is

fig. 7: diagram showing three transverse CTs installed in staggered form in a busbar

The return or adjacent phaseconductor (PR) creates a disturbingmagnetic field in the magnetic circuit(CM); this field is vectorially addedto the one created by the current I1to be measured of the conductorcrossing it normally (PA). Thisvectorial addition results inincreased induction in zone A.This induction increase depends on:- the current flowing in the disturbingconductor,- the distance between the magneticcircuit and this disturbing current.It then results in local saturationswhich increase the value of theexciting current (Ie), thus introducingerrors.

fig. 6: diagram showing a transverse CT with a looped primary circuit.

I1

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PR PA

A A

AA cross sectionBPA→

B→

BPR→

BPA→

B→

increased induction zone A

CM

BPR→

@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À@À


TC1TC2

TC3

TC1

TC3

TC2I1

I2

I3

increased induction zones


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therefore present in the CT magneticcircuit which is vectorially added to theinduction created by the current beingformed (see fig. 8). If both inductionshave the same sign and if the CT hasnot been designed to guarantee a givenaccuracy in the transient state, it ishighly likely that the secondary signaldelivered by this CT will be totallyunlike the primary current flowingthrough it.

c precautions to be taken with CTsv in steady state:- CTs must be designed for theirintended purpose.- the sum of the input impedances of allthe relays and/or measuringinstruments, to which the value of thewiring impedance must be added, mustbe less than or at the most equal torated burden. This impedance isobtained by dividing the rated output bythe square of the rated secondarycurrent.- installation conditions should not causehigh local saturation. It shall be doingaway with staggered installations (seefig. 7)v in transient state, for protectionsecondaries only- in the general case of constant timeprotections, to allow for all or part of thehysteresis phenomena, it is sufficient tocheck that the value of the interventionsetting current (of the protection)divided by the value of the CT ratedsecondary current, is less by twice theaccuracy limit factor of the secondary inquestion.- for time dependent protection devices(differential, zero sequence,...), ensurethat the CT specification complies withthe recommendations of the relaymanufacturer- if an accurate response is requiredduring this operating period, CTs mustbe specified and designed inaccordance with the various classesdefined in IEC 44-6 (see appendix 1).This standard always leads toconsiderable oversizing of the CTs.The need for a low remanent flux (caseof reclosings) results in use of magneticcircuit with airgap. This is how thelinearised CTs are obtained (see TPZin standard IEC 44-6).

t

t

t

Ip

Is

B

(primary)

(secondary)

(induction)

fig. 8: evolution of currents and induction in an unsaturated CT.

Measurement of zero sequencecurrent ( I o)

This is the current resulting from thevectorial sum of the three phasecurrents of a three-phase circuit. Thissum can be achieved in two ways.

c by adding up the secondary currentsof three CTs (Nicholson assembly).

For this, CTs with the same windingratio must be used, and the primaryand secondary connections mustrespect the polarities (winding direction)of the various primary and secondarywindings (see fig. 9).

Two phenomena limit the detectionthresholds in this method:v in steady state, the differences in

special applicationsMeasurement of residual currentsProtection of persons in LV distributionnetworks is frequently ensured bymonitoring residual current value. Thisprotection, generally provided by adevice incorporated in the LV circuit-breaker, is often autonomous: itsoperating energy is supplied by the CTdetecting the residual currents.Stipulated CT performances generallycall for use of ferromagnetic materialswith excellent relative permeability (µr)using nickel, which raises their cost.There is a quick method of sizing thisCT type (see appendix 3, bibliography:paper in the RGE review n° 4).


phase and winding ratio error meanthat the vectorial sum is not zero. Thisresults in a «false zero sequencecurrent» which may not be compatiblewith the required thresholds.

Pairing of CTs (in phase and module)enables practical thresholds to belowered:v in transient state, saturation andhysteresis of magnetic circuits generatethe same fault. Oversizing of the CTsdelays the moment of appearance ofthis phenomenon.

These solutions (pairing andoversizing) do not generally allowdetection of a current Io less than 6% ofthe phase currents.

c by adding up the fluxesTo avoid the inaccuracy of this firstmethod and find a way round itsconstraints, the Io currents can bemeasured using a single toroid CT or«toroid»: the three phase currents I1, I2,I3 of the three-phase network (seefig. 10) flow through its magnetic circuit.With a suitable design (ferromagneticmaterial, dimension and accuracy load)and taking certain toroid installationprecautions (grouping and centering ofcables, use of a ferromagnetic sleeve ifnecessary), this method enablesmeasurement of very low Io currentvalues with a high degree of accuracy(module error around 1% and phaseerror less than 60 angular minutes): afew hundred mA in HVA and adozen mA in LV.

c fault detectionIn HVA distribution networks, use offault detectors facilitates rapid faultlocation, thus minimising the part of thenetwork not supplied and reducingoutage time.

There are two possible means ofsignalling the fault current detected bythese devices:v using mechanical or electricalindicator lights placed at points easy toreach by operating staff (case of MV/LVsubstations in underground rural andurban networks).v by remote transmission to theoperating centre for fault detectorsplaced on remote-controlled switches ofpublic distribution networks.

These fault detectors are supplied byCTs for which no standards exist. Onlythe CT-fault detector combinations arespecified by operators.

EMC behaviourIn HVA, the EMC of the CTs can besaid to be satisfactory.

In HVB the mandatory electric fielddistribution shields of varying qualitymay result in unsatisfactory EMC ofCTs.

The coupling capacity between theprimary and secondary CT windingshelps transmit disturbances from theprimary to the secondary circuit. Thevalue of this capacity depends on theCT insulating voltage, the secondarycharacteristics and the insulationtechnology used.

Some company specifications, forvoltages Um > 123 kV, give a maximumvalue for the disturbance transmissionfactor. This value is measured in astandard test described in thespecification. The introduction of thisnotion into international standards iscurrently being discussed within CE 38.

a special riskOpening the secondary circuit of a CTis dangerous.

The magnetic induction flux flowing in themagnetic core is the sum of two fluxes ofopposite signs, one due to the presenceof a primary current and the other to thepresence of the secondary current.Cancellation of the latter by opening thesecondary circuit considerably increasesflux in the core, causing a very highvoltage rise at the secondary terminals.Peak or instantaneous voltages of over5 kV can be reached which may be fatalfor persons and cause severe equipmentdamage.

Practical conclusionsc on no account must the secondarycircuit of a current transformer inoperation be opened.c before carrying out interventions onthe load of a CT in operation, a veryhigh quality short-circuit must beinstalled between its secondaryterminals.

fig. 10: measuring current using a toroid.

metalsleeve

conductorgrouping andcentering

sleeve L > 2ø toroid

toroid

L

I1 I2

I3

fig. 9: connection of three CTs to measurezero sequence current (Nicholsonassembly).

P1 P2

S1 S2

P1 P2

S1 S2

P1 P2

S1 S2

I1

I2

I3

Io


Electromagnetic componentsA SS sensor is made up of five parts(see fig. 11).c a primary winding consisting of asingle copper conductor, the crosssection of which is determined by:v a primary rated continuous thermalcurrent,v a rated short-time thermal current;c a secondary winding supportgenerally toric and made of a non-ferromagnetic material;c a secondary winding supportgenerally toric and made of a non-ferromagnetic material;c a setting resistance connected to thesecondary winding;c a magnetic shielding protecting thewinding from any disturbances linked tothe magnetic fields outside the sensor.

Dielectric componentsc dielectric insulationJust like current transformers, theprimary and secondary of SS sensorsare insulated from each other by a soliddielectric resin in HVA.

c dielectric shieldIn order to improve the system's EMCbehaviour, an earthed dielectric shieldis placed between the primary and thesecondary winding.

ModellingIt is useful to design and use a model tostudy SS operation, in the same way asfor CTs.The model proposed below only appliesto standard frequencies. For highfrequencies (several hundred kHz), thedistributed capacitances of thesecondary winding must be introducedas well as the various primary-secondary, primary-frame andsecondary-frame coupling capacities.

Equivalent diagramTwo equivalent diagrams can be drawnup:c the first one (see fig. 12) is derivedfrom the CT diagram by the presenceof an ideal transformer, where:v L = inductance value of the wiringconnecting the sensor to its load M,

4. Rogowski coil current sensor

The principle of this current sensor wasdefined by ROGOWSKI in 1912. From1986 onwards this sensor, referred toas «SS» (specific sensor), has beendeveloped in industry for HVAnetworks.

operationPhysical principleApplication of Ampere's theorem to aRogowski coil (see fig. 3) shows thatthe voltage appearing at the terminalsof a load Z of very high value is afunction of the current I = i(t).

The current I to be measured createslocally, at each turn, an inductionb = µ0 h, where µ0 is the permeabilityof vacuum, the winding support notbeing made with a ferromagneticmaterial, and h the magnetic fieldcorresponding to current I. The fluxencompassed by the entire sensor iswritten as:

ø= π r 2 bturns∑

If all the N turns have identical crosssections and if their centres are placedon the same circle of diameter R whichcan be considered as very largecompared with their own radius r, thefollowing can be written:

ø= Nπ r 2 µ0 h

and by application of Ampere'stheorem:

ø= Nπ r 2 µ0i(t)

2 πRThe electromotive force developed inthe winding is written as:

e(t) = − dødt

=Nr

2 µ0

2R

didt

If i(t) = Ι 2 sin ωt + ϕ( ) thus

di / dt = ω Ι 2 cos ωt + ϕ( )and

e(t)= −Nr

2 µ0

2R

ω Ι 2 cos ωt + ϕ( )

= −K ω Ι 2 cos ωt + ϕ( ) fig. 12: equivalent CT type diagram of a SS sensor.

N1 N2

L1

L Lf Rt Ra

Zc

i2 = N1 I 1/N2

Ι1

fig. 11: cross section of a SS sensor for HVA.

secondary winding

secondary windingsupport

magnetic shielding

primary winding

dielectric shield

dielectric insulation

settingresistance


v Lf is the winding leakage inductance.The secondary winding of the SSsensors has touching turns withexcellent distribution on the support. Itsleakage inductance is very low andneed not thus be considered.v L1 ω = magnetising impedance of theequivalent current generator,v Rt = sum of the resistances of thewinding and connection,v Ra = setting resistance,v Zc = impedance of load M at theconsidered frequency.

c the second diagram (see fig. 13)includes a voltage generator derivedfrom the theoretical study. This is thediagram that will be used hereafter.

In this equivalent diagram:

E0 (t) = − K ω Ι 2 cos ωt + ϕ( )v E0(t) is a source of voltageproportional to the primary current. Ithas a π/2 lagging phase displacementwith respect to current i(t) where

K =N r 2 µ 0

2 R, where

v the product K ω represents thetransformation ratio and is expressed inVolt per Ampere (V/A).

standardsNo national or international standardscurrently define this type of sensor.Consequently SS sensors on themarket today comply with the IEC 185standard, except for the parametersconcerning the secondary signalsupplying very specific protection andcontrol/monitoring units. These unitswith their microprocessor technology,enable, by simple parameterisation, viaa keyboard or display, all the functions(protection, measurement, automationand communication) to be performedadapted to each installation.

Note:Today these SS sensors and protectionand control/monitoring units, SEPAM,are designed and marketed by onemanufacturer only (Merlin Gerin).

steady and transient stateoperationAs SS sensors have no magnetic circuitthey are not subjected to saturation or

Error analysisc ratio and phase errorThe perfect image of the primarycurrent for a SS sensor is the vector Ewith a laggin phase displacement of π/2with respect to current I1, i.e. in phasewith E0 in figure 15. The module of thisvector is given by E = K1 I1, where K1 isthe constant representing thetransformation ratio at a givenfrequency.In the same way as for all sensors, thesecondary signal of the SS sensors hasan error. This error is defined as thevector representing the differences ofvectors E and U: it is vector ε(nat) on thediagram in figure 15.

residual flux. Their perfect linearitymeans they give at the secondary analmost perfect image of steady andtransient states of the primary.

Manufacturing tolerances on windingsupport dimensions and on the value ofthe number of turns (severalthousands) are compensated for by asetting resistance (Ra).

EquationsThe vectorial diagram (see fig. 14) isdrawn up from the equivalent diagramin figure 13. This diagram yields thefollowing equations:

U(t) = E0 (t) − (RA + Rt + jL ω) i2 (t)

avec i2 (t) = U(t) / Zc

fig. 15: vectorial diagram, with error, of the SS sensor.

= setting range linked to Raoptimum setting = ε(reg) minimun

E

L ω i(t)

ε(reg) ε(nat)

E0

UA

Ra i(t) Rt i(t)i(t)

Ι1

ε(nat) = natural error (manufacturing)ε(reg) = error after setting.

fig. 14: vectorial diagram of a SS sensor.

ϕ ϕ

Ι1

ψ

E0

U

i2 (Ra + Rt) i2

L ω i2

fig. 13: equivalent diagram with voltage generator of a SS sensor.

L Rt Ra

Zc

Lf

UEo


At the end of manufacturing it isminimised by setting potentiometer Rato a value giving a minimumerror ε→(reg) vector. This sensor settingis adapted to the current inputs of theprotection and control/monitoring unitsfor which it was designed. Themaximum value of this error vector wasset at 1% of the value of the referencevector for all primary currentsbetween 0.2 and 10 times the ratedprimary current, and at 5% for a current200 times the rated primary current.

With these maximum errors,SS sensors with the same winding ratioare all completely interchangeable andare virtually identical in accuracy terms(see fig. 16) to CTs of class:v 0.5 for measurement,v 5P for protection.

c linearityThe SS sensor is linear:

e(t) = K ω Ι 2 cos ωt + ø( )This linearity gives it many advantages,the main ones being:v the possibility of reducing windingratio variety and thus of increasingstandardisation potential. Windingratios are imposed by the electronicsdynamics of the control and monitoringunit with which the sensor is associatedand by its required discrimination level.v an excellent response in transientstate. Absence of saturation, hysteresisand residual flux means these sensorsrespond perfectly in the transient state.Consequently, with no specialprecautions, this sensor type isinstalled on networks where protectiondevices need to take rapid actionduring transient states, and in particularon networks with long time constant orcontaining gas insulated metalcladequipment (with a risk of explosion).

External influencesThe response of these SS sensors, likethat of CTs, may be affected by theenvironment in certain conditions.

c modification of the secondary load ofthis sensor type causes an errorvariation. To reduce these variations,as a SS is a voltage source, its purelyresistive load must be as high aspossible (u 10 kΩ).

c frequencyThe signal sent by this sensor typedepends on the primary currentderivative (see the paragraph on«physical principle»). Freedom from theeffects of frequency is obtained byprocessing the SS sensor signal by anaccuracy integrator amplifier.

c primary conductor positionAmpere's theorem makes no referenceto the relative position of the current(primary conductor) and of the closedcontour (secondary winding) to which itis applied. This remark indicates thatthe sensor is theoretically unaffected bythe relative positioning of itscomponents. However, imperfections inmaking of the secondary winding maymake it slightly sensitive. Therefore,when installing sensors with non-integrated primary (LV), fairly preciserelative centering and azimuthingbetween primary and secondary mustbe performed. If this precaution is nottaken, errors of around 3% may occur.

c adjacent conductor positionAn adjacent conductor, through whichthe current from another phase orreturn for a loop (see fig. 6) flows,produces a magnetic field which isvectorially added to the one created bythe current to be measured, thusmodifying sensor response. SS sensorsmust be protected against this type ofdisturbance.

specification of Rogowskicoil current sensorsSS sensors and protection and control/monitoring units are supplied by thesame manufacturer, thus making SSspecification far simpler for the end-userwho does not have to specify, as for theCTs, the characteristics of the secondary(secondary current, accuracy levelpower, accuracy class and accuracy limitfactor). He only has to specify:c the insulation level of the sensordefined as for a CT;c the rated thermal short-circuitcurrent (I th) and the dynamiccurrent (Idyn) established according tothe same rules as for the CTs;c the operating range (rated primarycurrent and rated continuous thermalcurrent). For example, there are fouroperating ranges (30-300, 160-630,160-1600, 500-2500 A) for the SSsensors produced by Merlin Gerin.

EMC behaviourSS sensors have a small primary -secondary link capacity (≈ 20 pFin HVA). The presence of dielectricand magnetic shields, connected toearth, prevents transmission ofconducted (from MV primary network)and radiated disturbances. TheSS sensor and protection and control/monitoring unit thus has good EMCbehaviour.

fig. 16: comparison of accuracy of CT and SS sensorsPoint A positions the operation of a SS sensor which meets the accuracy requirements of CTsof class 0.5 and of class 1.

0

±30'±60'

±0.5 %

±1 %0

class 1

class 0.5

class CSp

= vector ε(reg)

A

A


5. hybrid sensors

The output signals of the CTs and SSsare directly used by the protection andcontrol/monitoring units. However, thesignals of certain other current sensorsmust be processed electronically beforethey can be used by these units: theseare the hybrid sensors. Their diagramresembles the one in figure 17.

DiagramIt may contain up to six elements:c primary sensing elementUses the various effects (optical,electronic or electrical) of the materialssubjected to a magnetic field created bythe current to be measured.c primary converterConverts the effect used by the sensingelement into a signal depending on theprimary current and adapted to thetransmission system.c transmission systemConveys the signal transmitted by theprimary converter over a distance ofvarying length.c secondary converterConverts this signal, representing theprimary current, into an electrical signalwhich can be used by the protectionand control/monitoring units.c primary supplySupplies necessary energy to thesensing element, the primary converterand, if necessary, the transmissionsystem.c secondary supplySupplies energy to the secondary

converter and, if necessary, to thetransmission system.

In certain sensors, these two supplies,primary and secondary, may be thesame.

The sensing elementsHybrid sensors have undergone majordevelopments in the course of recentyears. A number of magnetic fieldeffects have been used in primarysensing elements, in particular:c optical effectsUse of the effects of the magnetic fieldon the properties of light (optical currentsensor). Optics may also be usedsolely as a transmission system from aprimary sensing element of any type.Transmission then takes place byoptical fibre. The use of devicesobeying the laws of light physics(sensing element and transmissionsystem) gives the sensor its perfectgalvanic insulation. This advantage hasbeen made use of in manydevelopment programmes, some ofwhich resulted in the Faraday effectcurrent sensor.c electronic effectsInfluence of a magnetic field on asemiconductor (Hall effect currentsensor) and on a ferromagneticmaterial (resistivity variation used inmagneto-resistant current sensors).c electrical effectsThe flux created in a magnetic circuit bythe magnetic field coming from the

current to be measured, is cancelled bya magnetic flux generated andregulated by means of an auxiliarycurrent (zero flux current transformer).

Faraday effect opticalsensorsThe laws of light physics will be brieflyreviewed below to help understandingof the following sections.

Reminders

c polarisationA phenomenon specific to wavepropagation, in particular light waves,characterised by their vibrationdirection in a given plane, known as thepropagation plane, containing thepropagation direction.When this plane keeps a direction setin time, the light waves have a linearpolarisation. If the plane rotates aroundthe propagation direction at constantspeed, polarisation is elliptical or, in avery specific case, circular.

c birefringenceCertain natural bodies exhibit thephenomenon of birefringence. A flatlight passing through them is notpropagated at the same speedaccording to whether its polarisationplane is parallel to one or the other ofthe two perpendicular directionsspecific to the birefringent body.Birefringence may be intrinsic(anisotropic materials) or induced by astress:v mechanical stress or photo-elasticeffect,v electrical stress or Kerr or Pockelselectro-optical effect,v magnetic stress or Faraday magneto-optical effect.

Faraday effectIn 1845 Michael Faraday discoveredthat the polarisation plane of polarisedlight rotates as it passes through apiece of glass placed in a strongmagnetic field and propagated parallelto this field. The polarisation rotationangle (F) is proportional to the

fig. 17: hybrid sensor diagram.

primaryconverter

secondaryconverter

primarysensingelement

transmissionsystem secondary

signal(i or u)

Ι

primarysupply

secondarysupply

primarycurrent


circulation of the magnetic field (H)along the optical path L (see fig. 18).

F = V ∫ H dLIn this equation, V is a characteristic ofthe optical medium, known as Verdet'sconstant. Generally small, it has avarying dependence on temperature.As the Faraday effect is divergent, amonochromatic light (with singlefrequency) must be used.

In practiceThis effect is used with optical crystalsor fibres. In both cases a light source isrequired and the optical informationmust be processed so that it can beused by the protection and control/monitoring units.c light sourceFrequently a monomode laser diodewith a wavelength approaching780 nanometres: Verdet's constant isgreatest in this part of the wavelengthspectrum.c optical crystalOne or more crystals can be used,surrounding to a greater or lesserextent the conductor in which thecurrent to be measured flows (seefig. 19a). In free field opticalconfiguration, which is the mostfrequent case with crystals,mechanical-optical alignment problemsare particularly great.

c optical fibreThe controlled optics technique uses asa sensing element a monomode opticalfibre which can be wound several timesaround the primary conductor (seefig. 19b). In this case application ofAmpere's theorem gives:F = V N IThis technique ensures increasedsensitivity.

Optical fiber sensors are not sensitiveto external currents (return conductor,other phases, other circuits), whereasoptical crystal sensors are, to a greateror lesser extent, depending on theirconstruction technology.On the other hand, the opticalcharacteristics of the sensing element(crystal or fibre) are particularly affectedby variations in temperature andmechanical stresses.c converting the optical signal into anelectrical signal. This is achieved bycomparing the light beams emitted andreceived, generally using polarising-

fig. 19 : Faraday effect current sensors.

fig. 18: graphic representation of the Faraday effect.

a - exploded view of an optical crystal sensor b - diagram of an optical fibre sensor

LSR semi-reflecting bladeL Selfoc lensesF1 polarisation hold optical fibreF2 sensor fibreF3 multimode optical fibresI current to be measuredSSP separating/polarising system

at 45°P photodiodesPR reference photodiodes

yyyz||

~~~

yyy zz

zz

||||

~~

~~

~

Faraday effectoptical medium

polarisation planerotation angle

F

linear polarisationtransmitted light

linear polarisationincident light

magneticfield

H

L

4 crystals making up the sensing element

analyser

lens

optical fibreconnectors

polariser

primaryconductor

laserdiode

P

electronics

monomodeconnection

F2

F1

PR

F3

L

LSR

L

L

SSP

L

Ι

P P


a - in HVA (Merlin Gerin), with:

c in the foreground an optical sensor with itswound-on optical fibre conductor,

c in the background an equivalentconventional, but more space-consuming,CT sensor.

b - in HVB (Square D), with:

c in the foreground a conventional CT,

c in the background an equivalent, but lessspace-consuming, optical sensor.

fig. 20: optical sensor examples.

are perfectly linear. However, theprocessing electronics is limited in itsdynamics, for a given accuracy, by:v its pass-band;v its ability to detect to a 2 π therotation angle of the polarisation plane.However, digital signal processingtechniques can be used to correct this;v the supply voltages of thecomponents making up the primary andsecondary converter(s).

Despite these problems, present-daytechniques can produce optical currentsensors of an accuracy comparable tothose of CTs (see fig. 20).

EMC behaviourSince galvanic insulation between thecircuits (primary and secondary) isperfect (no coupling capacity), the EMCbehaviour (conducted disturbances) ofthis sensor is good.However, this behaviour may beaffected by that of its primary andsecondary converters sensitive toradiated disturbances (shieldings,relative positioning, etc...).

Note that this perfect galvanicinsulation is, as regards safety, a majorasset for this sensor type in that it doesaway with the explosion risks existingin HVB with oil insulated CTs.

Hall effect current sensorsHall effectA semiconductive wafer through whicha current i flows, immersed in amagnetic induction field B

→, develops

between two sides a potentialdifference known as Hall's voltage VHmeeting the equation:

VH = K i B

where K is the sensor's coefficient ofsensitivity.

This wafer forms the sensing elementof the Hall effect current sensor.

PrincipleThe Hall effect assumes that in a longwafer (see fig. 21), fitted with wideelectrodes injecting current i, all theelectrons move uniformly at speed V inthe opposite direction to current i.When a magnetic induction field B

→ is

applied perpendicularly to one of thelarge sides of the wafer, the -e chargedelectrons are deflected to one of the

separating prisms combined withphotodiodes which convert the lightsignal into an analog electrical signal.The latter is then processed andamplified so that it can be used by theprotection and control/monitoring units.

AccuracyOptical sensors (fibre or crystal) aresensitive to external conditions(temperature, auxiliary energy source),which thus affects their accuracy.

c influence of temperatureTemperature affects three parameters:v Verdet's constant Vv the birefringence of the opticalmedium,v the wavelength of the light emitted bythe laser diode.

In order to operate in the conditionsencountered on electrical networks,optical sensors must be temperaturecompensated (see appendix 3, [5]).Compensation can take the form of:v permanent action on the opticalsensing element (double twist of thefibre, return journey of light through thefibre, thermal compensation of crystal,etc...),v keeping the laser diode at atemperature compatible with therequired accuracy,v allowing for the real temperature ofeach element in the output signalshaping line.

c influence of mechanical constraints.The crystal or optical fibre must have avery low birefringence rate so as not toalter light polarisation in the absence ofmagnetic field. Mechanical stresses onthe crystal or fibre, linked totemperature variations, implementationand operation, must not change thisbirefringence rate.

c influence of signal conversionelectronics. The crystal and optical fibre

→EH

vΛΒ-e→

→EH-e

→v→ VH

d

i

fig. 21: theoretical diagram of the Hall effectsensor.


guarantee, in specified conditions of use,an accuracy compatible with the intendedpurpose (measurement or protection orboth). This results in the functionalelectronics diagram necessary for properoperation of this sensor (see fig. 22).

The pass-band of these sensors isrelatively large and it is possible tomeasure DC currents and currents withfrequencies of around 40 kHz. Pass-band width, for this sensor type,depends on the technology of themagnetic circuit, the electroniccomponents and the architecture usedfor signal processing.

EMC behaviourAbsence of galvanic insulation betweenthe sensor and the electronic elementsis a major handicap, particularly in HV.The EMC of the assembly (Hall effectsensor, protection and control/monitoring unit) may thus not be perfect.

zero flux current sensorPrincipleThe sensing element is a magneticcircuit (CM) (see fig. 4) in which the fluxcreated by the current to be measured(I1) is cancelled by a current (I2). Thevalue of this current is adjustedautomatically by an electronic power

small sides where they accumulate dueto the effect of Laplace's law force.F→

= − e V→

∧ B→

The load unbalance between the twosmall sides causes a Hall electric field,EH→

, to appear, which grows untilthe force (−e EH

→) balances that of the

induction field.

In these conditions, the electronsresume a uniform movement, andHall's electric field is written as:EH→

= − B→

j→

/ (N e)

where N is the number of chargecarriers (-e) and j

→ the current density

in the wafer: this results in Hall'svoltage:VH→

= K i→

B→

/ (N e d)

In practiceA practical solution for increasing sensorsensitivity is to increase B

→. This is

achieved by placing the Hall generatorin the airgap of a magnetic corethrough which the induction flux flowsdue to the magnetic field created by thecurrent to be measured (seefig. 5). Current is supplied and thesignal processed by means ofelectronic elements.

AccuracyThe response of Hall effect sensors isnot exactly proportional to B. This isdue to three factors:v offset voltage,v linearity error,v fluctuation with temperature.

c offset voltageAn error voltage linked to theproduction of the sensing element. Fora given temperature range it can becorrected by the secondary converter.

c linearity errorPresence of a magnetic circuit, evenwith a relatively high airgap, introducesa nonlinearity resulting from thesaturation phenomena. This sensor'sdynamics depends on the sizing of themagnetic circuit.

c fluctuation with temperatureTemperature influences in two ways:v by the coefficient of sensitivity Kwhich varies by roughly 0.01 % per °C,v by the mechanical constraints, furtherto temperature variations undergone bythe sensing element.

Sensor production must allow for allthese influencing factors and aim tocompensate them in order to obtain and

amplifier (A) controlled by probe (SD)voltage proportional to the flux flowingin the magnetic core (MC). Theresulting flux in this core is zero and thefollowing can thus be written:I2 = N1 I1 / N2, whereN1 = number of primary winding turns,N2 = number of secondary windingturns.

AccuracyThis system has an excellent accuracy.Current transformer error measuringbenches make use of this principle. Themodule error can be limited to very smallvalues (≈ 0.02 %), as can also be thephase error which may be less than 0.1angular minute (but which depends on theflux cancellation electronic circuits).

This sensor's performances mainlydepend on the performances of theamplifier, both as regards measuringrange and accuracy.

Note:Zero flux current transformers are usedto measure DC currents.

EMC behaviourThe signal for cancelling flux approachingzero is easily disturbed. This CT typemust therefore be placed in a highlyprotected electromagnetic environment(shields, filtered supplies, etc...).

fig. 22: functional diagram of the electronics of a Hall effect sensor.

differential amplifierand stabiliser of the

coefficient of sensitivityin temeraturep

probe

amplifier+

filter

powers supplies

residual inductioncompensationcircuit

thermistors

mechanical forcecompensationcircuit

B→

Ι

Uh = K Ι

i

DCgenerator


6. comparision table, synthesis

c poor CT SS optical zero fluxc c average conventional Rogowski sensors currentc c c good current coil sensor transformerc c c c very good transformer

performances:c linearity c c c c c c c c c c c cc exactness c c c c c c c c c c cc dynamics c c c c c c c c c c cc accuracy c c c c c c c c c c c c cc EMC c c c c c c c c c c c ccapacities:c measuring standard c c c c c c c c c cc supplying energy to protection c c c c c c c cand control/monitoring unitsc supplying the measuring signal to:v analog energy meters c c c c c c cv digital energy meters c c c c c c c c c cv digital protection and control/ c c c c c c c c c c cmonitoring units

relative cost compared with switchgear:c in HVA c c c c c c c c c cc in HVB c c c c c c c c cnumber of sensors installed each year:c present situation c c c c c c c c cc foreseeable evolution c c c c c c c c c


c EMCLinked to the increasing use ofelectronic technologies. In this field, theoptical sensor's behaviour is ideal.c linearityLinear CTs (TPZ type) are generallyspace consuming (airgap present) andtheir linearity is not perfect. When afault occurs, they transmit very highcurrent leading to very high thermalstresses to the equipment connected tothem. The SS sensor, with its perfectlinearity, has the best performancehere.c operating rangeA CT has a very narrow operatingrange which limits its use to one singleapplication. On the other hand, opticaland SS sensors, with wider operatingranges (≈ 10 times) have greaterpossibilities of use, only limited by theequipment to which they areconnected.Optical and SS sensors thus give thebest performances as regards the newtechnical constraints. A hybrid sensor,using a Rogowski coil as its sensingelement and optical fibres as itstransmission system, could prove theideal solution.In HVA, this solution is not yet feasiblefrom an economic point of view.

present solutionsToday most protection and control/monitoring equipment in operation useselectromagnetic or electronictechnologies. This equipment requiressufficiently powerful signals (≈ 5 to50 VA) from current sensors which areoften at a considerable distance (≈ 2 to150 m).In HV equipment, this power is suppliedby conventional current transformers.However, in HVA, several hundreddigital protection and control/monitoringunits are in operation. The majority,able to process signals with low energylevels, are associated with SS sensors.In HVB, experiments are currentlyunderway with Faraday effect sensorsusing optical crystals or fibres.Zero flux CTs are above all used in testbays and in DC transmission networks.

future solutionsThe very fast evolution of protectionand control/monitoring systems towardsdigital technologies has alreadyresulted in major changes to sensorspecifications. These specificationsgive priority to EMC, linearity andoperating range of sensors.

However, the SS solution, through itscost and associated advantages, is thesolution both for now and the future.In HVB the use of these new sensorsdepends on the development of digitalsolutions for the protection and control/monitoring units and on the creation ofdigital interfaces for existing units.Sensor evolution will take off as soonas this equipment becomes available.This evolution has already begun andsystems are already available, either«fully optical» or as in the ideal solutionrecommended above.Consequently, conventional CTs, withtheir limited performances andrelatively high costs, are doomed tovanish in the long term in HV, exceptperhaps those used for meteringenergy bills.

7. conclusion and future


limits of errors for «measurement» secondaries

± Percentage current (ratio) ± phase displacement at percentage of rated

accuracy classerror at percentage of rated current shown belowcurrent shown below minutes centiradians

5 20 100 120 5 20 100 120 5 20 100 120

0.1 0.4 0.2 0.1 0.1 15 8 5 5 0.45 0.24 0.15 0.150.2 0.75 0.35 0.2 0.2 30 15 10 10 0.9 0.45 0.3 0.30.5 1.5 0.75 0.5 0.5 90 45 30 30 2.7 1.35 0.9 0.91.0 3.0 1.5 1.0 1.0 180 90 60 60 5.4 2.7 1.8 1.8

Limits of error for «protective» secondaries

accuracy current error phase displacement at composite error at

class at rated primary rated primary current rated accuracy limitcurrent primary current

% minutes centiradians %

5 P ± 1 ± 60 ± 1.8 510 P ± 3 10

appendix 1: CT accuracy as in IEC 185


appendix 2: CT classification as in IEC 44-6

The various classes of currenttransformers for protection, defined byIEC 44-6 according to theirperformances, are listed in the tableopposite.

class performances

P Accuracy limit defined by the composite error ( ε c) with steady statesymmetrical primary current. No limit for remanent flux.

TPS Low leakage flux current transformer for which performance is defined bythe secondary excitation characteristics and turns ratio error limits. No limitfor remanent flux.

TPX Accuracy limit defined by peak instantaneous error ( ε ) during specifiedtransient duty cycle. No limit for remanent flux.

TPY Accuracy limit defined by peak instantaneous error ( ε ) during specifiedtransient duty cycle. Remanent flux not to exceed 10% of the saturationflux.

TPZ Accuracy limit defined by peak instantaneous alternating currentcomponent error ( ε ac) during single energization with maximum d.c. offsetat specified secondary loop time constant. No requirements for d.c.component error limit. Remanent flux to be practically negligible.

Réal.: Sodipe - Valence - Photo.: Merlin Gerin, Square DEdition: DTE - Grenoble03-95 - 2500 - Printing: ClercPrinted in France

appendix 3: bibliography

[1] Techniques de l'ingénieur:Transformateurs de mesure.D 4720 12-1990, D 4722 12-1990,D 4724 3-1991, R1016 10-1992.

[2] CEI 44-6 : First edition1992-03 : Transformateurs de mesure.Part 6 : Prescriptions concernant lestransformateurs de courant pourprotection pour la réponse en régimetransitoire.

[3] CEI 185 : Second edition 1987 andits amendment 1 1990-07.Transformateurs de courant.

[4] Méthode rapide deprédétermination des transformateursde courant,Pierre SCHUELLER (Merlin Gerin)page 41 to 45, RGE n°4 april 1990.

[5] Techniques de l’ingénieur:Capteurs de courant à fibresoptiques.R 1016 10-1992.

[6] La CEM: la compatibilitéélectromagnétique,Cahier Technique Merlin Gerin n° 149.F. VAILLANT.

[7] Le transformateur de courant pourla protection en HT,Cahier Technique Merlin Gerin n° 164.M. ORLHAC.