NEUTRAL GROUNDING REACTOR FOR MEDIUM VOLTAGE NETWORKS

P.Folliot – Alstom Petit Quevilly / FranceJM. Boyer – Alstom Montpellier / FranceS. Bollé – Alstom Petit Quevilly / France

Summary

The different modes of grounding the mediumvoltage power distribution networks differ fromcountry to another. In France, in the years 1980,the national utility EDF was committed in areflection on the policy of the neutral grounding ofthe MV networks in order to increase the powersupply availability.

EDF took the decision to define a new neutral pointreactor with variable compensating inductance andto replace the existing neutral reactors with fixedinductance values by this adjustable type.

In co-operation with their transformer suppliersthey elaborated the specifications for two deviceswith different maximum reactive fault currents of600 A and 1000 A. Both are designed to workindifferently in 20 kV or 15 kV networks. Afteradjustment of the compensating reactor, theresulting fault current under full homopolar tensionshould not exceed 40 A.

The homopolar capacity of the power distributionnetwork varies according to its topology and itsload characteristics. To profit from the advantagesof the compensated neutral, the balance betweenthe homopolar network capacity and theinductance of the coil must be maintained.

For the tuning of the reactor inductance, EDFdeveloped a new system. An additional currentsource is used to inject a homopolar current in thenetwork. Measurements before and during theinjection determine the parameters of adjustmentfor the compensating impedance.

Thus, ALSTOM, an approved EDF supplier,developed and tested a new reactor unit, whichgathers the following functions in only onecomprising module:

• Neutral point creation by means of a three-phase zigzag winding

• Variable compensating reactor, realisedthrough single-phase self-inductance coilswhich are coupled in parallel by a commutationsystem

• Resistor of fixed resistance value, connectedin parallel to the inductance coils to create anactive component of the fault current

• Single-phase current transformer, to inject thecurrent, which is needed for the system tuning

• Auxiliary cell, provided with load switches forthe parallel commutation of the single-phaseself-inductance coils. The load switches arecontrolled by a separated unit, which may beinstalled in the substation relay building.

ALSTOM underwent successfully the tests ofqualification imposed by the EDF-specification. Inparallel, an experimentation test phase is carriedout on several devices, already in service on site.

The industrial deployment of the compensatedneutral of the EDF network begins in 2001. About1000 to 1200 substations are concerned.

NEUTRAL GROUNDING REACTOR FOR MEDIUM VOLTAGE NETWORKS

P.Folliot – Alstom Petit Quevilly / FranceJM. Boyer – Alstom Montpellier / FranceS. Bollé – Alstom Petit Quevilly / France

Introduction

The different modes of grounding the mediumvoltage power distribution networks differ fromcountry to another. The physical characteristics ofthe networks, such as network extend, loaddensity, load nature, the quality of the earthingterminals, the network type – air, underground ormixed - led the operators of the various countriesto an independent choice of grounding theirparticular networks.

For the choice of the neutral grounding mode, theyhave to consider the criteria of tension controlduring the occurrence of single-phase ormultiphase fault currents, the reliability and thesensitivity of protections, the voltage level – simpleline-to-earth or phase-to-phase during of thenetwork voltage rise during the defect -, the qualityof supply as well as the security of goods andpeople.

The different modes of neutral grounding

Without being exhaustive, we recall in the followingthe principal types of medium voltage networkgrounding applied in various countries of the world:

Direct grounding:Two alternatives exist, characterised by thedifferent modes of grounding the earthingterminals:

a) direct grounding, neutral not distributed

The insulation level of networks without distributedneutral remains nearly equal to the line-to-earthvoltage, the ground potential rise in the proximity ofthe ground fault are conditioned by the quality ofthe earthing terminals which must have a very lowresistance.

b) direct grounding, neutral distributed

The networks with distributed neutral generallycomprise a grounding in many points.

The insulation level of the networks with distributedneutral remains very close to the line-to-earthvoltage and the fault current is of strong amplitude.

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Neutral point insulated :

The current appearing during a ground fault canonly circulate through the capacities between thephase conductor and the ground. It is of lowamplitude. Under single-phase fault conditions thevoltages of the healthy phases are increased to thephase-to-phase voltage of the network. This has tobe taken into account in the definition of theinsulation properties of the network.

Neutral grounding by a limiting resistor :

The limiting resistor is inserted between the MVneutral of the HV/MV transformer and the groundof the substation.

The maximum fault current results directly from theresistor value. This current must remain bigenough to ensure a sure and rapid fault detectionall while maintaining overvoltages in acceptablelimits due to personal security.

The detection of single-phase fault currents issimple. The insulation level in the case of neutralgrounding by a limiting resistor has to bedimensioned for the phase-to-phase voltage of thenetwork.

Neutral grounding by a fixed limiting impedance :

The limiting impedance can be inserted betweenthe MV neutral of the HV/MV transformer of thesubstation and the ground of the substation. To beindependent from the presence of a MV neutralpoint of the HV/MV transformer of the substation, itis possible to create an artificial neutral point.

This artificial neutral point is most generallyrealised by means of a three-phase transformer,whose primary winding is zigzag connected andwhose neutral is connected directly to the groundof the substation.

This mode of grounding is employed on the MVdistribution networks in France since the years1950. The specification HN 52-S50 of EDF, thenational utility of France, thus specified two typesof neutral reactors (zigzag winding) for the 5,5kv-10kv-15kv-20kv-30kv networks. A 300 A reactivefault current version is destined for the ruralnetworks with overhead lines, whereas the 1000 Aversion is gauged for urban networks mainly madeof underground cables.

These zigzag neutral point reactors compensatethrough a constant reactive current the capacitivecurrent of the network in the case of a ground fault.These fault values must remain big enough toensure a sure and rapid fault detection all while

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maintaining overvoltages in acceptable limits dueto personal security.

The detection of single-phase fault currents issimple.

The insulation level of the networks with neutralgrounding by a fixed limiting impedance has to bedimensioned for the phase-to-phase voltage.

Neutral grounding by connecting in series a fixedlimiting impedance and a limiting resistor :

Today, the MV networks of EDF are mainlyexploited with 20 kV assigned voltage.

In the years 1980, EDF was committed in areflection on the policy of the neutral grounding ofthe MV networks.

The company specifications HM-54-4054 and HM-54-4061 then had to be applied. They containrequirements on the quality factors, dimensions,masses, etc., and they precede already the level ofreliability which will be imposed on the futurehardware. These constraints had to be validatedby performance tests, which reproduce the defectswhich a zigzag neutral point reactor will meetlasting its life.

To give an example: For the qualification of thishardware, the " 300 A " and " 1000 A " neutralground reactors will undergo 1000 application testsof the full zero-sequence voltage with a maximumasymmetry of the current to validate theirmechanical behaviour at the generatedelectrodynamic stresses.

Neutral grounding by a compensating coils :

The compensating coils are variable limitingimpedances.The compensating coils can be connectedbetween the MV neutral point of the HV/MVtransformer of the substation and the substationgrounding.The compensating coils can also be connectedbetween the artificial neutral of a neutral reactorand the substation grounding.

The compensating coils are more known under thename Petersen coil, from the name of theirinventor.The compensating coils are adjustable, eithermanually for the use in fairly stable networks, orautomatically for disturbed networks. The ability ofadjustment makes it possible to compensate thecapacitive current almost completely. After a suchcompensation the network is “tuned".

The need of compensation adjustment isnecessary due to the frequent changes of thenetwork configuration (the load, the length of theexploited network and the share of undergroundcables of the network). The appropriate value isobtained by an analysis of the network state andthe use of an automatic tuning system.Overvoltages are limited to acceptable values dueto the tuning range.

The sensitivity and the selectivity of the groundfault detection and particularly the fault resistancemakes the fault detection more delicate. Thisdisadvantage often led the operators of this type ofnetwork to practise the maintenance of theconcerned part of the network during the searchand the elimination of the defect.The insulation level of the networks with neutralgrounding by a compensating coil has to bedimensioned for the phase-to-phase voltage.

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Grounding by a compensating coil has greatadvantages. It allows a ongoing evolution of thenetworks and makes it possible to limit the faultcurrent in the range of optimal values.It eliminates most of the single-phase fault currentsby automatic extinction thus avoiding short powerfailures.That’s why, for the network operator, acompensating coil is a major asset in view of thecontinuity and the quality of electric power supply.

The compensated neutral in EDF powerdistribution networks.

In the years 1980, EDF was engaged to getthrough a policy of quality improvement of theelectric power supply.

While preserving the structures and the conditionsof operating the current network, EDF definedobjectives such as:

• Fault number reduction• Reduction of the number and the duration of

power failures• Reduction of fault consequences• Reduction of voltage hollows• Increase of the security of people, goods and

equipment in the vicinity of the defect

The French rural networks consists of feeders withlengths ranging between 30 and 50 km. Underground fault condition the capacitive currents canreach a few hundreds amps. A reduction of thesevalues by means of a network topologymodification or a grounding improvement of theearthing terminals is not possible for financial andtechnical reasons.

The French rural networks have undergonesignificant evolutions during the last years.

To protect the environment, to respect theoriginality of certain sites, to ensure the safety ofthe people and the goods and to reduce thenetwork sensitivity in regard of weather events(lightning, storm), EDF transferred a lot of theirrural MV overhead lines to underground cables.

The emergence of underground cables in the ruralnetworks increased the capacitive current valuesunder ground fault conditions. In this context, theuse of neutral grounding by a fixed limitingimpedance shows up its limits. EDF launched acomplete inventory and undertook a profoundstudy on all the modes of MV neutral grounding.

An analysis of the advantages and disadvantagesof each system made it possible to release newdoctrines. The compensated neutral point seemed

to be an interesting solution and compatible withthe current networks, while enabling new conceptsfor fault control, the follow-up of faults and theoperational measures in case of a ground fault.

To profit from the advantages of the compensatedneutral, the balance between the homopolarnetwork capacity and the inductance of the coilmust be maintained.

The homopolar capacity varies according to thetopology and the load characteristics of thenetwork. The tuning systems available on themarket imply a voluntary modification of the coilinductance in order to generate procedures ofcalculation and position adjustment of the coil.

In view of the complexity of the automatic tuningsystems available and their implementation, EDFdecided to developed a new tuning system.An additional current source is used to inject ahomopolar current in the network. Measurementsbefore and during the injection determine theparameters of adjustment for the compensatingimpedance. This system has the followingadvantages:

• No inductance variation and thus no variationof compensating coil adjustment is necessaryduring the calculation.

• The transition time from one position toanother is very fast, when a compensatingimpedance made up of several commutablecoils of fixed inductances is used.

• The tuning system may even work during theappearance of a fault.

• The sequences of injection are short and canbe frequently repeated.

Having validated this concept, EDF wrote aspecification and consulted the manufacturers.

The company specification HN 52 S 25 gathers thecriteria, the constraints, the conditions ofacceptance and the procedures for testing thecompensating reactors.

The compensating reactors are designed to workindifferently in 20 kV or 15 kV networks.Two types are planned: one type with anadjustment range from 100 to 600 A reactive faultcurrent and another type with a range form 100 to1000 A.

After adjustment of the compensating reactor, theresulting fault current under full homopolar tensionshould not exceed 40 A.

To ensure the correct operation of wattmetric andvoltmetric homopolar protection systems, an activecomponent of the fault current higher than 20 A isnecessary.

The compensating reactor and its control unit(figure 1)

To answer to the EDF specification HN52S25,ALSTOM studied, developed and tested a 20kV–100/600 A and a 20kV–100/1000 A compensatingreactor for neutral grounding of medium voltagenetworks. This reactor gathers several features inonly one housing.

Neutral point creation

The creation of the neutral point is realised bymeans of a three-phase zigzag winding, whosephase inputs will be connected to the three lineconductors of the MV network of the substation.

The fixed reactor impedance "Zo" has to be verylow to avoid a considerable influence on the totalimpedance value.

Variable compensating reactor

The variable impedance is realised through single-phase self-inductance coils surrounded by amagnetic coat.These coils are coupled in parallel by means of acommutation system.

The use of 4 different coils allows to obtain 15adjustment positions which covers thecompensation range from 100 A to 600 A reactivefault current.

A whole of 5 coils makes it possible to obtain 31positions and thus to cover the compensatingrange from 100 A up to 1000 A reactive faultcurrent.

The impedance values of these single-phaseinductances have an approximate arithmeticprogression of 1-2-4-8 for the 100-600 A type andan approximate arithmetic progression of 1-2-4-8-16 for 100-1000 A type.

The coils are connected between the neutral pointof the three-phase neutral reactor and thesubstation ground. They are combined andconnected in parallel by the commutation system.

Active component of the fault current

A resistor of fixed resistance value is connectedbetween the neutral point of the zigzag winding

and the substation grounding. It creates an activecomponent of the fault current.

Current injection transformer

A single-phase current transformer injects thecurrent, which is needed for the automatic networktuning. The injected homopolar current allows tomeasure and parameterise the network state andto modify the adjustment of the compensatingreactor if necessary.

On the HV side, the current injection transformer isconnected between a line terminal and the groundterminal of the substation. On the LV side, thereare two LV bushings. The LV circuit is protected bya bipolar fuse disconnector. The bipolar fusedisconnector is situated in a switch box, locatedoutside on the front of the tank.

Auxiliary cell for automatic impedance adjustment

The parallel commutation of the single-phase self-inductance coils is carried out by load switches.The auxiliary cell for automatic adjustmentcomprises as many load switches as there aresingle-phase coils.

The auxiliary cell for automatic impedanceadjustment is provided with:

• A pre-heating system for starting the deviceunder extreme temperature conditions (-25°C).

• A terminal strip for control cable connection.

• Relays for the load switch interlocking andcontrol.

• Mechanical indicators representing the loadswitch position.

General constitution of de neutral groundingreactor

All these functions (three-phase neutral reactor,variable compensating reactor, active resistor andcurrent injection transformer) are gathered in onlyone comprising module. They are located in a tankwhich is provided with cooling fins and filled withdielectric liquid (mineral oil).

The auxiliary cell for automatic impedanceadjustment is positioned and bolted on the lid ofthis tank. The electrical connections between theauxiliary cell and the resistor are carried out byporcelain bushings inside the cell.

The neutral grounding reactor module is connectedto the MV network via plug bushings.

The control unit

The control unit of the neutral grounding reactorreceives orders from the automatic tuning system.The control unit itself creates the commands forthe commutation of the load switches, to couple orto disconnect the single-phase coils of the neutralgrounding reactor.

The control unit is a programmed device, which isinstalled in the substation relay building and whichcan be removed a hundred meters from the neutralgrounding reactor.

A 4-20 mA digital signal forwards all informationbetween the automatic tuning system and thecontrol unit of the neutral grounding reactor.

At automatic control operation mode, the controlunit receives only commands of the automatictuning system. In manual operation mode, thedevice is under full control of a technician. Thecommands have to be entered manually bypushbuttons.

The control unit receives information from andtransmits information to the neutral groundingreactor and the automatic tuning system.

The "augmente" command for a position changein order to increase the inductive current.

The ”diminue” command for a position change inorder to decrease the inductive current.

The control unit answers the automatic tuningsystem this kind of information:

"réglage en cours", indicating that the reactor iscarrying out a position change.

"position haute", indicating that the reactor arrivedat the position of maximal inductive current.

"position basse", indicating that the reactor arrivedat the position of minimal inductive current.

"commande sur place", indicating that the positioncontrol of the reactor is exclusively under manualcontrol in the substation control room.

"fonctionnement protection transformateurd’accord", indicating that the protecting fuse of thesecondary circuit of the current injectiontransformer burned through or that the secondarycircuit is inoperative due to the opening of thedisconnecting switch.

"équipement en défaut", indicating the absence ofsupply voltage or indicating that the reactor did notfinish a position change successfully.

The qualification and experimentation phase

A 20kV–100/600 A and a 20 kV–100/1000 Aneutral grounding reactor and their control unitsbuilt by ALSTOM underwent successfully the testsof qualification imposed by the EDF-specificationHN 52S25.

In parallel to these qualification tests, anexperimentation test phase is carried out onseveral devices, already in service on site.

Three 20kV–100/600 A and one 20kV–100/1000 Aneutral grounding reactors built by ALSTOM arecurrently in service on the EDF network.

The industrial deployment of the compensatedneutral of the EDF network begins in 2001. About1000 to 1200 substations are concerned.

Bibliographies:

M. Clément, B. Tromeur; évolution du neutre MT àEDF sur les réseaux ruraux – N.MT 95

P. Juston, D. Griffel, mise à terre du neutre dansles réseaux ruraux – NMT 95/REE 96