Neutral grounding reactor for medium voltage …

1 Neutral grounding reactor FOR medium voltage Alstom Petit Quevilly / FranceJM. Boyer Alstom Montpellier / FranceS. Boll Alstom Petit Quevilly / FranceSummaryThe 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 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 co-operation with their transformer suppliersthey elaborated the specifications for two deviceswith different maximum reactive fault currents of600 A and 1000 A.

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

3 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 underwent successfully the tests ofqualification imposed by the EDF-specification. Inparallel, an experimentation test phase is carriedout on several devices, already in service on industrial deployment of the compensatedneutral of the EDF network begins in 2001.

4 About1000 to 1200 substations are grounding reactor FOR medium voltage Alstom Petit Quevilly / FranceJM. Boyer Alstom Montpellier / FranceS. Boll Alstom Petit Quevilly / FranceIntroductionThe 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 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 different modes of Neutral groundingWithout being exhaustive.

5 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 distributedThe 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 ) direct grounding , Neutral distributedThe networks with distributed Neutral generallycomprise a grounding in many insulation level of the networks with distributedneutral remains very close to the line-to-earthvoltage and the fault current is of strong point insulated :The current appearing during a ground fault canonly circulate through the capacities between thephase conductor and the ground.

6 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 grounding by a limiting resistor :The limiting resistor is inserted between the MVneutral of the HV/MV transformer and the groundof 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 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 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.

7 To beindependent from the presence of a MV neutralpoint of the HV/MV transformer of the substation, itis possible to create an artificial Neutral 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 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 zigzag Neutral point reactors compensatethrough a constant reactive current the capacitivecurrent of the network in the case of a ground fault values must remain big enough toensure a sure and rapid fault detection all whileABCABCN51 ABCN=1 ABCN=LJ]DJ ZLQGLQJ maintaining overvoltages in acceptable limits dueto personal detection of single-phase fault currents insulation level of the networks with neutralgrounding by a fixed limiting impedance has to bedimensioned for the phase-to-phase grounding by connecting in series a fixedlimiting impedance and a limiting resistor.

8 Today, the MV networks of EDF are mainlyexploited with 20 kV assigned the years 1980, EDF was committed in areflection on the policy of the Neutral grounding ofthe MV 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 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 grounding by a compensating coils.

9 The compensating coils are variable compensating coils can be connectedbetween the MV Neutral point of the HV/MVtransformer of the substation and the compensating coils can also be connectedbetween the artificial Neutral of a Neutral reactorand the substation compensating coils are more known under thename Petersen coil, from the name of 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).

10 The appropriate value isobtained by an analysis of the network state andthe use of an automatic tuning are limited to acceptable values dueto the tuning 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 insulation level of the networks with neutralgrounding by a compensating coil has to bedimensioned for the phase-to-phase ]DJZLQGLQJ =1 ABCN=1 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 eliminates most of the single-phase fault currentsby automatic extinction thus avoiding short s why, for the network operator, acompensating coil is a major asset in view of thecontinuity and the quality of electric power compensated Neutral in EDF powerdistribution the years 1980, EDF was engaged to getthrough a policy of quality improvement of theelectric power preserving the structures and the conditionsof operating the current network, EDF definedobjectives such as.