Sepam - HV/MV Protection and control units (ENG) - Trinet

capacitor protectionintroductionCapacitor banks are used to compensate forreactive energy absorbed by electricalsystem loads, and sometimes to make upfilters to reduce harmonic voltage.Their role is to improve the quality of theelectrical system.They may be connected in star, delta anddouble star arrangements, depending on thelevel of voltage and the system load.A capacitor comes in the form of a case withinsulating terminals on top. It comprisesindividual capacitances which have limitedmaximum permissible voltages(e.g. 2250 V) and are series-mounted ingroups to obtain the required voltagewithstand and parallel-mounted to obtainedthe desired power rating.There are two types of capacitors:those with no internal protection,those with internal protection:a fuse is combined with each individualcapacitance.types of faultsgroupe 1groupe 2groupe 3groupe n(fig.1)Vn-1Vn-1VThe main faults which are liable to affectcapacitor banks are:overload,short-circuit,frame fault,capacitor component short-circuit.An overload is due to temporary orcontinuous overcurrent:continuous overcurrent linked to:raising of the power supply voltage,the flow of harmonic current due to thepresence of non-linear loads such as staticconverters (rectifiers, variable speed drives),arc furnaces, etc.,temporary overcurrent linked to theenergizing of a capacitor bank step.Overloads result in overheating which hasan adverse effect on dielectric withstandand leads to premature capacitor aging.A short-circuit is an internal or externalfault between live conductors, phase-tophaseor phase-to-neutral depending onwhether the capacitors are delta or starconnected.The appearance of gas in thegas-tight chamber of the capacitor createsoverpressure which may lead to the openingof the case and leakage of the dielectric.A capacitor component short-circuit isdue to the flashover of an individualcapacitance.with no internal protection: the parallelwiredindividual capacitances are shuntedby the faulty unit:the capacitor impedance is modifiedthe applied voltage is distributed to oneless group in the serieseach group is submitted to greater stress,which may result in further, cascadingflashovers, up to a full short-circuit.with internal protection: the melting of therelated internal fuse eliminates the faultyindividual capacitance:the capacitor remains fault-free,its impedance is modified accordingly.A frame fault is an internal fault between alive capacitor component and the framecreated by the metal chamber.Similar to internal short-circuits, theappearance of gas in the gas-tight chamberof the capacitor creates overpressure whichmay lead to the opening of the case andleakage of the dielectric.Merlin Gerin protection guide 47