Banka ya Kapasitora Parastina
Weha elektrîkî yên din ên, capacitor shunt dahiş da ku di navbera xerîdarên derandar û derandarek ji bîr re tekeşîne. Demê, pirantiya ku heta dabeş kirin têne. Dibejînên çend scheme yên li ser astn e ku protection of capacitor bank, lê dem îmeyan ku herî dikê bike şertekî, divê pêkhatîn ku girtina sernavî kapasitordan ji bîr re heye. Divê pêkhatîn ku girtina sernavî kapasitordan ji bîr re heye û maliyeya protection applying bikin. Hene 3 cûre protection arrangement ku bi banka capacitor hatine serkirin.
Element Fuse.
Unit. Fuse.
Bank Protection.
Element Fuses
Producerên capacitor unit ziyadetir fîseyan inbuilt fuse di her element de. Li vê rewşa, ger ên fault di her element de bigere, otomatîk disconect dike ji yekîtiya din. Li vê rewşa, uniti hala xebatake, lê bi output kêmtir. Di capacitor bank yên rate yên wekîn de, tenaha vê scheme yên inbuilt protection hatine serkirin ku parêzkariya protective equipments spécial evît bide.
Unit Fuse
Protection unit fuse ziyadetir werdike tu limit kirina duration of arc di navbera capacitor unit faulty de. Ji ber ku duration of arc limit e, çawa malpera mechanical deformation û gas production rojanî ne, û demê unitên neighborhood bank an jî hate nehatin. Ger her unit di capacitor bank de bekar tune ji fîse, li vê rewşa, capacitor bank hala xebatake dikare, ji ber ku faulty unit hate rakirin û wanazkirin.
Divê yekîna advantage yên guherrîn û protection û unit bank bekar tune ku nîşane dike location exact faulty unit. Lê di vê demê de, divê hesab bike ku size of the fuse for this purpose, divê qabul bike excessive loading ji harmonics system. Bi rêya vê current rating of the fuse element for this purpose is taken as 65% above the full load current. Herdemê individual unit of capacitor bank bekar tune ji fîse, divê discharge resistance bekar tune di her unit de.
Bank Protection
Herduhemîn fuse protection bekar tune di her capacitor units de, lê demî kapasitor unit under fault û associated fuse element blown out, voltage stress increase to the other capacitor units connected in series in same row. Generally, each capacitor unit is designed for withstanding 110% of its normal rated voltage. If any other capacitor unit further becomes out of service, in the same row where previously one unit is damaged, the voltage stress upon other healthy units of that row will increase further and easily crosses the limit of maximum allowable, voltage of these units.
Demî heta desirabî ye ku replace damaged capacitor unit from the bank as soon as possible to avoid excess voltage stress on the other healthy units. Demî, there must be some indicating arrangement to identify the exact faulty unit. As soon as the faulty unit is identified in a bank, the bank should be removed from the service for replacing the faulty unit. There are several methods of sensing unbalance voltage caused by failure of capacitor unit.
The figure below is showing the most common arrangement of capacitor bank protection. Here, the capacitor bank is connected in star formation. Primary of a potential transformer is connected across each phase. The secondary of all three potential transformers are connected in series to form an open delta and a voltage sensitive relay is connected across this open delta. In exact balanced condition there must not be any voltage appears across the voltage sensitive relay because summation of balanced 3 phase voltages is zero. But when there would be any voltage unbalancing due to failure of capacitor unit, the resultant voltage will appear across the relay and the relay will be actuated for providing an alarm and trip signals.
The voltage sensitive relay can be so adjusted that up to a certain voltage unbalancing only alarm contacts would be closed and for certain higher voltage level the trip contacts along with alarm contacts would be closed. The potential transformer connected across the capacitors of each phase also serves for discharging of the bank after being switched off.
In another scheme, the capacitors in each phase are divided into two equal parts connected in series. Discharge coil is connected across each of the parts as shown in the figure. In between the secondary of discharge coil and the sensitive voltage that unbalances the relay an auxiliary transformer is connected which serves to regulate the voltage difference between secondary voltages of discharge coil under normal conditions.
Here the capacitor bank is connected in star and the neutral point is connected to the ground through a potential transformer. A voltage sensitive relay is connected across the secondary of the potential transformer. As soon as there is any unbalance between the phases, the resultant voltage will appear across the potential transformer and hence the voltage sensitive relay will be actuated beyond a preset value.
Here, the capacitor bank of each phase is divided into two equal parts connected in parallel and the star points of both parts are interconnected through a current transformer. The secondary of the current transformer are connected across a current sensitive relay. In case any misbalancing occurs between the two parts of the bank, there would be a unbalance current flowing through the current transformer and hence the current sensitive relay will actuate. In this scheme for discharging the bank after switching off, discharge coil may be connected across the capacitors in each phase.
In another scheme of protection of capacitor bank, the star point of a three phase capacitor bank is connected to the ground through a current transformer and a current sensitive relay is connected across the secondary of the current transformer. As soon as there is any unbalancing between the phases of capacitor bank, there must be a current flowing to the ground through the current transformer and hence the current sensitive relay will be actuated to trip the circuit breaker associated with the capacitor bank.
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