Koila na Juyin Yawan Kasa ko Petersen Coil

Takaitaccen K'oyar Tashin

K'oyar tashin, wanda ake kira k'oyar Petersen, shine k'oyar inductance da ake amfani da shi don kawo daga cikin karamin k'oyar capacitive a jikin mai gaba a lokacin ta hankali.

Dalilin da Funtunshi

K'oyar ya kawo karamin k'oyar capacitive mai yawa a lokacin ta hankali ta hanyar haɗa da k'oyar inductive mai wasu tsari.

Funtunshin

K'oyar inductive da k'oyar ya haɗa da k'oyar capacitive, kuma ya kawo karamin k'oyar a matsayin ta hankali.

Karamin K'oyar Capacitive a Jikin Mai Gaba

Jikin mai gaba suna da karamin k'oyar capacitive daidai saboda dielectric insulation da ke cikin masu karamin da kasa.

Hesabunta Inductance

Voltage na biyu na system balanced three phase an samu a figure – 1.

A jikin mai gaba voltage da medium voltage da high voltage, har da karamin da ke cikin masu karamin da kasa, wanda ya haɗa da karamin k'oyar capacitive. Wannan k'oyar ya ci gaba da phase voltage da 90 degrees kamar yadda aka baka a figure – 2.

Idan ta hankali ta faruwa a yellow phase, voltage na yellow phase to ground ya zama zero. Point neutral na system ya zaɓe zuwa tashin vector na yellow phase. Saboda haka, voltage na red and blue phases (healthy) ya zama &sqrt;3 times the original value.

Bisa ga, k'oyar capacitive na red and blue phases (healthy) ya zama &sqrt;3 of the original as shown in figure-4, below.

Vector sum na resultanta da ke cikin biyu na k'oyar capacitive ya zama 3I, inda I ita ce k'oyar capacitive rated per phase a system balanced. Yana nufin, a lokacin healthy balanced condition of the system, I R = IY = IB = I.

Wannan an bayyana a figure- 5 below,

Wannan k'oyar resultanta ya zama ta hanyar path faulty to the earth as shown below.

Daga baya, idan muna sanya k'oyar inductive da inductance suitable (generally iron core inductor is used) between star point or neutral point of the system and ground, wannan ya ba da takamfi. A lokacin faulty, k'oyar through the inductor just equal and opposite in magnitude and phase of that of capacitive current through the faulty path. K'oyar inductive ya haɗa da path faulty of the system. K'oyar capacitive and inductive ya kawo karamin k'oyar a path faulty, hence there will not be any resultant current through the faulty path created due to capacitive action of the underground cable. The ideal situation is illustrated in the figure below.

Wannan concept an implemente da W. Petersen a shekarar 1917, saboda haka k'oyar inductor coil an amfani da shi, called Petersen Coil.

K'oyar capacitive component of the fault current is high in the underground cabling system. When earth fault occurs, the magnitude of this capacitive current through the faulty path becomes 3 times more than rated phase to earth capacitive current of healthy phase. This causes significant shifting of zero crossing of current away from zero crossing of voltage in the system. Due to presence of this high capacitive current in the earth fault path there will be a series of re-striking at fault location. This may lead unwanted over voltage in the system.

Inductance of the Petersen Coil is selected or adjusted at such value which causes the inductive current which can exactly neutralize the capacitive current.

Let us calculate the inductance of Petersen Coil for a 3 phase underground system. For that let us consider capacitance between conductor and earth in each phase of a system, is C farad. Then the capacitive leakage current or charging current in each phase will be

So, the capacitive current through the faulty path during single phase to earth fault is

After fault, the star point will have phase voltage as the null point is shifted to fault point. So the voltage appears across the inductor is Vph. Hence, the inductive current through the coil is

Now, for cancellation capacitive current of value 3I, IL must have same magnitude but 180o electrically apart. Therefore,

When the design or configuration of the system changes, such as length, cross-section, thickness, or insulation quality, the inductance of the coil must be adjusted. Therefore, Petersen coils often have a tap-changing arrangement.