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PETERSEN COILS Presented by Jawahar (131EE510)Petersen coilsPeterson coils are used to in ungrounded 3-phase grounding systems to limit the arcing currents during ground faults.The coil was first developed by W. Petersen in 1916.However the use of modern power electronics has revolutionized the performance of these classical solutions.
Basic PrincipleWhen a phase-to-earth fault occurs in ungrounded 3 phase systems, the phase voltage of the faulty phase is reduced to the earth potential as the capacitance of the faulty line is discharged at the fault location, the phase-to-earth voltage of the other two phases rises by 3 times.A charging current IC occurs between these phase-to-earth capacitances, which will continue to flow via the fault path while it remains.IC is three times the charging current of each phase-to-earth.
Isolated distribution system where IC, is the discharge current due to capacitance of healthy phases
IC= 3I IC = 3Vp(1/C)IC = 3VpCA modern step lessly adjustable Petersen coil consists of an iron-cored reactor connected between the star point of the substation transformer and earth in a three-phase system. In the event of a fault, the capacitive earth fault current (IC) is now neutralized by the current in the reactor as this is equal in magnitude, but 180 degrees out-of- phase.
Compensated system where Il=IC.
The Petersen coil may also be referred to as an Arc Suppression Coil (ASC).
When a phase to earth fault occurs in ungrounded 3 phase systems, the phase voltage of the faulty phase is reduced to the ground potential. This causes the phase voltage in the other two phases to rise by 3 times. This increase in voltage causes a charging current, Ic between the phase-to-earth capacitances..Diagram
The current Ic, which increases to three times the normal capacitive charging current, needs to complete its circuit. This causes a series of restrikes at the fault locations known as arcing grounds. This can also lead to overvoltages in the system.Vector Diagram
A Petersen coil consists of an iron-cored reactor connected at the star point of a three phase system. In the event of a fault, the capacitive charging current is neutralized by the current across the reactor which is equal in magnitude but 180 degrees out of phase. This compensates for the leading current drawn by the line capacitances.
The power factor of the fault moves closer to unity. This facilitates the easy extinguishing of the arc as both the voltage and current have a similar zero-crossingIC=3I=3Vp/(1/C) =3VpCWhere IC is the resultant charging current that is three times the charging current of each phase to ground.Consider a Petersen coil connected between the star-point and the ground with inductive reactance L, then
The current flowing through it is given by
IL =Vp/LTo obtain an effective cancellation of the capacitive charging currents, IL to be equal to IC.Therefore,Vp/L=3VpC From which we get,L=1/ (32C)The value of the inductance in the Petersen coil needs to match the value of the line capacitance which may vary as and when modifications in the transmission lines are carried out. Hence, the Petersen coil comes with a provision to vary the inductance.
When the value of L of arc suppression coil is such that the fault current If exactly balance the capacitive current Ic,it is called Resonant grounding. It is also called as Peterson coil grounding as the arc suppression coil used here is the Peterson coil which is an iron cored connected between the neutral and earth. The resultant current in the fault will be zero or can be reduced by adjusting the tappings on the Peterson coil.Advantages
The Peterson coil grounding has the following advantages:a. The Peterson coil is completely effective in preventing any damage by an arcingground.b. This coil has the advantage of ungrounded neutral system.
The Peterson coil grounding has following disadvantages:a. Due to varying operational conditions, the capacitance of the network changes fromtime to time. Therefore, inductance L of Peterson coil requires readjustment.b. The lines should be transposed.
Power system with resonance earthing (Petersen-coil)
Power systems with resonance earthing are widely in operation in Central European countries. The German power system statistic  indicates that 87 per cent of the MV-systems having nominal voltages Un = 10 ?30 kV and nearly 80 per cent of 110-kV-systems are operated with resonance earthing (Criteria: Total line lengths). Some MV-systems are operated with a combined scheme of resonance earthing under normal operating conditions and low-impedance earthing in case of earth- fault. Resonance earthing, therefore, is the dominating type of system earthing in Germany for power systems with voltage 10 kV up to 110 kV. In other countries such as India, South Africa and China, power systems with resonance earthing have gained an increasing importance during the last decades, however are still not so common as systems with low-voltage earthing.
Equivalent diagram in RYB-system and symmetrical components
Resonance earthing is realized by earthing of one or several neutrals of transformers through reactance (Petersen-coils), normally adjustable, which will be set in resonance to the phase-to-earth capacitances of the system. The principal arrangement of a power system with resonance earthing is outlined in Figure 5.10.The impedances of transformers and lines of the positive-sequence component can be neglected compared with those of the zero-sequence component due to the order of magnitude of the impedances. The admittance of the zero-sequence
ConclusionIn this contribution we have discussed the principle of resonant grounding and the effect of different disturbances on the measurements. A prerequisite for an automatic control operation of the Petersen coil is an unbalance in the network in order to get a non-zero neutral-to-earth voltage. To see the limits of this concept, we have elaborated the different disturbances, which may occur in every real-world network. According to these requirements we have presented a new algorithm to reduce the influence of the disturbances. The field test and the first practical experiences show the effectiveness of this new concept for the control of Petersen coils.