The theory and operation of Petersen coils

Abstract

The Introduction deals with the characteristics of systems having insulated and earthed neutral points.The elementary theory of the Petersen-coil system is then considered. The two main effects of the coil are neutralization of the leading component of the fault current and slowing down of the rate of rise of recovery voltage (r.r.r.v.) across the fault arc. The effect of system leakage resistance upon the residual fault current is also discussed.The author then considers the effect of unsymmetrical line capacitances in causing an excessive voltage on the neutral of a healthy system. Slight detuning of the coil may be necessary to reduce this voltage, and there is a possibility of unwanted operation of protective gear.Consideration of the effect of fault impedance on the fault and coil currents shows that normal values of fault resistance have a negligible effect upon the fault current, which is primarily determined by the tuning of the coil. Accurate tuning increases the sensitivity of the indicating gear to high-resistance faults and limits the power loss at the fault. The behaviour of reactive as distinct from resistive faults is discussed.The possibility of freak resonant effects during abnormal circuit conditions, such as the opening of one phase only, due to conductor breakage or non-simultaneous clearing of circuit breakers, is mentioned. Saturation of the iron core of the coil keeps unwanted resonance within safe limits, but abnormal operation of indicating gear may sometimes be attributed to these effects.The theory of arc extinction in Petersen-coil systems is then expounded, and the effect of resistance damping and tuning factor upon the r.r.r.v. across the fault is dealt with.A method of calculating the system capacitance to earth is given, together with formulae for 3-phase lines with and without earth wires. The effect of line series reactance, and the behaviour of electrically long lines, are considered. The Appendices contain general formulae.The application of Petersen coils to double-circuit and parallel lines is dealt with. A fault on one circuit impresses a voltage on the neutral of the adjacent healthy circuit. Probable values of this voltage are indicated and the advantages of compensating the mutual capacitance between circuits by means of “decoupling” and “suction” coils are analysed. Practical methods are suggested for tuning star-connected earthing and suction coils.