ELECTRICAL DESIGN1. FERRORESONANCEThe phenomenon of ferroresonance is the occurrence of highvoltages which may occur when a modest size capacitance is eitherin series or in parallel with nonlinear inductance, such as an ironcored transformer.In power systems, the most common place to find ferroresonance iswith a three phase distribution transformer energised through anunderground cable of moderate length. Under no load, or very lightload conditions, the capacitance of the cable is sufficient to precipitateferroresonant behaviour under single phase switching conditions (eg.the operation of an HV fuse or asynchronous operation of singlephase11kV switches such as a drop-out fuse unit.)The trend towards undergrounding of distribution assets and theincreasing installation of URD has resulted in a higher incidence ofsituations where single phase switching of the cable connectingtransformer could result in dangerous overvoltage due toferroresonance.The simplest form of occurrence of a ferroresonance circuit in a URDdistribution system is when the single-phase operating switchgear orswitch fuses are located some distance away from the transformeritself, with a length of cable joining the switchgear and transformer. Acircuit of this sort could occur, for example, where a substation issupplied from a set of EDO’s on a cable termination pole.In the case where single phase switching is performed directly at thetransformer terminals, there is no capacitance in circuit and as aresult no abnormal circuit. Since the equivalent circuit of a cableunder no load conditions is essentially a capacitive circuit, thepresence of the cable introduces a capacitance into the circuit andforms a series LC circuit consisting of the transformer winding,which under no load can be represented by an iron cored inductance, inseries with the core-sheath capacitance of the cable. [Note that this circuitapplies to 3-core screened and single core cables, ie. There is no core tocore capacitance.] The three-phase equivalent circuit is shown in Figure 1.Figure 1.1With one phase energised (R phase for example as shown in Figure 1.1) aseries circuit is formed consisting of the magnetised inductance Lm

between R and Y phases and the Y phase core-to-sheath cablecapacitance. In parallel with this circuit is a second identical series circuitconsisting of the magnetised inductance Lm between R and B phases andthe B phase core-to-sheath capacitance. Since each branch of this parallelcircuit is identical, the potential between the points Y and B is zero andtherefore the magnetising inductance Lm between Y and B phases doesnot enter into the circuit. Combining the circuit components results in anequivalent series circuit consisting of a capacitance in series with a nonlinearinductance which is therefore the ferroresonant circuit.