Cahier technique no 195 - Schneider Electric

A 15 VA-5P20 CT has a guaranteed error of less It is the magnetising current I m which generatesthan 5 % when it is subjected to 20 times its a metering error. If the CT were perfect,nominal current and delivers into its nominal load then I m = 0.(15 VA to I n ).The CT magnetisation curve represents theEach of the characteristics b, c, d is a function ofmagnetising current as a function of voltage Vthe two others.sdeveloped at the CT secondary. It can be dividedThe same CT can be affected a different power, into 3 zones (see fig. 3 ):a different accuracy class and a different ALF. 1 - non-saturated zone,However, a given CT only has one magnetising 2 - intermediate zone,curve and only one secondary winding3 - saturated zone.resistance (at a given temperature).When these last two elements are known (curve In zone 1, current I m is low and voltage V sand resistance), we can identify all theincreases almost proportionally to the primarynecessary correspondences between the various current.values a, b, and c to be assigned to the CT or Zone 2 is a vague zone between the nonsaturatedzone and the saturated zone. There israther between the various triplet combinations:(b 1 , c 1 , d 1 ) ⇔ (b 2 , c 2 , d 2 ) ⇔ (b i , c i , d i )no real break in the magnetisation curve. It ishard to locate a precise point on the curveAll the equivalences are deduced from the corresponding to the saturation voltage.simple laws of electricity, in particular Ohm’s law.In zone 3, the curve (V s I m ) becomes almostCT equivalent diagram (see fig. 2 ).horizontal. The error is considerable on the ratioand the secondary current distorted by thec CT ratio: I / I .n1 nsaturation.2c L m : CT equivalent inductive magnetisation A certain number of characteristic voltages are(saturable).highlighted for a CT: they correspond to zone 2 ;c I m : magnetising current.knowledge of these voltages is necessary whenc I 1 : primary current.another definition is given to a particular CT.c I 2 : secondary current corresponding to aIn2perfect CT, i.e. I 2 = I .1In13c I s : secondary current effectively crossing the2→ → →CT secondary: I2= Is + Im .V s1I 1 I 2 I s R ctI n1I n2I mL mV sR p1 - Non-satured zone2 - Intermediate zone3 - Satured zoneI mFig. 2 : a CT equivalent circuit.Fig. 3 : magnetisation curve (excitation) V s = f(I m )of a CT.Cahier Technique Schneider Electric no. 195 / p.5