Page 82 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 71

Fig. 2.44 Fluxes in 3-limb core.

Fig. 2.45 Residual fluxes in 3-limb core and tank.

quantities, they do not sum to zero. The residual flux F 0 can be visualized as flux that
is flowing in all three limbs at the same time. It must find a return path outside the
three main limbs. In Figure 2.45 the return path is through the space surrounding the
core and into the tank. Since the tank is not laminated it is liable to carry induced
eddy-currents which can cause it to overheat. The flux path for the residual flux has a
high reluctance and therefore a low inductance, because the flux must travel a long
way through the gap between the core and the tank. Consequently the zero-sequence
inductance is low, and the zero-sequence currents (which flow in the neutral wire) can
be large. If there is no neutral, the potential of the neutral point will oscillate (see
Figure 2.48).
In a five-limb core the return path is provided through two extra unwound limbs at
the ends of the transformer core. The reluctance of the zero-sequence or residual flux-
path is now low, so the zero-sequence inductance is high. This tends to limit the zero-
sequence current in the neutral connection to a low value. The residual flux does not
leakoutside the core and there is therefore no riskof overheating the tankby eddy-
currents.

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The term `zero-sequence' comes from the theory of symmetrical components, which is the mathematical
basis for analysis of unbalanced three-phase systems.