Auxiliary Current Transformers
Many circuit problems can be solved by connecting one current transformer to supply another current transformer, usually called an Auxiliary current transformer, shown in Figure below . Auxiliary current transformers perform like other current transformers, but there are certain problems.
First, the auxiliary current transformer constitutes an additional burden on the main current transformer, which usually increases the errors of the main current transformer. Second, the performance of auxiliary current transformers is not usually as good as the performance of main current transformers. This is because the auxiliary current transformer is designed to impose as small a burden as possible on the main current transformer. This means its own burden capacity must be relatively low. These two considerations mean that the errors of transformation, when an auxiliary current transformer is used, will typically be three times the value which might be expected with a single transformer. However, the special functions which can be performed by the auxiliary current transformer, as indicated in Figure , often dictate their use.
The Auxiliary current and voltage transformers change the overall ratio of main current transformers. They are connected in series with the secondary of the main current transformer when currents of various magnitudes or phase relationships must be matched. Special ratios in addition to those listed may be obtained by connecting the windings as auto transformers. The transformer may be used as a voltage transformer at voltages not exceeding 0.6 volts per turn. When used as a voltage transformer, the burden impedance must be at least 100 times the transformer impedance in order to keep the ratio error within 1%