Over-dimensioned CTs are capable of transforming fully offset fault currents without distortion. During voltage transformer supplier i.e. when there is no earth leakage current, the secondary of CBCT does not carry any current, as there is no net magnetic flux in the core. This allows the number of secondary turns to be chosen such as to optimize the effective primary pick up current.
The number of secondary turns does not need to be related to the cable/feeder rated current because no secondary current would flow under normal balanced conditions. They are different from normal protective and metering current transformers due to their performance requirement. At low primary current ratings it may be difficult to obtain sufficient output at the desired accuracy.
Many current transformers have a ring-shaped core, sometimes built up from annular stampings, but often consisting of a single length of strip tightly wound to form a close-turned spiral. It is used for auxiliary current transformers and for many low or moderate ratio current transformers used in switchgear of up to 11kV rating. This value is known as the ‘accuracy limit current’ and may be expressed in primary or equivalent secondary terms.
The values of the current error and phase error depend on the phase displacement between Is and Ie, but neither current nor phase error can exceed the vectorial error Ie. With a moderately inductive burden, resulting in Is and Ie approximately in phase, there is little phase error and the exciting component results almost entirely in ratio error. Figure 3 – Vector diagram for current transformer (referred to secondary) The secondary circuit must not be interrupted while the primary winding is energized.
The system is considered to be carrying rated current (300A) and the CT is feeding a burden of 10VA. Current & voltage in power circuits. The EZCT-2000C is Vanguard’s third-generation microprocessor-based current transformer test set.
3.Toroidal Current Transformer – This transformer does not contain primary windings. The current transformer is mainly classified into three types, i.e., wound current transformer, toroidal current transformer and bar-type transformers. The magnitude of the primary and secondary voltages depends on the number of turns on the windings.
Thus, the secondary of the CT may never be open when the primary is carrying the current. Thus, if the secondary winding is opened and the current flows through the primary windings, then there will be no demagnetizing flux due to the secondary current. When the current flows through the primary windings, it always flows through secondary windings and amperes turns of each winding are subsequently equal and opposite.
Thus, the current transformer operates on secondary circuit conditions. The secondary winding of the transformers has a large number of turns. The primary and the secondary windings are insulated from the cores and each other.
The current transformer is used for measuring the high voltage current because of the difficulty of inadequate insulation in the meter itself. The small Socomec split current transformer for easy retrofitting of cable connections.