1-Abnormally high primary fault currents, primary fault currents having a dc offset, residual flux, high secondary burden, or a combination of these factors results in the creation of high flux density in the CT iron core. When this density reaches or exceeds the design limits of the core, saturation results. At this point, the accuracy of the CT becomes poor, and the output waveform may be distorted by harmonics. Saturation results in the production of a secondary current lower in magnitude than would be indicated by the CT ratio. The severity of this transformation error varies with the degree of saturation. With total saturation, virtually no secondary current flows past the first quarter cycle.
For example, selective coordination of protective devices may not occur if CTs on a branch circuit saturate. Tripping of the branch circuit breaker may be delayed or may not even occur. Also high phase currents (e.g., due to motor starting inrush or phase faults) may cause unequal saturation of the CTs and produce a false residual current when applications have been made using the residual connection from three CTs. As a result, undesired tripping of the ground relay may occur, and the production or process may be jeopardized.
Such an event would result in the operation of the line-side main circuit breaker and result in an more extensive outage than should have occurred. Instantaneous over current relays may not even trip where the fault currents are high, and bus differential relays may falsely trip on through faults.
To avoid or minimize saturation effects, the secondary burden should be kept as low as possible.
Where fault currents of more than 20 times the CT nameplate rating are anticipated, a different CT, different CT ratio, or a lower burden may be required.
Generally saturation on symmetrical AC is most critical at the point of relay decision. Thus, in differential protection, the decision point is at the CT nearest to an external fault. A fault on one side of the current transformer is internal, for which the protection must operate, but the fault on the other side is external, and the protection must not operate. This external fault is often very large and requires good CT performance. AC saturation should not occur for this protection.
For over current protection of lines, the decision point is remote from the CT and generally not so critical because time is involved in the relay operation; hence, some saturation can be tolerated. For heavy close-in faults, these relays may be operating on the relatively flat part of their time curve, for which magnitude differences are not critical. This also applies to close-in heavy faults for instantaneous or distance-type relays.
Saturation by the DC offset of the primary AC current is a function of the power system, and it is not practical to avoid its effect by the CT design. This can be critical in differential protect ion where sever all CTs are involved in the fault determination. Differential relay designs use various techniques to prevent misoperation, generally keyed to no AC saturation.
In most other r applications, DC saturation is no t likely to b e too severe, or to significantly inhibit the protection from a practical standpoint. However, it should always be considered and checked.
Most faults tend to occur near the maximum voltage, at which the prefault current is low in the inductive power system. This minimizes the DC offset; therefore, it is seldom at its maximum possible value. In man y parts of the power system, the time constant will be short, such that when the DC offset occurs, it rapidly decays. Moreover, faults other than differential protect ion are not at the maximum value at the critical decision point of the protect ion.
For example, in line protect ion, the relay decision point is remote from the
CTs; consequently, the fault current is often lower, and line resistance is available to help mode rate the effect. In addition, the decision point may not be too critical, for time is often involved to clear remote faults. Generally, the close-in high-current faults will be greater than the relay pickup current, and with high-speed relays, operation may take place before DC CT saturation occurs. Should saturation occur before the line protection relays can operate, generally a delay in operation occurs until the CTs recover sufficiently to permit operation. Thus, the tendency for this type of protect ion usually is to ‘‘under-reach’’ momentarily, rather than ‘‘over-reach.’’
2- The equivalent circuit of sever saturated CT in high impedance comparison protections is short circuited of secondary side of saturated CT with its secondary winding resistance.