Since inrush current appears as operating current to a differential relay, the relay must have sufficient delay and insensitivity to the distorted wave or take advantage of the inrush’s distinctive waveform by using harmonic restraint or some other form of pattern recognition. The second harmonic predominates in inrush currents and is used in most transformer differential relays, either alone or in combination with other non-fundamental components.
The relays restrain if the harmonic(s) exceed(s) a percentage of the fundamental component. Historically, this percentage has been fixed by design. Some newer designs provide for a user setting.
Current transformer saturation also generates harmonics. Under symmetrical conditions, CT distortion produces only odd harmonics. Under asymmetrical conditions CT distortion produces both even and odd harmonics. CT saturation under asymmetrical conditions can delay a harmonically restrained element. Accordingly, an unrestrained element, set above the maximum inrush level complements the restrained unit. It is important to provide CTs with sufficient quality to provide good waveform long enough to allow either the restrained or unrestrained element operation.
Current Ip in below Figure shows sympathetic inrush current in transformer T1, resulting from the energizing of an adjacent transformer T2. The decaying dc component of current Ie flowing in
T2 develops a drop in the source resistance Rs, producing pulses of inrush current Ip on the alternate half cycles. Note the delayed buildup of Ip. The severity of the sympathetic inrush is a function of the level of dc voltage drop across the source resistance. A common set of differential relays should not be used to protect both T1 and T2 transformers in Fig. if they can be switched separately. The sum of the two transformer currents, Is, may not contain sufficient harmonics (Even Harmonics; because of closing of sum of two current to new symmetrical wave form. As we remember from Fourier analysis, the odd mathematical functions do not include even harmonics) to restrain the relays once transformer T1 saturates severely.