Current transformer ratio selection and performance require special attention when applying transformer protection. Unique factors associated with transformers, including its winding ratios, magnetizing inrush current, and the presence of winding taps or load tap changers, are sources of difficulties in engineering a dependable and secure protection scheme for the transformer.
Errors resulting from CT saturation and load-tap-changers are particularly critical for differential protection schemes where the currents from more than one set of CTs are compared.
CT Current Mismatch
Under normal, non-fault conditions, a transformer differential relay should ideally have identical currents in the secondaries of all current transformers connected to the relay so that no current would flow in its operating coil. It is difficult, however, to match current transformer ratios exactly to the transformer winding ratios.
This task becomes impossible with the presence of transformer off-load and on-load taps or load tap changers that change the voltage ratios of the transformer windings depending on system voltage and transformer loading.
The highest secondary current mismatch between all current transformers connected in the differential scheme must be calculated when selecting the relay operating setting. If time delayed overcurrent protection is used, the time delay setting must also be based on the same consideration.
The mismatch calculation should be performed for maximum load and through-fault conditions.
CT saturation could have a negative impact on the ability of the transformer protection to operate for internal faults (dependability) and not to operate for external faults (security).
For internal faults, dependability of the harmonic restraint type relays could be negatively affected if current harmonics generated in the CT secondary circuit due to CT saturation are high enough to restrain the relay. With a saturated CT, 2nd and 3rd harmonics predominate initially, but the even harmonics gradually disappear with the decay of the DC component of the fault current. The relay may then operate eventually when the restraining harmonic component is reduced.
These relays usually include an instantaneous overcurrent element that is not restrained by harmonics, but is set very high (typically 20 times transformer rating). This element may operate on severe internal faults.
For external faults, security of the differentially connected transformer protection may be jeopardized if the current transformers’ unequal saturation is severe enough to produce error current above the relay setting. Relays equipped with restraint windings in each current transformer circuit would be more secure.
Magnetizing Inrush (Initial, Recovery, Sympathetic)
When a transformer is energized after being de-energized, a transient magnetizing or exciting current that may reach instantaneous peaks of up to 30 times full load current may flow.
This can cause operation of overcurrent or differential relays protecting the transformer. The magnetizing current flows in only one winding, thus it will appear to a differentially connected relay as an internal fault.
Techniques used to prevent differential relays from operating on inrush include detection of current harmonics and zero current periods, both being characteristics of the magnetizing inrush current. The former takes advantage of the presence of harmonics, especially the second harmonic, in the magnetizing inrush current to restrain the relay from operation.
The latter differentiates between the fault and inrush currents by measuring the zero current periods, which will be much longer for the inrush than for the fault current.
Recovery Inrush //
Typically, this occurs upon removal of an external fault. The magnetizing inrush is usually less severe in this case than in initial energization as the transformer was not totally de-energized prior to voltage recovery.
Sympathetic Inrush //
A magnetizing inrush current can flow in an energized transformer when a nearby transformer is energized.
The offset inrush current of the bank being energized will find a parallel path in the energized bank. Again, the magnitude is usually less than the case of initial inrush. Both the recovery and sympathetic inrush phenomena suggest that restraining the transformer protection on magnetizing inrush current is required at all times, not only when switching the transformer in service after a period of de-energization.
For transformers with standard delta-wye connections, the currents on the delta and wye sides will have a 30° phase shift relative to each other. Current transformers used for traditional differential relays must be connected in wye-delta (opposite of the transformer winding connections) to compensate for the transformer phase shift.
Phase correction is often internally provided in microprocessor transformer protection relays via software virtual interposing CTs for each transformer winding and, as with the ratio correction, will depend upon the selected configuration for the restrained inputs.
This allows the primary current transformers to all be connected in wye.
Fault currents resulting from a turn-to-turn fault have low magnitudes and are hard to detect.
Typically, the fault will have to evolve and affect a good portion of the winding or arc over to other parts of the transformer before being detected by overcurrent or differential protection relays.
Through faults could have an impact on both the transformer and its protection scheme. Depending on their severity, frequency, and duration, through fault currents can cause mechanical transformer damage, even though the fault is somewhat limited by the transformer impedance.
For transformer differential protection, current transformer mismatch and saturation could produce operating currents on through faults. This must be taken into consideration when selecting the scheme, current transformer ratio, relay sensitivity, and operating time.
Differential protection schemes equipped with restraining windings offer better security for these through faults.
Backup protection, typically overcurrent or impedance relays applied to one or both sides of the transformer, perform two functions. One function is to backup the primary protection, most likely a differential relay, and operate in event of its failure to trip.
The second function is protection for thermal or mechanical damage to the transformer. Protection that can detect these external faults and operate in time to prevent transformer damage should be considered. The protection must be set to operate before the through-fault withstand capability of the transformer is reached.
If, because of its large size or importance, only differential protection is applied to a transformer, clearing of external faults before transformer damage can occur by other protective devices must be ensured.
Resource: Power System Protection – Arun Phadke, Virginia Polytechnic Institute