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Electrical Transformer Protection
High impedance:
In the High impedance differential relay the simple current or voltage based operated relay can be use as High impedance with the series of stabilizing resistor and withe parallel connection of metrosil.
in this the setting of stabilizing resistor is based on the required operating voltage in the relay and metrosil is provided for to avoid the relay to operate from through fault current.
for a High impedance differential protection we should have same CT ratios, same Knee point voltage, same class of CT s with same magnetizing current of the ct should be use. if any changes is there in the CTs causes different burden across the both end and it makes the change in set operating voltage across the relay.( in the case of ideal transformers we can go for High impedance).
Low impedance:
in the Low impedance relay nothing but biased differential relay in this we have the slopes to avoid relay operation from the through fault current and CT saturation condition also.
In the High Impedenc DPR the Protection relay will have only one summation current input and it will pick up based on the difference in the current sumation.
Low-impedance REF protection is provided with new numerical
or microprocessor-based protection relays. In most cases, operation of the low-impedance
REF protection is based on the fundamental current, after filtering
removes all harmonic currents .
The high-impedance REF relay is normally a current operated
relay with a resistor in series that provides stabilization.
Generally, it may be one of two different types. The first
type has internal resistors and has a voltage setting. In this
type, the resistors are effectively switched in and out to
change the setting and therefore the value of the stabilizing
voltage. The second type has an external variable resistor
where the setting is calculated in ohms and applied by changing
the resistance of the variable resistor.
Even with the use of percentage differential relays, the problem of the completely saturated CT for a close-in external fault still exists. To overcome this problem, the most commonly used bus differential relay, particularly on extra high voltage (EHV) buses, is the high-impedance voltage differential relay. This relay design circumvents the effects of CT saturation during external faults by assuming complete saturation for the worst external fault and calculating the error voltage across the operating coil. The relay discriminates between internal and external faults by the relative magnitudes of the voltage across the differential junction points.
One of the principal difficulties with shunt reactor protection is false relay operation during energizing or de-energizing the iron core. During these periods, DC offset with long time constants and low-frequency components of the reactor energization current cause most of the problems. High-impedance differential relays rather than low-impedance relays are recommended if this problem occurs.
