Comprehensive protection circuit for transformer

(1) Regardless of whether the overcurrent protection of the transformer adopts a current relay or a release, whether it is a definite time limit or an inverse time limit,
the composition and principle of the transformer overcurrent protection are exactly the same as those of the power line overcurrent protection described earlier.
    The setting of the transformer overcurrent protection operating current is basically the same as that of the power line overcurrent protection, which is the rated primary current of the transformer.
    The setting of the transformer overcurrent protection action time is also the same as that of the power line overcurrent protection, and is also set according to the "ladder principle". But
for the terminal substation of the power system, such as the transformer in the workshop substation, the action time can be set to the minimum value (0.5s).
    The sensitivity of the transformer overcurrent protection is calculated based on the primary
short-circuit current value I1rmt when a two-phase short circuit occurs on the secondary side bus of the transformer in the minimum operating mode of the system. For inspection, the sensitivity coefficient sp ≥ 1.5 is required. ． If sp fails to meet the requirements, low-voltage blocking over-current
protection can also be used.
    (2) Current quick-break protection of transformer The composition and principle of current quick-break protection of transformer are also
the same as the current quick-break protection of power lines described earlier.
    The setting calculation formula of the operating current (quick-break current) I of the transformer current quick-break protection is basically
the same The effective value of the three-phase short-circuit current period component of the secondary side bus of the transformer should be converted to the short circuit on the primary side. The current value, that is, the quick-
break current of the transformer current quick-break protection should be set to avoid the three-phase short-circuit current of the secondary side busbar.
    The sensitivity of the transformer current quick-break protection is tested based on the short-circuit current of a two-phase short circuit when the protection device is installed in the minimum system mode, and
sp ≥ 1.5 ~ 2 is required.
    The current quick-break protection of the transformer, like the current quick-break protection of the power line, also has a "dead zone". To compensate for the dead zone, it is also equipped with a
time-limited overcurrent protection.
    Considering that the transformer will have an impactful excitation inrush current when it is put into operation without load or when the voltage is suddenly restored, in order to avoid
the malfunction of , after the quick-break power-off E setting, the transformer can be put into trial operation several times when it is no-load. To check whether the current quick-break protection of the transformer malfunctions.
    (3) Overload protection of transformer The composition and principle of overload protection of transformer are exactly the same as the overload detection and protection of power lines.
    The setting calculation formula of the operating current of the transformer overload protection is basically the same as that of the power line overload protection. The action time limit of transformer overload protection
is generally 10-15s. The picture shows the comprehensive protection circuit of the transformer’s fixed-time overcurrent protection , current quick-break protection and overload protection
.