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IS220UCSAH1A Gas turbine card

E. Setting instructions Basic setting g setting Pick-up ratio v setting Operating char. switching point b Increased basic setting g-High Pick-up differential current I-Inst (no influence on inrush detection) Pick-up ratio of the inrush detector InrushRatio Inrush detection time InrushTime Amplitude compensation factors a1 a2 a3 Transformer circuit on winding 1 s1 Group of connection on windings 2 and 3 s2 s3 The purpose of the transformer differential protection is to detect phase faults in the protection zone. It may also detect earth faults and interturn faults. The protection is sensitive, fast and absolutely discriminative. Basic setting g The basic setting g defines the pick-up setting of the differential protection for internal faults. The lowest possible value should be chosen for "g" (high sensitivity) to enable it to detect transformer earth faults and interturn faults in addition to phase faults. The setting of "g" must not be too low, however, to avoid the danger of false tripping due to:
c.t. errors
the maximum off-load transformer current at the maximum short-time system voltage
tap-changer range. The off-load current of a modern power transformer is very low and usually lies between 0.3 and 0.5 % of rated current at rated voltage. During short-time voltage peaks, e.g. following load shedding, the off-load current can reach as much as 10 % or more.

The tap-changer voltage range is usually between ±5 % and ±10 %, but ranges of ±20 % and more do occur. Its influence has to be taken into account regardless of whether the tap-changer is manually operated or controlled by a voltage regulator.

Pick-up ratio v 

The pick-up ratio "v" is decisive for the stability of the protection during external phase and earth faults, i.e. in the presence of high through-fault currents. The value of "v" defines the ratio of the operating current to restraint current. The setting should be such that when operating under load conditions, weak faults causing only a low differential current can still be detected, but also there is no risk of maloperation during through-faults. A typical setting is v = 0.5. Restraint current b The restraint current b defines the point at which the characteristic is switched. The sloped section of the characteristic ensures that the relay remains stable during through-faults with c.t. saturation. The ability to switch between two different slopes enables the characteristic to adapt to different conditions. The recommended setting for "b" is 1.5. This provides high stability during high through-fault currents and sufficient sensitivity to detect fault currents in the region of the operating current. Factors a2 and a3 The full setting range for the factors a2 and a3 for compensating current amplitude only applies, if the reference value for the channels is set to 1.000. At all other settings, the permissible upper limit reduces in proportion to ratio of the channel reference values (transformer winding 2/winding 1, respectively winding 3/winding 1).

All three of these influences cause a differential current, which flows during normal system conditions. The setting for "g" must be chosen above the level of this differential current. A typical setting is g = 0.3 IN (i.e. 30 % IN).