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VMIVME-7750 General electric module

NrOfPhases defines whether single or three-phase measurement. CurrentInp defines the current input channel. All current I/P's may be selected. In the case of three-phase measurement, the first channel (R phase) of the group of three selected must be specified. BlockInp Binary address used as blocking input. (FFALSE, TTRUE, binary input or output of a protection function). Trip Output for signalling tripping (signal address). Start Output for signalling pick-up (signal address).

E. Setting instructions Settings: Current pick-up I-Setting Delay Delay Over or undercurrent MaxMin The definite time overcurrent function protects transformers, various parts of the auxiliaries supply system and possibly also generators against overcurrents caused by overloads, phase faults and earth faults. The filter at the input suppresses any DC component and harmonics so that the overcurrent function responds essentially just to the fundamental. The function is also suitable for sensitive current measurements such as are necessary, for example, to detect low earth fault currents. Connected across an external high-impedance circuit, the function can also be used as restricted earth fault (REF) scheme.

Current pick-up setting I-Setting

 The current setting I-Setting must be sufficiently high on the one hand to avoid any risk of false tripping or false signals under normal load conditions, but should be low enough on the other to detect the lowest fault current which can occur. The margin, which has to be allowed between the maximum short-time load current and the setting, must take account of:
the tolerance on the current setting
the reset ratio. The maximum short-time load current has to be determined according to the power system conditions and must take switching operations and load surges into account.

Fig. 3.5.3.1 Operating characteristic of the definite time overcurrent function Should the rated currents of c.t. IN1 and protected unit ITN differ, compensation is recommended. This is achieved with the aid of the reference value of the A/D channel or by correcting the overcurrent setting. For example, for ITN = 800 A and IN1 = 1000 A, the setting for a pick-up current of 1.5 ITN = 1200 A would have to be 15 15 800 1000 12 1 .. . I I TN N

CurrentInp An interposing c.t. in the input is essential for low current settings (< 0.2 IN, i.e. from 0.19 IN downwards). Delay The delay is used to achieve discrimination of the overcurrent function. It is set according to the grading table for all the overcurrent units on the power system. The zone of protection of our overcurrent function extends to the location of the next downstream overcurrent relay. Should the downstream relay fail to clear a fault, the overcurrent function trips slightly later as a back-up protection.