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HIMA K9202 Fan components

E. Setting instructions Settings: Current pick-up Current Delay Delay Undervoltage Hold-Voltage Reset delay Hold-Time The voltage controlled overcurrent function comprises a definite time overcurrent unit, which latches when the undervoltage unit responds. The protection is intended for generators and generator/transformer units, for which a fault current can fall below the pick-up of the overcurrent protection before it has opportunity to trip. Apart from the influence of a DC component, a decaying AC component can only occur on a generator, the steady-state fault current of which is very low, because of the large synchronous reactance Xd typical of modern generators (see Fig. 3.5.7.1). This function is largely insensitive to DC component and harmonics.

Overcurrent setting "Current" The current setting is chosen such that neither false tripping nor false signals can occur during normal operation and yet the minimum fault current is detected. The setting must therefore be between the maximum short-time load current and the minimum fault current and allow for the tolerance on the protection setting and also its reset ratio. The maximum short-time load current is a parameter of the power system concerned and must take account of switching operations, load surges and fast response excitation (Fig. 3.5.7.2).

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 the voltage controlled overcurrent function extends from the c.t's supplying it in the generator star-point to the location of the next downstream overcurrent relay. Undervoltage setting "Hold-Voltage" Providing the overcurrent unit has picked up and the undervoltage unit picks up as well, the undervoltage unit "Hold-Voltage" latches it in the pick-up state should the fault current fall below its pick-up setting. The setting of the undervoltage unit must be such that it can clearly distinguish between a normal load and a fault condition. Because of the different conditions prevailing during symmetrical and asymmetrical faults, the positivesequence component of the three-phase system is evaluated. A setting well below the lowest voltage, which can occur during normal load conditions is chosen (Fig. 3.5.7.3).

Reset time "Hold-Time" The reset time defined by the parameter "Hold-Time" determines how long the overcurrent unit remains latched to ensure a tripping signal of sufficient duration. The delay starts at the instant of tripping. Typical settings: Current 1.5 IN Delay 3 s Hold-Voltage 0.7 UN Hold-Time 0.5 s Should the rated currents of generator and c.t's differ appreciably, compensation of the overcurrent setting is recommended, if this has not already been done with the aid of the reference value of the A/D channel.