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HONEYWELL 51404170-175 51404172-175 framework

The underimpedance function serves as back-up protection for phase faults on the generator/transformer unit. It is faster and more sensitive than the overcurrent protection. Its disadvantage is that the zone of protection is shorter than the differential protection, which serves as the main protection. The underimpedance scheme is connected to the c.t's at the generator star-point and to the v.t's at the generator terminals. The underimpedance operating characteristic is a circle in the R/X plane, whereby the origin represents the location of the v.t's. The zone of protection covers the generator windings the cables and the step-up transformer.

The setting of the underimpedance function is determined by the short-circuit reactance of the step-up transformer. Otherwise, the distance between the step-up transformer and the HV circuitbreaker is mostly too short to be able to discriminate reliably with the impedance setting between faults in the generator/transformer unit zone and faults on the other side of the HV circuit-breaker. The impedance is thus set to 70 % of the transformer impedance, which at least includes the transformer winding on the generator side in the zone of protection. The setting of the underimpedance function is referred to rated voltage and current. The impedance of the protected zone is determined by the shortcircuit reactance of the step-up transformer and is given by: z1 = 0.7 xT [p.u.] The impedance seen by the underimpedance function depends on the c.t and v.t. ratios Ki and K

Underreactance MinReactance 

A. Application
detection of inadmissible operating conditions due to underexcitation of a synchronous generator. B. Features
circular operating characteristic (see Fig. 3.5.14.1)
selectable to operate inside or outside the circle
adjustable size and position of the operating characteristic
correction of phase errors caused by input circuit
adjustable time delay
insensitive to DC component in voltage and current
insensitive to harmonics in voltage and current
single or three-phase measurement
detection of the lowest impedance (distance from the centre of the circle)
underreactance measurement enabled by undercurrent unit (0.1 IN).

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. VoltageInp defines the voltage input channel. All voltage I/P's may be selected. In the case of three-phase measurement, the first channel (e.g. the phase-to-phase voltage R-S) of the group of three selected must be specified. Voltage and current channels must be sampled by the same 216EA61 unit. Angle For compensating phase errors of the analogue input signals caused by the input circuit. The setting can also be used to move the position of the impedance circle.