Protections; U/F Protection - WEG AVR-A-OPT-03 Manual De Instalación, Operación Y Mantenimiento

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Figure 11.1: Phase diagram
According to the phase diagram, the feedback voltage suffers an influence from the "S" phase current, which is add to the
"R" and "T" phase voltage. The influence is small in module and big in phase, which means that there is a good
compensation for reactive loads and a small influence of active loads.
The current transformer for reactive compensation shall be in "S" phase of the generator, and the sensing signal in "R" and
"T" phases.
To be sure that the compensation is in the correct way, follow the next steps:
a) Start the alternatorin single form (isolate from the current), apply a resistive charge of 20% of its capacity.
b) After turn the droop adjustment trimpot completely clockwise, this process will cause a voltage fall in the generator.
Turning back the trimpot in the anti-clockwise direction, the voltage must increase. If it doesn't happen, the CT (Current
Transformer) polarity should be inverted. When several machines are connected in parallel, these steps are necessary in
each machine to make sure that all the CT's are polarized the in the same way.

12 PROTECTIONS

12.1 U/F PROTECTION

On Figure 12.1.a and Figure 12.1.b a graph is shown with the alternatorvoltage variation as a function of the frequency
variation. For rated operating frequency the U/F is disabled. In the case of rotation slowdown (when shutting down, for
example), excitation diminishes, reducing the output voltage of the generator.
The voltage drop is on the average, 0 Volts for 0Hertz. For the case shown on Figure 12.1.a and Figure 12.1.b, the U/F
adjustment was done on the limit of the rated frequency.
U/F (50Hz) U/F (60Hz)
a) b)
F(Hz)
F(Hz)
Figure 12.1: Actuation point for U/F protection
* UR= Sensing Voltage
16
l Analog Voltage Regulator AVR-A-OPT-03