FAQ No. FAQ01600
Question
Why an SE Relay cannot be used in circuits containing thyristor control or
a rectifier?
Answer
A schematic diagram showing the operating principle for the SE is shown
below.
As shown in the above diagram, the waves for all three phases of the motor
current from the CT of the SET-3A (3B) are rectified, then converted to DC
voltage, which is necessary for the SE, by the resistor at each tap, and
output from the positive and negative terminals. The DC and AC component
ratios in this voltage differ between the normal three-phase condition and
conditions of phase-failure or imbalance.
For the overload factor, the DC average level is detected relative to the
set value. For the phase-failure factor, the proportion of the DC average
level and AC component is used to distinguish phase failure or imbalance.
When the motor is operated using thyristor phase control, the motor
current waveform is not a sine wave. The SE detects this as an adverse
condition and initiates unnecessary operations. The example below shows a
primary current and SET-3 output waveform. When there is a large AC
component in the output voltage, the SE series sometimes judges it to be a
phase-failure and initiates unnecessary operations.
Other Measuring & Monitoring Relay
FAQs
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