When setting the relay characteristic/torque angle for the three phase directional overcurrent element, a positive angle setting was specified. This was due to the fact that we consider as polarising voltage, the phase voltage value in quadrature of the current under fault conditions. With directional earth fault protection, the residual current under fault conditions lies at an angle lagging the polarising residual earth fault voltage.
When the value given by the formula is greater than 3KV peak, it is necessary to use
Metrosils. They are connected across the relay circuit and they allow to shunt the secondary
current output of the current transformer from the relay in order to prevent very high
secondary voltages.
Metrosils are externally mounted and have annular discs shape.
Their operating characteristics is according to the formula:
V = C.I0.25
Where
− V: Instantaneous voltage applied to the non-linear resistor (Metrosil)
− C: Constant of the non-linear resistor (Metrosil)
− I: Instantaneous current through the non-linear resistor (Metrosil)
With the sinusoidal voltage applied across the Metrosil, the RMS current would be
approximately 0.25 times the peak current. This current value can be calculated as follows:
4
52.0 2).(
⎭
⎬
⎫
⎩
⎨
⎧ = C
Irms rmsVs
Where
− Vs(rms): RMS value of the sinusoidal voltage applied across the Metrosil.
This is due to the fact that the current waveform through the Metrosil is not sinusoidal but
appreciably distorted.
For satisfactory application of the non-linear resistor (Metrosil), its characteristics should
comply with the following requirements:
− At the relay voltage setting, the non-linear resistor (Metrosil) current should be as
low as possible, but no greater than approximately 30mA rms for 1A current
transformers and approximately 100mA rms for 5A current transformer.
− At the maximum secondary current, the non-linear resistor (Metrosil) should limit
the voltage to 1500V rms or 2120V peak for 0.25 second. At higher relay voltage
settings, it is not always possible to limit the fault voltage to 1500V rms, so higher
fault voltage may have to be tolerated.
The following tables show the typical types of Metrosil that will be required, depending on
relay current rating, REF voltage setting etc.
