ABB Relion 670 Series Technical Reference Manual page 208

Section 6
Current protection
202
cycle). These samples correspond to the instantaneous voltage waveform
across the protected SCB and in further text will be marked with symbol u
• Equivalent rms voltage value based on Peak Voltage measurement. This value
is obtained as maximum absolute voltage sample value over last power system
cycle divided by √2 and in further text will be marked with symbol U
• Equivalent true RMS voltage value based on the following formula:
N
å
2
u
~ m
=
=
U m 1
TRMS
N
EQUATION2234 V1 EN
Where:
N is used number of samples in one power system cycle (for example, 20)
u are last N samples of the voltage waveform
~m
In further text this equivalent true RMS voltage quantity will be marked with
symbol U
TRMS
Some additional filtering of the calculated voltage quantities is additionally
performed within the function in order to avoid equivalent RMS voltage values
overshooting during capacitor switching.
In order to avoid dependence of the current integration on exact value of the
protected capacitor bank capacitance the whole integration process is done in per
unit system. In order to convert measured current in primary amperes into per unit
value the base current for the protected capacitor bank shall be known. This value
is set as parameter IBase and it represents the rated SCB current in primary
amperes at fundamental frequency. This value is calculated for a three-phase SCB
as follows:
[
×
1000 Q MVAr
=
IBase
[ ]
×
3 U kV
EQUATION2235 V1 EN
Where:
IBase
is base current for the function in primary amperes
Q[MVAr]
is shunt capacitor bank MVAr rating
U[kV]
is shunt capacitor bank rated phase-to-phase voltage in kV
]
1MRK 505 208-UEN B
~
peakRMS
(Equation 40)
(Equation 41)
Technical reference manual