Basler BE1-CDS220, Instruction Manual

Page: 463 / 476

BE1-CDS220 Settings Calculations D-5
Step 5: Calculate actual tap settings. Choose the tap that is out of range, and adjust to the nearest
valid tap setting.
Step 6: Calculate the opposite actual tap such that Equation D-5 is true.
2
1
2
1
ActualTap
ActualTap
IdealTap
IdealTap
=
Equation D-5
Calculate Minpu
The minimum pickup restraint setting (Minpu) adjusts the sensitivity of the relay. In non-numerical relays,
the Minpu was fixed at a typical value of 0.35 of the relay tap. In the BE1-CDS220 relay, the user can
choose lower or higher values to optimize the protection in each particular application. Selecting a lower
minpu setting will tend to raise the slope setting to maintain a given margin at the knee-point of the
differential tripping characteristic (for more information, see the Calculate Slope paragraphs later in this
section). Conversely, it is sometimes necessary to accommodate unmonitored loads in the differential
zone. In that case, the minpu setting may be higher. A setting of 0.25 per unit of transformer full load (FA)
rating is recommended for typical installations where no unmonitored load needs to be considered. This
value is well above the magnetizing current and provides a safe margin at the knee point of the slope
characteristic. If unmonitored loads such as station service or small capacitor banks are connected in the
differential zone, the minpu must be increased by the magnitude of the unmonitored current in multiples of
tap.
Step 1: Calculate desired minimum pickup setting in times tap. Choose the desired minimum pickup
in per unit on the transformer full load base (0.25). Multiply times the factor X calculated using
Equation D-6. If the ideal tap (TAP
nI
) is equal to the actual tap (TAP
nA
), the factor will be 1.
For this example, this is the case, so the minimum pickup of 0.25 per unit will equal a
minimum pickup of 0.25 times tap.
CTRn KVn TAP
COMP MVA
Tap
TAP
X
nA
n
nA
nI
∗ ∗ ∗
∗ ∗
= =
3
1000
Equation D-6
Step 2: Calculate the unmonitored load in times tap. Use Equation D-7.
CTRn TAP
COMPn pri Iunmon
Iunmon
n
∗
∗
=
0
=
Iunmon
Equation D-7
Step 3: Calculate the Minpu setting. Add the two to get the Minpu setting.
Minpu = 0.25 + 0
= 0.25
NOTE
Unmonitored capacitor banks should be avoided if possible, or they must be
treated very carefully to avoid false tripping due to inrush currents.