ABB RELION 620 Series Technical Manual page 487

1MAC504801-IB E
620 series ANSI
Technical Manual
2
U
kn
R 10
×
s
GUID-8C4CAB10-997F-4C14-9F30-5DB6F4CEC54E V1 EN
The actual sensitivity of the protection is affected by the protection relay setting, the
magnetizing currents of the CTs connected in parallel and the shunting effect of the
voltage-dependent resistor (VDR). The value of the primary current I
protection relay operates at a certain setting can be calculated using the formula:
I = × n ( I + I + × m I
prim rs u
GUID-2A742729-7244-4B1C-A4DF-404BDD3A68D9 V1 EN
I the primary current at which the protection is to start
prim
n the turn ratio of the current transformer
Operate value setting
I the value of the
rs
I the leakage current flowing through the VDR at the U
u
m the number of current transformers included in the protection per phase
I the magnetizing current per current transformer at the U
m
The I value given in many catalogs is the excitation current at the knee point voltage.
e
Assuming U ≈ 2 x U , the value of
kn s
The selection of current transformers can be divided into procedures:
1. The rated current I
n
high I is.
kmax
2. The rated primary current I
feeder.
The choice of the CT also specifies R
3. The required U is calculated using
kn
enough, another CT has to be selected. The value of U
in the case of Class X current transformers or it can be estimated using
4. The sensitivity I
prim
sufficient, the present CT is chosen. If a better sensitivity is needed, a CT with a
bigger core is chosen.
If other than Class X CTs are used, an estimate for U
)
m
s
I
e
≈
I
m gives an approximate value for
2
of the feeder should be known. The value of I
of the CT must be higher than the rated current of the
1n
.
in
Equation
is calculated using Equation
Protection functions
at which the
prim
voltage
voltage
s
also affects how
n
56. If U of the CT is not high enough,
kn
is given by the manufacturer
kn
59. If the achieved sensitivity is
is calculated using the equation:
kn
Section 4
(Equation 58)
(Equation 59)
Equation 59.
Equation 60.
481