ABB Relion REG670, Applications Manual

Page: 338 / 780

protection must be calculated. Taking this value as a base, the highest pickup current
setting can be written according to Equation
183 .
Ipu 0.7 Isc min
£ ×
EQUATION1263 V2 EN
(Equation 183)
where:
0.7
is a safety factor
Iscmin is the smallest fault current to be detected by the overcurrent protection.
As a summary, the operating current shall be chosen within the interval stated in
Equation
184 .
Im ax
1.2 Ipu 0.7 Isc min
k
× £ £ ×
EQUATION1264 V2 EN
(Equation 184)
The high current function of the overcurrent protection, which only has a short-delay
trip time, must be given a current setting so that the protection is selective to other
protection functions in the power system. It is desirable to have rapid tripping of faults
within a large part of the power system to be protected by the protection (primary
protected zone). A fault current calculation gives the largest current of faults, Iscmax,
at the most remote part of the primary protected zone. The risk of transient overreach
must be considered, due to a possible DC component of the short circuit current. The
lowest current setting of the fastest stage can be written according to
max
1.2
t sc high
I k I
³ × ×
EQUATION1265 V1 EN
(Equation 185)
where:
1.2
is a safety factor
k
t
is a factor that takes care of the transient overreach due to the DC component of the fault
current and can be considered to be less than 1.05
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate time of the phase overcurrent protection has to be chosen so that the fault
time is short enough that the protected equipment will not be destroyed due to thermal
overload while, at the same time, selectivity is assured. For overcurrent protection in
a radial fed network, the time setting can be chosen in a graphical way. This is mostly
used in the case of inverse time overcurrent protection. Figure
175 shows how the
time-versus-current curves are plotted in a diagram. The time setting is chosen to get
the shortest fault time with maintained selectivity. Selectivity is assured if the time
difference between the curves is larger than a critical time difference.
Section 9
1MRK 502 071-UEN -
Current protection
332 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual