Protection
8
Startup elements
The startup elements basically work as
sensitive detector to all types of fault. As
soon as fault or disturbance happens, the
highly sensitive startup elements will
operate immediately and initiate all
necessary
protection
functions
for
selective clearance of the fault.
The control circuit of tripping relays is
controlled by the startup elements. Only
when one of the startup elements is
triggered, the tripping relays can be
energized to trip. Thus, the maloperation,
due to fatal internal hardware fault, is
avoided in this way.
Based on different principle, there are four
kinds of startup elements listed below,
which are used to enhance the sensitivity,
and to guarantee the security in case of
IED’s internal hardware faults.
Sudden-change current startup
element
Sudden-change phase to phase or zero
sequence current elements are the main
startup element that can sensitively detect
most of faults. The criteria are as follows:
or
where:
Δi
=|| i
(K) - i
(K-T) | - |i
(K-T) - i
(K-2T) ||
: AB,BC or CA, e.g. iAB= iA-iB
K: The present sample
T: The sample quantity of one power cycle
Δ3i
0
: Sudden-change zero sequence current
I_Abrupt: The setting value of current
sudden-change elements
Zero sequence current startup
element
Zero sequence current startup element is
applied to improve the fault detection
sensitivity at the high resistance earth
faults. As an auxiliary startup element, it
operates with a short time delay.
Overcurrent startup element
If overcurrent protection function is
enabled, over current startup element is
used to improve the fault detection
sensitivity. As an auxiliary startup element,
it operates with short time delay.
Low-voltage startup element
When one end of the protected line is a
weak-source
system,
and
the
fault
sudden-change phase to phase current is
too low to startup the IED, low-voltage
startup element can be in service to startup
the tele-protection scheme with weak-echo
logic.
Phase selector
The IED applies different phase selectors
to determine the faulty phase to make
tripping
or
Auto-reclosing
initiation
correctly. There are three kinds of phase
selectors based on different principle for
different fault stages.
Sudden-change current phase
selector
It operates as soon as the sudden- change
current startup element starts up. It makes
a phase selection for fast tripping by