4
Some notes and
useful hints
Connection with
two phase current
transformers
If phase current transformers are used only in
two phases, a third current is recommended to
be summed from the currents of these phases.
This current is conducted to the input circuit
of the missing phase. This procedure has two
advantages, i.e. the phase failure protection does
not have to be disconnected and the measure-
ment of the load currents is more accurate than
in a two-phase measurement.
Stabilizing for virtual
earth-fault currents
The apparent neutral current caused by the dif-
ference of the phase current transformers con-
nected in parallel may cause an unnecessary op-
eration of the earth-fault unit, especially in an
overload situation. This can be avoided by us-
ing a stabilizing resistor R in the neutral cur-
rent circuit. The resistor must have a continu-
ous power rating of e.g. 30 W and can have a
resistance value of e.g. 100 Ω for the 1 A input
versus 10 Ω when the 5 A secondary input is
used. The value of the knee-point voltage must
be checked > 2 x U
stab
. The stabilizing resistor
will also slightly reduce the E/F sesitivity.
Problems with false
E/F or U/B trips
during start-up
If the unbalance or earth-fault units cause false
trippings during a start-up e.g. because of main
CT saturation or severe amounts of harmonics,
the units can be blocked by a control from the
starter over the external control input. If no
control signal is available, in some cases the start
information available from the realy itself can
be used for this blocking purpose by feedback
to the external control input.
A core balance transformer is recommended to
be used in networks with isolated neutral or in
high resistance earthed networks. The trans-
forming ratio of the core balance transformer
can be freely selected according to the earth-
fault current and, consequently, the sensitivity
of the earth-fault protection too. Due to the ex-
tremely small burden of the relay, very small
transforming ratios may be used in the cable
current transformers, in a KOLMA type trans-
former even as small as 10/1 A. A transforming
ratio of at least 50/1 A or 100/1 A is recom-
mended to be used. The setting of the earth-
fault unit is typically selected in the range
5...30% of the fully developed earth-fault cur-
rent and a typical trip time could be 0.5...2 sec.
The phase unbalance unit monitors the current
asymmetry of the network and protects the
motor against heavy network unbalance or sin-
gle-phasing. The phase unbalance unit is stabi-
lized against maloperation due to heavy currents
and also allows a higher degree of unbalance
when the motor is running at a load less than
the full load current. The operating time of the
unit follows an inverse time characteristic.
A separate unit checks for phase reversal condi-
tions and operates within a fixed time of 600
ms on a wrong phase sequence.
The unbalance and phase reversal units can be
separately selected or taken out of use. If e.g. a
motor is used also with a reversed direction of
rotation, the phase reversal is taken out of serv-
ice and does in this way not cause a trip when
the direction is reversed.
The undercurrent unit operates upon a sudden
loss of load. The unit is used e.g. for submers-
ible pumps, where the cooling is based on a con-
stant flow of liquid. If this flow is interrupted,
the cooling capacity of the motor is reduced.
This condition is detected by the undercurrent
unit, which trips the motor.
The start-up totalling counter constitutes an-
other way to control the number of start-up
attempts within a certain time and can be set to
comply with the motor manufacturers´ state-
ments on start-up number limiting.
By using the memorized fault parameters a com-
prehensive after-fault analysis can be performed
and a continuous follow-up of the start-up and
other parameters can give good indications
about the condition of the motor.
The serial communication facilities finally make
it possible to achieve all the measured and
memorized parameters in the easiest possible
way, direct to a control room or a similar place.
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