Operation - ABB HU Instruction Leaflet

41-347.12C
pick-up value of the switch.
I I
NDICATING NSTANTANEOUS
The instantaneous trip unit is a small ac operated
clapper-type device. A magnetic armature, to which
leaf-spring mounted contacts are attached, is
attracted to the magnetic core upon energization of
the switch. When the switch closes, the moving con-
tacts bridge two stationary contacts completing the
trip circuit. Also, during the operation, two fingers on
the armature deflect a spring located on the front of
the switch which allows the operation indicator target
to drop.
A core screw accessible from the top of the switch is
used to adjust for pickup.

OPERATION

The types HU and HU-1 are connected to the pro-
tected transformer as shown in figure 4. In such a
connection, the relays operate to protect the trans-
former for faults internal to the differential zone of the
transformer, but not for faults external to the zone.
Neither do the relays operate on magnetizing inrush
currents associated with energization of the trans-
former, even though these currents may appear as
an internal fault. To avoid these false operations,
each unit of the relay performs a separate function.
The differential unit (DU) prevents operation on
external faults, while the harmonic-restraint unit
(HRU) prevent operations on magnetizing inrush cur-
rents. Hence, the operation of the relay can best be
described under the headings of external fault cur-
rent, internal fault currents, and magnetizing inrush
currents.
E F C
XTERNAL AULT URRENTS
The types HU and HU-1 relays have a variable per-
centage characteristic. This means that the operating
current required to close the contact of the differen-
tial unit expressed in percent of restraint current var-
ies with the magnitude of the larger restraint current.
Figure 7 and figure 8 illustrate this characteristic. To
use these curves, divide each restraint current by the
appropriate tap and enter the horizontal axis using
the larger or largest restraint multiple. Then enter the
vertical axis, using the difference of the restraint mul-
tiples.
With the relay connected as shown in the schematic
6
T U (IIT)
RIP NIT
diagram of figure 9a, an external fault causes cur-
rents to flow in the air-gap restraint transformers of
the differential unit. If the line current transformers do
not saturate and the correct ratio matching taps
applied, no effective current flows in the operating
transformer of the relay. Hence, only a contact-open-
ing torque is produced on the differential unit.
On heavy external faults where a main current trans-
former saturates, current flows in the operating circuit
of the relay. With such a condition, the harmonic-
restraint unit may or may not close its contacts,
depending upon the harmonics present in the false
operating current. However, operation of the relay is
prevented by the variable percentage characteristic
of the differential unit, since a large differential cur-
rent is required to close its contacts during heavy
external faults.
I F
NTERNAL AULTS
In the case of an internal fault as shown in figure 9b,
the restraint of the differential unit is proportional to
the largest restraint current flowing. The sum of the
two restraint currents flows into the operating trans-
former and produces an excess of operating torque,
and the differential unit operates.
In the case of an internal fault fed from one source
only, the fault current flows in one restraint trans-
former and the operating transformer. An excess of
operating torque is produced on the differential unit
and it operates.
Faults normally appear as an offset sine wave with a
decaying dc component, and contain very few har-
monics. As a result, the harmonic-restraint unit will
operate during internal faults to permit the tripping of
the relay.
For heavy internal faults, the indicating instanta-
neous trip unit (IIT) will operate. Since this unit is
connected to an air-gap transformer, essentially only
the sine wave component of an internal fault is
applied to the IIT unit. The dc component of the fault
is bypassed by the transformer primary. For exam-
ple, an internal fault with a first peak of 28 times tap
value (includes fifty percent dc) is reduced to a first
peak of approximately 14 times tap value (dc compo-
nent absent) on the secondary transformer. The IIT
unit will just operate on this wave since it is set to
pick up at a peak current of 14.1 times tap (rms pick-
up value = 10 times tap).