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41-347.12C
Protected
Transformer
I
= 0
3
OP
3
I
5
(a) External Fault
Figure 9. Simplified Schematic of the Type HU Relay with Current Distribution for (a) External Fault (b) Internal Fault.
The varistor connected across the dc side of the
restraint rectifier of the harmonic restraint unit pre-
vents excessive voltage peaks from appearing
across the rectifiers. These peaks arise through
transformer action of the harmonic-restraint polar-
unit coils during heavy internal faults. The varistor
has a large value of resistance for low voltages, while
presenting a low value of resistance for high volt-
ages. This characteristic effectively reduces the volt-
age spikes on heavy internal faults while not
hampering performance during inrush, where the
voltage is considerably lower.
M
I
AGNETIZING
NRUSH
Magnetizing inrush current waves have various wave
shapes. A typical wave appears as a rectified half
wave with decaying peaks. In any case, the various
wave shapes are rich in harmonics with the second
harmonic predominant. Since the second harmonic is
always present in inrush waves and not in internal
fault waves, this harmonic is used to restrain the har-
monic-restraint unit during inrushes. The differential
unit may or may not close its contact, depending on
the magnitude of the inrush.
When a magnetizing inrush wave is applied to the
relay, the dc component of the wave is bypassed by
the air-gap operating transformer. The other compo-
8
External
Fault
87
R
R
5
7
I
7
C
URRENTS
Protected
Transformer
I
= I
+ I
3
5
7
OP
3
I
5
(b) Internal Fault
nents are fed into the filter circuits. The impedance
characteristics of these filters are such that the sec-
ond harmonic component flows into the restraint coil
of the polar unit, while the other harmonics flow into
the operating coil. The polar unit will not close its
contacts unless the second harmonic content is less
than 15 percent of the fundamental component.
The indicating instantaneous trip unit (IIT) will not
operate on inrush. The air-gap transformer will
bypass the dc component of the inrush thereby
reducing the magnitude of the wave applied to the IIT
unit. If the inrush has an initial peak of 16 times tap
value current, the air-gap transformer will reduce this
peak to approximately 8 times tap value on the sec-
ondary of the transformer. Since the IIT unit is set for
a peak value of 14.1 times tap (rms Pick-up value =
10 times tap), it will not operate on this inrush.
B
M
REAKER
AINTENANCE
Before some of the ct's are bypassed for breaker
maintenance the trip circuit, shown in figure 6, should
be opened. Otherwise the false-unbalanced current
will cause the relay to trip. It is not necessary to
short-circuit the relay operating circuit since it has an
adequate continuous-current rating. (See "Energy
Requirements".)
Internal
Fault
87
R
R
5
7
I
7
ESK00409
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