GE STD15C Instructions Manual page 13

GEK—45307
Any current of distorted, nonsinusoidal waveform may be considered as being composed
of a DC component plus a number of sine-wave components of different frequencies;
one of the fundamental system frequency, and the others, called "harmonics," having
frequencies
which are
2, 3, 4, 5,
etc.,
times the fundamental frequency. The
relative magnitudes and phase positions of
the harmonics with reference to
the
fundamental
determine the waveform. When analyzed in this manner, the typical
fault-current wave is found to contain only a very small
percentage of harmonics,
while the typical magnetizing-inrush—current wave contains a considerable amount.
The high percentage of harmonic currents in the magnetizing-inrush—current wave
afford an excellent means of distinguishing it electrically from the fault-current
wave. In the Type STD
relays, the harmonic components are separated
from
the
fundamental component by suitable electric filters.
The harmonic current components
are passed through the restraining circuit of
the
relay, while the fundamental
component is passed through the operating circuit.
The DC component present in both
the magnetizing-inrush— and offset-fault—current waves
is largely blocked by the
auxiliary differential-current transformer
inside
the relay, and produces only a
slight momentary restraining effect.
Relay operation occurs on differential-current
waves in
which
the
ratio of harmonics
to fundamental
is lower
than a given
predetermined value, for which the relay is
set (e.g. an internal fault—current
wave), and is restrained on differential—current waves in which the ratio exceeds
this value (e.g. magnetizing-inrush—current wave).
BURDENS
Burdens are shown in Table
III and IV. Burdens and minimum pickup values
are
substantially independent of the percent slope settings, and are all approximately
100% power factor. Figures given are burdens imposed on each current transformer at
5.0 amperes.
TABLE III
ZERO OPERATING CIRCUIT
* RESTRAINT CIRCUIT
TA
G
RESTRAINT 60 CYCLE RELAYS 60 CYCLE RELAYS
SETTIN
PICKUP
BURDEN IMPEDANCE BURDEN IMPEDANCE
AMPS
AMPS
VA
OHMS
VA OHMS
2.9
0.87
3.2 0.128 1.3 0.052
—
3.2 0.96
2.7 0.108 1.2 0.048
3.5 1.05 2.4
0.096 1.1 0.044
3.8
1.14
2.0 0.080 1.0 0.040
4.2 1.26
1.9 0.076 0.9 0.036
4.6 1.38 1.6
0.064 0.8
0.032
5.0 1.50 1.5 0.060 0.7
0.028
8.7 2.61
0.7 0.028 0.5 0.020
*
Burden of operating coil is 0 under normal conditions.
Burden of 50-cycle relay is the same or slightly lower.
TABLE IV
DC CONTROL CIRCUIT BURDEN
RATED VOLTS
48
125
250 48
110
220
MILLIANPS 140
105
88 140 96 80
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