3.2
SEL-587Z Instruction Manual Date Code 20020903
Relay Elements
High-Impedance Protection
High-Impedance Protection
The SEL-587Z Relay combines high-impedance protection principles with
advanced numerical technologies. The relay offers fast response during
internal faults and high security for through faults. Other features are
metering, sequential events reporting, digital fault recording, remote setting
capabilities, SEL
OGIC
®
control equations, and a high flexibility in adapting
the relay to required needs.
The SEL-587Z provides three independent high-impedance elements, one for
each phase. The relay also provides single- and three-phase overcurrent
elements with negative- and zero-sequence calculated elements. The low-
impedance overcurrent elements have instantaneous, definite time (SEL
OGIC
control equations timer), and time-overcurrent characteristics. All six
elements are independent, resulting in a completely flexible relay.
Two separate digital filters optimize the input quantities to the elements. The
half-cosine filter in the high-impedance elements primarily removes the dc
component, but also the odd harmonics as a result of the filter characteristic,
while the full-cosine filter in the overcurrent elements filters all but the
fundamental frequency.
Figure 3.1 Differential Relay Currents.
High-Impedance
Differential
The differential element compares the measured voltage with the differential
element setting. Because the calculation compares two values, the element
acts as a comparator, as shown in Figure 3.2 (A-phase) to Figure 3.4 on
page 3.3. Each phase has a separate differential element to provide phase-
segregated measurements. Each differential element has two levels, providing
six settings in total. When the measured voltage exceeds the setting, the relay
asserts the appropriate Relay Word bit. If the Relay Word bit was entered in
the trip equation and assigned to an output contact, the output contact closing
results in circuit breaker operation.
Figure 3.2 A-Phase High-Impedance Element.
87A
IA
87B
87C
IB
IC
LPF
and
ADC
Half-
Cosine
Digital
Filter
Full-
Cosine
Digital
Filter
Magnitude
Calculation
and Comparison
With Settings
Magnitude
Calculation
and Comparison
With Settings
50 Elements
51 Elements
87 Elements
_
+
_
+
87A1
87A2
Setting
87A1P
Setting
87A2P
87A
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