Schweitzer Engineering Laboratories SEL-587Z, Instruction Manual, Page: 40 / 276

3.8
SEL-587Z Instruction Manual
Date Code 20020903
Relay Elements
High-Impedance Protection
Figure 3.8 on page 3.7 shows two curves. Consider the areas under the curves
when selecting the number of MOVs to install: for an intersection point below
the bottom curve, select a single MOV; for an intersection point below the top
curve (between the two curves), select two MOVs. If the intersection point
falls above the top line, select a different CT ratio, CT class, or both.
In our example, the fault secondary current of 125 A and 432 V intersects at a
point between the two curves, i.e., a higher requirement than only one MOV,
but not exceeding the top requirement. For this case, order two MOVs. Most
applications should use the two MOV option.
CT Selection
Reference
It is vital that you select adequate current transformers for a high-impedance
differential application. Use the procedures outlined in ANSI/IEEE Standard
C37.110: 1996,
IEEE Guide for the Application of Current Transformers Used
for Protective Relaying Purposes. Connect the CT secondary windings in
parallel on the basis that all CTs are matched in ratio. Whereas CT saturation
is a possibility for through faults, CT saturation always occurs for in-zone
faults. Select, therefore, a CT core of sufficiently large area to provide enough
energy to the relay for fast operation.
Use bushing or toroidal CTs with fully distributed windings and low leakage
flux. For multiratio CTs, use the highest ratio in order to minimize the
secondary current and hence the voltage developed for the worst external
fault. Also, higher CT tap values generate progressively higher voltages
(autotransformer action); the voltage generated on the higher tap may exceed
the CT secondary insulation value during bus faults.
CT Arrangements
Connect the CT secondary windings in parallel on the basis that all CTs are
matched in ratio. Leads must be connected with the same polarity.
Figure 3.9
depicts a single-phase representation of an application for a single-zone, five
circuit breaker substation showing the CT polarity.
Figure 3.9 Single-Zone, Five Circuit Breaker Application (simplified).
CT Open Circuit Detection
Open circuiting a CT (faulty connector, removed test block, etc.) results in a
voltage appearing across the relay elements, the value of which is proportional
to the load current flowing through the missing CT input. For external faults
or heavy loads, this voltage may rise to a value higher than the relay setting,
resulting in relay misoperation. Set one level of the differential elements
(87n2, n = A, B, C) to a low value to detect this condition. You can activate an
alarm after a short time delay (minutes), block the trip elements after a longer
time delay (minutes or hours), or both.
Busbar
SEL-587Z
87A 87B 87C
52 52 52 52 52