GE Multilin 489 Instruction Manual page 168

5–58
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the step-up transformer connection. If this setting is chosen as "None", then it is assumed
that the transformer is Wye-Wye connected or that there is no step-up transformer. In this
case the element will use the following operating quantities.
Element
AB
BC
CA
If this setting is chosen as "Delta/Wye" then it is assumed that the transformer is Yd1 or
Yd11. In this case the following operating quantities are used.
Element
AB
BC
CA
The first zone is typically used to provide a backup protection for a step-up transformer
and generator system bus protection (generator impedance should not be included into
reach setting). The reach is set to cover the step-up transformer impedance with some
margin, say 25%. The time delay should be coordinated with step up transformer and bus
backup protection.
The second zone reach is typically set to cover the longest transmission line or feeder
leaving the generating station. Care must be taken for possible under-reaching effects due
to the fault contribution from other lines or generators. The element is intended for backup
protection and therefore time delay should always be included. This element is typically set
to coordinate with the longest operating time of the system distance relays protecting
lines leaving station.
The measuring point of the element is defined by the location of the VT and CT as shown in
FIGURE 5–11: Distance Element Setup on page –57. Therefore, the impedance of the step-
up transformer should be included and the impedance of the generator should not be
included. Care should also be taken to ensure the apparent impedance seen by the
element when the machine is operating at worst-case load and power factor does not
encroach into the operating characteristic. The reach setting is in secondary ohms. The
minimum operating time of the element is 150 ms to coordinate with VTFF operating time
(99 ms).
Voltage Current
Va – Vb Ia – Ib
Vb – Vc Ib – Ic
Vc – Va Ic – Ia
Voltage Current
× Ia
(Vab – Vca) /
3 3
(Vbc – Vab) / × Ib
3 3
(Vca – Vbc) / × Ic
3 3
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS