Current Elements - GE D90 Plus Instruction Manual

CHAPTER 7: PROTECTION

Current elements

Time overcurrent curves
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
For the sending bus relay, line-side VTs:
1. The net inductive reactance from relay into local system is –2 Ω + 3 Ω – 4 Ω = –3 Ω < 0.
An offset impedance ≥3 Ω must be used.
2. The net inductive reactance from relay through far-end busbar = 10 Ω – 3 Ω = 7 Ω. The
offset cannot be greater than 7 Ω.
3. Therefore, it is recommended to use a 5 Ω offset impedance.
For the receiving bus relay, bus-side VTs:
1. The net inductive reactance from relay into local system is –5 Ω + 7 Ω = 2 Ω > 0. There
is no need for offset.
2. The net inductive reactance from relay through far-end busbar is –3 Ω + 10 Ω – 4 Ω =
3 Ω; the offset cannot be greater than 3 Ω.
3. Therefore, it is recommended to use a 1.5 Ω offset impedance.
For the receiving bus relay, line-side VTs:
1. The net inductive reactance from relay into local system is –3 Ω – 5 Ω + 7 Ω = –1 Ω < 0.
An offset impedance ≥1 Ω must be used.
2. The net inductive reactance from relay through far-end busbar is 10 Ω – 4 Ω = 6 Ω. The
offset cannot be higher than 6 Ω.
3. Therefore, it is recommended to use 3.5 Ω offset impedance.
High-set overcurrent guidelines for protecting series compensated lines
Especially at low SIR values, fast fault clearance times are extremely important, from both
system stability and equipment damage viewpoints. The high-set overcurrent element,
when set appropriately, can be useful in achieving these goals. It helps the setting
calculations if the system impedances are reasonably well known.
Set the overcurrent pickup to the greater of the following values:
• The maximum infeed seen by the D90
• The maximum fault level seen by the D90
The maximum error of the phase overcurrent elements is less than 2%. Use a safety factor
of 1.25 to account for relay errors and system impedance uncertainty.
If CT saturation is an issue, such as close to a generation where long lasting DC
components are likely to saturate the CTs, note that the instantaneous overcurrent
elements require 1.33 cycles of data to operate for a multiple of pickup of 1.01. For higher
multiples of pickup, the relation between the multiple of pickup and the amount of data
required for operation before complete CT saturation is approximately linear. For example,
for a multiple of pickup of 4, approximately 1.33 / 4 = 0.332 of a power cycle is required by
the phase instantaneous overcurrent element to operate. Do not confuse this information
with the operating time, which includes some inherent delays, such as a trip rated output
contact.
This section outlines the current element settings.
The inverse time overcurrent curves used by the time overcurrent elements are the IEEE,
2
IEC, GE Type IAC, and I t standard curve shapes. This allows for simplified coordination with
downstream devices. If however, none of these curve shapes is adequate, FlexCurves can
be used to customize the inverse time curve characteristics. The definite time curve is also
an option that can be appropriate if only simple protection is required.
The following overcurrent curve types are available:
GROUPED PROTECTION ELEMENTS
Plus
, for a close in reverse fault
Plus
for a fault at 100% of the protected line
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