ABB RELION 670 Series Applications Manual page 140

Section 6
Differential protection
6.1.5 Summation principle
6.1.5.1 Introduction
A simplified bus differential protection for phase and ground faults can be obtained by using a
single, one-phase IED with external auxiliary summation current transformers. By using this
approach, more cost effective bus differential protection can be obtained. Such a solution makes
it feasible to apply bus differential protection even to medium voltage substations. The principal
differences between full, phase-segregated bus differential protection scheme and summation
type bus differential protection scheme are shown in figure 72.
Three one-phase
REB670
IEC06000126 V2 EN-US
Figure 72: Difference between phase segregated & summation type differential protection
In the full, phase-segregated design three, one-phase REB670 IEDs (that is, one per phase) are
used. However for the summation type only single, one-phase REB670 IED plus one auxiliary
summation CT per each main CT is required. These auxiliary summation CTs convert each main CT
three-phase currents to a single-phase output current, which are all measured by one REB670 IED.
The differential calculation is then made on a single-phase basis. By doing so, this more cost
effective bus differential protection can be applied. Due to this characteristic, this summation
type of bus differential protection can be applied for all types of stations arrangements as shown
section "Different busbar
As an example, the necessary equipment for the summation type, busbar differential protection
for a single busbar station with up to 24 bays, is shown in figure 73.
134
REB670
REB670
@
REB670
arrangements", for three, one-phase IEDs.
Single one-phase
REB670 type SLCE 8; 1/1A, 2/1A or 5/1A
REB670
Up to 18 pcs
+
with 1A
auxiliary CTs
CT inputs
Auxiliary Summation CT *)
*) One SLCE 8 per main CT
IEC06000126_2_en.vsd
Application manual
1MRK 505 337-UUS A
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