ABBDistributed busbar protection REB500 including line and transformer protection Product Guide

permissible. 

TPX, TPY and TPZ CTs may be mixed within 

one substation in phase-fault schemes. The 

relatively low CT performance needed for the 

busbar protection makes it possible for it to 

share protection cores with other protection 

devices.

Current transformer requirements for stability during external faults (Busbar protection)

The minimum CT requirements for 3-phase 

systems are determined by the maximum fault 

current. 

The effective accuracy limit factor (n') must be 

checked to ensure the stability of the busbar 

protection during external faults.

The rated accuracy limit factor is given by the 

CT manufacturer. Taking account of the burden and the CT losses, the effective accuracy 

limit factor n' becomes:

where:

n = rated accuracy limit factor 

PN = rated CT power

PE = CT losses

PB = burden at rated current

In the case of schemes with phase-by-phase 

measurement, n' must satisfy the following 

two relationships: 

where:

IKmax = max. primary through-fault current

I1N = rated primary CT current

Taking the DC time constant of the feeder into 

account, the effective n' becomes:

(2) n' 10 for TN 120 ms, or

n' 20 for 120 ms <TN 300 ms.

where: 

TN = DC time constant

Example: IKmax = 30000 A

I1N = 1000 A

TN  120 ms

Applying relationships (1) and (2): 

(2) n' 10

Selected: n' 10

The current transformer requirements for 

REB500sys for Line and Transformer protection are described in a separate publication 

[1].

Pick-up for internal faults

In the case of internal busbar faults, CT saturation is less likely, because each CT only 

conducts the current of its own feeder.

Should nevertheless CT saturation be possible, it is important to check that the minimum 

fault current exceeds the setting for Ikmin.

Note:

For systems that measure I0, the REB500 

questionnaire 1MRB520371-Ken should be 

filled in and submitted to ABB, so that the CT 

requirements can be checked in order to 

ensure proper I0 measurement.