ABB REL 356 Instruction Booklet Download

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ABB REL 356 Current Differential Protection

3-33

Settings and Application

Out of Step Block (OSB)

OSB = BOTH

This setting enables the out of step block logic that blocks the Zone 2 and/or Zone 3 distance units for out of step
conditions. The setting options are:

NONE

No OSB used

Out of step logic blocks Zone 2 impedance elements only

Out of step logic blocks Zone 3 impedance elements only

BOTH

Out of step logic blocks Zone 2 and Zone 3 impedance elements

Inner Blinder (RT)

RT = 1.15 ohms

This setting is the offset in the perpendicular direction to the line impedance on the R-X diagram in secondary ohms.
Apart from its function in the OSB and OST logic, it also performs load restriction of the three phase impedance units.

The resistance RT must have a setting sufficiently low (in ohms) to avoid operation on the minimum stable swing ohms
expected. Similarly it must have a setting sufficiently high to accommodate the maximum fault resistance that is likely
to be encountered for three phase faults on the protected line. Operation of the three phase distance function for all
zones can be avoided for swing angles (between the two equivalent sources) as great as 120

if the inner blinder is given

a resistance setting of 0.288 Z

or less, where Z

is the protected line positive sequence impedance plus the sum of the

lowest positive sequence impedance at each end of the line. The minimum setting should accommodate a three phase
fault resistance value of at least 0.1 Z

ohms based on an arc voltage of 400 volts per foot (primary) and typical phase

separation.

Using the average of these two conservative figures a reasonable value to use for the inner blinder resistance is 0.2 Z

RT = 0.2 x Z

= 0.2 (Z1SA + Z1SB + ZL1) = 0.2( 2.56 + 2.22 + 4.86) primary = 5.752 x 0.2 secondary = 1.15 ohms

secondary

Outer Blinder (RU)

RU = 3.45 ohms

This setting is the offset in the perpendicular direction to the line impedance on the R-X diagram in secondary ohms.

The outer blinder resistance setting must be chosen to assure proper distinction between faults and swing conditions.
Sustained load must not be allowed to operate the outer blinder. The recommended setting is 3 times the inner blinder
setting but not more than 80% of minimum load impedance.
RU = 3 x RT = 3 x 1.15 = 3.45 ohms < 80% of the minimum load impedance (10.6 ohms)

Out of Step Detection Timer (OST1)

OST1 = 5.00 cycles

This timer is started when the outer blinder, RU, has operated but the inner blinder, RT, has not operated. If the timer
times out, an out of step condition has been detected and OSB is active. The recommended setting is 3 cycles.
Out-of-step trip not used

Out of Step Trip Way In Timer (OST2)

OST2 = 5.00 cycles

This timer is started when the an out of step condition has been detected and the two blinders have operated. Once it
times out, a trip signal is issued for OST. The recommended setting is 2 cycles.
Out-of-step trip not used

Out of Step Trip Way Out Timer (OST3)

OST3 = 5.00 cycles

This timer is started when the an out of step condition has been detected and the OST2 timer has timed out and both
blinders are not operated. This permits controlling the time that the breaker opens. The recommended setting is 0.5
cycles.
Out-of-step trip not used