Voltage Memory; Impedance Zones; Single Pole / Three Pole Tripping (406 / 408 Only) - Siemens 7SG164 Ohmega 400 Series Manual

7SG164 Ohmega 400 Series Impedance Functions

1.3 Voltage Memory

The relay incorporates a voltage memory feature. This ensures directional security under closein three
phase fault conditions when there is little or no healthy phase voltage available for cross-polarising.
The relay monitors the positive sequence voltage of the system, and under fault conditions it adds a
replica of this voltage to the polarising voltage of each comparator, suitable phase-shifted to align it
with the ideal polarising voltage. 100% memory polarising is used, and is applied in conjunction with
cross-polarising.
Voltage memory is applied for a maximum of 200ms, after which time the zone 1 and, where fitted,
zone 4 comparators will be either inhibited from operating or have their operation latched until the fault
is cleared. Zone 2 and Zone 3 continue to operate to provide backup protection.

1.4 Impedance Zones

The relay has three zones of protection as standard; zone 1 & 2 are polarised to operate in the forward
direction while zone 3 can be set as either forward, reverse or as an offset zone. If the zone 3 is set as
an offset zone then the minimum reverse reach is the same as the minimum forward reach. A fourth
zone is available as an option in some models. This is a reverse-looking polarised characteristic. It is
commonly used as a non-tripping zone in conjunction with a blocking scheme.
The accuracy of the relay is ±5% or 0.1Ω, whichever is larger. The range for each of the impedance
elements is from 0.1Ω to 250Ω, regardless of the output of the current transformers. Obviously, the
settings used for 5A CTs will be smaller than those used for 1A CTs, on an equivalent circuit.
With 5A CTs the minimum advisable setting is 0.1Ω.
With 2A CTs the minimum advisable setting is 0.2 Ω.
With 1A CTs the minimum advisable setting is 0.5Ω.
For 2A and 1A, the settings can be reduced below these minimums, but the relay accuracy will be
reduced.

1.5 Single Pole / Three Pole Tripping (406 / 408 Only)

As the distance relay has separate comparators for each fault type there is an option for single pole
tripping. The tripping actions are determined by a choice of auto-reclose settings. There are three
auto-reclose actions available.
None
Internal
External
When none is selected then the relay will only issue a three pole trip. Regardless of fault type. With
Internal selected the relay allows either single pole or three pole trip dependent on the auto re-close
scheme settings. When external is selected the relay issues a single pole trip unless the three pole
trip select input is energised.
Single pole tripping only operates for a Zone 1 or aided trip fault. All other trips are always three pole.
©2010 Siemens Protection Devices Limited Chapter 2 Page 14 of 21