Out Of Step Protection Operataing Principle - GE MiCOM P40 Agile Technical Manual

Chapter 10 - Power Swing Functions
The OST principle uses positive sequence impedances. The positive sequence impedance is calculated as Z
V /I , where V and I are the positive sequence voltage and current quantities derived from the measured phase
1 1 1 1
quantities. The concentric quadrilaterals are designated Zone 5 and Zone 6. Zone 5 encompasses possible system
fault impedances and sits within Zone 6. Because OST and Predictive OST quadrilaterals are based on positive
sequence impedances, all OST conditions are covered by a single measurement. Both quadrilaterals are
independent and have independent reach settings.
All four resistive blinders are parallel, using the common angle setting (a) that corresponds to the angle of the total
system impedance (Z = Z
T
sending and receiving ends and Z
neither reactance line tilting nor esidual compensation is implemented.
In the figure, the purple solid impedance trajectory represents the locus for the non-recoverable power sawing,
known as a pole slip or Out Of Step condition. The dotted green impedance trajectory represents a recoverable
power swing.
4.2

OUT OF STEP PROTECTION OPERATAING PRINCIPLE

The Out of Step function has four different setting options, which are only visible if the PowerSwing Block is
enabled in the CONFIGURATION column:
Disabled: Disables the Out of Step function.
●
● Pred. OST: Splits the system in advance. It minimizes the angle shift between two ends and aids stability
in the split areas.
● OST: Splits the system when an out of step condition is detected, which is when a pole slip occurs.
Pred. OST or OST: Splits the system in advance or when an out of step condition is detected.
●
The Out-of-Step detection algorithm is based on the speed of the positive sequence impedance passing through
the characteristic. When the positive sequence impedance enters the outer quadrilateral (Zone 6) a timer is
started. The timer is stopped after the positive sequence impedance passes through the inner quadrilateral (zone
5). Let us call this time the zone 6 to zone 5 transition time.
If this time is less than 25 ms, the protection considers this to be a power system fault, not an Out-of-Step
condition. This 25 ms time is fixed and cannot be set. During a power system fault, the speed of change from a
load impedance to a fault impedance is fast, but the protection may operate slower for marginal faults close to a
zone boundary. This is particularly the case for high resistive faults inside the zone operating characteristic and
close to the Zone 5 boundary. The fixed time of 25 ms is implemented to provide sufficient time for a distance
element to operate and therefore to distinguish between a fault and an extremely fast power swing.
If the zone 6 to zone 5 transition time takes more than 25 ms but less than the set delta T time, this is treated as a
very fast power swing and the protection will trip if either the Pred. OST Trip or the Pred. & OST Trip
options are selected and the Out-of-Step tripping time delay (Tost) has expired. The minimum delta T setting is 30
ms, allowing 5 ms margin with the fixed 25 ms timer.
If the zone 6 to zone 5 transition time takes longer than the set delta T time, it is considered as a slow power
swing. On entering Zone 5, the protection records the polarity of the resistive part of the positive sequence
impedance. From this state, two outcomes are possible:
If the resistive part of the positive sequence impedance leaves Zone 5 with the same polarity as previously
●
recorded on entering Zone 5, it is considered to be a recoverable swing. In this case, the protection does not
trip.
If, when exiting Zone 5, the resistive part of the positive sequence impedance has the opposite polarity to
●
that of the recorded polarity on entering Zone 5, an Out-of-Step condition is recognised. This is followed by
tripping if either Pred. OST Trip or Pred. & OST Trip is selected.
As the tripping mode for the detected Out-of-Step condition is always three-phase, the Pred. OST and OST DDB
signals are mapped to the three-phase tripping signal in the default programmable scheme logic.
284
+ Z + Z ), where Z and Z
S L R S
positive sequence line impedance. The reactance lines are also parallel as
L
are equivalent positive sequence impedances at the
R
P543i/P545i
=
1
P54x1i-TM-EN-1