1. ZERO SETTING POWER SWING
BLOCKING PROTECTION
BY - JYOTIRMOY
CHANDRAKANTH
SURJIT
BADAL

2. DEFINITION : POWER SWING
A power swing is a system phenomenon that is observed when the phase
angle of one power source starts to vary in time with respect to another
source on the same network
 A power swing is stable when, following a disturbance, the rotation speed of all
machines returns to synchronous speed
 A power swing is unstable when, following a disturbance, one or more machines do
not return to synchronous speed. The generator torque angle reaches 180 degrees,
the machine is said to have slipped a pole, reached an out-of-step(OOS) condition,
or lost synchronism.

3. STABLE AND UNSTABLE POWER SWINGS

4. IMPEDANCE TRAJECTORIES SEEN BY A DISTANCE RELAY
• The impedance measurements
based on these varying voltages
and currents will also oscillate.
• If the measured impedance
becomes very small and enters the
distance relay zones it can lead to
an undesired trip of the distance
relay.

5. IMPEDANCE TRAJECTORIES FOR VARIOUS
RATIOS OF |ES/ER|

6. JUSTIFICATION FOR POWER SWING DETECTION
PSB signal ensures security of relay elements that are prone to
operate during power swings
OST signal separates power system at predetermined locations
to avoid network collapse during unstable power swings

7. CONVENTIONAL POWER SWING DETECTION METHODS
a) Rate of Change of Impedance Methods
 The rate of change of impedance methods are
based on the principle that the Z1 impedance
travels in the complex plane with a relatively
slow pace whereas during a fault,Z1 switches from
the load point to the fault location almost
instantaneously.

8. RATE OF CHANGE OF IMPEDANCE METHODS
Blinder Scheme
 Single Blinder : detects an unstable power
swing when the time interval required to cross
the distance between the right and left
blinders exceeds a minimum time setting
 Double Blinder : During a power swing, the
single-blinder element measures the time
interval ∆T that it takes the Z1 trajectory to
cross the distance between the outer and inner
blinders. When this measured time interval is
longer than a set time delay, a power swing is
declared.

9. CONVENTIONAL POWER SWING DETECTION METHODS
b) Concentric Characteristic Schemes
Principle:
After an outer characteristic has been crossed by
the Z1 impedance, a timer is started and the
interval of time before the inner characteristic is
reached is measured. A power swing is detected
when the time interval is longer than a set time
delay.

11. NON-TRADITIONAL METHODS FOR DETECTING POWER SWINGS
Continuous superimposed ΔI
The superimposed current method compares the
present values of currents with a buffer that is
taken two cycles earlier.
A delta current ΔI is detected if the difference
is greater than 5% of the nominal current. A
continuous ΔI measurement for three cycles
indicates a power swing condition and asserts
PSB
Can detect very fast power swings that are
hard to detect with conventional schemes
especially for heavy load conditions. On the
other hand, very slow slip rates below 0.1Hz,
where the ΔI between two cycles is less than the
threshold of 5% nominal current, are hard to
detect with the ΔI method

12. NON-TRADITIONAL METHODS FOR DETECTING POWER SWINGS
Continuous Impedance Calculation
 Monitors the progression in the complex plane
 Power swing is declared when the criteria for
all three loop impedances have been
fulfilled: continuity, monotony, and smoothness.
 Continuity verifies that the trajectory is not
motionless
 Monotony verifies that the trajectory does not
change direction
 smoothness verifies that there are no abrupt
changes in the trajectory

13. SWING CENTRE VOLTAGE METHOD
SCV is defined as the voltage at the location of a
two source equivalent system where the voltage
value is zero when the angles between the two
sources are 180 degrees apart.
Voltage Phasor Diagram of a Two-Machine System

14. FEATURES OF SCV
Under normal load conditions, the magnitude
of the SCV is constant.
Magnitude of the SCV changes between 0
and 1 p.u of system nominal voltage
SCV during an OOS condition

15. APPROXIMATION OF SCV
In a homogenous high-voltage power system
the impedance angle θ is close to 90° and the
diagram can be redrawn as shown
For the purpose of power swing detection, it is
the rate of change of the SCV that provides
the main information of system swings
Popular approximation of the SCV is:

16. The magnitude of the SCV is at its maximum when the
angular difference between the two sources is zero.
Conversely, it is at its minimum (or zero) when the
angular difference between the two sources is 180
degrees.
Power swing can be detected by calculating the rate of
change of the SCV.
Plot of SCV1 and its Rate-of-change

17. BASIC POWER-SWING BLOCKING LOGIC
FUNCTION

18. DEPENDABLE POWER-SWING BLOCKING
FUNCTION
The purpose of the dependable power-swing
detector is to supply a temporary DPSB signal that
will assert the PSB bit to compensate for the pickup
delay of the slope detector.
This will happen particularly after a lasting external
fault has been cleared and the network embarks
into a power-swing situation.
Eg: long-lasting fault right behind or at the remote
end of a transmission line on a marginally stable
network

19. SCV1 SLOPE DETECTOR FUNCTION
Rate of change of SCV1 must be above a minimum
threshold (Min_dSCV1)
The magnitude of the SCV1 must be within a
maximum (Thr_Hi) and a minimum (Thr_Low) threshold
The positive-sequence impedance measured by the
distance relay must reside within a starter zone.
Output of the slope detector is blocked any time
SCV1 is above a Max_dSCV1) or the absolute value
of the discontinuity detector is above Thr_d2SCV1.

20. PRINCIPLE OF SWING-CENTER VOLTAGE
DERIVATIVES
Used by the slope detector and swing
signature detector logic functions
Second-order time derivative takes a very
high value every time a discontinuity is
present in the signal
All values of derivatives are computed in
per-unit volts per cycle (V (pu)/cycle)

21. SWING-SIGNATURE DETECTION PRINCIPLE
Complements the slope detector
Based on the fact that if a fault detector picks up
during a power swing, no discontinuity will be
present on the SCV1 signal prior to the detection
because the fault detection is not the result of a
real fault
An algorithm that distinguishes between a power
swing and a real fault at the moment the outmost
distance element, to be blocked by the swing
detection, picks up

22. RESET LOGIC
The main condition indicating a disappearance
of the power swing is the rate of change of the
SCV1 signal taking a very small value (below
Min_dSCV1).

23. DETECTION OF THREE-PHASE FAULTS LOGIC
During a Three Phase fault, discontinuity will be
present on the corresponding SCV1 waveform
=> second derivative of SCV1 takes
a higher than usual value
=> the SCV1 will take a low value
=> its rate of change will be very
small

24. ADVANTAGES OF SCV METHOD
The SCV is independent of the system source and line impedances and is, therefore,
particularly attractive for use in a no-setting power-swing blocking function
The SCV is bounded with a lower limit of zero and an upper limit of one per unit,
regardless of system impedance parameters.
The magnitude of the SCV relates directly to δ