ZERO SETTING POWER SWING BLOCKING PROTECTION

BY - JYOTIRMOY

CHANDRAKANTH

SURJIT

BADAL

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.

STABLE AND UNSTABLE POWER SWINGS

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.

IMPEDANCE TRAJECTORIES FOR VARIOUS RATIOS OF |ES/ER|

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

CONVENTIONAL POWER SWING DETECTION METHODS

a) Rate of Change of Impedance Methods

The rate of change of impedance methods arebased on the principle that the Z1 impedancetravels in the complex plane with a relativelyslow pace whereas during a fault,Z1switches fromthe load point to the fault location almostinstantaneously.

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.

CONVENTIONAL POWER SWING DETECTION METHODS

b) Concentric Characteristic Schemes

Principle:

After an outer characteristic has been crossed bythe Z1 impedance, a timer is started and theinterval of time before the inner characteristic isreached is measured. A power swing is detectedwhen the time interval is longer than a set timedelay.

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

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

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

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

APPROXIMATION OF SCVIn a homogenous high-voltage power systemthe impedance angle θ is close to 90° and thediagram can be redrawn as shown

For the purpose of power swing detection, it isthe rate of change of the SCV that providesthe main information of system swings

Popular approximation of the SCV is:

The magnitude of the SCV is at its maximum when theangular difference between the two sources is zero.

Conversely, it is at its minimum (or zero) when theangular difference between the two sources is 180degrees.

Power swing can be detected by calculating the rate ofchange of the SCV.

Plot of SCV1 and its Rate-of-change

BASIC POWER-SWING BLOCKING LOGIC FUNCTION

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

SCV1 SLOPE DETECTOR FUNCTION

Rate of change of SCV1 must be above a minimumthreshold (Min_dSCV1)

The magnitude of the SCV1 must be within amaximum (Thr_Hi) and a minimum (Thr_Low) threshold

The positive-sequence impedance measured by thedistance relay must reside within a starter zone.

Output of the slope detector is blocked any timeSCV1 is above a Max_dSCV1) or the absolute valueof the discontinuity detector is above Thr_d2SCV1.

PRINCIPLE OF SWING-CENTER VOLTAGE DERIVATIVES

Used by the slope detector and swingsignature detector logic functions

Second-order time derivative takes a veryhigh value every time a discontinuity ispresent in the signal

All values of derivatives are computed inper-unit volts per cycle (V (pu)/cycle)

SWING-SIGNATURE DETECTION PRINCIPLE

Complements the slope detector

Based on the fact that if a fault detector picks upduring a power swing, no discontinuity will bepresent on the SCV1 signal prior to the detectionbecause the fault detection is not the result of areal fault

An algorithm that distinguishes between a powerswing and a real fault at the moment the outmostdistance element, to be blocked by the swingdetection, picks up

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).

DETECTION OF THREE-PHASE FAULTS LOGIC

During a Three Phase fault, discontinuity will bepresent 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

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