电力系统震荡与失步(POWER SWING AND OUT-OF-STEP CONSIDERATIONS )

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ＩＥＥＥ　电力系统震荡与失步(POWER SWING AND OUT-OF-STEP CONSIDERATIONS )

ER SWING AND OUT-OF-STEP CONSIDERATIONS )

DONGBEI

4.2 ADDITIONAL POWER SWING DETECTION METHODS
/ N/ ^6 Z$ }, C& b  c6 N5 n4.2.1 Continuous Impedance Calculation
This method determines a power swing condition based on a continuous impedance calculation.
Continuous here means, for example, that for each 5 ms step an impedance calculation is
performed and compared with the impedance calculation of the previous 5 ms. As soon as there is
a deviation, an out-of-step situation is assumed but not proven yet. The next impedance that
0 ]  n7 f! s$ L' u0 K( h4 Q  c% Eshould be calculated 5 ms later is predicted based on the impedance difference of the previous
measured impedances. If the prediction is correct, then it is proven that this is traveling impedance.
In this situation a power swing condition is detected. For security reasons additional predictive
2 K7 u4 c& b9 q* U$ O" D/ ucalculations may be required.
# k/ Q" t6 q9 h# BA delta impedance setting is not required anymore, because the algorithm automatically considers
any delta impedance that is measured between two consecutive calculations and sets the delta
impedance for the next calculation automatically in relation to the previous calculation. This leads
$ t$ {0 `' u) rto a dynamic calculation of the delta impedance and an automatic adaptation to the change of the
power swing impedance. Also the delta time setting is not required anymore because it is
+ x, g7 I2 l7 N; K/ [1 rdetermined by the calculation cycles of the algorithm.
R
X
Stable power swing
! b& q2 Z, g4 }9 s+ v9 Y* a$ limpedance trajectory
DZ1 DZ2 DZ3
7 F/ W" s) H/ l( Q2 @( o  dLoad
Figure 7 Power swing detection with continuous impedance calculation
* z% \0 |7 I, Z" f7 B) ^As long as the changing impedance vector is not approaching a tripping zone faster than the relay
can confirm the out-of-step condition (at least 3 calculations 10 ms) the detection will be successful.
5 W& c* L) L1 ePOWER SWING AND OUT-OF-STEP CONSIDERATIONS ON TRANSMISSION LINES
IEEE PSRC WG D6
$ v4 V" b9 i4 a% p# @4 ~( G17 / 59 2005-07-19
4.2.2 Swing-Center Voltage and its Rate of Change
0 ]$ ?4 y- |4 u( ?0 ^Swing-center voltage (SCV) is defined as the voltage at the location of a two-source equivalent
- Z" R) d3 y/ v- F3 l) A/ Ssystem where the voltage value is zero when the angles between the two sources are 180 degrees
apart. When a two-source system loses stability and goes into an OOS situation after some
disturbance, the angle difference of the two sources, d(t), will increase as a function of time. Figure
8 illustrates the voltage phasor diagram of a general two-source system, with the SCV shown as
$ a( V4 ?9 O* G) m4 O8 A( m. n/ ythe phasor from origin o to the point o'.
2 j8 e, [; [9 f* c! W, Ro'
# o3 J9 R5 `+ t; T9 Ao
6 K# V0 M! b/ ]* ao"
, l2 d9 j$ T4 P4 Q$ x7 nZ1S•I Z1L•I
. P( V# I2 n+ r  g1 S3 r, BVS
j ER
1 G+ Y# ^4 I( @8 vd
SCV
; Z4 [+ E/ P, {0 p( v9 K! b- D1 [Z1R•I
7 D# K* w# z6 X1 F& m+ H( x0 L4 d" N, Qq
I
0 \- ?5 Y9 k# MES
VR
0 M( j7 U4 }& c4 W4 cFigure 8 Voltage Phasor Diagram of a Two-Source System
An approximation of the SCV can be obtained through the use of locally available quantities as
3 }6 _3 O. y) K3 y! x9 i% X2 Qfollows:
4 u5 M' B+ t) ?  y2 USCV »| V | ×cosj S (6)
Where |VS| is the magnitude of locally measured voltage, and j is the angle difference between VS
and the local current as shown in Figure 9. In Figure 9, we can see that Vcosj is a projection of VS
0 W2 u( L) @& Monto the axis of the current, I. For a homogeneous system with the system impedance angle, q,
close to 90 degrees, Vcosj approximates well the magnitude of the swing-center voltage. For the
purpose of power-swing detection, it is the rate of change of the SCV that provides the main
information of system swings. Therefore, some differences in magnitude between the system SCV
' [% N4 T2 N* Wand its local estimate have little impact in detecting power swings. Ilar [6] first introduced the
! y% _# r9 v' h( j* J$ {* U  H3 Fquantity of Vcosj for power swing detection.

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