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 )

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4.2 ADDITIONAL POWER SWING DETECTION METHODS
! e: t: G$ |7 b$ a$ b4 N7 d; _9 K4.2.1 Continuous Impedance Calculation
This method determines a power swing condition based on a continuous impedance calculation.
2 z* F6 y+ x$ l: s5 v: k! m: T# Z" Y7 YContinuous here means, for example, that for each 5 ms step an impedance calculation is
: M3 X2 F& h2 Z) C/ P+ f' cperformed and compared with the impedance calculation of the previous 5 ms. As soon as there is
- r( F0 n; x) `" m. Ua deviation, an out-of-step situation is assumed but not proven yet. The next impedance that
5 ]3 [2 P8 e8 w, yshould be calculated 5 ms later is predicted based on the impedance difference of the previous
- _: c2 T9 i# T* z% fmeasured impedances. If the prediction is correct, then it is proven that this is traveling impedance.
6 Y( e/ u2 l0 R7 _% l4 E0 xIn this situation a power swing condition is detected. For security reasons additional predictive
calculations may be required.
" ^& d8 l- t7 o- B) FA delta impedance setting is not required anymore, because the algorithm automatically considers
3 ]7 E! U( T. v, s+ ]any delta impedance that is measured between two consecutive calculations and sets the delta
0 j# P. f$ j+ N) _impedance for the next calculation automatically in relation to the previous calculation. This leads
. _. L) B/ i: W, Z5 Lto 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
/ T7 q; W0 X8 S$ w9 Tdetermined by the calculation cycles of the algorithm.
R
5 t2 l/ K8 B6 ]) ZX
Stable power swing
impedance trajectory
DZ1 DZ2 DZ3
/ p/ b" v* B$ X7 _, t$ FLoad
- H  K8 ^1 g- t; u: u  DFigure 7 Power swing detection with continuous impedance calculation
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.
POWER SWING AND OUT-OF-STEP CONSIDERATIONS ON TRANSMISSION LINES
9 x, l% ^3 G2 I0 u0 o& R- TIEEE PSRC WG D6
$ `  l. ^+ d  L% }5 O; x( L17 / 59 2005-07-19
. r' o: P' m9 p, u' z4.2.2 Swing-Center Voltage and its Rate of Change
7 F; E9 w& l7 c* a2 Y1 ]Swing-center voltage (SCV) is defined as the voltage at the location of a two-source equivalent
% {2 f! Z3 c& A) a$ v- u& F2 p) \! vsystem 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
- V% y) `: S6 M) l6 Odisturbance, 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
" g- N4 ^: W0 f: B+ f2 H% zthe phasor from origin o to the point o'.
o'
o
o"
Z1S•I Z1L•I
& ~' J' v! B4 O! }VS
$ ^( m7 I6 B, Y# \1 H  @" Nj ER
# p1 u  s$ j4 ud
SCV
Z1R•I
9 j* ~7 \. O# E& Nq
I
: a$ m7 X" i' w, V. RES
, [0 L3 J. `# T5 cVR
Figure 8 Voltage Phasor Diagram of a Two-Source System
. E. ^: c1 a" C  X9 K7 mAn approximation of the SCV can be obtained through the use of locally available quantities as
follows:
SCV »| V | ×cosj S (6)
Where |VS| is the magnitude of locally measured voltage, and j is the angle difference between VS
. b9 y& \# G8 g, O5 @) uand the local current as shown in Figure 9. In Figure 9, we can see that Vcosj is a projection of VS
. R4 T, T! ?' G7 i$ H8 [5 p: Tonto the axis of the current, I. For a homogeneous system with the system impedance angle, q,
  l+ r8 k7 ~& \( E/ eclose to 90 degrees, Vcosj approximates well the magnitude of the swing-center voltage. For the
6 S& H, J0 m* E$ bpurpose 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
3 B8 V; c3 R% ~9 h2 P  w' n9 jand its local estimate have little impact in detecting power swings. Ilar [6] first introduced the
; i% B6 T  p: Pquantity of Vcosj for power swing detection.

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