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; r4 O/ ^5 S2 L- `4.2.1 Continuous Impedance Calculation
This method determines a power swing condition based on a continuous impedance calculation.
  r1 o, j+ s7 J$ K- s% D8 HContinuous 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
: f( H0 C# M0 J+ l3 H) Ga deviation, an out-of-step situation is assumed but not proven yet. The next impedance that
! }( A5 P+ S- ^; P( o, @9 tshould be calculated 5 ms later is predicted based on the impedance difference of the previous
7 w7 x. A  z9 s' y- jmeasured impedances. If the prediction is correct, then it is proven that this is traveling impedance.
6 K0 T1 Y" z3 W- H! d( h* sIn this situation a power swing condition is detected. For security reasons additional predictive
/ Z. K* N0 F) k& kcalculations may be required.
A 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
to a dynamic calculation of the delta impedance and an automatic adaptation to the change of the
# c5 h! {, G9 R" o4 R+ T6 mpower swing impedance. Also the delta time setting is not required anymore because it is
9 @6 I) A5 [8 c/ t0 Wdetermined by the calculation cycles of the algorithm.
R
X
Stable power swing
impedance trajectory
DZ1 DZ2 DZ3
  ^6 K" X/ s9 ]& ^* }Load
Figure 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
IEEE PSRC WG D6
1 ]' w& p5 D* Q17 / 59 2005-07-19
. I* |4 }- E6 ^/ A/ o" }: q4.2.2 Swing-Center Voltage and its Rate of Change
Swing-center voltage (SCV) is defined as the voltage at the location of a two-source equivalent
- a3 B% s! Q# [% {9 x) M, psystem where the voltage value is zero when the angles between the two sources are 180 degrees
4 t- s0 N" }8 N  P+ z& @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
9 G' m' |/ ~0 m. Fthe phasor from origin o to the point o'.
o'
o
o"
  j8 F$ X; t& B! o3 J2 Q. Z& [Z1S•I Z1L•I
( x, a. c1 e3 n5 H) pVS
j ER
4 l, \/ f3 l+ o# j. _) b4 l+ K  b; @d
SCV
Z1R•I
* s! C* _4 ^/ t7 f0 A- ~0 x0 iq
I
ES
VR
Figure 8 Voltage Phasor Diagram of a Two-Source System
% d9 O1 j9 ?) t5 v# v* N( H! eAn 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
and the local current as shown in Figure 9. In Figure 9, we can see that Vcosj is a projection of VS
( ^) M1 F6 ]! t- l  c0 qonto the axis of the current, I. For a homogeneous system with the system impedance angle, q,
1 l  L' K' f, j" G% d# |close to 90 degrees, Vcosj approximates well the magnitude of the swing-center voltage. For the
9 x& _  R0 j( n6 vpurpose of power-swing detection, it is the rate of change of the SCV that provides the main
& Q& O; l3 U. @information of system swings. Therefore, some differences in magnitude between the system SCV
and its local estimate have little impact in detecting power swings. Ilar [6] first introduced the
0 W9 B; H- f! r; k! _quantity of Vcosj for power swing detection.

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