Swing Mode Bifurcations Motivation & Background – Practical Goal: examine loss of stability mechanisms associated w/ heavily loaded transmission corridors. – Expect presence of low frequency, interarea swing modes across transmission corridor. – Can bifurcation tools developed for voltage analysis be adapted to this scenario (are voltage & angle instabilities really that different)?
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Network Structure in Swing Mode Bifurcations u Motivation & Background –Practical Goal: examine loss of stability mechanisms associated w/ heavily loaded.
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Network Structure in Swing Mode Bifurcations
Motivation & Background– Practical Goal: examine loss of stability
– Expect presence of low frequency, interarea swing modes across transmission corridor.
– Can bifurcation tools developed for voltage analysis be adapted to this scenario (are voltage & angle instabilities really that different)?
Key Ideas Voltage methods typically assume one degree of
freedom path in “parameter space” (e.g. load), or seek “closest” point in parameter space at which bifurcations occur
Alternative: leave larger # of degrees of freedom in parameter space, but constrain structure of eigenvector at bifurcation.
Key Questions
Is there a priori knowledge of form of eigenvector of interest for “mode” of instability we’re after?
Precisely what formulation for matrix who’s eigenvector/eigenvalue is constrained (e.g., what generator model, what load model, how is DAE structure treated, etc.)
Caveats (at present...)
Development to date uses only very simple, classical model for generators.
Previous work in voltage stability shows examples in which “earlier” loss of stability missed by such a simple model (e.g. Rajagopalan et al, Trans. on P.S. ‘92).
Review - Relation of PF Jacobian and Linearized Dynamic Model
This issue well treated in existing literature, but still useful to develop notation suited to generalized eigenvalue problem.
Structure in linearization easiest to see if we keep all phase angles as variables; neglect damping/governor; assume lossless transmission & symmetric PF Jac. Relax many of these assumptions in computations.
Review - Relation of PF Jacobian and Linearized Dynamic Model
Form of nonlinear DAE model
M• = L1(P
I - PN(, V))L1
• =
0 = L2(PI - PN(, V))
0 = (QI(V) - QN(, V))
Review - Relation of PF Jacobian and Linearized Dynamic Model
Requisite variable/function definitions:V n, V := vector of bus voltage magnitudes; n, := vector of bus voltage phase angles
relative to an arbitrary synchronousreference frame of frequency 0 (noreference angle is deleted);
m, := vector of generator frequencydeviations, relative to synchronousfrequency 0;
RIRI
RI
Review - Relation of PF Jacobian and Linearized Dynamic Model
variable/function definitions:
M mxm, M := diagonal matrix of normalizedgenerator inertias;
PI n, PI:= vector of net active power injection ateach bus; assumed constant;
QI: n-m n-m, QI(V) := vector valued function ofnet reactive power injection at load buses,normalized by voltage magnitude;
RI
RI
RI RI
Review - Relation of PF Jacobian and Linearized Dynamic Model
variable/function definitions:
L1 := rows 1 through m of an nxn identity matrix;L2 := rows m+1 through n of an nxn identity matrix;
PN(, V) := vector-valued function of active powerabsorbed by network at each bus
QN(, V) := vector-valued function of reactivepower absorbed by network at load buses,normalized by voltage magnitude
Linearized DAE/”Singular System” Form
Write linearization as:
~E •
x = ~Rx
Component Definitions where:
Mmxm 0 0
0 I mxm 0
0 0 02(n-m)x2(n-m)
E~
=
0 -Imxm 0
Imxm 0 0
0 0 I2(n-m)x2(n-m)
SR~
=
Component Definitions
and:
J: nx n-m (2n-m)x(2n-m), J =
Imxm 0 0
0 J11 J12
0 J21 J22
S =
RI RI RI
P
N
V P
N
Q
N
V {Q
N - Q
I}
L
L
Relation to Reduced Dimension Symmetric Problem
Consider reduced dimension, symmetric generalized eigenvalue problem defined by pair (E, J), where:
M 0
0 02(n-m)x2(n-m)
E =
Relation to Reduced Dimension Symmetric Problem
FACT: Finite generalized eigenvalues of (E, J) completely determine finite generalized eigenvalues of (
~E ~R)
Relation to Reduced Dimension Symmetric Problem
In particular,
(E, J ) has a finite generalized eigenvalue
(~E,
~R), has finite generalized eigenvalue
with = j .
Key Observation
In seeking bifurcation in full linearized dynamics, we may work with reduced dimension, symmetric generalized eigenvalue problem whose structure is determined by PF Jacobian & inertias.
When computation (sparsity) not a concern, equivalent to e.v.’s of M-1[J
11 - J
12J
22-1
J21
]
Role of Network Structure
Question: what is a mechanism by which might drop rank?
First, observe that under lossless network approximation, the reduced Jacobian has admittance matrix structure; i.e. diagonal elements equal to – {sum of off-diagonal elements}.
[J11
- J12
J22-1
J21
]
Role of Network Structure
Given this admittance matrix structure, reduced PF Jacobianhas associated network graph.
A mechanism for loss of rank can then be identified: branches forming a cutset all have weights of zero.
[J11
- J12
J22-1
J21
]
Role of Network Structure
Eigenvector associated with new zero eigenvalue is identifiable by inspection:
where is a positive real constant, and partition of eigenvector is across the cutset.
J11
- J12
J22-1
J21
1-1
= 0
Role of Network Structure
Returning to associated generalized eigenvalue problem, to preserve sparsity, one would have:
J11
J12
J21
J22
1–1 w
= 0
Role of Network Structure
Finally, in original generalized eigenvalue problem for full dynamics, the new eigenvector has structure [ 1 , – 1 ] in components associated with generator phase angles.
Strongly suggests an inter area swing mode, with gens on one side of cutset 180º out of phase with those on other side.
Summary so far...
Exploiting on a number of simplifying assumptions (lossless network, symmetric PF Jacobian, classical gen model...), identify candidate structure for eigenvector associated with a “new” eigenvalue at zero.
Look for limiting operating conditions that yield J realizing this bifurcation & e-vector.
Computational Formulation Very analogous to early “direct” methods of finding
loading levels associated with Jacobian singularity in voltage collapse literature (e.g., Alvarado/Jung, 88).
But instead of leaving eigenvector components associated with zero eigenvalue as free variables, we constrain components associated with gen angles.
Computational Formulation
Must compensate with “extra” degrees of freedom.
For example to follow, generation dispatch selected as new variables. Clearly, many other possible choices...
Computational Formulation
Final observation: while it is convenient to keep all angles as variables in original analysis, in computation we select a reference angle and eliminate that variable.
Resulting structure of gen angle e-vector components becomes [ 0 , 1 ]
Computational Formulation
Simultaneous equations to be solved:
Note that f tilde terms are power balance equations, deleting gen buses. Once angles & voltages solved, gen dispatch is output.
0 = f~
(~
, V)
0 = J~
((~
, V)[0, 1T, w~T]T
Computational Formulation
Solution method is full Newton Raphson.
Aside: the Jacobian of these constraint equations involves 2nd order derivative of PF equations. Solutions routines developed offer very compact & efficient vector evaluations of higher order PF derivative.
Case Study
Based on modified form of IEEE 14 bus test system.
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910
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19 20 - Transmission Line #'s# - Bus #'s
14 Bus Test System
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G
GG
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19 20 - Transmission Line #'s# - Bus #'s
14 Bus Test System
Cutset Here
Case Study
N-R Initialization: initial operating point selected heuristically at present. Simply begin from op. pnt. that loads up a transmission corridor, with gens each side.
Here choice has gens 1, 2, 3 on one side, gens 6, 8 on other side.
Model has rotational damping added as rough approximation to governor action.
Table 1: Original & Critical Operating Pnts.Original Op. Point Critical Op. Point
Key question 1: must systems inevitably encounter loss of stability via flux decay/voltage control mode (as identified in Rajagopalan et al) before this type of bifurcation?
Hypothesis: perhaps not if good reactive support throughout system as transmission corridor is loaded up.
Future Work
Key question 2: possibility of same weakness as direct point of collapse calculations in voltage literature - many generators hitting reactive power limits along the loading path.
Answer will be closely related to that of question 1!
Conclusions
Simple exercise to shift focus back from bifurcations primarily associated w/ voltage, to bifurcations primarily associated with swing mode.
Key idea: hypothesize a form for eigenvector, restrict search for bifurcation point to display that eigenvector.
Conclusions
While further is clearly development needed, method here could provide simple computation to identify a stability constraint on ATC across a transmission corridor.