Journal of Power and Energy Engineering, 2015, 3, 224-231
Published Online April 2015 in SciRes. http://www.scirp.org/journal/jpee
http://dx.doi.org/10.4236/jpee.2015.34031
How to cite this paper: Allaev, K.R., Mirzabaev, A.M., Makhmudov, T.F. and Makhkamov, T.A. (2015) Simplified Criterion of
Steady-State Stability of Electric Power Systems. Journal of Power and Energy Engineering, 3, 224-231.
http://dx.doi.org/10.4236/jpee.2015.34031
Simplified Criterion of Steady-State Stability
of Electric Powe r Sy ste ms
K. R. Allaev1, A. M. Mirzabaev2, T. F. Makhmudov3, T. A. Makhkamov4
1Power Engineering D epa rtm ent, Tashkent State Technical Un ive rsit y, Tashkent, Republic of Uzbekistan
2Mir Solar LLC, Tashkent, Republic of Uzbekistan
3Power Engineering D epa rtm ent, Tashkent State Technical Un ive rsit y, Tashkent, Republic of Uzbekistan
4Research and De velop ment Depar tment , Tec on Groups, Mosco w, Russian Fed erati on
Received January 2015
Abstract
In the paper the simplified criterion of a steady-state stability of electric power systems (EPS) is
justified on the basis of Lyapunov functions in a quadratic form ensuring necessary and sufficient
conditions of its performance. Upon that, the use of the node-voltage equations allows reducing
study of a steady-state stability of complex EPS to study of the generator-bus system. The obtained
results facilitate studies of a steady-state stability of the complex systems and have practical im-
port ance .
Keywords
Steady-State Stability, Matrix Method, Lyapunov Function, Node -Voltage Equations
1. Introduction
Study of EPS stability at small disturbances is based on known classical concepts of the General Theory of Sta-
bility of Motion [1]-[8].
As is known [1]-[4] [7], features of EPS are their continuous flow process, complexity, multiple connection of
system of facilities and their control devices. With increasing the capacity and complexity of EPS the problem
of automation of system state control both in normal operating conditions and during disturbances becomes
more acute. Development of the automated dispatch control systems (ADCS) for EPS and automatic control
systems require the new approach to development of the simplified mathematical models and the algorithms
meeting the practical requirements that form the basis of complex EPS control [1] [4] [7] [8].
There is a real-time calculation problem when the issue to ensure maximum operativeness of a current oper-
ating conditions stability calculation and timely definition of EPS approaching to stability limits is critical. The
most acceptable for their solution are the matrix computational methods grounded on use of the newest methods
to discover structural properties of dynamic systems which include modern electric systems [7]-[11].
Steady-state stability study of electric power systems consists in definition of a possibility of steady-state op-
eration under given values of electric power system’s parameters, an electricity generating sources operation
state, loads in nodal points and settings of automatic control equipment [1] [2]. The problem is usually solved

K. R. Allaev et al.
225
for the determined conditions.
In most cases the complex power system steady-state stability analysis is carried out under the supposition of
the lack of a self-oscillation in the electric power system considering that this requirement is ensured by appro-
priate setting of automatic regulators [3]. In this case the problem becomes simpler and is reduced to study of an
aperiodic steady-state stability of the system, i.e. to definition of dependence and a sign for the constant term of
the characteristic equation of the system upon the continuous variation of any parameter of the operation condi-
tion.
Now, except for classical, direct solution methods for calculation of a matrix spectrum of system are also used
for complete steady-state stability analysis, including an estimate of lack of eigenvalues in the right half-plane
by indirect criteria, dynamic simulation methods, etc. [11].
In the practice of calculations there is the popular method of so-called critical sections when the critical line
(section) is determined by high probability of loss of EPS stability because of overloading. Upon that, finally it
is not specified what generator or station are referred to the critical section, therefore definition of critical sec-
tion is ambiguous [12].
The devices of the synchronized phasor measurement units (SPMU) being developed create premises for de-
velopment of new analysis methods and algorithms of an electric power system state at small disturbances in-
cluding methods to detect weak tie-lines and critical sections that are based exclusively on operatively obtained
values of operating condition parameters [7].
2. Lyapunov Function in a Quadratic Form as the Tool for the Full Study of
Steady-State Stability in EPS
One of the most fruitful methods for study of EPS stability is application of a direct (second) Lyapunov’s me-
thod which require selection of special Lyapunov functions and obtaining of their derivatives taking into account
the perturbation equations [5] [6].
According to Lyapunov’s direct method which is applied to study of a dynamic systems stability including
electrical power systems it is generally supposed definition of special sign-definite function of state variables
( )
123
,?
n
Vх ххх
which derivative
ddVt
taken on account of system of differential equations describing
dynamics of the system, should be sign-definite with an opposite sign to V or be identically zero or strictly
sign-definite with an opposite sign to V. Under these requirements the system is, accordingly, stable or asymp-
totically stable [5] [6]. Construction of V for nonlinear systems is generally performed by a trial method and ob-
tained results ensure only sufficient conditions of stability for the explored system.
At the same time, in the case of linear autonomous systems there is a Lyapunov function in a quadratic form
ensuring both necessary and sufficient conditions of its stability [5]. Modern counting machines offer ample
opportunities for a solution of the higher order equations [13] and, accordingly, successful application of Lya-
punov functions in a quadratic form for study of a steady-state stability for complex EPS [14].
The essence of the method consists in the following. Let’s consider the linear time-invariant system described
in a state space by the system of differential equations:
ddx tAX=
(1)
where A is a square matrix with constant elements; X is a n—dimensional column vector with coordinates
1n
xx−
.
Following Lyapunov, we will define for this system function V in a quadratic form [5] [6]:
T
,1
,,1, 2,,
n
ij ij
ij
Vqx xXQXijn
=
= ==
∑

(2)
where Q is yet unknown square matrix of coefficients of a quadratic form; XT is transposed X (row-vector).
Owing to (1) the total derivative of V with time looks like:
( )
TT
ddVtXA QQAX= +
(3)
Let’s require that V should satisfy the condition:
ddVt W= −
(4)

K. R. Allaev et al.
226
where W is arbitrarily prescribed quadratic form of state variables.
Let’s denote:
T
A QQAC+=−
(5)
The main result consists in that the system (1) is asymptotically stable in only case when the (2) has positive
definite solutions Q at any positive definite matrix C [5].
The Equation (5) puts in correspondence to any symmetric matrix Q a matrix C and vise versa, and this cor-
respondence is linear [5] [17]. Elements of matrix Q are determined from (5) by a solution of
( )
12nn+
equa-
tions where n is a number of initial differential equations. If to set a positive definite symmetric matrix C (where
the determined from (5) matrix Q will be also positive definite) then due to linearity and stationarity of system
(1), according to the Lyapunov’s theorem, we will obtain an asymptotical stability of its equilibrium state. Upon
that, stability conditions should be strictly equivalent to the obtained on the basis of Routh-Hurwitz criterion [2].
There is a close connection between the Lyapunov’s theorem and other algebraic stability criteria: the Routh-
Hurwitz criterion [15], the Hermite stability criterion [16], the Shur-Kon criterion [17], and the constituent ma-
trix method [18] [19]. The main advantage of the Lyapunov’s second method for stability when studying stabil-
ity conditions is related to a possibility to operate in calculations with elements of a matrix A omitting calcula-
tions of coefficients of a characteristic polynomial for this matrix.
On the basis of Lyapunov’s functions in a quadratic form, we will carry out computational-experimental re-
search of a steady-state stability of both simplex and complex EPSs and compare results for them with the re-
sults obtained conventionally on the basis of the Routh-Hurwitz criterion.
2.1. Simplex EPS
The characteristic equation at small fluctuations of operating condition parameters at study of a steady-state sta-
bility of the simplex uncontrolled EPS (Figure 1(a)) taking into account transients in a field winding looks like
[2] [14]:
32
0 123
0apapap a+++=
(6)
where a0, a1, a2, a3 are the coefficients of a characteristic equation which are functions of operating condition
parameters and EPS.
In our case Lyapunov function in a quadratic form at
3 n=
according to (2) looks like:
T 222
11122233 3121 213 1 32323
222 VXQXqx qxqxqxxqxxqxx = =+++++
(7)
Let set C in the form of an identity matrix. Then
222
123
Wxxx =++
(8)
Upon that, the matrix of coefficients of a quadratic form (7) according to (2) looks like:
11 1213
21 2223
31 31 33
qqq
Qqqq
qqq


=


(9)
We have set negatively definite derivative W (positively definite symmetric C). If upon that the positive defi-
niteness requirements to the matrix Q of a quadratic form (7) are satisfied then initial system equilibrium state
asymptotic stability conditions (1) will be obviously provided.
For the positive definiteness of a quadratic form (7) according to Sylvester’s criterion [5] [6] it is necessary
and sufficient that principal diagonal minors of matrix Q were positive:
11 1213
11 12
1112321 2223
21 2231 31 33
0; 0;0
л лл
qqq
qq
q qqq
qq qqq


∆= >∆=>∆=>



(10)

K. R. Allaev et al.
227
T
1
G
L
T
2
70
80
90
electrical degrees
δ
U
0
= const
f
0
= const
ΔJli
Δ
π3
Δ
π2
Δ
π1
=q
11
(a) (b)
Figure 1. Behavior of minors of a quadratic form for the matrix Lyapunov equation (b) for
the EPS circuit design (a).
Let’s start checking violation of positivity of these minors with the first
1 11
лq∆=
. As disclosing ∆л1 shows,
the minor can become negative if following requirements [14] are broken:
10
q
E
P
C
δ
∂
= >
∂
(11)
dcd
XXX
′
>>
(12)
2 sin
arcsin
q ii
ii
Ea
aU δ >−
(13 )
where
q
E
P
is the real output power of the generator at
const
q
E=
., U is the terminal voltage of the generator,
q
E
is EMF of the generator,
d
X
,
d
X′
,
c
X
are synchronous and transient reactance of the synchronous ge-
nerator and reactance of the transmission line, accordingly,
δ
is the torque angle of the generator that deter-
mines stability of the generator and therefore, EPS,
ii
α
is a complementary angle [2] [3] [14].
The requirement (11) can be broken only in the case of overload which may cause an aperiodic instability of
EPS.
Violation of the inequality (12) is possible in the case of overcompensation of an reactance of the power line
by the direct compensation plants (series capacitances) that leads to electromagnetic instability in a power sys-
tem (self-excitation of generator).
The requirement (13) can be broken in operating conditions close to light load conditions and synchronous
generator operation with a transmitting line with appreciable resistances and thereby a stability violation process
has oscillatory behavior and is observed in the form of electromechanical oscillation of the generator rotor
(self -oscillation).
In other words, requirements to violation of positivity of the first minor in a square matrix (10) imply all
possible conditions that may lead to violation of an electrical power system steady-state stability, i.e., the com-
plete problem to define conditions of EPS instability is solved “in the small”.
The analysis has shown [14] [20] that positivity of
2Л
∆
,
3Л
∆
is reduced to satisfaction of the same re-
quirements (11)-(13).
So, conditions that may lead to violation of an electrical power system steady-state stability obtained by Lya-
punov’s second method coincide with earlier discovered on the basis of the generalized Routh-Hurwitz condi-
tions.
It is necessary to note that for the first time requirements of adequacy of the results obtained on the basis of
Lyapunov functions in a quadratic form and a Routh-Hurwitz criterion for an electrical power system have been
obtained in the work [20].
With a view of checking theoretical rules there were carried out computational-experimental researches of vi-
olation of principal minors positivity in a quadratic matrix (10). Calculations were carried out for the simplex
and complex EPS [14]. Calculations were also carried out on the basis of the Routh-Hurwitz criterion аor the
purpose of comparison.

K. R. Allaev et al.
228
Figure 1(b) shows variations of minors (10) at gradual increasing the load in EPS (increasing the transmitted
real power). The analysis shows that upon increasing the operation condition loads variations for all minors
iЛ
∆
from the matrix of a quadratic form of Lyapunov function (10) have the equal character, while variations
of characteristic equation coefficients (6) and Hurwitz determinants are absolutely different [14].
The result allows using positivity of the first minor, i.e .
11
0q>
, for complete analysis of a steady-state sta-
bility of the electrical power system because q11 contains all information on possible kinds of EPS instability “in
the small”. Upon that, positivity of the higher minors, i.e.
iЛ
∆
> 0 (
2, ,in=
; where n is the order of a diffe-
rential equation of initial EPS) may not be considered in the first approximation. Hence, it is possible to state
simplified (practical) criterion of a steady-state stability q11 > 0 which gives both necessary and sufficient condi-
tions of its performance. Traditionally [2], these requirements are obtained on the basis of positivity of characte-
ristic equation coefficients for the system (the necessary condition) and positivenesses of Routh-Hurwitz matrix
determinants (the sufficient condition).
2.2. Complex EPS
The calculation analysis of a steady-state stability for EPSs of various complexity shows [4] that the most strict
in theoretical aspect, convenient in computing aspect and effective aspect by the obtained results is use of two
fundamental methods: the method of Lyapunov functions in a quadratic form and the method of the nodal equa-
tions [14] [21]. The proposed stability research technique “in the small” has been suggested for the first time in
the work [22], and its essence consists in the following.
When studying a steady-state stability of complex EPS, calculation of the steady-state condition on the basis
of the node-voltage equations is carried out at first, voltages Uk for each node k and their arguments δk, and
further for each j-th generator are determined; then positivity of the first minor q11j (10) of the matrix of qua-
dratic form Q is checked using these data. Thereby the stability of the generator which is the fastest to come
closer to a limit at the given load is determined. In essence, stability study “in the small” of the complex EPS
using the proposed method turns to study of the generator-bus system.
Let’s consider steady-state stability conditions of complex EPS by the example of three-generator system
(Figure 2) since such model of the electrical power system as a whole adequately reflects properties and per-
formances of the complex electrical system [4] [23] [24].
Figure 3 represents characters of variations
11 j
q
(where
13j= −
) of the first elements of minors for ma-
trixes of quadratic forms
j
Q
for each generator of the system in question.
Changes (increasing) of minors of a quadratic form
I
ξ
for different generators in the case if loading of oper-
ating conditions of the electrical power system increases are different, that is clear from Figure 3.
Upon condition if the small deviations of EPS operating condition parameters are constant (∆П = constant) it
is possible to write:
32131 1
and hhh ξξξ ∆∆∆ 
(14)
where
j
h∆
are the incremental rates of first minors of quadratic form matrixes for each generator showing how
quickly a generator comes closer to the stability limit. The analytical expression
j
h∆
, for example, for the
second generator at i-th step looks like:
P1
P2
E1
E2
G2
G3
E3
P3
G1
Figure 2. The circuit design of three-generator electrical power system.

K. R. Allaev et al.
229
Δ
j
Δ3
11
Δ2
11
Δ1
11
=q
111
ξ3 = Δ
3i
−Δ
3(i + 1)
ξ2 = Δ
2i
−Δ
2(i + 1)
ξ1 = Δ
1i
−Δ
1(i + 1)
ΔΠ
i,i+1
Π
i+1
Π
Π
i
Π
Πpeд
Figure 3. Definition of the steady-state stability simplified criterion for the
complex EPS (П is the operating condition parameter of EPS).
( )
( )
( )
2, 21
2
22,
2, 121
ii
i
ii i
h
ξ
+
++

∆ −∆

∆= =∆

∆ΠΠ− Π

(15)
Generally (15) looks like:
( )
( )
( )
1
,1 1
ji ji
j
jji
jijjj i
h
ξ
+
++

∆ −∆

∆= =∆

∆ΠΠ−Π

(16 )
where П → U, f, I,
δ
, etc. are the operating condition parameters of EPS by which variations of minors for
generators of the explored system as a result of increasing the loads of operating conditions can be determined, j
is the generator for which (16) is calculated, i is a current step of increasing the loads for the given EPS operat-
ing condition parameter.
Let’s transfer to differentials under condition of small increments:
( )
jj
h dh∆= ∆
(17)
The condition
0
I
ξ >
for a quadratic form is always satisfied, therefore it is possible to be restricted to study
of strict performance of the inequality:
( )
( )
11 1,
1
d0
d
ji ji j
jji i
ji
q
h+
+

∆ −∆

∆=∆=

Π −ΠΠ


(18)
The condition (18) means that in order to provide a steady-state stability of j-th generator and, hence, EPS, the
fulfillment of the following condition is required:
11,
d0
d
j
i
q
Π
(19)
On the basis of the obtained results it is possible to propose the following algorithm for studies of a steady-
state stability of the complex electric power systems.
With increasing the load by the given parameter of operating condition П at each step i and for each generator
or the selected groups of generators the condition (19) is checked and compared with other similar conditions,
i.e., fulfillment of conditions is checked:
11,1 11,1 11,
ddd
ddd
n
qqq
ΠΠΠ
 
(20)

K. R. Allaev et al.
230
where n is a number of generators or stations checked on a steady-state stability. The generator which first minor
variation in a matrix of coefficients of a quadratic form is maximum will be the most critical from the point of
view of steady-state stability violation:
1 1,
dmax
d
j
q→
Π
(21)
for the considered series of generators.
Hence, j-th generator for which
1 1,
d dmax
j
qΠ→
will be the first which comes to a steady-state stability
limit of EPS.
Thus, that generator which tends to steady-state stability violation, and also possible sections (lines) which
represent the greatest danger from this point of view should be determined at first. The given factor is valuable
also in that it allows to define the corresponding parameter of operating conditions which is most preferable to
control the transient behavior of EPS in the case of regulation. This makes it possible to organize control of
transient process of the generator using automatic excitation control, automatic speed control and other control
systems and proactively ensure its steady-state stability. The significance of this result is obvious to practice of
maintenance of electric power systems.
This data imply efficiency of common application of the nodal equations and Lyapunov functions in a qua-
dratic form and prospectivity of the proposed method for study of a steady-state stability of complex EPS.
3. Conclusions
The obtained theoretical and computational results confirmed for systems of various complexity allow checking
stability of EPS “in the small” by study of positivity condition for the first minor of a Lyapunov function matrix
in a quadratic form
11 0 j
q>
and to consider it as the practical (simplified) criterion of EPS steady-state stabil-
ity providing its both necessary and sufficient conditions. Definition of the requirement
1 1,
d dmax
j
qΠ→
al-
lows the generator to reveal that represents the greatest danger from the point of view of stability violation.
Upon that, the study of a steady-state stability of the complex EPS turns to study of the “generator-bus” circuit
design that makes it possible to determine the particular generator or station which leads to violation of a system
stability and an asynchronous condition in system.
Thus, joint use of Lyapunov functions in a quadratic form and the node-voltage equations allows us to the
fullest extent to explore a steady-state stability of the complex electrical power system including both its elec-
tromechanical and electromagnetic violations.
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