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Academic Open Internet Journal ISSN 1311-4360 |
Volume 19, 2006 |
POWER SYSTEM STABILIZATION
IN MULTI AREA SYSTEM BY USING HVDC LINK
1Assistant Professor, Department of Electrical and
Electronic Engineering, Kumaraguru College of Technology, Coimbatore, Tamil
Nadu, India 680 006.
2Professor & Head, Department of Electrical and
Electronic Engineering, Coimbatore Institute of Technology, Coimbatore,
Tamil Nadu, India 680 036.
3P.G. Students, (PE&D) Department of Electrical
and Electronic Engineering, Kumaraguru College of Technology, Coimbatore,
Tamil Nadu, India 680 006.
ranipalamittam@yahoo.com ,mohanaprakashme@yahoo.co.in ,sureshonu@yahoo.co.in
Abstract: When an AC power
system is subjected to sudden load changes, system frequency will be considerably
disturbed and becomes oscillatory. This is predominant in interconnected power
system. In this paper new controller is designed to regulate the tie-line
power regulation through HVDC link in parallel with the existing AC_AC link
for stabilizing the frequency oscillation of AC System. The technique of Overlapping
Decomposition and Eigen value assignment are applied for the design of power
modulation controller. The linear model of AC-DC link is developed and the
system responses to various load changes are studied with the help of Mat-Lab
simulation. The study shows that the proposed controller is not only effective
in damping out the frequency oscillation but is also capable of eliminating
the transient frequency swing caused by large load distribution.
Keyword: High Voltage Direct Current
Link (HVDC-link), AC-DC Interconnected Power system, Stabilization of frequency
oscillation, Overlapping, Decomposition, Eigen Value Assignment Method, Load
Frequency Control (LFC).
I. INTRODUCTION
The
quality of power means consistency of frequency, voltage and level of
reliability. The main problem is balancing the total system generation against
the system load and losses so that the desired frequency and power interchange
(tie –line power flow) is maintained at economically and reliable as
possible.[1-3] Load frequency control (L.F.C) has gained importance with growth
of interconnected system. Thus greater reliance is being placed on the use of
special control aids to enhance system security, facilities economical design
and provide greater flexibility of system operation. However the classical load
frequency control (LFC) based on AREA control error is difficult to implement
in a deregulated environment. An Alternative concept is thus introduced where
selected units are automatically following load changes on the HVDC
connection.[6] In this anticipation of these circumstances advanced control
strategies are in much need. Recent development of power electronics devices in
AC power system provided attractive benefits have economical and innovation of
new technologies. [5]
When
an Ac power system is subjected to load disturbance, the system frequency may
be considerably perturbed from the operating frequency. The deviation of
frequency oscillation that exceeds the normal limit directly interrupts the
operation of power system. The frequency oscillation may experience serious
stability problem usually in the form of low frequency oscillation due to
in-sufficient system damping.
The conventional LFC system is not
very well suited to a deregulated energy market. HVDC connection will cause increased
operational strain, an alternative secondary control system can be introduced
in which special power station are selected to follow the HVDC load
automatically while the rest of the system deviation are handled by
conventional (LFC) method.
In this paper the advantages of power
modulation control by HVDC link to enhance the system damping is used which
also extend to stabilize frequency oscillation in AC system. The proposed
controller is also used in coordination with conventional governor control for greater efficiency.[7]
II.OBJECTIVE
The demand of power in
an area is increased due to installation of large loads with sudden change such
as installation of large steel mill / Magnetic levitation transportation / an
arc furnace etc.. This causes a serious problem of frequency oscillations in
that area. The capabilities of frequency control of governors in this area are
not enough. On the other hands other area of interconnected system has enough
frequency control capabilities to compensate to that area in the trouble as
shown in Figure 1. Hence an HVDC Link is installed in parallel with an AC tie
–line in order to supply more power to the area in need. This is shown with 2
areas AC-DC Link. The power modulation controller is designed to control the power
flow through the HVDC link.
The proposed
HVDC Link Power modulation controller is superior to the governor, the
conventional frequency control in terms of high-speed performance. When a
sudden load disturbance occur in an area, a HVDC link quickly starts the
control system to suppress the peak value of transient frequency deviation,
subsequently governor controller eliminates the steady state error of frequency
deviation. In design the HVDC link control, the dynamics of the governor in the
areas are neglected.
Fig.1.AC-DC Link for 2 Area Power Systems.
III. CONTROLLER DESIGN
The proposed
installation of the Power modulation controller is shown in figure 2, for
simplification and design purpose the dynamic of governor are not included. It
is implemented in 3 area-interconnected systems. The system is linearized to
include the dynamic Power modulation controller. ∆P is the total tie line
power deviation (∆PAC +
∆PDC).
Fig.2.HVDC Power Modulation Controller Implemented in
the System
To simplify the control
design the state equation of the system shown in Figure 02 can be expressed as
follows (for 3 area system) where subscript 1, 2 & 3 represent the Area1,
Area2 and Area3. The matrix ‘S’ can be Decomposed
(overlapping) and Eigen Value assignment can be applied.
Here the control scheme
for the power modulation controller (DPDC) is designed by the Eigen Value
assignment method, so that the dynamic aspects of the inter area oscillation
mode between areas are specified. This mode can be explicitly expressed after
applying the variable transformation.
(02)
Where ‘Y’ is the
transferred state vector, ‘W’ is transformation Matrix and ‘X’ is the state
vector in (01). Therefore the transferred system can be expressed as (03).
(03)
“W” consists of two diagonal blocks
with complex Eigen value a±jβ
and real value λ. The complex Eigen Value correspond to the inter area
oscillation mode, while the real Eigen value represents the system inertia
center mode. Hence it is concluded that HVDC link between the two areas is
effective to stabilize the inter-area mode only. Hence the input to terms
Δy3 and Δy4 are zero. This means that HVDC link
cannot control the inertia center mode. To solve this crux, it is expected that
governors in all the areas are responsible for suppressing the frequency
deviation due to the inertia mode.
In order to extract the sub system
where the inter-area oscillation mode is preserved between the areas from the
system ‘S’, the technique of Overlapping Decomposition is applied. The state
variables of original system ‘S’ are classified into X1= [Δf1]
and X2 =[ΔPac]. According to overlapping
decomposition, the system ‘S’ can be expanded as (04).
(04)
Where Z1=
[X1T , X1T]T and Z2
= [X2T , X2T]T.
The system ‘
’ can be written into two interconnected overlapping sub
systems as (05) and (06).
(05)
(06)
As a
result of decoupled system 
and 
can be expressed as
(07) and (08).
(07)
(08)
can be expressed as
(09)
(09)
Here the
control purpose of HVDC link is to damp the peak value of frequency deviation
after sudden load disturbance. The new percent overshoot MP(New) for
new damping ratio ζ(new) is calculated by (10).
(10)
The new un-damped natural
frequency is given (11)
(11)
As a result new Eigen value
αnew ± βnew can be calculated as in (12).
(12)
By Eigen value
assignment method the feed back control scheme of ΔPDC can be
expressed as (13).
(13)
The state
feed back scheme is constructed by two measurable signals ‘Δf’ changes in
the frequency of the area and ‘ΔPAC’ change in AC tie-line
power.
IV. SIMULATION MODEL AND
DATA OF THE SYSTEM
3-area delta
interconnected system (400 Mw, 2000Mw & 500 Mw) with reheat steam turbine
is used for the system studies. System data is given in the Appendix. The
Software’s like Mat lab, Simulink and Control System toolbox was used to carry
out the simulation study. As per the standard the change in transient frequency
should not exceed the ±0.50Hzs. Most preferable the deviation should lie
between ±0.02Hzs. For the case studies a large steel mill and an arc furnace
factory with a combine load demand of 4 Mw (0.01pu) is used as sudden load.
This sudden load was stimulated as shown in the Table 01.
The three-area delta
system was simulated with only AC-AC link alone with the conventional governor
control. Then the system was simulated with only AC-DC link alone. The last
simulation was done with AC-AC link & AC-DC link in parallel. Sudden
loading and unloading in different area solely and simultaneously was applied
for the study. All the above conditions are ideal case; hence loading of the
system was time invariant condition is also simulated.
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TABLE
I |
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DEMAND
OF SUDDEN LOAD |
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Sl No |
AREA1 |
AREA2 |
AREA3 |
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1.
|
4 Mw |
0 Mw |
0 Mw |
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2.
|
0 Mw |
4 Mw |
0 Mw |
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3.
|
0 Mw |
0 Mw |
4 Mw |
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4.
|
4 Mw |
-4 Mw |
0 Mw |
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5.
|
0 Mw |
4 Mw |
-4 Mw |
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6.
|
-4 Mw |
0 Mw |
4 Mw |
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7.
|
4 Mw |
4 Mw |
4 Mw |
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8.
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Periodical load change in
Area 1 |
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V. RESULT
All the above 8 case
were simulated. The results are tabulated in table II, table III and table IV.
(Tables II, III, IV are tabulated in the appendix). Case1 and case 7 are shown
in graphical representation figure III. It should be noted that the power
modulation output of HVDC link (ΔPDC) acting positively on an
area, reacts negatively on another area in an interconnected system. The time
constant TDC of the proportional controller is set appropriately at
0.05[sec] in the simulation study. From the graph [figure III (a)] it is noted
that with out the HVDC power modulation controller, the peak overshoot and
settling time is high. With HVDC power modulation controller and with the
absent of conventional governor controller the frequency is settling at a new
value. This is so because the controller will be effective to stabilize the
inter area oscillation mode only i.e. it cannot control inertia center
oscillation mode as we have derived in the equation (03). To solve this crux we
have included the conventional governor control that is responsible for
suppressing the frequency deviation due to inertia central oscillation mode.
