UPFC Influence to Transient Stability of Power System
Matus Novak, Richard Kravec, Martin Kanalik, Zsolt Conka, Michal Kolcun
Department of Electric Power Engineering
Teclmical university ofKosice
Kosice, Slovakia
matus.novak@tuke.sk, martin.kanalik@tuke.sk, zsolt.conka@tuke.sk, michal.kolcun@tuke.sk
Abstract- Large-scale
[I.
power systems operation is faced with
many challenges. One of many criteria, which should be fulfilled
In foreign countries, specialized devices are used with
many advantages. These specialized systems are known under
abbreviation FACTS - Flexible AC transmission system.
are stability criteria. Interconnecting separated power systems
into interconnected network results in large system complexity,
and
therefore
mixed
influences
from one
power
system
to
These devices are defined as alternating current
transmission systems based on power electronics and other
passive and active elements (capacitors, reactors, or
transformers), which provide regulation of one or more
parameters, such as current, voltage, impedance, phase shift.
[11]
another part. Also, new devices are introduced to improve power
system performance, like FACTS devices. Main goal of this paper
is to examine influence of one of these devices to power system
stability.
Keywords-power system, stability, PSLF, FACTS, UPFC.
I.
These devices bring many advantages and possibilities for
TSO as for example:
INTRODUCTION
Safety and reliability of power system operation is
discussed in present time very often. Power system stability of
interconnected power system, like ENTSO-E - European
power system, should be discussed very carefully, mainly due
to weak cross-border interconnections between countries, or
between control areas (CA). Because of these weak
interconnections, any fault, which appears in one CA will
influence neighboring CA. Latest trend of shutting down
nuclear power plants and building new renewable sources with
similar installed power without necessary changes in power
system bring new issues for power system stability. [10]
Since these problems appeared, great effort has been
expended to limit the impact of these problems to power
system operation. Previous research has shown that in some
operating conditions is power system without direct dispatcher
intervention put into risky conditions. Some protective actions
had to be taken previously to maintaein power systems safety
criteria, like re-dispatching, network reconfiguration, and
even "multilateral re-dispatch", which means, that one
transmission system operator (TSO) asks other TSO to change
configuration of their power system. Solution for these
problems can be provided by using new specialized devices
for power flow regulation and stability improvement. Since
these devices are very expensive, from economical point of
view their use is not very convenient, but from view of power
system operation, these devices can very well improve power
system operation performance and safety, mostly around
network bottlenecks. [8][[3]
[.
Transmission capacity increase for existing lines
to their limits without any other compensating
devices,
2.
Power flow regulation, loop flows reduction,
3.
Reliable interconnection of neighboring power
systems, and decreasing of needed electricity
production on both sides,
4.
Reduction of big shunt reactors and series
capacitors and therefore reduction of space in
power stations,
5.
Compactness and modality of new FACTS
systems allows their installation everywhere and
extending of their size,
6.
Power system stability improvement increasing
static and transient stability limits, damping of
generator electromechanical oscillations and
decreasing of short circuit currents.
These devices can be divided into four basic categories:
1.
Parallel regulators,
2.
Serial regulators,
3.
Combined serial-serial regulators,
4.
Combined serial-paralel regulators.
From this, different kinds of FACTS devices are known,
such as SVC (Static VAR compensator), STATCOM (Static
synchronous compensator), TCSC (Thyristor Controlled
Series Capacitor), PST (Phase Shifting Transformer), SSSC
(Static Synchronous Series Compensator), UPFC (Universal
Power Flow Controler), and we can also mention HVDC
This work was supported by The Ministry of Education, Science,
Research and Sport of the Slovak Republic and the Slovak Academy of
Sciences under the contract No. VEGA 1/0388/13.
978-1-4799-3721-9/14/$31.00 m014 IEEE
SPECIALIZED DEVICES
343
(High Yoltage Direct Current) transmiSSIOn system, which
have also possibility of controlling real and reactive power,
and is used to provide asynchronous connection between two
power systems with e.g. different frequency. From view of
which power can be regulated using this devices, we can
divide them into three categories:
I.
Reactive power regulation (SYC, STATCOM),
2.
Real power regulation (TCSC, PST, SSSC),
3.
Real and reactive power (HYDC, UPFC).
from UPFC most universal transmission device for both - real
and reactive power regulation and control.[3]
Transmission line
-------r1--l.u.J
Series
tTimsformer
Parallel
trallsfOlll1 er
e omrerter1
·
Converter2
-....�V,
..
Fig. 3 UPFC topology[2]
In next part, we will continue with description of power
system used for modeling of UPFC influence.
Fig. 1 STATCOM topology[7]
In following we will speak mostly about UPFC and its
principle. This device connects together STATCOM (Fig. I)
and SSSC (Fig. 2) device, and is therefore combination of
serial and parallel compensation, what on the other side
corresponds to technical and economic difficulties of this
device.
vJ
Fig. 4 Vector diagram of UPFC regulation possibilities[6]
III.
MODELLED POWER SYSTEM
For examination of UPFC influence to power system, 39bus New England power system was selected. Mainly its
complexity and installed power, has been taken into
consideration, when this power system was chosen. On the
same power system was previously researched influence of
primary and secondary power frequency control. For this
reason, power system was divided into three areas, with
generation and consumption almost in balance. All devices,
like exciter, exciter regulation, power system stabilizer (PSS)
and turbine with governor were modeled for all generators.
Fig. 2 SSSC topology[7]
For purpose of power system modeling, a Positive
Sequence Load Flow (PSLF) software was used, provided for
authors by Slovak TSO - Slovenska elektrizacmi a prenosova
sustava (SEPS) in version 17.05.
UPFC consists from parallel (excitation) and serial
(auxiliary) transformer. Both transformers are connected
through two YSC converters which are interconnected through
mutual DC intermediate circuit with condenser. Serial
converter injects AC voltage Upq (voltage on serial
transformer) which can be according input voltage UI (on left
side) freely rotated and can have amplitude freely set in range
from 0 :S Upq :S Upq max. Serial converter can therefore act in all
four quadrants and independently control real and reactive
power of line. Parallel converter can generate reactive current
in way to keep voltage U\ on set value. In this case, converter
acts in voltage control mode. Parallel reactive current can also
correspond to required inductive or capacitive reactive power,
and therefore converter acts in reactive power control mode.
Control parameters of UPFC are therefore amplitude and
phase shift of injected voltage Upq and amplitude of reactive
current Iq of parallel branch. This regulation possibility makes
In PSLF, UPFC is in load flow modeled by a transformer
with variable tap ratio and phase shift and by a generator at the
bus of line input, which supplies the reactive power for both
the shunt and series converters. For dynamic simulations,
UPFC is simulated using subroutine which acts as UPFC
regulator, and controls series voltage injection and shunt
MYAr injection into the network using current injections at
input and output bus of UPFC.[9]
344
Second fault, marked as B, was modeled in the middle of
the line from node 26 to node 27. This line was loaded by 316
MW and 84 MVAr, which represents about 32% of its rating.
Values of CCT for lines with and without UPFC connected are
in TABLE I. We can see that CCT difference for first fault is
only 35ms, for second, is 158 ms.
AREAl
<1>
TABLE I. CCT VALUES FOR AREA 1
Area
C7 '"
CCT (ms)
I
Without UPFC
With UPFC
Fault A (line 2-3)
277
312
Fault B (line 26-27)
207
365
<0>
B. AREA2
In area 2, we have chosen also two places of fault, as in
previous case. Since lines in this area are not heavily loaded,
we were not able to choose any line close to overload. First
fault C was modeled on line from bus 14 to 15, in the middle
of the line. To be clear, when we are speaking about fault, it is
always three phase solid short-circuit. This line was therefore
loaded at about 32% of rating. Other fault (D) was applied to
line from node 5 to node 8, also in the middle of the line. This
line was loaded at about 27,7 % of rated current. CCT
differences for situation with and without UPFC are more
significant for this area, 95, or even 200 ms. Curves of angle
of generator at bus 34 are on Fig. 7.
Fig. 5 39-bus power system splited to three areas with marked places where
faults were applied (from [12] , modified according description above)
SIMULATIONS
IV.
Using previously mentioned power system model, we have
chosen few faults, which have serious influence on stability,
and we performed some simulations for each of three areas, on
which power system was separated. UPFC was implemented
in Area 3, on line, which interconnects this area with Area I.
For all three areas, similar faults were chosen, and value of
critical clearing time (CCT) was evaluated for each place of
fault, comparing network with, and without UPFC
implemented. The Critical Clearing Time is the maximum
time during which a disturbance can be applied without the
system losing its stability.
A.
TABLE II CCT VALUES FOR AREA 2
Area 2
AREA 1
In area 1, two places of fault were chosen. First was
modeled in the middle of line from node 2 to node 3. This line
was before fault loaded by 671 MW and 144 MVAr, which
represents about 68% of its rating. This fault is marked as fault
A. Curves of angle for generator 34 and 37 are on Fig. 6.
Generator3
run out of
60
synchronisn
with UPFC,
t--fau1t--dur-ati
t�3J3ms
20
/
-�/
/
5
45
-20
-4
-40
With UPFC
Fault C (line 14-15)
297
392
Fault 0 (line 5-8)
247
447
Generator 34
runs out of
with UPFC,
II
I
./
F atilt applied a simulation tim t=55
fault duration
�r------
/
;A
)1 \
-60
-80
Without UPFC
synchronism -
80
0(")
40
CCT (ms)
t �393ms
Generator34
run out of
synchronism without UPFC,
fault duration
-Fault C Without
t� 278 ms
UPFC 297
-Fatlit A Without
time (s) 65
1\
\
'--.../
UPFC 278
UPFC 298
illS
-Fatlit A With
UPFC 3 12
UPFC 392
illS
illS
C With
UPF C 393 illS
Fig. 7 Curves of angle for generator 34 for fault C in area 2.
-Fault
illS
-Fatlit A Without
UPFC 227
illS
-Fault C With
-Fatlit A With
UPFC 3 13
illS
-Fault C Without
illS
Fig. 6 Curves of angle for generators 34 and 37 for fault A in area 1.
345
C.
cheap components - two transformers and two high power
converters, conclusion can be made, that other solutions will
be taken into consideration, when TSO will be choosing
solutions to improve power system stability and for power
flow control. This statement also supports the fact, that only
three UPFC were installed until present moment in whole
world. Therefore other, cheaper solutions have to be subject of
research for this purpose.
AREA 3
[n third area, UPFC is implemented, on line from node [6 to
[7 (Areal). For this area, also two faults were modeled, on
line from 23 to 24 (fault E) and for line from 2 [ to 22 (fault
F). First line was loaded to only [6% of rated current, other
one was loaded to 34% of rated current. Observed CCT
changes for this area were also significant for both faults.
Differences were 165, or even 175 ms. From this point of
view, control area with implemented UPFC can withstand
more serious faults, than other CA without UPFC. It can be
observed, that CCT for fault F was prolonged more than twice.
Curves of angle of generator at bus 34 and 33 are on Fig. 8.
REFERENCES
[I]
100
33 /
1) (')
34
/
80
�
----II -----60 �308 --I
/
40
133
/
/".
/
20
r------ /
/ V
F Without
5
75 -Fault
�
6,\ 65
.;
UPFC
-20
//
F Without
'\
/ � -Fault
-40
UPFC
\'n
-Fault F WithUPFC
-60
\ /
-80
-Fault F WithUPFC
�
-100
Genera or
Generator
runs ou of
synchr
run out of
ism -
synchronism -
with Ul Fe,
fault d
t
[2]
without UPFC,
ation
fault duration
fns
[3]
ms
F
[4]
time (s)
"....
Fau t applied
=
illS
133
illS
[5]
t
sim ration tine
t
132
307111S
s
[6]
308111S
Fig. 8 Curves of angle for generator 34 and 33for fault F in area 3.
[7]
TABLE III CCT VALUES FOR AREA 3
Area 3
[8]
CCT (ms)
Without UPFC
With UPFC
Fault E (line 23-24)
242
407
Fault F (line 21-22)
132
307
[9]
[10]
[11]
It has to be also stated, that without correct response of
UPFC regulator these improvements were not so significant.
Also, influence of UPFC on transient stability of more
complex power system can be subject of further research.
[12]
[13]
CONCLUSIONS
From previous simulations, significant positive impact of
UPFC on power system dynamic stability can be observed.
Because of UPFC price, which consists from four not really
346
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