Download Innovative power flow management and voltage control technologies

Power flow management
Innovative power flow
management and voltage
control technologies
Driven by ever increasing energy demands, environmental constraints, deregulation
and privatisation of the power supply industry, existing transmission systems are
often operated and stressed to the limit and occasionally beyond the performance
capability of their original design in order to maximise asset utilisation. To ensure
that under these conditions the economical, reliable and secure operation of the
grid is maintained, the need for various aspects of power flow management within
the power systems is becoming increasingly evident.
by E. Wirth and A. Kara
Power flow control concepts
ower flows can be influenced by
controlling the basic electrical
parameters, namely impedance of the
transmission line and system voltages,
as shown in eqn. 1:
P
US
PS = ReUS

U
= ImU 



QS
–
jXl
S
S
UR*
–
jXl

UR*

(1)
where
PS = active power across the transmission line
QS = reactive power at the sending end
US = sending end voltage
UR = receiving end voltage
Xl = impedance of the transmission line
To be able to control the flows of active power
P and/or reactive power Q, one or several of
these parameters can be controlled by power
equipment already available or under
development. The control of the basic electrical
parameters can be achieved using a shunt
control device, series control device, shunt
current injection device, series voltage
injection device or a combination of these.
POWER ENGINEERING JOURNAL JUNE 2000
In this article the
assumptions are used:
following
model
4 lossless transmission lines
4 sending and receiving ends are stiff nodes and
their voltages are equal in magnitude
4 performance characteristics are drawn for
midpoint location of control devices.
Shunt control device
The impact on power flow due to a capacitive
shunt device with a reactance of XC can be
investigated using the transmission model,
vector diagram and mathematical relations
shown in Fig. 1. Shunt devices basically impact
the voltage at the point of connection. When
connected to weak nodes in the power system,
for example in the midpoint or in the receiving
end of a long transmission line, the power flow
can be influenced substantially by the change of
voltage due to the shunt device.
Series control device
The voltage in series with the line can be
created by the natural voltage drop caused by
the line current across an impedance element
with a capacitive reactance of XC. The insertion
of a series compensation device in a
transmission line directly impacts the power
129
Power flow management
IS
Xl/2
Xl/2
Um
IR
UR
US
Xc
US
Ic
Um
( (
( (
US 1
PS = Re
US
Xl
2Xc
UR
*
UR
IS
IR
Xl
jXl (1
)
4Xc
Ic
*
US 1
QS = Im US
Xl
2Xc
jXl (1
I
Xl
)
4Xc
Xl /2
Xc
Um1
US
δ
UR
Xl /2
Um2
I
US
Um2
Um1
UR
UR
I
PS = Re US
QS = Im US
1 Transmission-line
model, power flow
equations and vector
diagram of the system
with a shunt control
device
2 Transmission-line
model, power flow
equations and vector
diagram of the system
with a series control
device
j(Xl
Xc )
*
US UR
j(Xl
*
US UR
Xc )
flow on the line. The influence of a capacitive
element providing the series voltage can be
investigated using the equations shown in
Fig. 2.
Shunt current injection device
Power flow control devices can utilise the
physical principles described above, or
depending on their construction and operating
mode, can be based on the concepts of
controllable shunt current injection and
controllable series voltage injection. The
concept of a device based on shunt current
injection can be demonstrated using the system
shown in Fig. 3. Ii is the controllable shunt
current injected to the midpoint of the
transmission system.
Series voltage injection device
As mentioned already, the series voltage can be
130
δ
provided by a controlled voltage source. The
series voltage device can be constructed such
that the injected voltage’s magnitude UT and/or
phase angle α can be varied. The impact on
power flow can be investigated by using the
transmission model, vector diagram and
equations shown in Fig. 4.
Impact of power flow control and reactive
power compensation devices on system
performance
By employing devices that can control the basic
electrical
parameters,
power
system
performance can be significantly improved.
One of the major aims of improving a
transmission system’s performance is to
increase its power transfer capability. By using
the concepts discussed above, it is possible to
quantify the impact that shunt and series
control devices have on power transfer
POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
capability and reactive power requirements in
transmission systems. The reactive power
balance is one of the many requirements that
enforces a practical limit on how much active
power P can be transferred over a system. Series
devices providing a specific amount of
compensation in general enable more active
power to be transferred with less sending end
reactive power supply requirements as
compared with a shunt device. Capacitive
series devices increase the transfer capability
(their reactive power output increases also with
line loading) and, in addition to increasing the
stability limit, the voltage regulation
capabilities of the system are significantly
improved.
transfer capability through reactive current
supply
Shunt devices help maintain the system voltage
when transferred power is varied. Shunt
reactors are used to compensate for the reactive
power surplus in case of reduced power transfer
or open transmission lines. In case of long
transmission lines, some of the shunt reactors
are permanently connected to the system to
give maximum security against overvoltages in
the event of sudden load rejection or opening of
lines. The conventional shunt capacitor
compensation provides the most economical
reactive power source for voltage control in
cases when additional voltage support is
required.
Conventional shunt control devices and
modern shunt current injection devices, e.g.
the STATCOM, can also control the power flow
Shunt compensation and control devices —
improving voltage maintenance and power
IS
Xl/2
Xl/2
Um
US
PS = Re US
US UR
Ii
jXl
2
QS = Im US
US UR
Ii
jXl
2
IS
4 Transmission-line
model, power flow
equations and vector
diagram of the system
with a series voltage
injection device. The
equations are based on
the concept with active
power PT drawn from
the network and
reactive power QT
generated locally,
shown in Fig. 6.
IR
Ii
UR
US
Um
*
IS
*
UR
IR
Ii
δ
X1
UT
Um1
US
3 Transmission-line
model, power flow
equations and vector
diagram of the system
with a shunt current
injection device
X2
Um2
IR
jX1I S
UR
US
UT
α
jX2 I R
Um2
UR
Um1
Um1 = UR + jX2 IR – UT
US = Um1 + jX1 IR +
IS = I R +
Re(UT IR*)
Um1*
Re(UT IR*)
Um1*
PS = Re (US I S*)
QS = Im (US I S*)
POWER ENGINEERING JOURNAL JUNE 2000
δ
131
Power flow management
UT
UT
PS
PS
QS
QS
PT,QT
PT,QT
7 Achievable
transmitted active
power for the different
series voltage injection
concepts and a
transmission angle δ
of 60°
8 Impact on power
transfer capability using
different series voltage
injection concepts for
transmission angles δ
between 0° and 90°
PT
5 Series voltage injection with P and Q
from network
6 Series voltage injection with P taken
from network, Q generated locally
through a transmission system in a limited
range by supplying or absorbing reactive
current at the point of connection to the
system.
devices,
further
system
performance
improvement can be achieved by providing
greater operational flexibility in addition to
increasing power transfer capability. There are
basically two ways of generating this series
voltage. One way is to draw all the active power
PT and reactive power QT requirements needed
to generate this voltage from the network, as
shown in Fig. 5. The other way is to draw only
the active power from the network and provide
the reactive power required locally as in Fig. 6.
The power flow control capabilities of
devices capable of coupling a series voltage
with a variable phase angle α are shown in
Fig. 7. The impact on power flow control of the
former concept is shown by the green curve and
of the latter by the purple one. Both the curves
are for series voltages of 20% of the nominal
sending end system voltage and a transmission
angle δ of 60° between the receiving and
sending end voltages. As the phase angle α of
the series injected voltage is varied between 0°
and 360°, the active power flowing through the
transmission system can theoretically be
controlled for a range from a maximum
through to minimum values. From Fig. 7, it can
also be seen that the locally provided reactive
power concept has a bigger impact on power
flow control compared with obtaining reactive
power from the network.
Fig. 8 shows the improvement and the limits
(α0° and α180°) in power transfer
capability with the different series voltage
injection concepts, for an injected voltage
magnitude of 20% of nominal system voltage,
over a range of transmission angles δ. The
purple band shows the operating capability of
the series voltage injection device with locally
supplied reactive power QT whilst the green
meshed band is due to a device drawing active
and reactive power from the power system. The
bands indicate the control ranges of devices for
α varying between 0° and 180°.
Series voltage injection devices — improving
flexibility and enhancing system performance
In the case of the series voltage injection
1600
PS, MW
1200
800
400
PT from network, QT local generated
PT, QT from network
0
90
180
270
360
α, deg
1600
PT from network, QT local generated
PT, QT from network
no control
PS, MW
1200
α 180°
800
α 0°
400
0
30
60
δ, deg
132
QT
90
POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
Xcmax compensation
1·2
Xcmin
compensation
1·0
PS
XC
0·8
X2
PS, pu
X1
UR
US
no compensation
0·6
0·4
0·2
a
0
0
20
40
60
80
100
120
140
160
180
δ, deg
b
Solutions to transmission system concerns
using power flow control technologies
Finding the most cost-effective solution to the
various
issues
limiting
transmission
performance is attracting ever growing interest
as utilities deregulate and a competitive
electrical supply environment is becoming a
norm rather than an exception. Power flow
control technologies can provide the key to
these solutions. An overview of the
transmission issues and the possible effective
solutions are summarised in Table 1. These
solutions include both conventional as well as
innovative technologies, though they are by no
means exhaustive. In must be noted that, due to
the wide range of network configurations and
9 (a) TCSC system and
(b) its performance
characteristics
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
power flow
issues
parallel line load sharing
post-fault sharing
power flow control
dynamic and
stability issues
lack synchronising torque
dynamic flow control
and transient stability
power oscillations
voltage stability
POWER ENGINEERING JOURNAL JUNE 2000
×
×
×
×
UPFC
×
×
×
low voltage at heavy load
high voltage at light load
voltage deviation
following outage
×
×
IPC
×
×
×
voltage and
reactive power
control issues
×
QBT
ASC
×
line overload
tripping of parallel circuit
BDV = breaker switched capacitor
BSC = breaker switched reactor
IPC = interphase power controller
QBT = quadrature boosting transformer
TCPAR
TCSC
×
thermal
issues
SVC
BSR
×
BSC
ASC = advanced series compensator
STATCOM
Table 1 Overview of transmission system limitations and possible solutions using control devices
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
STATCOM = static synchronous compensator
SVC = static VAr compensator
TCPAR = thyristor-controlled phase-angle regulator
TCSC = thyristor-controlled series capacitor
UPFC = unified power flow controller
133
Power flow management
10 (a) ASC system,
(b) its steady-state
operating and
(c) performance
characteristics
PS
X1
UT
X2
US
UR
a
1·6
I, pu
UT = 0·5 pu
capacitive compensation
1·4
UT = 0
no compensation
1·2
PS, pu
1·0
UT = 0·5 pu
inductive compensation
0·8
0·6
0·4
0·2
0
–0·2
–0·4
inductive
capacitive
UT, pu
0
b
20
40
60
80
100
120
140
160
180
δ, deg
c
system operation procedures, proper corrective
actions to deal with various issues are of
necessity application dependent.2
In steady-state conditions the total power
flow on all lines that connect two power
systems is determined by unbalance between
power production and load demand including
losses in the individual systems. On the other
hand, during transients the power flow control
equipment can also have an impact on the total
power exchange between the systems. Power
flow control technologies and equipment can
thus be generally categorised according to their
ability to solve steady-state or dynamic problem
domains.
Thermal issues are generally related to
thermal limits caused by a change in the
network configuration during outages and can
be overcome by rearranging the network or by
adding a power flow control equipment.
Voltage and reactive power control issues are
related to voltage constraints in the power
134
system. Low voltage at heavy load can be a
limiting factor under steady-state conditions.
The corrective actions include correcting the
power factor and compensating the reactive
losses in lines by supplying reactive power.
High voltage at light load is an undesirable
occurrence in the transmission and distribution
systems and may be diminished using
mechanically switched shunt capacitors or
reactors to supplement the action of
tapchangers. Low voltage as well as high
voltage following outages can exceed the
voltage limits so that corrective actions have to
be taken to avoid further equipment damage.
Power flow issues are generally related to
controlling the active power in the power
system for better utilisation of the transmission
assets, minimisation of losses, limit flows to
contract paths, post contingency strategies etc.
Dynamic and stability issues are related to
dynamic performance of the power system.
Transient stability describes the ability of the
POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
power system to survive the first few seconds
after a major disturbance and can be improved
by extracting energy from the sending end of
the network, supplying energy to the receiving
system respectively by increasing the
synchronising power between sending and
receiving ends. Power system oscillation
describes sustained or growing power swing
oscillations (generally in range below 1.5 Hz)
between generators or group of generators,
initiated by a disturbance (fault, major load
changes etc.). Solutions to this problem lie in
the use of equipment that permits dynamic
damping of these oscillations. Voltage stability
problem is a slow process caused by progressive
increase in load and can be improved by voltage
support, e.g. by using reserve devices, coordinating system load tapchangers, automatic
undervoltage load shedding or generator
control action.
Power flow control devices and their
performance characteristics
As Table 1 shows, solutions to the transmission
issues can be addressed by various power flow
control devices. Their application and
suitability to solve a particular problem depend
on many factors covering technical as well as
economical considerations. This section
provides brief descriptions of the technical
capability, technology and performance of each
of the devices listed in Table 1, allowing a first
estimate of device suitability for an intended
application.
Breaker switched capacitor and reactor (BSC,
BSR)
Shunt-connected equipment of these types
allow the reactive power to be supplied via
capacitor banks or absorbed via reactor banks
and thus have significant influence on the
11 (a) SVC system,
(b) its steady-state
operating and
(c) performance
characteristics
PS
X1
X2
Ush, pu
US
UR
capacitive
inductive
Ish, pu
b
a
1·2
SVC on capacitive limit
no control
1·0
PS, pu
0·8
SVC on inductive limit
0·6
0·4
0·2
0
0
20
40
60
80
100
120
140
160
180
δ, deg
c
POWER ENGINEERING JOURNAL JUNE 2000
135
Power flow management
voltage at the point of connection. Seriesconnected equipment allow the impedance
characteristics of the transmission system
where they are installed to be varied and thus
have direct impact on the power transfer
capability. These devices can be permanently
connected to a system or are connected through
circuit breakers. Breaker switched devices offer
greater operational flexibility in terms of
allowing the operators to adapt to changing
reactive power requirements of their power
systems. Their performance is limited by their
step-wise control characteristics.
PS
X1
X2
US
UR
a
Thyristor-controlled series capacitor (TCSC)
The thyristor-controlled series capacitor system
is shown in Fig. 9 together with its performance
characteristics. The variation of capacitance
can be achieved by varying the thyristorcontrolled reactance that is connected in
parallel to the capacitor. The reactance is
determined by the thyristor valve firing angle.
The controllable parameter influencing the
power flow is the capacitance of the TCSC.
Ush, pu
capacitive
inductive
Advanced series compensator (ASC)
In contrast to the TCSC where the reactive
power is produced or consumed by capacitors
and reactors, advanced series compensators use
power electronics elements with turn-off
capability such as integrated gate commutated
thyristors (IGCT). By proper repetitive
switching of the IGCTs, the phases of the
system are connected and/or disconnected
causing reactive power to flow among them.
The main difference from the TCSC is that the
injected series voltage UT of the ASC does not
depend on line current. The controllable
parameter here is the series injected voltage and
is coupled in general to the power system via a
booster transformer. Fig. 10 shows an ASC
system with its corresponding performance
characteristics.
Ish, pu
b
1·6
1·4
STATCOM on capacitive limit
1·2
PS, pu
1·0
0·8
no control
0·6
0·4
STATCOM on inductive limit
0·2
0
–0·2
–0·4
0
20
40
60
80
100
120
140
δ, deg
c
12 (a) STATCOM system, (b) its steady-state operating and
(c) performance characteristics
136
160
180
Static VAr compensator (SVC)
An SVC consists of a combination of fixed
capacitors, thyristor-switched capacitors and
thyristor-controlled reactors connected in
parallel with the power system in most cases via
a step-up transformer. The maximum SVC
reactive currents are dependent on SVC
terminal voltage. The reactive power produced
or consumed by an SVC is generated or
absorbed by passive reactive components. The
controllable parameter in this equipment is the
parallel capacitive or inductive susceptance.
POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
PS
UT
X1
US
UM
UR
a
1·0
UT
0·9
0·8
US
α=0
α
δ
UM
UR
PS, pu
0·7
α=40°
0·6
0·5
α=–10°
0·4
δ+α
0·3
0·2
0·1
b
0
–50
0
50
100
150
200
250
δ, deg
c
Within the SVC rating, its susceptance can be
continuously controlled. When the SVC
reaches its capacitive or inductive limit, it then
acts as a parallel capacitor or reactor,
respectively. Fig. 11 shows a SVC system, its
steady-state operating and performance
characteristics.
Static synchronous compensator (STATCOM)
By employing power electronics elements with
turn-off capability as in the case of the ASC, the
SVC system can be similarly improved to
become a static synchronous compensator
(STATCOM). The STATCOM basically consists
of an IGCT converter and a DC circuit. The
reactive power generation or absorption is
performed by the system itself and in balanced
conditions reactive elements are necessary for
energy storage during short periods between
power electronic switching. From the
STATCOM operating characteristics in Fig. 12,
it is evident that it can supply constant reactive
POWER ENGINEERING JOURNAL JUNE 2000
current almost over the entire range,
independent of the terminal voltage. The
STATCOM controllable parameter is its reactive
current.
13 (a) TCPAR system,
(b) its steady-state
operating and
(c) performance
characteristics
Thyristor-controlled phase angle regulator
(TCPAR)
Phase-shifting
transformers
(PST)
are
transformers with complex turn ratios. The
phase difference between the PST terminal
voltages is achieved by connecting a boosting
transformer in series with the transmission
line, as shown in Fig. 13. The active and
reactive powers that are injected into the
transmission line must be taken from the
network by the shunt transformer and
redirected to the boosting transformer. If losses
are neglected, the PST does not produce or
consume reactive power.
The thyristor-controlled phase angle
regulator is one type of PST with equal input
and output voltage magnitudes but with a
137
Power flow management
PS
UT
US
X1
UM
UR
a
1·2
UT
UT=0
US
UM
1·0
UR
β
UT=–0·5
UT=0·5
PS, pu
0·8
δ
0·6
0·4
0·2
b
0
–50
0
50
100
150
200
δ, deg
c
14 (a) QBT system,
(b) its steady-state
operating and
(c) performance
characteristics
phase shift between these voltages. The TCPAR
is controlled extremely quickly by a static
thyristor based on-load tapchanger. The
controllable parameter of the TCPAR is the
voltage phase shift angle α. Fig. 13 shows also
the steady-state operating and performance
characteristics of the TCPAR.
Quadrature booster transformer (QBT)
The quadrature booster transformer is another
type of PST where the phasor of the injected
voltage is shifted by a constant angle β with
respect to the input voltage vector. Various
types of QBT enable various β angles. The
controllable parameter of the QBT is the
magnitude of the injected voltage UT. Fig. 14
shows a QBT system with β=90°, its steady-
138
state
operating
characteristics.
and
performance
Interphase power controller (IPC)
The interphase power controller is a seriesconnected device, where the major
components in each of the phases are a reactor
and a capacitor subjected to individually phaseshifted voltages provided by two phase shifting
transformers PAR1 and PAR2. There are many
IPC configurations, depending on specific
application requirements and on the method
used to implement the internal phase shifts. In
the case where the reactor (XA) and the
capacitor (–XA) form a conjugate pair, each
terminal of the IPC will behave as a voltagedependent current source and provide the IPC
POWER ENGINEERING JOURNAL JUNE 2000
Power flow management
with the unique decoupling effect property, a
feature that is desirable. The controllable
parameters are the phase shift angles α1 and α2
of PAR1 and PAR2, respectively. Fig. 15 shows
an IPC system and its performance
characteristics.
PAR 1
XA
Innovative system solutions — the key to
cost-effective power flow control
Driven by ever increasing energy demands,
environmental constraints, deregulation and
privatisation of the power supply industry,
existing transmission systems are often
operated and stressed to the limit of, and
occasionally beyond, the performance
capability of their original design. To ensure
that under these conditions the economical,
reliable and secure operation of the grid is
maintained, power flow management concepts
employing innovative technologies have been
proposed.
Load sharing and loss minimisation,
regulating power flow through transmission
corridor, transient stability enhancement and
rapid power flow management to prevent
overloads as well as controlling power flow
patterns are transmission issues that are of
concern and interest to system operators
worldwide. Technical solutions for these
POWER ENGINEERING JOURNAL JUNE 2000
PS
α1
X1
PAR 2
US
UR
–XA
α2
a
XIPC<X1
1·2
XIPC=X1
1·0
αIPC=0
XIPC>X1
XIPC=X1
αIPC≠0
XA
XIPC =
0·8
PS, pu
Unified power flow controller (UPFC)
The basic structure of the unified power flow
controller and its performance characteristics
are shown in Fig. 16. It consists of shunt
(exciting) and series (boosting) transformers.
Both of these are connected by two IGCT
converters and a DC circuit represented by the
capacitor. One difference between the UPFC
and a PST is that the UPFC reactive power
injected into the line by the series branch does
not need to be transmitted from the parallel
branch. It is generated by the converter
connected to the series branch. The active
power injected into the system by the series
branch must be taken from the system by the
parallel branch and transmitted to the series
branch over the DC circuit. Additionally, the
reactive power of the parallel branch can be
controlled in the same manner as for the
STATCOM. The voltage UT can be of any phase
with respect to the input voltage US and can
have any magnitude ranging from 0 to UTmax
corresponding to the dimension of the UPFC.
The controllable UPFC parameters are phase
and magnitude of the injected voltage UT and
the magnitude of the parallel branch reactive
current.
2 sin
0·6
αIPC =
0·4
α 1 + α2
2
α 1 + α2
2
αIPC
0·2
0
–90
0
δ, deg
90
b
concerns have been proposed and discussed.
In Fig. 17 a 200MVA phase-shifting
regulating transformer for 240kV/132kV based
on a new compact concept is shown. The two
booster transformers for in-phase control and
quadrature control, normally connected in
series with the main transformer, are replaced
by only few extra windings inside the main
transformer tank. This considerably reduces
not only the investment costs but also the
operating costs. The main saving is in the
transformer cores and the copper windings.
Another key benefit is the significantly smaller
space that is required.
Utilities share many of the common energy
transmission problems yet have different
technical, economical and environmental
requirements. In order that their needs are
individually met, and cost-effective solutions
15 (a) IPC system and
(b) its performance
characteristics
139
Power flow management
16 (a) UPFC system
and (b) its performance
characteristics
PS
UT
X1
US
UR
a
1·2
UT=0·5 pu
P maximum
1·0
UT=0
PS, pu
0·8
0·6
0·4
UT=0·5 pu
P minimum
0·2
0
–50
0
50
100
150
200
δ, deg
b
17 200 MVA phaseshifting regulated
transformer for 240
kV/132 kV based on a
new compact concept
are provided, the key lies in the application of
innovative power flow control technologies.
Co-operation between the power industry
partners can develop optimised solutions
capable of meeting the performance
requirements demanded in the new and
evolving electrical utility environment.
References
1 DUNLOP, R. D., GUTMAN, R., AND MARCHENKO,
R. P.: ‘Analytical development of loadability
characteristics for EHV and UHV transmission lines’,
IEEE Trans., March/April 1979, PAS-98, pp.606–617
2 CIGRE TF 38-01-06: ‘Load flow control in high
voltage power systems using FACTS controllers’,
CIGRE, January 1996
3 WIRTH, E., and RAVOT, J. -F.: ‘Regulating
transformers in power systems — new concepts and
applications’ ABB Review, 4/1997
4 JAUCH, T., KARA, A., KIEBOOM, G., and WIRTH,
E.: ‘Operational aspects and benefits of interphase
power controllers with conventional or electronically
switched phase shifting devices — a robust FACTS
application’, CIGRE-Session, Paris, August 1998
5 LINDER, S. et al.: ‘A new range of reverse conducting
gate-commutated thyristors for high-voltage
medium-power applications’, Proceedings of the 7th
European Conference on Power Electronics and
Applications, Trondheim, Norway, September 1997
©IEE: 2000
The authors are with ABB High Voltage Technologies
Ltd., Dept. AET, PO Box 8546, CH8050, Zurich,
Switzerland.
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POWER ENGINEERING JOURNAL JUNE 2000