Download SVC for Mitigating 50 Hz Resonance of a Long 400 kV ac

SVC for Mitigating 50 Hz Resonance of a Long 400 kV ac Interconnection
Dr. A. Hammad *
Consultant, NOK
Baden, Switzerland
S. Boshoff
W.C. van der Merwe, C. van Dyk, W. Otto
R.P.A.
Trans-Africa projects
Johannesburg, South Africa
Summary
A new 400 kV ac intertie between NamPower
(Namibia) and Eskom (South Africa) is currently under
construction. The 890-km single circuit transmission
line is of a novel compact tower design. The existing
NamPower system has inherently a low first parallel
resonance frequency. This is due to its tree-like
configuration transmission network with very long
radial lines operated at Extra High Voltages (220 kV
and 330 kV) and remote generation. The system also
suffers from voltage stability problems.
With the introduction of the new 400 kV
interconnection to Eskom, the two problems inherent in
the NamPower system are accentuated. The large 400
kV line charging capacitance reduces the parallel
resonance frequency closer to 50 Hz or even lower and
makes the network more voltage sensitive as well. In
this case, both phenomena manifest themselves in the
form of extremely high and sustained overvoltages.
The paper explains the relationship between the 50-Hz
resonance and the voltage stability phenomena and
presents the approach for solving them. It also describes
the criteria used for the development of the
configuration and control functions of the new Static
VAr Compensator (SVC) at the 400 kV line terminal at
Auas. A brief summary of the SVC design features is
also given. Results of transient studies using EMTDC
simulations and frequency-domain analysis are
presented to demonstrate the problems and the
effectiveness of the adopted solutions.
Keywords: Resonances, Voltage Stability, Temporary
Overvoltages, SVC, Controls, Reliability.
_______________________________________________________
* NOK, CH-5401 Baden, E-mail: [email protected]
U.H.E. Kleyenstüber
NamPower
Windhoek, Namibia
1. INTRODUCTION
The Southern African Development Community
(SADC) is committed to the integration of its member
countries' energy sectors. Power demands in the SADC
are growing with a rate of 4-4.5% annually, with greater
growth ahead [1]. In Namibia at present, the power
network is built primarily along a 900-km 220 kV
double circuit backbone to South Africa in the south as
shown in Fig. 1. The bulk generation is supplied from
the Ruacana hydro station in the far north through a 520
km 330 kV transmission. Small coal fired generators at
Van Eck are used during emergencies to supply the load
centre in nearby Windhoek. To cope with the fast
growing economy and to provide a secure electric
energy to the mining and mineral industry in Namibia, a
new 400 kV ac transmission to interconnect the
Networks of NamPower and Eskom is planned to be in
operation in mid year 2000. The 890-km single circuit
400 kV line is of a novel compact tower design [2]. It
interconnects Eskom's Aries substation near Kenhardt in
South Africa to the new Auas substation near Windhoek
via the existing Kokerboom substation near
Keetmanshoop in Namibia as shown in Fig. 1.
In the past, due to the long 220 kV and 330 kV lines and
the relatively small and sparsely located loads with
respect to the generation sources, the NamPower system
suffered from voltage stability problems. For this
reason, the efficient use of the SVC at Omburu with a
dynamic range of +22.5/-37 MVAr supplemented with
60 MVAr of switched reactors and the skilful minute-tominute operation and supervision of the network,
through the use of small switched shunt reactors, have
ensured that the system narrowly survives.
Ruacana
330 kV
substations are equipped with breaker-switched shunt
reactors. Due to this unique network structure and
depending upon the number of generating units
connected to the network at Ruacana, the system has a
first natural parallel resonance frequency in the range of
55-70 Hz as depicted by curves 1&2 in Fig. 2.
520 km
1000
Omburu
1
800
3
2
SVC
600
Ohm
Van Eck
Auas
400
4
Hardap
470 km
200
0
Kokerboom
400 kV
0
50
100
150
200
Frequency, Hz
220 kV
Fig. 2 System Impedance/Frequency Characteristics
420 km
Harib
1000
1
5
Aries
800
2
6
Agganis
ESKOM
Ohm
600
Fig. 1 Schematic of Main Transmission of NamPower
400
With the introduction of the new 400 kV interconnection, the NamPower system should be
strengthened. However, the new line is also very long
with large charging capacitance which aggravates the
inherent problems in the NamPower system; namely
voltage stability and near 50-Hz resonance.
200
0
0
10
20
30
40
50
60
70
80
90
100
Frequency, Hz
The paper explains the relationship between the 50-Hz
resonance and the voltage stability phenomena and
describes the approach for solving them. It also presents
the optimisation process used for the development of the
configuration and control functions of the new SVC at
Auas. Results of transient studies using EMTDC
simulations and frequency-domain analysis are
presented to demonstrate the problems and the
effectiveness of the adopted solution.
Fig. 3 System Near 50-Hz Resonance
1. Existing transmission with 4 generators
2. Existing transmission with no generators
3. With 400 kV transmission & 4 generators
4. With 400 kV transmission & no generators
5. During 400 kV line energisation with 4 generators
6. During 400 kV line energisation & no generators
2. NEAR 50-HZ RESONANCE
With the addition of the new 400 kV line (AriesKokerboom-Auas) and its 4x100 MVAr shunt terminal
reactors, the system first resonance frequency shifts to
the range 60-75 Hz (curves 3&4 in Fig. 2). Note also the
remarkable reduction of system impedance at 50 Hz due
to the new line indicating the strengthening of the
system. At Kokerboom, two additional 100 MVAr
As shown in Fig. 1, the NamPower network has a treelike configuration with very long radial EHV lines. The
total length of the 132 kV and the 220 kV circuits is
about 1100 km and 2900 km respectively. Only the 330
kV line is shunt compensated, whereas the 220 kV
2
breaker switched reactors are used for very light load
conditions on the 400 kV transmission. The system
resonance, however, can shift towards 50 Hz or even
lower during system transients such as 400 kV line
energisation or recovery after clearing of line faults. Fig.
3 (curves 5&6) shows the network impedance seen at
Auas 400 kV bus at the instant of energising the 400 kV
line from the Auas side before closing the breaker on
the Kokerboom terminal.
The condition of a parallel resonance in a circuit is that
the system equivalent series reactance Xs equals the
inverse of the equivalent shunt susceptance Bs. When
this occurs at the fundamental frequency, the net system
self admittance defined as:
Yo = Go + j (Bs-1/Xs) = Go + j 1/Xo = 1/Zo
approaches zero and is limited only by the system loads
and generators. At light load conditions and/or no
generators connected at Ruacana, the net system
conductance (Go) becomes very small and Zo is very
large as shown in Fig. 5. To a good extent, this is
similar to the load flow Jacobian Q/U condition at
voltage instability. Moreover, a small variation in the
network configuration can cause an abrupt variation of
the system net reactance from very large inductive to
vary large capacitive at fundamental frequency as
illustrated in Fig. 5 (X2 to X1). Again, this is analogous
to the VSF condition at the bifurcation point for voltage
instability. Furthermore, when the resonance frequency
is below the fundamental frequency (case of X1) the
system will be self-excited as well known from the subsynchronous resonance phenomenon [4].
To further elucidate the phenomena of near
fundamental frequency parallel resonance in a power
system and its implications, analogy to the well-known
voltage stability phenomenon is made next.
3. ANALOGY BETWEEN 50-HZ RESONANCE
AND VOLTAGE INSTABILITY
Voltage stability phenomenon in transmission systems
is usually described by the nose-curve relationship
between voltage and power transfer as illustrated in Fig.
4. Voltage/reactive power sensitivity factor VSF
(U/Q) is one of the simple techniques used to assess
the proximity to voltage collapse [3]. In the stable
operation region VSF is positive whereas it is negative
in the unstable region as depicted in Fig. 4. As the
system approaches instability it becomes more sensitive,
i.e. VSF takes higher values. At the bifurcation point
(A), i.e. transition from stable to unstable conditions
VSF changes abruptly from + to -. This is because
its inverse (Q/U) passes through zero at A.
Therefore, a load flow computation employing NewtonRaphson iterative method diverges at that point since it
uses the system Jacobian matrix that contains the factor
Q/U which makes it singular. In the unstable region
the system behaves in a reverse sense, i.e. switching-in a
reactor will cause the voltage to rise.
1000
Zo
800
600
X0
X2
Ohm
400
200
X1
+
Y0
0
20 _
-200
30
40
50
60
70
80
Frequency, Hz
STABLE
Us
 U/ Q
Ur
-400
-600
Um
A
Fig. 5 Near 50-Hz Resonance Phenomenon
Um c
The impact of the near 50-Hz resonance problem in the
NamPower system can best be illustrated by simulating
the condition represented by curve 6 in Fig. 3 at Auas
substation during the 400 kV line energisation. As
shown in Fig. 6, at time t=0.5 the breaker at Auas
terminal is closed and it is assumed that the breaker at
Kokerboom is synchronised at t=0.7 s. At first, due to
the large charging capacitance of the line, the voltage
dips before it overshoots. The extreme high
overvoltages appearing at Auas with a peak value in
excess of 1.7 pu and a sustained TOV of over 1.5 pu is
attesting to the severity of the problem. It is clear that as
soon as the 50-Hz resonance is triggered, very high
UNSTABLE
+
0
_
 U/ Q
Pr
Fig. 4 Voltage Stability in a Transmission System
3
dynamic overvoltages appear with large time constant
depending on the system load and generation
conditions.
such a device is to be able to provide smooth VAr
absorption in order to continuously track the system
during steady state as well as during line energisation,
de-energisation, tripping, contingencies and faults. The
Auas substation proved to be the optimum location for
the device and in order to be most effective it should be
connected to the 400 kV bus.
600
400
Four different solutions have been considered as part of
a value engineering exercise. The criteria for developing
such solutions and for selecting the most appropriate
scheme are (in a descending order):
kV
200
0
0.4
0.5
0.6
0.7
1. Reliability, availability and maintainability (RAM)
2. Simplicity and Redundancy
3. Flexibility of operation practically during contingencies and degraded modes
4. Cost
5. Technology
0.8
-200
-400
The rating of the device was determined to be 250
MVAr inductive to be able to sufficiently mitigate the
near 50-Hz resonance with a good margin and to limit
the resulting dynamic overvoltages to an acceptable
level under worst conditions. The capacitive range was
determined to be 80 MVAr in order to maintain an
adequate margin of voltage stability during high power
transfers on the 400 kV intertie between NamPower and
Eskom similar to the existing margin on the 220 kV
interconnection.
-600
Fig. 6
Voltage at Auas during energisation of 400 kV
line Section to Kokerboom
4. SVC SOLUTION
The striking analogy between the near 50-Hz resonance
and voltage stability phenomena can in fact make them
dual where one produces overvoltage while the other
leads to voltage collapse. In fact this points to a
common solution method for both problems by means
of adequate reactive power compensation. For voltage
instability
employing
capacitive
shunt
VAr
compensation Bc moves the bifurcation point A to a
higher value as depicted in Fig. 4 [5 ]. It can be shown
that the increase in the voltage (Um) at bifurcation due to
insertion of Bc is such that:
The developed schemes are:
1. Statcom (Voltage source converters using gate-turnoff thyristors)
2. Single SVC unit in 12-pulse configuration
3. Single SVC unit in 6-pule configuration with 1-phase
transformers and built-in redundancies
4. Two SVC units each has a 75% rating and a 3-phase
transformer with no built-in redundancies.
2
(Umc/Um) = 1/(1 - Bc.Xs)
and Um/Um  0.5 Bc.Xs
The detailed optimisation process resulted in choosing
the scheme number 3 shown schematically in Fig. 7.
Similarly, the system first resonance frequency close to
50 Hz (fo) can be increased by introducing an inductive
compensation (-Bc) such that:
5. SVC DESIGN FEATURES
The SVC scheme (Fig. 7) consists of four 1-phase
transformers including a spare, four TCR identical units
(three plus one spare) and two identical damped doubletuned filters.
(foc/fo)2 = 1/(1 - Bc/Bs)
f/fo  0.5 Bc/Bs
Practically, however, the unpredictability of the
resonance occurrence presents another problem in that
very little preventive solutions could be safely utilised
while ensuring the reliable and continuous operation of
the NamPower system. This is crucial since only the
minimum amount of fixed and switched shunt reactors
could be used; otherwise the system voltage would
collapse due to its low short circuit capacity. Therefore,
the focus for finding a solution turned toward the use of
power electronics or FACTS devices. The function of
At present, the SVC technology is well established in
the power transmission environment with some
reasonable reliability and availability figures being
achieved on the average. However, classical approaches
to the application of SVCs in utility networks are in
principle focused on voltage control particularly voltage
support against collapse. In the NamPower system,
however, the Auas SVC has multiple functions; many
are unconventional, in the following priority order:
4
To cope with the sensitivity of the NamPower network
to both reactive power and harmonic current injections,
multiple TCR units (3x110 MVAr) are used. Each unit
consists of direct light triggered thyristor valves and has
its individual redundant control and cooling systems.
The three TCR units are controlled in a series sequence
(Master-Slave Mode) by the SVC redundant Master
Controller. The capacitive MVArs required are in the
form of 2x40 MVAr filter banks. Each filter is doubletuned to about 3rd/5th harmonics and provides
sufficient filtering and damping to achieve a good
performance particularly when one filter is out of
operation.
400kV Auas Substation
400kV/15 kV
SVC Transformer
(spare)
15 kV
Aux.
supply
TCR1 TCR2 TCR3 TCR4
The spare TCR unit fulfils the high reliability
requirement of the SVC that is considered an integral
part of the 400 kV transmission system. Also in case
one TCR unit fails no filter bank would unnecessarily
be switched off thus avoiding causing a double
contingency.
Filter 1 Filter 2
(spare)
Fig. 7 Schematic of Auas SVC Scheme
- Control the near 50-Hz resonance
Figures 8 and 9 depict the Auas 400 kV bus and the
TCR currents (at 15 kV) during the 400 kV line
energisation with the SVC using a classical voltage
controller. With the use of a special supplementary
overvoltage open-loop controller it is expected that the
SVC performance would be more superior.
- Limit temporary overvoltages during 400-kV line
energisation, system contingencies and post fault
conditions
- Avoid voltage collapse and provide sufficient voltage
stability margin
- Damp power oscillations on the 400 kV interconnection to Eskom.
600
Moreover, the weak NamPower network conditions add
various constraints such as low order harmonic
resonances and harmonic current injection limitations.
400
The fact that the entire NamPower system depends on
the successful, reliable and continuous operation of the
Auas SVC presents special high demands on the design,
quality, functionality and layout of the individual
components and sub-systems as well as the SVC
scheme as a whole. Furthermore, the Auas substation is
relatively remote from technical support and is planned
to be unmanned. Any outage time would, therefore,
impact severely on the entire system.
kV
200
0
0.4
0.5
0.6
0.7
0.8
-200
-400
The following are some highlights on some salient
aspects of the SVC design:
Time, s
-600
5.1 Single-Phase Transformer Units
It is well known from other SVC schemes that their
availability figures are much higher when 1-phase
transformers are used compared to 3-phase units since
the repair and transportation times for transformers are
very long.
Fig. 8 Voltage at Auas during 400 kV line energisation
with SVC
5.3 Maintainability
Even though it is acknowledged that reliability of SVC
devices are continuously improving, the maintainability
of the Auas SVC received special attention, based on
the experience gained in the operation and maintenance
of SVCs in South Africa [6]. Because of this, emphasis
has been placed on accessibility of maintenance
personnel to the Auas SVC scheme while it is energised.
The secondary voltage of 15 kV is chosen as an
optimum value for both thyristor valves and bulbar
designs.
5.2 TCR and Filter Units
5
The intent is to achieve maximum availability even
during equipment outages, by allowing on-line
maintenance, dual redundant equipment, monitoring and
By analogy to voltage stability phenomenon, a solution
can be achieved by variable reactive power absorption.
The -250/80 MVAr SVC scheme at Auas 400 kV
substation is developed through value engineering
criteria. Special features are implemented in the SVC
design and layout to economically meet the high levels
of
performance,
reliability,
availability
and
maintainability requirements.
8
TCR1
6
4
kA
2
References
0
0.4
0.6
0.8
[1] H. Henyon, "Integrating Southern
Independent Energy, Jan. 1999.
-2
-4
[2] H. White, F. Ritky, "Unique Suspension System
Conquers Rugged Terrain", Transmission and
Distribution World, Aug. 1997.
-6
Time, s
-8
6
[3] A. Hammad, W. Kuhn, "A Computation Algorithm
for Assessing Voltage Stability at AC/DC
Interconnections", IEEE Trans. on Power systems,
Feb. 1986, pp 209-216.
TCR2
4
kA
2
0
0.4
0.5
0.6
0.7
[4] A. Hammad, M. El-Sadek, "Application of a
Thyristor Controlled VAr Compensator for
Damping Sub-synchronous Oscillations in Power
systems", IEEE Trans. on Power apparatus &
Systems, Jan. 1984, pp 198-212.
0.8
-2
-4
Time, s
-6
[5] A. Hammad," Comparing the Voltage Control
Capabilities of Present and Future VAr
Compensating Techniques in Transmission
Systems", IEEE Trans. on Power delivery, Jan.
1996, pp. 475-484.
6
TCR3
4
kA
2
0
0.4
Africa",
0.5
0.6
0.7
0.8
[6] T. Magg, S. Boshoff, C. Hitchin, H. Mohabir, A.
Hammad, "Review of Performance and Operational
Experience of the Natal SVC's, Eskom, South
Africa", Proc. CIGRE SC14 Int. Colloquium on
HVDC & FACTS, Sept. 1997, paper 4.2.
-2
-4
Time , s
-6
Fig. 9 SVC TCR 15-kV currents during 400 kV line
energisation
controls wherever necessary and have safe access to all
equipment while the SVC is in operation.
It is also expected that thyristor valves employing stateof-the-art direct light triggered thyristors would be used
to achieve the required high levels of RAM as well as to
cope with the stringent controls for fast limiting the
extremely high system transient overvoltages.
5. CONCLUSIONS
The new 890 km 400 kV ac intertie between Namibia
and South Africa is essential to the economic
development and prosperity of the region.
Due to the unique structure of the NamPower network,
the new transmission line can cause a system near 50Hz parallel resonance. Such a phenomenon leads to
extremely high sustained overvoltages.
6