Bilag 13 reduceret version

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Institute of
Energy Technology
Aalborg University
Bilag 13
August 2 2006
Comprehensive use of High Voltage AC cables
in the Transmission Systems
Suggestion for research collaboration
and two Industrial PhD studies
1. Introduction
The transmission grid has up to present time been laid out as an
almost purely overhead line network except for crossing of water, in
densely populated areas and where overhead lines would affect
national nature interests seriously. The introduction of transmission
voltage level XLPE cables and the increasing political and public
interest in the environmental impact of overhead lines has resulted in
an increasing interest concerning the use of underground cables for
the transmission level.
As transmission level power cables possess a relatively large shunt
capacitance (compared to OHL’s) this sets a demand for steady-state
inductive reactive power compensation of the cables capacitive
reactive power production. This is today accomplished by means of
three-phase shunt reactors directly connected to the line(s). As the
cables possesses capacitance and the reactor inductance resonant
circuit behaviour becomes an important design feature, especially as
the quality factor of the power system is extremely high because of
the low overall resistance of the power lines. Switching operations and
autoreclosures in the network might easily initiate switching transient
voltages and currents leading to non-wanted conditions such as
overvoltages.
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It is seen as an unavoidable development of transmission system
layout and mode of operation that cable sections must be included to
some degree in future transmission systems and thereby will
contribute significantly to the technical performance of the
transmission network. Cables for the transmission voltage level
possesses significantly different electrical properties than overhead
lines with the same capacity, so both steady-state and transient
behaviour of such combined transmission systems with cable sections
included to a smaller or larger extent, ultimately as a pure cable
network, will be different than for todays existing overhead line based
transmission systems. This puts up an urgent need for analysis of such
combined OHL/Cable transmission systems in order to be able to plan,
design and operate such modern power systems, knowing their
behaviour in order to assess both costs and reliability of supply.
The motivation for the increasing need for insight into the use of AC
cables for transmission systems and thereby the motivation for this
project can be underlined in the following statements:
- Production pattern changes from large conventional power
plants to RES (renewable energy sources).
- RES is build, where the conditions for the technology are
best. For examples are wind farms build, where the wind
conditions are the best possible and the environmental
impact is small.
- Implementation of RES can cause need for reinforcement of
the transmission system to the new production units.
- There are large public opposition against more overhead
lines.
- Cables are considered for reinforcement of the transmission
system.
- The transmission grid is more likely to be used for transit of
electric power between countries than originally planned.
Todays state of the art for a comprehensive use of cables in the
transmission grid is rather limited, especially for the 400 kV voltage
level. Examples of existing 400 kV AC cable projects are:
Oresund strait 1
Berlin
Vienna
Vancouver
Oslo
Oresund strait 2
9 km
2 * 8,2 km
2 * 12 km
2 * 35 km
11,5 km
9 km
1973
1978/79
1979
1983 og 1984 (500 kV)
1981
1985
2
Copenhagen
Berlin
Madrid
Northern Jutland
London
33 km
12 km
13,5 km
2 * 14 km
20 km
1997 og 1999
1998 og 2000
2004
2004
2005
Transmission System Operator Energinet.dk are considering an
extensive use of 400 kV cables in Denmark to be a possible future
development, as can be seen from the table below:
400 kV AC cables
Southern ring, 1 system
Southern ring, 2. system
Northern ring, 1 system
Oresund ring, 1 system
Total amount
Land cable
[km]
202
202
60
464 km
Submarine cable
[km]
39
39
3
28
109 km
The amount of possible 400 kV AC cables is very large compared to
existing 400 kV cable projects.
From an introductory point of view one can foresee many possible
problems with the use of cables at transmission level:
-
Resonance circuits
o Impedance of overhead lines are series reactance and
some shunt capacitance
o Cables has large shunt capacitance and some series
reactance
o Shunt capacitance in cables are compensated by shunt
reactors. Resonance frequency
capacitance/compensation is app. 45-47 Hz.
o Capacitors and reactors forms various resonance circuits
during normal coupling/operation and faulty conditions.
-
The disconnection problem
o At disconnection time of a transmission line, there are
stored some energy in shunt capacitors and reactors as
well which may be in series connection, as configuration
changes during coupling.
o After disconnection this energy oscillates in the system
until they are damped by losses in the system.
o Changes in system configuration due to switching can
cause possible non-wanted resonant circuit conditions
unless they are damped.
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o Losses in 400 kV transmission system are low, and the
oscillations goes on for a relative long period of time
o During the oscillations dangerous over voltages can
occur in the system.
o Faulted conditions can create unexpected resonant
behaviour.
-
The inrush current problem
o When a transmission system is connected to an AC
source, the initial current and voltages in the system are
zero.
o The shunt capacitors are charged through some series
reactance and some oscillation phenomena with
significant over voltages can occur.
o If parts of the iron cores in transformers or reactors are
saturated during the inrush, then the transient behaviour
becomes very complicated.
o Autoreclosures
can
possibly
create
unexpected
behaviour.
The need for new research can be subdivided into two main
categories, which are closely interconnected. One package “A” of
work containing “close to component” modelling of dynamic
behaviour and another package “B” of work containing system
studies, i.e. the application of cables to the transmission system.
The initial problem of this research project becomes, as a
transmission system operator, to be able to perform the transition
from today’s OHL based transmission grid to a future transmission
grid containing a comprehensive amount of cables without losing the
technical confidence of the transmission system in any sense.
Is it from a technical point of view possible to include a
comprehensive amount of cables in the transmission grid and which
countermeasures can be used to control and limit overvoltages to
acceptable levels.
2. Description of PhD projects
This section describes in more details the ideas and the contents of
the research program and the individual PhD projects. This research
program is subdivided into packages called A and B. State of the art
and literature study for both package A and B will be carried out in a
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close collaboration between the two candidates in order to create a
common foundation for the research program. This includes a close
physical location and cooperation between the two candidates in the
first period of the research program period.
Package A: Modelling of system components
The main goal of this PhD project (package A) is to model the
components of the transmission system so that a precise
simulation model of the transmission system can be used (package
B) for planning and design analysis of the future possible integration of
cables in the transmission system
Recent experience (based on measurements during switching) within
Danish transmission system operator Energinet.dk is that such
compensated transmission level power lines containing both cables
(one or more sections or pure cable lines) and reactors for
compensation are capable of generating low-frequent oscillations with
a duration of seconds when switched off, see figure 1. Severe
switching overvoltages occur as parts of this dynamic oscillatory
behaviour.
Figure 1:
Switching overvoltages of the 400 kV line Trige-Ferslev when switching
the line section free of the rest of the grid
A Masters Thesis (Kim Søgaard, IET-AAU, 2005) for Energinet.dk has
shown that such switching overvoltages are due to resonant conditions
of the power lines during switching. The project shows that the
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dynamic modelling of the components such as reactor, cable and
overhead line must be based on an into deep physical modelling of the
components in question in order to achieve a sufficiently high degree
of precision from the simulations. Especially mutual coupling of the
three-phase system seems to play an important role. This precision
must be such that simulation results can be verified against transient
measurements of switching operations with good results.
Areas of work for PhD project in package A
- Worldwide state of the art analysis of used design criteria and
layout principles of transmission level cable implementations.
- An extensive literature study in order to reveal today’s level of
knowledge concerning the dynamic behaviour of transmission
system components with focus on systems with different
degrees of cables included.
- An into-deep physically
components such as:
based
dynamic
modelling
of
- Cable section (crossbonding, mutual coupling, screen
connection, physical laying in the cable trench, influence
of soil...., unsymmetry….)
- Compensating device (reactor with teaser, single
phase, three phase or FACTS device, unsymmetry,
mutual coupling)
- Overhead lines (layout, transposition, unsymmetry in
both series inductance and shunt capacitance)
- Circuit breaker (with focus on the switching of low
resonant circuit currents - does the circuit breaker
interrupt the current at zero crossing or is the current
forced to zero? Circuit breakers with pole spread shall be
considered. Measurements plays an important role!
- Modelling and verification of non-linear components
such as transformers and MOA (metal-oxide-arresters)
The modelling shall be closely connected to the actual real-life
design of the relevant components and are likely to include
elements of electric and magnetic field theory.
The models shall be able to reflect correct behaviour for any
real-life relevant switching condition. Therefore all possible
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switching conditions should be laid down as a part of the work
(e.g. single or three phase autoreclosure, inrush, switch-off,
pole discrepancy....) and thereby shape the basis for the
validity of the models.
Models should be implementable in commercially available
software packages such as PSCAD/EMTDC in order to make the
results of the project useful for Energinet.dk
- Transient measurements of switching on selected lines with
the purpose of a high quality verification of the models
described above. The goal is to be able to simulate any relevant
transient switching behaviour of the selected lines for
verification.
- An into-deep physically based study and explanation to the
precise origin of the dynamics of the system. It is not sufficient
to be able to simulate verified switching conditions. The
appearance and characteristics of the transient behaviour must
be explained so thorough that this can be communicated to
future practical designers of the transmission system.
- Countermeasures to lower/avoid non-wanted switching
transients should be considered and implemented in the
simulation model (e.g. the use of surge arresters, damping
countermeasures such as circuit breakers with resistors,
switching philosophies (three-phase, single-phase etc...)
This project (A) focuses on low-frequent switching behaviour and will
not include studies concerning high-frequency behaviour (e.g.
lightning transients etc.). Furthermore this project concentrates on the
component level and selected, existing OHL/cable transmission lines.
The future planning and consequences of an increased degree of
cables in the transmission network is not the scope of this project, but
is foreseen to be included in package B.
Package B: System Analysis
The work in package B will be about special design considerations in
high voltage transmission systems with a large share of cables. The
end goal will be a guideline to special precautions and solutions to
avoid resonance problems and dangerous over voltage problems in a
transmission network with future increased share of cables.
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Package B will take a top down approach to the problem. This means
that focus in the first hand will be on identifying, understanding and
describing physical phenomena that can be expected to cause voltage
problems. Preliminary calculations with simple network models shall
confirm the expected physical behaviour. The work shall form a basis
for specification of the necessary level of detail in models developed in
package A.
Suggestion of countermeasures and a final system study of a fictitious
large expansion of the Danish transmission system with cables are also
a part of package B.
Areas of work for PhD project in package B
- Worldwide state of the art analysis of used design criteria and
layout principles of transmission level cable implementations.
- An extensive literature study in order to reveal today’s level
of knowledge concerning the dynamic behaviour of
transmission system components with focus on systems with
different degrees of cables included.
- Identification and description of resonance phenomena in
transmission systems with cables.
- Development of future transmission system scenarios with
large amount of cables. The scenarios will be developed with
the existing Danish transmission system as starting point.
- Identifying state-of-the-art analysis methods in high voltage
cable projects.
- Identifying
investigate.
the
most
serious
resonance
problems
to
- Suggest various methods to reduce oscillation and over
voltage problems. A comprehensive brain storming and
valuation of ideas will be carried out.
- Investigate the need for new equipment modelling in existing
calculation tools and state needed specifications for new
models in close cooperation with package A.
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- List of practical possible
advantages and challenges.
countermeasures
including
- Building equipment models in close cooperation with package
A.
- System studies on the developed transmission system
scenarios with use of developed models and assessment of the
identified methods to reduce oscillation and over voltage
problems.
3. Expected results of the research programme
The most original contributions are as follows:
-
-
-
-
Precise dynamic models of transmission system components
validated against real-life dynamic measurements and
implemented in commercially available tools as eg. PSCAD. The
models should be able to predict the dynamic behaviour of
combined OHL/cable transmission systems precisely. (A)
An into-deep explanation of the physical origin of the
characteristic dynamic behaviour of transmission systems. The
explanations should be accessible mostly from a non-simulation
approach, i.e. from simple electrophysics.(A)
A comprehensive basis for creating a “handbook” for the design
and layout of future transmission systems at 400, 150 kV and
132 kV level. (B)
Analysis of relevant scenarios with large amount of cables
integrated and identification of possible problems and their
remedy. Examples are (B):
-
Harmonic propagation problems
Resonance circuit problems
The disconnection problem
Faulted conditions – autoreclosure 1/3 ph
Overvoltage protection design (TOV…?)
Inrush current problems
Compensation philosophies
It is expected that these PhD projects will result in subprojects suited
for Masters Thesis works and that the PhD student plays an active role
in the supervision of such students.
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4. Time schedule
It is suggested, that two industrial PhD studies is started in connection
with package A and B. The industrial PhD initiative is funded by
Ministry of Science, Technology and Innovation and the industrial
partner in common.
Time schedule:
Date
1/11 2006
1/2 2007
1/4 2007
31/10 2007
31/10 2008
Milestones A
Start
Identification of
components, which
influences resonances
Dynamic modelling
approach
Implementation of models
in simulation tools
1/7 2009
Measurements to be used
for validation
Validation of models and
explanations and
countermeasures
31/10 2009
End
Milestones B
Start
Identification of most
severe problems to
investigate
Chose of calculation
programs
Results with simple models
and suggestions to more
detailed models. Brutto
package of possible
countermeasures
Development and tests of
detailed models
System studies with
detailed models. Resulting
netto list of possible
countermeasures including
advantages and challenges
End
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