Simulation of distributed generation in network fault situations

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Simulation of distributed generation in network
fault situations

• Kimmo Kauhaniemi
• University of Vaasa VTT Processes
• Lauri Kumpulainen
• VTT Processes

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Simulation project
Simulation of distributed generation in network
fault situations
Co-operating partnersABB Substation Automation
Oy Helsinki Energy Wärtsilä Finland Oy / Power
Plants
Performed by VTT Processes Spring 2001
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Objectives

• To examine criteria for loss-of-mains protection.
  Upos, U0 Ineg, Z
• To check the operation of the normal protection
  of the network
• Cases faults in distribution network with
  different types of DG units.

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Loss-of-mains, LOM(Loss of grid, islanding)
Loss-of-mains is continued generation after the
host network fails. The generator is left
connected to a section of a network (island).

• Risks
• Safety of personnel (dead line is not dead)
• Damage of equipment due to asynchronous reclosure
• Quality of supply in the island

G
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Motivation
The main issue of electrical protection remains
effective and reliable loss-of-mains protection.
(CIRED, WG04 Dispersed Generation, Preliminary
Report presented at CIRED 99) It has been
recognised by many that the detection of loss of
mains (LOM), which is also known as islanding
condition as the single most challenging
protection problem of distributed generation.
(S.K. Salman Factors influencing the effective
integration of the rotating type distributed
generation into utilities distribution networks,
First International Symposium on Distributed
Generation Power System and Market Aspects,
Stockholm 2001.) Operation of dispersed
generation (DG) connected to the distribution
network presents several difficulties for a
reliable and safe operation of the power system.
Among these, protection against islanding appears
to be the most challenging aspect to be ensured
in order to allow DG a large diffusion.
(Caldon, R., Scala, A., Turri, R. Grid-connected
dispersed generation investigation on
anti-island protections behaviour, First
International Symposium on Distributed
Generation Power System and Market Aspects,
Stockholm 2001.) As islanding is still a very
controversial topic there are many open
questions. (Report IEA PVPS T5-01 Utility
aspects of grid connected photovoltaic power
systems, December 1998)
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Tools

• PSCAD/EMTDC V3 (simulations)
• TOP2000 output processor (graphical outputs)

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Background

• Simulation studies since 1996
• Various systems modelled
• urban cable network
• industrial network
• transmission systems
• DG units
• over 50 reports delivered to customers
• extensive custom component library

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What is PSCAD/EMTDC?

• EMTDC is a general purpose time domain simulation
  tool for studying transient behaviour of
  electrical networks
• PSCAD is a powerful graphical user interface
  which integrates seamlessly with EMTDC

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Solution method

• Solution method based on H.W. Dommels paper
  Digital Computer Solution of Electromagnetic
  Transients in Single and Multiphase networks
  from year 1969

R2L/?t
R?t/(2C)
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Why PSCAD/ETMDC?

• Accurate, versatile and detailed modelling
• from fast transients to machine dynamics
• all three phases modelled separately
• Custom models can be applied

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How?

• Modelling
• the model is constructed applying the component
  models from library
• Simulation
• simulation results are shown on the screen during
  the simulation
• results can be saved on files for further
  processing

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DG units studied

• Individual wind power plant with induction
  generator, 1 MW, connected to MV network
• Solar (photo-voltaic) unit, inverter, 50 kW,
  connected to LV network
• Diesel power plant, synchronous generator, 1 MW,
  connected to MV network

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Modelling

• Wind generator
• 1 MW wind generator (induction generator)
• Simplifications
• constant torque (no wind speed variation)
• no speed control

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Modelling

• Diesel generator
• 1 MW diesel generator (synchronous generator)
• Simplifications
• constant power output (no speed limits)

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Modelling

• Photo-voltaic unit
• Inverter model with constant output power of 50
  kW
• Filters to reduce harmonics

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Modelling

• Distribution network
• Urban cable network
• 110/20 kV substation, 20 kV lines down
  to secondary substations with load
• 0,4 kV network added in the case of
  photo-voltaic unit
• Simplifications
• Only two MV feeders were modelled in
  detail the rest of the network was
  modelled with corresponding earth capacitance

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Modelling

• Protection
• Feeder relays
• over-current
• earth fault
• Fuses
• 0,4 kV (photo-voltaic unit)
  fuses in LV network

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Fault cases

• MV network
• 3- and 2-phase short circuit
• single phase earth fault with fault resistances 0
  and 500 ohm
• LV network (case photo-voltaic unit)
• 3-phase short circuit
• 1-phase short circuit

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Location of the generator and fault locations (MV)
1. Generator along the feeder
Fault locations numbered
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Location of the generator and fault locations (MV)
2. Generator has its own feeder
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Location of the generator and fault locations (LV)
Photo-voltaic unit and fault locations
Case 1
Case 2
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Location of the generator and fault locations (LV)
Photo-voltaic unit and fault locations
Case 3
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Network load

• MV simulations with rather lightly loaded
  network.
• LV simulations with three different loading
  Load lt, , gt Punit

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Measurements

• Measurements using components of PSCAD master
  library and some custom made models.
• Currents, voltages, power, frequency, symmetrical
  components, network impedance.
• Recording of quantities into Comtrade format.

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Analysis - existing methods

• The most usual methods for LOM protection are
• probably
• ROCOF (rate of change of frequency, df/dt), the
  standard choice in the UK
• Voltage vector phase shift
• A number of other techniques have been proposed.
• The problem is often nuisance tripping.

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Analysis - possible other methods

• This study Evaluation of possible LOM criteria
• Positive sequence voltage (good experience in
  Denmark)
• Wind generator, diesel generator
• Zero sequence voltage and negative sequence
  current (not found in previous reports)
• Earth faults
• Network impedance (German practice for solar
  units)
• PV unit, LV connection

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Results - U undervoltage
Example
Positive sequence voltage at the MV side of the
transformer
Wind generator, 2-phase short circuit, location 8
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10
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Voltage (kV)
6
4
2
0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Time (s)
Fault occurs
Feeder relay trips
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Results - U undervoltage
Example
Positive sequence voltage at the MV side of the
transformer
Generaattorin muuntajan yläpuolelta mitattu
jännitteen myötäkomponentti
Wind generator, 2-phase short circuit, location 5
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Positive sequence voltage at the MV side of the
transformer
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Voltage (kV)
10
9
8
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Time (s)
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Time (s)
Fault occurs
Feeder relay trips
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Results - U0 and I- Earth faults
Example
U0
Wind generator, earth fault, 0 ohms, location 8
Ineg
Fault occurs
Feeder relay trips
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Results - U0 and I- Earth faults
U0
Example
Wind generator, earth fault, 0 ohms, location 5
Ineg
Fault occurs
Feeder relay trips
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Results - network impedance
Example
Photovoltaic unit, single phase short circuit,
location 7
1-phase short circuit
PV unit
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Results - network impedance
Example
Photovoltaic unit, single phase short circuit,
location 5
1-phase short circuit
PV unit
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Conclusions
The proposed methods (U,U0 I-,Z) seem to be
promising. Selective LOM-protection in the
examined fault cases may be possible. Loss-of-mai
ns protection is a complex issue. Extensive
further studies are needed.
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Topics for further studies

• Numerical analysis of simulation results --gt
  relay algorithms
• Different fault cases (2-line-to-earth fault,
  broken conductor)
• Case of healthy island (after e.g. malfunction of
  protection)
• Different types of networks (overhead, neutral
  point treatment, connection state etc.)
• Different loads (especially equal to the output
  of the generator)
• Different types of protection and automation
  schemes, auto-reclosings
• Operation of network protection in certain cases
  DG may cause unwanted effects unwanted tripping,
  prevention of tripping
• Several generators
• More accurate generator models
• Stability and protection of generators