INSTITUTE OF ENERGY TECHONOLOGY
GROUNDING FOR OFFSHORE WIND FARM
ELECTRICAL SYSTEM
GROUP WPS2 – 864,
SPRING SEMESTER 2010
Title:
Semester:
Project period:
Supervisor:
Project group:
Grounding for Offshore Wind Farm Electrical System
8th
Spring 2010
Zhe Chen
WPS2-864
SYNOPSIS:
_____________________________________
[Lucia Pereiro Estévez]
_____________________________________
[David Rodriguez Lamoso]
Copies:
Pages, total:
Appendix:
Supplements:
The main objective of the project is to model
and analyze different grounding philosophies
for the infield cable system of an offshore wind
farm. The grounding in wind farm is an
important point to treat. It is important provide
a correct grounding for the security of the
personal that work in this place and protection
of the equipment.
The main problems have the wind farms are
the single-line-ground faults (80%). In this
project will be analyzed the system behavior
with this disturbance using the selected
grounding. It will be obtained the voltage and
current in different points of the system. These
obtained parameters will be analyzed and
compared for obtain the best grounding.
4
111
3
4 CD
By signing this document, each member of the group confirms that all participated in the
project work and thereby that all members are collectively liable for the content of the report.
i
iv
Abbreviations
AC
Alternative current
DFIG
Doubly fed inductor generator
HV
High voltage
LV
Low voltage
MHV
Medium / high voltage
MV
Medium voltage
WT
Wind turbine
v
vi
Nomenclature
XGO
zero-sequence reactance of transformer
RN
resistance of grounding resistor
XN
reactance of grounding reactor
ørotor
diameter of the rotor
Estep50
step voltage for 50 kg of reference
Etouch50
touch voltage for 50 kg of reference
Cs(hs,k)
reduction factor for derating the nominal value of surface layer resistivity
ρs
wet resistivity of the surface
ts
shock duration in seconds
I’’K
short-circuit current
If
total zero sequence rms fault current (3I0)
Sf
split factor or current division factor
Df
decrement factor
Cp
corrective projection factor
Rg
grid resistance in transformer 150/33 kV
Em
mesh voltage
Es
step voltage
ρ
soil resistivity in Ω.m
Km
mesh voltage geometric correction factor
Ks
step voltage geometric correction factor
Ki
correction factor that into account the increase in current at the extremities
of the grid
Kii
corrective weighting factor that adjusts the effects of inner conductors on the
mesh
Kh
corrective weighting factor that emphasizes the effect of grid depth
XcoT
distributed capacitive reactance of the line
Xco
distributed capacitive reactance per phase
R
total grounding resistance
VLN
system line to neutral voltage
IR
current through the neutral resistance
Ico
capacitance charging current
IL
inductive current through the coil
vii
Ir
resistive current
XL
reactance of coil
f
frequency of the system
ZSC
short circuit impedance
Z0
zero sequence impedance
ZL
line impedance
Un
voltage in the neutral of the transformer
In
current in the neutral of the transformer
ZG
impedance of the grid
UG
rated voltage in the connection point
SGcc
initial short circuit apparent power of the external grid
RG
resistance of the grid
XG
reactance of the grid
ZL1
impedance in line 1
ZL2
impedance in line 2
ZT1
impedance of the transformer 150/33 kV
UKT
short circuit voltage in percent value of the primary winding
U
phase to phase voltage
Sn
apparent power of the transformer
XT1
reactance of the transformer 150/33 kV
RT1
resistance of the transformer 150/33 kV
ZL3
impedance in line 3
ZL4
impedance in line 4
ZT2
impedance of the three-windings transformer 33/0.69/3.3 kV
XT2
reactance of the three-windings transformer 33/0.69/3.3 kV
RT2
resistance of the three-windings transformer 33/0.69/3.3 kV
ZAS
doubly-fed induction generator impedance
Iras
rated current of generator
Uras
rated voltage of generator
Sras
rated apparent power of generator
ILR
locked-rotor current
RAS
doubly-fed induction generator resistance
XAS
doubly-fed induction generator reactance
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Table of contents
1
2
INTRODUCTION ......................................................................................... 1
1.1
Background ........................................................................................... 1
1.2
Problem formulation .............................................................................. 2
1.3
Objective ............................................................................................... 2
1.4
Project limitation .................................................................................... 3
1.5
Report structure .................................................................................... 3
GROUNDING PHILOSOPHIES AND REQUIREMENTS ............................ 4
2.1
Introduction ........................................................................................... 4
2.2
Methods of systems neutral grounding.................................................. 5
2.2.1
Ungrounded .................................................................................... 5
2.2.2
Solid grounding ............................................................................... 6
2.2.3
Resistance grounding ..................................................................... 6
2.2.4
Ground fault neutralizer .................................................................. 7
2.3
Comparison of different grounding systems .......................................... 8
2.4
Obtaining the system neutral................................................................. 9
3
MODELLING OF OFFSHORE WIND FARM............................................ 13
4
STATION GROUNDING SYSTEM ............................................................ 19
5
4.1
Design ground grid substation ............................................................. 19
4.2
Design ground grid wind turbine .......................................................... 27
MODELLING OF DIFFERENT TYPES OF GROUNDING ........................ 29
5.1
Ungrounded ........................................................................................ 29
5.2
Solid grounding ................................................................................... 30
5.3
Low-resistance grounding ................................................................... 31
ix
5.4
High resistance grounding................................................................... 32
5.5
Resonant grounding (Petersen coil) .................................................... 34
6
ANALYSIS................................................................................................. 37
6.1
Introduction to shorts circuits analysis ................................................. 37
6.2
Single-phase to ground short circuit .................................................... 38
6.3
Simulations with the chosen groundings ............................................. 40
6.3.1
Single-phase to ground short circuit on busbar 33A ..................... 41
6.3.2
Single-phase to ground short circuit on line 8 ............................... 68
6.4
Comparison of the results ................................................................... 93
7
ANALYSIS AND DISCUSSION ............................................................... 104
8
CONCLUSSIONS ................................................................................... 108
9
FUTURE WORK ..................................................................................... 110
10
REFERENCES ..................................................................................... 111
A.
Short Circuit Current from DIgSILENT ......................................................... I
B.
Equivalent Circuit of the wind farm .............................................................. V
C.
Tables of Short Circuit Currents on busbar 33A and on line 8 ................... IX
x
List of Figures
Figure 1-1 Global installed wind power capacity 08/09 MW ................... 1
Figure 2-1 Ungrounded. ..................................................................................... 5
Figure 2-2 Solid grounding ................................................................................. 6
Figure 2-3 Resistance grounded. ....................................................................... 7
Figure 2-4 Ground fault neutralizer..................................................................... 8
Figure 2-5 Grounding in generators ................................................................. 10
Figure 2-6 Connection wye-delta...................................................................... 10
Figure 2-7 Zigzag grounding transformers. ...................................................... 11
Figure 2-8 Wye-delta grounding transformers .................................................. 12
Figure 3-1 Model offshore wind farm, 2 string with 8 wind turbine each one .... 13
Figure 3-2 Distance between wind turbines. .................................................... 14
Figure 3-3 General scheme of DFIG ................................................................ 14
Figure 3-4 Transformation of mechanical energy to electrical energy in the wind
turbine. ............................................................................................................. 15
Figure 3-5 Pitch control .................................................................................... 16
Figure 3-6 Model wind turbine DFIG ................................................................ 17
Figure 4-1 Ground grid 60 x 60 m. ................................................................... 27
Figure 4-2 Ground grid 7 x 7 m with 2 rods. ..................................................... 28
Figure 5-1 Ungrounded .................................................................................... 30
Figure 5-2 Solid grounding ............................................................................... 31
Figure 5-3 Low-resistance grounding ............................................................... 32
Figure 5-4 High-resistance grounding. ............................................................. 33
Figure 5-5 Resonant grounding. ....................................................................... 35
Figure 6-1 Generic scheme of single-phase short circuit to earth .................... 39
Figure 6-2 Scheme impedance single-phase short circuit to earth ................... 39
Figure 6-3 Scheme with the measuring and simulation points ......................... 40
Figure 6-4 Waveform of fault current on busbar 33A. ........................... 42
Figure 6-5 Waveforms of voltage and current in HV side ................................. 42
Figure 6-6 Waveforms of voltage and current in LV side. ................................. 43
xi
Figure 6-7 Waveform of fault current on busbar 33A........................................ 44
Figure 6-8 Waveforms of voltage and current in HV side. ................................ 45
Figure 6-9 Waveforms of voltage and current in LV side. ................................. 46
Figure 6-10 Waveform of fault current on busbar 33A...................................... 46
Figure 6-11 Voltage and current in HV side...................................................... 47
Figure 6-12 Waveforms of voltage and current in LV side. ............................... 48
Figure 6-13 Waveform of Fault current on busbar 33A. ................................... 49
Figure 6-14 Waveform of voltage and current in HV side. ................................ 49
Figure 6-15 Waveforms of voltage and current in LV side. ............................... 50
Figure 6-16 Waveform of fault current with solid grounding ............................. 51
Figure 6-17 Waveforms of voltage and current (solid grounding) ..................... 52
Figure 6-18 Waveforms of voltage and current in LV side. ............................... 53
Figure 6-19 Fault current on busbar 33A. ......................................................... 54
Figure 6-20 Waveforms of voltage and current in HV side. .............................. 55
Figure 6-21 Waveforms of voltage and current in LV side. ............................... 55
Figure 6-22 Waveform of fault current on busbar 33A...................................... 56
Figure 6-23 Waveforms of voltage and current in HV side. .............................. 57
Figure 6-24 Waveforms of voltage and current in LV side. ............................... 57
Figure 6-25 Waveform of fault current on busbar 33A...................................... 58
Figure 6-26 Waveforms of voltage and current in HV side. .............................. 59
Figure 6-27 Waveforms of voltage and current in LV side. ............................... 59
Figure 6-28 Waveform of fault current on busbar 33A...................................... 60
Figure 6-29 Voltage and current in HV side...................................................... 61
Figure 6-30 Waveforms of voltage and current in LV side. ............................... 62
Figure 6-31 Waveform of fault current on busbar 33A...................................... 62
Figure 6-32 Waveforms of voltage and current in HV side. .............................. 63
Figure 6-33 Waveforms of voltage and current in LV side. ............................... 64
Figure 6-34 Waveform of fault current on busbar 33A...................................... 64
Figure 6-35 Waveforms of voltage and current in HV side. .............................. 65
Figure 6-36 Waveforms of voltage and current in LV side. ............................... 66
Figure 6-37 Waveform of fault current on busbar 33A. ......................... 66
Figure 6-38 Waveforms of voltage and current in HV side. .............................. 67
xii
Figure 6-39 Waveforms of voltage and current in LV side. ............................... 68
Figure 6-40 Waveform of fault current in the line 8........................................... 68
Figure 6-41 Waveforms of voltage and current in HV side .............................. 69
Figure 6-42 Waveforms of voltage and current in LV side ................................ 70
Figure 6-43 Waveforms of fault current in line 8 ............................................... 71
Figure 6-44Waveforms of voltage and current in HV side ................................ 72
Figure 6-45 Waveforms of voltage and current in LV side ................................ 72
Figure 6-46 Waveforms fault current ................................................................ 73
Figure 6-47 Waveforms of voltage and current in HV side ............................... 74
Figure 6-48 Waveforms of voltage and current in LV side ................................ 75
Figure 6-49 Waveform of fault current in line 8................................................. 75
Figure 6-50 Waveforms of voltage and current in HV side ............................... 76
Figure 6-51 Waveforms of voltage and current in LV side ................................ 77
Figure 6-52 Waveforms of fault current ............................................................ 77
Figure 6-53 Waveforms of voltage and current in HV side ............................... 78
Figure 6-54 Waveforms of voltage and current in LV side ................... 79
Figure 6-55 Waveforms fault current ................................................................ 79
Figure 6-56 Waveforms of voltage and current in HV side ............................... 80
Figure 6-57 Waveforms of voltage and current in LV side ................................ 81
Figure 6-58 Waveform fault current .................................................................. 81
Figure 6-59 Waveforms of voltage an current in HV side ................................. 82
Figure 6-60 Waveform of voltage and current in LV side ................................. 83
Figure 6-61 Waveform of fault current in the line 8........................................... 83
Figure 6-62 Waveforms of voltage and current in HV side ............................... 84
Figure 6-63 Waveforms of voltage and current in LV side ................................ 85
Figure 6-64 Waveform of fault current .............................................................. 85
Figure 6-65 Waveforms of voltage and current in HV side ............................... 86
Figure 6-66 Waveforms of voltage and current in LV side ................................ 87
Figure 6-67 Waveform of fault current .............................................................. 87
Figure 6-68 Waveforms of voltage and current in HV side ............................... 88
Figure 6-69 Waveforms of voltage and current in LV side ................................ 89
xiii
Figure 6-70 Waveform of fault current .............................................................. 89
Figure 6-71 Waveforms of voltage and current in HV side ............................... 90
Figure 6-72 Waveforms of voltage and current in LV side ................................ 91
Figure 6-73 Waveform of fault current .............................................................. 91
Figure 6-74 Waveforms of voltage and current in HV side ............................... 92
Figure 6-75 Waveforms of voltage and current in LV side ................................ 92
Figure 6-76 Fault currents with solid grounding (short circuit on busbar 33A).. 93
Figure 6-77 Distribution of fault current. ........................................................... 94
Figure 6-78 Fault currents with low-resistance grounding
(short circuit on
busbar 33A) ...................................................................................................... 94
Figure 6-79 Fault currents with Petersen coil (short circuit on busbar 33A) ..... 95
Figure 6-80 Fault currents with ungrounded (short circuit on busbar 33A) ....... 95
Figure 6-81 Fault current with solid grounding (short circuit on line 8). ............ 96
Figure 6-82 Fault currents with low-resistance grounding (short circuit on line 8).
......................................................................................................................... 97
Figure 6-83 Fault currents with Petersen coil (short circuit on line 8). .............. 97
Figure 6-84 Fault currents with ungrounded (short circuit on line 8)................. 98
Figure 6-85 Faut currents with both sectionalizers open. ................................. 98
Figure 6-86 Fault currents with both sectionalizers open. ................................ 99
Figure 6-87 Fault currents with both sectionalizers open. ................................ 99
Figure 6-88 Fault currents with sectionalizer 33 closed, left, and with
sectionalizer 150 closed, right. ....................................................................... 100
Figure 6-89 Tendency of short circuit current for different situations of
sectionalizers.................................................................................................. 102
Figure 7-1 Estimate of short circuit current for different number of strings ..... 104
Figure 7-2 Estimate of short circuit current for different number of strings ..... 105
Figure 7-3 Step voltage in WT1 with different groundings .............................. 105
Figure 7-4 Touch voltage in WT1 with different groundings ........................... 106
Figure 7-5 Step voltage in WT1 with different groundings .............................. 106
Figure 7-6 Touch voltage in WT1 with different groundings ........................... 107
Figure 7-7 Estimate of short circuit current for different number of strings ..... 107
Figure 10-1 Equivalent circuit of wind farm......................................................... V
xiv
List of Tables
Table 1 Advantages and disadvantages of grounding methods ......................... 9
Table 2 Parameters wind generator ................................................................. 15
Table 3 Parameters of the lines........................................................................ 17
Table 4 Parameters offshore substation transformer ....................................... 18
Table 5 Three phase short circuit in HV with the sectionalizer 150 closed. ...... 21
Table 6 Single phase to ground in LV side with both sectionalizers open. ....... 21
Table 7 Single phase to ground in LV side with the sectionalizer 33 closed. ... 22
Table 8 Single phase to ground in LV side with the sectionalizer 150 closed. . 22
Table 9 The percents of short circuit types ....................................................... 38
Table 10 Effective values and peak values of the fault current (short circuit on
busbar 33A). ................................................................................................... 100
Table 11 Effective values and peak values of the fault current (short circuit on
line 8). ............................................................................................................ 101
Table 12 Voltages and currents in the neutral with short circuit on busbar 33A.
....................................................................................................................... 102
Table 13 Voltages and currents in the neutral with short circuit on line 8. ...... 103
xv
xvi
1 INTRODUCTION
1.1 Background
As the demand for energy is constantly increasing, lot experts
expert have been
looking for new forms to reduce dependence of fossil energy. Wind energy is
cheaper compared with solar or renewable energy. In the past years wind
power has grown more rapidly than other renewable energy in electricity
generation.[1]
2.221
38.478
865
38.909
1.274
76.152
AFRICA & MIDDLE EAST
ASIA
EUROPE
LATIN AMERICA & CARIBBEAN
NORTH AMERICA
PACIFIC REGION
Figure 1-1 Global installed wind power capacity 08/09 MW
The offshore wind farms will become an important source of energy in the near
future. It is hoped that by the end of this decade wind farms of thousand of
megawatt will be installed in the sea of Europe.
Europe This would be equivalent to the
production of the traditional thermal power station. The offshore wind farms are
a solution to solve problems like the noise, visual pollution and problems of land
dispute.
Offshore
ore wind turbines are less obtrusive than turbines on land, as their
apparent size and noise is mitigated by distance. Because water has less
roughness surface than land (especially deeper water), the average wind speed
is usually considerably higher over open sea.
1
1.2 Problem formulation
This projects wants analyze and shows the different grounding configurations
for an offshore wind farm. Four different kind of grounding have been
considered: ungrounded, solid grounding, low resistance grounding and
Petersen coil.
The wind farm is formed of two strings of wind turbine each one. The model of
wind turbine is doubly fed inductor generator (DFIG).
For this purpose it has been decided to make the model of wind farm, using the
software DIgSILENT PowerFactory. DIgSILENT is a powerful and specialized
tool for simulating problems related to power systems. Different configurations
have to be analyzed when a single phase to ground short circuit in low voltage
side happens.
Different waveforms of voltage and current have been obtained for the
considered grounding and the behaviors in these waveforms have been
studied.
1.3 Objective
The main objective of the project is to model and analyze different grounding
philosophies for an offshore wind farm. DIgSILENT PowerFactory simulation
tool is used for simulations. The analysis compares and studies technical the
advantages and disadvantages to find the optimal grounding method.
The main goals of the project are summarized below:
•
Study of grounding philosophies.
•
Modeling of specific offshore wind farm.
•
Modeling the different types of grounding philosophies/configurations.
•
Analysis: Voltage, current and fault current.
•
Choice the best grounding.
2
1.4 Project limitation
The range of work on this project could be much wider. Unfortunately there are
some limitations. The most important limitations of this project are described
below.
•
DIgSILENT key license only can work for 100 nodes, for this reason we
simulated only 16 wind turbines.
•
In the short circuit analysis only single phase to ground has been
considered in low voltage side.
•
The lightning fault analysis has not been considered.
•
Evaluate the cost and requirement equipment has not been considered.
1.5 Report structure
The present report consists of 9 chapters. In the first chapter a presentation of
the report is made. At the beginning an introduction to the subject is presented.
The problem formulation, the objective and the project limitation are also
presented. The purpose of the second chapter is to describe the different
groundings and compared them. The model of offshore wind farm is presented
in chapter 3. Here the different parts of wind farm are described. Chapter 4
presents the ground grind design of the system and method of calculate. In
chapter 5 the different groundings are modeled. The short circuit analysis is
studied in the chapter 6. An estimation of real wind farm is realized in chapter
7. Chapter 8 shows the conclusions of the analysis. Future works are
enumerated in chapter 9.
3
2 GROUNDING PHILOSOPHIES AND REQUIREMENTS
In this chapter, the different types of grounding and the reasons of using the
system are presented.
2.1 Introduction
The grounding has in several functions, which have in common the use of earth.
There are two types of grounding:
•
Grounding of protection: to protect persons and equipment from
dangerous voltage.
•
Grounding of system: connection between the earth and an electric
system. Usually is realized in the neutral points.
Below they are the reasons why the grounding is used:
•
Security: protection of the persons and the equipments of high values of
voltage.
•
Set the grounding network to the earth potential: to avoid dangerous
voltage due to the capacitive coupling (parasitic capacitances phaseearth or capacitances between phases of systems at different voltage).
•
Reduce the currents of earth fault: the connection of earth system
through an impedance to limit the fault currents in case earth faults.
•
Reduce the overvoltages: the grounding can reduce the overvoltage by:
Transitional earth faults: the faults with arc generate overvoltages in
the healthy phases. These overvoltages are high in the isolated earth
system.
Increasing of the neutral potential: in an isolated system, an earth
fault causes that the neutral of the system has a voltage equal to the
phase voltage. The healthy phases increase in √3 times its voltage. If
the system is put to earth, the overvoltage will be lower if the
grounding is effective and the isolated level of the equipment can be
lower.
4
Manoeuvre transients and lightning: the grounding system, although
does not reduce the overvoltages from manoeuvre and lightning,
allows redistribute the voltage between the phases and reduce the
possibility of an isolated fault between phase and earth.
Simplify the location of the faults: a grounding system generates a
fault current which can be detected with easy for locating the fault
point.
2.2 Methods of systems neutral grounding
The grounding system can be classified according to the connections of neutral
of the earth system:
2.2.1 Ungrounded
The ungrounded system does not have an intentional connection of the neutral
to earth.
Really, the isolated systems are connected to earth trough of the capacitive
coupling between system conductors and ground
Figure 2-1 Ungrounded.
Advantage:
•
It is not necessary invest in equipment for the grounding. But for the
protection system is necessary.
5
Disadvantages:
•
•
High cost of isolating of equipments to earth. A fault causes that healthy
phases are increased √3 times their voltage.
High possibilities of transient overvoltages by faults with arc, resonances
or other causes.
Its use is restricted to medium voltage systems. This grounding requires
systems of fault detection.
2.2.2 Solid grounding
The system with solid grounding has a direct connection of the neutral to earth.
Figure 2-2 Solid grounding
Advantages:
•
Easy detection and localization of system earth faults.
•
Limitation of the overvoltage by earth faults and transients by
manoeuvres and lightning.
Disadvantages:
•
The earth faults are more energetic. The protections of high speed are
required for limiting the thermal and mechanical effects over equipments.
They are used in HV and MHV systems.
2.2.3 Resistance grounding
The system is connected to earth through resistance. The figure shows the
connection of the system.
6
Figure 2-3 Resistance grounded.
Depending of used value of the grounding resistance there are differentiated
two methods:
•
High resistance grounding.
Advantages of high resistance grounding:
It is not necessary an instant trigger against to a first fault to
earth.
Decrease of the damage by thermal effects and electrodynamics.
Decrease of the transient overvoltages by manoeuvres and
lightning.
Disadvantages of high resistance grounding:
•
Behaviour similar to ungrounded. The healthy phases increase
√3 times its voltage.
Low resistance grounding
The advantages and disadvantages of grounding with low resistance are similar
to solid grounding but with less harmful effects during the fault. This is because
it is decreased the earth current.
2.2.4 Ground fault neutralizer
The ground fault neutralizer system is to connect earth through a variable
reactance, such as Petersen coil.
7
Figure 2-4 Ground fault neutralizer
The induction coefficient of the coil is calculated to resonate with the earth
capacity of the system, so for an earth fault, the fault current decreases until
small resistive value.
Advantages:
•
During earth fault, the current is very low and is in phase with the
voltage, so the faults with arc are extinct easily.
•
An earth fault does not involve an instant trigger, then the continuity of
supply is better.
Disadvantages:
•
The healthy phases are composite voltage during the fault. Similar to
ungrounded.
•
The protection system is more complex.
This method is used in grounding of MV grid, especially in central Europe. [2][3]
2.3 Comparison of different grounding systems
In HV and MHV grids direct grounding systems are used for decrease the
solicitations by transient overvoltages and decrease the cost of the isolating.
In MV grids an optimum method of grounding does not exist. The choice of the
method is always a relation between costs of installation and operation.
In the following table the advantages and disadvantages of the different
methods of grounding are shown.
8
Table 1 Advantages and disadvantages of grounding methods
Grounding methods
Advantages
Disadvantages
Ungrounded
Limited the earth fault
currents (less than 1%)
Caused overvoltages more
complex
Solid grounding
The detection of earth
faults is easier
Caused high earth fault
currents
Resistance grounding (if it
Limited the earth fault
currents
Required protections more
complex
Decreased the
overvoltages
Caused earth fault currents
higher
Nearly zero fault current
Required protections more
complex
is compared with solid
grounding)
Reactance grounding (if it
is compared with
ungrounded)
Ground-fault neutralizer
2.4 Obtaining the system neutral
The normal form of obtaining a neutral point for grounding the system is use
transformer with wye-connected windings or the neutral of the generators. If it is
not possible, the grounding transformers or reactances are used.
The different forms of obtaining the system neutral are mentioned as follows:
•
Grounding in generators
The neutrals of generators usually are connected to earth through an
impedance to decrease the single phase fault current (most common fault),
either directly or through single phase transformer.
9
Figure 2-5 Grounding in generators
•
Grounding in transformers
They following may be used the neutral points of transformers with connection
wye-delta and the neutrals of the autotransformers and transformers wye-wye
with tertiary of compensation in delta.
An example of transformer with wye-delta connection is the following.
Figure 2-6 Connection wye-delta
Usually, the transformers with connection wye-wye are not used except in some
cases of high-resistance or resonant grounding.
•
Grounding transformers
10
Grounding transformers can be used to obtain a neutral. The two types of
grounding transformers more used are:
•
Zigzag grounding transformers
It is seen in the following scheme the internal connection of the transformer.
Figure 2-7 Zigzag grounding transformers.
When there is not fault in the system, a small magnetizing current flows in the
transformer winding. This is because the impedance of the transformer to
balanced three-phases voltages is high. Instead, the impedance of the
transformer to zero-sequence voltage is low so that the ground-fault current can
be high. The transformer divides the ground-fault current into three equal
components. These components flow in the three windings of the transformer.
•
Wye-delta grounding transformers.
11
An example of wye-delta grounding transformers can be the one shown
below.
TO UNGROUNDED
3 PHASE VOLTAGE
SOURCE
GROUND
FAULT
R
IG
IG
Figure 2-8 Wye-delta grounding transformers
This type of configuration is used for effective grounding or to accomplish
resistance-type grounding of an existing ungrounded system. To provide a path
for the zero-sequence current, the delta connection must be closed and the
delta voltage rating is fixed for any standard value.
In the figure it is shown a resistor between the primary neutral and ground, this
resistor limits the ground-fault current to a level satisfying for resistancegrounded systems. [2]
12
3 MODELLING OF OFFSHORE WIND FARM
The next figure 3-1 shows the offshore wind farm model.The model was done
with DIgSILENT PowerFactory software. [8]
Cable
Nexan
Sectionalizer 150
Offshore
Transformer
150/33 kV
Substation
Sectionalizer 33
Cable
JDR
DFIG
Figure 3-1 Model offshore wind farm, 2 string with 8 wind turbine each one
13
This model is consisting for 16 wind turbines of 5MW each, due to software
lomitation on the nodes. The distance between wind turbines is of 882 m, which
was calculated trough the next relation, ∅ × 7, where Ø is the rotor
diameter. 126 × 7 = 882 .
Figure 3-2 Distance between wind turbines.
For the wind turbine has been selected a variable speed wind turbine with a
doubly fed induction generator (DFIG) and blade pitch control.
Figure 3-3 General scheme of DFIG
This system consists in a gearbox and an asynchronous generator whose stator
is connecting directly to grid and whose rotor is connecting via two frequency
converters to grid.
The figure 3-4 shows the transformation of mechanical energy to electrical
energy in the wind turbine.
14
Figure 3-4 Transformation of mechanical energy to electrical energy in the wind turbine.
These wind turbines are more efficient then the wind turbines connected to the
grid directly. It is due to they could be run at variable speed. Also the indirect
connection to the grid can control the reactive power to improve power quality
for the electrical grid.
The disadvantages are the increase of price due to use a more complex control
system and the power electronic.
The next table 2 shows the wind turbine characteristics.
Table 2 Parameters wind generator
Electrical & Mechanical parameter
Rated power
Rated voltage
Number of poles pairs
Frequency
5 MW
0.95 kV
3
50 Hz
Stator resistance
0.00298989 p.u
Stator reactance
0.125 p.u
Rotor resistance
0.004 p.u
Rotor reactance
0.05 p.u
15
The applied control is the pitch control. The power control by pitch variation of
the blades is a mechanical process, then the reaction time of the change pitch
mechanism is a important point in the design of the turbine.
The generator slip begins to increase when is near of the rated power of the
turbine. There are two control power strategies depending on the generated
power is over or below the normal operating regime:
When the wind is strong, the obtained power is higher than rated power,
and then the slip increases and the rotor rotates faster. This occurs until
the pitch change mechanism of the blades takes over the problem,
guiding the blades and obtaining less wind power.
If the opposite happens, the wind suddenly drops, the control checks
several times per second the generated power, and how to get as much
power as possible, the pitch of the blades is changed.
The mechanism of pitch change is often with hydraulic motors or with
continuous machine accommodated in the nacelle.
Figure 3-5 Pitch control
When the wind reaches a speed over 15 m/s, the control regulates the pitch, to
obtain less power wind. Then, the obtained power is constant as shown in the
figure.
16
The figure 3-6 shows
ws the model of DFIG:
Figure 3-6 Model wind turbine DFIG
The wind turbines are connected through the cable JDR 36kV 3x500 mm2. The
cable from offshore substation to land is NEXANS TKVA 245 kV 3x1x400 mm2.
Table 3 Parameters of the lines
Parameters
Nexans TKVA 245 kV
JDR 36 kV
Longitud km
50
1.264 / 0.882
Rated Voltage kV
245
36
0.72
0.812
Nominal Frequency Hz
50
50
System Type
AC
AC
Rated Current (in
ground) kA
Parameters per Length
L
1,2 Sequence
Resistance R’ Ohm/km
0.09
0.0506
Reactance X’ Ohm/km
0.15
0.1072
17
Susceptance B’ uS/km
40.8407
87.02212
Parameters per Length Zero Sequence
Resistance R0’ Ohm/km
0
0.2829
Reactance X0’ Ohm/km
0
0.0971
Susceptance B0’ uS/km
36.76189
87.02212
Max End Temperature ºC
90
80
Another place very important in the last figure 3-1 is the offshore substation.
This substation are consisting in two transformer with the next characteristics.
Table 4 Parameters offshore substation transformer
Rated Power
160 MVA
Nominal frequency
50 Hz
Rated Voltage
HV-Side
150 kV
LV-Side
33 KV
Positive Sequence Impedance
Short-Circuit Voltage uk
12 %
SHC-Volatge (Re(uk))ukr
0.28125 %
Zero Sequence. Short citcuit voltage
Absolute uk0
1.6%
The connection of the transformer to earth is YNzn0. The resistance in the
transformer neutral in LV side will be changed to study the different grounding.
18
4 STATION GROUNDING SYSTEM
The grounding has two main objectives:
•
Protecting personnel from injury and damages. These connections are
made to parts of the system that are not usually energized but may
become energized due to an abnormal condition.
•
To provide means to carry electric currents into the earth under normal
and fault conditions without exceeding any equipment limits on continuity
of service.
These two objectives are obtained taking into account the following design
objectives.
•
Provide a low-impedance ground fault current return path in order to
activate the protection and clear the ground fault as soon as possible.
•
Limit to safe levels, the voltages on station and accessible equipment in
normal operations and during transitions electrical.
•
Minimize electrical noise interference in control and instrumentation
systems.
•
Minimize the effect of lightning strikes on personnel, equipment and
structures.
4.1 Design ground grid substation
Before start the calculations, it is important to mention that it has been followed
the guidelines given in the IEEE Guide for Safety in AC Substation
Grounding.[4]
The touch voltage and step voltage are important factors to design and insure a
safe design. The touch voltage and step voltage should be below the maximum
values, which are calculated with the next expressions [4]:
19
= [1000 + 6 ℎ, ]
%&' = [1000 + 1.5 ℎ, ]
0.116
#$
0.116
#$
(01)
(02)
Where
1000 is the body resistance (in Ω)
1.5
is the resistance of two feet in parallel
6
is the resistance of two feet in series
Cs(hs,K)
is 1 if there is no protective surface layer
ρs
is the wet resistivity of the surface
ts
is the shock duration (in s)
0.166 is a constant based on body weight of 50 kg
In this project was used the next parameters:
ρs =24.4 Ω.m because the substation are on the seabed, and the sand resistivity
is the previously given one. This value was retrieved from IEEE Guide for
Safety in AC Substation Grounding for gravel (type and size unknown) and
wetted with salt water.
ts= 0.115 s
= [1000 + 6 × 1 × 24.4]
%&'
0.116
= 392.14 +
√0.115
0.116
= [1000 + 1.5 × 1 × 24.4]
= 354.585 +
√0.115
(03)
(04)
The next step to calculate the short circuit current, this value was obtained by
software DIgSILENT PowerFactory, simulating the worst short circuit. Different
short circuits were simulated in HV side and in the LV side. Fault impedance
was considered zero as the resistance of neutral transformer, and the short
20
circuit durations is 0.115 seconds. The next tables provides the short circuit
currents obtained from DIgSILENT PowerFactory. The short circuit current
generated on the fault on HV side busbar is 150A and the short circuit current
generated on the LV side busbar is 33 A.
Table 5 Three phase short circuit in HV with the sectionalizer 150 closed.
Single phase to ground and three phase short circuit were simulated and it was
checked that the worst fault in HV side is the three phase short circuit with the
sectionalizer 150 closed, its values can be seen in the table 6.
It was also checked that the worst short circuits have been the single phase to
ground generates in LV side, like it is showed in the next tables.
Table 6 Single phase to ground in LV side with both sectionalizers open.
21
Table 7 Single phase to ground in LV side with the sectionalizer 33 closed.
Table 8 Single phase to ground in LV side with the sectionalizer 150 closed.
It has been possible to see that the worst fault is the single phase to ground
with the sectionalizer 33 closed generated on the busbar 33A which causes a
short circuit current of 27.49 kA. But to calculate the ground grid of the
substation the worst short circuit is the single phase to ground on the busbar
33A with the sectionalizer 150 closed. The short circuit current on the busbar
33A has a value of 20.60 kA, and the short circuit current in the transformer is
22
16.690 kA as it is shown in the table 9, this last value is used to calculate the
ground grid. Three phase short circuit has been calculated in the low voltage
side and the results have been put in the annex A.
I’’K= 16.690 kA
This parameter also can be calculated by the next expression [4]:
,-" = ,/ × 0/ × 1/ × Where
If
is the total zero sequence rms fault current (3I0)
Sf
is the split factor or current division factor
Df
is the decrement factor
Cp
is the corrective projection factor
(05)
The grid resistance to remote earth can be calculated by[4]
Where
1
1
61
;
;
× 61 +
23 = 02 × 5 +
<
7 √208
920
1
+
ℎ
×
4
8 ::
4
(06)
A
is the area of the grid (in m2)
L
is Lc+Lr for grids with few or no ground rods, and also for grids with
ground rods predominantly around the perimeter
Lc
in the total length of grid conductor (in m)
Lr
is the total length of ground rods (in m)
h
is the burial depth of the grid (in m)
SR
is the resistivity of the soil (in Ω.m)
The parameters used in this project are the next:
23
A
3600 m2 (60 m wide by 60 m long)
Lc
2520 m (length of grid conductor x number of grid conductor
60x21x2=2520 m)
Lr
0 m (rods was not used)
L
2520 m (L=Lc+Lr)
h
0.5 m
SR
24.4 Ω.m
1
1
6 1
;
; = 0.1882
23 = 24.2 × 5
+
× 61 +
<
2520 √20 × 3600
20
9
1 + 0.5 × 3600:
4
4
:
(07)
≈ 0.19 Ω
The next step to obtain the design grid is to calculate the mesh and step
voltages, theses voltages can be calculated with the next equations[4]:
? =
,′′- ? @
7& + 1.157
=
Where
,′′- @
7
(08)
(09)
ρ
is the soil resistivity (in Ω.m)
Km
is the mesh voltage geometric correction factor
Ks
is the step voltage geometric correction factor
Ki
is the correction factor that into account the increase in current at the
extremities of the grid
The coefficients Km, Ks and Ki can be obtained by the next expressions[4]:
=
? =
1 1
1
1
+ 1 − 0.5DEF G
B +
A 2ℎ 1 + ℎ 1
1 + 2ℎF
1
1F
ℎ
@@
8
Hln K
+
− M+
NO
P
2A
16ℎL
81L
4L
' A2O − 1
(10)
(11)
24
@ = 0.656 + 0.172O
Where
(12)
D
is the spacing between parallel conductors (in m)
d
is the diameter of the grid conductor (in m)
h
is the depth of the grid (in m)
n
is the number of parallel conductors in one direction
Kii
is the corrective weighting factor that adjusts the effects of inner
conductors on the mesh
Kh
is the corrective weighting factor that emphasizes the effect of grid depth
And these parameters can be calculated by[4]:
@@ =
1
2O
FQ
D
(13)
' = 91 + ℎQℎ
Where
ho
1m (reference depth of grid)
n
21
D
3m
d
0.02 m
h
0.5 m
@@ =
1
2 × 21
FQ
FR
= 0.7
' = 91 + 0.5Q1 ≈ 1.225
@ = 0.656 + 0.172 × 21 = 4.268
(14)
(15)
(16)
(17)
25
Once calculated the above variables, now it can be calculated Km and Ks
? =
3 + 2 × 0.05F
1
3F
0.5
Hln K
+
−
M
2A
16 × 0.5 × 0.02
8 × 3 × 0.02
4 × 0.02
=
0.7
8
P = 0.45
+
NO
1.225 A2 × 21 − 1
1
1
1
1
+
+ 1 − 0.5FREF G = 0.4092
B
A 2 × 0.5 3 + 0.5 3
(18)
(19)
and with this values it can be calculated Em and Es
? =
=
FS.S×RTTU×.S×S.FTV
FFWR.R×
= 311.035 V
24.4 × 16690 × 0.4092 × 4.268
= 282.27 +
2520
(20)
(21)
Now it can be checked that Em<Etouch50 (311.035<354.58) and Es<Estep50
(285.27<392.14) therefore our design is good. In the next figure 4-1 is possible
to look the grid design.[5] [6]
26
Figure 4-1 Ground grid 60 x 60 m.
4.2 Design ground grid wind turbine
The ground grid in the wind turbine transformer is calculated using the same
expressions that in the last case but with the next values. The worst fault has
been calculated when the three phase short circuit happens on busbar 33WT1.
I’’k
477 A (this current was obtained with DIgSILENT PowerFactory).
A
49 m2
Lc
42 m (length of grid conductor x number of grid conductor 8x6=48 m)
Lr
4 m (2 rods x 2 m each)
L
46m
n
3
D
3.5 m
d
0.02 m
27
h
0.5 m
Rg
1.9 Ω
Using the previous parameters it can be calculate Em and Es.
= 209.78 +
(22)
= 131.52 +
(23)
< $Z[\ℎ50
(24)
Now it is possible compare the Em with Etouch and Es with Estep50.
< $]^50
(25)
Therefore this model is correct. The next figure 4-2 the ground grid model can
be seen.
Figure 4-2 Ground grid 7 x 7 m with 2 rods.
28
5 MODELLING OF DIFFERENT TYPES OF
GROUNDING
This chapter is focused in computing the different parameters used in the
analyzed grounding types in this project. These grounding types were
mentioned in the chapter 2.
The grounding types used in this project will be the followings:
•
Ungrounded.
•
Solid grounding.
•
Low-resistance grounding.
•
High resistance grounding.
•
Resonant grounding (Petersen coil).
The grounding types are analyzed separately below. The grounding location is
the substation transformer (33/150 KV).
5.1 Ungrounded
In this case, there is not an intentional grounding connection of transformer. The
ungrounded system is in reality a capacitance grounding system like the figure
5-1 shows.
29
Figure 5-1 Ungrounded
In the previous figure, it is shown the distributed capacitive reactance to ground,
Xco, which is assumed to be balanced. To obtain the value, it is analyzed with
the DIgSILENT model.
•
R = 2.083 _` (positive-sequence capacity)
Then the distributed capacitive reactance is:
a&b = 2.083 _`
This value is the total distributed capacitive reactance of the line, so if this value
is divided per three to obtain the distributed capacitive reactance per phase. [2]
a& = 0.694 _`
5.2 Solid grounding
The solid grounding is obtained when there is a connection between the neutral
of a transformer or generator and ground. This connection is direct, without any
intentional intervening impedance.
The next figure 5-2 shows the solid grounding in the transformer of the high
voltage side.
30
Figure 5-2 Solid grounding
In this grounding type only the grounded resistance is considered, that is to say,
the resistance of ground grid calculated in the chapter 4. Then, the existing
resistance between the neutral of transformer and ground is: [2]
23 = 0.19 Ω
5.3 Low-resistance grounding
Low-resistance grounding is designed to obtain a ground-fault current between
100 A and 1000 A. To limit this current, it is used a neutral resistor which is
calculated with the next formula [2]:
2c =
Where:
•
+dc
,e
(26)
+dc is the system line to neutral voltage. In the model, the voltage in the
low side is 33 KV but it is line to line voltage, then:
+dc =
•
33
√3
= 19.05 +
(27)
,e is the desired ground-fault current. It is chosen 400 A for be a typical
value.
,e = 400 8
31
Using the equation 26, it is calculated the neutral resistance:
2c =
19.05 · 10g
= 47.62 Ω
400
(28)
To obtain the total ground resistance, it is should add the neutral resistance and
the ground grid resistance. The latter was calculated in the chapter 4.
2b = 2c + 23 = 47.62 + 0.19 = 47.81 Ω
(29)
When this resistance is used in the model, the ground-fault current is 0.44 KA, it
is checked that the current is limited between the values desired.
The figure 5-3 shows the low-resistance grounding in the transformer in the high
voltage side:
A
B
N
IR
C
R
N
V LN
=
IG
GROUND
FAULT
IG
Figure 5-3 Low-resistance grounding
In the figure 5-3:
•
IR is the current through the neutral resistance
•
IG is the ground-fault current before mentioned.
5.4 High resistance grounding
This grounding type is similar to low-resistance grounding but in this case the
resistance has a high ohmic value. The resistance is calculated to limit the
32
current Ir, current through of the neutral resistance. The value of Ir should be
equal or slightly greater than the total capacitance charging current, 3Ico.
The figure 5-4 shows the high-resistance grounding in the transformer of the
high voltage side. It also shows the current Ir mentioned before.
RN ≤
V LN
3· I G
Figure 5-4 High-resistance grounding.
The calculated resistance should be checked because usually when there is a
line-to-ground fault with a fault current greater than 10 A, this grounding type
should be avoided. The reason is the potential damage caused by an arcing
current greater than 10 A in a confined space.
So, it will be calculated the neutral resistance with the capacitance charging
current obtained of the model and after it will be checked if the ground-fault
current is lower than 10 A.
The formula [2], used to calculate the neutral resistance, shown in the figure 54, is:
2c ≤
Where:
•
+dc
3 · ,&
(30)
+dc is the line to neutral voltage.
33
•
,& is the capacitance charging current.
Replacing the values obtained from the DIgSILENT software into the last
formula:
33 · 10g
2c ≤ √3 = 476.31 Ω
40
(31)
If a neutral resistance of 476.31 ohm is chosen and it is adding the ground grid
resistance calculated in the chapter 5, the total resistance for introduce in the
model is obtained as.
2b = 2c + 23 = 476.31 + 0.19 = 476.50 Ω
(32)
This total resistance is introduced in the model and the ground fault current is
calculated.
,e = 0.06 i8
It is can say that it is not correct use this grounding type in the wind farm model
because the value is greater than 10 A and their use can be dangerous.
5.5 Resonant grounding (Petersen coil)
The resonant grounding systems are constituted by a variable reactance which
is connected between the neutral of substation transformer and ground. This
reactance is called also 'Petersen Coil', XL. The more important characteristic is
that during the ground faults, the inductive current of the reactance eliminates
the capacitive fault current produced by the grid. So, the current that flows for
the fault point is decreased to a small resistive current, Ir.
34
Figure 5-5 Resonant grounding.
The figure 5-5 shows the distribution of the fault current and the resistance “r”
represents the reactor losses. The capacitive fault current is annulated for the
inductive current of the coil as the next equation shows: [2]
,e = ,d + , + 3,& ≈ ,
(33)
This type grounding represents a great advantage to the appearance of singleline-ground fault in the lines, which are the most common faults. Supposing the
ground fault is in an insulator flashover, it may be self-extinguishing. This
method permits network operation during long time in these fault conditions, it
permits to decrease the transient triggers of the protections. This means an
improvement in the service and the decreasing of the maintenance in the
switches.
For correct operation of this system, the Petersen coil should be correctly tuned,
so the distributed capacitance of the grid (Xco) could be compensated by the
inductance of the coil. Due to continuous variations in the grid, the resonant of
the grounding systems need a tuning and control system that control them
dynamically.[2]
35
For obtaining the reactance value, XL, the formula following is used [7]:
ad =
Where:
•
•
1
3j
(34)
j = 2Ak, where k is the frequency of the system.
is the distributed capacitance of the system or Xco, with a value
obtained of the model. =
F.Vg lm
g
= 0.694 _` .
So the reactance value will be:
ad =
1
= 1,528.86 Ω
3 · 2 · A · 50 · 0.694 · 10ET
(35)
36
6
ANALYSIS
This chapter will present the short circuit calculations. The model analysis is
focused on the grounding methods selected in the chapter 5. With each
grounding type, the voltage and current graphics will be obtained. The different
of grounding types are compared and analyzed.
6.1 Introduction to shorts circuits analysis
The short circuit analysis is based on the calculation and determination of the
magnitudes of the fault currents and the contributions of each element to the
fault. These characteristics permit the breakers design and the adaption of the
protection mechanisms.
The short circuit current of the system, permits establish the characteristics of
the protection elements that should remove the fault current, then it is
necessary make the calculation for all system voltage levels.
From electric point of view, a short circuit is the accidental or unintentional
connection, through a resistance o impedance of low value, of two or more
points of a circuit that is working at normal conditions and different voltages. A
short circuit generates surges in the system currents, it can damage the
equipment.
The values of short circuit current to be considered are:
•
The short circuit maximum current.
•
The short circuit minimum current.
The short circuit maximum current is calculated for design the protection
equipment, protection adjustments and design of grounding.
In electric systems can produce different fault types, these are:
•
Three-phase short circuit.
•
Single-phase short circuit to earth.
•
Two-phase short circuit with or without earth contact.
37
The single-phase faults to earth can generate fault currents whose value can be
higher than the three-phase fault current. However, this more frequent happens
in the transmission or distribution systems in medium voltage, usually when the
fault appears near of the substation. The single-phase fault current seldom has
a value higher than the three-phase fault current.
The percentages of the short circuit types in a system are:
Table 9 The percents of short circuit types
SHORT CIRCUIT TYPES
IMPACT (%)
Single-phase short circuit to earth
80
Two-phase short circuit
15
Three-phase short circuit
5
However for simplify, this project is focused in the single-phase short circuit to
earth.
6.2 Single-phase to ground short circuit
The single-phase short circuit is responsible for the greatest number of short
circuits in the system (80% of the short circuits are single phase). This short
circuit causes short circuit currents that depend on the fault impedance and the
connection to earth of the transformers in the line.
This is the short circuit more frequent and violent, appearing more frequently in
solid grounding systems or through impedances with low value.
The computation is important due to the high currents and the connection to
earth. This permits to calculate the leaks to earth, the touch voltage or step
voltage to assess the interferences that these currents can cause.
38
The single-phase to ground short circuit is unbalanced and presents energy
losses, so it is necessary use the three sequences grid (positive, negative and
zero) for its calculation.
Figure 6-1 Generic scheme of single-phase short circuit to earth
Figure 6-2 Scheme impedance single-phase short circuit to earth
The short circuit current between a phase and earth has a value[9]:
oQ
√3
,n =
pqr + p
Where:
•
•
(36)
o is the line to line voltage.
pqr is the short circuit impedance, being the positive and negative
sequence.
•
p
is the equivalent impedance of return from the earth or zero
sequence impedance.
39
The equivalent circuit has been developed in more detail in the annex B.
6.3 Simulations with the chosen groundings
The short circuits have been realized in two different points of the system. One
is in the busbar 33A and the other point is on line 8, between WT7-WT8. A
single phase to ground short circuit has been calculated and the voltage and
current for the high and low voltage side have been obtained. The measuring
and simulation points (M1, M2) are showed in the figure 6-3.
Figure 6-3 Scheme with the measuring and simulation points
The two short circuits will be analyzed separately. The results with the short
circuit on busbar 33A and on the line 8 will be obtained, these will be analyzed
and compared.
When the short circuit is simulated, the following conditions were
chosen,
impedance fault is zero, the neutral cables resistance is zero. The simulation
time is 200 ms and the fault is cleaned in 115 ms as it was write in the chapter
4. As it was written in previous chapters the solid grounding, low resistance and
40
Petersen coil configuration are analyzed in the following simulations. The
parameters used in the software are as follows:
Solid grounding : Rg=0.19 ohms
Low resistance : RN=47.81 ohms
Petersen coil: Rg=0.19 ohms and XL=1528.86 ohms
Below, the behaviour of the system is analyzed against a single-phase to
ground short circuit on busbar 33A using the different groundings of the chapter
4. The system is analyzed in three different situations:
•
Both sectionalizers open
In this case the two strings of generators are connected to the grid
through the transformer and the connection between them does not
exist.
•
The sectionalizer 33 is closed and the sectionalizer 150 is open
Now the sectionalizer 33 is closed and the two strings of generators are
joined as seen in the figure 6-3.
•
The sectionalizer 150 closed and the sectionalizer 33 open.
This is the opposite case, the closed sectionalizer is the 150. The two
strings are connected after of the transformers.
The analysis is divided in these three states. In each situation the results with
the different groundings will be obtained and they will be discussed.
6.3.1 Single-phase to ground short circuit on busbar 33A
BOTH SECTIONALIZERS OPEN
•
Solid grounding
In the first place, the fault current is obtained when the system has a solid
grounding.
41
Figure 6-4 Waveform of fault current on busbar 33A .
The fault current reaches a value around 20 kA when the fault starts and then
its value decreases to 16.5 kA approximately.
The graphics of the voltage and current in the high voltage side (point M1) are
the following:
Figure 6-5 Waveforms of voltage and current in HV side
42
The voltage suffers a transient period, decreasing its value, while the fault is
kept until the time 0.115 seconds, in this moment the fault is cleared and the
voltage comes back to normal level. The current also suffers a transient period
but its value increases and reaches a value around 2 kA in the fault phase.
Theses transient periods are due to the high fault current which affects the high
voltage side.
In the other measurement point (line 1, LV side) the graphics of the voltage and
current are the following:
Figure 6-6 Waveforms of voltage and current in LV side.
In the low voltage side, the voltage in the fault phase has a value almost zero
but with variations, the phase C decreases its voltage until a value of 21.50 kV
approximately and the voltage in the phase B decreases until a value around
12.5 kV. When the fault is cleared the three phases suffers a transient period
and after they come back to normal level. The current when the fault appears
43
suffers a peak value of 3.15 kA in the fault phase. After the fault current goes
decreasing and the phases B and C goes increasing. When the fault is cleared
the current in the three phases starts to decrease until reach to its normal level.
It can be observed that the current in the phase C increases more than in the
fault phase, this may be due to the generator control.
•
Low-resistance grounding
The obtained fault current with this grounding is the following:
Figure 6-7 Waveform of fault current on busbar 33A.
When the fault appears, the fault phase suffers a peak current reaching a value
of 1.9 kA approximately. The peak current disappears very fast and the fault
current keeps a value around 0.65 kA while it is not cleared. The fault current
value is quite low if it is compared with the solid grounding.
To follow, they are shown the graphics of the voltage and current in the point
M1 (HV side).
44
Figure 6-8 Waveforms of voltage and current in HV side.
These graphics show that the voltage almost does not suffer any variation, it is
kept practically in its normal level. So, the current suffers a small transient
period while the fault is kept but practically without importance, returning to
normal level when the fault is cleared, this is due to the low fault current which
does not affect to the high voltage side.
Now, the graphics in the point M2 are obtained like the figure 6-9 shows.
45
Figure 6-9 Waveforms of voltage and current in LV side.
It can be observed that the voltage in the fault phase falls to zero and the
phases B and C increase √3 time their values until 49 kV approximately in the
moment that the fault is produced. When the fault is cleared, the three phases
come back to the normal level. The current in the fault phase suffers a peak in
the moment that the fault appears but during a short time period, after the
current increases its value to 1.0 kA approximately in the fault phase while the
fault continues. The phases B and C are not influenced by the fault.
•
Petersen coil
To continue, it is analyzed the system with Petersen coil. The following graphic
shows the fault current when it is used this grounding type.
Figure 6-10 Waveform of fault current on busbar 33A.
46
The fault current reaches a value around 1.45 kA when the fault is produced
and as time progresses, the current is decreased quite fast until a value around
0.03 kA. If it is compared this fault current with the two previous fault currents, it
can be said that this has a low value and its behaviour is good.
The voltage and current in the point M1 (HV side) are shown in the figure 6-11.
Figure 6-11 Voltage and current in HV side.
It can be said that both voltage and current do not suffer practically any
alteration. The fault has almost no influence on the phases.
The graphics of the voltage and current are shown in the next figure.
47
Figure 6-12 Waveforms of voltage and current in LV side.
In the figure 6-12 it can be observed that the voltage, when the fault is
produced, falls to zero in the fault phase and increases √3 times its value in the
healthy phases until a value around 50 kV. When the fault is cleared the voltage
in three phases is unbalanced. This is due to the behaviour of the wind turbine
control.
In contrast, the current in the fault phase in t=0 seconds has a peak value of
1.75 kA approximately and a transient period during a short time period, after
the current comes back to the normal level.
•
Ungrounded
The fault current with this grounding is shown in the figure 6-13.
48
Figure 6-13 Waveform of Fault current on busbar 33A.
In this case, the current reaches a value around 1.4 kA and it decreases with
the time. Its behaviour is very similar to the Petersen coil due to the
capacitances of the line.
The voltage and current in the point M1 (HV side) are in the next figure 6-14.
Figure 6-14 Waveform of voltage and current in HV side.
49
Its voltage and current is practically the same that the Petersen coil and its
values match.
In the point M2 (LV side), the obtained current and voltage are the following.
Figure 6-15 Waveforms of voltage and current in LV side.
The voltage, in the moment that the fault appears, falls to zero in the fault phase
and in the other two phases suffers a short transient period with a overvoltage,
after the voltage has a value of √3 times its initial values, around 46 kV. When
the fault is cleared the DIgSILENT software losses the reference point, but the
peak to peak voltage is the same before the fault. The current presents a peak
value and after it comes back the normal current.
50
SECTIONALIZER 33 CLOSED
•
Solid grounding
Below, it is obtained the waveform of fault current when it is used the solid
grounding.
Figure 6-16 Waveform of fault current with solid grounding
The fault current reaches a value around 32 kA when the fault starts and it goes
decreasing with the time until a value around 19 kA approximately. This current
has a value higher than the obtained when both sectionalizers are open. This is
due to the closing of the sectionalizer, as for this path the current flows until the
busbar 33A.
In the figure 6-17 are shown the voltage and current in the point M1 (HV side).
51
Figure 6-17 Waveforms of voltage and current (solid grounding)
When the fault starts, the three voltages suffer a transient period decreasing
their values until a value around 40 kV in the fault phase. In the moment that the
fault is cleared (t=0.115 sec), the three phases suffer again a transient period.
The current on the contrary, when the fault appears, increases its value in the
three phases. The fault phase reaches a value around 1.5 kA and when the
fault is cleared, the current suffers a transient period due to voltage fluctuations,
when the voltage is steady the current recoups its normal values.
The figure 6-18 shows the current and voltage in the other measurement point,
M2.
52
Figure 6-18 Waveforms of voltage and current in LV side.
The voltage of the fault phase, like in the case with both sectionalizers open,
takes a value almost zero while the fault is kept. The voltage in the others
phases decreases until that the fault is cleared, then every phases suffer a
transient period and come back to normal level.
The fault current presents a peak value of 3.10 kA when the fault happens and
after the current goes decreasing. The current increases its value in the other
two phases but also suffers a transient period when the fault is cleared. The
behaviour is very similar to the case with both sectionalizers open.
•
Low-resistance grounding
The fault current when the sectionalizer 33 is open is quite higher than when the
both sectionalizers are open.
53
Figure 6-19 Fault current on busbar 33A.
In this case, the current reaches a value around 3.8 kA when the fault appears,
this is because when the sectionalizer is closed, the fault current is the sum of
the current coming from the transformer and from the other string of wind
turbines through of the sectionalizer 33. After of the peak current, the current
decreases until a value around 1 kA and this value is kept while the fault exists.
The obtained voltage and current in the point M1 (high voltage side) are shown
to following.
54
Figure 6-20 Waveforms of voltage and current in HV side.
It can be said that the voltage is not affected by the fault and the current only
suffers a small transient period while the fault exists. This behaviour is similar to
the case with both sectionalizers open. The measurement point M1 is not
influenced by the closing of the sectionalizer.
To continue, the voltage and current in the other measurement point, M2, are
shown in the figure 6-21.
Figure 6-21 Waveforms of voltage and current in LV side.
55
In this point the voltage and current do not vary if it is compared with the
situation where the both sectionalizers are open.
•
Petersen coil
In this case, the peak current is higher than when both sectionalizers are open,
practically 2 times 1.5 kA. In the figure 6-22 is shown the fault current for this
case.
Figure 6-22 Waveform of fault current on busbar 33A.
The fault current in this situation reaches a value around 2.8 kA when the fault
appears. Like in the case with both sectionalizers open, the fault decreases with
the time to a value of 0.05 kA.
The voltage and current in the point M1 are shown to following.
56
Figure 6-23 Waveforms of voltage and current in HV side.
The obtained results are the same that when the both sectionalizers are open,
the voltage and current in this point are not influenced by the closing of
sectionalizer 33A.
In the same way that in the measurement point M1, the obtained voltage and
current in the point M2 (LV side) are very similar to that obtained with both
sectionalizers open like the figure 6-24 shows.
Figure 6-24 Waveforms of voltage and current in LV side.
57
These results show that the current and voltage in the point M2 are not
influenced by the closing of sectionalizer 33A.
•
Ungrounded
The fault current in this situation is shown in the figure 6-25.
Figure 6-25 Waveform of fault current on busbar 33A.
When the fault appears, the fault current reaches a peak value of 2.9 kA
approximately, with the time the current goes decreasingly until a value of 0.080
kA. This behaviour of the current is very similar to the Petersen coil and if it is
compared with the case with both sectionalizers open, it can be seen that the
fault current also is high.
The obtained voltage and current in the point M1 (HV side) are shown in figure
8-26.
58
Figure 6-26 Waveforms of voltage and current in HV side.
The obtained results are very similar to both sectionalizers open case and also
to the results of Petersen coil. Both voltage and current are almost not
influenced by the fault.
In the measurement point M2 (LV side), the obtained results are shown in the
figure 6-27.
Figure 6-27 Waveforms of voltage and current in LV side.
59
The voltage and current in this point is the same as that in the case with both
sectionalizers open. This measurement point is not affected by the current flows
through sectionalizer 33.
SECTIONALIZER 150 CLOSED
•
Solid grounding
The fault current, when the sectionalizer 150 is closed, is the following.
Figure 6-28 Waveform of fault current on busbar 33A.
In this situation, the fault current reaches a value around 24 kA, this current is
higher than when both sectionalizers are open and lower than when the
sectionalizer 33A is closed. The current also decreases with the time until a
value of 16 kA approximately.
Below, the graphics of the voltage and the current in the measurement point M1
are shown.
60
Figure 6-29 Voltage and current in HV side.
If it is compared the voltage in this case, figure 6-29, with the case which has
both sectionalizers open, figure 6-5, it can be seen that the voltage behaviour is
very similar. In contrast, the current behaviour changes. In this case the current
increases but not as much as in previous situations of the sectionalizers.
Now the graphics show the obtained current and voltage in the point M1.
61
Figure 6-30 Waveforms of voltage and current in LV side.
In this situation the current and voltage behaviour is the same that in the case of
both sectionalizer open and very similar to the case with sectionalizer 33
closed. It is due to that the current from the sectionalizer 33 does not flow for
the point M2.
•
Low-resistance grounding
The obtained fault current for this case is the following.
Figure 6-31 Waveform of fault current on busbar 33A.
When the sectionalizer 150 is closed the fault current almost does not vary
against the case with both sectionalizers open. It reaches a value around 1.95
kA in the moment that the fault appears and after decreases until a value of 0.6
kA approximately.
62
The following figures show the voltage and current in the point M1 (HV side).
Figure 6-32 Waveforms of voltage and current in HV side.
The graphics of the figure 6-32 show that the behaviour of the current and the
voltage is the same as that in the case with both sectionalizers open and in the
case with sectionalizer 33 closed.
The measured voltage and current in the point M1 are shown in the figure 6-33.
63
Figure 6-33 Waveforms of voltage and current in LV side.
In the point M2 happens the same happens as that in the point M1. The current
and voltage coincide with the case with both sectionalizers open and with
sectionalizer 33 closed .
•
Petersen coil
The fault current in this case where the Petersen coil is used reaches a value of
around 1.4 kA in the moment that the fault appears.
Figure 6-34 Waveform of fault current on busbar 33A.
The fault current is practically the same as that in the case with both
sectionalizers open and its variation also.
The voltage and current in the measurement point M1 are represented in the
figure 6-35.
64
Figure 6-35 Waveforms of voltage and current in HV side.
The obtained values are very similar with the result with both sectionalizers
open and with sectionalizer 33 closed. As the currents, the voltages almost are
not affected by the fault. Their behaviour is ideal.
The currents and voltages in the measurement point M2 are obtained, like that
the figure 6-36 shows.
65
Figure 6-36 Waveforms of voltage and current in LV side.
The obtained results are the same that in the case with both sectionalizers open
and with sectionalizer 33 closed. The current and the voltage are not affected
by the fault. This behaviour is ideal.
•
Ungrounded
Finally, it is obtained the fault current with the ungrounded system. The next
figure 6-37 shows the waveform of this current.
Figure 6-37 Waveform of fault current on busbar 33A .
The current value in the moment that the fault appears is the same as that in
the case with both sectionalizers open. Also coincide with the fault current in
Petersen coil.
The graphics of the voltage and current in the point M1 (HV side) are shown in
the figure 6-38.
66
Figure 6-38 Waveforms of voltage and current in HV side.
The obtained results coincide with the obtained in the other three situations of
the sectionalizers.
The results in the other measurement point, M2, are shown to following.
67
Figure 6-39 Waveforms of voltage and current in LV side.
The same happen in the point M2, LV side, because this measurement point is
not affected by the current flows through of both sectionalizers.
6.3.2 Single-phase to ground short circuit on line 8
BOTH SECTIONALIZERS OPEN
•
Solid Grounding
It is started with the analysis of the fault current in the line 8, the graphic is
9showed in the next figure 6-40.
Figure 6-40 Waveform of fault current in the line 8
In the previous figure 6-40 it is possible to look at that the current almost
reaches a value of 15 kA and it keeps this value while the fault is not cleaned.
68
The next figure 6-41 shows the waveforms voltage in high voltage side which
are measured in the point M1, it can be seen that the voltage in the phase A
and B are decreased but the phase C is increased, while the fault happens. It
can also be seen that the voltage comes back to the normal voltage after the
fault.
Figure 6-41 Waveforms of voltage and current in HV side
The waveform of current in HV side measured in point M1 shows that the
current in the phase A is increased and it reaches a superior value of 1 kA and
the other phases increase but to a lesser extent like it is seen in the last figure
6-41.
This behaviour is due to the high value of the fault current which influence in the
voltage and the current in the high voltage side.
If the current is compared with fault current case on busbar 33A it is possible to
see that the current is low in this case, this is normal due to the line resistance.
69
Now the voltage and the current in low voltage side are analysed in the next
graphics.
During the fault the voltage in the phase C has a little increment, but the other
phases are decreased their values at half voltage, and after the fault is cleaned
and the voltage come back to normal value.
Figure 6-42 Waveforms of voltage and current in LV side
The current obviously in the phase A is increased its values at 10 kA during the
fault. The phase B increases but in a little value and C is decreased in their
values.
•
Low Resistance
As in the solid grounding the fault current is showed in the figure 6-43. It is
possible to see the current and it is seen that it reaches a peak value of 1.5 kA
in the phase A when the fault happens, but this value is lower than the fault
current in the solid grounding case. It is seen that after of peak current, it keeps
a constant value during the fault.
70
Figure 6-43 Waveforms of fault current in line 8
The next figure shows that the value of voltage in the high voltage side does not
vary, this is a good behavior. If it is remembered the solid grounding case the
voltage in high voltage side varied its nominal values during the fault. Therefore
this configuration is better with regard the voltage in the solid grounding. The
current neither varies its values too much with regard its normal values.
71
Figure 6-44Waveforms of voltage and current in HV side
In low voltage side the voltage is increased, its values in the phases B and C √3
times their values reaching the values of 50 kV and the voltage in the phase A
is lower.
Figure 6-45 Waveforms of voltage and current in LV side
72
On the other hand the current in the phase B and C decreased their normal
values a little and the current in the phase A is decreased below value of 0.5 kA
and it is kept constant during the fault, this problem was analyzed in detail and it
was checked that this is due to the wind turbine controller.
•
Petersen Coil
The fault current in this case almost reaches a value of 1.5 kA in a peak current,
but these values are being decreased in the time, to reach extremely low
values, as shown in the next figure 6-46.
Figure 6-46 Waveforms fault current
The next figure shows that the waveforms of voltage and current in high voltage
side do not vary their values during the fault. This configuration is better with
regard to the current than in the low resistance configuration. There are not
differences between the voltage in the low resistance case and the Petersen
coil case.
73
Figure 6-47 Waveforms of voltage and current in HV side
In the low voltage side the voltage in the phases A and B is increased to almost
50 kV like the low resistance case but the difference is that after the fault is
cleaned the voltage is unbalance. This characteristic was analyzed and it was
checked that the voltage unbalance is due to the behavior of wind turbine
controller. This problem and the transients in the beginning did not exist with the
low resistance configuration.
74
Figure 6-48 Waveforms of voltage and current in LV side
The current too has transients during the short time when the fault happens,
after the current keeps the normal values, in this case the current has better
characteristics that the current in the low resistance case.
•
Ungrounded
The next waveform is similar to the waveform of fault current with the
configuration Petersen coil, peak values are practically the same too.
Figure 6-49 Waveform of fault current in line 8
The figure 6-50 shows that with this configuration the waveforms of voltage and
current are not affected by the short circuit in the line 8.
75
Figure 6-50 Waveforms of voltage and current in HV side
In the low voltage side the voltage is affected, the phase A is practically zero
and the other phases increase their values almost to 50 kA, when the fault is
cleared the DIgSILENT software losses the reference point, but the peak topeak
current is the same before the fault happens.By other hand the current suffers
some little transients when the fault happens, but it rapidly gets its normal
values during the fault.
76
Figure 6-51 Waveforms of voltage and current in LV side
SECTIONALIZERS 33 CLOSED
•
Solid Grounding
The fault current is similar to fault current in solid grounding case with the
sectionalizers open, it is obvious that the fault current in this case is higher than
the last case due to the fault current is the sum of fault current from transformer
and from second string of wind turbines.
Figure 6-52 Waveforms of fault current
The voltage in high voltage side is practically the same that in the solid
grounding with the sectionalizers open case. In the current there are
differences, it is possible to see that the current in the phases A and B are
77
increased and the phase C is decreased, but if this waveform of current are
compared with the waveform of current in the case with both sectionalizers
open, it is possible to see that the current increase a low value than with both
sectionalizers open, this is normal as the current from high voltage side is low.
Figure 6-53 Waveforms of voltage and current in HV side
In low voltage side the waveforms of voltage is the same that the waveforms of
voltage in the case with both sectionalizers open. The waveforms of current are
higher than the case both sectionalizers open, as in this case the waveform of
phase A above the 10 kA.
78
Figure 6-54 Waveforms of voltage and current in LV side
•
Low Resistance
In figure 6-55 it is possible to see that the peak current in the fault is higher than
the peak current in the case with both sectionalizers open, in this case the fault
current reaches the 2 kA, after the peak current the short circuit current has a
constant values during the fault until that the fault is cleaned.
Figure 6-55 Waveforms fault current
The waveforms of voltage and current in high voltage side are the same as the
waveforms of voltage and current with both sectionalizers open. It is possible to
see that the voltage and current almost do not vary.
79
Figure 6-56 Waveforms of voltage and current in HV side
The waveform of voltage in this case and the case with both sectionalizers open
is practically the same, but the waveform of current in this case is low in the
phase A than in the case with both sectionalizers open and also the phase A is
unbalance, this problem did not exist when the case with both sectionalizers
open was studied. As mentioned the phase A has a low value than the other
phases due to the wind turbine controller.
80
Figure 6-57 Waveforms of voltage and current in LV side
•
Petersen Coil
If comparing the fault current waveform in figure 6-58 with fault current when
the sectionalizers are open, it is possible to see that the waveform in figure 6-58
is higher than the other case. This is obvious as the fault current is the sum the
two fault currents one coming from the transformer and other coming from the
other string of wind turbines.
Figure 6-58 Waveform fault current
The waveform in figure 6-59 has the same values as the case with both
sectionalizers open. Therefore it can be seen that the waveform of voltage and
current are not affected with the changes in the sectionalizer 33.
81
Figure 6-59 Waveforms of voltage an current in HV side
The waveforms of voltage and current in the low voltage side are the same like
the case with both sectionalizers open. The only difference is that the next
waveforms have more transients when the fault happens.
82
Figure 6-60 Waveform of voltage and current in LV side
•
Ungrounded
This waveform has the same peak values that the last case, but the current
during the fault is higher than in the last case like the next figure 6-61 shows.
Figure 6-61 Waveform of fault current in the line 8
As when the sectionalizers are closed the waveform of voltage and current in
high voltage side are not affected by the short circuit in the line 8.
83
Figure 6-62 Waveforms of voltage and current in HV side
The below waveform shows the voltage and current has a similar behavior than
the case with the sectionalizers open, the only difference is that the current has
a peak current greater than the previous case.
84
Figure 6-63 Waveforms of voltage and current in LV side
SECTIONALIZERS 150 CLOSED
•
Solid Grounding
The fault current waveform in this case and the case with both sectionalizers
open are very similar, the fault current in this case is a little more high.
Figure 6-64 Waveform of fault current
The waveforms in high voltage side of figure 8-66 are the same that the case
with both sectionalizers open, the phases A and B decreased and the phase C
is increased.
85
Figure 6-65 Waveforms of voltage and current in HV side
Practically the voltage waveform in this case and the case with sectionalizer 33
closed is the same, the only difference is current value is a little smaller than
the case with sectionalizer 33 closed which reaches to 0.5 kA. When both
sectionaliters are open this current is higher.
86
Figure 6-66 Waveforms of voltage and current in LV side
The waveform of current in phase A in this case is smaller than in the case with
the sectionalizer 33 closed which reaches around 12.5 kA during the fault, also
it is possible to see that the waveform of current during the fault are
unbalanced.
•
Low Resistance
The next figure 6-67 shows fault current waveform and if it is compared with the
fault current when both sectionalizers are opens it is possible to see that the
waveform is practically the same.
Figure 6-67 Waveform of fault current
87
The next waveforms are the same to the different situations of the sectionalizers
(both sectionalizers open, sectionalizer 33 closed).
Figure 6-68 Waveforms of voltage and current in HV side
The next waveform of voltage and current were seen previously when the case
with both sectionalizer are opened.
88
Figure 6-69 Waveforms of voltage and current in LV side
•
Petersen Coil
The figure 6-70 shows the waveform of fault current and this current is the same
that when the configuration Petersen coil was analyzed with both sectionalizer
open.
Figure 6-70 Waveform of fault current
The waveforms of voltage and current have their normal values during the fault,
therefore the fault does not affect to the voltage and current like in the low
resistance case.
89
Figure 6-71 Waveforms of voltage and current in HV side
The next graphics are similar to the other cases studied with different situations
of sectionalizers for the Petersen coil configuration
90
Figure 6-72 Waveforms of voltage and current in LV side
•
Ungrounded
With the next figure 6-73 has been checked that the fault current is the same
regardless the position of the sectionalizers.
Figure 6-73 Waveform of fault current
The same thing happens with the waveforms of voltage and current in high
voltage side and in the low voltage side. The waveform of current in low voltage
side has a peak value lower than the seccionalizer 33 is closed.
91
Figure 6-74 Waveforms of voltage and current in HV side
Figure 6-75 Waveforms of voltage and current in LV side
92
6.4 Comparison of the results
In the first place, the fault currents have been compared in the different
situations which have been raised in the chapter 6.3.
•
Both sectionalizers open.
•
Sectionalizer 33 closed.
•
Sectionalizer 150 closed.
Below, the analysis has been performed when the fault appears on busbar 33A.
The different groundings have been analyzed in these situations in the same
graphic. The comparison has been started with the solid grounding.
Figure 6-76 Fault currents with solid grounding (short circuit on busbar 33A)
The figure 6-76 shows that the closing of the sectionalizer 33 produces an
increasing of the fault current. Instead, the closing of the sectionalizer 150 does
not produce any variation practically with regard to the case with both
sectionalizers open.
This increasing is due that the fault current has a component more, that is to
say, the fault current is now the sum of the current from the string of the wind
turbines, the current from the transformer and the current from the other string
of wind turbines like the figure 8-78 shows.
93
Figure 6-77 Distribution of fault current.
The same happens with the rest of groundings. The figures 6-78, 6-79 and 6-80
present the fault currents in the three situations of the low-resistance grounding,
Petersen coil and ungrounded respectively.
Figure 6-78 Fault currents with low-resistance grounding (short circuit on busbar 33A)
94
Figure 6-79 Fault currents with Petersen coil (short circuit on busbar 33A)
Figure 6-80 Fault currents with ungrounded (short circuit on busbar 33A)
It can be seen that the behaviour of the fault current in the cases with Petersen
coil and ungrounded is very similar. This is due that the fault current in the
ungrounded system goes decreasing its value due the capacitances of the line
and in the case of Petersen coil, the inductive current of the coil cancels the
capacitive fault current, so the intensity flows by the fault point is decreased to a
small component resistive.
95
In the other case, when the fault appears on the line 8, the system behaves of
similar form but the values of the fault current change as the short circuit
happens in another part of the wind farm.
The figure 6-81 shows the fault current in the three situations if the solid
grounding has been used.
Figure 6-81 Fault current with solid grounding (short circuit on line 8).
The fault current increases when the sectionalizer 33 is closed, it reaches a
value of 14.93 kA, instead, when both sectionalizers are open the fault current
reaches a value of 12.46 kA. This last value is very similar to the obtained with
the sectionalizer 150 closed, 13.01 kA.
The other three types of grounding have been presented in the next figures
(low-resistance grounding, Petersen coil and ungrounded).
96
Figure 6-82 Fault currents with low-resistance grounding (short circuit on line 8).
Figure 6-83 Fault currents with Petersen coil (short circuit on line 8).
97
Figure 6-84 Fault currents with ungrounded (short circuit on line 8).
The previous figures show that the fault current also increases when the
sectionalizer 33 is closed in every groundings. This is due to the current flows
by the sectionalizer 33.
In the second place a comparison has been realized between the different
grounding types with each position of the sectionalizers.
To begin, the fault currents have been obtained when both sectionalizers are
open and the fault appears on busbar 33A.
Figure 6-85 Faut currents with both sectionalizers open.
98
It is possible see that the fault current with solid grounding is more high than in
the other grounding three types.
With the other two situations, sectionalizer 33 closed or sectionalizer 150
closed, happen the same. The fault current with the solid grounding is more
high.
Figure 6-86 Fault currents with both sectionalizers open.
The graphic 6-86 presents the different fault currents with the sectionalizer 33
closed to the left and with the sectionalizer 150 closed to the right.
When the fault appears in the line 8 and both sectionalizers are open, the fault
current comes back to have the higher value with the solid grounding.
Figure 6-87 Fault currents with both sectionalizers open.
99
In the other two situations, sectionalizer 33 closed or sectionalizer 150 closed,
happen the same like the figure 6-88 shows.
Figure 6-88 Fault currents with sectionalizer 33 closed, left, and with sectionalizer 150 closed,
right.
The next table presents the effective values of the fault current and its peak
values when the fault appears on busbar 33A.
Table 10 Effective values and peak values of the fault current (short circuit on busbar 33A).
Maximum peak
fault current
(kA)
Fault current
(kA)
Both open
24.057
18.07
33 closed
37.128
26.85
150 closed
28.183
19.85
Both open
1.91
0.41
33 closed
3.819
0.81
150 closed
1.966
0.41
Both open
1.511
0.02
33 closed
3.025
0.05
150 closed
1.505
0.02
CONNECTIONS Sectionalizer
Solid grounding
Low-resistance
grounding
Petersen Coil
100
Ungrounded
Both open
1.513
0.04
33 closed
3.030
0.08
150 closed
1.508
0.04
As the table shows, the solid grounding is the worst case as it presents a high
value of fault current.
The next figure presents the effective values and peak values of the fault
current when the fault appears on the line 8.
Table 11 Effective values and peak values of the fault current (short circuit on line 8).
Maximum
peak fault
current (kA)
Fault
current
(kA)
Both open
13.787
10.29
33 closed
16.287
12.13
150 closed
14.407
10.75
Both open
1.576
0.40
33 closed
2.081
0.79
150 closed
1.594
0.40
Both open
1.417
0.03
33 closed
1.599
0.05
150 closed
1.434
0.03
Both open
1.419
0.04
33 closed
1.599
0.08
150 closed
1.434
0.04
CONNECTIONS Sectionalizer
Solid grounding
Low-resistance
grounding
Petersen Coil
Ungrounded
The conclusion is the same that in the previous case. The solid grounding has a
bad behaviour for the system.
101
To summarize, the next graphic shows the tendency of short circuit current and
the peak fault current, for the different situations of sectionalizer.
40
35
30
Short circuit in busbar 33A
Maximum peak fault current
(kA)
25
20
15
Short circuit in busbar 33A
Fault current (kA)
10
5
Solid
grounding
150 closed
33 closed
Both open
150 closed
33 closed
Both open
150 closed
33 closed
Both open
150 closed
33 closed
Both open
0
Short circuit in line 8 Maximum
peak fault current (kA)
Short circuit in line 8 Fault
current (kA)
LowPetersen Coil Ungrounded
resistance
grounding
Figure 6-89 Tendency of short circuit current for different situations of
sectionalizers
In last place, it is presented a table where have been compared the currents
and voltages in the neutral of the transformer with the different groundings,
except with the ungrounded as it does not have neutral connected to earth.
The table 16 shows the obtained values in each situation when the fault
appears on busbar 33A.
Table 12 Voltages and currents in the neutral with short circuit on busbar 33A.
Fault in Busbar 33A
Sectionalizers Open
Sectionalizer
Sectionalizer
33 Closed
150 Closed
Solid
Un
3.4336 kV
2.5511 kV
3.7718 kV
Grounding
In
18.0718 kA
13.4270 kA
19.8515 kA
Low
Un
19.3264 kV
19.3284 kV
19.3440 kV
Resistance
In
0.4092kA
0.4043kA
0.4046 kA
Un
19.3720 kV
19.3815 kV
19.3699 kV
In
0.0127kA
0.0127 kA
0.01277 kA
Petersen Coil
102
The results have been compared and it is possible to see that the solid
grounding has the higher current in the neutral, reaching a value of 19.85 kA
when the sectionalizer 150 is closed while the low-resistance grounding
reaches a value of 0.4046 and in the Petersen coil a value of 0.0127 kA.
In the other hand, the solid grounding has the lower voltage with the
sectionalizer 150 closed (the worst case) with a value of 3.77 kV. The lowresistance grounding and the Petersen coil have a value very similar, 19.34 kV
and 19.37 kV respectively.
The table 17 shows the voltages and currents in the neutral of the transformer
when the fault appears in the line 8, between WT7 and WT8.
Table 13 Voltages and currents in the neutral with short circuit on line 8.
Fault in line 8
Sectionalizers Open
Sectionalizer
Sectionalizer
33 Closed
150 Closed
Solid
Un
1.9562 kV
1.1522 kV
2.0422 kV
Grounding
In
10.2960 kA
6.0641 kA
10.7485 kA
Low
Un
19.1723 kV
18.8581 kV
19.188 kV
Resistance
In
0.4010 kA
0.3944 kA
0.4013kA
Petersen Coil
Un
19.5279 kV
19.5533 kV
19.5260 kV
In
0.0128kA
0.0128 kA
0.0128 kA
It is possible to see that the solid grounding has the higher current and the
lower voltage with regard to the other two grounding types. The same happens
when the fault appears on busbar 33A.
103
7 ANALYSIS AND DISCUSSION
The analyzed model is not the real system, due to the limitation of nodes in the
DIgSILENT software. The following points show an estimate of the short circuit
currents for the real system which has four strings with 8 wind turbines in each
busbar 33.
The studied model has only one string with 8 wind turbines in each busbar 33
as it is possible to see in the figure 3-1. The next estimate has been carried out
in the case of four strings when the fault happens on busbar 33A.
kA
Short Circuit current on Busbar 33A
30
25
20
15
10
5
0
Nº Strings in busbar
Short Circuit current on
Busbar 33A (kA)
1
2
3
4
16,56
19,9
23,24
26,58
Figure 7-1 Estimate of short circuit current for different number of strings
The figure 7-1 shows the increment of short circuit current in the busbar 33.
With each string the short circuit current increases 3.34 kA reaching a value of
26.58 kA for four strings.
The figure 7-2 presents the estimate of short circuit current in the transformer. It
is possible to see that the increase is of 0.35 kA with each string. When the
busbar has four strings the short circuit current is 14.62 kA. This could influence
in the design of ground grid realized in the chapter 4.
The touch voltage and step voltage have been obtained for this case and the
results are:
Vstep=247.26 V
Vtouch=272.45 V
104
These values are lower than the reference values Vstep=392.14 V and
Vtouch=354.58 V.
kA
Short Circuit current in transformer 160 MVA-A
16
14
12
10
8
6
4
2
0
Nº Strings in busbar
Short Circuit current in
transformer 160 MVA-A (kA)
1
2
3
4
13,57
13,92
14,27
14,62
Figure 7-2 Estimate of short circuit current for different number of strings
The touch voltage and step voltage have been checked for the different
grounding in the WT1, when the short circuit happens on busbar 33A.
Figure 7-3 Step voltage in WT1 with different groundings
105
Figure 7-4 Touch voltage in WT1 with different groundings
The figures 7-3 and 7-4 show that the touch voltage and step voltage do not
reach the reference touch voltage and step voltage. The short circuit currents
used to calculate these voltages are presented in the annex C.
When the fault happens on line 8 short circuit current is lower than the short
circuit current in the busbar 33A, so the touch voltage and step voltage also are
below of reference touch voltage and step voltage.
Figure 7-5 Step voltage in WT1 with different groundings
106
Figure 7-6 Touch voltage in WT1 with different groundings
The previous cases have been analyzed for one string in each busbar 33. An
estimate will be done for the case of four strings below . It is possible to see that
the increase is of 0.0016 kA with each string, these current was calculated
when the short circuit happens on busbar 33A. The obtained currents are lower
than the short circuit currents on busbar 33WT1, which is the worst case, as it
was calculated in the chapter 4. Therefore, the touch voltage and step voltage
are lower than the reference touch voltage and step voltage.
Short Circuit current in transformer of WT1
0,397
0,395
kA
0,393
0,391
0,389
0,387
Nº Strings in busbar
Short Circuit current in
transformer of WT1 (kA)
1
2
3
4
0,3901
0,3917
0,3933
0,3949
Figure 7-7 Estimate of short circuit current for different number of
strings
107
8 CONCLUSSIONS
As it has been checked in the previous chapter 6, there are two extreme cases
of grounding.
•
Solid grounding system presents a fault current very high with regard to
the others grounding.
•
Ungrounded system has a short circuit to ground current of capacitive
nature, which can produce a re-ignition of single-phase to ground fault.
The figures 6-27 (when the fault appears on busbar 33A and the
sectionalizer 33 is closed) and 6-63 (when the fault appears on line 8 and
the sectionalizer 33 is closed) show that in this grounding type an
overvoltage appears after the fault is cleared. This behaviour can be
harmful for the system.
For these reasons, these two groundings are not selected for the issue of this
project.
In the other hand, Petersen coil and low-resistance grounding have been
analyzed.
Petersen coil presents a good behaviour in every situations that have been
analyzed. This grounding also presents a low fault current and does not
produce overvoltages. But this method has some disadvantages.
•
Risk of Ferro-resonance.
•
More complex.
•
More expensive.
These disadvantages make that it may not be recommendable for the wind
farm.
Finally, low-resistance grounding also presents a good behaviour against the
single-line to ground short circuit. Its maximum fault current (with sectionalizer
33 closed) is of 3.819 kA when the fault appears on busbar 33A (the worst
case) and not presents important overvoltages.
108
To summarize, the best compromise for reducing earth fault currents and
transient overvoltages has been obtained with low-resistance grounding.
109
9 FUTURE WORK
Some additional tasks could be performed:
•
The short circuit analysis could be studied for three phases short circuit
or two phase short circuit.
•
Analyze the short circuits in wind farm with ring topology.
•
Study of different turbine types are also interesting (SCIG,...).
•
Economical study about the different groundings.
110
10 REFERENCES
[1]
Justin Wilkes and Jacopo Moccia, “Wind in power 2009 European
stadistics”,
February
2010
[Online].
Available:
http://www.ewea.org/fileadmin/ewea_documents/documents/statistics/10
0401_General_Stats_2009.pdf. Accessed : May 15, 2010.
[2]
Donald W.Zipse and Gene Strycula, “System grounding”, IEEE Std. 1422007. Grounding of industrial and Commercial power systems. 2007.
[3]
Douglas C. Dawson, “IEEE Guide for the Application of Neutral
Grounding in Electrical Utility Systems-Part I:Introduction” in IEEE Std
C62.92.1-2000. Guide for the application of neutral grounding in
electrical utility systems.2000.
[4]
John E. May, John P. Riganati and Sava I. Sherr, IEEE Std 80-1986
Guide for safety in AC Substation Grounding. 1985
[5]
E.G. “AL” Kiener, Donal C. Loughry and Andrew G. Salem, IEEE Std.
665-1995.Guide for Generating Station Grounding.1995.
[6]
A.P.Sakis Meliopoulos, Power system Grounding and Transitients, USA:
Marcel Dekker,1988.
[7]
B.M. Weedy and B.J. Cory,Electric Power Systems, Fourth edition,UK,
Wiley.1998.
[8]
DIgSILENT PowerFactory, DIgSILENT PowerFactory Version 13.1,
2004.
[9]
B. de Metz-Nublat, F.Duma and C. Poulain, “Cahier technique nº 158”,
Calculation of Short Circuit Currents.2005.
111
112
A. Short Circuit Current from DIgSILENT
The next tables have been obtained from DIgSILENT PowerFactory, and it is
showed the different short circuit (three phase short circuit and single phase to
ground short circuit) in high voltage side in the busbar 150A and low voltage
side in the busbar 33A with different situations of the sectionalizers.
High Voltage Side:
I
II
Low Voltage Side:
III
IV
B. Equivalent Circuit of the wind farm
The equivalent circuit of the wind farm is presented in the figure:
Figure 10-1 Equivalent circuit of wind farm.
The values determination of the different impedances are shown as follows the
equations and explanations from are used for computing [9].
•
External grid
For obtaining this impedance, the follow formula is used.
pe =
oe F
0e&&
(01)
Where oe is the rated voltage of the connection point, 0e&& is the initial short
circuit apparent power of the external grid.
Substituting the available values:
Like as showed from [34],
st
ut
150F
pe =
= 22.5 Ω
1000
≈ 0.1 at 150 KV and:
(02)
V
ae
2e F
= v1 − w x
pe
pe
(03)
ae
= 0.995
pe
(04)
ae ≈ pe
(05)
If the values are substituted
Then, it is possible to suppose that:
The obtained impedance will be:
pe = 2e + ae = 2.25 + 22.5y Ω
•
(06)
The line impedances of high voltage
The impedance value depend on the cable type used. In this case, the
impedance is obtained from the DIgSILENT software where a cable NEXANS
400 mm2 is used. The impedance is:
pdR = pdF = 9 + 15y Ω
•
(07)
The transformer impedance 150/33 KV
To calculate the transformer impedance of high voltage side the following
equation[9] is used:
pbR =
Where
onb o F
·
100 0D
(08)
onb is the short circuit voltage in percentage value of the primary
winding, U is the phase to phase voltage and 0D is the apparent power of the
transformer. The values of these parameters are:
onb = 12%
o = 33 +
0D = 160 |+8
Substituting the values of the transformer:
pbR = 0.816 Ω
VI
From [34], the resistance and reactance of the transformer are:
pbR ≈ abR
2bR = 0.2abR
(09)
(10)
Substituting the values, the value of transformer impedance is obtained:
•
pbR = 0.163 + 0.816y Ω
(11)
The line Impedances of the low voltage side
The obtained line impedance is between busbar 33 A or 33 B and the first wind
turbine, figure 8-1.
This impedance depends on the used cable, in this case JDR 500 mm2 and the
length of the cable. The impedance is obtained from DIgSILENT software.
pdg = 0.486 + 0.393y Ω
In second place the impedance between wind generators is obtained. The cable
is the same but the length is different.
pdS = 0.339 + 0.275y Ω
•
Transformer impedance of three-windings 33/0.69/3.3 KV
The calculation is the same as the transformer of the high voltage side. The
same equation is used.
pbF =
The values in this case are:
onb = 5.36 %
onb o F
·
100 0D
(12)
o = 33 +
0D = 5.6 |+8
Substituting the values in the equation, the transformer impedance is:
pbF = 10.42 Ω
From [9], the resistance and reactance of transformer are obtained:
VII
abF ≈ pbF
2bF = 0.2abF
(13)
(14)
The transformer impedance is:
pbF = 2.08 + 10.42y Ω
•
The doubly-fed induction generator impedance
To obtain the doubly-fed induction generator impedance, the following
expression [9] is used:
p}q =
1
o~
1
·
=
,ds
√3 ,~ ,dsQ
Q,
,~
~
o~ F
·
0~
(15)
Where ,~ , o~ and 0~ are respectively rated current, voltage and apparent
power of the generator, and ,ds is the locked-rotor current.
These parameters are obtained from the DIgSILENT software and the values
,ds
Q, = 7
~
are:
o~ = 3.3 +
0~ = 5891.47 +8
Substituting the values into the equation, the impedance is obtained:
Z€ = 0.264 Ω
That the relation between the resistance and reactance is:
2}q
= 0.43
a}q
(16)
And like the reactance may be comparable to impedance, a}q ≈ p}q , the
doubly-fed induction generator impedance will be:
p}q = 0.1135 + 0.264y Ω
VIII
C. Tables of Short Circuit Currents on busbar 33A and
on line 8
The step touch voltage are shown in the next figure for the different short circuit
currents, when the fault happens in the busbar 33A and on line 8.
Short Circuit on
Busbar 33A
SOLID GROUNDING
LOW RESISTANCE
PETERSENCOIL
UNGROUNDED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
Fault Current
(kA)
Vs (V)
Vt (V)
0,3901
107,49
171,49
0,3924
108,154
172,5
0,3921
108,072
172,37
0,0916
25,247
40,268
0,0951
26,211
41,806
0,0908
25,02
39,91
0,0827
22,794
36,355
0,0827
22,794
36,355
0,0827
22,794
36,355
0,0828
22,82
36,399
0,0828
22,82
36,399
0,0827
22,794
36,355
IX
Short Circuit on line 8
SOLID GROUNDING
LOW RESISTANCE
PETERSENCOIL
UNGROUNDED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
BOTH SECTONALIZERS
OPEN
SECTIONALIZER 33
CLOSED
SECTIONALIZER 150
CLOSED
Fault Current
(kA)
0,2654
Vs (V)
Vt (V)
73,15
116,672
0,2378
65,543
104,538
0,2599
71,6345
114,25
0,0916
25,2471
40,268
0,0954
26,294
41,938
0,0909
25,054
39,96
0,0827
22,794
36,355
0,0827
22,794
36,355
0,0827
22,794
36,355
0,0828
22,82
36,3995
0,0828
22,82
36,3995
0,0827
22,794
36,355
X