Grid Integration of Wind Energy in the Western Cape
Final Report
December 2009
Grid Integration of Wind Energy in the Western Cape
1
Commissioned by:
Deutsche Gesellschaft für Technische
Zusammenarbeit (GTZ) GmbH
Division Water, Energy, Transport
TERNA Wind Energy Programme
Dag-Hammarskjöld-Weg 1-5
D-65760 Eschborn
Contact:
Daniel Werner (daniel.werner@gtz.de)
Department of Environmental Affairs and
Development Planning (D:EA&DP)
Contact:
Dipolelo Elford (delford@pgwc.gov.za)
Birgit Moiloa (bandrich@pgwc.gov.za )
Eskom
Contact:
Kevin Leask (LeaskK@eskom.co.za)
Riaan Smit (Riaan.Smit@eskom.co.za)
Consultant:
DIgSILENT GmbH
Heinrich-Hertz-Str. 9
72810 Gomaringen
Germany
Web:http://www.digsilent.de
Contact:
Markus Pöller (mpoeller@digsilent.de)
Grid Integration of Wind Energy in the Western Cape
2
Table of Contents
1 Introduction ......................................................................................................................................... 5
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg ............................................................. 6
2.1 Grid Configuration ..................................................................................................................................6
2.2 Scope of Studies ....................................................................................................................................7
2.3 Considered Wind Farm Model ..................................................................................................................7
2.4 Analyzed Cases ......................................................................................................................................7
2.5 Impact on Thermal Limits in the Surrounding Subtransmssion Network.......................................................8
2.5.1 Probabilistic Assessment of the Non Delivered Energy in the Case of Wind Farm Limitation ......................9
2.5.2 Limit the Wind Farm Output in the Case of an Actual Line Failure ........................................................ 11
2.5.3 Dynamic Line Rating Systems ........................................................................................................... 11
2.6 Impact on Voltage Variations at the Connection Point and the Surrounding Subtransmission Network.......... 12
2.6.1 Reactive Power Control Modes.......................................................................................................... 12
2.6.2 Impact Studies................................................................................................................................ 13
2.7 Impact on Short Circuit Currents ........................................................................................................... 15
2.7.1 Special Modeling Considerations ....................................................................................................... 16
2.7.1.1 Short Circuit Currents of WTGs .................................................................................................... 16
2.7.1.2 Equivalent Synchronous Machine Model........................................................................................ 17
2.7.1.3 Model Aggregation...................................................................................................................... 18
2.8 Impact on Harmonics ........................................................................................................................... 19
2.8.1 Harmonic Current Injection Studies of Wind Farms ............................................................................. 19
2.8.1.1 Superposition ............................................................................................................................. 19
2.8.1.2 Wind Farms with Short Connection to the Grid .............................................................................. 20
2.8.1.3 Wind Farms with Long Cable Connection to the Grid ...................................................................... 20
2.8.1.4 Special Modelling Considerations.................................................................................................. 20
2.8.1.5 In-the-Field Measurements.......................................................................................................... 21
2.8.2 Results of Studies for the 150MW Wind Farm at Laingsburg ................................................................ 21
2.9 Impact on Flicker ................................................................................................................................. 21
2.9.1 General Methodology ....................................................................................................................... 21
2.9.2 Results of Studies............................................................................................................................ 22
2.10 Stage 1 – Summary ............................................................................................................................ 22
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid. ........................ 24
3.1 Scenario Definition ............................................................................................................................... 25
3.1.1 Base Case Scenarios ........................................................................................................................ 25
Grid Integration of Wind Energy in the Western Cape
3
3.1.2 High Wind Scenarios........................................................................................................................ 26
3.2 Series Compensation at Komsberg 400 kV .............................................................................................. 27
3.2.1 Modified Series Compensation at Komsberg 400 kV ............................................................................ 27
3.3 Impact on Voltage Variations at the Wind Farm Connection Point ............................................................. 28
3.4 Results of Contingency Analysis for Stage 2 ............................................................................................ 29
3.5 Stage 2 - Summary .............................................................................................................................. 31
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape.............. 32
4.1 Background ......................................................................................................................................... 32
4.2 Scenario Definition ............................................................................................................................... 34
4.2.1 Base Case Scenarios ........................................................................................................................ 34
4.2.2 High Wind Scenarios........................................................................................................................ 34
4.3 Results of Load Flow Studies ................................................................................................................. 36
4.4 Additional Considerations ...................................................................................................................... 36
4.4.1 Balancing of Wind Variations ............................................................................................................ 37
4.4.2 Reactive Power/Voltage Control ........................................................................................................ 37
4.4.3 System Stability .............................................................................................................................. 38
4.5 Stage 3: Summary ............................................................................................................................... 38
5 Conclusions and Recommendations.................................................................................................. 39
6 Referenced Documents...................................................................................................................... 41
List of Annexes...................................................................................................................................... 42
Grid Integration of Wind Energy in the Western Cape
4
1 Introduction
1 Introduction
The Western Cape is one of several South African regions with a particularly high wind regime. For this reason, a
large number of project developers and other investors from inside and outside South Africa have the intention to
develop these resources and to build and operate grid connected wind farms.
Besides the regulatory and legal framework, the electrical grid could represent a potential constraint with regard
to the extend up to which wind generation could potentially be exploited in the Western Cape.
For this reason, GTZ asked DIgSILENT in cooperation with the Western Cape Department of Environmental
Affairs and Development Planning (D: EA&DP) and ESKOM to carry out grid studies with the purpose of:
•
Demonstrating study approaches for the connection of wind farms to the subtransmission and
transmission grid.
•
Carry out a feasibility study for the integration of up to 2800MW of wind generation in the Western
Cape.
For this purpose, the following three stages have been defined for this project:
•
Stage 1: Connection of a 150MW wind farm at Laingsburg to 132kV grid (see chapter 2)
•
Stage 2: Connection of 750MW of wind farms in the Karoo area to the 400kV grid (see chapter 3)
•
Stage 3: High level feasibility studies for the integration of up to 2800MW of wind generation into the
Western Cape (see chapter 4).
This report provides a comprehensive overview about approaches and results of the studies that have been
carried out.
The studies are part of a joint project carried out by the Department of Environmental Affairs and Development
Planning (D:EA&DP), Eskom and the TERNA Wind Energy Programme of the German Technical Cooperation
(GTZ). TERNA is funded by the German Federal Ministry of Economic Cooperation and Development (BMZ).
Grid Integration of Wind Energy in the Western Cape
5
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
2 Stage 1:
1: Connection
Connection of a 150 MW Wind Farm at Laingsburg
For demonstrating relevant approaches for studying the grid connection of individual wind farms at
subtransmission levels (132kV), the example of a 150MW wind farm that shall be connected at the Laingsburg
132kV substation is described in the following sections.
2.1 Grid Configuration
The projected connection point of the 150 MW wind farm is stated to be approximately at 5.4 km from the
existing 132 kV transmission lines Laingsburg - Boskloof [1]. From this connection point one double circuit Wolf
line of 4 km length were considered to connect the 150 MW wind farm.
Figure 1 shows a schematic diagram of the wind farm connection near to Laingsburg. As shown in this diagram,
both parallel paths of the existing double circuit line have been opened and connected to a new substation at the
132kV wind farm connection point.
GEELB1_2
lod_70892..
lod_70892_1
RUITK1_2
KOUP1_2
GEMSB1_2
ANTJK1_2
2181WOLF
0,51
BAVIN1_2
ANTJIESK
2181WOLF
3,87
WHITH1_2
2181WOLF
2,08
BANTM2_2
2181WOLF
2,63
PIETM1_2
2181WOLF
2,07
QUARY1_2
KOUP
GEELBEK
BAVIANSK
BANTAM
BOSKLOOF 2
lod_70962_1
lod_72802_1
lod_72882_1
The cut-in of the wind farm into one circuit only has initially been considered too but even a very quick analysis
has shown that this would not be a feasible option for the connection of such a large wind farm. For this reason,
this option has not further been followed up on.
BOTES_2
WELTV1_2
2181WOLF
74,80
LEEUG1_2
2181WOLF
11,41
2181WOLF
11,17
Continued on
Bacchus 132kV diagram
2181WOLF
9,96
2181WOLF
12,93
2181WOLF
13,16
2181WOLF
14,20
2181WOLF
9,64
2181WOLF
24,09
2181WOLF
21,81
lod_70682_1
lod_72852_1
lod_72932_1
lod_70922_1
lod_70842_1
lod_70762_1
2181WOLF
12,41
lod_72962_1
2181WOLF
74,80
PIETM1_1
BANTM1_1
WHITH1_1
LEEUG1_1
2181WOLF
11,05
2181WOLF
12,93
BAVIN1_1
lod_70732_1
2181WOLF
21,81
2181WOLF
9,96
GEELB1_1
RUITK1_1
2181WOLF
13,16
2181WOLF
9,64
GEMSB1_1
KOUP1_1
2181WOLF
14,20
2181WOLF
24,09
BOTES_1
ANTJK1_1
WELTV1_1
To DROERIVIER
2181WOLF
11,45
QUARY1_1
2181WOLF
11,17
2181WOLF
12,40
2181WOLF
11,41
2181WOLF
22,49
BOSKLOOF 1
2181WOLF
11,05
trf_70682..
2181WOLF
22,49
2181WOLF
11,45
LEEUG22
LAIN132B1
lod_70686_1
LAIN132B2
LAINGSBURG
2161WOLF
WP51WOLF
14,45
14,45
LAIN WF Tr 1
LAIN132 WF
WP51WOLF
6,00
2161WOLF
6,12
M1311CH
28,11
LAIN33 WF
BUFPT1
LADIS13
trf_74002..
SWART1
trf_73002..
LAIN WF Tr 2
150 MW
Wind Park
2181WOLF
9,40
2181WOLF
9,40
LAIN0.69 WF
Ladismith
SWART22
BUFPT22
Voltage Levels
Laingsburg WF
400, kV
220, kV
132, kV
66, kV
33, kV
lod_73006_1
lod_74006_1
Figure 1: Wind farm connection near to the Laingsburg 132kV substation
Grid Integration of Wind Energy in the Western Cape
6
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
2.2 Scope of Studies
For analyzing the grid impact of a wind farm connection at 132kV, the following main aspects have to be studied:
•
Impact on thermal limits in the surrounding subtransmssion network
•
Impact on voltage variations at the connection point and the surrounding subtransmssion system:
•
Impact on short circuit levels
•
Impact on Power Quality aspects (Harmonics/Flicker, IEC 61400-21)
Each of these aspects requires different types of studies and modelling approaches, which are described in the
following sections.
2.3 Considered Wind Farm Model
Model
For studying the impact on thermal limits in the surrounding subtransmission network and for analyzing the
impact on voltage variations at the grid connection point, an aggregated model of the wind farm is typically used,
as shown in Figure 1. The wind farm is represented by the main step-up transformer, connecting the wind farm
to the 132kV grid connection point, a number of n parallel step-up transformers, stepping the LV-side of the wind
turbine generators (WTGs) up to the medium voltage level of the wind farm internal collector system and a
number of n parallel generators. The impedance of the wind farm internal cables can be neglected when just
looking at the impact on the surrounding 132kV network.
In the case of this particular example, it was assumed that the wind farm would consist of 75 WTGs having a
rating of 2MW each. However, for this type of grid impact study, the actual number and size of the individual
turbines is irrelevant as long as the total wind farm size at the grid connection point is correct.
2.4 Analyzed Cases
The studies described in the next section have been performed based on a model of the Western Cape network,
which includes the complete Western Cape transmission network and all 132kV subtransmission networks. The
basis of these studies is the “study year 2009”. The following operational scenarios were considered:
•
High Load in the Western Region Area
•
Low Load in the Western Region Area
Grid Integration of Wind Energy in the Western Cape
7
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
Table 1: List of analyzed cases
Name
High Load
Study Case description
Base Case, High Load
Low Load
HL+150 MW Wind Farm Laingsburg
LL+150 MW Wind Farm Laingsburg
Base Case, Low Load
High Load considering the
connection of a 150 MW wind farm
at Laingsburg 132 kV.
Low Load considering the
connection of a 150 MW wind farm
at Laingsburg 132 kV.
Project
0Western Region Master
Project_Stage 1 and Laingsburg
Wind Study_Stage 1
Wind speed diversity within the wind farm is initially not considered and all studies are based on the assumption
that up to the registered capacity of 150MW will potentially be generated at the grid connection point. Only when
encountering problems, such as overloaded lines under n-1 conditions, a more detailed assessment considering
wind speed diversity will be carried out.
This approach is generally recommended because the registered capacity is the only reliable information that is
typically available at this stage of the wind farm planning process and any other assumption would lead to results
without any margin. Only when encountering problems, it is advisable to look more into detail for avoiding
uneconomic results.
2.5 Impact on Thermal Limits in the Surrounding Subtransmssion Network
Network
In a first step, it is required to verify that the existing network capacity is able to take the additionally generated
power. For this purpose, credible contingencies, such as n-1 branch outages, have to be analyzed.
An n-1 analysis based on load flow calculations of all surrounding lines leads to the conclusion that there is a
thermal overload problem in the case that one circuit between Laingsburg and Boskloof is on outage and the
wind farm production is above 120MW.
For all other contingencies, no thermal problems can be identified.
The worst case situation with a wind farm production of 150MW and the outage of one of the parallel circuits is
depicted in Figure 2. As shown in this diagram, the circuit that will remain in service will carry a load of 127% of
its thermal rating.
The general options for mitigating such problems are:
•
Reinforce the existing line or build a new line.
•
Limit the wind farm during all times.
•
Limit wind farm output in case of actual line failure (manual or automatic inter-trip).
•
Consider dynamic line rating systems.
Grid Integration of Wind Energy in the Western Cape
8
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
Generally, the most economic solution under the constraint that a secure network operation is ensured should be
taken. However, the costs associated with each of the four options depend on
•
Length of the line
•
Number of hours per year during which the overload situation occurs (and during which the wind farm
output would have to be limited)
For this reason, only a probabilistic assessment considering the actual wind speed conditions at the site can
provide an answer to the question, which mitigation option should finally be applied.
GEELB1_2
RUITK1_2
KOUP1_2
GEMSB1_2
ANTJK1_2
2181WOLF
74,80 km
0,00 %
2181WOLF
22,49 km
0,00 %
2181WOLF
11,41 km
0,00 %
2181WOLF
11,17 km
0,00 %
Continued on
Bacchus 132kV diagram
2181WOLF
9,96 km
0,00 %
2181WOLF
12,93 km
20,17 %
2181WOLF
13,16 km
18,27 %
BOTES_2
2181WOLF
14,20 km
17,12 %
2181WOLF
9,64 km
17,76 %
2181WOLF
24,09 km
16,59 %
lod_72852_1
lod_72932_1
lod_70922_1
lod_70842_1
2181WOLF
12,41 km
0,00 %
lod_72962_1
2181WOLF
74,80 km
110,86 %
2181WOLF
11,41 km
112,44 %
2181WOLF
22,49 km
111,37 %
PIETM1_1
2181WOLF
11,17 km
115,48 %
BANTM1_1
WHITH1_1
2181WOLF
9,96 km
115,69 %
2181WOLF
11,05 km
20,68 %
2181WOLF
12,93 km
22,62 %
BAVIN1_1
GEELB1_1
RUITK1_1
2181WOLF
13,16 km
20,32 %
2181WOLF
9,64 km
17,88 %
GEMSB1_1
KOUP1_1
2181WOLF
14,20 km
17,53 %
To DROERIVIER
2181WOLF
11,45 km
22,97 %
Loading <=127 %
2181WOLF
24,09 km
16,33 %
BOTES_1
ANTJK1_1
2181WOLF
11,45 km
20,83 %
QUARY1_1
2181WOLF
12,40 km
120,99 %
BOSKLOOF 1
2181WOLF
11,05 km
19,67 %
2181WOLF
0,51 km
0,39 %
BAVIN1_2
2181WOLF
3,87 km
1,37 %
BOSKLOOF 2
WHITH1_2
2181WOLF
2,08 km
0,95 %
BANTM2_2
2181WOLF
2,63 km
6,05 %
PIETM1_2
2181WOLF
2,07 km
3,36 %
off
QUARY1_2
LAIN132B1
LAIN132B2
LAINGSBURG
2161WOLF
WP51WOLF
14,45
14,45km
km
15,97
15,72%
%
180,00 MVA
86,01 %
LAIN132 W F
WP51WOLF
6,00 km
14,48 %
2161WOLF
6,12 km
14,59 %
M1311CH
28,11 km
7,06 %
LAIN33 WF
BUFPT1
LADIS13
10,00 MVA
60,68 %
10,00 MVA
14,00 %
SWART1
2,50 MVA
81,36 %
150 MW
Wind Park
2181WOLF
9,40 km
31,04 %
2181WOLF
9,40 km
31,04 %
LAIN0.69 WF
Ladismith
SWART22
BUFPT22
Laingsburg WF
lod_73006_1
lod_74006_1
Figure 2: Outage of one of the parallel 132kV lines between Laingsburg and Boskloof
2.5.1 Probabilistic Assessment of the Non Delivered Energy in the Case of Wind Farm Limitation
For analyzing the second mitigation option; that is, limiting the wind farm output to 120 MW during all times (80
% of rated output), the probability that the actually generated power exceeds a level of 80% of installed capacity
has to be assessed. For this assessment, the following information is required in addition to a load flow model of
the zone of interest:
Grid Integration of Wind Energy in the Western Cape
9
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
•
Equivalent power curve of the wind farm
•
Average wind speed and, if available, the Weibull-shape factor of the wind speed distribution.
In the case that no more detailed information is available, which is usually the case in an early planning stage; a
typical power curve of a wind generator can be used as equivalent wind farm power curve. Effects, like wind-farm
internal losses etc. can be considered in the equivalent wind farm power curve.
Based on average wind speed and shape factor, the distribution of wind-speeds can be calculated, based on a
Weibull distribution. Together with the equivalent wind farm power curve, a wind generation duration curve for
one wind farm can be calculated showing the probability for the case that the generated wind power exceeds a
given level (see Figure 3).
The energy that cannot be delivered to the system can then be evaluated by calculating the area between that
part of the red curve that is above the blue line indicating the wind farm limit and the blue line itself. For a
monetary analysis, this area has to be multiplied by the assumed tariff.
In case of the wind farm at Laingsburg, there were no wind speed data available. For this reason, the
corresponding assessment has been carried out for an assumed average wind speed of 7 m/s and 8 m/s. this
assessment will provide a good idea about the order of magnitude of the monetary losses associated with a wind
farm limitation and at the same time show the sensitivity of this value against average wind speed.
Capping a single wind farm with 7m/s average wind speed at 80% results in a not delivered energy of
approximately 5% of the totally available energy. This would be equivalent to around 19 000 MWh/year, which
could not be delivered. Assuming a tariff of 1,25 Rand/kWh, the resulting economic losses to the wind farm
operators would be in the order of magnitude of 23 750 000 R/year.
If instead 8m/s average wind speed is assumed an considering also an 80% power output limit, the results
indicate a not delivered energy of approximately 7,5% of the totally available energy. This would be equivalent to
around 37 000 MWh/year, which could not be delivered, resulting in 46 250 000 R/year of losses.
Grid Integration of Wind Energy in the Western Cape
10
DIgSILENT
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
12,50
10,00
7,50
5,00
2,50
0,000
-2,50
0,500 1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500 9,500 10,50 11,50 12,50 13,50 14,50 15,50 16,50 17,50 18,50 19,50 20,50
x-Axis:
Windpark Analysis: Wind Speed in m/s
Windpark Analysis: Probability in %
160,00
Y =120,000 MW
16.624 %
120,00
80,00
40,00
0,00
-40,00
0,03
x-Axis:
DIGSILENT
20,03
Windpark Analysis: Cummulative Probability in %
Windpark Analysis: Generated Power in MW
40,03
60,03
80,03
High Load
Voltage at Laingsburg Wind Farm Connection Point
100,03
Plots
PV-Curve
Date: 7/23/2009
Annex: 1 /3
Figure 3: Wind speed distribution and power duration curve for an average wind speed of 7 m/s
2.5.2 Limit the Wind Farm Output in the Case of an Actual Line Failure
Alternatively, if network operation guidelines allow so, it would be much more economical to limit the wind farm
output only under situations, in which one of the circuits between Laingsburg and Boskloof is actually out of
service, e.g. because of a planned or an unplanned outage.
In cases, in which there is only a minor overload on the remaining circuit, like in the situation here, it would be
possible to limit the wind farm output by a manual post-fault action executed by the network operator.
Alternatively, or in situations in which there is a major overload (above the emergency rating of the line) it is
advisable to install an automatic intertrip-schemes that trips some of the wind turbines in the wind farm in the
case that the circuit breakers of one of the two parallel circuits are opened.
2.5.3 Dynamic Line Rating Systems
The rating of overhead lines depends on:
•
Ambient temperature
Grid Integration of Wind Energy in the Western Cape
11
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
•
Wind speed and wind direction
In case of cool weather and/or under high wind speeds, overhead lines experience a much better cooling and
hence have a much higher rating than under high temperature and/or low wind speed conditions.
Because of the positive correlation between line rating and wind farm output, the installation of a dynamic line
rating system at the critical double circuit line can be a very useful option for avoiding grid congestions.
Dynamic line rating systems are either based on:
•
Measurement of the conductor temperature
•
Measurement of the line sag
•
Measurement of ambient conditions
or combinations of some of the above listed measurements. Together with a thermal model of the line, it is
possible to predict the actual rating of the line at any moment in time.
In the example of the 150MW wind farm at Laingsburg it would be very likely that with the help of a dynamic line
rating system any limitation of the wind farm output could be avoided because the line rating would automatically
increase with increasing wind speeds and increasing wind farm outputs.
2.6 Impact on Voltage Variations at the Connection
Connection Point and the Surrounding
Subtransmission Network
Network
Besides the impact on thermal limits, load flow and contingency analysis studies allow analyzing the impact of
wind generation on voltage variations during normal operations and during contingencies.
In a wind generation impact study, the required level of reactive power support of wind farms need to be
identified in an early planning stage. Therefore, studies are typically carried out aiming at:
•
Identifying the required var-control mode (voltage/droop/const. power factor)
•
Identifying the required reactive power range
•
Identifying the need of additional reactive compensation devices in the system.
2.6.1 Reactive Power Control Modes
The least cost effective solution for a wind farm is the operation at constant power factor close to unity of each
individual WTG, for which no park-controller is required. This is the classical approach for small wind farms (only
a few turbines) connected to distribution grids (<=33kV)
Grid Integration of Wind Energy in the Western Cape
12
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
In case of wind farms connected to subtransmssion or transmission levels, the system operators usually asks the
wind farm operator to maintain a constant power factor, a constant reactive power setting or even the voltage at
the grid connection point.
Hardware costs of the WTGs depend on the required power factor range (or reactive power range) of each
individual WTG. In case of wind farms connected to a transmission or subtransmission level, the power factor
range required at the grid connection point is not equal to the power factor range of each WTG because the
reactive power need of the step-up transformers and the main grid-transformer has to be covered by the WTGs
too. For this reason, there is usually additional shunt capacitors connected near to the grid connection point for
complying with reactive power requirements of the system operator.
When requiring the operation with unity power factory at the grid connection point, the power factor range of
typical WTGs is usually sufficient for covering the reactive power demand of wind farm internal components.
When asking for a wider power factor range at the grid connection point, e.g. 0,95, WTGs with wider reactive
power range and/or additional shunt capacitors are usually required.
Voltage control requires a feed-back loop of the voltage and continuously varying reactive adjustment of the
WTGs. Depending on the dynamic requirements (speed of operation) and the required reactive power range of
the WTGs, it will be possible to realize voltage control by WTGs and additional shunt capacitors or even additional
dynamic reactive power compensation devices, such as SVC or STATCOM might be required, which can have a
very high cost impact.
Droop control refers to a combined voltage-reactive power control defined by a v-Q characteristic, according to
which reactive power is adjusted. Droop control is typically used for fast acting reactive power compensation
devices, such as STATCOMs or SVCs.
2.6.2 Impact Studies
The purpose of system impact studies should be to identify the required reactive power control mode, the
required reactive power range and dynamic performance requirements. It is important to consider cost aspects,
as described in the previous section for avoiding unnecessary investments.
The feasibility of a wind farm consisting of wind generators that operate with constant power factor (at MV-side
of WTG step-up transformers) can be evaluated by analyzing P-V-curves for normal operating conditions and for
a set of critical contingencies.
P-V curves can be used for analyzing voltage variations resulting from:
•
Varying wind speed (varying active power generation)
•
Contingencies
The P-V curves depicted in Figure 4 apply to normal operating conditions (red curve) and three contingencies
(line outages) in the neighborhood of the planned wind farm for the operation with a constant power factor of
cos(phi)=1 at the MV-side of each WTG step-up transformer.
Grid Integration of Wind Energy in the Western Cape
13
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
According to the results shown in Figure 4, the maximum voltage at the wind farm connection point would
exceed the usual +5% limit and the voltage variation against active power variations would be in the range of
approximately 5%. Hence operation with unity power factory would lead to unacceptable voltage variations.
It can further be observed from Figure 4 that voltage step changes because of line outages in the neighborhood
of the wind farm are within very reasonable limits (around 1%) and don’t cause any problem.
The typical approach for mitigating voltage problems, which are due to varying active power levels, is to operate
the wind farm with an active power dependent power factor (absorbing vars). This is a very cost effective
solution because such a reactive power or power factor characteristic can easily be implemented inside the
controller of each individual WTG and doesn’t require any voltage feed-back or global wind farm controller.
1,08
DIgSILENT
The resulting PV-curve is depicted in Figure 5. These curves are based on a wind farm with a minimum power
factor of 0,95 (absorbing) at the wind farm connection point. The voltage now stays within the required voltage
band and voltage variations against active power variations stay within acceptable limits of less than 2%.
X =150,000 MW
90.000
142.500 MW
1.070 p.u.
1.070 p.u.
1.065 p.u.
1,05
66.893 MW
59.690 MW
58.381 MW
57.418 MW
1.050 p.u.
Y = 1,050 p.u.
1,02
0,99
0,96
0,93
7,50
x-Axis:
47,50
87,50
Laingsburg WF: Active Power in MW
LAIN132 WF: Voltage in p.u. - Base Case
LAIN132 WF: Voltage in p.u. - Lain132kV_Laingsburg_Off
LAIN132 WF: Voltage in p.u. - Laingsburg_Boskloof_Off
LAIN132 WF: Voltage in p.u. - Laingsburg_Droerivier_Off
127,50
167,50
207,50
Figure 4: PV-curves for wind farm operation with unity power factor at the connection point
Grid Integration of Wind Energy in the Western Cape
14
DIgSILENT
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
X =150,000 MW
1,075
1.050 p.u.
1,050
Y = 1,050 p.u.
1.038 p.u.
1.037 p.u.
1.030 p.u.
1,025
1,000
0,975
0,950
7,50
x-Axis:
47,50
87,50
Laingsburg WF: Active Power in MW
LAIN132 WF: Voltage in p.u. - Base Case
LAIN132 WF: Voltage in p.u. - Lain132kV_Laingsburg_Off
LAIN132 WF: Voltage in p.u. - Laingsburg_Boskloof_Off
LAIN132 WF: Voltage in p.u. - Laingsburg_Droerivier_Off
127,50
167,50
207,50
Figure 5: PV Curves considering a cosphi(P) Characteristic
2.7 Impact on Short Circuit Currents
In the case of a grid impact study, the main purpose of short circuit studies is to verify that the short circuit
rating of existing equipment such as bus bars, circuit breakers, lines or cables remains within the required limits.
Additionally, short circuit studies are required for verifying the settings of line or transformer protection relays in
the network.
Grid Integration of Wind Energy in the Western Cape
15
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
2.7.1 Special Modeling Considerations
DIgSILENT
2.7.1.1 Short Circuit Currents of WTGs
0,30
ip
Fault Cleared
0,20
0,10
0,00
-0,10
-0,20
-0,30
-0,10
0,00
0,10
0,20
[s]
0,30
Tr2: Phase Current A/HV-Side in p.u.
Tr2: Phase Current B/HV-Side in p.u.
Tr2: Phase Current C/HV-Side in p.u.
Figure 6: Short circuit currents of a DFIG (HV-side of step-up transformer)
Modern, variable speed wind generators, which are either based on doubly-fed induction generators or
generators with fully rated converter, are difficult to model for standard, steady state short circuit analysis (e.g
IEC 60909 or ANSI C37) because of the following reasons:
•
If in the time frame of a few milliseconds only, these turbines are influenced by their controllers, making
their behavior nonlinear, even during these short time periods.
•
Doubly-fed induction generators are usually equipped with special protection mechanisms (e.g. crow
bar, Chopper-resistance) that trigger during voltage dips.
The curves depicted in Figure 6 show the currents of a doubly-fed induction generator in case of a short circuit.
The generator of this example is equipped with a crow-bar for protecting the rotor-side converter against high
currents and high DC-voltages.
Grid Integration of Wind Energy in the Western Cape
16
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
During the period, in which the crow-bar is inserted (first 50ms after fault insertion), AC-components of the short
circuit currents are very low because of the field-damping effect of the crow-bar. After the crow-bar has been
removed, the converter controllers support the voltage by increasing reactive currents. Therefore, AC components
of the short circuit current are again increasing but stay in the range of normal operating currents.
This means that doubly-fed induction machine wind generators have a considerable contribution to peak short
circuit currents.
In the longer term however, when the controllers have taken over again, fault currents will be in a range of
normal operating currents, depending on the reactive current control concept of the turbines. Because there are
many Grid Codes around the world that require the injection of reactive currents during situations of low voltages
and because voltage support usually helps the turbines to ride through low voltage conditions, many wind
generators inject reactive currents during situations of low voltages and thereby contribute to AC-components of
short circuit currents.
2.7.1.2 Equivalent Synchronous Machine Model
Because of the mentioned difficulties with the modeling of variable speed wind generators for steady state short
circuit analysis, simplified modeling approaches are required for the inclusion of variable speed wind generators in
IEC 60909 or ANSI C37-type short circuit calculations.
The typical approach, which is used in many places around the world is nowadays based on an “equivalent
synchronous machine” modeling approach, in which variable speed wind generators are approximated by an
equivalent circuit with subtransient and transient reactance:
•
The subtransient reactance is typically defined in a way, that the WTG contribution to peak short circuit
currents is reflected correctly.
•
The equivalent transient reactance typically defines the machine’s behavior during situations, in which
the controllers have taken over again.
Unfortunately, this approach is not able to provide very accurate results for the short circuit contribution of wind
farms. It should only be used for planning studies, when the short circuit rating of existing substations, lines and
transformers has to be verified.
In the case that accurate short circuit currents have to be calculated, e.g. for analyzing malfunctioning of
protection relays, time domain simulations based on accurate dynamic WTG models are required.
Grid Integration of Wind Energy in the Western Cape
17
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
DIgSILENT
2.7.1.3 Model Aggregation
Aggregation
External Grid
HV
Tr1
PCC
Windfarm Trf
Tr2
MV
N x Step-up Trf
WT1
G
~
N x WTG
G1
Figure 7: Aggregated Wind Farm Model for System Impact Studies
In case of grid impact studies, where the impact of a new wind farm on short circuit ratings of existing equipment
is analyzed, an aggregated wind farm model can be used. Typically, wind farms are modeled by one equivalent
generator and one equivalent step-up transformer having n times the size of each individual WTG and step-up
transformer (see Figure 7).
The impedance of wind farm internal cables can be neglected if maximum short circuit currents are in the centre
of interest of the studies.
We can therefore summarize the following:
•
Double Fed Induction Generator (DFIG):
- Considerable contribution to peak short circuit current.
- Contribution to thermal short circuit ratings: approx 1 p.u. shc-current
•
WTG with fully rated converter:
- Contribution to initial short circuit current: approx. 1 p.u. shc-current
Grid Integration of Wind Energy in the Western Cape
18
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
- Contribution to thermal short circuit ratings: approx 1 p.u. shc-current
In the case of a 150MW wind farm at Laingsburg this results in the following short circuit contribution from the
wind farm:
•
Contribution to initial shc-current (Ikss): approx 2 kA (at 132kV)
•
Contribution to peak shc-current (ip): 4,4 kA
•
Contribution to transient shc-current (Iks): 0,67 kA
The short circuit contribution of the wind farm is not critical in this particular example because the fault level at
the wind farm connection point is very low and the substation is operated far below its short circuit rating.
2.8 Impact on Harmonics
2.8.1 Harmonic Current Injection Studies of Wind Farms
Harmonic current injection studies are typically carried out based on wind turbine measurement report according
to IEC 61400-21, which lists the worst-case harmonic current injection at each harmonic frequency at which a
current injection >0,1% of rated current can be observed.
2.8.1.1 Superposition
Because it is very much unlikely, the all wind generators in a wind farm inject worst case harmonic currents with
the same phase angle at the same time, assumptions for the diversity with which harmonic currents are injected
into the grid by the different wind generators can be made.
The generally applied international standard that defines rules for these assumptions is IEC 61000-3-6. The basic
consideration of these rules is that:
•
Low order harmonics are typically resulting from grid commutated converters or saturation effects. Their
phase angle is therefore grid synchronized resulting in high correlation of the injected harmonic
currents.
•
High order harmonics are typically resulting from self commutated power electronics converters. Their
phase angel is therefore highly uncorrelated.
For considering these effects, IEC 61000-3-6 defines formulas allowing calculating the superposition of harmonic
voltage distortions resulting from harmonic current injection of different harmonic sources.
For wind generation, Summation Law 2 according to IEC 61000-3-6 is typically applied.
Summation Law 2 is based on the following formula for each harmonic order h and each harmonic source i:
Grid Integration of Wind Energy in the Western Cape
19
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
Uh = α
∑U α
hi
(1)
i
with:
•
α=1 for h<=4
•
α=1.4 for h<5 and h<=13
•
α=2 for h>13
The practical application of this formula requires the calculation of the harmonic voltage distortion at the
connection point caused by each harmonic source (wind generator) in the system and calculating the resulting
harmonic voltage distortion by evaluating the formula above.
2.8.1.2 Wind Farms with Short Connection to the Grid
For wind farms that are directly connected to the grid with a step-up transformer and without any considerable
HV-AC cables, a simplified version of this formula can be applied, as proposed in IEC 61400-21. According to
IEC61400-21, the harmonic current injections of the entire wind farm can be modelled by one equivalent
harmonic current source at the grid connection point. The harmonic current injection at every harmonic frequency
is calculated using (1) and by replacing voltage by current.
In these cases, harmonic voltage distortion can simply by calculated by taking the harmonic impedance at the
grid connection point with the highest magnitude and multiplying it with resulting harmonic current.
2.8.1.3 Wind Farms with Long Cable Connection to the Grid
Wind farms that are connected to the grid via considerable HV-AC cable (e.g. offshore wind farms) require more
sophisticated harmonic analysis because there is always a high risk for harmonic resonance problems because of
the cable capacitance. The actual location of resonance frequencies may vary with the operating conditions of the
transmission grid. Therefore, harmonic voltage distortion must be calculated for the full range of harmonic
impedances at the PCC, as it has been worked out by the system operator and by applying (1) for considering the
impact of all wind generators in the wind farm on harmonic voltage distortions.
The worst case harmonic voltage distortion is then reported and the basis of filter sizing, if a filter will be
required.
2.8.1.4 Special Modelling Considerations
In some turbine designs, local filters are used, which can have a considerable impact, especially in the case of
low order cable resonance problems in the system.
Measurement reports according to IEC 61400-21 only report magnitude of harmonic currents measured at the
LV-side. A worst case assumption consists of assuming that the entire harmonic contents appear either in positive
or negative sequence system. In case of 3rd, 6th, 9th, etc. harmonics, this definitely represents a worst case
assumption because the largest part of third order harmonic currents typically appears in the zero sequence. In
Grid Integration of Wind Energy in the Western Cape
20
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
the case that the studies show problems at these frequencies, this should be further analyzed, e.g. by looking at
harmonic currents measured at the HV-side of the wind turbine step-up transformers (MV-level).
2.8.1.5 InIn-thethe-Field Measurements
It is common practice to install a power quality meter in the grid connection point that monitors harmonic
voltages at the PCC and harmonic currents injected by the wind farm during wind farm operation.
2.8.2 Results of Studies for the 150MW Wind Farm at Laingsburg
Laingsburg
Because the actual type of wind generator that will be used at Laingsburg is not yet known at this stage of the
connection application, only a rough assessment using a harmonic current spectrum that is typical for modern,
variable speed wind turbines could be carried out.
The result of corresponding harmonic studies led to a total harmonic voltage distortion (THD) of 0,75% at the
132kV connection point and to individual harmonic distortion levels, which are far below any critical values. This
assessment confirms that no harmonic problems would have to be expected when connecting a large wind farm
to the 132kV bus bar at Laingsburg.
2.9 Impact on Flicker
2.9.1 General Methodology
Flicker, being a variation of the fundamental frequency voltage, can be a result of:
•
Continuous active power variations of the wind generators, e.g. resulting from wind turbulence.
•
Switching actions within in the wind farm.
Flicker assessment for wind farms is typically carried out according to IEC 61000-3-7 and IEC 61400-21. The
method requires the calculation of the minimum short circuit level and impedance angle at the PCC.
Based on the impedance angle, the maximum values for:
•
Flicker coefficient c
•
Flicker step-factor kf
•
Flicker step-factor ku
•
Max. number of switching operations N10
•
Max. number of switching operations N120
Grid Integration of Wind Energy in the Western Cape
21
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
for the relevant impedance angle at the grid connection point must be taken from a wind turbine measurement
sheet according to IEC 61400-21.
These values together with the minimum fault level at the grid connection point allow calculating:
•
Short-term and long term flicker value (Pst/Plt) during continuous operation.
•
Short-term and long term flicker value (Pst/Plt) due to switching actions.
for one wind generator.
For assessing the Flicker of an entire wind farm, IEC 61400-21 states that it can be assumed that the flicker
relevant components of wind turbulence and other flicker-relevant aspects are uncorrelated within the wind farm.
For this reason, the following formula can be applied for calculating the flicker of an entire wind farm with n
turbines based on the flicker of one individual WTG:
Pst = Plt = n Pstí
With Psti being the short-term flicker of one individual WTG.
This means in other words, that the total flicker of a wind farm with n WTGs is lower by a factor of
1 / n than
the flicker that would be caused by an individual WTG with n times the size.
2.9.2 Results of Studies
In this particular example, Flicker analysis was made using IEC 61400-21 data sheet of a typical variable-speed
wind generator.
The resulting Flicker levels are equal to:
•
Pst = 0,066
•
Plt=0,08
The significance of the actual results is very limited because the actual results will depend on the number of
turbines, their rating and the actual type of turbines that will be installed. However, because the calculated
numbers are far below any critical values it can be assumed that flicker won’t cause any problem in the case of a
150MW at Laingsburg, assuming that modern variable speed WTGs will be used.
2.10 Stage 1 – Summary
The example of a 150MW wind farm to be connected near to the Laingsburg 132kV substation is used for
demonstrating relevant aspects for the grid connection of wind farms at subtransmission levels.
Grid Integration of Wind Energy in the Western Cape
22
2 Stage 1: Connection of a 150 MW Wind Farm at Laingsburg
In this particular example, there is an issue with regard to thermal overloads of the surrounding overhead lines
under some n-1 situations. It is shown, that a probabilistic assessment of the power production of the planned
wind farm is required for deciding on the most economic mitigation option.
In this particular example, it would be recommended to cap the wind farm output under situations, in which only
one circuit between Laingsburg and Boskloof would be available (planned outage/maintenance or as a post-fault
action in case of a line trip). This could be achieved by an automatic active power limitation in the wind farm
controller or by tripping around 20% of the wind turbine generators (WTGs) under these conditions.
Voltage variations can potentially be an issue when operating the wind farm at a constant power factor of equal
to one at the grid connection point during all times. For keeping wind-dependent voltage variations within
reasonable limits (e.g. 2%), it is recommended in this particular case to operate the WTGs at an active power
dependent power factor, which means that increasing voltages because of increasing active power production
would automatically be compensated by increased absorption of reactive power. The example presented in this
chapter further shows that a power factory range of cos(phi)>0,95 at the wind farm connection point would be
sufficient for maintaining the voltage within a 2% limit. A feed-back voltage control would not be required in this
particular case.
The contribution of short circuit currents by the WTGs is difficult to model because relevant international
standards for the calculation of short circuit currents don’t make provision for variable speed WTGs yet. For this
reason, it is recommended to use an “equivalent synchronous generator” approach for assessing the required
rating of substation equipment and cables. If more detailed analysis is required, only a time-domain simulation
considering the actual behaviour of the planned WTGs during grid faults can lead to reliable results.
Because it can be assumed that modern, variable speed WTGs based on IGBT technology will be installed,
harmonics won’t cause any problem in this particular case. Harmonics emissions of modern IGBT based
converters are typically in high frequency ranges and can therefore be filtered locally. There is only a very minor
impact on low order harmonics.
Flicker is typically not an issue when connecting large wind farms to subtransmission or transmission grids.
Because flicker relevant frequencies of wind turbulence or other flicker-relevant effects (such as tower shadowing
etc.) are stochastically quite independent, the corresponding power variations at the grid connection point are
very well attenuated. This effect could be demonstrated by a flicker analysis according to IEC 61400-21. Only
when connecting a small wind farm (only a few turbines) to a weak distribution grid or a small island network,
flicker can be an important aspect that would require careful analysis.
Grid Integration of Wind Energy in the Western Cape
23
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
3 Stage 2:
2: Connection of 750 MW of Wind Farms
Farms in Karoo Area to
the 400 kV Grid.
Grid.
For demonstrating approaches and methodologies of connection studies for wind farms that will be integrated
into the main transmission system, the planned wind generation in the Karoo Area has been selected. The total
amount of wind generation that will be installed in that area, sums up to 750MW (see [1] and [2]).
The projected connection point of 750 MW of wind farms is set to be approximately at 71 km from the existing
Komsberg 400 kV series capacitor substation located in the Karoo area [1]. The connection of the wind farms to
the 400 kV System is proposed to be carried out by means of a new substation that will “cut trough” the corridor
Komsberg – Droerivier at the above mentioned distance. This new substation is from here on referred to as
Nuweveld 400 kV substation [1].
In this particular example, Nuweveld 400 kV substation is scheduled to connect 5 wind farms of around 150
MW/each that will add together 750 MW of wind generation in the Karoo area. The suggested location of the 5
wind farms is depicted in Figure 8 and described in Table 2. The substation arrangement was considered as a
double busbar configuration running solid.
Planned connection
point of the 750 MW
wind farm
Figure 8: Planned connection point of the 750 MW wind farms in the Laingsburg – Karoo Area.
As before in case of the Laingsburg studies, it has been assumed that all wind farms will be built by 2MW variable
speed wind generators. However, for this type of grid impact studies, only the total wind farm size is relevant, the
individual turbine size has almost no influence on the results.
Grid Integration of Wind Energy in the Western Cape
24
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
Table 2: Wind parks connected to the Nuweveld 400 kV substation
Distance from Nuweveld
Power
400kV Substation
Output
(MW)
20
150
Site 2
12
150
Site 3
35
150
Site 4
25
150
Site 5
24
150
Kronos
57,17 MW
82,31 Mvar
Nuweveld 750 MW
0,00
104,71
Aries 400 RX2
-147,67 MW-146,80 MW
4,02 Mvar 6,64 Mvar
-145,21 MW-146,27-146,37
MW
MW
11,40 Mvar 8,24 Mvar
7,92 Mvar
104,21
5,40
1,35
Helios 400 RX2 Helios 22 Load TX
Nuweveld
Droerivier-Muldersvlei 400_2 S1
27,82 %
336,08 MW
Droerivier-Muldersvlei 400_2 S2
27,85 %
308,41
-101,42
Droerivier-Muldersvlei 400 SCX1 Bypass
-334,89
17,98
-308,41 MW
101,42 Mvar
427,15
1,07
5,10
Bacchus-Droerivier 400 SCX1
Droerivier-Muldersvlei 400 SCX2
334,89 MW
-17,98 Mvar
420,08
1,05
10,54
Droerivier-Muldersvlei 400 SCX2 Bypass
-84,67 Mvar
309,52 MW
-136,28 Mvar
42,10 MW
66,13 Mvar
Bacc-Droe 400 2
Bacchus-Droerivier 400_1 S1
12,46 %
27,14 %
Droe-Mulde 400 2
12,46 %
-109,54 MW
30,44 Mvar
Victoria
109,54 MW
-27,66 Mvar
108,02 MW
-27,83 Mvar
Droerivier-Victoria 400_1 ..
9,66 %
L1-WF4
74,27 %
L1-WF5
74,21 %
L1-WF3..
73,79 ..
Nuweveld WF-..
Helios
Nuweveld WF-..
L1-WF1..
74,85 %
-56,60 MW
-177,57 Mvar
-108,02 MW
30,54 Mvar
Droerivier-Victoria 400_2 ..
9,74 %
Nuweveld WF..
Droerivier-Hydra 400_3 ..
12,66 %
Nuweve..
L1-WF2
74,46 %
Aries-Helios 400_1 ..
16,14 %
Nuweveld WF..
Aries 400 RX1
Hydra-Victoria 400_2
12,75 %
Aries
Hydra-Victoria 400_1 S1
(km)
Site 1
Hydra-Victoria 400_1 S2 ..
12,64 %
Wind Farm
-109,32 MW
-45,49 Mvar
-107,80 MW
-46,31 Mvar
42,10 MW
66,13 Mvar
-41,85 MW
-145,47 Mvar
-41,85 MW
-123,49 MW
-145,47 Mvar
71,41 Mvar
Droerivier
0,00
105,94
0,00
105,94
0,00 0,00
347,01 MW
105,94 105,94 -100,01 Mvar
66,60
16,69
Figure 9: Single line diagram showing the wind farms connected to the Nuweveld 400kV substation
Because the total amount of 750MW and the installation of a new substation represent a considerable impact on
the main transmission system of the Western Cape, a model of the complete ESKOM transmission network has
been used for studying the impact of up to 750MW on the Western Cape transmission system. This model has
been made available by ESKOM Tx.
3.1 Scenario Definition
3.1.1 Base Case Scenarios
The studies carried out in the next section have been performed based on the “study year 2009”, which means
that the planned 765kV line running into the Cape was not considered.
The studies were based on the following base-case scenarios (without wind generation), which were defined by
ESKOM Tx:
Grid Integration of Wind Energy in the Western Cape
25
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
•
High load, 1 Koeberg units in service
•
High load, 2 Koeberg units in service
Unfortunately, ESKOM couldn’t make available standard low load scenarios because ESKOM’s study approaches
are generally based on high load scenarios only.
However, because in case of wind generation impact studies, low load conditions might sometimes be more
problematic than high load situations, low load scenarios have been defined in cooperation of DIgSILENT and
ESKOM under the following assumptions:
•
Demand during low load conditions approximately 60% of high load. Equally distributed across the
whole system.
•
Generator dispatch defined by ESKOM
Because these assumptions don’t match with the real behaviour of the system and because the reactive power
dispatch, especially outside the cape, could not be defined in a way that all voltages under no-wind-low-load
conditions remain within the required limits, only a rough feasibility analysis based on thermal line loadings could
be carried out. The relevant low load scenarios are:
•
Low load, 1 Koeberg unit in service
•
Low load, 2 Koeberg units in service
It is therefore strongly recommended to setup realistic low load scenarios and to carry out additional load flow
and contingency analysis studies for credible low load scenarios.
3.1.2 High Wind Scenarios
Based on the base case scenarios described in the previous chapter, high wind scenarios have been defined. For
balancing the system, the addition of 750MW of wind generation must be compensated by a reduction of 750MW
of other generation. The corresponding scenarios have been defined based on the following rules, which were
given by ESKOM:
1.
Disconnection of Gas Turbine Generators (or running in SCO mode where possible)
2.
Reduction of pump storage generation at Palmiet.
3.
Reduction of coal power plants outside the Cape (Tutuka, Grootvlei, etc.)
On one additional note, if a reduction of coal plants outside the Cape is required, then a minimum dispatch level
of 65% of the rated power plant output should be considered.
Grid Integration of Wind Energy in the Western Cape
26
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
In the case of the stage 2 of the studies (750MW of wind generation in the Western Cape), the additional wind
generation could be balanced by the gas turbine generators only. The other steps were only required for Stage 3
studies (2800MW of wind generation in the Western Cape, see chapter 4).
3.2 Series Compensation at Komsberg 400 kV
In the analyzed study year (2009) several 400 kV series capacitors exist within the Western Cape area. In this
example of wind farm integration, the series compensation located at Komsberg 400 kV was of particular interest
because of the planned location of the Nuweveld 400 kV substation. Since this substation was assumed to “Cut Trough” the 400 kV Komsberg – Droerivier corridor, the degree of series compensation will be significantly
modified.
Under this assumption, several scenarios considering the actual series compensation as well as“resized” series
compensation were analyzed in order to investigate its impact. In Table 3 the existing Series Compensation of the
Komsberg – Droerivier corridor is detailed.
Table 3: Actual Series Compensation at Komsberg 400 kV
Total Line
Transmission Line
Series
Lenght
Reactance XL
Capacitor
Compensation degree
(km)
(Ohms)
Reactance Xc
(%)
(Ohms)
Droerivier-Bacchus
Droerivier-Muldersvlei
401.3
127.3
81.6
64.0
405.2
130.2
74.8
57.4
3.2.1 Modified Series Compensation at Komsberg 400 kV
As stated previously, the insertion of the Nuweveld 400 kV substation in the Komsberg – Droerivier corridor will
affect the degree of series compensation on the existing lines. In Table 4, a case comparison considering a
modified/unmodified series capacitor reactance is shown.
Table 4: Case Comparison of the Series Compensation in Komsberg 400 kV
Transmission Line
Lenght
(km)
Total Line
Series Capacitor
Reactance XL
Reactance Xc
(Ohms)
(Ohms)
Compensation degree
(%)
C0.Base Case
Droerivier-Bacchus
401.3
127.3
81.6
64.0
Droerivier-Muldersvlei
405.2
130.2
74.8
57.4
C1.Nuweveld; unmodified series capacitor reactance
Nuweveld - Bacchus
276,3
88,2
81,6
92,6
Nuweveld-Muldersvlei
280,2
90,6
74,8
82,5
C2.Nuweveld; modified series capacitor reactance
Nuweveld - Bacchus
276,3
88,2
56,3
63,9
Nuweveld-Muldersvlei
280,2
90,6
52,0
57,4
Grid Integration of Wind Energy in the Western Cape
27
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
Table 4 highlights that with the actual Komsberg series capacitor and considering the 400 kV Nuweveld
Substation (C1), the degree of series compensation for Nuweveld-Bacchus and Nuweveld-Muldersvlei
transmission lines would increase approximately to 92% and 82 % respectively. This would lead to high voltages
in the surrounding area.
For this reason, it is recommended to downsize the series compensation at Komsberg so that the compensation
degree remains the same as prior to the insertion of the new substation at Nuweveld.
Additionally, it is recommended to install around 400Mvar of shunt reactors at the 400kV connection point at the
Nuweveld substation in order to bring the voltages within the required limits.
Alternatively, it would be possible that the wind farms absorb reactive power. However, most WTGs can only
provide or absorb reactive power when the wind speed is above the cut-in wind speed. Otherwise the LV breaker
is opened and the generator is entirely disconnected from the system.
There are a few WTGs, which allow remaining connected if wind speed drops below the cut-in wind speed.
However, this feature is typically available at additional costs and additional losses (under no wind conditions).
For this reason, the connection of around 400Mvar of shunt reactors would probably be the better option.
3.3 Impact on Voltage Variations at the Wind Farm Connection Point
The P-V curves depicted in Figure 10 apply to normal operating conditions (red curve) and two contingencies (line
outages) in the neighborhood of the planned wind farm.
The PV-curve shows that the system is very strong at the 400kV connection point at that the voltage dependence
on active power production is very small even under the assumption that the wind farm operates at unity power
factor at the 400kV connection point.
Grid Integration of Wind Energy in the Western Cape
28
DIgSILENT
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
X =750,000 MW
1,08
613.272 MW
1,05
1.052 p.u.
1.050 p.u.
Y = 1,050 p.u.
1.045 p.u.
1.034 p.u.
1,02
0,99
0,96
0.950 p.u.
0,93
0,00
x-Axis:
200,00
400,00
Static Generator: Active Power in MW
Static Generator: Voltage in p.u. - Base Case
Static Generator: Voltage in p.u. Droerivier - Muldersvlei out
Static Generator: Voltage in p.u. Droerivier - Bacchus out
600,00
Y = 0,950 p.u.
800,00
1000,00
Figure 10: Voltage at the wind farm connection point against varying active power levels and
different contingencies
3.4 Results of Contingency Analysis for Stage 2
For verifying that a n-1 secure operation is possible when connecting up to 750MW of wind generation to the
Nuveweld 400kV substation, load flow calculations and n-1 contingency calculations have been executed and it
has been verified that no thermal overload of any line or no violation of voltage bands (except from those that
already existed under base case/no wind conditions) would occur.
The list of contingencies, which contains all 400kV lines in the Western Cape is listed in Table 5.
The results of n-1 contingency analysis show that up to 750MW of wind generation can be connected without
endangering the n-1 secure operation of the Western Cape transmission grid.
Grid Integration of Wind Energy in the Western Cape
29
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
Table 5: List of Contingencies
Name
Acacia-Koeberg 400
Acacia-Muldersvlei 400
Acacia-Philippi 400
Ankerlig-Aurora 400
Ankerlig-Koeberg 400
Aries-Helios 400
Aries-Kronos 400
Aurora-Juno 400
Bacc-Droe 400 2 [Droerivier]
Bacchus-Droerivier 400
Bacchus-Muldersvlei 400
Bacchus-Palmiet 400
Bacchus-Proteus 400
Droe-Mulde 400 2 [Droerivier]
Droerivier-Hydra 400
Droerivier-Muldersvlei 400
Droerivier-Proteus 400
Droerivier-Victoria 400
Gourikwa-Proteus 400
Helios-Juno 400
Hydra-Kronos 400
Hydra-Luckhof (Beta) 400
Hydra-Luckhof 400_2 (Perseus 2)
Hydra-Luckhof 400_3 (Perseus 3)
Hydra-Poseidon 400
Hydra-Victoria 400
Koeberg-Muldersvlei 400
Koeberg-Stikland 400
Muldersvlei-Stikland 400
Palmiet-Stikland 400
Grid Integration of Wind Energy in the Western Cape
30
3 Stage 2: Connection of 750 MW of Wind Farms in Karoo Area to the 400 kV Grid.
3.5 Stage 2 - Summary
The example of the connection of up to 750MW of wind generation in the Karoo area is used for demonstrating
relevant approaches for grid connection studies of large wind farms to the main transmission level.
In contrast to studies at subtransmission levels (see Stage 1/chapter 2), those studies have to consider the
impact of wind generation on the complete transmission system. Especially, the definition of relevant scenarios,
considering:
•
Load and wind generation levels;
•
Generator dispatch scenarios;
is essential for studying the impact of wind generation of thermal aspects and voltage variations.
With regard to wind generation levels, it is recommended to consider the maximum registered capacity at the
grid connection point as maximum wind generation level. This will lead to results with sufficient margin for
considering uncertainties and inaccuracies of model parameters and assumptions related to wind variations.
Only in the case that issues related to high wind generation are identified a probabilistic assessment should be
carried out for justifying potential mitigation options (see also, Stage 1 – section 2.5.1). This probabilistic
assessment should consider technical availability of the WTGs and wind variation.
With regard to the actual studies that have been carried out in this chapter, the following conclusions can be
drawn:
•
Voltage variations because of varying wind generation are very small, even in constant power factor
operation.
•
Operation with constant Q (var-control) is appropriate. (Slow) voltage control is possible and should be
considered but is not necessarily required.
•
Series compensation at Komsberg should be resized for avoiding overcompensation.
•
4x100Mvar shunt reactors are required at Nuweveld substation (or equivalent var-absorption of the wind
farms) because of the proximity of the Nuweveld substation to the Komsberg series compensation.
Power quality aspects such as flicker or harmonics are not relevant when connecting very large wind farms to
transmission levels, as it has already been shown for stage 1 studies. For this reason, a detailed power quality
assessment is not required in this case.
Grid Integration of Wind Energy in the Western Cape
31
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
4 Stage 3: Feasibility Studies for up to 2800MW of Wind
Generation in the Western Cape
4.1 Background
In stage 3 of the Western Cape wind impact studies, all wind farms for which applications existed in March this
year have been considered and a high level feasibility study about their impact on the ESKOM transmission grid
has been carried out.
The total amount of installed wind generation capacity that will feed into each HV substation is listed in Table 6.
Table 6: Totally installed wind generation per area and substation
Area
Substation
Application
(MW)
Vredendal-Juno
Juno (132 kV)
220
Aurora
Aurora (132 kV)
400
Darling
Koeberg (132 kV)
300
Overberg
Bacchus (132 kV)
242
Laingsburg-Karoo
Laingsburg (132 kV)
150
Laingsburg-Karoo
Nuweveld (400 kV)
750
Beaufort West-Loxton-Hydra
Droerivier (400 kV)
664
The main purpose of these studies was to analyse if the integration of up to 2800MW of wind generation in the
Western Cape would be feasible without any major network upgrade (line upgrade).
For this purpose, all wind generation projects, for which applications existed at the starting date of these studies
(end of March 2009) have been summed up, lumped together and connected to the nearest 400kV substation
(see Figure 11 to Figure 13). This means that the studies described in this chapter of this report explicitly exclude
all issues that might come up at the 132kV subtransmission levels. It is evident that substantial line upgrades will
be required for accommodating such a large amount of wind generation, wherever the distance to the next 400kV
substation is too far.
For studying the general feasibility of the planned wind farm projects, load flow calculations under base case and
contingency scenarios have been carried out and it has been verified that all cases are n-1 secure without any
thermal or voltage violations (except from those, which already existed in the base case/no wind scenarios).
Grid Integration of Wind Energy in the Western Cape
32
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
~
G
~
G
Nama 66 Load TX
Anke rlig-A urora 40 0_2
~
G
~
G
Aggeneis 66 Load
TX Aggeneis
Aggene
is 220 RX1
220 R X2
Aurora
Anker lig Gen 3 2 Ank erlig Ge n 4 2
Gromis 66 Load TX
Anker lig Gen 3A1nkerlig Ge n 41 Anker lig Gen 4 3
~
G
Helios-Juno 400_1 S2
Juno
Aurora-Juno 400_1
Anke rlig-A urora 40 0_1
Aurora WF
L1-WFJ
L1-WFA
Juno WF
Aurora 400 MW
Juno 220 MW
Ankerlig Power Station
Ankerlig-Koeberg 400_1
Darling WF
L1-WFD
Darling
G
~
G Ank erlig Ge n 1 2 G
A nkerlig Ge n 22
~
~
Ankerlig Ge n 21
Ankerlig Ge n 11
Darling 300 MW
Juno
Juno 400 RX1
Juno 400 RX2
Juno 66 Load TX
Aurora
132 LoadAurora
TX
Aurora 132Aurora
CX2 132 CX
1
400 RX1
Koeberg-Muldersvlei 400_2 S2(
Koeberg-Muldersvlei 400_2 S3
Koeberg-Muldersvlei 400_2 S3(1)
Koeberg-Muldersvlei 400_2 S 1
Muld esrvlei
Muldersvlei
132 L oad
66TX
Load TX
Muldersvlei
Mulde13
rsvlei
Mulde
2 C X3
13
rsvlei
2 C X2
13 2 CX1
Acac ia-M uldersvlei 40 0_1 S2
Acacia-Muldersvlei 400_1 S1
Acacia-Koeberg 400_1 S2
Bacchus-Muldersvlei 400_1 S1
Mulde
rsvlei Muldersvlei
S VCSVC SCX
Muldersvlei
SVC FCX
Koeberg-Stikland 400_1 S2
Koeberg Gen1
SVS
Koeberg 132 Load TX
Koeberg-Stikland 400_1 S3
Koeberg Gen2
Acacia-Koeberg 400_1 S1
G
~
Acacia-Koeberg 132_2
G
~
Muldersvlei
Koeberg-Stikland 400_1 S1
Koeberg Power Station
Bacchus-Droerivier 400_1 S4
G
~
Ankerlig-Koeberg 400_2
Aurora
Nuweveld
Nuweveld 750 MW
Nuweveld WF..
Helios 400 RX2Helios 22 Load TX
Nuweveld
Bacchus-Droerivier 400_1 S1
Droerivier-Muldersvlei 400_2 S2
Hydra-Poseidon 400_1
Victoria
Droerivier-Victoria 400_2
L1-WF4
L1-WF5
Nuweveld WF-..
L1-WF3
L1-WF1
Nuweveld WF-..
Helios
Droerivier-Hydra 400_3
Nuweve..
L1-WF2
Aries-Helios 400_1
Nuweveld WF..
Aries 400 RX2
Hydra-Victoria 400_2
Hydra-Victoria 400_1 S2
Kronos
Droerivier
Droerivier-Victoria 400_1
Aries
Hydra-Victoria 400_1 S1
Figure 11: Wind Generation in the Western Cape – Substations Darling, Aurora and Juo
Droerivier WF
L1-WFD
Droe-Mulde 400 2
Droerivier 664 MW
Bacc-Droe 400 2
Bacchus-Droerivier 400_1 S2
Komsberg Series Capacitors
Droerivier
Droerivier-Muldersvlei 400_2 S1
Bacchus-Droerivier 400 RX1
Droerivier 132 Load TX
Droerivier 22 Load TX
Droerivier 400 RX1
L1-WFL
Droerivier 400 RX2
Figure 12: Wind Generation in the Western Cape – Substations Nuweveld and Droerivier
Grid Integration of Wind Energy in the Western Cape
33
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
Muldersvlei
Komsberg Series Capacitors
Bacchus-Droerivier 400_1 S2
SVS
Droerivier-Muldersvlei 400_2 S1
Bacchus-Droerivier 400_1 S4
Bacchus-Muldersvlei 400_1 S2
Bacchus-Palmiet 400_1
Laingsburg
Droerivier 132 Load TX
Droerivier 22 Load TX
L1-WFL
Droerivier 400 RX1
Droerivier 400 RX2
Droerivier-Muldersvlei 400 RX2
Bacchus-Droerivier 400_1 S3
Muldersvlei-Stikland 400_1
Bacchus-Droerivier 400 RX1
Droerivier-Proteus 400_1
Muldersvlei
Muldersvlei
132
Muldersvlei
CX3
132 CX2
132 CX1
Bacchus-Muldersvlei 400_1 S1
Muldersvlei
SVCSVC SCX
Muldersvlei
Muldersvlei
SVC FCX
Droerivier
Laingsburg 150 MW
Proteus 400 RX
Bacchus-Proteus 400_1
ProteusProteus
66 Load
132
TXload TX
Proteus
Bacchus 132
Bacchus
CX1 Bacchus
132 CX2 132 Load TX
L1-WFO
Bacchus
Overberg
Proteus 132 CX1
Gourikwa-Proteus 400_1
Bacchus 400 RX1
Gourikwa-Proteus 400_2
Overberg WF
Gourikwa-Proteus 400_3
Ov erberg 242 MW
Gourikwa Power Station
G
~
G
~
G
~
G
~
G
~
Gen 13
Gourikwa GenGourikwa
12
Gourikwa Gen 11 Gourikwa Gen 22
Gourikwa Gen 21
Figure 13: Wind Generation in the Western Cape – Substations Overberg and Laingsburg
4.2 Scenario Definition
4.2.1 Base Case Scenarios
The base case scenarios have been defined in the same way as in case of stage 2 (see section 3.1.1). As already
stated in section 3.1.1, ESKOM could not make available approved low load scenarios. Therefore, approximate
low load scenarios have been derived from the high load scenarios and used for the studies. Hence, the accuracy
of the results obtained under low load conditions is very limited.
4.2.2 High Wind Scenarios
Based on the base case (no wind) scenarios, high wind scenarios have been defined considering all wind farms
according to Table 6.
It has further been assumed that under high wind conditions, all wind farms will operate at rated active power.
Additionally, losses that might occur at subtransmission levels, to which some of the wind farms will be
connected, have been neglected. Obviously, this represents an extremely conservative worst case assumption.
Grid Integration of Wind Energy in the Western Cape
34
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
Alternatively, the results can be interpreted in a way that up to 2800MW of wind generation can actually feed into
the Western Cape transmission grid and that the actually installed wind generation capacity might even be higher
when assuming realistic diversity factors.
The analyzed scenarios are listed in Table 7
Table 7: List of analyzed scenarios
Name
Study Case description
HL 1 Unit Koeberg
High Load considering only 1
Koeberg unit in service.
HL 2 Units Koeberg
High Load considering 2 Koeberg
units in service.
HL 1 Unit Koeberg + WF 2800 MW
High Load considering the
connection of all Western Cape wind
farms. 1 Koeberg unit in service.
HL 2 Units Koeberg + WF 2800 MW
High Load considering the
connection of all Western Cape wind
farms. 2 Koeberg units in service.
LL 1 Unit Koeberg
Low Load considering only 1
Koeberg unit in service.
LL 2 Units Koeberg
Low Load considering 2 Koeberg
units in service.
LL 1 Unit Koeberg + WF 2800 MW
Low Load considering the
connection of all Western Cape wind
farms. 1 Koeberg unit in service.
LL 2 Units Koeberg + WF 2800 MW
Low Load considering the
connection of all Western Cape wind
farms. 2 Koeberg units in service.
Project
Grid Planning High Load_Stage 2
and 3
Grid Planning High Load_Stage 2
and 3
For balancing the additional generation, the same procedure as described in section 3.1.2 was applied, which is
repeated here again:
4.
Disconnection of Gas Turbine Generators (or running in SCO mode where possible)
5.
Reduction of pump storage generation at Palmiet.
6.
Reduction of coal power plants outside the Cape (Tutuka, Grootvlei, etc.)
In contrast to the stage 2 studies described in chapter 3, it is necessary to reduce generation outside the Cape
for balancing up to 2800MW of wind generation in the Western Cape. This effectively reduces the power import
via the Cape Corridor and can even lead to power export from the Western Cape into the rest of the system via
the Cape Corridor under low load – high wind conditions as shown in Figure 14.
Grid Integration of Wind Energy in the Western Cape
35
238,53 MW
-344,31 Mvar
Hydra-Luckhof 400_3 (Perseus 3)
25,45 %
Hydra-Luckhof (Beta) 400_1
21,99 %
Hydra-Luckhof 400_2 (Perseus 2)
25,36 %
Beta-Hydra 765_2
6,42 %
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
250,16 MW
225,01 MW
253,87 MW
-169,79 Mvar -134,50 Mvar -166,20 Mvar
967,56 MW
-814,79 Mvar
Hydra
-458,95 MW
84,74 Mvar
-452,87 MW
81,11 Mvar
452,91 MW
-81,47 Mvar
Hydra-Victoria 400_1 S2 ..
39,90 %
-452,91 MW
81,47 Mvar
-1912,25 MW
407,80 Mvar
362,80 MW
-88,77 Mvar
373,31 MW
-94,58 Mvar
736,12 MW
-183,35 Mvar
Hydra-Poseidon 400_2
32,30 %
Hydra-Victoria 400_2
40,47 %
-635,08 MW
147,56 Mvar
Hydra-Victoria 400_1 S1
21,43 %
-365,36 MW
94,39 Mvar
Hydra-Poseidon 400_1
31,32 %
Figure 14: Hydra substation under low load - high wind conditions
4.3 Results of Load Flow Studies
For verifying that the ESKOM transmission grid could take up to 2800MW of wind generation without any major
network upgrade (new transmission line), load flow calculations under base case an n-1 conditions have been
carried out
The focus of these studies has been the transmission system in the Cape. Potential issues in the network outside
the Cape have been monitored but not analyzed in detail. Particularly operational issues resulting from situations,
in which power is exported from the Cape Corridor, have not been part of the scope of these studies.
The results of the load flow and n-1 studies that have been carried out for all scenarios according to Table 7
show that no thermal overload or voltage problems have to be expected and that a secure system operation with
up to 2800MW of wind generation will not cause any grid congestion at the main transmission levels.
4.4 Additional
Additional Considerations
Besides grid congestions, there are many other aspects that have to be considered when evaluating the feasibility
of up to 2800MW of wind generation in the Western Cape, which are mainly related to system operation.
Grid Integration of Wind Energy in the Western Cape
36
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
These aspects mainly relate to:
•
Balancing of wind variations
•
Reactive power/voltage control
•
System stability.
Even if none of these aspects have actually been analyzed as part of the presented studies, some general
aspects should be highlighted, which show that it is indeed very likely that also none of these aspects will finally
question the feasibility of up to 2800MW of wind generation in the Western Cape.
4.4.1 Balancing of Wind Variations
In the Western Cape there is a total of around 2000 MW of gas turbine generators installed, which are currently
used for balancing peak loads and for backing up the outage of the Koeberg nuclear power plant units.
These gas turbine generators are peak load power plants with excellent dynamic properties (fast ramp-up/rampdown times, fast start-up times etc.). Technically, these gas turbine generators can be very useful for balancing
the variations of wind generation in the Cape.
As soon as the new 765kV line running into the Cape will be available (probably after 2013), there will other,
potentially less expensive options for balancing wind fluctuations in the Cape using power plants outside the
Cape.
Additionally, there are around 400MW of pump storage capacity installed at Palmiet, which can additionally be
used for balancing wind variations.
4.4.2 Reactive
Reactive Power/Voltage Control
A high number of gas turbine generators in the Western Cape is equipped with clutches and can be operated in
synchronous condenser (SCO) mode. This means that the full reactive power/voltage control capability of those
synchronous generators is also available under low load/high wind hours.
The steady state studies that have been carried out for the integration of up to 2800MW of wind generation in
the Cape don’t identify any need for additional reactive power-control capability in the Cape. However, dynamic
studies are required for verifying that this also applies to short-term voltage stability aspects.
The impact of wind generation export to the rest of the South African power system has not been part of the
scope of these studies and it is strongly advisable to study those cases in more detail for identifying the impact of
Cape-export scenarios on voltage and other relevant aspects of northern part of the South African transmission
grid.
Grid Integration of Wind Energy in the Western Cape
37
4 Stage 3: Feasibility Studies for up to 2800MW of Wind Generation in the Western Cape
4.4.3 System Stability
Particularly during high import scenarios, the Cape transmission network is prone to voltage stability problems.
With the addition of wind generation in the Cape however, power imports will be reduced during high wind hours
and voltage stability problems will rather be reduced.
In situations, in which the Cape will export power to the rest of the system (high wind – low load), it is very much
unlikely that transient stability issues will represent a considerable constraints because modern, variable speed
wind generators are not sensitive to any kind of rotor angle stability problem.
4.5 Stage 3: Summary
High level feasibility studies for analyzing the impact of up to 2800MW of wind generation in the Western Cape on
the 400kV transmission grid have been carried out.
The results show that no considerable impact on the transmission grid has to be expected and that it will be
possible to accommodate such a high level of wind generation without any major upgrades of the 400kV
transmission grid.
At the same time it is understood that the transmission capacity of subtransmission and distribution grids (voltage
levels <=132kV) will be limited in some cases and that major network upgrades at these lower voltage levels will
be required.
For confirming the results of these initial feasibility studies, additional, more detailed studies will be required;
especially on basis of well approved low load scenarios.
Besides this, studies looking at operational aspects such as additional spinning reserve requirements, stability
aspects, dynamic performance requirements for peak load power plants etc. have to be carried out for confirming
the feasibility of up to 2800MW of wind generation in the Cape.
However, many general aspects of the Western Cape system, such as
•
the availability of peak-load power plants (gas turbine) in the Cape;
•
generators allowing for synchronous condenser operation and
•
the fact that wind generation in the Cape will reduce power imports into the Cape
can lead to the conclusion that the Western Cape transmission system is generally very well suited for the
integration of high amounts of wind generation even if some of these aspects still have to be confirmed by
additional, more detailed studies.
Grid Integration of Wind Energy in the Western Cape
38
5 Conclusions and Recommendations
5 Conclusions and Recommendations
The studies presented in this report describe general methodologies and approaches for studying the integration
of wind generation in subtransmission and transmission networks by means of three different examples:
•
Stage 1: Connection of a 150MW wind farm to a 132kV subtransmission grid
•
Stage 2: Connection of up to 750MW of wind farms to the 400kV transmission grid.
•
Stage 3: Feasibility studies about the integration of up to 2800MW of wind generation into the Western
Cape transmission system.
The results of these studies highlight a number of aspects that have to be considered when studying the grid
impact of wind generation in South Africa and show the general feasibility of the integration of up to 2800MW of
wind generation in the Western Cape.
For confirming the results of these feasibility studies, a number of additional studies will be required, as there
are:
•
Load flow and n-1 studies with additional scenarios, particularly low load scenarios
•
Stability studies under various different operating conditions. For these studies, appropriate dynamic
wind generator models are required.
•
Wind farm connection studies for each individual connection application
•
Studies about the operation of the ESKOM transmission system under scenarios in which the Cape
exports power to the rest of the system.
•
Studies related to the expected total power variations of wind generation (variations, ramp-up and
ramp-down speeds) for identifying additional reserve requirements have to be carried out.
•
Studies considering other renewable sources
•
Studies considering wind energy and other renewable energy sources outside the Western Cape
Province.
Besides these additional study requirements, the fact that
•
The Western Cape is an area having rather an import than an export problem under the present
situation.
•
There are a high number of fast peak-load power plants and pump storage schemes available in the
Western Cape with which it will be possible to balance wind variations.
Grid Integration of Wind Energy in the Western Cape
39
5 Conclusions and Recommendations
•
Some of the gas turbine generators in the Western Cape can operate in synchronous condenser mode
lead to the conclusion that the Western Cape transmission grid is very well suited for the integration of high
amounts of wind generation.
With regard to particular technical requirements of wind generators to be installed in South Africa, the following
general recommendations can be made:
•
Frequency and voltage range of operation should be defined analogously to the existing South African
practice.
•
A reactive power range at the grid connection point of (+/- 0,33 p.u./Prated, cos(phi)>0,95) under full
load and partial load conditions will be sufficient. It is recommended to decide on a case-by-case basis,
whether reactive power control capability during low and no wind hours will be required.
•
For wind farms that will be connected to subtransmission and distribution levels, operation with constant
power factory will be sufficient. If required, an active power dependent reactive power characteristic
should be foreseen (see section 2.6).
•
Wind farms that will be connected to the main transmission level should have the technical capability for
controlling the reactive power or the voltage (feed-back voltage control) at the grid connection point.
The required dynamic performance of such voltage control has to be identified with the help of
additional, dynamic studies.
•
Because of the overall amount of planned wind generation in the Cape, all wind generators should be
equipped with fault ride-through (FRT) capability for avoiding WTG trips in case of line faults.
For standardizing the technical characteristics of wind generators in South Africa, it is recommended to extend
the South African grid code by Connection Conditions for wind generation. These Connection Conditions should
consider existing standards with respect to the connection of thermal and hydro power plants and make provision
of the particular technical characteristics of wind generators.
Grid Integration of Wind Energy in the Western Cape
40
6 Referenced Documents
6 Referenced Documents
[1]
ESKOM – Riaan Smit. Grid Integration Workshop. Working Document.
[2]
ESKOM. Wind Generation Sites. (file: Wind Gen Sites 20090327.xls)
[3]
M. Pöller: TERNA- Project Workshop, (file: WorkshopSummary.ppt)
Grid Integration of Wind Energy in the Western Cape
41
ANNEXES
List of Annexes
Annexes
•
Annex A-1: PV-curves of Stage 1 – Studies
•
Annex A-2: Results of Contingency Analysis of Stage 2 – Studies
•
Annex A-3: Results of Contingency Analysis of Stage 3 – Studies
Grid Integration of Wind Energy in the Western Cape
42