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Analysis of requirements in selected Grid
Codes
Willi Christiansen & David T. Johnsen
Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Preface
This report has been submitted to Ørsted•DTU, Section of Electric power Engineering,
Technical University of Denmark (DTU). It has been carried out within the areas of
electrical power engineering and implementation of wind power. The Report is a preprojekt, which will be used as a guideline for a final Master thesis in spring 2006.
We, David T. Johnsen and Willi Christiansen, have carried out this project in a cooperation with Siemens Wind Power, Energi E2 and Ørsted•DTU, section of electric
power engineering (DTU). The project was developed in January 2006. We are thankful
to our supervisors Arne Hejde Nielsen from Ørsted-DTU, Kim Høj-Jensen and Jørgen
Nygaard Nielsen from Siemens Wind Power. A special thank also to Troels Sørensen
from Energi E2, who has followed the project with interest.
Lyngby den 20.01.2006
Willi Christiansen, [email protected]
David T. Johnsen, [email protected]
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Abstract
This Report contains studies about the connection requirements of wind power generating
units. Six Grid Codes from Canada, Denmark, Ireland, Germany, Scotland and UK have
been compared and analyzed. The subject is relevant due to the lack of information about
generic connection requirements for wind power generating units. The purpose with this
Report is to outline and analyze the most restringing wind power connection requirements.
The studies are divided into an analysis of the connection requirements regarding
continuous operation modus and a second part regarding operation during fault sequences.
The first part (static analysis) concentrates on the continuous load flow conditions. The
second part is an analysis of the dynamic requirements in the Grid Codes. This analysis
concentrates mainly on the requirement of the fault ride through capability of each wind
turbine generator.
In the first part, the most restringing power factor requirements are described. Due to the
worst case requirement, a wind turbine should be able to run continuously at full
production with a power factor of 0.90 lagging to 0.95 leading. Furthermore the maximum
required voltage and frequency range are outlined. The voltage range is more than ±10% of
the nominal voltage. The frequency requirement is within the range from 47.5Hz to 52Hz.
This requirement seems disproportionate high.
The result of the second part is a curve showing the required fault ride through capability. It
is required that a wind turbine under no circumstances is allowed to trip within the first
150ms. A voltage duration curve describes the voltage limits which define the voltage level,
in which a wind turbine has to continue operating.
The Report is completed with a theoretical review of different limiting load scenarios,
wherein the outlined restrictions can lead to problematic operation situations for the wind
power generating unit. The summarizing conclusion is that it is not sufficient only to obey
the requirements of the Grid Code individually. The requirements in the Grid Code should
rather be seen in its entirety and in relation to the given network. The studies have shown
that even if generic Grid Code requirements can be defined, detailed and challenging
network analyses have to be made for each wind power connection query, wherein
information about the type of wind turbine and of the given network is taken into
consideration.
3
Preface .................................................................................................................................................2
Abstract ...............................................................................................................................................3
1. Introduction ....................................................................................................................................5
2. Selection of Grid Codes..................................................................................................................6
3. Static regulations during continuous operation ..........................................................................7
3.1. Power factor regulations ...........................................................................................................7
3. 1. 1. The most restringing power factor regulation .................................................................11
3.2. Power Curtailment...................................................................................................................13
3.3. Voltage range and control .......................................................................................................14
3.4. Remote voltage control ............................................................................................................16
3.5. Frequency ................................................................................................................................17
3.6. Flicker ......................................................................................................................................18
3.7. Harmonics................................................................................................................................19
4. Dynamic regulation during fault sequences...............................................................................20
4.1. Fault ride trough requirements................................................................................................20
4. 1. 1. Exceptions from the fault ride through requirements......................................................23
4.2. Repeating fault sequences........................................................................................................23
5. Examples of limiting worst case scenarios .................................................................................24
5.1. Scenario 1, fast voltage changes in the transmission system...................................................24
5.2. Scenario 2, high voltage and pf requirements .........................................................................24
5.3. Scenario 3, weakening of the network .....................................................................................25
5.4. Scenario 4, active power recovering after a voltage dip .........................................................25
6. Discussion......................................................................................................................................26
7. Conclusion.....................................................................................................................................28
8. References .....................................................................................................................................30
Appendix ...........................................................................................................................................31
Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
1. Introduction
The aim of this Report is to describe the most essential conditions for connecting wind turbines to
the grid. The Grid Codes of selected countries with interesting technical and economical conditions
within the field of wind power are taken into consideration in the analysis.
The technical specifications of the chosen Grid Codes are divided into static and dynamic
requirements.
The static part of the Grid Code examination consists of the subjects regarding the continuous
operation of the wind farm. Following themes will be included in the static part: voltage control,
quality of voltage, pf requirements, power curtailment, frequency and flicker.
The dynamic part of the Grid Code examination consists of subjects regarding the operation of wind
turbines during fault sequences and disturbances in the Grid. Following themes will be included in
the dynamic part: fault ride through capabilities and fault recovery capabilities.
The most restringing conditions, seen from the perspective of the wind turbine, are outlined and
described. The most restringing conditions are outlined in the formulation of generic connection
requirements. The focus of this examination is on the dynamic requirements in the Grid Codes.
Worst case scenarios are presented to highlight sections wherein the generic Grid Code might not be
sufficient to ensure the stability and the quality of the electrical systems. The scenarios will be
discussed and suggestions as to how the connection queries of wind farms could be solved, are
briefly mentioned.
The purpose of outlining the requirements of a generic Grid Code is to give an idea of the technical
qualifications, which should be satisfied by a generic wind turbine model. This Report will
therefore be a summation of the requirements in selected Grid Codes. It has to be pointed out that
this Report does not consist of a complete generic Grid Code in itself.
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Analysis of requirements in selected Grid Codes
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2. Selection of Grid Codes
The selection of Grid Codes is based upon a number of preset considerations. It is important to
archive a broad variation of Grid Codes in order to obtain a realistic generic Grid Code. The
countries are selected in the view of interesting wind power aspects like technical possibilities and
geographical limitations. Furthermore following conditions must be fulfilled for the choices of Grid
Codes:
– Wind power potential
– Detailed section regarding wind power (or non-synchronous generators) within the
Grid Code
– Interesting network characteristics (island, weak/strong network, high penetrations of
wind power)
All together, six Grid Codes are selected for the analysis of a generic Grid Code.
Among the chosen Grid Codes is Denmark [2] due to the high penetration of wind power. Ireland
[4] is selected because it is an island-system and therefore of great interest. The Grid Code of EON
[3] (a German transmission system Operator (TSO)) is chosen due to the important wind power
market in Germany and due to detailed technical descriptions in the Grid Code of EON.
Furthermore the Grid Code from Scotland [5] and the UK [6] are analyzed because of the detailed
‘connection of non-synchronous generating unit’ section and because of the high wind potential in
UK and Scotland. Finally the Grid Code from the Canadian TSO [1], AESO, is taken into
consideration in order to archive a contribution from an oversee Grid Code. The Canadian Grid
Code is not included in the frequency analysis due to the fact that the Canadian system operates at
60Hz.
Parts of the Spanish and of the Australian Grid Codes have also been taken into account, but these
Grid Codes have not been analysed in detail.
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3. Static regulations during continuous operation
The first part of the Grid Code analysis concentrates on the static requirements. Some of the
requirements contains time limits or operates with time ranges. The values are however still
constant and the semi-static (time ranges) regulations are therefore included in this static analysis.
The second part (chapter 4. ) concerns the dynamic requirements in the Grid Codes. These
requirements do primarily concern the desired behaviour of the Wind Turbine Generator (WTG)
during faults and disturbances.
Unless described otherwise, the static requirements refer to the behaviour and to the power flow at
the connection point of the Wind Farm Power Station (WFPS) to the transmission grid. The
operation of a single WTG is therefore not of interest in this static requirements chapter. Contrary is
it in chapter 4. (dynamic requirements), where the operation and measurements of each WTG
applies.
3.1. Power factor regulations
The power factor regulation concerns the reactive power consumption of the Wind Farm Power
Station (WFPS). A simple induction generator, with no additional capacitors attached, will during
normal operation consume reactive power. This reactive power has to be produced somewhere in
the grid. It is preferred that the WFPS is reactive power neutral, since the distribution of reactive
power is relatively cost intensive. The requirements to the reactive power and to the power factor
are relatively similar in the different Grid Codes.
The static phase angel requirements are listed in the following Table 3.1. The Wind Farm Power
Stations (WFPS) shall be capable of operating at any point within the power factor ranges.
For the avoidance of doubt, a generating unit operating at lagging power factor delivers reactive
power into the transmission system.
Canada
Static, continues 0.90
power factor
lagging
to 0.95
leading
Denmark
Germany
–
Eon
Q/Prated
0.95 lagging to
0.95 leading
= 0 to
Q/Prated
for a rated
= 0.1 at
active power
full
capacity <
production 100MW
and
through a For a rated
straight
active power
capacity > 100
line to
7
Ireland
Scotland
UK
0.95 lagging
to 0.95
leading at full
production.
32.6MVAr
per 100MW
installed,
active
capacity from
100%
0.95
lagging
for
production
between
100% 20%. 0.95
leading
for
production
0.95
lagging to
0.95
leading
Analysis of requirements in selected Grid Codes
Q/Prated
= -0.1 to
Q/Prated
= 0 at zero
production
Willi Christiansen & David T. Johnsen
MW the
power factor is
voltage
dependent.
***
production to
50%
production.
0.95 lagging
to 0.95
leading from
50%
production to
idle.*
between
100% to
50%**
Table 3.1 power factor requirements of the selected Grid Codes.
*The requirements to the reactive power production in the Irish Grid Code are complex compared to
the other Grid Codes. The idea of varying the power factor dependent on the active production is
illustrated much clearer on Figure 3-1. It can be observed that the relation between reactive power
and active power must remain constant within the area from 100% to 50% active power production.
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Analysis of requirements in selected Grid Codes
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Figure 3-1 The Irish power factor requirements
The black triangle below the 10 % production line on Figure 3-1 indicates that the reactive power
output during operations below 10% must be altered, if the voltage limit is reached.
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
**The Scottish reactive power requirements diverse from the requirements in the UK when the
active power production gets below 20% of the rated active power. The Scottish case is illustrated
in the following Figure 3-2:
Figure 3-2. The Scottish power factor requirements
The inductive reactive power limits are reduced linearly below 50% Active Power output as shown
in Figure 3-2. The reactive power limits for active power output below 20 % shall be adjustable
within the area of Q = - (5% of rated MW output) to Q = 5% of rated MW output.
*** EON: With active power output, each generating unit with a rated power of 100 MW must
meet the range of reactive power provision shown in Figure 3-3 as a basic requirement at the
network connection point. Additional requirements may have to be met in individual cases.
In addition to any of the other Grid Codes, the German Grid Code [3] contains information on the
reactive power requirements during different voltage situations. The reason for including the actual
voltage at the bus is the fact that a high reactive power production induce higher voltages, which is
undesired if the initial voltage is at a high level already. Thereby the power factor control becomes a
part of the voltage regulations.
The case for the remaining Grid Codes is that it must be possible to operate between 0.95-0.9
lagging at full production, even if the voltage is at 1.1p.u. This could cause unnecessary stress to the
components due to the increasing voltage.
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Figure 3-3. The German power factor requirements for Prated above 100MW
When changing the reactive power output, step changes corresponding to a reactive power of more
than 2.5 % of the network connection capacity in the high voltage network and 5 % in the extra high
voltage network are not permissible. No step changes smaller than 500 kVAr will be required.
3. 1. 1. The most restringing power factor regulation
The most restringing power factor requirement (black line on Figure 3-4 and Figure 3-5) consists of
a combination of the power factor requirements listed in Table 3.1. The most restringing condition
at full output is the Canadian [1] 0.90 lagging to 0.95 leading. At lower production levels the
variable power factor of the Irish power factor requirements will surpass the restrictive Canadian
regulations. As seen in Figure 3-1, the reactive power production requirement of the Irish Grid Code
[4] surpasses the Canadian when the power factor gets below 0.95 leading and 0.90 lagging.
Following Figure 3-4 illustrates the Irish power factor requirements including the Canadian
intensifications. The main contribution of the Canadian Grid Code is the power factor of 0.90
lagging, which is applicable from 100% to 70% active production.
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Analysis of requirements in selected Grid Codes
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Figure 3-4, most restringing power factor settings
Figure 3-5, most restringing reactive power settings
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Analysis of requirements in selected Grid Codes
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The actual reactive production can be seen in the Figure 3-5. The required limits are defined by the
continuous black line.
The Wind Farm Power Stations shall be capable of operating at any point within the power factor
ranges. Furthermore it shall be possible to order reactive regulation requirements via remote control
and locally. Depending on the operational situation, the system operator changes the desired MVAr
or voltage reference. In general it must be possible, independent of the rated power, to run through
the agreed design range for the power factor at rated active power output within a few minutes. The
entire process must be possible as often as requested.
The issue regarding the generic power factor requirements is that the requirements are based on at
voltage level around 1.0p.u. In the transmission system a low voltage must be supported by an
increased reactive power production and vice versa. The opposite reaction will only cause further
voltage deviation.
Only the German Grid Code [3] includes the actual voltage level as a part of the power factor
requirements (see Figure 3-3).
3.2. Power Curtailment
The need for power curtailment occurs when it is not possible to compensate for the loss of wind
power by up rating conventional plants. To avoid this situation the wind power is curtailed. This
situation normally occurs during the daily load increase when the conventional plants have to
compensate for both the increasing load demand and for the declining wind production. Another
reason for curtailing wind is if the wind production increases beyond the load consumption. This
will increase the inertia of the transmission system which causes a frequency increase.
The curtailment amount is dependent on installed capacity, location, wind forecasting reliability and
economics and can not be outlined precisely. But it must in any case be possible to reduce the
power output in every operating condition and from any operating point to a maximum power value
which corresponds to a percentage value based on the network connection capacity. The reduction
of the power output to the signalled value must take place with at least 10% of the network
connection capacity per minute without the system being disconnected from the network.1 The main
indication of a production surplus is a frequency increase. Therefore the power output must be
reduced from a frequency of 50.5 Hz according to Figure 3. A gradient of 5 % per second applies.
When the frequency deviation decreases, the power output must be increased accordingly. The reestablishment of the active power supplied to the network must not exceed a maximum gradient of
10% of the network connection capacity per minute.
1
[3] E.ON Netz GmbH, Grid Code for high and extra high voltage, 1. August 2003 Page 23
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Figure 3-6, power curtailment requirements
The power curtailment concerns the right end of the curve in Figure 3-6 when the frequency
increases above 50.5Hz.
Furthermore it must be possible to constrain the active power gradient such that extreme increases
in active power during a short period of time are avoided. Again the parameters and range of the
active power gradients are of varying sizes and depends on the installed capacity, location etc. The
settings must apply with the TSO requirements.
A table containing the main issues regarding the active power curtailment is listed in Appendix 1.
The table is originally listed in the Danish Grid Code [2].
3.3. Voltage range and control
The Wind Farm Power Station (WFPS) must be able to run at rated voltage plus a specified voltage
range. The voltage range depends on the level of the voltage on the transmission system, which
varies from country to country.
Voltage range
Continuous
Limited
time
periods*
Denmark [2]
Germany [3]
Ireland [4]
-10%
5%
-3%
13%
-5%
10%
-20%
10%
-10%
20%
-10%
18%
-8%
10%
-13%
12%
-13%
12%
-13%
5%
-9%
12%
-10%
12%
400 kV
150kV
132kV
400 kV
220 kV
110 kV
400 kV
220 kV
110 kV
400 kV
150kV
X
132kV
14
X
Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Voltage range
Scotland [5]
Continuous
±5%
400 kV
±10%
±10%
Limited time ±10%
periods*
±15%
±20%
275 kV
132 kV
400 kV
275 kV
132 kV
UK [6]
-10%
5%
±10%
±10%
±10%
±10%
±10%
Canada [1]
400 kV
275kV
132kV
400 kV
275kV
132kV
The TSO will
provide
the
voltage
operating range
Table 3.2, permitted voltage ranges
* Denmark (1 hour), Scotland (15 min), UK (15 min)
It can be observed in Table 3.2 that the specified voltage range varies between the different
countries. Additionally some countries allow higher voltage ranges over a limited time period.
Following specified voltage ranges are the most restringing requirements:
Specified voltage range
Nominal Continuous
Voltages voltage range
400 kV
-13% +10%
275 kV
±10%
220 kV
-13%
150 kV
-3%
132 kV
±10%
110 kV
-13%
+12%
+13%
+12%
Limited time period
15 min
1 hour
-13% -20%
-13% -20%
-10%
-15%
X
& 10% 15%
%
X
-3%
-10% & -3%
-10% &
10% 20%
10% 20%
-10%
-20%
& 10% 18%
& 10% 20%
X
X
Table 3.3, specified worst case voltage range
Table 3.3 shows the continuous operating voltage range in regard to the nominal network voltage.
Normal operation of the WFPS should be possible within the specified range and time limit.
It has to be pointed out that exceptions are mentioned in most of the Grid Codes. Greater or lesser
variations in voltage to those set out above can bee agreed in relation to a particular connection site.
The following Figure 3-7 is a graphic display of the voltage range in the generic Grid Code. It can
be seen how the general requirement for all nominal voltages is continues operation at voltages
around ±10% of the nominal voltage.
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Voltage Range
25
20
V/Vnom in per cent
15
10
5
1 Hour
0
-5
15 min.
400
275
220
150
132
110
Continious Time
-10
-15
-20
-25
Nominal Voltage
Figure 3-7, specified worst case voltage range
- note that the operating interval for the 132V nominal voltage is 1 hour for voltages between 10%
above nominal voltage to 18% above nominal voltage.
3.4. Remote voltage control
Wind Farm Power Stations (WFPS) shall have a continuously-variable, continuously acting, closed
loop voltage regulation system with similar response characteristics to a conventional automatic
voltage regulator.
The voltage regulation system shall be adjustable by the TSO by signalling a voltage set point for
the voltage at the connection point. The voltage regulation system shall act to regulate the voltage at
this point by continuous modulation of the reactive power output within its reactive power range,
and without violating the voltage step emissions. The set-point shall be adjustable within 95% to
105% of rated voltage.
The response speed of the voltage regulation system, following a step change in voltage at the
connection point, shall be such that the wind farm power station achieves 95 % of its steady-state
reactive power response within 1 second.
It is only in the Irish [4] and the Canadian [1] Grid Code that these remote voltage control options
are specified in detail.
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
3.5. Frequency
The Following Table 3.4 shows the frequency requirements of the European Grid Codes:
Frequency range requirement
Frequency
(Hz)
52 Hz t to 53 Hz
51.5 Hz to 52 Hz
Minimum Time Delay
Denmark[2]
Germany[3] Ireland[4]
3 min
%
30 min
%
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Scotland[5] UK [6]
%
60 min
60 min
%
%
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
Continuous
Operation
51.0 Hz to 51.5 Hz
30 min
50.5 Hz to 51.0 Hz
30 min
49.5 Hz to 50.5 Hz
Continuous
Operation
49.5 Hz to 47.5
Hz
30 min
47.5 Hz to 47.0 Hz
3 min
%
20 sec
20 sec
20 sec
>47.0 Hz
%
%
20 sec
20 sec
20 sec
60 min
Continuous
Operation
60 min
Table 3.4, Frequency range requirement
The design of generator’s plant and apparatus must enable operation in accordance with the
frequency range in Table 3.4. Due to the frequency requirements in the table, wind farms are
required to be capable of operating continuously between 47.5Hz and 52Hz and time limited
between 47 and 47.5Hz likewise as between 52 and 53. This is a relative wide range in relation to
realistic events. Nevertheless a model of a wind turbine should be able to operate within this range.
Generic frequency range requirement
Frequency (Hz)
Code of operation
52 Hz to 53 Hz
3 min
51.5 Hz to 52 Hz
Continuous Operation
51.0 Hz to 51.5 Hz
Continuous Operation
50.5 Hz to 51.0 Hz
Continuous Operation
49.5 Hz to 50.5 Hz
Continuous Operation
49.5 Hz to 47.5 Hz
Continuous Operation
47.5 Hz to 47.0 Hz
20 sec
>47.0 Hz
20 sec
Table 3.5, Generic frequency range requirement
The case is different for Canada due to a system frequency of 60Hz. The Canadian frequency
requirements are not included in the analysis since there is no basis for comparison.
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
In addition the active power output may be reduced in the outside values within the frequency
ranges (see chapter 3.2. ). The WFPS should remain connected to the transmission system during
rate of change of transmission system frequency values of up to 0.5 Hz per second. Automatic
isolation from the network due to the frequency is only permissible at frequencies below 47Hz and
above 53Hz. Upon reaching 47Hz or 53Hz the unit must be automatically isolated from the network
without delay.
No additional WTG shall be started while the transmission system frequency is above 50.2Hz.
3.6. Flicker
Flicker is defined as a single, rapid change of the RMS voltage. Transmission system step changes
can occur due to switching in and out of capacitors, lines, cables, transformers and other plant.
Voltage fluctuations at a point of common coupling with a fluctuating load directly connected to the
transmission system shall not exceed the lines on following figure:
Figure 3-8, voltage flicker limits
Flicker shall not exceed 3% at any time. The maximum permissible values for rapid voltage
changes from wind farms in the connection point are shown on Figure 3-8. The red line indicates
the limits in the Danish Grid Code [2]. The black lines indicate the Canadian Grid Code [1].
The flicker contribution from the wind farm in the connection point shall be limited so that the short
term flicker (Pst), determined as a weighted average of the flicker contribution over ten minutes is
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
below 0.30. The long term flicker value (Plt), determined as a weighted average of the flicker
contribution over two hours shall be limited so that Plt is below 0.20.
The flicker contributions Pst and Plt are defined in IEC 61000-3-7 (Electromagnetic compatibility).
It is primarily the Danish Grid Code [2], which contains restrictive rules about flicker. The Irish and
the German Grid Codes do not mention flicker.
3.7. Harmonics
Wind Farm Power Stations connected to the transmission system shall be capable of withstanding
the levels of harmonic distortion liable to be present on the transmission system. These
electromagnetic compatibility levels are not directly specified in any of the Grid Codes but the
Danish. The Grid Codes from UK [6] and Scotland [5] refer to the standard ER G5/4 – “Planning
Levels for Harmonic Voltage Distortion and the Connection of Non-Linear Loads to the
transmission systems and Public Electricity Supply Systems in the United Kingdom”. The Canadian
Grid Code [1] refers to the IEEE standard 519-1992 “Recommended Practices and Requirements
for Harmonic Control in Electrical Power Systems”. The requirements of the mentioned standards
are not included in the report.
The requirements in the Danish Grid Code [2] regarding harmonics are outlined as follows:
The harmonic disturbance Dn for each individual harmonic shall be defined as:
The total harmonic effective distortion THD shall be defined as:
Dn shall be lower than 1 per cent for 1 < n < 51 in the connection point.
THD shall be smaller than 1.5%.
In general, the Danish Grid Code [2] limits the individual harmonics as a function of the nominal
voltage. Furthermore a limit of the THD is presented.
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Analysis of requirements in selected Grid Codes
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4. Dynamic regulation during fault sequences
4.1. Fault ride through requirements
Phase swinging or power oscillations must not result in triggering of the generating unit protection.
The turbine-generator unit control must not excite any phase swinging or power oscillations.
The Wind Turbine Generator (WTG) shall be equipped with voltage and frequency relays for
disconnection of the wind farm at abnormal voltages and frequencies. The relays shall be set
according to agreements with the regional grid company and the system operator. The protective
functions of the wind turbine shall include settings and time delays meeting the requirements
described in this section.
All countries have a fault ride through capability figure. These requirements do only concern short
circuits in the transmission system, and not short circuits within the Wind Farm Power Station
(WFPS).
Following fault ride through figures are partly the most restricting ones. In the following the
Scottish [5], the Irish [4] and the German [3] fault ride through figures are presented:
Figure 4-1, the Scottish fault ride through requirement
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
Figure 4-2, The Irish fault ride through requirement
Figure 4-3, The German (EON) fault ride through requirement for a near by generator fault
Figure 4-1 to Figure 4-3 show the fault ride through requirement from the Scottish, the Irish and the
German Grid Code. Altogether the three figures represent the most restringing requirements.
Following Figure 4-4 demonstrates the worst case Grid Code requirements. The fault ride through
requirement is defined as following:
A Wind Turbine Generator (WTG) shall remain connected to the transmission system for
transmission system voltage dips on any or all phases, where the transmission system voltage,
measured at the HV terminals of the grid connected transformer, remains above the heavy black line
in Figure 4-4:
U/Un
Fault ride through capability of wind farm power stations
90%
80%
70%
45%
15%
0%
0 150 625 700 (ms)
1500ms
2683ms
3 minutes
Figure 4-4, Fault ride through capability of wind farm power stations
Figure 4-4 shows that each generating unit shall remain transiently stable and connected to the
system without tripping for a close-up solid three-phase short circuit fault or any unbalanced short
circuit fault on the transmission system with a total fault clearance time of up to 150ms. Throughout
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
the operating range of the generating unit, these types of faults must not result in instability or
isolation from the network.
Furthermore, Figure 4-4 shows that wind turbine generator units shall be capable of continuous
operation down to 90% of rated voltage at the connection point.
In addition to the requirement that the WTG must remain connected to the transmission system, the
WTG shall have the technical capability to provide the following functions:
The wind WTG shall provide active power in proportion to retained voltage and maximize the
reactive current to the transmission system without exceeding WTG limits during the voltage dip in
the transmission system. The maximization of reactive current is described in detail in the below
Figure 4-5;
The wind farm power station shall provide its maximum available active power as quickly as the
technology allows with a gradient of at least 20% of the rated power per second. Within the grey
area in Figure 4-4 the active power increase can take place at 5% of the rated power per second.
This power increase should in any event occur within one second of the transmission system
voltage recovering to 0.90pu of the normal operating range.
The generating units must support the voltage within a disturbed network. If a voltage drop of more
than 10% of the root mean square (RMS) of the generator terminal voltage occurs, the generation
unit must be switched to voltage support, according to following figure:
Figure 4-5, Amount of reactive current feed for voltage support with a fault in the network
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
After the fault identification, the network voltage support must be provided within 20ms by
providing reactive power at the generator terminals with a factor of 2% of the rated current per
percent of the voltage drop. The maximization of reactive current shall continue for at least 600ms,
or until the transmission system voltage recovers to the normal operational range of the
transmission system; whichever is the sooner.
The transient phenomenon, with regard to the reactive power consumption after the voltage returns
to the normal operation range, must be completed after 400ms. After this time the reactive power
exchange must take place as it is specified on the basis of the normal operational schedule.
4. 1. 1. Exceptions from the fault ride through requirements
Wind Turbine Generator units are not required to ride through transmission system faults that cause
a forced outage of a radial line to the wind farm (isolation of the WFPS). Nor shall wind turbine
generator units ride through faults that occur on the lower voltage networks of the wind turbine.
The requirements described in Figure 4-4 and Figure 4-5 do also not apply when the wind turbine
power station is operating at:
less than 5% of its rated power;
during very high wind speed conditions, when more than 50% of the wind turbine
generator units in the power station have been shut down or disconnected under an
emergency shutdown sequence to protect the plant and apparatus.
4.2. Repeating fault sequences
A wind turbine generation unit shall have sufficient capacity to meet the above mentioned
requirements. Besides it shall be able to withstand the impacts from faults in the grid where
unsuccessful automatic reclosure takes place without necessitating disconnecting the generation
unit. The unit shall have capacity to meet following three independent sequences:
•
•
•
at least two single-phase earth faults within two minutes
at least two two-phase short circuits within two minutes
at least two three-phase short circuits within two minutes
Additionally, there shall be sufficient energy reserves (emergency power, hydraulics and
pneumatics) for the following three independent sequences:
•
•
•
at least six single-phase earth faults with five-minute intervals
at least six two-phase short circuits with five-minute intervals
at least six three-phase short circuits with five-minute intervals
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Willi Christiansen & David T. Johnsen
5. Examples of limiting worst case scenarios
The aim of this chapter is to analyze the consequences of the requirements in the Grid Codes during
different load scenarios. Suggestions on to how the wind turbines should operate and respond
during stressing load situations, without violating the requirements in the Grid Code, are given.
5.1. Scenario 1, fast voltage changes in the transmission system
A Wind Farm Power Station (WFPS) is connected to the transmission system through an automatic
tap changing transformer. During steady state load conditions, the transformer keeps the voltage on
the low voltage side of the transformer close to constant.
The transformer settings ensures a voltage of approx. 1pu at the connection point of the WFPS,
even if the pre-fault voltage on the transmission system is relatively low (0.9pu).
It is then assumed that a fault occurs on the transmission system. The fault is cleared by the
protection system within 200ms. The post-fault voltage on the transmission system is now relatively
high (1.1 pu) due to switching within the transmission system. The automatic tap-changing
transformer cannot tap fast enough to compensate for the sudden increase of the voltage. Therefore
the post-fault tap position is equally to the pre-fault tap position. This means that the post-fault
voltage on the low voltage side of the transformer is higher (in pu) than the voltage in the
transmission system. If the voltage gets above 1.2pu, the wind turbines within the Wind
Distribution System (WDS) may trip due to the settings of their protection system.
This example shows that relative simple occurrences can result in tripping of the wind turbines,
even though the Grid Code is obeyed. It can be summarized that the individual requirements in the
Grid Code are adapted to the local conditions of the transmission system. It is therefore necessary to
simulate extreme load conditions including the WFPS, analyzing how the requirements in the Grid
Code should be handled.
5.2. Scenario 2, high voltage and pf requirements
A requirement in the Grid Codes is that a WFPS has to be able to run continuously during high
voltage situations (above +10%). The reactive power requirements of a Wind Turbine Generator
(WTG) are simultaneously within the pf range of 0.9 lagging to 0.95 leading at full production. The
combination of a high voltage situation and a pf of 0.9 lagging (the wind turbine produces reactive
power) do not seem realistic. A production of reactive power will furthermore increase the voltage.
During a high voltage situation, the WTG should therefore prevent it self from producing at full
lagging.
This example shows again that it might not be sufficient to fulfil the individual Grid Code
requirements independently. The Grid Code requirements should be seen as a combination with
realistic network conditions.
This consideration is what can be observed on Figure 3-3. A pf of 0.95 lagging has only to be
obtained at low voltages and visa versa.
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Analysis of requirements in selected Grid Codes
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5.3. Scenario 3, weakening of the network
The third scenario consists of a WFPS with the installed capacity of several hundreds of megawatts,
which is installed in a remote end of the transmission system. A nearby transmission fault results in
the tripping of one of two parallel lines to that section of the system. The wind generation rides
through the fault and following the fault clearing, the wind generation increases back to or near the
original pre-disturbance megawatt value. However, with the weakened link to that part of the
system and the significant level of power interchange between that area and the rest of the
transmission network, there may not be enough dynamic reactive power reserve in the vicinity of
the wind farms to maintain voltage stability. As an example, such dynamic requirements might be
provided by the application of a static VAr compensator (SVC), to regulate transmission system
voltage immediately after a severe disturbance, and thereby ensuring a fast and stable voltage
recovery.
5.4. Scenario 4, active power recovering after a voltage dip
In most Grid Codes it is required that the active power returns after a fault occurrences within one
second, as soon as the voltage is above 0.90pu. During a fault ride through event the active power of
a wind turbine decreases. This is a consequence of the low voltage and of the decreasing mechanical
power input.
The voltage return after a fault sequence is dependent on the level of the short circuit power of the
transmission system. The voltage will return to the pre-fault value immediately in a robust system.
Areas with high penetration of wind power are typically not strong networks. This can be expected
especially if the fault clearance mechanism trips an OHL line (Scenario 3).
Due to the Grid Code requirement the wind turbine will increase its active power production as
soon as the voltage is above 0.9pu. The voltage can as a consequence of this get unstable and
significant voltage backswings can occur.
A suggested solution to this event is that the active power return should occur less rapid. If the
voltage is kept beneath 0.9pu at the terminals of the wind turbine, the active power recovery can be
spread out over a longer period of time. The oscillation, overshot and instability of the recovering
voltage can thereby be kept at a minimum. A consequence of this is a temporary loss of active
power.
With this strategy, the pre-fault conditions can be achieved within 10 seconds. This could be a
helpful strategy to satisfy the requirements in the Grid Code in weak networks, but it requires a
number a simulations.
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6. Discussion
The investigations of the requirements in the Grid Codes from selected countries have shown
several technical aspects, which have to be taken into consideration in wind turbine connection
queries.
The technical aspects of the requirements vary in the different Grid Codes. Some aspects reflect
regional differences between the transmission grids. As an example, the frequency range can be
mentioned. The worst case Grid Code requires a continuously operation between 47.5 Hz to 52 Hz.
This is a wide range, and it can hardly be imagined that any country in Europe operates
continuously within this range.
The explanation for the lower limit of the range is that Ireland has a relatively weak network, in
which the frequency can drop to 47.5Hz. The transmission systems of the "Union for the Coordination of Transmission of Electricity" (UCTE), which is the association of transmission system
operators in continental Europe, have a frequency which is relatively fixed to 50Hz. It does
certainly not drop down to 47.5Hz under normal operation. Therefore the frequency range of
47.5Hz to 52Hz reflects the technical differences between the transmission grids of Ireland and of
continental Europe.
Furthermore it was shown that the worst case requirement to the voltage range is above ±10%.
During limited time operation the voltage range is as wide as ±20% in some cases. This requirement
does not seem justifiable seen from the perspective of other electrical components within the
transmission system, which do not withstand such a wide voltage range.
It should therefore be called into question, why wind turbines should withstand voltage ranges that
are far above the ranges of other technical equipments within the transmission system.
The Grid Codes guidance for installation recommends the installation of an automatic tap-changing
transformer at the connection point of the Wind Farm Power Station (WFPS) to adjust for the
deviation in voltage. However the problem will only be solved in steady state. A tap-changing
transformer can not adjust for immediate voltage deviation, which may occur during disturbances
(see scenario 1 in chapter 5. ).
In the previous chapter a load scenario was presented wherein the issues regarding momentarily
active power recovery after a fault clearance were described. It was suggested to raise the post-fault
active power production slowly. This could prevent voltage oscillations and overshoots. The
requirements of the German Grid Code might be the solution to this problem. The German Grid
Code describes in detail an active power recovery procedure, which should initiate no later than 1
second after the voltage has returned to the pre-fault value. The procedure allows the Wind Turbine
Generator to delay the active power recovery.
This regulation is a notable advance compared to the requirements in other Grid Codes, who
demand the active power to return immediately after the voltage has reached 90% of its rated value.
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Analysis of requirements in selected Grid Codes
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This is an example of how some of the complicated requirements in the Grid Code can be fulfilled
concurrently while still taking the technical limits of the network into account.
The above example shows that detailed network analyses and problem solutions has to be included
in the connection queries of wind farms. If the Grid Code has to be satisfied in weak networks, a
secure and continuously operation of the wind farm could become a challenging issue.
This conclusion is one of the most important results from the analysis of the most restricting Grid
Code requirements. It is not sufficient to obey the regulations of the Grid Code individually; the
Grid Code should rather be seen in its entirety and in relation to the given network.
Therefore network analyses have to be made for each wind farm connection query and the technical
limits of the specific transmission system should be taken into account. The solutions will show
how a specific generating unit can comply with the Grid Code requirements.
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Analysis of requirements in selected Grid Codes
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7. Conclusion
This Report contains studies of wind power connection requirements, which are described in Grid
Codes from different countries. The countries are chosen in the view of interesting wind power
aspects like technical possibilities and geographical limitations. Denmark is chosen due to the high
penetration of wind power. Ireland is an island-system, and it is therefore of great interest. The Grid
Code of EON (a German transmission system Operator (TSO)) is chosen due to the important wind
power market in Germany and due to the detailed Grid Code of EON. Furthermore the Scottish and
the Grid Code from UK are analyzed. Finally the Grid Code from the Canadian TSO, AESO, is
taken into consideration.
The wind turbine connection requirements from the mentioned countries have been compared. The
most restringing regulations have been outlined with the purpose of demonstrating which technical
challenges generic wind turbine models should be able to handle during continuous operation and
during faults. The analysis has been divided into a static and a dynamic part. The static section
concentrates on the continuously operation while the dynamic part describes the behaviour during
fault sequences and disturbances in the grid.
Many of the technical aspects are only mentioned in some Grid Codes. The explanation for this is
that each Grid Code reflects the technical conditions of the respective transmission system. Every
country has its own special technical issues to deal with, and this speciality is reflected in the Grid
Code.
The static part has shown two important aspects. Firstly the significant requirement of the pf range
is conspicuously. The most restringing conditions is that a wind farm at its connection point to the
transmission system running at full active power production shall be able to vary its power factor
(pf) between 0.9 lagging to 0.95 leading. Lagging means feeding reactive power into the grid. It has
to be pointed out that the pf range is only between 0.95 leading to lagging, if just the European
requirements are taking into consideration.
The second significant requirement is the huge frequency range, wherein the wind farm has to be
able to operate continuously. A wind farm power station is required to run continuously between
47.5Hz and 52Hz. The lower limit reflects the Irish transmission system, whereas the upper limit of
52Hz reflects the German system. Beside this frequency range, continuous operation in a voltage
range of more than ±10% of the nominal voltage is required.
The analysis of the dynamic requirements has shown that a wind turbine under no circumstances is
allowed to trip during the first 150ms of a voltage drop sequence. This requirement reflects the
demand for keeping the generating wind turbine units in operation. This can avoid the loss of active
power production. In case of a longer duration of a fault sequence, the operating limit is described
with a voltage duration curve. The Curve describes in detail, when a wind turbine may trip.
There are given detailed descriptions on how the reactive current and the active power production
should act during and after a fault sequence. Especially the active power recovery following a fault
is important, since an immediate active power recovery could cause voltage fluctuations in a weak
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
network. Especially the German Grid Code has included detailed requirements on the active power
recovery.
Finally some worst case scenarios are presented. They demonstrate how complicated it is to avoid
violating some of the Grid Code requirement under different load conditions. All Grid Code
requirements should be satisfied independently of each other. Simultaneously most technical
requirements have a mutual influence on each other.
Summarized it can be concluded that it is not sufficient to obey the requirements of the Grid Code
individually. The Grid Code should rather be seen in its entirety and in relation to the given
network, in which the wind farm is going to be connected. It requires challenging planning and
connection studies if the general Grid Code requirements, which have been derived by comparing
the Grid Codes from different countries, have to be satisfied.
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Analysis of requirements in selected Grid Codes
Willi Christiansen & David T. Johnsen
8. References
[1] Wind Power Facility, Technical Requirements, Grid Code from Alberta Electric System
operator, Canada
[2] Wind turbines connected to grids with voltages above 100 kV, Technical regulations for the
properties and the regulation of wind turbines, Grid Code from the Danish TSO,
energinet.dk
[3] Grid Code, High and extra high voltage, EON Netz, German TSO
[4] WFPS1, Wind Farm Power Station Grid Code Provisions, ESB National Grid, Irish TSO
[5] Scottish Grid Code, Scottish Hydro-Electric Transmission Ltd, Scottish TSO
[6] The Grid Code, Revision 12, National Grid Electricity Transmission plc, TSO in UK
[7] Integration of Wind Energy into the Alberta Electric System, Electric System Consulting
ABB Inc
[8] Large Scale Integration of Wind Energy in the European Power Supply, analysis, issues and
recommendations, A report by EWEA, January 2006-01-18
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Willi Christiansen & David T. Johnsen
Appendix
Appendix 1. Power curtailment scheme
Type of
regulation
Primary
regulation aim
Purpose
Absolute production
constraint
Limit the wind farm'
s current power production in the connection point
to a maximum, specifically indicated MW value. Constraints
may be necessary to avoid overloading of the power grid.
Limit production to optional
MWmax
Delta production
constraint
It must be possible to reduce the power production of the wind farm by
a desired power value compared to what is possible at present, thereby
setting aside regulating reserves for the handling of critical power
requirements.
Limit production by
MWdelta
Balance
regulation
The power production of the wind farm must be adjusted to the current
power requirement with a view to maintaining the power balance of the
balance responsible market player and/or the system operator.
downward/upward regulation of production must be possible.
Change current production
by -MW/+MW with the set
gradient and maintain the
production at this level
Stop regulation
The wind farm must maintain the power production at the current level
(if the wind makes it possible). The function results in stop
for upward regulation and production constraints if the wind increases.
Maintain current production
Power gradient
constrainer
For system operational reasons it may be necessary for wind turbines to
limit the maximum speed at which the power output changes in relation
to changes in wind speed. The power gradient constrainer is to ensure
this.
Power gradients do not
exceed the maximum
settings
System protection
System protection is a protective function which must be able to
automatically downward regulate the power production of the wind
farm to a level which is acceptable to the power system. In the case of
unforeseen incidents in the power system (for instance forced outage of
lines), the power grid may be overloaded at the risk of power system
collapse. The system protection regulation must be able to rapidly
contribute to avoiding system collapse.
Downward regulate power
production automatically on
the basis of an external
system protection signal
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Willi Christiansen & David T. Johnsen
Appendix 2. Nomenclature
Grid Code
Lagging
Leading
OHL
P
pf
pu
Q
Description of the connection
conditions
The phase of the current is
behind the phase of the voltage
The phase of the current is
ahead the phase of the voltage
Over Head Line
Active power [MW]
Power factor
Per Unit
Reactive Power [MVAR]
RMS
SVC
TSO
UCTE
WDS
WFPS
WTG
THD
32
Root mean square
Static Var Compensator
Transmission System Operator
Union for the Co-ordination of
Transmission of Electricity
Wind Distribution System
Wind Farm Power Station
Wind Turbine Generator
The total harmonic effective
distortion