7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Comparison of high technical demands
on grid connected wind turbines
defined in international Grid Codes
Tobias Bublat, Phone: +49 (0)40 36149 7267, Fax: +49 (0)40 36149 1720, tobias.bublat@gl-group.com
Tobias Gehlhaar, Phone: +49 (0)40 36149 7577, Fax: +49 (0)40 36149 1720, tobias.gehlhaar@gl-group.com
Germanischer Lloyd Industrial Services GmbH, Business Segment Wind Energy,
Steinhöft 9, 20459 Hamburg, Germany
Abstract -- Since grid operators changed technical
standards for grid connected wind turbines by
publishing relevant Grid Codes, it is well known that
some of them are extra demanding and also have
influence on turbine development process. The article
shall give an overview on the highest demands given
in a purposive selection of regulations. A comparison
will show deviations in height and in the point of
reference of requirements. The necessity of straight
forward verification procedures will be shown. Such
procedures, based on this comparison, solve problems
and discrepancies partly coming from imprecise
definition or inadequately made explanations of some
requirements.
Index Terms -- Certification, FACTS, FRT, Grid
Code, Grid Code Compliance, Grid Connection
Compatibility, Grid Support, IGCC, LVRT, Wind
Turbine, Type Certificate (GCC)
I. INTRODUCTION
The number of grid connection regulations
mostly called Grid Codes has risen to an almost
unmanageable amount since the first TSOs
(Transmission System Operators) have defined and
published changed requirements for the connection
of wind turbines and wind farms to the high-voltage
TS (Transmission System) in the late 90’s. System
Operators and authorities of countries, where high
penetration of wind energy in the electricity system
is expected in the upcoming years, have to take the
challenge to ensure grid integration of wind
turbines without any difficulties.
Wind turbines shall be capable of showing a
behavior during normal operation but also during
grid failure similar as required for conventional
generating units such as coal fired power plants and
other synchronous generating units. Wind farms
shall ride through faults in the TS and they
moreover shall make a contribution to voltage
control for stability purposes.
Unfortunately connection conditions given in
international Grid Codes differ from one country to
another. System Operators define special
requirements depending on the respective TS
infrastructure and control area. This often causes
confusion for all parties concerned, as they are
possibly not aware of the different requirements or
unscheduled changes in some Grid Codes.
Manufacturers are constantly challenged to adapt
the design of their turbines with the latest
requirements of the network operators. Wind
turbines shall be configured in a way that they are
in compliance with the technical regulations.
Straight forward verification procedures have to
be improved and harmonized proving the wind
turbines grid connection compatibility and showing
compliance with the requirements given in the
respective Grid Codes. A uniform procedure helps
manufacturers, developers as well as System
Operators and should include the following steps:
1.
2.
3.
4.
5.
6.
7.
Evaluation of Grid Codes and extraction of
requirements
Comparison of Grid Code requirements
Creation of a test plan based on recognized
testing procedures
Measurement by accredited testing
laboratories (ISO 17025) and issuing of a
final measurement report.
Observation of steps 1 to 4 and assessment
of measurements by an accredited
certification body (DIN EN 45011) issuing
a Type Certificate (GCC) for Grid
Connection Compatibility afterwards.
Validation of a wind turbine model and
site assessment regarding Grid Code
Compliance of the wind farm.
Simplified grid connection negotiations on
basis of project certified wind farms based
on type certified wind turbines.
A uniform basis will lead to easier contract
negotiations with System Operators. The
verification and certification procedure for a turbine
type (steps 1 to 5) needn’t be repeated for every
site. Such a Project Certificate (GCC) will be
required for wind farms connected to the medium
voltage Distribution System in Germany after 2011
the latest [8]. In Spain, Project Certificates (GCC)
are required since January 01, 2008 [9].
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Details for a straight forward verification process are
given in section III of this paper.
II. COMPARISON OF GRID CODE REQUIREMENTS
relevant target markets is indispensable. The following
comparison shall give an overview on the latest connection
conditions given in a purposive selection of Grid Codes
from TSOs from all over the world (table no.1).
International Grid Code comparison (IGCC):
steps 1 and 2
For the verification and certification of the wind turbines
Grid Connection Compatibility (GCC) a determination of
connection requirements applicable to wind turbines in
Country / Region
TSO
Author
Title
Grid Code
Date of issue
GB
NGET
NGET
Germany
E.ON
E.ON
Scandinavia
All
Nordel
China
All
CEPRI
Poland
PSE
PSE
INSTRUKCJA RUCHU
I EKSPLOATACJI SIECI
PRZESYLOWEJ
2006
Australia
NEMMCO
AEMC
National Electricity Rules
2007
Grid Code – High and
extra high voltage
2008
April 2006
Nordic Grid Code
Technical
Rule
for
Connecting Wind Farm to
Power System
(Recommendations)
2005
Fig. 1. Grid Codes from different TSOs [10]
Only the most important and technically sophisticated
requirements shall be discussed. The comparison of
requirements is divided into the following topics:
A.
B.
C.
D.
E.
Operation during frequency and voltage deviations
Frequency bound power limitation
Active power control
Power factor correction and voltage control
Grid support including LVRT (Low Voltage Ride
Through)
The grid connection requirements for wind turbines given
in above listed Grid Codes are valid at the PCC (Point of
Common Coupling) with the TS. The PCC is mostly defined
on the high-voltage side of the wind farms step-up
transformer, but exceptions prove the rule.
Wind farms are divided into embedded and non-embedded
systems in Great Britain [1]. Non-embedded generating units
are directly connected to the TS whereas embedded units
only have an indirect connection to the TS by another user
(e.g. a wind farm or Distribution System). Embedded and
non-embedded units need to fulfill different requirements.
Furthermore different requirements have been defined for the
two TS areas Scotland and England/Wales, which are also
depending on the rated power output of the wind farm. The
PCC of a wind farm might also be defined on a remote TS
bus-bar in Great Britain [1].
Germany is divided into 4 control areas managed by the
TSOs EnBW, E.ON, Vattenfall Europe Transmission and
RWE. Smaller wind farms or single wind turbines being
connected to the Distribution System are currently not
covered by the connection conditions of the German TSOs.
But new approaches in the German energy sector are
going in that direction and a new Grid Code [8] for the
connection of wind turbines and other renewable energy
technologies to the medium voltage distribution system will
be finalized by the federal association of German power
industry bdew soon. Similar requirements will then apply
to the connection of wind turbines at lower voltage levels,
too.
The Australian Grid Code defines requirements for the
connection to both systems, the Distribution System and
the TS [6]. Prior to the year 2007 wind farms and wind
turbines had been classified as unscheduled units due to
their intermitted power source and were exempted from
most requirements. Since March 2007 new amendments for
wind turbines have been published by the Australian
Energy Market Commission AEMC and had been
assimilated in the Grid Code. These connection conditions
are also applicable to wind turbines and are similar to those
from other Grid Operators. The only difference is that
different levels of difficulty have been specified.
Requirements have been subdivided to an automatic access
standard, a minimum access standard and a negotiated
access standard. For wind farms complying with the
automatic access standard grid access won’t be denied [6].
A. Operation during frequency and voltage deviations
Wind turbines are required to continuously maintain
active power output, even if voltage and frequency levels
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
exceed values defined for the TS under normal operating
conditions. In Diagram 2 it can be seen that those operation
areas differ from one country to another. They can general
be divided in three main areas.
Fig. 2. Required wind farm operation depending on voltage, frequency and time
The first area is the frequency range around the nominal
value of grid frequency, where normal operation of the wind
farm and all single wind turbines is required without
limitations.
At lower frequency values (second area) wind farms shall
at least operate for specified minimum times which in most
cases are linked to system voltage, a prohibited reduction of
the active power output level or a combination of both.
Automatic reduction in active power output during low
frequency events should not be bigger than 5% of the
available power in Great Britain for instance [1]. Exceptions
have to be made for a change in wind speed.
B. Frequency bound power limitation
If the system frequency rises above the upper limit of the
normal operation area (third area) wind turbines are generally
required to reduce their active power output with a given rate
of change in most countries. In this power reduction mode
the rate of change (Gradient G) is either bound to the
respective change in frequency or shall be accomplished
within a specified time range.
G=
∆P  kW 
∆t  s 
or
G=
∆P
∆f
 kW 


 f 
The following gradients or times can be extracted from
Grid Codes:
0,4 PAvailable
(at each wind turbine)
Hz
E.ON
G=
CEPRI
Power reduction permitted, but not specified
PSE
Constant power reduction rate down to P= 0%
Pr at 50,5Hz and a rise up to 51,5Hz or by
disconnection of single wind turbines
NGET
G≥
0,2 PAvailable
Hz
(Limited
Frequency
Sensitive
Mode;
reduction must be achieved within t= 10s)
Nordel
Required as proportional power reduction as a
function of system frequency. Detailed
settings will be defined by the TSO.
AEMC
Minimum access standard for wind farms at
Transmission level and P = 30MW: Loading
level shall be reduced by an amount of P=
50% Pr
•
within t= 3s, and holding the output level
until frequency recovers to normal
operation frequency band
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
G≥
•
AEMC Q= ±0,395 Pr,
0,167 PAvailable
s
for 0 < P = Pr, U = Ur ±10%
Nordel Capability to operate with zero reactive power
exchange required at PCC
or by disconnecting the wind farm within
t= 1s
E. Grid support including Low Voltage Ride Through
(LVRT)
C. Active power control
Wind farms with a nominal capacity equal to 50 MW or
higher located in Great Britain or wind farms in Australia for
instance might be instructed to make a contribution to
frequency control if agreed upon [1] [6]. They will then be
required to provide Primary, Secondary and High Frequency
response. Wind farms are generally exempted from this
requirement in other countries, but might provide operating
reserve as an ancillary service.
For implementing such control area wind farms shall be
capable of varying their active power output accordingly
managed by signals provided by the System Operator.
Defined set points shall be adjusted with gradients similar to
those described under B., but the rate of change differs from
one country to another.
not less than G =
e.g. E.ON
∆S N  kVA 
∆t  min 
=
In the past wind farms and wind turbines have been
disconnected during grid failure as fast as possible. This
might lead to a collapse of the TS in circumstances due to
the high penetration of wind energy in some regions and a
resulting loss of thousands MW wind power generation.
For avoiding such scenarios grid operators decided that
wind farms shall run through grid faults without
disconnection as it is required for conventional power
plants.
Wind turbines shall stay connected for specified voltage
and time ranges as shown in Diagram 4. Disconnection is
generally not permitted for voltage values above the lines
shown in Diagram 4 and above limit line 1 in Diagram 5
for instance. In addition, some grid operators require
contribution to voltage stabilization and system recovery
during and after grid failure.
0,1 ⋅ S N
min
D. Power factor correction and voltage control
Wind farms are required to balance voltage deviations at
the PCC or the respective connection point by adjusting their
reactive power exchange and moreover by settling up
predetermined power factors. Voltage deviations due to load
flow changes shall be compensated. Examples are shown
below.
E.ON
Phase to phase
voltages [kV]
440
253
127
420
245
123
Fig. 4. LVRT requirements of TSOs
380
220
110
350
193
96
Power factor
1
0,95
0,95
inductive
capacitive
Fig. 3. Power factor control range E.ON [2]
Provided that
49,5 Hz < f < 50,5 Hz
P = Pr
CEPRI Depends on the wind turbines generator system and
grid configuration. Voltage regulation is required at
PCC
PSE
0,975ind - 0,975cap,
for 0 < P = Pr
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Dip duration above 140ms: Restoration of active
power output to 90% of PPre-disturbance within 1.0s
after the voltage returns to U= 90% of rated
system voltage.
AEMC Restoration of active power output to 95% of
PPre-disturbance within 100ms after fault clearance
(Automatic access standard).
III. STRAIGHT FORWARD VERIFICATION PROCEDURE
The 7 steps of this approach, named in the introduction
will be explained in the following section in more detail.
Fig. 5. LVRT requirement E.ON
With the exception of the Polish System Operator LVRT
capability is required for voltage dips down to zero voltage at
the PCC [5].
With respect to the older Grid Code E.ON has
dramatically changed the LVRT requirements in the Code of
2006 [2]. The voltage-time-diagram (Diagram 5) is now
divided in more areas defining the turbines behaviour during
grid failure. Below 1500ms and depending on the respective
area brief disconnection by the wind turbines might be
allowed, if defined re-synchronisation times are kept. This
will be agreed with the System Operator. If system voltage
does not recover to 80% of the highest phase-to-phase value
measured at the low voltage side of each turbine transformer
a quarter of all turbines in the wind farm shall be
disconnected by automatic protection devices after 1.5s, 1.8s,
2.1s and 2.4s [2].
Wind farms shall stay connected during 3-phase faults in
the TS until the primary protection system clears the fault in
Australia. For all other types of fault the wind farm shall
operate for the longest time expected for the respective
breaker fail protection to clear the fault or the highest of
430ms and the time needed by the relevant primary
protection system to separate the faulty grid element [6].
Additional support during faults is required in Australia,
Germany (E.ON) and the UK. Wind farms have to maximise
their reactive current generation for system voltage
stabilization. This has to be done with a defined reactive
current injection of minimum 2% INominal per %? U at the low
voltage side of each wind turbine transformer in Germany
and with the greater of Ib Pre-disturbance and 4% INominal per %? U
(Automatic access standard) at the PCC in Australia [2] [6].
Further Grid Support shall be given after fault clearance.
Active power has to be restored with a minimum gradient of
G= 0,2Pr / s (E.ON) or to a defined output level within a
specified time [2]. The latter applies to Great Britain and
Australia. Requirements are shown below.
NGET Dip duration up to 140ms: Restoration of active
power output to 90% of PPre-disturbance within 500ms
after the voltage returns to U= 90% of rated system
voltage.
The above discussed requirements of different Grid
Codes are examples only. All requirements have to be
translated, evaluated (step 1) and compared (step 2). The
comparison has shown differences between single demands
and their point of reference. Before testing turbine
capabilities it has to be ensured that the selected Grid
Codes are applicable to wind turbines, that requirements for
wind turbines have been determined properly and that the
point of reference for requirements has been clearly
defined. Success criteria have to be included to an
appropriate test plan to verify the turbines capability of
Grid Code Compliance.
Verification testing: steps 3 and 4
For the verification that a turbine complies with
requirements as given above full scale hardware tests have
to be performed according to specificly created test plans.
The test procedure applied has to be converted in a way
that different points within the given voltage-time curves of
TSOs will be tested to verify the LVRT capability of a
turbine. The same could be done to FACTS (Flexible
Alternating Current Transmission System). 3-phase and 2phase voltage dips with different duration have to be
performed and accomplished by the turbine. GL Wind–
Technical Note 066 [7] describes such a test series. An
accredited testing laboratory has to perform testing and
measure the test results, which have to be successful
accomplished by the turbine. All relevant values shall be
measured and reported in a measurement report. Now, an
independent certification body shall be involved at the
latest. The most appropriate and effective way to prove the
turbines conformity to regulations is seeking the assistance
of a certification body accredited for the Certification of
Grid Connection Compatibility for Wind Turbines
according to Grid Codes (DIN EN 45011). The very
expensive and time-consuming repetition of tests and
measurements can be avoided by a good test plan, which
already should be assessed by the certification body.
Type Certification (GCC): step 5
The final measurement report will be assessed and
results will be compared with the requirements given in
Grid Codes. If the results comply with all the requirements
a certificate will be issued stating the Grid Connection
Compatibility for a single wind turbine type for the
respective Grid Codes used as assessment criteria.
Project Certification (GCC): step 6 and 7
Simulations of the electrical behaviour of a wind farm
will be facilitated by using the measurements of type tests.
Most requirements given in Grid Codes refer to the PCC.
7th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms
Results of type test must be converted to this point regarding
the local conditions at site. Components such as transformers,
filters, FACTS and transmission lines have to be considered,
too. For an accurate simulation and the achievement of best
possible results the validation of a turbines simulation model
is indispensable. This calls for a uniform validation
procedure, containing both, testing and simulation.
changes. In the long run it will be very helpful having
validated simulation models.
IV. OUTLOOK TO EXPECTED CHANGES IN THE FUTURE
Line-to-line voltage
In the past most system operators did not check, whether
wind turbines connected to their grid complied with their
rules, neither if the wind farms did. Some started with asking
for some proving measurement, discussing the results with
the wind farm planner during the power purchase agreement.
More and more manufacturers of wind turbines started
testing and measuring the behaviour of their turbines during
full scale tests in the field. A way of testing with medium
voltage short circuit testing units became a widely accepted
method of proving the ability of wind turbines to ride through
low-voltage events without disconnection. Such equipment
can be seen in figure 6. The world wide amount of such
containers is rising. The most of them (6 in total) are
Medium
VoltageVoltage
Substatio
n
Literature
[1]
[2]
[3]
[4]
[5]
Time fault occurred
Fig. 5. LVRT requirement E.ON
in the possession of WINDTEST Kaiser-Wilhelm-Koog, one
is in Spain owned by Energy-to-Quality and some more are
direct property of several wind turbine manufacturers.
The outcome of many measurements is a lot of discussion on
the results. This is calling for standardisation. Germanischer
Lloyd issued 2 technical notes [7] and [11] with this aim,
enabling the Type Certification for Grid Code Compliance as
well as Project Certification for Grid Code Compliance
internationally.
Spanish government started requiring compliance with one
aspect of Spanish Grid Code (the part mentioned in P.O.12.3,
[13] which is LVRT only). AEE started defining a standard
test and measurement procedure to be used for project
certificates as required by Spanish government. This PVVC
[12] contains also a way showing compliance by using
simulation. This simulation needs to use models validated
with full scale tests at a wind turbine in the field. PVVC [12]
also contains a way of how validation can be done.
In Germany a new governmental requirement is on the way,
asking for Type Certification concerning more than only
LVRT. Furthermore a Project Certificate will be required for
each wind farm. This calls for simulation again, but it is also
stated clearly, that validation needs to be done.
As Grid Codes do not stop changing, simulation is being
more and more important. Harmonisation efforts for Grid
Codes are starting, e. g. by a group of experts at EWEA.
Unfortunately this will lead to additional Grid Codes, being
structural harmonised in a first step.
First changes in the grid leading to smart grid options,
FACTS related to new offshore grids and improved
cooperation between national grids will lead to more
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
National Grid “Grid Code”, Issue 3 Rev. 25, National Grid
Electricity Transmission, Warwick 2008
E.ON Netz “Grid Code High and extra high voltage“, E.ON Netz
GmbH, Bayreuth 2006
Nordel “Nordic Grid Code 2007 (Nordic collection of rules)”,
Helsinki 2007
CEPRI “Technical Rule for Connecting Wind Farm to Power
System
(Recommendations)”, GB/Z 19963-2005, Draft, China 2005
PSE “INSTRUKCJA RUCHU I EKSPLOATACJI SIECI
PRZESYLOWEJ”, Wersja 1.0, PSE Operator S.A., Warszawa
2005
AEMC “National Electricity Rules Version 19”, Australian
Energy Market Commission, Sydney 2008
GL Wind–Technical Note 066 ”Grid connection compatibility of
wind turbines according to Grid Codes (NAR) –Low Voltage
Ride Through (LVRT), Test procedure” , Germanischer Lloyd
Industrial Services GmbH, Hamburg 2005
VDN “Erzeugungsanlagen am Mittelspannungsnetz - Richtlinie
für den Anschluss und Parallelbetrieb von Erzeugungsanlagen am
Mittelspannungsnetz“, Entwurf von Juni 2007, Berlin 2007
RD 661/2007 „REAL DECRETO 661/2007, de 25 de mayo, por
el que se regula la actividad de producción de energía eléctrica en
régimen especial“, MINISTERIO DE INDUSTRIA, TURISMO
Y COMERCIO, Spain, 2007
http://www.gl-group.com/pdf/IGCC_list.pdf
GL Wind–Technical Note 065 ”Grid connection compatibility of
wind turbines according to Grid Codes (NAR) – certification
procedure” , Germanischer Lloyd Industrial Services GmbH,
Hamburg 2005
AEE “PROCEDURE FOR VERIFICATION VALIDATION
AND
CERTIFICATION OF THE REQUIREMENTS OF THE PO 12.3
ON THE RESPONSE OF WIND FARMS IN THE EVENT OF
VOLTAGE DIPS“ Madrid 2007
PO 12.3: “Requirements regarding wind power facility response
to grid voltage dips”, Ministerio de Industria, Turismo y
Comercio, Madrid 2006