Download Certification of advanced Electrical Characteristics

Certification of advanced Electrical
Characteristics based on Validation of WEC
Models and Simulation of Wind Farms
Bernhard Schowe-von der Brelie, Dr. Hendrik Vennegeerts, FGH e.V.,
Martin Schellschmidt, ENERCON GmbH
Abstract-- This paper describes the technical solutions
ENERCON has implemented at their wind turbines in order to
meet advanced requirement on dynamic voltage support. An
overview on the status quo of certification regulations and
implementation approaches in selected countries both on single
WEC types and on wind farms is provided. Furthermore, the
model validation and subsequent certification procedure FGH
Certification Office has developed and applied to first wind
farm certifications will be presented. A focus will be given to
the validation of the WEC model and the simulation of the
wind farm’s LVRT behaviour. Finally, potentials for
harmonisation of the certification and, subsequently, on the
underlying testing and validation schemes will be sketched.
Index Terms-- Power system stability, power system
dynamic stability, reactive power, voltage control, wind power
generation, certification, model validation
I. INTRODUCTION
Wind power remains one of the main drivers for ongoing
restructuring of German and European power systems. As
the number of Wind Energy Converters (WECs) being
integrated in the transmission and distribution system is
increasing and thus also their share of the overall energy
production, it is necessary for WECs to participate in
ensuring grid stability like conventional power plants do.
For this purpose, grid codes have been introduced by
system operators or respective associations in many
countries that list the requested electrical characteristics at
the point of common coupling to the power grid (medium
voltage and high voltage level).
The requirements stipulated in current national and
international grid codes call for sophisticated WEC
technologies. Today, generation and consumption of
reactive current during faults depending on residual voltage
are essential qualities requested from the WEC. ENERCON
has been a forerunner in serving these new requirements by
innovative control strategies since many years.
At the same time, more and more grid codes imply
specific schemes of proving the gride code compliance. In
some countries certificates have to be provided not only for
single WEC types but also for the system behaviour of
entire wind farms. Corresponding certification schemes are
highly in focus of all stakeholders worldwide and have been
foremost established in Europe. Respective testing
directives have been incorporated into the IEC standards
and on national levels. For the LVRT test a unique on-site
testing facility has been developed at FGH which is today
included in the IEC 61400-21 standard.
However, the behaviour of wind farms in terms of their
aggregated connection to the power grid at the PCC can not
be predicted based solely on the aggregation of
measurements results of single WECs as interactions have
to be accounted for. Neither a measurement at the PCC for
the entire wind farm is applicable due to the high rated
power of wind farms at the PCC with usually more than
10/20 MVA.
Hence, new approaches have to be established based on
simulation of the wind farm’s behaviour.
The validity of the WEC model has to be ensured by
model verification with existing field test results taking into
account appropriate inherent inaccuracies.
II. REQUIREMENTS OF GRID CODES
Along with the expansion of installed wind power
capacities the requirements of Grid Codes have increased in
terms of a demand for advanced capabilities of the WECs.
Typical requirements today are:
• Active Power Control
(point setting; frequency and voltage variable)
• Reactive Power Control
• in normal system operation (power factor setting)
• during system faults (Q-U-Mode; increased
reactive current injection during voltage dips)
• Low-Voltage-Ride-Through (LVRT) capabilities to
stay connected the power system in case of grid-wise
voltage dips
• System Disturbances acc. to IEC EN 61400-21
(flicker, switching operations, (inter)harmonics)
The LVRT capability provides an important contribution
to the system stability as an additional loss of gigawatts of
wind power in the system would increase serious blackout
potentials. In an extension, the reactive current injection
depending on the depth of the voltage dip provides a further
step to support the system recovering from the fault in terms
of backing the voltage. Modern WECs are capable of
providing this service by means of various options.
The requirements to provide reactive power during a grid
fault were first stipulated in the E.ON Netz GmbH Grid
Code issued 1 April 2006 [1]. (See Fig.2) In the following
years, these requirements were incorporated into further grid
codes. The principle of current injection during grid faults
has also been adapted in the BDEW Directive for Medium
Voltage of June 2008. As this directive is explicitly asking
for a certification of both single WEC units and WEC
clusters, e.g. wind farms, for all new installation as from
2009, there is an urgent need for respective certification
schemes in terms of this strongly dynamic behavior of
WECs. Generally the current injection is required for threephase failures as well as for asymmetrical ones.
immediately. This mode is used for riding through faults in
radial distribution systems without fault current
contribution.
The next stage was the development of the PHI Mode. In
selecting this mode a constant reactive current can be set for
the event of a dip in voltage.
Last years ENERCON focused on the development of the
QU-Mode as an enhanced UVRT (under-voltage-ridethrough) Mode. The WEC feeds in reactive current
depending on residual voltage according to the requirements
of the Grid Codes. A series of measurements were taken
with four different voltage dips in field tests on a test
turbine with a UVRT container (Tests A to D, see Fig. 3).
The validation of this feature is demonstrated in Fig. 6.
Fig. 3: Validation of the requirements for reactive current injection during
under- or overvoltage
Fig. 1 Different LVRT requirements from selected grid codes [FGH]
Current Grid Codes in Germany state that in the event of
a voltage dip of more than 10% of the generator's effective
voltage value, reactive current of a minimum of 2% of the
rated current must be provided per percentage of voltage
dip. Furthermore, once voltage has returned, voltage support
must be maintained for another 500ms as shown in Fig. 2
[1].
In another trial run (Test E), the WEC was tested during
overvoltage. At an approximate rated voltage of 119%
where voltage is situated above the deadband, the WEC
injects reactive current into the grid in order to reduce
voltage. This trail run is illustrated by Test E (Fig. 4).
Another Grid Code requirement states that after voltage
has returned to the deadband range, voltage support must be
maintained for a further 500ms in accordance with the
specified characteristic. (Fig. 1)
Fig. 4: Results of the overvoltage test
Fig. 2 Principle of voltage support in the event of a grid fault [1]
III. ENERCON SOLUTIONS
Since 2002 ENERCON has done extensive work to
develop the power plant characteristics of its WECs in
thousands of tests. The advantage of ENERCON WECs
with fault ride through performance is the high flexibility to
adapt to the technical and formal needs of the system.
ZERO POWER Mode (ZPM) was developed in the first
stage. During a fault the WEC immediately stops feeding in
current but stays in full operation (rotor keeps rotating). If
voltage returns, the WEC recommences current injection
Fig. 5: Maintaining voltage support for another 500ms after returning to
normal voltage band
Fig. 6: Results of the undervoltage tests
As shown in Fig. 5, after a voltage dip, higher grid
voltage was determined by the test set up. After fault
clearance, voltage increases to 100% above rated voltage, so
that the WEC switches from generation mode (voltage
support) for undervoltage, to absorption mode for
overvoltage. This operating point is maintained for 500ms
before the WEC continues in normal operation.
The WEC’s behaviour during grid faults was commented
by the FGH Certification Office as follows in early 2009:
“Generation and consumption of reactive current during
three-phase faults is one of the crucial requirements in the
present German Grid Codes”
“A steady dependency of the reactive current on the
extent of a voltage dip is an essential condition for a stable
reaction of a wind turbine in the power system“
“The ‘QU-Mode’ solution presented by ENERCON is the
most promising alternative to fulfil these requirements”
“FGH Certification Office will examine the conformity of
the WEC performance with the Grid Code requirements in
detail, when the final test report is available”
New international specifications require flexible reaction
to faults. Voltage may dip even further during a grid fault
and the WEC then has to react to this new dip. In this case,
ENERCON provides a solution, as shown in Fig. 7.
ZERO POWER Mode can be underplayed with an
adjustable trigger threshold between 0 and 45% Urated. If
voltage drops below the set threshold, the WEC stops
injection. If voltage recovers without reaching the rated
value, it is supported through reactive current injection. For
varying voltage dips, this allows the WEC to react
accordingly in the event of a fault.
Fig. 7: UVRT-Mode with underplayed ZP-Mode
IV. CERTIFICATION
Since 2003 ENERCON validates the power plant
characteristics of its WECs. ENERCON has ordered FGH to
certify all of its new types to minimize the effort for
commissioning and compliance tests on site and to provide
reliable planning data to system operators. FGH, an
renowned research association on power systems in
Germany, has established a certification office in 2004 for
“Power Generation Characteristics” of dispersed generation
units. This accredited office certifies type specific electrical
characteristics of power generation units according to the
respective Grid Codes.
In the past so called product certificates have been issued
by FGH Certification Office. These certificates are based on
a conformity check of the results of field tests at a single
representative of that WEC type to be performed by
accredited testing institutes [7].
As of January 2009 the BDEW Directive for Medium
Voltage stipulates that dispersed power generation units and
plants connected to the medium voltage grid have to be
certified. Furthermore, a newly published directive to the
German Renewable Energy Act (EEG) of 2008 is asking for
a certification of WECs as well. Hence, a new approach had
to be developed as a purely assessment of the clusters’
behaviour at point of common coupling (PCC) based on an
aggregation of individual characteristics as proved by type
certificate would totally neglect the various dynamical
feedbacks between WECs and other assets in the generation
cluster. Therefore the grid codes are asking for a stationary
and dynamical power flow calculation based on validated
models of the single generation units.
In the following, different working groups have been set
up under the patronage of the German Association for Wind
Power Promotion FGW to elaborate the respective technical
guidelines: for field measurement, for model validation and
for the certification process itself. FGH has chaired the
working group on defining the certification process [9].
Modelling WECs’ electrical characteristics with highly
transient dynamics under failure conditions in terms of a
reliable validation and certification process posed a
challenge to all stakeholders involved. The envisaged
schemes have to be
i) valid for different kind of WECs and their
correspondent models,
ii) reliable in terms of the utilized simulation software
and finally
iii) trustful as some internal information on the control
algorithms have to be provided.
The now developed scheme provides two types of
certification: a) a unit certificate likely to the product
certificate based on a conformity test of a single unit but
enhanced with a model validation of that respective unit
type. b) a cluster certificate for the entire generation cluster,
e.g. wind farm, consisting of the same or different kind of
generation units, which is based on a simulation of the
cluster’s behaviour taking into account the validated models
of each unit involved. While the unit certificate proves the
electrical characteristics of that single unit under laboratory
conditions, i.e. at the artificial reference point to the cluster
internal grid right behind the testing equipment, the cluster
certificate provides information about the cluster’s
behaviour at the PCC as requested by most Grid Code.
Under the chair of FGH the technical guideline for power
production units, Section 8 (FGW TR8), has elaborated the
certification scheme for the unit certification in summer
2009 [8]. FGH Certification Office has positively concluded
an accreditation audit in August 2009 in order to be entitled
to carry out inspections for unit and plant certification.
In contrast to cluster certification which has to be
requested by e.g. the wind farm owner, unit certification is
in the responsibility of the WEC manufacturer.
V. MODEL VALIDATION
The model validation can be seen as the most crucial step
in the certification scheme. On the one hand the certifier has
to be provided with the most detailed and comprehensive
information about the WEC control in order to verify the
accurate mapping of the control processes onto the model.
On the hand the manufactures naturally want to hide as
much internal information about the WEC control as
possible in order to keep competitive advantages. The
following process is foreseen in the TR8:
The manufacturer has to provide a comprehensive,
computer-based model of the generation unit, which may be
– and presumably will be in most cases – encapsulated as a
black box model. The model must be executable in
commercial grid analysis applications and thus be capable of
describing the electrical characteristics upon which the
certificate is based on by simulating the testing procedures
as performed in the field test, hence enabling a verification
of simulation results against real field test results.
In addition, an open, where necessary simplified, model
of the unit must be provided. The open model must allow
the certifier to follow the logical links between control loops
in the relevant system controls. The degree of detail of the
open model may be clarified in advance between the
certification authority and the manufacturer. In some cases it
may be sufficient to present block diagrams.
All models are confidential and are not passed on by the
certification authority.
Model
Validated Model
Validation
measurement
Generation
Unit
unit
grid
Simulation
Fig. 8: Model validation via verification t
hrough measurements
Due to the fact that most manufacturers can only provide
models to describe the positive sequences in these days,
these models are not capable to display the WECs reaction
to asymmetrical failures yet. Therefore, it is necessary to
comprehensively reveal the unit’s fault detection scheme in
order to at least describe the correct switching into different
performance modes in the respective fault situations.
The model may be subdivided into several models
specifically for verifying particular characteristics.
The models must be capable of modelling all
characteristics relevant to power station operation for
normal grid operation and for a fault situation in a network
analysis program. It has been agreed that models capable to
root square means calculations are sufficient with regard to
te characteristics to be validated.
Model validation is performed by the certification office
based on the comprehensive, computer-based unit model
i) by comparing simulation results to the measured data
given in the test report,
ii) as well as on the basis of simulation results for test
specifications for a variety of defined setpoint and/or
grid conditions.
In terms of the comparison a complex scheme of tolerated
deviations between the measurement and the simulation has
been developed.
The validated model will be clearly identified with the unit
certificate. It will be stored at the certification office to
provide highest confidentiality for its further application in
wind farm simulation in the course of cluster certification.
VI. WIND FARM SIMULATION
FGH has set up a certification scheme for generation
clusters like wind farms, that will apply the results of the
unit certification in order to evaluate the cluster’s electrical
characteristics at the PCC. The evaluation is principally
based on four different kinds of examinations:
i) Stationary power flow calculations to verify:
a. reactive power supply in normal operation
according to the prerequisites of the Grid Code
b. current carrying capacity with respect to thermal
stress of assets
c. impact of cable capacitances
d. impact of additional assets in the cluster‘s internal
grid
e. relative voltage fluctuations at the PCC and
adherence to admissible voltage bands (EN
50160) within the cluster‘s internal grid (to avoid
unintentional protection releases)
ii) Dynamical simulations of the cluster’s behaviour in
failure situations (symmetrical and asymmetrical) to
evaluate the LVRT capabilities based on the
validated unit’s model. The reactive current injection
has to be calculated with respect to the PCC taking
into account all the cluster’s assets in terms of
inductive or capacitive interaction as well as the
system’s protection measures.
iii) Further straight forward calculation with respect to
a. characteristics for System Pertubations of single
Units must be transferred to cluster according to
IEC 61400-21
b. flicker
c. harmonics
d. fast voltage fluctuations
iv) Finally, the application of fixed characteristics of the
single units like the active power control – by setpoint or frequency-dependent – leads to a
consideration of the respective behaviour of the
entire cluster by simple aggregation.
For calculation purposes the public grid shall be
represented by its short-circuit power and the impedance
angle, which has to be provided by grid operator. All cluster
grid internal data must be provided by the plant’s operator.
Based on these calculations the yielded electrical
characteristics of the wind farm will undergo a conformity
check with respect to the underlying grid code, thus
providing a vote for or against issuing a cluster certificate.
VII. SUMMARY AND CONCLUSIONS
The requirements stipulated in national and international
Grid Codes require sophisticated WEC technology.
Generation and consumption of reactive current during
three-phase faults depending on residual voltage are the
essential qualities required of modern power plants.
Certification of WECs by independent and accredited
third parties like FGH Certification Office is essential to
provide reliable planning data for system operators and
customers. The requirements for wind farm certification
includes the validation of WEC models for dynamic or
transient power system studies. The newly developed
certification schemes at FGH form a reliable basis to meet
this challenges.
VIII. REFERENCES
[1]
[2]
[3]
[4]
Grid Connection Regulations for High and Extra High Voltage, E.ON
Netz GmbH, Apr. 2006
TransmissionCode 2007 Netz- und Systemregeln der deutschen
Übertragungsnetzbetreiber, VDN, Aug. 2007
Technische Richtlinie Erzeugungsanlagen am Mittelspannungsnetz,
BDEW, Ausgabe Juni 2008
S. Wachtel, S. Adloff, J. Marques, M. Schellschmidt, “Certification of
Wind Energy Converters with FACTS Capabilities” in Proc.
European Wind Energy Conference; Brussels, 2008
[5]
[6]
[7]
[8]
[9]
S. Wachtel, J. Marques, E. Quittmann, M. Schellschmidt, “Wind
Energy Converters with FACTS Capabilities and the benefits for the
integration of wind power into power systems”, 7th International
Workshop on Large-Scale Integration of Wind Power into Power
Systems as well as on Transmission Networks for Offshore Wind
Farms, Madrid, Spain, 2008
M. Schellschmidt, M. Kruse, Dr.-Ing. K-H Weck, „Certification of
Wind Energy Converters with enhanced UVRT options“, 9th German
Wind Energy Conference, Bremen, 2008
T. Smolka,, et. Alt: Grid Integration of Wind Energy Converter –
Experiences of Measurements and Status Quo of Certification
Procedures; IEEE-CIGRE Conference, Calgary, Canada, 2009
B. Schowe-von der Brelie, Th. Smolka, P. Siemes, H. Vennegeerts,
A.Schnettler.: Certification of wind farms with respect to their power
generation unit characteristics – first experiences, obstacles and
challenges of model validation and testing procedures; EWEC 2009,
Marseilles, France, 2009
FGW: Technische Richtlinie Teil 8: Zertifizierung der elektrischen
Eigenschaften von Erzeugungs-einheiten und –anlagen am MittelHoch- und Höchstspannungsnetz, Revision 0; FGW, Kiel, Germany,
2009 (english version soon available)
IX. BIOGRAPHIES
Bernhard Schowe-von der Brelie (1971)
graduated in physics at Philipps-Universität
Marburg in 1997. Since 2006 he is responsible for
the business development at the Institute for High
Voltage Engineering at RWTH Aachen University
and for the Research Association for Power
Systems and Power Economics (FGH) e.V. in
terms of acquisition and project management.
Since 2008 Mr. Schowe-von der Brelie is the
deputy Head of FGH Certification Office. He is active in German national
working groups on certification schemes for proofing the conformity of
single wind energy converters and entire wind farms with respect to the
new German grid codes.
Dr. Hendrik Vennegeerts (1973) is with
Forschungsgemeinschaft für Elektrische Anlagen
und Stromwirtschaft e.V. (FGH) since August
2004, first as consultant in and then head of the
system technique section. Since August 2006 he is
head of the department system studies / software
development / training. Here, among other things,
various studies and research projects on the
electric power supply networks are performed and
the software product INTEGRAL, a network
calculation tool, is further developed. Mr. Vennegeerts is actively involved
in many national and international advisory and steering bodies. He is
heading the institutes studies of WEC model implementation and
validation.
Martin Schellschmidt was born 1974 in
Hannoversch Münden, Germany. He finalized his
studies in electrical engineering with a degree as a
Dipl.-Ing. in electrical engineering at the
University
of
Applied
Sciences
of
Friedberg/Hessen, Germany. Since 2001 he is
working with ENERCON in the R&D division.
He manages there the R&D Group of validation
and funtional testing at the ENERCON head
office in Aurich, Germany. Focus of these
workings is the validation of wind turbine generators and especially their
power plant capabilities. Mr Schellschmidt is active in German national
working groups on measuring and certification schemes for wind energy
converters and customer generation power plants.