European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
Modelling of Distributed Energy Resources with
ATP-EMTP
Thomas Degner, Alfred Engler, Oleg Osika
ISET e.V., Germany
Department Engineering and Power Electronics
Koenigstor 59
34119 Kassel, Germany
Phone +49 561 7294 232 / Fax +49 561 7294 400
tdegner@iset.uni-kassel.de
Abstract - Distributed Energy Resources (DER), e. g. wind turbines, combined heat and
power units, photovoltaic and inverter controlled DER, gain importance in today’s power
system. Their fluctuating behaviour and their specific control characteristics require
manifold investigations. Due to the nature of ATP-EMTP with regard to simulating supply
structures it is a suitable tool for such investigations, but requires the addition of models of
DER. ISET has utilised ATP-EMTP for different power system studies with consideration of
DER. The developed DER-models will be introduced in this contribution. A compilation of the
different DER-models into a library is planned.
Keywords: Power system analysis, distributed energy resources, system control, wind
turbines, inverter control
1 Introduction
Distributed Energy Resources (DER), e. g. wind turbines, combined heat and power units,
photovoltaic and inverter controlled DER, gain importance in today’s power system. Their
fluctuating behaviour and their specific control characteristics require manifold
investigations.
ATP-EMTP is an established analysis tool for power systems. It is also a suitable tool to
study the integration of DER into power systems. Due to the license conditions it is especially
attractive for universities and education. However, suitable models for DER need to be
developed.
ISET has utilised ATP-EMTP for different power system studies with consideration of DER.
In the following sections we present some of the developed models and give a few application
examples.
2 DER Models
2.1 Wind turbine
The developed model represents stall controlled wind turbines with induction generator. In
detail the model includes the rotor, the mechanical drive including elasticities, damping and
gear drive, the induction generator, the reactive power compensation unit, the grid
interconnection unit, and the transformer to the grid.
The model was implemented in ATP-EMTP (see Figure 1). An ATP-EMTP universal
machine UM3 is used to simulate the induction generator. The other main components are:
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
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Compensation unit consisting of 12 condenser banks and the control logic. The
different steps of the compensation unit are automatically connected with a certain
time delay.
Logical block which evaluates the rotor speed
Logical block which evaluates the voltage
Measurement of rotor speed (signal required to limit rotor speed via a brake)
Group to simulate a mechanical brake
Group to simulate wind speed including tower shadow effect
Group for grid connection and for measurement of active, reactive and apparent
power, power factor, current, and voltage.
Output transformer connected over a line
Figure 1: ATP-Draw representation of the wind turbine model implemented in ATP-EMTP. The
different functional groups are mentioned in the text.
2.2 Diesel motor generator set
At present diesel motor generator sets (DGS) still play an important role in the power supply
of island grids. The developed model represents a diesel generator set which includes a
synchronous generator with voltage and frequency control loops. Figure 2 shows the
implementation in ATP-EMTP with the different functional groups.
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
Figure 2: ATP-Draw representation of the DGS model using a synchronous machine model
SM59_FC. (1): voltage control loop, (2) frequency measurement (3) frequency control loop.
2.3 Bi-directional battery inverters
A model for bi-directional battery inverters, which are able to form grids, has been developed.
The inverters are represented by a frequency and voltage controlled three-phase voltage
source (Fig. 3).
The “Models”-language, which is a feature of ATP-EMTP, enables the direct encoding of
voltage source inverters (VSIs), see Figure 4. The parameters of the “Models” script can be
set via a graphical user interface (Figure 5). The Input to the “Models” script is the current of
each phase which is provided by ATP-EMTP TACS measuring devices („T-elements” in
Figure 3). The output are voltage signals (red) which provide instantaneous voltage values for
the phases A, B and C. The output inductance of the inverter is represented by the RLC-block.
Principally, the control strategy for VSIs is based on frequency and voltage variable operation
and power acquisition via instantaneous current and voltage. Thus, the frequency and voltage
control are realised via the measured active and reactive power. The measured power is used
to adjust the output frequency and the output voltage via droops. The droops are similar to
those in utility grids. This control approach is named as SELFSYNCTM [1]. The graphical
interface window gives the following parameter set:
• f
rated frequency of the VSI (Hz)
• Amp rated magnitude of the output voltage (V)
• tau
time delay for the active and reactive power acquisition (s)
• kp
frequency droop coefficient (Hz/W)
• kq
voltage droop coefficient (V/VAr)
• kph
phase correction (phase feed forward) (rad/W)
U U U
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
Figure 3: Graphical representation of the voltage source inverter (VSI)
Figure 4: Executable part of ATP-EMTP “Models” description for the frequency and voltage variable
three-phase battery inverter
3 Application in power system studies
The developed models have been used for different power system studies. Some examples are
given in the following paragraphs.
3.1 Integration of wind turbine generators into small power systems
The integration of wind energy into the power supply of a small island was investigated in
this study. The considered island is connected to the mainland via a submarine cable. A diesel
generator is available on the island to ensure the power supply in the case of a loss of the
submarine cable.
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
Figure 5: Graphical work interface of the voltage source inverter (VSI)
ATP-EMTP has been used to perform load flow analyses, to get voltage profiles and for the
determinations of the loading of the lines. The achieved results compare well to results
obtained with dedicated load flow analysis programs like PowerFactory from DIgSILENT.
A transient analysis has been performed to study the effect of wind power on voltage
variations, and to study the response to short circuits followed by a short-term interruption.
Figure 6 shows the power output of the simulated wind turbine (RMS time domain). The
response of the power output to the wind speed variations as well as the function of the
compensation unit can be seen. The model was successfully applied to study the voltage
variations in the power supply system of the island.
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(2) Active and recative power (W) of wind turbine vs time
Figure 6: (1) Wind speed and (2) corresponding power output of wind turbine. The turbine is connected
at the time t=6s to the grid. The peaks in the power output result from the operation of the compenstation
unit. The small ripple on the power output is due to the assumed tower shadow effect.
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
Figure 7: Simulation of inverter dominated distribution system (ATP-EMTP)
3.2 Inverter-dominated micro-grids
3.2.1 Simulation of multi-inverter grids
The transient behaviour of three battery inverters coupled via a distribution system (15kV) is
simulated. The inverters are represented by frequency and voltage controlled three-phase
voltage inverters and the distribution system consists of switches, overhead lines (π-blocks),
transformers and a load (Figure 7). The three inverters operate in parallel via the MV
distribution system and supply the resistive load with total power 100 kW.
By means of the implemented control functions, the contribution of each inverter is
determined by the applied droops. In the case of Figure 8 the contribution of the inverters are
set to 20, 30 and 50 kW by the slope of the frequency droop of each inverter. The slope of the
frequency droop can be used to account for the size of the inverter. The setting of the rated
frequency f0 in combination with the droop can be used for the control of the energy flow.
The rated frequencies of the three inverters are set to 50 Hz. Due to the loading, the
system’s frequency decreases to 49 Hz (Figure 9). The frequencies of all inverters oscillate
around a common a common mean frequency. A supervisory control, which is normally not a
part of the inverter’s controller, would be able to restore the frequency to 50 Hz by changing
the rated frequencies of the inverters.
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
Figure 8: Active power of the inverters (W). The
load of 100 kW is shared between the three
inverters.
Figure 9: Frequency of the island system (Hz).
The frequency change results from a load step
(100 kW)
3.2.2 Laboratory validation of ATP-EMTP simulations
The computer simulations are compared to laboratory measurements. For the laboratory
experiments two three-phase inverter clusters with the bi-directional battery inverter Sunny
Island 4500 from SMA Technology AG are used. They are connected with 14 kWh battery
banks. The Sunny Island battery inverter can be operated in three different modes. In the
experiments a droop mode with a frequency droop and a voltage droop is used. Several Sunny
Islands in parallel connection act as grid forming devices. Due to this functionality, it is
possible to expand a supply system and to enable power sharing by using different slopes for
the droops.
Figure 10 Island grid system configuration with two 3-phase inverter clusters and different loads
Although, the Sunny Island is only available as a single-phase version, three Sunny Islands
can be connected together in order to form a three-phase inverter cluster (Figure 10).
A load step of 18 kW resistive load is applied to the parallel connected inverter clusters with
different settings of the frequency droops (Figure 11 and 12).
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
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Figure 11: Output currents of the Sunny-Island inverters with equal frequency droops (ratio kp1/kp2=1)
and a load step 18 kW: a) ATP-EMTP simulation; b) laboratory results
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Figure 12: Output currents of the Sunny-Island inverters with unequal frequency droops (ratio
kp1/kp2=2) and a load step 18 kW: a) ATP-EMTP simulation; b) laboratory results
Figures 11 and 12 show the same qualitative results for the laboratory experiments and the
computer simulations during transients and steady state. Even without the consideration of the
dynamics of the semi-conductor switches the model of the inverter represented by an ideal
voltage source shows sufficient accuracy.
4 Conclusion
ATP-EMTP has been successfully used for the investigation of power systems including
distributed energy resources (DER). Suited models for some DER have been developed to
enable these investigations. These models include specific control functions of the devices
e.g. “active power – frequency” and “reactive power – voltage” droops of the battery
inverters. The models were implemented using standard ATP-EMTP components and features
like the ATP-EMTP “Models” language. The ATP-Draw “Group” feature is used to organise
European ATP-EMTP Users Group Meeting and Conference, 12.-14. September 2005, Warsaw, Poland
the specific DER models into a single block. The performed investigations are focused on
steady state analysis (load flow), RMS time domain and transients. The simulation
environment has also been used to support the development of control techniques for
inverters. For the support of the analysis, suited measuring blocks have been compiled. The
developed models are shown in Figure 10. A compilation of the different models into a library
is planned.
Generators
Wind turbine generator
Three phase voltage source inverter
Diesel generator set
Grid with adjustable impedance
Measuring devices
Computation of rectangular components for one phase
systems
Calculation of RMS
Calculation of active, reactive and apparent power in
single phase systems
Computation of rectangular components for threephase system
Figure 10: Developed DER models and measurement devices
Literature
[1] A. Engler: Regelung von Batteriestromrichtern in modularen und erweiterbaren
Inselnetzen. Dissertation.de, Berlin, 05/2002, ISBN 3-89825-439-9
[2] A. Engler, O. Osika: Simulation of Inverter Dominated Minigrids. 2nd European PVHybrid and Mini-Grid Conference, 25./26.09.2003, Kassel
Acknowledgement:
We would like to express our thanks to the European Commission for their support in the
MicroGrids project ENK5-CT-2002-00610 and the DISPOWER project ENK5-CT-200100522.