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Aspects of Grid Integration of Renewable Energy Sources in Weak
Power Systems
Dipl.-Wirtsch.-Ing. Sönke Grunau, Dipl.-Wirtsch.-Ing. Jan Reese, Lars Jessen M. Eng., Prof. Dr.-Ing. Friedrich W.
Fuchs
Christian-Albrechts-University of Kiel, Technical Faculty, Institute for Power Electronics and Electrical Drives, Kaiserstraße 2, 24143 Kiel, Germany, E-Mail: sgr@tf.uni-kiel.de, jre@tf.uni-kiel.de, lje@tf.uni-kiel.de, fwf@tf.uni-kiel.de
Abstract
In this paper some special aspects of grid integration of renewable energy systems (RES) and improving system behavior under weak grid conditions are analyzed. At first a principle to measure frequency dependent grid impedances by
current injection is explained and field measurements are presented. Secondly, the economic efficiency of short time
energy storage systems for wind turbines (WT) is under investigated. With these systems it is possible to provide additional active power to support the grid to maintain stable and it is shown that economical advantages for WT operators
can be achieved. Finally the system response of an electric grid with RES at transient faults is presented and optimized.
The key aspect is the reactive power exchange between different energy resources and the grid during grid faults. Results show that damping this power exchange is possible by modeling and adapting the synchronization algorithm.
1
Introduction
The structure of the electrical power system changes due
to the progressive decentralization of energy production.
The importance of renewable energy systems (RES) will
continue to increase significantly in the future. Further it
will influence the voltage in the power system significantly [1, 2]. Hence, the impact of RES requires an active
contribution of each RES to ensure power quality and grid
stability, which are defined in national and international
standards and guidelines [3-6]. In most cases RES are
connected to the grid by using converters based on power
electronics. The challenges and opportunities for the use
of modern power electronic generator systems for improvement of power quality are presented in [1] and [7].
A system approach with parallel power generators of different power is not found at present. However, a system
approach is necessary for power system stability and a
secure and reliable energy supply. The dependence of
energy production on weather conditions is an increasing
challenge with an increasing amount of RES connected to
the grid. Hence, traditional renewable energy sources are
hardly qualified for grid control. Therefore grid development, short- and long-term energy storage systems (ESS)
and the development of new requirements regarding a
RES contribution for securing power quality and grid stability is necessary.
This paper gives a summary of research activities at the
University of Kiel in grid integration of RES. Three aspects are discussed, approaches to solutions are identified
and validated by measurements and simulations. The focus lies on improving the system behavior of a power grid
with decentralized energy sources.
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The system behavior of a power grid with parallel inverters is strongly influenced by the frequency dependent
impedance at the PCC [7, 8]. Furthermore the injected
harmonics by power electronic systems depend on the
frequency dependent grid impedance. In this paper a method to measure the grid impedance by injecting currents
in a range from 80 Hz to 6 kHz is presented and field
measurements are shown. The second part of this paper is
about short term ESS for wind turbines (WT). The use of
ESS in RES is currently under wide discussion [9, 10]. It
is possible to distinguish between long-term and shortterm ESS [11]. Long-term ESS can be applied to compensate fluctuating injection of power caused by changing
primary energy supply. They also offer the possibility to
influence power flows in the grid. However, the use of
short-term ESS is currently less considered. With the increasing intensification of the grid codes (GC) concerning
the active power feed, short-term ESS can become economical. This is demonstrated in this paper based on studies. The third aspect is about the optimization of the system behavior of decentralized parallel generators during
transient faults. Especially at the point of fault declaration
there is a reactive power exchange between the parallel
generators and the grid. The damping of this power exchange depends on the strength of the grid. To optimize
the system behavior a control-loop for the output impedance of the inverters is proposed. This causes the attenuation of the reactive power exchange in a decentralized
fault clearing. Measurements in the laboratory show the
improved system performance of parallel operating power
producers especially in weak networks.
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2
Frequency depended Grid Impedance Measurement
2.1
Fundamentals
The impedance of the power grid is needed for assessing
the change in voltage before connecting an energy source
to the power grid. Therefore, it is one of the basic information in network calculation [12]. Since renewable
energy resources like WTs and photovoltaics are nonlinear they feed in fundamental and harmonic currents [13].
According to Ohm's law, these harmonic currents evoke
harmonic voltages in the grid. To handle the adverse effects of harmonic voltages there are standards to limit voltage and current harmonics [3, 5]. Usually, the grid impedance is only known at fundamental frequency by performing short circuit calculations. While resistiveinductive extrapolation is wrong at higher frequencies because of resonances, it is possible to estimate the first resonance. Nevertheless, even the estimation is not accurate
because of the little-known and time-varying capacitance
[14]. Therefore the calculation of voltage harmonics provoked by the injected current of renewable energy sources
is often wrong.
While calculating the frequency depended grid impedance
is often doubtful it is possible to measure it. There are two
different categories, active and passive, to classify the different methods [15].
Active Methods can be subcategorized in frequency response and impulse response methods [15]. Frequency
response methods base on the injection of a current with a
defined frequency, amplitude and phase angle. The impedance can be calculated with Ohm’s law by measuring
the injected current and the resulting voltage. If the voltage in the power grid is already polluted with harmonics
it is necessary to perform two measurements with different currents at the polluted frequencies. The difference of
the two voltages and the two currents can be applied for
calculation of the impedance (1). The impedance over a
frequency spectrum can be obtained by performing several measurements at different frequencies [16].
Figure 1: Measurement setup
thod depends on the existing distortion in the power grid.
Often the background distortion is not appropriate for frequency dependent impedance estimation due to low voltage levels and repetition rate [15].
2.2
Field Measurements
The presented measurements are performed in a public
low voltage power grid by harmonic current injection
with an inverter in a frequency range from
to
. Fig. 1 shows the power grid where exemplary
measurements have been carried out.
Node N1 is the busbar of the MV-LV transformer that
connects the 0.4 kV low voltage grid to the 20 kV medium voltage grid. Node N2 is connected to node N1 by a
383 m long line. The load and generator symbols mark
the cumulated load and generation of the nodes. For simplification the impact of loads and generators were neglected for network calculation.
The frequency dependent impedance of node N2 is shown
in Fig. 2. It illustrates the volatility of the impedance during a working day. In addition the calculated impedance
based on the inductive and resistive grid parameters from
short-circuit network calculation is plotted (black line).
The second measurement (Fig. 3) illustrates the influence
of the load and generation on the frequency dependent
grid impedance of node N1. The blue curve shows the
impedance of the node with the influence of load and
generation in the low voltage grid. The red curve shows
the series connection of the transformer and the medium
voltage level. While the calculated frequency depending
(1)
The impulse response method is based on equation (1),
too. The difference is that the grid is subjected to impulses by a switched load [17]. The grid impedance can
be calculated with the currents and voltages in frequency
domain before and after the switching. In the simplest
case the load is resistive and only switched one time. This
leads to the problem that the impedance can only be identified at frequencies with existing harmonic voltages in
the grid that cause harmonic currents on the switched
load. To overcome this problem it is possible to apply a
pulsing load [18].
As mentioned above there are also passive methods without influencing the currents and the voltages in the grid.
The idea is to utilize the voltage and current fluctuations
caused by equipment of grid customers. Hence, this me-
,6%1
Figure 2: Measurement of the frequency depended grid
impedance of node N2 (blue, red, green) and calculated
grid impedance with
and
from network calculation (black)
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Figure 4: Active power injection restrictions, left: PCurtailment, center: Ramp-Rate-Limitation, right: DeltaControl, available (black) and injected (red) active power, blue: lost energy (deficit)
Figure 3: Measurement of the frequency depended grid
impedance of node N1 (blue), impedance of transformer
and medium voltage layer (red) and the calculated grid
impedance with
and
from network calculation (black)
impedance is similar to the impedance of the transformer
and the medium voltage grid the impedance of the whole
node is strongly influenced by grid customers, especially
above 1 kHz. Due to the resonance the impedance is approximately
higher at 1.3 kHz and approximately
lower at 5 kHz than calculated.
sults. Depending on the limiting rule it can be distinguished between temporary and permanent deficits.
Temporary deficits can occur due to the prioritized reactive power injection at voltage sags or when active power
injection has to be reduced due to over frequency. Also
injection can be limited due to a curtailment (Fig. 4, left).
Further rules are specific to the particular GC. A GC
overview is given in [23]. For example Ireland, Denmark
or the ENTSO-E GC [24] require a ramp rate limitation of
active power injection (Fig. 4, center). Resulting deficits
are not highly significant due to their temporarily occurrence. By assessing typical wind data these deficits could
be estimated. Delta-Control, Fig. 4 right, is required by
the GCs of Denmark, Ireland, Spain and the ENTSO-E
GC in some cases. WTs work in a suboptimal operating
point in this mode and inject the power
(2).
(2)
(3)
3
Short Term ESS for WT
The impact of wind-power injection into the grid will increase with a higher amount of WTs installed. Since the
wind is a fluctuating power source, grid quality and stability can be affected [19]. As a consequence WT have to
fulfill GC which underlay a permanent process of revision
[20]. Currently such a process occurs in Europe by the
ENTSO-E. There is a discussion to combine WT with
ESS to enhance the grid integration of WTs. An overview
about storage technologies for wind power applications is
given in [11]. Applications for ESS at WTs are discussed,
for example for an active power smoothing [10] or to
support WTs during grid faults [21].
3.1
Injection Deficits due to Grid Codes regarding Active Power Injection
Variable reactive power injection is generally possible by
means of active power converters. A contribution to voltage level or quality regulation can be done by WTs [22],
as claimed by some GCs. Due to the dependency of the
wind, active power injection of WTs cannot take place
completely freely. By means of a maximum power point
tracking it is sought to achieve the maximum possible active power output. But in the GCs some rules regarding
the active power injection can be found.
If active power injection is limited, total available power
cannot be injected and a deficit of injection reward re-
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is the available active power, is the Delta-Control factor, its size is defined by the TSO. A typical range is
shown in (3). In the occurrence of a low frequency, power
injection can be raised by
to help the frequency to
rise again [25]. Such permanent deficits are significantly
higher and investigated in the following chapter.
3.2
Break Even Analysis
If Delta-Control is made compulsory to provide frequency
regulation, this energy could also be provided by an ESS.
It is assumed that this power has to last until secondary
grid regulating is in action, typically after 15 to 30 minutes. ESS costs for Lead-Acid and Li-Ion batteries depending on this storage time are set in contrast to the
permanent deficit, a Delta Control would cause.
3.2.1 Assumptions
To numerate the permanent deficit of Delta-Control,
German feed-in tariffs are chosen as calculation base. The
deficits depend on WT’s full-load hours (VLS), which are
supposed to be between 2.600 and 3.000 hours for modern on- and offshore WTs.
Total ESS costs per kW (
) are assumed to increase
linearly with the storage time t, see (4), taking the power
costs per kW (
) and the capacity costs per kWh
(
) into account.
(4)
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Figure 5: ESS costs per kW and storage time, upper
(solid) and lower (dashed) battery prices
Based on [26] and [27] upper and lower prices are determined, shown in Fig. 5.
Storage’s life-time strongly depends on the way, it is operated. It depends on the operating conditions like operational temperature, the driven depths of discharge or the
number of cycles driven. A complete charging and discharging process is normally defined as one cycle. If the
battery is only partially charged and discharged, a partial
cycle can be calculated respectively.
To determine the required cycles for Delta Control operation, the number of events of frequency deviations in the
grid has to be estimated. Such deviations normally occur
hourly, as a result of the power trading and occasionally
due to power plant outages or fast changes of big loads.
Primary frequency control methods can compensate these
deviations in most cases within a few minutes [28].
If the ESS is rated to store
for 30 minutes, 0.7 – 1
cycles are assumed here as daily stress. For a total life
time of 20 years, approximately 5100 – 7300 cycles result. If operating conditions are ideal, life time of modern
batteries for industrial applications is assumed to depend
only on the number of cycles driven. Modern Lead-Acid
batteries reach maximum cycle numbers between 2000
and 4000, modern Li-Ion batteries between 5000 and
10000 [26].
For the desired operation Lead-Acid batteries have to be
renewed at least once, Li-Ion batteries can remain the total time. Storage prices tend to decrease in the future [26],
here a reduction to 75% within 10 years is assumed. If
other hardware parts, like the inverter and the grid connection remain the whole life, the total cost for a LeadAcid ESS will be up to 1.6 times greater than the initial
installation costs. The efficiencies of the inverters are assumed with 97 %, of the Li-Ion batteries with 85 % and of
Figure 6: Amortization depending on storage time of
ESS with modern Lead-Acid batteries (left) and of modern Li-Ion batteries (right) for a life time of 20 years,
solid: upper battery prices, dashed: lower battery prices
,6%1
Figure 7: Norton equivalent of a current controlled
voltage source inverter
the Lead-Acid batteries with 70 %.
3.2.2 Results
In Fig. 6 the effect of the ESS rating (capacity, in terms of
how long
can be stored) on the ratio of total injection deficits (
) and total ESS costs (
) is illustrated.
All ESS systems with a rating of up to 30 minutes storage
time can be economical for WT operators, if they are
forced to remain a constant power reserve of
with
Delta Control for up to this time. 30 minutes is a crucial
time limit, because after that secondary frequency control
mechanisms with a much more powerful power reserve
begin to work. It can be seen, that both storage technologies show an equal behavior under given considerations,
because the smaller prices for Lead-Acid systems are
compensated by the need of their partly reinvestment. The
efforts of achieving ideal conditions for the operation (e.g.
cooling) are neglected and could affect a technology selection. It is obvious that the economic efficiency can be
raised, if more VLS can be achieved by a WT or the ESS
size can be decreased. Especially if prices of batteries decrease in the future and maximum cycle number can be
optimized, economical efficiency will further increase.
The results do not consider that ESS can further be used
to optimize WT efficiency, for example to reduce temporarily deficits or to provide ancillary services like black
start capability or backup power supply for the WT or
wind park.
4
Influence of the Synchronization
Algorithm on the Power Injection
during Grid Faults
Low voltage ride through (LVRT) describes the ability of
renewable energy systems stabilizing the grid voltage
amplitude and grid recovery at the point of common
coupling (PCC) during symmetrical and asymmetrical
transient events [29]. This capability is defined by several
guidelines especially for medium voltage grids [4, 6, 29].
As the grid impedance of medium voltage grids can be
assumed to be inductive [30] the voltage amplitude at the
PCC is stabilized by feeding in inductive current in case
of voltage sags. In this case, the voltage drop across the
grid impedance
, Fig. 7, results in a higher voltage
level at the PCC.
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(8)
(9)
Insert (9) in (6) and setting
closed loop transfer function of the PLL
by (10), Fig. 9:
the
is given
(10)
Figure 8: Block-diagram of a current controlled voltages
source inverter (VSI) synchronized with voltage
In case of parallel inverters, there are interactions between the inverters especially during transient events of
the grid voltage
. The dynamic behavior of each inverter depends on the equivalent input impedance [31].
Since the equivalent input impedance depends on the implemented control and the synchronization algorithm
(PLL), the influence of the PLL regarding the LVRT capability is examined in the contribution.
The PLL is used to determine the grid voltage angle,
which is necessary for the grid synchronized power infeed. Since the synchronization is achieved when the qcomponent of the measured voltage
is zero [32], the
active and reactive power infeed can be done using d and
q component of the current respectively.
4.1
Model of the PLL in global dqcoordinates
A typical system structure is shown in Fig. 8. Due to measurement equipment, there is a delay
given between
the physical and measured voltage
. It can be described with (5).
(5)
The order transfer function can be described by its resonant frequency
and the damping factor [33], (11):
(11)
Equation (11) clarifies, that damping and bandwidth are
dependent on the operating point Upcc,0. Especially in case
and
should be updated to
of deep voltage sags,
achieve sufficient dynamic and robust behavior during
voltage amplitude variations.
4.2
Results
Fig. 10 shows the detection of a 3-phase voltage sag to
0.12 per unit and the reactive current injection during the
grid fault as it is claimed by the grid codes.
It can be seen that the deviation
is less in case of updating
and
depending on the operation point, especially during the phase-jump in the grid voltage, generated by load shedding. Moreover, the deviation is better
damped. Thus the LVRT performance of a single inverter
is improved, since the current injection during grid vol-
depends on the dynamics of the PLL which reacts on
small deviations of
, Fig. 9.
is determined with (6):
Thus
(6)
where
is the closed loop transfer function of the
PLL shown in Fig. 9. Hence a difference between measured and physical voltage
results. In rotating coordinates and using the Taylor approximation, the invertermeasured voltage
is given by (7-9):
(7)
Figure 9: Block-diagram
ing reference frame
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, considering global rotat-
Figure 10: Results of current injection behaviour during voltage sag (12% UN);
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tage variation is enhanced. With regard to the LVRT capabilities of parallel operating inverters, this results in a
lower excitation of low frequency oscillations, since these
oscillations occur due to small deviations of the current
injection of different current sources. This is further examined in research projects at the University of Kiel.
5
Conclusion
Due to the volatile operation conditions and unknown capacitances in the power grid, the estimation of the grid
impedance is not precisely, especially at higher frequencies. Since loads and generators have a large impact on
the frequency depending impedance in a frequency range
above
, a correct calculation of voltage harmonics
caused by inverter introduced current harmonics are not
possible without measurements. Exemplary measurements are shown.
Due to the increasing impact of wind-power injection, the
grid stability and power quality can be affected negatively. As a result a trend to increase regulations in the GCs
regarding active power injection is noticeable, for example in the ENTSO-E GC. By means of ESS permanent
deficits can be reduced and efficiency of WTs can be enhanced. In this paper permanent deficits by delta control
are investigated and a break even analysis reveals that
ESS, rated for 30 min storage of
of nominal power,
can enhance the overall profitability of a WT. Both storage technologies analyzed show an equal economic behavior.
Regarding the LVRT capability of parallel operating inverters, the influence of the synchronization algorithm is
exposed in this contribution. It can be concluded, that an
adaption of the voltage amplitude in the control improves
the behavior of each connected inverter during voltage
sags and thus it enhances the overall LVRT reaction.
6
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