Download UPEC 2010 - Preparation of Papers in Two

Effects of Distributed Generators from Renewable
Energy on the Protection System in Distribution
Networks
Ahmad AbdelMajeed
Robert Viereck
Fred Oechsle
Martin Braun
Stefan Tenbohlen
University of Stuttgart
[email protected]
DIgSILENT GmbH
EnBW Regional
AG
[email protected]
om
University of Stuttgart
University of Stuttgart
[email protected]
[email protected]
[email protected]
Abstract-- The increasing number of distributed generators
especially from renewable energy requires investigating the
impact of their integration on the protection system.
This paper evaluates the effects of distributed generators on
protection relays functionalities (main and backup protection) in
the medium voltage grids. In order to obtain profound results,
simulations were done on several realistic scenarios for existing
medium voltage grid. As all short circuits were simulated in the
time domain by using the software Power Factory from
DIgSILENT GmbH, dynamic models for distributed power
generation had to be implemented. The results were analyzed
graphically and tested for negative influences on the protection
functions. In addition, simulation was also done to study the
effects of the realistic requirements for the increasing number of
distributed power generation in the future. The results from
these simulations are presented and discussed.
Index Terms— Power distribution protection, distributed
generators, simulation.
I.
INTRODUCTION
In Germany, the number of small scale distributed generation
(DG) units especially from renewable energy has increased
sharply in the recent years and will grow up significantly in
the near future especially the share of wind and solar energy
in the total amount of energy generation through a targeted
support from the federal government. Due to the low
production capacities from DG units from renewable energy,
they are normally connected to the medium or low voltage
network, this results in problems of power quality and
compliance with the voltage range. Currently, most of these
DG units are not contributing to the short circuit current in
case of fault, but they are disconnected from the network in
case of high voltage drops, but the new directive from the
Federal Association of German Energy (BDEW) states that
the future installed systems must also contribute to the fault
current in case of faults. So the target of this paper is to
investigate this case and to study the side effect on the
protection relays functionalities through simulations on a real
existing medium voltage grid. Different worst case scenarios
were defined. Dynamic models for the small scale DG units
were used.
The effects of integrating the DG units to the network were
analysed mathematically in theory [1, 2, 3, 4] for a simple
assumed small networks, these DG units could change the
level and the direction of the short circuit current. This causes
an increment for the short circuit currents, the direction of the
short circuit currents varies depending on the location and
penetration of the DG units. However this paper discusses in
details the effects of integrating the DG units in a real existing
medium voltage grid taking into account the current and the
expected future expansion of integration of DG units from
renewable energy.
In this paper a real existing medium voltage network called
“Freiamt“ is introduced with it’s protection relays, then the
simulation setup is explained, afterwards simulation results
are introduced based on three different scenarios; the first
scenario shows the simulation results without integrating the
current expansion of DG units, this scenario forms the basic
for a later comparison, the second scenario shows the
simulation results with integrating the current expansion of
DG units, the third scenario shows the simulation results for
double expansion of DG units. Later on a comparison figures
show the effect of the DG units on the network protection.
II. MEDIUM VOLTAGE GRID MODEL AND PROTECTION
SYSTEM
The simulations shown in this paper are performed in the time
domain by using the software Power Factory from
DIgSILENT GmbH [5] on a real existing medium voltage
network called “Freiamt“. This network is one of the
experimental networks of EnBW regional AG, it covers an
area around 52 km² which has 4000 inhabitants, the
expansion of renewable power generation and DG units
exceeds the load in this network approximately by 1,5 GWh
every year, this extra power is usually fed back into the
110 kV network level. More details about power production
and consumption are shown in Table 1.
Table. 1. Power production and consumption under the current expansion of
renewable energy and users.
Type
Users
Photovoltaic
Wind Turbines
Power
4,7 MW
2,5 MW
1,8 MW
This network is monitored and protected by 5 relays; one
DMT (Definite Minimum Time) directional over current relay
monitors the two “Sexau” main feeders as shown in Fig. 1.
The backup protection for these two feeders forms a distance
protection for “Denzlingen”, another distance protection relay
monitors the “Sexau” substation on the other direction. The
reserve power supply of the substation “Sexau” is also
monitored with a distance protection relay but this relay was
not considered in this paper.
The protection coordination settings for the relays are
shown in Table. 2.
for the medium voltage level. Thus, the conventional power
plants will have enough time to start. This so-called "faultride-through" ability is the base of all models in this paper.
A generic model of a Doubly Fed Induction Generator
(DFIG) is used to model wind turbines. The model was
designed by DIgSILENT GmbH and used widely. A single
induction generator is controlled by a model which simulates
the mechanical and electrical properties, as well as the
controllers and protection equipment.
The PV inverter was modeled as self-guided, pulse-width
modulated inverter. This allows active and reactive power to
be controlled and thus makes it possible to run through
voltage dips without disconnection from the grid.
III. SIMULATION SETUP
Fig. 1. “Freiamt” medium voltage network diagram
Table. 2. Relays Coordination Settings
Relay Position
Denzlingen in
Direction
Sexau (1)
Sexau in
Direction
Helgenreuthe (2)
Sexau in
Direction
Rathaus (3)
Sexau in
Direction
Denzlingen (4)
Activation
Current
300 A
1st
level
0,1 s
2nd
level
0,6 s
3rd
level
1,0 s
End
time
1,7 s
440 A
0,3 s
-
-
-
440 A
0,3 s
-
-
-
300 A
0,1 s
-
-
0,5 s
Originally, DG Units were not meant to contribute to grid
faults. For this reason their protection equipment will
disconnect them in case of voltage drop in short circuit
situations (i.e. grid faults). Due to the enormous growth in
renewable energy, especially wind energy, a fault in the
transmission grid during periods of weak conventional
generation would lead to a high unbalance of load demand
and generation, which in turn could lead to wide spread
outage.
To prevent the network from such cascaded results and
failures, the German Association of Energy Economics
(BDEW) has released a grid code [6,7] allowing distributed
generators connected to the high voltage grid (> 110 kV) to
be separated from the network in the earliest 150 ms after
fault inception . A similar grid code is now coming into effect
To view the contribution of DG units to short circuit currents
in the medium voltage gird in case of fault, simulations in the
time domain were done. The fault applied to the network was
a 3-phase fault in all cases.
The main goal of the simulations is to see the effect of
distributed generation on the protection system. Hence the
fault location had to be varied in order to see different fault
scenarios. This meant a large number of simulations which
would take a lot of time to conduct and to analyze the results.
To automate this tedious task, the built-in scripting language
of Power Factory DPL (DIgSILENT Programming
Language) was used to run a series of simulations
automatically and provide the combined results for analysis.
The basic structure of the script can be seen in the flow
chart in Fig. 2. In this context, a path is a set of lines and
nodes between two points in the grid. In this case the grid was
separated into three paths to reduce the calculation time. The
fault location was calculated based on predefined step size.
The resulting set of files was merged (by superposition
method) and plotted to obtain graphical information for the
analysis.
Fig.2. Flow chart illustrating the basic structure of
the script
IV. SIMULATION RESULTS
Simulation results were introduced based on three different
scenarios; the first scenario shows the simulation results
without integrating the current expansion of DG units, this
scenario forms the basis for a later comparison, the second
scenario shows the simulation results with integrating the
current expansion of DG units, the third scenario shows the
simulation results for double expansion of DG units.
A. Scenario “1”
To obtain results for the impact of DG units production on the
protection coordination, several simulations were carried out
without feeding from DG units. These figures form the basis
for later comparison.
The following Figures show the behavior of the phase
current IL1 (phase A) representing the symmetrical shortcircuit current I”K in case of three-phase short circuit. The
overcurrent relay located in “Sexau” was deactivated during
these simulations in order to observe the levels of backup
protection.
Fig. 3. shows the short circuit current flow as would be
measured from the backup protection relay in “Denzlingen”
for the complete feeder in the direction of “Helgenreuthe”.
Fig.4. and Fig.5. show the current flow, as it would be seen
from the DMT relay in “Sexau” in the direction of
“Helgenreuthe” and “Rathaus” feeders respectively. As
expected, the short circuit current decreases with increasing
distance from the short circuit position but never goes to a
value below 1 kA. Fig. 3. clearly shows the different stages
and levels of the distance protection which acts as backup
protection. The simulation were also compared with IEC
60909 standards, the simulations and the calculations have a
very low deviations from each other.
Fig. 3. I”K in the path between “Denzlingen” and “Sexau” as a function of
time and distance (without DG)- Relay :Denzlingen
Fig. 4. I”K in the path between “Sexau” and “Helgenreuthe” as a function of
time and distance (without DG)- Relay :Helgenreuthe
Fig. 5. I”K in the path between “Sexau” and “Rathaus” as a function
of time and distance (Without DG)- Relay: Rathaus
B. Scenario “2”
As a first comparison to the case without integration of DG
units, all DG models which are used in the simulations are
assumed to operate in their nominal operational power, this
would be a cold, sunny winter day in accordance with a
strong wind, this is only a hypothesis as a "worst-case
scenario". Furthermore, all distributed generation units were
treated as if they would meet the new guidelines for
generating units in the medium voltage network. The
protection relays have also been disabled for the following
charts to visualize the influence of the network over the time.
As seen from the following Figures (6-8), the lowest short
circuit current is also within the range around 1 kA; and the
activation condition for the over current relay is met. The
short circuit current is no longer constant at a fixed value but
it has small oscillations, this leads to a non-ideal sharp
boundaries in the backup protection triggering, as the
measured fault impedance is swinging in the same rate.
Fig. 6. I”K in the path between “Denzlingen” and “Sexau” as a function of
time and distance (with DG)- Relay :Denzlingen
Fig. 7. I”K in the path between “Sexau” and “Helgenreuthe” as a function of
time and distance (with DG)- Relay :Helgenreuthe
Fig. 8. I”K in the path between “Sexau” and “Rathaus” as a function of time
and distance (With DG)- Relay :Rathaus
For the over current protection there is no negative
influence because the short circuit current sometimes is
slightly larger than the case without integrating of DG units.
This is caused by the interaction of different circumstances;
for short circuits near “Sexau” substation the voltage at
“Sexau” substation is almost zero, the same as in the case
without integration of DG units. Because of that the same
short circuit current is delivered by the feeder. Added to this a
small contribution from the healthy path (without faults) can
be observed. This behavior is similar to the short circuit
behavior of a transmission line with tap. Additionally the
power consumption of loads in the healthy path is mainly
provided by the distributed generation in this path. Since the
load is reduced by the short circuit, due to the voltage drop,
part of the current generated in the healthy path will also flow
towards the short circuit. The effect is only observed at short
circuits near the central substation “Sexau” and if the
generation in the healthy feeder outweighs the generation in
the affected feeder.
By the current integration of DG units from renewable
energy, the difference can be only considered in case of a
fault in the “Rathaus” feeder because the short circuit current
contribution in case of fault in “Helgenreuthe” feeder is small
and can be neglected.
Different behavior of the back-up protection for short
circuits between the two paths can be explained by the
imbalance of the distributed generation units. The path
“Helgereuthe” contains about 70% of the installed capacity of
distributed generators and can therefore feed in much more
current in case of short circuit in the “Rathaus” path than the
other way around. This implies that distributed generators
play only a minor role in reducing the short circuit current in
the faulty path. This is particularly the case of three-phase
short circuits as they occur here. The short circuit separates
the corresponding feeder into two separate zones and all the
DG units behind the short circuit (as seen from the relay in
“Sexau”) cannot participate in the short-circuit current. Even
if all currently installed distributed generation units would
meet the new standards for generating facilities, only a
slightly different behavior for the protection system would be
observed. However, this would not lead to a failure of
protection. Another point for this would be supporting the
producer's own protection systems; in case of a sharp voltage
drop when three-phase faults occur, the producers would be
allowed to separate their units from the grid before the
backup protection could occur as such in action. For all the
faults which could be affected by the DG units are at least in
the second zone of distance protection, with a release time of
0.6 s. This time is significantly higher than the allowed time
for them to remain connected to the grid (0.3 s for V<0.45
p.u). This would be 0.6 s after all DG units are already
disconnected [6, 7].
Fig. 9. and Fig. 10. show the comparison of the short
circuit current for the backup protection with and without the
integration of DG units. Comparing the short circuit current
for both cases, then the aforementioned oscillations on the
short circuit current can be seen clearly. The reason for the
oscillations is the converter model of the PV–models. As a
contribution to short-circuit current, these oscillations are
negligible, as they swing within the range of ±50 A.
Furthermore, for the backup protection the short circuit
current is reduced but this reduction is so small in comparison
to the short circuit current and can only show a minor delay in
tripping, the delayed trips are shown as white areas.
Fig. 9. I”K Difference in case of short circuit in “Helgenreuthe” pathRelay: Denzlingen
Fig. 10. I”K Difference in case of short circuit in “Rathaus” pathRelay: Denzlingen
A. Scenario “3”
The simulations with the current expansion already show a
slight influence on the protection as the DG units from
renewable energy are increasing and expanding
significantly. Simulations were also done to check the
effect of double expansion of DG units on the protection
system. The nominal power output of all PV- systems was
doubled, as well as the output of the wind turbines, the
resulting generation values can be found in Table 3. This
configuration can be used to represent an estimated growth
in a range of 5 – 10 years. A scaling factor of 1.1 was used
for the loads to the take increasing load demand into
account.
Fig. 13. Scenario “2” Back up protection levels in the path “Denzlingen”“Sexau”-“Helgenruthe”
Table. 3. Power production and consumption under expected expansion
of renewable energy and users
Type
Users
Photovoltaic
Wind Turbines
Power
5,1 MW
5,0 MW
3,6 MW
Fig. 11. and Fig. 12. show the resulting back up
protection levels in the paths of “Helgenreuthe” and
“Rathaus” respectively.
Fig. 14. Scenario “2” Back up protection levels in the path “Denzlingen”“Sexau”-“Rathaus”
Fig. 11. I”K difference in case of short circuit in “Helgenreuthe” pathRelay: Denzlingen
Fig. 15. Scenario “3” Back up protection levels in the path
“Denzlingen”-“Sexau”-“Helgenruthe”
Fig. 12. I”K difference in case of short circuit in “Rathaus” pathRelay: Denzlingen
The simulation results show that the increased
generation capacity increases the already described effects
in the current expansion of renewables. Erratic behaviour
of the protection is not observed. The impact on primary
and backup protection will remain small so that they can be
neglected. The following Figures (13-16) show similar
behaviours of short circuit current, the changes of the
current flow in the various paths, as well as the shift of the
levels of backup protection as in the previous section.
Fig. 16. Scenario “3” Back up protection levels in
the path “Denzlingen”-“Sexau”-“Rathaus”
V. CONCLUSION
In this paper, simulations in the time domain on an existing
MV network were performed (in the time domain) with
dynamic models to investigate the side effect of integration
DG units from renewable energy on the protection system.
The simulation shows that the small scale distributed
generation has no significant impact on the protection in the
medium voltage network. While the expected reduction of
short-circuit current due to the infeed by the small scale
generation can be observed, the effects remain minimal.
These simulations were assumed as “Worst case scenario”
so that all the renewable energy sources were feeding with
their nominal power at the same time. But the fluctuation of
wind energy and the geographical position of Germany
(relatively few sunny days) lead that the simulated scenarios
can probably be found in reality.
For symmetrical short circuit, there was no effect on the
overcurrent protection, the operating ranges of the distance
protection tend to under reach only slightly which can be
ignored.
Unbalanced faults, which are much more common in
medium voltage grids, could not be simulated due to
inappropriate models. The lower short-circuit current, and
different fault detection by the protection relays could lead to
malfunction in those cases. For a full assessment of the
impact of small scale distributed generation on medium
voltage protection, new models for the generation have to be
developed.
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Martinez, J.A. ; Martin-Arnedo, J., “ Impact of distributed generation
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[4]
Kauhaniemi, K. ; Kumpulainen, L., “Impact of distributed generation
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DIgSILENT Power Factory is a leading high-end power system
analysis tool for applications in transmission, distribution, generation,
industrial and railway systems, wind power and Smart Grids.
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BDEW: Erzeugungsanlagen am Mittelspannungsnetz - Richtlinie für
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BDEW:Regelungen und Übergangsfristen für bestimmte Anforderungen
in Ergänzung zur technischen Richtlinie:
Erzeugungsanlagen am
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Erzeugungsanlagen am Mittelspannungsnetz, July, 2010.