Download IV. Switching-induced voltage changes

Grid Connection and Integration of Wind
Turbines
Bernhard ARNDT
University of Applied Science Würzburg-Schweinfurt, Germany
[email protected]
Abstract—The use of wind turbines sharply increased
in the last decade. This poses problems for the
MV/HV-grid. System perturbations are in the focus to
maintain grid stability. In brief, sources of voltage
fluctuations are shown and the current concepts of
wind turbines are compared concerning this aspect.
The recently modified standards in Germany are
presented and a sample of network calculations for
the integration of a wind park is given.
I.
INTRODUCTION
In recent years the energy produced by wind
turbines increased sharply and in Germany we
expect in 2020 a 20-25% share of wind of the total
electricity produced. This poses new challenges for
planning the grid: in Germany most inland wind
turbines are located in very small wind farms of
two to five turbines of 1.5 to 2 MW each. This
gives a total of 3 to 10 MW at the connection point,
which is typically not in the vicinity of HVsubstations, but mostly at the end of low-power
MV-lines. This has an impact on mains stability.
This is different to big wind farms we have offshore
and inshore with a larger number of wind turbines.
Also in other European countries the harvesting of
wind is concentrated in bigger farms, fig. 1 shows
the situation near Vejer in southwest Spain.
II. To maintain grid stability and keep system
perturbations low, careful planning is necessary
So, what are the perturbations:
-
harmonics
voltage fluctuations
flicker
unbalanced loads
frequency changes
voltage dips
short breaks
The contributors are:
static converters and inverters
all other power electronics
nonlinear
loads
like
transformers,
fluorescent tubes, arc-furnaces
appliances with dynamic behaviour like
automatic welding machines
Fig. 1: Wind turbines near Vejer, Spain
When tracing the sources of system perturbations
caused by wind turbines at the connection point you
will always end up with one input parameter: the
wind and its fluctuating power delivered, as the
power produced is proportional to the cube of the
wind speed, which is definitely not constant. But
also the construction of the wind turbine affects the
wind flow: from the tower itself and when a blade
passes the tower the wind flow is disturbed and also
the power generated varies as the blades turn and
pass different heights. Both contribute to voltage
fluctuations and flicker.
But also the technical concept of the generating unit
affects the grid. Turbines with synchronous
generators are connected today via static converters
(direct connection isn’t state of the art anymore). In
normal operation the converters produce harmonics,
which have to be limited where necessary. Doubleinverter-fed asynchronous generators use a
converter with lower power, so there are less
harmonics produced.
Comparing both concepts, the synchronous
generator has a better efficiency and reactive power
can be controlled by the field excitation, but it has a
higher harmonics level due to the converter
normally used. The asynchronous generator is
cheap and robust, but requires power factor
correction and has higher flicker and a slightly
lower efficiency.
Power frequency
(mean value of
fundamental
measured over 10 s)
Voltage magnitude
variations
Rapid voltage changes
Supply voltage dips
Harmonics
±1% (49.5 - 50.5 Hz)
for 99.5% of week
-6%/+4% (47- 52 Hz)
for 100% of week
±10% for 95% of week,
mean 10 minutes rms
values
4% normal
6% infrequently
Plt ≤ 1 for 95% of week
Majority: duration <1s,
depth <60%.
Locally limited dips
caused by load
switching on:
10 - 15%
See fig. 3
Fig. 2: for the permissible perturbations, a quick look at
the relevant standard EN 50160 (values for MV
networks)
: nominal active power of the generator
: nominal apparent power of the generator
In case of a single CP in a system this condition is
always complied to, if
≥ 50.
If the grid’s source impedance is highly inductive,
this calculation should be done using the complex
source impedance. But his is still an approximation.
phase shift between current and voltage of the
generating unit in degrees at the maximum apparent
power
.
: phase angle of the mains source impedance.
If you search for the maximum voltage lift at a
given power level, you can change the formula into
You can write above formulae also as
and
Fig. 3: Harmonic levels EN 50160
Furthermore in Germany the transmission system
operators have additional requirements laid down in
the “Technische Regeln zur Beurteilung von
Netzrückwirkungen (2. Edition 2007)” (Technical
regulations for the evaluation of system
perturbations).
III.
PERMISSIBLE SYSTEM PERTURBATIONS
1.
Permissible voltage increase at the connection
point: Δv ≤ 2%.
With only one connection point, this can be easily
estimated with the short-circuit ratio:
: system short circuit power at the connection
point (CP)
: sum of the maximum apparent power of
all generating sources at this CP.
SAmax of a wind turbine can be calculated from the
maximum apparent power of a single generator,
where p1min is the relative maximum of the active
power in one minute
respectively.
From the formula, you can easily see, that the
voltage dip at the connection point can be negative.
With an appropriate R/X and a sufficiently small
(
for inductive reactive power) the
voltage at the connection point decreases even
without connecting a generating unit. This mode of
operation leads to increasing losses and a reduced
power transfer capability. If you have a weak
connecting point or if your generated power is too
high and the next strong point is too far, you may
also go for this mode of operation.
In intermeshed networks and/or operating your grid
with multiple distributed generating units you have
to check the voltage increase doing a complex loadflow analysis. You have to ensure, that the
condition for
is fulfilled at the most adverse
connection point.
The
shown
formulae
present
practical
approximations. It is assumed that the angle
between the voltage at the busses of the substations
and the voltages at the connection points is
negligible. Also the backlash of the voltage changes
on voltage and current at the connection point are
not cared for. The voltage changes calculated from
above formulae are always slightly higher than the
exact values.
From many case studies you can assume that the
limits of relevant standards (especially EN 50 160)
for the voltage fluctuations are met both in MV and
LV networks, if the voltage increase in the MV
network is limited to the above mentioned 2%. In
special cases of the network, the transmission
system operator may require a lower voltage
increase than the 2% mentioned.[1]
IV.
SWITCHING-INDUCED VOLTAGE CHANGES
The voltage changes from switching generating
units on and off the network must also be limited to
2%. This may not happen more frequent than every
1.5 minutes at maximum generator apparent power.
If it is lower than half this power a time lag of
minimum 12 seconds is permissible. For very
infrequent operating cycles, i.e. once per day, the
grid operator may permit a higher voltage change.
You can estimate the voltage change by
network dependent current factor
: nominal apparent power of a single generating
unit
: network short circuit power
The factor
not only evaluates the amount, but
also the time characteristics of the current during
the switching operation. It is determined in the test
report according FGW TR 3 [2] in relation of the
network source impedance angle for every single
generating unit and has to be provided by the
turbine manufacturer. Concurrent switching
operations are to be avoided.
These technical regulations also govern flicker,
harmonics, ripple control and protection schemes.
The first edition was introduced in 1998, when the
total installed wind power was 2,900 MW. Ten
years later we had 23,903 MW, an increase by a
factor of eight. Due to this massive increase in 2008
these regulations were tightened. The current values
are in brief:
 Voltage fluctuations
 Flicker long term
or
(flicker factor and long term flicker
amount)
 Harmonics the 5th order was cut by half to
0.058 for 10 kV and 0.029 for 20 kV
networks and for the other harmonic levels
see fig. 3.
 Ripple control, a maximum lowering of the
audio level by 5% is now permissible, as
10 to 20% previously.
V.
SECURITY OF ENERGY SUPPLIES
Wind turbines share 7% of the electricity produced
in Germany today and the forecast is for 2020
around 20% (30% all renewables). The problem is,
that the power delivered is not controllable but
arbitrarily, just as the wind blows or the sun shines.
But also the sudden switch-off of a large amount of
generating units due to a short circuit in the higher
network (as it has happened in 1998) causes
frequency fluctuations. In the 2009 edition of the
Renewable Energy Law (EEG 2009) in Germany
some changes were introduced to help the
transmission system operators:
Equipment with a feeding power of more than 100
kW must have the option to reduce the active power
in the case of network overload by remote control
and the capability of remote reading the actual
values of the power delivered.
VI. GERMAN ORDER ON SUPPLY OF SYSTEM
SERVICES (SYSTEMDIENSTLEISTUNGSVERORDNUNG
SDLWINDV)
With the enormous increase of wind power, these
systems need to be more “grid-friendly”. In general,
this covers the behaviour in case of failure,
provisioning of reactive power, frequency control
and voltage control. As this is involved with
financial efforts, a bonus for every grid-friendly
delivered kWh was introduced. The intention was
mainly to upgrade also existing turbines over 100
kW. This bonus is currently at 0.5 cent/kWh.
For pitch-controlled turbines, this can easily be
achieved whereas for stall-controlled turbines it is
only possible to switch them off.
Either ripple-control or telephone-networks (fixed
or mobile) are used to control the turbines.
VII. Practical example:
Wind farm Buchbrunn near Würzburg: 4 turbines
with dual-speed asynchronous generators rated at
1.5 MW each and one turbine at 2.0 MW with a
double-fed asynchronous generator. In this wind
farm only the 2 MW turbine is fully controllable
whereas the 1.5 MW turbines can only be switched
off. The upgrade for enhanced system services
consisted of changing the control-parameters for the
2.0 MW turbines and the installation of enhanced
capacitor banks for the additional provision of
reactive power. The remote-control
was
implemented through a fixed telephone line.
Fig. 4 Wind farm Buchbrunn with 5 turbines
VIII. CONCLUSION
The use of wind power sharply increased in the last
decade by the Renewable Energy Law (EEG) in
Germany and other countries. Power system
stability is now also in the focus for wind farms, as
the share of wind power of the total electricity
generation will increase to more than 25% in the
next decade. We need to face further tightening of
standards to maintain power system quality. In the
future not only the amount produced but also the
quality supplied will be in the focus. Wind turbines
are becoming “conventional power generation” and
need to be considered as real power plants with full
controllability.
REFERENCES
[1]
[2]
[3]
Eigenerzeugungsanlagen
am
Mittelspannungsnetz;
Richtlinie für den Anschluss und Parallelbetrieb von
Eigenerzeugungsamlagen
am
Mittelspannungsnetz.
VDEW, 2. Ausgabe 1998
Technische
Richtlinien
für
Erzeugungsanlagen,
Forschungsgemeinschaft Wind (FGW), 2010.
BDEW
(Bundesverband
der
Energieund
Wasserwirtschaft e.V., German Federal Association for
Energy and Water) www.bdew.de