Download Power Quality issues: Weak Grids

Wind Engineering
Module 7.2: Power Quality Issues
Lakshmi N. Sankar
[email protected]
Electromagnetism
http://www.windpower.org/en/stat/emag/emagn.htm
Electromagnetism
• The current magnetizes the iron core and
creates a pair of magnetic poles, one
North, and the other South.
• The two compass needles consequently
point in opposite directions.
Induction
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The light bulb flashes the moment you
connect the switch to the battery.
The explanation is, that the magnetic
field coming from the upper
electromagnet flows through the lower
iron core.
The change in that magnetic field, in
turn induces an electric current in the
lower coil.
The current in the lower coil ceases
once the magnetic field has stabilized.
If you switch off the current , you get
another flash, because the magnetic
field disappears. The change in the
field induces another current in the
lower core, and makes the light bulb
flash again.
Simple AC Generator
Source: http://www.tpub.com/content/doe/h1011v3/css/h1011v3_111.htm
Simple AC Generator
• In this type of generator, a DC source
is supplied to the rotating field coils.
• This produces a magnetic field around
the rotating element.
• As the rotor is turned by the prime mover,
the magnetic field will cut the conductors
of the stationary armature, and an EMF
will be induced into the armature windings.
Alternating Current
• With an alternating current in the electrical grid,
the current changes direction very rapidly, as
illustrated on the graph above.
• Ordinary household current in most of the world
is 230 Volts alternating current with 50 cycles
per second = 50 Hz
– "Hertz" named after the German Physicist H.R. Hertz
(1857-1894).
• The number of cycles per second is also called
the frequency of the grid.
• In USA household current is 130 volts with 60
cycles per second (60 Hz).
Phase Angle
http://www.windpower.org/en/stat/unitsac.htm
• Since the voltage in an alternating current system varies with time
one cannot connect a generator safely to the grid, unless the current
from the generator oscillates with exactly the same frequency, and is
exactly "in step" with the grid,
– i.e. that the timing of the voltage cycles from the generator coincides
exactly with those of the grid.
– Being "in step" with the grid is normally called being in phase with the
grid.
• If the currents are not in phase, there will be a huge power surge
which will result in huge sparks, and ultimately damage to the circuit
breaker (the switch), and/or the generator.
• In other words, connecting two live AC lines is a bit like jumping onto
a moving seesaw.
– If you do not have exactly the same speed and direction as the seesaw,
both you and the people on the seesaw are likely to get hurt.
Three-Phase AC Generators
• The principles of a three-phase generator
are basically the same as that of a
single-phase generator.
• There are three equally-spaced windings
and three output voltages.
• These are all 120° out of phase with
one another.
Stationary Armature 3 Phase
Generator
Power Quality
• The term "power quality" refers to the
voltage stability, frequency stability, and
the absence of various forms of electrical
noise (e.g. flicker or harmonic distortion)
on the electrical grid.
• More broadly speaking, power companies
(and their customers) prefer an alternating
current with a nice sinusoidal shape.
Starting and Stopping
• Most electronic wind turbine controllers are programmed
to let the turbine run idle without grid connection at low
wind speeds.
• If it were connected to the grid at low wind speeds,
energy will flow from the grid to the turbine and it would
run as a motor.
– The motor may over-speed and be damaged.
– There are several safety devices, including fail-safe brakes, in
case the correct start procedure fails.
• Once the wind becomes powerful enough to turn the
rotor and generator at their rated speed, the turbine
generator becomes connected to the electrical grid at the
right moment.
– This is done using electrical controllers.
Effects of Sudden Starts
• If you switched a large wind turbine on to the
grid with a normal switch, the neighbors would
initially see a brownout
– This is because of the current required to magnetize
the generator.
• This is followed by a power peak due to the
generator current surging into the grid.
• Another unpleasant side effect of using a "hard"
switch would be to put a lot of extra wear on the
gearbox, since the cut-in of the generator would
work as if you all of a sudden slammed on the
mechanical brake of the turbine.
Soft Starting with Thyristors
• To prevent this situation, modern wind turbines are soft
starting.
• They connect and disconnect gradually to the grid using
thyristors, a type of semiconductor continuous switches
which may be controlled electronically.
– You may in fact have a thyristor in your own home, if you own a
modern light dimmer, where you can adjust the voltage on your
lamps continuously.
• Thyristors waste about 1 to 2 per cent of the energy
running through them.
• Modern wind turbines are therefore normally equipped
with a so called bypass switch, i.e. a mechanical switch
• This is activated after the turbine has been soft started,
and the thyristor is bypassed.
Power Quality issues:
Weak Grids
• If a turbine is connected to a weak electrical grid, (i.e. it
is vary far away in a remote corner of the electrical grid
with a low power-carrying ability), there may be some
brownout / power surge problems of the sort mentioned
above.
• In such cases it may be necessary to reinforce the grid,
in order to carry the fluctuating current from the wind
turbine.
• Local power companies have experience in dealing with
these potential problems, because they are the exact
mirror-image of connecting a large electricity user, (e.g.
a factory with large electrical motors) to the grid.
Power Quality Issues: Flicker
• Flicker is an engineering expression for short
lived voltage variations in the electrical grid
which may cause light bulbs to flicker.
• This phenomenon may be occur if a wind turbine
is connected to a weak grid, since short-lived
wind variations will cause variations in power
output.
• There are various ways of dealing with this issue
in the design of the turbine:
– mechanically, electrically, and using power
electronics
Power Quality issues: Islanding
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Islanding is a situation which may occur if a section of the electrical grid
becomes disconnected from the main electrical grid, e.g. because of
accidental or intended tripping of a large circuit breaker in the grid (e.g. due
to lightning strikes or short circuits in the grid).
If wind turbines keep on running in the isolated part of the grid, then it is
very likely that the two separate grids will not be in phase after a short while.
Once the connection to the main grid is re-established it may cause huge
current surges in the grid and the wind turbine generator.
It would also cause a large release of energy in the mechanical drive train
(i.e. the shafts, the gear box and the rotor of the wind turbine) much like
"hard switching" the turbine generator onto the grid would do.
The electronic controller of the wind turbine will therefore constantly have to
monitor the voltage and frequency of the alternating current in the grid.
In case the voltage or frequency of the local grid drift outside certain limits
within a fraction of a second, the turbine will automatically disconnect from
the grid, and stop itself immediately afterwards.
– Normally by activating the aerodynamic brakes
Current Approaches for
Improving Power Quality
• Field Data
– http://www.nrel.gov/wind/pdfs/39477.pdf
• Modeling
– http://www.nrel.gov/wind/pdfs/39510.pdf
– http://www.nrel.gov/wind/pdfs/40851.pdf
Sample Field Data
http://www.nrel.gov/wind/pdfs/39477.pdf
Conclusions based on NREL Studies
http://www.nrel.gov/wind/pdfs/39870.pdf
• From the power fluctuation perspective, the larger the area of the
wind farm the more diverse the wind profile that drives each turbine.
Thus, there is a greater chance that the fluctuation in one turbine will
be out of phase with another on the other side of the wind farm.
• NREL studies show that the more groups used to represent a wind
farm, the smaller the fluctuation. The same conclusion can be drawn
that a wind farm with more small turbines creates fewer
power/voltage fluctuations on the power grid than a few large
turbines.
• As the number of turbines in a wind farm increases over a large
area, the characteristics of the wind farm are masked by the
collective impact. Thus the impact of tower shadow and wind
turbulence on the wind farm will be leveled out.
Conclusions based on Field Data
http://www.nrel.gov/wind/pdfs/39477.pdf
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The generating rating of the Blue Canyon Wind Power Project is about 6%
of WFEC’s peak load, and during light load periods it may approach 14% of
the load.
At such levels, the data show that on average the fluctuations of wind power
only increase the short-time frame variability of system apparent load by
8%.
For a longer-time frame, the increase in system apparent load variability is
even less.
The available data show that wind generation has less impact on system
regulation requirements than system load.
Field data Conclusions (Continued)
http://www.nrel.gov/wind/pdfs/39477.pdf
• The uncertainty of wind power availability complicates the dayahead resources scheduling and hour-ahead adjustment processes.
• Better wind power forecasting can help improve the system
performance.
– Improve how system operators will incorporate the information into scheduling
and operating decision processes.