Features - Bodo's Power

following bulky capacitor. The distorted waveform can be overcome if
a PFC input stage is used. If we apply a Fourier analysis to this signal
we would see a lot of harmonic components.
For example the harmonic content related to the fundamental wave is
20%, the apparent power is calculated as follows:
P
VA
= PW
+ PVAR
P W = Active Power [W]
P VA = Apparent Power [VA]
P VAR = Reactive Power [VAR]
1
0.
2
This results in a 2% lower efficiency of the generator. These extra
losses are produced from the higher current in the copper of the generator.
Beside the copper losses there are also iron losses which are
affected by higher order harmonics.
To control the PMSG normally sinusoidal commutation or field oriented
control is used. The former has limited gain and frequency
response. The time-variant perturbations to the current control loop
cause phase lag in the motor current. This results in less torque off
the generator. Therefore more current is required to maintain the
Figure 5: Power stage
Figure 6: Controller part
Figure 7: Voltage and current waveform
=
2
+
2
= 1.
019
RENEWABLE ENERGY
same torque. To solve this problem filed oriented control is used. The
current space vector is fixed in direction with respect to the rotor,
independent of rotation. The resulting flux is controlled for optimal
torque with minimal phase lag. Due to this reason, more active power
and less apparent power is transferred from the generator to the
inverter which leads to a higher efficiency.
Feeding power into the public grid
The next step is now to feed the power into the public grid. For the
highest efficiency of the whole system the operating principle of the
line inverter should be understand and optimized.
To show the operating principle the simulation model in figure 5 and
6 is used. The controller part is based on sinusoidal commutation.
The inverter topology is a single phase voltage source inverter.
To feed energy into the public grid, normally a DC-AC converter with
buck capability is used. The current feed to the public grid should
have, for low emission and high efficiency, a sinusoidal shape. With
the appropriate control algorithm it is possible to minimize the reactive
power. Nearly a power factor of -1 can be obtained. Figure 7
shows the simulated wave form feed into the public grid. The disadvantage
of the above buck converter is the need of the line inductors
and some bulky capacitors. The size of the inductors is determined
by the switching frequency of the power stage and the operation
mode (continuous or discontinuous). The result of a lower inductance
value (same switching frequency) is a smaller size. But a lower
inductance value leads to a higher ripple current which affects the
EMC behaviour.
All these aspects should be considered during the design phase to
get an optimised solution.
The future
As mentioned above when working with the back to back converter
there are two main disadvantages.
The line side needs power inductors. Normally the switching frequency
of line inverters is in the range between 5-20 kHz. Therefore, the
buck inductors are relatively large.
The BBC needs electrolytic capacitors on the DC side. The capacitors
are large in their volume and are reducing the life time of the
inverter.
To reduce cost and volume and increase lifetime it could be considered
to develop a generator inverter combination. Therefore, the
inverter is a part of the generator housing. This is only possible with
a very compact inverter without the need of bulky capacitors and
inductors. This is possible with the use of a matrix converter. We calculated
that it would be possible to design a 20kW matrix converter
with a volume of only 3.3dm3 (including EMI Filter).
Conclusion
In summary, understanding of the different parts (generator, inverter
and control part) of small wind power stations can lead to better
designs. Better designs are more reliable and the most important
point, they have a higher overall efficiency. Several points and strategies
are described and must be considered during the design phase
of inverters for small wind power stations in the kilo watt range to
reach the highest overall performance.
Abbreviation used in the text
PMSG Permanent magnet synchronous generator
BBC Back to Back converter
LUT Look up table
www.negal.ch
www.bodospower.com August 2009 Bodo´s Power Systems ®
www.bodospower.com August 2009 Bodo´s Power Systems ®
www.bodospower.com August 2009 Bodo´s Power Systems ®
35