Welcome to the third learning unit in the chapter "Solar Inverters".
This unit will deal with solar inverter topologies.
In the last two learning units, the two core functions
of a solar inverter have been explained: The MPP tracking
and the conversion from direct to alternating current.
Here you can see a simplified representation of the circuit
that we worked out in the last unit.
The direct current of the PV modules
is converted into a sinusoidal alternating voltage
by a PWM bridge circuit
and the signal is then smoothed.
You can also see a capacitor between
the solar generator and the bridge circuit.
This capacitor serves as a buffer.
Among other things, it compensates for fluctuations
in the solar generator.
But where does MPP tracking take place in this inverter?
And how is the PWM bridge controlled?
These functions are controlled via two control loops.
The MPP of the solar generator is searched
for in the first control loop - for this purpose, for example,
the current and voltage of the solar generator are measured.
The second control circuit
ensures that the voltage and frequency of the output signal
are synchronous with the mains voltage.
For this purpose, grid - current, voltage and frequency
are measured on the alternating current side
and the clocking of the PWM bridge is controlled accordingly.
The inverter shown here
is a single-stage, transformerless inverter.
The voltage is only converted once
- in the PWM bridge.
Since every voltage conversion is accompanied by losses,
this topology has its advantages.
However, there are restrictions regarding the voltage with this topology:
The PWM circuit can be used to reduce the voltage,
but not to increase it.
The voltage of the solar generator must therefore be higher
than the peak of the grid-side alternating voltage.
In Germany and several other countries
the mains single-phase voltage is 230V plus minus 10%.
However, this is only the average value of the voltage -
the peak value is 325 V.
However, it can also change and be up to 10% higher - that is 358 V.
The solar generator must supply this 358 V at any time of operation
with a single-stage inverter.
Typical solar modules deliver round about 34-40 V at MPP under STC conditions,
Hence, around 10 modules connected in series would therefore be required.
However, it must be noted
that the voltage decreases with increasing temperature.
This means that even more modules connected in series would be required.
Furthermore, shading of one or several modules in the string is of importance.
As we have seen, the MPP voltage shifts significantly when there is shading.
Under certain circumstances, the generator cannot be operated
in its MPP in order to achieve the necessary voltage
for feeding into the public grid.
As an alternative,
two-stage inverters are used:
Here, the voltage is converted to a specific DC voltage by a boost converter.
This direct voltage is then converted
into alternating current by the PWM bridge.
The additional voltage level
gives much more freedom for the design of the solar generator,
since the input voltage can now also be lower.
Most of the transformerless inverters installed today contain such a boost converter.
The advantage of the larger input voltage range, however, is bought
at the cost of a somewhat lower degree of efficiency,
since every voltage conversion is associated with losses.
For this reason, modern solar inverters do have the option
to by-pass the boost converter in case that the DC
output voltage of the solar generator is high enough.
The inverter we just discussed is a transformerless inverter.
This means that the solar generator is electrically connected
to the public grid via the inverter.
So we have a closed circuit:
electrons flow from the negative pole of the solar generator
through the inverter circuit into the public grid and from there
through the inverter back to the positive pole of the solar generator.
If there is a fault in the system - for example due to defective insulation -
another current path can arise:
If an earthed person touches
such a defective contact, a current flows through the person to earth.
The only other earthed point
in this circuit is the output of the inverter.
The current must therefore flow back into the system through this output.
There it can be measured as a fault current.
An AC fault current of 30mA can be life-threatening for people.
Therefore, the inverter must permanently monitor the residual current Delta I
and interrupt the circuit immediately
if 30mA is exceeded.
An alternative to residual current monitoring is the galvanic separation
between the solar generator and the public grid.
With galvanic isolation, a transformer is built into the circuit.
The power is transmitted inductively through the transformer
and there is no conductive connection between the circuits.
As a result,
the module contacts are never connected to a live network line,
not even in the event of a short circuit.
In some countries, galvanic isolation
by means of a transformer is required for safety reasons.
In the case of an inverter with a line transformer,
a 50 Hz transformer is usually connected
between the output filter and the grid.
This transformer can also be used for voltage adjustment
so that the bridge circuit is operated at voltages other than the mains voltage.
However, a 50 Hz transformer is very large and heavy.
In addition to galvanic isolation,
the use of a transformer offers another advantage.
And that has to do with the potential-induced degradation - called PID.
To understand how the inverter topology affects the PID,
let's take a look at the potential on the solar generator.
To understand this,
we have to differentiate between the terms potential and voltage:
The electrical potential describes the voltage in relation to a defined zero point.
Usually the earth potential is chosen as a reference point.
The voltage, on the other hand, describes
the potential difference between two points.
As an example, consider a module string with 300 volts
- this corresponds to around 8 modules in series.
The plus and minus poles of the PV generator
are ideally symmetrical to the potential
of the earthed neutral conductor.
The PV module at the negative end of the string
then has a potential of minus 150 volts to earth, while the module at the
positive end of the string has a potential of plus 150 volts.
With some module types, a negative potential to earth
can lead to a leakage current flowing through the glass,
the encapsulation material and the solar cells to the earth.
This can lead to potential-induced degradation.
If you want to prevent the PID,
you have to shift the potential of the modules into the range.
But how do you do that?
In the case of an inverter with a transformer, grounding the negative pole
shifts all modules of the PV system to positive potential,
and grounding the positive pole shifts them to negative potential.
Hence, to avoid PID, the negative pole must be earthed.
However, this is not possible with a transformerless inverter.
The plus and minus poles of the generator are alternately
connected to one phase of the network by the bridge circuit,
while the neutral conductor is at ground potential.
If you were to earth one of the generator poles,
you would create a short circuit in one of the switching states.
Since the N conductor is usually earthed,
the DC voltage on the generator inevitably
swings around the earth potential.
If, on the other hand, an inverter with a transformer is used,
there is no direct conductive connection between the modules and the grid.
With such an inverter, the modules can therefore be earthed.
Let's summarize the advantages
and disadvantages of the different topologies.
Transformerless inverters are light, comparatively inexpensive
and have the highest levels of efficiency at 96% and more.
On the other hand, they cannot be used with modules that have to be grounded.
For safety reasons, residual current monitoring is also necessary here.
In addition, galvanic isolation is required in some countries
so that they cannot be used in these countries.
Inverters with transformers provide this galvanic isolation
and also offer the option of grounding the PV modules.
This is an advantage with thin-film modules,
which often require the negative pole to be grounded.
Disadvantages are the size, the weight and the additional costs of the transformer.
And last but not least, a transformer causes additional losses,
so that the maximum efficiency is around 2-4%
lower than with a transformerless inverter.
I thank you for your attention.