Transformerless Inverters with Thin Film Technology

Is it possible to use only one Technology of Inverters capable of using any module technology either thin film or crystalline without causing the modules to decay or causing TCO corrosion / PID .

All TLX versions of the manufacturer Danfoss can handle back ( rear) contacted modules or thin film modules without causing PID or other issues.

1PID:

There are 3 possibilities - TCO corrosion

-Polarisation fr. rear contacted modules

-siliconbased shifting of “standard “ modules

All in common is the polarity and the leakage current ( caused from the potential @cell level, modules compared to the soil, and the Insulation of the modules)

Furtherone is the cell structure , the AR coating as spoken from many companies like Solon, Sun Power.

With some rear contacted cells the PID effect starts almost in the beginning ( immediate 30% loss ) others may take month , even years .

This is one reason why module manufacturers are storing “black listed inverter topologies” or give green light. Depending on which technology are implemented on module level , either + or – pole should be grounded.

2. Earth Concept/Bonding vs Safety :

Even though + or – polegrounding seems to be a easy task, practically it can cause a lot of headache. The Maintenance team mostly is not aware of a “grounded system”. As most of the time Maintenance work is done during operation, and the modules are always “under full load” it is wise to make the Persons in charge sensitive regard this matter.

Especially in Europe it is not needed to ground systems by law, with the only exception of thin film modules.Theres also lot of differences regards Spanish, French and German legislation regards grounding methods ( some Arab countries and Israel e.g use “german settings regards inverter set ups )

The next problem is to ground the system via a circuit breaker capable of switching off according Voltage and Current ( in Europe up to 1000V DC is possible since 2012 theres talks even about 1500 V DC systems ).

Always a DC Main switch should be included to easily remove the earth connection. ( during scheduled maintenance work )

Also should be thought of how the system can detect wether the circuit breaker went off or how easily the inverter can recognize that the system is online again to not cause the modules to decay unrecognized.

( if the circuit breaker is still not replaced – so no grounding occures)

2.1Performance Issues:

Not only safety features also a lot of inverters cannot handle the recommendations. Out of physical reasons a transformerless inverter cannot be choosen anymore.Some central Grid Inverters are capable via transformer kit with galvanic isolation to run with a pole ( + or - ) earthed ( without trouble ).

There are several Central grid Inverters ( transformerless ) which have their own Medium Voltage transformer or similar a isolated integrated transformer included.

Otherwise those systems cannot handle the full potential of the PV Plant in comparison to transformerless inverters.( cause is the bonding towards Earth )

So with the already minimized efficiency of thin film modules, and transformer based inverters this solution gives also less performance , so the already “weaker system “ will loose even more PR.

Since years ( transformerless inverter topologies ) the manufacturing companies trying to find a way to not bond via Earth but rather via the Neutral to ground “ much smoother “.

With different topologies been used like:

Balance Mode, Quiet Rail or Flying Inductor topology thin film modules only work in a tiny little window of the MPPT range , or/and to much components desperately needed will increase the price of the unit , by that again will not give you lot of cost advantages.

Other solutions prefer Earthing via a high resistance ( Suntechnics/ Conergy ) so called Drain Technology or active driven boxes which try to solve the issues (arising) again called Performer Unit @ Night.

In Germany ( VDE 0100/712 ) e.g doesn `t allow to separate AC/DC only with a simple separation ( when using a high resistor ), it has to be electrically isolated.

3 Solution ( GVC ):Ground Voltage Control

With the patented Topology ( GVC ) Danfoss set a new standard using thin film modules , poly or mono crystalline modules , even rear contacted cells can be used with a single module of inverter ( TLX ).

The big advantages in that is that the topology is making use of the technology that AC and DC are not separated ( galvanically ). A IT Network. See picture 1

Therefore any grounding wether on DC or AC side supplied will affect the other Part of the installation ( as not usually with galvanically isolated inverters ) The inverters built into the PV Plant ( IT Network ) will only be separated at the Medium or Low Voltage side of the transformer Station. see picture 2

It can be a Transformerstation “as usual” only that the Low Voltage side shouldn t be earthed at all. With the AC supply towards the Distribution Boards, the only thing to worry about is that the cabling has to be done via a 5 core cabling , separating Earth and Neutral from the supply line at the beginning already.

So a Neutral / 3 Phase / Earth supply line is mandatory , the GVC unit has to be placed inside the transformerstation once, and the 5 core cabling will supply the inverters , the GVC will “introduce” some Voltage in between Neutral and Earth. ( so it s not equal anymore ).

The cause of this virtually grounding method will shift the potential arising also between Phases and Soil /Earth because there s no separation between AC and DC side ( no galvanic isolation ) the shift at the DC side will be similar. That s the big advantage about this solution ( compared to transformer based inverters this would have been impossible to implement) the only solution is to earth via the Soil / Earth.

Voltage but also Current can be individually adjusted ( according module specification ) , with the change in polarity inside the device the module technology been used doesn t count anymore.

Anything wether plus or minus related problems can be “healed” , that s absolutely georgious as this can be achieved with one single inverter.

To be fully balanced all the leakage currents , cables , modules and inverters must be ZEROED OUT.Not only the installation or modules itself , also components , rain and Insulation as well degradation must be cancelled out.

A 18 month old system with a leakage current of 17mA will easily reach during a rainy period 34mA (double the amount ).Those datas are publically available and several times discussed . With one unit ( a GVC unit ) you can handle up to 100 inverters max, another good reason as with transformer based units each single inverter has to be checked during maintenance work.( here only 1 item )

And also the grounding must be done ( in this case 100 times ) traditionally while here with grounding solution The power been “used” is determined from the total leakage current from the whole PV Plant.

In this case when the 30mA will be reached it would accumulate to 20 W ( only ! ) roughly 175kWh been used /lost per anno.

With a total production Northern Europe of 950 kWh/kWp 142 5000 kWh can be achieved that s 0,012% so very less compared to loose constantly 2-3 % with transformerbased inverters instead.

3.2 System Safety/Security:

After All ( VDE confirmed ) solutions also security and safety issues regards maintenance workers were implemented , several tests in Denmark, Germany and Spain were conducted. ( Hardware )

Also different sizes of testsites were choosen ranging from 60kW towards 1 MW in a 18 month durabilityTest different configurations were tested and datas collected will be shared in the near future with module suppliers. PICTURE 4:Shows a testsite with 1 MW installed modules with a sampling rate of 20 periods per 30 seconds.

Picture 5 :Shows the Voltage levels picking up from 4am to 10 pm with a sampling rate of 30 seconds.

The whole concept was finalized and tested from ( BGETM ) and it s according VDE for Photovoltaic Systems.

The only thing which has to be implemented is a Neutral bar which is completely sealed and can t be touched

As the floating in between Earth and Neutral is according GVC Off Set Voltage and not Zero anymore.

The other safety issue the Danfoss ( TLX ) will take care of , in case of an Earth fault he will shutdown immediately.( DC side will remain open and no leakage current will flow )

Having an Earth Fault online with any of the Strings involved will cause the device to reduce the Voltage and current involved, further one the inverters will shut down.That s another reason why no Earth Fault could damage ( or further damage ) modules in the DC String involved ( high leakage current ).

During ground or stray currents/ or faults with the GVC Box the inverters will automatically shut down. During an Earth fault the current monitoring will detect a fault minimize the current and also Voltage will be reduced in this case it doesn t matter if it s related towards Plus or Minuspole. ( also a shortcut GVC Voltage is possible )

4 Results :

Thanks to the TLX inverter and the GVC solution also big PV Plants can be built with decentralized invertertechnology , so that means theres a solution to use small , compact inverter systems instead of Central Grid inverters with their problematic tracking capability.

The Central Grid MPPT Tracker is never the best choice regards kWh/kWp produced as he will go for the global instead of locally based MPPT values , therefore loose out if compared with a inverter based with 3 trackers e.g

3 x 6 kW tracked individually compared to a Central inverter “draw” which has to work from 1 kW towards 100kW , by that in between he will loose out on performance compared to single tracked 6kW units in the whole PV Plant.

The possibility to relate towards temperature/ Voltage and irradiation can t be beaten via centralized Inverters also the price continuously went down and pricewise are PV Plants similar already.

This solution can also solve PID issues with “standard” modules.

Written from Lars Fischer Technical PM from Danfoss Solar inverters EP Photovoltaic

Translated Andreas Iliou Techn.Customer Engineer/ Lecturer Suntech Power Munich 2012