How to choose the right inverter?

There are thousands of inverters available in the market, and every manufacturer is coming up with new models each year. The following are the types of inverters available in the market

1. Central inverters 
2. String inverters - TL (transformerless)
3. String inverters - with transformer 
4.  Micro inverters

Problem can arise when modules based on different technology is connected with an incompatible inverter. In this blog i will be discussing the issues related to thin film modules with transformerless inverters available in the market.

PV module market is dominated by crystalline modules (c-Si) in the market. These c-Si are 0.2mm thickness of silicon wafers, whereas thin film modules are made of very less amount of semiconductor materials. The other type of thin film modules are CdTe and CIGS used as active material. In thin film modules Transparent Conductor Oxide (TCO) acts as conductive material.

Challenges in Thin Film modules: 

-          TCO corrosion

Sodium Ions (Na+) in glasses during moisture condition can penetrate into the PV module and attack the TCO layer usually in thin film - CdTe modules. This first attacks the edges of the modules and the module edges turn milky white. This can result to loss of conductivity and this will permanently affect the modules, finally it will affect the entire module.

TCO Corrosion

-          Polarization 

Polarization effect is usually experienced in crystalline modules (c-Si). Here the sodium ions in the glass get stuck in the laminated film and these cannot greatly affect the performance of the modules. This does not affect the performance too much if proper precaution measures are taken. This same effect can also be observed in thin film modules with CIGS as active material because a film is laminated before the TCO layer. 

-        Leakage Current

According to the fundamentals of physics a PV panel acts as a capacitor. This capacitance is proportional to the area of the panel and inversely proportional to the distance between two capacitor plates. Therefore thin film or flexible modules can produce higher amount of stray capacitance. The relationship between stray capacitance, module area and thickness can be given as follows,

C = (Eo*Er * A)/d

Parasitic capacitance is higher for thin film or flexible panels, this can lead to higher leakage currents. These leakage currents can be deadly when the structures are not grounded and a human comes in contact with the panel structures. Hence grounding of these thin film modules is necessary and when grounded this can affect the operation of transformerless inverters. Understand of Transformerless is very critical to come up with a safe and efficient design. The leakage current is very less in mA but when a short circuit happens in a solar panel string we can expect this to rise significantly and can cause bad injuries to human when in contact.

The stray leakage currents from PV array can increase also due to

1. Moisture - in early mornings
2. Rain
3. change in soil/earth conditions due to rain (i.e. this will reduce the earth resistance)
4. Dust on panels

Effect of earth resistance (from above point 3, consideration -3kWp system, with capacitance 0.7uF)

This is a very serious issue but most of the EPC and system integrators ignore this issue. Similar problems where also addressed in countries like USA, where the system integrator refused to make changes to the ground the panels for transformerless inverters.

The effect of leakage currents are very less in a inverter with transformer and are negligible. This is due to galvanic isolation of AC and DC it never affect the performance of the inverter connected to the grid. Where as in transformerless inverters the leakage currents can affect the operation of inverter and hence a leakage current monitoring system are usually installed in the inverters (RCMU - Residual Current Monitoring Unit). This system will monitor and give a alert during higher discharge of leakage currents for safety reasons, but these are not protection devices.

TT Grounding connection and Leakage current path

Why RCD? Usually most of the grid standards (Germany, France, USA, UK etc..) recommend the use of RCD (Residual Current Device) which monitor these leakage currents and disconnect the AC side of the inverter from the grid. But in developing countries like India is there is no grid codes or regulations for such protection in PV systems and there is no study regarding this issues. Are the EPC aware of the safety issues related to leakage currents? Inverter manufacturers like SMA recommend RCD protection of 100mA for their TL type inverters. A detailed analysis will be required to select the right type of RCD (Type A or Type B) and other parameters. I will be discussing more about the issues, sizing and selection of RCD in my next blog.

Simulation Results: The results obtained from simulations indicate that the leakage current strongly depends on the inverter typology(3kWp PV system with module capacitance: 1μF and R_earth = 30 ohm ):

- for inverter with transformer, leakage current is negligible

- for transformerless half bridge inverter, leakage current is very small (9mA)-So we note that leakage currents are not necessarily linked with all transformerless inverters, indeed a proper choice in the inverter typology will strongly reduce this current. This comes from the reduced voltage variation amplitude of the polarities.

- for TL inverter, with a PV array capacitance of 0.8 mF , the leakage current can cause the trigger actions of the RCD 

Conclusion:  

 In case where the leakage current is important (> 30 mA), following solutions can be carried-out:
- Using inverters with transformer or transformerless half bridge inverter
- Using PV mono or polycrystalline modules (these modules with small tray capacitance)
- Using an advanced control mode inverter (External RCD's as per regulations)

Similarly care should be take care when we choose a thin film module to avoid TCO corrosion and polarization. Its better to choose a inverter with transformers while going with thin films or if TL transformers are used we must make sure that the stray capacitance is kept low to avoid high leakage currents.

Reference: 
- SMA

- Concept paper: TRANSFORMERLESS INVERTERS AND RCD: WHAT'S THE PROBLEM?
T. Tran-Quoc1, H. Colin2, C. Duvauchelle3, B. Gaiddon4, C. Kieny1, C. LE Thi Minh1, S. Bacha5, S. Aissanou5, G. Moine6, Y. Tanguy6