Common problems in PV plant - Part 3

27th Nov 17  

(To be read in conjunction with our previous article "Common problems in PV plant - Part 1& 2)

In our previous article "Common problems in PV plant - Part 2" we explained you the solar PV module specific problems which are evident on field after significant amount of time. All such problems lead to reduction in power output from the power plant. There are however few more problems leading to reduced power output from power plant. This article would try explaining you such additional but important (common) problems.

One of the crucial parts in the power plant is their interconnection which is done through cables. Cables are used to carry the generated DC power from solar module to inverter (via junction boxes) and evacuate AC power to grid. While they are usually of highly conductive material, optimizing its length (and the area) is very important. The resistance (R) of the cableis given by the formulae

R = ρ * L / A

Where ρ is the resistivity of the material, L is the length of the cable and A is the cross section area of the cable. As evident from the formulae above, increasing the cable length would increase the resistance of the cable leading to increased power loss.Also evident is that increasing the area would lead to decrease in the cable resistance but would lead to an increase in cost (cost of wire increases its diameter/area increase).Hence it is necessary to have a balance between power loss and the cost of wire. Additionally, it is also important to optimize the position of junction boxes and inverter so that the power loss for a given cross section area of cable is minimized. The figure below shows theplanned power plant at one of our sites with an optimized(as simulated by the software)wiring diagram. The interconnection between modules (and hence the strings) is in a manner that the output voltage of the string is maximized while optimizing the cable length. The position of junction box (shown as boxes in figure) and the inverter (shown as a pointer in the figure) is considered in such a way that the cable length is optimized to minimize energy losses.

1 : Optimized wiring diagram (Source: HelioScope)

Operation & Maintenance (O&M) of a solar plant is one of the most important contributors for its efficient running. Solar plant (as is made up of different components) requires maintenance at regular intervals (ranging from days to weeks) throughout the year. While there are best practices in place,issues leading to degradation of power output from power plant are fairly visible. Solar modules as we know generate power when sunlight falls on it,the top covering (transparent) glass (which allows light to fall on the solar cell) needs to be cleaned periodically with demineralized soft water. While this is viable in large scale solar plants, the small scale solar plants either don't clean the modules at all or do it with the available tap water. This leads to accumulation of salts on the glass and/or scratch on the glass if there are particles in water leading to degradation of the glass (Figure 2).

The small (or medium) scale plants don't always have string level monitoring. It is hence necessary to check the voltage across these strings (by clamp meter) to ensure the inter-modular connectivity.The solar power plant uses combiner box to combine the power output of various solar modules. A combiner box in addition also has fuse (or MCB). The string of solar panel due to high irradiance may (sometimes) produce more current leading to shorting of fuses which needs immediate replacement to ensure continuous operation of the plant. While easily monitored in large scale plants such problem cannot be found out easily in small scale plants leading to monetary loss (for few days till the fuse are replaced).

2 : Accumulation of salts on solar module (left) & combiner box (on right)

An energy meter (or net meter in rooftop plant) used in solar power plants records the amount of energy generated and hence exportedto the grid. Additionally, the billing cycle settlement (and hence the financial settlement) is based on the reading of net meter. However, the reading of the meter happens once only in two months (or monthly). While being an important part of the system, there have been many cases where due to various technical reasons that the meter is either faulty or not working at all. This would lead to various problems as there is no record of amount of energy generated (for partial or sometimes the entire billing cycle) and also the amount of energy consumed (if the spare meter is not installed). This would also lead to monetary losses.

3 : Pictorial representation ofa utility meter

Moreover, the inverter converts the generated DC power to AC power and feeds it to the grid. The feeding of the generated AC power to the grid however depends on grid availability. This means that the generated power would not be fed to the grid if there is power outrage. This is because the inverter has a special feature in it called anti-islanding which would prevent the plant to feed the power considering the safety of the grid (and the worker who is working on the line). This feature while is important for safety of the grid would lead to a monetary loss for solar plant owners. Additionally, in India where there are still outrages of power in certain towns and/or cities for more than 2~3 hours (during daytime) may lead to viability issues for the solar power plants (in few cases).

Figure 4: Pictorial representation of power outrage

A solar module as we know generates variable DC while our grid is AC in nature. Inverter, which converts DC power to grid compatible AC power, is hence considered to be at heart of the plant. It is (mostly) made up of electronic components which requiremaintenance at a controlled temperature for its efficient operation. It has inbuilt fans which maintains temperature by continuously circulating air. While (mostly) the inverter installed for utility scale power plants have a special conditioned room, the inverters at rooftop plant are either installed below a shade or (sometimes) outdoors. The climate conditions hence play an important role as during summer the fans would not be able to appropriately cool the inverter leading to reduced power output from it. Additionally in (few) places the fans may be stressed over a limit causing temporary or permanent failure and hence a cascading effect which may lead to critical problems in inverter. Solving such problem may take time ranging from few hours to days leading to power and hence monetary loss.

Figure 5: Thermographic image of inverter (Source: Google images)

We at Waaree have a total EPC experience of more than 300 MW with more than 500 MW of solar modules installed globally. We have executed more than 4000 projects solarizing the remotest corners. We also have adequate experience and provide both Annual Maintenance and Preventive Maintenance for the solar power plants.

Let us all pledge to make solar energy the primary source of energy in the near future.