Solar hybrid installation in India

Hello Readers,

I have been recently quoted in SolarNovus magazine regarding a hybrid installation in Lonavala, near Mumbai.

You can read the entire article here - Click here

Diesel Generator loading and renewable systems integration

Abstract: 

Diesel Generators are widely used in India and most of the developing countries where the grid is unstable. The cost of diesel fuel has been growing rapidly in the past few years. The demand for diesel is only expected to raise day by day and can no way come down. How equipped and aware is the Indian/developing community regarding their dependency on such a fuel? Should we think about conservation and smart usage of these non-renewable fuel like Diesel, as these are the primary source of fuel for automobile industry?

In this article I will be focusing on DG sets loading profiles, effects of low loading and how to make your DG more efficient. As discussed in my previous article on smart power management, regarding synchronization of DG with solar inverters, I will be adding more flavor to this article by also discussing on integration of renewable energy systems to the existing DG systems to make it economically viable option.

"Any creative comments or ideas from the readers are welcome, since this is just a platform for me and peers in the same field to make people aware of these technologies"

Diesel cost has doubled in the last decade: 

The cost of diesel was INR 23.29 per liter in 2002. Now the cost has doubled to INR 52.45 (Check graph below). Can the cost of diesel increase more? I leave the answer to the experts!

Source: moneycontrol.com

I have the following questions in my mind.

In this current situation what must be the steps taken by the Indian central and state governments? Is there any restriction in fuel consumption or purchase by a individual.? Do we have any restrictions on DG sets purchase or usage? The DG usage increase the burden on a common man indirectly? should the government think about imposing special prices on diesel for DG usage.? 

Lastly,  how do we act smart in this scenario and contribute as individuals and also benefit financially?

Diesel Generator working and Loading profile: 

In actual practice the diesel fuel consumption (kWh/litre) is never in exact proportion to the load and reduces non-linearly with reducing load. Figure below indicates maximum energy production occurring at the rated capacity and the production rate reducing with the load.

While selecting a diesel generator it is always better to understand its loading profile. If loading can vary drastically over a limited time period, it is always better to consider two or more DG's synchronized in modular fashion. The savings can be considerably high on the long run, please find some calculations below under Economics heading.

At idling conditions, ie., at no load, the diesel engine consumes approximately 2.5 times more fuel to produce a unit of energy (or generates only 40% of the energy for a litre of diesel) than when operating at the rated capacity. At a loading of 40%, the fuel consumption is 23% higher than at full load or the energy generation is 19% less than at the rated capacity. Thus efficiency of diesel generator always reduces with the decrease in load.[2]

DG with renewable energy technologies: 
As discussed in my previous articles and blogs, it is possible to run renewable energy technologies like solar and wind synchronized with diesel generators. Usually in developing countries like India diesel generators are used in almost all commercial and industrial buildings, this is due to the unstable grid.

The following are the options to be considered

1. Have 2 or more DGs based on load requirements (sync one by one based on load - in modular fashion) this can help in fuel reduction

2. Have solar PV or wind 20-30% of the load requirements

Economics or Cost savings:  

1. Cost of 100kVA diesel generator - 12lakhs (supply+installation) 
2. Cost of 100kW (Solar grid tie system) - 100 lakhs (Supply+installation) 

The payback from solar is completely dependent on generation and the FiT or savings from conventional electricity tariff. If generation or usage (load) is not optimized the payback can be prolonged. These is a misconception that running DG under idle condition just for solar operation is profitable.

The design engineer or project developer should understand that loading DG below 40-50% can always make the system consume more fuel. Here is a example,

Here is a cost comparison (using simulation): (please check reference 2 for more details)

As per the research paper published [2] at Murdoch University, WA., the comparison has been generated using Hybrid2 simulation.

Here two scenario's have been considered

• Under Normal Load conditions 
• Under Modified load conditions 

Obeservation from above table: 

Under Normal Loading condition - 

• Using a single 60kW diesel with a additional 20kW wind can increase the dump load % upto 10% (this is too high) and the % of fuel saving is 9.3%. This leads to a average cost/kWh as 14.30 $cents. 
• Whereas under the same loading condition but with two generators (40+20kW) with 20kW turbines can bring down the overall average cost upto 13.36 $cents. This is due to the modular fashion generators and reduction in dump %

Under variable loading condition -

• Under this condition the with 40+20kW DG in modular fashion and 20kW wind turbine, the system seems to perform much economical fashion (12.87 $cents)

Potential in Micro grids: 
Lots of isolated grid installations have happened in the past and the potential is huge. For example islands like Maldives after being affected by Tsunami in December 2004, have been completely dependent on micro grid systems with renewable energies. 

Similarly many developing countries like India, countries in south America and Africa can greatly benefit using these systems. In my future blogs/articles I will be discussing a case study on these micro grid systems.

Conclusion: 

The above results show the commercial advantage on using two diesel generators instead of a single big diesel generator. In a two-diesel generator modular system the flexibility in choosing the diesels to meet the load has resulted in better operational efficiencies. Thus the smaller diesel along with the wind generator can take care of the low load situations leading to reduced fuel use and lower operating costs. The load management strategy resulted in better overall cost savings and reduced generation cost in the dual diesel scenario.

Reference: 

1. http://www.moneycontrol.com/news/market-edge/chartthe-day-how-diesel-prices-have-doubledlast-decade_757880.html  

2.  Effect of load management and optimal sizing on the economics of a wind-diesel hybrid power system. P. S. Panickar*, S. M. Islam** and T. L. Pryor* * Murdoch University Energy Research Institute, Murdoch University, WA.

My article on a internation solar magazine from USA (Global Solar Technology)

Hello all,


My exclusive article on power management during islanding condition (PV sync with diesel generator) has been published in a international Solar magazine - Global Solar Technology, USA. 

You can have a look at my article here - Click here 

Download Link - http://globalsolartechnology.com/documents/global_solar_6.1_low_res.pdf 

"In a grid tie solar PV system, no energy is wasted, i.e. any excess electricity is given to the electrical grid. Grid tie solar systems are particularly worthwhile during periods where there is absence of load, that is weekends or holidays when your solar system continues to give out electrical power. Hence there is very good potential for grid tie inverters in India, but is Indian grid infrastructure, strong enough for these latest distribution generation technologies? This article will speak more about the issues associated with the above concern and explain the methods to overcome these conditions by integrating/synchronizing the grid connected inverter with diesel generator during a power cut/grid failure."

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 

My review on "The India Blackout/grid disturbance" - 30,31st July 2012

My presentation on review of July 2012 blackout in India has been published.

You can view the presentation here:

30, 31st July 2012 - The India Blackout from Giridaran Srinivasan

"In July 2012, India faced one of the worst blackouts, where almost 48,000 MW of load got disconnected from the New Grid. More than 600 million people were affected by the blackout.
What led to the blackout which lasted two days? Did it happen because of bad grid infrastructure? or failure in following the set processes?

Central Electricity Regulatory Commission(CERC) investigated the causes, recommendation and future course of action. We found the report from CERC very interesting and thought that it would be useful to you as well. We have put the information in a more easily digestible form – in a power point slide deck. The ppt is shared below. Do let us know your comments."

My views about solar PV grid connection issues

Hello all,

I have been quoted on Solar Novus Today, a US solar magazine. 

Here is the link: Click here

"Technical issues to be addressed

The main technical issues to be addressed for off-grid systems will be correct sizing by the installer and proper installation by the local work force in off-grid areas. Also, there needs to be proper standardization for indigenous equipment such as the battery and inverters used in systems.

“Smart metering for solar system integration with grid will take a lot of time to be accepted since people see solar power as a solution for frequent power cuts in India” says Giridaran Srinivasan, Project Engineer, RESolve Energy Consultants. He adds that unstable grids across various states in India (where power failures are frequent) don’t accept decentralised renewable energy power and make net metering impossible during power cuts as grid tie inverters disconnect immediately when a grid failure occurs.

During power cuts, the owners can only use solar power by using off grid changeover switch or hybrid inverters (expensive hence used for large rooftops), although they are forced to consume all the power themselves, which is not always feasible, hence it is always good to have a battery backup for evening and night power requirements, which will also be helpful during blackouts and emergencies."

Sync diesel generator with solar grid inverters - during Islanding condition

Abstract: As mentioned in my previous blog on solar grid interactive inverters, there are various issues involved with solar grid inverter for Indian conditions. Now in this blog I will be discussing more about the issues and methods to integrate/synchronize a grid connected inverter with diesel generator during a power cut/grid failure. In states like Tamil Nadu we have power-cuts every day, so the solar rooftop owner will try to consume all the power generated from solar and reduce its dependence on diesel generators.

The inverters do not start without the reference voltage and frequency, so during this islanding condition the rooftops owners cannot use their solar power generated. The owners usually consider solar as a backup for power cuts and also consider payback by selling excess generated. Now the only solution in the market is the hybrid inverters, but these are very expensive compared to normal grid inverters. But what about installations with existing grid inverters? Most of the rooftop owners are not aware of these islanding conditions and the EPC vendors just install existing grid inverters without any alteration proposed below.

What I propose here is, the grid inverters needs to be connected to a common bus bar. In the common Bus-bar all the sources and loads are connected. During the islanding condition the DG switch ON to generate reference voltage and frequency to start up the solar grid inverters (we fool the inverters here). Once the inverter is ON the load will be supplied by both inverter and DG sets (at low RPM). Is it practically possible to synchronize a grid tie inverter with a diesel generator (DG sets) and make it work? ie. during a power failure, when grid power is not available.

I feel its possible only under the following condition's 

• We have a stable/good quality generator which gives good stable voltage and frequency. 
• The other condition is the generator should not sag, dip or create any surge current when the inverter turns ON. 

Lets now consider that the inverter has turned ON using the DG sets reference voltage and current. I see two possible conditions now,

1. The load is always more than the PV generator supply
2. Possibility that load can be less than PV generator supply

During the condition 1, i do not see any issues if everything works fine. 

But during the condition 2, the power generated from PV will be greater than the load requirements and hence there is a possibility that this current will be fed to the ac line of of the bus bar / DG sets (stator of DG sets). What will happen during this condition? We can expect a rise in AC voltage and frequency- above critical level. If this happens, this condition can trip the inverter due to over-voltage/frequency protection circuits. Now the rooftop owner cannot consume power from the solar and therefore he is forced to operate the DG sets. This condition can also damage the vital parts of the generator, here the protection of generators become very important. 

How to custom design the distribution panel board to use the solar generator during the condition 2 and also protect the DG sets? I also foresee very high requirement of a effective control system, which can handle all the above and below conditions

- Using a dump load to remove the excess power generated. 
- Effective control of loads  (To increase or decrease the load as per requirements using PLC based controls) 

I can also see that there will be no commercial benefit in running a generator just for reference voltage and current. This is because when we operate the DG sets running at lower efficiency (20-30%) or less than its rated value can consume more fuel than during rated power (ie. 85-100% efficiency). This can be solved by using a smaller DG sets (Just for the emergency load) than the required full load size, but the disadvantage is during power failure in the evenings or night.