Grid connected PV rooftop (Import/Export)

In a grid tie solar PV system no energy is wasted – any excess electricity is contributed to the grid. Grid tie solar systems is particularly worthwhile during periods of absence of load, during week-ends or holidays for example when your modules continue to churn out electricity.

This is how a grid tie system is connected to the grid,

There are innumerable technical issues/constraints involved in grid connected solar PV rooftop, a list highlighting a few are given below.

1. Grid Synchronization – the inverter will automatically sync its voltage and frequency as per the grid requirements. In India, the grid is never stable, so will can exp lots of sync issues.

2. Over voltage condition – Grid inverter has to isolate from the grid automatically during this condition

3. Low voltage below the preset value – inverter has to automatically switch off/disconnect within few milliseconds.

4. Islanding: There could always be possibility of something being wrong with the inverter and it continues to put electricity to the grid in the event of grid failure. “Grid failure” detection is very important with inverter. So additional AC disconnect should be provided after the inverter (near metering panel) on top of the automatic changer over switch which disconnect from ongrid to off grid during grid power failure.

5. Injection of DC power into the grid – Isolation of DC is very important. If the inverters are transformerless there is a possibility of DC injection into the AC side. To avoid this external insolation transformers must be installed on the AC side/output side of the inverter.

6. THD – Total Harmonic Distortion level should be kept below a certain value (Voltage-THD should be less than 5% as per CEA regulations). These harmonics are caused by non-linear load and proper measures must be taken by the DISCOM by installing harmonic measurement meters and harmonic filters to maintain it below the regulation level.

7. Voltage level of primary side of distribution transformer (step down transformer)- usually 11KV in TN. So there is a requirement for additional transformer which has to collect the output from the big rooftop output (usually >100kWp) and step us to the 11kV level. Therefore, lots of additional infrastructure will be required – this is additional cost for the developer/ rooftop owner.

But small rooftops are directly connected to the distribution panel point at the household with a Net meter / separate meter (http://www.gsv-microtech.com/products/datasheets/pqm3.pdf) and a AC disconnect is provided for emergency other than the automatic inverter disconnect, as mentioned in point 4. The voltage levels are usually 3-phase 415V or 1-phase 220V and no separate transformers are required in this case when connected to the grid. Usually most of the above issues mentioned are taken care by the latest grid tie inverters and these inverters also come with inbuilt protection and surge arrests.  

A grid tie inverter disconnects automatically when grid failure happens, hence net metering/metering is not possible during this event. This is done in order to protect the electricians working on the line and the inverter, as per the grid regulations. Such inverters are more suitable for countries like Germany and the USA where grid failure or power failure is a rare event. But this could cause serious issues in developing countries like India where power cuts are frequent.  

Solution: 

Change over switch:During this condition, the rooftop owner is unable to use the solar power generated even for his own load. Hence there is a need for an off-grid change over switch. This is done along with some changes to the distribution panel (sync with generator for creating reference voltage and current). 

Hybrid Inverter: In a hybrid inverter, various sources like PV panels, wind turbines, diesel generators and batteries can be connected simultaneously. These inverters then choose the best possible solution for a particular situation as programmed. However hybrid inverters are preferred for large rooftop systems because of their high cost 

Now latest hybrid inverters are available in the Indian market which can solve the above islanding issue and make the design more flexible. These systems are available from manufactures like Zigor  (from Spain now available in India). A detailed study about these inverters will be discussed in my future blogs.

Please check the power generation graph below for a site in Mumbai - using Zigor grid tie inverter (only grid import, no export)

Reference: 
India - Smart grid task force
ZIGOR India Pvt. Ltd

Load issues (starting current & harmonics ) with PV systems

Start up current:
Generally high powered equipments consume very high current more than two times its rated value during start. This will greatly affect the PV system when connect to such loads. Usually refrigerators, heaters, water pump and air conditioners consume such high current during start. Given below is the starting current of a refrigerator versus time. It is clearly visible that the current gets stabilized only after t=4.5.

Similarly, when a 2000W water pump is switched on its instantaneous power during start up increase more than two time its rated power i.e. 5000W. This is clearly shown in the image below, the top channel is the current and below is the voltage.

Solution:
Soft starter: The loads affected should have a starter, which could “soften” the start-up.
Use permanent magnets in pumping systems to assist start-up.

Harmonic distortion
Connecting non lineal loads to the inverter cause voltage deformation. These non linear loads cause over heating of appliance which may result in burning insulation material and also leads to short lifetime of equipments.

When a non linear load is connected to the inverter the voltage deformation happens due to the irregular current consumption by the load. This is clearly shown in the image below,

Solution:
Use high switching frequency inverter or using harmonic filters.

Reference: PROBLEMS RELATED TO APPLIANCES IN AUTONOMOUS PV APPLICATIONS. Task 3 experts of the IEA. Website: www.task3.pvps.iea.org.

The Energy Guzzlers!

India has more than 500 million square meters of commercial built up area, which is expected to reach around 2000 million square meters by 2030 (assuming 5 to 6% growth rate), according to the USAID ECO III project report. Chennai alone has a lot of shopping malls, commercial complexes, Five-star hotels and theater complexes. According to TNEB sources the total energy requirement for Chennai is 2200MW out of which shopping areas / shopping malls like T.Nagar, Pondy bazaar etc. consume around 600MW and commercial buildings (office and software companies in the IT corridor) consume around 500MW of power. Here is a sample list:

Shopping complex - EA, Spencer’s, Ampa skywalk, Ispahani center, Fountain plaza, Ramee

Theater complex -Satyam cinemas Abirami complex, Citi center, Sangam cinemas

Commercial building -Olympia Tech Park, Thamarai Park, Asendas,

Hotels -Taj, Hilton, Hyatt, The Park, Le Royal meridian,

The energy (in Kilowatt-Hour) used per square meter per year in shopping centers in India varied between 103-532 kWh/Sq.m./year with an average of 252 kWh/Sq.m./year. Considering that, Chennai is currently estimated to have approximately 20,57,600 Sq.ft. of mall space, meaning that the average total power consumption of malls in Chennai is 4,81,71,617 kWh/Year.

Existing commercial one shift buildings in India consume around 149 units per square meter annually, therefore the total power consumption of a building per can be around 20lakh units a year. Similarly the average area of a 4 or 5 start hotel is projected to be around 19,136 Sq.mt, which consumes a enormous 48, 65, 711 units a year.

For our case study here I have considered the biggest shopping mall in south India, which is the Express Avenue (EA) at whites’ road in Chennai. EA is spread around 10 acres of land with a built area of 3.75Acres, which is equal to 1, 63, 350 sq. ft. This is 400 times the size of an average middleclass (single bedroom -400 Sq.ft) household in Chennai. 

The total retail space of the EA mall is 900,000 Sq.ft; therefore the consumption will be more than 2 crore units of electricity each year. These units of electricity are equivalent to lighting 18,000 household with an annual consumption of 1200 units each. 

 Source: The Hindu (Dated: December 2009)

If we just consider the 3.75 acres of rooftop area, it alone can carry a whopping 0.7 to 1 MWp of solar PV power capacity, which can generate around 12 to 16 lakh units a year and this can replace the grid electricity for around 1300  houses, with an average consumption of 100 units a month per household. But the energy generated is only a very small portion of the consumption which will be self consumed by the mall itself, but this can greatly bring down the load on TNEB grid.

3D Shadow simulation – based on avaiable data from Google

Upcoming malls in Chennai:

With reference to the Asicpac report on malls in Chennai, it is very clear that the local population is still shopping hungry and the demand still tops the supply. More than 62, 86,000 Sq.ft of malls (13 malls) are expected to be commissioned by end of 2013 at various parts of Chennai, mostly in the developing areas of the city. This will greatly bring down the shopping hunger demand, but at what cost? Do we have enough planning and power infrastructure for the further addition?

Conclusion:

If all the above shopping malls, commercial building, star hotels and cinema complex are considered with solar PV rooftop installation, it can generate enough amount of electricity for the nearby neighborhood/houses or at least for the own building consumption and excess can be sent to the grid using the net metering concept.  

If the concept of energy efficiency and net metering/distributed generation is adopted by the above commercial complex / buildings, the nearby communities can greatly benefit and reduce the load on the unstable TNEB grid.  And this will also significantly reduce the power cuts in the urban/rural areas in Tamil nadu other than Chennai.

The recent Tamil Nadu solar state policy has implemented net metering for the very first time along with the GBI (Generation Based Incentives). As per the policy, commercial establishments, colleges, government buildings, residential schools, IT parks, industries, and buildings with a built-up area of 20,000sqm come under the Solar Purchase Obligation (SPO). They can meet this 6% SPO by generating their own power, purchasing solar power from the state electricity board or by purchasing power from private solar power producers. Recently TEDA has announced that all colleges in Tamil Nadu should have solar PV systems in their rooftops, you can find the information here - Must for all colleges by 2014

This policy can be a game changer for the rooftop segment as the GBI details below show. GBI can easily make the payback or return on investment faster, provided the GBI is implemented quickly along with existing subsidy. The question, however, is can TNEB recover from its big losses and implement SPO effectively?

Rooftop GBI	(projects commissioned before 31 Mar - 2014)
Year 1	Year 2	Year 3	Year 4	Year 5	Year 6
Rs. 2/kWh	Rs. 2/kWh	Rs. 1/kWh	Rs. 1/kWh	Rs. 0.5/kWh	Rs.0.5/kWh

- Giridaran Srinivasan