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Monday, 29 June 2015

Optimize the electrical loads for a successful solar PV system

As per a recent survey, India is second among the countries in terms of growing electrical demand. To meet this ever increasing electrical demand in an economically feasible and environmentally friendly way, Renewable Energy and Energy Conservation are the best options. In this regard, the Indian government is planning at a fast pace and the result is schemes and programs like Jawahar Lal Nehru National Solar Mission (JNNSM) and many more. Many states in the country have implemented solar schemes such as roof top net metering arrangements and so on.

The current scenario in India is that the roof top solar PV system along with major solar PV plants is coming up at a rapid pace. A good population is keen to know and eager to install a roof top solar PV module. Here are some of the vital basics needed as a priori to install a roof top solar PV system for the residential sector in India.

What should be the ideal capacity of my solar PV system? (a FAQ)


I hope, the following paragraphs will answer most of your queries.  
   
First and foremost thing is to know the key elements of any solar PV system. Any solar PV system, I am talking of stand-alone solar PV system, consists of the below given equipments.

1.  Solar PV module,
3.  Battery,
4.  Inverter,
5. Miscellaneous items such as supporting frame, wires,   switches, change-over etc.

Of the above listed electrical equipments and elements each has its own rated capacity, and to get a proper matching between them is very important. Prior to working out the optimum capacity of the solar PV system, one has to determine the right electrical loads which can be connected to make this solar PV system viable with a low pay back period. The steps in planning for a solar PV system are:

Step 1: Determination of proper electrical load


Make a list of electrical appliances that you are planning to run from the solar PV system. Prior to this one has to find out how much power each piece of equipment draws for its operation. The electrical power needed by the equipment is given on its name plate from where one can note down and prepare a list. The electrical wattage of some of the commonly used appliances is:

Sr no
Equipment
Electrical load (in Watts)
1
Microwave oven
1200- 2000 W
2
Electric Geyser
2000-3000 W
3
Washing Machine
300-500 W
4
Electric Iron
600-800 W
5
Water pump (domestic, 1-phase)
375-750 W
6
Ceiling fan
80 W
7
PC
100-150 W
8
Color TV
150 W
9
Tubelight (including choke)
45 W
10
Freezer
150-250 W
11
Room Cooler
150-250 W
12
AC
1000-2000 W

The electrical load given above is for reference only. Actual rating may be obtained from the equipment name plate or manufacturers specification only, as the rating or electrical load of any equipment may vary according to its capacity, features, etc.

Step 2: Optimize your electrical load

As per the given list one can very well judge that appliances such as microwave ovens, geysers, AC, hotplates etc are equipments capable of drawing heavy current and hence power. To run these equipments or appliances on solar you have to go for a much higher values of installed solar capacity, which is going to increase the capital cost and the pay-back period (period necessary to get back the investment). So it is advised not to connect or run these equipments on solar PV system.

(If you an inverter technology based refrigerator or AC, then the possibility of running them on solar PV system is there.)

 In fact equipments such as Tubelight, CFL, fan, room cooler, TV, laptop etc, and in some emergency situations, electric iron, should be connected and run on solar PV system.

To separate the equipments requiring heavy power, moderate and low power one has to re-wire the distribution system or separate the circuits from the distribution mains (MCB). One can use a change-over switch also.

Step 3: Battery size

Appropriate battery size is the key element in making your PV system a success. The life of a PV module is around 25 years as claimed by the manufacturer, whereas the expected battery life is 3 to 5 years. Also the battery cost is significant. As one goes for a higher Ampere-hour (Ah; the rating of battery is given in Ah) battery, its cost increases. 

For example; recently I have installed a 400 W solar PV system costing around 52 thousand INR in Bhopal, India, the battery of 150 Ah capacity, with 5 years warranty, alone was of 14 thousand INR.     

So the summary is:
Battery is a vital element of your solar PV system whose life is less as compared to other equipments of the system and is comparatively costlier. The equipments which you have selected to get connected to the solar PV system, if works mostly during day time, is the optimum load as per the solar PV system design. This in turn will permit you to have a battery with lower Ah capacity which in turn will reduce the overall system cost.

If your load is ‘switched off’ mostly during the day time, then you have to keep a large battery to store the whole energy produced by the PV module during the day. Thus, to optimize your electrical requirements you have to look into your usage pattern and the criticality of your application.

For a ready reference, the back-up time for a particular capacity battery is given below
Load
Inverter capacity
Battery capacity/ Back-up time
100 Ah
150 Ah
200 Ah
Full Load
Half Load
Full Load
Half Load
Full Load
Half Load
2 TL + 2 F+ 1 PC+ 3 CFL
650 VA
1 hr.
2 Hr, 40 m
1 Hr, 50 m
4 Hr, 20 m
2 Hr, 40 m
6 Hr, 10 m
2 TL + 4 F+ 1 PC+ 3 CFL
850 VA
40 m
1 Hr, 50 m
1 Hr, 10 m
3 Hr
1 Hr, 50 m
4 hr, 20 m

TL and F stands for Tube-light and fan respectively.     

The charge controller is an equipment that controls the charging of the battery and thus helps in improving the battery health and life. The capacity of charge controller is in ampere.  The charging current ( CC ) of a battery is given in manufacturer’s specification, but for ready reference the CC  of a 100 Ah battery is 10 A,  150 Ah battery is 15 A and so on. So for a 150 Ah battery a charge controller of 20 or 30 A is sufficient.

Step 4: Inverter size

Inverter is the equipment which converts the DC voltage of the battery into AC 230 V, so that your normal AC appliances can be connected to the solar PV system. 

Selecting a proper inverter size is very important. Use only the inverter which provides a pure sine wave otherwise your equipments are going to suffer. The output capacity of inverter is given in Volt-Ampere (VA), whereas the appliances are rated in watts. We know that VA multiplied by power factor is watts. So you have to know the power factor of commonly used electrical gadgets for a precise calculation. For simplicity you can assume that the power factor is 0.8, which is the value for most commonly used inductive household equipments.

So a 850 VA inverter is of 850 x 0.8 = 680 W only. So the total electrical loads which can be connected to a 850 VA inverter can be about 600 W. This does not mean that for better utilization of resources one has to keep an inverter of higher capacity. The answer is:

The efficiency of an inverter is about 80% to 90% i.e. 10% to 20% of the energy given to an inverter is consumed by the inverter itself. So higher the inverter capacity higher the losses. Also a higher capacity inverter is useless unless the battery is also appropriately sized.

For a better understanding, have a look into my roof top solar PV system (photo below).


I have installed a 400 W solar PV module (4 panels of 100 W each, make Topsun) along with a 40 A charge controller. The battery used is of Luminous make, 150 Ah, 5 year warranty and the inverter is of 850 VA sine wave of Su-Kam make. 

The whole system is working quite satisfactorily since May 2015. On this system I have used a 150 W room cooler and one 80 W fan the whole day during the summer with occasional load of a 150 W  color TV. I have tried to operate a 375 W water pump and a 600 W automatic iron also. Both the equipments worked nicely, but one at a time. The motor during starting draws a higher current (which is natural for motors), thanks to the in-built feature of the inverter which permits a 300% plus over-current for a few ms to cater such loads. 

Recently I have connected the entire Light & Fan load of 2 bedrooms and a hall. The total connected load on the solar inverter system is now 850 W (4 x 40 tubelights, 4 x 80 W ceiling fan, 1 x 150 TV, 2 x 150 room cooler) but the maximum load at a time is restricted to 400 W.             

Saturday, 13 June 2015

SF6 CIRCUIT BREAKERS FOR MODERN POWER SYSTEM PROTECTION

Electrical faults give rise to abnormal operating conditions and can damage or disrupt the power system in many ways. It is necessary that the faulty section should be immediately disconnected so that the normal operation of the rest of the system is maintained. The protective relay should immediately detect the fault and initiate the operation of circuit breaker or breakers.

Properties of Sulphur hexafluoride (SF6) gas:

Sulphur hexafluoride (SF6) is a chemically stable, odourless, inert, non-inflammable and non-toxic gas. This gas has a high dielectric strength and outstanding arc quenching properties. At atmospheric pressure, the dielectric strength of SF6 gas is about 2.5 times of air and may increase up to 5 times. 

SF6 and its decomposition products are electro-negative. This property permits electron capture at relatively higher temperature. The ability of an atom to attract and hold electrons is called “electro-negativity”. Thus, the dielectric strength rises rapidly which enables the breaker to withstand the recovery voltage even under extreme switching conditions.

Construction of SF6 circuit breaker:

Double pressure breaker is the early design of SF6 circuit breaker and its operating principle is the same as that of air blast circuit breaker. Because of its complicated construction and the need for various auxiliary equipments such as compressors, control device etc., this type of SF6 circuit breaker has become obsolete.

The puffer type or single pressure type SF6 circuit breaker is the most popular and is available in the voltage range 3.6 kV to 765 kV. In such SF6 breakers, the SF6 gas is compressed by a moving cylinder and is released through a nozzle to quench the arc. Figure shows the working principle of single pressure type SF6 circuit breaker. 

The operating mechanism, may be pneumatic or hydraulic, and is installed on the base. This operating mechanism is connected to the movable contacts located in the interrupter with the help of insulated rod of fibre glass. The interrupter and support insulator are filled with SF6 gas at a pressure of about 5 kg/cm2.


Figure shows the interrupter of a puffer type SF6 circuit breaker in fully closed and a position in which contacts are separating. The moving cylinder or puffer cylinder and the moving contacts are coupled together. As a result, when the contacts are separated, the trapped SF6 gas is compressed. This compressed gas is released axially through a nozzle. The gas removes the heat of the arc by axial convection and radial dissipation. The arc diameter reduces with the decrease of current and becomes very small during current zero and thus the arc is extinguished. Due to the electro-negativity and low arc time constant of SF6 gas, it rapidly regains the dielectric strength after final current zero.     

Application of SF6 circuit breaker

SF6 circuit breaker has the ability to interrupt high fault currents, magnetizing and capacitive currents without too much over-voltages. Thus, it can perform duties like clearing line faults, and switching of capacitors, transformers and reactors. Because of the various advantages mentioned above, SF6 circuit breakers are preferred for voltages above 132 kV.