Use of Vegetable oil as dielectric medium in Power Transformers

Electric transformers while in operation produce heat due to iron and copper losses (although stray losses are also there). The heat thus produced must be carried away swiftly to avoid excessive temperature rise in various parts of the transformer such as winding and insulation. The cooling medium used must prevent excessive rise in temperature in any portion of the transformer and should avoid formation of “hot spots” within the transformer.

Mineral oil is normally used as an insulating and cooling medium in power transformers. The oil covers the core and coil assembly completely and fills small voids in the insulation to enhance the transformer performance. 

Advantages of Vegetable Oil:

Over the years mineral or silicone oil has been used as insulating and cooling medium in the transformers. Vegetable oil such as rapeseed, sunflower oils etc. are bio-degradable and have a much higher flashover point, are environment friendly and less inflammable. Vegetable oil has higher flash and fire point when compared to mineral oil. Similarly the dielectric strength is also higher. 

Properties of  Vegetable Oil-based Envirotemp Insulating fluid:

The flash point and dielectric strength of “Envirotemp FR3” vegetable oil-based insulating fluid is 330^oC and 56 kV at 25 ^oC whereas for mineral oil the values are 147 ^oC and 45 kV respectively.  Transformers using vegetable oil will require lesser fire safety systems. Since transformers with vegetable oil are better in terms of fire hazard protection, hence can be used in environment sensitive and densely populated areas.

First EHV class Power Transformer with Vegetable oil:

Siemens has successfully produced and commissioned, in 2014, the world’s first EHV class power transformer that uses vegetable oil as the dielectric medium. The transformer, which is a 380/110 kV power transformer, uses nearly 100 tons of vegetable oil and is commissioned in Bruchsal-Kandelweg substation in Germany. 

Although using vegetable oil in power transformers is not new. Siemens have produced and commissioned more than 30 transformers that use vegetable oil as a dielectric medium up to 69 kV class transformers with individual capacity of 30 MVA.   

Energy conservation in Residential and Commercial buildings

"Residential and Commercial buildings account for a significant portion of the total energy consumption in India." 

These buildings use electricity and other energy sources such as natural gas etc. Electricity is used for lighting and operating other useful appliances. The potential for energy conservation in buildings remains large despite of the improvements in energy efficiency and house keeping. For any building, the envelope that is walls, roofs, floors, windows and doors has a significant impact on the energy consumption.

Recommended Energy Conservation Measures:
The commonly recommended energy conservation measures are:

• The energy use of a building is dominated by weather, especially in extreme weather conditions. Heat gain and loss from direct conduction of heat or from air passage are significant. Addition of thermal insulation can be cost effective. CO₂ based ventilation controller can be implemented in various commercial buildings including cinemas, classrooms, retail stores and establishments to reduce the energy requirement. Recently several materials such as selective glasses or chromogenic glazing have been used to improve energy efficiency of buildings.
•  Simple and inexpensive measures to improve the efficiency of a lighting system include the use of energy efficient lamps and ballasts, reflective devices, de-lamping and maximum use of day light. Sensors and controllers can be used to reduce electrical lighting consumption. Energy efficient equipments of standard make should be used to comply with the efficiency standards and to save money.
• The energy use due to Heating Ventilation and Air-conditioning (HVAC) may be significant particularly in the light of increasing living standards.  30 to 40% of the total energy consumption in any commercial building is because of HVAC. Measures to improve energy performance include appropriate setting of thermostat, retrofitting of central heating and cooling plants, installation of heat recovery system etc.
• An automated energy management and control system (EMCS) can control the building energy use by continuously monitoring the energy consumption of various equipments and making necessary adjustments.   
• Regular energy audits should be conducted to identify opportunities of energy saving. The short term and cost effective measures should be implemented immediately. 

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,

2.  Charge controller,

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.             

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.

What is Circuit Breaker?

A circuit breaker is an automatic switch designed to protect an electrical circuit from damage caused by heavy currents, mainly caused due to overloading, short circuit, or earth fault. The basic function of a circuit breaker is to interrupt the current flow and isolate the faulty section from the rest of the network. Circuit breakers are made in different sizes ranging from the low voltage and low current circuit breaker used in low voltage residential, commercial and industrial premises to very high voltage and high current breakers used in electrical power stations and generating stations.

When a heavy current is interrupted, an electric arc is initiated. This arc must be quenched in a safe and reliable manner so that the breaker contacts are safe and the gap between the contacts can regain the dielectric strength in the desired time span. Different circuit breakers use different arc quenching medium and are accordingly classified as oil circuit breakers, air blast circuit breakers, vacuum circuit breakers and SF₆ circuit 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.    

Know your Solar Batteries

Lead-acid batteries are commonly used as an energy storage device for the stand-alone or off-grid solar PV system. Batteries are rated according to their voltage, Ampere-hour (Ah) storage capacity and their ability to deliver the stored energy over a given period of time called the C-rating. Figure 1 shows a 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.

   

Fig. 1: 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.

When a voltage, higher than the battery, is applied to the battery through an inverter or a charge controller of the solar PV system, a charging current will flow into the battery and will charge it. The flow of charging current is opposite to the flow of load current supplied by the battery. The rate of charge or current that will flow into the battery will depend on the difference between the battery voltage and the applied voltage (the voltage supplied by the solar PV system etc.).

Once the battery is fully charged, it is necessary to stop the charging otherwise it will damage the battery. Its here the charge controller comes into picture. A charge controller, usually used with the solar PV system, is thus essential to ensure that the battery is never overcharged.

Overall efficiency of charging and discharging of LA battery:

The temperature of operation and the rate of charge/discharge affect the performance of the battery. Because of the electrical resistance (internal resistance of the battery and the electrical path), some of the electrical power (I²R) and hence energy supposed to charge the battery is converted into heat. During the charging process, some of the hydrogen ions combine with free electrons and are converted into gaseous hydrogen. When hydrogen is lost during charging, energy is also lost. 

Thus, the charging efficiency is about 95%. 

Similarly, energy is also lost during discharging of battery and hence, 

the overall efficiency of charging and discharging of a lead-acid battery is nearly 90%.

Since the battery loss due to resistance is proportional to the square of the current, therefore 

high current charging or discharging will result in higher internal losses and reduced overall efficiency.

Effect of Temperature on Battery performance:

There is also an increase in the battery temperature during charging and discharging, particularly at high rates, due to higher I²R losses. If the battery remains too warm for a longer duration its life is reduced. Thus, charging and discharging rates should not exceed the specified values given by the manufacturer for a given type of battery. Increase in temperature accelerates the chemical reaction process in the battery. Thus, in high temperature the battery is liable to get self-discharged without any external load and hence has to be kept in a cool place with preferable temperature of 27 ^oC.

A fully charged 24 V battery will have a voltage of around 25.5 V. As the electrical load, connected to the battery through the inverter, draws the current, the level of charge in the battery is reduced and thus the voltage of the battery. The battery voltage will fall down quickly at first, but as the discharging continues, the rate of voltage drop slows down and will reach 24 V when the battery has drained to half of its rated charge capacity. As the battery approaches the fully discharged state, the voltage starts to fall at a greater pace again. When the electrical load is drawing a heavy current from the battery, the voltage will drop. The larger the battery, smaller is the voltage drop.

What is different in a Deep discharge flooded lead-acid battery?

Deep discharge flooded lead-acid battery, commonly used with solar PV system in India, uses antimony to strengthen the lead and can be used down to 20% of their initial capacity. The plates are thicker, with less area and hence are designed for sustained lower level currents. Flooded batteries need water topping at least twice a year. Although deep discharge batteries are capable of deep discharge, but their life depends on the depth of discharge incurred every time during the operation. Deep discharge i.e. 70 to 80% discharge of the rated charge leads to decrease in battery life.            

Washing of Solar PV Arrays

A solar PV array is a set of solar PV modules mounted together and connected in series, parallel or series-parallel combination to get the desired electrical output i.e. voltage, current and hence the DC power.

Why washing of Solar PV arrays is important?

Solar PV arrays are usually called maintenance free, but they require regular cleaning and washing to maintain the rated power output. The glass covers are cleaned regularly to remove dirt and other deposits such as bird droppings. Cleaning of Solar PV system is regular and a natural process during the rainy season, but in other seasons, regular washing of the same with simple water is needed to enhance the system performance. Figure 1 shows the manual cleaning of a small roof top solar PV system.

Fig.1: Manual cleaning of a small roof top solar PV system.

Wrong cleaning practices, bad quality water and use of in-appropriate cleaning agents may damage the PV modules or other components. It may also reduce the system performance. Certain Solar PV sites require specific cleaning procedures. Procedure also depends on the water quality and the cleaning mechanism used.


What precautions to be taken while cleaning of Solar PV system?

The water used for cleaning of PV panels should be clean, soft and low in mineral content. If the water is comparatively harder, the water must be squeezed off to avoid building up of scale. In any case, the water hardness should not be more than 200 ppm. 

One can use lukewarm water and soft sponge or micro-fibre cloth also if the deposits are thick and stubborn. If necessary, a mild, non-abrasive, non-caustic detergent may be used. Situations, more prone to diesel fumes and deposits, may require a mild de-greaser agent along with the soap.

Cleaning of solar PV arrays should be carried out when they are not excessively hot i.e. the arrays should preferably be cleaned in the morning or evening time. Thus, cleaning of small solar PV arrays is similar to cleaning of any ordinary glass window. 

But in large Solar PV systems, several PV modules are connected in series, producing enormous DC voltage. In PV systems having higher DC voltages, cracks in modules, cables or joints may pose dangerous conditions to cleaning personnel. Hence, it is adviced to inspect the modules and cables thoroughly for any cracks or other damages before the commencement of cleaning. Workers engaged in cleaning should wear proper protective equipments while working with larger and higher voltage PV plants.  

Visual inspection of the PV modules should also be done after washing to check for any cracks, chips or discolouration. If any defect is observed, it should be monitored regularly for any further deterioration. During the inspection, condition of the mounting frame should be noted for any loose nut/bolt or rusting. 

Pachmarhi a small town in Madhya Pradesh with so many waterfalls

Pachmarhi or Panchmarhi, also known as “Satpura ki Rani (Queen of Satpura)”, is a hill station in Madhya Pradesh state of Central India. It is situated at a height of 1100 m in the Satpura hill range in the Hoshangabad district of Madhya Pradesh. 

Blessed with very rich flora and fauna, lush greenery, waterfalls, rich and rare wildlife, Pachmarhi is well connected from Bhopal, Nagpur, Hoshangabad and Pipariya. The nearest railway station is Pipariya (on the Itarsi-Jabalpur section) which is nearly 55 km away. From Bhopal, one can reach Pachmarhi in 5 hours by road and the distance is 200 km. Pachmarhi is a popular tourist retreat and one can visit the place through-out the year. 

The pleasant time to visit the place is from October to December, but during this period the water in the falls is too cold to get a bathe. Even in the March the water in the falls is a bit colder to some people.

The highest point in Pachmarhi is Dhoopgarh which is 1350 m, an ideal place to see sun rise and sun set, although watching sun-rise is now banned by the forest department and the forest barrier opens up only after 9 am. On way to Dhoopgarh, one can see Bison and Peacock, particularly during rainy season. Pachmarhi once served as the summer capital of the Central Province. Pachmarhi is also listed among the Biosphere reserves in the UNESCO list.

Pachmarhi is a small town dependent mainly on tourism and most of the area is under the administration of Pachmarhi Cantonment Board serving the Indian Army. The only petrol pump is the police petrol pump. There are many budget hotels along with hotels and resorts run by M.P.Tourism Department.

Most interesting places to visit in Pachmarhi are:

  Rajat Prapaat or Silver fall,

  Apsara Vihar and Paanchali kund,

  Bee fall,

  Reechgarh,

  Dhoopgarh,

  Rajendra Giri,

The places of devotional interest (pilgrimage site) are:

  Jata Shankar,

  Bada Mahadev,

  Gupt Mahadev,

  Chouragarh,

  Old Church which is now closed to visitors.

Near to Pachmarhi are 3 wildlife sanctuary/park namely, Bori sanctuary, Satpura National park and the Pachmarhi Sanctuary. These three are collectively called Satpura tiger reserve. The jungle nearby has tigers, panthers, bear, bison, apart from wide variety of birds and small animals such as monkeys (they are also visible in the town and nearby your hotel).

To see the waterfalls and other similar locations one has to hire a Maruti Gypsy, which is the best or the only vehicle available in Pachmarhi to go through the ghats. A full Gypsy can be hired for 1200 INR; one can also pay per person which goes to about 250 INR per head. Apart from this one has to pay the forest fee and guide hiring fee of 900 INR /vehicle/ day to the forest department. In day one, you can visit Pandav caves, Silver falls, Apsara Vihar, Paanchali kund, Bee fall, Reechgarh and Dhoopgarh.

Silver fall

Sunset on a cloudy day at Dhoopgarh

The next day one can go to Duchess fall, although for Duchess fall there is a trekking path of nearly 2 hours (to and fro), so one has to be mentally prepared to visit the Duchess fall. But it is a really nice waterfall to visit, calm and peaceful as compared to Bee fall.

Above the Duchess fall is the AC fall where one can see water in bulk pouring down through a small gap of some 3 feet width. Unlike some of the falls, all the waterfalls in Pachmarhi has waterfall throughout the year.  Similarly for Chouragarh, one has to walk through the hills for some 2 or 3 hours. Other devotional places can be approached by your own vehicle.

Pachmarhi is perhaps among the highest visited hill station in Central India. It is a very nice place for a 2-3 days break at an affordable price. The people are cooperative and easy and special preparation is needed.