String size calculation in Grid tied Solar PV system

Solar modules are connected in series or parallel or series-parallel combination to get the required voltage, current and power. Series connection will increase the string voltage while keeping the current constant. On the other hand parallel connection will increase the current whereas the voltage remains the same. Now what about the voltage, current and power in the series-parallel connection? Yes  you are correct, all the three will increase.

The capacity of the solar PV plant depends on the roof space available and the individual requirement. Once the PV module capacity is decided we can size the inverter. Normally the inverter size is smaller than the rated output of the PV array at Standard Test Conditions (STC). This is due to the losses or de-rating factors such as panel dissimilarity, dust, losses in cables etc. An inverter of 80 % size of the PV array is quite normal.  

What is a string in the solar PV system?

Number of modules connected in series is called the “string”. Size of the string determines the voltage input to the inverter. The maximum and minimum number of modules in the string depends on the maximum and minimum voltage of the inverter.

The output voltage generated from the PV should never damage the inverter that’s why we have to calculate the maximum system voltage. The maximum system voltage should not be more than the highest acceptable inverter voltage.

Similarly we have to calculate the minimum number of modules in the string so that in worst case scenario the PV system’s output voltage is sufficient enough to turn the inverter ‘ON’.

Relationship between Ambient Temperature and String Output:

As expected, a certain relation exists between the ambient temperature and the string voltage, which is to be considered while calculating or designing the string size.  The PV output is inverse to the ambient temperature i.e. with the decrease in ambient temperature; there is a certain increase in string voltage and power. The vice-versa happens during summer season. With a correctly sized PV array, the DC output will remain within the optimum operating range of the inverter under different working and ambient conditions.

What should be the minimum string size in the solar PV system?

Softwares are also used in the design, construction, operation and maintenance of Solar PV plants. These softwares help to optimize the design configuration and system layouts.

In the following paragraphs, manual calculations of string size is given for the easy understanding of concepts. They are easy enough to be done manually.

For the calculation we need some data from the data-sheet of the module. Suppose the;

Voltage at Open Circuit, V_OC = 43.4 V,

Temperature at Standard Test Condition (STC) = 25^oC,

Temperature Coefficient at V_OC = (-) 0.15 V/ ^oC,

Voltage at Maximum Power, V_mp = 35.4 V,

Temperature Coefficient at V_mp = (-) 0.17 V/ ^oC,

Now from the inverter data sheet, we have to get the maximum input DC voltage and the start ( or strike) voltage of the inverter. From the environmental data we have to collect the hottest day-time temperature and coldest day-time temperature of the location where the PV system is to be installed.

Suppose that the effective cell temperature during the hottest day is 70 ^oC, which is 45 ^oC above the temperature at Standard Test Condition (STC) of 25^oC.

Therefore, the V_mp voltage would be changed (reduced) by 45 x (-) 0.17 = (-) 7.65 V

Hence the V_mp at 70 ^oC cell temperature would be 35.4 + (– 7.65) = 27.75 V.

Next we should consider the de-rating factor which is due to the earlier mentioned factors such as module mismatch, dust & dirt, cable loss etc. Let it be 0.88.

So the effective minimum V_mp for each module at the inverter input would be 27.75 x 0.88 = 24.4 V.

From the above results we can calculate the minimum permissible number of modules in the string.

Assume that the minimum start (or strike) voltage for the inverter is 140 V. Taking a safety margin of 10%, i.e. 140 x 1.1 = 154 V. This means that the string size should be so selected that a minimum of 154 V is maintained at the inverter input terminals in the worst case scenario.

Hence the minimum number of modules in the string to surpass the start voltage is

154/ 24.4 =  6.31 rounded up to 7 modules.

What should be the maximum string size in the solar PV system?

As mentioned earlier, the output voltage of a solar PV module increases as the ambient temperature drops below the temperature at STC.  At the coldest day-time temperature the V_OC of the array shall never be greater than the maximum allowed input voltage of the inverter. Now we have to calculate the maximum number of modules in the string. Oversizing the string can damage the inverter, cancel warranties and violate the safety codes.

Let’s suppose that during the coldest day, the effective cell temperature is 15 ^oC, which is 10 ^oC below the temperature at STC of 25^oC. 

Therefore the V_OC should be increased by (15 – 25) x (-) 0.15 = 1.5 V. You can say that the Voc at 15 ^oC is 43.4 + 1.5 = 44.9 V.

Assume that the maximum safe working voltage allowed by the inverter is 400 V. Then the maximum number of modules permissible in the string is 400/ 44.9 = 8.9 rounded down to 8 modules.

The goal of this article was to convey the basic process for sizing the PV string for a grid connected system. Let’s promote Solar PV system in our vicinity.

Under-estimated small Off-grid Roof top solar PV systems to be re-examined

The utility scale solar PV projects have grown up significantly in India over the last 5 years. Focus is shifting on roof top (RT) solar PV systems, as is evident in the Jawaharlal Nehru National Solar Mission (JNNSM). Under the mission 40,000 MW of RT solar PV systems are to be installed by 2022.  In the light of falling prices of solar PV modules, both off-grid as well as grid connected roof top solar PV systems seem to be a viable and workable solution in addressing nation’s energy and environmental issues.

Potential of Roof Top Solar PV systems:

As per the 2011 census, over 140 million houses in India has proper roof top spaces that can accommodate up-to 3 kW of solar PV system. I am talking of residential houses not about the commercial buildings, shopping malls, educational institutions etc. RT solar PV systems can be vital in rural areas. Even a 300 W roof top Solar PV system on each of these 140 million houses will tremendously add to the Renewable Energy portfolio. 

Issues with Grid connected Solar PV systems, which are yet to be streamlined:

Looking into the extent of execution of solar PV systems in India, the weightage seems to be primarily on the grid connected RT solar PV systems, particularly on the government establishments and buildings. The rising electricity tariff has already given RT solar PV the much required niche and made it viable even without the support of any subsidy. 

Lack of clarity  on rules and regulations, technical and safety issues, power quality, metering and bill related issues pose tiresome proposition to both utility and owners of grid connected RT solar PV system. This is true particularly in the state of Madhya Pradesh, where the net metering has just started. Even the Discom officials are unaware of the procedures. Apart from that a grid connected RT solar PV system needs the approval of two government bodies i.e. the Urja Vikas Nigam and the related Distribution Company to get it installed; which as we know a time consuming and tiring exercise.

Potentials of Small scale Off-grid solar PV system needs to be Re-evaluated:

Small scale off-grid solar PV system is seen as underdog. In my opinion, such PV system has a huge potential in this country, provided the awareness is created. A properly worked out small RT off-grid solar PV system on affordable initial capital may lead to better utilization of unused roof top space. These RT solar PV systems can be seen as a sustainable remedy to frequent power cuts and peak power shortage. It’s better to promote small off-grid solar PV systems of say 200-300 W on every possible roof top. It hardly needs 25 square feet of space and can be arranged in a variety of fashion (refer fig1).

Fig.1: Small scale Off-grid Roof top Solar PV system of 300 W

Such small off-grid solar PV systems are capable of energizing the essential light and fans, TV and computer requirement of a three room house. The initial cost is also affordable, say around 40,000 INR with battery back-up, even less than 25,000 INR when an existing battery-inverter set is converted into a solar PV-battery-inverter system. There is no need to go for cheap popular gimmick as subsidy etc, provided the common mass is made aware. Such small solar PV systems are easy to install, and budget friendly.

Even a 20-25% of the residential energy requirements is fulfilled by these small but effective RT solar PV systems, it may be seen as a boon. A solar PV system is more worthy if the self-consumption is about 50% or more. The same energy can be routed to industries or commercial establishments and that too at a better tariff.   

Manufacturing of Power Cables

Power cables are insulated conductors used to carry electrical power from one point in the electrical circuit to another point or to any electrical equipment. They may be installed as permanent wiring within buildings, buried in ground or provided overhead depending on the site specific requirements. 

Modern power cables come in a variety of sizes, materials and types, each particularly adapted to its use. These cables comprise of conductors, insulation, inner sheath, armour and outer sheath.

The construction and materials of cables are determined by three main factors:

1.       Working voltage, which determines the thickness of the insulation,

2.       Current carrying capacity, determining the cross-sectional area of conductors, and

3.       Environmental conditions such as temperature, water, chemical, mechanical impact etc.

Power Cables have aluminium or copper conductors and polymer insulation. Conductors may be solid, stranded circular or shaped. Stranding makes the cable flexible and easy to handle whereas shaping is done to make them compact. All multi-core cables of 16 sq.mm size or above are sector shaped. The conductor is manufactured in equal segments and compacted, and then laid together. This also reduces losses due to skin and proximity effect.

Conductors:

Cables with copper conductors have higher tensile strength, better conductivity and better flexibility. They are usually used in providing power connections to electrical equipments in industries, instrumentation, submarine and ship wiring and in mining applications also. These cable conductors are manufactured in accordance to IS: 8130 (Indian specifications) and IEC: 60228/ BS: 6360 (International standards). Manufacturing process of both copper and aluminium conductor cables are the same.

Insulation:

Cables are usually made with polymer dielectrics as insulation, so as to have better thermal and thermo-mechanical properties under both normal and abnormal conditions. Usually PVC (thermo-plastic dielectric) and XLPE (thermo-setting dielectric) insulation are used. Cables with PVC insulation use PVC compound whereas XLPE insulated cables use XLPE compound with certain addition to improve the electrical and mechanical characteristics. Insulation process in cable manufacturing are governed by IS: 5831/ IS: 7098 and IEC: 60502/ BS: 6746/ BS: 5467.

Screening:

XLPE cables with voltage rating exceeding 3.3 kV are provided with both conductor and insulation screening. Conductors are screened with extruded screen of semi-conducting compound. Insulation screening has a non-metallic screen part combined with a metallic part. Non-metallic part consists of semi-conducting compound tape applied directly over the insulation. Over this copper tape is applied helically which forms the metallic part of the insulation screen.

Perfect bonding of insulation and screening is essential to avoid cavities and void formation in dielectric. Conductor screen, insulation and the non-metallic part of the insulation screen, all the three, are usually applied in one operation to ensure perfect bonding.

Inner-sheath:

Inner-sheath of PVC acts as bedding for steel armouring. Sometimes filler cords are also provided to maintain the circular shape. Inner-sheaths are compatible with temperature rating of cables.

The inner sheath is applied either with extrusion or by wrapping. The dimensions of the inner sheath are maintained as per IS: 1554/7098.

Armouring:

Armouring provides the mechanical protection to the cable and are made of low resistivity material. Armouring of single core cables are either wires or strips of Aluminium or aluminium alloy to avoid hysteresis losses. Multi-core cables are provided with galvanized steel wire or strips. Galvanized wire armouring is used where cables are subjected to stresses. Armouring is done as recommended in IS:3975.

Outer Sheath:

Power cables   are usually provided with outer sheath made up of PVC/ polymer. These outer sheaths are harder than inner sheath and manufactured with various characteristics of sheathing compounds for example general purpose, heat resistant, fire retardant, flame retardant, UV radiation resistant, anti rodent/termite compound etc. Compounds for outer sheath are supposed to meet IS: 5831 specifications. 

Inverter technology based Refrigerators and Air-conditioners

"Regular or traditional refrigerators and air-conditioners usually have a single speed compressor that runs on a fixed speed. These compressors are either ‘ON’ or ‘Off’ state depending on the temperature inside the refrigerator or room and the thermostat’s setting." 

Basically the compressor of traditional refrigerator has induction motor. Because of the high pressure of compressor, the motor acts as "a motor with blocked rotor" at start, and therefore draws a heavy current (usually 3 to 4 times the normal running current) during starting. These compressors start and stop depending on the temperature inside the refrigerator.

The thermostat switches the compressor ‘ON’ when the temperature inside the refrigerator or ambient temperature inside the room rises above the desired temperature and vice-versa. 

" For example, a regular 1.5 ton fixed speed Air-conditioner requires a starting current of approximately 30 A which is 3 times its normal running current of 10 A." 

Hence regular refrigerators and air-conditioners are not suitable for running on inverters or solar PV systems. Or in other words they require an inverter of very high capacity. 

" For example a 1.5 ton normal air-conditioner will need an inverter of at-least 5 kVA capacity."

How Inverter technology based refrigerators and Air-conditioners are different?

The inverter technology based refrigerators and Air-conditioners have variable speed motors that start up gradually resulting in a much reduced starting current. These compressors are always ‘ON’ and change their speed depending on the temperature requirement.

An inverter technology based air-conditioner of 1.5 ton needs only 6 to 7 A during normal running and the starting current is also much less compared to the traditional air-conditioner. Thus an inverter of 2 or 3.5 kVA would be enough for operating a 1.5 ton air-conditioner. Reduced initial current results in reduced wear and tear of parts i.e. increased component life and hence of the refrigerators and air-conditioners. This also reduces the sharp current fluctuations that the compressor places on the power supply. In addition, the inverter technology based compressor will have higher power factor which results in lower electricity consumption. 

Initial cost and Energy saving with Inverter technology Refrigerators and Air-conditioners:

Although the initial cost of inverter technology based refrigerator and air-conditioner is approximately 40% higher than the traditional one but there is a 30% reduction in energy bill. These refrigerators and air-conditioners are more suitable to be operated on a solar PV system as the required inverter capacity will be very low.  

Soft starters for Induction Motors

Starting method significantly affects the starting current, torque, mechanical stress and hence the life of an induction motor. The soft starter electronically controls the starting torque and current of an Induction motor. Their field of application includes HVAC fans and pumps, industrial fans, pumps, conveyors and other processing equipments etc. Soft starters can be used with both 3-phase and 1-phase induction motors. The following material is intended to acquaint the reader with the theory and operation of solid state soft starter and motor controller.

It is necessary to understand the load characteristics requirements and the motor capability when used with a soft starter and controller. Induction motors can be classified based on the locked rotor torque and current, breakdown torque, pull up torque, and the percentage slip. General purpose Induction motors has the highest share in terms of sale. They have a typical slip of 3 to 5% and are used in applications requiring low starting torque such as industrial fans, blowers, centrifugal pumps, compressors etc.

Starting the induction motor with Normal motor starter:

The rotor of an induction motor can be squirrel cage or wound rotor. The wound rotor induction motor allows controlling the speed and torque and is generally started with a secondary resistance in the rotor. As the resistance is reduced, the speed of the motor increases. Thus the motor can develop substantial torque while limiting the locked rotor current.

It is common to start an Induction motor using a motor starter which directly connects the motor to the utility supply causing the motor to draw a high starting current or inrush current. The inrush current drawn by a motor when started with a normal starter is of apprehension as it causes the supply voltage to dip causing an impact on other sensitive electrical loads. When the starting current is large, the magnetic forces to which the motor winding is subjected, are also large. The mechanical shock thus created can damage the winding insulation, motor shaft, belt etc. leading to premature failure of motor and the associated system.

Theory and Operation of solid state soft starter:

A soft starter is a form of reduced voltage starter used for starting of Induction motor. These starters are similar to resistance or reactance starter and are connected in series with the motor. These starters use solid state switching devices such as TRIAC or SCR, to control the voltage/current fed into the motor. The TRIAC or SCR are turned ‘ON’ for a part of each cycle. The average voltage is controlled by varying the conduction angle of these SCRs/TRIACs. Thus the voltage to the motor can be easily changed according to the required starting conditions and this can be done automatically with the help of a control circuit. The control circuit can be pre-programmed to provide a particular output voltage profile based on a time sequence.  The circuit can also dynamically control the output voltage to get a voltage profile based on the measurements of current and speed of the motor. The earlier type of controlling is an example of 'open loop control' while the later is called ‘closed loop control’.

Advantages of solid state soft starter:

Solid state soft starters and controllers can control the starting characteristics such as-

Ø  acceleration and deceleration time,

Ø  starting and overload current and

Ø  motor torque, to match the load requirements.

Suppose a motor takes a starting current of 6 times the normal current. The soft starter, when used, can be set to limit this current up to 3 times (as shown in fig.1). Reduction in current also reduces the torque (as shown in fig.2) which in turn reduces the mechanical stress. The torque available from the motor is proportional to the square of the current. As the starting current, in this case, is reduced to 50%, the torque reduces to 25% of the value, produced when the motor is started with a normal starter.   

    

Fig.1: Motor current-speed characteristics for Normal/ Soft starter

Fig.2: Motor torque-speed characteristics for Normal/ Soft starter

The SCR or TRIAC must be able to control the current applied to the motor at line voltage. To get a high degree of reliability, these SCRs or TRIACs must be rated 3 times the line voltage. Thus the SCR-diode or SCR-SCR combination is usually used. The SCR-SCR method provides a symmetrical output and thereby reduced harmonics, whereas the SCR-diode combination gives an inferior output, but is cheaper in cost and easy to implement. The 3-pulse technology uses a SCR-diode combination whereas a 6-pulse technology uses a SCR-SCR combination.   

Large fuel imports a threat to Indian Energy Security

A country must be able to reliably meet the energy demands of all sectors for different needs with safe, convenient and competitive energy in a sustainable manner to have an energy secure future. Nearly 85% of the primary energy comes from non-renewable and fossil fuels which are continuously diminishing.

Adversity on the Energy front:

The Indian economy has been facing great adversity on the energy front. India’s dependence on imported fossil fuels reached to 38% in 2012, despite of the fact that we have sizeable domestic fossil fuel resources. We were ranked as the fourth largest energy consumer in the world in 2011, following China, the United States and Russia. Our country imports more than 75% of the oil demand. The import bill for crude has been rising steadily and was 160 billion $ in the year 2012-13. Even with a significant coal deposits, we are importing nearly 25% of our total coal usage. The import is mainly from Australia, Indonesia and South Africa. The increasing coal shortage is because of a lack of competition among producers, insufficient investments, and other problems in its mining industry.

Country’s major share of electricity generation is from coal based power plants. Currently coal fired thermal power plants (TPP) contribute over 60% of India’s installed capacity and 66% of the electricity generation. In the last decade, coal based power plant generation capacity was doubled and substantial capacity addition is in the pipeline. Coal shortages are a major contributor to shortfalls in electricity generation and the consequent blackouts in the country.

Future of Coal based Power Plants:

Although, the Coal fired power plants are discouraged due to increasing pressure to reduce carbon emission, import dependence and increasing fuel price, reducing price of Renewable Energy etc., it is expected that the installed capacity of these plants will be about 270 GW by 2032. India’s existing coal fired thermal power plants are currently based on sub-critical technology which is inefficient. Super-critical boiler technology is being adopted at a significant scale in the 12^th FYP program. A properly integrated and automated coal management system is also required to ensure uninterrupted power generation and unnecessary piling up of inventory.

However, the development and deployment of these efficient technologies is sluggish due to Indian coal having ash content and low calorific value (CV). Government of India (GoI) plans to stop sub-critical power plants addition after 2017. It is expected that ultra-super-critical technology will be commercialized after 2017 and IGCC based power plants after 2017. Some speculations say that ultra-supercritical technology will be commercialized only by 2022, anyway time will tell what is the future of coal based power plants.

Outcome of large dependence on Imported Fuels:

The large dependence on fuel imports and the inability to reverse this trend has impacted the development of Indian economy. The unpleasant effects include depleting foreign exchange reserves, price jolts because of volatility of global energy markets etc. With a large share of imported energy sources, domestic prices of not only energy, but the entire value chain get affected by the volatile international prices. We are also acknowledged for subsidizing energy sources. Therefore, the term “energy security” has a large sense for the country including economic stability and ensuring the well being of the people.

                                        

Ways to achieve Sustainable Energy development:

The country should consider all forms of available and emerging energy sources and technologies to achieve Sustainable Energy Development. A greater investment in R & D in alternate and renewable energy sources can make their price competitive with that of conventional energy. We should focus on Energy Efficiency and lower energy intensive routes for the development. 

The cost of protecting the environment and un-doing the environmental damage caused by the energy supply and use should also be included in the energy cost. Elimination of subsidies and other special treatments, to certain segments of the society and consumer, are necessary. Manufacturers should be encouraged to accelerate R & D efforts for bringing out more energy efficient equipments. Minimum efficiency standards for all equipments must be fixed. The government should provide incentives and enhance other market development strategies, for promoting energy efficiency. However these approaches require large upfront funding and a robust policy framework to ensure adequate success over long periods of time.

Demand Side Management (DSM) is also a very successful tool to reduce the overall energy demand. Although we are lagging far behind in a DSM and I personally feel that we must have a strong framework in implementing DSM. Efficient public transport system, electric vehicles and fuel substitution will also play a crucial role. Therefore, India’s energy strategy would necessarily comprise of action on both demand and supply sides with due consideration to policy, finance and technology.

Energy storage system is of essence for Solar PV systems?

Energy storage is a vital element in power sector particularly when a country has large volume of Renewable Energy (RE). Cheap, practical and easily available energy storage systems can address some of the critical issues associated with RE sources particularly the Solar PV systems. It will build an atmosphere of confidence in the Utility working, particularly utilities with large scale RE penetration; reduce the investment in transmission systems and development of reserve capacity. 

Widespread deployment of battery storage would mitigate the intermittency phenomenon associated with solar PV systems. In fact, in some of the countries utilities have mandated that all new PV systems should have energy storage systems to smoothen the power variations. Solar PV systems with battery storage could also help to manage unplanned voltage fluctuations, particularly in areas with high penetration of solar PV systems.

Figure 1 shows the main components of a simple Roof top Solar PV system with Battery back up.

Fig.1: Layout of a simple Roof top Solar PV system with Battery back-up

Example of Energy Storage at the Utility end:

Utilities too have promoted Energy Storage systems at their end. Duke Energy has installed a hybrid energy storage system comprising of battery bank and ultra-capacitor at its North Carolina based substation (www.technologyreview.com). In this hybrid storage system, the ultra-capacitor helps the power system during large but short duration power surges, for example when the Solar output dips during a cloudy day. Battery bank, as customary, will help to recover during large duration shortages. 

It is expected that the two storage systems along with smart electronic controls will help the power system in mitigating the power shortage over periods ranging from seconds to several hours. This hybrid storage is supposed to provide a more economical solution as compared to each of the storage systems when used separately. 

According to officials at Maxwell Technologies, the ultra-capacitor storage system can completely eliminate the output fluctuation of less than 30 seconds and provide sufficient smoothening of fluctuations of up to 5 minutes. In recent years the ultra-capacitor energy storage systems have become more accepted as high power shock absorbers for industrial and transportation applications in combination with Lead-acid batteries or advanced chemical batteries. Ultra-capacitor has high power capability due to very low internal resistance, wide operating temperature range of -40^oC to 65^oC, minimum maintenance, high cycling ability and reasonable price.

The latest utility scale battery storage technology emerged in the commercial market is the 8 MWh capacity Vanadium Redox Battery bank installed at Everett Sub-station, Washington state, and which is to be commissioned in January 2017. This battery system,  compact and concealed in container, is non-flammable and can be discharged upto 100%, i.e. the Depth of Discharge (DoD) can be 100%.    

Essence of Energy Storage to End User:

At the end user level the energy storage system will provide the user with the much needed back up during night hours when the grid is out. In countries like India the Time-of-Day (TOD) tariff is not implemented in the residential and commercial sector. I am sure that in the near future Utilities have to come up with the TOD tariff in these sectors also. Then in such a case a solar PV system with energy storage will be very beneficial. 

TOD tariff is an effective tool to reduce the peak hour kWh shortage. Shifting of some portion of the load to the storage system will be of great relief to the utilities reeling under stress. 

Fig 2: A Roof Top Solar PV system with battery back-up.

In some countries, the feed-in-tariff has been rigorously cut down or eliminated at all. In such a condition, the pay back periods of solar PV systems are highly dependent on the percentage of solar energy used for “self-consumption”. 

"Experts are in the opinion that a solar PV system is worth only if the self-consumption is about 50% or more." 

Energy storage systems enable owners of solar PV systems to increase their self-consumption. Although installing energy storage system increases the overall cost of the system and hence the electricity produced, but still it is not as expensive as many would think of. Lithium-ion batteries are considered as the most affordable and dependable energy storage systems. These batteries can discharge bigger burst of power and can eliminate the need of ultra-capacitor but are costly.