Maximum Power Point Tracking Technique

A PV Panel consists of several photovoltaic cells in series and parallel connections. Series connections are responsible for increasing the voltage of the module whereas the parallel connection is responsible for increasing the current in the array. Typically, Solar Panel has converting efficiency of 8 to 15 % of the Incident Solar Irradiance into electrical energy.

I-V characteristics of a Solar panel is as shown in the fig 1. below.

At the open circuit voltage VOC and the short circuit current ISC, the power generated is zero.

The Maximum power (Pm) is generated at a point where the product Vm*Im is maximum and this point is called as Maximum Power Point. Maximum power point tracking technique is used to improve the efficiency of the solar panel.

There are different techniques used to track the maximum power point, few of the most popular techniques are

1) Perturb and Observe (hill climbing method)

2) Incremental Conductance method

3) Fractional short circuit current

4) Fractional open circuit voltage

5) Neural Network

6) DC-Link Capacitor Droop Control Technique

Perturb & Observe Method

The P&O algorithm is also called “hill-climbing”.  Hill-climbing involves a perturbation on the duty cycle of the power converter and P&O a perturbation in the operating voltage of the DC link between the PV array and the power converter. In this method, the sign of the last perturbation and the sign of the last increment in the power are used to decide what the next perturbation should be.   As shown in fig.2 on the left of the MPP incrementing the voltage increases the power whereas on the right decrementing the voltage increases the power.   If there is an increment in the power, the perturbation should be kept in the same direction and if the power decreases, then the next perturbation should be in the opposite direction. Based on these facts, the algorithm is implemented. The process is repeated until the MPP is reached. This technique holds good when irradiance is constant and it can track MPP in the wrong direction for dynamic changes in atmospheric condition.

Fig.2 PV panel characteristic with MPP operating points

Incremental Conductance Method

In this method, the MPP can be tracked by comparing the instantaneous conductance I/V to incremental conductance dI/dV

At MPP the slope of the PV curve is 0 (zero).

(dP/dV) MPP=d(VI)/dV

0=I+VdI/dVMPP

dI/dVMPP = – I/V at MPP

dI/dVMPP > – I/V at left of MPP

dI/dVMPP < – I/V at right of MPP

Here we are sensing both the voltage and current simultaneously. Hence the error due to change in irradiance is eliminated. However, the complexity and the cost of implementation increases.

The techniques so far discussed are the most popular one and presently in use. The selection adapting any of these differ in many aspects such as required sensors, complexity, cost, range of effectiveness, convergence speed, correct tracking when irradiation and/or temperature change, hardware needed for the implementation or popularity, among others.

–Divya Darshini : Engineer , Technical Solution Group

Thermography-A Quick Analysis

Solar energy is one of the cleanest forms of energy. With increase in power demand and subsidies available from local governments, countries like India are opting more for solar energy; Availability of sun shine and vast land area is an added advantage. With this the number of challenges in operation and maintenance services has also increased. One such challenge is optimal performance of each Photo Voltaic module (and other related equipments) for failure free operation, optimal generation and high returns on investment. To monitor the performance of each PV module at a lower maintenance cost is becoming more difficult. Hence, we need a holistic and evidence-based approach to do predictive maintenance, and one such feasible solution is thermography.

 

The selection of thermal imaging is purely our choice. We can use a hand held Thermal Imaging camera for better detailing or a UAV/Drone for larger imaging, depending on level of analysis we need to do. All PV module have a glass front surface. Thermal measurement of glass surface is not easy due to its thermal emissivity. Glass has a property of reflecting the temperature of things around it. To avoid measurement error, we need to position the thermal imager between 5˚ to 60˚ (keeping 0˚ as perpendicular) which comes best by practice (initially trial and error will help). The emissivity for any type of Solar PV module can be set to 0.95 and reflected temperature to 20˚C in the device for optimal thermal recording. The distance of measurement must be set manually, one or two meters for a hand-held device is best. For correct and informative thermal recording, shadowing and reflections must be prevented. One more way to get better results from thermography is to perform inspection on rear of the module as well, hence avoiding most of the interfering factors.

After recording the warmer area will be clearly visible; if we observe a certain or entire part of a solar panel to be hotter than its neighbor, it indicates an anomaly. Depending on the shape and location, they indicate various problems like

 

  • If an entire module is warmer than usual that might indicate interconnection problem
  • If individual cells or strings of cells are showing up as a hot spot or a warm patchwork pattern, then the cause can usually be found either in defective bypass diodes, in internal short-circuit.
  • Shadowing and cracks in cells show up as hot patches in the thermal image.
  • The temperature rise of a cell or part of a cell indicates a defective cell or shadowing.

 

Thermal images obtained under load, no-load, and short-circuit conditions should be taken for analytical comparison. A comparison of thermal images of the front and rear faces of the module might also give valuable information. The further analysis is performed on software tool provided along with the data obtained and is compared with the data sheet of the module. The module can be observed on hourly basis for two to three days if needed and then conclusion is drawn.

 

The thermographic inspection of PV modules usually indicates potential defects at the cell and module level or electrical interconnection issues. The inspections can be carried out when Solar Power plant is in operating condition and do not require a system shut down. Thermal imaging cameras are primarily used for capturing abnormalities in the form of heat and does not detect faults. It records data which we must carry out analysis for further conclusions. Classification and assessment of the abnormality, if any detected, requires a sound understanding of solar technology, knowledge of the inspected system and additional electrical measurements. Documentation is a must and should contain all inspection conditions, additional measurements, and other relevant data. This will help to maintain the solar panel functionality and to extend their lifetime.

— Harsha – Electrical Engineer

Wireless Sensor Network – Delivery Reliability

The utility scale solar power plants require various equipment such as inverter, string combiner box, Weather station, Transformer, Multi-Function Meter (MFM), Tri-Vector Meter (TVM), Trackers, Fire alarms and Vacuum Circuit Breaker (VCB). All equipment in the plant should be monitored to increase plant availability, functionality check of each equipment and for datalogging.

Modbus is a standard that is open and is widely-used network protocol in the industrial manufacturing environment. It’s a common link that has been implemented by hundreds of vendors for integration on thousands of different manufacturing devices to transfer discrete/analog I/O and register data between control devices. A MODBUS communication is always initiated by the master node to the slave node.

Our EagleSunTM sense node interfaces with Modbus output of equipment, the EagleSunTM sense node converts the sensed data packets to 867MHz RF signal. The sense nodes form self-organising/self-healing cluster to relay the data packets to the Base station/Mesh master. Base station/Mesh master is a central component in a wireless SCADA system, as it collects  equipment data, processes the collected data and sends the processed data to the server.

EagleSunTM SCADA Architecture

 

In network/cluster formation, one of the sense nodes acts as the master and rest of the sense nodes act as slaves.

A network/cluster consists of following layers.

  1. Lowest layer: All sense nodes in this layer acts as slaves
  2. Middle layer: All nodes in this layer acts as a master to lowest layer and sends the data to the Base station/Mesh master
  3. Top layer: Consists of Base station/Mesh master, which collects all the sensor data through master nodes, and this collected data is processed to form a single data packet, which is then sent to the server.

A master in each layer controls many slave nodes in the lower layer by polling mechanism, to gather data. The master node transmits broadcast message to the slave nodes. When all master nodes finish broadcasting message, slave nodes select a master node as the most preferred master node, second most preferred master node and so on depending upon the received signal strength. In normal conditions each slave node relays its data to the most preferred master node. In case of low signal strength or low battery power on a master node, the slave node finds the other path (Self-healing) through second most preferred master node to relay its data packet to the Base station/Mesh master. So, the slave nodes have the capability of self-organising and self-healing which improves reliability of the entire system.

Delivery reliability can be achieved by proper selection of the Master node count. If the number of sensors in Hierarchy “i” is NSi , the number of elected Masters NMi in this hierarchy will be set to more than sqrt(NSi) so that each slave node can find other paths to relay its data packet to the Base station/Mesh master even in adverse conditions.

 

— BY RAJU KAKI

Product Engineer-Avi Solar Energy

Temperature Effect On Solar Power Generation

Solar Energy is one of the booming fields in the renewable energy sector. Due to effects of climate changes, Green energy summits across the globe have shown enthusiasm about solar energy policies.  Land owners and industrial giants are willing to invest in the solar energy sector with an eye on profitable returns on investment and positive image created by reducing carbon footprint.

General belief is that more sun light gives more generation irrespective of weather condition, which is apparently true, but fact is that we cannot ignore the importance of environmental factors such as, temperature and wind speed that significantly influence solar energy generation. Generating solar power may seem facile but, when you start analysing each aspect of a solar plant one could find it quite riveting.

Generally, we expect good generation from solar panels during summer, because the insolation received over the panels will be higher. Analysis, shows it is not too beneficial; Plants sometimes struggle to meet expected generation numbers. Here temperature comes into the picture. While expecting higher generation with higher insolation we often ignore the fact of higher module temperatures which lead to lower than expected Solar energy generation.

 

Most commonly used PV module technology is Polycrystalline, and this material has a negative temperature coefficient.  This means, with increase in temperature, generation efficiency of the module (which is typically 15-16%) will decrease. Typically, this temperature coefficient is around -0.4%/° C, this indicates with each degree rise from 25°C (which is the cell temperature at STC) the efficiency of module will reduce by 0.4%. Physics behind this phenomenon is that, with increase in temperature open circuit voltage decreases and reverse saturation current increases. Open circuit voltage(Voc) is the measure of amount of recombination in a Silicon cell or in other words it corresponds to the amount of forward bias, as Voc depends on saturation current and light generated current of solar cell. For a Si solar cell, saturation current becomes double with 10°C rise in temperature and brings down the open circuit voltage.  The equation between these parameters is

According to climatic conditions in India, Polycrystalline modules are preferred in most of the regions. Hence, as per the working principle of polycrystalline modules one cannot expect higher generation numbers during high temperature days.  Hence, temperature correction is important while evaluating generated units. Generally solar power plant performance is measured by a parameter called “Performance Ratio” (PR). This is the ratio of total generation for a time-period to maximum generation possible for the available DC load with available insolation during the same period.

One needs to apply temperature correction factor while calculating Performance Ratio of a plant. A theoretical approach to correct PR value is as follows:

 

PR % = (Energy AC) / (Energy DC STC X Insolation) % —— (1)

Temperature Corrected PR% = Normal PR (equation1) / (1 + µ(TCELL–25°C)) % ——(2)

µ is the Temperature coefficient provided by module manufacture

There is another method which NREL has suggested to calculate PR more accurately, but it requires average cell temperature for a whole year to calculate PR. The NREL suggested method is given below:

PR % = Normal PR (equation 1) / (1–µ (TCELL Avg–TCELL[i]))

TCELL Avg = Average module cell temperature

TCELL[i] = Temperature of Cell/Module at the time instant

Here we are giving one-month PR and corrected PR value for a 60MW plant (calculated by eqn 1 and 2), and the average difference is about 4% between Normal PR and Temperature corrected PR. This difference may vary month to month depending on weather condition.

From the above data we can infer that it is very much required to evaluate Solar Power plant performance according to the weather condition. In this study we have only considered the main aspect of weather i.e. Temperature, but other weather parameters like wind speed, wind direction, humidity have their own influence on the solar panel performance. Hence, if major influencers among these weather parameters are taken care off while evaluating generation units then it will give a more a realistic and accurate PR value.

— By Madhushree Sinha

Electrical Engineer

Solar Seasonal Tilt Part 2

Studies done in certain solar PV plants in the Central India revealed that shadow effects were resulting in generation losses.  Losses were more at the 35 degrees tilt angle whereas after moving to the 5 degrees tilt angle, at the onset of spring season, the shadow effects were negligible. To tide over the problem, either the shadow causing objects had to be removed or the tilt angle altered during the winter season. A limited exercise to move panels to 18 degrees of certain Solar arrays during January (this tilt angle is usually scheduled for July) was done but shadow effects were still seen.

One can do a cost benefit analysis with respect to losing generation at an unintended angle for that season and loss due to shadow effects. In this case, the exercise was futile to move the panels to 18 degrees to counter shadow effects. Following on the generation closely for all the 6 months at the 35 degrees tilt, it was noticed that in January the panels which were not affected by shadows were performing lesser than the forecasted generation itself. Probably another tilt angle could be tried for January month alone.  However, it is not feasible once Module mounting structure (MMS) design considerations are frozen for 3 specific tilt angles.

Two equal sized and co-located Solar plants were compared on different aspects basing it against the forecasts – i) the generation differences at different tilt angles ii) Effect of shadows across both plants. Following were the findings.

  1. The generation differences for the 2 plants at the 18 degrees tilt angle was higher for one plant against the forecasts.
  2. The benefits of generation in the 35-degree tilt angle was the least in both the plants as compared to the 18 degrees and 5 degrees tilt.

The effect of shadows at both the plants had a major impact on generation at the 35 degrees tilt, and it was more in one of the plants. It was evident that the terrain and its compounded impact on shadow effects resulted in generation loss at the 18 & 35 degrees angle and thereby the gains of seasonal tilt were limited.

By Ganesh H.

AVP Analytics

Solar Seasonal Tilt-A Quick Analysis

PV Solar Plants are of 3 types principally – i) Fixed Tilt Plant ii) Tracker based plants (most common are with Single Axis trackers) iii) Seasonal Tilt Plants.

Fixed Tilt Plants are installed at a fixed tilt angle, mostly linked to the latitude of the place and facing south (for plants installed in the northern hemisphere). Single Axis tracker plants are installed with trackers aiding the movement of the Solar panels, that face East as the day starts and progressively moves towards the west as the day ends. Trackers are expected to boost energy generation by a conservative ~15% against Fixed Tilt Plants.

Many a time there is a discussion on how much seasonal tilt of Solar Modules offers as generation improvement against a fixed tilt PV Plant. As seasons change, the benefits also change. On an average there is a 5-6% increase in generation /year, with certain seasons offering an improvement of >10% with the right element of seasonal tilt.

Usually Developers opt for 3 tilt angles for the solar panels in a year (within the tropics in the northern hemisphere) –:  typically, 3 to 5 degrees from April to June, 17 to 20 degrees from July to September and 27 to 35 degrees from October to March depending on the latitude of the site.

It has been assessed that the seasonal tilt benefit during the winter season is minimal; since the tilt angle remains the same, say at 35 degrees for 6 months with no further angle changes planned, there are less chances of accrual of more benefits than in summer months.

The seasonal tilt angles to be used are decided at the design stage after assessing the terrain, latitude and so on. Based on the design, the structures are fabricated with slots to move to the desired tilt angle according to the use in different seasons. Seasonal tilting activities are done manually.

Importance Of Ground Fault Monitoring

Among some glaring issues in Solar Plant Operation and Maintenance, Ground fault a is very common phenomenon, specifically arising during monsoon and wet conditions. Occurrence of ground faults affect the generation as well as lead to safety issues.

In accordance with the IEC60364, earthing systems are configured as TT, TN or IT networks. The DC field is susceptible to fire and the solar inverters are earthed in IT type configuration. In IT network configuration the inverter DC side has resistance grounding and the connected components of inverter have direct earthing. A special Ground Fault Monitoring Device(GFMD) is installed to protect the system from high leakage current in case of ground faults.

For DC side ground fault monitoring, the negative or positive pole of DC input of the inverter is grounded with high resistance. The entire connection is called ground fault monitoring device(GFMD). For DC ground fault, the leakage current flows through the GFMD. If the leakage current crosses the pre-set value which usually less than 32mA, the GFMD breaks the path for leakage current and passes the ground fault trip signal to the inverter and eventually inverter trips. Once the ground fault on DC side is rectified, the inverter starts automatically.

Ground Fault Monitoring

The inverters in solar PV plants has a AC system which are typically ungrounded. For AC side ground faults, a separate mechanism trips the inverter with current imbalance. The internal current transducers measure the line currents of the inverter. If the Phasor sum of line currents crosses 8% of the actual measured sum value, the inverter receives current imbalance trip signal and eventually inverter trips.

Phasor sum of phase currents = Ir + Iy + Ib = 0A   (balance network)

If the sum of phase currents is non-zero, the circuit is imbalance due to external AC fault. If this non-zero value crosses the 8% of the numerical sum, the inverter trips with current imbalance.

Hence during design phase it is recommended to choose inverters with ground fault monitoring device.

— By Raju kaki- Electrical Engineer

Failure Analysis Of Central Inverters

The solar power industry has grown enormously in the last decade. At present, the installed capacity of India is 15.6 GWp which is expected to reach 20 GW by 2017-18. Amidst the huge number of projects being planned, it is possible to predict the number of central inverters in solar sector to be installed.

If we delve in a solar power plant, the most important equipment to come across are Inverters. These Inverters convert DC power from solar array to AC power which is fed to transformer for stepping up followed by the grid. Generally, solar inverters come with 5 years warranty with full load efficiency of around 98%. However, buyers must be cautious while selecting inverter for their solar plant as reliability of inverters for solar generation is utterly important. Let’s go through a case study which throws light on keeping a tab on reliability of inverters.

Case:

The study focuses on reliability of Inverters installed in a Solar Power generation plant. A set of inverters have been studied and errors/issues have been analysed for enhancement and future references.

Description:

Study of Identical Inverters across the solar plants were carried out. Considering the performance of 150 identical central inverters with 1MW capacity across 5 regions, of which, at least 80% of the inverters are working for over a year on the field.

This reliability study is based on the statistical measures of Mean Time to Failure (MTTF), Mean Time Between Failure (MTBF), Mean Time To repair (MTTR) and Failure rate.

Terms & Definitions:

  • MTBF: Average time difference between two consecutive failures.
  • MTTR: Average time taken for repair of a specific failure.
  • MTTF: The anticipated timespan for a device/product to last in an operation.

It is like MTBF, only difference being MTTF is used for non-repairable product and MTBF is used for repairable ones.

  • FAILURE RATE: Number of failures over a time-period. Failure rate is reciprocal of MTBF.

RESULT  

MTBF (hrs)59.14
MTTR (hrs)1.18
MTTF (hrs)60.32
FAILURE RATE/hr0.02

FAILURE RATE/day

0.41
FAILURE RATE/month12.17

Study shows a pretty reliable number for this inverter as the failure rate is moderate.

Choosing right inverters and maintaining them by O&M team is extremely important for overall plant performance. There is also an external factor called ground fault which impacts the inverters performance.

— By Madhushree Sinha- Electrical Engineer

Defining new Trends in Solar PV Plant Monitoring– Avi Solar launches – EagleSun SCADA

EAGLESUN SCADA OFFERS SEAMLESS MONITORING OF SOLAR PV PLANTS WITH A RANGE OF CUSTOM OPTIONS, IT IS ADAPTIVE TO THE USER’S DEVICE.

The world is moving towards utilization of renewable energy and with installations of numerous solar PV Power Plants, it has become a challenge for the plant owners to manage their assets precisely. Solar Power plants are spread across vast landscapes in remote areas and deploying technical personnels to monitor the plant is not a viable option anymore. Avi Solar has indigenously developed advanced SCADA system to manage and monitor Solar PV plants. SCADA introduced by Avi Solar is tested and applauded by numerous Solar Plant owners for its accuracy and intuitive design.

Real time monitoring of power plant with an efficient SCADA system can save a fortune for plant owners. The losses due to one underperforming PV string may seem very small, but if unattended this can have a multiplier effect and can have a large negative impact on yearly profit.


A Typical EagleSun™ SCADA Interface for Real Time Monitoring

EagleSun SCADA system comprises of RF based Communication hardware and cloud based software which needs a simple 2G/3G datacard at the site to monitor the plant. The hardware interfaces with all major equipment in field like string combiner box, Inverters, Weather station, MFM, VCB panel and all other sensors. A simple user friendly graphical interface allows plant owners to see their generation data remotely on their smartphones or PCs.

Robust Hardware (SCBs and RF Communication)

An engineer can monitor breakdowns, manage alerts and generate reports for in-depth performance analysis. For owners of multiple plants, Avi Solar can provide customized dashboard to monitor and compare multiple plants on single login. Since EagleSun SCADA is developed inhouse, it is very cost effective and is also backed up by Avi Solar’s best in class service guarantee.

Avi Solar Energy Pvt Ltd is India’s leading independent O&M service provider for solar PV plants. With the field experience from over 700MWp assets, company has started developing SCADA and other monitoring products that are customized for PV plants. The development team comprises of highly accomplished & passionate product designers who have worked in MNCs, and released several global products. These products were deployed & tested in all projects for which the company performed EPC. The flagship product EagleSun SCADA is now available for power plant owners across the country.
With over 10GWp of solar PV plants commissioned in India, several SCADA systems are either non-functional or the data is unavailable remotely or the owners have not renewed the hefty annual license fee! Avi Solar is not only providing solutions to new plants but also has started replacing some of the existing SCADA systems with EagleSun SCADA which needs simple 2G/3G data connectivity at site.

— Sayed Sahid- Marcom Executive