Case Study_Optimization of Design Method, Validation and Performance Analysis of 100MW Solar PV System

Abstract??:??The design of the utility-scale solar PV project should be balanced in terms of maximizing energy yield and minimizing the cost. The main objective of this paper is to present and enhance the different technical proficiency methodologies of selection of module, inverter, and mounting system and balance the system component for the plant location . Various parameters take place to develop the realization of a project of this nature, such as land terrain, plant layout, technology selection, and system configuration, which play an important role covered by design criteria. Subsequently, the different parameters obtained are compared with those obtained in the literature and with those obtained by means of specialized PVSyst, Metenorm software and plant SCADA data. As a result, the tilt and pitch of the plant have been optimized to 8 and 6 meters, respectively. Based on the different temperature profiles (10 o, 25 o, and 70 o) and the inverter MPPT band, the module string configuration is finalized. For this study, 30 modules in the series are used to prepare one string, and one string is used for the entire plant. The system has been designed for 1500V dc max with a required cable current rating for the DC cable and AC cable in the ratings of 33 kV and 600V. Thus, the average DC ohmic loss calculated will be 0.252% with validated cable insulation resistance of the earthing system. Thus, careful consideration of other design parameters like module selection, orientation, string configuration, system type, and atmospheric factors such as prolonged cloud cover and mist results in better performance of the plant.

Keywords—PVdesign, Validation, Performance, Losses, Energy Yield.

???????????????????????????????????????????????????????????????????????????????I.Introduction

The primary requirement for the design of any solar power project is accurate solar radiation data. It is essential to know the method used for measuring data for accurate design. Data may be instantaneously measured (irradiance) or integrated over a period of time (irradiation), usually one hour or day. India has enormous potential for solar energy generation, owing to its geographical location, which means it receives solar radiation almost all the year, which amounting to 3000 hours of sunshine. This is equal to more than 5000 trillion kWh. Almost all parts of India receive 4–7 kWh of solar radiation per sq metre per day.[1]

The design of utility-scale solar PV plants needs technical proficiency to maintain a balance between performance and cost. With maximising energy yield and minimising project cost being the focal point, various factors such as land terrain, plant layout, technology selection and system configuration play an important role. For delivering a sustainable power plant, developers put their confidence in multidisciplinary teams that provide complete plant design and engineering services. A Solar PV plant consists of two parts: a DC PV array and an AC system. This paper focused on design and engineering involved with the DC side of the plant, validation, performance analysis of photovoltaic energy system. The major DC components in a Solar PV plant are Photovoltaic modules, PV Connectors, string combiner boxes, DC cables, and inverters. The analysis and evaluation of solar resources, climatic conditions, and site-specific losses also come under the scope of DC engineering.?[2].

???????????????????????????????????????????????????????????????????????????????I.Design Basis Input

The design inputs fall under the following areas of the Solar PV plant engineering:

l???Solar Resource data for the finalised site

l???Weather and climatic conditions (ambient temperature, wind velocity, etc.)

l???Technology of PV modules and Inverters

l???Compatibility between PV modules and Inverters

l???PV plant losses

o Optical losses

o Module losses

o Electrical losses (AC and DC)

o System Losses

2.1 Solar Resource Mapping

The most important factor that determines the generation of a solar PV plant is the solar radiation available at that location. The total solar radiation available at any point is quantified as global solar radiation, technically called Global Horizontal Irradiation (GHI). Global Solar Radiation is essentially the sum of Direct Irradiation and Diffused Irradiation.

The final site location mentioned is located near the village of Ettankulam, Taluk-Mannur, District-Tirunelveli, Tamilnadu. The geographical coordinates are:

Latitude – 8.85°N

Longitude – 77.63°E

Altitude – 120 m

All energy yield estimations are done based on the above data. Solar radiation and ambient temperature data also available in PVsyst report and are attached as Annexure A.

Our system design has been carried out using 10°C as the Min Temp and 50°C as the Max Temp. However, for PV module string sizing and calculations, we have used a broader temperature range.

2.2 Technology

Photovoltaic modules are generally categorised into crystalline and thin-film technologies. Crystalline technology modules have higher efficiency and require less land per MW compared to thin films. Since land availability is limited, polycrystalline technology is selected. This report has been prepared considering GCL 325 Wp and Astronergy 320Wp & 325Wp modules. The chosen inverter is the GE Make Solar Central Inverter with a capacity 1100 kW at 50°C at Unity PF

2.3 Photovoltaic Modules And Inverter compatibility

Annual energy yield estimation is done using PVSyst simulation software, which uses PAN and OND files to represent the characteristics of a module and an inverter. It has also accounted for losses according to the standard benchmark, recommendation guidelines, and calculations. The PV module and inverter compatibility means that it should achieve maximum generation at safe operating conditions.[3][3]

The module selection and design are compatible with the GE-made 1.1 MW inverter. The plant would be divided into blocks of 4.4 MW capacity. The 4.4 MW block consist of one inverter room with four 1100kW inverters. The design is carried out in such a way that it minimises components’ losses and meets all standards and safety precautions.[4]

The module selection and design are compatible with the GE-made 1.1 MW inverter. The plant would be divided into blocks of 4.4 MW capacity. The 4.4 MW block consist of one inverter room with four 1100kW inverters. The design is carried out in such a way that it minimises components’ losses and meets all standards and safety precautions.[4].

?????????????????????????????????????????????????????????????????????????????????I.?Design Criteria?

3.1 Capacity

The contracted capacity of the project is 50 MWac. Solar modules are being used with a total DC capacity of 55 MWp to achieve the contracted generation and performance ratio.

3.2 Type Of System

PV plants can be broadly classified into fixed, seasonal tracking, single-axis tracking, and double-axis tracking plants. The modules can also be mounted in a portrait or landscape fashion. This report is based on a ground- mounted fixed tilt plant. A single table structure consists of 60 modules, with 2 in portrait layout. The tilt and pitch of the plant has been optimised for this kind of system, and land availability is at 8° and 6.0 meters.?[5].

3.3 String Configuration

The 320/325wp modules are connected in a series of 30 modules to prepare one string. All configurations selected to match the inverter are done using module parameters assumed at worst conditions as a precautionary measure. The formation of one string and the connections are shown in Dc single line diagram (dc sld).

TABLE?: 3 : ARRAY?STRING?CURRENT?CALCULATION

According to above temperature profile and inverter MPPT band, module string configuration is finalized. We have taken 30 modules in the series to prepare one string.?[6]

3.4 Design Input For SMB

The DC system design will require SMB’s of 14 strings. Spare inputs will be available in consideration future expansion, ease of O&M and specification requirement. Each string will have input fuse protection. The rating of fuses will be based on Isc value of modules at Y Connectors. The system has been designed for 1500 VDC max.

3.4a??Sizing Calculation of SMB

i .Sizing Calculation for Input fuse

Input fuse rating = 1.25 *1.25* (Isc per channel)

ii.Sizing Calculation for Output DC Disconnector Switch Output DC Disconnector Switch rating = 1.25 * 1.25*(Isc at SMU Output)

3.5 Earthing Calculations Of Solar Array

Considering steel as the material to be used in earthing strip.Therefore, Calculation for strip size has been done with steel material which is shown as below-

K= 79.14 , Isc= 1200 kA Considering Maximum DC Short Circuit Current at Inverter DC side. S= I √ t / k

I = 1.200 kA = 1200 Amp

t = 1Sec

S = 15.00 Sqmm

Corrosion factor= 15 %

Minimum cross section of earth strip is required 17.25Sqmm We have taken 25 X 3 GI strip for DC Array earthing.

Earthing of smb is required to earth surge protection devices with adequate wire or strips. earthing of smb is done using flexible cu wire / gi strip/ gi rod which will be directly connected to gi strip nearest to smb.?[7]

3.6 Dc Cable Sizing Calculation

Required cable current (Amp-city) rating ≥ Current at STC x safety factor (1.25)

As per module current at STC is 8.77. Branch connectors are used to connect 2 strings. Hence,

Current at STC: 8.77 x 2 = 17.54 Amp;

Max current using safety factor 17.54 x 1.25 = 21.925 Amp

DC Cable Ampacity (Siechem) =70 A @ 60°C(6 sq.mm), 55 A @ 60°C(4 sq.mm)

However, the cable ampacity decreases due to derating factor specified above Applying derating factor of selected cable makes,?

Ampacity after Derating

=??70 x 0.5 =35.0 A(6 sq.mm)

55 x 0.5 = 27.5 A(4 sq.mm

Thus for Safe operation and durability we will use 4 or 6 sq.mm cable for connection of PV array string to SMB Final cable size will be intimated after actual Power Loss Calculation based on cable scheduling.?[8]

3.6a Power Loss Calculation

The power loss calculation for 6 sq mm cable is tabulated below:

The maximum string current is for 325 Wp modules at 8.77 Amps (Imp) and current after branch connector will

8.77 x 2 = 17.54 A

The Max DC resistance at 20°C is 3.39 Ω/km and at 50°C 3.80 Ω/km

Average string to SMU length (L) is 42 meter.

Power loss in cable has been derived with P= I2?X R x L (W).Thus Avg. DC Ohmic loss from String to SMB input will be 49.10 W which is approx. 0.252%.?[9]

?????????????????????????????????????????????????????????????????I.Monitoring Stsrem Output

The overall plant monitoring system is shown in figures V.1 and V.2. It displays the real time data of weather monitoring station such as reading of irradiation in horizontal plan on array angle, surface module temperature and atmospheric temperature. It also shows the string level voltage and current, inverter wise power, AC equipment of Transformer, HT panel, poling switchyard operation status and measured reading.?

Based on site level irritation data and generated energy injected into the grid, the plant performance ratio and capacity utilization factor will be calculated.

?????????????????????????????????????????????????????????????????????????????I.Result and Discussion?

This report presents the results of 50MW design and validation, which were tested and characterized at the installed site. The performance of the solar power plant??matches??the estimated generation.

The report concludes performance CUF (Capacity Utilization Factor) depends on several considerations like???solar light intensity, temperature, air flow through the modules apart from the type of module and quality of module, tilt angle (or tracking), design aspects to avoid cable losses and the performance of inverters (inverter efficiency) and transformer efficiency. There are some built-in losses which can be minimised through correct design, but they are??not avoided completely.?[15], [16]

In high temperature areas, thin film modules will give better efficiency than crystalline modules. The capacity factor varies from 16 to 20% in various zones of the country. [18].In most areas in Rajasthan and Gujarat, it is around 20%. Overall, in most places it is around 19%. In some places where the CUF is around 18%, it is required to increase to 19% by adding extra 50kWp of modules for every 1MW of capacity to compensate for the built-in losses in the system.

??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????II.?Conclusion

The modules show a reduction in power output through years of operation. It is observed that the quality of modules is necessary to determine the amount of reduction.??Innovations in solar technology and quality assurance could reduce this power reduction simultaneously. Based on the results of past work and technology, one can fairly assume power reduction is maximum 0.5% per year from the 3rd year of deployment. This can also be compensated by the addition of 50 kW of modules for 1MW per year from 4th year to the 24th year of operation. The performance of”solar power plants is best defined by the Capacity Utilization Factor CUF), which is the ratio of the actual electricity output from the plant to the maximum possible output during the year. The estimated CUF for this solar power plant is 19.8% and actual CUF is 18.1% and it also depends on the design parameters. (But since there are several variables that??contribute to the final output from a solar plant, the CUF varies over a wide range. These could be on account of??derating of modules at higher temperatures, other design parameters like ohmic loss, and atmospheric factors such as prolonged cloud cover and mist. Therefore, it is essential to consider the uncertainty factor of metronome meteo data in order to achieve the expected CUF and the better performance of the plant.