Photovoltaic Systems – Utility ScalePart 1

April 7, 2014

Learning Outcomes

•A comparison of the design process for utility scale PV projects vs smaller scale projects

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Value to participants

• A review of the importance of technical vs non-technical components of utility scale projects

• A review of utility scale projects both commissioned and in development

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Design Steps in a Utility-Scale System

1. Examination of site and estimation of performance

2. Determining financing model3. Carrying out PV system engineering and design4. Securing relevant permits5. Construction6. Inspection7. Connection to the grid8. Performance monitoring

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Grid-Connected Utility-Scale PV System

Grid-Connected Utility-Scale PV Systems

Comparison of PV system engineering and design for different scales

oEvaluation of space availability, solar availability, and electrical consumption

oPV array sizing

oModule selection

oInverter selection

oBalance of system

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Grid-Connected Utility-Scale PV Systems

System sizing by AC Power

•Small: Up to 10kW (Residential)oTypically, 240V AC, single phase

•Medium: 10kW to 500kW (Commercial)

•Large: 500kW to 5MWoTypically, 208V AC, three phase

•Very Large: 5MW to 1GW (Utility)oTypically, 480V AC, three phase

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Grid-Connected Utility-Scale PV Systems

Properties of a 3-phase system:

• The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to reduce the size of the neutral conductor because it carries little to no current; all the phase conductors carry the same current and so can be the same size, for a balanced load.

• Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.

• Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors

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Grid-Connected Utility-Scale PV Systems

Recall the circuit diagram for a 3-phase system

(Y-connected)

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Grid-Connected Utility-Scale PV Systems

Before looking at a utility-scale system, consider a smaller (21kW) 3-phase system, with these components:

1.Modules with nameplate 305 Wp output

a. VOC = 64.2 V; VOC(max) = 73.7 V

b. VP = 54.7 V; VP(min) = 47.0 V

c. ISC = 5.96 A

2.Inverters with 7000 W outputa. VIN(max) = 600 V

b. 250 V < VMPPT < 480 V

c. IIN(max) = 30 A

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Grid-Connected Utility-Scale PV Systems

3. Module upper and lower bounds:a. VIN(max)/VOC(max) = 600/73.3 = 8.13 8 modules

b. VMPPT(min)/VP(min) = 250/47 = 5.32 6 modules

4. Array Power

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NMODULES 6 7 8

NSOURCE_CIRCUITS

1 1830 W 2135 W 2440 W

2 3660 W 4270 W 4880 W

3 5490 W 6405 W 7320 W

4 7320 W 8540 W 9760 W

Grid-Connected Utility-Scale PV Systems

5. Best Choice:a.3 source circuits, 8 modules in each circuit

Higher voltage operation

b.3 invertersOne for each phase

c.3 sets of 3 source circuits 9 source circuits, 72 modules72 x 305W = 21,960W

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Grid-Connected Utility-Scale PV Systems

21kW three-phase PV system

12THWN – Thermoplastic Heat and Water Resistant Nylon-Coated

Grid-Connected Utility-Scale PV Systems

To build a utility-scale system, one can construct subarrays (similar to the 21kW system), then combine them to achieve a much larger power output.So to construct a 250kW system, we can use:

1.The same modules with nameplate 305 Wp output• VOC = 64.2 V; VOC(max) = 73.7 V• VP = 54.7 V; VP(min) = 47.0 V• ISC = 5.96 A; IP = 5.6 A

2.An inverter with 250 kW outputa. VIN(max) = 600 V

b. 320 V < VMPPT < 600 V

c. IIN(max) = 814 A

d. VOUT = 480 V

e. IOUT(max) = 301 A 13

Grid-Connected Utility-Scale PV Systems

3. More on the invertera. Large inverters are not quite as efficient as smaller units,

and may have a inversion efficiency of 96%

b. Therefore the array power should have:

PARRAY = 250kW/0.96 = 260kW

4. Using again an 8 module source-circuita. PSOURCE-CIRCUIT = 2440W

b. Therefore the number of source-circuits

PARRAY = 260,000/2440 = 106.6

5. Even more on the invertera. It is preferable to combine source circuits to produce a

balanced set of output currents, matching (or compatible with) inputs on inverter

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Grid-Connected Utility-Scale PV Systems

6. Dividing the source-circuitsa. 100 source-circuits is a nice round number with many

ways for division (10x10, 20x5), but the produced power would be less than 260kW

b. A better choice for this example is 108 source-circuits, producing 263.5kW, and dividable into:1. 12 groups of 9 source-circuits

2. 9 groups of 12 source-circuits

c. Any division must match the inverter current and voltage ratings: 1. Each source circuit has a maximum voltage output of 8x73.3 =

586V

2. Each source circuit has a peak current of 5.6A, so 108x5.6 = 603A, which is less than the inverter maximum of 814 A

d. Source-circuit combiner boxes that can combine 12-source circuits are available

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Grid-Connected Utility-Scale PV Systems

250kW three-phase PV system:

physical layout

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Grid-Connected Utility-Scale PV Systems

250kW three-phase PV system:

electrical layout

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Grid-Connected Utility-Scale PV Systems

250MW three-phase PV system:

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