presentations for the project of solar energy

Solar Power Plant
Design
BY:
KAPIL JOSHI

Topics
 What are the different topologies and interconnection methods for a PV
system?
 What are the various components that are used in a rooftop PV system?
 How do we do preliminary sizing of a grid-connected PV System?
 How do we arrive at detailed sizing of a grid-connected PV system?
 What should be the sizing and specifications of these components?
 How to make the necessary drawings/ diagrams of a grid-connected rooftop
PV system?
Topic : SOLAR POWER PLANT DESIGN

Stand-alone PV system.
Hybrid PV system.
Grid-connected PV system.
Types of PV System

Metering Arrangements for Rooftop PV Systems
A typical metered home. Gross-metered system.
Net-metered system (hybrid).
Net-metered system.

List of Equipment & Material
 PV Module
 DC Cable (PV modules to SJB)
 String Junction Box (SJB)
 DC Cable (SJB to Inverter)
 Cable Tray/ Conduit
 DC Isolator*
 Inverter
 AC Cable
 Isolation Transformer*
 AC Distribution Box
 Lightning Arrestor*
 Earthing Cable
 Earthing Strip
 Earth Pit (2 or 4 or more)
 Generation Meter
 Bi-directional Net-meter
 DC Fuse
 DC Surge Protection Device (SPD)
 AC Surge Protection Device (SPD)
 Isolator: MCB/ ELCB/ RCCB
 Labels

2-Step System Sizing
 Step 1: PV System Capacity
 Where we specify:
 DC Capacity
 AC Capacity
 Battery Capacity
 Capacity is based on:
 Energy requirement
 Space limitation
 Budget limitation
 Policy limitation
 Step 2: Specific Equipment
Specifications
 Where we specify:
 Make and model
 Quantities & Sizing
 Dimensions (e.g. cable length)
 Repute Brand
 Selection is based on:
 Specification
 Availability
 Cost

Step 1 (Given)
 Case study:
 Type of system: Grid-connected
 Capacity constraint: Available space
 Capacity for which space is available: 30 kW
 That’s it!!!

Step 2.1: Site Survey
 Undertake a detailed site survey (if not already done so) to identify:
 Shadow-casting features and shadow-free area
 Location for mounting inverter(s) and AC Junction Boxes, typically on terrace/ roof
 Location for interconnection point (distribution panel of house/ building), typically
on ground floor
 Location where earth pits will be installed
 (Check if distribution panel has extra feeder/ interconnection point of sufficient
capacity)
[Note: These are in addition to other steps involved in a site survey.]

Room, 3 meter up
Site Survey Drawing
Location for PV Arrays Location for:
• String (DC) Junction Box
• Inverter(s)
• AC Junction Box
DC Cable Routing
Earth Pit Location
AC Cable Routing
Earthing Strip Routing
17.77 m X 13.10 m
= 232.79 m2
(~19.4 kW)
22.07 m X 5.45 m = 120.28
m2
(~10.02 kW)

Step 2.2: Grid-connected Inverter Selection

Modules and Strings (1/5)
 Let’s count PV modules…
 PV Module (DC) Capacity:
 20 kW; 10 kW
 Estimate the total number of PV Modules:
 20,000 / 300 = 67; 10,000 / 300 = 33
 Which inverters can be used? (Ref. Max. PV Power)
 17 kW; 10 kW

 How many modules in series (i.e. string size) can the inverter accept?
 What is the inverter’s MPPT range?
 410 … 850 V; 460 … 850 V
 What is the maximum number of modules per string that the inverter can accept
within the ‘MPPT range’?
 850 / VMP = 850 / 36.72 = ~23; 850 / VMP = 850 / 36.72 = ~23
 What is the maximum number of modules per string without exceeding the inverter’s
‘Max. DC Voltage’?
 1000 / VOC = 1000 / 45.50 = ~21; 1000 / VOC = 1000 / 45.50 = ~21
 What is the maximum number of modules per string acceptable to the inverter?
 21; 21

 Let’s converge…
 Can we have 1 string per inverter?
 i.e. 67 modules per string? i.e. 33 modules per string?
 NO NO
 Can we have 2 strings per inverter?
 i.e. 34 modules per string? i.e. 17 modules per string?
 NO Maybe!!! Let’s freeze this for now.
 i.e. 22 modules per string?
 NO
 i.e. 17 modules per string?
 Maybe!!! Let’s freeze this for now.

 Let’s check…
 What is the VMP of the string be within MPPT range at STC?
 VMP: 17 x 36.72 V = 624.24 V; VMP: 17 x 36.72 V = 624.24 V
 YES; YES;
 Will the VMP of the string be within MPPT range on a bright sunny day?
 Bright sunny day: 1000 W/m2; Max. TAMB: 42°C  Max. TMOD: 73°C
 VMP: 17 x 31.07 V = 528.19 V; VMP: 17 x 31.07 V = 528.19 V
 YES; YES
 Will the maximum string voltage be less than inverter’s ‘Max. DC Voltage’?
 VOC: 17 x 45.50 V = 773.50 V; VOC: 17 x 45.50 V = 773.50 V
 Hence, 17 modules per string is suitable for the inverter.
 DC Capacity: DC Capacity:
 300 W x 17 x 4 = 20.4 kW; 300 W x 17 x 2 = 10.2 kW

 Let’s try to push the string size further…
 Can we use 18 modules per string instead of 17?
 DC Capacity for 20 kW system:
 300 W per module x 18 modules per string x 4 strings
 = 21.6 kW
 NO! This is more than inverter’s ‘Recommended Max. PV Power’.
 DC Capacity for 10 kW system:
 300 W per module x 18 modules per string x 2 strings
 = 10.8 kW
 YES! This is within the inverter’s ‘Recommended Max. PV Power’.

String Junction Box (SJB) Specification (1/3)
 How many inputs and outputs are required in the String Junction Box…
 For 20 kW system? For 10 kW system?
 If the number of PV strings are greater than number of input DC connections to the
inverter, then we will have to connect strings in parallel in the SJB.
 No. of PV strings:
 4 2
 No. of input DC connections to the inverter:
 6 3
 Hence, are the strings required to be connected in parallel?
 NO NO
 SJB input-output specification:
 4-in 4-out 2-in 2-out
 These can be combined as 6-in 6-out

 Let’s specify the 4-in 4-out SJB…
 Cable Gland/ Connector
 Incoming DC: 4 pairs of male/ female panel-mount MC4 connectors
 Earthing: 1 Nos. of (M16) Cable Gland for 6 mm2 earthing cable.
 Fuse-holders and Fuses
 Fuse rating > ISC x 1.25 x 1.25 = 8.65 x 1.25 x 1.25 = 13.51 A  15A, 1000 VDC
 Fuse-holder (Fuse-base) > Fuse rating  30 A, 1000VDC
 8 Nos., each
 Surge Protection Device (SPD) (i.e. Surge Arrestors)
 1 Nos. Type II, 1000 VDC, 40 kA
 Terminal Blocks
 Required when connecting SPD are connected in parallel.

 Internal cables:
 DC (positive and negative): 4 mm2 PV cables
 Earthing Cable: 6 mm2 copper cable
 Enclosure
 Thermoplastic Polycarbonate
 Degree of protection: IP65
 Design Temperature > 50 °C Ambient

DC Cable Sizing
 DC Cables need to conform to 2 requirements:
 Ampacity
 Ampacity is the current-carrying capacity of the conductor
 Ampacity of conductor should be more than ISC x 1.25 x 1.25 of the string
 Voltage Drop
 Should be less than 2%
 Note that these values should be calculated at the:
 Actual PV module temperatures (E.g. 70°C), and
 Actual/ ambient (E.g. 45°C) temperature of PV cable.

DC Cable Resistivity & Ampacity

Ampacity Deration due to Temperature

Check for Ampacity
 Step 1: For PV string:
 ISC = 8.65 A
 Hence, Ampacity of cable should be more than 13.51 A.
 Step 2: For 4 mm2 PV cable:
 Ampacity @ 20°C = 44 A
 (Because multiple cables will be laid together in the cable tray.)
 Ampacity deration factor @ 41-45°C ambient temperature = 0.87
 Hence, Ampacity of the PV cable is 38.28 A.
 Ampacity of the PV cable is more than the PV string current.
 Hence, first condition is met.

Check for Voltage Drop
 Step 1: Maximum length of the PV cable is from the farthest PV module
 (17 + 13 + 3 + 3 + 22 + 12 + 5.5 + 3 + 4.5) m + Extra 10%
 = ~100 m
 Step 2: Resistance of the PV cable
 R @ 20°C = 5.09 Ω/ km x 0.1 km = 0.51 Ω
 R @ 70°C = R20°C x [ 1 + α ( T Cable – 20 ) ] = 0.61 Ω
 Where,
 For copper, α = 0.393 % / °C
 Assume PV cable is at 70°C
 Step 3: Voltage drop = IMP x R = 8.17 A x 0.61 Ω = 5 V
 Step 4: VMP, String @ 70°C = 17 x 31.43 V = 534.3 V
 Step 5: Voltage Drop in % = 0.93%, which is less than 2%
 Hence, second condition is also met.

AC Cable Sizing
 AC cables are required from:
 Inverter to nearby AC Distribution Box
 AC Distribution Box to Main AC Distribution Box
 Note: Same ampacity and voltage drop rules apply as in DC cables.

AC Cable: Inverter to nearby AC Distribution Box
 Let’s calculate for two inverters:
 20 kW System; 10 kW System
 What is the maximum output AC current?
 3 x 29 A (i.e. 3φ); 3 x 16 A (i.e. 3φ)
 After safety factor…
 3 x 36.25 A; 3 x 20 A
 Which 4-C AC cable from the datasheet can carry this current?
 Note: Usually, the length of this cable is not substantial, and hence, voltage drops
are very minor.
 Aluminium,10 mm2; Aluminium, 4 mm2
 Note: Often for uniformity and convenience of procurement, we may use the
same AC Cables for both inverters:
 Aluminium,10 mm2; Aluminium, 10 mm2

AC Cable: ACDB to ACDB (Ampacity)
 Once all the inverter outputs are combined in the ACDB near the inverter, it will add up the
currents and evacuate power using a single AC cable.
 In our case, combined AC current: 3 x 45 A (i.e. 3φ)
 Hence, after considering safety factor: 3 x 56.25 A
 Which AC cable is suitable from the datasheet?
 Aluminium, 25 mm2
 Note: For long distances, it is preferred to used armoured cables.

AC Cable: ACDB to ACDB (Voltage Drop)
 Let’s assume that:
 The installation (including inverter) is on the terrace/ roof above the 3rd floor of the building,
and main ACDB of the building is on the ground floor, and
 The surveyor determines this cable routing distance to be 25 m.
 From datasheet, resistance of 25 mm2 Aluminium cable, R:
 R @ 20 °C = 1.2 Ω per km x 0.025 km = 0.03 Ω
 Therefore:
 R @ 75°C = R20°C x [ 1 + α ( T Cable – 20 ) ] = 0.036 Ω
 Voltage drop = I x R = 45 A x 0.036 Ω = 1.6 V
 Voltage drop in % = 1.6 / 230 = 0.7%
 This is within the 2% limit, hence 25 mm2 Aluminium cable is acceptable.

AC Distribution Box (ACDB)
 How many in? How many out?
 2-in, because two separate inputs will come from the two inverters
 In our example, each wire is 10 mm2
 1-out, because we combine the above two and evacuate in a single wire.
 In our example, this wire is 25 mm2
 What do we need in the AC Junction Box?
 MCB (Miniature Circuit Breaker) for each incomer from inverter
 To individually disconnect each inverter, and protect from overcurrent.
 From datasheet: Rated current: 40 A, 240 V, Breaking Capacity: 10 kA
 SPD (Surge Protection Device)
 From datasheet: 400 VAC, Type 2 (4-pole)
 RCCB (Residual Current Circuit Breaker)
 To protect inverter from residual current and earth leakage faults
 From datasheet: 63 Amp, 100 mA (4-pole))

Drawings
 Single Line Diagram
 Equipment/ Module/ Project Layout Diagram
 Cable Route Plan
 Earthing Layout Diagram

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