solar power
Rakesh Bohra
Project Engineer, Solar PV
Projects, Malpani Group
Best Practices before and during Project
Execution – Grid connected solar PV Project
A solar PV project engineering is well diversified into different segments viz. civil, electrical,
mechanical, electronics, data acquisition, weather monitoring etc. The paper tries to incorporate
all the desired details, minute yet very important to be taken care of while constructing a solar
PV project.
I
ndia is on verge to solar revolution in
coming time. With growing demand
of clean energy, the solar energy sector is also growing fast. Project Developer
companies are investing a huge finance to
fetch the finalized good quality SPV plant.
EPC (Engineering Procurement and construction) companies are also not leaving
any stone unturned to maintain their credibility and is now getting global maintaining all international practices in SPV Projects. There are lot many things that are
essentially to be kept in mind before finalizing the execution and during execution.
In solar power plant to get the desired
result it is must to know that the design
and the execution you are going through
is well versed and well done.
Preliminary basic things to be
kept in concern
• SPV Plants are put under following 3
modes in India: JNNSM, State Policies
& REC mechanism.
• Which policy (international, national,
state or other) you want to set up your
project in keeping in mind the well
availability of Recourses and pay back
tariff scheme.
• The geography and the terrain you
are looking ahead for your future project must be receiving best irradiation
preferably > 6.5 kwh/m2/day . Yet depending on the states with favorable
tariffs irradiation may be considerably
acceptable. We must consider past 10
years weather data before working
upon the designs.
• We need to keep in mind few very important things viz. approach road to
4
the project plot, distance of nearest
power pumping station and the most
important is land strata.
• It is a known fact that single axis and
dual axis tracking technologies provide more than 15% additional generated power. However technical experts
choose to operate without both single
axis and dual axis tracking systems,
simply to avoid any moving part in the
plant. They want to avoid the risk of a
breakdown ever. It is also a bit costlier
however if you have enough space, it
is better to install more PV modules to
cover the desired power generation.
• At times some land brokers buy agricultural land and sell and want to sell
it off as agricultural land itself to avoid
short term capital gain tax. It is advisable for the potential developers to
buy the converted land by sharing the
short term capital gains tax with land
broker
Civil and Structural Engineering
1. Site Topography and Contour Survey
• The most important here in this segment is to review the strata of the land
and thus design the structure foundations as per it. Say if you are planning
a project in rocky terrain, make sure
that the excavation for foundation of
building or array structure will be easily
done up to the designed depth.
• If the terrain is undulated or with dune
soil strata, it is recommended to properly water bound it and compact the
land before starting work. To avoid
undulations the soil cutting can be
done yet it is a cost effective step. If
the arrays are not properly designed on
undulated land plant will not generate
power effectively.
2. Soil Testing
• The foremost step is to examine the
soil strength by testing procedures under IS 2720 and then design the mix
concrete ratio as per the result.
• The concrete shall be designed for
moderate to severe exposure condition
as suggested in Table no. 3 of IS: 456
-2000.
• Generally , soils that are a bit alkaline
(pH = 7.5 – 7.9), are slightly corrosive to
uncoated steel, and are very slightly corrosive to concrete materials as well, they
may adversely effect the life of foundations and Module Mounting structures.
3. MMS Marking and Foundation Work
• The first step is to mark true North on
the plane and with respect of it only
start marking the foundations and
structures.
• Main step and the first one is to mark
the foot/stub with surveying total station, it is highly important to mark and
excavate/auger it with accuracy because as if this first step will be wrong
the whole structure will be misaligned
at the end and there are chances for PV
modules may not fit into the structure.
• It is always recommended to constantly
verify each stub/foot for accurate alignment, horizontal and vertical both. We
can use threading technique or Laser
technique to avoid inclination of stubs
from actual axis. If by mistake some of
the stub slightly misaligned, during the
energetica
india
· JUL14
solar power
Fig. 1: Misalignment in Stub from The Very Initial stage of Project.
concrete casting process itself you can
use hammer (ensure to stroke lightly
from bottom of stub only) to straighten it back to its position.
• Make a habit during project to constantly check and measure the distance
(Horizontal and vertical) between stubs
is as per drawing or not. Use calibrated spirit level and measuring tape for
accuracy.
• Some important tests to ensure
strength of concrete mixture are
strongly recommended viz. cube test
, hammer test , slump test etc. strict
test procedures as per IS 516-1959
must be followed. With This pull out
test of stubs/structure foot is also recommended to ensure the strength of
casted stub.
• Proper water curing with specific frequency needs to be ensured.
4. Inverter/MCR
• Ensure the mix design ratio of foundations of control rooms.
• If the Land strata there is comprised
with loose/dune soil structure, first go
for compaction following rubble filling
then put the foundations.
• The foundation height of the transformers and inverter rooms must be
design in such a way that there may
not occur any problem in cable laying
and termination.
• There must be a wall isolating the inverters to nearby duty transformers.
Fig 2: Zinc coating Thickness of MMS testing by
Alcometer.
• Sliding Ramp must be there to easily
take heavy equipments in and out of
the room.
• The Iron rods to be used in Reinforcement concrete must be specifically TMT
and ISI marked, construction grade Fe
500 (PRIMEAR) and must be certified
as per IS 1786-2000
Mechanical/ Array Structure:
• Make sure that the structure are designed to bear the wind load, dead
load, seismic load etc. as per weather
history and the design must be assured
by software like Staad Pro etc.
• It is recommended to ensure if the
structure is properly galvanized or
not. Use instruments like alcometer to
check at site.
• Never allow any sort of drilling in array
structure specifically if the MMS (module mounting structure) segment is hot
dip galvanized, as drilling may allow
the inward area to get rusted in future.
In case of segment that are cold rolled
pre-galvanized you can allow drilling and
paint the drilled area with zinc rich paint
as the cold rolled sheet has inherent
quality where-in the Zinc seeps in and
automatically covers the cut portion as
the section is very thin .In reference to
BS 4652, The thickness of zinc coating is
in accordance with the values laid down
in BS 729 APPENDIX D The repaired area
should in general not be more than 0.5
% of surface area of component.
Date
Coating
Observed
(Microns)
Spec.
Status
21.01.13
132
65 micron
(Min)
OK
23.01.13
95
65 micron
(Min)
OK
31.01.13
105
65 micron
(Min)
OK
06.02.13
156
65 micron
(Min)
OK
• Make sure that the fasteners installed
in the structure are as per design i.e.
it is strictly to be observed that washers are installed or not, where ever required in case of chances of thermal
expansion/contraction spring washers
should also be used. Also it is recommended to use stainless steel fasteners
instead of GI fasteners.
• The fasteners must be tightened with
the help of torque wrench only; ensure
proper torque, it is strongly recommended to put a mark on the tightened fastener to guide the torque limit
and loosening of fasteners in future.
• The tightened structure prior to module mounting must be checked with
inclinometer to ensure that it is perfectly giving exact PV Module tilt angle
and alignment.
• It is highly important to confirm that
the diagonal alignment of PV array
structure is perfect.
• Make sure that the free flow of string
cables must not be hindered by Bolts
in between the purlin , bolts should
Concrete Cube Testing Parameters
Cube Design Mix
Area
Cube Made on Date
Cube tested on
Date
Actual Load(N)
Strength(N/mm2)
Average
Status
7 Days
7 Days
7 Days
7 Days
1
410
18,22
2
395
17,56
17,56
Ok
3
380
16,89
1
385
17,11
2
380
16,89
17,11
Ok
3
390
17,33
Cube No.
7 Days
M25
Stub Piles
Control Room
(Footing)
M25
10.02.13
10.02.13
17.02.13
17.02.13
Table1: Depicting Concrete Cube Test Under Different Design Ratios
energetica
india
· JUL14
5
solar power
be outward in such case, if design
does not allow separate cable trays for
strings to be laid on.
• For any of the Movable electromechanical parts say in Breakers, Transformers,
and Isolators etc. Ensure proper greasing /lubricating movable part
Electrical Engineering
This is the most important segment in a
solar PV projects, and requires high attention to work within this segment very since
during drawing preparation itself. The performance of the plant is highly dependent
on this part. All designs must meet the international standards like IEC 62548 etc.
To understand better we are further dividing this category into two parts: DC Electrical and AC Electrical Engineering.
1. DC Electrical Engineering
• PV Modules are the Soul of a solar PV
plant and are source of DC power,
High level of care with specific skilled
resources are required while handling
and mounting them on the structures.
• Ensure the orientation of modules
while mounting otherwise at the
end it will be difficult to make string
connections.
• Ensure at initial stage itself if any physical defects (as defined in clause 10.1 of
IEC 61215 For crystalline modules and
IEC 61646 for Thin film PV Modules)
must not be there in PV modules .
• It is better to have least inter-connections in DC system. No any joint at
string level is recommended.
• Make sure that while mounting the
modules , the terminals of modules
must be kept open.
• Ferruling in strings and other cables
must be done in R-C-T-P (Row/column/Table/Polarity)Way with practical
checking (Continuity checking/Fuse
in-out method etc.) and not to rely
only on what mentioned in the drawings ;as there are chances of manipulating the string connections by workers during execution. Also Labeling on
Module Mounting structures in field
must be essentially done.
• DC Cable sizing is recommended to
be variable for JBs at different distances from inverters to minimize the
losses.
6
Fig 3: DC Cable Laying in Trenches and Module Mounting Structrures.
• While interconnecting the strings, it is
highly recommended to take care of
the positive /negative terminal connections as it may cause fuse failure problems afterwards keeping the strings
out of generation.
• Ensure that exact wattage (indicated
on Module spec. sheet) of modules are
mounted or not.
• For safe side you can also record and
compare the initial parameters viz.
Vmp ,Imp, Voc , Isc , ff , temp. (Module & Ambient) and Radiation Readings before connecting them on load
with the parameters recorded in flash
test reports.
• Make sure that DC cables are TUV certified and UV protected and designed
as per IEC 60364- 712.
• All cables must be laid strictly as per
drawings and any change during project must be recorded and revised
drawings must be made to refer in future whenever required.
• Make sure that the DC string cables
are properly dressed in well placed
trays, they must not be hanging, UV
protected cable ties must be used for
dressing them properly.
• Heat Shrinkable sleeves must be applied to cables at Junction Box end up
to lugs.
• It is always better to have different
color DC cables say Red for Positive
and Black for Negative.
• The string cables must be laid within
PVC Conduits in the trenches. Bricks
and granular sand with safety tape
should be laid in proper way through
the whole trench is a basic practice.
• The string combiner boxes must be of
IP 65 ratings ,and must be kept at a
height of atleast 500 mm above FGL
to keep it safe in case of floods and
water accumulation. All spare openings must be sealed with Puff seal to
avoid rodents to enter into it.
• Make sure that only bimetallic lugs
must be used at SMB (+/-) termination if the cables and bus-bar bar of
dissimillar metals say if cables are of alluminium and bus-bar is of copper, to
avoid electrolysis phenomenon.
• Module tilt angle recommended
in design must be checked with
inclinometer.
2. AC Electrical Engineering :
• The most important point to remember here is to take extensive care while
working with H-Poles at both the ends
(Plant side and Substation side). All
material used in the system viz Fuses,
CT/PT/ Disc insulators, lightening arrestors ,earthling mechanism etc. must be
of high quality and of specific ratings
as any minor fault in this system may
lead huge power generation loss.
• It is recommended to ensure the availability of distance protection relay at
either side of the project.
• Conductors/ earthling wire /Transmission tower angles & Braces must
be properly tightened with torque
energetica
india
· JUL14
solar power
PV Module Tilt Angle
Tilt angle
(Measured)
Spec.
Result
19.8°
20°±1
OK
20.6°
20°±1
OK
20.9°
20°±1
OK
18.1°
20°±1
NOT OK
22.2°
20°±2
NOT OK
Fig 4: Module tilt angle measurement (sampling must
be done continuously during project execution) ,Structure Must not be in Rectangular form and not in Trapezoidal form.
•
•
•
•
•
•
•
wrenches. Phase markings must be
provided on towers and H-Poles.
Isolator and Earth switch must be there
inclusive of Gang operating switch on
H-Pole switchyard.
All CTs (current transformer) & PTs (Potential Transformer) must be tested
thoroughly and must be compliance
with state utility.
All types of relays must be tested prior
to commission the plant.
The fuses to be used on H-Pole and
Grid (11/33/66 KV) must be a pure
AlNiCo fuse with specified ratings AS
PER DESIGNS.
All recording meters must be kept safe
in box in the switch yard.
Ensure Phase sequence matching at
each terminal from substation side to
plant side before commissioning itself
Ensure if the arrived electrical equipments are as per rated designs or not
with reference to SLD.
energetica
india
· JUL14
• It is recommended to have a design
mechanism equipped with RMU (Ring
Main Unit, SF6 recommended) for big
plants to avoid generation loss during
failure in cables, transformers or during maintenances as well. RMU is best
recommended to provide redundancy
during faults and is used to provide
uninterrupted power distribution.
• It is must to take extensive care while
planning the schedule of cable lying
in trenches (say which cable will lay
first in the same trench), as unplanned
laying may impose unnecessary material losses and even create defects in
delicate communication cables.
• Ensure the availability of testing instruments like IR tester, Micro ohm
Meter, Clamp On Meter, Leakage
Clamp On Meter, 3 Ph Variac , Phase
sequence meter, & DC HV kit.
• The most important task is to ensure following tests on transformer
to be done strictly as per IEC 60050
Chapter 421 & IS 2026 etc. Viz. Insulation resistance (IR) measurement
BETWEEN HV TO LV, LV TO EARTH &
HV TO EARTH (IEC 60137) ; VOLTAGE
RATIO TEST; Excitation/Magnetizing
current test; Magnetic Balance Test;
Vector group Relationship; winding
resistance etc.
• IS 1225-1983 rules and guidelines
should be strictly followed during
power cables and communication
cables lying. LV/HV Cables must be
buried at depth of 1.5 b below the
ground, with proper brick work.
• IS 1225-1983 ;clause 4.2.1.3 Route
indicators - Power cable route Indicators should be provided at an interval
not exceeding 200 M and also at turning points of the power cable route
wherever practicable.’
• Meggering of cables must be essentially done before laying and
connecting.
• Cables in trenches at road crossings
must be laid within Hume pipes (NP-2
grade) only.
• Always leave at least 1-1.5 Meter cable loop on both LV/HV Cables in case
to be used during any fault in future.
• Newly laid HV & LV cables after
straight through joint / end termination shall be tested as per IS 1255.
• If all HV/ LV and communication cables
are to be laid in same trench then the
minimum separation distance between
HT & LT cables should be 75-100 mm
strictly separated by brick wall to avoid
total harmonic distortions in signal
transmission. Communication cable
must be separated by a distance of
at least 400 mm and it should also be
separated by brick wall.
• All LV cables shall be fire resistant and
in accordance with IEC 60331.
• The depth of the cable trench must be
at least 1000-1200 mm.
• Try to avoid unnecessary joints in HT
Cables as they may get failed during
rainy season. Jointing must be done by
skilled engineers only.
• Lying of cables and dragging them
from cable drum must be done giving support (sand/plastic sheet etc.) to
the cables at ground to avoid cutting/
scratching of the cables.
• If you are using single core cable ,ensure color coding for separate phases
• Cable terminations should be done
by skilled engineers only, lugs should
be properly crimped. In cables with
shielding required, the shielding must
be done neatly.
• Insulation Resistance, High-Pot and
contact resistance test must be performed on the Bus Bar of HT breaker.
• HV test; IR test; Contact Resistance;
BREAKER TIMINGS ETC. tests must be
conducted on Breakers
Electronics Engineering:
• The inverters make should be finalized before confirming its temperature
coefficient and sustainability to the
weather conditions of the site. Also it
needs to be ensured that the vendor‘s
post installation service is satisfactory
or not.
• Inverters to be used must have AC/DC
isolation slot.
• It is must to check a few main specifications viz. reference voltage, conversion
efficiency, IP protection islanding mode
(compliance to IEEE 1547 & UL 1741
Safety standards) mechanism etc.
• Make sure to install GFDI Kit in inverters (Else As recommended by module
manufacturer) so as to avoid PID Loses
in future if any.
7
solar power
Fig 5: Cable terminations at inverter end.
• Make sure to design the inverter rooms
outdoor/indoor as per IP protection
standards specified for inverters and
other equipments.
• The AC supply of inverter is meant to
be kept un-interrupted during running,
hence suitable auxiliary supply system
needs to be ensured. You can use UPS
systems but the best recommended is
to use Inverter output itself to provide
auxiliary supply to the inverter (closed
loop system, tapping from LT of transformer for auxiliary supply) .
• There should be a gland plate at bottom of the inverter to support the cables at bus-bar. For convenience it is
recommended to use single core cables.
• The communication cables must be
CAT 6 OFC. Modes of the cables need
to be ensured for different purposes
to be used, viz. for CCTV it should be
single mode and for SCADA it must be
multimode.
• The data sending/receiving speed /
bandwidth should be proper enough
to handle required data transmission
on remote access as well.
• The jointing /splicing of the OFC cables should be done very precisely and
by skilled engineers only, try to avoid
the number of joints in OFC cables as
much as possible.
• Try to avoid the bending of OFC cables
as much as possible; it should be done
within permissible radii specified by
manufacturer only.
• Signal cables shall be routed separate
from HV, MV and LV cables. Minimum
distance should be 600mm to HV and
MV cables and 300mm to LV cables.
Instrument cables and communication cables can generally be routed
together. Signal cables crossing at a
right angle with LV cables is acceptable
without further segregation.
8
Solar PV projects are
always thought to be
simplest power projects
to construct; people
often perform project
activities as taken for
granted and does not
give much attention to
the execution work
• The saving memory of data should be
enough to save at least one year data
(CCTV data can be rewrite on every 15
days interval).
• SCADA must be designed for string
level monitoring; if string is 2x1 type,
then separate monitoring should be
provided by SCADA System for single
string.
• The SCADA application enables dynamic configuration, stand-alone and
client-server configurations, redundancy for data protection, and historical
and real-time trends analysis, as well
as advanced alarm management.
• The support of such protocols as IEC
61850 and DNP3 are considered an asset if you have to communicate with
various electric substation devices.
• Weather monitoring station must be
well equipped with sensors and gauges like pyranometer (both on horizontal tilt and POA tilt), anemometer, temperature sensor (ambient & module ) ,
rain gauge ,hygrometer, air mass sensor etc.
Earthing /Grounding of AC/DC &
Communication System
Photovoltaic (PV) power systems are capable of producing hazardous voltages and
Currents for decades. To ensure the safety
of the public for these extended periods
of time, PV systems must be properly designed and installed with respect to earthling/grounding using the highest standards of workmanship.
• The overall grid/mesh in DC FIELD must
possess earthling value less than 1 ohm
as per IEEE-80 equations.
• Yet earthling values are dependent on
soil resistivity ,soil strata varies at different geographies , even though the
design does not recommends the termination of DC Field earthing mesh ;it
is highly recommended to terminate
the earthing mesh at different places
in dedicated earthing pits (earth rods)
• Earthing design must follow the practices strictly mentioned in IS 3043.
• In case you are using RS-485 cables
for communication (say different JBs
are in communication loop), ground
the shielding at one end only keeping
the other end ungrounded so that the
noise if any will not travel in a continuous path.
• Make sure to provide separate earthing mechanism of String Combiner
Boxes, if possible try to provide separate earthing for Power and Communication elements.
• It is highly recommended to ground
the shielding strip of HT Cable at secondary end and not primary ; where
the HT Breaker is connected , so that in
case any graund fault occur it may not
flow via transformer itself .
• Neutral of transformer (where ever
star connection is made, mostly secondary) must be grounded to avoid
unbalancing of phase voltages and
energetica
india
· JUL14
solar power
•
•
•
•
•
•
•
•
•
•
•
•
thus causing major faults.Also the
body of transformer must be grounded.Neutral and ground on the transformet can be tied together.
Body of HT Breakers along with neutral must also be grounded.
Avoid connecting neutrals of different
transformers to common ground, it
may violate.
Ensure dedicated earthling pits to be
drawn for inverters and transformers,
carbon black paint should be applied
where Earth strips are welded /jointed
in mesh.
All module frames and mounting
racks must be properly grounded. The
grounding wire must be
-- Properly fastened to the module
frame to assure good electrical
contact. Use the recommended
-- type, or an equivalent, connector
for this wire.
If the support structure is made of
metal, the surface of the structure
must be electroplated and
-- have excellent conductivity. The
grounding conductor must then
make a connection to earth using
a suitable earth ground electrode.
We recommend the lay-in lug
when grounding. The rack must
also be grounded unless it is mechanically connected by nuts and
bolts to the grounded PV modules.
Proper grounding is achieved by connecting the module frame(s) and
structural members continuously using a suitable grounding conductor
It is recommended to use maintenance free earthing system inclusive
of minimum 15 mm dia. 3000 mm
long cu rod.
Earthing of SCADA system should be
done separately.
Separate dedicated earthing Pits are
recommended to each inverter in the
system.
All transmission line lattice towers
must be equipped with counter poise
earthing system.
If simple pole tower structure is there,
earthing rod can be buried nearby
and connected with the tower leg.
Separate dedicated earthling pits
must be provided for Main H-Pole in
switch yard. The earthling grid must
energetica
india
· JUL14
Fig: 6 Earthing values in permissible limits at various terminals in PV System.
be made up of 50X6 MM aluminum
strips.
• The fencing of switchyard and other
transformer yards must also be earthed
connected to nearby earthling pit.
• All Lightening arrestors must be of
ESE (Early Streamer Emission) type
and must be provided with dedicated
earthing mechanism.
Miscellaneous
• Before deputing the engineers and supervisors to start work, a firm training
should be delivered them regarding
what things to be kept in mind regarding each and every respective job.
• Every deputed engineer/ in-charge
must be having handy drawings during execution.
• Any changes deviation from drawing
in execution must be done within permissible tolerance limit.
• Since solar projects are fast pace projects, proper lighting arrangements
should be done for working during
night as well.
• If the project plot is having good
contour with less frequent rainfall,
it is recommended to plan well designed Rain Water Harvesting system,
harvested water post treated with
RO Plant can be used to clean PV
Modules.
• It is must to construct proper drainage system otherwise Water may accumulate in the plant premises during
heavy rain and may cause hazards.
Conclusion
Solar PV projects are always thought to
be simplest power projects to construct;
people often perform project activities as
taken for granted and does not give much
attention to the execution work. This paper very strongly concludes that SPV project execution should not be taken in very
lightly. Be specific and précised while finalizing each activity. The points drawn &
recommendations made here are not limited to, activity engineers are ought to be
much focused and knowledge oriented
about their respective field 7
9