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Solar PV system Grounding, Faults and Protection

Proper earthing/Grounding of the solar PV system is very important for safety and reliability. Earthing of the PV system, fault current calculation and protections are related. Solar PV power generation is different from rest of the power generation. Solar PV System is spread over large areas, Solar PV has a mix of  AC DC system and PV cell does not store energy like a battery, or a AC generator. Grounding is required for Lightning protection, Fault current Path and detection, Equipotential bonding, Prevent of Corrosion and to carry leakage current

Different Types of Grounding are Protective earthing, System Grounding/Earthing, DC unearthed system, DC earthed system, Positive pole grounded, Negative pole grounded, Mid point grounded (Very rare), Solid earthed, Resistance earthed and Active grounding in PV system. Earthing in PV system depends of the type of cells.

Combiner box receives both positive and negative cables from each string. These box will also have fuse. Earthing conductor of recommended size has to be connected between this earthing bolt and the plant earth grid.

SOLARLOK 5-String Combiner Box

Picture courtesy: TE Connectivity Ltd 

SolKlip Grounding Clips are used for grounding of Solar Panel using 10 AWG/6 Sqmm or 12 AWG / 4 Sqmm bare copper wire. Ground Bolts made of Stainless steel is used to connect sold copper wire 6AWG (16 Sqmm) to 12 AWG (4 Sqmm). The bolts earth the aluminium frame of the module.

 

Picture courtesy: TE Connectivity Ltd 

EATON Crouse Hinds series lugs

Picture Courtecy : Eaton.com

Deterioration of the module back cover, failure of insulation in cable, Rat bites, Plastic material exposure to UV rays, pollution deposit, rain, bad ingress protection are some of the causes of the fault.

The Magnitude of short circuit current in a PV system is very close to the load current.

Fault in an Ungrounded PV system will also produce a fault current of small magnitudes which can also cause fire and safety hazard for people if left undetected. In a grounded PV system, most of the string current from healthy module will get diverted to the fault location.

PV System protection is different from the AC power system because the PV system consist of DC power from PV modules, Inverter and Step up transformer with Grid.

Eaton, BRL215CAF, 2 pole breakers with inbuilt AFCI (AC or DC)

(Picture courtesy: EATON)

Insulation monitoring is continuous process done by certain devises. These devises measure the insulation resistance continuously.

Picture Courtecy : Bender GmbH & Co. KG
(www.bender.de)

Insulation measurement is done intermittently for a grounded system.  Insulation resistance measurement can only be done when the system is shutdown unlike insulation monitoring which is down when system is running.

Picture Courtesy: KYORITSU Electrical Instruments Works Ltd
www.kew-ltd.co.jp/en

RCD (Residual current device) and RCMU (Residual current monitoring unit) are used to trip an inverter in case of excessive leakage currents in transformer less system and for protection of the distribution system fed from solar PV system.

Picture Courtesy: Littelfuse, Inc
www.littelfuse.com

Sensing and measuring DC current is done with help of hall effect current transducers.

Picture Courtecy: Electrohms Pvt Ltd
www.electrohms.com

Strings, sub arrays and arrays can also be protected by DC circuit breakers instead of fuse. Combiner box will house a circuit breaker or isolator instead of the fuse.

OTDC
800...1600 A
HPMCB
S800
MCCB Tmax
T PV

Picture Courtecy: ABB

Combiner box are just like a distribution board or a small panel. Circuit breakers or fuse are located inside the combiner box.

Picture Courtesy: ABB

In ungrounded DC system the fault current magnitude will be very less. It is not possible to detect and clear fault with fuse. Special sensitive relays are required.

Picture Courtecy: Littelfuse
www.littelfuse.com

Fire in Solar PV systems like any other electrical fire has chances of electrocution to fire fighters. There is additional risk because the plant cannot be shut down like any other power plant. Solutions such as PVSTOP are available to mitigate these risks.   PVSTOP is a non-conductive, non-flammable polymer coating that is sprayed on to the PV panels to make the panel face opaque which can later be removed without damaging the PV panel. This stops the generation of power and thus cuts power at the source.

Pictures Courtesy: PVSTOP
www.pvstop.com.au

When there is a fault, the current flows into the ground and raises the potential of the non-conducting metal parts.  When a person comes in contact with these metal parts, current will enter the body. If the Metal part is grounded, the magnitude of current entering person will be lesser.

Detailed guidelines on Solar PV system Grounding, Faults and Protection consist of 28 chapters with 73 pages. This detailed guideline gives many practical examples, calculations, illustration which will help an application engineer to actually design a plant. This full guideline can be accessed in below link.

1 Introduction
2 Scope
3 Exclusion
4 How is solar PV power generation different from rest
5 Why Grounding is required
6 Types of Grounding
6.1 Protective grounding
6.2 System grounding
6.3 DC unearthed system
6.4 DC earthed system
6.5 Negative pole grounding
6.6 Positive pole grounding
6.7 Potential induced degradation
6.8 TCO Degradation
6.9 Surface polarisation
6.10 PV offset box
6.11 Detection of PID
6.12 Resistance earthed
6.13 Midpoint Grounding
6.14 Transformer Less Inverter and Grounding
6.15 Grounding Kit
6.16 Earthing at inverter side or PV side
7 What is Grounded in a PV system
7.1 Junction box
7.2 Connector
7.3 Module frame and support Grounding
7.4 Cable armour grounding
7.5 Bonding
7.6 Combiner box
7.7 Cable routing
7.8 Interconnection of Solar PV system and AC substation earthing
8 Grounding Accessories
8.1 Ground clips
8.2 Ground Bolt
8.3 Grounding lugs (Lay in type)
8.4 Grounding lugs (Crimping type)
9 Faults
9.1 Cause of faults in Solar PV system
9.2 Different places where fault occurs
9.3 Different types of fault
9.4 Short circuit current in PV system
9.5 DC and AC Arc
9.6 Series fault and parallel arc fault
9.7 PV module equivalent circuit
9.8 Fault current in Ungrounded PV system
9.9 Current Back feed or Reverse current in PV system
9.10 Fault current in Grounded PV system
9.11 Blindspot Fault
9.12 AC Side fault and PV Contribution
10 Fault current in PV System
10.1 Fault current in single string
10.2 Fault current in solar power plant
10.3 Fault current Calculation
11 Earthing of PV system with inverter built in module
12 Protection
12.1 Different devises to detect faults
12.2 GFPD
12.3 AFCI
12.4 Insulation monitoring
12.5 Insulation Measurement
12.6 RCMU and RCD
12.7 IDMT Earth fault relay (51G) for AC system fed from PV System
12.8 DC differential relay
12.9 Fuse
12.10 Fuse sizing and selection
12.11 Fuse coordination in PV system.
12.12 SSTDR
12.13 DC fault detection and Hall effect current sensor
12.14 Circuit breakers and Switch Disconnectors
12.15 Combiner box
12.16 DC ground fault monitoring in ungrounded solar PV system
12.17 Blocking diode
12.18 Reverse current overload
12.19 Does protection stop the fault current completely
12.20 Fault current carrying capability of PV system components
12.21 Line to Line Fault
12.22 Protection system application in PV system versus AC system
12.23 PV cell over voltage protection
13 Sizing of earthing conductor
14 PV cell characterises with respect to temperature
15 Sizing of Power cable
16 Fault current Calculation in Solar PV system
17 Fire Safety
18 Electrical Safety
19 Earthing/Grounding of Racks with piles
20 Protection by Extra Low voltage
21 Corrosion
22 Leakage current
23 Parasitic capacitance
24 Typical PV Ratings
25 Lightning Arrestor and Surge Protection devise in Solar PV System
26 Measurement of resistivity
27 Installation of PV Panels in Petro chemical installations and Industries
28 Reference

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