Surge Protection Devices(SPD) for Solar PV plant

Why SPD is required in Solar PV plant

• The  outdoor installation  location  of solar PV makes them directly exposed to harsh conditions like rain, wind and dust. Among the weather conditions lightning strikes require specific attention as they can severely affect the safety and performance of a PV plant 
  
• Then lightning strike hits the ground, it discharges energy, affecting the electrical field on the ground. For the solar PV plant this poses two risks:

1. A direct impact which can physically destroy the solar equipment on a rooftop
2. Transitory over voltages passing through cables by magnetic coupling, which can lead to the damage of sensitive components such as printed circuit boards (PCB).

• The potential impact of lightning events increases with PV system size. At locations with frequent lightning, unprotected PV systems will suffer repeated and significant damages. This results in substantial repair and replacement costs, system downtime and loss of revenue.
  
• The lightning arrester provides the  protection against direct impact of lightning as per IEC62305.
• For transitory over voltages(due to lightning surges that are produced or propagated by the building’s electrical system )SPD in parallel with the circuit to be protected is required to deliver the energy to the ground and limit the overvoltages.
  

• Surge Protective Devices (SPDs) provide protection against electrical surges and spikes, including those caused directly and indirectly by lightning.
• Properly installed surge protective devices (SPDs) will minimize the potential impact of lightning events.
• Sensitive electrical equipment's of PV system like AC/DC Inverter, monitoring devices and PV array must be protected by surge protective devices (SPD).

Location of SPD in solar PV plant

1. DC Protection at the Input to the Inverter

•     The inverter is one of the most expensive and fragile components of a PV system, and it must be protected from electrical surges originating at the solar panels or from within the feeder cables.

•  If the combiner box and inverter are more then 30 feet apart than an SPD is recommended at both ends of the run.
• In case of string inverter,The DC SPD is provided inside the inverter.In that case there is no need to install DC SPD at inverter Incoming side.

         2. DC Protection at the Combiner Box

• The combiner box is the ideal location for the first tier of protection. A correctly sized SPD can divert surges before they impact the switching function of the combiner box and prevent the surge from propagating down stream.

     3. AC Protection at the Output of the Inverter

• To protect the inverter from surges originating in the facility, an SPD is highly recommended on the output or AC side of the inverter.

     4. A high percentage of surges originate from utility power

• To protect the entire facility and the PV system, a service entrance SPD is essential as Per UL1449 3rd Edition, this SPD should be a UL Type 1 listed device installed on the line side of the main disconnect.
  

Selection of SPD ratings

• The Size, performance and specification of SPD depend on following characteristics

Current characteristic of SPD

• I:Surge Current Rating (KA),
• In: Nominal Discharge Current (In),
• Imax: Maximum discharge Current (Imax)

Voltage characteristic of SPD

• Uc: Maximum Continuous Operating Voltage (MCOV),
• Up: Voltage Protection Rating (VPR) or surge voltage rating (SVR) or Clamping Voltage
  

Types of SPD

Type 1 SPD

• Protection against transient overvoltages due to direct lightning strokes.
  
• If there is LA installed in the vicinity of Solar PV plant,installation of Type-I SPD is must.
  
•  It can discharge the voltage from lightning spreading from the earth conductor to the network conductors.
  

Type 1 SPD is characterized by a 10/350µs current wave.

Type 2 SPD

• Protection against transient overvoltages due to switching and indirect lightning strokes.
  
• The Type 2 SPD is the main protection system for all low voltage electrical installations.
  
• To protect the inverter from surges originating in the facility, Type-II SPD is highly recommended on the output or AC side of the inverter.

Type 2 SPD is characterized by an 8/20µs current wave.

Type 3 SPD

• Type3 SPD is used for local protection for sensitive loads and these SPDs have a low discharge capacity.
  
• The sensitive SCADA and monitoring system circuits  in Solar PV plant should be protected by Type-III SPD. 

Type 2 SPD is characterized by an 1.2/50µs current wave.

Type of losses in Solar PV plant & their % consideration in Simulation Softwares

A.     DC side Losses

1. Module array mismatch losses: Maximum 1% and we have to consider 0.2 to 0.3%. These losses is due to the performance difference in the module parameters. (Example: A same capacity module with the same tilt & irradiance falling on it will show the difference in current band voltage value.

     Suggested Values:

     2% for most modules and systems with long strings

    1% for modules that have tight wattage tolerances

    0% is automatically used on modules with DC optimizers or microinverters

1. Module quality loss:0.2 to 0.3% and it should be less than 1%.this is related to the module manufacturing process and its material chemical composition.
2. LID losses: Light-induced degradation (LID) is a less-well-known phenomenon that impacts a large segment of the crystalline-silicon cell market. In short, it is the degradation that occurs in a solar cell over the first few days after the installation as a result of exposure to sunlight. This can lead to losses of 0.5% - 1.5%.

• To understand the causes of LID, and why certain types of modules are affected, one must first understand two factors that differentiate solar cells: their crystal structure (monocrystalline or multi-crystalline) and their electrical properties (P-type or N-type).
• Crystal structure refers to differences in the structure of a solar cell resulting from how it is produced:
  Monocrystalline - solar cells that are grown using a process (the Czochralski process) that produces a uniform crystal structure that is sliced to make solar cells. These tend to have better electrical properties. They also tend to have somewhat higher oxygen concentrations, which is important for LID.
  Multicrystalline - solar cells that are produced by some form of vapor deposition, which grows silicon onto a substrate. These will have many crystalline sections, which show up as different reflective edges in a solar cell. These are less efficient at producing electricity compared to equivalently-sized monocrystalline cells but are cheaper and faster to produce. They also have less oxygen present in the material.
• Electrical properties refer to properties of silicon wafers (which make up a solar cell) that are needed to create a voltage the difference in the cell when exposed to sunlight:
• P-type: a p-type silicon wafer contains a controlled quantity of impurities, referred to as doping elements, that accept electrons more readily and let a PV module create a voltage difference to produce power under sunlight. Most p-type cells use boron as the doping element, while some others use gallium. Boron plays an important role in LID.
• N-type: these silicon wafers contain impurities that have the opposite effect; they release, rather than accept, electrons. N-type silicon wafers do not exhibit LID.
• LID is typically caused by the formation of boron-oxygen compounds in the silicon wafers that make up the solar cell. This means that monocrystalline solar cells that are p-type with boron will exhibit the most LID, and p-type multi-crystalline cells will also exhibit LID, but to a lesser extent due to a smaller oxygen concentration. The LID process is usually not accounted for in the lab testing of modules, so it won’t be included in the PV module datasheet.

         Suggested Values:

       1.5% for most crystalline solar modules

       0.5% for most multi-crystalline solar modules

       0% for n-type modules,

      3.  Losses due to temperature:8% to 15% as per the site condition.

     4. Soiling Losses: Maximum 3% and depends on the type of soil like loose, hard, soft, etc. and                type of installation like ground-mounted or rooftop.

    5.DC Ohmic Loss: Losses in DC cable from Module to inverter and it should be less than 2% and         it will depend on the size and length of cable.

B.      AC side Losses

1.Inverter efficiency losses:

1.5% but depends on atmospheric temperature and cooling arrangement for the inverter.

These losses can be observed up to 3% if proper ventilation is not available.

2. Transformer losses: It should be maximum of 2% and have two parts:

i. Core loss: Depends on the material.

ii. Copper loss: Depends on current and loading.

3.  AC ohmic losses: Losses due to AC cable voltage drop from the inverter to injection point and depends on the resistance and reactance of AC cable. These should be less than 3% or depends on the Distribution company or utility rules and regulations. These losses depend on reactance and resistance values and length of cable