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ALLTEC Protection Pyramid™ for Photovoltaic Solar Field

Photovoltaic systems are inherently exposed to both direct and indirect lightning events. For a large capacity solar farm, the installation of solar arrays requires extensive open area. Solar array panels and infrastructure are extremely exposed to lightning from thunderstorms and potential damage to both solar panels and critical equipment.

Direct lightning events are not only a threat to these photovoltaic (PV) facilities, but also indirect lightning events which generate an electromagnetic force, induced overvoltage, and transients upon AC and DC conductors. These events disrupt the operation of critical electronic equipment in solar modules, inverters, battery chargers, and control circuits. Since transient surges exhibit voltage amplitudes ranging from a few hundred to several thousand volts, there can be significant challenges associated with implementing a comprehensive grounding, and surge protection system throughout a facility.

1. TIER 1: Photovoltaic (PV) System Ground System Design and Analysis

The ground system of a PV field should be designed for effective dissipation of lightning energy as well as fault current at the inverter stations. In addition, the ground system design and analysis should identify the size and spacing of cables and ground rods for the plant grounding grid, as well as touch and step potentials. Proper and in-depth soil resistivity measurement is the first step for effective design of a PV grounding system.

1.1 Soil Resistivity Measurement

Often, at a site where a grounding system is to be installed, extensive civil engineering work must be carried out. This work usually involves geological analysis, which results in considerable information on the nature and configuration of the site soil. Regarding bore sample resistance testing, the determination of soil resistivity value from a laboratory is not recommended since the unknown interfacial resistance of the soil sample and the electrode are included in the measured value [Ref. Section 7.2, IEEE Std. 81-1983].

To properly determine the resistivity of the soil, all soil resistivity field testing should be conducted with the Wenner 4-point method corresponding to IEEE Std. 81. The Wenner 4-point measurement test employs 4 test probes, spaced apart from each other at equal distances. It is considered to be the most reliable soil resistivity test available today.

Alltec personnel can conduct a site visit with in-depth soil resistivity testing including both medium and short run spacing. In this sample case, a total of twenty-four (24) tests were performed within the entire solar field, with test spacing from 6-inches to 120 feet. The equipment used for the tests was a special direct-current (DC) induced polarization (IP) geological meter, with an 800-volt p-p and 500mA power supply.

Soil Resistivity Measurement

Fig: Soil Resistivity Measurement Setup with Special Instrument and External 12V Battery Source

1.2 Ground System Design and Analysis

The Solar Field Grounding System was modeled in integrated grounding software. Using the validated model, the plant grounding system was analyzed to determine whether it meets the safety requirements of the IEEE 80 Standard. The safety evaluation is based on the analysis of the plant grounding system performance under worst case fault conditions at the inverter station. Using the computer model, a comprehensive fault analysis was performed to determine the fault that causes maximum ground potential rise at the solar field. Safety step and touch voltage evaluation is performed for the entire solar farm including solar arrays, inverter/transformer stations, and perimeter fence layout.

Touch Voltage Analysis
Step Voltage Analysis

2. TIER 2: Surge Protection System

In addition to proper grounding and bonding, it is crucial to install both AC and DC surge protection devices (SPD) at key points throughout a solar facility to adequately protect panel module circuits, inverter stations, and critical control circuits at the combining switchgear box.

It is strongly recommended that a comprehensive network of quality SPDs be installed throughout the solar farm’s AC and DC power distribution to protect critical circuits against hardware damage and from operational disruptions resulting from lightning and non-lightning related transients.

Transient surges are categorized as either externally generated impulses; a lightning induced electrical energy burst, for example, and as internal switching transient irregularities.  A decaying oscillatory ringing transient attributed to power factor correction activities is an example of the latter anomaly.

Although lightning generated transients are, by far, the most intense surge events, surges originating elsewhere can be at high voltage levels, as well. For example, lightning induced surges initiated by direct lightning strikes can produce momentary voltages up to 75 kV at their point of impact. Indirectly coupled surge impulses originating from nearby lightning activity, on the other hand, generate voltage bursts up to 25 kV. Power source generated surge activity initiated by power factor correction capacitors and from load shedding operations can produce surge voltages up to 15 kV. Internally generated capacitive load generated transient activity, ground potential differences, and the power cycling of inductive loads can each produce surge voltages as high as 10kV.

A cascaded network of DC SPDs, designed to work in tandem with each other, should be installed on combining equipment per line and on the input of the inverter circuit (Alltec Model: AD-PM50-600-3C). In addition, it is recommended to install AC SPDs on the output of the inverter (Alltec Model: ADSx2F-200-480W), as well as, medium voltage input to the step-up transformer in the photovoltaic combining switchgear (Alltec Model: AMV13800Y). Any medium voltage transformer load that has either suffered from premature insulation breakdowns, or is deemed to be vulnerable to this type of damage, should be individually protected by quality stand-alone Station Class arrestor fortified SPDs – even if they are already equipped with Normal or Heavy Duty class surge arresters.

 

In addition, critical data circuits and UPS in the switchgear room should be protected with correctly coordinated AC SPDs based on location and configuration. These SPDs should incorporate common mode suppression components to shunt lightning induced surge current to the electrical distribution’s ground circuit. They should also employ normal mode suppression circuits to distribute internally generated surge current between power phases, and between the phase and neutral conductors on the AC power service. They should be equipped with a first order noise filter to attenuate disrupting transient related noise voltages to equipment safe levels. Careful consideration should be given regarding protecting equipment control and data interfaces wherever operational disruptions would cause significant downtime or work disruptions. At the very least, all control and data lines entering and exiting a structure should incorporate proper SPD protection.

3. TIER 3: Lightning Risk Analysis and Structural Lightning Protection System

The National Fire Protection Association NFPA 780 and International Electro-Technical Commission (IEC)-62305 standards refer to lightning risk analysis in order to establish a technical compliance base for selection of levels and requirements of lightning protection systems. In addition, it is part of the concerned authority’s responsibility to determine by cost-benefit analysis or other method of consideration if possible damage to the installed solar panels, inverter station, and photovoltaic combining switchgear (PVCS) justifies the installation of a code compliant lightning protection system to prevent potential damage. The probability of direct lightning strike to the solar farm is attributed to the regional isokeraunic level (number of flashes/sq.km/year) and the facility’s collection volume.

The risk associated with loss of service should be of primary concern in determining the need for an effective lightning protection system for any solar facility. NFPA 780 Annex L (Table L.5.1.2) notes “continuity of facility service” as one of the major consequences in its simplified risk analysis, unfortunately, no specific loss component values are referenced in the detailed risk analysis for any facility where continuity of service is required.

In any instance where possible damage to any installed system (either ground mounted solar panel units and/or inverter station/PVCS stations) is considered as “unacceptable loss of service”, it is recommended that International Electro Technical Commission (IEC) 62305 2010-2 Table C.8 should be considered to select the typical loss values for power supply.

Alltec Lightning Risk Assessment for large solar array panels with multiples inverters and PVCS stations reflects the necessity of an effective lightning protection system for 100% continuous service from all installed components.

3. Lightning Protection System (LPS) Design

External lightning protection system design & installation is not a common practice for solar farms. The lightning protection of solar arrays by traditional franklin air terminals is still subject to concern regarding its effectiveness, shadow effect, installation cost, and appearance. Although the NFPA 780-2014 edition provides guidelines for lightning protection requirements for roof-mounted or ground-mounted solar (photovoltaic and thermal) panels, it will take some time for industry to embrace the advancing change to current practice.

As an alternative to the traditional franklin rod system, Early Streamer Emitter (ESE) type lightning air terminal may be utilized to protect a solar farm area. The TerraStreamer® ESE terminal provides large protection zones as per National French standard NF C 17-102. TerraStreamer® ESE terminals can be installed on the independent masts with radial lightning protection ground systems with single point reference to the electrical grid. This configuration provides necessary protection zones, effectively dissipate the lightning energy to ground, and helps to insulate the solar panels and inverter/PVCS stations from damage. The other benefit of this system is that there will likely be significantly lower cost of labor, and material to install the entire system as compared to traditional franklin rod system.

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ALLTEC offers a strategic approach to meeting our clients needs, the ALLTEC Protection Pyramid™. This approach looks at all aspects of a facility and works in a holistic fashion to make sure all areas are protected with an effectively interlocking defense. If you have an existing or planned facility, be sure to give ALLTEC a call or email. We can make sure your operation is the safest possible facility, backed by our team of dedicated risk-mitigation experts. If you have a current facility or future project that needs grounding/bonding solutions, surge suppression or lightning protection please contact ALLTEC at either 1-828-646-9290 or online-info@alltecglobal.com

Rev. 20150226

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