System Operating Voltage for Cat® PVT110, PVT115, PVT117, and PVT120 Photovoltaic Module {1471} Caterpillar

Photovoltaic

PVT110 (S/N: TF21-UP)

PVT115 (S/N: TF51-UP)

PVT117 (S/N: TF71-UP)

PVT120 (S/N: GH81-UP; TF81-UP)

Introduction

The National Electrical Code (NEC) defines that the maximum system voltage for a Photovoltaic (PV) System is to be determined using the open circuit voltage (V_OC) of PV modules. Due to the performance characteristics of Cat PVT 110 & PVT 110 ARC Series thin film modules, increased open circuit voltage may occur at low operating temperatures. This document provides a summary of the method used to calculate a low temperature correction factor to be applied to open circuit voltage for Cat PVT 110 & PVT 110 ARC Series thin film photovoltaic modules.

Do not perform any procedure in this Special Instruction until you have read the information and you understand the information.

Safety Section

Cleaning activities create risk of damage to the modules and array components, as well as the potential for electric shock

Only properly trained personnel who understand the risks of applying water to electrical components should clean modules. Trained personnel shall wear appropriate electrically insulating Personal Protective Equipment (PPE) during cleaning, inspection operations, or when working near modules.

Professional cleaning services trained to work on live electrical systems are available for hire.

The module encapsulating material and glass provide protection from electrical shock hazard when intact. However, when cracked or chipped, the module’s integrity is compromised (i.e., potential loss of electrical insulating properties,) thereby creating an electrical shock hazard.

Note: Cracked or broken modules represent a shock hazard due to leakage currents, and the risk of shock is increased when modules are wet. Before cleaning, thoroughly inspect modules for cracks, damage, and loose connections.

Note: The voltage and current present in an array during daylight hours are sufficient to cause a lethal electrical shock.

Acceptable module cleaning methods include spraying the modules with low-pressure water that is closely matched in temperature to the temperature of the module or to use a dry brushing technique. The following guidelines minimize impact to plant power generation, reduce safety hazards, and minimize risk of module damage.

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Contact with loose connectors can result in electrical shock and may cause injury or death.

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Do not operate or work on this product unless you have read and understood the instruction and warnings in the relevant Operation and Maintenance Manuals and relevant service literature. Failure to follow the instructions or heed the warnings could result in injury or death. Proper care is your responsibility.

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Before servicing/performing maintenance on the machine, electrical power must be physically disconnected; battery plugs must be disconnected from the batteries, or the trailing cable must be unplugged, and warning tags and padlocks shall be applied by a certified electrician. Certified electricians shall perform or direct any electrical work, including any energized testing, repair work in controllers, motors, or other approved compartments, and shall insure that all compartments are properly closed and inspected prior to re-energization. All applicable lock out and tag out procedures must be followed.

Discussion

Article 690.7 of the NEC directs the maximum system voltage to be calculated as:

1. The sum of the rated open-circuit voltage of the series-connected photovoltaic modules corrected for the lowest expected ambient temperature.

2. Where other than crystalline or multicrystalline silicon photovoltaic modules are used, the system voltage adjustment shall be made in accordance with the manufacturer’s instructions.

In the 2011 version of the NEC, Article 690.7 added a clause to further define the calculation method to be used:

1. When open-circuit voltage temperature coefficients are supplied in the instructions for listed PV modules, they shall be used to calculate the maximum photovoltaic system voltage as required

This document summarizes Cat manufacturer instructions to be used when calculating the maximum system voltage in compliance with the NEC (editions 2011 and earlier).

Effects of Temperature on V_OC

Illustration 1	g06363510

The temperature dependence of V_OC at low cell temperatures is weaker than that observed at the higher (typical) operating ranges traditionally used to determine temperature coefficients. This is apparent in the non-linearity of normalized V_OC vs. module temperature refer to Illustration 1. For this reason, applying a temperature coefficient derived from a linear regression to data over typical operating temperatures (example 25-50°C) will overestimate V_OC at low module temperatures. By deriving a temperature coefficient specifically across the coldest temperature range of -45°C to +25°C using a linear regression a more accurate approximation of cold temperature behavior is obtained.

V_oc Correction Factor

To determine the final factor to be used as the multiplying factor for V_oc per the method outlined in the 2011 edition of the NEC, the measured V_oc data in Illustration 1 was fit with a least-squares 4th order polynomial function. This function was then evaluated to obtain a factor for each of the temperature intervals below. The lowest temperature in each 5 degree interval was used in the calculation as a conservative assumption. To determine the corrected V_oc according to the 2011 NEC, apply the V_oc correction factor from Table 1 to the module’s V_oc at standard test conditions (STC), as given in the PVT 110 & 110 ARC Series datasheets

Conclusion

By applying the appropriate V_oc correction factor from Table 1 to a module’s nameplate V_oc, Maximum System Voltage calculations compliant with the method defined in the 2011 edition of the NEC can be performed.