Reverse PV String Connection Scenario Analysis

Introduction

This article is mainly about the analysis on the reverse PV string connection scenario.

1. When the SUN2000 is grid-tied, do not maintain the DC input power cable, for example, connect or disconnect a PV string or a PV module in a PV string. Otherwise, electric shocks may occur.

2. If the DC input power cable is reversely connected, do not operate the 'DC SWITCH' and the positive and negative connectors immediately. Wait until the solar irradiance decreases at night and the PV string current decreases to less than 0.5 A. Then, Set the two 'DC SWITCH' to 'OFF', remove the positive and negative connectors, and correct the DC input power cable polarity.

Prerequisites

Taking the sun2000-50ktl-c1 as an example, the analysis process of various scenarios of series positive and negative pole reverse connection of the SUN2000 is as follows:

1. In the same MPPT route, the two strings are reversely connected.

2. In the same MPPT route, two strings are connected, one string is correct and the other string is in reverse.

3. In the same MPPT route, only one string is connected in the reverse direction.

The following figure shows the two PV strings and DC switch connected to the same MPPT circuit.

Scenario #1

Analysis: In the preceding figure, the two PV strings connected to the same MPPT are reverse. After the DC switch is turned on, each PV string forms a short-circuit loop with the IGBT inverted diode of the BOOST circuit through the DC switch, as shown by the red solid line in the figure. If the DC switch is turned off, as a result, the DC switch is arced and damaged, and the MPPT cannot work properly.

Note: In this scenario, if the DC terminal is directly removed and inserted, arcing occurs.

Handling suggestion: When the DC input power cable is reversely connected, do not operate the DC switch or the positive and negative connectors immediately. Wait until the solar irradiance decreases at night and the PV string current decreases to less than 0.5 A. Then turn off the two DC switches, remove the positive and negative connectors, and correct the polarity of the DC input power cable.

Scenario #2

Analysis: As shown in the preceding figure, two PV strings are connected to the same MPPT circuit. One is correct and the other is reverse. The two PV strings are short-circuited without passing through the DC switch, as shown by the red solid line in the figure. In this case, the input voltage is 0, the PV string current is the short-circuit current of the PV module, it has no damage to the inverter and damage to the PV module.

Note: In this scenario, because the two strings are directly short-circuited. If the DC terminals are directly removed and inserted, it will cause arcing and cause electric shock. However, operating the DC switch has no effect.

Handling suggestion: Do not operate the DC switch or the positive and negative connectors immediately if the DC input power cable is reversely connected. Wait until the solar irradiance decreases at night and the PV string current decreases to less than 0.5 A. Then turn off the two DC switches, remove the positive and negative connectors, and correct the polarity of the DC input power cable.

Scenario #3

Analysis: As shown in the preceding figure, one MPPT circuit is connected to only one string in the reverse direction. After the DC switch is turned on, the string and the IGBT diode of the BOOST circuit form a short-circuit loop through the DC switch, as shown by the red solid line in the figure. In this case, if the DC switch is turned off, the DC switch is arced and damaged, and the MPPT cannot work properly.

Note: In this scenario, if the DC terminal is directly removed and inserted, arcing will occur.

Handling suggestion: Do not operate the DC switch or the positive and negative connectors immediately if the DC input power cable is reversely connected. Wait until the solar irradiance decreases at night and the PV string current decreases to less than 0.5 A. Then turn off the two DC switches, remove the positive and negative connectors, and correct the polarity of the DC input power cable.

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