Inspection of String Circuit Current Tests for Solar PV Systems
Diagram 1 shows IV diagram of the power generation area. An IV curve is a curve drawn on a graph that measures the current-voltage characteristics of a PV cell and takes current on the vertical axis and voltage on the horizontal axis. Using the obtained IV curve, abnormalities in power generation can be identified. Here are some terms that are used in the IV curve's diagram.
- Open-circuit Voltage (Voc): Voltage when the solar panel is not carrying current.
- Short-circuit current (Isc): Current flowing when the negative and positive electrodes of the solar cell are short-circuited.
- Maximum Power Point (Pmax): The maximum value of the product of current and voltage on the IV curve. The inverter is controlled so that the solar cell always operates at this point.
- Voltage at Maximum Power (Vpm): Voltage at maximum operating point.
- Current at Maximum Power (Ipm): Current at maximum operating point.
String current test according to IEC62446-1 standard
The standard IEC62446-1 describes the measurement of string currents in photovoltaic systems. This test verifies the functionality of strings and that no significant issues exist. For PV string current tests, there are short-circuit and operational current tests.
String short-circuit current test
The short-circuit current of a string, Isc is the current that flows when the positive and negative terminals of the string are shorted together, and is the maximum current value of the string.
When a solar panel is connected to a device such as an inverter or solar charge controller, the Isc value is used to determine the maximum amount of current the panel can take. Due to the risk of flying arcs, direct measurement using the current terminal of a digital multimeter (DMM) is not recommended.
An AC/DC clamp meter can be used to measure the Isc of the PV module. However, solar cells have a high measured current when solar light is present and a high voltage when a large number of cells are connected in series, which can be dangerous. To short-circuit solar cells, it is necessary to use the right tools, such as high-capacity circuit breakers.
With the Diode Bypass Tester FT4310, you can measure Isc without the need for a circuit breaker, together with the bypass diode open test.
The test can be done at the strings of the junction boxes, so there is no need to walk up to the roof. This makes work much more efficient. However, you need to disconnect the string being measured from the interconnect prior to measurement.
String operational current test
Measurement of the operating current of the string is performed at the time of inspection of the string. By measuring the operational current, it is possible to check if the string is in the operating state. For a system with multiple strings composed of the same number and the same type of modules, the current values of the strings are almost the same. If a string has an anomaly, the current value is significantly different from the rest of the strings. By comparing the current values of each string, it is possible to identify an irregularity.
Installing the Wireless Adapter Z3210 on an AC/DC Clamp Meter from Hioki allows Bluetooth® connections. By using a free application called GENNECT Cross, you may evaluate measurement results using the comparison feature. The string operation's current can be easily judged by this function.
The value of Ipm, which is the highest output operating point when the inverter is operating, is also important. This value can also be measured by using a clamp meter.
Instruments used for string current measurement
| Product model | Isc measurement | String operational current and Ipm | Max. Current | Resolution | Max. Voltage | Compatible with the DC High Voltage Probe | Bluetooth® Communications |
|---|---|---|---|---|---|---|---|
| Bypass diode tester FT4310 | ✔ | - | 15 A | 0.1A | 1000 V | - | ✔ |
| AC/DC Clamp Meter CM4371-50 | ✔ | ✔ | 600 A | 0.01A | 1000 V | ✔ | *✔ |
| AC/DC Clamp Meter CM4375-50 | ✔ | ✔ | 1000 A | 0.1A | 1000 V | ✔ | *✔ |
| AC/DC Clamp Meter CM4373-50 | ✔ | ✔ | 2000 A | 0.1A | 1000 V | ✔ | *✔ |
- *With the Wireless Adapter Z3210
BYPASS DIODE TESTER FT4310
AC/DC CLAMP METER CM4371-50
AC/DC CLAMP METER CM4373-50
AC/DC CLAMP METER CM4375-50
In addition to measuring current, a clamp meter can also measure voltage. It is capable of measuring both the open-circuit voltage, Voc and the voltage at the inverter's maximum output operating point, Vpm. The DC High Voltage Probe P2000 can be used to measure up to DC 2000 V. With this, high-voltage PV systems may be measured with ease. Also, the above-mentioned instruments can send the measurement results to mobile devices using Bluetooth®. This saves time and effort when making a report.
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