To verify the correct operation of the module chains, current-voltage characteristics are determined. For measuring the current-voltage characteristics, specialized instruments are used, such as the Metrel MI 3108 Eurotest PV multifunction meter, which is shown in Fig. 1.
The meter allows for testing photovoltaic installations according to the PN-EN 62446 standard, which at the same time meet the requirements of the PN-EN 61557 and PN-EN 61010 standards. The instrument combines the functions of an electrical installation parameter meter and photovoltaic installation tester. It allows, among others, to create current-voltage characteristics, to convert the parameters to STC values (standard test conditions) and to measure the power on AC and DC sides of the inverter. An important aspect in the measurement of PV installations is the simultaneous measurement of environmental parameters such as panel temperature and solar radiation intensity.

Measurement of continuity of protective and equipotential connections

In accordance with PN-EN 62446-1 standard, the continuity measurement of protective (earthing) and equipotential bonding is recommended first. In Fig. 2 a scheme for measuring the continuity of protective connections using the MI 3108 Eurotest PV instrument, which was connected to the PV panel frame and to the inverter housing, is shown.

Testing the polarity of the wires and strings

An important step in verifying the correct operation of the PV installation is to test the polarity of the wires and to check the correct connection of the module strings. The procedure for the polarity test is described in detail in the standard PN-EN 62446-1, while the wiring diagram of the MI 3109 Eurotest PV meter is presented in Fig. 3. The polarity measurement consists of connecting all wires of negative polarity and checking the voltages between all wires of positive polarity to the first reference wire in turn. The resulting voltage value of approximately 0 V indicates correct connections, and twice the voltage rating will indicate a reverse connection.

Open circuit voltage measurement \( V_{oc} \)

Measurement of the open circuit voltage \( V_{oc} \) is one of the most important tests that allows to quickly verify the correctness of the connections of the modules in the chain. The measurement is carried out using the MI 3109 Eurotest PV meter with the following connection diagram ( Fig. 4 ). When measuring \( V_{oc} \), the PV system must not be connected to the load and the meter is connected to the wires coming out of the PV modules.
The voltage value of a given string allows you to verify that the right number of modules are connected in the right way. The ability to enter environmental parameter values into the meter allows the open circuit voltage value to be converted to STC conditions. In the test, either the data from the module data sheet or the measured values \( V_{oc} \) for a single module are taken as reference values. Obtained results that deviate from expected results may suggest:

• incorrect connection of system components such as missing connection or reversed polarity of modules,
• short-circuit of some bypass diodes,
• damage to the insulation or the insulation of the connecting components.

Measurement of short circuit current Isc or operating current

The test recommended by the PN-EN 62446-1 standard is the measurement of the short-circuit current \( I_{sc} \), which can be performed with the MI 3108 meter. It is also possible to measure the operating current using the Metrel A 1391 clamps. On Fig. 5 there is a diagram of how to connect the meter to the PV installation in order to measure the DC current. The short-circuit current is measured by connecting the wires coming out of the PV module to each other and putting the clamps on the wire. On the other hand, the operating current is measured while the installation is running.

Current-voltage I(U) characteristics test

An important step in verifying the correct operation of a photovoltaic installation is to check the current-voltage characteristics of the module chains. This is an extended version of measurements \( V_{oc} \) and \( I_{sc} \), whereby plotting the entire I(U) characteristic makes it possible to provide information about the voltage of the maximum power point \( V_{sc} \), the current of the maximum power point \( I_{mpp} \) and the maximum power obtained just at that point. By using the solar irradiance meter in the measurement, the current-voltage characteristic curve can be recalculated to STC conditions, as a result of which it is possible to make a real comparison of the efficiency of the installation with the manufacturer's catalog data. The shape of the characteristics also allows to diagnose errors in the operation of the installation. The changes of the I(U) characteristic and the factors causing the anomalies are shown in Fig. 6 and Fig. 7.

In the test it is important to determine the current value of solar irradiance in the plane of the modules. This is important because the PN-EN 62446-1 standard defines the minimum irradiance at which the current-voltage characteristics test is reliable and can be converted to STC conditions. In case of using Metrel devices, radiation intensity should be above 500 \( \frac{W}{m^{2}} \). PN-EN 61829 standard defines the way of performing an effectiveness test of photovoltaic modules and an exact conversion of measurements to STC conditions defining the minimum value of solar radiation intensity as 700 \( \frac{W}{m^{2}} \). Solar irradiance testing is possible in several ways, namely using Metrel's A 1378 PV Remote Unit external temperature and solar irradiance recorder, or using the pyranometer shown in Fig. 8.

In measuring environmental parameters, it is important that the solar intensity sensor be placed in the plane of the module so that it is not shaded or exposed to reflected light. The temperature sensor should be placed on the back of the module and closest to the center of either cell and module.