Test Specification 5.02

5.02

TC5

MPPT accuracy under unbalanced/partial shading conditions

During the operation of a PV system, it is possible for asymmetries in the operation of the PV array to happen. A case in point is the partial shading of PV modules. This results in a distorted I-V characteristic that leads to more than one maximum point in the P-V curve. The scope of this test is to assess the ability of the inverter, in particular the MPPT to detect the operation point that leads to the maximum power yield under such conditions.

The setup for this test is a reduced version of the previous test (efficiency test). For this test only the input (DC) power and voltage are necessary together with the detailed characteristic that the I-V tracer module provides. The components used in this test system are listed below:

• One LV AC bi-directional source that plays the role of the AC grid
• One PV array which supplies with DC current the PV inverter. The PV array’s behaviour is determined by the environmental conditions.
• One data logger device which is used to monitor and record the power values
• One I-V tracer module required for measuring the I-V characteristic of the array
• One transducer
• One PV inverter which is the System (Device) under Test

MPPT accuracy value(s)

Input parameters

• Solar irradiance (W/m2)
• Ambient temperature (oC)
• PV modules parameters (Voc(V), Vmp(V), Isc(A), Imp(A), CTI(A/oC), CTV(V/oC), Rs(Ω))
• PV array configuration

Output parameters

• Input (DC) power (W)
• I-V characteristic of the PV array

The test is organized and executed in two steps in which a stepwise change in irradiance conditions is implemented on part of the PV array. The behaviour of the MPPT is recorded before and after the change in order to provide useful information about its response to the unbalanced conditions. The test can be repeated several times for the same or different conditions for a better understanding of the MPPT behaviour.

• The PV inverter is connected to both the AC source and the PV array
• The PV module parameters are selected so that the power/voltage are well within the operating range
• The PV array is configured in a connectivity that allows power/voltage values well within the operating range
• The PV array receives sufficient irradiance produce a voltage/power level within the required limits
• The inverter is in stable operation and injects power to the AC source
• The monitoring system is initialized and running

• The irradiance on a selected number of PV modules changes stepwise from the initial to a lower value. The final value can be equal or not for all the shaded modules
• The inverter stabilizes at a new operating point
• The DC power and voltage are recorded with a sampling time of <=1 s and for at least 10 s. Also, the I-V characteristic is recorded
• The above steps are repeated as many times as the different partial shading combinations are

N/A

A sampling time of <=1 s for the instantaneous values is sufficient for this test to provide the necessary information since the values of all quantities are averaged within a 10-second interval

Uncertainties may be introduced due to the controllability of irradiance and temperature or the stability of the MPPT algorithm under the specific test conditions.

The test should be suspended and restarted if one of the quality attributes described in the TC is not met. Also, stability should be ensured during each measurement. If for some reason (e.g., irradiance variations, severe MPPT oscillations) there is not stable operation of the inverter the test results should be partly or completely discarded.