Experiment Specification 5.01.01

5.01.01

TS5.01 and TS5.02

Power efficiency and MPPT accuracy under various operating conditions

PV & DG Lab: Centre for Renewable Energy Sources and Saving (CRES)

For the specific experiment a mixed hardware/simulation setup is selected because of the nature of the SuT and the two PoIs. It is worth mentioning that the hardware equipment used for the AC grid and the PV array are emulators of their actual counterparts because fully controllable conditions are required, which can only be achieved by emulating the behaviour of them instead of using a real PV array and a connection to the actual AC grid. In this way, the AC (V, f) as well as the PV parameters (irradiance, temperature, etc.) are fully controllable throughout the experiment and reduce the risk of uncertainty and test failure.

The selected setup consists of the following devices:

• One 3-phase grid simulator (nominal values 400 V, 50 Hz, 12 kVA)
• One DC amplifier (400 V, 25 A)
• One computer/data logger
• One 3-phase resistive load (up to 12 kW)
• One digital power meter
• Two circuit breakers, one on the AC and one on the DC side
• One PV inverter as the SuT

The use of a grid simulator in this setup is crucial because it ensures constant operating conditions for the inverter on the AC side. The specific grid simulator is coupled with an AC load because it does not allow absorption of power. Therefore, the load is used to absorb the power generated by the PV inverter. The DC amplifier, on the other hand, is used to emulate the DC output of the PV array. In order to control the amplifier with an I-V characteristic a computer equipped with data logging and control capabilities is used. The specific computer monitors the instantaneous current and voltage on the DC side and controls the output voltage/current of the amplifier based on the selected I-V curve. The computer allows a number of possibilities including the selection of PV module characteristics, connectivity, irradiance and temperature. In addition, the computer is used as a monitoring device (I-V tracer and oscilloscope) which shows in real time the DC operating point of the inverter in conjunction with the theoretical I-V characteristic of the PV array. In this way, the user can identify the theoretical maximum power as well as the I-V curve under partial shading conditions. Last but not least, for the efficiency measurement a Digital Power meter is used. The specific device is simultaneously connected to the DC and AC sides of the inverter and directly provides fully synchronised and averaged measurements of input and output power for the efficiency calculation. The recording period of each measurement is 20 s.

For the efficiency and MPPT mapping test the steps are described in detail in TS5.01. Here an overview of these steps is provided from the viewpoint of the specific laboratory implementation:

1. All connections are as shown in the experiment setup diagram with the two circuit breakers open
2. The computer application is launched and specific parameters for the PV arrays are set, including the PV module characteristics, the connectivity of the modules in the array, the irradiance and the temperature. The values are selected based on the procedure described in TS5.01
3. The DC amplifier is started
4. The DC breaker is closed in order to connect the PV simulator to the PV inverter’s input. Normally, under the initial conditions the PV inverter should not be able to start converting power due to low power on the DC side.
5. The grid simulator is initiated with the required voltage and frequency values (230 V for 1-phase and 400 V for 3-phase operation, 50 Hz)
6. The AC load is adjusted via its keypad controller in order to consume an active power equal to at least 130% of the maximum potential generation from the PV inverter. The safety margin of 20% is necessary in order to avoid any uncertainties in the values of the resistors due to heating.
7. The AC breaker is closed
8. The irradiance and temperature parameters in the PV simulator are adjusted to the first measuring point as described in TS5.01
9. A sufficient amount of time (i.e., 180 s) is allowed in order for the inverter to start converting power. This happens only the first time. Once the irradiance levels are high enough, and remain above this limit, the PV will remain in conversion mode. The specific time may vary based on the inverter specifications but the typical value is that of 3 min=180 s.
10. The values on the DPM display are quality checked and recorded. Also, the MPP value on the computer screen is recorded for the MPPT efficiency calculation
11. The irradiance and temperature values are adjusted so that the complete operating range is obtained as described in TS5.01

After the completion of this first round of measurements, the second round regarding the three efficiency curves is obtained. The steps for this procedure are exactly the same as before. The only difference is that the MPP measurement is not necessary and that the user must fine-tune the temperature/irradiance values in order to keep the DC voltage constant throughout each measurement.

Lastly, the MPPT efficiency under partial shading conditions is implemented in a similar way. Specifically, steps 1-7 are implemented exactly as described above. The irradiance level after step 7 is selected to a sizable amount in order to allow for a significant AC power at the output of the inverter (i.e., 50-100%). Once the inverter is stable, the user implements a step change in the irradiance level on a subset of the PV modules. The profile of this change must have been calculated in advance based on the experiment requirements (e.g., how lower the new power must be or how many maxima the power must show). The behaviour of the inverter is recorder and stored in real time with the computer against the theoretical I-V curve. This test may be repeated several times for different forms of the I-V curve.

For the measurement of the DC current and voltage via the computer/data logger the precision is determined by the resolution of the analogue input of the data logger. In this case, the used data acquisition card provides a 16-bit conversion resolution for both the current and the voltage measurements. Considering that the maximum operating values are 400 V and 25 A this results in an accuracy equal to 6mV and 0.4 mA respectively.

Furthermore, the DPM used allows a measurement accuracy equal to 0.04% of reading + 0.04% of range for the power measurements.

All data are stored in ‘CSV’ file format either manually (DPM) or automatically (computer/data logger).