9.3 Intermittency data analysis - CSIRO 22 kW PV system

To analyse PV power output ramp rates for a small-scale solar system, high-resolution 5-second data was gathered for a 22 kW PV system located on the roof of the CSIRO Energy Centre office building in Newcastle. The data was collected for a period of two weeks and comprised solar irradiance (global horizontal irradiance) and PV output power.

Occurrences of continuous ramping of the 22 kW PV system’s output power for periods of five, ten, fifty and sixty seconds were analysed using the 5-second resolution data collected. The distributions of ramp events over the various timescales were obtained using the same method as that for the DKASC. Figure 107 shows the distribution of ramp-up rates for variations recorded in five seconds. A total of 247 occurrences of output power ramp-up events with rates of at least 0.5kW/s over 5 seconds were recorded in the two-week period. Of these occurrences 54 were over 0.9kW/s, corresponding to increases in output power of at least 4.5kW (20.5% of plant rating) in five seconds. The analysis also showed there were 253 occurrences of ramp-down rates of 0.5kW/s or more for five second periods throughout the two weeks. The distribution of ramp-down events with rates greater than 0.8kW/s can be seen in Figure 108. Ramp-down rates of over 1.3kW/s were observed to occur ten times, which corresponds to a power drop of at least 6.5kW (~30% of plant rating) in just five seconds.

Figure 107 Distribution of ramp-up events in 5-second periods for 22 kW PV system

Figure 108 Distribution of ramp-down events in 5-second periods for 22 kW PV system

Figure 109 and Figure 110 show the distribution of ramp-up and ramp-down events respectively in 10-second periods with rates of 0.6kW/s or more. A total of 30 occurrences of ramp-up events with rates of more than 0.8kW/s were observed translating to an increase in the PV system’s power output of at least 8kW (36% of rating) in ten seconds. The corresponding number of occurrences of ramp-down events with rates of more than 8kW/s recorded was 28.

Figure 109 Distribution of ramp-up events in 10-second periods for 22 kW PV system

Figure 110 Distribution of ramp-down events in 10-second periods for 22 kW PV system

Figure 91 shows the distribution of ramp-up event occurrences over 50-second periods with rates greater than 0.1kW/s, corresponding to output power increase of at least 5kW in 50 seconds. There were 68 events of increase in power output of at least 10kW (45% of plant rating) in 50 seconds recorded. Similarly, Figure 112 shows the distribution of output power ramp-down events over 50-second periods and five events of output power drops of more than 15kW (68% of plant rating) in 50 seconds were recorded.

The distribution of ramp-up event occurrences over 60-second periods with rates greater than or equal to 0.1kW/s is shown in Figure 113. The total number of such events in the two-week period was found to be 189, with nine occurrences of ramp-up events with rates greater than 0.24kW/s, corresponding to output power increases of more than 14.4kW (65% of plant rating) in 60 seconds. Figure 114 shows the distribution of output power ramp-down events over 60-second periods for rates greater than 0.1kW/s, the total number of which was found to be 117. The number of times the output power dropped at least 14.4kW in 60 seconds was found to be ten.

Figure 111 Distribution of ramp-up events in 50-second periods for 22 kW PV system

Figure 112 Distribution of ramp-down events in 50-second periods for 22 kW PV system

Figure 113 Distribution of ramp-up events in 60-second periods for 22 kW PV system

Figure 114 Distribution of ramp-down events in 60-second periods for 22 kW PV system

The number of occurrences of ramp-up events over various time periods for the 22kW PV system is summarised in Table 20. It can be seen that the majority of the fluctuations observed over all time periods are small variations with ramp rates of less than 0.2kW/s. A significant number of rapid increases in output power were observed with ramp rates of more than 0.8kW/s over 5- and 10-second periods which correspond to increases of more than 4kW (18% of plant rating) and 8kW (36% of plant rating) respectively in a very short time frame.

Table 20 Number of occurrences of ramp-up events with various ramp rates for different timescales 22 kW PV

Timescale 0-
0.1kW/s
0.1-
0.2kW/s
0.2-
0.3kW/s
0.3-
0.4kW/s
0.4-
0.5kW/s
0.5-
0.6kW/s
0.6-
0.7kW/s
0.7-
0.8kW/s
0.8-
0.9kW/s
0.9
kW/s+
5s 66428 1041 472 250 132 97 45 24 27 54
10s 34626 968 433 197 125 48 40 15 14 16
50s 3767 197 66 2 0 N/A N/A N/A N/A N/A
60s 2748 155 34 0 N/A N/A N/A N/A N/A N/A

Note: ‘N/A’ refers to ramp-rate events that are not feasible - such ramp rates over those timescales will exceed the capacity of the plant.

Table 21 presents a summary of occurrences of ramp-down events over various time periods. Similar to ramp-up events, majority of the total number of fluctuations over all time periods are small variations with ramp rates of less than 0.2kW/s. A considerable amount of rapid drops in output power with ramp rates greater than 0.8kW/s, corresponding to at least 4kW (26%) and 8kW (51%) drops in output power for 5- and 10-second time periods respectively, were observed.

Table 21 Number of occurrences of ramp-down events with various ramp rates for different timescales 22 kW PV

Timescale 0-
0.1kW/s
0.1-
0.2kW/s
0.2-
0.3kW/s
0.3-
0.4kW/s
0.4-
0.5kW/s
0.5-
0.6kW/s
0.6-
0.7kW/s
0.7-
0.8kW/s
0.8-
0.9kW/s
0.9
kW/s+
5s 67085 985 463 288 150 93 61 39 12 48
10s 35520 847 468 216 107 56 36 27 13 15
50s 3780 139 54 5 0 N/A N/A N/A N/A N/A
60s 2791 87 30 0 N/A N/A N/A N/A N/A N/A

Note: ‘N/A’ refers to ramp-rate events that are not feasible - such ramp rates over those timescales will exceed the capacity of the plant.