An important consideration in modern power systems is the impact of non-linear loads and sources. Non-linear loads can introduce harmonics in the voltage and current which causes deterioration in the power quality. Since the load current is used to calculate the compensating current reference, load current harmonics can introduce harmonic distortion into the current controller reference signals. This can lead to harmonics in the output current of the PV inverter. In case 3 the balanced rectifier load shown in Figure 6.1 is connected to the system to test the performance of the inverter control scheme in the presence of load current harmonics. In the first simulation for case study 3 the proposed HCDFT method of sequence extraction from Chapter 4 is used. The performance of the system using the HCDFT to measure the negative sequence current is compared to that obtained using AP filters and a DSOGI extraction technique.
In case study 3, switch Sub is open and the load current remains balanced through the
Under these conditions the inverter output should be the same both with and without the negative sequence compensation loop enabled. In case study 3, switch Snl is initially open
and only linear balanced loads are connected to bus B1. At t = 0.25 s Snl is closed
connecting a three-phase rectifier load to B1. The rectifier has a 15 µF output capacitor and supplies a 27.1 kW resistive load rnl = 15 Ω. When the non-linear load is connected the load
current THD is 5.3%. The load current harmonics increase the harmonic distortion in the voltage at bus B1, shown in Figure 6.11(a). Prior to the connection of the non-linear load the voltage has a THD of 0.15%. Once the rectifier is connected, the THD increases to 2.0%. The load current is shown in Figure 6.11(b) and the grid current is shown in Figure 6.11(c).
(a)
(c)
Figure 6.11: (a) Voltage at the inverter bus and (b) load current, and (c) grid current when the HCDFT is used for case study 3
Within one half cycle of the fundamental after the non-linear load is connected, the HCDFT reference current calculator settles to zero, as shown in Figure 6.12. Even in the presence of harmonically distorted inputs the HCDFT retains a fixed settling time and nearly zero steady state error. The inverter current, is shown in Figure 6.13(a). After the non-linear load is connected, the THD increases from 1.9% to 2.5% due the distortion in the voltage at bus B1. To demonstrate the negligible impact of the compensation loop on performance under balanced harmonic load currents, the same test has been carried out with the compensation loop disabled. The PV inverter current for this case is shown in Figure 6.13(b). By comparing Figure 6.13(a) and Figure 6.13(b) it can be seen that there is no additional distortion in the inverter current due to the compensation current reference when the HCDFT is used.
Figure 6.12: Negative sequence amplitude calculated by the HCDFT when a three-phase rectifier load is connected
(a)
(b)
Figure 6.13: (a) Current injected by the PV inverter when the HCDFT method is used to calculate the compensation current and (b) inverter current when the compensation loop is
disabled
Next, the compensation loop is re-enabled, and two conventional methods, AP filters and the SOGI, are used to calculate the negative sequence load current. Results for the AP filter and SOGI are shown in Figure 6.14 and Figure 6.15 respectively. The amplitude of the compensating current reference in Figure 6.14(a) should be zero because the system is balanced. However, the amplitude shown in Figure 6.14(a) has a non-zero output once switch Snl closes. After the non-linear load is connected the inverter current using AP filters, shown
(a)
(b)
Figure 6.14: (a) Amplitude of the negative sequence reference current and (b) inverter current when AP filters are used to calculate the compensation current
Comping Figure 6.15(a) and Figure 6.14(a) shows that the error in the reference compensation current amplitude is lower using SOGI filters. However, compared to using the HCDFT ,the SOGI method produces a higher overshoot and steady-state error in the compensating current reference amplitude. This results in a higher THD in the inverter current, shown in Figure 6.15(b). The inverter current, grid current, and load voltage THDs are listed in Table 6.2 for the HCDFT, AP filters, SOGI, and with the compensation reference disabled.
(a)
(b)
Figure 6.15: (a) Amplitude of the negative sequence current reference and (b) inverter current when SOGI filters are used to calculate the compensation current
Table 6.2: Current and voltage THDs for case study 3
Sequence calculation method
Compensation
disabled HCDFT APF SOGI
Inverter Current THD 2.5% 2.5% 15% 4.1%
Grid Current THD 7.5% 7.5% 10.0% 8.1%
Voltage at B1 THD 2.0% 2.0% 2.4% 2.1%
When the HCDFT is used, current and voltage THDs are the same as when the inverter is controlled in power conversion mode without compensation. These values are listed in
columns 2 and 3 of Table 6.2. Because of the resonant element in the SOGI filters, this method showed significant improvement over the AP filters. However, it could not entirely attenuate the harmonics in the load and Figure 6.15shows increased current distortion when the rectifier load was connected. The HCDFT method has a comparable settling time to that of the SOGI reference technique, however it also shows improved steady state harmonic rejection compared to the SOGI.
Case study 3 demonstrates the performance of the HCDFT method in the presence of load current harmonics caused by non-linear loads. When the HCDFT reference calculator is employed there is no additional harmonic distortion compared to the case where no compensation current reference is added.