AREA DE ELECTRICIDAD

The proposed a.c. side voltage-sensor-less control operating under unbalanced grid voltage is then tested. The effectiveness of the new symmetrical component decomposition method on the current control is evaluated in this section.

The new symmetrical component decomposition method has its advantage in term of faster response. It is proposed to shorten the time spent on generating accurate current reference for power control if it is applied to a conventional current control system during unbalanced grid conditions. This effect is firstly verified. The comparison is made between the proposed symmetrical component decomposition method and other methods in the dq reference frame. The parameters of the L filter and other components are listed in Table 7.3:

Parameters Values

Grid voltage 230V (RMS)

Grid frequency 50Hz

Coupling inductance 10mH

Series resistance 0.1

Target modulation depth 0.8

Power reference 3000W

Switching Frequency 10kHz

Table 7.3 Parameters of system under study

The waveform of the a.c. side voltage is shown in Figure 7.28. The single phase voltage dip is applied and Phase C voltage is dropped to 50% of its nominal value. The control objective is to inject negative sequence current to balanced the power output on three phases. The current response is shown in Figure 7.29 (a). The output power is constant, verified by Figure 7.29 (b).

Figure 7.28 Grid voltage with single phase voltage dip

(a) (b)

Figure 7.29 Response of the current control equipped with voltage sensors and using new proposed symmetrical components decomposition method against single phase voltage dip:

(a) current waveform; (b) output power at PCC

Another simulation is carried out to provide comparison using conventional notch-filtering method to decompose symmetrical components. Same current control parameters, grid voltage and single phase voltage dip are applied. The current and power response is shown in Figure 7.30. By comparing response shown in Figure 7.29 and Figure 7.30. The symmetrical component decomposition method proposed in this research has little effect on system damping

as the delay introduced in much smaller than the conventional method thus more suitable to be applied to the a.c. voltage-sensor-less control system.

(a) (b)

Figure 7.30 Response of the current control equipped with voltage sensors and using notch filter to decompose symmetrical components against single phase voltage dip:

(a) current waveform; (b) power output at PCC

The simulation of the proposed a.c. voltage-sensor-less control is carried out. The control strategy is first select to be the balanced current strategy. The a.c. side voltage is still the same as shown in Figure 7.28. The three phase current during the voltage dip is shown in Figure 7.31. As introduced in Chapter 4, the current control objective could vary depending on the configuration and roll of the VSI. The constant current strategy aims at a balanced utilization of the power modules on each phase. The negative sequence current is not desired for causing unbalanced current output on different phases. Therefore, the current reference for positive sequence is generated based on the estimated grid-side voltage to keep the desired average power output shown in Figure 7.32 (a), while the current reference for negative sequence is kept 0 regardless what the negative sequence voltage is as shown in Figure 7.32 (b). The power of negative sequence will be absorbed by the d.c. link capacitor. The relevant voltage control therefore is disabled.

Figure 7.31 Current response to single phase voltage dip aiming at balanced current

The system is assumed to be starting up with accurate initial grid synchronisation achieved. The current response reaches its steady state with in three fundamental cycles after start. It is slower than the simulation result shown in 6.25 due to the band-width of the system is restricted for avoiding interference between controllers of the positive and negative sequence. The current response in term of Idand Iqshown in Figure 6.32 indicates that the slower response is

mainly caused by the reduced bandwidth of the current control stage. The voltage estimation is swift enough to reach steady state within one fundamental period which can be verified by inspecting the curves of the current reference while the actual current takes time to settle. As the current reference for negative sequence is set to zero, the current is still balanced during the single phase voltage dip.

(a) (b)

Figure 7.32 Idand Iqresponding to the single phase voltage dip,

(a) positive sequence; (b) negative sequence

It can be verified from Figure 7.33 that the power of the negative sequence during the unbalanced fault causes a 100Hz oscillation on the output power which will affect the d.c. link voltage. The amplitude of the oscillation is related to the current output of the positive sequence and the voltage of the negative sequence as introduced in Chapter 4. The average power output is kept 3kW as its reference.

Another control objective associated with unbalanced grid fault-ride-through is to compensate the active power oscillation by injecting negative sequence current. The current reference can be generated using Equation 4.18 with Q0, P2ndand Q2ndto be 0. During the single phase voltage

dip, the current is also unbalanced. The current references are generated according to the voltage of positive and negative sequence respectively as shown in Figure 7.35. After the fault cleared, the current returns to balance as normal. The three phase current waveform is shown in Figure 7.34. The power output is plotted and shown in Figure 7.36. It can be verified that after a short transient stage of approximate two and half fundamental cycles, the power output is returned to its reference value and constant. The d.c. bus voltage will not be affected active power oscillation in this case.

(a) (b)

Figure 7.35 Idand Iqresponse to the single phase voltage dip,

(a) positive sequence; (b) negative sequence

7.6 Chapter summary

This chapter presents simulated case studies to check or verify the proposed control scheme and controller design. The initial start-up and synchronisation to the grid, the steady state operation under balanced grid voltage and the system performance during an unbalanced voltage dip are investigated. The initial start-up process is able to regulate the current injected without synchronisation to the grid. This feature is highly desirable to ensure the safety of the power modules. The transient response of the grid voltage estimation during the start-up process is swift enough to minimise the use of the hysteresis current control and its harmful effect of harmonic injection. The steady-state operation of the a.c. voltage-sensor-less current control with PWM output is proved to be able to track the grid voltage variation while keeping the desired power output in a swift manner. The newly design symmetrical component decomposition algorithm is able to be adopted by the a.c. voltage-sensor-less controller, and plays a vital role in riding-through unbalanced grid fault.

The simulation results has verified the design objective is able to be reached, given the suitable conditions such as sufficient d.c. bus voltage, and well-tuned control parameters. The system can be further verified by experiments.