voltage droop mode is denoted the Q D mode. All the possible operation modes are shown in Table 1. Figure 12 depicts the experimental results of a selfsynchronized synchronverter with the grid frequency lower than 50 Hz. It can be seen that the synchronverter frequency is much smoother than the grid frequency obtained by a PLL, which means the performance is much worse if a PLL is adopted to provide the frequency reference. Table 1. The operation modes of a self-synchronized synchronverter. switch Sc switch SP switch SQ Mode 1 ON ON N/A 1 ON OFF Self-synchronization 1 OFF ON N/A 1 OFF OFF N/A 2 ON ON P mode, Q D mode 2 ON OFF P mode, Q mode rectifiers without a Dedicated synchronization Unit Like synchronverters, a self-synchronized synchronverter can also be operated as a rectifier [15]. The controller for threephase PWM-controlled rectifiers is shown in Figure 13. Compared to Figure 3, similarly, three major changes are made: 1) a virtual current i s generated from the error between the grid voltage v and the control signal e is introduced, and the current fed into the controller can be either the virtual current i s or the grid current i; 2) the synchronization unit to provide the grid frequency reference and the reset Frequency (Hz) Frequency (Hz) 50 0 10 20 30 40 Time (Seconds) (a) 50 PD mode, Q mode 50.1 50 0 10 20 30 40 Time (Seconds) (b) 50 60 0 10 20 30 40 Time (Seconds) (d) 50 60 0.8 0.6 1.1 Error (V) Normalized E (p.u.) OFF 49.9 60 1.2 1 0.4 0.2 0.9 0 10 20 30 40 Time (Seconds) (c) 50 0 60 100 80 80 60 60 40 Q (Var) P (W) ON OFF 50.2 50.1 40 20 20 0 -20 0 -20 OFF 2 signal is removed from the controller; and 3) a PI controller is added to generate the reference frequency/speed io r for the virtual synchronous motor while driving the error between the reference speed io r and the virtual speed io to zero. 50.2 49.9 2 PD mode, Q D mode 0 10 20 30 40 Time (Seconds) (e) 50 60 0 10 20 30 40 Time (Seconds) (f) 50 60 FIG 12 The experimental results of a self-synchronized synchronverter [5]. (a) The synchronverter frequency f. (b) The grid frequency fg from a three-phase PLL for comparison (not used for control). (c) The amplitude E of the generated voltage e. (d) The amplitude of v - vg. (e) The real power at the terminal. (f) The reactive power at the terminal. December 2016 z IEEE PowEr ElEctronIcs MagazInE 25