A Study on Stability Control of Grid Connected DC Distribution System Based on Second Order Generalized Integrator-Frequency Locked Loop (SOGI-FLL)

Abstract

This paper studies a second order generalized integrator-frequency locked loop (SOGI-FLL) control scheme applicable for 3-phase alternating current/direct current (AC/DC) pulse width modulation (PWM) converters used in DC distribution systems. The 3-phase AC/DC PWM converter is the most important power conversion system of DC distribution, since it can boost 380 Vrms 3-phase line-to-line AC voltage to 700 Vdc DC output with various DC load devices and grid voltages. The direct-quadrature (d-q) transformation, positive sequence voltage extraction, proportional integral (PI) voltage/current control, and phase locked loop (PLL) are necessary to control the 3-phase AC/DC PWM converter. Besides, a digital filter, such as low pass filter and all pass filter, are essential in the conventional synchronous reference frame-phase locked loop (SRF-PLL) method to eliminate the low order harmonics of input. However, they limit the bandwidth of the controller, which directly affects the output voltage and load of 3-phase AC/DC PWM converter when sever voltage fluctuation, such as sag, swell, etc. occurred in the grid. On the other hand, the proposed control method using SOGI-FLL is able to do phase angle detection, positive sequence voltage extraction, and harmonic filtering without additional digital filters, so that more stable and fast transient control is achieved in the DC distribution system. To verify the improvement of the characteristics in the unbalanced voltage and frequency fluctuation of the grid, a simulation and experiment are implemented with 50 kW 3-phase AC/DC PWM converter used in DC distribution.

New SOGI-FLL Grid Frequency Monitoring with a Finite State Machine Approach for Better Response in the Face of Voltage Sag and Swell Faults

“…Consequently, the fixed upper 51 Hz and lower 49 Hz saturation bounds employed in Figure 6 are replaced by the new saturation variables SAT up and SAT down determined by (8) and 9, respectively, which are stored in inner variables.…”

“…The behavior of the SOGI-FLL is well studied in the literature. In [7][8][9], the SOGI-FLL was presented as the coupling between its two main parts, the SOGI and the FLL, whose dynamics correspond to that of a second-order system. However, the behavior of the SOGI-FLL is deeply perturbed by voltage sags, which produce high distortion peaks in the FLL response [10][11][12].…”

Frequency-Tracking Algorithm Based on SOGI-FLL for Wireless Power Transfer System to Operate ZPA Region

“…Figure 10 shows the block diagram of the SOGI-FLL. The transfer function is expressed in Equation ( 20) [18][19][20][21].…”

“…The transfer function is expressed in Equation ( 20) [18][19][20][21]. ( ) = ( ) = 2 +`2 2 + 2`+`2 (20) is the error between the input signal and the output signal of the d-axis, 2 is gain and the center frequency is `. Using Equation (20), the Bode diagram is shown in Figure 10.…”

“…( ) = ( ) = 2 +`2 2 + 2`+`2 (20) is the error between the input signal and the output signal of the d-axis, 2 is gain and the center frequency is `. Using Equation (20), the Bode diagram is shown in Figure 10. When the frequency of the input signal is lower than the center frequency, and ` are the same phase.…”