With increasing demand for clean and infinite energy resources, renewable energy sources, like photovoltaic, fuel cell and wind power, have experienced rapid growth over the past few years [1, 2]. One of the prospective uses for renewable energy sources is the installation of distributed power systems to the existing power grid for supplying adequate energy. The distributed power systems include a power electronics converter to improve a system’s performance and reliability.
In a single-phase grid-connected power system, consisting of a DC/DC converter and a DC/AC converter, the converter has a pulsated instantaneous output power as shown in Fig. 1. If the DC/AC converter has a large enough dc-link capacitance, the capacitor can smoothen the dc-link voltage, but the input current will have low frequency ripple components, where the frequency corresponds to the second-order frequency. This low frequency current ripple flows into a renewable energy source, and finally gives large negative effects on the energy harvesting and energy source’s expectancy.
Fig. 1.Two-stage grid-connected inverter under single-phase electrical utility
In order to reduce the low frequency current ripple at renewable energy sources, several-schemes such large electrolytic capacitors, usually in the range of mF , extra power decoupling circuits [4-7] and active power filters  have been proposed in prior studies, and some of them are popularly used because of their simplicity and effectiveness. However, the schemes that use extra circuits grapple with the problems of a short system lifespan, large volume or high cost. Active control schemes using existing dc-link capacitors in a two-stage dc-dc-ac converter, can suppress these difficulties. Some authors proposed a feed forward compensator , and the compensator produced a ripple cancellation duty ratio based on the correlation between the instantaneous output power and the dc-link voltage. However, this has the drawbacks of poor disturbance rejection and robustness in terms of parameter variations. These are inherent problems commonly found in most feed forward compensators. These demerits were solved by the proper design of a two-loop feedback compensator . However, this scheme leads to the problem of slow dynamics in the outer voltage loop. This slow dynamics causes a slow transient response of the dc-link voltage. Moreover, the dc-link capacitor may experiences large overshoot or undershoot voltages unless the dc-link capacitor is sufficiently large.
In this paper, a novel low frequency current reduction scheme is presented. The proposed scheme is based on the common two-loop feedback scheme, but reconstructs the control structure by changing the feedback control configuration. The superiority of proposed control mechanism especially in terms of the faster dynamics of the outer voltage loop is verified by investigating the fundamental mechanism. A quasi-notch filter is adapted to improve the control structure and to alleviate hardware implementation concerns. A proper control design of the two-stage DC/DC/AC converter is presented based on small-signal modelling, and several experiments are performed to verify that the proposed scheme achieves a ripple-free dc current from renewable energy sources. The experiments also verify that it has much faster dynamics in terms of the dc-link voltage loop with a fast settling time. In addition, maximum power point tracking (MPPT) experiments are performed to demonstrate the superiority of the propose scheme.
2. Principle of Proposed and Conventional Scheme
2.1 Principle of proposed scheme
Fig. 2 represents a two-stage DC/DC/AC grid-connected power system with the proposed low frequency current reduction scheme for a single-phase electrical utility. A non-isolated type DC/DC boost converter is used to maintain a sufficiently high dc bus voltage, vd, even when renewable energy sources drop to low voltage levels. For the second-stage converter, a single-phase DC/AC fullbridge inverter topology is used and the output current, is, is regulated with the output inductor, Ls. The input current of the DC/DC boost converter, ig, is controlled by the dc-dc converter’s control loop. In the DC/AC inverter control loop, the current compensator, Cis, regulates is in accordance with its reference, is*. The dc-link voltage, vd, is regulated by the outer voltage loop, denoted by a dotted line.
Fig. 2.Proposed second-order harmonic current reduction scheme
As seen from Fig. 2, the current reference, ig*, of the DC/DC converter is given externally in the proposed scheme. Thus the current control loop achieves total isolated from the dc-link voltage. With the isolated control structure, the interference, caused by the dc-link voltage, can be eliminated against a ripple-less dc current regulation. Thus the control structure provides more a favorable control platform for achieving fast dynamics in the voltage loops, while providing low frequency ripple current reduction.
2.2 Comparison of conventional and proposed scheme
Fig. 3 shows the control structure for the conventional scheme introduced in . In this scheme, the current loop is coupled with the voltage output loop. Even with a large capacitor, the dc-link voltage, vd, contains some low frequency ripple components. As a result, the outer voltage loop is easily disturbed by these low frequency components, unless the voltage loop is designed with a strong attenuation in the low frequency region. In a previous study of , in order to achieve a strong attenuation, the voltage compensator was designed with a low crossover frequency at the expense of the dynamic response.
Fig. 3.Block diagram of conventional current ripple reduction scheme
Fig. 4 shows a simulation comparison between the conventional and proposed schemes under the same crossover frequency of 16 Hz in the voltage control loop. It is clearly seen that the input current of the proposed scheme shows a smaller ac ripple due to the isolated control structure even under the same crossover frequency.
Fig. 4.Performance comparison of current reduction scheme
Following sections describe the detailed controller structure and compensator design for performance verification.
3. Compensator Design of Proposed Scheme
3.1 Properties of the quasi-notch filter
The current loop of the dc-dc converter requires a proper design with an extremely high loop gain at the second-order frequency to achieve an effective low frequency current ripple reduction. With a sufficiently high control loop gain, the current, ig, becomes a ripple-less dc current by suppressing the impact of the low frequency ripple current.
However, designing with a high gain is impractical due to several difficulties such as component tolerances, temperature variations, and parasitic effects. An alternative way to achieve a current ripple reduction is to design the voltage loop gain of the DC/AC inverter so that it is much smaller than the current loop gain of the DC/DC converter. As a promising solution, this paper adapts a quasi-notch filter, which provides strong attenuation in a specific frequency range, to the outer voltage loop of the DC/AC inverter, and consequently, it provides a sufficiently small loop gain of the voltage loop, while permitting a high crossover frequency with fast dynamics.
CQRF(s), shows the transfer function of the quasi-notch filter, and its attenuation level at ωo, Q. They are given as (1) and (2), where Qz is the quality factor of the resonant zero, Qp is the quality factor of the resonant pole, and ωo is the resonant frequency. Eq. (2) can be derived from (1) by letting s=jωo, and expressing in decibels , where j is the square root of −1.
The quasi-notch filter attenuates the gain in a specific frequency range, like the standard notch filter. However, its phase drop is adjusted by the strength of the attenuation. With this adjustable phase drop providing a flexible compensator design, it is possible to increase the crossover frequency even beyond the second-order frequency. Fig. 5 presents the frequency response of the quasi-notch filter under Qp =10 with several Qzs.
Fig. 5.Quasi-notch filter with varying resonant zero under fixed resonant pole
3.2 Control loop and compensator design
Closed-loop block diagrams of the DC/DC converter and the DC/AC inverter are illustrated in Fig. 6, where the small-signal transfer function blocks: Gig — duty-to-input current, Gis — duty-to-output current, Gvdis — output current-to-dc link voltage transfer function, Cig — input current compensator, Cis — output current compensator, Cvs — dc link voltage compensator, Hig — input current sensor gain, His — output current sensor gain, Hvs — dclink voltage sensor gain, and Fm — PWM gain. The small-signal transfer functions of Gig, Gis, and Gvdis, can be easily derived by using the averaged PWM switch model in .
Fig. 6.Closed-loop control block diagram of the proposed scheme
After deriving the small-signal transfer functions, proper compensators are designed in the frequency domain. Fig. 7 and Fig. 8 show the compensator design results of the dc-dc converter and the dc-ac inverter in gain/phase plots. The compensators are designed with the following circuit parameters: vg = 50~70V, vd = 250V, vs = 110V, Lg =3.3mH, Cd = 1880μF, and Ls = 3mH, which correspond to the 1kW hardware prototype specifications used in the experiments.
Fig. 7.Loop gain and closed-loop response of the current loop
Fig. 8.Gain / phase plots of the voltage loop including current loop of dc-ac converter
For the DC/DC converter, the current loop is designed with the one-zero two-pole PID compensator shown in (3) with Cig(0) = 27000, ωpg = 3000 rad/sec, and ωzg = 270 rad/sec. The resultant loop gain, shown in Fig. 7 (a), shows that the phase margin is higher than 160 degrees and the crossover frequency is about 100 Hz. For the current loop of the DC/AC inverter, the proportional-resonant type (PR) compensator shown in (4) is used to achieve a desirable high gain at a line frequency of 60 Hz, where Cis(0) = 970, kps = 0.2, and ωps = 380 rad/sec. As seen in Fig. 7(b), the phase margin of the current loop is higher than 170 degrees and the crossover frequency is about 4 kHz. The voltage loop of the dc-ac inverter is composed of the two-pole one zero PID compensator in (5) with Cvs(0) = 1.5, ωpv = 27000 rad/sec, and ωzv = 76 rad/sec and the quasi-notch filter in (1) designed with Qz = 500, Qp= 10, and ωo = 753.98 rad/sec. Fig. 8 shows the results of the gain/phase plots of the loop gain. As in the figure, it is clear that the attenuation is more than −20dB with a double fundamental frequency of 120Hz.
4. Experimental Result
A 1 kW hardware prototype has been built to verify the proposed control scheme, as shown in Fig. 9, and a digital signal processor is used to implement the proposed compensators. A programmable dc power supply equipped with solar array simulation (SAS) control software from Regatron is used to emulate the renewable energy output of a PV array. The dc-link command is set to 250V.
Fig. 9.Experimental setup of two-stage single-phase grid-connected inverter
Figs. 10 and Fig. 11 show the steady-state operation waveforms under a 7A input current command. Without the low frequency current reduction scheme, the low frequency ripple current is exhibited in the input current as shown in Fig. 10. The Total Harmonic Distortion (THD) is measured at 2.9 %.
Fig. 10.Steady-state waveforms without the proposed scheme
Fig. 11.Steady-state waveforms with the proposed scheme
On the other hand, with the proposed low frequency current reduction scheme, shown in Fig. 11, the input current achieves a ripple-free dc current since the dc-link capacitor takes over most parts of the ripple components. This is due to sufficient attenuation of voltage loop gain by the quasi-notch filter. In the proposed scheme, the THD of the output current is slightly higher at 4.8%. However, this still satisfies international standards and regulations.
Fig. 12 shows the transient responses of the proposed and conventional schemes. Fig. 12(a) is the experimental result under the conventional scheme with a 2 Hz crossover frequency, and Fig. 12(b) is the test result under the proposed scheme. It is clearly seen that the proposed scheme of Fig. 12(b) exhibits improvements in terms of a fast transient response, due to the high crossover frequency of the voltage control loop.
Fig. 12.Dynamic transient waveforms under increasing input current.
Fig. 13 shows the maximum power point tracking(MPPT) experimental results using SAS control software from Regatron, where x denotes the maximum and minimum operating points during 2 seconds and o denotes the averaged operating point. Figure 13(a) shows the case without the low frequency current reduction scheme and Figure 13(b) illustrates the experimental result with the proposed low frequency current reduction scheme. It can be seen that the range of the operating point with the proposed scheme is closer to the averaged operating point, due to the reduction of the low frequency current ripple. As a result, the MPPT efficiency with the proposed scheme is 96.43%, which is about 3% higher than the efficiency without the scheme at 93.57%. The performance results of the conventional and the proposed schemes are summarized in Table 1.
Fig. 13.Experimental waveform of the maximum power point tracking
Table 1.Performance Summary of conventional scheme and proposed scheme
In this paper, a novel low frequency current reduction scheme is proposed for a two stage DC/DC/AC gridinterconnected power system. By isolating the current closed loop of the DC/DC converter from the dc-link capacitor and incorporating a quasi-notch filter into the voltage loop of the DC/AC inverter, a fast dc-link voltage control can be achieved. The fundamental theory of the proposed control scheme was introduced, and its superiority in terms of achieving fast dynamics was clearly explained. The experimental results show that the novel current reduction scheme achieves a ripple free dc input current with a fast dc-link voltage regulation, and thus reduces the settling time of the dc-link voltage control.