An effective hybrid wind-photovoltaic system including battery energy storage with reducing control loops and omitting PV converter
Introduction
Nowadays, hybrid systems have attracted attention, due to the economic and environmental advantages. Among these systems, hybrid PV-wind turbine systems have become popular because of their high reliability, complementary energy production profiles, and suitability for installation in different weather conditions [1], [2], [3], [4], [5]. Studies have shown that these two types of renewable resources have an alternative and evolutionary property [6], [7], [8]. In general, during the day, when the photovoltaic production is more, the wind turbine power production is less, and during the night period, when the generation of photovoltaic units is negligible and close to zero, due to the increase in wind speed compared to the day, the generated power of wind turbines increases dramatically. This complementary feature is also seen in different seasons. For example, in the summer, when the intensity of the sun is more, less wind is available, whereas in the winter, when the wind mean velocity is higher, the effective radiation of the sun is weaker. This feature has motivated researchers to seek ways of combining the two energy sources. However, few studies have explored the possibility of augmenting pre-installed wind turbines with PV system with little structural modifications. Rather, most studies have focused on introducing new architectures. Daniel et al. have used a three-phase square wave converter to combine photovoltaic and wind turbine in a hybrid solar-wind system [9]. In [10] a new three-input dc–dc boost converter has been proposed to feed DC loads by a hybrid system. However, the addition of these converters to a hybrid system is not affordable. Some research works have been dedicated optimizing the existing systems [11], [12], [13], [14], [15]. Some typical hybrid systems consisting of a photovoltaic systems, wind turbines and storage units are presented in [11] and [12], and the optimal capacity of each unit is calculated using different numerical algorithms. A grid-connected PV/Wind power generation system focusing on smoothing of the DC-link voltage fluctuations and reduction of the DC-link capacitor bank size has been introduced in [13]. Reference [14] presented a laboratory scale of a hybrid, which integrates a photovoltaic system, a wind turbine, an energy storage, and an AC load, each one connected to a DC-link through a suitable power converter. Some studies have also investigated remedial measures for power fluctuations through energy storages [16], [17], [18], [19], [20]. In [16,17], the state of charge (SOC) of the battery storage system is controlled in hybrid systems to reduce the output power fluctuations. Sebastián [18] and Hayes et al. [19] have introduced new methodologies for analyzing and optimizing hybrid structures by using battery systems. Also, several studies have made efforts to introduce new control schemes in hybrid system [21], [22], [23], [24]. Nian et al. [21] presents a modified control scheme to smooth the active and reactive power output of a hybrid system by focusing on the fifth and seventh grid voltage harmonics. In [22] a novel two-layer constant-power control scheme has been presented for a wind farm equipped with doubly-fed induction generators (DFIGs), where the DFIG DC-link is interfaced with a supercapacitor energy storage system. A system with the same structure as the one discussed in [22], but featuring virtual inertia to enhance the stability and dynamic behavior of the system, is presented in [23]. Reference [25] presented a new energy management strategy for a hybrid DFIG-SC-BESS system. In this paper the coordinated operation of BESS and SC is done through the implementation of an energy management algorithm and let the authors to remove the battery voltage control loop. In [26] a new control and energy management of a hybrid system which consists of DFIG, PV system, and a battery bank is presented. In this paper the battery bank benefits of only a current control loop in its control structure. Another embedded energy share method between the high energy storage system (battery) and the auxiliary energy storage system such as supercapacitors (SC) is introduced in [27]. The SC modules are dimensioned for peak power requirement, and the battery's module ensures the average power. The battery module is connected to the dc-bus through a dc/dc converter for the first topology (which let the authors to remove the battery dedicated voltage control loop) and without a converter for the second configuration. Also, buck-boost converters are used between the SC and the dc-bus to manage the available energy. An approach of coordinated and integrated control of solar PV generators with the MPPT control and battery storage control to provide voltage and frequency support to an islanded microgrid has been introduced in [28]. In this configuration, the battery energy storage is controlled only by a control loop. Also, active and reactive power control with solar PV, MPPT and battery storage is proposed for the grid connected mode. The control strategies show effective coordination between inverter control, MPPT control, and energy storage charging and discharging control.
The aforementioned articles, have not taken a holistic approach in taking into account efficiency, optimality of the structures, and cost effectiveness. Rather, they concentrate on the control and coordination of the two power sources. Also, there is not much effort to optimize and control the hybrid system to increase efficiency.
Wandhare et al. [6] proposed a promising architecture, where the dedicated photovoltaic converter has been omitted in a hybrid system with a DFIG. The conventional wisdom is that the photovoltaic capacity should be the same as the photovoltaic converter rating. However, Wandhare and Agarwal [6] has proved that the photovoltaic capacity can indeed be far greater than the rating of the grid-side converter (GSC). In other words, after merging the PV converter and GSC, the utilization of the new converter increases. It is worth mentioning that, in the configuration proposed in [6], MPPT can be implemented by controlling the DC-link voltage. Despite these advantages, the hybrid system presented in [6] has a number of shortcomings: It has a large power output fluctuation, and DC-link voltage exhibits significant excursions under some operating conditions. Moreover, if the solar irradiation and wind speed are both large simultaneously, the PV output power has to be curtailed due to the rating limitation of its GSC. Consequently, this configuration is unable in harvesting the maximum solar energy. In addition, its GSC is not used efficiently all the time.
This paper intends to address the aforementioned shortcomings by proposing a cost-effective and efficient configuration. Thus, a battery energy storage system (BESS) with a reduced control scheme has been added to a PV-DFIG system. In the proposed hybrid system, the GSC power and its DC-link voltage fluctuations have been smoothed to a large extent. Moreover, the size of the DC-link capacitor has been reduced, and the GSC has been utilized more efficiently. Additionally, in the proposed configuration, the GSC rating does not impose any limitation on the PV output power. The operation and effectiveness of the proposed hybrid system have been demonstrated and compared with those of the conventional hybrid system presented in [6], by off-line simulations.
Section snippets
Proposed system description
Fig. 1 shows the configuration of the purposed system. To illustrate the concepts, the capacity of the DFIG is assumed to be 1 MW along with a PV system with a capacity more than GSC rating. Typically, the ratings of the RSC and GSC are considered to be from 25% to 30% of the generator`s rating [29]. So, the ratings of the RSC and GSC are chosen as 250 kVA and 340 kVA, respectively, while the rating of the PV system is assumed to be 380 kW. To highlight the advantage of the proposed system, the
Various power flow scenarios
In this Section, power flow in the proposed hybrid system is studied to highlight the relationship between PPV and PGSC and to determine their ratings. As it was mentioned, the periodic and complementary natures of solar irradiation and wind speed permits a smaller rating for the GSC than the PV source capacity. From (4) and (6), one has:
Based on (29), if the rotor speed is more (less) than the synchronous speed, the rotor power will be positive (negative) and the rotor circuit
Simulation results
A model of the proposed hybrid system of Fig. 1 is simulated for operation under various scenarios and the results are presented and discussed in this section. Tables 1 and 2 list the control and circuit parameters of the hybrid system. Thus, the rotor speed is considered in both sub-synchronous and super-synchronous regimes of operation, as shown in Fig. 6(a). The stator breaker (SS) is excited at t = 0 s. From this moment until t = 1 s, the generator speed is near the synchronous speed,
Conclusion
An effective hybrid DFIG-PV-BESS system is proposed to provide an economical solution for combining PV and grid. In this configuration the PV converter is merged with wind turbine converters. By removing the PV converter, the number of system converters is reduced, leading to decreasing the total cost of the system and using optimally of DFIG converters. In comparison with conventional hybrid systems, by considering a battery energy storage and optimal control of converters in the proposed
Declaration of Competing Interest
None.
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