In this chapter, a photovoltaic system integrated into the building is investigated. The studied structure includes also a battery energy storage system. The overall system is connected to a four-wire AC bus, with the possibility to supply single-phase and three-phase loads. Each equipment is interfaced with a dedicated power converter. This chapter examines the technical operation of all structure components and gives a detailed mathematical study of the DC/AC power converter control in case of two modes, namely, grid connected mode and standalone mode. The investigated control is based on resonant controller. The resonant controller parameters tuning, which is based on the generalized stability margin criterion, is detailed in this chapter. To prove the performance of the proposed control algorithm, several simulation tests developed under PSIM software were performed and then validated by experimental results.
- photovoltaic systems
- battery energy storage system
- building microgrid
- DC/AC power converter control
- standalone mode
- grid connected mode
Nowadays, rooftop photovoltaic systems (PV) and building-integrated photovoltaic (BIPV) systems are becoming well known and commonly used. The growth of these installations is due to their environmental advantages in addition to their social and economic benefits. Indeed, since building electricity consumption accounts for a large proportion of a country’s overall consumption, and tends to increase further for the coming years, local generation offers an ideal solution [1, 2, 3, 4].
Regarding the obstruction of PV systems’ fluctuating aspect, it can be derived in various ways. The integration of battery energy storage system (BESS) is considered as efficient and complementation solution, mainly for standalone microgrids . Urban photovoltaic systems are usually connected to the distribution network, but the operation in standalone is also possible [6, 7].
In order to ameliorate the PV system efficiency, an adequate control strategy should be introduced. In the literature, several control techniques are developed: integral proportional regulators, resonant correctors, hysteresis correctors, sliding mode controls, predictive controls, and so on [8, 9, 10, 11].
This chapter investigates the operation of PV system devoted to building application. It gives an overview of the control of all integrated power converters and then explains in details the control of the DC/AC power converter in both operation modes, namely, standalone mode and grid connected mode. For the grid connected mode, the control must ensure that the AC bus voltage remains within the acceptable range, and for standalone mode the DC/AC converter is controlled to inject generated PV power into the AC-link.
This chapter first outlines overall system description, followed by a review of each power converter control. A detailed mathematical study is dedicated to the DC/AC converter control in grid connected and autonomous modes. Simulation results and experimental validation are subsequently presented.
2. System description
The building solar system structure is given in Figure 1. It is composed of a PV panels in parallel with a battery energy storage system which are linked to a DC bus, a DC/AC power converter, and an LCL filter interfacing between DC and AC bus. Single- and three-phase linear and nonlinear loads are connected to the AC bus.
The linear building loads are modeled by a resistive load, and the nonlinear ones are modeled by a rectifier connected to a capacitive filter at the DC side. This model is conformed to many building loads, similar to televisions, personal computers, and fluorescent lamp ballast . In case of three-phase balanced loads, the neutral current is zero, but since several building loads are single phase and include electronic converters, their waves include harmonics which induce a nonzero neutral current. Regarding neutral wire, the more common considered structures are presented in Figure 2. The first structure is based on DC-link neutral point where the neutral wire is generated via two identical capacitors (Figure 2a). In the second structure, the neutral wire is generated through a Delta/Star grounded transformer as shown in Figure 2b . As to the third configuration, it is based on four-leg power converter (Figure 2c) [14, 15, 16]. In this chapter the structure with transformer is adopted.
The PV system presents two operating modes according to the grid state:
Grid connected mode: this mode is activated when the grid is available. In this case, the power surplus is injected into the grid, and if the consumption is superior to local generation, the power flow will be directed from the grid to loads and eventually to charge batteries according to their stat of charge (SOC).
Standalone mode: this mode is activated when the grid is absent. In this case, building loads are supplied first by the PV system then if necessary by the BESS. In case of power deficit, the shedding of non-priority loads is carried out.
3. Power converter control
An overview of the control of each converter presented in Figure 2b is subsequently presented.
3.1 BESS DC/DC converter control
Batteries are frequently integrated to PV systems thanks to their special energy characteristics. Indeed, batteries have a high energy density, which ensure long time of stable operation. The charging time and number of cycles depend on the adopted technology.
The battery power flow is bidirectional. In discharge mode, the power is supplied by battery, and in charging mode, the power is absorbed by battery. For both modes, the state of charge limits should be respected to not affect the battery lifetime.
The BESS incorporates a DC/DC power converter that manages battery operation modes according to the appropriate control. A cascade control is adopted, the inner loop regulates the battery current, and the external one regulates the DC-link voltage.
3.2 PV DC/DC converter control
The structure of the two-stage power conversion is adopted for the PV system; this configuration is commonly privileged in the majority of the PV systems. The difference with the conventional structure is that the
3.3 DC/AC converter control
3.3.1 Modeling of the DC/AC converter
The output of the DC/AC and the LCL filter are modeled in single phase as shown in Figure 5. According to this figure, the obtained results are expressed as follows:
Based on Eq. (3), the transfer function between the current
3.3.2 Control of the DC/AC converter
188.8.131.52 Standalone mode
In standalone mode, the DC/AC converter control ensures that the LCL filter capacitor voltages are equal to their references. In that case, the converter control includes two cascade loops as shown in Figure 7. The external loop is based on a resonant controller
184.108.40.206.1 Tuning of the external loop resonant controller
For simplification reasons, it is assumed that the internal current loop is faster than the external voltage loop. Thus, it can be approximated equal to the unity by associating it with the PWM function. The following block diagram is then obtained for the determination of the external voltage loop resonant controller parameters as presented in Figure 8.
According to Figure 4, the open and closed-loop system transfer functions are expressed by Eqs. (7) and (8), respectively. Note that the transfer function of the resonant controller
The method chosen for the resonant controller parameters tuning is based on the generalized stability margin criterion [17, 18]. The reference polynomial
According to the generalized stability margin criterion, the resonant controller parameters are tuned by identifying the characteristic polynomial of the closed-loop system
The identification of
For the obtained resonant controller parameters, Figure 9 shows the pole maps of
220.127.116.11.2 Tuning of the inner loop gain
The inner current loop must ensure a response time much smaller than the external voltage loop. To this purpose, the gain
18.104.22.168 Grid connected mode
In grid connection operation mode, the DC/AC converter controls power exchange with grid. In this mode, only the current loop is controlled. This loop is based on a resonant controller as shown in Figure 16.
The simplified block diagram of the current loop is given by Figure 17.
For the tuning of the internal loop resonant controller, the generalized stability margin criterion is considered. The system characteristic polynomial
The identification between the system characteristic polynomial
We choose ri equal to 100 and
For the obtained resonant controller parameters, Figure 18 shows the pole map of
4. Simulation results
Several simulation tests developed under PSIM software were done. Figure 21 presents the LCL filter capacitor voltage in islanded mode for different values of voltage reference
5. Experimental results
Figure 24 shows the experimental test bench. The used AC/DC converter is from SEMIKRON. Currents and voltages are censored via LEM LV25 and LEM 55LP, respectively, as given in Figure 24. The control algorithm is implemented on the STM32F4 Discovery. The acquisition time is set to 100 μs. Figure 25 presents the LCL filter capacitor voltage in islanded mode for different values of voltage reference
In this chapter, the control of power converters integrated in building solar system is investigated. The studied system is composed of a PV panel in parallel with a battery energy storage system which are linked to a DC bus, a DC/AC power converter, and an LCL filter interfacing between DC and AC bus. Single- and three-phase linear and nonlinear loads are connected to a four-wire AC bus. The neutral wire is generated through a Delta/Star grounded transformer. An overview of the control of each power converter is presented. This chapter focuses on the control of the DC/AC power converter. The resonant controller is adopted. A set of simulation and experimental tests were done to show the efficiency of the studied control algorithm.
This work was supported by the Tunisian Ministry of Higher Education and Scientific Research under Grant LSE-ENIT-LR11ES15.
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