Control strategies for a hybrid renewable energy

Renewable and Sustainable Energy Reviews 42 (2015) 597–608

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Control strategies for a hybrid renewable energy system: A review P.G. Arul n, Vigna K. Ramachandaramurthy, R.K. Rajkumar Power Quality Research Group, Universiti Tenaga Nasional, Putrajaya Campus, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 15 January 2014 Received in revised form 8 August 2014 Accepted 20 October 2014 Available online 8 November 2014

This paper presents a review of a standalone and grid-connected hybrid renewable energy system (HRES) to supply AC loads. The configuration of the HRESs and interfacing power converters for connecting the energy sources to the AC bus is extensively discussed. An overview of the control concepts in an HRES and the application of the appropriate control schemes for system stabilization, effective injection of high quality power and proper load sharing are discussed. The different approaches for HRES design and control strategies for power converters in the recently published literature are also briefly addressed. Finally, this paper highlights the future developments in HRESs to increase the utilization of power generated from renewable energy sources (RESs). & 2014 Elsevier Ltd. All rights reserved.

Keywords: Hybrid renewable energy system Standalone and grid-connected system Power converters Control strategies

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Interfacing configuration of HRES sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Control concepts in HRESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Power converter topologies and control strategies for a single inverter interfaced HRES . . 5. Power converter topologies and control strategies for a parallel inverter interfaced HRES. 6. Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Power generation by RESs is becoming more popular and economical than the traditional generation systems to supply reliable power in areas not served by conventional power grids [1,2]. RESs are unpredictable and fluctuating in nature and also typically produce low power compared to traditional generation. Hence, some means of integrating multiple sources are required to provide a more reliable and sustainable energy [3]. The integration of various RESs forms a hybrid renewable energy system (HRES), which provides continuous power to the consumers versus a system based on a single source [4,5]. The HRES sources require power converters for the efficient and flexible interconnection of RESs to work either in a standalone or grid-connected mode.

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Corresponding author: Tel.: þ60 123980722/60 3 89212020; fax: þ 60 3 89212116. E-mail address: [email protected] (P.G. Arul).

http://dx.doi.org/10.1016/j.rser.2014.10.062 1364-0321/& 2014 Elsevier Ltd. All rights reserved.

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597 599 599 601 606 607 607 607

However, the HRES with an unpredictable nature for PV and wind cannot supply sufficient and stable power to meet the power demand [6]. To ensure the dynamics of the HRES, several stable power sources, such as batteries, fuel cells (FC) [7], supercapacitors [8], or diesel generators, must be integrated into the HRES especially in standalone mode and into the utility in gridconnected mode. In addition to the various benefits, the HRES has numerous technical challenges on the system power quality [9], such as power fluctuation because of the presence of a new source or plug-and-play feature of RESs, voltage and frequency deviation caused by the transition from grid-connected to standalone mode and vice versa. Therefore, the HRESs must have the ability to mitigate the power quality issues to supply high-quality and more reliable steady power. The power quality and system stability can be achieved by an appropriate control technique embedded into the power converter control circuit. However, the main challenge is to design suitable control strategies for the HRES to overcome the above challenges. The aim of this paper is to

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P.G. Arul / Renewable and Sustainable Energy Reviews 42 (2015) 597–608

Fig. 1. Typical configuration of the DC and AC bus linked HRES.

Fig. 2. Modified configuration of the DC and AC bus linked HRES.

Fig. 3. Typical configuration of the AC bus linked HRES.


review the power conversion and control strategies of the HRES for future developments in generating power from RESs. However, the HRES in this paper is for self-consumption and the power grid acts as a source of power support.

2. Interfacing configuration of HRES sources The HRES can be interfaced to an AC bus line and then to the utility directly or via a common DC bus by using the appropriate power converters [10]. The typical configuration of a DC and AC bus linked HRES is shown in Fig. 1. In this HRES configuration, a group of PV panels are interfaced through a DC–DC converter to regulate their fluctuating DC output. The wind turbine coupled with a permanent magnet synchronous generator (PMSG) generates a three-phase AC voltage, and its amplitude and frequency varies with rotor speed. Therefore, the wind turbine generator is connected to the DC bus via a rectifier and DC/DC converter [11]. Furthermore, the wind turbine coupled with a brushless doubly fed machine (BDFM) can work with a fluctuating wind speed and generates a three-phase AC voltage with a constant frequency by controlling the exciting converter frequency [12]. Hence, the wind turbine coupled with the BDFM can be directly connected to the AC bus as shown in Fig. 2. The storage battery is connected to the DC bus through a bidirectional DC–DC converter to maintain a stable supply–demand balance at its rated capacity. The common DC bus collects the regulated power from various RESs to supply DC loads and maintain a constant DC voltage at the input terminal of the DC–AC inverter. A single DC–AC inverter is used to interface the common DC bus to the AC bus. The AC bus feeds the inverted power to the AC loads and also has the flexibility to integrate with the utility when the power generated from the HRES sources is not sufficient to meet the power demand. In the modified HRES configuration as shown in Fig. 2, the DC sources and loads are connected to the DC bus, whereas the AC sources and loads are connected to the AC bus. The DC and AC buses are integrated with a single bidirectional inverter. The HRES configuration illustrated in Figs. 1 and 2 has the following advantages. The power converters in the system require a simple control algorithm because source side converters are only respon-

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sible for harvesting the maximum power from the RESs, controlling the coordination of the real power output among the RESs or regulating the DC bus voltage. In addition, the load/utility side converters are only responsible for controlling the voltage, frequency, real and reactive power and harmonics for maintaining the power quality. The output of the RESs can be flexibly regulated (either buck or boost) by the DC–DC converter to meet the requirement of the DC–AC inverter input [13,14]. Most of the RESs and storage devices are DC based and can easily be connected to the DC bus line [15]. Although the HRES configurations illustrated in Figs. 1 and 2 have significant benefits, the efficiency of the system is reduced because of losses in a number of power converter stages. Moreover, the DC–DC converter must be cut off during low power levels to prevent inefficient operation. Thus, the generation by the RESs remains unutilized. This may be acceptable for grid-connected applications but could be a matter of concern for standalone applications [16]. If the function of the source side DC–DC converter is merged with the inversion stage, even a small fraction of the power generated by RESs can be utilized [17]. In addition, if the single DC–AC inverter fails, the whole HRES will face blackout. Fig. 3 shows the typical configuration of the HRES, connecting various RESs directly to the AC bus line and/or utility through individual DC–AC inverters. In this configuration, if any one of the inverters fails, the HRES can still supply the required amount of power from the remaining sources [18]. Therefore, the HRES configuration illustrated in Fig. 3 is more reliable and economical and improves the system's efficiency [19] compared to the HRES shown in Figs. 1 and 2. However, this system has the disadvantage of requiring a complicated control algorithm for the DC–AC inverters to regulate the power and current injected into the AC bus and/or utility.

3. Control concepts in HRESs The HRES sources must be properly controlled by specific power converter control schemes to perform voltage and power regulation in standalone and grid-connected mode. The control concept of the DC and AC bus linked HRES is shown in Fig. 4. In standalone mode, one of the sources producing stable

Fig. 4. Control concept of the DC and AC bus linked HRES.

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Fig. 5. Control concept of the AC bus linked HRES.

Fig. 6. Proposed HRES configuration in [37].

output power must be operated in voltage-controlled mode to regulate the DC bus voltage, and the remaining sources should be operated in current-controlled mode to control the coordination of power among the RESs [20]. In addition, the inverter on the AC side must be operated in voltage-controlled mode to keep the voltage and frequency of the AC bus constant. Therefore, the voltage and frequency (Vf) control scheme must be implemented in the inverter control circuit to perform the voltage and frequency regulation [21–23]. In grid-connected mode, the inverter must be operated in the current control mode [24] to regulate the active and reactive power injected into the grid and to supply the AC loads. Therefore, the inverter control circuit must be adopted with an active and reactive power (PQ) control scheme [22,23]. The control concept of the AC bus linked HRES is shown in Fig. 5. In an AC bus linked HRES, all of the sources are connected in parallel to the AC bus. In standalone mode, the HRES sources are responsible for maintaining the AC bus voltage constant, sharing the load according to their ratings, and controlling the availability of power from their energy source [25]. To maintain a constant AC bus voltage, at least one of the sources producing a stable output power must be operated in a voltage-controlled mode [26]. Because all of the HRES sources are connected to the same AC bus, the remaining sources will follow the AC bus voltage and should be able to operate in the current-controlled mode to supply high quality power [20]. Therefore, the control circuit of the stable source must be implemented with a Vf control scheme to perform voltage and frequency regulation, and the control circuit of the

Table 1 Details of the HRES presented in [37]. Source Power converter topology Controller(s) Control technique Control device Implementation topology Simulation tool Operating mode(s)

PV (450 250 V), Wind (80–200 V) Multi-input DC–DC buck converter, Single-phase full bridge inverter Not specified SPWM Microcontroller PICI8F4431 Experimental Not applicable Grid-connected

remaining sources can be adopted with the following control schemes to regulate the current injected into the AC bus. (i) PQ control scheme (ii) Active Power and Voltage (PV) control scheme In grid-connected mode, the AC bus voltage and frequency reference is fixed by the grid. Therefore, the converter control circuit may be embedded with any of the PQ, PV, or Vf control schemes. However, in an AC bus linked standalone HRES, the source operated in voltage-controlled mode behaves as the master and the other sources behave as the slave [18,19]. The problem with the master–slave operation is that the whole HRES must be shut down when there is a disturbance or fault in the master unit [27]. The AC bus voltage of the HRES can be maintained stable if more than one


source is operated in voltage-controlled mode. As an example, the stable slave source with the highest output power can automatically become the master source [28]. Moreover, proper load sharing

601

among the parallel-connected RESs can be achieved by the current/power sharing control (CSC) [29] or droop control concepts [30]. The CSC strategy has the advantage of good load sharing and dynamic response and can reduce the circulating currents between the converters. However, the CSC strategy requires a communication link to synchronize the sources for sharing the load, lowers the system reliability and limits the flexibility (i.e., plug and play) of the system. The droop control concept for load sharing does not require any communication interface among the sources [31], and it also increases the reliability and flexibility of the system [32]. However, the droop control concept has some drawbacks, such as slow dynamic response, frequency and voltage deviations and a lack of system stability due to line impedance especially in a low voltage standalone system [33,34]. The independent control concept of energy sources as shown in Figs. 4 and 5 allows the HRES to connect or disconnect several sources without restructuring the system and eliminates the requirement of a complex control algorithm [35,36].

4. Power converter topologies and control strategies for a single inverter interfaced HRES Fig. 7. Proposed voltage change-trend hysteresis band control st.

Table 2 Details of the HRES presented in [38]. Source Power converter topology Controller(s) Control technique Control device Implementation topology Simulation tool Operating mode(s)

PV 1 (2 kW), PV 2 (3 kW) DC–DC boost converter, Three-phase VSI PI PWM, Hysteresis PWM Not applicable Simulation Matlab/Simulink Grid-connected

The HRES configuration proposed in [37] is shown in Fig. 6, and the details are listed in Table 1. A multi-input DC–DC converter with maximum power point tracking (MPPT) control transfers the maximum power from the RESs to the DC bus individually or simultaneously. If one of the sources fails to generate power, the multi-input DC–DC converter can still transfer the maximum power from the other source. The full bridge inverter converts the total available DC power into AC power and injects a sinusoidal AC current into the utility. According to this topology, the filter at the DC bus regulates the DC bus voltage and improves the power quality of the utility. The function of the DC bus filter is to buffer an energy balance when the energy between the source and load is not exactly the same. The DC bus capacitor voltage change is

Fig. 8. Overall power converter control scheme for the standalone, (a). Inverter control strategy for the grid-connected mode

602


where ΔVdc is the change in the DC bus capacitor voltage. Based on (1), the voltage across the DC bus capacitor changes when there is a difference between the input and output active powers of the DC bus filter. Fig. 7 shows that the DC bus voltage Vdc is regulated within the hysteresis band limits by adjusting the injected AC current command iac, ref. It can be seen that the change in the injected AC current command only occurs in a specific time segment. Therefore, the power quality of the utility can be improved by injecting a stable AC current. This control strategy is implemented in the microcontroller, which produces the gate control signals for the full bridge inverter to generate the injected AC current and regulate the DC bus voltage. A similar HRES approach is presented in [38,39]. In [38], a single-phase current hysteresis PWM control strategy is proposed for the three-phase DC–AC inverter, which effectively reduces the switching losses and injects low harmonic distorted three-phase sinusoidal currents with a unity power factor into the grid.

Furthermore, hysteresis control provides a fast dynamic response and good accuracy [40]. However, the major problem with hysteresis control is the resultant variable switching frequency of the converter [24,41]. The details of this HRES are listed in Table 2. In [39], the three-phase full bridge inverter with the sinusoidal pulse width modulation (SPWM) control technique eliminates the side band harmonics and supplies a low harmonic, distorted threephase AC output voltage by limiting the modulation index mi to an optimal value (i.e., mi o1). However, it should be noted that the DC link capacitor is suitable only for transient stability, and the integration of stable sources in the DC link is more appropriate for the long term to ensure the stability of the HRES. Ref. [42] presented the DC and AC bus linked HRES. The overall power converter control scheme in standalone mode is shown in Fig. 8, whereas the inverter control scheme in grid-connected mode is shown in Fig. 8(a). In this HRES configuration, all of the sources are connected in parallel to the common DC bus through their individual DC–DC converter. The MPPT feature is also realized. The storage battery is directly connected to the DC bus to smooth the power fluctuation and the stability of the DC bus

Table 3 Details of the HRES presented in [42].

Table 4 Details of the HRES presented in [45].

expressed as: Z 1 P in P out dt C dc ΔV 2dc ¼ 2

Source Power converter topology Controller(s) Control technique Control device Implementation topology Simulation tool Operating mode(s)

ð1Þ

PV (400 W), Wind (600 W) DC–DC buck converter, Single-phase full bridge inverter PI PWM Dspace Experimental Not applicable Standalone and grid-connected

Source Power converter topology Controller(s) Control technique Control device Implementation topology Simulation tool Operating mode(s)

Fig. 9. Control strategies for the HRES presented in [45].

PV (750 W), Wind (1 kW), FC (1.2 kW) DC–DC boost converter, Single-phase full bridge inverter PI PWM Not applicable Simulation PSIM Grid-connected


voltage. Supervisory energy management [43,44] with a real-time control system monitors and controls the power supply–demand balance to ensure the stable operation of the entire HRES. The single-phase full bridge inverter connects the DC bus to the AC bus and operates in two modes. During standalone mode, the inverter with the voltage control strategy regulates the magnitude and frequency of AC bus voltage by controlling the load current in the inner loop and the bus voltage in the outer loop. During gridconnected mode, the inverter with the current control strategy regulates the current injected into the utility by controlling the inverter output current Io. It was observed that the proposed HRES provides a reliable and continuous power supply with a smooth transition from the standalone to the grid-connected mode and vice versa even if there are interruptions or fluctuations. The details of this HRES are listed in Table 3. Ref. [45] also presented the DC and AC bus linked HRES, and the complete converter control strategy is illustrated in Fig. 9. In this configuration, the DC–DC converters of the PV and wind source are incorporated with the voltage-based MPPT control technique to extract the maximum power from the sources. An additional DC–DC boost converter is used to control the FC to balance power fluctuation. The output voltage from each source is controlled independently by using a voltage controller. A single-phase current controlled inverter connecting the DC and AC bus controls the current injected into the grid and also regulates the DC bus voltage. The phase angle of the grid voltage is obtained by using phaselocked loop (PLL) to control the grid current in phase with the grid voltage to maintain the unity power factor. The LC filter at the inverter output eliminates the switching harmonics and reduces the system losses [46]. The proposed HRES supplies continuous high quality power with better reliability than a system with a single source. The details of this HRES are listed in Table 4. Ref. [47] presented a similar DC and AC bus linked HRES. The details of this HRES configuration are listed in Table 5. In this

603

configuration, a three-phase current controlled voltage source inverter interfaces with the common DC bus to the utility through a series impedance, and its control strategy is shown in Fig. 10. In this control strategy [48], the DC bus voltage is controlled to ensure sufficient injection of the active power into the utility. The output of this controller produces the reference active power for inverter control. The inner current control loops control the active and reactive power injected into the utility by independently controlling the d-axis and q-axis components of the inverter output currents id and iq in a dq rotating reference frame. It was observed that the power factor of the system is maintained at unity and that the total harmonic distortion of the injected current is reduced. Moreover, the decoupled control will enhance the system's performance and stability. An HRES combining a variety of sources is also presented in [49], and the details of this configuration are listed in Table 6. Its control strategy shown in Fig. 11 is employed in the wind turbine to extract the maximum power by controlling the pitch angle and speed of the generator. The pitch angle control uses wind speed signals and the power output of the generator as the inputs. The speed control is realized through field orientation by setting idr ¼0, and the q-axis current is used to control the rotational speed of the PMSG according to the variation in the wind speed. The converter circuit of the storage units is implemented with the current control strategy as shown in Fig. 12 for both charging and discharging. The three-phase voltage source inverter interfacing the common DC bus is employed with a similar control strategy as in [48] to regulate the active and reactive power flow. In Ref. [50], a standalone microgrid model is presented by combining three HRESs. An active power and voltage (PV) control scheme as shown in Fig. 13 is adopted for the standalone single three-phase inverter interfacing the common DC voltage bus linking the diversity of the RESs and energy storage units. This control strategy consists of two cascade loops to regulate the

Table 6 Details of the HRES presented in [49]. Table 5 Details of the HRES presented in [47]. Source Power converter topology Controller(s) Control technique Control device Implementation topology Simulation tool Operating mode(s)

PV (15 kW), Wind (20 kW), FC (10 kW) DC–DC boost converter, Three-phase voltage source inverter PI PWM Not applicable Simulation PSIM Grid-connected

Source Power converter topology

PV (1 kW), Wind (2 kW), battery and supercapacitor DC–DC boost converter for PV, Three-phase controlled rectifier for wind, Bi-directional DC–DC boost converter for storage units and Three-phase voltage source inverter Controller(s) PI Control technique PWM, SVPWM Control device Not applicable Implementation topology Not specified Simulation tool Not applicable Operating mode(s) Standalone

Fig. 10. Control strategy for the inverter proposed in [48].

604


Fig. 11. Wind turbine controller proposed in [49].

Table 7 Details of the HRES presented in [55]. Source

Fig. 12. Control circuit for the storage devices proposed in [49].

Fig. 13. Active power–voltage control scheme proposed in [50].

Fig. 14. Voltage control scheme proposed in [34].

PV (5 kW), wind (1 kW), battery (5 kW/20 kWh), FC (1.2 kW/4.8 kWh) and DG (8 kW) Power converter DC–DC buck–boost converter for PV, Full-wave bridge topology rectifier & DC–DC boost converter for wind, Bi-directional inverter. Controller(s) PI Control technique PWM Control device PLC Implementation Simulation model Simulation tool PSCAD/EMTDC Operating mode(s) Standalone

active power injection and also to maintain the magnitude of the AC bus voltage. The inner current control loops independently regulate the d-axis and q-axis components of the inverter output currents id and iq in the dq rotating reference frame. The reference value of the inverter output currents is obtained from the controlled active power and voltage in the outer loop. The compensated outputs of the two current controllers are used to generate the gate control signals of the inverter switches. PLL is used to control the angular position of the dq reference frame by using a feedback loop, which forces the q-axis component to zero. The Pf droop control is used to adapt to the load change. Ref. [51] proposed a fuzzy and decoupled dq based PV control strategy for the standalone three-phase voltage source inverter interfacing the DC bus linked HRES. In this control strategy, fuzzy rules are used to set the parameters of the PI controllers to achieve a greater response for large signal disturbances. Ref. [34] presented three individual HRESs connected in a microgrid, and each system consists of RESs, an energy storage system and a grid interfacing inverter with virtual inductance at its output. The voltage control scheme used in the single threephase inverter control circuit is shown in Fig. 14. The virtual inductance in the proposed control strategy effectively decouples and can accurately control the real and reactive power in both standalone and grid-connected mode. A concern for the virtual inductor voltage control scheme is the differentiation of the line


605

current Iline. Differentiation can cause high frequency noise amplification, which in turn may destabilize the voltage control scheme especially during a transient. A general approach to avoid noise amplification is to add a low pass filter [52,53] or high pass filter [54] to avoid the introduction of excessive noise. However, this approach is the tradeoff between the overall control scheme stability and the virtual inductor control accuracy.

According to the configuration in Fig. 2, a similar HRES is presented in [55], and the details are listed in Table 7. In this HRES, RESs and storage devices are integrated into the DC bus to supply the required amount of power, and a diesel generator connected to the AC bus is used as an emergency backup if all of the sources are exhausted. A bi-directional inverter cascaded with a buck-boost converter and single-phase controlled rectifier links the DC and AC bus. The synchronization of the inverter output voltage and the AC system voltage is achieved through the PLL. The control algorithm implemented in the programmable logic controller (PLC) allows the HRES to operate in different modes to meet the power demand.

Fig. 15. HRES configuration proposed in [33].

Fig. 18. Active and reactive power control scheme proposed in [50].

Fig. 16. Modified PQ control strategy proposed in [33].

Fig. 17. Pf and QV droop scheme [49,57,58].

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5. Power converter topologies and control strategies for a parallel inverter interfaced HRES Ref. [33] investigated the control strategies of an AC bus linked standalone HRES combining the wind, diesel and storage battery as shown in Fig. 15. The storage battery supports the system stability by controlling the voltage and frequency during a sudden change in load and wind speed variation [56]. The excess power

from the wind source is used to charge the battery. When the battery is fully charged, the excess power from the wind source is consumed by the dump load. During low wind speed, the diesel engine provides the deficit power to keep the frequency of the system constant. The bidirectional voltage source converter adopted with the modified pq control strategy as shown in Fig. 16 controls the charging and discharging of the battery. In addition, the voltage and frequency is maintained at a constant value under varying linear loads and variable wind speeds. The active power filter (APF) compensates for the harmonics generated by nonlinear loads. It was observed that the modified pq control strategy maintains the system voltage and frequency constant under different dynamic conditions. Ref. [49] also discussed the droop control scheme for parallel inverters connecting HRES sources to the AC power system. The flow of the active and reactive power between the sources and AC bus can be determined by the vector relationship between the inverter output voltage V and power angle δ along with the inductor's reactance. The mathematical relationship for P and Q is shown in Eqs. (2) and (3): P¼

Fig. 19. Microgrid model proposed in [62].

3VE sin δ 2 ωL

ð2Þ

3V V E cos δ 2ωL

ð3Þ

Q¼

Fig. 20. Vf control strategy proposed in [62].

From Eqs. (2) and (3), P is dependent on the power angle and δ and Q are dependent on the inverter output voltage V. Therefore, P and Q can be independently controlled by using the Pf and QV droop scheme as illustrated in Fig. 17 for by sharing the currents [57,58]. Controlling the power using the conventional Pf and QV droop will introduce a significant coupling between P and Q especially during transients. In [34], the power control and sharing scheme for two parallel inverters in microgrid interfacing HRESs is presented. The active and reactive powers are effectively decoupled by connecting the virtual inductance at the inverter output as shown in Fig. 14. The decoupled power control improves the system's stability. However, this approach

Fig. 21. Power control strategy proposed in [62].


may increase the reactive power control and sharing error because of an increased impedance voltage drop. By incorporating the line voltage drop effect into the power control scheme, the reactive power control and sharing can be improved. This can be realized by adding the ΔV/Q slopes into the voltage droop control. The control scheme for parallel three-phase inverters interfacing two individual HRESs in standalone mode is presented in [50]. One inverter is operated in voltage control mode to keep the bus voltage constant, and the other inverter is operated in power control mode to regulate the active and reactive power from the energy sources. The PQ control scheme as shown in Fig. 18 is adopted in one of the parallel inverters. The total active power is shared by two inverters by adapting the power–frequency (Pf) droop control scheme. It should be noted that when the load undergoes a step change, the bus voltage is regulated at its nominal value after a short transient. The PV and PQ control scheme combined with the droop control concept is an efficient method for the parallel operation of inverters [59]. The parallel operation of inverters in the AC bus linked standalone HRES is presented in [60,61]. The single-master and multi-master approaches are used in the inverter control strategies. In the single-master approach, one of the inverters is operated in voltage-controlled mode, and the other inverters are operated in PQ mode. In the multi-master approach, more than one inverter is operated in voltage-controlled mode, and the other PQ inverters may also coexist. An AC bus linked microgrid model proposed in [62] is shown in Fig. 19. The Vf control and power control strategies as shown in Figs. 20 and 21 are integrated into the voltage source inverter for the microgrid to operate in both standalone and grid-connected modes. When there is no grid support (standalone mode), the Vf control strategy is enabled to regulate the voltage and frequency at the point of common coupling (PCC) and also to provide power balance for the isolated system. When the state of charge (SOC) of the battery reaches its minimum limit, the power control strategy is enabled to charge the battery from the grid. After the battery gets charged, the power control strategy switches from charging mode to discharging mode or PQ control mode to minimize the grid consumption.

6. Future trends The trend in developing countries is to install a standalone HRES for electrifying rural regions that do not have the access to the power grid. However, many of these installed generation units are not connected to the existing power grid. Further research and development for HRESs is required for the future power grid mix with generation from an installed standalone HRES to address the following:

Minimize the power conversion losses as much as possible by

using an appropriate design for the HRES configuration and for advancement in power converter topologies. Enhance the performance of the HRES through the design and implementation of advanced control strategies [63] to supply quality power. The existing power grids based on conventional power generation systems are constructed without considering the integration of HRESs. Therefore, the standalone HRES must rely on storage batteries to store excess power and mitigate several technical issues, such as voltage fluctuations, voltage flicker and fluctuating power loads. Moreover, the grid-connected HRES requires storage batteries to store the excess power because not all of the utilities buy power back from these power generation units. The use of storage batteries may have unexpected

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environmental consequences. To expand the utilization of the HRES sources with the conventional power generation systems, the stability of the HRES should be further improved to overcome the constraints of the existing power grid.

7. Conclusion In this paper, several aspects of the HRES configuration and control strategies for standalone and grid-connected HRESs are specifically reviewed. It is important for the HRES to have appropriate interfacing power conversion circuits and controllers. The AC bus-linked HRES configuration reduces the number of power conversion stages and losses in power transferred to the load/ utility. The control strategy based on a communication link increases the control complexity and affects the expandability of the HRES. The master-slave control with the droop concept does not require a communication link and provides good load sharing. In addition, the master–slave concept adds features, such as the flexibility, expandability and modularity of the HRES. However, enhancements in the HRES's efficiency, quality, stability and reliability are a few of the future research needs and require advances in power electronic devices and control techniques. This review will help supply viable solutions to enhance the HRES.

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