*2.1. Definitions of Ancillary Service*

In this section, a brief introduction to ancillary services has been given with standard definitions from the literature. An insight to Reactive Power (Q) being an ancillary service is provided. In order to understand the concept of ancillary services, a few definitions from the literature have been listed here.


### *2.2. Popular Ancillary Services in Electric Power Market*

Figure 2 shows some popular ancillary services in electric power market. They are:


**Figure 2.** Popular ancillary services in electric power market suggested by FERC.

### *2.3. Additional Services in Electric Power Market*

Figure 3 shows additional services in electric power market suggested by FERC. They are:

	- • Transmission system monitoring and control
	- • Transmission reserves
	- • Repair and maintenance of the transmission network
	- • Metering, billing and communications.
	- a. Planning services:
	- • Black-start studies
	- • Load-flow analysis
	- • Planning for bulk-power system expansion.
	- • Scheduling
	- • Billing
	- • Contract administration
	- • Reporting to several regulatory bodies.

**Figure 3.** Additional services in electric power market suggested by FERC.

### *2.4. Q Injection to Grid*

One of the primary ancillary services that is necessary for a power system operator is Q injection to grid [5]. In Figure 4, the red curve indicates the capability of the PV inverter to provide Q. Furthermore, based on the voltage at point of common coupling (PCC), freedom of having higher current distortion is permissible. Several countries have added Reactive power injection to grid into the countries' standard grid code (GC) requirements. In general, if a country follows standard GC, power generation by PVs is required to cease immediately when there is a fault occurring in the grid. However, because of high level of penetration of PVs into grid, a sudden and quick power interruption due to a fault in the grid would cause severe problems. For to this reason, many countries like Spain, Italy, Germany and Japan have modified their GCs [6–9].

**Figure 4.** PV inverter reactive power capability based on current limits.

There are numerous services that can be extracted with the use of PVs. Figure 5 shows some of the important ancillary services involving solar PVs. It can be noted that ancillary services provided by PV systems open an important pathway in electric power market and Q injection to grid has been area of research for the last three decades [10–22].

**Figure 5.** Services provided by PV systems.

Solar-PV panels do not possess Q, since they provide electric power by using PV effect. The power conversion from DC of solar panels to AC injected to grid takes place due to inverter circuitry. This inverter has the capability of providing Q support in fault/normal conditions. Inverters could provide various other ancillary services. Some of these such as lowvoltage ride-through (LVRT) and MPPT have become necessary. Although, Q support has not been made mandatory for grid connected PV systems, the higher penetration levels of PVs indicate more accessibility to control of P and Q. Hence, it would become a code included in GCs of all countries using more renewable form of power conversion. In general, for PV-grid topologies, the inverter converts the DC of PV panels to alternating current (AC) that is to be supplied to grid. Figure 6 shows a single-phase PV-grid system that can be used for requirements up to 7 kW. There are many types of inverters that are used in a PV-grid scenario. In the following section, a brief summary of inverter topologies for use in grid-connected systems is provided.

**Figure 6.** A sketch of single-phase PV-grid system.

### **3. PV-Grid Inverters—A Summary of Di**ff**erent Topologies**

Numerous works have been proposed in literature to illustrate various topologies of inverters including state-of-art review [23]. Traditional inverters such as voltage source inverter (VSI) and current source inverter (CSI) have a major drawback, i.e., voltage buck and boost actions cannot take place simultaneously. In order that buck and boost actions take place collectively, an additional converter has to be added in the circuitry, making the whole system more expensive. Popular impedance source inverters (ZSIs) have been discussed in the literature; they have the ability to overcome the major disadvantage of involving a two-stage topology in power conversion. Both boosting and bucking actions are possible with this topology. ZSI is a combination of VSI and CSI. Boosting of voltage takes place at the DC link with the help of a unique technique called shoot-through [24,25]. In recent years, an interesting inverter topology namely admittance source inverter (YSI) was introduced. The following section gives an overview on different inverter topologies available in literature.

### *3.1. Traditional Inverters Vs Multilevel Inverters*

One of the traditional configurations of inverters that is connected to power grid is VSI (shown in Figure 7). In VSIs, the output voltage is always lesser than the input voltage. VSIs have the ability to introduce currents with low harmonics into the grid. When a CSI (shown in Figure 8) is used instead of VSI, current injection to grid can take place without the need of an additional converter. The output from a VSI and CSI comprises of two unique levels of voltage, but it suffers from higher switching losses. The rate of change of voltage (dv/dt) is higher for traditional two-level inverters. The frequency of switching is also high. They are most suited for low voltage applications.

**Figure 7.** Voltage source inverter.

Multilevel inverters (MLIs) were introduced to overcome the drawbacks of traditional inverters. The classification of MLIs is given in Figure 9. Switching losses are a main factor of concern in two level inverters. Using MLIs, they can be minimized.MLIs aid to reduce switching losses and harmonics. They can be used for high voltage applications. The rate of change of voltage (dv/dt) is lesser for MLIs. The levels of voltage could be increased to greater than two. Hence, apure sinusoidal waveform is obtained as the output of the inverter. The harmonics in the output are mitigated and losses could be reduced largely. With the introduction of multilevel topology in CSI (shown in Figure 10), low harmonic currents are obtained. The frequency at which the switching action takes place is reduced with the introduction of a multilevel topology for a current source inverter. A brief comparison between traditional inverters and multilevel inverters is presented in Table 1. Table 2 summarizes the state of art PV grid inverter topologies of MLIs.

**Figure 8.** Current source inverter.

**Figure 9.** Classification of multilevel inverter topologies.

**Figure 10.** A multilevel CSI topology.


**Table 1.** Traditional two-level inverters Vs MLI.



#### *3.2. Concept of Z Source and Its Application in Solar Industry*

Even though multilevel inverters have shown better performance than traditional inverters, they still have drawbacks. The number of switches is quite high in an MLI. Although the switches required need smaller rating, the number of required switches is high, thus making the circuit complex and costly. Thus, ZSIs with several advantages over the aforementioned inverters were introduced. Figure 11 shows a voltage fed ZSI.

A ZSI is a combination of inductors and capacitors. A ZSI would operate as a VSI or CSI depending on the application. The output voltage ranges from zero to infinity. Many researchers have adapted impedance source topologies and many advances in the topologies have been listed in literature like YSIs and their advancements [26,27] and ZSIs and their advancements [28–65]. Figure 12a–c give an overall classification of topologies of impedance source networks. A summary of these topologies, as presented in different literature works, is presented in the following section.

**Figure 11.** A voltage fed ZSI.

Solar modules are widely preferred in both residential and commercial applications. PV cells are connected in parallel and series in order to form one module. Many such modules in combination is a panel. To develop economical and e fficient PV systems, MPPT algorithms are used. Generally, the inverter portion of the PV-inverter-grid structure comprises of a boost circuit and a filter. MPPT algorithms may or may not be used depending upon the application. In PV systems, in order to obtain dc-ac conversion, ZSI is an intelligent choice [66]. ZSIs can boost the voltage levels with a very compact structure. For a 10 kilowatt (kW) PV system, 20 kW inverter is required with a traditional inverter but by using ZSIs, a 10 kW inverter is enough for a 10 kW PV system with same kilo volt-ampere (KVA) maintained. Traditional inverters pose challenges in their control and modulation mmechanisms. These issues are eradicated using ZSIs.

The boost factor for a simple boost control method can be obtained from Equations (1)and (2) where M is the modulation index, and B is the Boost factor, T is the total time-period, which is one complete cycle. T0 is the time-period for which the output waveform is obtained.

$$\mathbf{B} = 1/(2\mathbf{M} - \mathbf{1})\tag{1}$$

$$1 - \mathbf{M} = \mathbf{T}0/\mathbf{T} \tag{2}$$

**Figure 12.** *Cont*.

**Figure 12.** *Cont*.

**Figure 12.** (**a**) Broad classification of Z source network topologies. (**b**) Classificztion of Z source transformerless topologies. (**c**) Classification of Z source topologies with transformer/coupled inductor.

Summaries of stateof the art PV-grid inverter topologies of Z source networks without transformer and with transformer/coupled inductor arepresented in Tables 3 and 4 respectively. The features of each structure with components used, including passive elements and semiconductor devices peer reviewed from different literature works are listed. Detailed topological figures can be obtained from the respective reference papers cited for each structure listed in the tables.




**Table 4.** State of art PV grid inverter topologies of Z source networks with transformer/coupled inductor.

In Tables 3 and 4, the following abbreviations were used


### *3.3. Grid Integration Configurations, Synchronization& Standards*

Grid-integrated PV systems could be of various power levels and sizes. They are designed for specific applications and needs, with a scope ranging from one PV module to over 100 MW [69]. Hence, a generic PV-inverter-grid structure, as shown in Figure 13, could vary for each plant.

**Figure 13.** A generic structure of a PV-inverter-grid structure (Picture courtesy of **ASEA** Brown Boveri).

In order to make things seem less complex, PV-grid systems are divided based on power rating into


Table 5 gives a summary of PV-grid-inverter configurations along with pros and cons of each configuration to provide a clear-cut guidance in choosing the type of system depending upon the requirements.



**Figure 14.** PV-Grid Synchronization methods. Reproduced from [70], 14th European Conference on Power Electronics and Applications (EPE): 2011.

Synchronization of the inverter with the grid is a major challenge in grid integration. Typically, inverters operate like current sources that inject the current in phase with grid voltage [71]. Therefore, pf needs to be maintained at unity or near to unity while the grid is connected to an inverter system. The most important thing is the synchronization of the inverter with the grid voltage. The rule of thumb for synchronization is that the total real power of the grid must be equal to the voltage of the grid and current of the inverter summed. Based on the synchronization rule, the Equation (3) is derived.

$$\mathbf{P(grid)} = \mathbf{V(grid)} + \mathbf{I(inverter)}\tag{3}$$

Several methodologies can be studied from literature for synchronization of grid and PV inverter. Figure 14 gives a brief of literature works surveyed in this regard.Grid integration and the injection of current into the grid play a critical role in the operation of a grid connected PV system. Different works have highlighted current injection into the grid in accordance with recommended standards [72–87].

Due to the increase in PV-grid applications, many standards and GCs are proposed in order to have secure transmission of power into grid. Some of the well-known bodies that develop the standards are Institute of Electrical and Electronic Engineers (IEEE) of USA, IEC of Switzerland and Deutsche Kommission Elektrotechnik (DKE) of Germany. A summary of these standards and GCs is given in Table 6.


**Table 6.** A Summary of International codes for PV applications.

#### **4. A Summary of Intelligent Algorithms & Optimization Techniques in Grid-Tied Inverters**

Due to a rapid increase in complexity, optimization has become necessary in the design of every system. When PVs are involved, it means that there is going to be intermittency in the output power. In order that the load is fed without any fluctuation, optimization techniques must be incorporated to ge<sup>t</sup> smoother and better output. In order to understand modern intelligent algorithms and optimization techniques, one must have an understanding on the computational intelligence, which is used along with optimization techniques. Figure 15 lists the computational intelligence platforms that are discussed briefly in the following section.


**Figure 15.** Computational intelligence techniques.

Figure 16a shows the classification of exact optimization depending on treatment of uncertainties. Figure 16b shows the classification of heuristic optimization. Table 7 lists the optimization techniques used in transmission and distribution systems with Q as one of the control variables. Table 8 summarizes various Q control techniques applied to the different sets of surveyed configurations.

 **Figure 16.** (**a**) Classification of exact optimization depending on treatment of uncertainties. (**b**) Classification of Heuristic optimization.





### **5. Conclusions and Future Scope**

Grid-tied inverter topologies are important components for the interface between the RER and the utility grid. Now, single-phase, transformerless configurations of range 1–10 kW are gaining interest. When compared to transformer-based configurations, the main advantages of transformerless configurations are:


Thanks to the technological advancements in the area of power electronics, numerous transformerless inverters derived from conventional H-bridge topology have been developed. These inverters o ffer high e fficiency and reliability. They also have lower electromagnetic interference, since transformers or coupled inductors are not involved in the design. In recent times, low-e fficiency PV arrays have been widely used. In order to achieve maximal e fficiency, the materials involved in fabrication of PV panels need to be carefully investigated and used. In this paper, a critical review of grid connected PV systems was performed. The definition of ancillary services and the reactive power market with reactive power as an ancillary service was examined. A review of the di fferent topologies of inverters with special reference to state of art topologies such as y source inverter derivatives was presented. Unique aspects of each topology in terms of structure and functional merits/demerits were presented in detail. In the coming era, a basic understanding of power converters becomes necessary for the successful integration of PVs with grid. Fulfilling the GC requirements also becomes a major challenge. Hence, in this paper, the synchronization between the inverter and the grid was examined, with the aim of outlining important concepts in grid synchronization and standards. Finally, intelligent algorithms and optimization techniques surveyed from di fferent literature works were listed. A summary of di fferent works available in the literature has been presented with the aim of providing researchers with an overview ofgrid-connected architectures. With the advent of Perovskite material used in solar cells, solar technology has seen tremendous advances. Future work may focus on the manufacturing side of solar cells, since this is currently an area of grea<sup>t</sup> discussion.

**Author Contributions:** All authors contributed equally in this research activities for its final presentation as full research article.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
