Communication Systems in Distributed Generation: A Bibliographical Review and Frameworks

The exhaustion of natural energy and oil reserves has initiated the concept of renewable energy systems (RESs). This has expanded the vision of energy sector towards a diversified power grid while introducing the distributed energy resources (DERs) and distributed generation (DG). Though, this diversification is achieved by adding new energy generation sources and a two-way power flow, it opens the channel of production and trading with alternating current (AC) and direct current (DC) energy formats. But DC-based energy, due to its sporadic nature, can be further stored easily by energy storage devices. However, in recent years, a compelling need has arisen to understand the communication systems in distributed generation (DG) for better performance management, control and parallel power transfer. In this article, a bibliographic review on communication systems in distributed generations (DGs) is provided. The study identifies various communication technologies, standards, and protocols used in AC and DC-based DGs. Moreover, it contains the classification of different frameworks and methods involved. The methodology of different approaches and their likely combination are discussed for different types of communication networks. This study also represents useful information for readers, thereby demonstrate the complete life-cycle of digital data in sensors/actuators, transmitter, receiver, filter, decoder for control of DG elements and identifies future challenges as well. A comprehensive list of publications to date are compiled to provide a complete picture of different developments in this area.

Renewable energy systems (RESs) are getting more importance during the last two decades [1]. This is due to the hazardous concerns of: 1) depletion of traditional energy resources such as coal, diesel, oil, gas, 2) getting clean and healthy environment, and 3) increasing demand of energy with the population growth and changing life style. Since most of the RES are inherently DG-based, the resources such as photovoltaic (PV) panels, wind turbines, e-plants, energy storage systems (ESSs) can be directly connected to a DG [2]- [9]. In DER, voltage levels can be easily managed. On the other hand, in conventional AC grid system, transformers are used to step down the voltage levels, DC voltage levels can be changed by using DC-DC converter [10]. Moreover, DC power systems require no reactive power compensation and bear no skin and proximity effects [11]. A conceptual view of distributed power generation system can be seen in Fig. 1. 1 A DER is generating DC or AC power and 1 In this figure, PEC is the acronym of power electronics converter.
doing energy interaction with renewable energy resources and AC grid [12] through distribution transformers and power electronic converter (PEC). The concept invites integration of all elements of electricity systems to improve operations, efficiency and resilience while reducing conversion and distribution losses [13], [14]. This involves integration with: 1) centralized power and heat generation units, which provide power to DERs using AC to AC conversion, 2) renewable energy resources using DC to DC conversion, 3) bi-directional integration with smart transmission and distribution, EMS, ESS, transportation electrification using DC to DC conversion, 4) storage devices such as hydro-storage [15] and batteries using DC to DC conversion. Although DG systems do not get instant power from AC grid, a communication system for instant information exchange may still be handy during power intermittency to decide on its reconnection with AC source for power exchange. Based on instant communication, another component which could tackle the sporadic nature of renewable energy sources and difference in demand-supply is the ESS [16]. ESS can easily store the produced DC-based energy in DC batteries. This is to tackle and control the power fluctuation during irregular periods and RES connection in a DER [17]. Despite of the feasibility provided by ESS towards the intermittency of RES, there are some limitations which could be faced due to: 1) the impact of environment, 2) aging-cost, 3) technology at hand [18]. The alternates to overcome these limitations should be utilized. For large-scale RES plants, such as wind and solar farms, the pumped storage hydro-electricity station and lead acid batteries are respectively the best alternates to energy storage [15], [19]. Note an alternate to the pumped storage hydroelectricity could be a combination of fuel cells and hydrogen tanks electrolyzers [19]. The alternate technologies and instant power availability from AC grid can enhance the coordination of DERs by maintaining acceptable levels of voltage and frequency stability in power distribution systems [20]. However, it requires a reliable communication infrastructure to optimize the use of renewable energy systems for high penetration levels, which is the main motivation of this article.
In the earlier era, this communication was handled in power systems by supervisory control and data acquisition (SCADA) to transfer data between field devices, control units, and computers in the SCADA central host. A ring system was also introduced to connect DGs to the consumers by a grid [21]- [23]. Later, PMUs were also introduced for SCADA enhancement in smart power grid [24], [25].
Information and communication technologies (ICTs) can be used in a DGs and DER systems for optimal and secure bi-directional flow of power with dynamic loads [26]. However, communication systems in DC microgrid and smart grid systems should meet some specific requirements based on grid applications such as reliability, latency, bandwidth and security [27], [28]. The selection of proper communication network is a big challenge in DG due to many variables and different component requirements, which depend on The main contribution of this article is to explore and identify the development of communication infrastructures in DG systems. The article aims to bridge the gap of different applications of communication frameworks in DGs by: 1) communication infrastructures, 2) mathematical representation of such an infrastructure, 3) the respective networks and technologies, and 4) analysis of different validation approaches carried out by different applications. From the perspective of DG, a bibliographic review on communication systems is provided covering technologies, standards, protocols and classification of different frameworks.
The rest of the article is structured as follows: Section II introduces the communication system and standards. The infrastructure and mathematical representation is explained in Section III. Section IV and V discuss the different communication networks and technologies respectively. Finally, the concluding remarks and future challenges are illustrated in Section VI.

II. COMMUNICATION SYSTEM AND STANDARDS
The delivery of energy to remote geographical locations has urged the development of communication system 2 in distributed power generation systems. A comprehensive communication system can be seen in Fig. 3. It is showing a DG power system with different available communication technologies and networks accompanied by a DG control center. It particularly comprises of the following components: a) a communication infrastructure, b) communication networks, and c) communication technologies. All these components effectively contribute towards the control, monitoring and management of the DG system for a reliable delivery of energy to customers and industry end-users.
From the perspective of communication standards, different RESs-based DGs can use different communication standards based on: 1) requirements, 2) applications, and 3) available resources. For instance, the IEC61850-7-410 is a communication standard considered for monitoring and control using different logical nodes classes and data objects [37]. Some other communication standards with their utilization and applications are summarized in Tables 1 and 2 respectively.

III. COMMUNICATION INFRASTRUCTURE AND MATHEMATICAL MODELING
In DER-based grids, communication infrastructures are considered to be the backbone of all the information exchange and telecommunication services. There are some articles which describe the need of communication infrastructures, their required characteristics and traffic requirements [38]- [42]. Authors in [43] have covered various available wired and wireless communication technologies with their possible use in smart grid applications. In DER-based grid systems, though the information subsystem (e.g. smart meter and sensors) will be different than the traditional AC system, VOLUME 8, 2020 same communication infrastructure can be used in both AC and DC systems.
An insight of communication infrastructure will be represented with a mathematical model.

1) Communication Infrastructure of a DG -Mathematical Representation:
The mathematical representation of a communication infrastructure and its connection with networks and technologies is expressed. Consider communication infrastructure of a DG as shown in Fig. 3. A set of various renewable energy resources are represented as base stations. The base stations are equipped with energy storage devices, which can be utilized when the conventional system cannot provide sufficient power. All the base stations are communicating with DG control center. A two-way communication is facilitated with communication technologies.
2) Communication Channel Model: For the wireless communication technologies like cellular, Zigbee, Wi-Fi, Wi-Max, and bluetooth, consider transmission and reception with W L,T and W L,R antennas respectively. W L,T is deployed at each RES. A sample of information transferred through communication channel can be represented as: where s t,i is the i-th information received at time-instant t, z t,i ∈ R W L,R is the combiner to scale the received information, C t is the multi-communication channel. B t,i ∈ R W L,T is the beamformer for directional signal transmission for transmitter T t,i ∈ R. W t,i ∈ R W L,R is the independent and identically distributed noise across space and time.

3) Observation Model:
To monitor the communication channel in (1), an observation model for a state x t is represented. This representation requires transformation from a complex number to a real number, which involves property of Kronecker product 3 as: where vec represents the vectorization. This gives observation model as: where y t ∈ R m is the observation output, and v t ∈ R m is the white-Gaussian observation noise.
For wired communication technologies, the beamformer and the antennas are not considered in the communication infrastructure.
Once the base of a communication system is defined by its infrastructure, the role of communication network and technology comes in. The selection of a particular communication network in particular depends on the required data rate and coverage range of any specific application.

B. NEIGHBORHOOD AREA NETWORKS (NAN)
The NAN is utilized for DER monitoring information. The high coverage area (100 m-10 km) and data rate (1-100 kpbs) allows control signals at smart meters to be relayed to distribution system operators (DSOs) and transmission system operators (TSOs). The technologies used by NAN are PLC, Zigbee, mesh-network, Wi-Fi, cellular, digital subscriber line (DSL) and Wi-Max [44]- [50], [53]- [57].

V. COMMUNICATION TECHNOLOGIES
In this section, communication technologies with DG will be reviewed for better performance management, control and parallel power transfer with different real life applications. A framework of this section can be seen in Fig. 5 [66] or controller area network (CAN) protocol [67]. ESS is an essential part of any DER for energy storage. This is due to the intermittent nature of renewable energy sources and difference in demand-supply [68]. Different communication techniques have been proposed for energy management system (EMS) to reduce cost and resource wastage [69], [70]. Energy efficiency of different communication systems (cellular, WSNs etc.) connected to a DG, can also be increased with the concept of energy harvesting [71], [72]. Different communication techniques are being proposed and implemented for DG with different benefits from power line communication (PLC), wireless communication, internet of energy (IOE) [73]- [79] etc.
PLC is considered to be a strong candidate in DG system followed by visible light communication (VLC), Wi-Fi, Wi-Max, Zigbee, internet of energy (IOE), fiber optic communication (FOC) or combination of any of these communication techniques.

A. WIRED COMMUNICATION -POWER LINE COMMUNICATION (PLC)
PLC is used in different applications for low voltage power system, such as automatic meter reading, demand side networks (DSN) [80] etc. It is due to this property that PLC is considered to be the most commendable option for DERs. Its structural flexibility in expansion and readily available infrastructure has allowed it to deal with the bi-directional flow of power, varying nature of DC/AC loads, and for better signaling in DERs [81]- [90]. A DC transmitter and receiver block diagram is shown in Fig. 6. 1) Utilization and Architecture: The major advantage of using PLC in a grid is its low installation cost because power lines are already deployed and no amount has to be paid to any VOLUME 8, 2020  communication service provider [91]. Though PLC suffers from different noises, attenuation and distortion problems [92], it is considered to be more secure from cyber-attacks as compared to wireless communication systems [93]. A coupler/de-coupler in a PLC transceiver is used to inject or extract information signal from a grid [94]. A coder and decoder will improve the bit error rate (BER) [69] at the cost of transceiver complexity. A modulator along with carrier signal is used to map the signal properties to communication channel properties. Moreover, the interaction of PLC with: 1) different DERs, such as solar panels, wind turbines, e-plant and hydrogen fuel cell [95]- [97], 2) DC bus, 3) data logger, and 4) processor, are shown in Fig. 7.
2) Applications and Advantages: Table 3 lists PLC with its applications and advantages. In [81], PLC is used for control signal communication in a DER-based grid for load sharing. The authors have proposed switching frequency modulation (SFM) techniques to overcome power convertor limitations and also to enhance PLC performance. The performance is also verified by using 3.3 kW dual active bridge prototype. In [98], an intelligent PV module was proposed with PLC by using frequency shift keying (FSK) modulation. This is to reduce the electricity losses by full monitoring and to help in predictive maintenance of a PV system. This work was further extended in [99] for residential consumer with home plug communication architecture in which orthogonal frequency division multiplexing (OFDM) modulation technique is used to enhance noise immunity [100] in PLC. A low cost solution was discussed in [101] in which PLC module is used by using amplitude shift keying (ASK) without communication modem. A smart graphical user interface was also designed and tested with sixteen panel each with monitoring module. This work was further extended in [102] by having one monitoring module for four PV panels to reduce the cost. A scheme was also discussed to localize the faulty panel with the help of data gathered through PLC and synchronization of monitoring time. To enhance the performance of DC-DC power optimizer (DCPO) in a DG system with PV panels, PLC was used with differential phase shift keying (DPSK) modulation technique along with discrete fourier transform (DFT) [103]. By sharing the data of PV panels connected in series in string using existing DC cables, an algorithm is run to achieve maximum power from PV panels because current can be reduced in a string due to non-uniform ageing, shading or manufacturing differences of PV panels. The proposed technique performance is verified with string of six PV panels connected in series. DG system for EV is discussed with PLC: 1) in-vehicle, 2) between vehicle and grid [104], and 3) using multi-carrier modulation technique [105]. Using PLC in EV will reduce weight and space and it will also make the maintenance and diagnosis, easy. Channel modeling is done in: 1) in-vehicle, and 2) grid-to-vehicle. Noise modeling in time and frequency domains, produced by motor drives and AC/DC converters, is also proposed. PLC can be used for trip information, entertainment, vehicle diagnosis in DC grid and Plug-in EV. Small scale radial distribution system (for industrial applications) is implemented with photo voltaic (PV) DER-based grid system in [67] by using PLC between energy management system (EMS) and several battery management systems (BMS). Single carrier is generated and then modulated by the Bus bar impedance to have different carrier signals. The proposed system can be used with different applications such as road signs, street lightening or parking meter systems. Street lightening system can work smoothly with maximum 10kb/sec. In [106], noise power spectral densities (PSDs) are derived to enhance data transmission in LVDC based grid. Electro-magnetic (EM) noises are also predicted while using PLC in a DERs-based grid to optimize the management and performance of the system. These EM noises are usually generated by house hold devices [107]. In [108], PLC is deployed to analyze data transmission over pulse width modulation (PWM) network. This is utilized by using a PWM-based filter. PMW is also used with PLC in [109] to exchange information between invertor and a motor in a grid. PLC is widely used to exchange information for control and coordination among different convertors in DRES based grids [110], [111]. To deal with information signal attenuation issue and also to design an economical PLC transceiver, a fractional harmonic domain based technique is proposed in [112]. The primary control loop and modulation algorithm of the convertor is used for encoding and decoding of information data.

B. WIRELESS COMMUNICATION -OPTICAL AND RADIO FREQUENCY
Wireless communication can be classified into: a) optical, and b) radio-frequency (RF) wireless communication.

1) Optical Wireless Communication (OWC):
Light-emitting diodes (LEDs) have already captured the conventional lightening devices usage market due to its low energy consumption. LEDs are also used for communication purposes along with illumination at the same to achieve high data rates in the range of GHz as compared to conventional RF communication [113]. Note OWC is also called as visible light communication (VLC) or light-fidelity (Li-Fi) [114]. It is considered to be harmless for human body and more secure because it cannot penetrate in walls. The data rate can further be increased by using Visible light laser diodes (LDs) [115] or by developing multi-input multi-output (MIMO) communication architecture [116]. The major limitation of VLC is that its data rate can decrease significantly with the increase in distance between transmitter and receiver.
Applications of VLC: A simple block diagram of a VLC transceiver is shown in Fig. 8. Information signal is first modulated and then amplified according to the channel conditions. A photo diode is used at receiver side to detect the modulated signal and then demodulation is done to estimate the information signal. A solar powered home with VLC is shown in Fig. 9. Little modifications are required in LED bulbs and other user's devices such as smart phones, laptops and smart sound system to get full advantage of this high data rate communication technique. Approximately 8% of the total energy consumption is used for lightening purposes in commercial and residential buildings [117]. Table 4 summarizes the applications of VLC with its advantages. VLC is proposed in [118] for personalization and localization using LEDs for building management, considering indoor environment. The variable pulse position modulation (VPPM) was used for secure communication and location information transmission and authorization with different dimming levels of LEDs. DG system with smart DC LED-based intelligent lighting system named EDISON was discussed with different VOLUME 8, 2020   communication techniques including VLC [119]. VLC was used for control signal transmission with high Bandwidth efficiency as compared to other communication techniques. The modulation technique even works when the LEDs are almost dimmed and appeared to be off to human eye.

2) Radio Frequency (RF) Wireless Communication:
In RF wireless communication, electromagnetic waves are used to carry the information. So many different communication architecture have been proposed with their advantages such as easy installation, low cost and flexibility and disadvantages such as information security breach, limited available spectrum and interference from other users or devices [120], [121]. In a MIMO communication architecture, band width and power efficiency can be increased by increasing number of antennas at transmitter and/or at receiver side (See Fig. 10). To further improve the system performance large scale or massive MIMO communication systems are also been proposed with large number of antennas [122]. Table 5  Applications and Advantages: A scalable MIMO communication system architecture is shown in Fig. 11. This system has a MIMO energy management system (EMS). Table 6 lists the application of RF wireless communication. RF wireless communication technologies will get a place in DERs by using the concept of simultaneous information and power transfer (SWIPT) [70] to reduce energy resources wastage. A super capacitor was used with multi directional power flow to store energy for wireless sensor nodes. WSNs in a DG can be used for information and control signal communication inside DG system or among different set of DG systems [123]. A smart personal WSN was proposed and implemented inside a building for DC powered LED based lightening system. The energy optimization is achieved by controlling the illuminance using WSN [124]. A smart street lightening system was tested by F. Leccese in which each pole has a transceiver to form a Zigbee communication architecture in a mesh topology. Then all the information is transmitted and processed in a central control unit by using a low cost with good computational performance Raspberry-Pi processor [125]. This central control unit is connected to a grid through Wi-Max to overcome the distance limitations of commercially available Wi-Fi networks [126]. By using the motion and light sensors inside the building, 55% energy saving was achieved. LEDs were controlled and illumination was monitored by using Zigbee architecture, keeping in mind the user satisfaction [127]. A public street lightening system performance was evaluated in [128] with WPAN by using digital addressable lightening interface (DALI) to digitally control light bulbs ballasts. Another smart lightening system was proposed with Brute-Force algorithm to optimize the energy consumption. Among the lightening poles, PLC transceivers were used for monitoring and sharing the information, then the Wi-Max architecture was used for segment and supervisor monitoring and control [129]. Industrial WSN was used for information sensing and exchange to do strategy estimation and event-triggered control [130] in a DG system. A wind power farm (WPF) with wireless RF communication architecture was proposed in [131] according to IEC 61400-25 standard [132] for remote monitoring with scalable area coverage and capacity. The network performance was   evaluated considering different wireless technologies like ZigBee, WiFi and WiMAX in view of end-to-end delay, wireless channel capacity, and data loss.

C. EXISTING RESEARCH IN COMMUNICATION TECHNOLOGIES -SUMMARY
All the communication technologies discussed in this section are contributing individually or in combination. This is based on the respective technology requirements, such as: 1) cost, 2) data rate, 3) reliability, 4) easy expansion, 5) information security, and 6) interference from other users or devices. In principle, PLC technology has low installation cost with options of flexible expansion. From the perspective of data rates, VLC is significant. It can provide high data rates as compared to conventional RF communication. The massive MIMO communication-based architectures are also VOLUME 8, 2020  considered to be a strong candidate for DGs communication networks. This is due to their higher spectral efficiency with low energy consumption.

VI. CONCLUSION AND FUTURE CHALLENGES
The communication system in DGs have gained a lot of attention due to the increasing trend of utilizing renewable energy resources. Currently, the outlook of renewable as a source of energy is too optimistic. This has raised bars with high expectations on the value of technology. This article provided a survey on the utilization of communication technology in DG system while discussing the recent applications and frameworks. It also expressed the advantages of communication infrastructures in DG systems. However, to successfully implement the framework of communication in DGs, significant challenges of integration of various technologies and digital layers will be encountered.
Though the frameworks and review on communication infrastructures show promising achievements in future DG systems, the full deployment of the infrastructure could face numerous number of challenges. This can be seen in Table 7. These challenges are due to the fusion of various protocols and technologies, which could lead to constraints of standardization and optimization [133]- [135].