Wireless Technologies

There is a set of wireless technologies that can be exploited within smart grids, with specific advantages but also some limitations, when used in the last-mile communications network segment, where devices such as sensors and controllers are deployed in critical points of the electric grid. Among these devices

are smart meters, concentrators or traffic aggregators in transmission and distribution networks, which can use either wired and wireless solutions to convey data.

There are some advantages in considering wireless communications for the distribution segment such as ease of installation, maintenance and future expansion, allowing a greater degree of flexibility, which may represent a significant leverage when different requirements may exist in current or future vision of SGs. Another advantage is related with the potential low cost, especially when mature and well disseminated technologies with massive production are employed, like the case of Wi-Fi. They are also able to support different and less restrictive device locations when compared with wired solutions and they can support node mobility and interoperability mechanisms. The wireless characteristic can allow the operation of the communications network in the event of a disturbance in the electric grid that impairs communications via power lines. This is especially relevant when considering the operation of isolated systems.

The aforementioned advantages can also be seen as a double-edged sword in issues such as location where radio planning is usually involved and node placing strategies may need to be carefully considered due to the limited range characteristic of wireless links. The adverse propagation conditions of the communication medium often lead to higher information losses when compared to guided media, which is aggravated by time-varying conditions of the communication channel. The coverage is dependent on the allowable power emission, the usable spectrum (free or licensed bands) and the presence of obstacles among other factors. The particular shared nature of the medium is often pointed out as the main disadvantage since it is prone to eavesdropping and it is exposed to interference from different wireless communications systems. Despite the beneficial properties of wireless implementations challenges such as performance and security need to be carefully addressed. The associated cost benefit characteristic may not be so advantageous if new solutions have to be designed to be employed within smart grids.

2.8.2.1 IEEE 802.11 / Wi-Fi

One the most well-known IEEE standards is the 802.11 that defines Wireless Local Area Networks (WLAN) with similar services and behavior of wired Ethernet. There are two modes of communication:

in the infrastructure mode, stations communicate through an Access Point (AP) typically connected to a backbone network, usually wired; in the ad hoc mode, stations are able to communicate directly with each other without involving other devices, like an AP.

This family of standards defines the physical and medium access control layers. Multiple non-interoperable alternatives are defined for the PHY layer, using either Direct Sequence Spread Spectrum (DSSS) or Orthogonal Frequency Division Multiplexing (OFDM) over the 2.4 and 5 GHz non-licensed Industrial, Scientific and Medical (ISM) radio bands. At MAC layer a CSMA/CA protocol is adopted.

The major characteristics of the most relevant 802.11 variants are summarized in Table 2.7.

The IEEE 802.11n introduced improvements at the PHY layer to enable a higher throughput and enhance the network coverage, using Multiple Input Multiple Output () technology. Additional improve-ments were introduced in terms of cyber-security by the 802.11i, Quality of Service (QoS) in 802.11e, Wireless Mesh Network (WMN) support in 802.11s and vehicular communications by 802.11p.

Table 2.7: IEEE 802.11 Networks Characteristics

Characteristic 802.11 Legacy 802.11a 802.11b 802.11g 802.11n

Bandwidth 20 MHz 20 MHz 20 MHz 20 MHz 20/40 MHz

Frequency

Band 2.4 GHz 5 GHz 2.4 GHz 2.4 GHz 2.4/5 GHz

Number of

Channels 3 12/13 3 3 20/40 MHz

Modulation BPSK, QPSK, DSSS, FHSS

BPSK, QPSK, MQAM, OFDM

BPSK, QPSK, DSSS

BPSK, QPSK, MQAM, OFDM

BPSK, QPSK, MQAM Max. Data

Rate 1.2 Mbps 54 Mbps 11 Mbps 54 Mbps 600 Mbps

Max. Range - 30 m 75-100 m 75-100 m 150-180 m

MAC

Protocol CSMA/CA

Wi-Fi is the commercial designation of the IEEE 802.11 family and it is promoted by the Wi-Fi Alliance⁶, which has contributed for the widespread availability and implementation of this technology.

It has led to the mass production of devices with the natural competition between manufacturers allowing costs to be reduced and favoring their usage in the deployment of wireless LANs. This has turned it into a ubiquitous technology used as a basic communications infrastructure. Newer applications in SG also consider the use of Wi-Fi as a candidate technology in certain segments, namely in last-mile scenarios.

Distribution substation automation and DER monitoring and control LANs can be implemented through Wi-Fi solutions. In fact, some IEC 61850 based applications consider the use of Wi-Fi [69]:

ˆ Enhanced transformer differential protection - the use of spread spectrum radio in protection and control schemes opens the possibility of having wireless solutions for on-line monitoring of equipment such as sensors and actuators;

ˆ Redundant link for distribution automation system - wireless LANs are pointed as an alternative for IEC 61850 protection and control applications in MV substations;

ˆ Communications-aided line protection - using Wi-Fi supporting the interconnection between substa-tions for line differential protection applicasubsta-tions. Laboratory experiments using wireless LAN with repeaters for range extension are referred to as a successful validation for this kind of applications;

ˆ Control and monitoring of remote DER - point-to-point systems are considered as an alternative for distribution automation and DER management, especially in rural areas were other technologies are deemed economically impractical.

2.8.2.2 IEEE 802.16 / WiMAX

In 2001, IEEE released a standard for wireless metropolitan area networks. The IEEE 802.16 standard, as it was designated, defines the PHY and MAC layers of the radio interface of combined fixed and

6“Wi-Fi Alliance” - http://www.wi-fi.org/

mobile point-to-multipoint broadband wireless access [70]. The PHY layer defines two frequency bands for specific operational environments. The 10 to 66 GHz band targets Line-of-Sight LoS communications;

a single carrier modulation is adopted and channel bandwidths are around 25 MHz, enabling raw data rates up to 120 Mbps. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes are supported. In the frequency bands below 11 GHz, LOS is not required and both non-licensed and licensed bands are available. The 5.8 GHz band is used for fixed communications, where issues such as interference and compatibility and radiated power constraints need to be considered, since this is the preferred scenario. The 3.5 GHz band is used for fixed and mobile communications but it requires licensing, like the bands in the 2.3 and 2.5 GHz, which are used for mobile communications only. In these bands both OFDM and OFDMA respectively for fixed and mobile access, both allowing FDD and TDD duplexing techniques. Available channel bandwidths vary from 10 to 20 MHz and the channel rates depend on such factors as radio technology, duplexing mode, channel bandwidth and distance;

nonetheless, 140 Mbps is an achievable upper band value. The 802.16m amendment for Advanced Air Interfaces introduced enhancements at the PHY layer and antenna design, achieving more than the double of the original data rates.

The Worldwide interoperability for Microwave Access (WiMAX) is a designation promoted by the WiMAX Forum⁷, which certifies and promotes the compatibility and interoperability of broadband wire-less devices based on the IEEE 802.16 standard. Commercially WiMAX is described as an alternative technology to cable and ADSL for the last-mile wireless broadband access. Some of the applications targeted by WiMAX include Internet access, backhaul support, triple play and, recently, smart grids and AMI. In [69] other applications supported by WiMAX are defined within the smart grids concept:

ˆ Wireless automatic meter reading - as an application defined between AMI systems of utilities, which may also interact with communications systems of service providers;

ˆ Real-time pricing - as another application over AMI systems that allows real-time price models based on real-time energy consumption, enabling customer awareness, and provides the exchange of incentive-based services;

ˆ Outage detection and restoration - the geographical coverage of WiMAX solutions allows the estab-lishment of a two-way communication channel to implement fast outage detection and restoration procedures.

2.8.2.3 IEEE 802.15.4 / ZigBee

The IEEE 802.15 family of standards defines the technologies and solutions for Wireless Personal Area Networks (WPAN). They target the consumer market and ease of connectivity for personal and hand-held devices. Specifically, the IEEE 802.15.4 standard [71] defines the specification for rate low-power and low-complexity and short-rage WPANs, which are networks designed to convey small amounts of information over relatively small distances. These networks are composed of Full-Function Devices (FFD) that may operate as coordinators, which are devices capable of relaying messages and talk to any other device, and Reduced-Function Devices (RFD), which are intended to run simple applications, and

7 “WiMAX Forum” - http://www.wimaxforum.org/

can only talk to an FFD. The RFDs are usually power constrained devices. The standard defines the PHY and MAC layers functionalities. The MAC layer uses CSMA/CA and the PHY layer uses a DSSS technique combined with different modulation schemes over three different unlicensed ISM frequency bands:

ˆ From 868.0 to 868.6 MHz - in Europe (1 channel with up to 20 kbps in legacy and 100 kbps in the latest version);

ˆ From 902 to 928 MHz - in North America (10 channels with up to 40 kbps and 250 kbps in the latest version);

ˆ From 2.4 to 2.4835 GHz - Global use (16 channels with up to 250 kbps).

Additional PHY variants were introduced using Direct Sequence - Ultra Wide Band and Chirp Spread Spectrum. Available subcarrier modulations schemes include BPSK, ASK and Offset-QPSK (O-QPSK).

One recent variant was introduced by the IEEE 802.15 Task Group 4 along with the concept of Smart metering Utility Network (SUN). According to [72] SUNs are communications networks that provide metering and control capabilities to a utility system, supporting low-power and large scale applications through point-to-point connections. This amendment defines multiple data rates in different frequency bands ensuring the support of IEEE 802.15.4 legacy devices. One of the objectives is to ensure that a SUN is able to support interoperability in multi-regional unlicensed frequency bands considering different scenarios and traffic patterns. Particularly interesting are the outdoors scenarios in which support for utility SG applications for in the last-mile segment is expected [73].

ZigBee specifies a set of higher layer network and application protocols over IEEE 802.15.4. These layers enable functionalities like initialization, device association/disassociation, security management and address management, among others. ZigBee defines three types of devices: PAN Coordinator (FFD), Router (FFD), End Device (RFD). It also defines three different topologies at the network level: star, tree and mesh. The ZigBee Alliance⁸ is responsible for the definition of the ZigBee protocol stack and also promotes the use of networks based on this technology. ZigBee targets applications like building automation, embedded sensing, wireless sensor networks, to mention a few. For smart grids the following applications are considered [69]:

ˆ Control of domestic appliances - as a communications infrastructure for HANs, ZigBee allows ap-pliances to communicate with the network coordinator to control the operating state of devices;

ˆ Direct Load Control - ZigBee allows grid interconnecting networks like AMI to interact with the domestic HAN manager and deliver different degrees of load control to a service operator or utility.