CHAPTER- 2 WIRELESS TECHNOLOGIES FOR POWER SYSTEM APPLICATIONS/MANAGEMENT

Transcription

1 16 CHAPTER- 2 WIRELESS TECHNOLOGIES FOR POWER SYSTEM APPLICATIONS/MANAGEMENT 2.1 INTRODUCTION Generally the power system network is divided into Transmission and distribution networks. The transmission network involves the stepping up of the generated voltage in the generating station and transmit power to the substations through high voltage over head transmission lines. On the other hand the distribution network involves the step-down of voltages into different levels and distributed to different consumers through low voltage power lines. The transmission and distribution lines are the back bones of power system network. Therefore monitoring and protection of lines is very important. The complex power system network is shown in Fig It is divided into two infrastructures based on protection and monitoring. Power infrastructure Information infrastructure Fig. 2.1 Power system network.

2 17 The power infrastructure includes electric power system, which is a network of electrical components used to generate, supply, transmit and use electric power. An example of electric power system is the network that supplies power to the Residential and Industrial with power for sizable regions, this power system is known as grid. It can be broadly divided into generators that supply power, transmission system that carries power from generating centers to the load centers and the distribution system that feeds power to the nearby homes and industries. In this scenario, the protection and monitoring of the network plays a major role. When any fault occurs in complex network, the information is communicated through information infrastructure. Fault sensing protective relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted equipment can be quickly de-energized and the balance of the system is restored. There are many existing protective schemes and technologies are available to identify and transfer information regarding disturbances occurred in power network. These techniques have been quite successful but are not adequate for the present time varying network configurations. This can be achieved by providing fast and effective communication system, discussed in the next section. The following section discusses various power system applications achieved through standardized wired communication technologies, e.g. PLC, SCADA, Fiber Optic [23] and wireless communication technologies, e.g. IEEE based wireless LAN, IEEE WiMAX, 3G/4G cellular, ZigBee based on IEEE , IEEE based MobileFi, Blue tooth-ieee etc. and its challenges [40].

3 DIFFERENT TECHNOLOGIES FOR POWER SYSTEM PROTECTION AND MONITORING The technologies that can be used for protection and monitoring of network are wired and wireless communications Wired communication Wired communication refers to the transmission of data over a wire-based communication technology. It offers more secure and reliable connection since data is not being transmitted through the air. The traditional wired communications are: Supervisory control and Data Acquisition (SCADA) which has limited bandwidth, 75bits/s to 2400bits/s [24]. Higher bandwidth is necessary for monitoring (detection of abnormality), control and management tasks [23]. When feeders are considered, PLC is well-suited, because it is a medium that is available throughout the distribution system. PLC has potential to transmit data at a maximum rate of 11 Kbit/s; when the PLC has sufficient robustness and reliability, this maximum data rate can be achieved only in a narrow frequency range of 9-95 khz [25]. This low rate of communication is not ideal for secure communication. Therefore, if more information has to be sent from all the components in a feeder, higher bandwidth is required [23]. Dedicated wired communication is another option. Interference and attenuation is one of the main problems with copper wire connections. A fiber optic cable is the solution for interference but increases the cost [23]. In addition to above, this will have some drawbacks like, require physical connections and will reduce the flexibility. Further when a pole goes down, the communication link will be broken and may result in poor performance.

4 Wireless communication The work considers wireless communication as a medium for feeder level communication. Leon et al have proposed a wireless sensor network for secure energy infrastructure and overcoming the limitations of wired communication [26]. A wireless sensor network for distribution automation is proposed by Muthu Kumar, N. Suresh Kumar and M.A.B Narayan in [27]. Latest trend to reduce energy consumption by mobile computing is elaborately discussed [28, 29]. Wireless communication will transfer the information over a long distance without the use of electrical conductors or wire. It provides more flexibility than wirebased means of communication and continues to play a significant role in the modernization of the electric power system. Examples of modernization efforts related to increased communications in the electric power system to improve reliability and efficiency. Accurate location and prediction of T & D disturbances and failures are still in early stage of development. More reliable approach can be developed by improving communication infrastructure; this would results in better asset management. Wireless technologies have significant benefits over wired, such as improved protection, control, speed outage restoration, substation monitoring and management [30], power system operation analysis [31], maintenance, planning and also have low installation and maintenance cost, rapid deployment, mobility, etc. Several activities are going on in many areas of power system using this technology [41]. Interference in the presence of buildings and trees is the disadvantage of wireless communication which could result in multi-path; using improved receivers and directional antennas can overcome this drawback, which will increase the cost.

5 20 Another problem is security issues. This can overcome by using secure protocol, encryption and decryption technologies [23]. 2.3 WIRED TECHNOLOGIES This section presents the opportunities and challenges with different wirebased communication technologies being used in various power system applications PLC Power Line Communication (PLC) represents an economic, versatile and dependable tool for power system applications. All power line communication systems operate by impressing a modulated carrier signal on the wiring system. Different types of power line communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Applications Power-Line Carrier has been using in many power system applications such as distribution area load control and even in many household applications for control of alarming, lighting, a/c and heating. The major application is on Transmission Lines in Protective relaying, power line protection relaying to clear faults. It can also be used to provide remote tripping functions for shunt reactor protection, transformer protection, remote load control and remote breaker control ARSEL PLC based AMR System ARSEL AMR (automatic meter reading) system shown in Fig.2.2. Each customer khh meter is equipped with a PLC transmitter that converts the meter disk movement into an equivalent reading. Then the meter reading is added by a customer code and sent to a central receiver located in the distribution transformer [33]. The system deploys Direct Sequence Spread Spectrum DSSS modulation technique working in the frequency range from 9 khz to 95 khz [34, 35].

6 21 The generated PLC signal is coupled to the line through coupling circuit. It allows the communication signals of higher frequency and blocks the line frequency [35]. Finally, the receiver has to extract the transmitted information carried by the weekend and distorted signal that is completely embedded in noise. Fig.2.2. ARSEL PLC based AMR System SCADA SCADA stands for Supervisory Control and Data Acquisition. It focuses on the supervisory level consists of one or more computers connected by a communications system to a number of RTUs placed at various locations to collect data. It is positioned on top of hardware to which it is interfaced, in general via Programmable Logic Controllers (PLCs), or other commercial hardware modules. It provides following critical functions Data Acquisition Supervisory control Alarm Display and Control Supports operator control of remote (or local) equipment

7 22 Applications: SCADA can be considered for various power system applications, such as monitoring and control of frequency, generation, load and transformer oil level. It can also use for substation automation monitoring, protection and control of distributed energy resources. A composite generation and transmission system reliability evaluation technique is used to perform the numerical analysis of the joint SCADA and power system model [36] SCADA based power system The SCADA based power system shown in Fig.2.3 is used to capture and record changes through a complex communication network. MV currents are monitored by the outgoing protection digital relays and LV side is monitored by a special Transformer Inelegant Module (TIM) [34, 37]. Fig. 2.3.SCADA System Block diagram Fiber-Optic An optical fiber is a flexible, transparent fiber made of a pure glass (silica) not much thicker than a human hair. It functions as waveguide or light pipe to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. Optical fibers are widely used in fiber-optic communications, which permits

8 23 transmission over longer distances and at higher band widths than other forms of communication. Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal. Applications Optical fiber is used by many telecommunication companies to transmit telephone signals, Internet communication, and cable television signals. Due to much lower attenuation and interference, optical fiber has large advantages over existing copper wire in long-distance and high-demand applications. However, infrastructure development within cities was relatively difficult and time-consuming, and fiber-optic systems were complex and expensive to install and operate. Due to these difficulties, fiber-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. 2.4 WIRELESS TECHNOLOGIES This section presents the opportunities and challenges with different wireless communication technologies for achieving various power system protection applications [23] Wireless LAN IEEE based wireless LAN provides high speed communication [37]. It is having spread spectrum technology [38]. IEEE covering three technologies: Frequency Hopping Spread spectrum [FHSS], Direct Sequence Spread spectrum

9 24 [DSSS], Infrared (IR) at 1 & 2 Mbps data rates. IEEE b also known as Wi-Fi. It operates on 2.4GHz frequency. IEEE i also known as the cyber security in wireless LANs using Advanced Encryption Standard (AES) [39, 40]. Wireless LAN offers various benefits over wired LAN as it is less expensive, easy to install and provides Mobility of devices [41]. Applications: Wireless LAN can be considered for various power system applications. IEC standard has proposed substation automation and protection applications [42]. It has also proposed Ethernet based communication networks to achieve interoperable Substation Automation Systems (SAS) [43]. Wireless LAN technologies are also known as wireless Ethernet; and therefore, it can be considered for these applications Transformer differential protection Fig.2.4 shows the based wireless LAN. It can be used to enhance the distribution substation automation and protection, Transformer differential protection, temperature of the oil, transformer oil level and breaker failure. Fig.2.4 can also shows load Tap Changer (LTC) sensor with wireless feature can enhance transformer differential protection by on-line monitoring LTC position. It can also used for protection of switchyard equipments using sensors and wireless communication Redundant link for distribution automation system To improve the reliability of critical operations, Wireless LAN can be used to provide redundant wireless link in parallel with optic fiber as shown in Fig.2.4. G. Thonet and B. Deck [44] has presented wireless LAN applications for protection and control of IEC based low and medium-voltage substation by introducing e- breaker platform.

10 25. Fig.2.4 Wireless LAN communication for distribution substation Power line protections using wireless communication Fig.2.5 shows the application of wireless LAN for power line protection between two substations. In the laboratory environment wireless LAN is demonstrated for transmission line differential protection by K.M. Abdel-Latif et al [45]. The distance coverage has enhanced by using repeater between both ends: however, this would increase end to end delay Monitoring and control of remote DERs Wireless LAN link can be used with distributed energy resources, as shown in Fig.2.5. It is not economical and difficult to maintain fiber-optic communication for rural area distribution system with more dispersed DERs. Hence, wireless communication technologies are more feasible. The monitoring, control, protection, and metering between DAS and DERs have been studied in [46].

11 26 Fig.2.5 Wireless LAN communications for inter-substation and remote DERs WiMAX Microwave Access (WiMAX) technology is a part of Wireless Metropolitan Area Network (WMAN) [47]. It is having wide operating range of 10-66GHz for communication. It provides data rate up to 70Mbps and distance up to 48km [40]. Applications: Wireless Automatic Meter Reading (WMAR) Fig.2.6 shows the WAMR system based on WiMAX. Implementation of WAMR for revenue metering offers several advantages to electric utilities and/or service provider in the smart grid environment by reducing the need for human meter readers and errors in reading On -line real-time pricing WiMAX technology more suitable for Wireless Automatic Meter Reading (WAMR) due to large distance coverage and high data rates. Fig.2.6 shows the WAMR system based on WiMAX. It offers several advantages in the smart grid environment by reducing the need for human meter readers and errors in meter reading.

12 Outage Detection and Restoration Currently, distribution network has almost negligible outage detection mechanism especially for residential customers and rural areas, which result into low reliability of power supply and less utilization factor. With the help of two-way communication using WiMAX fast outage detection & restoration can be implemented and utilization factor can be improved. Fig. 2.6 WiMAX communication for WAMR RF Communication RF communication works by creating electromagnetic waves at a source and being able to pick up those electromagnetic waves at a particular destination. These electromagnetic waves travel through the air at near the speed of light. The wavelength of an electromagnetic signal is inversely proportional to the frequency; the higher the frequency, the shorter the wavelength. Frequency is measured in Hertz and radio frequencies are measured in khz, MHz and GHz. Higher frequencies result in shorter wavelengths. The wavelength for a 900 MHz device is longer than that of a 2.4 GHz device. In general, signals with longer wavelengths travel a greater distance and penetrate through, and around objects better than signals with shorter wavelengths.

13 28 Applications: There is a wide range of products and application areas that fit into this category safety, security and control of homes and commercial buildings, mobile services within business and healthcare, industrial automation, distribution automation, smart metering, asset management and much more Cellular The 3G / 4G cellular technology operates on the spectrum range of /1900MHz [49]. These are licensed frequency bands. Data transmission rate of this technology is Kbps and distance coverage is depending upon the cellular service [40]. This technology offers extensive data coverage, no maintenance costs and network fully maintained by carrier [50]. The advantage with cellular technology is that the existing infrastructure can be used, maintenance is less. Also, with the recent growth in 3G / 4G cellular technology, the data rate and QoS are improving very fast. Types of cellular technologies and its applications: CDMA a) Code Division Multiple Access (CDMA) technology for distribution substation Due to mobile phone users, cellular coverage is available even in very remote locations. X. Zhang, Y. Gao, G. Zhang, and G. Bi [51] has demonstrated the use of Code Division Multiple Access (CDMA) technology for power system Supervisory Control and Data Acquisition (SCADA), by providing cellular communication between substation Remote Terminal Unit (RTU) and SCADA server. Fig.2.7 shows the application of cellular technology for SCADA interface of remote distribution substation site. Several technologies for GPRS and CDMA networks are developed in [52].

14 29 Fig.2.7 Cellular technology for SCADA and power grid monitoring GPRS a) Monitoring and metering of remote DERs by GPRS Cellular technology can be used for monitoring and metering of remotely located DERs is shown in Fig.2.7. Information exchange can be carried out in the form of SMS, General Packet Radio Service (GPRS) technology for monitoring application of remote substation is explained in [50, 53] GSM GSM stands for Global System for Mobile Communication and is an open, digital cellular technology used for transmitting mobile voice and data services. The GSM is a circuit-switched system that divides each 200 khz channel into eight 25 khz time-slots. It was developed using digital technology. It is having an ability to carry 64 Kbps to 120 Mbps of data rates. Each GSM digitizes and compresses data, then sends it down through a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1,800 MHz frequency band. The GSM standard is the most widely accepted standard and is implemented globally.

15 30 a) GSM based system for smart home applications GSM module was used for receiving Short Message Service (SMS) from user s mobile phone that automatically enable the controller to take any further action such as to switch ON and OFF the home appliances such as light, air conditioner etc. The system is integrated with microcontroller and GSM network interface using assembly language. After receiving the SMS command, the microcontroller unit automatically controls the electrical home appliances by switching ON or OFF the device according to the user order [26]. b) GSM based WAMR system Wireless automatic meter reading system (WAMRS) is the widely used GSM network. It provides two way communications. Fig.2.8 shows the structure of wireless automatic meter reading system (WAMRS). The GSM network, which enables communication between the AMR interface and the center [27]. Fig.2.8 Structure of diagram of WAMRS c) GPRS and GSM network for Distributed Load-Shedding Wireless Distributed Load-Shedding Management system (WDL-SMS) shown in Fig.2.9. It consists of a GPRS modem that sends SMS messages over the GSM

16 31 network from the control center (CPM location) to the transformer site (DCM Site) and vice versa. This is designed to handle remotely distributed transformers that are not connected to a SCADA system. Challenges and solutions of cellular technology Call establishment takes time delay, and moreover, call dropout can affect the large data exchange but it provides a cost-effective, wireless, always-connected, twoway communication and vast coverage and hence this technology may be used in most of the power system applications. Fig.2.9 System architecture showing the CPM, DCM with the GPRS modems and the GSM network d) Green Radio (Energy saving on proposed GSM Technology) Recent analysis made by manufactures and network operators that, current wireless networks are not very energy efficient, particularly the base stations. As part of the international efforts for energy conservation and CO 2 reduction, migration to an energy-efficient mobile infrastructure is of high importance to the mobile communications industry.

17 32 The Green Radio program provides reduction in power consumption over current designs for wireless communication networks without significantly compromising the quality of service to the users. The Green Radio project is pursuing energy reduction from two different perspectives. The first is to examine alternatives to the existing cellular network structures to reduce energy consumption. The second approach [28, 29], is to study novel techniques that can be used in base stations or handsets to reduce energy consumption in the network ZigBee ZigBee is a cost effective, reliable and low power wireless network. It offers a data rate of Kbps. It operates on the frequency range of 868MHz, 15 MHz and 2.4 GHz with DSSS modulation technique. Its coverage area is m. ZigBee employs 128-bit AES encryption for security [54]. It is widely used in remote meter reading, remote control, security systems, building automation and computer peripheral application. Applications: 1) Control of home appliances Due to low power consumption, low data rate and low cost, ZigBee is very much suitable for Wireless Sensor Networks (WSN). Smart Home Area Network (HAN) can be formed using advanced Smart Home Area Network (HAN) can be formed using advanced ZigBee network. Fig.2.10 shows the star configured ZigBee HAN for control of home appliances [55, 56]. 2) Direct load control The local HAN can be automatically controlled locally with the help of controller or remotely using utility AMI infrastructure. J.Cheng, M.Hung and P.Kadar have proposed the implementation of direct load control for home appliances [55, 56].

18 33 Fig.2.10 ZigBee technologies for smart home area network Other Potential Wireless Technologies Mobile Broadband Wireless Access (MBWA) IEEE MBWA provides high mobility, high Bandwidth and low latency in the licensed frequency bands below 3.5GHz. It is also known as MobileFi. It offers real time peak data rate of 1Mbps to high speed data rate of 20Mbps. This standard is optimized for full mobility up to vehicular speed of 250km/h [57]. MBWA may be used for smart grid applications, electric grid monitoring and SCADA systems. IEEE (MBWA) is new emerging technology, and hence, communication infrastructures for this technology are not readily available. Currently, use of this technology may be costly solution compare to cellular technology Digital Microwave Technology Digital microwave operates on licensed frequency band of 2-40GHz, and provides the data rate up to 155Mbps. Microwave technology provides very long distance coverage up to 60 kilometers. It accepts data from Ethernet or ATM port and transmits it to the other as microwave radio. Digital microwave can support point to point communication for smart grid applications, e.g. transfer trip between DER and distribution substation feeder protection relay.

19 34 Microwave radio is susceptible to two types of signal fading, precipitation and multi-path interference. Encryption for security may result in to additional latency as it takes larger message sizes [40] Bluetooth Bluetooth is a low power, short range radio frequency communication standard. It operates on GHz unlicensed ISM band. It is having a data rate of 721Kbps [54]. Depending upon the communication configuration its distance coverage is between 1m-100m. Bluetooth technology can be used for local online monitoring applications in substation automation systems [58]. These devices are highly influenced by surrounding communication link and may interfere with IEEE based wireless LAN network. The Bluetooth offers weak security compare to other standards. 2.5 COMPARISON AND DETAILS OF DIFFERENT WIRE LESS COMMUNICATION TECHNOLOGIES Essential Features The following are the essential features for end to end communication in wireless communication: Low latency: Due to critical working of grid, it requires fast and real time reaction during abnormal conditions. The wireless technology should be important for high priority low latency alerts during abnormality. Security: As the electric grid is large, protecting, monitoring and its data confidentiality is critical. Therefore it requires secure communication.

20 35 Low overload: Excessive use of control packets and multiple nodes trying to send same information leads to overload. This reduces the band width available for data traffic. Therefore it is necessary to keep the overhead low. Fixed stations: All nodes are fixed in smart grid. Therefore communication architecture does not require any mobility. Scalability: The network of wireless nodes is expected to be large considering the scale of towers from the substation to residential places. The communication architecture must work equally for large network as well as small network Comparison between various wireless technologies Details of various wireless technologies Wireless LAN, WiMAX, Cellular, ZigBee, MobileFi, Digital Microwave and Bluetooth are given in Table 2.1. Based on Table 2.1, GSM seems to be the superior technology for high power applications as it covers wide range and available even remote areas. Table 2.1: Summary of wireless communication technologies Wireless Technology Data Rate Wireless LAN 1-54Mbps 100m WiMAX 70 Mbps 48km Approximate Coverage Cellular Kbps km ZigBee Kbps m Mobile-Fi 20 Mbps Vehicular Std Potential Power System Applications Distribution protection and automation[38,42-46] Wireless Automatic Meter Reading[27,48] SCADA and monitoring of power system network[26-29,50-53] Direct load control of home applications[55, 56] Communication for PEVs and remote monitoring Digital Microwave 155 Mbps 60 km Transfer trip (point-topoint)[40] Bluetooth 721 Kbps 1-100m Local online monitoring applications[58]

21 CONCLUSIONS The introduction section of this chapter discusses the importance of power and information infrastructure in complex power system network for its effective protection and energy monitoring. It mainly focuses on power system network and challenges related to wire and wireless communication infrastructure. The section-2 presents the different technologies that are used for protection and monitoring of the power network. The section-3&4 completely deals with the existing wired PLC, SCADA, Fiber Optics technologies and wireless technologies LAN, WiMAX, ZigBee, 3G/4G cellular, MobileFi, digital microwave, Bluetooth and its potential power system applications. The last section discuses the comparison and details of different wireless communication technologies.