A Laboratory-Based Three-Phase Smart Grid Sensor Network Testbed

A laboratory-based three-phase sensor network testbed for the Smart Grid was developed at the Smart Grid and High-Power System Laboratory of The University of Hong Kong. The setup is featured by a scaled three-phase transmission-line model, visualization of sensor measurement, optical communication network, and integration with global positioning system (GPS). The three-phase transmission-line model consists of three power cables and two transmission towers in which various types of sensors, including magnetic sensors, infrared sensors, strain gauges, and accelerometers, are installed to monitor the condition of the transmission lines and the transmission towers. Magnetic sensors and infrared sensors are employed as advanced sensors, which can provide more accurate and comprehensive information of the transmission line. The sensor data is transferred to a computer for analysis and visualization. A graphical user interface (GUI) was designed in LabVIEW to integrate the data acquisition and display the measurement results, including cable position, inclination and vibration of the tower, and frequency and waveform of the cable current. The host computer also forms an IP network with five remote computers, via optical fibers and an optical interface card, for testing various communication protocols. The topology and connectivity of the network are graphically displayed. The sensor network is integrated with GPS and can perform synchronized measurement with the GPS timing. This sensor network testbed provides a platform for the implementation testing, experimentation, and feasibility evaluation of new sensor applications under test in the Smart Grid.


Introduction
The Smart Grid is an idea of modernizing power grids to overcome the existing limitations, shortcomings, and challenges of power systems. Owing to environmental issues, the expansion of transmission grids has been stagnant for decades, which largely constrains the delivery of renewable energy. The ever-increasing cost of fossil fuel is pushing the usage of alternative energy. More and more customers are voicing out the need for transparency and liberty in the power market, high quality/price ratio of electricity, and freedom to interact with the grid. The existing infrastructure is aging, whereas the load demand continues to increase. To prevent a possible decline in reliability, the grid needs to be equipped with the latest technology, such as wide-area measurement and self-healing capability. (1) As a result, many countries are pushing forward the Smart Grid. Slootweg and Enexis (2) define the Smart Grid as a common denominator for a wide range of developments that make medium-and low-voltage grids more intelligent and flexible than they are nowadays. The development of the Smart Grid can improve the utilization of renewable energy, promote the interactive power market, and enhance compatibility and reliability.
Sensors play an important role in making the grid smarter. (3) The Smart Grid will need information more than simply voltage and current. A variety of sensors will be used to detect the environment around the facilities and condition of the infrastructure. (4) Advanced sensors are necessary because of their superior sensitivity, accuracy, and compatibility. Since developmental testing may present an interruption or a reliability threat to a live power system, a testbed for experimenting on various types of sensing technologies and communication protocols in the Smart Grid is needed for testing in an isolated fashion. Gang et al. (5) designed a Smart Grid testbed to enable the research community to analyze their designs and protocols in a laboratory environment. Qiu et al. (6) built a real-time cognitive radio network for the Smart Grid. Nevertheless, their testbeds do not provide a real transmission model for experiments. To the best of our knowledge, we are the first to develop a testbed for studying three-phase transmission lines in the Smart Grid. Our testbed is designed to perform sensor testing, concept verification, simulation on sensor network, (7) intrasystem communication, and wide-area synchronization.
Our design features a three-phase power transmission system, visualization, multisensor network, use of advanced sensors, networking control, and global positioning system (GPS) monitoring. Various types of sensors are employed to provide multiple types of information to the host computer. The uses of optical communication and IP network enhance compatibility and scalability of the setup. The control system processes the sensor data to visualize the transmission line infrastructure and display complicated charts in real time. GPS is deployed to perform synchronization. All the components of this testbed are commercially available, and researchers can build their own setup easily.

Platform with multiple advanced sensors
A complete sensing solution for all the transmission system conditions requires multiple types of sensor. (8) Table 1 shows the responses of the accelerometer, temperature sensor, strain gauge, and magnetic sensor in sagging, tilting, explosion, tower collapse, and electrical fault. Different types of sensors are sensitive to different types of scenarios. For example, an electrical fault can only be sensed by a magnetic field sensor while a tower collapse can be sensed by both a strain sensor and an accelerometer. Therefore, a sensor network with multiple types of sensors is essential for the comprehensive monitoring of transmission lines. Our setup includes a multisensor network consisting of strain gauges, accelerometers, infrared sensors, and magnetic sensors. The advanced magnetic sensing technology is adopted because a magnetic sensor is sensitive to all types of abnormal events. As shown in Table 1, the magnetic field waveform and distribution are sensitive to sagging, galloping, explosion, collapse, and electrical fault. Thus, the magnetic field can signify these abnormal events.
Owing to its extensive sensing ability, it can be used as a universal sensor. In addition, the magnetic field measured using the magnetic sensor can be used to determine the cable position and perform power quality analysis, which is difficult to achieve using conventional sensors. The advanced temperature sensing technology is also applied in the setup. The infrared sensor can measure the cable temperature without physical contact with a high-voltage cable, which makes installation and maintenance easier and safer. Furthermore, it is equipped with a laser tracking device to measure the temperature of a galloping cable.

Visualization of measurement information
Visualization of measurement information is a key component of the setup because it conveys abstract information in intuitive ways and thus enables users to perceive and understand a large amount of information instantaneously. (9) The monitoring part of our setup is highly visualized to enhance situational awareness and reduce cognitive demand on the operators. The graphical user interface (GUI) displays the measurement and analysis results of the electrical and spatial information of a transmission line and transmission towers in the forms of images, diagrams, and charts. The magnetic field waveform is presented in graphs while the power quality analysis calculated by performing Fourier transform on the magnetic waveform is presented in a spectrum.

Integration with GPS
Our setup is integrated with GPS for providing position and time. As a result, the synchronization accuracy of the sensor network can be boosted from 1 ms up to 1 μs, which is 1000 times better than the current standard. (10) The time provided by the GPS

Three-phase three-cable transmission lines, sensors, and electrical system
The laboratory setup of the testbed is established in the Smart Grid and High-Power System Laboratory at The University of Hong Kong. In this three-phase three-cable power transmission system, the positions of sensors and the GUI are the same for each cable. Figure 1(a) shows a schematic structural diagram of the three-phase transmission lines. The sensor network includes several types of sensors: magnetic sensors, infrared sensors, strain gauges, and accelerometers [ Fig. 1(b)]. Each end of the transmission line is installed with one set of strain gauges and one accelerometer. Another accelerometer is mounted on each tower. An infrared sensor is placed 0.15 m horizontally from the transmission line. The distance between towers is 3 m and the ends of the cables are suspended on the towers 1 m from the ground, and the separation between adjacent cables is 200 mm. Each cable is connected to each phase of the three-phase 50 Hz mains power supply. Figure 2 shows a schematic diagram of the electrical connection of this three-phase power transmission system. Sensors are installed on the three-phase power transmission lines suspended by the two transmission towers. The three-phase power cables are connected to the power switch via a three-phase connector. The phase separation among the three phases is 120°. The power switch is connected to the mains switch box via the Neutral Line and Ground Line. A miniature circuit breaker (MCB) protects the circuit from damage caused by overloading or short circuit. (11) The MCB can be reset to resume normal operation after a faulty condition. A residual-current device (RCD) protects users from electric shock owing to electricity leakage. (12) MCB and

Sensor data transmission and network connection
A data-acquisition (DAQ) card (National Instruments NI USB 6211 M) establishes a link between the host computer (monitoring station) and the sensors [ Fig. 3(a)]. It  collects the analog data from the sensors, converts them into digital form, and sends them via a USB hub to the host computer, which plays the role of a monitoring station that analyzes the information from sensors and monitors the transmission line. There are 16 analog input channels with ±10 V voltage range (16 bits) with a high sampling rate of 250k samples per second. To provide a platform for testing and implementing communication protocols, there are five computers simulating other monitoring stations and they are connected with the actual monitoring station by optical communication links. Figure 3

Testbed characterization and performance in operation
The operation of this three-phase transmission-line model was characterized using a 3-phase power quality analyzer (Fluke 434). The phase current in one phase line can be varied from 0 to 43 A (Fig. 4) without affecting the others (Fig. 5). The phase separation of 120° can be observed from Fig. 5(d).
The GUI of the monitoring station is shown in Fig. 6. The diagram labeled by (1) is the position chart indicating the position of the mid-span of the transmission line. The position is derived from the magnetic field measured by two magnetic sensors based  on the Biot-Savart Law. The performance of the magnetic sensors was evaluated for the scenarios of sagging and galloping of one of the cables. The position of the cable before and after sagging or galloping can be determined accurately with an error of less than 5%, as one can observe from Figs. 7 and 8, respectively. As the transmission line is monitored using two magnetic sensors, there are two magnetic waveforms displayed in the graphs labeled by (2) in Fig. 6. The magnetic waveforms are essentially the waveforms of the electrical currents in the transmission lines as governed by the Biot-Savart Law. The inclination diagrams (3) show the front and side views of Towers 1 and 2 and users can observe if there is any inclination for the transmission towers. The inclination is calculated from the tilting angles measured using the accelerometers mounted on the transmission towers. The tower inclination was tested as shown in Fig.  9. The test result shows that the inclination graph can well reflect the tower tilting. Diagram (4) is the three-dimensional monitoring of the transmission line derived from the tilting angles measured using the accelerometers mounted on the transmission line. The catenary of the cable before and after sagging can be observed in real-time with this 3D diagram (Fig. 10). Spectrum (5) is the power quality analysis by taking the Fourier transform with the magnetic field waveform measured using the magnetic sensors as shown in (2). The GUI also provides numerical measurement information in the panel labeled by (6). The numerical information includes the cable temperature measured using the infrared sensor, cable tension measured using the strain gauges, magnitude of the electric current carried by the transmission line as determined by inverse calculation from the magnetic field measured using the two magnetic sensors, and the exact values of the tower inclinations. If these values exceed certain predetermined safety levels, the corresponding warning indicators are lit up to alert the operators. When there is strong vibration to the tower owing to, for example, explosion by terrorist sabotage, the oscillation is sensed by the accelerometers mounted on the transmission towers and the corresponding "Shocked" indicator is lit up to alarm the operators. As such, all the abnormal events listed in Table 1 can be effectively monitored in this sensor network testbed.
As discussed earlier, this testbed is equipped with the features of networking control and GPS monitoring. In the setup, the host computer is connected to five remote computers via optical fibers. The network is visualized in the interface of the host computer (Fig. 11). The network can be rearranged from the star configuration to another configuration that suits the need of any experiments and simulations. The interface can also be integrated with various protocols to perform synchronization, protective relaying, etc. The IP address and connection status of each computer are shown in the interface. The interface could correctly indicate the network activity in the testing. The GPS monitoring is performed in another interface (Fig. 12). The map obtained from Google Earth correctly shows the geographical location of the testbed with accuracy within 2.2 m. The GPS of the system can track satellites in parallel with twenty channels for acquisition (1 s for hot start), reacquisition (0.1 s average), and synchronization (1 μs).

Conclusions
The laboratory-based three-phase transmission-line sensor network testbed is featured using a platform with multiple advanced sensors, a GUI visualizing the measurement information, and integration with GPS. The monitoring station has access to all the sensor data and it can carry out further analysis to provide monitoring of the transmission  line. The system can detect various electrical and spatial abnormal events of the transmission line, including cable sagging and galloping, tower collapse and explosion, and electrical faults. Moreover, it is equipped with a GPS receiver and can perform time synchronization for wide-area measurement. The performance of the testbed was tested and verified in this work. Compared with the other Smart Grid testbed systems that focus on energy flow (5) and communication, (6) this setup offers the unique advantage that it was particularly designed for implementing sensor networks on transmission grids. It provides a three-phase transmission-line model for experimenting on various sensor network facilities in Smart Grid research, for example, synchronized wide-area measurement. It is a comprehensive platform for experimenting on various sensor network schemes and protocols for the Smart Grid. On the basis of this Smart Grid testbed, two scientific works have already been carried out and published. (13,14)