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1 PHASOR MEASUREMENT UNIT : An Overview Vishal Wadkar, Pavan Salunkhe, Ganesh Bhondave *1(PG Student of Electrical Department, R.H.Sapat COE College, Nashik, India) *2(PG Student of Electrical Department, SND COE College, Nashik, India) *3(UG Student of Electrical Department, SND COE College, Nashik, India) vishalwadkar9@gmail.com*1, pavansalunkhe@gmail.com*2, ganesh.bhondave95@gmail.com*3 ABSTRACT This paper provides a tutorial introduction to phasor measurement units (PMU). A phasor measurement unit (PMU) is a device which measures the electrical waveform on an electricity grid, using a common time source for synchronization. PMUs measure positive sequence voltages and currents on the transmission grid, and when a sufficient number of PMUs are installed. Time synchronization allows synchronized realtime measurements of multiple remote measurement points on the grid. In power prevalent blackouts, implementation of state-of- engineering, these are also commonly referred to the-art technologies, such as a state estimation of as synchrophasors and are considered one of the most important measuring devices in the future of power systems. Phasor measurement units (PMUs) Positioning System Sattelite (GPS). A PMU can be a dedicated device, or the PMU function can be incorporated into a protective relay or other device. Synchronized phasor measurement selevate the standards of power system monitoring, control, and protection to a new level. Keywords: Phasor measurement unit, Power system control, Power system monitoring, Power system protection, Real-time measurement. Introduction In many countries around the world are affected by power failures, which are caused by factors such as lack of investment into power system infrastructure, inadequate asset maintenance, and continuous increase in electricity consumption that overstresses the power transmission and distribution system. Consequently, power companies suffer from losses of billions of dollars, and inconvenience to private and business customers. In order to prevent the the transmission network, is required to achieve better controllability, higher reliability and stability of the power system. The Phasor Measurement are power system devices that provide Unit (PMU) is a device that is employed to detect synchronized measurements of real-time phasors of the voltage and current waveform that is voltages and currents. Synchronization is achieved by same-time sampling of voltage and current waveforms using timing signals from the Gobal synchronized with a clocking signal obtained continuously from the global positioning system (GPS). What is Phasor? A phasor is a complex number that represents both the magnitude and phase angle of the sine waves found in electricity. Phasor measurements that occur at the same time are called "synchrophasors", as are the PMU devices that allow their measurement. In typical applications phasor measurement units are sampled from widely dispersed locations in the power system network and synchronized from the common time source of a global positioning system 1

2 (GPS) radio clock. Synchrophasor technology provides a tool for system operators and planners to measure the state of the electrical system and manage power quality. Synchrophasors measure voltages and currents at diverse locations on a power grid and can output accurately time-stamped voltage and current phasors. Because these phasors are truly synchronized, synchronized comparison of two quantities is possible, in real time. These comparisons can be used to assess system conditions. Fig:- Sinusoidal waveform and its phasor representation Here the phase or phase angle is the distance between the signal s sinusoidal peak and a specified reference and is expressed using an angular measure. Here, the reference is a fixed point in time (such as time = 0). The phasor magnitude is related to the amplitude of the sinusoidal signal. Fig-Using a PMU, it is simple to detect abnormal waveform shapes. A waveform shape described mathematically is called a phasor. What is phasor measurement unit? A PMU is an electroinc device that uses state-ofthe-art digital signal processors that can measure 50/60Hz AC waveforms (voltages and currents) typically at a rate of 48 samples per cycle (2880 samples per second). The analog AC waveforms are digitized by an Analog to Digital converter for each phase. A phase-lock oscillator along with a Global Positioning System (GPS) reference source provides the needed high-speed synchronized sampling with microsecond accuracy. Additionally, digital signal processing techniques are used to compute the voltage and current phasors. Fig. - Phasor Measurement Unit Block Diagram Line frequencies are also calculated by the PMU at each site. This method of phasor measurement yields a high degree of resolution and accuracy. The resultant time tagged phasors can be transmitted to a local or remote receiver at rates up to 60 samples per second. Technical overview 2

3 A phasor is a complex number that represents both the magnitude and phase angle of the sine waves found in electricity. In typical applications phasor measurement units are sampled from widely dispersed locations in the power system network and synchronized from the common time source of a global positioning system (GPS) radio clock. Synchrophasor technology provides a tool for system operators and planners to measure the state of the electrical system and manage power quality. Synchrophasors measure voltages and currents at diverse locations on a power grid and can output accurately time-stamped voltage and current phasors. Because these phasors are truly synchronized, synchronized comparison of two quantities is possible, in real time. These comparisons can be used to assess system conditions. The technology has the potential to change the economics of power delivery by allowing increased power flow over existing lines. Synchrophasor data could be used to allow power flow up to a line's dynamic limit instead of to its worst-case limit. Phasor networks A phasor network consists of phasor measurement units (PMUs) dispersed throughout the electricity system, Phasor Data Concentrators (PDC) to collect the information and a Supervisory Control And Data Acquisition (SCADA) system at the central control facility. Such a network is used in Wide Area Measurement Systems (WAMS), the first of which was begun in 2000 by the Bonneville Power Administration. The complete network requires rapid data transfer within the frequency of sampling of the phasor data. GPS time stamping can provide a theoretical accuracy of synchronization better than 1 microsecond. Clocks need to be accurate to ± 500 nanoseconds to provide the one microsecond time standard needed by each device performing synchrophasor measurement. For 60Hz systems, PMUs must deliver between 10 and 30 synchronous reports per second depending on the application. The PDC correlates the data, and controls and monitors the PMUs (from a dozen up to 60). At the central control facility, the SCADA system presents system wide data on all generators and substations in the system every 2 to 10 seconds. PMUs often use phone lines to connect to PDC, which then send data to the SCADA and/or Wide Area Measurement System (WAMS) server. Functions The availability of synchronized phasor measurements has given rise to the possibility of two categories of new and improved applications. One category has been broadly referred to as Wide- Area Control. It is in the same family as all existing automatic control and protection, which are mostly local, that is, the actuating signal source and the control signal destination are in the same substation. Synchronized measurements and fast communication now make it possible for such control to be wide-area or regional. Special and unique examples of such wide-area control already exist and are called Special Protection Schemes (SPS). The increasing availability of phasor measurements will make the development of more wide-area controls easier. Some example possibilities are provided below. The other category is the enhanced control center functions. The main functions of the control center SCADA, State Estimator, Contingency Analysis, etc. are for the system operator to monitor the power system and make operational changes, either using supervisory control or by telephone, to ensure the reliable and efficient operation of the 3

4 system. The availability of phasor measurements at faster rates can improve these functions. Secure operation of power systems requires close monitoring of the system operating conditions. This is traditionally accomplished by the state estimator which resides in the control center computer and has access to the measurements received from numerous substations in the monitored system. By collecting analog measurements and the status data of the circuit breakers from remotely monitored and controlled substations and feeding them as input into state estimation function, state estimation can provide an estimate for all metered and unmetered electrical quantities and network parameters of the power system, detect and filter out gross errors in the measurement set and detect the topology errors in the network configuration. Until recently, available measurement sets did not contain phase angle measurements due to the technical difficulties associated with the synchronization of measurements at remote locations. Global positioning satellite (GPS) technology alleviated these difficulties and lead to the development of phasor measurement units (PMU). The phasor measurement system and phasor measurement process are shown in the figure. Fig. - Phasor Measurement Process Fig. - Phasor measurement system Optimal placement of pmu Synchronized Phase Measurement Unit (PMU) is a monitoring device, which was first introduced in mid-1980s. Phasor measurement units (PMU) are devices, which use synchronization signals from the global positioning system (GPS) satellites and provide the phasors of voltage and currents measured at a given substation. As the PMUs become more and more affordable, their utilization will increase not only for substation applications but also at the control centers for the EMS applications. One of the applications, which will be significantly affected by the introduction of PMUs, is the stat estimator. TVA is in the process of installing phasor measurement units (PMU) for enhanced monitoring of the TVA system. In order to avoid redundant use of PMUs, the optimal locations for the new PMUs must be determined. The objective this project is to make use of a minimum number of PMUs in order to make the system fully observable. Installation of PMUs will be a gradual process, requiring decisions on the best possible locations for a limited number of PMUs at the beginning. Hence, a systematic method is needed for finding the best locations for new PMUs in the presence of other already placed PMUs and/or conventional measurements. This project investigates this issue and provides a practical solution for the PMU placement problem. 4

5 How does phasor technology compare with SCADA? ATTRIBUTE SCADA PHASOR Standards The IEEE 1344 standard for synchrophasors was completed in 1995, and reaffirmed in In 2005, it was replaced by IEEE Standard C , which was a complete revision and dealt with issues concerning use of PMUs in electric power systems. The specification describes standards for measurement, the method of quantifying the measurements, testing & certification requirements for verifying accuracy, and data transmission format and protocol for real-time data communication. The standard is not yet comprehensive- it does not attempt to address all factors that PMUs can detect in power system dynamic activity. Measurement Analog Digital Resolution 2-4 Up to 60 Samples samples per Observability per sec Steady state sec Dynamic/T ransient Monitoring Local Wide area Phase angle No Yes measurement Measurement Magnitud Magnitude( Quantity e-(rms)- RMS) and MW,MV phase AR offset from common Fig. - IEEE 57 Bus System: Location of 14 PMUs referencefor complete system observability (considering MW,MVA zero injection busses) R and Angle Difference Testing of PMUs A unique property of PMUs is their capability of achieving high accuracy synchronized measurement. In order to test PMUs used for a specific WAMPAC application, a laboratory testing platform including the use of GPS signal is needed. The platform should be capable of testing the PMUs behavior under transient conditions. One of critically important feature of the platform is that test signals must be synchronized. The required synchronization accuracy can be validated by simulating an arbitrary system transient and by using two different simulated signals with known 5

6 properties (e.g. amplitude, phase angle, frequency, etc.) in the testing procedure: two signals are forwarded to two different test devices, synchronized by GPS. The hardware should have the capability of achieving synchronism using GPS. The simplest way to check the quality of the synchronization is to generate simulated (and synchronized) test signals, and to check if they are also synchronized at the output of the test devices used for PMU testing. The phase difference of the signals in the measuring device can be compared with the phase difference of simulated signals. For the purpose of testing PMUs, a suitable library of computer simulated test signals was established. Such a library can be particularly important for a formal assessment of PMU transient properties. The next important element of the Laboratory for Synchronized Measurement is the library of signals obtained through a simulation of a power system. The simulation software tool used here is the ATP- EMTP. Here it is critical to simulate realistically transient processes in networks relevant for the assessment of PMUs. The test signals must be amplified in order to be used for PMU synchronized testing. In order to synchronize amplifiers, two sets of Omicron CMC-256 test set with GPS functions are used. Furthermore, the following measuring equipment used for basic testing are: laboratory oscilloscope and the National Instrument s Data Acquisition System. In Fig. a global block diagram of the synchronized testing is presented. Here two typical software packages (ATP-EMTP and Dig silent) are used for the purpose of system simulation. The simulated signals (voltages and currents) are transferred to the Omicron test set according to the Comtrade format. The Omicron test set was consisting of two GPS synchronized amplifiers, which amplified simulated signals and were forwarded to PMUs, Data Acquisition System and Oscilloscope. One of critical issues here is to check the accuracy of the synchronized laboratory setup from Fig. Each Omicron amplifier was used to amplify the simulated busbar voltage/current waveforms. Two sets of synchronized Omicron CMC-256 are here used. The synchronization was achieved by using Omicron CMGPS system, which can synchronize the internal time of the both test sets, thus achieving the PMU functionality. The file format that was imported into Omicron CMC-256 is COMTRADE, created by ATP-EMTP. The COMTRADE is the standard format for transient data exchange. Fig. - Global block diagram of testing Implementations 1)The Bonneville Power Administration (BPA) is the first utility to implement comprehensive adoption of synchrophasors in its wide-area monitoring system. Today there are several implementations underway. 2) The FNET project operated by Virginia Tech and the University of Tennessee utilizes a network of approximately 80 low-cost, high-precision Frequency Disturbance Recorders to collect syncrophasor data from the U.S. power grid. 3) In 2006, China's Wide Area Monitoring Systems (WAMS) for its 6 grids had 300 PMUs installed mainly at 500 kv and 330 kv substations and power plants. By 2012, China plans to have PMUs at all 500kV substations and all powerplants of 6

7 300MW and above. Since 2002, China has built its own PMUs to its own national standard. One type has higher sampling rates than typical and is used in power plants to measure rotor angle of the generator, reporting excitation voltage, excitation current, valve position, and output of the power system stabilizer (PSS). All PMUs are connected via private network, and samples are received within 40 ms on average 4) The North American Synchrophasor Initiative (NASPI), previously known as The Eastern Interconnect Phasor Project (EIPP), has over 120 connected phasor measurement units collecting data into a "Super Phasor Data Concentrator" system centered at Tennessee Valley Authority (TVA). This data concentration system is now an open source project known as the open PDC Applications 1) Power system automation, as in smart grids 2) Load shedding and other load control techniques such as demand response mechanisms to manage a power system. (i.e. Directing power where it is needed in real-time) 3) Increase the reliability of the power grid by detecting faults early, allowing for isolation of operative system, and the prevention of power outages. 4) Increase power quality by precise analysis and automated correction of sources of system degradation 5) For power system voltage stability. 6) Power system operation control, monitoring and planning. 7) Application of PMUs for fault detection and protection against fault. 8) Wide Area measurement and control, in very wide area super grids, regional transmission networks, and local distribution grid. Conclusion A PMU is a state of the art tool that has already proven that when used correctly and within its known limitations, it can help solve some of the existing problems and give us a better understanding of the overall behavior of power systems. Implementation of phasor measurement technology requires investment and commitment by utilities and system operators on an enterprise system level. The investments include: studies, equipment purchase and upgrade, maintenance, resource allocation and training. For utilities and system operators to make a step toward systemwide implementation of phasor measurement technology, it is desirable to identify and select key applications that would benefit the individual systems and the interconnected grid overall. Smart grid benefits for Advanced smart metering, high power quality, accommodates generation options, load adjustment, wide area measurement and control with PMUs and SCADA system, consumer participation, Demand response support, cyber security and many more for fulfilling consumers demand. Synchrophasor technology has the potential to greatly improve operators ability to conduct real time grid operations and detect and respond to potential disturbances. Phasor systems and data will help operators and planners to improve accuracy. The optimal PMU Placement decreases number of PMUs that reduces cost of system. Using PMU in smart grid increases reliability of power system stability. Therefore it is possible to monitor the power system observability by using PMU. References [1] Optimal Multistage Scheduling of PMU Placement: An ILP Approach 7

8 Devesh Dua, Sanjay Dambhare, Rajeev Kumar Gajbhiye, IEEE, and S. A. Soman [2] Phasor Measurement Units: Functionality And Applications D. THOLOMIER, H KANG, B CVOROVIC AREVA T&D(CAN), AREVA T&D (UK) [3] Setup of the Laboratory for Synchronized Measurement for PMU s Testing Vladimir Terzija, Senior Member, IEEE, Shawn Shihao Wu, MIET, John Fitch [4] Applications of phasor measurement units (PMUs) in electric power system networks incorporated with FACTS controllers Bindeshwar Singh, N.K. Sharma, A.N. Tiwari, K.S. Verma, and S.N. Singh [5] Wide-Area Monitoring, Protection, and Control of Future Electric Power Networks Vladimir Terzija, Gustavo Valverde, Deyu Cai, Pawel Regulski, Vahid Madani, John Fitch, Srdjan Skok, Miroslav M. Begovic, and Arun Phadke [6] New Smart Grid Applications for Power System Operations Anjan Bose, Fellow, IEEE [7] Communication Needs For Wide Area Measurement Applications A.G. Phadke, Life Fellow IEEE, and J.S. Thorp, Life Fellow, IEEE [8] Optimal Placement of PMUs for Power System Observability Using Topology Based Formulated Algorithms B. Mohammadi-Ivatloo 8