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Procedia Technology 25 ( 2016 ) 718 – 725

Global Colloquium in Recent Advancement and Effectual Researches in Engineering, Science and Technology (RAEREST 2016) Wide Area Measurement Technology in Power Systems

R.B. Sharmaa*, Dr. G.M.Dholeb

aGovernment college of Engineering ,Chandrapur-444203,India bR.H.Sapat College of Engineering, Management Studies and Research, Nashik-422005,India

Abstract

Deregulation, competitive , expansion of transmission and distribution network, reliable electricity supply, increased applications of power electronics make the power system more complex. Therefore, many times power system has to operate closure to their maximum limits so as to use existing assets more efficiently. To operate under these constraints and still maintain high reliability is a very challenging task for power system generation, operation and planning requiring new intelligent system, technology and solutions. For decades, traditional supervisory control and data acquisition system (SCADA) measurements have been providing information on bus voltage, transmission line, generator, and flows, transformer taps, breaker status as well as others system parameters. However, monitoring, operate and control of such large power system grid only steady state information may not be sufficient. Synchrophasor measurement technology (Wide area measurement technology) is a new term, which provides dynamic view of the grid. This paper introduces the role and advantages of using wide area measurement technology by answering three fundamental questions: what is wide area measurement technology; why is it needed and where does it best fit.

©© 2016 2015 The The Authors. Authors. Published Published by byElsevier Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAEREST 2016. Peer-review under responsibility of the organizing committee of RAEREST 2016

Keywords:Synchrophasor measurement technology;synchrophasor; ; Supervisory control and data acquisition system(SCADA); phasor data concentrator; Synchrophasor standard

* Corresponding author. Tel.: 07172-227274; fax: 07172-227334. E-mail address : [email protected], [email protected]

2212-0173 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAEREST 2016 doi: 10.1016/j.protcy.2016.08.165 R.B. Sharma and G.M. Dhole / Procedia Technology 25 ( 2016 ) 718 – 725 719

1. Introduction

In last two-three decades, power industry have been deregulated, expanding transmission and distribution network day by day to serve the increasing energy demand of the nation, and increased applications of power electronics devices make the power system more complex. As a result of these changes power industry is constantly facing challenges and contingencies such as whispered blackouts, which led to huge economic losses [1-3]. The requirements of a proper power system operation cannot be accomplished without a supervisory control and data acquisition system (SCADA) [4].For decades, traditional SCADA measurement has been providing power system information to system operators. These measurements are typically updating once every 4 to 10 seconds offering a steady state view of the power system behavior. However, for monitoring, operation and control of such large grid only steady state information is not being sufficient [5, 6].As a consequence of these challenges, new intelligent system should be introduced and established in the power system in order to tackle such challenges. Synchronize measurement technology (Wide area measurement system ) is a new term considered to be one of the most important technologies in the future of power systems due to its unique ability to sample analogue voltage and current waveform data in synchronism with a Global positioning system (GPS) clock and compute the corresponding frequency component from widely dispersed locations [7]. Recently, they are commercially available for purpose of monitoring, operation and control [8-12]. Phasor measurement units (PMUs) are the most widely used synchronized measurement devices for power system applications. Measurements from PMUs are obtained from widely dispersed locations, synchronized with respect to a GPS clock. This allows the direct measurement of voltage, phase angles and increases significantly the accuracy and speed of energy management system applications [13-17].PMU technology maturating rapidly, either as standalone PMUs, or with integrated PMUs. Synchrophasor measurement technology (SMT) provides dynamic view of the grid along with information bus voltages, line, generator and transformer flows, as well as other system parameters. The signals measured by PMUs have been used in power system monitoring, operation and control. The objective of this paper is to provide an introduction to wide area measurement technology, with a focus on phasor, synchrophasor, phasor measurement unit (PMU), synchrophasor standards and number of synchrophasor technology applications. First phasor, synchrophasor is defined and how GPS provides the time tag is discussed. An overview of wide area measurement system is described then evolution of synchrophasor with synchrophasor standards is also presented. Two other essential questions are then answered. Why is synchrophasor technology needed? Where does synchrophasor technology fit best? Finally, this paper concludes with the importance of the wide area measurement technology in electrical networks.

2. What is synchrophasor technology?

Fig. 1. (a)Sinusoidal phasor representation ; (b) Wide area measurment system

The Stinmetz concept of phasor calculation [18] has evolved into calculation of real – time phasor measurements synchronized to an absolute time reference. A phasor is a complex number with magnitude and phase angle that is used to represent the magnitude of the sinusoidal signal under specific frequency at a specific point of time as shown 720 R.B. Sharma and G.M. Dhole / Procedia Technology 25 ( 2016 ) 718 – 725

in Fig.1 (a) the phase angle is the distance between sinusoidal peak of the signal and a specified reference point of time (such as t=0) and is expressed using an angular measure. The phasor magnitude is related to the amplitude of the sinusoidal signal. When the phasor measurement is time stamped against the coordinated Universal time (UTC) by GPS, it is called synchrophasor. This allows measurement in different locations to be synchronized and time aligned, then combined to provide a precise, comprehensive view of a region or interconnection. Usually GPS receiver is used to provide UTC time to synchronize the phasor measurement. Thus the synchrophasor estimation means phasor measurements that occurs at the same time is based on the same concept of the phasor estimation, with only difference that is calculated from data samples using UTC as the time reference for the measurement.

3. Wide area measurement system

Currently measurements are provided by the SCADA system. The SCADA system includes three components: the Remote terminal units (RTU), the communication links between RTUs and master stations or SCADA front end computers. The main function of the SCADA is receiving and processing the information, forming the real time bases and archives, displaying the information, documenting the data and finally solving the dispatching task. The RTUs collect various types of measurements from the field and are responsible for transmitting them to control centre [19, 20].In this case the measurements are analog and represent voltage amplitudes, injected power at a node or line power flows. During dynamic event on the network such as a load or topology change, time skew errors can occur on measurements provided by the SCADA system this is because the snapshot of measurements is collected over a few seconds by the SCADA system. With the invention and development of PMU based wide area measurement system (WAMS) as shown in Fig. 1 (b), which utilizes the one pulse per second time synchronized pulse provided by the GPS it is possible to measure both magnitude and phasor angle using phasor estimation algorithm in power system [21-33].The algorithm for phasor calculation is the key element that determines the quality of phasor measurements. Nowadays, most of the PMU manufactures adopt recursive DFT algorithm. The phasor estimation algorithm with better dynamic characteristics is still needed. From a practical point of view, handling large amount of data coming from number of PMUs requires development of a phasor data concentrator (PDC).The PDC is a node in a grid where phasor packets from number of PMUs or other PDCs is correlated and fed out a single stream to other applications. Using GPS time tag the PDC correlates PMU data and to create a system wide measurement set. The PDC provides various quality checks on received phasor data and inserts appropriate quality flags into the correlated data stream. PDCs may provide additional functions, like monitoring the overall measurement system and displaying or reporting the measurement performance and it can also provide specialised outputs, such as direct interface to SCADA/EMS. The data measured from PMU is sent to PDC using communication links which include telephone link, microwave and fibre optics cable. Among these communication links the optic fibre communication links have the least total communication time delay as per synchrophasor standards[34, 35]. These measurements can further utilised for wide area monitoring, protection and control applications for secure operation of the power system.

4. Evolution of synchrophasor technology and standards

Phasor technology has not evolved over a night; it has taken several other developments before phasor measurement technology could be developed as shown in Fig. 2. In early 1970 the invention of the microprocessor based symmetrical component distance relay (SCDR) [36, 37] which makes accurate real time measurement of symmetrical components of voltages and currents for a transmission line is the major milestone in the development of the PMU. The GPS technology developments in early 1980 led to the next step in PMU development by allowing the sequence of voltage and /or current measurements from across the wide area electrical network to be time stamped and synchronized measurements. PMU using the GPS technology was first invented in the power system research laboratory by Virginia Tech. In 1988 and first commercial PMU is developed in 1992 by Macrodyne technologies Inc.[38]. To demonstrate how synchrophasor can improve the accuracy in measurements, to examine differences between exact phasor values, calculations based on synchronized phasor measurements and based on traditional technology. Also in order to R.B. Sharma and G.M. Dhole / Procedia Technology 25 ( 2016 ) 718 – 725 721 achieve interoperability among PMUs made by different manufacturers it is essential that all PMUs perform to a common standard. The IEEE has published the standards for synchrophasors and its several revisions. A short account of the development of these standards is explained as below.

Fig. 2. Evolution of synchophasor technoloy and standards

4.1. Standard IEEE 1344-1995

The concept of synchronized phasor with the power system was introduced in the 1980s and the first synchrophasor standard, IEEE 1344 was introduced in 1995[39]. It was created to introduce synchrophasors to the power industry and set basic concepts for the measurement and methods for data handling. It introduced a PMU which is a device and estimates synchrophasor equivalent quantities for an AC input. It has also introduced synchronized measurement using precise timing sources, formalized an extension for IRIG-B which has been adopted by industry [40].

4.2. Standard IEEE C37.118-2005

IEEE C37.118-2005 standard provides a method to qualify the measurement, tests to make sure the measurement conforms to the definition, and error limits for the test. It also defines a data communication protocol, including messages formats for communicating this data in a real time system, explanation, examples, and supporting information are also provided. In the first major improvement, C37.118 -2005[41] added a method for evaluating a PMU measurement and requirements for steady-state measurement. Total vector error (TVE) compares both magnitude and phase of the PMU phasor estimate with the theoretical phasor equivalent signal for the same instant of time. TVE provides an accurate method of evaluating the PMU measurement. Second C37.118-2005 expanded the communication method to include higher order collection and improved identifications. The basic status was improved to include indications of data quality.PMU identification was also added to all messages. The concept of PDC which included data from several PMUs introduced. Data type and classes were indentified. The standard C37.118-2005 has been very successful. It is pointed out that in IEEE C37.118-2005 only steady state performance tests are described and dynamic measurements were not addressed in it. In addition, frequency measurements have always been a part of the data reporting, but the standard has no requirements for them. These issues and the communication compatibility with standard IEC61850 [42] need to be addressed. 722 R.B. Sharma and G.M. Dhole / Procedia Technology 25 ( 2016 ) 718 – 725

4.3. Synchrophasor Standards IEEE C37.118.1-2011 and IEEE C37.118.2-2011

The original synchrophasor standard was IEEE 1344-1995. It was replaced by IEEE Std C37.118-2005 in which the test signals are held constant in magnitude, angle and frequency during each test. In general, power system voltage and current waveforms are not static sinusoids. The working groups of IEEE are updating the old standards with adding the dynamic performance requirements. This has now been split into two standards, IEEE Standard 37.118.1-2011, covering measurement provisions, and IEEE Standard 37.118.2-2011 covering data communication. Both the standards contain the previous material with updates and additional provisions.

4.3.1 Standard IEEE C37.118.1-2011

In this standard, additional clarification is provided for the phasor and synchronized phasor definitions [43]. The concepts of total vector error (TVE) and compliance tests are retained and expanded, tests over temperature variation have been added, and dynamic performance tests have been introduced. In addition, limits and characteristics of frequency measurement and rate of change of frequency (ROCOF) measurement have been developed.

4.3.2 Standard IEEE C37.118.2-2011

IEEE Standard 37.118.2-2011 describes the data communication requirements [44]. A method for real-time exchange of synchronized phasor measurement data between power system equipment is defined. This standard specifies messaging that can be used with any suitable communication protocol for real-time communication between phasor measurement units, phasor data concentrators, and other applications. It defines message types, contents, and use. Data types and formats are specified. A typical measurement system is described; communication options and requirements are also described. In 2014 an amendment of the standard, called IEEEC37.118.1a-2014 was published in order to fix some inconsistencies and to relax some constrains difficult to meet [45].

5. Why synchrophasor measurement technology

Since last three decades, the traditional current and voltage were widely installed worldwide by power utilities. These devices descend/ provide the voltage and current values from hundreds of thousands of volts and several amperes that can be measured directly. At the same time the traditional SCADA system is used to measure the real time bus voltage/ current magnitude, phase angle and power flows. The control and protection decision were made upon these measurements. The SCADA system obtains these measurements using RTUs though these units have enough accuracy level. There are major shortcomings in using these devices. Firstly, these devices are installed at different places all around a power system, the control centre collects, and long information gathered by the field sites, display information to HMI, and generates actions based upon detected events. So far devices at different locations, the time for signals to send forward or back were different. Secondly without a reference signal, it is hard to obtain the simultaneous measurements even though all precautions were carefully considered. Thus the calculations based on these measurements, may not be accurate as they were supposed to be. The advantages of PMU are depicted below; x Current technology only allows for direct measurements of the voltage and current magnitude, but not the phase angle. Phase angles are calculated by a computer running state estimation software. These calculated angles are not always the most accurate because of computer processing delays errors of the state estimator algorithm. PMU produce accurate time stamped measurements of voltage and current magnitude as well as phasor angles. x Another advantage of using synchrophasors is calculated quickly. Most PMUs can calculate up to 30 to 60 samples /cycle with the GPS time stamp provided by GPS hardware that is accurate to millisecond or better. In contrast, SCADA system scan in the range 2 to 4 samples/second range. x One of the major advantages of using synchrophasors the ability to provide coherent data from different parts of the network. System engineers and operators require knowing the trends of voltage phase angle differences among coherent groups of generators and major interconnections to monitor the stability of the power system. R.B. Sharma and G.M. Dhole / Procedia Technology 25 ( 2016 ) 718 – 725 723

The phase angle difference also provides knowledge of the system to operators on the available power transfer margin. The asynchronous and slow rate of SCADA system does not provide sub second power system information to state estimator. Therefore, a SCADA/EMS does not provide the dynamic state measurement of the power system. x PMU measurements offer solutions to number of critical power system problems. These include the protection of multi terminal lines, protection of series compensated lines.

6. Where does it fit best?

As power system expands and develops more and more, the dynamic behaviour of the system introduces more and more difficult operation, generation and control problems. The traditional SCADA/EMS systems are based on steady state power flow analysis whose measurement data resolution are in order of seconds, and consequently are insufficient to capture the fast power system dynamics. Today synchrophasors are no longer an academic interest or curiosity; synchrophasors moved beyond the demonstration project and have left the laboratory. The rapid progress of synchrophasor technology greatly promotes many applications of electrical power system. There are many currently implemented and proposed future applications of synchrophasor technology. The major applications reported the literatures like IEEE transactions, CIGRE reports, conferences and various catalogues of the vendors can be classified into three categories power system monitoring (WAMPS), power system protection (WAPPS ), and power system control(WAPSC) in real time and off-line situations as shown in fig.3.

Fig. 3. Synchophasor technoloy applications

6.1. Wide area Power system monitoring(WAMPS)

The state of the power system is defined as the collection of the positive sequence voltages at all the network buses obtained simultaneously. State estimation plays a key part in the real time monitoring and control of power systems. The technology of state estimation currently in use was developed in 1970s and is based on unsynchronized measurements .Due to the low scanning rates and relatively slow computations present technology is incapable of providing information about the dynamic state of the power system. The synchronized phasor measurements provide a completely new opportunity to re-cast the entire state estimation process [46, 47].

6.2. Wide area Power system protection(WAPPS)

The Synchronized phasor measurements offer solutions to a number of complex protection problems. In general, phasor measurements are particularly effective in improving protection functions, which have relatively slow response times. The existing protection system has redundant primary protection coupled with multiple backup schemes. The resulting system is highly dependable in that virtually every fault is ultimately cleared. This involves equipment and system protection as well as remedial action schemes, an additional protection area where phasor 724 R.B. Sharma and G.M. Dhole / Procedia Technology 25 ( 2016 ) 718 – 725

measurements can play a role is that of adaptive security and dependability, such as out of step relays, line relays, and reclosing. Promising application of PMUs is accurate measurement line impedance a key input for fault location and clearing applications. PMUs could also be used for direct fault calculation using data for both ends of transmission line [48-50].

6.3. Wide area Power system Control(WAPSC)

Prior to the introduction of phasor measurements all control in power system used local measurements and a mathematical model of the larger system. It was recognized that such controllers were rarely optimum, and could produce completely unacceptable responses to system phenomena when the models were inaccurate. The idea of using phasor measurements to improve such control of power system elements has been investigated for a number of years [51-53]. Typical applications were to problems where the control objectives were global in nature: forexample an HVDC controller may be called upon to damp electromechanical oscillations between two widely separated areas of a power system. Synchronized measurements offer a unique opportunity to bring in remote measurements of system state vector to be controller, and thus remove from control loop the uncertainty associated with the mathematical model. Thus, the controller becomes primarily feedback-based, rather than model-based, in its implementation.

7. Conclusion

The purpose of phasor measurement is to produce a simplified representation of power system signals that are accurately represents the actual system status. This paper has introduced wide area measurement technology known as synchrophasor technology. By answering the questions “what is synchrophasor technology”, “why is it needed”, and “where does it fit best”, the reader is better prepared to understand how synchrophasor technology can contribute to present and future power system online and offline applications. With the growing interest in Phasor technology (PMUs) throughout the world, it is clear that these systems will be implemented in most major electrical networks.

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