Table of Contents

Abstract..........................................................................................................................................3
1.0 Introduction .............................................................................................................................4
2.0 History…………………….................................................................................................................5
3.0 Measurement Techniques .......................................................................................................6
4.0. Phasor Measurement Units.....................................................................................................8
4.1. Standards……………………………………………………............................................................10 4.1.1. COMTRADE Standard - IEEE C37.111……………………………………………………….11 4.1.2. Synchrophasor Standard -IEEE C37.118…………………………………………………..11
5.0. Optimization - Spreading and Placements.............................................................................12
5.1. Applications - State Estimation .................................................................................13
5.2 Applications - Control and Protection.........................................................................14
6.0. Conclusions............................................................................................................................16
References……………………………………………………………………………………………………………………………….17

ABSTRACT
Phasor Measurement Unit (PMU) is an emerging technology in Electric Power Systems which has many advantages in Wide Area Disturbance Analysis, Control and Protection. Phasor Measurement Unit technology provides phasor information (both magnitude and phase angle) in real time. The advantage of referring phase angle to a global reference time is helpful in capturing the wide area snap shot of the power system. Effective utilization of this technology is very useful in mitigating blackouts and learning the real time behavior of the power system. This paper will summarize the overview of the history of the PMU, the measurement techniques of the PMU, the basic of the PMU diagram, current PMU standards and basic applications of the PMUs.

1.0 INTRODUCTION
The origins of today’s Phasor Measurement Systems can be traced as early as 1970s when the first Symmetrical Component Distance Relay (SCDR) was developed. The microcomputers in those decades did not meet the required classifications of a distance relay algorithm’s computations. The invention of SCDR was first intended to resolve the computational effort. By applying symmetrical components of voltages and currents, the SCDR was able to convert the six fault equations of a three phase power systems into a single equation [1]. One development was the Symmetrical Component Discrete Fourier Transform (SCDFT). In particular, it became apparent that a technique for measuring positive sequence voltages and currents very accurately and with measurement response time of one cycle of the fundamental frequency was of great interest in other applications, provided that the measurements could be synchronized across the power system [1]. This led to the next stage of development: synchronization of the sampling clocks used in sampling voltage and current signals. However, as the computer technology advances, the innovation of line relaying is no longer necessitated. Yet, the thought and analysis of SCDR utilizing symmetrical components led to many other Power System applications: load flow, stability, short circuit, optimum power flow, state estimation, contingency analysis, etc.
In the mid-1980s, the first Phasor Measurement Unit (PMU) prototype was developed in the Power System Research Laboratory at Virginia Tech as GPS satellites were being deployed in significant numbers in early 1980s. It became clear that by using GPS time signals as inputs to the sampling clocks in the measurement system of digital relays, one would have a very powerful measurement tool, which would be able to provide instantaneous picture of the state of the power system. Since then, the PMUs’ ability to accurately and instantaneously calculate synchronized phasors of voltages and currents has promoted their persistent propagation in power systems throughout the world. More and more applications are being researched, studied, and implemented to meet measurement, protection, and control requirements in the increasingly stressed market-deregulated power systems.

2.0 HISTORY
Wide-area measurements in power systems have been used in EMS functions for a long time. Economic Dispatch, tie line bias control, etc. all require wide area measurements. However, the birth of modern wide-area measurement systems can be traced back to a very significant event which took place in 1965. The 1965 catastrophic failure of the North- Eastern power grid in North America can be said as a trigger of the modern EMS systems based upon state estimators [1]. Numerous numbers of researches were conducted in techniques for determining the state of the power system in real time based upon real time measurements. By then, the feasibility of achieving synchronized measurements was far and beyond; yet, a technique was developed in which the measurements were done by sequential scan. The state of the power system was estimated by a non-linear state-estimator [1.] Then, the first PMU was born in Virgina Tech (1982-1992) utilizing the GPS transmissions to synchronize the sampling clocks in an effort for the collected phasors data to have a common reference point. AEP, DOE, BPA, and later NYPA funded the development, and the first assembled prototype units were installed on the BPA, AEP, NYPA systems [1]. Due to the incomplete deployment of early GPS satellites, early PMUS relied on precise, yet expensive, internal crystal clocks to keep time accurately. Therefore, one can say early PMU developments were heavily dependent upon the limitation of those of GPS system. Once the GPS system advanced and the number of satellites boomed around the globe, today’s PMUs only require a chip set of the GPS receivers to stamp time upon the collected data.
Figure 1: First experimental PMUs developed at Virgina Tech

3.0 MEASUREMENT TECHNIQUES
Conventionally, the state of the power system is estimated using system models and traditional measurements of power flows, current magnitudes, and voltage magnitudes. At least several seconds are required for the estimator to process the raw measurements before the result of system state is available. On the contrary, PMUs are able to provide immediate state of the buses they are installed on and, if the system parameters are accurately known, calculate in real time the state of neighboring buses through one simple linear step [2]. The PMUs’ ability of real-time system state measurement can be attributed to the algorithms for phasor calculation from sampled data implemented in the unit. Discrete Fourier Transform (DFT) is the most commonly known methods for phasor calculation [2]. A simplified phasor calculation can be obtained from the following equation: where, X is the calculated phasor; N is the total number of samples in one cycle; and Xk is the kth sample of waveform.

Figure 2: Estimation of phasors from sampled data using Discrete Fourier Transform.

A computationally more efficient method to calculate phasors is to recursively compute phasors by adding the new sample to, and discarding the oldest sample from, the data set. With the recursive procedure, only two multiplications need to be executed with each new sample point [2]. This simpler algorithm allows implementation with most digital devices currently used in power system as long as time synchronization is performed. This increased availability of synchronized data with little additional cost has a great potential to simplify and improve real-time analysis programs, such as power system state estimation and adaptive relaying.

Figure 3: Recursive estimation of phasors from sampled data with moving window DFT.

4.0 PHASOR MEASUREMENT UNITS
The ability to synchronize measurements system-wide is an innovative feature that sets PMUs apart from traditional measurement devices. Traditional measurements (real and reactive powers, voltage magnitudes, and current magnitudes) have very limited time synchronization and the time skews among measurements vary according to devices, distances, and communication channel conditions between substations and data centers. In the past, various communication systems, like leased lines, optical fibers, microwave, or AM radio broadcasts, have been considered for synchronization of measurement devices. However, most of them failed to provide high enough precision signals or to be reasonably economical.
With the success in GPS, modern PMUs use precise time signals. In fact, the modern PMUs use one pulse per second signals provided by the GPS satellite receivers. Therefore, GPS satellite transmissions are utilized as the preferred method of achieving synchronization of sampling clocks in PMUs. GPS timing is so precise that the timing pulse accuracy is better than 1µs [2]. One microsecond in a 60 Hz system corresponds to an angle error in the measured phasor of less than 0.02 degrees, which is more precise than what is required by most advanced power system applications. Now, the measurements through the clock will be simultaneous for the purpose of estimation and analysis of the power system state. Figure 4 below shows how the two voltages of the two far apart substations can be compared on the same phasor diagram thanks to the synchronized accurate time stamp.

Figure 4: Synchronizing the sampling processes for different signals of miles apart and putting their phasors on the same phasor diagram.

A functional block diagram of a PMU is provided in figure 5. The sampling clock pulses are created in a phase locked loop by using the GPS 1 pps signal [2]. The generated sampling clock pulses are in turn used for sampling the analog signals. As mentioned previously, phasors of phase voltages and currents are computed by the recursive DFT algorithms. The calculated phasors are combined to form positive sequence measurements. The microprocessor tags the resulted data with the timing information provided by the clock and the Second of Century (SOC) count provided by the GPS receiver. Lastly, all the information packages are sent via either the moderns or other communication means to the data center.
Figure 5: Functional block diagram of the elements in a Phasor Measurement Unit. The general structure is similar to many power system relays and digital fault recorders.

4.1 STANDARDS
So far, two IEEE standards have been established associated with the phasor measurement units. The first one is known as COMTRADE which is for a general transient data recording file format. The second is the standard applicable to the PMU technology named as SYNCHROPHASOR.
4.1.1 COMTRADE – IEEE STANDARD C37.111
COMTRADE (Common Format for Transient Data Exchange) standard is also referenced as IEEE C37.111. This is originated from a working group report of CIGRE study committee SC34. This standard is designed for the playback of digital fault recorder data to analyze relay response to power system disturbances. The use of COMTRADE files for protection testing is still limited due to the users' lack of familiarity with the standard. Nonetheless, they provide the user a means to playback the data to the relay and provide sufficient power to perform the tests. With standard data format and modern test instruments, the user is now able to test the complete protection scheme with the playback of actual power system events at the user's location.
4.1.2 SYNCHROPHASOR – IEEE STANDARD C37.118
SYNCHROPHASOR standard is also known as IEEE Standard C37.118. This standard was actually initiated by the PMU users in order to have inter-operability of PMUs supplied by the different manufactures. Formulated by the Power System Relaying Committee of IEEE Power & Energy Society, the standard specifies the protocol for communicating the PMU data to the Phasor Data Concentrator (PDC) [3]. The recent IEEE C37.118 standard on the synchrophasors outlines certain stringent requirements in terms of how to precisely measure the phase angle with respect to the global time reference – the coordinated universal time (UTC), and how to report the phasor information. The standard also specifies the Total Vector Error (TVE) allowed in evaluating the phasor for different compliance level to allow interoperability between different vendor PMUs. [4]

5.0 OPTIMIZATION - SPREADING and PLACEMENT
As a new class of measurement, synchronized phasor measurements greatly elevate the availability as well as the quality of data and information useful to improve system monitoring, protection, control, and operation of the increasingly stressed power systems. Though currently, the number of PMUs installed in the system is not large enough to make significant differences, the many power utilities have acknowledged the importance of PMUs to prevent any future blackouts; hence, such utilities and consultants are endeavoring with research institutions to search for the practical PMU installation strategies.
PMU placement strategies have been studied in the past to meet the requirements of specific applications. Different studies have been performed and different algorithms are tested for optimal placement in fact. However, PMU placement algorithms are only beneficial and useful to most monitoring and control purposes applications, but not other applications. Though widely installing PMUs on every bus will allow almost all possible application to be implemented, this installation strategy will require a major economic undertaking; thus, deploying on all buses of the system instantaneously is economically unsound or unwise. Nonetheless, at least 1/4 to 1/3 of the system buses must be equipped with PMUs to achieve complete observability of the grid with pure phasor measurements [5]. Therefore, the intention for accelerating the PMU proliferation also stimulates the research of the applications that make the most of the synchronized phasor measurements, among which, state estimation is the ones that may achieve the greatest positive impact from PMUs.

5.1 APPLICATIONS – STATE ESTIMATION
The concept of state estimation is to produce the best possible estimate of the true state of the system using the available imperfect information. After being proposed and introduced by Fred Schweppe in the late 1960’s, state estimation broadened the capabilities of Supervisory Control and Data Acquisition (SCADA) systems by taking advantage of the measurement redundancy [7]. Now, all Energy Management Systems (EMS) are equipped with a State Estimator (SE) to provide the latest information on the operating state of the power system. The techniques that evolved depended upon measuring active and reactive power flows and voltage magnitudes at substations, and then communicating them to a central site for processing. This is still the technology in use today in most power systems. The fact that the data is scanned over a considerable period (seconds to minutes) means that the calculated state is at best an approximation to averaged system state rather than the actual real time state of the system [7]. The estimates that are produced are referred to as “Static State Estimates”.
Synchronized phasor measurements of positive sequence bus voltages (and currents) directly, are a natural solution for state estimation or state measurement applications. If there were no existing state estimation software in an EMS center, a PMU only system would be a logical choice. Positive sequence voltage and currents lead to a linear state estimator [7].
One significant advantage or feature of the state estimation the PMU can offer is that it is not necessary to have the completely network before the state estimation can be performed. Very often, it is possible that the PMUs are installed at key locations of the network, and the measurements can be performed either directly or indirectly of the full network.

Figure 6: Current measurements used to provide indirect voltage measurements.[6]

5.2 APPLICATIONS – CONTROL and PROTECTION
Before the birth of phasor measurements, all control in power systems were based on local measurements and some modeling of a larger system. However, such control system is considered as rarely optimum, and could result unpredictable and unacceptable responses if the models used were inaccurate. Once the PMUs come along, ways to improve controlling the power system components are researched and investigated, for example controlling HVDC terminals, FACTS devices and so on [8].
The idea of using phasor measurements does not stop at control devices, but continue to protection systems. One crucial protection area where phasor measurements can play a role is that of Adaptive Security and Dependability. Many of today’s power utilities follow redundancy methods where redundant primary protection is coupled with multiple back up schemes such as Local Breaker Failure Back-up schemes. Consequently, today’s system is very reliable and dependable in that virtually every fault will be cleared. However, the trade-off is that false trips will occur, and this could aggravate the disturbance in turn. One solution to this is using remote phasor measurements to determine the system balance during times of stress. The mechanism is to alter the relaying logic which normally lets any relay that “sees” a fault to trip the breaker to logic that demands a vote such as two out of three [8].

Figure 7: Adaptive control of security and dependability of a protection system.

Another advantage of using phasor measurements is adaptive Out-of-Step Relaying that it allows the system to predict the state of the system by itself in the case of out-of-setp protection schemes. Figure 9 shows how the two PMUs communicate over the fault and predict if the fault would result out-of-step based on the phase angle of the measurement. Figure 8: Adaptive Out-of-Step Relaying

6.0 CONCLUSION
In today’s age, Phasor Measurement Units (PMUs) are widely acknowledged as one of the most promising developments in the field of real-time monitoring of power systems. Since PMU measurements are taken at much higher speed than that of conventional technology, a detailed and precise scanning of the grid can be achieved. Each and every time-stamped measurement allows synchrophasors from different utilities to be time-aligned or “synchronized” and combined together providing a precise and comprehensive view of the entire interconnection. Synchrophasors also enable a better indication of grid stress, and can be used to trigger corrective actions to maintain reliability before the major collapse happens. In summary, great advancement in technology comes with the complexity in full implementation. Optimal placement or optimization of Synchrophasor PMUs is still being researched, tested and yet to be fully explored, but the contributions of PMUs are already realized, and can eventually lead to a more reliable and smarter power grid.
References
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[2] A. G. Phadke, "Synchronized phasor measurements in power systems," Computer Applications in Power, IEEE, vol. 6, pp. 10-15, 1993.
[3] “IEEE Standard for Synchrophasors for Power Systems”, IEEE C37.118 – 2005.
[4] K. Narendra, T. Weekes and M. Hydro, “Phasor Measurement Unit (PMU) Communication Experience in a Utility Environment,” ERLPhase Power Technologies Ltd.
[5] T. L. Baldwin, L. Mili, M. B. Boisen, Jr., and R. A. Adapa, "Power system observability with minimal phasor measurement placement," Power Systems, IEEE Transactions on, vol. 8, pp. 707-715, 1993.
[6]. R.F. Nuqui and A.G. Phadke “Phasor Measurement Unit Placement Techniques for Complete and Incomplete Observability, IEEE Trans. On Power Delivery, Vol. 20, No. 4, October 2005, pp 2381-2388.

[7] A. Abur, "Optimal Placement of Phasor Measurement Units for State Estimation,"
[8] R.D. Quint and M.K. Thomas “Implementation of an Adaptive Voting Scheme using synchronized phasor measurements,” Power Systems, 2012 Proceedings of IEEE