International Conference on Power, Signals, Controls and Computation (EPSCICON), 8 – 10 January 2014

SYNCHRONISED PHASOR MEASUREMENT UNITVipin Krishna R, S. Ashok, Megha G Krishnan

Department of Electrical EngineeringNational Institute of Technology, Calicut, Kerala

vipinkrishna.r@gmail.com, ashoks@nitc.ac.inmeghagkrishnan@yahoo.com

Abstract—With the advent of real time Phasor MeasurementUnits (PMUs), synchronised phasor measurements are possiblewhich allows monitoring of dynamic phenomena. PMU terminalsinstalled in proper nodes of power systems enable increasing thetransmission capacity. Also it improves the operational safety ofthe power system. The better measurement performance andsystem wide data for monitoring and presentation of powersystem dynamics provide the operator with real time phasorinformation for remedial actions. The design of a SynchronisedPhasor Measurement Unit (PMU) based on a Digital SignalProcessor is explained in this paper. The standard temporalreference of this system is generated with a signal of 1 pulse persecond (1PPS) from a global positioning system (GPS).Thealgorithm used for the implementation of proposed system isRecursive Discrete Fourier Transform (DFT) algorithm. Thesimulation for the proposed algorithm in LabVIEWsoftware is explained in this paper and the results obtainedafter simulation were also given. The developed PhasorMeasurement Unit (PMU) provides phasor information (bothmagnitude and phase angle) of a given signal in real time. Theinformation can be utilized to estimate and calculate the powersystem state.

Keywords— 1pps, CCStudio, DSP, GPS Receiver, DFT, Algorithm,Matlab, Synchronised Phasor Measurement Unit, LabVIEW.

I. INTRODUCTIONYNCHRONISED phasor measurements are becoming animportant element of wide area measurement systems used

in advanced power system monitoring, protection and controlapplications. Phasor Measurement Units (PMUs) are powersystem devices that provide synchronised measurements ofreal time phasors of voltages and currents. Synchronisation isachieved by same-time sampling of voltage and currentwaveforms using timing signals from the Global PositioningSystem (GPS) Satellite. Synchronised phasor measurementselevate the standards of power system monitoring, control andprotection to a new level.

A number of PMUs are already installed in several utilitiesaround the world for various applications such as adaptiveprotection, system protection schemes, and state estimation.One of the most important issues that need to be addressed inthe emerging technology of PMUs is site selection. The intended system application influences the required number ofinstallations. The cost of PMUs limits the number that will beinstalled although an increased demand in the future isexpected to bring down the cost. The placement sites are also

limited by the available communication facilities, the cost ofwhich may be higher than that of the PMUs. A judiciouschoice of PMU locations is necessary to meet the criteria ofcost and the intended PMU applications. PMUs become moreand more attractive to power engineers because they canprovide synchronised measurements of real-time phasors ofvoltage and currents. As the sole system monitor, stateestimator plays an important role in the security of powersystem operations. Optimal placement of PMUs in powersystems to enhance state estimation is a problem that needs tobe solved.

II. PHASOR MEASUREMENT UNITA. Phasors

Phasors are basic tools of ac circuit analysis, usuallyintroduced as a means of representing steady state sinusoidalwave forms of fundamental power frequency. Measuring thesevoltage phasors in real time allows operators to see andrespond to approaching grid stability problems. Even when apower system is not quite in a steady state, phasors are oftenuseful in describing the behavior of the power system. Forexample, when the power system is undergoingelectromechanical oscillations during power swings, the waveforms of voltages and currents are not in steady state andneither is the frequency of the power system at its nominalvalue. Under these conditions, as the variations of the voltagesand currents are relatively slow, phasors may still be used todescribe the performance of the network, the variations beingtreated as a series of steady state conditions.

Figure 1. A sinusoid and its representation as a phasor

Phasor is a quantity with magnitude and phase (with respectto a reference) that is used to represent a sinusoidal signal. A

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78-1-4799-3612-0/14/$31.00©20149 IEEE

International Conference on Power, Signals, Controls and Computation (EPSCICON), 8 – 10 January 2014

sinusoid and its phasor representation are illustrated in figure 1. The phase angle of the phasor is arbitrary, as it depends upon the choice of the axis t = 0. Note that the length of the phasor is equal to the RMS value of the sinusoid. Here the phase or phase angle is the distance between the signals 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. The word phasor indicates a measurement of both the signal magnitude and the angle. B. Phasor Representation

Consider a pure sinusoidal quantity given by

(1.1) ω being the frequency of the signal in radians per second, and being the phase angle in radians. Xm is the peak amplitude of the signal. The root mean square (RMS) value of the input signal is (Xm /2). Recall that RMS quantities are particularly useful in calculating active and reactive power in an AC circuit. Equation (1.1) can also be written as

(1.2) It is customary to suppress the term ej(ωt) in the expression above, with the understanding that the frequency is ω. The sinusoid of Eq. (1.1) is represented by a complex number X known as its phasor representation: (1.3) The phasor representation is only possible for a pure sinusoid. In practice a waveform is often corrupted with other signals of different frequencies. It then becomes necessary to extract a single frequency component of the signal and then represent it by a phasor. Extracting a single frequency component is often done with a Fourier transform calculation. In sampled data systems, this becomes the discrete Fourier Transform (DFT) or the Fast Fourier Transform (FFT). The phasor definition also implies that the signal is unchanging for all time. However, in all practical cases, it is only possible to consider a portion of time span over which the phasor representation is considered. This time span, also known as the data window, is very important in phasor estimation of practical waveforms.

Figure 2. PMU at two substations [8] Phasor technology is considered to be one of the most important measurement technologies in the future of power systems due to its unique ability to sample analog voltage and current waveform data in synchronism with a GPS clock and compute the corresponding 50 Hz phasor component (i.e. complex numbers representing the magnitude and phase angle of a 50 Hz sinusoidal waveform) from widely dispersed locations as shown in figure 2. This synchronised sampling process of the different waveforms provides a common reference for the phasor calculations at all the different locations.

Figure 3. Various phase angles The phase angle difference between two sets of phasor

measurements is independent of the reference. Typically, one of the phasor measurements is chosen as the reference and the difference between all the other phase angle measurements (also known as the absolute phase angle) and this common reference angle is computed and referred to as the relative phase angles with respect to the chosen reference as shown in figure 3.

C. Phasor Measurement Unit (PMU) The complex angle of a phasor depends on the frequency of

the waveform and an arbitrary time reference. Since the time reference for each phasor measured in a large system is arbitrary, the complex angle of each phasor measurement cannot be compared across the system and is used only for local protection and control. However, with the use of Phasor Measurement Units (PMUs) it is possible to make synchronised phasor measurements for system wide applications. PMUs can measure the phase angle at a bus directly and therefore directly measure the state of the network. Synchronised phasors have a fixed frequency (the nominal power system frequency) and the time reference is synchronised across the system, typically using global positioning system (GPS) technology. Synchronised phasor measurements supplement power system protection, metering, and control and can be applied to mitigate current system wide problems such as blackout prevention, visualization, reliability, standards monitoring, security assessment, and integration of renewable energy resources. PMUs are placed at points on the

International Conference on Power, Signals, Controls and Computation (EPSCICON), 8 – 10 January 2014

interconnected grid and the measured data is sent to a Phasor Data Concentrator (PDC).

D. Design of Phasor Measurement Unit

The whole system consists of two modules, the sensor module and the PMU module. The measurements and calculations are done for three phase system. Next is the communication part which uses serial communication to transmit data to the utility computer. The system structure of the PMU using the DSP and the GPS is shown in figure.4.

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Figure 4.Basic block diagram of PMU

1) Sensor Module: Sensor module constitutes the PTs (Potential Transformer) and CTs (Current Transformer) along with the GPS Receiver. The three-phase instantaneous values of voltage and current are inputted to the A/D converter though the PT (Potential Transformer) and CT (Current Transformer). There the analog LPF is used as antialiasing filter. The GPS receiver receives lPPS (Pulse Per Second) clock signal synchronising to the UTC with high accuracy from the GPS satellites. In such a way the synchronised measurement at multi-points in the power system can be realized.

2) PMU Module: The processor and all other electronic circuitry comes under the PMU module. The recursive DFT algorithm is used for reduction of computing time. The phasor calculation is done only by executing the MAD (Multiply and Add) operations, and the DSP is good for such calculation. Since the DSP is the high-speed signal processor, if necessary, other state variables can be computed in the each phasors every sampling intervals. DFT is a method of calculating the Fourier transform of a small number of samples taken from an input signal x(t). The Fourier transform is calculated at discrete steps in the frequency domain, just as the input signal

is sampled at discrete instants in the time domain. Recursive and nonrecursive algorithms can be applicable. Non-recursive is the best but in the computation point of view recursive is widely used.

3) Global Positioning System (GPS): The GPS system has 24 active satellites. The satellites describe orbits that let any point in the Earth to have line sight 24 hours a day with different satellites. The GPS system proportionate services as the position in geographical coordinates, the altitude above the sea, velocity and direction of a moving object, the magnetic derive in degrees, the time by the UTC (Universal Coordinated Time), the signal 1PPS and others[7].

Figure 5. Representation of the GPS satellite disposition These are arranged in six orbital planes displaced from each other by 60 and having an inclination of about 55 with respect to the equatorial plane. The satellites have an orbital radius of 16,500 miles, and go around the earth twice during one day. They are so arranged that at least six satellites are visible at most locations on earth, and often as many as 10 satellites may be available for viewing. The most common use of the GPS system is in determining the coordinates of the receiver, although for the PMUs the signal which is most important is the one pulse per second. This pulse as received by any receiver on earth is coincident with all other received pulses to within 1 microsecond. The figure 5 shows the representation of the GPS satellite disposition. There are four satellites in each of the six orbits, which orbit around the earth with a period of half a day. The GPS satellites keep accurate clocks which provide the one pulse per second signal. The time they keep is known as the GPS time which does not take into account the earth’s rotation. Corrections to the GPS time are made in the GPS receivers to account for this difference so that the receivers provide UTC clock time.

A terrestrial GPS receives the signal from one or more satellites (for example, to calculate the geographical coordinates it is required the reception of the signal from three satellites). Once the receptor detects the signal with good strength from at least one satellite, it decodes the UTC and transmits it in synchronisation with the 1PPS signal from each one of the instruments that measure the phasor (PMU) [7]. Each PMU uses the 1PPS signal to synchronise its own measurement system. The 1PPS signal has a TTL level and is generated in the GPS receiver system. By omission, each time the receiver is energized, the 1PPS signal is generated even if the receiver doesn’t detect the signal from a satellite. Once the

International Conference on Power, Signals, Controls and Computation (EPSCICON), 8 – 10 January 2014

receiver detects a satellite, the receiver synchronises the rise time of the 1PPS with the reception of the UTC. The 1PPS signal has a pulse width of 10 microseconds, the rise time takes usually less than 20 nanoseconds and a period of exactly one second 100 nanoseconds. In the other hand, UTC is synchronised with a reference from a Cesium atomic clock of high stability that is contained in each satellite. The clock of each satellite is synchronised with a master clock, so each satellite sends the UTC signal at the same time. For any application where a GPS is required for synchronisation, the rise time of the 1PPS signal has to be used because this is synchronised at each repetition of the UTC [7].

4) Processor: A DSP processor is used to calculate the phasor. Also DSP provides diagnostic monitoring with FFT of spectrum analysis effectively and enables enhanced real time algorithms. Since recursive computation is easy and non-recursive algorithm requires costly high end processors, DSP processor is selected for the calculation of phasor [9].

5) Standard Protocols: The latest PMU/PDC protocol is the IEEE C37.118 that was developed in the last few years and approved late 2005. It will replace the IEEE 1344 Synchrophasor protocol which has been in use as the PMU standard since its development in 1998. Before these standards were developed, the defacto standard for PMU to PDC communication has been the Macro dyne type 1 and type 2 protocols developed by Macro dyne Corporation. Some of the PDC to PDC protocols include the PDC data exchange format, the PDC stream, second level PDC using NTP time and the PDC stream, second level PDC using native time. These standards address issues like synchronisation of data sampling, data to phasor conversions, and formats for timing input and phasor data output [7].

III. DEVELOPMENT OF PMU A. Algorithm Developed

Figure 6. Algorithm

B. Simulation Of PMU

The simulation of proposed algorithm in LabVIEW software is done. The Figure 7 shows the simulation window of the phase measurement using LabVIEW software and figure 8 shows the front panel in LabVIEW. The input is given through National Instruments signal generation. The input is given through NATIONAL Instruments PC-6251 data acquisition (DAQ) board. The figure 9 shows the simulation setup for the Labview.

Figure 7. Block diagram in Labview

International Conference on Power, Signals, Controls and Computation (EPSCICON), 8 – 10 January 2014

Figure 8. Front panel in LabVIEW

Figure 9. Simulation setup

C. Simulation Results

The Simulation for the proposed model in Labview is done. The input is given through the DAQ which have a maximum values range from +10 to -10. A voltage of 9 is applied through the DAQ and the reading where noted for 100 samples. Figure.10 shows the Simulation test result of the phasor measurement for a single phase analog input given to the system.

Figure 10. Simulation Test result

IV. CONCLUSION

The algorithm for the development of Synchronised phasor measurement unit was developed and the simulation results were observed. The Synchronised phasor measurement unit has been implementing using Simulink model which have to be dumped in TMS320F2812 trainer kit using CCStudio software. The phasor measurement using the 1pps input is in the developing stage. The reference sine wave is generated using Simulink blocks and the calculation is done. The communication part is also in the developing stage.

REFERENCES

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[2] A.G. Phadke, "Synchronized Phasor Measurements in Power Systems", IEEE Computer Applications in Power, Vol. 6, NO. 2, April 1993, pp 10-15

[3] Yutaka Ota, Hiroyuki Ukai, Koich Nakamura, Hideki Fujita, ”Evaluation of Stability and Electric Power Quality in Power System by using Phasor Measurements”, in Proceedings 2000 International Conference Power System Tecnology (PowerCon2000), vol.3, December 2000, pp.1335-1340.

[4] R. O. Burnett, M. M. Butts, T. W. Cease, V. Centeno, G. Michel, R. J. Murphy, and A. G. Phadke, “Synchronized phasor measurement of a power system event,” IEEE Trans. Power Syst., vol. 9, no. 3, pp. 1643–1650, Aug. 1994.

[5] Katsuyasu Nakano, Yutaka Ota, Hiroyuki Ukai, Koichi Nakamura, and Hideki Fujita,” Frequency Detection Method Based on Recursive DFT Algorithm” Power Systems Computations Conference 14th, 24-28 June 2002, Sevilla .

[6] Megha G Krishnan, S Ashok, “Implementation of Recursive DFT algorithm for Phasor Measurement

International Conference on Power, Signals, Controls and Computation (EPSCICON), 8 – 10 January 2014

Unit (PMU)” IEEE International Conference on Engineering Education: Innovative Practices and Future Trends (AICERA 2012).

[7] IEEE Standard 1344-1995. “IEEE Standard Synchrophasors for Power Systems.”

[8] http://www.phasor-rtdms.com/phaserconcepts/phasor_adv_faq.html

[9] VI Microsystems PVT Ltd, “MICRO – 2812 Technical Reference Version 3.0”